Histidine-rich glycoprotein binding to activated human platelets.

Specific binding of purified histidine rich glycoprotein (HRGP) to human platelets stimulated with either bisdiazoniumbenzidine-crosslinked immunoglobulin G (BDB-IgG), with thrombin or with collagen was dose- and divalent cation dependent. A 5-10-fold increase of platelet bound 125I-HRGP was obtained when 0.5-0.8 x 10(9) platelets/ml were activated with 100 micrograms BDB-IgG/ml, 0.1 U thrombin/ml or 15 micrograms collagen/ml. At maximal binding tested 16,000 molecules of HRGP became bound per platelet, but saturation was not achieved. Such platelet inhibitors as acetylsalicylic acid, prostaglandin E1 and cytochalasin B reduced the capacity of platelets to bind ligand, and by kinetic experiments involving enzymatic digestion of radiolabelled bound HRGP the ligand revealed to remain surface bound rather than being taken up to inner parts of the cell.