Brûlet P, McConnell H M
Biochemistry. 1977 Mar 22;16(6):1209-17. doi: 10.1021/bi00625a028.
Three different phospholipid haptens have been synthesized, in which the haptenic group is the paramagnetic nitroxide (spin-label) group. These lipid haptens differ from one another in the length and composition of the molecular chain linking the 2,2,6,6-tetramethylpiperidinyl-N-oxy moiety to the phosphodiester group of the lipid. These lipid haptens have been incorporated at low molar concentrations (0.01 to 0.5 mol %) in liposomes containing various proportions of cholesterol and dipalmitoylphosphatidylcholine (DPPC). A study has been made of specific antinitroxide IgG (and Fab) binding to these liposomes, and the fixation of complement. From these studies we conclude: (a) For lipid haptens whose possible extension above the bilayer plane is limited (e.g., approximately 10-20 A), antibody binding and complement fixation depend strongly on the hapten structure and host lipid composition, because of steric limitations on the accessibility of lipid haptens to the binding sites in the protein. (b) Complement fixation by specific IgG antibodies directed against the nitroxide group as part of a lipid hapten depends strongly on the lateral mobility of the lipid hapten when its molar concentration in the plane of the membrane is of the order of 0.1 mol % or less. It is likely that this conclusion applies to many lipid haptens, and possibly other membrane components. (c) The inclusion of cholesterol in lipid membranes has at least two distinct effects on complement fixation involving lipid haptens. Through a steric effect on bilayer structure (probably involving lateral molecular ordering) cholesterol in phosphatidylcholine bilayers can enhance hapten exposure to antibody binding sites, enhance antibody binding, and thereby enhance complement fixation. It is likely that cholesterol also affects complement fixation at low hapten concentrations through a modification of membrane fluidity.
已经合成了三种不同的磷脂半抗原，其中半抗原基团是顺磁性氮氧化物（自旋标记）基团。这些脂质半抗原在将2,2,6,6 - 四甲基哌啶基 - N - 氧基部分连接到脂质的磷酸二酯基团的分子链的长度和组成上彼此不同。这些脂质半抗原已以低摩尔浓度（0.01至0.5 mol％）掺入含有不同比例胆固醇和二棕榈酰磷脂酰胆碱（DPPC）的脂质体中。已经对特异性抗氮氧化物IgG（和Fab）与这些脂质体的结合以及补体的固定进行了研究。从这些研究中我们得出以下结论：（a）对于其在双层平面上方的可能延伸受到限制（例如，约10 - 20埃）的脂质半抗原，抗体结合和补体固定强烈依赖于半抗原结构和宿主脂质组成，这是由于脂质半抗原与蛋白质中结合位点的可及性存在空间限制。（b）针对作为脂质半抗原一部分的氮氧化物基团的特异性IgG抗体介导的补体固定在很大程度上取决于脂质半抗原在膜平面中的摩尔浓度为0.1 mol％或更低时的横向流动性。这个结论可能适用于许多脂质半抗原，也可能适用于其他膜成分。（c）在脂质膜中加入胆固醇对涉及脂质半抗原的补体固定至少有两种不同的影响。通过对双层结构的空间效应（可能涉及横向分子排列），磷脂酰胆碱双层中的胆固醇可以增强半抗原暴露于抗体结合位点，增强抗体结合，从而增强补体固定。胆固醇也可能通过改变膜流动性在低半抗原浓度下影响补体固定。
