Human fetal CYP3A7 and human NADPH-cytochrome P450 reductase were coexpressed in insect cells, TN-5, infected with a recombinant baculovirus carrying both cDNAs. The expression of reductase in TN-5 cells was shown to be sufficient for the CYP3A7 dependent 16 alpha-hydroxylation of dehydroepiandrosterone. However, the extra addition of cytochrome b5 and phospholipid was necessary to obtain a maximal activity of CYP3A7 catalyzing the reaction. CYP3A7 expressed in TN-5 cells was capable of metabolizing testosterone, cortisol and dehydroepiandrosterone 3-sulfate as well as dehydroepiandrosterone. The apparent Vmax for 6 beta-hydroxylations of testosterone was similar to that obtained for 6 beta-hydroxylation of cortisol (2.9 versus 2.5 nmol/nmolP450/min). In contrast, the apparent Vmax for 16 alpha-hydroxylation of dehydroepiandrosterone and its 3-sulfate were 20 and 2 times greater than those observed for steroid 6 beta-hydroxylations, respectively (67.5 and 5.8 versus 2.5-2.9 nmol/nmol P450/min). On the other hand, the apparent K(m) for 6 beta-hydroxylations of testosterone and cortisol were greater than those for 16 alpha-hydroxylations (120 and 860 versus 46-58 microM). Thus, CYP3A7 was active for steroid 6 beta-hydroxylations and 16 alpha-hydroxylations, but there were greater differences in Vmax/K(m) ratios between these reactions.