Platelet Membrane Glycoproteins Implicated in Ristocetin-Induced Aggregation

Ristocetin induces aggregation of platelets in the presence of Willebrand Factor. Certain preparations of ristocetin when preincubated with washed normal platelets render the platelets less responsive to the addition of Willebrand Factor. It has been demonstrated that such preparations are contaminated with a proteolytic enzyme(s). Platelets treated in this way with different ristocetin preparations were analysed using disc gel electrophoresis. Membrane glycoproteins I and II were depleted only when using contaminated ristocetin preparations. Platelets labelled with 3H-adenine when treated with this ristocetin did not demonstrate leakage of 3H-labelled material, thus showing that the platelet membrane remained intact. Platelets were treated with proteolytic enzymes of known specificities and following further washing were tested in an aggregating system consisting of ristocetin and Willebrand Factor. The aggregation response was markedly reduced compared to control platelets. These results point to the glycoproteins of the platelet membrane being part of the receptor for the ristocetin-Willebrand Factor complex.