Role of coenzyme Q in the mitochondrial respiratory chain. Reconstitution of activity in coenzyme Q deficient mutants of yeast.

The reduction of cytochrome c by the reduced form of the 6-decyl analogue of coenzyme Q follows first-order kinetics with respect to cytochrome c and increases in a linear manner with added mitochondrial protein. The activity is completely sensitive to antimycin A in whole cell extracts of yeast as well as in isolated mitochondria and fractionates with markers for the mitochondrial electron-transport chain. The presence of both cytochrome b and c1 in an approximately 2:1 ratio appears essential for enzymatic activity. Reduced coenzyme Q-cytochrome c reductase obeys Michaelis-Menten kinetics when assayed in mitochondria obtained from a yeast strain lacking coenzyme Q. Both reduced nitotinamide adenine dinucleotide and succinate:cytochrome c reductase activities were not detectable in six coenzyme Q deficient strains tested, but were restored after addition of the oxidized form of the coenzyme Q analogue. No marked difference in the concentration of the analogue required to restore the two activities was observed.