Parkes C, Gagnon J, Kerr M A
Biochem J. 1983 Jul 1;213(1):201-9. doi: 10.1042/bj2130201.
Human complement components C2 and Factor B each contain one free thiol group/molecule. Reaction with p-chloromercuribenzoate destroyed the haemolytic activity of C2 but had no effect on Factor B. Reaction of C2 with I2 gave a 16-fold enhancement of its haemolytic activity. The pH optimum for the reaction was 7.0. The I2 reacted at the thiol group in C2 with a stoicheiometry of 1 mol of I2/mol of C2. The product of the reaction was unaffected by millimolar concentrations of dithiothreitol; however, azide and cyanide were inhibitory. Reaction with azide did not result in re-expression of the thiol group. Mild oxidation of the thiol group with m-chloroperbenzoic acid did not enhance the haemolytic activity. The results suggest that reaction with I2 causes intramolecular covalent, but not disulphide, bond formation. I2 reacted with Factor B at the free thiol group without affecting the haemolytic activity. A CNBr-cleavage peptide from C2a (obtained by cleavage of C2 by subcomponent C1s) containing the free thiol group was isolated. Automated Edman degradation of the peptide showed that it was the N-terminal peptide of C2a. The free thiol group was identified at position 18.
人补体成分C2和B因子各自含有一个游离巯基/分子。与对氯汞苯甲酸反应破坏了C2的溶血活性，但对B因子没有影响。C2与碘反应使其溶血活性增强了16倍。该反应的最适pH为7.0。碘与C2中的巯基反应，化学计量比为1摩尔碘/摩尔C2。反应产物不受毫摩尔浓度二硫苏糖醇的影响；然而，叠氮化物和氰化物具有抑制作用。与叠氮化物反应不会导致巯基重新表达。用间氯过苯甲酸对巯基进行轻度氧化不会增强溶血活性。结果表明，与碘反应导致分子内共价键而非二硫键的形成。碘与B因子的游离巯基反应而不影响其溶血活性。分离出了来自C2a（通过亚成分C1s切割C2得到）的含游离巯基的溴化氰裂解肽。对该肽进行自动埃德曼降解表明它是C2a的N端肽。游离巯基位于第１８位。
