The mechanism of the elaboration of ring b in ergosterol biosynthesis

Methods for the preparation of [3α−³H]ergosta-7,22-dien-3β-ol (5,6-dihydro-ergosterol), [5,6−³H₂]ergosta-7,22-dien-3β-ol and [3α−³H]ergosta-7,22-diene-3β,5α-diol are described. It is shown that 5,6-dihydro[3α−³H]ergosterol on incubation under aerobic conditions with whole cells of Saccharomyces cerevisiae LK₂G₁₂ is efficiently converted into ergosterol. Studies carried out with dihydro[5α,6α−³H₂]-ergosterol demonstrate that the introduction of the 5,6-double bond in ergosterol biosynthesis is attended by an overall cis-elimination of two hydrogen atoms. To differentiate between a hydroxylation–dehydration mechanism and a dehydrogenation mechanism, the metabolism of [3α−³H]ergosta-7,22-diene-3β,5α-diol was studied. It was shown that this diol is converted into ergosterol only under aerobic conditions. It is therefore suggested that the introduction of the 5,6-double bond of ergosterol does not occur through a hydroxylation–dehydration mechanism.