Competitive product inhibition of aromatase by natural estrogens

• Published in: Journal of steroid biochemistry and molecular biology Vol. 44; no. 4; pp. 651 - 656
• Main Authors: Shimizu, Yukiko, Yarborough, Carol, Osawa, Yoshio
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Oxford Elsevier Ltd 01.03.1993; Elsevier Science
• Subjects: Analytical, structural and metabolic biochemistry; Androstenedione - metabolism; Aromatase Inhibitors; Binding, Competitive; Biological and medical sciences; Enzymes and enzyme inhibitors; Estrogens - metabolism; Estrogens - pharmacology; Fadrozole - pharmacology; Female; Fundamental and applied biological sciences. Psychology; Humans; Kinetics; Microsomes - enzymology; Oxidoreductases; Placenta - enzymology; Pregnancy; Substrate Specificity; Testosterone - metabolism; Human; Aromatization; Placenta; Enzymatic activity; Enzyme; Estrogen; Oestrogen synthase; Sex steroid hormone
• ISSN: 0960-0760; 1879-1220
• DOI: 10.1016/0960-0760(93)90274-Z

In order to better understand the function of aromatase, we carried out kinetic analyses to asses the ability of natural estrogens, estrone (E 1), estradiol (E 2), 16α-OHE 1, and estriol (E 3), to inhibit aromatization. Human placental microsomes (50 μg protein) were incubated for 5 min at 37°C with [1β- 3H]testosterone (1.24 × 10 3 dpm 3H/ng, 35–150 nM) or [1β- 3H,4- 14C]androstenedione (3.05 × 10 3 dpm 3H/ng, 3H/ 14C = 19.3, 7–65 nM) as substrate in the presence of NADPH, with and without natural estrogens as putative inhibitors. Aromatase activity was assessed by tritium released to water from the 1β-position of the substrates. Natural estrogens showed competitive product inhibition against androgen aromatization. The K i of E 1, E 2, 16α-OHE 1, and E 3 for testosterone aromatization was 1.5, 2.2, 95, and 162 μM, respectively, where the K m of aromatase was 61.8 ± 2.0 nM ( n = 5) for testosterone. The K i of E 1, E 2, 16α-OHE 1, and E 3 for androstenedione aromatization was 10.6, 5.5, 252, and 1182 μM, respectively, where the K m of aromatase was 35.4 ± 4.1 nM ( n = 4) for androstenedione. These results show that estrogens inhibit the process of andrigen aromatization and indicate that natural estrogens regulate their own synthesis by the product inhibition mechanism in vivo. Since natural estrogens bind to the active site of human placental aromatase P-450 complex as competitive inhibitors, natural estrogens might be further metabolized by aromatase. This suggests that human placental estrogen 2-hydroxylase activity is catalyzed by the active site of aromatase cytochrome P-450 and also agrees with the fact that the level of catecholestrogens in maternal plasma increases during pregnancy. The relative affinities and concentration of androgens and estrogens would control estrogen and catecholestrogen biosynthesis by aromatase.