title: The heparin-binding exosite of factor IXa is a critical regulator of plasma thrombin generation and venous thrombosis.
abstract: The role of the factor IXa heparin-binding exosite in coagulation was assessed with mutations that enhance (R170A) or reduce (R233A) stability of the protease-factor VIIIa A2 domain interaction. After tissue factor (TF) addition to reconstituted factor IX-deficient plasma, factor IX R170A supported a 2-fold increase in velocity index (slope) and peak thrombin concentration, whereas factor IX R233A had a 4- to 10-fold reduction relative to factor IX wild-type. In the absence of TF, 5 to 100 pM of factor IXa increased thrombin generation to approach TF-stimulated thrombin generation at 100% factor IX. Factor IXa R170A demonstrated a 2- to 3-fold increase in peak thrombin concentration and 5-fold increase in velocity index, whereas the response for factor IXa R233A was blunted and delayed relative to wild-type protease. In hemophilia B mice, factor IX replacement reduced the average time to hemostasis after saphenous vein incision, and the time to occlusion after FeCl(3)-induced saphenous vein injury. At 5% factor IX, the times to occlusion for factor IX wild-type, R170A, and R233A were 15.7 minutes, 9.1 minutes (P </= .003), and more than 45 minutes. These data support the role of the factor IXa heparin-binding exosite as a critical regulator of coagulation and novel antithrombotic target.
authors.affiliation: Departments of Medicine/Hematology-Oncology and Pathology, University of Wisconsin-Madison, USA.
authors.forename: Yang
authors.initials: Y
authors.lastname: Buyue
authors.forename: Herbert C
authors.initials: HC
authors.lastname: Whinna
authors.forename: John P
authors.initials: JP
authors.lastname: Sheehan
mesh_terms: D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D015949:Factor IXa / Q000737:chemistry* / Q000378:metabolism; D002836:Hemophilia B / Q000235:genetics; D006487:Hemostasis; D006801:Humans; D051379:Mice; D008954:Models, Biological; D009154:Mutation; D017434:Protein Structure, Tertiary; D012501:Saphenous Vein / Q000293:injuries / Q000473:pathology; D013917:Thrombin / Q000096:biosynthesis* / Q000737:chemistry; D013997:Time Factors; D020246:Venous Thrombosis / Q000097:blood* / Q000235:genetics*
chemical_list: D015949:Factor IXa; D013917:Thrombin