Kishore N, Shah D, Skanes V M, Levine R P
James S. McDonnell Department of Genetics, Washington University School of Medicine, St. Louis, MO 63110.
Mol Immunol. 1988 Sep;25(9):811-9. doi: 10.1016/0161-5890(88)90117-4.
Covalent binding of the fourth complement protein, C4, to immune complexes is an important first step in the complement mediated processing of the complexes. Many of the initial encounters between the proteins of the complement system and antigen and antibody occur in solution, and prior to this report, studies of the interactions between them have focused on complement binding to preformed immune precipitates that most likely are not found in vivo. We have characterized the covalent binding of C4b to immunoglobulin molecules in a fluid-phase system consisting only of antibody in solution and purified C4 and C1s. We demonstrate that human C4b binds to IgG in the fluid phase, that its covalent binding is predominantly to the heavy chain of IgG, and that the covalent linkage is by either amide or acyl ester bonds. In addition, we compare the covalent binding efficiencies of two genetic variants of C4, C4A3 and C4B1, to IgG. C4A3 binds 3-4 times more IgG than C4B1 over a range of C4 concentrations, and C4A3 has a higher binding efficiency than C4B1 for IgM, IgA, IgG2a and F(ab')2 as well as for a protein antigen, BSA. Furthermore, we found that whereas C4A3 is bound to immunoglobulins in the fluid-phase predominantly by amide linkage, C4B1 is bound by either amide or acyl ester bonds. The results presented here suggest that the covalent binding efficiency of C4A3 and C4B1 to IgG is similar to that reported for their covalent binding to small molecules.
补体系统的第四种补体蛋白C4与免疫复合物的共价结合是补体介导处理这些复合物的重要第一步。补体系统的蛋白质与抗原和抗体之间的许多初始相互作用发生在溶液中，在本报告之前，对它们之间相互作用的研究集中在补体与预先形成的免疫沉淀物的结合上，而这些沉淀物很可能在体内并不存在。我们在一个仅由溶液中的抗体以及纯化的C4和C1s组成的液相系统中，对C4b与免疫球蛋白分子的共价结合进行了表征。我们证明人类C4b在液相中与IgG结合，其共价结合主要发生在IgG的重链上，并且共价连接是通过酰胺键或酰基酯键。此外，我们比较了C4的两种基因变体C4A3和C4B1与IgG的共价结合效率。在一系列C4浓度范围内，C4A3结合的IgG比C4B1多3 - 4倍，并且C4A3对IgM、IgA、IgG2a和F(ab')2以及蛋白质抗原牛血清白蛋白（BSA）的结合效率高于C4B1。此外，我们发现虽然C4A3在液相中与免疫球蛋白的结合主要通过酰胺键，而C4B1则通过酰胺键或酰基酯键结合。此处呈现的结果表明，C4A3和C4B1与IgG的共价结合效率与它们与小分子的共价结合效率相似。
