Tannic acid has been shown to decrease mutagenicity and/or carcinogenicity of several amine derivatives and polycyclic aromatic hydrocarbons in rodents. The purpose of this study was to evaluate the effect of tannic acid on cytochrome P450 (CYP)-catalyzed oxidations using rat liver microsomes (RLM) and human liver microsomes (HLM) as the enzyme sources. In RLM, tannic acid showed a non-selective inhibitory effect on 7-methoxyresorufin O-demethylation (MROD), 7-ethoxyresorufin O-deethylation (EROD), tolbutamide hydroxylation, p-nitrophenol hydroxylation and testosterone 6 beta-hydroxylation activities with IC50 values ranged from 14.9 to 27.4 mu M. In HLM, tannic acid inhibited EROD, MROD and phenacetin O-deethylation activities with IC50 values ranged from 5.1 to 7.5 mu M, and diclofenac 4-hydroxylation, dextromethorphan O-demethylation, chlorzoxazone 6-hydroxylation and testosterone 6 beta-hydroxylation with IC50 values ranged from 20 to 77 mu M. In baculovirus-insect cell-expressed human CYP 1A1 and 1A2, the IC50 values of tannic acid for CYP 1A1- and 1A2-catalyzed EROD activities were 23.1 and 2.3 mu M, respectively, indicating that tannic acid preferably inhibited the activity of CYP 1A2. Tannic acid inhibited human CYP1A2 non-competitively with a Ki value of 4.8 mu M. Tannic acid was also found to inhibit NADPH-CYP reductase in RLM and HLM with IC50 values of 11.8 and 17.4 mu M, respectively. These results suggested that the inhibition of CYP enzyme activities by tannic acid may be partially attributed to its inhibition of NADPH-CYP reductase activity. (C) 2007 Elsevier Ltd. All rights reserved.
