Abstract
The inhibition of S-adenosylmethionine:Δ 24-sterol methyltransferase (EC 2.1.1.41) activity by endogenous cellular components has been studied in vitro. The principal inhibitors were Na + and K +; Cs +, NH 4 + and Li + were also shown to inhibit the reaction. The possible significance of inhibition by sodium and potassium is discussed. Evidence is presented for the presence of more than one enzyme capable of methylating sterols in cell-free extracts of yeast. Three enzymatic activities are described which differ in their respective Michaelis constants for S- adenosyl- l-methionine , pH optima, and affinity for zymosterol. Based on differences in the apparent Michaelis constants for zymosterol, it appears that only one may be responsible for in vivo methylation of this ergosterol precursor.
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