Androgen synthesis and aromatization by equine corpus luteum microsomes

Whereas mare corpus luteum does not produce androgens or estrogens in vivo, the incubation of mare corpus luteum microsomes with progesterone and NADPH resulted in 17α-hydroxyprogesterone and estrogen production with a small yield of androstenedione. In the presence of an aromatase inhibitor (4-hydroxyandrostenedione), 17α-hydroxyprogesterone and androstenedione were accumulated. Aromatization of testosterone and androstenedione occurred via stereospecific loss of the 1β, 2β hydrogen atoms and was inhibited by MgCl2, KCl, and EDTA. The Km of estrogen synthetase from equine corpus luteum for testosterone was 18.5 ± 2.7 nM and for androstenedione was 11.5 ± 1.5 nM. 19-Norandrogens were aromatized with a slightly higher efficiency than were androgens, but the affinity of the aromatase was lower for 19-norandrogens than for androgens. Our results suggest that aromatases from equine testis and corpus luteum are closely related enzymes. On the other hand, the question arises as to the relationship among the cell origin, the synthetizing abilities, and in vivo production of the corpus luteum in different mammalian species.