Studies on Δ8–Δ7 Isomerization and Methyl Transfer of Sterols in Ergosterol Biosynthesis of Yeast1

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Abstract
The formation of cholesta-7,24-dien-3β-ol and its activity as a substrate for the sterol sidechain methyltransferase in yeast have not previously been studied. Experiments with acetonepowder extracts of yeast showed that the sterol is formed from zymosterol by Δ8-Δ7 isomerization. However, direct conversion of cholesta-7,24-dien-3β-ol into zymosterol could not be demonstrated. The reversibility of the reaction was proved by the detection of 3H-incorporation into cholesta-8-en-3β-ol (with lathosterol as a carrier) from [3H]H2O in the medium. Incubation of cholesta-7,24-dien-3β-ol and S-adenosyl-L-[methyl-14C]methionine with the acetone-powder extract resulted in methylation of the sterol to form episterol. Similar incubation of zymosterol gave fecosterol and episterol, suggesting that fecosterol initially formed by the methylation was isomerized to episterol. In intact cells, however, an alternative pathway (zymosterol → cholesta-7,24-dien-3β-ol → episterol) may also operate. The relative importance of the two pathways is not known.