Effective lysis of model thrombi by a t-PA mutant (A473S) that is resistant to alpha(2)-antiplasmin

This study used two mutants of tissue-type plasminogen activator (t-PA) wit h resistance to inhibitors of fibrinolysis to define the contribution of pl asminogen activator inhibitor (PAI)-1 and alpha (2)-antiplasmin (alpha (2)- AP) to the control of fibrin lysis. Wild-type t-PA was compared with KHRR29 6-299AAAA, which is resistant to PAI-1, and with A473S, which is resistant to alpha (2)-AP. We examined these forms of t-PA in model systems that are physiologically relevant. Neutralization of alpha (2)-AP was essential for lysis of plasma clots, irrespective of their platelet content, by either wi ld-type t-PA or KHRR296-299AAAA. In marked contrast, A473S lysed plasma clo ts without neutralization of alpha (2)-AP. Model thrombi, with structures s imilar to in vivo thrombi, were lysed slowly by wild-type t-PA; the rate an d extent of lysis were enhanced by the addition of antibodies to alpha (2)- AP or PAI-1. A473S was more effective than wild-type t-PA without the addit ion of antibodies by virtue of its resistance to alpha (2)-AP. This resista nce was remarkable, in that no complex formed between A473S t-PA and alpha (2)-AP, even after extended incubation, when 50% of wild-type t-PA could be converted to complex. Comparison of A473S and KHRR296-299AAAA mutants show ed their similar effectiveness in lysis of platelet-rich model thrombi. Thu s, PAI-1 and alpha (2)-AP contribute approximately equally to the inhibitio n of thrombus lysis. This study underlines the functional significance of a lpha (2)-AP as a direct inhibitor of t-PA and further explains the basis of the accepted role of alpha (2)-AP as a regulator of fibrin persistence and thrombus resistance to lysis.