Diminished inhibition of mitochondrial electron transfer from succinate to cytochrome c by thenoyltrifluoroacetone induced by antimycin.

Thenoyltrifluoroacetone inhibits succinate-cytochrome c reductase activity of resolved succinate*cytochrome c reductase complex. Dixon plots of [succinate-cytochrome c reductase activity]-’ or [succinateubiquinone reductase activity]-’ versus concentration of inhibitor are consistent with there being a single site of inhibition by thenoyltrifluoroacetone in resolved reductase complex. This agrees with results obtained with succinate-ubiquinone reductase complex by Mowery and co-workers (Mowery, P. C., Steenkamp, D. J., Ackrell, B. A. C., Singer, T. P., and White, G. A. (1977) Arch. B&hem. Biophys. 178,495-506). When electron transfer from succinate to cytochrome c is partially inhibited by antimycin, thenoyltrifluoroacetone at concentrations up to 500 pM does not inhibit the remaining succinate-cytochrome c reductase activity as effectively as in the absence of antimycin. The efficacy of thenoyltrifluoroacetone progressively decreases with increasing amounts of antimycin, until, at concentrations of antimycin which inhibit electron transfer more than 60%, addition of thenoyltrifluoroacetone causes a 2to t-fold stimulation of the rate of cytochrome c reduction. The antimycin-dependent increased rate of cytochrome c reduction which occurs on addition of thenoyltrifluoroacetone is abolished by superoxide dismutase. These results indicate that inhibition of respiration by thenoyltrifluoroacetone occurs by a mechanism in which the inhibitor causes the reduced form of a low potential oxidation-reduction component to react with oxygen, thus generating superoxide anion, and that antimycin enhances this effect. The oxidation-reduction component whose reactivity with oxygen is promoted by thenoyltrifluoroacetone is most likely ubisemiquinone. If this interpretation is correct, our results indicate that thenoyltrifluoroacetone does not inhibit the reduction of ubiquinone to ubisemiquinone but does inhibit the subsequent reduction of ubisemiquinone to ubiquinol. It follows that reduction of ubisemiquinone to ubiquinol is catalyzed by electron transfer from a donor which is an oxidation-reduction component of the respiratory chain, possibly the high potential iron-sulfur center S-3 of succinate dehydrogenase, as opposed to the possibility that this reaction might occur by a non-