Seya T, Nagasawa S
J Biochem. 1981 Feb;89(2):659-64. doi: 10.1093/oxfordjournals.jbchem.a133243.
Human C3 is composed of two disulfide-linked polypeptide chains, termed alpha chain (with a molecular weight of 110,000) and beta chain (with a molecular weight of 75,000). When 3C was heated at above 50 degrees C and at neutral pH, a single peptide bond in the alpha chain was selectively cleaved to yield two alpha chain fragments with molecular weights of 75,000 and 44,000. The two alpha chain fragments and intact beta chain are originally connected by disulfide linkages but are gradually dissociated upon prolonged heat treatment. The dissociation seems to be caused by thiol-disulfide interchange reactions, since the dissociation was prevented by the addition of monoiodoacetic acid and only 1 mol of thiol group was determined to be newly generated upon heat treatment of C3. The heat-induced C3 cleavage reached a plateau when almost 1 mol of thiol group appeared. In addition, the heat-induced C3 cleavage was prevented by pretreatments with C3 convertase and methylamine, which are known to cleave the thioester linkage in the side chain of C3. Thus, the thioester linkage, which is the latent reactive site in C3 and which, upon activation of C3, forms an ester linkage with cell surfaces, seems to make a specific peptide bond extremely heat-labile.
人补体C3由两条通过二硫键连接的多肽链组成，称为α链（分子量为110,000）和β链（分子量为75,000）。当C3在50℃以上及中性pH条件下加热时，α链中的一个肽键被选择性裂解，产生分子量分别为75,000和44,000的两个α链片段。这两个α链片段和完整的β链最初通过二硫键相连，但在长时间热处理后会逐渐解离。这种解离似乎是由硫醇 - 二硫键交换反应引起的，因为加入碘乙酸可防止解离，并且在C3热处理后仅测定到1摩尔新生成的硫醇基团。当出现近1摩尔硫醇基团时，热诱导的C3裂解达到平台期。此外，用C3转化酶和甲胺预处理可防止热诱导的C3裂解，已知这两种物质可裂解C3侧链中的硫酯键。因此，硫酯键作为C3中的潜在反应位点，在C3激活后与细胞表面形成酯键，似乎使特定的肽键对热极度不稳定。
