Cytochromes P450 (CYPs) catalyze the 4-hydroxylation of all-trans-retinoic acid (ATRA), an agent used in the treatment of certain malignancies. Literature studies have implicated several CYPs in this reaction, but the relative importance of individual CYPs is unclear. Human microsomal CYPs that contribute to the activity were evaluated by correlation with activities of hepatic drug-metabolizing CYPs, the capacity of cDNA-derived CYPs to catalyze the reaction, and inhibition of the microsomal activity by chemicals. 4-HydroxyATRA formation in microsomes varied 7-fold (8.7 to 61 pmol/mg protein/min) and correlated partially with activities mediated by CYPs 3A, 2C, and 1A (p = 0.53 to 0.66). cDNA-derived CYPs 2C8, 2C9, and 3A4, but not 1A1 or 1A2, catalyzed ATRA 4-hydroxylation (2.53, 4.68, and 1.29 pmol/pmol CYP/hr). The Km for the reaction was 9 +/- 3 microM in hepatic microsomes (N = 3) and 6 microM in microsomes containing cDNA-derived CYP2C8; by comparison, Km values for the activity mediated by CYPs 2C9 and 3A4 were 100 and 74 microM, respectively. Inhibition of microsomal ATRA 4-hydroxylation was elicited by chemicals that interact with CYP2C8 (paclitaxel and diclofenac), but not those that interact with CYP2C9 (sulfaphenazole, tolbutamide, and torasemide). The CYP3A inhibitor troleandomycin and an anti-CYP3A IgG inhibited the activity slightly. Greater inhibition was produced by the less selective CYP3A inhibitors parathion, quinidine, and ketoconazole; CYP1A inhibitors were ineffective. These findings suggest that CYP2C8 is a major contributor to ATRA 4-hydroxylation in human liver and that 3A subfamily CYPs may be minor participants. Individual variation in CYP2C8 and 3A4 expression may influence ATRA pharmacokinetics and drug interactions during therapy.