NADPH-cytochrome P-450 reductase: Preferential inhibition by ellipticine and other type II compounds having little effect on NADPH-cytochrome c reductase

Abstract

Ellipticine (5,11-dimethyl-[6H]-pyrido[4,3b]carbazole) binds with an affinity greater than most other compounds known to interact with P-450. Control and 3-methylcholanthrene-induced aryl hydrocarbon (benzo[ a]pyrene) hydroxylase (EC 1.14.14.2) and acetanilide 4-hydroxylase and control and phenobarbital-induced ethylmorphine N-demethylase activities are all markedly inhibited by ellipticine to about the same extent. Ellipticine and other Type II compounds (metyrapone, octylamine-1, pyridine and aniline) preferentially inhibit NADPH-cytochrome P-450 reductase activity, while affecting NADPH-cytochrome c reductase activity very little. Butanol-1, a compound having pure Reverse Type I character, does not block P-450 reductase activity like these Type II compounds. These data suggest that Type II compounds bind to P-450 ferric iron in the sixth coordinate position in such a way as to impede transfer of the first electron from the hydrophobic binding site of the reductase to the P-450-substrate complex, while leaving unencumbered any transfer of electrons from the hydrophilic binding site of the reductase to soluble electron acceptors such as cytochrome c. These data indicate that ellipticine may be very useful in attempting to understand the mechanism by which electrons are transferred from the reductase to the cytochrome(s) P-450.