Metabolism of benzo[a]pyrene and benzo[a]pyrene-7,8-diol by human cytochrome P450 1B1

Abstract

Benzo[a]pyrene (B[a]P), a ubiquitous environmental, tobacco and dietary carcinogen, has been implicated in human cancer etiology. The role of human cytochrome P450 1B1 in the metabolism of B[a]P is poorly understood. Using microsomal preparations of human P450 1A1, 1A2 and 1B1 expressed in baculovirus-infected insect cells, as well as human and rat P450 1B1 expressed in yeast, we have determined the metabolism of B[a]P, with and without the addition of exogenous epoxide hydrolase, and B[a]P-7,8-dihydrodiol (7,8-diol), each substrate at a concentration of 10 microM. HPLC analysis detected eight major metabolites of B[a]P and four metabolites of the 7,8-diol. The results of these studies indicate that cytochrome P450 1B1 carries out metabolism of B[a]P along the pathway to the postulated ultimate carcinogen, the diol epoxide 2, at rates much higher than P450 1A2 but less than P450 1A1. The rates of formation of the 7,8-diol metabolite in incubations with epoxide hydrolase are 0.17 and 0.38 nmol/min/nmol P450 for human P450 1B1 and 1A1, respectively, and undetectable for 1A2. The rates of total tetrol metabolite formation from the 7,8-diol, which are indicative of diol epoxide formation, are 0.60, 0.43 and 2.58 nmol/min/nmol P450 for 1B1, 1A2 and 1A1 respectively. In agreement with other reports of rat P450 1B1 activity, our data show this rat enzyme to be very active for B[a]P and 7,8-diol, with rates higher than human P450 1B1. In addition to the established role of P450 1A1 in B[a]P metabolism, P450 1B1 may significantly contribute to B[a]P and 7,8-diol metabolism and carcinogenesis in rodent tumor models and in humans.

Cancer Risk Assessment, Indicators, and Guidelines for Polycyclic Aromatic Hydrocarbons in the Ambient Air

“…This step may be carried out by CYP1A1, peroxidase-catalyzed epoxidation, or through cooxidation by simultaneous exposure to B[a]P and sulfur or nitrogen oxides (Cavalieri and Rogan 1995;Constantin et al 1994;Petruska et al 1992;Thakker et al 1985). In addition to CYP1A1, cytochrome P450 isoenzymes such as CYP1A2, CYP1B1, and CYP3A4 may also participate in the metabolic activation of PAHs (Guengerich 1993;Kim et al 1998;Nelson et al 1996). This seems to be of particular importance with respect to the metabolic activation of fjord-region PAHs.…”

Understanding the human health effects of chemical mixtures.

“…CYP1B1 appears to be more active than CYP1A1 in the conversion of a number of PAHs to genotoxic intermediates (269). In the presence of epoxide hydrolase, both CYP1A1 and CYP1B1 catalyze the conversion of the archetypal PAH, benzo[a]pyrene, to its 7,8-dihydrodiol, and both enzymes can in turn metabolically activate this benzo[a]pyrene metabolite to a mutagenic form (269,271,272). Covalent adducts, formed by the reaction of PAH diol epoxide metabolites with guanine in mutational hotspots of critical genes such as that of the p53 tumor suppressor (273,274), may initiate tumorigenesis if not efficiently repaired.…”

The relationship between DNA adduct formation by benzo[a]pyrene and expression of its activation enzyme cytochrome P450 1A1 in rat

“…BaP is, however, oxidized also to other metabolites such as the other dihydrodiols, BaPdiones and hydroxylated metabolites (Bauer et al, 1995;Chun et al, 1996;Kim et al, 1998;Jiang et al, 2001;Zhu et al, 2008). Even though most of these metabolites are detoxification products, BaP-9-ol is a precursor of 9-hydroxy-BaP-4,5-epoxide which can form another adduct with deoxyguanosine ( Fig.…”