Ubiquinol:cytochrome c oxidoreductase (complex III). Effect of inhibitors on cytochrome b reduction in submitochondrial particles and the role of ubiquinone in complex III.

Two sets of studies have been reported on the electron transfer pathway of complex III in bovine heart submitochondrial particles (SMP). 1) In the presence of myxothiazol, MOA-stilbene, stigmatellin, or of antimycin added to SMP pretreated with ascorbate and KCN to reduce the high potential components (iron-sulfur protein (ISP) and cytochrome c(1)) of complex III, addition of succinate reduced heme b(H) followed by a slow and partial reduction of heme b(L). Similar results were obtained when SMP were treated only with KCN or NaN(3), reagents that inhibit cytochrome oxidase, not complex III. The average initial rate of b(H) reduction under these conditions was about 25-30% of the rate of b reduction by succinate in antimycin-treated SMP, where both b(H) and b(L) were concomitantly reduced. These results have been discussed in relation to the Q-cycle hypothesis and the effect of the redox state of ISP/c(1) on cytochrome b reduction by succinate. 2) Reverse electron transfer from ISP reduced with ascorbate plus phenazine methosulfate to cytochrome b was studied in SMP, ubiquinone (Q)-depleted SMP containing </=0.06 mol of Q/mol of complex III, and Q-replenished SMP. The results showed that Q was not required for electron transfer from ISP to b, a reaction that was inhibited by antimycin (also by myxothiazol or MOA-stilbene as reported elsewhere). It was also shown that antimycin did not inhibit electron transfer from b (b(H)) to Q, in clear contrast to the assumption of the Q-cycle hypothesis regarding the site of antimycin inhibition.