EXPRESSION OF FUNCTIONAL CYTOCHROME P4501A1 IN HUMAN EMBRYONIC HEPATIC TISSUES DURING ORGANOGENESIS

Investigations with chemical inhibitors and with inhibitory antibodies specific for cytochrome p4501A-catalyzed ethoxyresorufin (ethoxypheno xazone) O-deethylation and 2-acetylaminofluorene (N-2-fluorenylacetami de) ring hydroxylation indicated that cytochrome(s) P450 of the 1A sub family was functionally expressed in human embryonic hepatic tissues a t very early stages (days 50-60) of gestation. Lack of detectable capa city of hepatic microsomal enzymes to catalyze either N-hydroxylation of 2-acetylaminofluorene or O-demethylation of methoxyresorufin indica ted that functional cytochrome P4501A2 is expressed minimally or negli gibly in human embryonic hepatic tissues. By contrast, profound inhibi tion of the ring hydroxylation of 2-acetylaminofluorene and of the O-d eethylation of ethoxyresorufin by 7,8-benzoflavone as well as by anti- cytochrome P4501A1 antibodies indicated the presence of significant le vels of functional cytochrome P4501A1 in hepatic microsomes of human e mbryos. Using the reverse transcriptase-linked polymerase chain reacti on with specific oligonucleotide primers, we also detected significant expression of cytochrome P4501A1 mRNA in human embryonic livers. Poly merase chain reaction amplification, cloning and sequencing of the cor responding cDNA provided evidence that the cytochrome P4501A1 mRNA exp ressed in human embryonic tissues was identical to that expressed in a dult human tissues. The results of the study have important implicatio ns in terms of the embryotoxic effects of chemicals that are known to be substrates, inhibitors or inducers of cytochrome P4501A1 and to whi ch pregnant women are exposed.