Isenman D E, Young J R
J Immunol. 1984 Jun;132(6):3019-27.
Human C4 displays a structural polymorphism which is consistent with there being two closely linked genetic loci coding for this protein. These give rise to two C4 isotypes, designated C4A and C4B, which can be distinguished by charge and apparent m.w. differences in their respective alpha-chains and by the presence or absence of the Chido/Rodgers blood group antigens. Previous qualitative studies of C4 immune hemolysis activity in whole plasma had suggested that the C4B isotype was functionally more active. By using purified C4A and C4B isolated from individual donors known serologically to possess only one of the C4 isotypes, we examined the molecular basis for the differences in their respective hemolytic activities. It was found that the C4B:C4A hemolytic activity ratio was approximately 4:1. This fourfold difference could not be accounted for by a commensurate difference in the cleavage rate of the two isotypes by C1s by differences in the kinetics of assembly or intrinsic decay of the respective C3 convertase enzymes, or by differences in the rate of isotypic C4b cleavage by factor I in the presence of C4bp . However, the fourfold greater deposition efficiency of nascent C4b of the C4B isotype onto the surface of C1-bearing sheep erythrocytes quantitatively accounted for the observed difference in immune hemolysis function. It was further found that the thioester bond of nascent C4b of the C4A isotype preferentially transacylates onto amino group nucleophiles, whereas in the C4B isotype, acylation of hydroxyl groups is strongly preferred. Thus, the difference in immune hemolysis activity between the two C4 isotypes does not necessarily indicate an impairment of function in C4A; it may merely be a reflection of the relative abundance at the surface of a C1-bearing target of hydroxyl and amino groups capable of being acyl acceptors for nascent C4b. Finally, we also present evidence showing that the apparent m.w. difference between the alpha-chains of the C4A and C4B isotypes is not due to differences in protein glycosylation.
人类C4表现出一种结构多态性，这与存在两个紧密连锁的编码该蛋白的基因座相一致。这些基因座产生两种C4同种型，分别命名为C4A和C4B，它们可以通过各自α链的电荷和表观分子量差异以及是否存在Chido/Rodgers血型抗原来区分。先前对全血浆中C4免疫溶血活性的定性研究表明，C4B同种型在功能上更具活性。通过使用从血清学上已知仅拥有一种C4同种型的个体供体中分离出的纯化C4A和C4B，我们研究了它们各自溶血活性差异的分子基础。发现C4B:C4A溶血活性比约为4:1。这四倍的差异不能通过C1s对两种同种型的裂解速率的相应差异、各自C3转化酶组装或内在衰变动力学的差异，或在存在C4bp时I因子对同种型C4b的裂解速率差异来解释。然而，C4B同种型新生C4b在携带C1的绵羊红细胞表面上四倍更高的沉积效率定量地解释了观察到的免疫溶血功能差异。进一步发现，C4A同种型新生C4b的硫酯键优先转酰基到氨基亲核试剂上，而在C4B同种型中，羟基的酰化强烈优先。因此，两种C4同种型之间免疫溶血活性的差异不一定表明C4A功能受损；它可能仅仅反映了能够作为新生C4b酰基受体的羟基和氨基在携带C1的靶标表面上的相对丰度。最后，我们还提供证据表明，C4A和C4B同种型α链之间的表观分子量差异不是由于蛋白质糖基化的差异。
