Effective lysis of model thrombi by a t-PA mutant (A473S) that is resistant to α₂-antiplasmin

This study used two mutants of tissue-type plasminogen activator (t-PA) with resistance to inhibitors of fibrinolysis to define the contribution of plasminogen activator inhibitor (PAI)-1 and α₂-antiplasmin (α₂-AP) to the control of fibrin lysis. Wild-type t-PA was compared with KHRR296-299AAAA, which is resistant to PAI-1, and with A473S, which is resistant to α₂-AP. We examined these forms of t-PA in model systems that are physiologically relevant. Neutralization of α₂-AP was essential for lysis of plasma clots, irrespective of their platelet content, by either wild-type t-PA or KHRR296-299AAAA. In marked contrast, A473S lysed plasma clots without neutralization of α₂-AP. Model thrombi, with structures similar to in vivo thrombi, were lysed slowly by wild-type t-PA; the rate and extent of lysis were enhanced by the addition of antibodies to α₂-AP or PAI-1. A473S was more effective than wild-type t-PA without the addition of antibodies by virtue of its resistance to α₂-AP. This resistance was remarkable, in that no complex formed between A473S t-PA and α₂-AP, even after extended incubation, when 50% of wild-type t-PA could be converted to complex. Comparison of A473S and KHRR296-299AAAA mutants showed their similar effectiveness in lysis of platelet-rich model thrombi. Thus, PAI-1 and α₂-AP contribute approximately equally to the inhibition of thrombus lysis. This study underlines the functional significance of α₂-AP as a direct inhibitor of t-PA and further explains the basis of the accepted role of α₂-AP as a regulator of fibrin persistence and thrombus resistance to lysis.