Van Schravendijk M R, Dwek R A
Mol Immunol. 1982 Sep;19(9):1179-87. doi: 10.1016/0161-5890(82)90328-5.
With the aim of clarifying the relationship between the activation of the first component of complement (C1) by immunoglobulin and by polyanions, the mode of interaction of C1q with DNA was investigated by structural and inhibition studies. DNA inhibits C1q binding to IgG immune complexes (ICs) through binding to C1q rather than to IgG, as seen from two lines of evidence. Firstly, DNA does not bind to ICs under conditions where full binding to C1q is observed. Secondly, at I - 0.15, DNA inhibits more strongly when mixed first with C1q rather than with ICs. The inhibition of C1q-IgG binding by DNA is subject to kinetic factors. Firstly, DNA is not an effective inhibitor if added after C1q has bound to ICs. This at least in part reflects a portion of the IgG-bound C1q that exchanges only very slowly with free C1q. Secondly, the relative rates of association of C1q to DNA and ICs at different ionic strengths are important in determining whether inhibition is observed. The existence of a kinetic effect in the inhibition by DNA means that inhibition experiments cannot be used to establish whether DNa binds to the same site on C1q as IgG. This question was therefore approached by structural studies. Precipitation of C1q with DNA was greatly diminished by heat or pH 4.45 denaturation of C1q, by pepsin digestion to remove the globular heads, and by limited modification with 1, 2-cyclohexanedione. In contrast, extensive modification with methyl acetimidate only had a limited effect. In these respects the structural requirements for C1q-DNA precipitation were similar to those for C1q-Igg binding, as would be consistent with binding of DNA and IgG to nearby or overlapping sites on C1q. In view of residual DNA-precipitating activity in the pepsin fragment preparation of C1q, there is the possibility that there are additional DNA sites on the collagenous tails.
为了阐明免疫球蛋白和多阴离子对补体第一成分（C1）的激活之间的关系，通过结构和抑制研究对C1q与DNA的相互作用模式进行了研究。从两方面证据可知，DNA通过与C1q结合而非与IgG结合来抑制C1q与IgG免疫复合物（ICs）的结合。首先，在观察到DNA与C1q完全结合的条件下，DNA不与ICs结合。其次，在离子强度I-0.15时，若先将DNA与C1q混合而非与ICs混合，其抑制作用更强。DNA对C1q-IgG结合的抑制作用受动力学因素影响。首先，如果在C1q与ICs结合后添加DNA，则其不是有效的抑制剂。这至少部分反映了与IgG结合的一部分C1q与游离C1q的交换非常缓慢。其次，在不同离子强度下C1q与DNA和ICs的相对结合速率对于确定是否观察到抑制作用很重要。DNA抑制作用中存在动力学效应意味着抑制实验不能用于确定DNA是否与IgG结合在C1q的同一位点上。因此，通过结构研究来探讨这个问题。C1q的热变性或pH 4.4进行的变性、用胃蛋白酶消化以去除球状头部以及用1,2-环己二酮进行的有限修饰，均可大大减少C1q与DNA的沉淀。相比之下，用甲基乙酰亚胺进行的广泛修饰仅具有有限的影响。在这些方面，C1q-DNA沉淀的结构要求与C1q-IgG结合的结构要求相似，这与DNA和IgG结合到C1q上附近或重叠位点一致。鉴于在C1q的胃蛋白酶片段制剂中存在残留的DNA沉淀活性，有可能在胶原尾部存在额外的DNA结合位点。
