ABSTRACT
A mutant PAD-1 (D514-->Q) of the recombinant fragment PAD-1 comprising Leu469-Ser723 of the A1 domain of bovine von Willebrand factor (vWF) neither inhibited the binding of [125I]vWF to platelets nor the agglutination of human platelets induced by bovine vWF. PAD-1, on the other hand, inhibited human platelet agglutination induced by bovine vWF and [125I]vWF binding to human platelets. Collagen binding properties of the mutant, however, were indistinguishable from those of PAD-1. These results suggested that Asp514 within the A1 domain of vWF is required for interaction of bovine vWF with GPIb receptor on human platelets.
Subject(s)
Aspartic Acid , Platelet Membrane Glycoproteins/metabolism , von Willebrand Factor/chemistry , Animals , Binding Sites , Cattle , Collagen/metabolism , Computer Simulation , Electrochemistry , Models, Molecular , Mutagenesis , Point Mutation , Protein Conformation , Protein Structure, Secondary , Recombinant Proteins/metabolism , Structure-Activity Relationship , von Willebrand Factor/genetics , von Willebrand Factor/metabolismABSTRACT
Echicetin, a new protein isolated from Echis carinatus venom by reverse phase and ion exchange chromatography specifically inhibited agglutination of fixed platelets induced by several platelet glycoprotein Ib (GPIb) agonists, such as bovine von Willebrand factor (vWF), alboaggregins, and human vWF in the presence of botrocetin. Unlike alboaggregins, echicetin bound to GPIb but did not induce agglutination of washed or fixed platelets. In contrast to disintegrins, it did not block adenosine 5'-diphosphate (ADP)-induced platelet aggregation in the presence of fibrinogen. The apparent molecular weight of echicetin measured on sodium dodecyl sulfate (SDS) gel electrophoresis was 26 Kd under nonreducing conditions. On reduction, echicetin showed 16 and 14-Kd subunits suggesting that the molecule is a dimer. Reduced echicetin retained its binding activity and its inhibitory effect on the agglutination of fixed platelets induced by bovine vWF. 125I-echicetin bound to fixed platelets with high affinity (kd = 30 +/- 1.8 nmol/L) at 45,000 +/- 2,400 binding sites per platelet. The binding was selectively inhibited by a monoclonal antibody to the 45-Kd N-terminal domain of platelet GPIb, but not by monoclonal antibodies to other regions on GPIb. Binding of 125I-bovine vWF to fixed platelets was strongly inhibited by echicetin. In contrast, bovine vWF showed a much weaker inhibitory activity on binding of 125I-echicetin to platelets. The half life of echicetin in blood was approximately 170 minutes with no detectable degradation. Echicetin significantly prolonged the bleeding time of mice, suggesting that it may inhibit vWF binding to GPIb in vivo as well as in vitro.