Koppelman S J, van't Veer C, Sixma J J, Bouma B N
Department of Haematology, University Hospital, Utrecht, The Netherlands.
Blood. 1995 Oct 1;86(7):2653-60.
The complement protein C4b-binding protein plays an important role in the regulation of the protein C anticoagulant pathway. C4b-binding protein can bind to protein S, thereby inhibiting the cofactor activity of protein S for activated protein C. In this report, we describe a new role for C4b-binding protein in coagulation. We observed inhibition of the intrinsic factor X activating reaction by the complex of C4b-binding protein and protein S. At the plasma concentration of protein S, the factor X activation was inhibited for 50% and addition of C4b-binding protein led to a potentiation of the inhibition to almost 90%. Because C4b-binding protein alone had no effect on the activation of factor X, we hypothesized that binding of C4b-binding protein to protein S was a prerequisite for optimal inhibition of factor X activation. C4b-binding protein lacking the beta-chain, which is unable to bind to protein S, did not potentiate the inhibitory effect of protein S. In an earlier study, we observed that C4b-binding protein increased the binding affinity of protein S for factor VIII. Therefore, a possible interaction of C4b-binding protein with factor VIII was investigated. C4b-binding protein bound to factor VIII and to thrombin activated factor VIII in a saturable and specific way. Also, factor VIII in complex with von Willebrand factor was able to bind C4b-binding protein. The beta-chain of C4b-binding protein was not required for the interaction with factor VIII because C4b-binding protein lacking the beta-chain also bound to factor VIII. Monoclonal antibodies directed against the alpha-chain of C4b-binding protein inhibited the binding to factor VIII, whereas monoclonal antibodies directed against the beta-chain had no effect on the binding to factor VIII. This finding indicates that the binding site for factor VIII on C4b-binding protein is localized on the alpha-chains of C4b-binding protein. The potentiation by C4b-binding protein of the inhibition of the factor X activation by protein S was blocked by a monoclonal antibody directed against the alpha-chain of C4b-binding protein. This finding indicates that the potentiation of the inhibitory effect of protein S was mediated via an interaction of C4b-binding protein with factor VIII. C4b-binding protein did not bind to factor V and was not able to potentiate the inhibitory effect of protein S on prothrombinase activity.(ABSTRACT TRUNCATED AT 400 WORDS)
补体蛋白C4b结合蛋白在蛋白C抗凝途径的调节中起重要作用。C4b结合蛋白可与蛋白S结合，从而抑制蛋白S对活化蛋白C的辅因子活性。在本报告中，我们描述了C4b结合蛋白在凝血中的新作用。我们观察到C4b结合蛋白与蛋白S的复合物对内在因子X激活反应有抑制作用。在蛋白S的血浆浓度下，因子X激活被抑制50%，添加C4b结合蛋白导致抑制作用增强至近90%。由于单独的C4b结合蛋白对因子X激活无影响，我们推测C4b结合蛋白与蛋白S的结合是最佳抑制因子X激活的先决条件。缺乏β链而无法与蛋白S结合的C4b结合蛋白不会增强蛋白S的抑制作用。在早期研究中，我们观察到C4b结合蛋白增加了蛋白S对因子VIII的结合亲和力。因此，研究了C4b结合蛋白与因子VIII的可能相互作用。C4b结合蛋白以可饱和且特异性的方式与因子VIII和凝血酶激活的因子VIII结合。此外，与血管性血友病因子形成复合物的因子VIII也能够结合C4b结合蛋白。与因子VIII相互作用不需要C4b结合蛋白的β链，因为缺乏β链的C4b结合蛋白也能与因子VIII结合。针对C4b结合蛋白α链的单克隆抗体抑制其与因子VIII的结合，而针对β链的单克隆抗体对其与因子VIII的结合无影响。这一发现表明C4b结合蛋白上因子VIII的结合位点位于C4b结合蛋白的α链上。针对C4b结合蛋白α链的单克隆抗体阻断了C4b结合蛋白对蛋白S抑制因子X激活的增强作用。这一发现表明蛋白S抑制作用的增强是通过C4b结合蛋白与因子VIII的相互作用介导的。C4b结合蛋白不与因子V结合，也不能增强蛋白S对凝血酶原酶活性的抑制作用。（摘要截短于400字）
