ENZYME DISTRIBUTION IN POTATO MITOCHONDRIA

Enzyme distribution in potato mitochondria was investigated by selectively disrupting the outer and inner membranes with digitonin. Antimycin-insensitive NADH-cytochrome c reductase, an outer membrane marker, was released at low digitonin concentrations (0.1 mg mg-1 mitochondrial protein). Soluble matrix enzymes, fumarase and malate dehydrogenase were released at 0.3-0.4 mg digitonin mg-1 protein, as the inner membrane ruptured. Very little (about 10%) cytochrome oxidase activity was released, even at higher digitonin concentrations, in accord with this enzyme being an integral inner membrane protein. By this criterion adenylate kinase is also firmly bound to the inner membrane. Evidence indicates that it faces the intermembrane space.Malic enzyme activity was released by the same digitonin concentration that released fumarase and malate dehydrogenase, indicating that malic enzyme is a soluble matrix enzyme. No activity was released at low digitonin concentrations which selectively break the outer membrane, showing that malic enzyme is not present in the intermembrane space.Considerable catalase activity (20-40 μmol O2 min-1 mg-1 protein) was associated with washed mitochondrial preparations, but 95% of this was lost upon purification of mitochondria. The remaining activity was firmly bound to the mitochondrial membranes. © 1979 Oxford University Press.