title: Residual factor VIII-like cofactor activity of thioredoxin and related oxidoreductases.
abstract: BACKGROUND
  Factor VIII is the cofactor for Factor X activation by Factor IXa. Activated Factor X, Factor Xa, in turn activates prothrombin in a sequence that leads to fibrin clot formation at the site of vascular injury. Although the biochemistry of the cascade has been well studied, the molecular mechanism underlying the cofactor role of Factor VIII is not understood.
  
  
  METHODS
  We screened a bacterial peptide display library with Factor IXa and Factor X co-immobilized on tosylactivated Dynabeads which were then used as platelet surrogates. Validation of peptide selection procedure and comparison of Factor VIII-like cofactor activity of oxidoreductases was performed using COATEST assays. Determination of Factor VIII as a folding catalyst with potential disulphide isomerase activity was determined using the RNase A renaturation assay.
  
  
  RESULTS
  We set out to identify the cofactor requirements of the Factor IXa/Factor X procoagulant complex by random peptide display, and isolated a peptide with the active-site sequence, CGPC, of thioredoxin. This peptide was able to activate Factor X in a Factor IXa-dependent manner. Redox catalysts or oxidoreductases with homologous active-site vicinal cysteines such as PDI and DsbA also mimicked Factor VIII in their requirement of Factor IXa in Factor X activation. However, the cofactor activity of these peptides was up to a 1000-fold lower than that of Factor VIII and they were therefore unable to catalyse blood coagulation. Factor X activation by PDI and by Factor VIII was abolished by oxidation in an isolated system, which implies a possible role for thiol-disulphide exchange in the activity of the tenase complex. Using scrambled RNase A as a surrogate substrate, we also found that Factor VIII could renature this enzyme.
  
  
  CONCLUSIONS
  Our findings suggest that Factor VIII may be a specialized folding catalyst with disulphide isomerase activity. We suggest that it is this activity that may underlie its cofactor function in Factor X activation, and that this function is interchangeable with classical oxidoreductases.
  
  
  CONCLUSIONS
  The possible involvement of thiol-disulphide interchange as a mechanism underlying Factor VIII cofactor activity may provide some insight into the biochemistry of the intrinsic tenase complex.
authors.affiliation: Department of Haematology, Royal Free and University College Medical School, University College London, Hampstead Campus, Rowland Hill Street, London NW3 2PF, UK. bayele@biochem.ucl.ac.uk
authors.forename: Henry K
authors.initials: HK
authors.lastname: Bayele
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Murdock
authors.forename: K John
authors.initials: KJ
authors.lastname: Pasi
mesh_terms: D001777:Blood Coagulation; D001792:Blood Platelets / Q000502:physiology; D002384:Catalysis; D003067:Coenzymes / Q000378:metabolism; D004220:Disulfides / Q000378:metabolism; D005169:Factor VIII / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D015951:Factor Xa / Q000235:genetics / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D017510:Protein Folding; D011994:Recombinant Proteins / Q000378:metabolism; D013879:Thioredoxins / Q000235:genetics / Q000378:metabolism*
chemical_list: D003067:Coenzymes; D004220:Disulfides; D011994:Recombinant Proteins; D013879:Thioredoxins; D005169:Factor VIII; D010088:Oxidoreductases; D015951:Factor Xa; D005978:Glutathione