The role of cytochrome b5 in mixed function oxidations: effect of microsomal binding of the hemoprotein on hepatic N-demethylations.

Incubation of rat cytochrome b5 (D-b5) with rat liver microsomes resulted in specific binding of the hemoprotein. The bound hemoprotein was rapidly reduced by NADH. The NADH cytochrome c reductase activity in these preparations increased in proportion to the amount of cytochrome. In contrast to D-b5, which inhibited N-demethylation and the NADH synergism, the binding of cytochrome b5 preparations, reconstituted from heme and apocytochrome b5 had no effect on either the NADPH-dependent N-demethylation of aminopyrine or ethylmorphine or the NADH synergism observed with rat liver microsomes. In addition, manganese protoporphyrin-apocytochrome complex, when bound to microsomes in amounts equilvalent to D-b5, showed no effect on N-demethylation activity. These results suggest that homogeneous cytochrome b5 contains contaminating amounts of tightly bound detergent which presumably is removed during the extraction of the heme from the apocytochrome.