Studies on the activation and inactivation of the branched chain alpha-keto acid dehydrogenase in the perfused rat heart.

Evidence for a reversible process resulting in stable activated and inactivated states of the mitochondrial branched chain alpha-keto acid dehydrogenase complex in isolated perfused rat heart is presented. The inactivation process is mediated by pyruvate infusion, while activation (up to 18-fold) is facilitated by branched chain alpha-keto acid substrates. The low activity state of the branched chain complex characteristic of freshly excised rat hearts could be maintained by infusion of either pyruvate or glucose. Activation of the complex in the perfused rat heart was achieved slowly by substrate-free perfusion, while rapid activation was accomplished by infusion of branched chain alpha-keto acids. The fully activated enzyme complex resulting from branched chain alpha-keto acid infusion subsequently could be inactivated maximally by infusion of pyruvate alone or intermediate degrees of inactivation could be produced by certain ratios of co-infused pyruvate and branched chain alpha-keto acid. alpha-Ketoisocaproate was an order of magnitude more effective than alpha-keto isovalerate either in preventing inactivation or in stimulating the opposing activation process when co-infused with pyruvate. The mitochondrial pyruvate transport inhibitor, alpha-cyanocinnamate, effectively prevented inactivation of the complex by infused pyruvate. Differential changes in the activation states of the branched chain alpha-keto acid dehydrogenase and pyruvate dehydrogenase complexes were evident when the two complexes were compared in apparently similar flux-inhibited (via octanoate infusion) and flux-stimulated (via dichloroacetate infusion) metabolic conditions. The differential effect of pyruvate concentration on the activity states of the two complexes was also well-defined. The results of the present study suggest distinct systems for the regulation of the activity of the two multienzyme complexes of interest. While our results argue neither for nor against an inactivation of the branched chain alpha-keto acid dehydrogenase complex by a protein kinase, the regulatory properties of such an intramitochondrial protein kinase may not be similar to the pyruvate dehydrogenase kinase. The mechanistic nature of the suggested novel regulatory system concerned with the pyruvate-mediated inactivation of the branched chain alpha-keto acid activation cannot be inferred at the present time.