Basis for decreased D-beta-hydroxybutyrate dehydrogenase activity in liver mitochondria from diabetic rats

Liver mitochondria from rats made diabetic with streptozotocin have a reduced level of D-beta-hydroxybutyrate dehydrogenase (BDH) activity and decreased ratios of oleic/stearic and arachidonic/linoleic acids in the phospholipids of the mitochondrial membrane. This altered activity and lipid environment result from insulin deprivation since maintenance of the diabetic rats on insulin leads to normal characteristics (J.C. Vidal, J.O. McIntyre, P.F. Churchill, and S. Fleischer (1983) Arch. Biochem, Biophys. 224, 643-658). In the present study, the basis for the reduced enzymatic activity of this lipid-requiring enzyme was analyzed using three approaches: (i) Purified D-beta-hydroxybutyrate, dehydrogenase was inserted into membranes from mitochondria, submitochondrial vesicles, and mitochondrial lipids extracted therefrom. The activation was the same and optimal irrespective of whether the preparations were derived from normal or diabetic rat liver. Therefore, the decreased activity does not appear to be referable to an altered lipid composition. (ii) BDH activity can be released from the mitochondria by phospholipase A2 digestion. The released activity was proportional to the endogenous activity in the submitochondrial vesicles from normal and diabetic membranes. (iii) The BDH activity in submitochondrial vesicles was titrated by inhibition with specific antiserum. Less enzyme was found in mitochondria from diabetic rats as compared with those from normal animals. Hence, the lowered enzymatic activity is due to decreased enzyme in the mitochondrial inner membrane and not to the modified lipid environment.