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Probing the Dynamics of Clot-Bound Thrombin at Venous Shear Rates.
Laura M Haynes1, Thomas Orfeo1, Kenneth G Mann2
1Department of Biochemistry, Robert Larner M.D. College of Medicine, University of Vermont, Colchester, Vermont.
Fibrin-bound thrombin remains stable under venous flow, showing resistance to antithrombin/heparin but susceptibility to dabigatran. This highlights the complex dynamics of thrombin inhibition within a fibrin matrix.
Area of Science:
- Biochemistry
- Hematology
- Biophysics
Background:
- Exosite-mediated thrombin binding to fibrin enhances clot stability.
- Fibrin-bound thrombin exhibits differential inhibition susceptibility to antithrombin/heparin versus active-site inhibitors.
- Fibrin possesses both low-affinity (Kd = 2.8 μM) and high-affinity (Kd = 0.15 μM) thrombin binding sites.
Purpose of the Study:
- To evaluate the stability of fibrin-bound thrombin under simulated venous flow conditions.
- To determine the accessibility and susceptibility of clot-bound thrombin to inhibition.
Main Methods:
- Development of a parallel-plate flow chamber to study thrombin adhesion to fibrin under venous shear rates (46-184 s⁻¹).
- Utilized a thrombin-specific fluorogenic substrate (SN-59) to monitor enzyme activity.
- Employed a mathematical model to simulate thrombin-fibrin interactions.
Main Results:
- A subpopulation of thrombin remained stably adhered to fibrin for over 30 minutes under venous flow.
- Thrombin adhesion was saturable at >500 nM and dependent on initial fibrinogen concentration.
- Antithrombin/heparin inhibited 72% of clot-bound thrombin, while dabigatran showed reversible inhibition (50-93%).
Conclusions:
- Clot-bound thrombin exhibits significant stability due to dynamic rearrangement within the fibrin matrix.
- The binding affinity and accessibility of thrombin to inhibitors are influenced by the fibrin structure.
- Understanding these interactions is crucial for developing targeted anticoagulant therapies.


