Interaction Of Plasminogen With Human Platelets

血栓的溶解(纤溶)需要纤溶酶，这是一种由组织纤溶酶原激活剂(TPA)激活纤溶酶原而产生的蛋白酶。纤溶酶原和组织型纤溶酶原激活剂都能结合到血小板表面，在那里它们的相互作用会大大加速。因此，血小板是纤溶系统的重要促进剂。我们实验室一直在用两种方法来研究纤溶酶原与血小板的相互作用：平衡结合实验和血小板依赖的纤溶酶原激活率测量。我们发现凝血酶对血小板的刺激增加了纤溶酶原与血小板表面的高亲和力结合，并极大地加快了纤溶酶原转化为纤溶酶的速度。经过去除C-末端赖氨酸的羧基肽酶B处理的血小板缺乏高亲和力的纤溶酶原结合，并且显示出支持纤溶酶原转化的能力大大降低。纤维蛋白原抗体的F(ab‘)2片段也能减少纤溶酶原的结合和转化，但其作用不如羧基肽酶那样明显。我们的目标有三个：(1)确定对羧基肽酶和抗纤维蛋白原敏感的结合部位的数量和亲和力；(2)确定这类结合部位是否彼此不同；以及(3)确定这些结合部位在促进纤溶酶原转化为纤溶酶方面的相对重要性。

Dissolution of blood clots (fibrinolysis) requires plasmin, a protease derived from the activation of plasminogen by tissue plasminogen activator (tPA). Both plasminogen and tPA are known to bind to the surface of platelets, where their interaction becomes greatly accelerated. Therefore, platelets are important promoters of fibrinolysis. Our laboratory has been examining the interaction of plasminogen and platelets with two types of assays: equilibrium binding experiments and rate measurements of platelet-dependent plasminogen activation. We have found that stimulation of platelets with thrombin increases high-affinity binding of plasminogen to the platelet surface and greatly accelerates the rate of plasminogen conversion to plasmin in the presence of platelets. Platelets that have been treated with carboxypeptidase B, which removes C-terminal lysines, lack high-affinity plasminogen binding and demonstrate much less ability to support plasminogen conversion. Plasminogen binding and conversion are also reduced by F(ab')2 fragments of anti-fibrinogen antibodies, but the effect is not as marked as the carboxypeptidase effect. Our goal is three-fold: (1) to determine the number and affinity of binding sites sensitive to carboxypeptidase and to anti-fibrinogen; (2) to establish whether these classes of sites are different from each other; and (3) to determine the relative importance of these classes of sites in promoting plasminogen conversion to plasmin.