Lynn L. Barron

The effect of acetate on the regulation of the branched chain α-keto acid and the pyruvate dehydrogenase complexes in the perfused rat liver

Abstract The regulatory consequences of acetate infusion on the pyruvate and the branched chain α-keto acid dehydrogenase reactions in the isolated, perfused rat liver were investigated. Metabolic flux through these two decarboxylation reactions was monitored by measuring the rate of 14 CO 2 production from infused 1- 14 C-labeled substrates. When acetate was presented to the liver as the sole substrate the rate of ketogenesis which resulted was maximal at concentrations of acetate in excess of 10 m m . The increase in hepatic ketogenesis during acetate infusion was not accompanied by an alteration of the mitochondrial oxidation-reduction state as measured by the ratio of β-hydroxybutyrate/ acetoacetate in the effluent perfusate. While acetate infusion did not affect the rate of α-keto[1- 14 C]isocaproate decarboxylation, the rate of α-keto[1- 14 C]isovalerate decarboxylation was stimulated appreciably upon acetate addition. No change was observed in the amount of extractable branched chain α-keto acid dehydrogenase during acetate infusion. The rate of [1- 14 C]pyruvate decarboxylation was stimulated in the presence of acetate at low ( m ) but not at high (>1 m m ) perfusate pyruvate concentrations. The stimulation of the metabolic flux through the pyruvate dehydrogenase reaction upon acetate infusion was accompanied by an increase in the activation state of the pyruvate dehydrogenase complex from 25.7 to 35.6% in the active form. In a liver perfused in the presence of the pyruvate dehydrogenase kinase inhibitor, dichloroacetate, at a low concentration of pyruvate (0.05 m m ) the infusion of acetate did not affect the rate of pyruvate decarboxylation. As the rate of mitochondrial acetoacetate efflux is increased during acetate infusion the stimulation of pyruvate and α-ketoisovalerate decarboxylation is attributed to an accelerated rate of exchange of mitochondrial acetoacetate for cytosolic pyruvate or α-ketoisovalerate on the monocarboxylate transporter.