Regulation of a-Ketoisocaproate Oxidation in Liver Mitochondria

Oxidation of a-ketoisocaproate (KIC, 4-methyl-2-oxopentanoate) by coupled liver mitochondria was inhibited 60-70% by 1 m~ AMP, ADP, or its analogue, adenosine-5’-0-(2-thodiphosphate); inhibition was blocked by atractyloside. AMP inhibition was mediated by ADP generated via adenylate kinase. ADP inhibition was timeand temperature-dependent and required mitochondrial integrity. Oxidation of a-ketoisovalerate (KIV, 3-methyl-2-oxobutyrate), when added as the only substrate, was not affected or mildly inhibited by ADP (15%). In the presence of equimolar KIC, ADP markedly inhibited KIV oxidation. ATP (with or without the uncoupler carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP)) did not inhibit oxidation of KIV or KIC while FCCP inhibited KIC but not KIV decarboxylation by intact liver mitochondria. In the presence of FCCP and oligomycin, ATP partially relieved the FCCP-induced inhibition of KIC oxidation. At low substrate concentrations, inhibition of KIC oxidation by ADP or FCCP was attenuated by carnitine. Inhibition of KIC oxidation by ADP or AMP correlated with incpeased intramitochondrial ADP. Under identical conditions, pyruvate oxidation by intact liver mitochondria was stimulated by ADP and inhibited by ATP. These effects persisted after freeze-thawing. ADP did not affect KIC oxidation by coupled heart mitochondria, while ATP in the presence of FCCP inhibited