DEMONSTRATION OF A FUNCTIONALLY ACTIVE TPA-LIKE PLASMINOGEN-ACTIVATORIN HUMAN PLATELETS

The mechanism of platelet-enhanced fibrinolysis is unclear. We therefo re investigated the fibrinolytic activity of human platelets and demon strated that they contain a tissue plasminogen activator (tPA)-like pl asminogen activator, abbreviated as tPA-like-PA. This activator was de tected by ELISA in platelet incubation medium and in platelet Triton e xtracts. Plasminogen activation assays' showed that this tPA-like-PA c ould induce plasminogen activation to form plasmin. Western blots of T riton extracts incubated with anti-tPA antibody demonstrated a major 6 4-kD protein band, compared to a 70-kD band for standard single chain tPA, plus a minor 118-kD band corresponding to a complex of tPA-like-P A and plasminogen activator inhibitor (PAI-1). Western blots of Triton extracts incubated with anti-PAI-1 antibody produced an approximately similar high-molecular-weight (118 kD) protein band. Fibrin zymograph ic analysis of affinity-purified tPA-like-PA demonstrated a major and a minor fibrin lysis zone, which approximately corresponded to the tPA -like-PA and its complex with PAI-1 observed by Western blots. Immunog old labelling and electron microscopy demonstrated that platelet activ ator, either as the free form or co-localized with PAI-1, was present in granules and in channels of the open canalicular system. We conclud e that platelets contain a functionally active tPA-like-PA, whose low fibrinolytic activity might be due to its readily forming a complex wi th PAI-1. This functionally active tPA-like-PA might contribute to the enhanced fibrinolytic activity of platelets observed in platelet-rich thrombi.