Properties of the factor Xa binding site on human platelets

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Coagulation Factor Xa (FXa) plays a central role in the coagulation cascade.
• Platelets are known to accelerate coagulation reactions.
• The precise mechanism and significance of FXa interaction with platelets require further elucidation.

Purpose of the Study:

• To quantify the affinity of human FXa for activated platelets.
• To determine the impact of platelet binding on FXa enzymatic activity.
• To investigate the role of platelet-derived factors in FXa-platelet interaction.

Main Methods:

• Equilibrium binding studies using 125I-labeled FXa.
• Measurement of FXa enzymatic activity in prothrombin to thrombin conversion.
• Competition assays with related coagulation factors.
• Effect of calcium ions (Ca2+) on FXa-platelet interaction.
• Inhibition studies using an antibody against coagulation Factor V.

Main Results:

• High affinity (Ka = 3-4 x 10^10 M^-1) of FXa for thrombin-activated platelets.
• Platelet binding increases FXa activity by 300,000-fold in thrombin generation.
• Platelet-associated FXa activity exceeds that with optimal Factor V and phospholipids.
• Ca2+ is essential for FXa-platelet binding (optimum 2.5 mM).
• FXa bound to platelets is resistant to antithrombin III inactivation.
• An antibody against Factor V inhibits FXa binding and subsequent thrombin generation, implicating platelet Factor V.

Conclusions:

• Platelets provide a high-affinity binding site for coagulation Factor Xa.
• Platelet surface dramatically enhances FXa catalytic efficiency in thrombin formation.
• Platelet Factor V appears to mediate the interaction between FXa and activated platelets.
• This interaction is crucial for rapid thrombin generation and may be significant in hemostasis.