Aromatization of testosterone is catalyzed by the microsomal enzyme, cytochrome P450 aromatase (P450arom). Once substrate binding occurs, there is a sequential hydroxylation, oxidation, and removal of the C19 carbon, followed by aromatization of the A ring of the steroid. The entire aromatase reaction uses three molecules of oxygen and three electrons donated by NADPH (62-64). In the testis, aromatase activity is detectable in Leydig cells, Sertoli cells, and germ cells (65-69). The relative contributions of each of these testicular cell types to testicular aromatase activity varies with age and between species (70). In the rat and mouse, germ cells are a significant site of activity during adulthood (68). Sertoli cells contribute to testicular aromatase activity only in immature animals (71). However, in humans, Leydig cells are the only source of testicular estrogens at all ages (39). Sertoli cell cultures from juvenile monkeys expressed follicle-stimulating hormone (FSH) driven aromatase (72).
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