Plasminogen binding by α2-antiplasmin and histidine-rich glycoprotein does not inhibit plasmiogen activation at the surface of fibrin

Abstract

α 2-antiplasm ( α 2-AP) exerts its inhibitory effect on fibrinolysis by rapidly inhibiting the plasmin evolved; in addition, it has been suggested that interference with the binding of plasminogen to fibrin, a function shared with histadine-rich glycoprotein (HRGP), may also be sinnificant in inhibition of fibrinolysis. To elucidate if plasminogen binding by these two α 2-globulins may decrease the generation of plasmin by tissue-type plasminogen activator (t-PA) at the surface of fibrin, a system mimicking the fibrin/plasma interface was used. Attempts were made to differentiate the plasminogen binding from the plasmin inhibitory function of α 2-AP. The activation of human Glu-plasminogen (native plasminogen with NH2-terminal glutamic acid) by fibrin-bound t-PA was performed in a plasma environment using either normal plasma, α 2-AP- HRGP-depleted plasmas supplemented with increasing amounts of the lacking protein, or in a reconstituted system with purified plasminogen and various concentrations of α 2-AP and HRGP. The activation of Glu-plasminogen in α 2-AP-depleted plasma containing a normal concentration of HRGP produced a time-dependent increase in the generation of plasmin. The addition of 1 μM- α 2-AP to this plasma prevented the formation of Lys-derivatives and produced a marked decrease (42%) in the number of plasminogen-binding sites. In contrast, the addition of 1.5 μM-HRGP to HRGP-depleted plasma containing a normal amount of α 2AP produced only a modest (17%) decrease in the amount of plasmin(ogen) bound. Moreover, in a purified system the amount of plasminogen-binding sites and thereby of plasmin generated at the surface of fibrin in the presence of both α-2 globulins was similar to the amount generated in the presence of α 2-AP alone. These results indicate clearly that the formation of reversible complexes between plasminogen and α 2-AP does not interfere with the binding and activation of plasminogen at the fibrin surface. In contrast, the inhibition of plasmin by α 2-AP decreases importantly the number of plasminogen-binding sites (carboxy-terminal lysines) and inhibites thereby the accelerated phase of fibrinolysis. It can be concluded that interference of the binding of plasminogen to fibrin by α 2-AP during plasminogen activation, does not play a significant role in inhibition of fibrinolysis, and that the plasminogen-binding effect of HRGP, if any, is obscured by the important inhibitory effect of α 2-AP.