EQUINE OVARIAN AROMATASE - EVIDENCE FOR A SPECIES-SPECIFICITY

Mare granulosa cells and cyclic corpus luteum microsomes are reported to aromatize 19-norandrogens more efficiently than androgens. However, 16-alpha-hydroxytestosterone and epitestosterone were not aromatized by the equine corpus luteum microsomal estrogen synthetase. These resu lts indicate that the equine aromatase system would be different from the human placental microsomal estrogen synthetase, which aromatizes 1 6-alpha-hydroxyandrogens and epitestosterone but not 19-norandrogens. Furthermore, our data show that the rates of aromatization of androgen s and 19-norandrogens were not additive and that 19-norandrogens compe titively inhibited the aromatization of androgens, suggesting that a s ingle enzymic system would be involved in the aromatization of androst enedione, 19-norandrostenedione, testosterone, and 19-nortestosterone. Our findings, which are identical to those previously reported for st allion testis and mare placental estrogen synthetases, provide evidenc e for a strong species specificity of the equine aromatase system.