18472-36-1Relevant articles and documents
Stereochemical fate of C-26 and C-27 during the conversion of isofucosterol to sitosterol and of 24-methylenecholesterol to campesterol and dihydrobrassicasterol in Oryza sativa cell cultures
Nasu, Kozue,Takahashi, Kyoko,Morisaki, Masuo,Fujimoto, Yoshinori
, p. 381 - 385 (2007/10/03)
Administration of pro-R-methyl-13C-labeled isofucosterol to cultured cells of Oryza sativa revealed that the pro-R and pro-S methyls at C-25 become the pro-R and pro-S methyls at C-25 of sitosterol, respectively. Similar administration experime
Site-directed mutagenesis of the sterol methyl transferase active site from Saccharomyces cerevisiae results in formation of novel 24-ethyl sterols
Nes, W. David,McCourt, Brian S.,Marshall, Julie A.,Ma, Jianzhong,Dennis, Allen L.,Lopez, Monica,Li, Haoxia,He, Ling
, p. 1535 - 1542 (2007/10/03)
Δ(24(28))-Sterols are end products of a mono C-methylation pathway catalyzed by the native Δ(24(25))- to Δ(24(28))-sterol methyl transferase (SMT) enzyme from Saccharomyces cerevisiae. Using a Tyr81 to Phe mutant SMT enzyme of S. cerevisiae, generated by site-directed mutagenesis of a highly conserved residue in the sterol binding site, we found that several Δ(24(25))- and Δ(24(28))-sterols, which are not substrates for the native protein, were catalyzed to mono- and bis-C24-alkylated side chains. The mutant protein behaved similarly to the native protein in chromatography and in binding zymosterol, the preferred substrate. Zymosterol was converted to fecosterol by the Y81F mutant protein with similar turnover efficiency as the native protein (K(m) = 12 μM and k(cat) = 0.01 s-1); trace 24-ethyl sterols were detected from these incubations. 4α-Methyl zymosterol, which is not a normal substrate for the wild-type SMT enzyme, was converted to 4α- methy fecosterol in high yield. When fecosterol and 4α-methyl fecosterol were assayed individually at saturating concentrations only fecosterol served as an effective substrate for the second C-transfer step (K(m) = 38 μM and k(cat) = 0.002 s-1), suggesting that successive C-methylation of Δ(24(28))-substrates is limited by product release and that molecular recognition of sterol features involves hydrogen bond formation. Isomeric 24- ethyl sterol olefins generated from 24(28)-methylene cholesterol were characterized by chromatographic (GC and HPLC) and spectral methods (MS and 1H NMR), viz., fucosterol, isofucosterol, and clerosterol. Changes in rate of C-methylation and product distributions resulting from deuterium substitution at C28 were used to establish the kinetic isotope effects (KIEs) for the various deprotonations leading to C24-methylene, C24-ethylidene, and C24-ethyl sterols. An isotope effect on C28 methyl deprotonation generated during the first C1-transfer was detected with zymosterol and desmosterol paired with AdoMet and [2H3-methyl]AdoMet. A similar experiment to test for a KIE generated during the second C1-transfer reaction with AdoMet paired with 24(28)-methylenecholesterol and [28-2H2]24(28)-methylene cholesterol indicated an inverse isotope effect associated with C27 deprotonation. Alteration in the proportion of the C24 alkylated olefinic products generated by the pure Y81F mutant resulted from the suppression of the formation of Δ(24(28))-ethylidene sterols (C28 deprotonation) by a primary deuterium isotope effect with a compensating stimulation of the formation of 24-ethyl sterols (C27 deprotonation). Kinetic study on the rate of product formation indicated a normal KIE of k(H)/k(D) = 2.62 for the first C1-transfer. Alternatively, an inverse KIE was established with k(H)/k(D) = 0.9 for the second C1-transfer resulting from conversion of the 24(28)-double bond (sp2 hybridization) to a 24β-ethyl group (sp3 hybridization). From the structures and stereochemical assignments of the C-ethyl olefin products, the stereochemistry of the attack of AdoMet in the second C1-transfer was found to operate a Si-face (backside) attack at C24, analogous to the first C1- transfer reaction.
Partial Synthesis of "Sargasterol" and (20S)-Cholesterol
Sucrow, Wolfgang,Nooy, Michael van
, p. 1897 - 1906 (2007/10/02)
The data of the synthetic (20S)-3β-hydroxy-5-cholestan-24-one (5b) and its acetate 5a differ significantly from those given for the degradation product of "sargasterol".The compounds 5a,b were converted into the E/Z-isomeric (20S)-stigmasta-5,24(28)-dien-3β-ols 8b, 9b, into their acetates 8a, 9a, and into (20S)-cholesterol (10b). (20S)-cholesterol acetate (10a) shows a lower melting point than that which is given in literature.
