Insulin regulation of branched chain alpha-keto acid dehydrogenase in adipose tissue

The enzyme which oxidizes alpha-keto[1-14C]isocaproate to 14CO2 is activated by incubation of adipose tissue segments with insulin. A 3-fold reduction in the apparent Km of the enzyme for alpha-ketoisocaproate was observed when homogenates of adipose tissue segments treated with insulin were compared to homogenates of control tissues. The enzyme was assayed at various times after homogenization of adipose tissue segments. Relatively small changes were observed in the activity from control or insulin-treated tissues for 30 min after homogenization. The persistence of the insulin effect after homogenization suggests that insulin may cause a covalent modification of the enzyme. The possibility that alpha-ketoisocaproate is oxidized by pyruvate dehydrogenase, which is also stimulated by insulin, is unlikely since the enzyme responsible for oxidation of 14C-labeled branched chain alpha-keto acids can be inactivated by heat at a rate distinct from that of pyruvate dehydrogenase. Moreover, unlabeled branched chain alpha-keto acids inhibit the oxidation of alpha-keto[1-14C]isocaproate but not that of [1-14C]pyruvate. Branched chain alpha-keto acid hydrogenase can be activated by incubation of adipose tissue homogenates in the presence of magnesium chloride and in the absence of ATP. The addition of ATP plus an ATP-regenerating system reverses the activation of the enzyme. The apparent Km of the enzyme is reduced and the Vmax is increased by incubation of tissue extracts under appropriate conditions.