Insulin as a probe of mitochondrial metabolism in situ.

Our previous studies of insulin action have led us to the finding that insulin acts specifically on the mitochondrial Krebs cycle to stimulate, by 30%, the oxidation of carbons 2 and 3 of pyruvate to CO2. Insulin also stimulates the oxidation of both carbons of acetate. These carbons can be converted to CO2 only after passing through all of the reactions of the Krebs cycle more than once. Carboxyl groups, such as number 1 of pyruvate, are oxidized to CO2 without any effect of insulin, and can be converted to CO2 by extramitochondrial enzyme. We conclude that insulin must act on the complete intramitochondrial cycle and not on the four enzymes of the Krebs cycle which are present in the cytoplasm. The path taken by those carbons affected by insulin is traced through the complete Krebs cycle, and the necessity for this effect to be mitochondrial has been verified by demonstration of the same specific effect of insulin on the oxidation of the 2 and 3 carbons of succinate. The use of this phenomenon is proposed for the study not only of human diabetes, but of all mitochondrial disorders, by using 14C specifically labeled tracers in culture or biopsy material, or 13C labeled tracer material in vivo.