Nitidine chloride-induced CYP1 enzyme inhibition and alteration of estradiol metabolism

Abstract

The cytochrome P450 (P450) 1 family is an important phase I enzyme involved in carcinogen activation. Nitidine chloride (NC) is a pharmacologically active alkaloid with polyaromatic hydrocarbon found in the roots of Zanthoxylum nitidum (Roxb.) DC, a traditional medicinal herb widely used in China. We examined the inhibitory effects of NC on CYP1A1, 1B1, and 1A2. NC significantly inhibited CYP1A1- and 1B1-catalyzed ethoxyresorufin O-deethylation activity (IC50 5 0.28±0.06 and 0.32±0.02 mM, respectively) in a concentration- dependent manner, but only showed slight inhibition of CYP1A2 activity (IC50 > 50 mM). Kinetic analysis revealed that NC competitively inhibited CYP1B1 with a Ki value of 0.47±0.05 mM, whereas NC caused a mixed type of inhibition on CYP1A1 with Ki and KI values of 0.14±0.04 and 0.19±0.09 mM, respectively. The observed enzyme inhibition neither required NADPH nor revealed time dependency. Molecular docking manifested the generation of strong hydrogen-bonding interactions of Ser116 in CYP1A1 and Ser127 in CYP1B1 with methoxy moiety of NC. Additionally, NC-induced alteration of estradiol (E2) metabolism was also investigated in the present study. Hydroxyestradiols, including 2-hydroxyestradiol [(2-OHE2) nontoxic] and 4-hydroxyestradiol [(4-OHE2) genotoxic] generated in recombinant enzyme incubation systems and cultured MCF-7 cells were analyzed, and NC was found to preferentially inhibit the nontoxic 2-hydroxylation activity of E2 mediated by CYP1A1. In conclusion, NCwas a mixed type inhibitor of CYP1A1 and a competitive inhibitor of CYP1B1. The remarkable inhibition on E2 2-hydroxylation might increase the risk of 4- OHE2-induced genotoxicity.