Vaughan D E, Mendelsohn M E, Declerck P J, Van Houtte E, Collen D, Loscalzo J
Center for Thrombosis and Vascular Research, University of Leuven, Belgium.
J Biol Chem. 1989 Sep 25;264(27):15869-74.
Cell surface binding sites for the constituent proteins of the fibrinolytic system may play a role in the localization and regulation of fibrinolysis. In the present study, specific binding of recombinant human tissue-type plasminogen activator (rt-PA) to human blood platelets was identified and characterized. 125I-labeled rt-PA was found to bind specifically, saturably, and reversibly to the surface of gel-filtered platelets, reaching equilibrium within 5 min at 22 degrees C. Scatchard analysis revealed a single class of binding sites. Unstimulated platelets bound 120,000 +/- 24,000 (mean +/- S.D.) molecules/platelet with an apparent Kd of 340 +/- 25 nM, whereas thrombin-stimulated platelets bound 290,000 +/- 32,000 molecules/platelet with an apparent Kd of 800 +/- 60 nM. Binding of 0.1 microM 125I-rt-PA was greater than 90% reversible by a 50-fold excess of unlabeled rt-PA. Binding was not inhibited by fibrinogen or single chain urokinase-type plasminogen activator, but plasminogen partially competed for binding of 125I-rt-PA to platelets (up to 40% displacement). These findings indicate that the platelet surface possesses a large number of specific, low affinity binding sites for t-PA and provide further evidence for the role of platelets in localization and regulation of fibrinolysis.
纤溶系统组成蛋白的细胞表面结合位点可能在纤溶作用的定位和调节中发挥作用。在本研究中，鉴定并表征了重组人组织型纤溶酶原激活剂（rt-PA）与人血小板的特异性结合。发现125I标记的rt-PA能特异性、饱和性和可逆性地结合到凝胶过滤血小板表面，在22℃下5分钟内达到平衡。Scatchard分析显示存在一类结合位点。未刺激的血小板以340±25 nM的表观解离常数结合120,000±24,000（平均值±标准差）个分子/血小板，而凝血酶刺激的血小板以800±60 nM的表观解离常数结合290,000±32,000个分子/血小板。0.1μM的125I-rt-PA的结合在加入50倍过量的未标记rt-PA后有超过90%的可逆性。纤维蛋白原或单链尿激酶型纤溶酶原激活剂不抑制结合，但纤溶酶原部分竞争125I-rt-PA与血小板的结合（高达40%的置换）。这些发现表明血小板表面具有大量针对t-PA的特异性低亲和力结合位点，并为血小板在纤溶作用的定位和调节中的作用提供了进一步证据。
