Dudeja P K, Brasitus T A
Biochim Biophys Acta. 1987 Jun 23;919(3):307-10. doi: 10.1016/0005-2760(87)90270-0.
An earlier study (Biochim. Biophys. Acta 46 (1961) 205-216) failed to detect the enzymatic synthesis of phosphatidylcholine (PC) from phosphatidylethanolamine (PE) via a transmethylation pathway in rat small-intestinal microsomal membranes. This pathway was therefore assumed to be absent from this organ. Recently, however, in our laboratory it has been demonstrated that this pathway for the synthesis of phosphatidylcholine is present in rat colonic brush-border and basolateral membranes. It was therefore of interest to examine whether phospholipid methylation activity was present in rat small-intestinal brush-border membranes. The results of the present experiments demonstrate for the first time that this pathway for the synthesis of phosphatidylcholine exists in these plasma membranes. Evidence to support the enzymatic nature of this reaction include: loss of activity by heat denaturation and at 0 degree C, significant inhibition by S-adenosyl-L-homocysteine and saturation kinetics. The predominant product of this brush-border membrane phospholipid methyltransferase is phosphatidyl-N-monomethylethanolamine. This enzymatic activity has an apparent Km for S-adenosyl-L-methionine of 40 microM, a Vmax of 8.4 pmol/mg protein per 5 min, and a pH optimum of 8.0.
一项较早的研究（《生物化学与生物物理学学报》46卷（1961年）205 - 216页）未能在大鼠小肠微粒体膜中检测到通过转甲基化途径由磷脂酰乙醇胺（PE）酶促合成磷脂酰胆碱（PC）的过程。因此，人们认为该器官不存在这一途径。然而，最近在我们实验室已证明，磷脂酰胆碱的这一合成途径存在于大鼠结肠刷状缘膜和基底外侧膜中。因此，研究大鼠小肠刷状缘膜中是否存在磷脂甲基化活性就变得很有意义。本实验结果首次证明，磷脂酰胆碱的这一合成途径存在于这些质膜中。支持该反应酶促性质的证据包括：热变性和在0℃时活性丧失、S - 腺苷 - L - 高半胱氨酸的显著抑制以及饱和动力学。这种刷状缘膜磷脂甲基转移酶的主要产物是磷脂酰 - N - 单甲基乙醇胺。这种酶活性对S - 腺苷 - L - 甲硫氨酸的表观Km为40μM，Vmax为每5分钟每毫克蛋白质8.4 pmol，最适pH为8.0。
