Metabolism of Δ24-sterols by yeast mutants blocked in removal of the C-14 methyl group

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Abstract
The metabolism of exogenously provided .DELTA.24-sterols by whole cell cultures of a polyene-resistant mutant (D10) of Candida albicans blocked at removal of the C-14 methyl group was investigated. Comparison of the relative efficiencies of transmethylation at C-24 of selected sterol substrates revealed the following substrate preferences of the Candida .DELTA.24-sterol methyltransferase (EC 2.1.1.41): zymosterol > 4.alpha.-methylzymosterol > 14.alpha.-methylzymosterol. Exogenous 4,4-dimethylzymosterol was not transmethylated by mutant D10. Incorporation of the 14C-labeled methyl group of S-adenosyl-L-[methyl-14C]methionine into the sterols of a D10 culture preloaded with zymosterol indicated that zymosterol was a better (40 .times.) substrate than endogenous lanosterol. Feeding zymosterol to D10 and a polyene-resistant strain of Saccharomyces cerevisiae (Nys-P100) that was also blocked at removal of the C-14 methyl group gave 24-methyl sterols possessing .DELTA.22 and ring B unsaturation. Mutant D10 was able to produce ergosterol from zymosterol whereas Nys-P100 produced ergosta-7,22-dienol. When grown in the presence of 3 .mu.M 25-aza-24,25-dihydrozymosterol, a known inhibitor of the .DELTA.24-sterol methyltransferase, Nys-P100 accumulated 14.alpha.-methylzymosterol, a minor metabolite in this mutant under normal growth conditions and hitherto unidentified as a yeast sterol.