Effect of ubiquinone extraction on the reaction of the mitochondrial bc1 complex with ferricyanide.

Depletion of endogenous ubiquinone by pentane extraction of mitochondrial membranes lowered succinate-ferricyanide reductase activity, whereas quinone reincorporation restored the enzymatic activity as well as antimycin sensitivity. The oxidant-induced cytochrome b extrareduction, normally found upon ferricyanide pulse in intact mitochondria in the presence of antimycin, was lost in ubiquinone-depleted membranes, even if cytochrome c was added. Readdition of ubiquinone-2 restored the oxidant-induced extrareduction with an apparent half saturation at 1 mol/mol bc1 complex saturating at about 5 mol/mol. These findings demonstrate a requirement for the ubiquinone pool of the cytochrome b extrareduction. Since the initial rates of cytochrome b reoxidation upon ferricyanide addition, in the presence of antimycin, did not saturate by any ferricyanide concentration in ubiquinone-depleted mitochondria, a direct chemical reaction between ferricyanide and reduced cytochrome b was postulated. The fact that such direct reaction is much faster in ubiquinone-depleted mitochondria may explain the lower antimycin sensitivity of the succinate ferricyanide reductase activity after removal of endogenous ubiquinone.