Activation of coagulation factor V by a platelet protease

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Factor V (FV) is essential for prothrombin activation, requiring conversion to its active form, Factor V(a) (FVa).
• Previous research suggested human platelets contain both activated FV and an FV activator.

Purpose of the Study:

• To investigate the form in which Factor V is stored in platelets.
• To identify and characterize the platelet Factor V activator.
• To determine the binding characteristics of activated Factor V peptides to the platelet surface.

Main Methods:

• Platelets were disrupted (freezing/thawing, sonication) to release Factor V.
• Coagulation assays were used to measure Factor V activation.
• Purified Factor V was incubated with Factor V-deficient platelets and/or thrombin.
• Platelet Factor V activator activity was assessed using a thrombin inhibitor.
• Factor V proteolysis was examined using radiolabeled Factor V and thrombin-stimulated platelets.
• Subcellular fractionation was performed to localize the platelet protease.
• Binding of activated Factor V peptides to platelets was studied using radiolabeled peptides and displacement assays.

Main Results:

• Platelet-released Factor V is activated 3- to 10-fold by thrombin, indicating storage as a procofactor.
• A platelet Factor V activator, not inhibited by a thrombin inhibitor, was identified.
• No significant Factor V proteolysis was detected when thrombin-stimulated platelets were incubated with Factor V, suggesting limited release of platelet protease by thrombin.
• The platelet protease activating Factor V resides in the soluble fraction.
• Specific peptides (105,000, 87,000, and 78,000 M(r)) derived from Factor V(a) bind to the platelet surface.
• The 78,000 M(r) peptide exhibits high affinity binding and likely accounts for most FVa activity.

Conclusions:

• Human platelets store Factor V as an inactive procofactor.
• A distinct platelet protease, not thrombin, activates Factor V.
• Specific Factor V-derived peptides bind to platelet surfaces, contributing to prothrombin activation.
• The role of this platelet protease in hemostasis requires further investigation.