Plant polyphenols, such as gallic acid, have been reported to have a range of biological activities including antimutagenic effects. Previously, we reported that gallic acid (3,4,5-trihydroxybenzoic acid), an agent found in wine and tea, inhibits androstenedione 6beta-hydroxylase activity (Ki 70 microm), a cytochrome P450 (CYP3A) marker in human liver microsomes. The pre-incubation of gallic acid (100 microM) with human liver microsomes in the absence of NADPH, as compared with the presence of NADPH, before assay of androstenedione 6beta-hydroxylase activity significantly increased the inhibitory effects of the gallic acid (0.03 +/- 0.03 nmol (mg microsomal protein)(-1) min(-1) compared with 0.20 +/- 0.06 nmol (mg microsomal protein) (-1) min(-1) (P < 0.05)). The antioxidant ascorbic acid and the radical scavenger glutathione prevented this observed increase in inhibition. Removal of gallic acid-derived products from the incubation completely restored CYP3A activity. In contrast, the activities of CYP1A and CYP2E, and non-CYP mediated reductive microsomal 17beta-hydroxysteroid dehydrogenase activity were refractory to inhibition by gallic acid.