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:1.4.1.2 (glutamate dehydrogenase)
4,380 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. Radioactively labelled 4-methyl-2-oxopentanoate was taken up by isolated pancreatic islets in a concentration- and pH-dependent manner and led to the intracellular accumulation of labelled amino acid and to a decrease in the intracellular pH. Uptake of 4-methyl-2-oxopentanoate did not appear to be either electrogenic or Na+-dependent. The islet content of 2-oxo acid radioactivity was not affected by either 2-cyano-3-hydroxy-cinnamate (10mM) or pyruvate (10mM), although both these substances inhibited the oxidation of [U-14C]4-methyl-2-oxopentanoate by islet tissue. 2. 4-Methyl-2-oxopentanoate markedly stimulated islet-cell respiration, ketone-body formation and biosynthetic activity. The metabolism of endogenous nutrients by islets appeared to be little affected by the compound. 3. Studies with the 3H- and 14C-labelled substrate revealed that 4-methyl-2-oxopentanoate was incorporated by islets into CO2, water, acetoacetate, L-leucine and to a lesser extent into islet protein and lipid. Carbon atoms C-2, C-3 and C-4 of the acetoacetate produced were derived from the carbon skeleton of the 4-methyl-2-oxopentanoate, but the acetoacetate carboxy group was derived from the incorporation of CO2. These results, and consideration of the relative rates of 14CO2 and acetoacetate formation from 1-14C-labelled as opposed to U-14C-labelled 4-methyl-2-oxopentanoate, led to the conclusion that the pathway of catabolism of this 2-oxo acid in pancreatic islets is identical with that described in other tissues. The amination of 4-methyl-2-oxopentanoate by islets was attributed to the presence of a branched-chain amino acid aminotransferase (EC 2.6.1.42) activity in the tissue. Although glutamate dehydrogenase activity was demonstrated in islet tissue, the reductive amination of 2-oxoacids did not seem to be of importance in the formation of leucine from 4-methyl-2-oxopentanoate. 4. The results of experiments with respiratory inhibitors and uncouplers, and the finding that 14CO2 production and islet respiration were linked in a 1:1 stoicheiometry suggested that 4-methyl-2-oxopentanoate catabolism was coupled to mitochondrial oxidative phosphorylation. The catabolism of 4-methyl-2-oxopentanoate in islet tissue appeared to be regulated at the level of the initial 2-oxo acid dehydrogenase (EC 1.2.1.25) reaction.
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PMID:The metabolism of 4-methyl-2-oxopentanoate in rat pancreatic islets. 4 43

High aminotransferase activities catalyzing the reactions between L-glutamate and L-glutamine and the aliphatic ketomonocarboxylic acids 2-ketoisocaproate, 2-ketocaproate, and 2-ketoisovalerate were observed in pancreatic B-cell mitochondria. While maximal rates of transamination with L-glutamate were observed in the presence of micromolar concentrations of keto acid, maximal rates of transamination with L-glutamine were recorded only in the presence of millimolar concentrations of keto acid. The insulin secretagogue 2-ketoisocaproate was the most effective transamination partner for L-glutamate, while the insulin secretagogue 2-ketocaproate was the most effective transamination partner for L-glutamine. Since B-cell mitochondria are well supplied with L-glutamate and L-glutamine, 2-ketoglutarate generation in the presence of these two neutral 2-keto acids may be an important prerequisite for their insulin secretory potency. High rates of transamination of 2-ketoglutarate were observed in the pancreatic B-cell mitochondria with the branched-chain amino acids L-leucine and L-valine, but not with L-norleucine. In connection with the ability of L-leucine to activate glutamate dehydrogenase, this high activity of the branched-chain amino acid aminotransferase in pancreatic B-cell mitochondria may provide an explanation for the insulin secretory potency of this amino acid.
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PMID:Transamination of neutral amino acids and 2-keto acids in pancreatic B-cell mitochondria. 286 44

Hepatocytes isolated from livers of fed rats were incubated with a mixture of glucose (10 mM), ribose (1.0 mM), acetate (1.25 mM), alanine (3.5 mM), glutamate (2.0 mM), aspartate (2.0 mM), 4-methyl-2-oxovaleric acid (ketoleucine) (3.0 mM), and, in paired flasks, 10 mM-ethanol. One substrate was 14C-radiolabelled in any given incubation. Incorporation of 14C into glucose, glycogen, CO2, lactate, alanine, aspartate, glutamate, acetate, urea, lipid glycerol, fatty acids and the 1- and 2,3,4-positions of ketone bodies was measured after 20 and 40 min of incubation under quasi-steady-state conditions. Data were analysed with the aid of a realistic structural metabolic model. In each of the four conditions examined, there were approx. 77 label incorporation measurements and several measurements of changes in metabolite concentrations. The considerable excess of measurements over the 37 independent flux parameters allowed for a stringent test of the model. A satisfactory fit to these data was obtained for each condition. There were large bidirectional fluxes along the gluconeogenic/glycolytic pathways, with net gluconeogenesis. Rates of ureagenesis, oxygen consumption and ketogenesis were high under all four conditions studied. Oxygen utilization was accurately predicted by three of the four models. There was complete equilibration between mitochondrial and cytosolic pools of acetate and of CO2, but for several of the metabolic conditions, two incompletely equilibrated pools of mitochondrial acetyl-CoA and oxaloacetate were required. Ketoleucine was utilized at a rate comparable to that reported by others in perfused liver and entered the mitochondrial pool of acetyl-CoA directly associated with ketone body formation. Ethanol, which was metabolized at rates comparable to those in vivo, caused relatively few changes in overall flux patterns. Several effects related to the increased NADH/NAD+ ratio were observed. Pyruvate dehydrogenase was completely inhibited and the ratio of acetoacetate to 3-hydroxybutyrate was decreased; flux through glutamate dehydrogenase, the citric acid cycle, and ketoleucine dehydrogenase were, however, only slightly inhibited. Net production of ATP occurred in all conditions studied and was increased by ethanol. Futile cycling was quantified at the glucose/glucose 6-phosphate, glycogen/glucose 6-phosphate, fructose 6-phosphate/fructose 1,6-bis-phosphate, and phosphoenolpyruvate/pyruvate/oxaloacetate substrate cycles. Cycling at these four loci consumed about 22% of cellular ATP production in control hepatocytes and 14% in ethanol-treated cells.
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PMID:Quantitative analysis of intermediary metabolism in rat hepatocytes incubated in the presence and absence of ethanol with a substrate mixture including ketoleucine. 293 May 1

Rat pancreatic endocrine tumours were induced by administration of streptozotocin plus nicotinamide. Fifteen to eighteen months later tumours with wet weights of 0.1 to 224 mg were isolated. These tumours were compared with normal rat pancreatic islets. Insulin release from perifused tumours was stimulated by D-glucose, L-leucine, 2-ketoisocaproate, and D-glyceraldehyde, potentiated by theophylline and inhibited by norepinephrine. Compared with isolated rat pancreatic islets, however, insulin secretory responsiveness to glucose stimulation and insulin content were reduced in tumour tissue. Hypoglycaemia in tumour bearing rats and impaired diffusion of insulin out of the tumours may explain this difference. The pattern of enzyme activities observed in tumour tissue was typical for pancreatic endocrine tissue. The activities of succinate dehydrogenase, the two types of the monoamine oxidase, and alpha-glucosidase were in the normal range in tumour tissue. Only the activities of 5'nucleotidase and glutamate dehydrogenase were decreased. Immunocytochemical analysis of the tumours revealed that they contained an average of 91% B-cells. In addition 8% of D-cells were encountered. Proportions of A-cells and PP-cells ranged below 1%. Thus this endocrine tumour of the pancreas with a high proportion of functionally intact B-cells is an interesting model for studying regulation of secretion and endocrine tumour development.
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PMID:Secretory, enzymatic, and morphological characterization of rat pancreatic endocrine tumours induced by streptozotocin and nicotinamide. 299 5

The various neutral amino acids and aliphatic 2-keto acids exhibit differential effects on insulin secretion. The common denominator for all these effects is the 2-ketoglutarate generation in the pancreatic B-cell mitochondria. The neutral amino acids L-leucine and L-norvaline and the aliphatic ketomonocarboxylic acids 2-ketoisocaproate, 2-ketocaproate, 2-ketovalerate, and 2-keto-3-methylvalerate all stimulate insulin secretion and increase 2-ketoglutarate generation in pancreatic B-cell mitochondria through activation of glutamate dehydrogenase and transamination with L-glutamate and L-glutamine, respectively. The neutral amino acids L-valine, L-norleucine, and L-alanine and the aliphatic 2-keto acids 2-ketoisovalerate and pyruvate do not stimulate insulin secretion and do not increase 2-ketoglutarate generation in pancreatic B-cell mitochondria. Inhibition of 2-keto acid induced insulin secretion by L-valine and L-isoleucine is accompanied by reduced 2-ketoglutarate generation in pancreatic B-cell mitochondria. Thus intramitochondrial 2-ketoglutarate generation in pancreatic B-cells may regulate the insulin secretory potency of amino acids and 2-keto acids.
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PMID:2-ketoglutarate generation in pancreatic B-cell mitochondria regulates insulin secretory action of amino acids and 2-keto acids. 352 57

A method for branched-chain amino acid aminotransferase is described which is based on running the reaction in the reverse of the usual direction with glutamate and alpha-ketoisocaproate as substrates. The alpha-ketoglutarate generated is reduced with glutamate dehydrogenase and NADH. For sensitivity in the nanomole range, the NAD+ generated is measured directly by converting to the highly fluorescent strong alkali product. For smaller samples, down to the 0.2- to 2-pmol range, the NAD+ is amplified by enzymatic cycling.
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PMID:A method for branched-chain amino acid aminotransferase activity in microgram and nanogram tissue samples. 402 4

In the absence of another exogenous nutrient, L-leucine but not L-norleucine stimulates insulin release from rat pancreatic islets, although the corresponding keto acids, 2-ketoisocaproate and 2-ketocaproate, are equally potent secretagogues. Such a situation cannot be ascribed to the preferential transamination of L-leucine as compared to L-norleucine in islet homogenates. Indeed, in the presence of a suitable activator of glutamate dehydrogenase, L-leucine and L-norleucine stimulate secretion to the same extent. It is concluded that the rate of transamination of these amino acids in intact islet cells depends on the availability of a 2-keto acid partner rather than on the assayed amino acid aminotransferase activity.
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PMID:Does leucine- and norleucine-induced insulin release depend on amino acid aminotransferase activity? 634 80

L-Leucine and its nonmetabolized analogue, 2-aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH) activate glutamate dehydrogenase in pancreatic islets, whether the reaction velocity is measured in the direction of glutamate synthesis or glutamate deamination. The rate of glutamate oxidative deamination is increased by ADP and inhibited by 2-ketoglutarate, NH4+ and GTP. The islet homogenate catalyzes the transamination between L-glutamate and either 2-ketoisocaproate or pyruvate, and between 2-ketoglutarate and L-leucine, L-aspartate, L-alanine, L-isoleucine, L-valine, L-norvaline or L-norleucine, but not b (+/-) BCH. The glutamate-aspartate transaminase is preferentially located in mitochondria relative to other transaminases. The parallel effects of L-leucine and BCH on glutamate dehydrogenase and their vastly different abilities to act as transamination partners may account for both analogies and discrepancies in the metabolic and functional responses of the islets to these two branched-chain amino acids.
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PMID:The stimulus-secretion coupling of amino acid-induced insulin release. XI. Kinetics of deamination and transamination reactions. 675 75

L-Glutamine markedly enhances insulin release evoked by L-leucine in rat pancreatic islets. The metabolic situation found in the islets exposed to both L-glutamine and L-leucine was investigated. L-Leucine slightly decreased the rate of L-glutamine deamidation, inhibited the conversion of glutamate to 2-ketoglutarate by transamination, increased the oxidative deamination of L-glutamate, stimulated the recirculation of 2-ketoglutarate to glutamate and inhibited the further oxidative metabolism of 2-ketoglutarate. L-Glutamine slightly decreased the rate of L-leucine conversion to 2-ketoisocaproate, but inhibited more severely the conversion of 2-ketoisocaproate to acetoacetate and CO2. Several of these findings appeared attributable to activation of glutamate dehydrogenase by L-leucine. When allowance was made for the influence of exogenous amino acids on the oxidation of endogenous fatty acids, a close parallelism was found between the rate of generation of reducing equivalents or O2 uptake and the insulin secretory response to L-leucine and/or L-glutamine. These findings reinforce the view that the process of nutrient-stimulated insulin release coincides with and may be attributable to an increase in catabolic fluxes in the islet cells.
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PMID:The stimulus-secretion coupling of amino acid-induced insulin release. Metabolic response of pancreatic islets of L-glutamine and L-leucine. 704 26

In rat pancreatic islets, D-glucose in concentrations exceeding 5.6 mM caused a concentration-related decrease of the mitochondrial NADH/NAD+ ratio, as judged from the changes in the islet content of glutamate, NH4+, and 2-ketoglutarate, and assuming that the glutamate dehydrogenase reaction is near equilibrium with the mitochondrial NAD system. The concentration dependency of the response to D-glucose was vastly different in islet and parotid cells, respectively. L-Leucine, 2-ketoisocaproate, BCH (a nonmetabolized but insulinotropic analog of L-leucine) and 3-phenylpyruvate also lowered the mitochondrial NADH/NAD+ ratio. In the presence of D-glucose, the latter ratio was also decreased by NH4+ or the absence of extracellular Ca2+, but dramatically increased by aminooxyacetate. Taking into account prior metabolic findings, the nutrient-induced fall in the mitochondrial redox state is thought to reflect an increased clearance of mitochondrial NADH through both the respiratory chain and malate-aspartate shuttle. The nutrient-induced decrease in the mitochondrial NADH/NAD+ ratio might favor both the circulation of metabolites in the Krebs cycle and the exit of Ca2+ from the mitochondria.
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PMID:Hexose metabolism in pancreatic islets: regulation of the mitochondrial NADH/NAD+ ratio. 755 20


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