title: Complex formation of platelet thrombospondin with histidine-rich glycoprotein.
abstract: Thrombospondin and histidine-rich glycoprotein are two proteins with diverse biological activities which have been associated with human platelets and other cell systems. Using an enzyme-linked immunosorbent assay, we have demonstrated that purified human platelet thrombospondin formed a complex with purified human plasma histidine-rich glycoprotein. The formation of the thrombospondin-histidine-rich glycoprotein complex was specific, concentration dependent, and saturable. Significant binding was detected when histidine-rich glycoprotein was incubated with thrombospondin immobilized on anti-thrombospondin IgG-coated plates, indicating that the observed complex formation was not due to a thrombospondin interaction with the plastic surface. Sucrose-density-gradient ultracentrifugation of a mixture of thrombospondin and histidine-rich glycoprotein also revealed the formation of fluid-phase complexes, with an estimated stoichiometry of 1 thrombospondin: 3.5 histidine-rich glycoprotein. Fibrinogen, which has been previously shown to bind to absorbed thrombospondin, did not inhibit the formation of the thrombospondin-histidine-rich glycoprotein complex. Histidine-rich glycoprotein complexed with thrombospondin was capable of binding heparin and neutralizing the anticoagulant activity of heparin in plasma. Specific complex formation between thrombospondin and histidine-rich glycoprotein may play a significant role in influencing platelet blood vessel wall interactions as well as modulating the association of various cells with the extracellular matrix.
mesh_terms: D001665:Binding Sites; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism*; D001798:Blood Proteins; D002450:Cell Communication; D002499:Centrifugation, Density Gradient; D004347:Drug Interactions; D004797:Enzyme-Linked Immunosorbent Assay; D005340:Fibrinogen / Q000502:physiology; D006023:Glycoproteins / Q000097:blood* / Q000378:metabolism; D006493:Heparin / Q000097:blood / Q000378:metabolism; D006801:Humans; D011506:Proteins / Q000378:metabolism; D019699:Thrombospondins; D000099053:Histidine-Rich Glycoprotein
chemical_list: D001798:Blood Proteins; D006023:Glycoproteins; D011506:Proteins; D019699:Thrombospondins; D000099053:Histidine-Rich Glycoprotein; D005340:Fibrinogen; D006493:Heparin