title: Platelet-binding of the von Willebrand factor.
abstract: The aggregation of platelets by the antibiotic, ristocetin, requires a plasma cofactor (VIII:vWF) and one or more specific binding sites on the platelet membrane. The interaction between VII:vWF and the platelet was examined using VIII:vWF labelled with 125I. In the presence of ristocetin (1.5 mg/ml), from 70 to 90% of the 125I-VIII:vWF became platelet-bound. By contrast, only 21% was bound with thrombin (2.5 microgram/ml), and 2.2% with buffer alone. Fractionation of the platelets revealed that peak radioactivity was present in the membrane fraction. Treatment of ristocetin-reacted platelets with either chymotrypsin, 100 microgram/ml, or trypsin, 75 microgram/ml, resulted in the partial release of the membrane-bound radioactivity. It is concluded that VIII:vWF binds to the platelet membrane in the presence of ristocetin.
mesh_terms: D001665:Binding Sites; D001779:Blood Coagulation Factors; D001792:Blood Platelets; D002462:Cell Membrane; D002918:Chymotrypsin / Q000494:pharmacology; D005169:Factor VIII / Q000378:metabolism; D006801:Humans; D007457:Iodine Radioisotopes; D010974:Platelet Aggregation; D012310:Ristocetin / Q000494:pharmacology; D014357:Trypsin / Q000494:pharmacology; D014841:von Willebrand Factor / Q000378:metabolism
chemical_list: D001779:Blood Coagulation Factors; D007457:Iodine Radioisotopes; D014841:von Willebrand Factor; D012310:Ristocetin; D005169:Factor VIII; D002918:Chymotrypsin; D014357:Trypsin