Pownall H J, Pao Q, Brockman H L, Massey J B
J Biol Chem. 1987 Jul 5;262(19):9033-6.
Model high density lipoproteins containing human apolipoprotein A-I, cholesterol, and a variety of phosphatidylcholines (PCs) have been prepared and tested. The PCs included 1-palmitoyl-2-oleoyl PC (POPC) and its diether analog 1-O-hexadecyl-2-oleyl PC (POPC ether), 1,2-diphytanoyl PC (DPhPC), 1-palmitoyl-2-phytanoyl PC, and 1-phytanoyl-2-palmitoyl PC. All ester PCs were good acyl donors for the transesterification of cholesterol catalyzed by human lecithin-cholesterol acyltransferase except DPhPC, which showed no reactivity. The PCs containing one phytanoyl chain donated an acyl chain to cholesterol as fast as non-branched fatty acyl chains. However, the competitive inhibition of lecithin-cholesterol acyltransferase by POPC ether and DPhPC was similar, and both lipids formed a macromolecular matrix that supported the reactivity of other ester PC substrates. The bulk of physicochemical properties of model high density lipoproteins composed of DPhPC were indistinguishable from those of POPC ether. These properties included 1) alpha-helical content of the apoprotein as assessed by circular dichroism, 2) microviscosity as determined from the fluorescence polarization and lifetime of the probe 1,6-diphenyl-1,3,5-hexatriene, 3) macromolecular weight based upon analytical gel filtration chromatography, and 4) surface polarity revealed by the fluorescence of 6-propionyl-2(dimethylamino)naphthalene. The only major difference in a physicochemical property was that the molecular surface area of DPhPC (area = 69 A2 at collapse pressure) determined by monolayer methods was 17 A2 greater than that of POPC (area = 53 A2 at collapse pressure) at all surface pressures measured. We suggest that the properties of DPhPC in being enzymatically nonreactive but a competitive inhibitor are due to its much larger size and that the active site of lecithin-cholesterol acyltransferase cannot bind phospholipid substrates in a catalytically productive way if they have surface areas of 70 A2 or more.
含有人类载脂蛋白A-I、胆固醇和多种磷脂酰胆碱（PCs）的模型高密度脂蛋白已被制备并测试。这些PCs包括1-棕榈酰-2-油酰磷脂酰胆碱（POPC）及其二醚类似物1-O-十六烷基-2-油酰磷脂酰胆碱（POPC醚）、1,2-二植烷酰磷脂酰胆碱（DPhPC）、1-棕榈酰-2-植烷酰磷脂酰胆碱和1-植烷酰-2-棕榈酰磷脂酰胆碱。除了无反应性的DPhPC外，所有酯型PCs都是人类卵磷脂胆固醇酰基转移酶催化胆固醇酯交换反应的良好酰基供体。含有一条植烷酰链的PCs向胆固醇提供酰基链的速度与非支链脂肪酰链一样快。然而，POPC醚和DPhPC对卵磷脂胆固醇酰基转移酶的竞争性抑制作用相似，并且这两种脂质都形成了一个大分子基质，支持其他酯型PC底物的反应性。由DPhPC组成的模型高密度脂蛋白的大部分物理化学性质与POPC醚的性质没有区别。这些性质包括：1）通过圆二色性评估的载脂蛋白的α-螺旋含量；2）根据探针1,6-二苯基-1,3,5-己三烯的荧光偏振和寿命确定的微粘度；3）基于分析凝胶过滤色谱法的大分子重量；4）由6-丙酰基-2（二甲基氨基）萘的荧光揭示的表面极性。物理化学性质的唯一主要差异在于，通过单层方法测定的DPhPC的分子表面积（在崩塌压力下面积 = 69 Å2）在所有测量的表面压力下都比POPC的分子表面积（在崩塌压力下面积 = 53 Å2）大17 Å2。我们认为，DPhPC在酶促反应中无反应性但具有竞争性抑制作用的性质是由于其尺寸大得多，并且如果磷脂底物的表面积为70 Å2或更大，卵磷脂胆固醇酰基转移酶的活性位点就无法以催化有效的方式结合磷脂底物。
