PT - JOURNAL ARTICLE AU - Kaoru Kobayashi AU - Miki Nakajima AU - Kanako Oshima AU - Noriaki Shimada AU - Tsuyoshi Yokoi AU - Kan Chiba TI - Involvement of CYP2E1 as A Low-Affinity Enzyme in Phenacetin <em>O</em>-Deethylation in Human Liver Microsomes DP - 1999 Aug 01 TA - Drug Metabolism and Disposition PG - 860--865 VI - 27 IP - 8 4099 - http://dmd.aspetjournals.org/content/27/8/860.short 4100 - http://dmd.aspetjournals.org/content/27/8/860.full SO - Drug Metab Dispos1999 Aug 01; 27 AB - Phenacetin O-deethylation (POD) exhibits biphasic kinetics in human liver microsomes. Although cytochrome P-450 (CYP) 1A2 is responsible for the high-affinity component of POD, the enzyme(s) that catalyzes the low-affinity reaction is still unknown. We examined the roles of human CYPs in POD by using human liver microsomes and recombinant CYPs from baculovirus-infected insect cells. Of the recombinant CYPs studied, CYP1A2 showed the highest POD activity. CYP1A1, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 also showed POD activity at 500 μM phenacetin. KM values of recombinant CYP1A2 and CYP2E1 (28 ± 2 μM and 785 ± 125 μM, respectively) were similar to those of the high- and low-affinity components of POD in pooled human liver microsomes (15 ± 5 and 894 ± 189 μM, respectively). Fluvoxamine (10 μM) and anti-CYP1A2 antibodies potently inhibited POD activity at 500 μM phenacetin in pooled human liver microsomes to 22.8 and 34.2% of controls, respectively. CYP2E1 inhibitors diethyldithiocarbamate and aniline also reduced POD activity. The combination of fluvoxamine (10 μM) and aniline (1 mM) further inhibited the residual POD activity not inhibited by fluvoxamine alone. Microsomal POD activity in 12 human livers in the absence of fluvoxamine was correlated with immunoquantified CYP1A2 levels (r = 0.961,p &lt; .001) and, in the presence of 10 μM fluvoxamine, was correlated with immunoquantified CYP2E1 levels (r = 0.589, p &lt; .01) or chlorzoxazone 6-hydroxylase activity (r = 0.823,p &lt; .001). These results suggest that CYP2E1 is responsible for the low-affinity component of POD in human liver microsomes. The American Society for Pharmacology and Experimental Therapeutics