UBIQUINOL-CYTOCHROME-C OXIDOREDUCTASE - THE REDOX REACTIONS OF THE BIS-HEME CYTOCHROME-B IN UNENERGIZED AND ENERGIZED SUBMITOCHONDRIAL PARTICLES

The redox reactions of the bis-heme cytochrome b of the ubiquinol:cyto chrome c oxidoreductase complex (complex III, bc(1) complex) were stud ied in bovine heart submitochondrial particles (SMP), It was shown tha t (i) when SMP were treated with the complex III inhibitor myxothiazol (or MOA-stilbene or stigmatellin) or with KCN and ascorbate rep reduc e the high potential centers of complex III (iron-sulfur protein and c ytochromes c + c(1)), NADH or succinate reduced heme b(L) slowly and i ncompletely, In contrast, heme b(H) was reduced by these substrates co mpletely and much more rapidly, Only when the complex III inhibitor wa s antimycin, and the high potential centers were in the oxidized state , NADH or succinate was able to reduce both b(H) and b(L) rapidly and completely, (ii) When NADH or succinate was added to SMP inhibited at complex III by antimycin and energized by ATP, the bis-heme cytochrome b was reduced only partially, Prereduction of the high potential cent ers was not necessary for this partial b reduction, but slowed down th e reduction rate, Deenergization of SMP by uncoupling (or addition of oligomycin to inhibit ATP hydrolysis) resulted in further b reduction. Addition of ATP after b was reduced by substrate resulted in partial b oxidation, and the heme remaining reduced appeared to be mainly b(L) . Other experiments suggested that the redox changes of cytochrome ZI effected by energization and deenergization of SMP occurred via electr onic communication with the ubiquinone pool, These results have been d iscussed in relation to current concepts regarding the mechanism of el ectron transfer by complex III.