Spooner P J, Clark S B, Gantz D L, Hamilton J A, Small D M
Housman Medical Research Center, Department of Medicine, Boston University School of Medicine, Massachusetts 02118.
J Biol Chem. 1988 Jan 25;263(3):1444-53.
A reproducible, fairly narrow-sized population of rat lymph chylomicrons, approximately 100 nm, was isolated by centrifugation and combined with low levels of [1-13C]oleic acid for NMR studies. The carboxyl chemical shift was monitored as a function of aqueous pH to characterize the ionization behavior of the fatty acid in these particles. The titration curves were very similar to those for oleic acid in equivalent-sized emulsion particles composed of egg phosphatidylcholine and triolein. A simple partition-ionization model was fitted to the data to derive values for apparent ionization constant, expressed as pKapp, of 7.4-7.5 and the "true" surface to core partition coefficient of approximately 7 for oleic acid in chylomicrons. The fatty acid in chylomicrons thus appeared to be largely associated with the surface regions of these particles. Addition of bovine serum albumin to the samples showed that near physiologic pH much of the fatty acid was bound to the albumin at fatty acid to albumin-binding stoichiometries as high as 5.1 and with mass ratios of greater than 2 in favor of the lipid or lipoprotein particles. Lowering the pH of the medium shifted the distribution of fatty acid away from albumin so that at pH 5 with the emulsion, virtually all the fatty acid was associated with the lipid. The behavior observed under physiologic conditions is consistent with the rapid clearance and redistribution of fatty acid generated in these particles by lipolytic processes. However, under conditions of severe acidosis, hyperlipidemia, and hypoalbuminemia a significant portion of fatty acids might be retained in triglyceride-rich lipoproteins and their remnants and affect subsequent metabolism.
通过离心分离出了一群可重现的、大小相当窄的大鼠淋巴乳糜微粒，其直径约为100纳米，并与低水平的[1-13C]油酸混合用于核磁共振研究。监测羧基化学位移随水相pH值的变化，以表征这些颗粒中脂肪酸的电离行为。滴定曲线与由卵磷脂和三油酸甘油酯组成的等大小乳液颗粒中油酸的滴定曲线非常相似。将一个简单的分配-电离模型拟合到数据中，得出乳糜微粒中油酸的表观电离常数（以pKapp表示）为7.4-7.5，“真实”的表面到核心分配系数约为7。因此，乳糜微粒中的脂肪酸似乎主要与这些颗粒的表面区域相关。向样品中添加牛血清白蛋白表明，在接近生理pH值时，大部分脂肪酸以脂肪酸与白蛋白结合化学计量比高达5.1且质量比大于2（有利于脂质或脂蛋白颗粒）的形式与白蛋白结合。降低介质的pH值会使脂肪酸的分布从白蛋白转移开，以至于在pH为5的乳液中，几乎所有脂肪酸都与脂质相关。在生理条件下观察到的行为与这些颗粒中通过脂解过程产生的脂肪酸的快速清除和重新分布一致。然而，在严重酸中毒、高脂血症和低白蛋白血症的情况下，相当一部分脂肪酸可能会保留在富含甘油三酯的脂蛋白及其残粒中，并影响后续代谢。
