A possible restriction of ferro- and ferricyanide oxidoreductase activities of rat liver mitochondria by the outer membrane

Enzymatic reduction of labile iron by organelles of the rat liver. Superior role of an NADH-dependent activity in the outer mitochondrial membrane

The enzymatic system mainly responsible for the reduction of labile iron ions in mammalian cells is still unknown. Using isolated organelles of the rat liver, i.e. mitochondria, microsomes, nuclei and the cytosol, we here demonstrate that Fe(III), added as Fe(III)-ATP complex, is predominantly reduced by an NADH-dependent enzyme system associated with mitochondria (65% of the overall enzymatic Fe(III) reduction capacity within liver cells). Microsomes showed a significantly smaller Fe(III) reduction capacity, whereas the cytosol and nuclei hardly reduced Fe(III). Studying the mitochondrial iron reduction, this NADH-dependent process was not mediated by superoxide, ascorbic acid, or NADH itself, excluding low-molecular-weight reductants. No evidence was found for the involvement of complex I and III of the respiratory chain. Submitochondrial preparations revealed the highest specific activity reducing Fe(III) in the outer membrane fraction. In conclusion, an NADH-dependent mitochondrial enzyme system, most likely the NADH-cytochrome c reductase system, located at the outer membrane, should decisively contribute to the enzymatic reduction of labile iron within liver cells, especially under pathological conditions.

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.