Yabusaki Y, Nishino T, Ariga N, Katsuki H
J Biochem. 1979 Jun;85(6):1531-7. doi: 10.1093/oxfordjournals.jbchem.a132483.
The formation of cholesta-7,24-dien-3 beta-ol and its activity as a substrate for the sterol side-chain methyltransferase in yeast have not previously been studied. Experiments with acetone-powder extracts of yeast showed that the sterol is formed from zymosterol by delta8-delta7 isomerization. However, direct conversion of cholesta-7,24-dien-3 beta-ol into zymosterol could not be demonstrated. The reversibility of the reaction was proved by the detection of 3H-incorporation into cholesta-8-en-3 beta-ol (with lathosterol as a carrier) from [3H]H2O in the medium. Incubation of cholesta-7,24-dien-3 beta-ol and S-adenosyl-L-[methyl-14C]methionine with the acetone-powder extract resulted in methylation of the sterol to form episterol. Similar incubation of zymosterol gave fecosterol and episterol, suggesting that fecosterol initially formed by the methylation was isomerized to episterol. In intact cells, however, an alternative pathway (zymosterol yields cholesta-7,24-dien-3 beta-ol yields episterol) may also operate. The relative importance of the two pathways is not known.
此前尚未对胆甾-7,24-二烯-3β-醇的形成及其作为酵母中甾醇侧链甲基转移酶底物的活性进行研究。用酵母丙酮粉提取物进行的实验表明，该甾醇是由酵母甾醇通过δ8-δ7异构化形成的。然而，未能证明胆甾-7,24-二烯-3β-醇能直接转化为酵母甾醇。通过检测培养基中[3H]H2O中的3H掺入到胆甾-8-烯-3β-醇（以羊毛甾醇为载体）中，证明了该反应的可逆性。将胆甾-7,24-二烯-3β-醇与S-腺苷-L-[甲基-14C]甲硫氨酸与丙酮粉提取物一起孵育，导致甾醇甲基化形成表甾醇。对酵母甾醇进行类似的孵育得到麦角甾醇和表甾醇，这表明最初通过甲基化形成的麦角甾醇异构化为表甾醇。然而，在完整细胞中，可能还存在另一条途径（酵母甾醇生成胆甾-7,24-二烯-3β-醇生成表甾醇）。这两条途径的相对重要性尚不清楚。
