ИСТИНА

Background: Binding of factors X (fX) and Xa to activated platelets is important for the formation of prothrombinase and intrinsic tenase, respectively. The mechanism of this binding remains unclear. Aims: Aims of this work was investigate interaction of X or fXa with the membrane of activated platelets or purified phospholipid membranes Methods: Flow cytometry, surface plasmon resonance and mathematical modeling were used to investigate interaction of X or fXa with the membrane of activated platelets or purified phospholipid membranes. Confocal microscopy was used to study binding of fXa to platelets thrombi formed in flowing whole blood under shear conditions. Results: In agreement with previous reports, fX/fXa binding to phospholipids and procoagulant activated platelets was high-affinity and calcium-dependent, but unexpectedly only partially reversible. Dissociation of fX(fXa) was a two-step process ending with a plateau that was up to 10-fold greater than expected from association binding studies. In other words, the overall binding process was hysteresis-like, demonstrating a kind of ‘memory’. The same hysteresis-like kinetics was observed for annexin V, and the overall phenomenon was shown to be explained by trimerization of the protein monomers on the membrane. Examination of fXa binding to platelet aggregates formed in a parallel-plate flow chambers coated with collagen revealed that hysteresis phenomenon persisted in such realistic thrombi with naturally occurring fXa concentrations suggesting its physiological importance in preventing fXa wash-out by flow. Conclusion: Binding of fX (fXa) to either purified phospholipid membranes or activated platelets (either washed or in thrombi) had hysteresis- like behavior, related to multimerization of coagulation factor monomers on the phospholipid surface. Disclosure of Interest: None declared.