Thromb Haemost 1994; 71(03): 347-352
DOI: 10.1055/s-0038-1642441
Original Article
Schattauer GmbH Stuttgart

Platelet-Bound Prekallikrein Promotes Pro-Urokinase-Induced Clot Lysis: A Mechanism for Targeting the Factor XII Dependent Intrinsic Pathway of Fibrinolysis

Jean-Pierre Loza
The Vascular Research Laboratory, Department of Medicine, Institute for the Prevention of Cardiovascular Disease, Deaconess Hospital, Harvard Medical School, Boston, MA, USA
,
Victor Gurewich
The Vascular Research Laboratory, Department of Medicine, Institute for the Prevention of Cardiovascular Disease, Deaconess Hospital, Harvard Medical School, Boston, MA, USA
,
Michael Johnstone
The Vascular Research Laboratory, Department of Medicine, Institute for the Prevention of Cardiovascular Disease, Deaconess Hospital, Harvard Medical School, Boston, MA, USA
,
Ralph Pannell
The Vascular Research Laboratory, Department of Medicine, Institute for the Prevention of Cardiovascular Disease, Deaconess Hospital, Harvard Medical School, Boston, MA, USA
› Author Affiliations
Further Information

Publication History

Received: 17 June 1993

Accepted after revision 10 November 1993

Publication Date:
06 July 2018 (online)

Summary

Clots formed from platelet rich plasma were found to be lysed more readily by low concentrations of pro-urokinase (pro-UK) than clots formed from platelet poor plasma. This was not a non-specific effect since the reverse occurred with tissue plasminogen activator. A mechanical explanation due to platelet-mediated clot retraction was excluded by experiments in which retraction was inhibited with cyto-chalasin B. Therefore, a platelet-mediated enzymatic mechanism was postulated to explain the promotion of fibrinolysis. Casein autography of isolated platelets revealed a ≈ 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 XIIa, induced activation of pro-UK and of chromomgenic substrate for kallikrein (S-2302). This activity corresponded to approximately 40-200 pM kallikrein per 10 8 washed and gel filtered platelets per ml. The activation of pro-UK by PPA-pretreated platelets was dose-dependent and inhibited by soybean trypsin inhibitor but not by bdellin, a specific inhibitor of plasmin, nor by the corn inhibitor of factor XIIa. Kinetic analysis of pro-UK activation by kallikrein showed promotion of the reaction by platelets. The KM of the reaction was reduced by platelets by ≈ 7-fold, while the kcat was essentially unchanged. In conclusion, PK was shown to be tightly associated with platelets where it can be activated by factor 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 have also been shown to be associated with platelets, the present findings suggest a mechanism by which the factor Xlla-dependent intrinsic pathway of fibrinolysis can be localized and targeted to a thrombus.