Mitochondrial complex I (CI) deficiency represents a common biochemical pathophysiology underlying Leigh syndrome spectrum (LSS), manifesting with progressive multi-system dysfunction, lactic acidemia, and early mortality. To facilitate mechanistic studies and rigorous screening of therapeutic candidates for CI deficient LSS, we used CRISPR/Cas9 to generate an ndufs2 -/- 16 bp deletion zebrafish strain. ndufs2 -/- larvae exhibit markedly reduced survival, severe neuromuscular dysfunction including impaired swimming capacity, multiple morphologic malformations, reduced growth, hepatomegaly, uninflated swim bladder, yolk retention, small intestines, and small eyes and pupils with abnormal retinal ganglion cell layer. Transcriptome profiling of ndufs2 -/- larvae revealed dysregulation of the electron transport chain, TCA cycle, fatty acid beta-oxidation, and one-carbon metabolism. Similar transcriptomic profiles were observed in ndufs2 -/- missense mutant C. elegans (gas-1(fc21)) and two human CI-disease fibroblast cell lines stressed in galactose media. ndufs2 -/- zebrafish had 80% reduced CI enzyme activity. Unbiased metabolomic profiling showed increased lactate, TCA cycle intermediates, and acyl-carnitine species. One-carbon metabolism associated pathway alterations appear to contribute to CI disease pathophysiology, as folic acid treatment rescued the growth defect and hepatomegaly in ndufs2 -/- larvae. Overall, ndufs2 -/- zebrafish recapitulate severe CI deficiency, complex metabolic pathophysiology, and relevant LSS neuromuscular and survival phenotypes, enabling future translational studies of therapeutic candidates.

BackgroundZagociguat (zag) is a CNS-penetrant, soluble guanylate cyclase (sGC) stimulator that has been evaluated in phase 2a, with phase 2b ongoing, clinical studies of primary mitochondrial disease (PMD) subjects with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS). To explore its utility in a broader array of PMDs and secondary mitochondrial disorders, we performed prfeclinical modeling of zag across larval and adult zebrafish models with biochemical deficiencies in diverse respiratory chain (RC) complexes or dihydrolipoamide dehydrogenase (Dldh).MethodsZag was evaluated for tissue uptake, gross toxicity, protection from RC toxin-induced brain death, neuromuscular dysfunction, heartbeat loss, and biochemical dysfunction in transgenic or toxin-exposed zebrafish with mitochondrial enzyme deficiencies in complex I (ndufs2−/− or rotenone-exposed wild type (WT)), complex IV (surf1−/− or azide-exposed WT), multiple RC complexes (fbxl4−/−), or pyruvate dehydrogenase complex (dldh−/−). Zag effects were also studied on the whole-body oxygen consumption capacity (MO2) and swimming activity of WT and complex IV disease adult zebrafish.ResultsSimilar zag levels were observed in adult brains and tail muscle. No morphological or functional toxic effects of zag were observed on larvae viability. Zag provided neuromuscular protection in complex I deficient genetic and pharmacologic inhibitor models. In complex IV deficient models, prevention from brain death occurred at 100 nM zag in high-dose azide-exposed WT larvae; however, no rescue of swimming or neuromuscular phenotypes in low-dose azide-exposed surf1−/− larvae was observed. A total of 100 nM zag rescued MO2 and maximum swimming speed in adult surf1−/− zebrafish. Larval swimming activity was also preserved with 10 nM zag treatment in azide-stressed fbxl4−/− larvae but not at 10 nM, 100 nM, or 1 µM zag in dldh−/− larvae. Zag (10 nM) enhanced complex I enzyme activity that is suggestive of mitochondrial biogenesis and key aspects of mitochondrial physiology in azide-exposed surf1−/− and fbxl4−/− larvae.ConclusionPreclinical evaluation of zag demonstrated its safety, significant protection of neuromuscular dysfunction and/or acute RC stressor-induced decompensation, and improved mitochondrial physiology across multiple different genetic and/or pharmacologic models of RC-deficient PMD. Thus, zag may yield therapeutic potential for an array of diseases with mitochondrial dysfunction beyond MELAS, potentially including Leigh syndrome spectrum disorder and primary mitochondrial myopathies.

N-linked glycoproteins function in numerous biological processes, modulating enzyme activities as well as protein folding, stability, oligomerization, and trafficking. While N-glycosylation of mitochondrial proteins has been detected by untargeted MS-analyses, the physiological existence and roles of mitochondrial protein N-linked glycosylation remain under debate. Here, we report that MRS2, a mitochondrial inner membrane protein that functions as the high flux magnesium transporter, is N-glycosylated to various extents depending on cellular bioenergetic status. Both N-glycosylated and unglycosylated isoforms were consistently detected in mitochondria isolated from mouse liver, rat and mouse liver fibroblast cells (BRL 3A and AFT024, respectively) as well as human skin fibroblast cells. Immunoblotting of MRS2 showed it was bound to, and required stringent elution conditions to remove from, lectin affinity columns with covalently bound concanavalin A or Lens culinaris agglutinin. Following peptide:N-glycosidase F (PNGase F) digestion of the stringently eluted proteins, the higher Mr MRS2 bands gel-shifted to lower Mr and loss of lectin affinity was seen. BRL 3A cells treated with two different N-linked glycosylation inhibitors, tunicamycin or 6-diazo-5-oxo-l-norleucine, resulted in decreased intensity or loss of the higher Mr MRS2 isoform. To investigate the possible functional role of MRS2 N- glycosylation, we measured rapid Mg2+ influx capacity in intact mitochondria isolated from BRL 3A cells in control media or following treatment with tunicamycin or 6-diazo-5-oxo-l-norleucine. Interestingly, rapid Mg2+ influx capacity increased in mitochondria isolated from BRL 3A cells treated with either N-glycosylation inhibitor. Forcing reliance on mitochondrial respiration by treatment with either galactose media or the glycolytic inhibitor 2-deoxyglucose or by minimizing glucose concentration similarly reduced the N-glycosylated isoform of MRS2, with a correlated concomitant increase in rapid Mg2+ influx capacity. Conversely, inhibiting mitochondrial energy production in BRL 3A cells with either rotenone or oligomycin resulted in an increased fraction of N-glycosylated MRS2, with decreased rapid Mg2+ influx capacity. Collectively, these data provide strong evidence that MRS2 N-glycosylation is directly involved in the regulation of mitochondrial matrix Mg2+, dynamically communicating relative cellular nutrient status and bioenergetic capacity by serving as a physiologic brake on the influx of mitochondrial matrix Mg2+ under conditions of glucose excess or mitochondrial bioenergetic impairment.

Dihydrolipoamide dehydrogenase (DLD) deficiency is a recessive mitochondrial disease caused by variants in DLD, the E3 subunit of mitochondrial α-keto (or 2-oxo) acid dehydrogenase complexes. DLD disease symptoms are multisystemic, variably manifesting as Leigh syndrome, neurodevelopmental disability, seizures, cardiomyopathy, liver disease, fatigue, and lactic acidemia. While most DLD disease symptoms are attributed to dysfunction of the pyruvate dehydrogenase complex, the effects of other α-keto acid dehydrogenase deficiencies remain unclear. Current therapies for DLD deficiency are ineffective, with no vertebrate animal model available for preclinical study. We created a viable Danio rerio (zebrafish) KO model of DLD deficiency, dldhcri3. Detailed phenotypic characterization revealed shortened larval survival, uninflated swim bladder, hepatomegaly and fatty liver, and reduced swim activity. These animals displayed increased pyruvate and lactate levels, with severe disruption of branched-chain amino acid catabolism manifest as increased valine, leucine, isoleucine, α-ketoisovalerate, and α-ketoglutarate levels. Evaluation of mitochondrial ultrastructure revealed gross enlargement, severe cristae disruption, and reduction in matrix electron density in liver, intestines, and muscle. Therapeutic modeling of candidate therapies demonstrated that probucol or thiamine improved larval swim activity. Overall, this vertebrate model demonstrated characteristic phenotypic and metabolic alterations of DLD disease, offering a robust platform to screen and characterize candidate therapies.

PDF file, 1661K, Supplemental Figure S1- Chemical Structures. Supplemental Figure S2-Oxygen Consumption in MCF-7 Cells treated with or without betalapchone and dicoumarol. Supplemental Figure S2 legend. Supplemental Figure S3-Survival assay for normal mammary epithelial cells treated with betalapachone with or without dicoumarol. Supplemental Figure S3 legend. Supplemental Figure S4-Survival assay for MDA-MB231 cells treated with or without quinones and ROS scavengers. Supplemental Figure S4 legend. Supplemental Figure S5-Westernblot of MCF-7 cells treated with betalapachone with or without catalase. Supplemental Figure S5 legend. Supplemental Figure S6-Proteolysis of PARP in MCF-7 cells treated with betalapachone or staurosporine. Supplemental Figure S6 legend. Supplemental Figure S7- NQO1 mediated futile cycle of betalapachone. Supplemental Figure S7 legend. Supplemental Figure S8- Western blot and analysis of NQO1 to Catalase ratios in Breast Cancer Cell lines. Supplemental Table 1- NQO1 enzymatic activity in breast cancer cell lines. Supplemental Table 1 legend.

SURF1 deficiency (OMIM # 220110) causes Leigh syndrome (LS, OMIM # 256000), a mitochondrial disorder typified by stress-induced metabolic strokes, neurodevelopmental regression and progressive multisystem dysfunction. Here, we describe two novel surf1(-/-) zebrafish knockout models generated by CRISPR/Cas9 technology. While gross larval morphology, fertility, and survival into adulthood appeared unaffected, surf1(-/-) mutants manifested adult-onset ocular anomalies and decreased swimming activity, as well as classical biochemical hallmarks of human SURF1 disease, including reduced complex IV expression and enzymatic activity and increased tissue lactate. surf1(-/-) larvae also demonstrated oxidative stress and stressor hypersensitivity to the complex IV inhibitor, azide, which exacerbated their complex IV deficiency, reduced supercomplex formation, and induced acute neurodegeneration typical of LS including brain death, impaired neuromuscular responses, reduced swimming activity, and absent heartrate. Remarkably, prophylactic treatment of surf1(-/-) larvae with either cysteamine bitartrate or N-acetylcysteine, but not other antioxidants, significantly improved animal resiliency to stressor-induced brain death, swimming and neuromuscular dysfunction, and loss of heartbeat. Mechanistic analyses demonstrated cysteamine bitartrate pretreatment did not improve complex IV deficiency, ATP deficiency, or increased tissue lactate but did reduce oxidative stress and restore glutathione balance in surf1(-/-) animals. Overall, two novel surf1(-/-) zebrafish models recapitulate the gross neurodegenerative and biochemical hallmarks of LS, including azide stressor hypersensitivity that was associated with glutathione deficiency and ameliorated by cysteamine bitartrate or N-acetylcysteine therapy.

Dihydrolipoamide dehydrogenase (DLD) deficiency is a recessive mitochondrial disorder caused by depletion of DLD from α-ketoacid dehydrogenase complexes. Caenorhabditis elegans animal models of DLD deficiency generated by graded feeding of dld-1(RNAi) revealed that full or partial reduction of DLD-1 expression recapitulated increased pyruvate levels typical of pyruvate dehydrogenase complex deficiency and significantly altered animal survival and health, with reductions in brood size, adult length, and neuromuscular function. DLD-1 deficiency dramatically increased mitochondrial unfolded protein stress response induction and adaptive mitochondrial proliferation. While ATP levels were reduced, respiratory chain enzyme activities and in vivo mitochondrial membrane potential were not significantly altered. DLD-1 depletion directly correlated with the induction of mitochondrial stress and impairment of worm growth and neuromuscular function. The safety and efficacy of dichloroacetate, thiamine, riboflavin, 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), l-carnitine, and lipoic acid supplemental therapies empirically used for human DLD disease were objectively evaluated by life span and mitochondrial stress response studies. Only dichloroacetate and thiamine showed individual and synergistic therapeutic benefits. Collectively, these C. elegans dld-1(RNAi) animal model studies demonstrate the translational relevance of preclinical modeling of disease mechanisms and therapeutic candidates. Results suggest that clinical trials are warranted to evaluate the safety and efficacy of dichloroacetate and thiamine in human DLD disease.

The breaking of RNA strands by 2'-O-transphosphorylation is a ubiquitous reaction in biology, and enzymes that catalyze this reaction play key roles in RNA metabolism. The mechanisms of 2'-O-transphosphorylation in solution are relatively well studied, but complex and can involve different transition states depending on how the reaction is catalyzed. Because of this complexity and the lack of experimental information on transition-state structure, pinning down the chemical details of enzyme-catalyzed RNA strand cleavage has been difficult. Kinetic isotope effects (KIEs) provide information about changes in bonding as a reaction proceeds from ground state to transition state, and therefore they provide a powerful tool for revealing mechanistic detail. Application of kinetic isotope analyses to RNA 2'-O-transphosphorylation faces three fundamental challenges: synthesis of RNA substrate isotopomers with 18O substitutions at the 2'-O, 5'-O and nonbridging phosphoryl oxygens; determination of the 18O/16O ratios in the residual unreacted substrate or product RNAs; and analyzing these data to allow calculation of the KIEs for use in evaluating different mechanistic scenarios. In this chapter, we outline methods for surmounting these challenges for solution RNA 2'-O-transphosphorylation reactions, and we describe their initial application to understand nonenzymatic solution reactions and reactions catalyzed by the enzyme ribonuclease A.

Solvent D2O and (18)O kinetic isotope effects on RNA 2'-O-transphosphorylation catalyzed by Zn(2+) demonstrate an altered transition state relative to specific base catalysis. A recent model from DFT calculations involving inner sphere coordination to the non-bridging and leaving group oxygens is consistent with the data.

The specificity of enzymes for their respective substrates has been a focal point of enzyme kinetics since the initial characterization of metabolic chemistry. Various processes to quantify an enzyme's specificity using kinetics have been utilized over the decades. Fersht's definition of the ratio kcat/Km for two different substrates as the “specificity constant” (ref [7]), based on the premise that the important specificity existed when the substrates were competing in the same reaction, has become a consensus standard for enzymes obeying Michaelis–Menten kinetics. The expansion of the theory for the determination of the relative specificity constants for a very large number of competing substrates, e.g. those present in a combinatorial library, in a single reaction mixture has been developed in this contribution. The ratio of kcat/Km for isotopologs has also become a standard in mechanistic enzymology where kinetic isotope effects have been measured by the development of internal competition experiments with extreme precision. This contribution extends the theory of kinetic isotope effects to internal competition between three isotopologs present at non-tracer concentrations in the same reaction mix. This article is part of a special issue titled: Enzyme Transition States from Theory and Experiment.

We developed an isotopic technique to assess mitochondrial acetyl-CoA turnover (approximate to citric acid flux) in perfused rat hearts. Hearts are perfused with buffer containing tracer [C-13(2),H-2(3)]acetate, which forms M5 + M4 + M3 acetyl-CoA. The buffer may also contain one or two labeled substrates, which generate M2 acetyl-CoA ( e. g. [C-13(6)] glucose or [1,2-C-13(2)] palmitate) or/and M1 acetyl-CoA ( e. g. [1-13C] octanoate). The total acetylCoA turnover and the contributions of fuels to acetyl-CoA are calculated from the uptake of the acetate tracer and the mass isotopomer distribution of acetyl-CoA. The method was applied to measurements of acetyl-CoA turnover under different conditions ( glucose +/- palmitate +/- insulin +/- dichloroacetate). The data revealed ( i) substrate cycling between glycogen and glucose- 6-P and between glucose-6-P and triose phosphates, ( ii) the release of small excess acetyl groups as acetylcarnitine and ketone bodies, and ( iii) the channeling of mitochondrial acetylCoA from pyruvate dehydrogenase to carnitine acetyltransferase. Because of this channeling, the labeling of acetylcarnitine and ketone bodies released by the heart are not proxies of the labeling of mitochondrial acetyl-CoA.

We recently reported that levulinate (4-ketopentanoate) is converted in the liver to 4-hydroxypentanoate, a drug of abuse, and that the formation of 4-hydroxypentanoate is stimulated by ethanol oxidation. We also identified 3 parallel β-oxidation pathways by which levulinate and 4-hydroxypentanoate are catabolized to propionyl-CoA and acetyl-CoA. We now report that levulinate forms three seven-carbon cyclical CoA esters by processes starting with the elongation of levulinyl-CoA by acetyl-CoA to 3,6-diketoheptanoyl-CoA. The latter γ-diketo CoA ester undergoes two parallel cyclization processes. One process yields a mixture of tautomers, i.e., cyclopentenyl- and cyclopentadienyl-acyl-CoAs. The second cyclization process yields a methyl-pyrrolyl-acetyl-CoA containing a nitrogen atom derived from the ε-nitrogen of lysine but without carbons from lysine. The cyclic CoA esters were identified in rat livers perfused with levulinate and in livers and brains from rats gavaged with calcium levulinate ± ethanol. Lastly, 3,6-diketoheptanoyl-CoA, like 2,5-diketohexane, pyrrolates free lysine and, presumably, lysine residues from proteins. This may represent a new pathway for protein pyrrolation. The cyclic CoA esters and related pyrrolation processes may play a role in the toxic effects of 4-hydroxypentanoate.

Abstract Improving patient outcome by personalized therapy involves a thorough understanding of an agent's mechanism of action. β-Lapachone (clinical forms, Arq501/Arq761) has been developed to exploit dramatic cancer-specific elevations in the phase II detoxifying enzyme NAD(P)H:quinone oxidoreductase (NQO1). NQO1 is dramatically elevated in solid cancers, including primary and metastatic [e.g., triple-negative (ER−, PR−, Her2/Neu−)] breast cancers. To define cellular factors that influence the efficacy of β-lapachone using knowledge of its mechanism of action, we confirmed that NQO1 was required for lethality and mediated a futile redox cycle where ∼120 moles of superoxide were formed per mole of β-lapachone in 2 minutes. β-Lapachone induced reactive oxygen species (ROS), stimulated DNA single-strand break-dependent poly(ADP-ribose) polymerase-1 (PARP1) hyperactivation, caused dramatic loss of essential nucleotides (NAD+/ATP), and elicited programmed necrosis in breast cancer cells. Although PARP1 hyperactivation and NQO1 expression were major determinants of β-lapachone–induced lethality, alterations in catalase expression, including treatment with exogenous enzyme, caused marked cytoprotection. Thus, catalase is an important resistance factor and highlights H2O2 as an obligate ROS for cell death from this agent. Exogenous superoxide dismutase enhanced catalase-induced cytoprotection. β-Lapachone–induced cell death included apoptosis-inducing factor (AIF) translocation from mitochondria to nuclei, TUNEL+ staining, atypical PARP1 cleavage, and glyceraldehyde 3-phosphate dehydrogenase S-nitrosylation, which were abrogated by catalase. We predict that the ratio of NQO1:catalase activities in breast cancer versus associated normal tissue are likely to be the major determinants affecting the therapeutic window of β-lapachone and other NQO1 bioactivatable drugs. Mol Cancer Ther; 12(10); 2110–20. ©2013 AACR.

Ribonuclease A (RNase A) is a widely used model system illustrating basic principles of enzyme catalysis. Yet, ambiguity regarding the transition state it stabilizes precludes a complete understanding of its mechanism. Here, we present kinetic isotope effect analyses that provide direct evidence for a late, anionic transition state for RNase A catalyzed RNA 2′‐O‐transphosphorylation similar to catalysis by hydroxide in solution, but with dramatically less charge accumulation on the 5′O. Molecular simulations indicate stabilization of negative charge on the NPOs by H‐bonding with a protonated His12 and Lys41. Quantum chemical calculations consistent with the experimental KIEs provide a unified mechanistic interpretation whereby P‐O5′ bond cleavage is concerted with proton transfer from a protonated His119. Thus, theory and experiment provide strong support that RNase A exploits both direct electrostatic stabilization of the anionic transition state and general acid catalysis, resulting in an altered transition state relative to that observed in solution. This work was supported by grants NIH GM096000 to M.E.H., NSF MCB0717850 to V.E.A., NIH AI081987 to J.A.P. and NIH GM064288 to D.M.Y. D.L.K was supported by NIH training grant T32‐GM008056.

Enzymes function by stabilizing reaction transition states; therefore, comparison of the transition states of enzymatic and nonenzymatic model reactions can provide insight into biological catalysis. Catalysis of RNA 2′- O -transphosphorylation by ribonuclease A is proposed to involve electrostatic stabilization and acid/base catalysis, although the structure of the rate-limiting transition state is uncertain. Here, we describe coordinated kinetic isotope effect (KIE) analyses, molecular dynamics simulations, and quantum mechanical calculations to model the transition state and mechanism of RNase A. Comparison of the 18 O KIEs on the 2′O nucleophile, 5′O leaving group, and nonbridging phosphoryl oxygens for RNase A to values observed for hydronium- or hydroxide-catalyzed reactions indicate a late anionic transition state. Molecular dynamics simulations using an anionic phosphorane transition state mimic suggest that H-bonding by protonated His12 and Lys41 stabilizes the transition state by neutralizing the negative charge on the nonbridging phosphoryl oxygens. Quantum mechanical calculations consistent with the experimental KIEs indicate that expulsion of the 5′O remains an integral feature of the rate-limiting step both on and off the enzyme. Electrostatic interactions with positively charged amino acid site chains (His12/Lys41), together with proton transfer from His119, render departure of the 5′O less advanced compared with the solution reaction and stabilize charge buildup in the transition state. The ability to obtain a chemically detailed description of 2′- O -transphosphorylation transition states provides an opportunity to advance our understanding of biological catalysis significantly by determining how the catalytic modes and active site environments of phosphoryl transferases influence transition state structure.

A novel high-throughput sequencing kinetics approach is used to measure functional binding of the apparently nonspecific RNA-binding protein C5 to all possible sequence variants in its substrate binding site; C5 binds different substrate variants with affinities varying widely, and with a similar affinity distribution to that of highly specific nucleic-acid-binding proteins, but it does not bind its physiological RNA targets with the highest affinity. The interaction of 'specific' binding proteins with nucleic acids is attributed to high-affinity interaction with favoured sequences or structures. Proteins that lack recognized cognate sites are considered to bind 'non-specifically'. This paper presents a new and unexpected view on the concept of nonspecificity/specificity for nucleic acid-binding proteins. Eckhard Jankowsky and colleagues show that the nonspecific RNA-binding protein, C5, which is a subunit of RNase P, exhibits an underlying specificity. Using a novel high-throughput kinetic approach, they measure affinities of C5 for different sequences that vary over several orders of magnitude. Compared with specific binding protein affinities, C5 binds with affinities that are more moderate, suggesting that the only distinction between specific and nonspecific binding proteins is the region of the affinity distribution that is used. Nucleic-acid-binding proteins are generally viewed as either specific or nonspecific, depending on characteristics of their binding sites in DNA or RNA1,2. Most studies have focused on specific proteins, which identify cognate sites by binding with highest affinities to regions with defined signatures in sequence, structure or both1,2,3,4. Proteins that bind to sites devoid of defined sequence or structure signatures are considered nonspecific1,2,5. Substrate binding by these proteins is poorly understood, and it is not known to what extent seemingly nonspecific proteins discriminate between different binding sites, aside from those sequestered by nucleic acid structures6. Here we systematically examine substrate binding by the apparently nonspecific RNA-binding protein C5, and find clear discrimination between different binding site variants. C5 is the protein subunit of the transfer RNA processing ribonucleoprotein enzyme RNase P from Escherichia coli. The protein binds 5′ leaders of precursor tRNAs at a site without sequence or structure signatures. We measure functional binding of C5 to all possible sequence variants in its substrate binding site, using a high-throughput sequencing kinetics approach (HITS-KIN) that simultaneously follows processing of thousands of RNA species. C5 binds different substrate variants with affinities varying by orders of magnitude. The distribution of functional affinities of C5 for all substrate variants resembles affinity distributions of highly specific nucleic acid binding proteins. Unlike these specific proteins, C5 does not bind its physiological RNA targets with the highest affinity, but with affinities near the median of the distribution, a region that is not associated with a sequence signature. We delineate defined rules governing substrate recognition by C5, which reveal specificity that is hidden in cellular substrates for RNase P. Our findings suggest that apparently nonspecific and specific RNA-binding modes may not differ fundamentally, but represent distinct parts of common affinity distributions.

Enolization of acetyl‐CoA on CS had been identified in Eggerer¡s experiments with [3H]acetyl‐CoA (Biochem. Z. 343: 111, 1965). To test whether CS catalyzes sequential cycles of acetyl‐CoA enolization, we incubated [2H3, 13C2]acetyl‐CoA with pig heart CS + oxaloacetate (OAA) (added as either a bolus, a bolus of OAA/malate, or malate + NAD + malate dehydrogenase). We measured the mass isotopomer distribution (MID) of acetyl‐CoA and citrate. The productions of M4 + M3 + M2 acetyl‐CoA, and of M4 + M3 + M2 citrate revealed up to 3 cycles of acetyl‐CoA enolization. This was confirmed when condensation of unlabeled acetyl‐CoA + OAA was conducted in 50% D2O. Acetyl‐CoA was up to M3 labeled and citrate was M1 labeled. Thus, up to 3 cycles of acetyl‐CoA enolization were catalyzed by CS.We hypothesized that ACL could also catalyze enolization of acetyl‐CoA. We incubated [2,2,4,4‐2H4]citrate + Mg++, CoA, ATP and rat liver ACL. The production of M2 + M1 + M acetyl‐ CoA, and the progressive de‐labeling of acetyl‐CoA demonstrated multiple cycles of enolization of acetyl‐CoA on ACL. This was confirmed when cleavage of unlabeled citrate was conducted in 50% D2O, by the accumulation of M1 + M2 + M3 acetyl‐CoA. Thus, multiple cycles of acetyl‐CoA enolization were catalyzed by ACL.(Supported by the NIDDK Metabolomic Initiative grant R33DK070291).