Degradation of branched-chain amino acids and 2-oxo acids in human and rat muscle

Abstract

Studies were made at physiological substrate concentrations on the transamination and oxidative decarboxylation of branched-chain amino acids and on the oxidation of branched-chain 2-oxo acids in homogenates, 600 g supernatants, and mitochondria of various types of human skeletal muscle and of rat m. quadriceps and liver. l-Carnitine increased the oxidation of 4-methyl-2-oxopentanoate and 3-methyl-2-oxobutanoate in mitochondria of human muscle, but to a lesser degree than it did in mitochondria of rat muscle. It had no effect on the oxidation of 3-methyl-2-oxopentanoate. Half-maximal rates of α-decarboxylation of the 2-oxo acids were observed at similar concentrations in muscle mitochondria of man and rat, but the maximal rates were much higher in rat muscle mitochondria. Rates of transamination of valine and leucine were about similar in four types of human muscle, but lower than those in rat m. quadriceps. The oxidation rate of the branched-chain 2-oxo acids appears to vary for the different types of human muscle, but is markedly lower than that in rat muscle. In homogenates of human muscle CO 2 was only produced from 4-methyl-2-oxopentanoate by oxidative decarboxylation and the oxidation appears to proceed no further.