The oxidation of leucine by hemidiaphragms of control and diabetic rats was studied in vitro. Rats were rendered diabetic with streptozotocin. Hemidiaphragms of diabetic rats produced approximately 50% more 14CO2 during incubation with 0.1 mM [l-14C]leucine than did control muscles. This was observed during incubation with or without glucose and in the presence or absence of a full complement of plasma amino acids. The concentration of leucine in the tissue water of hemidiaphragms from diabetic rats was greater tlian that in the control muscles before incubation. The specific activity of leucine at the end of 60 min incubation was not significantly different in diabetic and control muscles, indicating that the increased 14CO2 production represented stimulation of leucine oxidation. Hemidiaphragms of diabetic rats released more leucine into the medium during incubation than did control muscles. The stimulating effect of diabetes on leucine oxidation in vitro was reversible by insulin therapy prior to sacrifice. The addition of 5 mM pyruvate to a medium containing glucose inhibited 14CO2 production from [14C]leucine in control muscles, but stimulated leucine oxidation by hemidiaphragms of diabetic rats. Leucine oxidation by hemidiaphragms of diabetic rats was markedly stimulated by the addition of an electron acceptor, 0.02 mM methylene blue, suggesting that the NADH/NAD ratio may be ratelimiting for branched chain amino acid oxidation in muscles of diabetic rats, but not in muscles of controls. We suggest that the accelerated oxidation of branched chain amino acids by muscles may play a role in the acceleration of the muscle protein catabolism and gluconeogenesis which develop during insulin deficiency. The restraining effect of the cellular redox potential on branched chain amino acid oxidation may play a role in the eventual deceleration of protein catabolism during a prolonged fast.
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