Covalent binding properties of the C4A and C4B isotypes of the fourth component of human complement on several C1-bearing cell surfaces.
Covalent binding properties of the C4A and C4B isotypes of the fourth component of human complement on several C1-bearing cell surfaces.
复制标题
人类补体第四种成分的 C4A 和 C4B 同种型在几个 C1 携带细胞表面上的共价结合特性。
DOI：
10.4049/jimmunol.136.7.2542
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发表时间：
1986
影响因子：
4.4
通讯作者：
J. Young
中科院分区：
文献类型：
--
作者：
D. Isenman;J. Young
在之前的一项研究中，我们证明硫酯介导的人 C4B 同种型转酰基到绵羊红细胞 (ES) 上的效率大约是 C4A 的四倍。此外，虽然C4B主要形成酯键，但C4A显示出偏好形成酰胺键。因此，我们认为观察到的两种同种型的相对功能活性将是它们的亲核偏好和 C1 承载靶标的表面组成的综合反映。本研究检验了这一假设。用乙酰亚胺乙酯对 Es 上的氨基进行化学修饰，使 C4A 同种型的 C1 依赖性结合减少两倍，而对 C4B 结合的影响可以忽略不计。此外，对于人类红细胞和两种人类白细胞系 K562 和 U937，C4B 与 C4A 沉积比率从 ES 时的大于 4 降至 1.5 至 2 之间。无论靶标如何，C4A 和 C4B 分别保持其对形成酰胺键和酯键的偏好。有趣的是，共价沉积在各种细胞上的放射性标记的 C4A 和 C4B 的 SDS-PAGE 谱表明，转酰化特异性程度更高，因为两种同型 α 链有时与不同的膜成分结合。这些差异不容易通过给定表面成分上每种同种型的首选亲核试剂丰度的简单差异来解释，也不是由于一种同种型与致敏抗体的优先结合所致。我们推测新生的 C4B 可能含有一个底物结合位点，该位点有助于含有每种同种型优选的亲核试剂类别的分子对硫酯羰基进行有效的攻击。
In a previous study we demonstrated that the thioester-mediated transacylation of the human C4B isotype onto sheep erythrocytes (ES) was approximately fourfold more efficient than that of C4A. Moreover, although C4B formed predominantly ester linkages, C4A displayed a preference for amide bond formation. We therefore suggested that the relative functional activity observed for the two isotypes would be a combined reflection of their nucleophilic preference and the surface composition of the C1-bearing target. The present study tests this hypothesis. Chemical modification of amino groups on Es with ethylacetimidate produced a twofold decrease in the C1-dependent binding of C4A isotype, while having a negligible effect on C4B binding. Furthermore, with human erythrocytes and two human leukocyte cell lines, K562 and U937, the C4B to C4A deposition ratio decreased from greater than 4 with ES to between 1.5 and 2. Irrespective of the target, C4A and C4B maintained their preference for forming amide and ester bonds, respectively. Interestingly, SDS-PAGE profiles of radiolabeled C4A and C4B, which had been covalently deposited on the various cells, suggested a further degree of transacylation specificity, as the two isotypic alpha-chains sometimes bound to different membrane components. These differences were not easily accounted for by simple differences in the abundance of the preferred nucleophile for each isotype on a given surface constituent, nor were they due to the preferential binding of one isotype to the sensitizing antibody. We speculate that nascent C4B may contain a substrate binding site that facilitates productive attack on the thioester carbonyl by molecules containing the class of nucleophile preferred by each isotype.