Abstract

Cell-free extracts of yeast convert 14C-mevalonic acid into squalene, lanosterol, zymosterol, and sterols more polar than ergosterol. Formation of two of the more polar sterols (Es1 and Es2) has also been demonstrated with methyl-labeled methionine and with 14C-adenosylmethionine as precursors. The two polar sterols, Es1 and Es2, formed from radioactive methyl donors yield labeled formaldehyde on chemical degradation. Es2 is reduced to a mixture of ergosterol and ergostanol on partial reduction with Raney nickel and hydrogen. The tentative structure 5,7,22,24(28)-ergostatetraene-3β-ol is assigned to one of the sterols (Es2). Intact yeast cells convert zymosterol to Es2 anaerobically, and aerobically they convert Es2 to ergosterol. Evidence for the role of Es1 and Es2 as intermediates in ergosterol biosynthesis is discussed, and it is suggested that a C27 sterol is the acceptor in the transmethylation reaction.

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