1. The possible involvement of 5-pregnene-3β,20β-diol in 16-unsaturated C19 steroid biosynthesis has been investigated. 2. 5,16-Androstadien-3β-ol (andien-β) formation from [4-14C]pregnenolone (3β-hydroxy-5-pregnen-20-one), 5-pregnene-3β,20α-diol and 5-pregnene-3β,20β-diol was studied in homogenates of boar testis and the mean yields obtained were 25.6, 2.7 and 16.0% respectively. 3. Short-term kinetic studies with pregnenolone and 5-pregnene-3β,20β-diol separately and together suggested that the latter compound might be an intermediate in the biosynthesis of andien-β. 4. In agreement with this interpretation, radioactive 5-pregnene-3β,20β-diol has been isolated during andien-β biosynthesis from [4-14C]pregnenolone in the presence of NADPH, more radioactivity being trapped under limiting conditions of andien-β formation with NADH present as cofactor. 5. Further, 5-pregnene-3β,20β-diol and andien-β have been shown to inhibit the formation of the 16-unsaturated C19 steroid from [4-14C]pregnenolone, the yield of radioactive 5-pregnene-3β,20β-diol increasing in the presence of added unlabelled andien-β. 6. It is concluded that there may be two pathways leading to 16-unsaturated C19 steroid formation from pregnenolone, one of these involving 5-pregnene-3β,20β-diol as an intermediate. Possible mechanisms are presented and discussed.
Concerning the Role of Lanosterol and Cycloartenol in Steroid Biosynthesis
