Tissue - Type Plasminogen Activator Increases the Binding of Glu - Plasminogen to Clots Chien

Porcine tissue-type plasminogen activator (t-PA) increases the binding of 125I-glu-plasminogen to clots made from human plasma or purified fibrinogen in a time and t-PA concentration dependent fashion. The accumulation of plasminogen was faster and greater on noncrosslinked plasma clots than on clots which had been crosslinked by Factor XIIIa. Furthermore, the uptake of plasminogen to crosslinked fibrin clots occurred at a slower rate in the presence of a2-plasmin inhibitor (a2PI) than in its absence. The kinetics of the uptake of '251-plasminogen were analyzed using SDS-polyacrylamide gel electrophoresis and radioautography of solubilized plasma clots formed in the presence of t-PA. During the initial phase there was a decrease of clot-bound glu-plasminogen; simultaneously, there was a slight increase in clot-bound glu-plasmin and in plasmin complexed to a2PI that was crosslinked to a-chain polymers of fibrin. This was followed by a marked increase in clot-bound plasminogen having glutamic acid as NH2-terminal (glu-plasminogen) and gluplasmin. t-PA-induced enhancement ofglu-plasminogen uptake appears to be mediated by plasmin but does not require the conversion of glu-plasminogen to plasminogen having lysine or methionine as NH2-terminal. The described mechanism assures an adequate supply of clot-bound plasmin, which is the enzyme ultimately involved in the degradation of fibrin. This work was presented in part at the Satellite Symposium "Fibrinogen and its Degradation Products-Structure and Function" at the IXth Congress of the International Society on Thrombosis and Haemostasis, Stockholm, Sweden, 1983. Address correspondence to Dr. Tran-Thang. Received for publication 28 March 1984 and in revised form 21 August 1984. Introduction During blood coagulation, 30% (0.2 ,mol/l) of plasma a2-plasmin inhibitor (a2PI)' are crosslinked to a-chain polymers of fibrin (1); an equimolar amount of plasminogen having glutamic acid as NH2-terminal (glu-plasminogen) is bound to fibrin (10% of plasma concentration or 0.2 Mmol/l) (2). The amount of fibrin-bound a2PI thus suffices to inhibit all plasmin generated from fibrin-bound plasminogen (2). Tissue-type plasminogen activator (t-PA) mediates specific and efficient thrombolysis (3-8). This process is dependent on fibrin-bound plasmin which is protected against the inhibitory effect of circulating a2PI (9-1 1). An excess of plasminogen or of plasmin is therefore needed at the fibrin surface to bring about fibrin degradation. The rapid complexation of circulating plasmin by a2PI precludes accumulation of active plasmin on fibrin (9-11). Several authors postulated that the conversion of glu-plasminogen into plasminogen having lysine or methionine as NH2-terminal (lys-plasminogen), which adsorbs to fibrin to a greater extent than the glu-form (12-18), would ensure an additional supply of plasminogen onto the clot. In this report, we have investigated an alternative hypothesis, i.e., whether porcine t-PA promotes the uptake of glu-plasmin-