Cytochrome P450, CYP3 in the human fetal liver
Previous work (Barnes et al, 1989) suggested that a different major im-munorelated form of CYP3 existed in adult and fetal human liver. Northern blot analysis was conducted, therefore, using an adult CYP3 cDNA probe, to determine how similar the fetal CYP3 messenger RNA species was to the adult. Two major messenger RNA species of similar length (2.1kb and 3.0kb) in both the adult and fetal liver were observed, indicating a high degree of homology between the two. A human fetal cDNA expression library in Agtll was prepared and screened using both an adult cDNA probe and monoclonal antibodies raised against a human adult liver CYP3, cytochrome P450 form (Barnes et al., 1987). This resulted in the isolation of a number of cDNA clones, one which was similar to (90% nucleotide sequence homology) but distinct from the adult cDNA sequences in the C7P3 gene family. Therefore, we have isolated and sequenced a partial cDNA cone which is unique to the human fetus and it appears that a different immunorelated form of C7P3 does exist in adult and fetal liver, with a distinct protein being expressed in the fetus. It is thought that one of the main physiological roles of this CFP3 protein in the fetus, is its involvement in production of estriol, required for the maintenance of pregnancy. In order to gain some insight into which specific amino acid changes, between the fetal and adult CYP3 proteins, might contribute to the altered substrate specificities observed in the CYP3 enzyme (Cresteil et al., 1982; Kitada et al., 1987), the positions of these changes were analyzed with respect to the tertiary structure of P450cam. This allowed speculation concerning which specific amino acid changes may be important in affecting the metabolic profile of the CYP3 protein and furthermore, the amino acids that may be important in the substrate access channel. On comparison of protein CYP3 levels in both adult and fetal livers (Western blot analysis) with clinical data, the OT3 protein expression was noted to be higher in an adult patient treated with an anticonvulsant drug, than in patients receiving no drug treatments. Furthermore, large interindividual differences in CYP3 protein expression levels were seen in adult livers, although this was not as pronounced in fetal livers. This perhaps reflects the need for more constant levels of the CYP3 protein in all human fetuses, due to the vital role it plays in the production of estriol.