The binding of phytohemagglutinins to human platelet plasma membranes.

The binding of phytohemagglutinins from Phaseolus vulgaris (erythroagglutinating phytohemagglutinin (E-PHA) and leukoagglutinating phytohemagglutinin (L-PHA)) and from Lens culinaris (lentil-PHA) to human platelet cell surfaces has been demonstrated. Each platelet binds on an average 500,000 to 600,000 molecules of E-PHA with an apparent dissociation constant of 0.5 X 10m7 M. These values were 250,000 to 350,000 molecules bound for L-PHA and 300,000 to 400,000 molecules bound for lentil-PHA with dissociation constants of 4 X lo-’ M and 1.4 X 10e7 M, respectively. Both E-PHA and lentil-PHA can be released from the platelet surface using appropriate oligosaccharide haptene inhibitors, indicating that the phytohemagglutinins do not enter the cells. When E-PHA and L-PHA bind to platelets the cells are aggregated, adenylate cyclase is inhibited, and a protein designated as thrombin-sensitive protein is released from the particulate fraction of the cell. Thus, binding of these compounds to the platelet surface mimics thrombin-induced aggregation and the release reaction. Lentil-PHA binds tightly to platelets but the cells do not aggregate and there is no inhibition of adenylate cyclase or release of thrombin-sensitive protein. Thrombin induces platelet aggregation, adenylate cyclase inhibition, and thrombin-sensitive protein release even when platelets are saturated with lentil-PHA. After incubation of platelets with either thrombin or L-PHA, there is an apparent Z-fold increase in the number of receptor sites for lentil-PHA, suggesting that these compounds produce a conformational change in the platelet surface exposing increased numbers of lentil-PHA receptor sites.