Higazi A A, Bdeir K, Hiss E, Arad S, Kuo A, Barghouti I, Cines D B
Department of Clinical Biochemistry, Hadassah University Hospital and Hebrew University-Hadassah Medical School, Jerusalem, Israel.
Blood. 1998 Sep 15;92(6):2075-83.
Single-chain urokinase plasminogen activator (scuPA), the unique form secreted by cells, expresses little intrinsic plasminogen activator activity. scuPA can be activated by proteolytic cleavage to form a two-chain enzyme (tcuPA), which is susceptible to inhibition by plasminogen activator inhibitor type I (PAI-1). scuPA is also activated when it binds to its cellular receptor (uPAR), in which case the protein remains as a single chain molecule with less susceptibility to PAIs. Fibrin clots are invested with PAI-1 derived from plasma and from activated platelets. Therefore, we compared the fibrinolytic activity of complexes between scuPA and recombinant soluble uPAR (suPAR) to that of scuPA, tcuPA, and tcuPA/suPAR complexes. scuPA/suPAR complexes mediated the lysis of plasma-derived fibrin clots 14-fold more extensively than did equimolar concentrations of scuPA and threefold more extensively than did tcuPA or tcuPA/suPAR, respectively. The enhanced catalytic activity of scuPA/suPAR required that all three domains of the receptor be present, correlated with its PAI-1 resistance, was not dependent on fibrin alone, and required a plasma cofactor that was identified as IgG. Human IgG bound specifically to suPAR and scuPA/suPAR as determined by using affinity chromatography and immunoprecipitation. Plasma depleted of IgG lost most of its capacity to promote the fibrinolytic activity of scuPA/suPAR, and the activity of the complex was restored by adding plasma concentrations of purified IgG. These studies indicate that scuPA/suPAR can function as a plasminogen activator in a physiological milieu.
单链尿激酶型纤溶酶原激活剂（scuPA）是细胞分泌的独特形式，其内在纤溶酶原激活剂活性很低。scuPA可通过蛋白水解切割被激活，形成双链酶（tcuPA），tcuPA易受I型纤溶酶原激活剂抑制剂（PAI-1）的抑制。scuPA与细胞受体（uPAR）结合时也会被激活，在这种情况下，该蛋白仍为单链分子，对PAI的敏感性较低。纤维蛋白凝块中含有来自血浆和活化血小板的PAI-1。因此，我们比较了scuPA与重组可溶性uPAR（suPAR）形成的复合物、scuPA、tcuPA以及tcuPA/suPAR复合物的纤溶活性。scuPA/suPAR复合物介导的血浆源性纤维蛋白凝块溶解比等摩尔浓度的scuPA广泛1四倍，比tcuPA或tcuPA/suPAR分别广泛三倍。scuPA/suPAR增强的催化活性要求受体的所有三个结构域都存在，与它对PAI-1的抗性相关，不单独依赖于纤维蛋白，并且需要一种被鉴定为IgG的血浆辅因子。通过亲和层析和免疫沉淀测定，人IgG特异性结合suPAR和scuPA/suPAR。去除IgG的血浆失去了大部分促进scuPA/suPAR纤溶活性的能力，通过添加血浆浓度的纯化IgG可恢复复合物的活性。这些研究表明，scuPA/suPAR在生理环境中可作为纤溶酶原激活剂发挥作用。
