Involvement of cytochrome P-450 in the synthesis of 5,16,androstadien-3 beta-ol from pregnenolone in pig testes microsomes.

The conversion of pregnenolone to 5,16,androstadien-3 beta-ol, the first intermediate in the biosynthetic pathway of the androst-16-ene steroids, is catalysed by a microsomal enzyme system in the testes of the pig. This reaction is analogous to the conversion of pregnenolone to dehydroepiandrosterone in the biosynthesis of the androgens, since both systems involve the conversion of C21 steroids to C19 steroids by removal of the 2-carbon side chain. Cytochrome P-450SCCII catalyses the formation of the first C19 androgen intermediates, while the enzyme system that catalyzes the formation of the first androst-16-ene intermediates, so called andien-beta synthase, has not previously been well characterized. Andien-beta synthase and cytochrome P-450SCCII activities have been measured in an in vitro assay system with boar testes microsomes using [14C]pregnenolone as substrate. Both enzyme systems require NADPH and oxygen for maximal activity and are inhibited by carbon monoxide when oxygen levels are low. Classical inhibitors of cytochrome P-450 including SKF-525A, metyrapone and alpha-naphthoflavone inhibited both enzyme systems to a similar extent. In addition, inhibitory antibodies against NADPH cytochrome P-450 reductase also inhibited both enzyme activities in testes microsomes. It is concluded that the formation of 5,16,androstadien-3 beta-ol from pregnenolone in pig testes microsomes is catalyzed by cytochrome P-450.