HEXOSE METABOLISM IN PANCREATIC-ISLETS - SUCCINATE-DEHYDROGENASE ACTIVITY IN ISLET HOMOGENATES

Succinate dehydrogenase activity was measured in rat pancreatic islet homogenates incubated in the presence of [1,4-C-14]succinate, the reac tion velocity being judged through the generation of (CO2)-C-14 in the auxiliary reactions catalysed by pig heart fumarase and chicken liver NADP-malate dehydrogenase. In the presence of 1.0 mm succinate, the r eaction velocity averaged 5.53 +/- 0.44 pmol min-1 mug-1 islet protein . The K(m) for succinate was close to 0.4 mm and the enzymic activity was restricted to mitochondria. These kinetic results indicate that, u nder the present experimental conditions, the activity of succinate de hydrogenase does not vastly exceed that of either NAD-isocitrate dehyd rogenase or the 2-ketoglutarate dehydrogenase complex, at least when t he latter enzymes are activated by ADP and/or Ca2+ . Nevertheless, the activity of succinate dehydrogenase is sufficient to account for the increase in O2 uptake evoked in intact islets by the monomethyl ester of succinic acid. It could become a rate-limiting step of the Krebs cy cle in models of B-cell dysfunction.