Triton X-100 soluble proteins from 125I-labeled human platelets were studied by crossed immunoelectrophoresis employing a multispecific rabbit antibody raised against whole normal platelets. Emphasis was placed upon an analysis of immunoprecipitates containing 125I-labeled major membrane glycoproteins, and in particular, a prominent immunoprecipitate containing a glycoprotein antigen (s) previously designated as protein 16. SDS-polyacrylamide gel electrophoresis of protein 16 precipitated by a monospecific alloantibody. IgG L . . . , confirmed the presence of both glycoproteins IIb and IIIa. 125I-IgG L . . . , at concentration below that capable of precipitating protein 16 by itself, bound specifically to the precipitate containing protein 16 produced by the multispecific rabbit antibody. No other precipitates formed by the rabbit antibody contained either glycoprotein IIb or IIIa. When platelet proteins, incubated with optimum concentrations of ethylenediamine tetraacetic acid (EDTA) or ethyleneglycol bis (B- aminoethylether) NN1-tetraacetic acid (EGTA), were electrophoresed against the rabbit antibody, previously unobserved immunoprecipitates that contained either free glycoprotein IIb or free glycoprotein IIIa were detected. Upon readdition of excess Ca++, but not Mg++, to the same protein samples, a single immunoprecipitate containing both glycoproteins was once again observed. It is thus demonstrated that glycoproteins IIb and IIIa can form Ca++-dependent complexes (protein 16) in Triton X-100 extracts of normal platelets. The potential significance of the reversible association of these glycoproteins to normal platelet function is discussed.
In agreement with previous studies, we observed that incubation of washed human platelets with EDTA at 37 degrees C for short periods caused an irreversible loss of their aggregation response to adenosine diphosphate and markedly diminished their capacity to bind fibrinogen. AP-2 is a monoclonal antibody that reacts with a determinant specific to the glycoprotein (GP) IIb-IIIa complex. We now report that in a direct binding assay, the number of sites for AP-2 on platelets incubated with EDTA at 37 degrees C fell to approximately 30% of those present on control platelets. This effect of EDTA was not observed at room temperature. Analysis of the treated platelets by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed normal amounts of GP IIb and GP IIIa. However, studies using crossed immunoelectrophoresis with 125I-AP-2, 125I-Tab (anti-GP IIb), or 125I- AP-3 (anti-GP IIIa) in intermediate gels showed that at 37 degrees C, EDTA was inducing an irreversible change in GP IIb-IIIa complexes. A reduction in size and probable dissociation of the GP IIb-IIIa precipitate was accompanied by the appearance of precipitates having the characteristics of those given by free GP IIb and free GP IIIa and the location of a major new cathodal precipitate, which bound Tab and AP-3 but not AP-2. Membrane modifications associated with the loss of antigenic determinants on GP IIb-IIIa may explain EDTA-induced loss of platelet aggregability at 37 degrees C.
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