Observations on the sterol side-chain alkylation mechanism in a Trebouxia species. (Short Communications)

Abstract

The sterols of most higher plants and algae are C28 and C29 compounds in which the additional alkyl group at C-24 is derived by transmethylation from methionine Ergosterol* biosynthesized by Neurospora crassa in the presence of [Me-2H3]methionine contained two deuterium atoms in the 24-methyl group (Jaureguiberry et al., 1965), demonstrating the intermediacy of a 24-methylene sterol (Scheme 1, III lII4V). In the chrysophyte Ochromonas malhamensis four deuterium atoms derived from [Me-2H3]methionine were found in poriferasterol (Smith et al., 1967), showing that a 24-ethylidene sterol is formed and subsequently reduced (Scheme 1, I-II-m-M.V-VI-VII-VIII) In contrast the chlorophytes Chlorella vulgaris and Chlorella ellipsoidea incorpor- ated three and five deuterium atoms into the C28 and C29 sterols respectively when grown in media con- taining [Me-2H3]methionine (Tomita et al., 1970, 1971). Similarly a slime mould, Dictyostelium discoideum, incorporated five deuterium atoms into 5ao-stigmast-22-en-3P-ol (Lenfant et al., 1969). In these organisms a 24-methylene sterol is not reduced to give a 24-methyl sterol, nor is a 24-ethylidene compound (VI) an intermediate in 24-ethyl sterol formation. Several alternative mechanisms have been proposed for the stabilization of cations (II) and (V) as indicated in Scheme 1 (Castle et al.,