==== Front Front Physiol Front Physiol Front. Physiol. Frontiers in Physiology 1664-042X Frontiers Media S.A. 10.3389/fphys.2020.543564 Physiology Review From Glucose to Lactate and Transiting Intermediates Through Mitochondria, Bypassing Pyruvate Kinase: Considerations for Cells Exhibiting Dimeric PKM2 or Otherwise Inhibited Kinase Activity Chinopoulos Christos * Department of Medical Biochemistry, Semmelweis University, Budapest, Hungary Edited by: Paolo Bernardi, University of Padua, Italy Reviewed by: Eric Fontaine, Université Joseph Fourier, France; Ajit Divakaruni, UCLA Health System, United States *Correspondence: Christos Chinopoulos, chinopoulos.christos@eok.sote.huThis article was submitted to Mitochondrial Research, a section of the journal Frontiers in Physiology 01 12 2020 2020 11 54356417 3 2020 02 11 2020 Copyright © 2020 Chinopoulos.2020ChinopoulosThis is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.A metabolic hallmark of many cancers is the increase in glucose consumption coupled to excessive lactate production. Mindful that L-lactate originates only from pyruvate, the question arises as to how can this be sustained in those tissues where pyruvate kinase activity is reduced due to dimerization of PKM2 isoform or inhibited by oxidative/nitrosative stress, posttranslational modifications or mutations, all widely reported findings in the very same cells. Hereby 17 pathways connecting glucose to lactate bypassing pyruvate kinase are reviewed, some of which transit through the mitochondrial matrix. An additional 69 converging pathways leading to pyruvate and lactate, but not commencing from glucose, are also examined. The minor production of pyruvate and lactate by glutaminolysis is scrutinized separately. The present review aims to highlight the ways through which L-lactate can still be produced from pyruvate using carbon atoms originating from glucose or other substrates in cells with kinetically impaired pyruvate kinase and underscore the importance of mitochondria in cancer metabolism irrespective of oxidative phosphorylation. cancerglycolysismitochondriametabolomicsWarburg effectoncometabolismlactate dehydrogenase ==== Body Glucose and Lactate in Cancer: Background It is a well-known fact that most cancers exhibit increased rates in glucose consumption (Bose and Le, 2018). This is clinically exploited by following radionuclide-labeled glucose analogs for the purpose of tumor imaging in living human beings (Feng et al., 2019). The very same cancers are also known to be major lactate producers, which is important for their survival (de la Cruz-Lopez et al., 2019). The combination of an increased consumption of glucose with an increase in lactate output led to the assumption that cancers exhibit an increase in glycolysis; although this is true, serving the purpose of generating glycolytic metabolites which are diverted toward biosynthetic processes (DeBerardinis et al., 2008) and NADPH by the pentose phosphate pathway (Icard and Lincet, 2012), most tumors express a dimeric form of the M2 isoform of pyruvate kinase which has been reported to be much less active than that found in healthy cells; furthermore, numerous posttranslational modifications and mutations have been reported for this gene product, leading to a much reduced activity but still fueling cancer aggression (see section “Pyruvate Kinase”). Even more so, tumor cells with undetectable levels of pyruvate kinase still producing lactate can be found in vivo (Israelsen et al., 2013). On one hand, the decrease in pyruvate kinase activity is important for maintaining a metabolite “traffic jam,” forcing upstream metabolites toward biosynthetic pathways; on the other hand, it points to a metabolic conundrum because L-lactate may only originate from pyruvate, a metabolite arising from phosphoenolpyruvate (PEP) through pyruvate kinase in glycolysis (see Figure 1). The purpose of this review is to highlight the pathways that can lead to pyruvate and lactate—even commencing from glucose—bypassing pyruvate kinase. This is important because (i) carbon-labeled atoms in glucose may appear in lactate without net ATP production from glycolysis and (ii) hints on the possibility that other pathways leading to pyruvate/lactate could be crucial for cancer cell survival that are perhaps amenable to pharmacological and/or genetic manipulation. The list of pathways appearing below has been assembled by mining the following databases: Kyoto Encyclopedia of Genes and Genomes1 (Kanehisa and Goto, 2000), BRaunschweig ENzyme Database2 (Jeske et al., 2019), Metabolic Atlas3 (Robinson et al., 2020), Biochemical, Genetic, and Genomic knowledge base4 (King et al., 2016), MetaNetX5 (Moretti et al., 2016), Human Metabolome Database6 (Wishart et al., 2018), and Virtual Metabolic Human7 (Noronha et al., 2019). FIGURE 1 Biochemical pathways connecting glucose or other metabolites to pyruvate and L- or D-lactate. The box in magenta represents a mitochondrion. Glycolysis is highlighted in green. Metabolites found both inside and outside the mitochondria that are not connected with an arrow are highlighted in matching striped colors (to avoid arrow clutter). For abbreviations, see Table 1. Pyruvate Kinase Pyruvate kinase generates ATP at the “substrate level” in the absence of oxygen by catalyzing the dephosphorylation of PEP to pyruvate (see Figure 1). There are four isoforms denoted as L, R, M1, and M2. For details regarding kinetic properties, tissue distribution, and regulation, the reader is referred to the review by Israelsen and Vander Heiden (2015). In the present review, the PKM2 isoform will be specifically examined; for a more thorough evaluation, the reader is referred to Li et al. (2014, 2018), Wong et al. (2015); Yang and Lu (2015), Dayton et al. (2016b), Hsu and Hung (2018), and Alquraishi et al. (2019). The non-enzymatic functions of PKM2 are examined elsewhere (Hoshino et al., 2007; Stetak et al., 2007; Luo et al., 2011; Yang et al., 2012; Yang and Lu, 2013). Basically, PKM2 exhibits lower enzymatic activity compared to that by PKM1 (Yamada and Noguchi, 1999) and is allosterically regulated by fructose-1,6-bisphosphate (FBP); it exists either as a dimer with low affinity for PEP or as an FBP-bound tetramer with high affinity for PEP (Mazurek et al., 2005; Zhang et al., 2019). Although PKM2 has been branded as “the predominant isoform in cancer cells” (Altenberg and Greulich, 2004; Mazurek et al., 2005), further scrutiny in 25 human malignant cancers, six benign oncocytomas, tissue-matched controls, and several cell lines showed that “PKM2 dominance was not a result of a change in isoform expression, since PKM2 was also the predominant PKM isoform in matched control tissues.” Therefore, a switch from PKM1 to PKM2 isoform expression during malignant transformation may not be taking place, as previously postulated (Christofk et al., 2008). Mindful of the controversy surrounding the proposed functions of PKM2 (Hosios et al., 2015; Harris and Fenton, 2019), the group of Vander Heiden characterized the effects of cancer−associated PKM2 mutations on enzyme kinetics and allosteric regulation and reported that a decrease in PKM2 activity supports the rapid proliferation of cells (Liu V. M. et al., 2020). This is in line with earlier reports showing that a decrease in PKM2 activity due to posttranslational modifications (Lv et al., 2011) or inhibition by oxidative stress (Anastasiou et al., 2011) promotes tumor growth (Prakasam et al., 2018). Alternatively, exposure to small molecule PKM2 activators or expression of the constitutively active PKM1 thwarts cancer cell proliferation (Anastasiou et al., 2012). Finally, it has been also shown that PKM2 is not even required for the growth of many cancers (Cortes-Cros et al., 2013; Israelsen et al., 2013; Wang et al., 2014; Lunt et al., 2015; Dayton et al., 2016a, 2018; Lau et al., 2017; Tech et al., 2017; Hillis et al., 2018). In aggregate, the consensus seems to be that the lower the pyruvate kinase activity, the greater the stimulation of tumor growth. As discussed in the section below entitled “Evidence Showing That Pyruvate Kinase Inhibition Does Not Lead to a Proportional Decrease in Pyruvate/Lactate Formation,” even those cells exhibiting low—or even undetectable—pyruvate kinase activity still produce lactate, which begs the question: where does this lactate come from? Evidence Showing That Pyruvate Kinase Inhibition Does Not Lead to a Proportional Decrease in Pyruvate/Lactate Formation In Cortes-Cros et al. (2013), it was shown that knockdown of both PKM1 and PKM2 (PKM2 knockdown was on the order of > 95%) leading to an approximately fivefold decrease in overall pyruvate kinase activity yielded only a ∼50% decrease in the appearance of 13C originating from glucose to lactate. In Chaneton et al. (2012), silencing of both PKM1 and PKM2 to an extent greater than 90% led to only a ∼30% decrease in pyruvate and lactate production, while PEP concentration increased by 100%. In Vander Heiden et al. (2010), it was shown that cancer cell lysates expressing no pyruvate kinase activity produced 50% of pyruvate from PEP compared with the total cell lysates. Although in this work it was postulated that phosphate from PEP is transferred to the catalytic histidine on human PGAM1, this claim was subsequently rejected by the same authors, attributing their earlier findings to contaminating ATP−dependent protein kinases (Hosios et al., 2015). In all of the abovementioned studies, it was assumed that, in view of severely diminished pyruvate kinase activity, pyruvate and lactate production is attributed to carbon sources other than glucose. Indeed Yu et al. (2019), determined that, in pancreatic ductal adenocarcinoma cells with PKM1 and PKM2 knockdown, cysteine catabolism generated ∼20% of intracellular pyruvate. The purpose of the present review is to not only outline these pathways but also show additional ways for obtaining 13C labeling in pyruvate or lactate originating from glucose; furthermore, since some of these pathways involve intermediates that transit through the matrix, the role of the mitochondria is emphasized, which is unrelated to the concept of oxidative phosphorylation. Pathways Leading to Pyruvate Commencing From Glucose: Intermediates Not Transiting Through the Mitochondria The pathways shown in this section refer to Figure 2 (lavender arrows). Multiple arrows imply multiple biochemical steps. FIGURE 2 Pathways leading to pyruvate commencing from glucose, highlighted in lavender: intermediates not transiting through the mitochondria. For abbreviations, see Table 1. (1) Glc + PEP → Glc-6-P + pyruvate: This reaction is catalyzed by glucose-6-phosphatase (G6PC) (Nordlie, 1974; Colilla et al., 1975) (for abbreviations, see Table 1). In humans, G6PC expression was reported to be elevated in GBM when compared with normal brain (Abbadi et al., 2014), while in rodent hepatomas it was found to be decreased (Weber and Cantero, 1955). TABLE 1 Abbreviations. 2-Oxoglrm 2-oxoglutaramate (a-ketoglutaramate) 2-PG 2-Phosphoglycerate 3-OH-pyr 3-hydroxypyruvate 3-PG 3-Phosphoglycerate 4-OH-proline 4-hydroxyproline 5-10 mTHF 5-10 methylene-Tetrahydrofolate ACLY ATP Citrate Lyase ACO Aconitase ADH Alcohol Dehydrogenase AGXT Alanine-glyoxylate Aminotransferase aKG a-ketoglutarate Ala Alanine ALXT Alanine-Ketomalonate Transaminase Aml Aminomalonate Asn Asparagine Asp Aspartate cADC cis-Aconitate Decarboxylase CLYBL Citramalyl-CoA Lyase CS Citrate Synthase CYB5D1 Cytochrome B5 Domain-Containing Protein 1 Cys Cysteine D2HGDH D-2-Hydroxyglutarate Dehydrogenase DAAO D-amino acid Oxidase D-LDH D-Lactate Dehydrogenase FAHD Acylpyruvase FH Fumarate Hydratase Fum Fumarate G6PC Glucose 6 phosphatase GALK N-acetylgalactosamine Kinase GalNAc N-Acetylgalactosamine GalNAc-1-P N-Acetylgalactosamine-1-Phosphate GAPDH Glyceraldehyde 3 Phosphate Dehydrogenase Glc Glucose Glc-6-P Glucose-6-phosphate Gln Glutamine Glu Glutamate GLUD Glutamate Dehydrogenase Gly Glycine Gly-3-P Glyceraldehyde-3-Phosphate GLYCTK Glycerate Kinase GOT Aspartate Aminotransferase GPAT Glutamine-Pyruvate Transaminase GPT Alanine Aminotransferase GRHPR Glyoxylate Reductase HAO Hydroxyacid Oxidase IDH Isocitrate Dehydrogenase Ile Isoleucine KGDHC a-Ketoglutarate Dehydrogenase Complex LAAO L-amino-acid Oxidase LDH Lactate Dehydrogenase Leu Leucine Mal Malate MDH Malate Dehydrogenase ME Malic Enzyme MGTK Methylglutaconase MPC Mitochondrial Pyruvate Carrier mTHF methyl-Tetrahydrofolate OAA Oxaloacetate Oml Oxomalonate PCK Phosphoenolpyruvate Carboxykinase PCK Pyruvate Carboxylase PDHC Pyruvate Dehydrogenase Complex PEP Phosphoenolpyruvate PGM Phosphoglucomutase PGPase 2-phosphoglyceric acid Phosphatase Phe Phenylalanine PHGDH Phosphoglycerate Dehydrogenase Php Phosphohydroxypyruvate PKM2 Pyruvate Kinase isoform M2 PL Phospholipids PPP Pentose Phosphate Pathway PSAT Phosphoserine Aminotransferase Pser Phosphoserine PSPH Phosphoserine Phosphatase pyr Pyruvate Q Quinone QH2 Quinol SDH Succinate Dehydrogenase SDS Serine Dehydratase Ser Serine SHMT Serine Hydroxymethyltransferase SUCL Succinate-CoA Ligase THF Tetrahydrofolate Thr Threonine TR-Pase Tartrate-resistant acid Phosphatase Tyr Tyrosine Val Valine (2) Glc →→→ methylglyoxal →→→ pyruvate: This may occur through four different routes involving aldehyde dehydrogenase 9, zinc binding alcohol dehydrogenase domain containing two [more recently renamed to prostaglandin reductase 3 (Yu et al., 2013)] and at least two oxoaldehyde dehydrogenases; for details, see Vander Jagt and Hunsaker (2003). Methylglyoxal has been reported to trigger metastasis in breast, anaplastic thyroid, and colorectal cancer (Chiavarina et al., 2017; Antognelli et al., 2019; Nokin et al., 2019). (3) Glc →→→ PEP → pyruvate: the terminal reaction is catalyzed by tartrate-resistant acid phosphatases (TRAP), the molecular identity of which remained unknown well after their biochemical characterization (Helwig et al., 1978; Chen and Chen, 1988; Hayman et al., 1989); they are most likely substantiated by a metalloprotein enzyme with the ability to catalyze the hydrolysis of orthophosphate monoesters under acidic conditions (Bull et al., 2002). The expression of this enzyme (TRAP) is a marker of bone disease in cancer patients (Nguyen et al., 1991; Koizumi and Ogata, 2002; Mose et al., 2003; Terpos et al., 2003; Chao et al., 2005). (4) Glc →→→ PEP; PEP + GalNAc → GalNAc-1P + pyruvate: Terminal reaction catalyzed by N-acetylgalactosamine kinase isoforms 1 or 2 (Pastuszak et al., 1996). These enzymes are implicated in many signaling pathways inherent to carcinogenesis (Zeidan and Hart, 2010). (5) Glc →→→ 3-PG → 2-PG (by phosphoglucomutase 1 or 2) → glycerate [probably through 2-phosphoglyceric acid phosphatase (Baranowski et al., 1968)] → 3-OH-pyr [by glyoxylate reductase (Mdluli et al., 2005)]; 3-OH-pyr + Ala (or glyoxylate) → Gly + pyruvate (or Ser): the terminal reaction is catalyzed by alanine-glyoxylate aminotransferase (Danpure et al., 2003). The mitochondrial isoform of the latter enzyme (alanine-glyoxylate aminotransferase isoform 2, AGXT2) has been reported to form glycine and pyruvate from alanine and glyoxylate; this reaction has been confirmed in normal tissues (Holmes and Assimos, 1998) and HepG2 cancer cells (Baker et al., 2004). The same reaction has been reported to take place in peroxisomes (Poore et al., 1997). On the other hand, loss of alanine-glyoxylate aminotransferase (AGXT) expression has been reported to accelerate the progression of hepatocellular carcinoma (Sun et al., 2019). A “futile cycle” may exist between 3-PG and glycerate through 2-phosphoglyceric acid phosphatase and glycerate kinase 1 and 2; glycerate kinase 2 is also found in the mitochondria (Guo et al., 2006). (6) Glc →→→ 3-PG → phosphohydroxypyruvate (Php), catalyzed by phosphoglycerate dehydrogenase; Php + Ala → phosphoserine (Pser) + pyruvate, catalyzed by phosphoserine aminotransferase (PSAT) (Hirsch and Greenberg, 1967): PSAT overexpression is associated with increased tumorigenicity in human esophageal squamous cell carcinoma (Liu et al., 2016) and colon carcinomas (Yoon et al., 2015) and a poor outcome on tamoxifen therapy in recurrent breast cancer (De Marchi et al., 2017); conversely, its selective loss suppresses migration, invasion, and experimental metastasis in triple negative breast cancer (Metcalf et al., 2020). (7) Glc →→→ 3-PG →Php (catalyzed by phosphoglycerate dehydrogenase); Php + Ala (or Glu) → Pser + pyruvate (or →Kg); the latter reaction is catalyzed by phosphoserine aminotransferase; Pser → Ser → pyruvate, catalyzed by serine dehydratase (Ogawa et al., 2006) or serine dehydratase-like (SDSL) (Ogawa et al., 2006). Notably, SDS was reported to be absent from human colon carcinomas (Snell et al., 1988). (8) Glc →→→ Glyoxal →→→ glyoxylate (Lange et al., 2012); glyoxylate + 3-OH-pyr (or Ala) → Gly + pyruvate (or Ser): the terminal reaction is catalyzed by AGXT (for considerations related to cancer, see pathway no. 5). (9) Glc →→→ 3-PG →Php (catalyzed by phosphoglycerate dehydrogenase); Php + Glu → Pser + →Kg; latter reaction catalyzed by phosphoserine aminotransferase; →Kg + Ala → Glu + pyruvate, catalyzed by alanine aminotransferase (GPT; for considerations related to cancer, see pathway no. 6). Pathways Leading to Pyruvate Commencing From Glucose: Intermediates Transiting Through the Mitochondria These pathways depend on one or more of three critical parameters: (1) glyoxylate entry into the mitochondria, (2) reversibility of the matrix phosphoenolpyruvate carboxykinase (PCK2), and (3) reversibility of the mitochondrial pyruvate carrier (MPC). Regarding glyoxylate, I was unable to find information on its transport across the inner mitochondrial membrane; however, it is known that it can be processed by the matrix-localized AGXT2 (Kakimoto et al., 1969). PCK2 expression and activity level are critical for many cancer types: in tumor-initiating enriched prostate cancer cell clones, PCK2 was overexpressed, and this correlated with more aggressive tumors and lower survival rates (Zhao et al., 2017); in lung cancer cell lines and in non-small cell lung cancer samples, PCK2 expression and activity were enhanced under low-glucose conditions (Leithner et al., 2015); finally, it was reported that PCK2 is required for glucose-independent cancer cell proliferation and tumor growth in vivo (Vincent et al., 2015). Regarding PCK2 reversibility, the enzyme has been shown to operate in the reaction toward OAA synthesis in mitochondria from rabbit liver (Carlsen et al., 1988), pigeon and rat liver (Wiese et al., 1996), guinea pig liver (Garber and Ballard, 1970; Garber and Salganicoff, 1973), rabbit enterocytes (Wuensch and Ray, 1997), chicken liver (Hebda and Nowak, 1982; Makinen and Nowak, 1983; Wilson et al., 1983; Erecinska and Wilson, 1984), and bullfrog liver (Goto et al., 1980). However, in Vincent et al. (2015), it was shown that a fraction of pyruvate originated from glutamine from PEP through PCK2. With respect to the reversibility of the MPC, this is a working hypothesis because there are no data showing pyruvate release from normally polarized mitochondria. Nevertheless, this is not a far-fetched hypothesis: succinate and other metabolites are effluxed from the mitochondria for non-metabolic roles against a hyperpolarized membrane potential (Mills et al., 2016), demonstrating that this is possible under appropriate conditions. It may be also relevant that pyruvate catabolism through the pyruvate dehydrogenase complex is associated with suppression of tumor growth in vitro and in vivo (Michelakis et al., 2008); relevant to this, genes coding for both the pyruvate dehydrogenase complex and pyruvate carboxylase in certain cancers are usually downregulated (Yuen et al., 2016); furthermore, pyruvate is found in blood plasma, urine, and cerebrospinal fluid, and its presence there is not associated with damage of plasma membranes. Of course, this does not mean that extracellular pyruvate originated from the mitochondria, but it indicates that it can cross the plasma membrane through monocarboxylate transporters, some of which are distributed both in plasma and in the inner mitochondrial membrane (Hussien and Brooks, 2011); indeed monocarboxylate transporter 1, which is one of the four known pyruvate transport mechanisms, was recently shown to export pyruvate from the cell (Hong et al., 2016); however, mitochondrial pyruvate export remains hypothetical especially in view of the fact that its exit is influenced by the membrane potential and →pH. It was also recently reported that loss of an MPC isoform prior to a tumorigenic stimulus doubled the frequency of adenoma formation and produced higher-grade tumors, and this was associated with a glycolytic metabolic phenotype and increased expression of stem cell markers (Bensard et al., 2020). Mindful of the above, these pathways are as shown in Figure 3 (yellow arrows). FIGURE 3 Pathways leading to pyruvate commencing from glucose, highlighted in yellow: intermediates transiting through the mitochondria. For abbreviations, see Table 1. (10) Glc →→→ glyoxal →→→ glyoxylate: Glyoxylate enters the mitochondria; glyoxylate + Ala → Gly + pyruvate through AGXT2. Pyruvate may exit the mitochondria through the MPC (for considerations related to cancer, see pathway no. 5). (11) Glc →→→ PEP which enters the mitochondria; PEP transport across the inner membrane of mammalian mitochondria has been demonstrated to occur by the tricarboxylate carrier by Robinson (1971) and the group of Soling et al. (1971) and Kleineke et al. (1973) and to a lesser extent by the adenine nucleotide carrier, shown by the Shug and Shrago (1973); Sul et al. (1976) and in Drahota et al. (1983) and reviewed in Passarella et al. (2003). The possibility of a PEP/pyruvate transporter has also been put forward (Satrustegui et al.,2007). More recently, PEP cycling via mitochondrial PEPCK evoking PEP transport across the inner mitochondrial membrane has also been demonstrated by the group of Kibbey (Stark et al., 2009); PEP → OAA by PCK2; OAA → pyruvate by reverse operation of PC. However, this is expected to be a very minor path. Pyruvate may exit the mitochondria through the MPC. (12) Glc →→→ PEP; PEP enters the mitochondria through the means outlined in pathway 11. PEP → OAA by PCK2; OAA → pyruvate by FAHD1 (Pircher et al., 2011, 2015). FAHD1 also converts 3-acylpyruvate, acetylpyruvate, and fumarylpyruvate to pyruvate (Pircher et al., 2011). It is not known where acetylpyruvate comes from, but its existence is known since Krebs reported it (Krebs and Johnson, 1937). Pyruvate may exit the mitochondria through the MPC. FAHD1 depletion has been shown to induce premature senescence in human endothelial cells by inhibiting mitochondrial metabolism (Petit et al., 2017); however, this might be a double-edged sword since OXPHOS capacity has been inversely correlated with malignancy in several cell types (Zhou et al., 2003; Matoba et al., 2006; Hu et al., 2012; Hall et al., 2013; Bartesaghi et al., 2015; Nicolay et al., 2015; Capala et al., 2016; Smith et al., 2020). (13) Glc →→→ PEP; PEP enters the mitochondria through the means outlined in pathway 11; PEP → OAA by PCK2; OAA → Mal by MDH2; Mal → pyruvate by ME2,3 (Zelewski and Swierczynski, 1991). Pyruvate may exit the mitochondria through the MPC. ME2 knockdown suppresses tumor growth in lung cancer (Ren et al., 2014), while ME2,3 deletions confer lethality in pancreatic cancer (Dey et al., 2017). (14) Glc →→→ PEP; PEP enters the mitochondria through the means outlined in pathway 11; PEP → OAA by PCK2; OAA → Mal by MDH2; Mal exits the mitochondria; Mal → pyruvate by ME1 (Zelewski and Swierczynski, 1991; Loeber et al., 1994). ME1 knockdown inhibits the growth of colon cancer cells (Murai et al., 2017), and its overexpression is associated with larger breast tumor size, higher incidence of lymph node metastasis, and higher incidence of lymph–vascular invasion (Liu C. et al., 2020). In the same line, ME1 is associated with tumor budding—a phenomenon representing epithelial to mesenchymal transition—in oral squamous cell carcinomas (Nakashima et al., 2020). (15) Glc →→→ PEP; PEP enters the mitochondria through the means outlined in pathway 11; PEP → OAA by PCK2; OAA + acetyl-CoA → citrate by CS; citrate exits the mitochondria through the dicarboxylate carrier; citrate + ATP + CoASH → acetyl-coA + ADP + Pi + OAA by ACLY (Chypre et al., 2012); OAA → Mal by MDH1; Mal → pyruvate by ME1 (for considerations related to cancer, see pathway no. 14). (16) Glc →→→ PEP; PEP enters the mitochondria through the means outlined in pathway 11; PEP → OAA by PCK2; OAA + Glu →→Kg + Asp by GOT2; Asp exits the mitochondria; Asp + →Kg → Glu + OAA by GOT1; OAA → Mal by MDH1; Mal → pyruvate by ME1 (for considerations related to cancer, see pathway no. 14). (17) Glc →→→ PEP; PEP enters the mitochondria through the means outlined in pathway 11; PEP → OAA by PCK2; OAA + acetyl-CoA → citrate by CS; citrate → cis-aconitate, intermediate of ACO2 reaction; cis-aconitate → itaconate by cADC; itaconate + CoASH + ATP (or GTP) → itaconyl-CoA + Pi + ADP (or GDP) by SUCL (Nemeth et al., 2016); itaconyl-CoA → citramalyl-CoA by methylglutaconase (MGTK); citramalyl-coA → acetyl-CoA + pyruvate by CLYBL (Shen et al., 2017). Pyruvate may exit the mitochondria through the MPC. CLYBL has been reported to be associated with colorectal cancer metastasis (Li and Peng, 2013). Furthermore, CLYBL was reported to be overexpressed in 465 out of 38,258 tumor samples in the COSMIC database8. Pathways Leading to Pyruvate But Not Commencing From Glucose: Intermediates Not Transiting Through the Mitochondria These pathways are shown in Figure 4 (green arrows). FIGURE 4 Pathways leading to pyruvate but not commencing from glucose, highlighted in green: intermediates not transiting through the mitochondria. For abbreviations, see Table 1. (18) Ser → pyruvate, catalyzed by SDS or SDSL (for considerations related to cancer, see pathway no. 6). (19) Ser →→→ PEP; PEP → pyruvate; terminal reaction catalyzed by tartrate-resistant acid phosphatase (TR-Pases; for considerations related to cancer, see pathway no. 3). (20) Ser →→→ PEP; PEP + GalNAc → GalNAc-1P + pyruvate. The terminal reaction is catalyzed by N-acetylgalactosamine kinase isoforms 1 or 2 (for considerations related to cancer, see pathway no. 4). (21) Ala → pyruvate, catalyzed by L-amino-acid oxidases (LAAO) (Nakano et al., 1967): Several mammalian LAAOs have been described, of which the enzyme “interleukin-4 induced gene 1” (IL4I1) is the best characterized (Castellano and Molinier-Frenkel, 2017); IL4I1 expression was reported to be associated with poor prognosis in human breast cancers (Finak et al., 2008). (22) Ala + 2-oxoglrm → Gln + pyruvate, catalyzed by glutamine-pyruvate transaminase (GPAT) (Cooper and Meister, 1972; Cooper and Kuhara, 2014). GPAT is upregulated in many cancers in a MYC-dependent manner (Dong et al., 2020). (23) Ala + 2-Oml → Aml + pyruvate, catalyzed by alanine-ketomalonate transaminase (ALXT) (Nagayama et al., 1958). I was unable to find relevant literature on ALXT expression or aminomalonate levels and cancer. (24) Ala + αKg → Glu + pyruvate, catalyzed by GPT: GPT—similar to GPAT—is upregulated in many cancers in a MYC-dependent manner (Dong et al., 2020). (25) Ala + OAA → Asp + pyruvate; enzyme unknown (Rowsell, 1956). (26) Ala + Glyoxylate → Gly + pyruvate, catalyzed by alanine-glyoxylate aminotransferase (for considerations related to cancer, see pathway no. 5). (27) Ala + 3-OH-pyr → Ser + pyruvate, catalyzed by alanine-glyoxylate aminotransferase (for considerations related to cancer, see pathway no. 5). (28) Thr → Gly + acetaldehyde, catalyzed by SHMT1 (Garrow et al., 1993; Pinthong et al., 2014); Gly + 5,10 mTHF → THF + Ser, catalyzed by serine hydroxymethyltransferase 1; Ser → pyruvate, catalyzed by SDS or SDSL. SHMT1 knockdown induces apoptosis in lung cancer cells (Paone et al., 2014), and SHMT inhibitors block the growth of many human cancer cells (Ducker et al., 2017). Patients with high SHMT2 expression exhibit a shorter overall survival rate compared with patients with low expression (Koseki et al., 2018; for further considerations related to SDS or SDSL and cancer, see pathway no. 6). (29) Asp + αKg → Glu + OAA, catalyzed by GOT1; OAA → Mal by MDH1; Mal → pyruvate by ME1 (for considerations related to cancer, see pathway no. 14). (30) 4-OH-proline →→→ pyruvate, through glyoxylate formation (see pathway no. 26). (31) Cys →→→ pyruvate through the sulfinate pathway (Stipanuk, 1979, 2020). Notably, in pancreatic cancer cells exhibiting PKM1/2 knockdown, 20% of intracellular pyruvate originated from cysteine (Yu et al., 2019). The contribution of cysteine catabolism to cancer has been extensively reviewed by Serpa (2020). (32) Cys → 3-sulfino-L-alanine catalyzed by aspartate 4-decarboxylase (Liu et al., 2012); 3-sulfino-L-alanine is transaminated to 3-sulfinopyruvate by either aspartate aminotransferase or deaminated to the same product by cysteine sulfinic acid deaminase; 3-sulfinopyruvate is non-enzymatically converted to sulfite and pyruvate (Stipanuk, 2020; for considerations related to cancer, see pathway no. 31). (33) Cys →→→ H2S + pyruvate through the 3-mercaptopyruvate pathway (Nagahara and Sawada, 2006). Cys can also transaminate with →-ketoglutarate to form glutamate and 3-mercaptopyruvate though GOT1, exhibiting cysteine transaminase activity. The catabolism of 3-mercaptopyruvate toward pyruvate is outlined in the reactions below (pathway no. 34; for considerations related to cancer, see pathway no. 31). (34) L-cysteine is isomerized to D-cysteine by cysteine racemase (2-amino-3-mercaptopropionic acid racemase) (Soda and Osumi, 1969); D-Cys is converted to 3-mercaptopyruvate by D-amino acid oxidase and, in turn, to pyruvate and H2S by 3-mercaptopyruvate sulfurtransferase (3MST) (Shibuya et al., 2013) or thiosulfate sulfurtransferase (TST) (Pallini et al., 1991). The possibility of conversion of D-Cys to pyruvate by D-cysteine desulfhydrase (Nagasawa et al., 1985) in mammalian cells is yet to be reported. 3-Mercaptopyruvate can also react with hydrogen cyanide, forming pyruvate and thiocyanate in a reaction catalyzed by 3MST or TST; obviously, this is only a very minor route of pyruvate production due to cyanide toxicity (Bhandari et al., 2014; for further considerations related to cancer, see pathway no. 31). (35) Ser → dehydroalanine (2-aminoacrylate) by serine dehydratase (SDS), serine dehydratase-like protein (SDSL), or serine racemase (SRR): Dehydroalanine can further hydrolyze to NH3 and pyruvate through SDS, SDSL, or SRR (Kashii et al., 2005); sometimes this reaction is referred to as hydrolysis by “2-aminoacrylate aminohydrolase.” Dehydroalanine can also spontaneously hydrolyze to NH3 and pyruvate through the intermediate 2-iminopropanoate; the latter later part of this spontaneous hydrolysis can be accelerated by 2-iminopropanoate deaminase (Lambrecht et al., 2012). Dehydroalanine can also be derived from 2 3,5-diiodo-L-tyrosine or 3,5-diiodo-L-tyrosine by thyroid peroxidase in the process of forming thyroxine and triiodothyronine, respectively (Gavaret et al., 1980). The crucial importance of serine metabolism for the growth and survival of proliferating cells is extensively reviewed in Yang and Vousden (2016) and Newman and Maddocks (2017). (36) Se-methyl-L-selenocysteine (SeMSC, Se-methylselenocysteine, methyl selenocysteine) can be deaminated to methaneselenol, NH3, and pyruvate by selenocysteine lyase (Esaki et al., 1982). SeMSC can be found in many edible plants, including garlic, onions, and broccoli, as well as in dietary supplements (Yang and Jia, 2014). SeMSC was shown to exhibit anticarcinogenic properties (Ip et al., 1991; Medina et al., 2001) and even potentiate the antitumor activity of anticancer drugs (Cao et al., 2014). (37) Val →→→ 2-methyl-3-oxopropanoate; 2-methyl-3-oxopropanoate can get transaminated with alanine by AGXT2 to D-3-amino-isobutanoate + pyruvate (Kakimoto et al., 1969). The overexpression of enzymes participating in valine catabolism is associated with poor prognosis in prostate cancer (Mayers et al., 2016) and tumors of the colon (Shan et al., 2019). The role of valine in cancer has been extensively reviewed in Ananieva and Wilkinson (2018) and Lieu et al. (2020). (38) Leu →→→ 3-methylbutanoyl-CoA; the latter compound is converted to isobutyryl-CoA through branched-chain fatty acid metabolism (many steps); isobutyryl-CoA →→→ 2-methyl-3-oxopropanoate; 2-methyl-3-oxopropanoate can get transaminated with alanine by AGXT2 to D-3-amino-isobutanoate + pyruvate (Kakimoto et al., 1969). Because leucine catabolism shares many steps with that of valine, for considerations related to cancer, see pathway no. 37. (39) Ile →→→ 2-methylbutanoyl-CoA; the latter compound is converted to isobutyryl-CoA through branched-chain fatty acid metabolism (many steps); isobutyryl-CoA →→→ 2-methyl-3-oxopropanoate; 2-methyl-3-oxopropanoate can get transaminated with alanine by AGXT2 to D-3-amino-isobutanoate + pyruvate (Kakimoto et al., 1969). Because isoleucine catabolism shares many steps with that for valine, for considerations related to cancer, see pathway no. 37. (40) Pro + αKg + O2 → CO2 + succinate + trans-4-hydroxy-L-proline, catalyzed by prolyl 4-hydroxylase subunit alpha (isoforms 1, 2, or 3); trans-4-hydroxy-L-proline is then converted to L-1-pyrroline-3-hydroxy-5-carboxylate, also yielding NAD(P)H, by either pyrroline-5-carboxylate reductase (isoforms 1, 2, or 3) or left–right determination factor 1 (LEFTY1), a member of the TGF-→ family of proteins; L-1-pyrroline-3-hydroxy-5-carboxylate can be converted to L-erythro-4-hydroxyglutamate, also yielding NAD(P)H, by aldehyde dehydrogenase 4 family member A1; in turn, L-erythro-4-hydroxyglutamate is transaminated with either OAA by GOT2, yielding 4-hydroxy-2-oxoglutarate + aspartate, or →Kg by GOT1 or GOT2, yielding 4-hydroxy-2-oxoglutarate + glutamate; finally, 4-hydroxy-2-oxoglutarate is converted to glyoxylate and pyruvate by 4-hydroxy-2-oxoglutarate glyoxylate-lyase. It is relevant that increased proline catabolism has been recently reported to support metastasis (Elia et al., 2017). Arg, through either interconversion to metabolites as for proline catabolism or through citrulline/ornithine and the fumarate nucleotide cycle will also lead to pyruvate formation; however, this probably requires inter-organ communication and, thus, may not be found within a single cell. The crucial role of proline catabolism in tumor growth and metastatic progression is extensively reviewed in Phang (2019) and D’Aniello et al. (2020). Pathways Leading to Pyruvate But Not Commencing From Glucose: Intermediates Transiting Through the Mitochondria These pathways are shown in Figure 5 (blue arrows). FIGURE 5 Pathways leading to pyruvate but not commencing from glucose, highlighted in blue: intermediates transiting through the mitochondria. For abbreviations, see Table 1. (41) Thr →→→ acetyl-CoA; acetyl-CoA + OAA → citrate, catalyzed by CS; citrate exits the mitochondria through the dicarboxylate carrier; citrate + ATP + CoASH → Acetyl-coA + ADP + Pi + OAA by ACLY (Chypre et al., 2012); OAA → Mal by MDH1; Mal → pyruvate by ME1. The potential role of threonine catabolism in cancer is reviewed in Tsun and Possemato (2015) and Lieu et al. (2020) (for further considerations regarding ME1 and cancer, see pathway no. 14). (42) Thr →→→ acetyl-CoA; acetyl-CoA + OAA → citrate, catalyzed by CS; citrate → cis-aconitate, intermediate of ACO2 reaction; cis-aconitate → itaconate by cADC; itaconate + CoASH + ATP (or GTP) → itaconyl-CoA + Pi + ADP (or GDP) by SUCL; itaconyl-CoA → citramalyl-CoA by MGTK; citramalyl-coA → acetyl-CoA + pyruvate by CLYBL. Pyruvate may exit the mitochondria through the MPC (regarding threonine and cancer, see pathway no. 41; regarding CLYBL and cancer, see pathway no. 17). (43) Asn →→→ Asp; Asp + αKg → Glu + OAA by GOT2; OAA by PCK2; OAA → pyruvate by reverse operation of PC. However, this is expected to be a path of a very minor flux. Pyruvate may exit the mitochondria through the MPC. The crucial role of asparagine availability in cancer is explored in Panosyan et al. (2014); Krall et al. (2016), and Knott et al. (2018). However, more emphasis on asparagine availability for anabolic, rather than catabolic, purposes is given. (44) Asn →→→ Asp; Asp + αKg → Glu + OAA by GOT2; OAA → pyruvate by acylpyruvase (FAHD1). Pyruvate may exit the mitochondria through the MPC (for considerations related to cancer, see pathways no. 12 and 37). (45) Asn →→→ Asp; Asp + αKg → Glu + OAA by GOT2; OAA → Mal by MDH2; Mal → pyruvate by ME2,3. Pyruvate may exit the mitochondria through the MPC (for considerations related to cancer, see pathways no. 13 and 37). (46) Asn →→→ Asp; Asp + αKg → Glu + OAA by GOT2; OAA → Mal by MDH2; Mal exits the mitochondria; Mal → pyruvate by ME1 (for considerations related to cancer, see pathways no. 14 and 37). (47) Tyr, Phe →→→ Fum; Fum → Mal by FH; Mal → pyruvate by ME2,3 (for considerations related to cancer, see pathway no. 13). (48) Tyr, Phe →→→ Fum; Fum → Mal by FH; Mal exits the mitochondria; Mal → pyruvate by ME1 (for considerations related to cancer, see pathway no. 14). (49) Tyr, Phe →→→ Fum; Fum → Mal by FH; Mal → OAA by MDH2; OAA → pyruvate by acylpyruvase (FAHD1). Pyruvate may exit the mitochondria through the MPC (for considerations related to cancer, see pathway no. 12). (50) Thr →→→ acetyl-CoA; acetyl-CoA + OAA → citrate, catalyzed by CS; citrate exits the mitochondria through the dicarboxylate carrier; citrate + ATP + CoASH → acetyl-coA + ADP + Pi + OAA by ACLY; OAA → PEP by PCK1; PEP enters the mitochondria; PEP → OAA by PCK2; OAA → pyruvate by acylpyruvase (FAHD1). Pyruvate may exit the mitochondria through the MPC (for considerations related to cancer, see pathway no. 12). (51) Thr →→→ acetyl-CoA; acetyl-CoA + OAA → citrate, catalyzed by CS; citrate exits the mitochondria through the dicarboxylate carrier; citrate + ATP + CoASH → acetyl-coA + ADP + Pi + OAA by ACLY; OAA → PEP by PCK1; PEP enters mitochondria; PEP → OAA by PCK2; OAA → Mal by MDH2; Mal → pyruvate by ME2,3. Pyruvate may exit the mitochondria through the MPC (for considerations related to cancer, see pathway no. 13). (52) Thr →→→ acetyl-CoA; acetyl-CoA + OAA → citrate, catalyzed by CS; citrate exits the mitochondria through the dicarboxylate carrier; citrate + ATP + CoASH → acetyl-CoA + ADP + Pi + OAA by ACLY; OAA → PEP by PCK1; PEP enters the mitochondria; PEP → OAA by PCK2; OAA → Mal by MDH2; Mal exits the mitochondria; Mal → pyruvate by ME1 (for considerations related to cancer, see pathway no. 14). (53) Thr →→→ acetyl-CoA; acetyl-CoA + OAA → citrate, catalyzed by CS; citrate exits the mitochondria through the dicarboxylate carrier; citrate + ATP + CoASH → Acetyl-coA + ADP + Pi + OAA by ACLY; OAA → PEP by PCK1; PEP + GalNAc → GalNAc-1P + pyruvate. Terminal reaction catalyzed by N-acetylgalactosamine kinase isoforms 1 or 2 (for considerations related to cancer, see pathway no. 4). (54) Thr →→→ acetyl-CoA; acetyl-CoA + OAA → citrate, catalyzed by CS; citrate exits the mitochondria through the dicarboxylate carrier; citrate + ATP + CoASH → acetyl-coA + ADP + Pi + OAA by ACLY; OAA → PEP by PCK1; PEP → pyruvate; the terminal reaction is catalyzed by tartrate-resistant acid phosphatases (for considerations related to cancer, see pathway no. 3). Incompletely Characterized Reactions Forming Pyruvate In the literature, some reactions have been described to produce pyruvate but are incompletely characterized. These are collectively listed below: (55) O-carbamoyl-L-serine + H2O → pyruvate + 2 NH3, catalyzed by carbamoyl-serine ammonia lyase (Copper and Meister, 1973). O-Carbamoyl-L-serine is a weak inhibitor of a phosphate-dependent glutaminase (Shapiro et al., 1979); mindful of the crucial importance of glutamine catabolism through glutaminases in many cancer types, this route of pyruvate provision is probably minor. (56) L-Cysteine-S-conjugate + H2O → a thiol + NH3 + pyruvate, catalyzed by cysteine S-conjugate →-lyases (Cooper and Pinto, 2006). The possibility of cysteine S-conjugate β-lyases metabolizing anticancer agents is reviewed in Cooper et al. (2011). (57) cystathionine + H2O → L-homocysteine + pyruvate + NH3 or cysteine + H2O → sulfide + NH3 + pyruvate or cystine → thiocysteine + pyruvate + NH3, all catalyzed by cystathionine gamma-lyase (Stipanuk et al., 2006; Chiku et al., 2009). Cystathionine gamma-lyase was reported to be upregulated in bone−metastatic PC3 cells, and its knockdown suppressed tumor growth and metastasis (Wang et al., 2019). In the same line, this enzyme was shown to be upregulated and played a crucial role in the proliferation and migration of breast cancer cells (You et al., 2017). (58) L-Serine O-sulfate + H2O → pyruvate + NH3 + sulfate catalyzed by serine-sulfate ammonia-lyase (Tudball and Thomas, 1972). I was unable to find relevant literature on serine-sulfate ammonia-lyase expression or L-serine O-sulfate levels and cancer. (59) N-Acetylneuraminate → N-acetyl-D-mannosamine + pyruvate catalyzed by N-acetylneuraminate lyase (Brunetti et al., 1962); relevant to this, treatment of HL-60 cells by phorbol esters leads to a marked increase in the activity of this enzyme (Warren, 1986). (60) D-Alanine + H2O + O2 → pyruvate + NH3 + H2O2 catalyzed by DAAO (Nagata et al., 1992; Abe et al., 2005; Fuchs et al., 2005; Smith et al., 2009). The interaction of D-alanine (and other D-amino acids) with tumors is reviewed in Bastings et al. (2019). (61) L-Alanine → pyruvate + NH3 catalyzed by glutamate dehydrogenase; this reaction exhibits a weak activity (Silverstein, 1974). The role of glutamate dehydrogenase in cancer cells has been extensively reviewed in Moreno-Sanchez et al. (2020). (62) 2-Oxosuccinamic acid + Ala → Asn + pyruvate, catalyzed by asparagine aminotransferase (Cooper, 1977; Maul and Schuster, 1986). The origin of 2-oxosuccinamic acid is not known (Cooper et al., 1987). I was unable to find relevant literature on 2-oxosuccinamic acid levels and cancer. (63) Pyruvate oxime + acetone → pyruvate + acetone oxime, catalyzed by oximinotransferase (Omura et al., 1956). Due to acetone volatility, this is probably a very minor pathway for pyruvate production. (64) Methylmalonyl-CoA + pyruvate → propionyl-CoA + oxaloacetate catalyzed by methylmalonyl-CoA carboxytransferase (Swick and Wood, 1960). This reaction is reversible and thus may yield pyruvate. I was unable to find relevant literature on methylmalonyl-CoA carboxytransferase and cancer. (65) L-Alanine + 3-oxopropanoate → pyruvate + →-alanine, catalyzed by either →-alanine-pyruvate transaminase (Ito et al., 2001) or alanine-glyoxylate aminotransferase isoform 2 (Lee et al., 1995) (for considerations related to cancer, see pathway no. 5). (66) Phenylpyruvate + L-alanine → L-phenylalanine + pyruvate catalyzed by phenylalanine (histidine) transaminase (Minatogawa et al., 1977). Phenylpyruvate has been reported to inhibit pyruvate kinase activity in human brain (Weber, 1969), thus enhancing PK-bypassing pathways. Phenylpyruvate levels were also found to be increased in ovarian cancers (Fong et al., 2011). (67) 2-Oxoisohexanoate + L-alanine → L-leucine + pyruvate, catalyzed by the mitochondrial branched-chain L-amino acid aminotransferase (Schadewaldt et al., 1995). The role of branched-chain L-amino acid aminotransferase in cancer has been reviewed in Ananieva and Wilkinson (2018). (68) PCK1, ME1, and ME2,3 may also convert OAA to CO2 and pyruvate (Sauer, 1973; Carlson et al., 1978; Bukato et al., 1995; Lee et al., 1995) (for considerations related to cancer, see pathway nos. 13 and 14). (69) Salsolinol can be converted to salsolinol-1-carboxylate by salsolinol synthetase which can then be catabolized to dopamine and pyruvate (by an unknown enzyme); salsolinol is an endogenous catechol isoquinoline detected in humans derived from dopamine metabolism (Sandler et al., 1973; Collins et al., 1979). Salsolinol has been implicated in the initiation and promotion of alcohol-related breast carcinogenesis (Murata et al., 2016). Pathways Leading to L-Lactate and D-Lactate Including Those Not Going Through Lactate Dehydrogenase These pathways are shown in Figure 6 (brown arrows). FIGURE 6 Pathways leading to L- and D-lactate, including those not going through lactate dehydrogenase, highlighted in brown. For abbreviations, see Table 1. Lactate—unlike pyruvate—exhibits chirality; thus, it exists in L- or D- configuration. In humans, a putative D-lactate dehydrogenase is known to exist (Flick and Konieczny, 2002; Ewaschuk et al., 2005; Chen et al., 2015). In metabolomics experiments, it is uncommon to distinguish between L- and D-lactate even although it is possible by using special columns. In this section, D- and L-lactate-forming pathways are outlined, including those not going through LDH: (70) D-lactate formation by methylglyoxal and intestinal flora (Chen et al., 2015) (for considerations related to cancer, see pathway no. 2). (71) Pyruvate + QH2 → D-lactate + Q, catalyzed by D2HGDH in the mitochondrial matrix (Cammack, 1969, 1970). Mutations in D2HGDH have been reported to be involved in multiple types of cancers but render the enzyme hypoactive or inert (Ye et al., 2018); thus, it is unlikely for this route to be important regarding pyruvate production. (72) D- (or L-) Lactate + 2 ferricytochrome → 2 ferrocytochrome C + 2 H+ + pyruvate, catalyzed by D-lactate dehydrogenase; this reaction is mentioned in several databases, but no reference is given. (73) D- (or L-) Lactate + 2 ferricytochrome → 2 ferrocytochrome C + 2 H+ + pyruvate, catalyzed by cytochrome B5 domain-containing protein 1; this reaction is mentioned in several databases, but no reference is given. (74) Pyruvate + NADPH → NADP+ + L-lactate, catalyzed by ADH (Bosron and Prairie, 1972). The many roles of ADH in malignant neoplasms have been extensively reviewed in Orywal and Szmitkowski (2017). (75) Pyruvate + H2O2 → L-lactate + O2, catalyzed by hydroxyacid oxidases (HAO1,2,3) (Fry and Richardson, 1979; Vignaud et al., 2007). However, in Jones et al. (2000), no HAO activity was reported. In primary pancreatic tumors, HAO3 is strongly downregulated (Thakur et al., 2008). HAO2 was reported to inhibit the malignancy of clear cell renal cell carcinoma cells. Overall, it is unlikely for this to be a substantial pathway in yielding pyruvate in cancer. (76) Protein deglycase (E.C. 3.5.1.124) may form D-lactate from proteins (Richarme et al., 2015; Richarme and Dairou, 2017). Relevant to this, the deglycase DJ-1/Park7 is important for cancer cell survival (Vasseur et al., 2009). (77) Methylglyoxal spontaneously forms a hemithioacetal adduct with GSH; subsequently, glyoxalase I (lactoylglutathione lyase; EC 4.4.1.5) produces S-D-lactoylglutathione from this adduct (Thornalley, 1990), and glyoxalase II (hydroxyacylglutathione hydrolase; EC 3.1.2.6), in turn, hydrolyzes S-D-lactoylglutathione to D-lactate + GSH (Cordell et al., 2004) (for considerations related to cancer, see pathway no. 2). Finally, it is worth mentioning that LDH may process substrates other than pyruvate and lactate, interconverting glyoxylate + NAD+ to oxalate + NADH or α-ketobutyrate to →-hydroxybutyrate or L-glycerate to hydroxypyruvate (Dawkins and Dickens, 1965; Kim and Whitesides, 1988). Pathways Leading to Pyruvate Commencing From Glutamine (Glutaminolysis) It is a well-known fact that most cancer cells grow much better when feeding media contain glutamine; this spurred from the pioneering studies of Eagle et al. (1956), showing the dependence of cancer cells growing in monolayer cultures on glutamine. The many critical roles of glutamine in tumor metabolism is reviewed in Altman et al. (2016). From the energetic point of view it were Reitzer et al. (1979) who first showed that glutamine, not sugars, is the main energy source in cultured HeLa cells and that carbon atoms from glutamine incorporate into lactate, but not more than 13%. Zielke et al. (1980), likewise reported that human diploid fibroblasts metabolize up to 13% of media glutamine to lactate. In the same line of thought, Scott et al. (2011), showed that, in human melanoma cell lines, glutamine did not significantly label lactate, in agreement with the data of Ta and Seyfried (2015) reporting that, in a murine glioblastoma cell line, minimal amounts of lactate derived from glutamine were detected. Le et al. (2012), as well as Son et al. (2013) likewise showed that 13C-labeled atoms in glutamine appear in lactate also to a minimal extent. However, in a study published by DeBerardinis et al. (2007), ∼60% of the glutamine metabolized by SF188 cells was claimed to be converted to lactate, although they seemed to combine this percentage with that of alanine production. The pathway of converting glutamine to pyruvate (and lactate), referred to by McKeehan (1982) as “glutaminolysis,” has been considered a hallmark of tumor metabolism; however, this is a misconception: in normal tissues, ∼18% of glutamine carbons appear in lactate (Windmueller and Spaeth, 1974), as opposed to ∼10–13% (or less) in tumor cells (see the references above). Thus, if anything, cancer cells exhibit a decrease in glutamine-to-lactate conversion exactly as anticipated, mindful that glutamine provides both energy and building blocks for several biosynthetic processes of cancer. Although glutaminolysis was originally attributed to the pathway Gln → Glu → aKg → succinyl-CoA → succinate → fumarate → malate (exiting the mitochondria) → pyruvate (through malic enzyme), several other routes may also contribute (outlined below; see Figure 7). FIGURE 7 Pathways leading to pyruvate commencing from glutamine (glutaminolysis), highlighted in red. For abbreviations, see Table 1. (78) (For the sake of completion, the glutaminolysis pathway proposed by McKeehan (1982) is repeated in the present entry) Gln → Glu → aKg → succinyl-CoA → succinate → fumarate → malate; malate exits the mitochondria → pyruvate; this last step is catalyzed by cytosolic malic enzyme (ME1). (79) Gln → Glu → aKg → isocitrate → cis-aconitate → itaconate by cADC; itaconate + CoASH + ATP (or GTP) → itaconyl-CoA + Pi + ADP (or GDP) by SUCL (Nemeth et al., 2016); itaconyl-CoA → citramalyl-CoA by methylglutaconase (MGTK); citramalyl-coA → acetyl-CoA + pyruvate by CLYBL (Shen et al., 2017). Pyruvate may exit the mitochondria through the MPC. (80) Gln → Glu → aKg → isocitrate → cis-aconitate → citrate, exiting the mitochondria → citrate + ATP + CoASH → acetyl-coA + ADP + Pi + OAA by ACLY (Chypre et al., 2012); OAA → Mal by MDH1; Mal → pyruvate by ME1. (81) Gln → Glu → aKg → isocitrate → cis-aconitate → citrate, exiting the mitochondria → citrate + ATP + CoASH → acetyl-coA + ADP + Pi + OAA by ACLY; OAA → PEP by PCK1; PEP + GalNAc → GalNAc-1P + pyruvate. The terminal reaction is catalyzed by N-acetylgalactosamine kinase isoforms 1 or 2. (82) Gln → Glu → aKg → isocitrate → cis-aconitate → citrate, exiting the mitochondria → citrate + ATP + CoASH → acetyl-coA + ADP + Pi + OAA by ACLY; OAA → PEP by PCK1; PEP → pyruvate; the terminal reaction is catalyzed by tartrate-resistant acid phosphatases. (83) Gln → Glu → aKg → succinyl-CoA → succinate → fumarate → malate → pyruvate by ME2,3; pyruvate may exit the mitochondria through the MPC. (84) Gln → Glu → aKg; aKg transaminates with Asp forming Glu and OAA, by GOT2; OAA → pyruvate by FAHD1 (Pircher et al., 2011, 2015); pyruvate may exit the mitochondria through the MPC. (85) Gln → Glu → aKg; aKg transaminates with Asp forming Glu and OAA, by GOT2; OAA → Mal by MDH2; Mal exits the mitochondria; Mal → pyruvate by ME1 (Zelewski and Swierczynski, 1991; Loeber et al., 1994). (86) Gln → Glu → aKg; aKg transaminates with Asp forming Glu and OAA, by GOT2; OAA → Mal by MDH2; malate → pyruvate by ME2,3; pyruvate may exit the mitochondria through the MPC. Energetics of Glycolysis With Kinetically Inactive PK Glycolysis yields a net of two ATP molecules per glucose molecule; however, in view of an inactive PK while pyruvate is made through PK-bypass pathways, net ATP production from glycolysis is expected to be zero. Although the importance of high-energy phosphate generation has been downplayed in cancer tissues (Vander Heiden et al., 2009), it cannot be ignored that—according to the BRENDA database—among the 336 enzymatic reactions requiring ATP in a cell (without even considering quantitatively important, non-enzymatic mechanisms such as Na+/K+ ATPase), 125 of them occur in the cytosol. Clearly, while it is imperative to prevent phosphofructokinase and hexokinase from ATP-dependent feedback inhibition and allow a high flux of glycolysis for the sake of generating intermediates shuttled toward other pathways, ATP is still needed for many other reactions. Crunching the numbers regarding cytosolic energetics is a daunting task, but what is definite is that a cell with nearly zero ATP production from glycolysis may not harbor ATP-consuming mitochondria, for whatever reason (hypoxia, mtDNA mutations, etc.). This can be solved by maintaining the adenine nucleotide translocase in “forward” mode, i.e., providing ATP to the cytosol which is made by SUCL supported by glutaminolysis (Chinopoulos et al., 2010). Production of pyruvate and, therefore lactate is still maintained by the PK-bypassing pathways so as to thwart a reductive stress as pyruvate-to-lactate by LDH maintains a low NADH/NAD+ ratio. Finally, it is important to emphasize that this lack of ATP generation by glycolysis due to PK inhibition does not only occur in neoplastic tissues, but it seems to be a more general pathophysiological mechanism also present in tissue ischemia: it was recently reported that during acute kidney injury, PK was inhibited by oxidative/nitrosative stress for the purpose of diverting glycolytic intermediates toward the pentose phosphate pathway which, in turn, yielded reducing equivalents and mounted a better response during the reperfusion phase where ROS are formed, thus increasing the chances for organ survival (Zhou et al., 2019). Conclusion The above considerations aim to (i) highlight that L-lactate can still be produced from pyruvate using carbon atoms originating from glucose or other substrates in cells with kinetically impaired pyruvate kinase and (ii) show that the mitochondria may contribute to cancer metabolism irrespective of oxidative phosphorylation by providing means of contributing to pyruvate production. Having said that, it is important to emphasize that none of the aforementioned reactions take into account the potential regulatory effects of metabolites on other reactions such as those occurring on PK by amino acids (Chaneton et al., 2012; Yuan et al., 2018). In addition, each enzyme probably exhibits different kinetic and thermodynamic constraints which control the overall flux, which also means that many of these pathways may not operate simultaneously. Such exponentially increasing complexity of a system precludes the possibility of predictions and modeling, though I would be happy to be proven wrong. Author Contributions CC wrote and edited the manuscript. Conflict of Interest The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Funding. This work was supported by grants from NKFIH (FIKP-61822-64888-EATV, VEKOP 2.3.3-15-2016-00012, 2017-2.3.4-TET-RU-2017-00003, KH129567, and K135027) to CC. I am indebted to Dr. Dora Ravasz, Dr. David Bui, Jeonghyoun Lee, and Seung Won Jeong for assistance with database mining and to Prof. Thomas N. Seyfried for helpful discussions. 1 https://www.genome.jp/kegg/ 2 www.brenda-enzymes.org 3 https://www.metabolicatlas.org/ 4 http://bigg.ucsd.edu/ 5 http://www.metanetx.org/ 6 www.hmdb.ca 7 https://www.vmh.life/ 8 https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=CLYBL ==== Refs References Abbadi S. Rodarte J. J. Abutaleb A. Lavell E. Smith C. L. Ruff W. (2014 ). Glucose-6-phosphatase is a key metabolic regulator of glioblastoma invasion. Mol. Cancer Res. 12 1547 –1559 . 10.1158/1541-7786.mcr-14-0106-t 25001192 Abe H. Yoshikawa N. Sarower M. G. Okada S. (2005 ). Physiological function and metabolism of free D-alanine in aquatic animals. Biol. Pharm. Bull. 28 1571 –1577 . 10.1248/bpb.28.1571 16141518 Alquraishi M. Puckett D. L. Alani D. S. Humidat A. S. Frankel V. D. Donohoe D. R. (2019 ). Pyruvate kinase M2: a simple molecule with complex functions. Free Radic. Biol. Med. 143 176 –192 . 10.1016/j.freeradbiomed.2019.08.007 31401304 Altenberg B. Greulich K. O. (2004 ). Genes of glycolysis are ubiquitously overexpressed in 24 cancer classes. Genomics 84 1014 –1020 . 10.1016/j.ygeno.2004.08.010 15533718 Altman B. J. Stine Z. E. Dang C. V. (2016 ). From Krebs to clinic: glutamine metabolism to cancer therapy. Nat. Rev. Cancer 16 619 –634 . 10.1038/nrc.2016.71 27492215 Ananieva E. A. Wilkinson A. C. (2018 ). Branched-chain amino acid metabolism in cancer. Curr. Opin. Clin. Nutr. Metab. Care 21 64 –70 . 10.1097/mco.0000000000000430 29211698 Anastasiou D. Poulogiannis G. Asara J. M. Boxer M. B. Jiang J. K. Shen M. (2011 ). Inhibition of pyruvate kinase M2 by reactive oxygen species contributes to cellular antioxidant responses. Science 334 1278 –1283 . 10.1126/science.1211485 22052977 Anastasiou D. Yu Y. Israelsen W. J. Jiang J. K. Boxer M. B. Hong B. S. (2012 ). Pyruvate kinase M2 activators promote tetramer formation and suppress tumorigenesis. Nat. Chem. Biol. 8 839 –847 .22922757 Antognelli C. Moretti S. Frosini R. Puxeddu E. Sidoni A. Talesa V. N. (2019 ). Methylglyoxal Acts as a Tumor-Promoting Factor in Anaplastic Thyroid Cancer. Cells 8 547 . 10.3390/cells8060547 31174324 Baker P. R. Cramer S. D. Kennedy M. Assimos D. G. Holmes R. P. (2004 ). Glycolate and glyoxylate metabolism in HepG2 cells. Am. J. Physiol. Cell Physiol. 287 C1359 –C1365 .15240345 Baranowski T. Wolna E. Morawiecki A. (1968 ). Purification and properties of crystalline 2-phospho-D-glycerate hydro-lyase from human muscle. Eur. J. Biochem. 5 119 –123 . 10.1111/j.1432-1033.1968.tb00345.x 5660677 Bartesaghi S. Graziano V. Galavotti S. Henriquez N. V. Betts J. Saxena J. (2015 ). Inhibition of oxidative metabolism leads to p53 genetic inactivation and transformation in neural stem cells. Proc. Natl. Acad. Sci. U.S.A. 112 1059 –1064 . 10.1073/pnas.1413165112 25583481 Bastings J. van Eijk H. M. Olde Damink S. W. Rensen S. S. (2019 ). d-amino acids in health and disease: a focus on cancer. Nutrients 11 :2205 . 10.3390/nu11092205 31547425 Bensard C. L. Wisidagama D. R. Olson K. A. Berg J. A. Krah N. M. Schell J. C. (2020 ). Regulation of tumor initiation by the mitochondrial Pyruvate Carrier. Cell Metab. 31 284 –300.e7 .31813825 Bhandari R. K. Oda R. P. Petrikovics I. Thompson D. E. Brenner M. Mahon S. B. (2014 ). Cyanide toxicokinetics: the behavior of cyanide, thiocyanate and 2-amino-2-thiazoline-4-carboxylic acid in multiple animal models. J. Anal. Toxicol. 38 218 –225 . 10.1093/jat/bku020 24711295 Bose S. Le A. (2018 ). Glucose metabolism in cancer. Adv. Exp. Med. Biol. 1063 3 –12 .29946772 Bosron W. F. Prairie R. L. (1972 ). Triphosphopyridine nucleotide-linked aldehyde reductase. I. Purification and properties of the enzyme from pig kidney cortex. J. Biol. Chem. 247 4480 –4485 .4402936 Brunetti P. Jourdian G. W. Roseman S. (1962 ). The sialic acids. III. Distribution and properties of animal N-acetylneuraminic aldolase. J. Biol. Chem. 237 2447 –2453 .13874013 Bukato G. Kochan Z. Swierczynski J. (1995 ). Purification and properties of cytosolic and mitochondrial malic enzyme isolated from human brain. Int. J. Biochem. Cell Biol. 27 47 –54 . 10.1016/1357-2725(94)00057-3 7757881 Bull H. Murray P. G. Thomas D. Fraser A. M. Nelson P. N. (2002 ). Acid phosphatases. Mol. Pathol. 55 65 –72 .11950951 Cammack R. (1969 ). Assay, purification and properties of mammalian D-2-hydroxy acid dehydrogenase. Biochem. J. 115 55 –64 . 10.1042/bj1150055 5359443 Cammack R. (1970 ). Mammalian D-2-hydroxy acid dehydrogenase. Effect of inhibitors and reaction sequence. Biochem. J. 118 405 –408 . 10.1042/bj1180405 5528639 Cao S. Durrani F. A. Toth K. Rustum Y. M. (2014 ). Se-methylselenocysteine offers selective protection against toxicity and potentiates the antitumour activity of anticancer drugs in preclinical animal models. Br. J. Cancer 110 1733 –1743 . 10.1038/bjc.2014.85 24619073 Capala M. E. Pruis M. Vellenga E. Schuringa J. J. (2016 ). Depletion of SAM50 Specifically Targets BCR-ABL-expressing leukemic stem and progenitor cells by interfering with mitochondrial functions. Stem Cells Dev. 25 427 –437 . 10.1089/scd.2015.0151 26855047 Carlsen B. D. Lambeth D. O. Ray P. D. (1988 ). Synthesis of malate from phosphoenolpyruvate by rabbit liver mitochondria: implications for lipogenesis. Biochim. Biophys. Acta 965 1 –8 . 10.1016/0304-4165(88)90143-2 2831992 Carlson G. M. Colombo G. Lardy H. A. (1978 ). A vicinal dithiol containing an essential cysteine in phosphoenolpyruvate carboxykinase (guanosine triphosphate) from cytosol of rat liver. Biochemistry 17 5329 –5338 . 10.1021/bi00618a002 728403 Castellano F. Molinier-Frenkel V. (2017 ). An overview of l-amino acid oxidase functions from bacteria to mammals: focus on the immunoregulatory phenylalanine oxidase IL4I1. Molecules 22 :2151 . 10.3390/molecules22122151 29206151 Chaneton B. Hillmann P. Zheng L. Martin A. C. L. Maddocks O. D. K. Chokkathukalam A. (2012 ). Serine is a natural ligand and allosteric activator of pyruvate kinase M2. Nature 491 458 –462 . 10.1038/nature11540 23064226 Chao T. Y. Yu J. C. Ku C. H. Chen M. M. Lee S. H. Janckila A. J. (2005 ). Tartrate-resistant acid phosphatase 5b is a useful serum marker for extensive bone metastasis in breast cancer patients. Clin. Cancer Res. 11 544 –550 .15701839 Chen C. H. Chen S. C. (1988 ). Evidence of acid phosphatase in the cytoplasm as a distinct entity. Arch. Biochem. Biophys. 262 427 –438 . 10.1016/0003-9861(88)90394-3 3364974 Chen C. M. Chen S. M. Chien P. J. Yu H. Y. (2015 ). Development of an enzymatic assay system of D-lactate using D-lactate dehydrogenase and a UV-LED fluorescent spectrometer. J. Pharm. Biomed. Anal. 116 150 –155 . 10.1016/j.jpba.2015.07.018 26265307 Chiavarina B. Nokin M. J. Bellier J. Durieux F. Bletard N. Sherer F. (2017 ). Methylglyoxal-mediated stress correlates with high metabolic activity and promotes tumor growth in colorectal cancer. Int. J. Mol. Sci. 18 :213 . 10.3390/ijms18010213 28117708 Chiku T. Padovani D. Zhu W. Singh S. Vitvitsky V. Banerjee R. (2009 ). H2S biogenesis by human cystathionine gamma-lyase leads to the novel sulfur metabolites lanthionine and homolanthionine and is responsive to the grade of hyperhomocysteinemia. J. Biol. Chem. 284 11601 –11612 . 10.1074/jbc.m808026200 19261609 Chinopoulos C. Gerencser A. A. Mandi M. Mathe K. Torocsik B. Doczi J. (2010 ). Forward operation of adenine nucleotide translocase during F0F1-ATPase reversal: critical role of matrix substrate-level phosphorylation. FASEB J. 24 2405 –2416 . 10.1096/fj.09-149898 20207940 Christofk H. R. Vander Heiden M. G. Harris M. H. Ramanathan A. Gerszten R. E. Wei R. (2008 ). The M2 splice isoform of pyruvate kinase is important for cancer metabolism and tumour growth. Nature 452 230 –233 . 10.1038/nature06734 18337823 Chypre M. Zaidi N. Smans K. (2012 ). ATP-citrate lyase: a mini-review. Biochem. Biophys. Res. Commun. 422 1 –4 . 10.1016/j.bbrc.2012.04.144 22575446 Colilla W. Jorgenson R. A. Nordlie R. C. (1975 ). Mammalian carbamyl phosphate : glucose phosphotransferase and glucose-6-phosphate phosphohydrolase: extended tissue distribution. Biochim. Biophys. Acta 377 117 –125 . 10.1016/0005-2744(75)90292-2 164220 Collins M. A. Nijm W. P. Borge G. F. Teas G. Goldfarb C. (1979 ). Dopamine-related tetrahydroisoquinolines: significant urinary excretion by alcoholics after alcohol consumption. Science 206 1184 –1186 . 10.1126/science.505002 505002 Cooper A. J. (1977 ). Asparagine transaminase from rat liver. J. Biol. Chem. 252 2032 –2038 .14957 Cooper A. J. Krasnikov B. F. Niatsetskaya Z. V. Pinto J. T. Callery P. S. Villar M. T. (2011 ). Cysteine S-conjugate beta-lyases: important roles in the metabolism of naturally occurring sulfur and selenium-containing compounds, xenobiotics and anticancer agents. Amino Acids 41 7 –27 . 10.1007/s00726-010-0552-0 20306345 Cooper A. J. Kuhara T. (2014 ). alpha-Ketoglutaramate: an overlooked metabolite of glutamine and a biomarker for hepatic encephalopathy and inborn errors of the urea cycle. Metab. Brain Dis. 29 991 –1006 . 10.1007/s11011-013-9444-9 24234505 Cooper A. J. Pinto J. T. (2006 ). Cysteine S-conjugate beta-lyases. Amino Acids 30 1 –15 . 10.1007/s00726-005-0243-4 16463021 Cooper A. J. Raps S. P. Meister A. (1987 ). Fluorometric determination of alpha-ketosuccinamic acid in rat tissues. Anal. Biochem. 167 312 –320 . 10.1016/0003-2697(87)90170-9 3442326 Cooper J. L. Meister A. (1972 ). Isolation and properties of highly purified glutamine transaminase. Biochemistry 11 661 –671 . 10.1021/bi00755a001 5059882 Copper A. J. Meister A. (1973 ). Enzymatic conversion of O-carbamyl-L-serine to pyruvate and ammonia. Biochem. Biophys. Res. Commun. 55 780 –787 . 10.1016/0006-291x(73)91212-6 4761084 Cordell P. A. Futers T. S. Grant P. J. Pease R. J. (2004 ). The Human hydroxyacylglutathione hydrolase (HAGH) gene encodes both cytosolic and mitochondrial forms of glyoxalase II. J. Biol. Chem. 279 28653 –28661 . 10.1074/jbc.m403470200 15117945 Cortes-Cros M. Hemmerlin C. Ferretti S. Zhang J. Gounarides J. S. Yin H. (2013 ). M2 isoform of pyruvate kinase is dispensable for tumor maintenance and growth. Proc. Natl. Acad. Sci. U.S.A. 110 489 –494 . 10.1073/pnas.1212780110 23267074 D’Aniello C. Patriarca E. J. Phang J. M. Minchiotti G. (2020 ). Proline metabolism in tumor growth and metastatic progression. Front. Oncol. 10 :776 . 10.3389/fonc.2020.00776 32500033 Danpure C. J. Lumb M. J. Birdsey G. M. Zhang X. (2003 ). Alanine:glyoxylate aminotransferase peroxisome-to-mitochondrion mistargeting in human hereditary kidney stone disease. Biochim. Biophys. Acta 1647 70 –75 . 10.1016/s1570-9639(03)00055-4 12686111 Dawkins P. D. Dickens F. (1965 ). The Oxidation of D- and L-Glycerate by Rat Liver. Biochem. J. 94 353 –367 . 10.1042/bj0940353 14346088 Dayton T. L. Gocheva V. Miller K. M. Bhutkar A. Lewis C. A. Bronson R. T. (2018 ). Isoform-specific deletion of PKM2 constrains tumor initiation in a mouse model of soft tissue sarcoma. Cancer Metab. 6 :6 . Dayton T. L. Gocheva V. Miller K. M. Israelsen W. J. Bhutkar A. Clish C. B. (2016a ). Germline loss of PKM2 promotes metabolic distress and hepatocellular carcinoma. Genes Dev. 30 1020 –1033 . 10.1101/gad.278549.116 27125672 Dayton T. L. Jacks T. Vander Heiden M. G. (2016b ). PKM2, cancer metabolism, and the road ahead. EMBO Rep. 17 1721 –1730 .27856534 de la Cruz-Lopez K. G. Castro-Munoz L. J. Reyes-Hernandez D. O. Garcia-Carranca A. Manzo-Merino J. (2019 ). Lactate in the regulation of tumor microenvironment and therapeutic approaches. Front. Oncol. 9 :1143 . 10.3389/fonc.2019.01143 31737570 De Marchi T. Timmermans M. A. Sieuwerts A. M. Smid M. Look M. P. Grebenchtchikov N. (2017 ). Phosphoserine aminotransferase 1 is associated to poor outcome on tamoxifen therapy in recurrent breast cancer. Sci. Rep. 7 :2099 . DeBerardinis R. J. Lum J. J. Hatzivassiliou G. Thompson C. B. (2008 ). The biology of cancer: metabolic reprogramming fuels cell growth and proliferation. Cell Metab. 7 11 –20 . 10.1016/j.cmet.2007.10.002 18177721 DeBerardinis R. J. Mancuso A. Daikhin E. Nissim I. Yudkoff M. Wehrli S. (2007 ). Beyond aerobic glycolysis: transformed cells can engage in glutamine metabolism that exceeds the requirement for protein and nucleotide synthesis. Proc. Natl. Acad. Sci. U.S.A. 104 19345 –19350 . 10.1073/pnas.0709747104 18032601 Dey P. Baddour J. Muller F. Wu C. C. Wang H. Liao W. T. (2017 ). Genomic deletion of malic enzyme 2 confers collateral lethality in pancreatic cancer. Nature 542 119 –123 . 10.1038/nature21052 28099419 Dong Y. Tu R. Liu H. Qing G. (2020 ). Regulation of cancer cell metabolism: oncogenic MYC in the driver’s seat. Signal Transduct. Target. Ther. 5 :124 . Drahota Z. Rauchova H. Mikova M. Kaul P. Bass A. (1983 ). Phosphoenolpyruvate shuttle–transport of energy from mitochondria to cytosol. FEBS Lett. 157 347 –349 . 10.1016/0014-5793(83)80573-0 6862029 Ducker G. S. Ghergurovich J. M. Mainolfi N. Suri V. Jeong S. K. Hsin-Jung Li S. (2017 ). Human SHMT inhibitors reveal defective glycine import as a targetable metabolic vulnerability of diffuse large B-cell lymphoma. Proc. Natl. Acad. Sci. U.S.A. 114 11404 –11409 . 10.1073/pnas.1706617114 29073064 Eagle H. Oyama V. I. Levy M. Horton C. L. Fleischman R. (1956 ). The growth response of mammalian cells in tissue culture to L-glutamine and L-glutamic acid. J. Biol. Chem. 218 607 –616 .13295214 Elia I. Broekaert D. Christen S. Boon R. Radaelli E. Orth M. F. (2017 ). Proline metabolism supports metastasis formation and could be inhibited to selectively target metastasizing cancer cells. Nat. Commun. 8 :15267 . Erecinska M. Wilson D. F. (1984 ). Relationship of the intra- and extramitochondrial adenine nucleotide ratios during synthesis of phosphoenolpyruvate using extramitochondrial ATP. J. Biol. Chem. 259 10904 –10906 .6088521 Esaki N. Nakamura T. Tanaka H. Soda K. (1982 ). Selenocysteine lyase, a novel enzyme that specifically acts on selenocysteine. Mammalian distribution and purification and properties of pig liver enzyme. J. Biol. Chem. 257 4386 –4391 .6461656 Ewaschuk J. B. Naylor J. M. Zello G. A. (2005 ). D-lactate in human and ruminant metabolism. J. Nutr. 135 1619 –1625 . 10.1093/jn/135.7.1619 15987839 Feng H. Wang X. Chen J. Cui J. Gao T. Gao Y. (2019 ). Nuclear imaging of glucose metabolism: beyond (18)F-FDG. Contrast Media Mol. Imaging 2019 :7954854 . Finak G. Bertos N. Pepin F. Sadekova S. Souleimanova M. Zhao H. (2008 ). Stromal gene expression predicts clinical outcome in breast cancer. Nat. Med. 14 518 –527 .18438415 Flick M. J. Konieczny S. F. (2002 ). Identification of putative mammalian D-lactate dehydrogenase enzymes. Biochem. Biophys. Res. Commun. 295 910 –916 . 10.1016/s0006-291x(02)00768-4 12127981 Fong M. Y. McDunn J. Kakar S. S. (2011 ). Identification of metabolites in the normal ovary and their transformation in primary and metastatic ovarian cancer. PLoS One 6 :e19963 . 10.1371/journal.pone.0019963 21625518 Fry D. W. Richardson K. E. (1979 ). Isolation and characterization of glycolic acid oxidase from human liver. Biochim. Biophys. Acta 568 135 –144 . 10.1016/0005-2744(79)90281-x 444540 Fuchs S. A. Berger R. Klomp L. W. de Koning T. J. (2005 ). D-amino acids in the central nervous system in health and disease. Mol. Genet. Metab. 85 168 –180 . 10.1016/j.ymgme.2005.03.003 15979028 Garber A. J. Ballard F. J. (1970 ). Regulation of phosphoenolpyruvate metabolism in mitochondria from guinea pig liver. J. Biol. Chem. 245 2229 –2240 .4315147 Garber A. J. Salganicoff L. (1973 ). Regulation of oxalacetate metabolism in liver mitochondria. Evidence for nicotinamide adenine dinucleotide-malate dehydrogenase equilibrium and the role of phosphoenolpyruvate carboxykinase in the control of oxalacetate metabolism in intact guinea pig and rat liver mitochondria. J. Biol. Chem. 248 1520 –1529 .4144388 Garrow T. A. Brenner A. A. Whitehead V. M. Chen X. N. Duncan R. G. Korenberg J. R. (1993 ). Cloning of human cDNAs encoding mitochondrial and cytosolic serine hydroxymethyltransferases and chromosomal localization. J. Biol. Chem. 268 11910 –11916 .8505317 Gavaret J. M. Nunez J. Cahnmann H. J. (1980 ). Formation of dehydroalanine residues during thyroid hormone synthesis in thyroglobulin. J. Biol. Chem. 255 5281 –5285 .7372636 Goto Y. Shimizu J. Shukuya R. (1980 ). Purification and molecular characteristics of mitochondrial phosphoenolpyruvate carboxykinase from bullfrog (Rana catesbeiana) liver. J. Biochem. 88 1239 –1249 . 10.1093/oxfordjournals.jbchem.a133092 6970195 Guo J. H. Hexige S. Chen L. Zhou G. J. Wang X. Jiang J. M. (2006 ). Isolation and characterization of the human D-glyceric acidemia related glycerate kinase gene GLYCTK1 and its alternatively splicing variant GLYCTK2. DNA Seq. 17 1 –7 . 10.1080/10425170500476665 16753811 Hall A. Meyle K. D. Lange M. K. Klima M. Sanderhoff M. Dahl C. (2013 ). Dysfunctional oxidative phosphorylation makes malignant melanoma cells addicted to glycolysis driven by the (V600E)BRAF Oncogene. Oncotarget 4 584 –599 . 10.18632/oncotarget.965 23603840 Harris R. A. Fenton A. W. (2019 ). A critical review of the role of M2PYK in the Warburg effect. Biochim. Biophys. Acta Rev. Cancer 1871 225 –239 . 10.1016/j.bbcan.2019.01.004 30708038 Hayman A. R. Warburton M. J. Pringle J. A. Coles B. Chambers T. J. (1989 ). Purification and characterization of a tartrate-resistant acid phosphatase from human osteoclastomas. Biochem. J. 261 601 –609 . 10.1042/bj2610601 2775236 Hebda C. A. Nowak T. (1982 ). The purification, characterization, and activation of phosphoenolpyruvate carboxykinase from chicken liver mitochondria. J. Biol. Chem. 257 5503 –5514 .7068603 Helwig J. J. Farooqui A. A. Bollack C. Mandel P. (1978 ). Purification and some properties of tartrate-sensitive acid phosphatase from rabbit kidney cortex. Biochem. J. 175 321 –329 . 10.1042/bj1750321 736900 Hillis A. L. Lau A. N. Devoe C. X. Dayton T. L. Danai L. V. Di Vizio D. (2018 ). PKM2 is not required for pancreatic ductal adenocarcinoma. Cancer Metab. 6 :17 . Hirsch H. Greenberg D. M. (1967 ). Studies on phosphoserine aminotransferase of sheep brain. J. Biol. Chem. 242 2283 –2287 .6022873 Holmes R. P. Assimos D. G. (1998 ). Glyoxylate synthesis, and its modulation and influence on oxalate synthesis. J. Urol. 160 1617 –1624 . 10.1097/00005392-199811000-00003 9783918 Hong C. S. Graham N. A. Gu W. Espindola Camacho C. Mah V. Maresh E. L. (2016 ). MCT1 Modulates Cancer Cell Pyruvate Export and Growth of Tumors that Co-express MCT1 and MCT4. Cell Rep. 14 1590 –1601 . 10.1016/j.celrep.2016.01.057 26876179 Hoshino A. Hirst J. A. Fujii H. (2007 ). Regulation of cell proliferation by interleu-induced nuclear translocation of pyruvate kinase. J. Biol. Chem. 282 17706 –17711 . 10.1074/jbc.m700094200 17446165 Hosios A. M. Fiske B. P. Gui D. Y. Vander Heiden M. G. (2015 ). Lack of Evidence for PKM2 Protein Kinase Activity. Mol. Cell 59 850 –857 . 10.1016/j.molcel.2015.07.013 26300261 Hsu M. C. Hung W. C. (2018 ). Pyruvate kinase M2 fuels multiple aspects of cancer cells: from cellular metabolism, transcriptional regulation to extracellular signaling. Mol. Cancer 17 :35 . Hu Y. Lu W. Chen G. Wang P. Chen Z. Zhou Y. (2012 ). K-ras(G12V) transformation leads to mitochondrial dysfunction and a metabolic switch from oxidative phosphorylation to glycolysis. Cell Res. 22 399 –412 . 10.1038/cr.2011.145 21876558 Hussien R. Brooks G. A. (2011 ). Mitochondrial and plasma membrane lactate transporter and lactate dehydrogenase isoform expression in breast cancer cell lines. Physiol. Genomics 43 255 –264 . 10.1152/physiolgenomics.00177.2010 21177384 Icard P. Lincet H. (2012 ). A global view of the biochemical pathways involved in the regulation of the metabolism of cancer cells. Biochim. Biophys. Acta 1826 423 –433 . 10.1016/j.bbcan.2012.07.001 22841746 Ip C. Hayes C. Budnick R. M. Ganther H. E. (1991 ). Chemical form of selenium, critical metabolites, and cancer prevention. Cancer Res. 51 595 –600 .1824684 Israelsen W. J. Dayton T. L. Davidson S. M. Fiske B. P. Hosios A. M. Bellinger G. (2013 ). PKM2 isoform-specific deletion reveals a differential requirement for pyruvate kinase in tumor cells. Cell 155 397 –409 . 10.1016/j.cell.2013.09.025 24120138 Israelsen W. J. Vander Heiden M. G. (2015 ). Pyruvate kinase: function, regulation and role in cancer. Semin. Cell Dev. Biol. 43 43 –51 . 10.1016/j.semcdb.2015.08.004 26277545 Ito S. Ohyama T. Kontani Y. Matslida K. Sakata S. F. Tamaki N. (2001 ). Influence of dietary protein levels on beta-alanine aminotransferase expression and activity in rats. J. Nutr. Sci. Vitaminol. 47 275 –282 . 10.3177/jnsv.47.275 11767207 Jeske L. Placzek S. Schomburg I. Chang A. Schomburg D. (2019 ). BRENDA in 2019: a European ELIXIR core data resource. Nucleic Acids Res. 47 D542 –D549 .30395242 Jones J. M. Morrell J. C. Gould S. J. (2000 ). Identification and characterization of HAOX1, HAOX2, and HAOX3, three human peroxisomal 2-hydroxy acid oxidases. J. Biol. Chem. 275 12590 –12597 . 10.1074/jbc.275.17.12590 10777549 Kakimoto Y. Taniguchi K. Sano I. (1969 ). D-beta-aminoisobutyrate:pyruvate aminotransferase in mammalian liver and excretion of beta-aminoisobutyrate by man. J. Biol. Chem. 244 335 –340 .5773299 Kanehisa M. Goto S. (2000 ). KEGG: kyoto encyclopedia of genes and genomes. Nucleic Acids Res. 28 27 –30 .10592173 Kashii T. Gomi T. Oya T. Ishii Y. Oda H. Maruyama M. (2005 ). Some biochemical and histochemical properties of human liver serine dehydratase. Int. J. Biochem. Cell Biol. 37 574 –589 . 10.1016/j.biocel.2004.08.004 15618015 Kim M. J. Whitesides G. M. (1988 ). L-Lactate dehydrogenase: substrate specificity and use as a catalyst in the synthesis of homochiral 2-hydroxy acids. J. Am. Chem. Soc. 110 2959 –2964 . 10.1021/ja00217a044 King Z. A. Lu J. Drager A. Miller P. Federowicz S. Lerman J. A. (2016 ). BiGG Models: a platform for integrating, standardizing and sharing genome-scale models. Nucleic Acids Res. 44 D515 –D522 .26476456 Kleineke J. Sauer H. Soling H. D. (1973 ). On the specificity of the tricarboxylate carrier system in rat liver mitochondria. FEBS Lett. 29 82 –86 . 10.1016/0014-5793(73)80531-9 4719206 Knott S. R. V. Wagenblast E. Khan S. Kim S. Y. Soto M. Wagner M. (2018 ). Asparagine bioavailability governs metastasis in a model of breast cancer. Nature 554 378 –381 . 10.1038/nature25465 29414946 Koizumi M. Ogata E. (2002 ). Bone metabolic markers as gauges of metastasis to bone: a review. Ann. Nucl. Med. 16 161 –168 . 10.1007/bf02996296 12126040 Koseki J. Konno M. Asai A. Colvin H. Kawamoto K. Nishida N. (2018 ). Enzymes of the one-carbon folate metabolism as anticancer targets predicted by survival rate analysis. Sci. Rep. 8 :303 . Krall A. S. Xu S. Graeber T. G. Braas D. Christofk H. R. (2016 ). Asparagine promotes cancer cell proliferation through use as an amino acid exchange factor. Nat. Commun. 7 :11457 . Krebs H. A. Johnson W. A. (1937 ). Acetopyruvic acid (alphagamma-diketovaleric acid) as an intermediate metabolite in animal tissues. Biochem. J. 31 772 –779 . 10.1042/bj0310772 16746397 Lambrecht J. A. Flynn J. M. Downs D. M. (2012 ). Conserved YjgF protein family deaminates reactive enamine/imine intermediates of pyridoxal 5’-phosphate (PLP)-dependent enzyme reactions. J. Biol. Chem. 287 3454 –3461 . 10.1074/jbc.m111.304477 22094463 Lange J. N. Wood K. D. Knight J. Assimos D. G. Holmes R. P. (2012 ). Glyoxal formation and its role in endogenous oxalate synthesis. Adv. Urol. 2012 :819202 . Lau A. N. Israelsen W. J. Roper J. Sinnamon M. J. Georgeon L. Dayton T. L. (2017 ). PKM2 is not required for colon cancer initiated by APC loss. Cancer Metab. 5 :10 . Le A. Lane A. N. Hamaker M. Bose S. Gouw A. Barbi J. (2012 ). Glucose-independent glutamine metabolism via TCA cycling for proliferation and survival in B cells. Cell Metab. 15 110 –121 . 10.1016/j.cmet.2011.12.009 22225880 Lee I. S. Muragaki Y. Ideguchi T. Hase T. Tsuji M. Ooshima A. (1995 ). Molecular cloning and sequencing of a cDNA encoding alanine-glyoxylate aminotransferase 2 from rat kidney. J. Biochem. 117 856 –862 . 10.1093/oxfordjournals.jbchem.a124787 7592550 Leithner K. Hrzenjak A. Trotzmuller M. Moustafa T. Kofeler H. C. Wohlkoenig C. (2015 ). PCK2 activation mediates an adaptive response to glucose depletion in lung cancer. Oncogene 34 1044 –1050 . 10.1038/onc.2014.47 24632615 Li X. Peng S. (2013 ). Identification of metastasis-associated genes in colorectal cancer through an integrated genomic and transcriptomic analysis. Chin. J. Cancer Res. 25 623 –636 .24385689 Li Y. H. Li X. F. Liu J. T. Wang H. Fan L. L. Li J. (2018 ). PKM2, a potential target for regulating cancer. Gene 668 48 –53 . 10.1016/j.gene.2018.05.038 29775756 Li Z. Yang P. Li Z. (2014 ). The multifaceted regulation and functions of PKM2 in tumor progression. Biochim. Biophys. Acta 1846 285 –296 . 10.1016/j.bbcan.2014.07.008 25064846 Lieu E. L. Nguyen T. Rhyne S. Kim J. (2020 ). Amino acids in cancer. Exp. Mol. Med. 52 15 –30 .31980738 Liu B. Jia Y. Cao Y. Wu S. Jiang H. Sun X. (2016 ). Overexpression of Phosphoserine Aminotransferase 1 (PSAT1) Predicts Poor Prognosis and Associates with Tumor Progression in Human Esophageal Squamous Cell Carcinoma. Cell. Physiol. Biochem. 39 395 –406 . 10.1159/000445633 27372650 Liu C. Cao J. Lin S. Zhao Y. Zhu M. Tao Z. (2020 ). Malic enzyme 1 indicates worse prognosis in breast cancer and promotes metastasis by manipulating reactive oxygen species. Onco Targets Ther. 13 8735 –8747 . 10.2147/ott.s256970 32922044 Liu P. Ge X. Ding H. Jiang H. Christensen B. M. Li J. (2012 ). Role of glutamate decarboxylase-like protein 1 (GADL1) in taurine biosynthesis. J. Biol. Chem. 287 40898 –40906 . 10.1074/jbc.m112.393728 23038267 Liu V. M. Howell A. J. Hosios A. M. Li Z. Israelsen W. J. Vander Heiden M. G. (2020 ). Cancer-associated mutations in human pyruvate kinase M2 impair enzyme activity. FEBS Lett. 594 646 –664 . 10.1002/1873-3468.13648 31642061 Loeber G. Dworkin M. B. Infante A. Ahorn H. (1994 ). Characterization of cytosolic malic enzyme in human tumor cells. FEBS Lett. 344 181 –186 . 10.1016/0014-5793(94)00386-6 8187880 Lunt S. Y. Muralidhar V. Hosios A. M. Israelsen W. J. Gui D. Y. Newhouse L. (2015 ). Pyruvate kinase isoform expression alters nucleotide synthesis to impact cell proliferation. Mol. Cell 57 95 –107 . 10.1016/j.molcel.2014.10.027 25482511 Luo W. Hu H. Chang R. Zhong J. Knabel M. O’Meally R. (2011 ). Pyruvate kinase M2 is a PHD3-stimulated coactivator for hypoxia-inducible factor 1. Cell 145 732 –744 . 10.1016/j.cell.2011.03.054 21620138 Lv L. Li D. Zhao D. Lin R. Chu Y. Zhang H. (2011 ). Acetylation targets the M2 isoform of pyruvate kinase for degradation through chaperone-mediated autophagy and promotes tumor growth. Mol. Cell 42 719 –730 . 10.1016/j.molcel.2011.04.025 21700219 Makinen A. L. Nowak T. (1983 ). 3-Mercaptopicolinate. A reversible active site inhibitor of avian liver phosphoenolpyruvate carboxykinase. J. Biol. Chem. 258 11654 –11662 .6619135 Matoba S. Kang J. G. Patino W. D. Wragg A. Boehm M. Gavrilova O. (2006 ). p53 regulates mitochondrial respiration. Science 312 1650 –1653 . 10.1126/science.1126863 16728594 Maul D. M. Schuster S. M. (1986 ). Kinetic properties and characteristics of mouse liver mitochondrial asparagine aminotransferase. Arch. Biochem. Biophys. 251 585 –593 . 10.1016/0003-9861(86)90367-x 3099645 Mayers J. R. Torrence M. E. Danai L. V. Papagiannakopoulos T. Davidson S. M. Bauer M. R. (2016 ). Tissue of origin dictates branched-chain amino acid metabolism in mutant Kras-driven cancers. Science 353 1161 –1165 . 10.1126/science.aaf5171 27609895 Mazurek S. Boschek C. B. Hugo F. Eigenbrodt E. (2005 ). Pyruvate kinase type M2 and its role in tumor growth and spreading. Semin. Cancer Biol. 15 300 –308 . 10.1016/j.semcancer.2005.04.009 15908230 McKeehan W. L. (1982 ). Glycolysis, glutaminolysis and cell proliferation. Cell Biol. Int. Rep. 6 635 –650 . 10.1016/0309-1651(82)90125-4 6751566 Mdluli K. Booth M. P. Brady R. L. Rumsby G. (2005 ). A preliminary account of the properties of recombinant human Glyoxylate reductase (GRHPR), LDHA and LDHB with glyoxylate, and their potential roles in its metabolism. Biochim. Biophys. Acta 1753 209 –216 . 10.1016/j.bbapap.2005.08.004 16198644 Medina D. Thompson H. Ganther H. Ip C. (2001 ). Se-methylselenocysteine: a new compound for chemoprevention of breast cancer. Nutr. Cancer 40 12 –17 . 10.4324/9781410608000-4 11799917 Metcalf S. Dougherty S. Kruer T. Hasan N. Biyik-Sit R. Reynolds L. (2020 ). Selective loss of phosphoserine aminotransferase 1 (PSAT1) suppresses migration, invasion, and experimental metastasis in triple negative breast cancer. Clin. Exp. Metastasis 37 187 –197 . 10.1007/s10585-019-10000-7 31630284 Michelakis E. D. Webster L. Mackey J. R. (2008 ). Dichloroacetate (DCA) as a potential metabolic-targeting therapy for cancer. Br. J. Cancer 99 989 –994 . 10.1038/sj.bjc.6604554 18766181 Mills E. L. Kelly B. Logan A. Costa A. S. H. Varma M. Bryant C. E. (2016 ). Succinate Dehydrogenase supports metabolic repurposing of mitochondria to drive inflammatory macrophages. Cell 167 457 –470.e13 .27667687 Minatogawa Y. Noguchi T. Kido R. (1977 ). Species distribution and properties of hepatic phenylalanine (histidine):pyruvate aminotransferase. Hoppe Seylers Z. Physiol. Chem. 358 59 –67 . 10.1515/bchm2.1977.358.1.59 14070 Moreno-Sanchez R. Marin-Hernandez A. Gallardo-Perez J. C. Pacheco-Velazquez S. C. Robledo-Cadena D. X. Padilla-Flores J. A. (2020 ). Physiological Role of Glutamate Dehydrogenase in Cancer Cells. Front. Oncol. 10 :429 . 10.3389/fonc.2020.00429 32328457 Moretti S. Martin O. Van Du Tran T. Bridge A. Morgat A. Pagni M. (2016 ). MetaNetX/MNXref–reconciliation of metabolites and biochemical reactions to bring together genome-scale metabolic networks. Nucleic Acids Res. 44 D523 –D526 .26527720 Mose S. Menzel C. Kurth A. A. Obert K. Breidert I. Borowsky K. (2003 ). Tartrate-resistant acid phosphatase 5b as serum marker of bone metabolism in cancer patients. Anticancer Res. 23 2783 –2788 .12926113 Murai S. Ando A. Ebara S. Hirayama M. Satomi Y. Hara T. (2017 ). Inhibition of malic enzyme 1 disrupts cellular metabolism and leads to vulnerability in cancer cells in glucose-restricted conditions. Oncogenesis 6 :e329 . 10.1038/oncsis.2017.34 28481367 Murata M. Midorikawa K. Kawanishi S. (2016 ). “Chapter 25 - Molecular Link Between Alcohol and Breast Cancer: the Role of Salsolinol ,” in Molecular Aspects of Alcohol and Nutrition , ed. Patel V. B. (San Diego, CA : Academic Press ), 315 –324 . 10.1016/b978-0-12-800773-0.00025-2 Nagahara N. Sawada N. (2006 ). The mercaptopyruvate pathway in cysteine catabolism: a physiologic role and related disease of the multifunctional 3-mercaptopyruvate sulfurtransferase. Curr. Med. Chem. 13 1219 –1230 . 10.2174/092986706776360914 16719781 Nagasawa T. Ishii T. Kumagai H. Yamada H. (1985 ). D-Cysteine desulfhydrase of Escherichia coli. Purification and characterization. Eur. J. Biochem. 153 541 –551 . 10.1111/j.1432-1033.1985.tb09335.x 3908101 Nagata Y. Masui R. Akino T. (1992 ). The presence of free D-serine, D-alanine and D-proline in human plasma. Experientia 48 986 –988 .1426150 Nagayama H. Muramatsu M. Shimura K. (1958 ). Enzymatic formation of aminomalonic acid from ketomalonic acid. Nature 181 417 –418 . 10.1038/181417a0 13504217 Nakano M. Tsutsumi Y. Danowski T. S. (1967 ). Crystalline L-amino-acid oxidase from the soluble fraction of rat-kidney cells. Biochim. Biophys. Acta 139 40 –48 . 10.1016/0005-2744(67)90111-8 4962138 Nakashima C. Kirita T. Yamamoto K. Mori S. Luo Y. Sasaki T. (2020 ). Malic enzyme 1 is associated with tumor budding in oral squamous cell carcinomas. Int. J. Mol. Sci. 21 :7149 . 10.3390/ijms21197149 32998265 Nemeth B. Doczi J. Csete D. Kacso G. Ravasz D. Adams D. (2016 ). Abolition of mitochondrial substrate-level phosphorylation by itaconic acid produced by LPS-induced Irg1 expression in cells of murine macrophage lineage. FASEB J. 30 286 –300 . 10.1096/fj.15-279398 26358042 Newman A. C. Maddocks O. D. K. (2017 ). Serine and functional metabolites in cancer. Trends Cell Biol. 27 645 –657 . 10.1016/j.tcb.2017.05.001 28601431 Nguyen M. Bonneterre J. Hecquet B. Desoize B. Demaille A. (1991 ). Plasma acid and alkaline phosphatase in patients with breast cancer. Anticancer Res. 11 831 –833 .2064338 Nicolay B. N. Danielian P. S. Kottakis F. Lapek J. D. Jr.Sanidas I. Miles W. O. (2015 ). Proteomic analysis of pRb loss highlights a signature of decreased mitochondrial oxidative phosphorylation. Genes Dev. 29 1875 –1889 . 10.1101/gad.264127.115 26314710 Nokin M. J. Bellier J. Durieux F. Peulen O. Rademaker G. Gabriel M. (2019 ). Methylglyoxal, a glycolysis metabolite, triggers metastasis through MEK/ERK/SMAD1 pathway activation in breast cancer. Breast Cancer Res. 21 :11 . Nordlie R. C. (1974 ). Metabolic regulation by multifunctional glucose-6-phosphatase. Curr. Top. Cell. Regul. 8 33 –117 . 10.1016/b978-0-12-152808-9.50009-2 4370737 Noronha A. Modamio J. Jarosz Y. Guerard E. Sompairac N. Preciat G. (2019 ). The Virtual Metabolic Human database: integrating human and gut microbiome metabolism with nutrition and disease. Nucleic Acids Res. 47 D614 –D624 .30371894 Ogawa H. Gomi T. Nishizawa M. Hayakawa Y. Endo S. Hayashi K. (2006 ). Enzymatic and biochemical properties of a novel human serine dehydratase isoform. Biochim. Biophys. Acta 1764 961 –971 . 10.1016/j.bbapap.2006.02.010 16580895 Omura H. Shimamura M. Yamafuji K. (1956 ). Measurement of transoximase action. Enzymologia 17 359 –362 .13397521 Orywal K. Szmitkowski M. (2017 ). Alcohol dehydrogenase and aldehyde dehydrogenase in malignant neoplasms. Clin. Exp. Med. 17 131 –139 . 10.1007/s10238-016-0408-3 26886278 Pallini R. Guazzi G. C. Cannella C. Cacace M. G. (1991 ). Cloning and sequence analysis of the human liver rhodanese: comparison with the bovine and chicken enzymes. Biochem. Biophys. Res. Commun. 180 887 –893 . 10.1016/s0006-291x(05)81148-9 1953758 Panosyan E. H. Wang Y. Xia P. Lee W. N. Pak Y. Laks D. R. (2014 ). Asparagine depletion potentiates the cytotoxic effect of chemotherapy against brain tumors. Mol. Cancer Res. 12 694 –702 . 10.1158/1541-7786.mcr-13-0576 24505127 Paone A. Marani M. Fiascarelli A. Rinaldo S. Giardina G. Contestabile R. (2014 ). SHMT1 knockdown induces apoptosis in lung cancer cells by causing uracil misincorporation. Cell Death Dis. 5 :e1525 . 10.1038/cddis.2014.482 25412303 Passarella S. Atlante A. Valenti D. de Bari L. (2003 ). The role of mitochondrial transport in energy metabolism. Mitochondrion 2 319 –343 . 10.1016/s1567-7249(03)00008-4 16120331 Pastuszak I. Drake R. Elbein A. D. (1996 ). Kidney N-acetylgalactosamine (GalNAc)-1-phosphate kinase, a new pathway of GalNAc activation. J. Biol. Chem. 271 20776 –20782 . 10.1074/jbc.271.34.20776 8702831 Petit M. Koziel R. Etemad S. Pircher H. Jansen-Durr P. (2017 ). Depletion of oxaloacetate decarboxylase FAHD1 inhibits mitochondrial electron transport and induces cellular senescence in human endothelial cells. Exp. Gerontol. 92 7 –12 . 10.1016/j.exger.2017.03.004 28286170 Phang J. M. (2019 ). Proline metabolism in cell regulation and cancer biology: recent advances and hypotheses. Antioxid. Redox Signal. 30 635 –649 . 10.1089/ars.2017.7350 28990419 Pinthong C. Maenpuen S. Amornwatcharapong W. Yuthavong Y. Leartsakulpanich U. Chaiyen P. (2014 ). Distinct biochemical properties of human serine hydroxymethyltransferase compared with the Plasmodium enzyme: implications for selective inhibition. FEBS J. 281 2570 –2583 . 10.1111/febs.12803 24698160 Pircher H. Straganz G. D. Ehehalt D. Morrow G. Tanguay R. M. Jansen-Durr P. (2011 ). Identification of human fumarylacetoacetate hydrolase domain-containing protein 1 (FAHD1) as a novel mitochondrial acylpyruvase. J. Biol. Chem. 286 36500 –36508 . 10.1074/jbc.m111.264770 21878618 Pircher H. von Grafenstein S. Diener T. Metzger C. Albertini E. Taferner A. (2015 ). Identification of FAH domain-containing protein 1 (FAHD1) as oxaloacetate decarboxylase. J. Biol. Chem. 290 6755 –6762 . 10.1074/jbc.m114.609305 25575590 Poore R. E. Hurst C. H. Assimos D. G. Holmes R. P. (1997 ). Pathways of hepatic oxalate synthesis and their regulation. Am. J. Physiol. 272 C289 –C294 .9038835 Prakasam G. Iqbal M. A. Bamezai R. N. K. Mazurek S. (2018 ). Posttranslational Modifications of Pyruvate Kinase M2: tweaks that benefit cancer. Front. Oncol. 8 :22 . 10.3389/fonc.2018.00022 29468140 Reitzer L. J. Wice B. M. Kennell D. (1979 ). Evidence that glutamine, not sugar, is the major energy source for cultured HeLa cells. J. Biol. Chem. 254 2669 –2676 .429309 Ren J. G. Seth P. Clish C. B. Lorkiewicz P. K. Higashi R. M. Lane A. N. (2014 ). Knockdown of malic enzyme 2 suppresses lung tumor growth, induces differentiation and impacts PI3K/AKT signaling. Sci. Rep. 4 :5414 . Richarme G. Dairou J. (2017 ). Parkinsonism-associated protein DJ-1 is a bona fide deglycase. Biochem. Biophys. Res. Commun. 483 387 –391 . 10.1016/j.bbrc.2016.12.134 28013050 Richarme G. Mihoub M. Dairou J. Bui L. C. Leger T. Lamouri A. (2015 ). Parkinsonism-associated protein DJ-1/Park7 is a major protein deglycase that repairs methylglyoxal- and glyoxal-glycated cysteine, arginine, and lysine residues. J. Biol. Chem. 290 1885 –1897 . 10.1074/jbc.m114.597815 25416785 Robinson B. H. (1971 ). Transport of phosphoenolpyruvate by the tricarboxylate transporting system in mammalian mitochondria. FEBS Lett. 14 309 –312 . 10.1016/0014-5793(71)80287-9 11945784 Robinson J. L. Kocabas P. Wang H. Cholley P. E. Cook D. Nilsson A. (2020 ). An atlas of human metabolism. Sci. Signal. 13 :eaaz1482 . Rowsell E. V. (1956 ). Transaminations with pyruvate and other alpha-keto acids. Biochem. J. 64 246 –252 . 10.1042/bj0640246 13363834 Sandler M. Carter S. B. Hunter K. R. Stern G. M. (1973 ). Tetrahydroisoquinoline alkaloids: in vivo metabolites of L-dopa in man. Nature 241 439 –443 . 10.1038/241439a0 4705752 Satrustegui J. Pardo B. Del Arco A. (2007 ). Mitochondrial transporters as novel targets for intracellular calcium signaling. Physiol. Rev. 87 29 –67 . 10.1152/physrev.00005.2006 17237342 Sauer L. A. (1973 ). An NAD- and NADP-dependent malic enzyme with regulatory properties in rat liver and adrenal cortex mitochondrial fractions. Biochem. Biophys. Res. Commun. 50 524 –531 . 10.1016/0006-291x(73)90871-1 4144006 Schadewaldt P. Wendel U. Hammen H. W. (1995 ). Human branched-chain L-amino acid aminotransferase: activity and subcellular localization in cultured skin fibroblasts. Amino Acids 9 147 –160 .24178815 Scott D. A. Richardson A. D. Filipp F. V. Knutzen C. A. Chiang G. G. Ronai Z. A. (2011 ). Comparative metabolic flux profiling of melanoma cell lines: beyond the Warburg effect. J. Biol. Chem. 286 42626 –42634 . 10.1074/jbc.m111.282046 21998308 Serpa J. (2020 ). Cysteine as a Carbon Source, a Hot Spot in Cancer Cells Survival. Front. Oncol. 10 :947 . 10.3389/fonc.2020.00947 32714858 Shan Y. Gao Y. Jin W. Fan M. Wang Y. Gu Y. (2019 ). Targeting HIBCH to reprogram valine metabolism for the treatment of colorectal cancer. Cell Death Dis. 10 618 . Shapiro R. A. Clark V. M. Curthoys N. P. (1979 ). Inactivation of rat renal phosphate-dependent glutaminase with 6-diazo-5-oxo-L-norleucine. Evidence for interaction at the glutamine binding site. J. Biol. Chem. 254 2835 –2838 .429321 Shen H. Campanello G. C. Flicker D. Grabarek Z. Hu J. Luo C. (2017 ). The human knockout gene CLYBL connects Itaconate to Vitamin B12. Cell 171 771 –782.e11 .29056341 Shibuya N. Koike S. Tanaka M. Ishigami-Yuasa M. Kimura Y. Ogasawara Y. (2013 ). A novel pathway for the production of hydrogen sulfide from D-cysteine in mammalian cells. Nat. Commun. 4 :1366 . Shug A. L. Shrago E. (1973 ). Inhibition of phosphoenolpyruvate transport via the tricarboxylate and adenine nucleotide carrier systems of rat liver mitochondria. Biochem. Biophys. Res. Commun. 53 659 –665 . 10.1016/0006-291x(73)90712-2 4716993 Silverstein E. (1974 ). Equilibrium kinetic study of bovine liver glutamate dehydrogenase at high pH. Biochemistry 13 3750 –3754 . 10.1021/bi00715a021 4368692 Smith A. L. Whitehall J. C. Bradshaw C. Gay D. Robertson F. Blain A. P. (2020 ). Age-associated mitochondrial DNA mutations cause metabolic remodelling that contributes to accelerated intestinal tumorigenesis. Nat. Cancer 1 976 –989 . 10.1038/s43018-020-00112-5 33073241 Smith S. M. Uslaner J. M. Yao L. Mullins C. M. Surles N. O. Huszar S. L. (2009 ). The behavioral and neurochemical effects of a novel D-amino acid oxidase inhibitor compound 8 [4H-thieno [3,2-b]pyrrole-5-carboxylic acid] and D-serine. J. Pharmacol. Exp. Ther. 328 921 –930 . 10.1124/jpet.108.147884 19088300 Snell K. Natsumeda Y. Eble J. N. Glover J. L. Weber G. (1988 ). Enzymic imbalance in serine metabolism in human colon carcinoma and rat sarcoma. Br. J. Cancer 57 87 –90 . 10.1038/bjc.1988.15 3126791 Soda K. Osumi T. (1969 ). Crystalline amino acid racemase with low substrate specificity. Biochem. Biophys. Res. Commun. 35 363 –368 . 10.1016/0006-291x(69)90507-5 5788493 Soling H. D. Walter U. Sauer H. Kleineke J. (1971 ). Effects of synthetic analogues of phosphoenolpyruvate on muscle and liver pyruvate kinase, muscle enolase, liver phosphoenolpyruvate carboxykinase and on the intra-/extra-mitochondrial tricarboxylic acid carrier transport system. FEBS Lett. 19 139 –143 . 10.1016/0014-5793(71)80498-2 11946196 Son J. Lyssiotis C. A. Ying H. Wang X. Hua S. Ligorio M. (2013 ). Glutamine supports pancreatic cancer growth through a KRAS-regulated metabolic pathway. Nature 496 101 –105 . 10.1038/nature12040 23535601 Stark R. Pasquel F. Turcu A. Pongratz R. L. Roden M. Cline G. W. (2009 ). Phosphoenolpyruvate cycling via mitochondrial phosphoenolpyruvate carboxykinase links anaplerosis and mitochondrial GTP with insulin secretion. J. Biol. Chem. 284 26578 –26590 . 10.1074/jbc.m109.011775 19635791 Stetak A. Veress R. Ovadi J. Csermely P. Keri G. Ullrich A. (2007 ). Nuclear translocation of the tumor marker pyruvate kinase M2 induces programmed cell death. Cancer Res. 67 1602 –1608 . 10.1158/0008-5472.can-06-2870 17308100 Stipanuk M. H. (1979 ). Effect of excess dietary methionine on the catabolism of cysteine in rats. J. Nutr. 109 2126 –2139 . 10.1093/jn/109.12.2126 512701 Stipanuk M. H. (2020 ). Metabolism of Sulfur-Containing Amino Acids: how the Body Copes with Excess Methionine, Cysteine, and Sulfide. J. Nutr. 150 (Suppl. 1 ), 2494S –2505S .33000151 Stipanuk M. H. Dominy J. E. Jr.Lee J. I. Coloso R. M. (2006 ). Mammalian cysteine metabolism: new insights into regulation of cysteine metabolism. J. Nutr. 136 (6 Suppl. ), 1652S –1659S .16702335 Sul H. S. Shrago E. Shug A. L. (1976 ). Relationship of phosphoenolpyruvate transport, acyl coenzyme A inhibition of adenine nucleotide translocase and calcium ion efflux in guinea pig heart mitochondria. Arch. Biochem. Biophys. 172 230 –237 . 10.1016/0003-9861(76)90071-0 1252077 Sun Y. Li W. Shen S. Yang X. Lu B. Zhang X. (2019 ). Loss of alanine-glyoxylate and serine-pyruvate aminotransferase expression accelerated the progression of hepatocellular carcinoma and predicted poor prognosis. J. Transl. Med. 17 :390 . Swick R. W. Wood H. G. (1960 ). The Role of Transcarboxylation in Propionic Acid Fermentation. Proc. Natl. Acad. Sci. U.S.A. 46 28 –41 . 10.1073/pnas.46.1.28 16590594 Ta N. L. Seyfried T. N. (2015 ). Influence of Serum and Hypoxia on Incorporation of [(14)C]-D-Glucose or [(14)C]-L-Glutamine into Lipids and Lactate in Murine Glioblastoma Cells. Lipids 50 1167 –1184 . 10.1007/s11745-015-4075-z 26537505 Tech K. Tikunov A. P. Farooq H. Morrissy A. S. Meidinger J. Fish T. (2017 ). Pyruvate Kinase Inhibits Proliferation during Postnatal Cerebellar Neurogenesis and Suppresses Medulloblastoma Formation. Cancer Res. 77 3217 –3230 . 10.1158/0008-5472.can-16-3304 28515149 Terpos E. de la Fuente J. Szydlo R. Hatjiharissi E. Viniou N. Meletis J. (2003 ). Tartrate-resistant acid phosphatase isoform 5b: a novel serum marker for monitoring bone disease in multiple myeloma. Int. J. Cancer 106 455 –457 . 10.1002/ijc.11247 12845688 Thakur A. Bollig A. Wu J. Liao D. J. (2008 ). Gene expression profiles in primary pancreatic tumors and metastatic lesions of Ela-c-myc transgenic mice. Mol. Cancer 7 :11 . 10.1186/1476-4598-7-11 18218118 Thornalley P. J. (1990 ). The glyoxalase system: new developments towards functional characterization of a metabolic pathway fundamental to biological life. Biochem. J. 269 1 –11 . 10.1042/bj2690001 2198020 Tsun Z. Y. Possemato R. (2015 ). Amino acid management in cancer. Semin. Cell Dev. Biol. 43 22 –32 .26277542 Tudball N. Thomas P. (1972 ). The enzymic degradation of L-serine O-sulphate, Mechanism of the reaction. Biochem. J. 128 41 –46 . 10.1042/bj1280041 4673572 Vander Heiden M. G. Cantley L. C. Thompson C. B. (2009 ). Understanding the Warburg effect: the metabolic requirements of cell proliferation. Science 324 1029 –1033 . 10.1126/science.1160809 19460998 Vander Heiden M. G. Locasale J. W. Swanson K. D. Sharfi H. Heffron G. J. Amador-Noguez D. (2010 ). Evidence for an alternative glycolytic pathway in rapidly proliferating cells. Science 329 1492 –1499 . 10.1126/science.1188015 20847263 Vander Jagt D. L. Hunsaker L. A. (2003 ). Methylglyoxal metabolism and diabetic complications: roles of aldose reductase, glyoxalase-I, betaine aldehyde dehydrogenase and 2-oxoaldehyde dehydrogenase. Chem. Biol. Interact. 14 341 –351 . 10.1016/s0009-2797(02)00212-0 Vasseur S. Afzal S. Tardivel-Lacombe J. Park D. S. Iovanna J. L. Mak T. W. (2009 ). DJ-1/PARK7 is an important mediator of hypoxia-induced cellular responses. Proc. Natl. Acad. Sci. U.S.A. 106 1111 –1116 . 10.1073/pnas.0812745106 19144925 Vignaud C. Pietrancosta N. Williams E. L. Rumsby G. Lederer F. (2007 ). Purification and characterization of recombinant human liver glycolate oxidase. Arch. Biochem. Biophys. 465 410 –416 . 10.1016/j.abb.2007.06.021 17669354 Vincent E. E. Sergushichev A. Griss T. Gingras M. C. Samborska B. Ntimbane T. (2015 ). Mitochondrial phosphoenolpyruvate carboxykinase regulates metabolic adaptation and enables glucose-independent tumor growth. Mol. Cell 60 195 –207 . 10.1016/j.molcel.2015.08.013 26474064 Wang Y. H. Huang J. T. Chen W. L. Wang R. H. Kao M. C. Pan Y. R. (2019 ). Dysregulation of cystathionine gamma-lyase promotes prostate cancer progression and metastasis. EMBO Rep. 20 :e45986 . Wang Y. H. Israelsen W. J. Lee D. Yu V. W. C. Jeanson N. T. Clish C. B. (2014 ). Cell-state-specific metabolic dependency in hematopoiesis and leukemogenesis. Cell 158 1309 –1323 . 10.1016/j.cell.2014.07.048 25215489 Warren L. (1986 ). Sialic acid lyase in human promyelocytic leukemic cells (HL-60) during phorbol-ester-induced differentiation. Biochim. Biophys. Acta 888 278 –281 . 10.1016/0167-4889(86)90226-0 3463364 Weber G. (1969 ). Inhibition of human brain pyruvate kinase and hexokinase by phenylalanine and phenylpyruvate: possible relevance to phenylketonuric brain damage. Proc. Natl. Acad. Sci. U.S.A. 63 1365 –1369 . 10.1073/pnas.63.4.1365 5260939 Weber G. Cantero A. (1955 ). Glucose-6-phosphatase activity in normal, pre-cancerous, and neoplastic tissues. Cancer Res. 15 105 –108 .14352196 Wiese T. J. Wuensch S. A. Ray P. D. (1996 ). Synthesis of citrate from phosphoenolpyruvate and acetylcarnitine by mitochondria from rabbit, pigeon and rat liver: implications for lipogenesis. Comp. Biochem. Physiol. B Biochem. Mol. Biol. 114 417 –422 . 10.1016/0305-0491(96)00035-1 8840517 Wilson D. F. Erecinska M. Schramm V. L. (1983 ). Evaluation of the relationship between the intra- and extramitochondrial [ATP]/[ADP] ratios using phosphoenolpyruvate carboxykinase. J. Biol. Chem. 258 10464 –10473 .6885788 Windmueller H. G. Spaeth A. E. (1974 ). Uptake and metabolism of plasma glutamine by the small intestine. J. Biol. Chem. 249 5070 –5079 .4605420 Wishart D. S. Feunang Y. D. Marcu A. Guo A. C. Liang K. Vazquez-Fresno R. (2018 ). HMDB 4.0: the human metabolome database for 2018. Nucleic Acids Res. 46 D608 –D617 .29140435 Wong N. Ojo D. Yan J. Tang D. (2015 ). PKM2 contributes to cancer metabolism. Cancer Lett. 356 184 –191 . 10.1016/j.canlet.2014.01.031 24508027 Wuensch S. A. Ray P. D. (1997 ). Synthesis of citrate from phosphoenolpyruvate and acetylcarnitine by mitochondria from rabbit enterocytes: implications for lipogenesis. Comp. Biochem. Physiol. B Biochem. Mol. Biol. 118 599 –605 . 10.1016/s0305-0491(97)00242-3 9467872 Yamada K. Noguchi T. (1999 ). Nutrient and hormonal regulation of pyruvate kinase gene expression. Biochem. J. 337 (Pt 1 ), 1 –11 . 10.1042/0264-6021:3370001 9854017 Yang H. Jia X. (2014 ). Safety evaluation of Se-methylselenocysteine as nutritional selenium supplement: acute toxicity, genotoxicity and subchronic toxicity. Regul. Toxicol. Pharmacol. 70 720 –727 . 10.1016/j.yrtph.2014.10.014 25444999 Yang M. Vousden K. H. (2016 ). Serine and one-carbon metabolism in cancer. Nat. Rev. Cancer 16 650 –662 . 10.1038/nrc.2016.81 27634448 Yang W. Lu Z. (2013 ). Nuclear PKM2 regulates the Warburg effect. Cell Cycle 12 3154 –3158 .24013426 Yang W. Lu Z. (2015 ). Pyruvate kinase M2 at a glance. J. Cell Sci. 128 1655 –1660 . 10.1242/jcs.166629 25770102 Yang W. Zheng Y. Xia Y. Ji H. Chen X. Guo F. (2012 ). ERK1/2-dependent phosphorylation and nuclear translocation of PKM2 promotes the Warburg effect. Nat. Cell Biol. 14 1295 –1304 . 10.1038/ncb2629 23178880 Ye D. Guan K. L. Xiong Y. (2018 ). Metabolism, Activity, and Targeting of D- and L-2-Hydroxyglutarates. Trends Cancer 4 151 –165 . 10.1016/j.trecan.2017.12.005 29458964 Yoon S. Kim J. G. Seo A. N. Park S. Y. Kim H. J. Park J. S. (2015 ). Clinical implication of serine metabolism-associated enzymes in colon cancer. Oncology 89 351 –359 . 10.1159/000439571 26439504 You J. Shi X. Liang H. Ye J. Wang L. Han H. (2017 ). Cystathionine- gamma-lyase promotes process of breast cancer in association with STAT3 signaling pathway. Oncotarget 8 65677 –65686 . 10.18632/oncotarget.20057 29029463 Yu L. Teoh S. T. Ensink E. Ogrodzinski M. P. Yang C. Vazquez A. I. (2019 ). Cysteine catabolism and the serine biosynthesis pathway support pyruvate production during pyruvate kinase knockdown in pancreatic cancer cells. Cancer Metab. 7 :13 . Yu Y. H. Chang Y. C. Su T. H. Nong J. Y. Li C. C. Chuang L. M. (2013 ). Prostaglandin reductase-3 negatively modulates adipogenesis through regulation of PPARgamma activity. J. Lipid Res. 54 2391 –2399 . 10.1194/jlr.m037556 23821743 Yuan M. McNae I. W. Chen Y. Blackburn E. A. Wear M. A. Michels P. A. M. (2018 ). An allostatic mechanism for M2 pyruvate kinase as an amino-acid sensor. Biochem. J. 475 1821 –1837 . 10.1042/bcj20180171 29748232 Yuen C. A. Asuthkar S. Guda M. R. Tsung A. J. Velpula K. K. (2016 ). Cancer stem cell molecular reprogramming of the Warburg effect in glioblastomas: a new target gleaned from an old concept. CNS Oncol. 5 101 –108 . 10.2217/cns-2015-0006 26997129 Zeidan Q. Hart G. W. (2010 ). The intersections between O-GlcNAcylation and phosphorylation: implications for multiple signaling pathways. J. Cell Sci. 123 13 –22 . 10.1242/jcs.053678 20016062 Zelewski M. Swierczynski J. (1991 ). Malic enzyme in human liver. Intracellular distribution, purification and properties of cytosolic isozyme. Eur. J. Biochem. 201 339 –345 . 10.1111/j.1432-1033.1991.tb16291.x 1935931 Zhang Z. Deng X. Liu Y. Liu Y. Sun L. Chen F. (2019 ). PKM2, function and expression and regulation. Cell Biosci. 9 :52 . Zhao J. Li J. Fan T. W. M. Hou S. X. (2017 ). Glycolytic reprogramming through PCK2 regulates tumor initiation of prostate cancer cells. Oncotarget 8 83602 –83618 . 10.18632/oncotarget.18787 29137367 Zhou H. L. Zhang R. Anand P. Stomberski C. T. Qian Z. Hausladen A. (2019 ). Metabolic reprogramming by the S-nitroso-CoA reductase system protects against kidney injury. Nature 565 96 –100 . 10.1038/s41586-018-0749-z 30487609 Zhou S. Kachhap S. Singh K. K. (2003 ). Mitochondrial impairment in p53-deficient human cancer cells. Mutagenesis 18 287 –292 . 10.1093/mutage/18.3.287 12714696 Zielke H. R. Sumbilla C. M. Sevdalian D. A. Hawkins R. L. Ozand P. T. (1980 ). Lactate: a major product of glutamine metabolism by human diploid fibroblasts. J. Cell. Physiol. 104 433 –441 . 10.1002/jcp.1041040316 7419614