title: Single-chain factor XII exhibits activity when complexed to polyphosphate.
abstract: BACKGROUND
  The mechanism underpinning factor XII autoactivation was originally characterized with non-physiological surfaces, such as dextran sulfate (DS), ellagic acid, and kaolin. Several 'natural' anionic activating surfaces, such as platelet polyphosphate (polyP), have now been identified.
  
  
  OBJECTIVE
  To analyze the autoactivation of FXII by polyP of a similar length to that found in platelets (polyP70 ).
  
  
  RESULTS
  PolyP70 showed similar efficacy to DS in stimulating autoactivation of FXII, as detected with amidolytic substrate. Western blotting revealed different forms of FXII with the two activating surfaces: two-chain αFXIIa was formed with DS, whereas single-chain FXII (scFXII; 80 kDa) was formed with polyP70 . Dissociation of scFXII from polyP70 abrogated amidolytic activity, suggesting reversible exposure of the active site. Activity of scFXII-polyP70 was enhanced by Zn(2+) and was sensitive to NaCl concentration. A bell-shaped concentration response to polyP70 was evident, as is typical of surface-mediated reactions. Reaction of scFXII-polyP70 with various concentrations of S2302 generated a sigmoidal curve, in contrast to a hyperbolic curve for αFXIIa, from which a Hill coefficient of 3.67 was derived, indicative of positive cooperative binding. scFXII-polyP70 was more sensitive to inhibition by H-d-Pro-Phe-Arg-chloromethylketone and corn trypsin inhibitor than αFXIIa, but inhibition profiles for C1-inhibitor were similar. Active scFXII-polyP70 was also able to cleave its physiological targets FXI and prekallikrein to their active forms.
  
  
  CONCLUSIONS
  Autoactivation of FXII by polyP, of the size found in platelets, proceeds via an active single-chain intermediate. scFXII-polyP70 shows activity towards physiological substrates, and may represent the primary event in initiating contact activation in vivo.
authors.affiliation: Faculty of Biological Sciences, University of Leeds, Leeds, UK.
authors.forename: R
authors.initials: R
authors.lastname: Engel
authors.forename: C M
authors.initials: CM
authors.lastname: Brain
authors.forename: J
authors.initials: J
authors.lastname: Paget
authors.forename: A S
authors.initials: AS
authors.lastname: Lionikiene
authors.forename: N J
authors.initials: NJ
authors.lastname: Mutch
mesh_terms: D000590:Amino Acid Chloromethyl Ketones / Q000737:chemistry; D000838:Anions / Q000737:chemistry; D001120:Arginine / Q000737:chemistry; D001792:Blood Platelets / Q000378:metabolism; D020134:Catalytic Domain; D016264:Dextran Sulfate / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry; D004610:Ellagic Acid / Q000737:chemistry; D004792:Enzyme Precursors / Q000737:chemistry; D005174:Factor XII / Q000737:chemistry*; D006487:Hemostasis; D006801:Humans; D007616:Kaolin / Q000737:chemistry; D010940:Plant Proteins / Q000737:chemistry; D011122:Polyphosphates / Q000737:chemistry*; D011288:Prekallikrein / Q000737:chemistry; D011485:Protein Binding; D012313:RNA / Q000737:chemistry; D013499:Surface Properties
chemical_list: D000590:Amino Acid Chloromethyl Ketones; D000838:Anions; D004220:Disulfides; D004792:Enzyme Precursors; D010940:Plant Proteins; D011122:Polyphosphates; C052160:trypsin inhibitor, Zea mays; D004610:Ellagic Acid; D007616:Kaolin; D012313:RNA; D005174:Factor XII; D016264:Dextran Sulfate; D011288:Prekallikrein; D001120:Arginine
keywords: blood coagulation; factor XII; hemostasis; polyphosphates; zymogens