Electron transport in beef heart muscle preparation The inhibition of ubiquinone-stimulated succinate oxidase activity by antimycin and a quinoline-N-oxide derivative

Abstract

1. The succinate oxidase activity of acetone extracted heart muscle preparation could be restored by the addition of ubiquinone homologues.2. Where short chain ubiquinone homologues were employed to restore succinate oxidase activity, the reconstituted system was found to be no longer fully inhibited by antimycin.3. When the acetone extracted preparation was preincubated with ubiquinone homologues and then washed free of unbound quinone, the succinate oxidase of the reconstituted particle was found to be completely inhibited by antimycin.4. Previous reports had proposed that the quinone mediated antimycin insensitive electron was due either to an electronic by-pass around the antimycin sensitive site or to the displacement of inhibitor by the added quinone.These two alternatives were investigated by measuring the sensitivity of acetone extracted heart muscle preparation at different concentrations of inhibitors in the presence of quinone.5. The results appear to support an electron by-pass mechanism of electron flow mediated by hydrophilic quinones Q-o and methyl-Q-o. The pattern of the reversal of antimycin inhibition by the higher ubiquinone homologues Q-2 and Q-3 suggests that displacement of inhibitor is the controlling factor.6. The Q-o and methyl-Q-o mediated antimycin insensitive rate of succinate oxidation of both extracted and unextracted heart muscle was compared and results suggest that this pathway of electron flow does not involve the endogenous ubiquinone of the respiratory chain.7. These findings are further considered in relation to the hydrophilic nature of short chain ubiquinone homologues.