A Study of the Mechanism of Inhibition of Fibrinolysis by Activated Thrombin-activable Fibrinolysis Inhibitor*
- From the ‡Departments of Biochemistry and Medicine, Queen’s University, Kingston, Ontario K7L 3N6, Canada, the§Department of Biochemistry, University of Vermont, Burlington, Vermont, 05405-0068, and the ¶Hamilton Civic Hospitals Research Centre, 711 Concession Street, Hamilton, Ontario, L8V 1C3, Canada
Abstract
TAFI (thrombin-activable fibrinolysis inhibitor) is a recently described plasma zymogen that, when exposed to the thrombin-thrombomodulin complex, is converted by proteolysis at Arg92 to a basic carboxypeptidase that inhibits fibrinolysis (TAFIa). The studies described here were undertaken to elucidate the molecular basis for the inhibition of fibrinolysis. When TAFIa is included in a clot undergoing fibrinolysis induced by tissue plasminogen activator and plasminogen, the time to achieve lysis is prolonged, and free arginine and lysine are released over time. In addition, TAFIa prevents a 2.5-fold increase in the rate constant for plasminogen activation which occurs when fibrin is modified by plasmin in the early course of fibrin degradation. The effect is specific for the Glu- form of plasminogen. TAFIa prevents or at least attenuates positive feedback expressed through Lys-plasminogen formation during the process of fibrinolysis initiated by tissue plasminogen activator and plasminogen. TAFIa also inhibits plasmin activity in a clot and prolongs fibrinolysis initiated with plasmin. We conclude that TAFIa suppresses fibrinolysis by removing COOH-terminal lysine and arginine residues from fibrin, thereby reducing its cofactor functions in both plasminogen activation and the positive feedback conversion of Glu-plasminogen to Lys-plasminogen. At relatively elevated concentrations, it also directly inhibits plasmin.
Footnotes
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↵* This work was supported by Grant MT-9781 from the Medical Research Council of Canada, Grant T-2631 from the Heart and Stroke Foundation of Ontario, and Grant PHSHL46703-6 from the National Institutes of Health.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‖ To whom correspondence should be addressed. Tel.: 613-545-2957; Fax: 613-545-2987; E-mail:nesheimm{at}post.queensu.ca.
- Abbreviations:
- TAFI
-
thrombin-activable fibrinolysis inhibitor
- TAFIa
-
activated TAFI
- t-PA
-
tissue plasminogen activator
- VFK-CMK
-
d-Val-Phe-Lys chloromethyl ketone
- DAPA
-
dansylarginineN-(3-ethyl-1,5-pentanediyl) amide
- Glu1-Plg(S741C-fluorescein)
-
recombinant human (S741C) plasminogen labeled at Cys741 with fluorescein
- Lys78-Plg(S741C-fluorescein)
-
the Lys78derivative of Glu1-Plg(S741C-fluorescein).
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- Received November 19, 1997.
- Revision received July 28, 1998.
