Studies with Ubiquinone‐Depleted Submitochondrial Particles

1 The effects of extraction and reincorporation of ubiquinone on the NADH and succinate dehydrogenase activities of submitochondrial particles from beef heart have been studied. 2 Extraction of ubiquinone had no effect on the maximal velocity or the Km for NADH of the NADH dehydrogenase of the particles measured with ferricyanide as electron acceptor. 3 The succinate dehydrogenase activity of the particles, measured with phenazine methosulfate as electron acceptor, was diminished by approximately 50% upon the extraction of ubiquinone, and was restored when ubiquinone was reincorporated into the particles. Concomitantly, the Km for succinate decreased and increased upon the extraction and reincorporation of ubiquinone. 4 Thenoyltrifluoroacetone, piericidin A, or pretreatment of the particles with cyanide, caused an approximately 50% decrease of the succinate dehydrogenase activity of the ubiquinone containing particles, while that of the ubiquinone-depleted particles was virtually unaffected by these agents. 5 Addition of mitochondrial phospholipids stimulated slightly the succinate dehydrogenase activity of both ubiquinone-containing and ubiquinone-depleted submitochondrial particles. 6 The data indicate that the succinate dehydrogenase of ubiquinone-depleted particles resembles in several respects preparations of isolated, soluble succinate dehydrogenase. It is concluded that the removal of ubiquinone causes a decrease in the rate of dissociation of fumarate from the enzyme and an inactivation of the interaction of its non-heme iron moiety with phenazine methosulfate. 7 It is pointed out that ubiquinone not only serves as an electron-transfer catalyst between succinate dehydrogenase and the cytochrome b–c1 complex, but may also exert a regulatory effect on these entities.