The oxidation of external NADH by an intermembrane electron transfer in mitochondria from the ubiquinone-deficient mutant E3-24 of Saccharomyces cerevisiae.

Cells of the E3-24 mutant of the strain D273-10B of Saccharomyces cerevisiae, grown in a fermentable substrate not showing catabolite repression of respiration (2% galactose), are able to respire, in spite of their ubiquinone deficiency in mitochondrial membranes. Mitochondria isolated from these mutant cells oxidize exogenous NADH through a pathway insensitive to antimycin A but inhibited by cyanide. Addition of methanolic solutions of ubiquinone homologs stimulates the oxidation rate and restores antimycin A sensitivity in both isolated mitochondria and whole cells. Mersalyl preincubation of isolated mitochondria inhibits both NADH oxidation and NADH-cytochrome c oxido-reductase activity (assayed in the presence of cyanide) with the same pattern. Electrons resulting from the oxidation of exogenous NADH reduce both cytochrome b5 and endogenous cytochrome c. The increase in ionic strength stimulates NADH oxidation, which is also coupled to the ATP synthesis with an ATP/O ratio similar to that obtained with ascorbate plus N,N,N',N'-tetramethyl-p-phenylendiamine (TMPD) as substrate. The effect of cyanide on these activities and on NADH-induced endogenous cytochrome c reduction is also comparable. These results support the existence in vivo and in isolated mitochondria of a energy-conserving pathway for the oxidation of cytoplasmatic NADH not related to the dehydrogenases of the inner membrane, the ubiquinone, and the b-c1 complex, but involving a cytochrome c shuttle between the NADH-cytochrome c reductase of the outer membrane and cytochrome oxidase in the inner membrane.