Antimycin-resistant alternate electron pathway to plastocyanin in bovine-heart complex III

Bovine-heart Complex III can catalyze the reduction of spinach plastocyanin by a decyl analog of ubiquinol-2 at a rate comparable with the rate of plastocyanin reduction by plastoquinol as catalyzed by the cytochrome b6-f complex purified from spinach leaves. This plastocyanin reduction as catalyzed by Complex III was almost completely inhibited by myxothiazol at stoichiometric concentrations, partially inhibited by UHDBT (5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole) and funiculosin, and was relatively insensitive to antimycin and HQNO (2-n-heptyl-4-hydroxyquinoline-N-oxide). Cytochrome c reduction as catalyzed by Complex III displayed a residual, inhibitor-insensitive rate of 5% of the uninhibited rate for each of the three inhibitors, antimycin, myxothiazol, and UHDBT. However, the residual rate that was insensitive to each of the inhibitors added singly was inhibited further by addition of the remaining two inhibitors. From these results it is concluded that plastocyanin reduction involves an electron-transfer pathway through Complex III that is distinct from the pathway utilized for reduction of cytochrome c.