Cytochrome b5 shifts oxidation of the anticancer drug ellipticine by cytochromes P450 1A1 and 1A2 from its detoxication to activation, thereby modulating its pharmacological efficacy

Abstract Ellipticine is a pro-drug, whose activation is dependent on its oxidation by cytochromes P450 (CYP) and peroxidases. Cytochrome b 5 alters the ratio of ellipticine metabolites formed by isolated reconstituted CYP1A1 and 1A2, favoring formation of 12-hydroxy- and 13-hydroxyellipticine metabolites implicated in ellipticine–DNA adduct formation, at the expense of 9-hydroxy- and 7-hydroxyellipticine that are detoxication products. Cytochrome b 5 enhances the production of 12-hydroxy and 13-hydroxyellipticine. The change in metabolite ratio results in an increased formation of covalent ellipticine–DNA adducts, one of the DNA-damaging mechanisms of ellipticine antitumor action. This finding explains previous apparent discrepancies found with isolated enzymes and in vivo , where CYP1A enzymatic activation correlated with ellipticine–DNA-adduct levels while isolated CYP1A1 or 1A2 in reconstituted systems were much less effective than CYP3A4. The effect of cytochrome b 5 might be even more pronounced in vivo , since, as we show here, ellipticine increases levels of cytochrome b 5 in rat liver. Our results demonstrate that both the native 3D structure of cytochrome b 5 and the presence of the heme as an electron transfer agent in this protein enable a shift in ellipticine metabolites formed by CYP1A1/2.