title: Zn²(+) -containing protein S inhibits extrinsic factor X-activating complex independently of tissue factor pathway inhibitor.
abstract: BACKGROUND
  Protein S (PS) has direct anticoagulant activity, independently of activated protein C (APC). The mechanisms underlying this activity remain unclear, because PS preparations differ in activity, giving rise to conflicting results. Some purification procedures result in loss of intramolecular Zn²(+) , which is essential for inhibition of prothrombinase.
  
  
  OBJECTIVE
  To investigate the inhibition of extrinsic factor (F)Xase by Zn²(+) -containing PS.
  
  
  METHODS
  Purified component extrinsic FXase assays were used to determine FXa generation in the presence and absence of PS and/or tissue factor pathway inhibitor (TFPI). Binding assays, immunoblots and thrombin generation assays in plasma supported the FXase data.
  
  
  RESULTS
  Zn²(+) -containing PS potently inhibited extrinsic FXase in the presence of saturating phospholipids, independently of TFPI, whereas inhibition of extrinsic FXase by Zn²(+) -deficient PS required TFPI. Immunoblots for FXa and functional assays showed that Zn²(+) -containing PS inhibited primarily the quantity of FXa formed by tissue factor (TF)-FVIIa, rather than FXa amidolytic activity. Zn²(+) -containing PS, but not Zn²(+) -deficient PS, bound to TF with high affinity (K(dapp) = 41 nm) and targeted TF function. Binding of PS to FVIIa was negligible, whereas PS showed appreciable binding to FX. Increasing FX concentrations 10-fold reduced PS inhibition five-fold, suggesting that PS inhibition of FXase is FX-dependent. PS also exhibited TFPI-independent and APC-independent anticoagulant activity during TF-initiated thrombin generation in plasma.
  
  
  CONCLUSIONS
  PS that retains native Zn²(+) also retains anticoagulant functions independently of APC and TFPI. Inhibition of extrinsic FXase by PS at saturating levels of phospholipids depends on PS retention of intramolecular Zn²(+) , interaction with FX, and, particularly, interaction with TF.
authors.affiliation: Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, CA, USA.
authors.forename: N
authors.initials: N
authors.lastname: Fernandes
authors.forename: L O
authors.initials: LO
authors.lastname: Mosnier
authors.forename: L
authors.initials: L
authors.lastname: Tonnu
authors.forename: M J
authors.initials: MJ
authors.lastname: Heeb
mesh_terms: D000925:Anticoagulants / Q000737:chemistry / Q000627:therapeutic use; D005170:Factor X / Q000737:chemistry*; D015951:Factor Xa / Q000737:chemistry; D006801:Humans; D007700:Kinetics; D008074:Lipoproteins / Q000737:chemistry*; D010743:Phospholipids / Q000737:chemistry; D011485:Protein Binding; D011486:Protein C / Q000737:chemistry; D017293:Protein S / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry; D013925:Thromboplastin / Q000737:chemistry; D013997:Time Factors; D015032:Zinc / Q000737:chemistry / Q000494:pharmacology*
chemical_list: D000925:Anticoagulants; D008074:Lipoproteins; D010743:Phospholipids; D011486:Protein C; D017293:Protein S; D011994:Recombinant Proteins; C051928:lipoprotein-associated coagulation inhibitor; D005170:Factor X; D013925:Thromboplastin; D015951:Factor Xa; D015032:Zinc