Binding of thrombin-activated human factor VIII to platelets.

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Factor VIII is a crucial protein in the blood coagulation cascade.
• Platelets play a significant role in hemostasis and thrombosis.
• Understanding the interaction between factor VIII and platelets is vital for comprehending coagulation.

Purpose of the Study:

• To investigate the association of factor VIII with platelets.
• To determine how platelet activation affects factor VIII binding.
• To explore the role of factor VIII binding in the formation of coagulation complexes on platelet surfaces.

Main Methods:

• Iodine-125 labeling of purified factor VIII using the Bolton-Hunter reagent.
• SDS-polyacrylamide gel electrophoresis and autoradiography to analyze factor VIII polypeptides.
• Quantification of factor VIII binding to platelets using radiolabeled factor VIIIa.
• Assessment of binding in the presence of thrombin, antibodies, and platelet agonists.

Main Results:

• Activated factor VIII (VIIIa) shows distinct polypeptide changes upon thrombin activation.
• Platelets bind factor VIIIa, with binding increasing significantly (3-15 fold) upon thrombin activation.
• Platelet binding of factor VIIIa is not saturated at high concentrations and is independent of certain platelet glycoproteins.
• Binding of factor VIIIa to platelets is normal in a patient with deficient factor Xa binding, suggesting distinct mechanisms.

Conclusions:

• Platelets actively bind activated factor VIII (VIIIa).
• Platelet activation significantly enhances factor VIIIa binding.
• The binding of factor VIIIa alone does not solely determine the assembly of active proteolytic complexes on the platelet surface.