The microsomal enzyme 17P-hydroxysteroid dehydrogenase (17P-HSD) catalyzes the interconversion of dehydroepiandrosterone and androstenediol, or androstenedione and testosterone. The 17P-dehydrogenase reaction is reversible, in contrast to the previous reactions in testosterone biosynthesis (60). Following the A5 pathway in humans, the C19 product, dehydroepiandrosterone, is converted to androstenediol by 17P-HSD, and then androstenediol is converted to testosterone by 3P-HSD. In the A4 pathway occurring in rodents, 17P-HSD uses androstenedione as substrate to produce testosterone (61). Testosterone is the principle steroid end product secreted by adult Leydig cells, but it can be further metabolized to 17P-estradiol (E2) and dehydrotestosterone (DHT) before secretion.
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