Aromatization of testosterone and 19-nortestosterone by a single enzyme from equine testicular microsomes. Differences from human placental aromatase.

A single enzyme in the stallion testis was able to aromatize both testosterone and nortestosterone. This enzyme had a much lower affinity for nortestosterone than for testosterone. In contrast to human placental estrogen synthetase, this enzyme aromatized testosterone and 19-nortestosterone with similar efficiency. The differences observed (effects of monovalent cations, inhibition of androstenedione aromatization by testosterone and 19-nortestosterone and, above all, rate of norandrogen aromatization) suggest that the aromatase in the horse testis is not the same as that in the human placenta.