Preissner K T, Wassmuth R, Müller-Berghaus G
Biochem J. 1985 Oct 15;231(2):349-55. doi: 10.1042/bj2310349.
S-protein, the main inhibitor of the assembly of the membrane attack complex of complement, was isolated from human plasma by a simple purification procedure, which includes barium citrate adsorption, ammonium sulphate precipitation, chromatography on DEAE-Sephacel and Blue Sepharose and gel filtration on Sephacryl S-200. The homogeneous protein (sedimentation coefficient 4.6 S) was obtained in approx. 5% yield relative to its concentration in plasma, which was found to be 0.3-0.5 mg/ml. The final product did not cross-react with antisera against complement proteins or other proteinase inhibitors of human plasma. On polyacrylamide-gel electrophoresis in the presence of sodium dodecyl sulphate, S-protein migrated as a single-chain band with an apparent Mr of 74000 under non-reducing conditions and as a doublet of Mr 78000 and 65000 upon reduction. In plasma or serum S-protein also existed in two forms of corresponding Mr values, as was evidenced by an immunoblot enzyme-linked immunosorbent assay technique. S-protein was found to be an acidic glycoprotein with 10% (W/W) carbohydrate content and several isoelectric points in the range pH 4.75-5.25, and it contained one free thiol group per molecule of protein. The functional properties of S-protein in the complement system were demonstrated by its ability to inhibit complement-dependent cell lysis in a concentration-dependent manner (Ki 0.6 microM) and by its incorporation into the nascent SC5b-7 complex. A new function for S-protein could be revealed in the blood coagulation system. The slow progressive inhibition of thrombin by antithrombin III was not affected by S-protein, whereas the purified protein interfered with the fast inactivation of thrombin clotting as well as amidolytic activity by antithrombin III-heparin complex. The acceleration of this inhibition reaction by heparin was counteracted by S-protein, indicating the ability of S-protein to neutralize heparin activity.
S蛋白是补体膜攻击复合物组装的主要抑制剂，通过一种简单的纯化程序从人血浆中分离得到，该程序包括柠檬酸钡吸附、硫酸铵沉淀、DEAE-葡聚糖凝胶和蓝色葡聚糖凝胶色谱以及Sephacryl S-200凝胶过滤。相对于血浆中0.3 - 0.5 mg/ml的浓度，以约5%的产率获得了均一的蛋白质（沉降系数4.6 S）。最终产物与针对补体蛋白或人血浆其他蛋白酶抑制剂的抗血清不发生交叉反应。在十二烷基硫酸钠存在下进行聚丙烯酰胺凝胶电泳时，S蛋白在非还原条件下以单链条带迁移，表观分子量为74000，还原后为分子量78000和65000的双峰。免疫印迹酶联免疫吸附测定技术证明，在血浆或血清中S蛋白也以两种相应分子量形式存在。发现S蛋白是一种酸性糖蛋白，碳水化合物含量为10%（W/W），有几个等电点在pH 4.75 - 5.25范围内，且每个蛋白质分子含有一个游离巯基。S蛋白在补体系统中的功能特性通过其以浓度依赖方式抑制补体依赖性细胞裂解的能力（Ki 0.6 μM）以及其掺入新生的SC5b - 7复合物得以证明。在血液凝固系统中可能揭示出S蛋白新的功能。抗凝血酶III对凝血酶的缓慢渐进性抑制不受S蛋白影响，而纯化的S蛋白会干扰抗凝血酶III - 肝素复合物对凝血酶凝血活性以及酰胺水解活性的快速灭活。肝素对这种抑制反应的加速作用被S蛋白抵消，表明S蛋白具有中和肝素活性的能力。
