Fibrinogen-Binding Properties of the Human Platelet Glycoprotein IIb–IIIa Complex: A Study Using Crossed-Radioimmunoelectrophoresis

Abstract

Fibrinogen-binding platelet proteins have been examined by crossed-immunoelectrophoresis of solubilized, washed platelets followed by the incubation of the immunoplates with /sup 125/I-fibrinogen and exposure to x-ray films. Incubation with 0.1 mg/ml of /sup 125/I-fibrinogen revealed the binding of fibrinogen to the immunoprecipitates representing the glycoprotein IIb-IIIa complex, factor XIIIa chain, a granule membrane protein termed G4, fibrinogen, and albumin. Only the glycoprotein IIb-IIIa precipitate and the fibrinogen precipitate showed significant binding when the concentration of /sup 125/I-fibrinogen was lowered to 0.01 mg/ml. Thi indicates that the binding of fibrinogen is specific. The binding of /sup 125/I-fibrinogen to the precipitates representing the glycoprotein IIb-IIIa complex, the factor XIIIa chain, and G4, but not to the albumin precipitate, was significantly lowered in the presence of EDTA. This effect of EDTA increased with increasing pH, with no binding at pH 8.7. The results indicate that the glycoprotein IIb-IIIa complex, but not the separate glycoproteins IIb and IIIa, can act as Ca/sup 2 +/ or Mg/sup 2 +/-dependent fibrinogen receptor, under proper physiologic conditions.