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

Article abstract of DOI:10.1021/tx990189w

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 M₃ and M₈) 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 ~ 1B1 > 2E1. OPZ M₃ also inhibited P450s 1A2, 1A1, 1B1, and 3A4 but not more effectively than OPZ. OPZ Ms was not inhibitory. OPZ behaved as a competitive inhibitor of P450 1A2, with a K(i) of 1.5 μM. 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 M₃, 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 μM 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 (IC₅₀ < 1 μM) but not appreciably by 40 μM 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.