Heparin-binding platelet proteins demonstrated by crossed affinity immunoelectrophoresis

Platelet proteins that interact with heparin were studied using crossed affinity immunoelectrophoresis. Platelet proteins solubilized in Triton X-100 were applied to crossed immunoelectrophoresis against anti-platelet antibodies, and an intermediate gel containing heparin covalently linked to Sepharose 4B was inserted. Six immunoprecipitates were absent or showed an altered position compared to control immunoplates, indicating that the corresponding antigens were bound to the immobilized heparin. These precipitates represented platelet factor 4, thrombospondin, glycoprotein Ib, and three antigens termed G4, 17 and 25. The subcellular location of the heparin-binding proteins was either in the surface membrane (glycoprotein Ib and the antigens 17 and 25), or in the alpha-granules (platelet factor 4, thrombospondin and G4). both forms of platelet factor 4 appearing after crossed immunoelectrophoresis, i.e. a line-form and a peak-form, bound strongly to the heparin. Glycoprotein Ib showed a weak binding whereas its proteolytic split product glycocalicin did not significantly bind to the heparin in the present system. It is concluded that the platelets contain at least six heparin-binding proteins which are present on the cellular surface or are able to be exposed to the extracellular medium after the release-reaction has occurred.