The markers of pig heart mitochondrial sub-fractions. II. - On the association of malate dehydrogenase with inner membrane.

Malate dehydrogenase, reputed to be a soluble matricial enzyme, is shown to be also strongly associated with the inner membrane, in pig heart mitochondria. Repeated sonications, water washes, freezing-thawing cycles are not very effective to remove malate dehydrogenase activity from inner membranes, which whatever the treatment, remains important. This activity is only partly solubilized by the substrates, malate or oxaloacetate. High ionic strength treatments by either NaCl-carbonate or 3M KCl have a strong effect, but they also remove cytochrome c oxidase and rotenone-sensitive NADH-cytochrome c reductase, reputed inner membrane intrinsic enzymes, thus strongly damaging the inner membrane. After the action of phospholipase A from Naja Naja Venom, the residual activity is about twenty per cent and only phosphatidyl choline and phosphatidyl ethanolamine decreased significantly, the other phospholipids being unchanged. It is suggested that the enzyme is deeply buried in the membrane and mainly interacts with phosphatidyl choline.