Biosynthesis of Sterols by a Yeast Homogenate

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-3β-ol and 5α-cholesta-7,24-dien-3β-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-3β-ol and to 5α-cholesta-7,24-dien-3β-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.

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