Potent inhibition of recombinant human cytochrome P-450 1A1 by pentamethoxystilbene

Previously it was reported that various hydroxystilbene compounds strongly inhibit human cytochrome P-450 1 enzymes and were postulated as candidate chemopreventive agents. In this study, the inhibitory potential of P-450 1 enzyme activities by 3,5,3 4 5-pentamethoxystilbene (PMS), a synthetic stilbene compound, was evaluated with the Escherichia coli (E. coli) membranes of recombinant human cytochrome P-450 1A1, 1A2, or 1B1 coexpressed with human NADPH-P-450 reductase. PMS produced a significant inhibition of ethoxyresorufin O-deethylation (EROD) activities with IC50 values of 0.14, 934, and 3.2 M for 1A1, 1A2, and 1B1, respectively. PMS did not significantly inhibit EROD activities in human liver microsomes. To elucidate the mechanism of inhibition by PMS, kinetic studies were performed. Analysis of the mode of inhibition indicated a mixed-type inhibition of P-450 1A1. The inhibition of P-450 W-mediated EROD activity by PMS was not irreversible-mechanism based. The loss of EROD activity of P-450 W with PMS was blocked by trapping agents such as glutathione, N-acetylcysteine, or dithiothreitol. Moreover, PMS significantly suppressed P-450 W-mediated TROD activity and P-450 W gene expression in HepG2 cells induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Taken together, the results suggested that PMS is a potent and selective inhibitor of human P-450 W and may be considered for use as a cancer chemopreventive agent in humans.