Reprint of: Ubisemiquinone Is the Electron Donor for Superoxide Formation by Complex III of Heart Mitochondria.

Much evidence indicates that superoxide is generated from O in a cyanide-sensitive reaction involving a reduced component of complex III of the mitochondrial respiratory chain, particularly when antimycin A is present. Although it is generally believed that ubisemiquinone is the electron donor to O, little experimental evidence supporting this view has been reported. Experiments with succinate as electron donor in the presence of antimycin A in intact rat heart mitochondria, which contain much superoxide dismutase but little catalase, showed that myxothiazol, which inhibits reduction of the Rieske iron-sulfur center, prevented formation of hydrogen peroxide, determined spectrophotometrically as the HO-peroxidase complex. Similarly, depletion of the mitochondria of their cytochrome c also inhibited formation of HO, which was restored by addition of cytochrome c. These observations indicate that factors preventing the formation of ubisemiquinone also prevent HO formation. They also exclude ubiquinol, which remains reduced under these conditions, as the reductant of O. Since cytochrome b also remains fully reduced when myxothiazol is added to succinate- and antimycin A-supplemented mitochondria, reduced cytochrome b may also be excluded as the reductant of O. These observations, which are consistent with the Q-cycle reactions, by exclusion of other possibilities leave ubisemiquinone as the only reduced electron carrier in complex III capable of reducing O to O.