Ripoche J, Sim R B
Biochem J. 1986 May 1;235(3):815-21. doi: 10.1042/bj2350815.
Complement receptor type 1 (CR1) is a glycoprotein of Mr about 250 000 present on erythrocytes and other cell types. CR1 acts as a cofactor in the factor I-mediated breakdown of complement fragment C3b to form iC3b. Using an assay of cofactor activity, a wide variation in mean CR1 levels between erythrocytes from individual donors is observed. CR1 levels also decrease on ageing of erythrocytes in vivo, and again the rate of loss is widely variable between individuals. However, variable loss of CR1 during ageing of erythrocytes is likely to make only a minor contribution to the observed variation in mean CR1 levels. CR1 is very sensitive to proteolysis, and random proteolytic removal of CR1 from erythrocytes is likely to be an important factor in loss of CR1 on ageing of red cells in vivo. In vitro, mild trypsin treatment, plasmin or thrombin digestion of erythrocytes results in the loss of the factor I cofactor activity from the cell surface, and appearance of this activity in the supernatant. We conclude that an active fragment of CR1 is released from the cell surface on proteolysis. Subsequent prolonged trypsin treatment destroys most of the activity of this fragment. Proteolytic removal of CR1 from red cells may account not only for loss on ageing of cells, but also for the acquired CR1 deficiencies observed by others in systemic lupus erythematosus.
补体受体1（CR1）是一种存在于红细胞和其他细胞类型上的糖蛋白，分子量约为250000。CR1作为一种辅助因子，参与因子I介导的补体片段C3b降解形成iC3b的过程。通过辅助因子活性检测发现，不同个体供体红细胞的平均CR1水平存在很大差异。在体内，随着红细胞老化，CR1水平也会下降，而且个体之间的丢失速率差异很大。然而，红细胞老化过程中CR1的可变丢失可能仅对观察到的平均CR1水平差异有较小贡献。CR1对蛋白水解非常敏感，红细胞中CR1的随机蛋白水解去除可能是体内红细胞老化时CR1丢失的一个重要因素。在体外，用温和的胰蛋白酶处理、纤溶酶或凝血酶消化红细胞会导致细胞表面因子I辅助因子活性丧失，并在上清液中出现这种活性。我们得出结论，蛋白水解时CR1的一个活性片段从细胞表面释放。随后长时间的胰蛋白酶处理会破坏该片段的大部分活性。红细胞中CR1的蛋白水解去除不仅可能导致细胞老化时的丢失，还可能解释其他人在系统性红斑狼疮中观察到的获得性CR1缺陷。
