title: Complex formation of platelet thrombospondin with plasminogen. Modulation of activation by tissue activator.
abstract: Thrombospondin (TSP), a multifunctional alpha-granule glycoprotein of platelets, binds fibrinogen, fibronectin, heparin, and histidine-rich glycoprotein and thus may play an important role in regulating thrombotic influences at vessel surfaces. In this study we have demonstrated that purified human platelet TSP formed a complex with purified human plasminogen (Plg). Complex formation was detected by rocket immunoelectrophoresis of mixtures of the purified radiolabeled proteins. Significant complex formation of fluid-phase Plg with adsorbed TSP was also demonstrated by enzyme-linked immunosorbent assay (ELISA). The complex formation was specific, saturable, and inhibited by excess fluid-phase TSP, with an apparent KD of approximately 35 nM. In both ELISA and rocket immunoelectrophoresis systems, complex formation was inhibited by 10 mM epsilon-amino-n-caproic acid, implying that there is a role for the lysine binding sites of Plg in mediating the interaction. TSP also formed a complex with plasmin as detected by ELISA but did not directly inhibit plasmin activity measured with a synthetic fluorometric substrate or with a 125I-fibrin plate assay. TSP, when incubated with Plg before addition to 125I-fibrin plates significantly inhibited the generation of plasmin activity by tissue plasminogen activator (TPA) in a manner that was calcium dependent. A kinetic study of Plg activation by TPA in the presence of TSP demonstrated that Michaelis-Menten kinetics were followed and that TSP acted as a noncompetitive inhibitor. These studies support the hypothesis that TSP, acting as a multifunctional regulator in focal areas of active hemostasis, could serve as a prothrombotic influence, leading to increased deposition of fibrin.
mesh_terms: D001665:Binding Sites; D001792:Blood Platelets / Q000378:metabolism*; D004789:Enzyme Activation; D005342:Fibrinolysis; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008239:Lysine; D046911:Macromolecular Substances; D010958:Plasminogen / Q000378:metabolism*; D010960:Plasminogen Activators / Q000378:metabolism*; D019699:Thrombospondins
chemical_list: D006023:Glycoproteins; D046911:Macromolecular Substances; D019699:Thrombospondins; D010958:Plasminogen; D010960:Plasminogen Activators; D008239:Lysine