ROLES OF CYTOCHROMES P450 1A2 AND 3A4 IN THE OXIDATION OF ESTRADIOL AND ESTRONE IN HUMAN LIVER-MICROSOMES

Of seven cDNA-expressed human cytochrome P450 (P450) enzymes (P450s 1A 2, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4) examined, P450 1A2 was the most active in catalyzing 2- and 4-hydroxylations of estradiol and estrone. P450 3A4 and P450 2C9 also catalyzed these reactions although to less er extents than P450 1A2. P450 1A2 also efficiently oxidized estradiol at the 16 alpha-position but was less active in estrone 16 alpha-hydr oxylation; the latter reaction and also estradiol 16 alpha-hydroxylati on were catalyzed by P450 3A4 at significant levels. Anti-P450 1A2 ant ibodies inhibited 2- and 4-hydroxylations of these two estrogens catal yzed by liver microsomes of some of the human samples examined. Estrad iol 16 alpha-hydroxylation was inhibited by both anti-P450 1A2 and ant i-P450 3A4, while estrone 16 alpha-hydroxylation was significantly sup pressed by anti-P450 3A4 in human liver microsomes. Fluvoxamine effici ently inhibited the estrogen hydroxylations in human liver samples tha t contained high levels of P450 1A2, while ketoconazole affected these activities in human samples in which P450 3A4 levels were high. alpha -Naphthoflavone either stimulated or had no effect on estradiol hydrox ylation catalyzed by liver microsomes; the intensity of this effect de pended on the human samples and their P450s. Interestingly, in the pre sence of anti-P450 3A4 antibodies, cr-naphthoflavone was found to be a ble to inhibit estradiol and estrone 2-hydroxylations catalyzed by hum an liver microsomes. The results suggest that both P450s 1A2 and 3A4 h ave major roles in oxidations of estradiol and estrone in human liver and that the contents of these two P450 forms in liver microsomes dete rmine which P450 enzymes are most important in hepatic estrogen hydrox ylation by individual humans. P450 3A4 may be expected to play a more important role for some of the estrogen hydroxylation reactions than P 450 1A2. Knowledge of roles of individual P450s in these estrogen hydr oxylations has relevance to current controversies in hormonal carcinog enesis [Service, R. F. (1998) Science 279, 1631-1633].