Influence Of Fibrin And Fibrinogen On The Activation Of Plasminogen By Urokinase

Human plasma was clotted with thrombin or Ca⁺⁺ in the presence of urokinase (UK), and the activity of plasmin was measured by hydrolysis of S-2251. As a control, the hydrolysis of S-2251 was measured without the addition of thrombin (in the absence of clot). The hydrolysis of S-2251 was higher in the clot than in the plasma. When euglobulin solution was clotted by thrombin in the presence of UK, the hydrolysis of S-2251 was higher in the presence than in the absence of clot. Thus the increased hydrolysis of S-2251 in the clot after the addition of UK was not due to the interaction of plasmin with inhibitors (∝₂AP or∝₂M) in the presence of clot, but plasminogen (plg) seems to be better activated by UK in the presence of fibrin. When purified Glu-plg was used, Glu-plg was better activated by UK in the presence of fibrin than fibrinogen. The addition of AP to the mixture of Glu-plg and UK resulted in complete inhibition of plasmin activity but the presence of fibrin partially prevented its inactivation by ∝₂AP. When Glu-plg was incubated with plasmin,resuiting Lys-plg was far better activated by UK than Glu-plg, but the presence of fibrin slightly enhanced its activation rate. When fibrinogen or fibrin was degraded by plasmin, early FDP enhanced the activation more than early FgDP. Early FgDP still better activates Glu-plg than fibrinogen. Extensive degradation products (D and E) resulted in less activation of Glu-plg than in the absence of these products. The addition of these degradation products to already activated plasmin had no effect on the enzymatic activity of plasmin.

In conclusion, Glu-plg was better activated by UK when clot was formed. Binding of Glu-plg with fibrin through lysine binding sites protected it from inactivation by AP. Early degradation also resulted in better activation of Glu-plg, but extensive degradation resulted in less activation which may be an important feedback mechanism of fibrinolysis.