The conversion of [3H]estradiol to 2-hydroxyestradiol (2-OH-E2) by homogenates of porcine ovarian follicles was assayed in vitro in the presence and absence of 10 and 100 μM concentrations of the following potential substrates or inhibitors of estrogen- 2 4-hydroxylase (E- 2 4-H): (1) estrogens; estrone (E1), estriol (E3) and 17α-estradiol (17α-E2), (2) catecholestrogens; 2-hydroxyestradiol (2-OH-E2), 4-hydroxyestradiol (4-OH-E2) and 2-hydroxyestrone (2-OH-E1); (3) 2-methoxyestradiol (2-MeO-E2); (4) halogenated estrogens; 2-bromoestradiol, (2-Bromo-E2) 4-bromoestradiol and 2,4-dibromoestradiol; (5) androgens; testosterone (T), dihydrotestosterone (DHT) and androstenedione; (6) progesterone; (7) epinephrine; (8) inhibitors of steroid aromatase; aminoglutethimide and 4-hydroxyandrostenedione and (9) SKF 525A, an inhibitor of cytochrome P-450. Progesterone and 2-Bromo-E2 were the two most effective inhibitors (2-OH-E2 formation =4 and 5% of control at 100 μM and 29.6 and 17.4% at 10 μM of progesterone and 2-Bromo-E2, respectively). 2-MeO-E2 at 100 μM was nearly as effective as progesterone in inhibiting E- 2 4-H activity but only caused about 50% inhibition at 10 μM. The three catecholestrogens reduced 2-OH-E2 formation to about the same degree (21-23% of control at 100 μM). The 2,4-dibromo-E2 was equipotent with the catecholestrogens while 4-bromo-E2 was about half as effective. The phenolic estrogens, potential substrates for the enzyme, reduced 2-OH-E2 formation to different degrees, with E3 being the most effective. Among the androgens, DHT was almost as effective an inhibitor as the catecholestrogens, T was about half as effective while androstenedione had no effect. Epinephrine and the two inhibitors of aromatase did not inhibit E- 2 4-H activity. SKF 525A inhibited E- 2 4-H activity but with a potency only about 1 10th that reported for liver.
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