Biosynthesis of sterols by a yeast homogenate. Incorporation of mevalonic acid into cholesta-5,7,24-trien-3beta-ol and 5alpha-cholesta-7,24-dien-3beta-ol.

Incubation of (3RS,2R)-[2-14C,2-3H]mevalonic acid and (3RS, 2S)-[2-14C,2-3H]mevalonic acid with mechanically disrupted yeast cells resulted in C27-metabolites. Two (14C5, 3H4)-metabolites, cholesta-5,7,24-trien-3beta-ol and 5 alpha-cholesta-7,24-dien-3beta-ol, were isolated and characterized. The impairment of the 24-methyl transferase system was confirmed by the lack of incorporation of 14C into the sterol fraction on incubation of S-adenosyl-L-[methyl-14C]methionine with the yeast homogenate. The results indicate that interference with the (C-24)-alkylating system did not prevent the transformation of lanosterol to the cholesta-5,7,24-trien-3beta-ol and to 5 alpha-cholesta-7,24-dien-3beta-ol. It can therefore be inferred that transformations of the nucleus and of the side chain can function independently. However our results do not provide a definition of the actual sequence of the metabolic events between lanosterol and ergosterol.