Kewitz H, Pleul O
Proc Natl Acad Sci U S A. 1976 Jul;73(7):2181-5. doi: 10.1073/pnas.73.7.2181.
Specific radioactivities of choline, acetylcholine, phosphocholine, lecithin, lysolecithin, and glycerophosphorylcholine have been measured in brain, blood, liver, and muscle after the intravenous injection of three labeled precursors: choline, methyl-labeled methionine, and ethanolamine. In relation to the specific activity of free choline in blood there was significantly more radioactivity in the free choline of brain after administration of methyl-labeled methionine and labeled ethanolamine than after labeled choline. Since the choline moiety of lipids, which returns back to the choline pool, contained less radioactivity after methyl-labeled methionine and labeled ethanolamine than after labeled choline, it is the most likely interpretation of the finding that choline, in brain can be formed by methylation of free ethanolamine. Data from liver confirm that lecithin is formed in the liver by methylation of phosphatidylethanolamine. No indication was found for the synthesis of choline in muscle. Rates of transfer and transport of choline in brain have been calculated as nmol x g-1 x min-1 as follows: turnover rate of choline, 36.5; rate of synthesis of choline by methylation and net loss of choline into the bloodstream, 6.3; inflow from the blood 6.2; outflow into the blood, 12.5; transfer into lipids and vice versa, 20; transfer to acetylcholine and vice versa, 4.
静脉注射三种标记前体（胆碱、甲基标记的甲硫氨酸和乙醇胺）后，已测定了脑、血液、肝脏和肌肉中胆碱、乙酰胆碱、磷酸胆碱、卵磷脂、溶血卵磷脂和甘油磷酸胆碱的比放射性。与血液中游离胆碱的比活性相比，给予甲基标记的甲硫氨酸和标记的乙醇胺后，脑中游离胆碱的放射性明显高于给予标记胆碱后。由于脂质的胆碱部分返回胆碱池，甲基标记的甲硫氨酸和标记的乙醇胺后其放射性低于标记胆碱后，因此最有可能的解释是脑中的胆碱可由游离乙醇胺甲基化形成。肝脏的数据证实卵磷脂是由磷脂酰乙醇胺甲基化在肝脏中形成的。未发现肌肉中有胆碱合成的迹象。脑中胆碱的转移和运输速率以nmol×g-1×min-1计算如下：胆碱周转率36.5；通过甲基化合成胆碱和胆碱向血液中的净损失率6.3；从血液流入6.2；流出到血液中12.5；转移到脂质中及反之亦然20；转移到乙酰胆碱及反之亦然4。
