Mitochondrial cytochrome b-c complex: its oxidation-reduction components and their stoichiometry.

A cytochrome b-c₁ complex was isolated from pigeon breast muscle mitochondria and purified to a content of 3 nmol of cytochrome c₁ per milligram of protein. Anaerobic suspensions of the preparation were titrated with reducing equivalents (NADH) and oxidizing equivalents (ferricyanide). The oxidation-reduction components of the complex were measured by the number of reducing equivalents accepted or donated per cytochrome c₁ and compared with the stoichiometries of the known redox components as measured by independent methods. The preparation accepts or donates 5.2 ± 0.3 equivalents per cytochrome c₁, while the measured content of cytochrome c₁, cytochrome b₅₆₁, cytochrome b₅₆₅, Rieske iron-sulfur protein, ubiquinone, and succinate dehydrogenase accounts for 5.0 ± 0.2 equivalents per cytochrome c₁. It is concluded that there are no unknown redox components in the cytochrome b-c₁ complex. The cytochrome b-c₁ complex (energy transduction site 2) appears to be a structural unit containing equal amounts of cytochrome c₁, cytochrome b₅₆₁, cytochrome b₅₆₆, and the Rieske iron-sulfur protein.