Seya T, Nagasawa S
Department of Immunology, The Center for Adult Diseases, Osaka.
J Biochem. 1988 May;103(5):792-6. doi: 10.1093/oxfordjournals.jbchem.a122348.
The third component of human complement, C3 is composed of two disulfide-bridged polypeptide chains of Mr 120,000 (alpha chain) and Mr 70,000 (beta chain). C3 has a thioester bond that serves as a binding site for targets when C3 is activated. Heat treatment of C3 induces autolytic peptide bond cleavage at the thioester site in the alpha chain as well as rupture of the thioester bond. The alpha chain fragments are linked to each other and beta chain via disulfide bonds. This study, however, documented that prolonged heating gave rise to liberation of several fragments including beta and the larger fragment of alpha chain. Using a fluorescent thiol reagent and [14C]iodoacetamide, we analyzed thiol residues present on each fragment, and elucidated that the thiol residue exposed by rupture of the thioester bond shifts in turn to another fragment resulting in the liberation of the fragments. The results were compatible with those on C4, and suggested that the generated thiol residue induces thiol-disulfide interchange reaction. On heating of plasma, fragments of C3 were not released, while the cleavage of the alpha chain occurred more effectively. The heated C3 (56 degrees C, 15 min) became insusceptible to C3b inactivator (I) and factor H, suggesting that additional conformational change is accompanied with cleavage of the thioester bond.
人类补体的第三个成分C3由两条通过二硫键相连的多肽链组成，一条分子量为120,000（α链），另一条分子量为70,000（β链）。C3具有一个硫酯键，当C3被激活时，该键作为靶点的结合位点。对C3进行热处理会诱导α链硫酯位点处的自溶肽键断裂以及硫酯键的破裂。α链片段通过二硫键相互连接并与β链相连。然而，本研究记录了长时间加热会导致包括β链和α链较大片段在内的多个片段的释放。我们使用荧光硫醇试剂和[14C]碘乙酰胺分析了每个片段上存在的硫醇残基，并阐明硫酯键破裂后暴露的硫醇残基依次转移到另一个片段，从而导致片段的释放。结果与C4的结果一致，并表明生成的硫醇残基诱导了硫醇 - 二硫键交换反应。在加热血浆时，C3的片段未被释放，而α链的裂解更有效。加热后的C3（56℃，15分钟）对C3b灭活剂（I）和因子H不敏感，这表明硫酯键的裂解伴随着额外的构象变化。
