Inhibition and inactivation of human cytochrome P450 isoforms by phenethylisothiocyanate

The inhibition and mechanism-based inactivation potencies of phenethyl isot hiocyanate (PEITC) for human cytochrome P450 (CYP) activities were investig ated using microsomes from baculovirus-infected insect cells expressing spe cific human CYP isoforms. PEITC competitively inhibited phenacetin O-deethy lase activity catalyzed by CYP1A2 (K-i = 4.5 +/- 1.0 muM) and coumarin 7-hy droxylase activity catalyzed by CYP2A6 (K-i = 18.2 +/- 2.5 muM). Benzyloxyr esorufin O-dealkylase activity catalyzed by CYP2B6 was most strongly and no ncompetitively inhibited (K-i = 1.5 +/- 0.0 muM). Paclitaxel 6 alpha -hydro xylase activity catalyzed by CYP2C8 was not affected by PEITC up to 100 muM . PEITC noncompetitively inhibited S-warfarin 7-hydroxylase activity cataly zed by CYP2C9 (K-i = 6.5 +/- 0.9 muM), S-mephenytoin 4'-hydroxylase activit y catalyzed by CYP2C19 (K-i = 12.0 +/- 3.2 muM), bufuralol 1'-hydroxylase a ctivity catalyzed by CYP2D6 (K-i = 28.4 +/- 7.9 muM), and chlorzoxazone 6-h ydroxylase activity catalyzed by CYP2E1 (K-i = 21.5 +/- 3.4 muM). The inhib ition for testosterone 6 beta -hydroxylase activity catalyzed by CYP3A4 was a mixed-type of competitive (K-i = 34.0 +/- 6.5 muM) and noncompetitive (K -i = 63.8 +/- 12.5 muM) inhibition. Furthermore, PEITC is a mechanism-based inactivator of human CYP2E1. The k(inact) value was 0.339 min(-1) and K-i was 9.98 muM. Human CYP1A2, CYP2A6, CYP2B6, CYP2D6, and CYP3A4 were not ina ctivated. The present study directly proved that the chemopreventive effect s of PEITC for nitrosamine-induced carcinogenesis are due to the inhibition of CYP by an in vitro study. The possibility that PEITC would affect the p harmacokinetics of clinically used drugs that are metabolized by these CYP isoforms was also suggested.