Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:2.3.3.1 (citrate synthase)
4,488 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. The activities of citrate synthase and NAD+-linked and NADP+-linked isocitrate dehydrogenases were measured in nervous tissue from different animals in an attempt to provide more information about the citric acid cycle in this tissue. In higher animals the activities of citrate synthase are greater than the sum of activities of the isocitrate dehydrogenases, whereas they are similar in nervous tissues from the lower animals. This suggests that in higher animals the isocitrate dehydrogenase reaction is far-removed from equilibrium. If it is assumed that isocitrate dehydrogenase activities provide an indication of the maximum flux through the citric acid cycle, the maximum glycolytic capacity in nervous tissue is considerably greater than that of the cycle. This suggest that glycolysis can provide energy in excess of the aerobic capacity of the tissue. 2. The activities of glutamate dehydrogenase are high in most nervous tissues and the activities of aspartate aminotransferase are high in all nervous tissue investigated. However, the activities of alanine aminotransferase are low in all tissues except the ganglia of the waterbug and cockroach. In these insect tissues, anaerobic glycolysis may result in the formation of alanine rather than lactate.
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PMID:Activities of citrate synthase, NAD+-linked and NADP+-linked isocitrate dehydrogenases, glutamate dehydrogenase, aspartate aminotransferase and alanine aminotransferase in nervous tissues from vertebrates and invertebrates. 0 Oct 3

1. The activities of citrate synthase, NAD+-linked and NADP+-linked isocitrate dehydrogenase were measured in muscles from a large number of animals, in order to provide some indication of the importance of the citric acid cycle in these muscles. According to the differences in enzyme activities, the muscles can be divided into three classes. First, in a number of both vertebrate and invertebrate muscles, the activities of all three enzymes are very low. It is suggested that either the muscles use energy at a very low rate or they rely largely on anaerobic glycolysis for higher rates of energy formation. Second, most insect flight muscles contain high activities of citrate synthase and NAD+-linked isocitrate dehydrogenase, but the activities of the NADP+-linked enzyme are very low. The high activities indicate the dependence of insect flight on energy generated via the citric acid cycle. The flight muscles of the beetles investigated contain high activities of both isocitrate dehydrogenases. Third, other muscles of both vertebrates and invertebrates contain high activities of citrate synthase and NADP+-liniked isocitrate dehydrogenase. Many, if not all, of these muscles are capable of sustained periods of mechanical activity (e.g. heart muscle, pectoral muscles of some birds). Consequently, to support this activity fuel must be supplied continually to the muscle via the circulatory system which, in most animals, also transports oxygen so that energy can be generated by complete oxidation of the fuel. It is suggested that the low activities of NAD+-linked isocitrate dehydrogenase in these muscles may be involved in oxidation of isocitrate in the cycle when the muscles are at rest. 2. A comparison of the maximal activities of the enzymes with the maximal flux through the cycle suggests that, in insect flight muscle, NAD+-linked isocitrate dehydrogenase catalyses a non-equilibrium reaction and citrate synthease catalyses a near-equilibrium reaction. In other muscles, the enzyme-activity data suggest that both citrate synthase and the isocitrate dehydrogenase reactions are near-equilibrium.
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PMID:Activities of citrate synthase and NAD+-linked and NADP+-linked isocitrate dehydrogenase in muscle from vertebrates and invertebrates. 0 36

Azotobacter beijerinckii was grown in ammonia-free glucose/mineral salts media in chemostat culture under oxygen or nitrogen limitation. Selected enzymes of the tricarboxylic acid cycle and poly-beta-hydroxybutyrate metabolism were monitored in relation to oxygen supply for both steady and transition states. Two dissolved oxygen concentrations were used for the nitrogen-limited steady state to investigate the possible effects of respiratory protection of nitrogenase on these enzymes. The levels of NADH oxidase, isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase increased markedly on relaxation of oxygen limitation while pyruvate dehydrogenase and citrate synthase were relatively unaffected. beta-Ketothiolase and acetoacetyl-CoA reductase levels decreased as oxygen limitation was relaxed. Respiratory activity, as measured by the QO2 value, increased with oxygen supply rate. Imposition of oxygen limitation on a nitrogen-limited culture caused an immediate increase in the NADH/NAD ratio but this rapidly readjusted to its previous steady-state value. These changes are discussed in relation to respiratory protection of nitrogenase and poly-beta-hydroxybutyrate metabolism in A. beijerinckii.
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PMID:Regulation of the tricarboxylic acid cycle and poly-beta-hydroxybutyrate metabolism in Azotobacter beijerinckii grown under nitrogen or oxygen limitation. 1 43

A protease from Tetrahymena pyriformis inactivated eight of nine commercially available enzymes tested, including lactate deyhdrogenase, isocitrate dehydrogenase (TPN-specific), glucose-6 phosphate dehydrogenase, D-amino acid oxidase, fumarase, pyruvate kinase, hexokinase, and citrate synthase. Urate oxidase was not inactivated. Inactivation occurred at neutral pH, was prevented by inhibitors of the protease, and followed first order kinetics. In those cases tested, inactivation was enhanced by mercaptoethanol. Most of the enzyme-inactivating activity was due to a protease of molecular weight 25,000 that eluted from DEAE-Sephadex at 0.3 M KCl. A second protease of this molecular weight, which was not retained by the gel, inactivated only isocitrate dehydrogenase and D-amino acid oxidase. These two proteases could also be distinguished by temperature and inhibitor sensitivity. Two other protease peaks obtained by DEAE-Sephadex chromatography had little or no no enzyme inactivating activity, while another attacked only D-amino acid oxidase. At least six of the enzymes could be protected from proteolytic inactivation by various ligands. Isocitrates dehydrogenase was protected by isocitrate, TPN, or TPNH, glucose-6-dehydrogenase by glucose-6-P or TPN, pyruvate kinase by phosphoenolypyruvate or ADP, hexokinase by glucose, and fumarase by a mixture of fumarate and malate. Lactate dehdrogenase was not protected by either of its substrates of coenzymes. Citrate synthase was probably protected by oxalacetate. Our data suggest that the protease or proteases discussed here may participate in the inactivation or degradation of a least some enzymes in Tetrahymena. Since the inactivation occurs at neutral pH, this process could be regulated by variations in the cellular levels of substrates, coenzymes, or allosteric regulators resulting form changes in growth conditions or growth state. Such a mechanism would permit the selective retention of enzymes of metabolically active pathways.
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PMID:Enzyme inactivation by a cellular neutral protease: enzyme specificity, effects of ligands on inactivation, and implications for the regulation of enzyme degradation. 1 68

Aconitase and NAD linked isocitrate dehydrogenase were present in Ascaris lumbricoides muscle at only very low activities, whilst there were significant levels of citrate synthase, NADP linked isocitrate dehydrogenase, 2-oxoglutarate dehydrogenase and succinic thiokinase. Pyruvate dehydrogenase was present in A. lumbricoides muscle at levels comparable with mammalian tissues and results suggest that it is modulated via a phosphotransferase/phosphatase system. The tricarboxylic acid cycle intermediates, citrate, isocitrate and 2-oxoglutarate were all detected in freeze clamped muscle, but their steady state levels were considerably lower than those found in mammalian tissues.
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PMID:Pyruvate and citrate metabolism in the muscle tissue of Ascaris lumbricoides. 2 88

2-Methylcitrate was tested in vitro on enzymes which interact with citrate and isocitrate. It was found to inhibit citrate synthase, aconitase, the NAD+- and NADP+-linked isocitrate dehydrogenase. This inhibition was competitive in nature except in the case of aconitase, and the Ki for all the enzymes was in the range of 1.5-7.6 mM. Phosphofructokinase was also inhibited by 2-methylcitrate with 50% inhibition achieved at 1 mM. ATP-citrate lyase and acetyl-CoA carboxylase were not inhibited by this compound. 2-Methylcitrate was not a substrate for ATP-citrate lyase. Acetyl-CoA carboxylase was activated by 2-methylcitrate with a Ka of 2.8 mM. The apparent Km (3.3 mM) for 2-methylcitrate for the mitochondrial citrate transporter was about 10-fold higher than the apparent Km (0.26 mM) for citrate. The tricarboxylase carrier can also be inhibited by low concentrations (0.2 mM) of 2-methylcitrate when the concentration of citrate is close to the apparent Km. Accumulation of 2-methylcitrate inside the mitochondrion, therefore, might lead to inhibition of enzymes in the citric acid cycle and thereby contribute to the ketogenesis and hypoglycemia seen under these conditions.
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PMID:Effect of 2-methylcitrate on citrate metabolism: implications for the management of patients with propionic acidemia and methylmalonic aciduria. 12 73

Closed aorta working hearts perfused with 1 mM pyruvate were subjected to a 4-fold increase in work load by raising the left atrial filling pressure. Citric acid cycle flux, pyruvate uptake, and oxygen consumption rose 3-fold when cardiac output was increased. In the first 40 sec after the transition tissue glutamate and citrate fell by 22 and 45%, respectively, and there were reciprocal decreases in malate and aspartate. The ratio of creatine phosphate/creatine declined by 50% within 30 sec, with a corresponding increase in inorganic phosphate, but the fall in the ATP/ADP ratio was only 10%. During the first 10 sec the surface fluorescence from cardiac pyridine nucleotides fell by 30% and this change was synchronous with a sharp decline in the calculated adenine nucleotide phosphate potential. This suggests that heart mitochondrial respiration is controlled by the cytosolic phosphate potential, and that a state 4 to state 3 transition occurs when cardiac output is increased. Apparent disequilbrium of creatine phosphokinase can be explained by the compartmentation of most of the cardiac ADP within the mitochondria. Citric acid cycle flux was coordinated by activational interactions at citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase, but a transient imbalance between the individual cycle steps leads to a sharp peak of lactate production shortly after the work transition.
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PMID:Regulation of myocardial energy metabolism. 17 15

A realistic metabolic model of the tricarboxylic acid cycle in the perfused rat heart was constructed to help explain the sequence of biochemical events regulating the metabolism of exogenous pyruvate following a large increase in work load. The unchelated Mg2+ level was the most important controlling factor. The resulting mixture of chelated and unchelated nucleotides and tribasic acids effected coordinated control of citrate synthase, aconitase, isocitrate dehydrogenase, succinyl CoA synthetase, fumarase, and nucleoside diphosphokinase, because Mg2+-chelates are generally substrates whereas unchelated species are inhibitors. Succinate dehydrogenase is largely controlled by the ubiquinone redox potential. The fluxes through alpha-ketoglutarate and malate dehydrogenases are largely dependent on thepyridine nucleotide redox potential, but the succinyl CoA-to-CoASH ratio strongly affects the former enzyme as well. The model predicts an accumulation of succinate during the transition to higher work output.
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PMID:Computer simulation of metabolism in pyruvate-perfused rat heart. II. Krebs cycle. 22 18

Levels of several intermediary metabolites were measured in cells grown in acetate medium in order to test the hypothesis that the glyoxylate cycle is repressed by phosphoenolpyruvate (PEP). Wild-type cells had less PEP than either isocitrate dehydrogenase - deficient cells (which had greater isocitrate lyase activity than the wild type) or isocitrate dehydrogenase - deficient, citrate synthase-deficient cells (which are poorly inducible). Thus induction of the glyoxylate cycle is more complicated than a simple function of PEP concentration. No correlation between enzyme activity and the level of oxaloacetate, pyruvate, or citrate was found either. Citrate was synthesized in citrate synthase-deficient mutants, possibly via citrate lyase.
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PMID:Acetate metabolism in Escherichia coli. 34 78

1. The contents of some intermediates of glycolysis, the citric acid cycle and adenine nucleotides have been measured in the freeze-clamped locust flight muscle at rest and after 10s and 3min flight. The contents of glucose 6-phosphate, pyruvate, alanine and especially fructose bisphosphate and triose phosphates increased markedly upon flight. The content of acetyl-CoA is decreased after 3min flight whereas that of acetylcarnitine is decreased markedly after 10s flight, but returns towards the resting value after 3min flight. The content of citrate is markedly decreased after both 10s and 3min flight, whereas that of isocitrate is changed very little after 10s and is increased by 50% after 3min. The content of oxaloacetate is very low in insect flight muscle and hence it was measured by a sensitive radiochemical assay. The content of oxaloacetate increased about 2-fold after 3min flight. A similar change was observed in the content of malate. The content of ATP decreased about 15%, whereas those of ADP and AMP increased about 2-fold after 3min flight. 2. Calculations based on O(2) uptake of the intact insect indicate that the rate of the citric acid cycle must be increased >100-fold during flight. Consequently, if citrate synthase catalyses a non-equilibrium reaction, the activity of the enzyme must increase >100-fold during flight. However, changes in the concentrations of possible regulators of citrate synthase, oxaloacetate, acetyl-CoA and citrate (which is an allosteric inhibitor), are not sufficient to account for this change in activity. It is concluded that there may be much larger changes in the free concentration of oxaloacetate than are indicated by the changes in the total content of this metabolite or that other unknown factors must play an additional role in the regulation of citrate synthase activity. 3. The increased content of oxaloacetate could be produced via pyruvate carboxylase, which may be stimulated during the early stages of flight by the increased concentration of pyruvate. 4. The decreases in the concentrations of citrate and alpha-oxoglutarate indicate that isocitrate dehydrogenase and oxoglutarate dehydrogenase may be stimulated by factors other than their pathway substrates during the early stages of flight. 5. Calculated mitochondrial and cytosolic NAD(+)/NADH ratios are both increased upon flight. The change in the mitochondrial ratio indicates the importance of the intramitochondrial ATP/ADP concentration ratio in the regulation of the rate of electron transfer in this muscle.
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PMID:Changes in the contents of adenine nucleotides and intermediates of glycolysis and the citric acid cycle in flight muscle of the locust upon flight and their relationship to the control of the cycle. 43 78


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