1. The Arg-Gly-Asp (RGD) recognition site of platelet glycoprotein IIb- IIIa on nonactivated platelets is accessible to high-affinity macromolecules
- Author
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Yoshiaki Tomiyama, Tsubakio T, Rs, Piotrowicz, Kurata Y, Jc, Loftus, and Tj, Kunicki
- Subjects
Blood Platelets ,Binding Sites ,Platelet Aggregation ,Molecular Sequence Data ,Immunology ,Antibodies, Monoclonal ,Fibrinogen ,Platelet Membrane Glycoproteins ,Cell Biology ,Hematology ,Biochemistry ,Epitopes ,Immunoglobulin Fab Fragments ,Mice ,Immunoglobulin G ,Animals ,Humans ,Amino Acid Sequence ,Oligopeptides - Abstract
We have characterized a murine IgG monoclonal antibody, OP-G2, specific for platelet glycoprotein (GP) IIb-IIIa (alpha IIb beta 3). OP-G2 Fab fragments inhibit fibrinogen-mediated platelet aggregation and competitively inhibit adenosine diphosphate-induced binding of 125I- fibrinogen to washed platelets. OP-G2 binding to GPIIb-IIIa is specifically inhibited by RGD-containing peptides but not the fibrinogen gamma-chain carboxy-terminal peptide, and OP-G2 Fab fragments, like RGD-containing peptides, alter the conformation of GPIIb-IIIa resulting in the expression of a ligand-induced binding site (LIBS) recognized by PMI-1. OP-G2 fails to bind to the recombinant Cam variant of GPIIb-IIIa (alpha III beta 3Cam) wherein an Asp119 to Tyr119 substitution in GPIIIa abrogates the ability to recognize RGD. These data indicate that OP-G2 recognizes an epitope at or in very close proximity to the RGD recognition site of GPIIb-IIIa and that, in every aspect tested, OP-G2 behaves like a macromolecular RGD ligand. Interestingly, two-color flow cytometry shows that OP-G2 IgG can bind to nonactivated platelets. Quantitative binding assays indicate that nonactivated platelets bind approximately 50,000 125I-OP-G2 molecules/platelet. Furthermore, the affinity of OP-G2 for platelets activated with thrombin is roughly fivefold higher (nonactivated, kd = 24.8 nmol/L; activated, kd = 4.9 nmol/L). These results suggest that the RGD recognition site of GPIIb-IIIa is available to macromolecules that contain RGD even on nonactivated platelets, provided that the affinity of the ligand is adequate.
- Published
- 1992