Abstract

The human P450III family has been shown to be composed of at least four members, P450IIIA3 (HLp), P450IIIA4 (P450NF), P450IIIA5 (HLp3), and P450IIIA6 (HLp2). Due to the lack of probes that specifically recognize the individual members of this family, little is known about their relative expression. We prepared a form-specific antibody to P450IIIA5 by immunoabsorption of anti-P450IIIA5 IgG against Sepharose 4B upon which microsomes that did not contain P450IIIA5 or purified P450IIIA3 had been bound. Immunoblot analyses demonstrated that P450IIIA5 was expressed at detectable levels in only 19 of 66 (29%) human livers. The expression of P450IIIA5 was not influenced by the gender or medical history of the patients. When the expression of P450IIIA5 in different age groups was examined, it was observed that P450IIIA5 was detected in a statistically significantly higher percentage of children and adolescents (19 years old and under), as compared with the remaining population (8 of 17, 47%, versus 11 of 46, 24%, respectively). Furthermore, P450IIIA5 was detected in 1 of 10 human fetal livers. Of the large number of compounds identified as substrates of P450III family members, P450IIIA5 was found to actively metabolize nifedipine, testosterone, estradiol, dehydroepiandrosterone 3-sulfate, and cortisol, whereas it metabolized poorly or did not metabolize erythromycin, quinidine, 17 alpha-ethynylestradiol, and aflatoxins. The acetylenic steroid gestodene was found to be an effective mechanism-based inhibitor of both P450IIIA4 and P450IIIA5. Immunoblots of microsomes isolated from untreated and dexamethasone-, phenobarbital-, or 3-methylcholanthrene-treated HepG2 cells that were developed with an antibody that recognizes all the P450III family members demonstrated that no proteins in the P450III family were expressed by the HepG2 cells. In conclusion, our studies indicate that P450IIIA5 is polymorphically expressed at all stages of human development and is more limited in its metabolic capabilities than is P450IIIA4.