Effects of benzyl isothiocyanate on rat and human cytochromes P450: Identification of metabolites formed by P4502B1

Naturally occurring isothiocyanates, such as benzyl isothiocyanate (BITC), are potent and selective inhibitors of carcinogenesis induced by a variety of chemical carcinogens. These effects appear to be mediated through favora ble modification of both phase I and II enzymes involved in carcinogen meta bolism. The inactivation of rat and human cytochromes P450 (P450s) in micro somes and the reconstituted system by BITC was investigated. BITC is a mech anism-based inactivator of rat P450s 1A1, 1A2, 2B1, and 2E1, as well as hum an P450s 2B6 and 2D6. BITC was most effective in inactivating P450s 2B1, 2B 6, 1A1, and 2E1, whereas the activities of human P450 2C9 and rat P450 3A2 were not altered. The concentrations required for half-maximal inactivation (K-I) of P450s 1A1, 1A2, 2B1, and 2E1 were 35, 28, 16, and 18 muM, respect ively. The corresponding values for k(inact) were 0.26, 0.09, 0.18, and 0.0 5 min(-1), respectively. Sodium dodecyl sulfate-polyacrylamide gel electrop horesis of P450 2B1 inactivated by [C-14]BITC indicated specific and covale nt modification of the P450 apoprotein by a metabolite of BITC. High-perfor mance liquid chromatography analysis of the BITC metabolites revealed that benzylamine was the major metabolite and there were lesser amounts of benzo ic acid, benzaldehyde, N,N'-di-benzylurea, and N,N'-di-benzylthiourea. Pres umably, BITC was metabolized to the reactive benzyl isocyanate intermediate that covalently modified the P450 apoprotein or hydrolyzed to form benzyla mine. BITC was an efficient inactivator of P450 2B1 with a partition ratio of approximately 11:1. This irreversible inactivation of P450s by BITC coul d contribute significantly to its chemopreventative action.