Kowluru A, Rana R S, MacDonald M J
Arch Biochem Biophys. 1985 Oct;242(1):72-81. doi: 10.1016/0003-9861(85)90481-3.
Pancreatic islet homogenates contain a Mg2+-requiring phospholipid methyltransferase activity, the activity of which was doubled by calcium (K0.5 less than 5 microM). Other divalent metal ions stimulated the activity from 11 to 35%, but zinc and strontium were inhibitory. Cyclic AMP had no effect on the enzyme activity and cyclic GMP inhibited it slightly. Calcium increased the Vmax of the enzyme without affecting its Km with respect to S-adenosylmethionine (6 microM). Chlorpromazine, trifluoperazine, and dibucaine inhibited the calcium-stimulatable activity without affecting the activity in the absence of calcium. Phosphatidylserine stimulated, and arachidonic acid and palmitic acid inhibited, the basal enzyme activity. The methylated products were found to be primarily mono- and dimethylphosphatidylethanolamine (30%) and phosphatidylcholine (43%) and an, as yet unidentified, nonpolar lipid fraction (27%), as judged by thin-layer chromatography. In the presence of calcium, incorporation of methyl groups into phosphatidylcholine, mono- and dimethylphosphatidylethanolamine, and nonpolar lipids was increased by 131, 60, and 46%, respectively. Based on the localization of the enzyme activity in the insulin secretory granule fraction, it is proposed that phospholipid methylation plays a role in coupling the stimulus to the initial events in insulin secretion, leading to the exocytosis of insulin.
胰岛匀浆含有一种需要镁离子的磷脂甲基转移酶活性，该活性在钙离子存在下增加了一倍（半最大反应浓度小于5微摩尔）。其他二价金属离子刺激该活性增加11%至35%，但锌和锶具有抑制作用。环磷酸腺苷对该酶活性无影响，环磷酸鸟苷则稍有抑制作用。钙离子增加了该酶的最大反应速度，而不影响其对S-腺苷甲硫氨酸（6微摩尔）的米氏常数。氯丙嗪、三氟拉嗪和丁卡因抑制了钙离子刺激的活性，而不影响无钙离子时的活性。磷脂酰丝氨酸刺激基础酶活性，花生四烯酸和棕榈酸则抑制该活性。通过薄层层析判断，甲基化产物主要是单甲基和二甲基磷脂酰乙醇胺（30%）、磷脂酰胆碱（43%）以及一种尚未鉴定的非极性脂质组分（27%）。在钙离子存在下，甲基掺入磷脂酰胆碱、单甲基和二甲基磷脂酰乙醇胺以及非极性脂质中的量分别增加了131%、60%和46%。基于该酶活性在胰岛素分泌颗粒组分中的定位，有人提出磷脂甲基化在将刺激与胰岛素分泌的初始事件偶联中起作用，从而导致胰岛素的胞吐作用。
