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 results indicate that the equine aromatase system would be different from the human placental microsomal estrogen synthetase, which aromatizes 16 alpha-hydroxyandrogens and epitestosterone but not 19-norandrogens. Furthermore, our data show that the rates of aromatization of androgens and 19-norandrogens were not additive and that 19-norandrogens competitively inhibited the aromatization of androgens, suggesting that a single enzymic system would be involved in the aromatization of androstenedione, 19-norandrostenedione, testosterone, and 19-nortestosterone. Our findings, which are identical to those previously reported for stallion testis and mare placental estrogen synthetases, provide evidence for a strong species specificity of the equine aromatase system.