Sim E, Sim R B
Biochem J. 1983 Feb 15;210(2):567-76. doi: 10.1042/bj2100567.
The C3b receptor of human erythrocytes is known to act as a cofactor for the cleavage of the complement protein C3b by the serine proteinase C3b/C4b Ina. The same cofactor activity is shown to be present on human tonsil B-lymphocytes. The cofactor activity of the C3b receptor can be assayed, on intact cells or in solubilized extracts of cells, by determining the rate of C3b cleavage in the presence of fixed concentrations of C3b and of C3b/C4b Ina. This assay method was used to compare the characteristics and relative quantities of C3b receptors on erythrocytes and lymphocytes. The cofactor activities associated with these two cell types resemble each other, but are distinct from the serum cofactor proteins, C4bp and Factor H, in antigenicity and in pH- and ionic-strength-dependence, and are distinct from Factor H in substrate specificity. Assay of cofactor activity in intact cells indicates that there are about 80-fold more receptors per cell on the lymphocyte surface than on erythrocytes. Assays with cells made permeable by detergent show that, whereas essentially all of the receptors on erythrocytes are on the cell surface, B-lymphocytes contain a large internal receptor pool, which makes up more than 80% of the total cofactor activity of the cell.
已知人类红细胞的C3b受体可作为丝氨酸蛋白酶C3b/C4b Ina裂解补体蛋白C3b的辅助因子。研究表明，人类扁桃体B淋巴细胞也具有同样的辅助因子活性。C3b受体的辅助因子活性可以通过在固定浓度的C3b和C3b/C4b Ina存在的情况下测定C3b的裂解速率，在完整细胞或细胞溶解提取物中进行检测。该检测方法用于比较红细胞和淋巴细胞上C3b受体的特性和相对数量。与这两种细胞类型相关的辅助因子活性彼此相似，但在抗原性、pH值和离子强度依赖性方面与血清辅助因子蛋白C4bp和因子H不同，并且在底物特异性方面与因子H不同。完整细胞中辅助因子活性的检测表明，淋巴细胞表面每个细胞的受体数量比红细胞多约80倍。对经去污剂处理而具有通透性的细胞进行检测表明，红细胞上基本上所有的受体都在细胞表面，而B淋巴细胞含有大量的细胞内受体库，其占细胞总辅助因子活性的80%以上。
