HUMAN-PLATELET SURFACE BINDING OF ENDOGENOUS SECRETED FACTOR VIII-VONWILLEBRAND FACTOR AND PLATELET FACTOR-4

Abstract

Washed human platelets in buffers containing either 2 mM Ca2+ or 4 mM EDTA were stimulated by human .alpha.-thrombin to induce secretion. The binding of 2 endogenous secreted proteins, factor-VIII-related protein (VIII-R) (von Willebrand factor) and platelet factor 4, was measured by reacting thrombin-treated and control platelets with specific antibodies to these proteins, then quantifying antibody binding with 125I-staphylococcal protein A. Both of these granule proteins were associated with the platelet membrane surface by a Ca2+-dependent mechanism after thrombin-induced secretion. This ability to bind endogenous secreted proteins to the plasma membrane surface may provide a mechanism by which the platelet can concentrate and organize its secreted proteins for subsequent physiologic reactions.