Pannell R, Black J, Gurewich V
Department of Biomedical Research, St. Elizabeth's Hospital, Boston, Massachusetts 02135.
J Clin Invest. 1988 Mar;81(3):853-9. doi: 10.1172/JCI113394.
Tissue plasminogen activator (t-PA) and/or pro-urokinase (pro-UK) induced lysis of standard 125I-fibrin clots suspended in plasma was studied. Doses were kept below the concentration at which a nonspecific effect was seen, i.e., where fibrinogenolysis and major plasminogen consumption were observed. Small amounts of t-PA potentiated clot lysis by pro-UK by attenuating the lag phase characteristic of pro-UK, and causing a much earlier transition to the rapid phase of lysis. Similar promotion of the fibrinolytic effect of pro-UK was obtained when clots were pretreated with UK or with a little plasmin (less than 1% clot lysis). Promotion by plasmin was nullified by a subsequent treatment of the clot with carboxypeptidase B, indicating that the plasmin effect was related to the exposure of carboxy terminal lysine residues on fibrin. These lysine termini, absent in undegraded fibrin, are known to be essential for the high affinity binding of plasminogen to fibrin. In contrast, clot lysis by t-PA was unaffected by plasmin pretreatment and little affected by carboxypeptidase B treatment of the fibrin substrate. Therefore, plasminogen bound to lysine termini on fibrin, although found to be essential for pro-UK, did not appear to serve as a substrate for t-PA. Selective activation of fibrin bound plasminogen has been attributed to the conformational change in Glu-plasminogen that occurs as a result of binding. The present findings suggest that this conformational change occurs when plasminogen is bound to a terminal lysine but not to an internal lysine. Plasminogen bound to the latter site on fibrin was activated by t-PA and therefore is involved in the ternary complex. This initiates lysis of the undegraded clot and exposes the plasminogen binding sites required by pro-UK. By their complementary activation of fibrin bound plasminogen, t-PA followed by pro-UK induces efficient and synergistic fibrinolysis, whereas each is relatively inefficient when used alone.
研究了组织型纤溶酶原激活剂（t-PA）和/或尿激酶原（pro-UK）诱导悬浮于血浆中的标准125I-纤维蛋白凝块溶解的情况。剂量保持在未观察到非特异性效应的浓度以下，即观察到纤维蛋白原溶解和大量纤溶酶原消耗的浓度以下。少量t-PA通过减弱pro-UK的特征性延迟期并导致更早地转变为快速溶解期，增强了pro-UK的凝块溶解作用。当凝块用尿激酶（UK）或少量纤溶酶（小于1%凝块溶解）预处理时，也获得了类似的pro-UK纤溶作用促进效果。纤溶酶的促进作用在随后用羧肽酶B处理凝块后消失，这表明纤溶酶的作用与纤维蛋白上羧基末端赖氨酸残基的暴露有关。这些赖氨酸末端在未降解的纤维蛋白中不存在，已知它们对于纤溶酶原与纤维蛋白的高亲和力结合至关重要。相比之下，t-PA引起的凝块溶解不受纤溶酶预处理的影响，且受纤维蛋白底物羧肽酶B处理的影响很小。因此，结合在纤维蛋白赖氨酸末端的纤溶酶原，虽然被发现对pro-UK至关重要，但似乎不是t-PA的底物。纤维蛋白结合的纤溶酶原的选择性激活归因于结合导致的谷氨酰胺纤溶酶原的构象变化。目前的研究结果表明，这种构象变化发生在纤溶酶原结合到末端赖氨酸而非内部赖氨酸时。结合在纤维蛋白后者位点上的纤溶酶原被t-PA激活，因此参与三元复合物。这启动了未降解凝块的溶解，并暴露了pro-UK所需的纤溶酶原结合位点。通过它们对纤维蛋白结合的纤溶酶原的互补激活，t-PA随后是pro-UK诱导高效和协同的纤维蛋白溶解，而单独使用时每种都相对低效。
