Stereochemistry of yeast Delta(24)-sterol methyl transferase
Stereochemistry of yeast Delta(24)-sterol methyl transferase
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DOI：
10.1016/s0968-0896(97)00010-2
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发表时间：
1997-05-01
影响因子：
3.5
通讯作者：
Czyzewska, EK
中科院分区：
文献类型：
--
作者：
AcunaJohnson, AP;Oehlschlager, AC;Czyzewska, EK
S-腺苷-1-甲硫氨酸：δ（24）-甾醇甲基转移酶（24-SMT）介导酵母甾醇C-28碳的引入。已经表明，亚甲基化反应的假定阳离子中间体的锍类似物是该过程的有效体内和体外抑制剂。在这些抑制剂的存在下，酵母菌的培养物产生的酵母甾醇，酶的天然底物的比例增加，而麦角甾醇和麦角甾四烯醇的比例下降。根据处理的培养物中C-24甲基化甾醇与C-24非甲基化甾醇的比例确定类似物的体内抑制能力[I-50（μ M）]，其顺序如下：25-硫代胆甾醇碘（0.07）> 24（S）-甲基-25-硫代胆甾醇碘（0.14）> 24（R）-甲基-25-硫代胆甾醇碘（0.25）。放射性标记的S-腺苷-1-甲硫氨酸（SAM），粗酶和25-硫代胆甾醇碘揭示了动力学抑制，这种抑制剂是非竞争性的酵母甾醇和竞争性SAM。与24（R）-甲基-25-硫代胆甾醇基碘相比，24（S）-甲基-25-硫代胆甾醇基碘的抑制能力更大，这表明甲基从Si面供给到Delta（24）。当结合Arigoni以前的工作考虑时，本结果推断由酵母24-SMT介导的亚甲基化通过从Delta（24）的si面烷基化，随后氢从C-24迁移到C-25穿过表面，并最终从表面上的C-28损失氢来进行。(C)1997 Elsevier Science Ltd.
S-Adenosyl-1-methionine:Delta(24)-sterol methyl transferase (24-SMT) mediates introduction of the C-28 carbon of yeast sterols. It has been shown that sulfonium analogues of the presumptive cationic intermediates of the methylenation reaction are potent in vivo and in vitro inhibitors of this process. In the presence of these inhibitors, cultures of yeast produced increased proportions of zymosterol, the natural substrate of the enzyme, while proportions of ergosterol and ergostatetraenol were decreased. New C27-sterol metabolites were also found. The in vivo inhibitory power of the analogues [I-50 (mu M)] was determined from the proportion of C-24 methylated sterols to C-24 nonmethylated sterols in treated cultures to be in the following order: 25-thiacholesterol iodide (0.07) > 24(S)-methyl-25-thiacholesteryl iodide (0.14) > 24(R)-methyl-25-thiacholesteryl iodide (0.25). Kinetic inhibition as revealed by radiolabeled S-adenosyl-1-methionine (SAM), crude enzyme and 25-thiacholesteryl iodide revealed this inhibitor to be uncompetitive with respect to zymosterol and competitive with respect to SAM. The greater inhibitory power of 24(S)-methyl-25-thiacholesteryl iodide compared to 24(R)-methyl-25-thiacholesteryl iodide suggests that methyl donation to Delta(24) occurs from the si face. When considered in conjunction with Arigoni's previous work, the present results infer the methylenation mediated by yeast 24-SMT proceeds by alkylation from the si face of Delta(24) followed by migration of a hydrogen from C-24 to C-25 across the re face and final loss of a hydrogen from C-28 on the re face. (C) 1997 Elsevier Science Ltd.