Studies on the regulation of the branched chain alpha-keto acid dehydrogenase in the perfused rat liver.

The regulation of the branched chain alpha-keto acid dehydrogenase multienzyme complex was investigated in the isolated, perfused rat liver. The metabolic flux through the branched chain alpha-keto acid dehydrogenase was monitored by measuring the production of 14CO2 from infused 1-14C-labeled branched chain alpha-keto acid substrates. The rate of decarboxylation of alpha-keto[1-14C]isocaproate exceeded that of alpha-keto[1-14C]isovalerate at all concentrations of the substrates infused. Coinfusion of either alpha-ketoisovalerate or alpha-keto-beta-methylvalerate inhibited the rate of alpha-keto[1-14C]isocaproate decarboxylation. The rate of alpha-keto[1-14C]isovalerate decarboxylation ws enhanced during coinfusion of L(--)carnitine, while alpha-keto[1-14C]isocaproate decarboxylation was unaffected. The presence of pyruvate in the perfusion medium resulted in an inhibition of the flux through the branched chain complex with either alpha-ketoisocaproate or alpha-ketoisovalerate as the substrate. DL-beta-hydroxybutyrate infusion inhibited alpha-keto[1-14C]isocaproate decarboxylation by 18% but resulted in nearly a 100% stimulation of alpha-keto[1-14C]isovalerate decarboxylation. The evidence presented indicates that (alpha) the metabolic flux through the branched chain alpha-keto acid dehydrogenase complex can be monitored effectively in a continuous fashion in the perfused liver by following the release of 14CO2 from infused 1-14C-labeled substrates and (b) the changes observed in the metabolic flux through the branched chain complex during coinfusion of alternative substrates and other compounds may be entirely different depending upon which branched chain alpha-keto acid substrate is utilized to monitor this reaction.