title: Proteolytic cleavage of protein S during the hemostatic response.
abstract: BACKGROUND
  Protein S is a vitamin K-dependent protein with anticoagulant properties. It contains a so-called thrombin-sensitive region (TSR), which is susceptible to cleavage by coagulation factor Xa (FXa) and thrombin. Upon cleavage, the anticoagulant activity of protein S is abolished.
  
  
  OBJECTIVE
  The aim of the present study was to determine whether protein S is cleaved within the TSR during activation of the coagulation system under near physiological conditions.
  
  
  RESULTS
  In a reconstituted coagulation system containing apart from protein S only procoagulant constituents and synthetic phospholipid vesicles, protein S was cleaved at Arg60 by the FXa generated (3 mol min(-1) mol(-1) enzyme). FXa-catalyzed cleavage of protein S, however, was inhibited by factor Va and prothrombin by more than 70%. During clotting of recalcified citrated plasma in the presence of a synthetic lipid membrane, no FXa-catalyzed proteolysis of protein S was observed. Substituting platelets for phospholipid vesicles resulted both in the reconstituted system and in plasma in cleavage of the TSR. Cleavage was at Arg60 and was observed upon platelet activation, irrespective of the presence of FXa (13 pmol min(-1) 10(-8) platelets). No cleavage by thrombin was observed in either the reconstituted coagulation system or clotting plasma.
  
  
  CONCLUSIONS
  These findings suggest that in vivo the anticoagulant activity of protein S is not down-regulated by FXa or thrombin during activation of coagulation. Our results rather suggest a role for a platelet protease in down-regulating the anticoagulant activity of protein S during the hemostatic response.
authors.affiliation: Department of Plasma Proteins, Sanquin Research, Amsterdam, The Netherlands. h.brinkman@sanquin.nl
authors.forename: H J M
authors.initials: HJ
authors.lastname: Brinkman
authors.forename: K
authors.initials: K
authors.lastname: Mertens
authors.forename: J A
authors.initials: JA
authors.lastname: van Mourik
mesh_terms: D001665:Binding Sites; D001777:Blood Coagulation; D001792:Blood Platelets / Q000166:cytology / Q000502:physiology; D002478:Cells, Cultured; D004730:Endothelium, Vascular / Q000166:cytology; D015951:Factor Xa / Q000378:metabolism*; D006487:Hemostasis; D006801:Humans; D007700:Kinetics; D008081:Liposomes / Q000494:pharmacology; D017293:Protein S / Q000378:metabolism* / Q000502:physiology; D013917:Thrombin / Q000378:metabolism*
chemical_list: D008081:Liposomes; D017293:Protein S; D013917:Thrombin; D015951:Factor Xa