Interaction of annexin V and platelets: Effects on platelet function and protein S binding

Intact annexin V but not its fragments bound specifically to platelets in the presence of calcium. Maximal binding observed was 0.87 pmole annexin V per 10⁸ platelets with an apparent dissociation constant of 2.13 × 10¹¹ M. This represents approximately 5000 binding sites per platelet. Platelet stimulation by thrombin increased the binding of annexin V by 40-fold. Addition of phospholipid vesicles or treatment with phospholipase C inhibited annexin V binding to platelets, suggesting that phospholipid is the binding site for annexin V and that phosphatidylcholine forms at least part of this site. Binding of annexin V did not cause platelet aggregation and did not affect thrombin or collagen induced aggregation. However annexin V significantly inhibited the binding of protein S to the platelet surface.