Investigation of an S-Adenosylmethionine:Δ24-Sterol Methyltransferase in Ergosterol Biosynthesis in Yeast: SPECIFICITY OF STEROL SUBSTRATES AND INHIBITORS

Abstract

Abstract The role of an S-adenosylmethionine:Δ24-sterol methyltransferase (hereafter referred to as methyltransferase) in ergosta-5,7,22-trien-3β-ol (ergosterol) biosynthesis in yeast has been investigated. Sterol substrate specificity studies indicate that 5α-cholesta-8,24-dien-3β-ol (zymosterol) is the best methyl group acceptor in the methyltransferase assay. 4-Methyl sterols are very poor substrates; sterols with a fully reduced side chain (i.e. no double bond at C-24) are not methylated. A corresponding 3-ketosteroid, 5α-cholesta-8,24-dien-3-one, was methylated at a slower rate; similarly, sterols with nuclear double bonds in positions 5 or 5 and 7 were poorer substrates than zymosterol. Inhibition studies indicate that sterols with a saturated isooctyl side chain are competitive inhibitors of zymosterol in the methyltransferase reaction. Sterols that possess an alkylated side chain markedly altered the rate of methyltransfer; at low concentrations of substrate, addition of 24-alkyl-substituted sterols stimulated the methyltransferase, whereas at higher concentrations of substrate the 24-alkyl sterols were inhibitory. Conclusions regarding the sequences of nuclear and side chain reactions in ergosterol biosynthesis and the control of the methyltransferase activity are outlined.