Conformational changes in von Willebrand's factor protein: effects on the binding of factor VIII-coagulant

The effects of conformational changes in purified canine von Willebrand's factor (VWF) were investigated to explore the relationship between its factor VIII-related antigen (VIII:AG) and ristocetin cofactor (RCoF) properties and the factor VIII-coagulant (VIII:C) binding site(s). Binding of VIII:C from canine von Willebrand's disease (VWD) plasma by VWF was used to measure the combining reaction of these proteins. The VWF was denatured to varying degrees by exposure to temperature and pH extremes, low ionic strength, and 6 M urea. Various treatments resulted in three types of change: elimination of RCoF, VIIIR:Ag, and VIII:C binding, removal of RCoF activity alone; or elimination of RCoF and retarded elution of VIII:Ag and VIII:C. As long as VIIIR:Ag reactivity was maintained, binding of VIII:C could be demonstrated; but in the absence of VIIIR:Ag, neither RCoF activity nor VIII:C binding remained. These results suggest that VIII:C binding and RCoF sites are separate on VWF and that interaction between VIII:C and VWF is possible even after significant structural changes occur in VWF. Furthermore, RCoF is more vulnerable to denaturation than the antigenic site. The spectrum of conformational changes that affect the properties of VWF may parallel the various recognized subtypes of VWD.