Studies on the Mechanism of Estrogen Biosynthesis. VIII. The Development of Inhibitors of the Enzyme System in Human Placenta1

Abstract

Structural features of over 100 steroids and some non—steroids were evaluated systematically as inhibitors of estrogen biosynthesis in human placental microsomes. The microsomes were preincubated with and without potential inhibitors, and then were incubated with [4–14C] androst–4–ene–3,17–dione or [4–14C]3P—hydroxyandrost— S—en—17–one (DHA) and an NADPH generating system. After adding tritiated estrogens as recovery markers, the estrogens were isolated by thin—layer chromatography and were assayed using dual—label scintillation counting to determine percent conversion and inhibition produced. C10 A4–3–keto compounds were better inhibitors than the corresponding ring A reduced (5<x or 5P) compounds or A4–3–keto 19–nor compounds. Further conjugation with the A4—3–keto system was also beneficial so that A1.4'6–3–one, A4–6–3–one and A4– 3,6–dione compounds were effective. The 17–keto was superior to the 17P—hydroxy group in providing inhibitory properties in all cases, while the 17P—acetate and formate groups approached the effectiveness of the ketone in instance. Appropriate kinetic tests on the more effective compounds, androsta–l,4,6–triene–3,17–dione, androst– 4–ene–3,6,17–trione and estr—4–ene–3,17–dione, showed that they inhibited the conversion of androstenedione to estrogen competitively. The reaction product compounds, estrone and estradiol– 17β, were poor inhibitors. Compounds which inhibited steroid 3β–hydroxydehydrogenase were only marginally effective against the aromatase enzyme system as judged by conversion of androstenedione to estrogen. Using such compounds, there was no evidence that DHA was C–19 hydroxylated, indicating that this is not a significant alternative pathway to estrogens in human placenta. (Endocrinology92: 866, 1973)