BIOSYNTHESIS OF ETHYL SIDE CHAINS ON STEROLS OF A MYXOMYCETE DICTYOSTELIUM DISCOIDEUM

Previous work of several laboratories has shown that the two carbon atoms C‐28 and C‐29 of the ethyl or ethylidene side chains of phytosterols are introduced on a C‐24 unsaturated side chain by two successive C‐methylation steps. A scheme, based on previous work of Nes et al. and Goodwin et al., gives the various intermediates which can be formed by stabilisation of the intermediate carbonium ion (II). It was usually thought that ethylidene compounds such as (III) were the precursors of ethyl compounds such as (VIII) and this has been proved recently in the case of poriferasterol (IX) which contained four deuterium atoms when Ochromonas malhamensis was grown in presence of [Me‐2H3] methionine [8]. In the present paper it is reported that the sterol of the slime mold Dictyostelium discoideum [Δ 22‐stigmasten‐3‐β‐ol (Xa)] contains five deuterium atoms, when extracted from D. discoideum fed on cells of a methionine‐less strain of E. coli grown in presence of [Me‐2H3] methionine. The unsaturated sterol (Xa) is accompanied by the saturated sterol (XIa) which was not previously found in this organism. Mass spectrometry of the pentadeuterated sterols (Xa) and (XIa) seemed to show conclusively that all five deuterium atoms are located in the ethyl side chain [11], but it was thought necessary to confirm this conclusion by a detailed analysis of the side chain; thus, the sterol (Xa) was ozonised and part of the side chain isolated as a crystalline p‐phenylphenacyl ester (XIIIa). Mass spectrometry of this compound and of a synthetic non‐labelled (XIIIb) and the corresponding monodeuterated derivative (XIIIc) confirms that all five deuterium atoms of the Dictyostelium sterol (Xa) are in the ethyl side chain. These results exclude an ethylidene compound (such as III) as a precursor of the sterol (Xa); they do not allow us to choose between other possible routes illustrated in the scheme. Our experiments also show that there is no hydrogen shift during the second methylation step. Copyright © 1969, Wiley Blackwell. All rights reserved