Cytochrome oxidase extracted from mitochondria of wild‐type baker's yeast by means of bile salts and fractionated with ammonium sulfate had a specific activity about 18 times that of the mitochondria. When mitochondria of a cytoplasmic “petite” mutant were subjected to the same procedure, the resulting preparation exhibited virtually no cytochrome oxidase activity. Gel filtration resolved the active enzyme preparation into five protein peaks, the first of which contained copper and cytohemin and showed cytochrome oxidase activity. Bio‐Gel P‐300 filtration of the inactive preparation from mitochondria of the “petite” mutant likewise gave five peaks. The first peak, however, while containing copper, was free from cytohemin and devoid of cytochrome oxidase activity. When the material recovered from the first gel filtration peak of the active cytochrome oxidase preparation was subjected to disc electrophoresis on polyacrylamide, there appeared two protein bands. Two bands characterized by the same mobilities were also observed upon disc electrophoresis of the first peak material derived from mutant mitochondria. The evidence for the presence in “petite” mutant mitochondria of a protein which is identical with or closely similar to the apoprotein of wild‐type yeast cytochrome oxidase was confirmed by the finding that the incubation of submitochondrial particles of the “petite” mutant with cytohemin produced cytochrome oxidase activity. The enzymatic activity obtained was about 30% of that encountered in submitochondrial particles of wild‐type yeast. Copyright © 1969, Wiley Blackwell. All rights reserved
