Progesterone metabolism in vitro by rabbit testes at different stages of development.

Testicular homogenates from white rabbits of 0.4, 1, 3, 4, 5, 8, 18, and 24 months of age were incubated with [3H]progesterone and NADPH. At 12 days of age, major C21-17-OH-and C19- steroids formed from progesterone were 17alpha-hydroxyprogesterone and testosterone. However, at 4-24 months of age, 17alpha-hydroxyprogesterone, 3beta,17alpha-dihydroxy-5alpha-pregnan-20-one, and 5alpha-reduced C19-steroids such as 17beta-hydroxy-5alpha-androstan-3-one, 3beta-hydroxy-5alpha-androstan-17-one, and 5alpha-androstane-3beta,17beta-diol were the major products. The formation of significant quantities of 5alpha-reduced C19-steroids, which had been demonstrated previously only in prepubertal testes of rats and mice, was present in prepubertal as well as adult testes of rabbits. These findings clearly indicate that the metabolic patterns of progesterone in the adult rabbit differ from those in the adult rat and mouse. The major C19-steroids formed by testes are testosterone in the adult rat and mouse, but 5alpha-reduced C19-steroids in the adult rabbit.