title: Coagulation procofactor activation by factor XIa.
abstract: BACKGROUND
  In the extrinsic pathway, the essential procofactors factor (F) V and FVIII are activated to FVa and FVIIIa by thrombin. In the contact pathway and its clinical diagnostic test, the activated partial thromboplastin time (APTT) assay, the sources of procofactor activation are unknown. In the APTT assay, FXII is activated on a negatively charged surface and proceeds to activate FXI, which activates FIX upon the addition of Ca(2+). FIXa feeds thrombin generation through activation of FX. FIXa is an extremely poor catalyst in the absence of its FVIIIa cofactor, which, in the intrinsic FXase complex, increases FXa generation by approximately 10(7). One potential APTT procofactor activator in this setting is FXIa.
  
  
  OBJECTIVE
  To test the hypothesis that FXIa can activate FVIII and FV.
  
  
  METHODS
  Recombinant FVIII and plasma FV were treated with FXIa, and the activities and integrities of each procofactor were measured using commercial clotting assays and sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE).
  
  
  RESULTS
  Kinetic analyses of FXIa-catalyzed activation and inactivation of FV and FVIII are reported, and the the timing and sites of cleavage are defined.
  
  
  CONCLUSIONS
  FXIa activates both procofactors at plasma protein concentrations, and computational modeling suggests that procofactor activation during the preincubation phase of the APTT assay is critical to the performance of the assay. As the APTT assay is the primary tool for the diagnosis and management of hemophilias A and B, as well as in the determination of FVIII inhibitors, these findings have potential implications in the clinical setting.
authors.affiliation: Department of Biochemistry, University of Vermont College of Medicine, Burlington, VT, USA.
authors.forename: M F
authors.initials: MF
authors.lastname: Whelihan
authors.forename: T
authors.initials: T
authors.lastname: Orfeo
authors.forename: M T
authors.initials: MT
authors.lastname: Gissel
authors.forename: K G
authors.initials: KG
authors.lastname: Mann
mesh_terms: D001665:Binding Sites; D001780:Blood Coagulation Tests / Q000379:methods / Q000592:standards; D002384:Catalysis; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation; D005165:Factor V / Q000378:metabolism*; D005169:Factor VIII / Q000378:metabolism*; D015945:Factor XIa / Q000502:physiology*; D006801:Humans; D007700:Kinetics; D010314:Partial Thromboplastin Time / Q000379:methods / Q000592:standards
chemical_list: D005165:Factor V; D005169:Factor VIII; D015945:Factor XIa
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