PLATELET-BOUND PREKALLIKREIN PROMOTES PRO-UROKINASE-INDUCED CLOT LYSIS - A MECHANISM FOR TARGETING THE FACTOR-XII DEPENDENT INTRINSIC PATHWAY OF FIBRINOLYSIS

Clots formed from platelet rich plasma were found to be lysed more rea dily by low concentrations of pro-urokinase (pro-UK) than clots formed from platelet poor plasma. This was not a non-specific effect since t he reverse occurred with tissue plasminogen activator. A mechanical ex planation due to platelet-mediated clot retraction was excluded by exp eriments in which retraction was inhibited with cytochalasin B. Theref ore, a platelet-mediated enzymatic mechanism was postulated to explain the promotion of fibrinolysis. Casein autography of isolated platelet s revealed a almost-equal-to 90 kDa band of activity which comigrated with plasma prekallikrein (PK)/kallikrein, a known activator of pro-UK . Furthermore, treatment of platelets with plasma PK activator (PPA), consisting essentially of factor XII(a), induced activation of pro-UK and of chromomgenic substrate for kallikrein (S-2302). This activity c orresponded to approximately 40-200 pM kallikrein per 10(8) washed and gel filtered platelets per ml. The activation of pro-UK by PPA-pretre ated platelets was dose-dependent and inhibited by soybean trypsin inh ibitor but not by bdellin, a specific inhibitor of plasmin, nor by the com inhibitor of factor XIIa. Kinetic analysis of pro-UK activation b y kallikrein showed promotion of the reaction by platelets. The K(M) o f the reaction was reduced by platelets by almost-equal-to 7-fold, whi le the k(cat) was essentially unchanged. In conclusion, PK was shown t o be tightly associated with platelets where it can be activated by fa ctor XIIa during clotting. The activation of pro-UK by platelet-bound kallikrein provides an explanation for the observed platelet mediated promotion of pro-UK-induced clot lysis. Since pro-UK and plasminogen h ave also been shown to be associated with platelets, the present findi ngs suggest a mechanism by which the factor XIIa-dependent intrinsic p athway of fibrinolysis can be localized and targeted to a thrombus.