Abstract

Cytochrome P4503A7 (CYP3A7) is the primary CYP isoform expressed in human fetal hepatic microsomes, and its potential role in human embryotoxicity has attracted considerable investigative attention. In this study, we investigated the 4-hydroxylation of highly embryotoxic and teratogenic retinoic acids (RA) as catalyzed by human fetal liver microsomes (HFLM) and demonstrated that CYP3A7 is an efficient RA hydroxylase. When all-trans-retinoic acid(tRA), 9-cis-retinoic acid (9cRA), or 13-cis-retinoic acid (13cRA) were incubated with HFLM (54–109 gestational days) plus NADPH, each of these three retinoic acids was rapidly converted to its corresponding 4-hydroxy and 4-oxo metabolites. The reactions were strongly inhibited by CO (CO:O₂, 80:20) and were NADPH-dependent, indicating that the reactions were catalyzed by P450 isoenzymes. At 54 to 89 gestational days, 4-hydroxylase activities were relatively low. However, at gestational days 96 to 109, activities were much higher. Selective inhibitors were employed for elucidation of the roles of individual CYP isoenzymes in HFLM. α-Naphthoflavone, paclitaxel, and diethyldithiocarbamate showed little or no effects on HFLM-catalyzed reactions, indicating that CYP1A1, CYP1A2, CYP1B1, CYP2C8, and CYP2E1 did not play significant roles in the catalysis. By contrast, troleandomycin strongly inhibited the reaction (70–75% inhibition), suggesting that CYP3A7 was primarily responsible for the observed catalysis. It was also discovered that CYP3A7 SUPERSOMES efficiently catalyzed the 4-hydroxylations of tRA, 9cRA, and 13cRA. Because 4-hydroxylated metabolites of RA are much less potent embryotoxins and teratogens, the results indicated that the 4-hydroxylation of RA, catalyzed prenatally by CYP3A7, might play an important role in protecting the human fetus against RA-induced embryotoxicity.