Phenotypic Comparison of Platelet- and Plasma-Derived Factor Va: Tyrosine Phosphorylation Differentiates the Two Cofactor Pools.

Factor Va, the essential cofactor for thrombin formation via the Prothrombinase complex, exists in two pools in whole blood: 75–80% is found in the plasma, while platelet alpha-granules contain the remaining 20–25%. Platelet-derived factor Va possesses several modifications not present in its plasma counterpart, such as an O-glycosylation on Thr⁴⁰², and is the more procoagulant molecule, exhibiting resistance to inactivation by activated protein C (APC) and plasmin when bound to the activated platelet membrane surface. As the platelet-derived cofactor pool originates from endocytosis of plasma-derived factor V by megakaryocytes, it can be hypothesized that, subsequent to its endocytosis, factor V is post-translationally modified, proteolytically processed, and packaged into alpha-granules to form the unique platelet-derived molecule. Platelet- and plasma-derived factor Va were purified from three individual donors. Platelets were isolated from whole blood and lysed with Triton X-100 in the presence of the tyrosine phosphatase inhibitors sodium orthovanadate and okadaic acid to prevent dephosphorylation by any released tyrosine phosphatases. Factor V in both the platelet lysates and the platelet-poor plasma was activated with thrombin, adsorbed onto a resin-coupled monoclonal antibody against human factor V, washed extensively, and eluted using 10% SDS. Western blotting analyses, using a monoclonal antibody against phosphotyrosine residues, revealed the presence of tyrosine phosphorylation on the light chain of plasma-derived, but not platelet-derived, factor Va. No such modification was observed on the heavy chain of either cofactor. Monoclonal antibodies against the heavy and light chains of factor V were used to verify that similar levels of factor V were present in all samples. This differential modification was observed in all three donors. Subsequent MALDI-TOF analyses of trypsin-digested plasma- and platelet-derived factor Va light chains were performed to identify the site(s) of tyrosine phosphorylation. These data identified phosphorylation of the C-terminal tyrosine residue (Tyr²¹⁹⁶) in plasma-derived, but not platelet-derived, factor Va. The phosphorylated fragment (residues 2188–2196, m/z = 1152.5) is consistently present in spectra of plasma-derived factor Va, while only the non-phosphorylated fragment (m/z = 1072.5) has been observed in the platelet-derived cofactor. To demonstrate that the phosphatase inhibitors used were effective, the platelets were lysed in the presence of added plasma-derived factor Va. Tyrosine phosphorylation, as determined by immunoblotting, was still present on the light chain, indicating that the dephosphorylation was occurring prior to platelet lysis. Tyrosine phosphorylation of other proteins has been implicated in regulation of endocytosis. Further experiments will need to be performed to determine if the modification has a similar role in megakaryocyte endocytosis of factor V.