Demonstration of variable forms of the platelet factor 4 immunoprecipitate using crossed immunoelectrophoresis

Proteins with different electrophoretic properties were precipitated by a monospecific antiserum to platelet factor 4 either as a "line" or as a "peak" precipitate. The "line" form seen on crossed immunoelectrophoresis of whole platelets was retained when immobilized thrombin was included in the intermediate gel. The retention was partially abolished when thrombin had been blocked at the active serine site or at the fibrinogen binding site. The "peak" form seen on analysis of material secreted from platelets passed unaffected through thrombin-Sepharose. It is suggested that platelet factor 4 exists in the platelets in a state different from that observed extracellularly after platelet secretion.