The mechanism of the elaboration of ring B in ergosterol biosynthesis.

Methods for the preparation of [3alpha-(3)H]ergosta-7,22-dien-3beta-ol (5,6-dihydro-ergosterol), [5,6-(3)H(2)]ergosta-7,22-dien-3beta-ol and [3alpha-(3)H]ergosta-7,22-diene-3beta,5alpha-diol are described. It is shown that 5,6-dihydro[3alpha-(3)H]ergosterol on incubation under aerobic conditions with whole cells of Saccharomyces cerevisiae LK(2)G(12) is efficiently converted into ergosterol. Studies carried out with dihydro[5alpha,6alpha-(3)H(2)]-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 [3alpha-(3)H]ergosta-7,22-diene-3beta,5alpha-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.