4-HYDROXYLATION OF ESTRADIOL BY HUMAN UTERINE MYOMETRIUM AND MYOMA MICROSOMES - IMPLICATIONS FOR THE MECHANISM OF UTERINE TUMORIGENESIS

Estradiol is converted to catechol estrogens via 2- and 4-hydroxylatio n by cytochrome P450 enzymes, 4-Hydroxyestradiol elicits biological ac tivities distinct from estradiol, most notably an oxidant stress respo nse induced by free radicals generated by metabolic redox cycling reac tions. In this study, we have examined 2- and 4-hydroxylation of estra diol by microsomes of human uterine myometrium and of associated myoma ta. In all eight cases studied, estradiol 4-hydroxylation by myoma has been substantially elevated relative to surrounding myometrial tissue (minimum, 2-fold; mean, 5-fold), Estradiol 2-hydroxylation in myomata occurs at much lower rates than 4-hydroxylation (ratio of 4-hydroxyes tradiol/2-hydroxyestradiol, 7.9 +/- 1.4) and does not significantly di ffer from rates in surrounding myometrial tissue. Rates of myometrial 2-hydroxylation of estradiol were also not significantly different fro m values in patients without myomata, We have used various inhibitors to establish that 4-hydroxylation is catalyzed by a completely differe nt cytochrome P450 than 2-hydroxylation. In myoma, alpha-naphthoflavon e and a set of ethynyl polycyclic hydrocarbon inhibitors (5 mu M) each inhibited 4-hydroxylation more efficiently (up to 90%) than 2-hydroxy lation (up to 40%), indicating > 10-fold differences in K-i (< 0,5 mu M vs, > 5 mu M). These activities were clearly distinguished from the selective 2-hydroxylation of estradiol in placenta by aromatase report ed previously (low K-m, inhibition by Fadrozole hydrochloride or ICI D 1033). 4-Hydroxylation was also selectively inhibited relative to 2-hy droxylation by antibodies raised against cytochrome P450 IB1 (rat) (53 vs. 17%), These data indicate that specific 4-hydroxylation of estrad iol in human uterine tissues is catalyzed by a form(s) of cytochrome P 450 related to P450 IB1, which contribute(s) little to 2-hydroxylation , This enzyme(s) is therefore a marker for uterine myomata and may pla y a role in the etiology of the tumor.