The antioxidant functions of cytochrome c

Low (C 1/2=1.5×10 −7 M) concentrations of horse cytochrome c strongly inhibit H 2O 2 production by rat heart mitochondria under conditions of reverse electron transfer from succinate to NAD +. The effect is abolished by binding of cytochrome c with liposomes and is not prevented by SOD. Yeast cytochrome c is much less effective than the horse protein whereas acetylated horse cytochrome c is without effect. H 2O 2 formation stimulated by antimycin A is resistant to added cytochrome c. In inside-out submitochondrial vesicles, H 2O 2 production is suppressed by all three cytochrome c samples tested, but at higher concentrations (C 1/2 is about 5×10 −7 M). In vesicles, SOD abolishes the cytochrome c inhibition. We conclude that extramitochondrial cytochrome c is competent in down-regulation of the Complex I H 2O 2 production linked to the reverse electron transfer. Such an effect is absent in the inside-out submitochondrial vesicles where another antioxidant cytochrome c function can be observed, i.e. the oxidation of O 2 − to O 2. A possible role of cytochrome c in the antioxidant defence is discussed.