Protective effect of magnesium and potassium ions on the permeability of the external mitochondrial membrane

The data reported are fully consistent with the well-known observation that exogenous cytochrome c (cyto- c) molecules do not permeate through the outer membrane of mitochondria (MOM) incubated in isotonic medium (250 mM sucrose). Cyto- c is unable to accept electrons from the sulfite/cyto- c oxido-reductase (Sox) present in the intermembrane space, unless mitochondria are solubilized. Mitochondria incubated in a very high hypotonic medium (25 mM sucrose), in contrast to any expectation, continue to be not permeable to added cyto- c even if Sox and adenylate kinase are released into the medium. The succinate/exogenous cyto- c reductase activity, very low in isotonic medium, is greatly increased decreasing the osmolarity of the medium but in both cases remains insensitive to proteolysis by added trypsin. In hypotonic medium, magnesium and potassium ions have a protective effect on the release of enzymes and on the reactivity of cyto- c as electron acceptor from both sulfite and succinate; results which are consistent with the view that MOM preserves its identity and remains not permeable to exogenous cyto- c. This report strengthens the proposal, supported by previously published data that in isotonic medium the exogenous NADH/cyto- c electron transport system is catalyzed by intact mitochondria, not permeable to added cyto- c.