Binding of bovine factor VIII-coated colloidal gold particles to receptors on platelet membranes

Bovine factor VIII/platelet aggregating factor was adsorbed into gold granules and the protein-gold complex added to either formalin-fixed or fresh washed human platelets. Following aggregation, binding of gold granules to the platelets was measured by monitoring the optical density of colloidal gold remaining in the supernatant. Scatchard analysis of binding data indicated that multiple classes of binding sites were present. The number of high affinity binding sites per formalin-fixed platelet depended on the concentration of ristocetin: 420 gold granules were calculated to bind at 1.4 mg/ml of ristocetin, 610 at 0.6 mg/ml of ristocetin and 875 when no ristocetin was added. Fresh washed platelets bound 1350 granules per cell in the absence of ristocetin. We conclude that during platelet aggregation, induced by bovine factor VIII, the binding sites on the platelet surface are only partially occupied.