(Repu le 7 mai/l2 octobre 1971) Several laboratories have shown that the two carbon atoms C-28 and C-29 of the ethyl or ethylidene side chains of phytosterols are introduced on to a C-24 unsaturated side chain by two methylation steps. A scheme based on previous work suggests the various intermediates which can be formed by stabilisation of the carbonium ion 3. We have shown previously that in the biosynthesis of 50i-stigmastan-3p-01 ( l l a ) and 5a-stigmast-22-en-3/3-01 (12a), sterols of the myxomycete Dictyosteliurn discoideurn, routes h and i could be excluded.I n the present paper we describe the in vivo conversion, by the myxomycete, of differently labelled lanosterol (13), stigmastanol ( l l c ) , and 5a-stigmast-22-en-3p-01 (12c), into the sterols 12a and l l a , and discuss the possible biosynthetic routes for the two sterols.After incorporation of a mixture of [26,27-14C2]lanosterol and [24-3H]lanosterol (3H/14C = 5.60 & 0.25), the isolated sterols l l a and 12a were converted into l l b (3H/14C = 5.54 0,28) and 14b (3H/14C = 5.61By ozonisation followed by oxidation in neutral or basic medium, 12a was converted into the esters 15 (3H/14C = 5.54 0.3), respectively. These results show that during the C-24 methylation, H-24 is not lost; the hydrogen which is lost is located neither a t C-24 nor a t C-23 and must therefore be a t C-25. This excludes routes a and b for sterol 12.After incorporation of a mixture of [26,27-14C2] These results show that during the conversion of 13 into 12a, 50°/, of the H-23 atoms migrate from C-23 to (2-24 and 25O/, remain on C-23 ; the other 25O/, are eliminated. During the conversion of 13 into l l a both H-23 atoms are retained. These results excluded route g for the two sterols. Since we have demonstrated that the conversion of l l c into 12a is possible in high yields (4.60/,) with an efficiency eight times greater than the conversion of 12c into l l a , we propose that the sterol l l a is synthesized in vivo according to route e with migration of H-23 from C-23 to C-24. The sterol 12a is biosynthetized either by route d, f, the 22,23 double bond being introduced during the alkylation step, or by desaturation of the sterol l l a , the C-22, C-23 double bond being introduced after the C-24 alkylation.
0.28).0.28) and 16 (3H/14C = 5.55 0.15).Les etudes de biosynthese des chaines ethyle et Bthylidbne en C-24 des phytosterols ont montre que les deux atomes de carbone proviennent de la mkthionine [1-3,3a].Plusieurs mecanismes ont kt6 proposes pour expliquer cette double methylation [I-81 (voir schirma) : la premiere methylation du prdcurseur sterolique 1 conduit B la formation d'un intermediaire methylenique 2, qui peut servir de substrat B une deuxiitme methylation pour donner le carbocation intermediaire 3. Le carbocation 3 peut, apres stabilisation, soit par elimination d'un proton, soit par fixation d'un ion hydrure, conduire B des composes portant deux atomes de carbone derivant du groupe methyle de la methionine. Si