Mechanism of activation of hepatic branched-chain alpha-ketoacid dehydrogenase by a muscle factor.

Abstract

We recently reported that a skeletal muscle factor specifically activates the branched-chain alpha-ketoacid (BCKA) dehydrogenase in liver mitochondria (Paul, H. S., and Adibi, S. A. (1982) J. Biol. Chem. 257, 12581-12588). The evidence suggested that the muscle factor may be a phosphatase or may stimulate the phosphatase in liver mitochondria, thus converting the inactive BCKA dehydrogenase into an active form. In the present study we have investigated these possibilities. The muscle factor did not increase the activity of BCKA dehydrogenase which was previously activated in vitro by preincubation of mitochondria or by the addition of Triton or Ca2+ to the incubation medium. The muscle factor had little or no stimulatory effect on BCKA dehydrogenase when this enzyme was activated in vivo by treatment of rats with streptozotocin or clofibrate. The addition of NaF (an inhibitor of phosphatase) to the incubation medium entirely abolished the stimulatory effect of the muscle factor on BCKA dehydrogenase. Elimination of phosphatase by partial purification of BCKA dehydrogenase resulted in loss of stimulation of the dehydrogenase by the muscle factor. These results suggest that the muscle factor activates the BCKA dehydrogenase in liver mitochondria by stimulating its associated phosphatase.