CYTOCHROME-P-450-DEPENDENT BIOTRANSFORMATION OF QA-208-199 IN CULTURED RAT CONCEPTUSES

It has been shown, using the method of rat post-implantation embryo cu lture, that the rat conceptus metabolizes the lipoxygenase inhibitor - hydroxy-N-methyl-7-propoxy-2-naphthalenethanamine (QA 208-199, QAB) in vitro. The capacity to metabolize QAB and accumulate its main metabol ites depends on the developmental stage and length of exposure. In the present study, pregnant Han Wistar dams were given ip injections of e ither Aroclor 1254 (AC), 3-methylcholanthrene (3-MC) or phenobarbital (PB) before the dissection and culture of 9.5- and 10.5-day-old concep tuses in order to identify possible inducible conceptal cytochrome P-4 50 species. Conceptuses preinduced in utero were exposed to QAB in vit ro for 48 hr, after which metabolites in the culture medium and concep tual tissues were determined with HPLC. Embryotoxic effects were evalu ated in the same conceptuses. Preinduction in utero with AC, 3-MC or P B revealed that pretreatment with PB led. after 48 hr of culture, to a statistically significantly increased impairment of embryonic differe ntiation in 9.5-day-old conceptuses in vitro in comparison with vehicl e-induced QAB-exposed conceptuses. Preinduction with PB led to higher levels of QAA, the main metabolite in vivo, in the culture medium. In the conceptal tissues, metabolite levels were significantly lower afte r preinduction with AC. In vehicle-induced 10.5-day-old embryos QAB ca used less embryotoxicity in comparison with 9.5-day-old ones, In all p reinduced conceptuses cultured from 10.5 to 12.5 days, yolk-sac vascul arization was less affected and fewer embryonic anomalies were found i n comparison with conceptuses cultured from 9.5 to 11.5 days. AC and P B pretreatment caused a decreased morphological score without changes in overall growth. Analysis of QAB and its metabolites in culture medi a and conceptal tissues at 12.5 days showed different levels of metabo lites depending on the inducer used. To find further evidence for the involvement of cytochrome P-450 enzymes in the conceptal metabolism of QAB, conceptuses preinduced in utero (3-MC or PB) were exposed to QAB in vitro and gassed during the second half of the culture period with a mixture containing 35% carbon monoxide (CO), an inhibitor of cytoch rome P-450 enzymes. CO treatment led to an inhibition of conceptal met abolism of QAB in comparison with that in conceptuses cultured under n ormal gassing conditions (without CO). These results strongly suggest the involvement of cytochrome P-450-dependent monooxygenases in the co nceptal metabolism of QAB in vitro. 10.5-day-old conceptuses, after pr eexposure in utero to PB, were less susceptible to QAB embryotoxicity in vitro than were 9.5-day-old ones. In addition, the response to prei nduction appeared to be age dependent.