Vol. 29, Issue 4, 389-393, April 2001

Mechanism-Based Inhibition of Human Cytochrome P450 1A1 by Rhapontigenin

Young Jin Chun, Shi Yong Ryu, Tae Cheon Jeong, and Mie Young Kim

College of Pharmacy, Chungang University, Seoul, Korea (Y.J.C., M.Y.K.); Korea Research Institute of Chemical Technology, Taejon, Korea (S.Y.R.); and College of Pharmacy, Yeungnam University, Kyungsan, Korea (T.C.J.)

Recently we reported that resveratrol (trans-3,4',5-trihydroxystilbene) showed selective inhibition of recombinant human cytochrome P450 (P450) 1A1 in a concentration-dependent manner. The inhibition of recombinant human P450 1A1, 1A2, or 1B1 by various hydroxystilbene compounds having a similar structure to resveratrol was investigated using bacterial membranes from a human P450/NADPH-P450 reductase bicistronic expression system to find new candidates for cancer chemopreventive agents. Of seven compounds tested, rhapontigenin (3,3',5-trihydroxy-4'-methoxystilbene) exhibited a potent and selective inhibition of human P450 1A1 with an IC₅₀ value of 0.4 µM. Rhapontigenin showed 400-fold selectivity for P450 1A1 over P450 1A2 and 23-fold selectivity for P450 1A1 over P450 1B1. Rhapontigenin did not show any significant inhibition of ethoxyresorufin O-deethylation (EROD) activity in human liver microsomes, the other human P450s such as P450 2E1, P450 3A4, P450 2D6, P450 2C8, and P450 2C9, or human NADPH-P450 reductase. We have further investigated the inhibition kinetics of P450 1A1 by rhapontigenin. Rhapontigenin inhibited EROD activity of expressed human P450 1A1 in a competitive manner. The loss of EROD activity was time- and concentration-dependent. The values for K_i and k_inactivation were 0.09 µM and 0.06 min¹, respectively. The loss was not blocked by the trapping agents glutathione, N-acetylcysteine, or dithiothreitol. These results suggest that rhapontigenin is a potent mechanism-based inactivator of human P450 1A1 and may be considered as a good candidate for a cancer chemopreventive agent in humans.