Studies on the initial reactive sites for platelet activation. Cationized ferritin- and wheat germ agglutinin-binding sites on the cells.

The binding sites for cationized ferritin or ferritin-conjugated wheat germ agglutinin on the cell surface were studied on platelets before or after fixation in glutaraldehyde. The effects of neuraminidase on the binding sites were also demonstrated after fixation of the platelets. Changes in the binding sites and distribution pattern due to exposure to these ligands were further investigated in the unfixed platelets under a variety of conditions such as incubation time and medium. The fixed platelets incubated with either ligand showed an even and continuous distribution of particles on the cell surface. In the unfixed platelets, the ligands were rapidly moved and aggregated probably by lateral migration after binding to the cell surface. The ligands were also bound to the membrane surface and simultaneously appeared in the interior of the open canalicular system. As the binding sites were moved on the cell surface as well as into the open canalicular system, morphological changes suggestive of platelet secretion occurred. The binding sites of either ligand were redistributed on the platelet cell surface. Glycoprotein Ib, thought to be the receptor site for wheat germ agglutinin on the cell surface, contains sialic acids that contribute to the negative charge of platelets. Therefore, glycoprotein Ib may play an important role as the initial reactive site for thrombotic stimuli.