Graduate Thesis Or Dissertation
 

The role of S-adenosylmethionine in the biosynthesis of ergosterol and related sterols in Saccharomyces cerevisiae

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  • An enzyme system was prepared from Saccharomyces cerevisiae, strain MCC, which carries out the transfer of the methyl group from S-adenosylmethionine or methionine to the side chain of yeast sterols. The cell-free system has been shown to be incapable of synthesizing ergosterol but carries out the synthesis of two as yet unidentified sterols which contain the methyl group of S-adenosylmethionine. Whole cells were shown to synthesize both ergosterol and the two sterols found in the cell-free system. By time course studies of pulsed methionine-methyl-C¹⁴ incorporation and by feeding experiments, it has been possible to show that the two sterols synthesized by the cell-free homogenate are precursors of ergosterol. The cell-free system requires for maximal activity ATP, MnCl₂, K₂CO₃, ascorbate, acetate and glucose-6-phosphate. When S-adenosylmethionine is the methyl donor, ATP is not necessary. The function of K₂CO₃, since it is not incorporated into sterol to any extent, was tested as a possible allosteric effector for this enzyme. Neither heating nor treatment with HgCl₂ produced a preparation which was insensitive to carbonate. Evidence was found that be two precursors have a double bond at C-22, as does ergosterol, but differ in the unsaturation of the ring system.
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