Human factor XII (Hageman factor) autoactivation by dextran sulfate. Circular dichroism, fluorescence, and ultraviolet difference spectroscopic studies.
The first event leading to the activation of the plasma kallikrein-kinin system is the surface-dependent conversion of factor XII to an active enzyme. Factor XII autoactivation was investigated using dextran sulfate as a soluble activating surface, and the significance of aggregation and the nature of the conformational change were examined by ultraviolet difference spectroscopy, fluorescence and circular dichroism. Results indicate that DS500 (500-kDa dextran sulfate) induces aggregation of factor XII. Analysis of the binding data suggests that 165-192 factor XII molecules can bind to one DS500 chain, while a 1:1 stoichiometry is observed with 5-kDa dextran sulfate. The interaction of factor XII and dextran sulfate is a biphasic process. It is initiated by a fast contraction of the molecule upon binding, as revealed by an apparent increase in organized secondary structures, and then followed by a slow relaxation process during cleavage and subsequent activation. Overall, the results are consistent with a model in which factor XII undergoes conformational changes upon binding to the activating surface. The rapidity of autoactivation in the presence of DS500, as opposed to 5-kDa dextran sulfate, implies that aggregation provides a special mechanism whereby proteolytic cleavage is accomplished efficiently when factor XII molecules are bound side by side on the DS500 molecule.
Samuel
M
M
Department of Biochemistry and Molecular Biology, New York Medical College, Valhalla 10595.
Pixley
R A
RA
Villanueva
M A
MA
Colman
R W
RW
Villanueva
G B
GB
D002942:Circular Dichroism; D016264:Dextran Sulfate / Q000378:metabolism*; D004789:Enzyme Activation; D005174:Factor XII / Q000378:metabolism* / Q000648:ultrastructure; D006801:Humans; D066298:In Vitro Techniques; D046911:Macromolecular Substances; D011485:Protein Binding; D011487:Protein Conformation; D012542:Scattering, Radiation; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet
D046911:Macromolecular Substances; D005174:Factor XII; D016264:Dextran Sulfate