Photosynthesis in the Higher Plant Vicia faba: V. Role of Malate as a Precursor of the Tricarboxylic Acid Cycle

In the higher plant Vicia faba, anomalous labeling patterns in the organic acids and related amino acids of the tricarboxylic acid cycle which result from photosynthetic ¹⁴CO₂ fixation (in conjunction with an enzyme localization pattern unique to plant mitochondria) suggest that the tricarboxylic acid cycle functions primarily as a pathway leading to glutamic acid biosynthesis during autotrophic growth. The distribution of isotope in citrate indicates little recycling of oxaloacetate for the resynthesis of citrate. Rather, malate appears to provide both the C₂ and C₄ fragments for the synthesis of citrate, and [³H]formate and ¹⁴CO₂-labeling patterns implicate serine as the ultimate C₃ precursor of malate.     

Differences between mouse and rat pancreatic islets: succinate responsiveness,malic enzyme,and anaplerosis

Succinic acid methyl esters are potent insulin secretagogues in rat pancreatic islets, but they do not stimulate insulin release in mouse islets. Unlike rat and human islets, mouse islets lack malic enzyme and, therefore, are unable to form pyruvate from succinate-derived malate for net synthesis of acetyl-CoA. Dimethyl-[2,3-(14)C]succinate is metabolized in the citric acid cycle in mouse islets to the same extent as in rat islets, indicating that endogenous acetyl-CoA condenses with oxaloacetate derived from succinate. However, without malic enzyme, the net synthesis from succinate of the citric acid cycle intermediates citrate, isocitrate, and alpha-ketoglutarate cannot occur. Glucose and other nutrients that augment alpha-ketoglutarate formation are secretagogues in mouse islets with potencies similar to those in rat islets. All cycle intermediates can be net-synthesized from alpha-ketoglutarate. Rotenone, an inhibitor of site I of the electron transport chain, inhibits methyl succinate-induced insulin release in rat islets even though succinate oxidation forms ATP at sites II and III of the respiratory chain. Thus generating ATP, NADH, and anaplerosis of succinyl-CoA plus the four-carbon dicarboxylic acids of the cycle and its metabolism in the citric acid cycle is insufficient for a fuel to be insulinotropic; it must additionally promote anaplerosis of alpha-ketoglutarate or two intermediates interconvertible with alpha-ketoglutarate, citrate, and isocitrate.     

Physiology and metabolism of pathogenic neisseria: tricarboxylic acid cycle activity in Neisseria gonorrhoeae.

Tricarboyxlic acid cycle activity was examined in Neisseria gonorrhoeae CS-7. The catabolism of glucose in N. gonorrheae by a combination of the Entner-Doudoroff and pentose phosphate pathways resulted in the accumulation of acetate, which was not further catabolized until the glucose was depleted or growth became limiting. Radiorespirometric studies revealed that the label in the 1 position of acetate was converted to CO2 at twice the rate of the label in the 2 position, indicating the presence of a tricarboxylic acid cycle. Growth on glucose markedly reduced the levels of all tricarboxylic acid cycle enzymes except citrate synthase (EC 4.1.3.7). Extracts of glucose-grown cells contained detectable levels of all tricarboxylic acid cycle enzymes except aconitase (EC 4.2.1.3), isocitrate dehydrogenase (EC 1.1.1.42), and a pyridine nucleotide-dependent malate dehydrogenase (EC 1.1.1.37). Extracts of cells capable of oxidizing acetate lacked only the pyridine nucleotide-dependent malate dehydrogenase. In lieu of this enzyem, a particulate pyridine nucleotide-independent malate oxidase (EC 1.1.3.3) was present. This enzyme required flavin adenine dinucleotide for activity and appeared to be associated with the electron transport chain. Radiorespirometric studies utilizing labeled glutamate demonstrated that a portion of the tricarboxylic acid cycle functioned during glucose catabolism. In spite of the presence of all tricarboxylic acid cycle enzymes, N. gonorrhoeae CS-7 was unable to grow in medium supplemented with cycle intermediates.     

The tricarboxylic and acid pathway in Desulfovibrio.

Strains of two species of Desulfovibrio were examined for enzymes of the tricarboxylic acid cycle and related pathways. Pyruvate carboxylase (EC6.4.1.1) is present, and alpha-ketoglutarate is formed via the tricarboxylic acids. Glutamate, but not succinyl-CoA, arises from alpha-ketoglutarate. A pathway exists from pyruvate by malic enzyme (EC 1.1.1.39) activity to malate, then fumarate and succinate, again with no evidence of succinyl-CoA formation. The enzymes concerned with metabolism of these dicarboxylic acids show greater activity in the strains that can grow by fumarate dismutation. Glutamate (or glutamine), alpha-ketoglutarate, and yeast extract repress the enzymes that metabolize the tricarboxylic acids. There appears to be no glyoxylate cycle in Desulfovibrio vulgaris or D. desulfuricans.     

The role of the tricarboxylic acid cycle in citric acid accumulation by Aspergillus niger

Summary Determinations of the momentary levels of various intermediates related to the activity of the tricarboxylic acid cycle have been made during citric acid production in high-accumulating (manganese deficient) and lowaccumulating (manganese supplemented) mycelia of Aspergillus niger. During the growth period the levels of almost all TCA cycle acids, with the exception of 2-oxo-acids, were unusually high; during the induction phase of citrate accumulation malate, fumarate, and isocitrate decreased, whereas pyruvate, oxalacetate, and citrate increased. The presence of succinate could not be demonstrated. The interrelations of the momentary concentrations of the intermediates mainly demonstrate a lack in activity of 2-oxoglutarate dehydrogenase, representing a block in the TCA cycle concomitant with a strongly operating glycolysis as a prerequisite for citrate accumulation. Inhibition studies with crude enzyme preparations suggest that an inhibition of malate dehydrogenase by citrate and also inhibition of isocitrate dehydrogenase by citrate and 2-oxoglutarate occur during the production phase as additional factors.     

Nonfunctional Tricarboxylic Acid Cycle and the Mechanism of Glutamate Biosynthesis in Acetobacter suboxydans

Acetobacter suboxydans does not contain an active tricarboxylic acid cycle, yet two pathways have been suggested for glutamate synthesis from acetate catalyzed by cell extracts: a partial tricarboxylic acid cycle following an initial condensation of oxalacetate and acetyl coenzyme A. and the citramalate-mesaconate pathway following an initial condensation of pyruvate and acetyl coenzyme A. To determine which pathway functions in growing cells, acetate-1-(14)C was added to a culture growing in minimal medium. After growth had ceased, cells were recovered and fractionated. Radioactive glutamate was isolated from the cellular protein fraction, and the position of the radioactive label was determined. Decarboxylation of the C5 carbon removed 100% of the radioactivity found in the purified glutamate fraction. These experiments establish that growing cells synthesize glutamate via a partial tricarboxylic acid cycle. Aspartate isolated from these hydrolysates was not radioactive, thus providing further evidence for the lack of a complete tricarboxylic acid cycle. When cell extracts were analyzed, activity of all tricarboxylic acid cycle enzymes, except succinate dehydrogenase, was demonstrated.     

Derivatization of the tricarboxylic acid intermediates with O-benzylhydroxylamine for liquid chromatography–tandem mass spectrometry detection

The tricarboxylic acid (TCA) cycle is an interface among glycolysis, lipid metabolism, and amino acid metabolism. Increasing interest in cancer metabolism has created a demand for rapid and sensitive methods for quantifying the TCA cycle intermediates and related organic acids. We have developed a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method to quantify the TCA cycle intermediates in a 96-well format after O-benzylhydroxylamine (O-BHA) derivatization under aqueous conditions. This method was validated for quantitation of all common TCA cycle intermediates with good sensitivity, including α-ketoglutarate, malate, fumarate, succinate, 2-hydroxyglutarate, citrate, oxaloacetate, pyruvate, isocitrate, and lactate using a 8-min run time in cancer cells and tissues. The method was used to detect and quantify changes in metabolite levels in cancer cells and tumor tissues treated with a pharmacological inhibitor of nicotinamide phosphoribosyl transferase (NAMPT). This method is rapid, sensitive, and reproducible, and it can be used to assess metabolic changes in cancer cells and tumor samples.     

Amino acid and lactate catabolism in trimethylamine oxide respiration of Alteromonas putrefaciens NCMB 1735

The nonfermentative Alteromonas putrefaciens NCMB 1735 grew anaerobically in defined media with trimethylamine oxide as external electron acceptor. All amino acids tested, except taurine and those with a cyclic or aromatic side chain, were utilized during trimethylamine oxide-dependent anaerobic growth. Lactate, serine, and cysteine (which are easily converted to pyruvate) and glutamate and aspartate (which are easily converted to tricarboxylic acid cycle intermediates) were metabolized at the fastest rate. Growth with lactate as growth-limiting substrate gave rise to the formation of 40 mol% acetate, whereas serine and cysteine were nearly completely oxidized to CO2. Molar growth yields with the latter substrates were the same and were 50% higher than with lactate. This showed that more ATP was formed when acetyl coenzyme A entered the tricarboxylic acid cycle than when it was converted via acetyl phosphate to acetate. Also, growth with formate as substrate indicated that the reduction of trimethylamine oxide to trimethylamine was coupled with energy conservation by a respiratory mechanism.     

Amino acid and lactate catabolism in trimethylamine oxide respiration of Alteromonas putrefaciens NCMB 1735.

The nonfermentative Alteromonas putrefaciens NCMB 1735 grew anaerobically in defined media with trimethylamine oxide as external electron acceptor. All amino acids tested, except taurine and those with a cyclic or aromatic side chain, were utilized during trimethylamine oxide-dependent anaerobic growth. Lactate, serine, and cysteine (which are easily converted to pyruvate) and glutamate and aspartate (which are easily converted to tricarboxylic acid cycle intermediates) were metabolized at the fastest rate. Growth with lactate as growth-limiting substrate gave rise to the formation of 40 mol% acetate, whereas serine and cysteine were nearly completely oxidized to CO2. Molar growth yields with the latter substrates were the same and were 50% higher than with lactate. This showed that more ATP was formed when acetyl coenzyme A entered the tricarboxylic acid cycle than when it was converted via acetyl phosphate to acetate. Also, growth with formate as substrate indicated that the reduction of trimethylamine oxide to trimethylamine was coupled with energy conservation by a respiratory mechanism.     

Biochemical and structural studies of malate synthase from Mycobacterium tuberculosis

Establishment or maintenance of a persistent infection by Mycobacterium tuberculosis requires the glyoxylate pathway. This is a bypass of the tricarboxylic acid cycle in which isocitrate lyase and malate synthase (GlcB) catalyze the net incorporation of carbon during growth of microorganisms on acetate or fatty acids as the primary carbon source. The glcB gene from M. tuberculosis, which encodes malate synthase, was cloned, and GlcB was expressed in Escherichia coli. The influence of media conditions on expression in M. tuberculosis indicated that this enzyme is regulated differentially to isocitrate lyase. Purified GlcB had K(m) values of 57 and 30 microm for its substrates glyoxylate and acetyl coenzyme A, respectively, and was inhibited by bromopyruvate, oxalate, and phosphoenolpyruvate. The GlcB structure was solved to 2.1-A resolution in the presence of glyoxylate and magnesium. We also report the structure of GlcB in complex with the products of the reaction, coenzyme A and malate, solved to 2.7-A resolution. Coenzyme A binds in a bent conformation, and the details of its interactions are described, together with implications on the enzyme mechanism.     

Regulatory characteristics of phosphoenolpyruvate carboxylase from the extreme thermophile, Thermus aquaticus.

Phosphoenolpyruvate carboxylase from the extremely thermophilic bacterium, Thermus aquaticus YT-1, exhibits a virtually absolute requirement for acetyl CoA and there is strong positive cooperativity in the interaction of this activator with the enzyme. Several tricarboxylic acid cycle intermediates inhibit the enzyme. These findings suggest an anaplerotic role for the enzyme and an allosteric modulation of its activity by acetyl CoA and tricarboxylic acid cycle intermediates.     

Quantification of carbon fluxes through the tricarboxylic acid cycle in early germinating lettuce embryos

A method involving labeling to isotopic steady state and modeling of the tricarboxylic acid cycle has been used to identify the respiratory substrates in lettuce embryos during the early steps of germination. We have compared the specific radioactivities of aspartate and glutamate and of glutamate C-1 and C-5 after labeling with different substrates. Labeling with [U-14C]acetate and 14CO2 was used to verify the validity of the model for this study; the relative labeling of aspartate and glutamate was that expected from the normal operation of the tricarboxylic acid cycle. After labeling with 14CO2, the label distribution in the glutamate molecule (95% of the label at glutamate C-1) was consistent with an input of carbon via the phosphoenolpyruvate carboxylase reaction, and the relative specific radioactivities of aspartate and glutamate permitted the quantification of the apparent rate of the fumarase reaction. CO2 and intermediates related to the tricarboxylic acid cycle were labeled with [U-14C]acetate, [1-14C] hexanoate, or [U-14C]palmitic acid. The ratios of specific radioactivities of asparate to glutamate and of glutamate C-1 to C-5 indicated that the fatty acids were degraded to acetyl units, suggesting the operation of beta-oxidation, and that the acety-CoA was incorporated directly into citrate. Short-term labeling with [1-14C]hexanoate showed that citrate and glutamate were labeled earlier than malate and aspartate, showing that this fatty acid was metabolized through the tricarboxylic acid cycle rather than the glyoxylate cycle. This was in agreement with the flux into gluconeogenesis compared to efflux as respiratory CO2. The fraction of labeled substrate incorporated into carbohydrates was only about 5% of that converted to CO2; the carbon flux into gluconeogenesis was determined after labeling with 14CO2 and [1-14C]hexanoate from the specific radioactivity of aspartate C-1 and the amount of label incorporated into the carbohydrate fraction. It was only 7.4% of the efflux of respiratory CO2. The labeling of alanine indicates a low activity of either a malic enzyme or the sequence phosphoenolpyruvate carboxykinase/pyruvate kinase. After labeling with [U-14C]glucose, the ratios of specific radioactivities indicated that the labeled carbohydrates contributed less than 10% to the flux of acetyl-CoA. The model indicated that the glycolytic flux is partitioned one-third to pyruvate and two-thirds to oxalacetate and is therefore mainly anaplerotic. The possible role of fatty acids as the main source of acetyl-CoA for respiration is discussed.     

Reversible transdominant inhibition of a metabolic pathway. In vivo evidence of interaction between two sequential tricarboxylic acid cycle enzymes in yeast

The enzymes of the Krebs tricarboxylic acid cycle in mitochondria are proposed to form a supramolecular complex, in which there is channeling of intermediates between enzyme active sites. While interactions have been demonstrated in vitro between most of the sequential tricarboxylic acid cycle enzymes, no direct evidence has been obtained in vivo for such interactions. We have isolated, in the Saccharomyces cerevisiae gene encoding the tricarboxylic acid cycle enzyme citrate synthase Cit1p, an "assembly mutation," i.e. a mutation that causes a tricarboxylic acid cycle deficiency without affecting the citrate synthase activity. We have shown that a 15-amino acid peptide from wild type Cit1p encompassing the mutation point inhibits the tricarboxylic acid cycle in a dominant manner, and that the inhibitory phenotype is overcome by a co-overexpression of Mdh1p, the mitochondrial malate dehydrogenase. These data provide the first direct in vivo evidence of interaction between two sequential tricarboxylic acid cycle enzymes, Cit1p and Mdh1p, and indicate that the characterization of assembly mutations by the reversible transdominant inhibition method may be a powerful way to study multienzyme complexes in their physiological context.     

Compartmentation of acetyl CoA studied by analysis of tricarboxylic acid cycle acids and 3-hydroxybutyrate in bile of rats given [2,2,2-2H3]ethanol.

Acetate, 3-hydroxybutyrate, pyruvate, lactate, citrate, 2-oxoglutarate, succinate, fumarate and malate were analysed in rat bile by gas chromatography and gas chromatography/mass spectrometry of their O-melthyloxime-t-butyldimethylsilyl derivatives. The concentration of acetate increased to about 1.8 mmol/l after administration of [2,2,2-2H3]ethanol. Acetate was formed from ethanol to an extent of about 82% and retained all of the 2H at C-2, whereas 15% of the 2H had been lost in the tricarboxylic acid cycle intermediates and 24% in 3-hydroxybutyrate. Thus the exchange of 2H for 1H takes place after formation of acetyl CoA. For citrate and 3-hydroxybutyrate, 41% and 11% respectively was formed from [2,2,2-2H3]ethanol. These results indicate that different pools of acetyl CoA are used for the synthesis of ketone bodies and citrate, with the latter being derived from ethanol to a much larger extent. Smaller fractions of 2-oxoglutarate (16%) and succinate (5%) were derived from [2,2,2--2H3]ethanol, indicating significant contributions from amino acids.     

The role of amino acid catabolism in the formation of the tricarboxylic acid cycle intermediates and ammonia in anoxic rat heart

Amino acid catabolism, the tricarboxylic acid cycle intermediates and ammonia formation were studied in isolated perfused rat heart under anoxia. The total net anaplerosis due to amino acid degradation in anoxia was equal to that in oxygenation (6.29 and 6.09 mumol/g dry weight per h, respectively) as a result of the increased transamination of glutamic and aspartic acids. During anoxic perfusion, the rate of catabolism of glutamic and aspartic acids was 1.5-times higher than in normoxia, while depletion of branched-chain amino acids, lysine, proline, arginine and methionine, was inhibited. Alanine was the product of excessive degradation of glutamic and aspartic acids. Under anaerobic conditions, in spite of inhibition of amino acid deamination, ammonia formation was increased 2.7-fold as compared to oxygenation. The principal amount of ammonia (96%) was produced at degradation of adenine nucleotides. A 2.5-fold increase in the pool of the tricarboxylic acid cycle intermediates under anoxia was associated mainly with accumulation of succinate. The data suggest that the coupling of alanine- and aspartate amino transferases is a mechanism controlling the tricarboxylic acid cycle pool size in anoxic heart.     

Properties of an inducible C 4 -dicarboxylic acid transport system in Bacillus subtilis

The transport of the tricarboxylic acid cycle C(4)-dicarboxylic acids was studied in both the wild-type strain and tricarboxylic acid cycle mutants of Bacillus subtilis. Active transport of malate, fumarate, and succinate was found to be inducible by these dicarboxylic acids or by precursors to them, whereas glucose or closely related metabolites catabolite-repressed their uptake. l-Malate was found to be the best dicarboxylic acid transport inducer in succinic dehydrogenase, fumarase, and malic dehydrogenase mutants. Succinate and fumarate are accumulated over 100-fold in succinic dehydrogenase and fumarase mutants, respectively, whereas mutants lacking malate dehydrogenase were unable to accumulate significant quantities of the C(4)-dicarboxylic acids. The stereospecificity of this transport system was studied from a comparison of the rates of competitive inhibition of both succinate uptake and efflux in a succinate dehydrogenase mutant by utilizing thirty dicarboxylic acid analogues. The system was specific for the C(4)-dicarboxylic acids of the tricarboxylic acid cycle, neither citrate nor alpha-ketoglutarate were effective competitive inhibitors. Of a wide variety of metabolic inhibitors tested, inhibiors of oxidative phosphorylation and of the formation of proton gradients were the most potent inhibitors of transport. From the kinetics of dicarboxylic acid transport (K(m) approximately 10(-4) M for succinate or fumarate in succinic acid dehydrogenase and fumarase mutants) and from the competitive inhibition studies, it was concluded that an inducible dicarboxylic acid transport system mediates the entry of malate, fumarate, or succinate into B. subtilis. Mutants devoid of alpha-ketoglutarate dehydrogenase were shown to accumulate both alpha-ketoglutarate and glutamate, and these metabolites subsequently inhibited the transport of all the C(4)-dicarboxylic acids, suggesting a regulatory role.     

Formation and dissimilation of oxalacetate and pyruvate Pseudomonas citronellolis grown on noncarbohydrate substrates.

Metabolism of lactate as a carbon source by Pseudomonas citronellolis occurred via a nicotinamide adenine dinucleotide (NAD)-independent L-lactate dehydrogenase, which was present in cells grown on DL-lactate but was not present in cells grown on acetate, aspartate, citrate, glucose, glutamate, or malate. The cells also possessed a constitutive, NAD-independent malate dehydrogenase instead of the conventional NAD-dependent malate dehydrogenase instead of the conventional NAD-dependent enzyme in the tricarboxylic acid cycle. Both enzymes were particulate and used dichlorophenolindo-phenol or oxygen as an electron acceptor. In acetate-grown cells, the activity of pyruvate dehydrogenase and NAD phosphate-linked malate enzyme decreased, cells grown on glucose or lactate. This was consistent with the need to maintain a supply of oxalacetate for metabolism of acetate via the tricarboxylic acid cycle. Changes in enzyme activities suggest that gluconeogenesis from noncarbohydrate carbon sources occurs via the malate enzyme (when oxalacetate decarboxylase is inhibited) or a combination of the NAD-independent malate dehydrogenase and oxalacetate decarboxylase.     

Metabolism of propionate by sheep liver: Pathway of propionate metabolism in aged homogenate and mitochondria

Experiments were conducted with aged nuclear-free homogenate of sheep liver and aged mitochondria in an attempt to measure both the extent of oxidation of propionate and the distribution of label from [2-¹⁴C]propionate in the products. With nuclear-free homogenate, propionate was 44% oxidized with the accumulation of succinate, fumarate, malate and some citrate. Recovery of ¹⁴C in these intermediates and respiratory carbon dioxide was only 33%, but additional label was detected in endogenous glutamate and aspartate. With washed mitochondria 30% oxidation of metabolized propionate occurred, and proportionately more citrate and malate accumulated. Recovery of ¹⁴C in dicarboxylic acids, citrate, α-oxoglutarate, glutamate, aspartate and respiratory carbon dioxide was 91%. The specific activities of the products and the distribution of label in the carbon atoms of the dicarboxylic acids were consistent with the operation solely of the methylmalonate pathway together with limited oxidation of the succinate formed by the tricarboxylic acid cycle via pyruvate. In a final experiment with mitochondria the label consumed from [2-¹⁴C]propionate was entirely recovered in the intermediates of the tricarboxylic acid cycle, glutamate, aspartate, methylmalonate and respiratory carbon dioxide.     

Intracellular acetyl unit transport in fungal carbon metabolism

Acetyl coenzyme A (acetyl-CoA) is a central metabolite in carbon and energy metabolism. Because of its amphiphilic nature and bulkiness, acetyl-CoA cannot readily traverse biological membranes. In fungi, two systems for acetyl unit transport have been identified: a shuttle dependent on the carrier carnitine and a (peroxisomal) citrate synthase-dependent pathway. In the carnitine-dependent pathway, carnitine acetyltransferases exchange the CoA group of acetyl-CoA for carnitine, thereby forming acetyl-carnitine, which can be transported between subcellular compartments. Citrate synthase catalyzes the condensation of oxaloacetate and acetyl-CoA to form citrate that can be transported over the membrane. Since essential metabolic pathways such as fatty acid β-oxidation, the tricarboxylic acid (TCA) cycle, and the glyoxylate cycle are physically separated into different organelles, shuttling of acetyl units is essential for growth of fungal species on various carbon sources such as fatty acids, ethanol, acetate, or citrate. In this review we summarize the current knowledge on the different systems of acetyl transport that are operational during alternative carbon metabolism, with special focus on two fungal species: Saccharomyces cerevisiae and Candida albicans.