The binding of bovine platelet aggregating factor to human platelets

Bovine platelet-aggregating factor was labeled with 125I by the solid phase lactoperoxidase method with full retention of biological activity. The unlabeled platelet-aggregating factor was shown to compete with labeled platelet-aggregating factor in binding to fresh or formalin-fixed platelets. Scatchard saturation analysis revealed two classes of binding sites with dissociation constants of 0.4 and 6.5 nM and estimated 3,000 high affinity and 12,000 low affinity sites. Ristocetin (1.25 mg/ml) enhanced the binding of PAF to both washed and formalinized platelets by increasing the apparent number of both high and low affinity binding sites, while stimulating the rate of aggregation induced by platelet-aggregating factor on both washed and fixed platelets. Binding of platelet-aggregating factor to platelets was rapid with maximal binding at ten minutes. Rabbit antibody to platelet-aggregating factor formed a rapid (one minute) complex with platelet-aggregating factor which sedimented following ten minutes' centrifugtion at 8,700 g. This complex was soluble in antibody excess.