Mechanism of inhibition by trinitrophenylation of the ristocetin cofactor activity of von Willebrand factor.

In the presence of ristocetin, von Willebrand factor is capable of agglutinating washed platelets. Modification of only a small percentage of amino groups of von Willebrand factor with trinitrobenzenesulfonic acid markedly inhibits this platelet agglutinating activity. 90% of the platelet agglutinating activity is lost after modification of only 10% of the von Willebrand factor amino groups. Since only the higher molecular weight forms of the heterogeneous von Willebrand factor polymers possess this platelet agglutinating activity, it was important to demonstrate that trinitrophenylation did not alter the degree of von Willebrand factor polymerization. This was accomplished by agarose gel electrophoresis. Subsequent direct binding and competitive binding studies demonstrated that trinitrophenylation markedly impairs the ability of von Willebrand factor to bind to the platelet surface. Thus the loss of platelet agglutinating activity upon modification of only a small fraction of the amino groups of von Willebrand factor is attributable to impaired binding of the modified von Willebrand factor to the platelet surface.