Cytochrome b5-like hemoprotein of outer mitochondrial membrane: OM cytochrome b. II. Contribution of OM cytochrome b to rotenone-insensitive NADH-cytochrome c reductase activity.

The participation of OM cytochrome b in the rotenone-insensitive NADH-cytochrome c reductase activity of rat tissues was investigated in comparison with that of cytochrome b5, by using antibodies against these two cytochromes. The specificity of each antibody was confirmed by inhibition studies of NADH-cytochrome c reductase activities reconstituted from purified cytochromes and NADH-cytochrome b5 reductase. OM cytochrome b-mediated NADH-cytochrome c reductase activity was found in various tissues including liver, kidney, heart, and brain. The contribution of this activity to the total rotenone-insensitive NADH-cytochrome c reductase activity was high in heart and brain cells. NADH-cytochrome c reductase activity mediated by OM cytochrome b was principally localized in mitochondrial outer membrane. Immunoadsorption studies using antibody-coated polyacrylamide beads showed that significant OM cytochrome b-mediated activity is present in the microsomal membrane, and that cytochrome b5-mediated activity also exists in the mitochondrial outer membrane.