Occurrence of Fatty Acid Esters of Sterol Intermediates in Ergosterol Synthesis by Yeast during Respiratory Adaptation

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Abstract
Free sterols and their fatty acid esters in Saccharomyces cerevisiae were separated by TLC, and the sterol contents and compositions of both fractions were analyzed by GLC. During respiratory adaptation of anaerobically grown yeast cells, larger amounts of esterified sterols were accumulated than of free sterols. The sterol compositions of both fractions were quite different. Ergosterol was the predominant component in the free sterol fraction, whereas the esterified sterol fraction contained significant amounts of sterol intermediates in ergosterol synthesis, such as zymosterol, lanosterol, and two unidentified sterols, tentatively named x and y. During respiratory adaptation, the radioactivity from L-[methyl-24C]methionine was incorporated into both free and esterified sterols. In the early period of incubation, incorporation of the radioactivity into free sterol y was marked but subsequently the radioactive sterol y was rapidly converted into ergosterol. In contrast, conversion of radioactive sterol intermediates to ergosterol in the esterified sterol fraction was much slower. With the cell-free extracts of yeast, radioactivities of [12Clmevalonate and S-adenosyl-[14C]-methionine were incorporated mainly into the free sterol fraction. Active 14C interconversion between free and esterified sterols was not found in the cell-free system. Sterol ester did not serve as a substrate for partially purified S-adenosylmethionine: Δ24-sterol methyltransferase from yeast, while the free sterols obtained from the sample of sterol ester proved to be good methyl acceptors.

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