Localization of cytochromes in intact and fragmented mitochondrial membranes

Differences in the reactivity of cytochrome c toward external oxidants ferricyanide and cytochrome c peroxidase enzyme-substrate complexes in the intact and fragmented membranes may be easily explained on the basis of different locations of cytochrome c. The oxidation of cytochrome c is nearly 100% in the intact mitochondria, suggesting its location to be on the outside of the membrane permeability barrier and freely accessible to the external oxidants. In the sonicated membrane fragments, only 40% of cytochrome c reacts with ferricyanide or ES complex. Cytochromes c, c(1), a, and a(3) are oxidized roughly in the same proportion by the external oxidants. These data suggest the existence of complete cytochrome chains on both sides of the membrane permeability barrier in the submitochondrial particles, and serve to explain a number of hitherto unresolved complexities concerning their electron transfer and energy coupling reactions. Energy coupling and controlled electron flow occurs in sets of cytochromes c(1), a, a(3) on the same side of the membrane permeability barrier.