Interaction of lectins with human platelets. Effects on platelet stimulation by thrombin and ristocetin

Wheat germ agglutinin induced aggregation and secretion of fresh platelets. Aggregation, but not secretion of serotonin by platelets in plasma, by the lectin was inhibited by 5 mM EDTA. Further, the lectin-induced stimulation of fresh platelets was blocked by prostaglandin E1. Thus, this lectin stimulates platelets by a mechanism which closely mimics thrombin activation and is independent of intercellular crosslinking. Lentil lectin did not stimulate platelets. Each platelet contained about 6 . 10(-5) binding sites for the lectins with an apparent dissociation constant of 3.0 . 10(-7) M. Wheat germ agglutinin, which binds mainly to glycoprotein I (Mr 150 000), increased the subsequent binding of thrombin to fixed platelets while lentil lectin was without effect. It appears that thrombin and wheat germ agglutinin bind to independent but interacting sites. Wheat germ agglutinin, but neither thrombin nor lentil lectin, inhibited the agglutination of platelets by ristocetin. Further, rat platelets were not aggregated by either ristocetin or wheat germ agglutinin. It appears that the interaction sites of ristocetin and wheat germ agglutinin on platelets are overlapping.