Cytochrome P450-dependent bioactivation of prodysmorphogens in cultured conceptuses

These investigations were undertaken to determine the extent to which tissues of cultured rat conceptuses contain cytochrome P450 isoforms in sufficient quantities to significantly influence the capacity of certain chemicals to elicit dysmorphogenic effects in vitro. Investigations with highly sensitive probe substrates/inhibitors and with immunologic methods enabled the detection of at least four separate P450 isoforms in tissues of the visceral yolk sac, ectoplacental cone, and embryo proper. One of the isoforms was identified as P450IA1 and was found to be inducible by polycyclic aromatic hydrocarbons in all three tissues. Other isoforms exhibited properties differing from characterized adult rat hepatic isoforms. Each of the isoforms was detectable in conceptuses on gestational days 10, 11, 12, and 14 and was present in the highest concentrations in the visceral yolk sac. Conceptal P450IA1 catalyzed the conversion of dysmorphogenically inactive 2-acetylaminofluorene to 7-hydroxy-2-acetylaminofluorene, a proximate dysmorphogen. Investigations with microinjections suggested that visceral yolk sac hydroxylation was largely responsible for the bioactivation reaction in vitro. The same isoform exhibited no capacity to influence the dysmorphogenic activity of cyclophosphamide. The results demonstrated that tissues of cultured rat conceptuses may contain P450 isoforms in sufficient amounts to markedly influence the dysmorphogenic activity of substrates of the corresponding isoforms.