Characteristics of estrogen-2/4-hydroxylase of porcine ovarian follicles: Influence of steroidal and non-steroidal agents on the activity of the enzyme In vitro

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.