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Mechanisms regulating adipose-tissue pyruvate dehydrogenase.

ABSTRACT: 1. Isolated rat epididymal fat-cell mitochondria showed an inverse relationship between ATP content and pyruvate dehydrogenase activity consistent with competitive inhibition of pyruvate dehydrogenase kinase by ADP. At constant ATP concentration pyruvate rapidly activated pyruvate dehydrogenase in fat-cell mitochondria, an observation consistent with inhibition of fat-cell pyruvate dehydrogenase kinase by pyruvate. Pyruvate dehydrogenase in fat-cell mitochondria was also activated by nicotinate (100mum) and by extramitochondrial Na(+) (replacing K(+)) but not by ouabain or insulin. 2. In rat epididymal fat-pads incubated in vitro pyruvate dehydrogenase was activated by addition of insulin in the absence of substrate or in the presence of glucose (10mm) or fructose (10mm). Glucose and fructose activated the dehydrogenase in the absence or in the presence of insulin, and pyruvate also activated in the absence of insulin. It is concluded that extracellular glucose, fructose and pyruvate may activate the dehydrogenase by raising intracellular pyruvate and that insulin may activate the dehydrogenase by some other mechanism. 3. Ouabain (300mum) and medium in which K(+) was replaced by Na(+), activated pyruvate dehydrogenase in epididymal fat-pads. Prostaglandin E(1) (1mug/ml), 5-methylpyrazole-3-carboxylate (10mum) and nicotinate (10mum), which are as effective as insulin as inhibitors of lipolysis and which like insulin lower tissue concentration of cyclic AMP (adenosine 3':5'-cyclic monophosphate), did not activate pyruvate dehydrogenase. Higher concentrations of prostaglandin E(1) (10mug/ml) and nicotinate (100mum) produced some activation of the dehydrogenase. 4. It is concluded that the activation of pyruvate dehydrogenase by insulin is not due to the antilipolytic effect of the hormone and that the action of insulin in lowering adipose-cell concentrations of cyclic AMP does not afford an obvious explanation for the effect of the hormone on pyruvate dehydrogenase. The possibility that the effects of insulin, ouabain and K(+)-free medium may be mediated by Ca(2+) is discussed.

SUBMITTER: Martin BR

PROVIDER: S-EPMC1174178 | biostudies-other | 1972 Sep

REPOSITORIES: biostudies-other

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Project description:1. In epididymal adipose tissue synthesizing fatty acids from fructose in vitro, addition of insulin led to a moderate increase in fructose uptake, to a considerable increase in the flow of fructose carbon atoms to fatty acid, to a decrease in the steady-state concentration of lactate and pyruvate in the medium, and to net uptake of lactate and pyruvate from the medium. It is concluded that insulin accelerates a step in the span pyruvate-->fatty acid. 2. Mitochondria prepared from fat-cells exposed to insulin put out more citrate than non-insulin-treated controls under conditions where the oxaloacetate moiety of citrate was formed from pyruvate by pyruvate carboxylase and under conditions where it was formed from malate. This suggested that insulin treatment of fat-cells led to persistent activation of pyruvate dehydrogenase. 3. Insulin treatment of epididymal fat-pads in vitro increased the activity of pyruvate dehydrogenase measured in extracts of the tissue even in the absence of added substrate; the activities of pyruvate carboxylase, citrate synthase, glutamate dehydrogenase, acetyl-CoA carboxylase, NADP-malate dehydrogenase and NAD-malate dehydrogenase were not changed by insulin. 4. The effect of insulin on pyruvate dehydrogenase activity was inhibited by adrenaline, adrenocorticotrophic hormone and dibutyryl cyclic AMP (6-N,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate). The effect of insulin was not reproduced by prostaglandin E(1), which like insulin may lower the tissue concentration of cyclic AMP (adenosine 3':5'-cyclic monophosphate) and inhibit lipolysis. 5. Adipose tissue pyruvate dehydrogenase in extracts of mitochondria is almost totally inactivated by incubation with ATP and can then be reactivated by incubation with 10mm-Mg(2+). In this respect its properties are similar to that of pyruvate dehydrogenase from heart and kidney where evidence has been given that inactivation and activation are catalysed by an ATP-dependent kinase and a Mg(2+)-dependent phosphatase. Evidence is given that insulin may act by increasing the proportion of active (dephosphorylated) pyruvate dehydrogenase. 6. Cyclic AMP could not be shown to influence the activity of pyruvate dehydrogenase in mitochondria under various conditions of incubation. 7. These results are discussed in relation to the control of fatty acid synthesis in adipose tissue and the role of cyclic AMP in mediating the effects of insulin on pyruvate dehydrogenase.

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Mechanisms regulating adipose-tissue pyruvate dehydrogenase.

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