Metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in human hepatocytes: 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid is a major detoxification pathway catalyzed by cytochrome P450 1A2.

Metabolic pathways of the mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) remain incompletely characterized in humans. In this study, the metabolism of MeIQx was investigated in primary human hepatocytes. Six metabolites were characterized by UV and mass spectroscopy. Novel metabolites were additionally characterized by 1H NMR spectroscopy. The carcinogenic metabolite, 2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline, which is formed by cytochrome P450 1A2 (P450 1A2), was found to be transformed into the N(2)-glucuronide conjugate, N(2)-(beta-1-glucosiduronyl)-2-(hydroxyamino)-3,8-dimethylimidazo[4,5-f]quinoxaline. The phase II conjugates N(2)-(3,8-dimethylimidazo[4,5-f]quinoxalin-2-yl)sulfamic acid and N(2)-(beta-1-glucosiduronyl)-2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, as well as the 7-oxo derivatives of MeIQx and N-desmethyl-MeIQx, 2-amino-3,8-dimethyl-6-hydro-7H-imidazo[4,5-f]quinoxalin-7-one (7-oxo-MeIQx), and 2-amino-6-hydro-8-methyl-7H-imidazo[4,5-f]quinoxalin-7-one (N-desmethyl-7-oxo-MeIQx), thought to be formed exclusively by the intestinal flora, were also identified. A novel metabolite was characterized as 2-amino-3-methylimidazo[4,5-f]quinoxaline-8-carboxylic acid (IQx-8-COOH), and it was the predominant metabolite formed in hepatocytes exposed to MeIQx at levels approaching human exposure. IQx-8-COOH formation is catalyzed by P450 1A2. This metabolite is a detoxication product and does not induce umuC gene expression in Salmonella typhimurium strain NM2009. IQx-8-COOH is also the principal oxidation product of MeIQx excreted in human urine [Turesky, R., et al. (1998) Chem. Res. Toxicol. 11, 217-225]. Thus, P450 1A2 is involved in both the metabolic activation and detoxication of this procarcinogen in humans. Analogous metabolism experiments were conducted with hepatocytes of untreated rats and rats pretreated with the P450 inducer 3-methylcholanthrene. Unlike human hepatocytes, the rat cell preparations did not produce IQx-8-COOH but catalyzed the formation of 2-amino-3,8-dimethyl-5-hydroxyimidazo[4,5-f]quinoxaline as a major P450-mediated detoxication product. In conclusion, our results provide evidence of a novel MeIQx metabolism pathway in humans through P450 1A2-mediated C(8)-oxidation of MeIQx to form IQx-8-COOH. This biotransformation pathway has not been detected in experimental animal species. Considerable interspecies differences exist in the metabolism of MeIQx by P450s, which may affect the biological activity of this mutagen and must be considered when assessing human health risk.

Inhibition of human cytochrome P450 enzymes by 1,2-dithiole-3-thione, oltipraz and its derivatives, and sulforaphane.

Recent studies have demonstrated that two chemoprotective agents, oltipraz (OPZ), a synthetic derivative of the natural compound 1, 2-dithiole-3-thione (D3T), and sulforaphane (SF), an isothiocyanate, are not only inducers of glutathione S-transferases but also inhibitors of some major cytochrome P450 enzymes (P450s) involved in xenobiotic metabolism. We examined P450 inhibition by the two compounds and compared two OPZ metabolites (OPZ M(3) and M(8)) and D3T using human P450s expressed in Escherichia coli membranes. OPZ was a more potent inhibitor than D3T or SF, in the following order of inhibition: P450 1A2 > 3A4 > 1A1 approximately 1B1 > 2E1. OPZ M(3) also inhibited P450s 1A2, 1A1, 1B1, and 3A4 but not more effectively than OPZ. OPZ M(8) was not inhibitory. OPZ behaved as a competitive inhibitor of P450 1A2, with a K(i) of 1.5 microM. Incubation of P450 1A2 with OPZ and NADPH led to a first-order loss of the P450 spectrum, and the loss was not blocked by glutathione. No such time-dependent loss of P450 was seen with P450 1A2 and D3T, P450 1A2 and OPZ M(3), P450 1A2 and SF, P450 3A4 and OPZ, P450 3A4 and D3T, P450 2E1 and OPZ, or P450 2E1 and D3T. The time- and concentration-dependent loss of P450 1A2 activity in the presence of OPZ was characterized with a K(i) of 9 microM and a k(inactivation) of 0.19 min(-)(1). The activation of 2-amino-3,5-dimethylimidazo[4, 5-f]quinoline (MeIQ) in an E. coli lac-based mutagenicity tester system containing functional human P450 1A2 was inhibited by OPZ (IC(50) < 1 microM) but not appreciably by 40 microM D3T. Our results indicate that OPZ is a competitive and mechanism-based inhibitor of P450 1A2, and the extent of this inhibition is significantly greater than that of other chemoprotective chemicals with P450 1A2 or other human P450s.