Graduate Thesis Or Dissertation
 

S-adenosylmethionine : [triangle]²⁴-sterol methyltransferase and the regulation of ergosterol biosynthesis in Saccharomyces cerevisiae

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  • The inhibition of S-aden.osylmethionine: Δ²⁴-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⁺, NH₄⁺ and Li⁺ were also shown to inhibit the reaction. The inhibition by potassium cation has been studied, and we have found that energy dependent transport and accumulation of K⁺ inhibits the sterol methyltransferase activity in intact respiring mitochondria. Evidence is presented for three enzymatic activities capable of methylating sterols in cell-free extracts of yeast. These differ in their respective Michaelis constants for S-adenosylmethionine, pH optima, and affinity for zymosterol. An apparent cooperative-type relationship with S-adenosylmethionine has been proposed based on observed deviations from theoretical Michaelis-Menten kinetics. However, all three enzymatic activities were similarly inhibited by monovalent cations, had the same subcellular location, and gave identical sterolic products. The subcellular location of the sterol methyltransferase activity has been determined. All three activities reside in yeast mitochondrial and promitochondrial structures. This provides a direct relationship between ergosterol biosynthesis and respiration, since the mitochondria are the site of respiration in the cell. The sterol methyltransferase deviates from many other mitochondrial enzymes in that it is present at low levels during anaerobic growth and is not subject to catabolite repression by high levels of glucose.
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