Predominant 4-hydroxylation of estradiol by constitutive cytochrome P450s in the female ACI rat liver.

The ACI rat is extremely sensitive to estrogens as mammary carcinogens, whereas the Sprague-Dawley strain is relatively resistant. Comparison of the disposition and effects of estrogens in these two strains should provide insights into the mechanisms of estrogen carcinogenicity. We have begun this investigation by comparing the metabolism of [(3)H]17beta-estradiol (E2) by liver microsomes prepared from female rats from each strain. Both strains produce estrone (E1) as the major product at E2 concentrations >1 microM, with smaller amounts of 2-hydroxy-E2 formed. As the E2 concentration is decreased, however, aromatic hydroxylation becomes a more dominant pathway for both strains. At starting E2 concentrations as low as 3 nM, Sprague-Dawley liver microsomes produced comparable yields of 2-hydroxy-E2 and E1. In contrast, ACI liver microsomes yielded a profound shift to aromatic hydroxylation as the dominant pathway as E2 concentrations dropped below 1 microM, and this shift reflected the production of 4-hydroxy-E2 as the predominant product. The apparent K(m) for 4-hydroxylation of E2 is <0.8 microM, as opposed to approximately 4 microM for 2-hydroxylation, suggesting that different cytochrome P450s (CYPs) are responsible. Western immunoblotting of the liver microsomal preparations from ACI and Sprague-Dawley rats for CYPs known to catalyze 2- and 4-hydroxylation of E2 revealed that both strains contained comparable amounts of CYP 2B1/2 and 3A1/2, but no detectable amounts of CYP 1B1, the proposed E2 4-hydroxylase. Although this enzyme is not a constitutive CYP in Sprague-Dawley rat liver, its presence in ACI liver could provide a ready explanation for the predominance of 4-hydroxy-E2 as a product. The identity of the estradiol 4-hydroxylase in ACI rat liver and the role of this unique reaction in the heightened sensitivity to E2 carcinogenicity remain to be elucidated.