Possible existence of a CYP3A protein in liver microsomes from female rats.

CYP3A proteins are P450 monooxygenases involved in the metabolism of steroids, retinoic acid and several important drugs. In rats, the number of CYP3A genes and proteins, and therefore important aspects of their inducibility, developmental regulation and sex specificity are not known for certain. Using triacetyloleandomycin-metabolite complex formation, testosterone hydroxylase assays and immunoblots from peptide maps, we obtained results suggesting that in liver microsomes from mature rats, at least three, possibly four CYP3A proteins are expressed: one is present in females, another is male-specific, and one or two additional CYP3A proteins are inducible by phenobarbital, steroids, and triacetyloleandomycin.