Cytochrome P-450-dependent biotransformation of a series of phenoxazone ethers in the rat conceptus during early organogenesis: evidence for multiple P-450 isoenzymes.

Using highly sensitive probe-substrate analyses, investigations of drug biotransformation in tissues of the rat conceptus during an early stage of organogenesis revealed that three separate tissue components each contained P-450 isozymes capable of catalyzing the monooxygenation of foreign organic chemicals. Tissues of the embryo proper contained constitutive P450(s) that catalyzed readily measurable O-depentylation and O-debenzylation of pentoxyphenoxazone and benzyloxyphenoxazone, respectively, but no measurable O-demethylation of methoxyphenoxazone and barely detectable O-deethylation of ethoxyphenoxazone. Higher specific activities for the O-depentylation and O-debenzylation reactions were measured in preparations of the yolk sac and this organ also appeared to contain constitutive P450(s) for the readily detectable O-deethylation of ethoxyphenoxazone. The O-demethylation of methoxyphenoxazone could not be detected in the yolk sac. Only the O-debenzylation reaction could be detected in tissues of the ectoplacental cone. Treatment of conceptuses in utero with 3-methycholantherene (MC) resulted in significantly increased rates of O-deethylation reactions in preparations of yolk sac and embryo but not ectoplacental cone. Demethylation was not detectable in the same preparations. Treatment with phenobarbital, pregnenolone-16 alpha-carbonitrile, or isosafrole produced no observable effect on any of the reactions studied. Carbon monoxide (CO:O2 = 80:20 versus N2:O2 = 80:20) markedly inhibited all reaction rates and inhibition could be reversed by replacement of CO with N2. Deethylation and debenzylation were inhibited by anti-P450IA1 IgG after MC induction but were not affected by the same IgG fraction in untreated conceptuses. Depentylation reactions were not inhibited by anti-P450IA1 or anti-P450IIB1/2 antibodies under any of the conditions used. Deethylation was strongly inhibited by 1.0 microM 7,8-benzoflavone in tissues from MC-treated but not untreated conceptus. Metyrapone (0.1 mM) failed to significantly inhibit any of the measurable conceptus-catalyzed depentylation reaction. The results indicated the presence of four (or more) functional P450 isozymes in tissues of the conceptus during organogenesis, a constitutive depentylase(s) in the yolk sac and embryo, a constitutive deethylase(s) present in the yolk sac, an MC-inducible deethylase(s) in the embryo and yolk sac, and constitutive debenzylase(s) present in all three tissues. No O-demethylation was detectable in any of the three tissues, even after in utero exposure to inducers.(ABSTRACT TRUNCATED AT 400 WORDS)