Lysine Binding Sites Of Plasminogen Research Articles

The use of tranexamic acid in reducing bleeding complications

Abstract Tranexamic acid inhibits fibrinolysis by competitively blocking the lysine binding sites of plasminogen, inhibiting binding between fibrin and plasminogen, and activation of plasminogen. It also competitively inhibits tissue plasminogen activator and inhibits plasmininduced platelet activation. The synthesis of tranexamic acid was first reported in 1962. It is used to treat or prevent excessive blood loss from trauma, surgery, and in various medical conditions including haemophilia and heavy menstrual bleeding. As a medicine affecting coagulation, it is listed in the World Health Organization’s List of Essential Medicines. Tranexamic acid remains a versatile and inexpensive agent with potential benefit in health services from the lowest to highest income countries. This pharmacy review focuses on the evidence base to support the use of tranexamic acid in acquired and inherited bleeding disorders to reduce bleeding complications.

Residues Essential for Plasminogen Binding by the Cation-Independent Mannose 6-Phosphate Receptor

The 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR) is a multifunctional protein that binds diverse intracellular and extracellular ligands with high affinity. The CI-MPR is a receptor for plasminogen, and this interaction can be inhibited by lysine analogues. To characterize the molecular basis for this interaction, surface plasmon resonance (SPR) analyses were performed using truncated forms of the CI-MPR and plasminogen. The results show that the N-terminal region of the CI-MPR containing domains 1 and 2, but not domain 1 alone, of the receptor's 15-domain extracytoplasmic region binds plasminogen (K(d) = 5 +/- 1 nM) with an affinity similar to that of the full-length receptor (K(d) = 20 +/- 6 nM). In addition to its C-terminal serine protease domain, plasminogen contains lysine binding sites (LBS), which are located within each of its five kringle domains, except kringle 3. We show that kringles 1-4, but not kringles 1-3, bind the CI-MPR, indicating an essential role for the LBS in kringle 4 of plasminogen. To identify the lysine residue(s) of the CI-MPR that serve(s) as an essential determinant for recognition by the LBS of plasminogen, site-directed mutagenesis studies were carried out using a construct encoding the N-terminal three domains of the CI-MPR (Dom1-3His) which contains both a mannose 6-phosphate (Man-6-P) and plasminogen binding site. The results demonstrate two lysine residues (Lys53 located in domain 1 and Lys125 located in the loop connecting domains 1 and 2) of the CI-MPR are key determinants for plasminogen binding but are not required for Man-6-P binding.

Interaction with human plasminogen system turns on proteolytic activity in Streptococcus agalactiae and enhances its virulence in a mouse model

Interactions of several microbial pathogens with the plasminogen system increase their invasive potential. In this study, we show that Streptococcus agalactiae binds human plasminogen which can be subsequently activated to plasmin, thus generating a proteolytic bacterium. S. agalactiae binds plasminogen via the direct pathway, using plasminogen receptors, and via the indirect pathway through fibrinogen receptors. The glyceraldehyde-3-phosphate dehydrogenase is one of the S. agalactiae proteins that bind plasminogen. Presence of exogenous activators such as uPA and tPA are required to activate bound plasminogen. Results from competitive inhibition assays indicate that binding is partially mediated through the lysine binding sites of plasminogen. Following plasminogen binding and activation, S. agalactiae is able to degrade in vitro fibronectin, one of the host extracellular matrix proteins. Moreover, incubation of S. agalactiae with either plasminogen alone, or plasminogen plus fibrinogen, in the presence of tPA enhanced its virulence in C57BL/6 mice, suggesting that acquisition of plasmin-like activity by the bacteria increase their invasiveness.

Induction of catalytic activity of plasminogen by monoclonal antibody IV-Ic in the presence of divalent metal cations and α2-antiplasmin

Investigation of the influence of divalent metal cations on the induction of plasminogen catalytic activity by monoclonal antibody IV-Ic showed that the presence of metal cations in the reaction medium changes the induction by slowing down or accelerating the process. Ions of Zn(2+), Mn(2+), and Cu(2+) completely inhibit activation. Ions of Co(2+) and Ni(2+) decrease the rate of the first and second phases of the reaction more than 2 times. Ca(2+) ions do not have any effect on the activation rate. Ions of Mg(2+), Ba(2+), and Sr(2+) increase the rate of the first phase of the reaction by 1.5, 2.0, and 2.0 times and the rate of the second phase by 2.0, 3.8, and 4.7 times, correspondingly. Sr(2+) ions have the strongest stimulating effect on plasminogen activation by monoclonal antibody IV-Ic. Investigation of the dose dependent effect of Sr(2+) on the rate of plasminogen activation by monoclonal antibody IV-Ic showed stimulating effect of Sr(2+) at concentrations from 0.1 to 1.0 mM with half maximum at 0.6 mM. However, Sr(2+) ions do not affect amidolytic activity of plasmin and activation of plasminogen by streptokinase. Sr(2+) ions also do not affect monoclonal antibody IV-Ic binding to plasminogen. The effect of Sr(2+) is specific and mediated by the IV-Ic component. The presence of metal cations affects conformational changes in the process of active site formation. Metal cations also affect structure of the plasminogen molecule active site in the complex with monoclonal antibody IV-Ic and enzyme-substrate interaction. The effect of alpha(2)-antiplasmin on the induction of plasminogen catalytic activity by monoclonal antibody IV-Ic in range of concentrations from 5 to 30 nM has been studied. alpha(2)-Antiplasmin at concentration 30 nM almost completely inhibits induction of plasminogen catalytic activity by monoclonal antibody IV-Ic at the ratio plasminogen/alpha(2)-antiplasmin of 3 : 1. This can be explained by competition of alpha(2)-antiplasmin and monoclonal antibody IV-Ic for the lysine-binding sites of plasminogen and inhibition of the active center in activated complex plasminogen*-mAB IV-Ic. Divalent metal cations and alpha(2)-antiplasmin are important factors in induction of plasminogen catalytic activity by monoclonal antibody IV-Ic.

A central role for plasminogen in the inflammatory response to biomaterials

The inflammatory response to implanted biomaterials severely limits their deployment in patients. Plasminogen has been shown to play a central role in cell migration, and therefore could regulate this inflammatory response. We sought to determine if plasminogen influences recruitment of inflammatory cells to a biomaterial implanted into plasminogen-deficient (Plg(-/-)) mice. Small disks of polyethylene terephthalate, a material used in vascular grafts, were surgically implanted into the peritoneum of wild-type and Plg(-/-) mice. Recruitment of neutrophils and monocytes/macrophages into the peritoneum and onto the disks was measured, primarily at 18 h. Monocyte/macrophage recruitment was markedly blunted in Plg(-/-) mice compared with wild-type mice. Unexpectedly, neutrophil recruitment was also markedly decreased in the Plg(-/-) mice. While recruitment of leukocytes into the peritoneum was plasminogen-dependent, the adhesion of the emigrating cells to the implants was not. In contrast, adhesion but not recruitment was reduced in fibrinogen-deficient mice. Reconstitution of Plg(-/-) mice with intravenous or intraperitoneal plasminogen differentially restored monocyte/macrophage and neutrophil recruitment. Tranexamic acid, an inhibitor of the lysine binding sites of plasminogen, suppressed leukocyte recruitment in wild-type mice, but aprotinin, a plasmin inhibitor, did not. Plasminogen exerts a marked influence on both neutrophil and monocyte/macrophage recruitment to implanted biomaterials. This role is distinct from that of fibrinogen, and the two inflammatory cell types use plasminogen in different ways. Plasminogen represents a therapeutic target for controlling the inflammatory response to implanted materials.

Plasminogen interaction and activation on Streptococcus mutans surface.

A number of pathogenic microorganisms have been previously shown to bind plasminogen. The subsequent activation of plasminogen into plasmin can contribute to their virulence. In this study, we have shown that Streptococcus mutans is able to bind both human plasminogen and plasmin. Binding of plasminogen to S. mutans was inhibited by L-lysine and epsilon-aminocaproic acid, indicating that binding is mediated via lysine-binding sites of plasminogen. S. mutans enhanced the activation of plasminogen by tissue plasminogen activator but not by urokinase. This enhancement turned out to be dependent on cell concentration. Zymogram analysis showed that the plasmin activity acquired after plasminogen binding and activation is the most important proteolytic activity in the strain tested. These results suggest a mechanism involving acquisition of a host protease that might contribute to the infective process of this microorganism.

Anti-plasminogen autoantibodies from plasma of patients with systemic lupus erythematosus having anti-phospholipid antibody syndrome: isolation and some immunochemical properties.

Blood plasma samples from patients with systemic lupus erythematosus having the anti-phospholipid antibody syndrome were found to contain anti-plasminogen antibodies of the IgG class. The titers of anti-plasminogen autoantibodies of the IgG class were elevated in these patients compared with normal controls. Part of the pool of IgG anti-plasminogen antibodies reacts with an epitope in the lysine-binding sites of plasminogen. Anti-plasminogen IgG isolated from patients' blood plasma is specific only for a native epitope of human plasminogen passively adsorbed on immunosorbent micro-titration plate. As shown by enzyme immunoassay, autoantibodies to plasminogen of the IgG class cross-react with human fibrinogen.

The blockage of the high-affinity lysine binding sites of plasminogen by EACA significantly inhibits prourokinase-induced plasminogen activation

Prourokinase-induced plasminogen activation is complex and involves three distinct reactions: (1) plasminogen activation by the intrinsic activity of prourokinase; (2) prourokinase activation by plasmin; (3) plasminogen activation by urokinase. To further understand some of the mechanisms involved, the effects of epsilon-aminocaproic acid (EACA), a lysine analogue, on these reactions were studied. At a low range of concentrations (10–50 μM), EACA significantly inhibited prourokinase-induced (Glu-/Lys-) plasminogen activation, prourokinase activation by Lys-plasmin, and (Glu-/Lys-) plasminogen activation by urokinase. However, no inhibition of plasminogen activation by Ala 158-prourokinase (a plasmin-resistant mutant) occurred. Therefore, the overall inhibition of EACA on prourokinase-induced plasminogen activation was mainly due to inhibition of reactions 2 and 3, by blocking the high-affinity lysine binding interaction between plasmin and prourokinase, as well as between plasminogen and urokinase. These findings were consistent with kinetic studies which suggested that binding of kringle 1–4 of plasmin to the N-terminal region of prourokinase significantly promotes prourokinase activation, and that binding of kringle 1–4 of plasminogen to the C-terminal lysine 158 of urokinase significantly promotes plasminogen activation. In conclusion, EACA was found to inhibit, rather than promote, prourokinase-induced plasminogen activation due to its blocking of the high-affinity lysine binding sites on plasmin(ogen).

114 The effect of argatroban on staphylokinase activity in the presence of endothelial cell injured by thrombin

When Glu-plasminogen (Glu-plg) was incubated with urokinase (UK) in the presence of various concentrations of tranexamic acid and S-2251, the hydrolysis of S-2251 increased in the presence of 1 mM of tranexamic acid, but decreased in the presence of more than 10 mM of tranexamic acid. Use of SDS-polyacrylamide gel electrophoresis indicated: 1) Glu-plg was converted to plasmin better in the presence of 1 mM of tranexamic acid, but in the presence of more than 10 mM of tranexamic acid it was less converted. 2) Plg I (of higher molecular weight) was more easily converted than plg II by UK, but binding of tranexamic acid with lysine binding sites of plasminogen caused almost equal rate of activation of plg I and II.

112 Effect of heparin and hirudin on endotoxin-induced PAI-1 mRNA expression by human umbilical vein endothelial cells

When Glu-plasminogen (Glu-plg) was incubated with urokinase (UK) in the presence of various concentrations of tranexamic acid and S-2251, the hydrolysis of S-2251 increased in the presence of 1 mM of tranexamic acid, but decreased in the presence of more than 10 mM of tranexamic acid. Use of SDS-polyacrylamide gel electrophoresis indicated: 1) Glu-plg was converted to plasmin better in the presence of 1 mM of tranexamic acid, but in the presence of more than 10 mM of tranexamic acid it was less converted. 2) Plg I (of higher molecular weight) was more easily converted than plg II by UK, but binding of tranexamic acid with lysine binding sites of plasminogen caused almost equal rate of activation of plg I and II.

Acceleration of Plasminogen Activation by Tissue Plasminogen Activator on Surface-bound Histidine-proline-rich Glycoprotein

Histidine-proline-rich glycoprotein (HPRG), also known as histidine-rich glycoprotein, is a major plasminogen-binding protein. In this work we characterized extensively the circumstances under which HPRG accelerates plasminogen activation and the specificity of this effect. Soluble HPRG did not significantly influence plasminogen activation. In contrast, native HPRG bound to hydrazide or nickel chelate surfaces strongly stimulated the activation of plasminogen by tissue plasminogen activator, but not by urokinase or streptokinase. The efficiency of activation on surface-bound HPRG was increased for Glu-plasminogen (41-fold), Lys-plasminogen (17-fold), and cross-linked Glu-plasminogen (11-fold) but not for mini-plasminogen, and was mainly due to a decrease in the apparent Km. A reduced susceptibility to inhibition by chloride ions contributed to the higher activation rate of Glu-plasminogen on an HPRG surface. The immobilized N- and C-terminal domains, but not the histidine-proline-rich domain of HPRG, also bound plasminogen and stimulated its activation. HPRG-enhanced plasminogen activation was proportional to the quantity of HPRG immobilized and was abolished by anti-HPRG antiserum, by low concentrations of epsilon-aminocaproic acid, by methylation of lysine residues in HPRG, and by treatment of HPRG with carboxypeptidase B. Soluble HPRG and a plasminogen fragment, kringle 1-2-3, acted as competitive inhibitors by binding to plasminogen and immobilized HPRG, respectively. The interaction of the conserved C-terminal lysine of HPRG with the high affinity lysine binding site of plasminogen is necessary and sufficient to accelerate plasminogen activation. Unlike other stimulators of plasminogen activation, the effect of HPRG on fibrinolysis is modulated by factors that influence the equilibrium between solution and surface-bound HPRG.

Binding of plasminogen to corneal fibroblasts and their extracellular matrix. Evidence for a receptor in cell membranes

Several components of the fibrinolytic system play an important role in tissue remodelling after wounding of the cornea. Secretion of tissue-type and urokinase-type plasminogen activators by corneal fibroblasts has been described previously. In this work, we report the existence of a plasminogen binding protein on the surface of cultured corneal fibroblasts and in the extracellular matrix produced by these cells. This receptor was characterized on ligand-blots and localized by fluorescence cytochemistry using biotinylated plasminogen and labelled streptavidin. Plasminogen binding to cells was specific, saturable, dose dependent and inhibited by ε-aminocaproic acid, indicating that the lysine binding sites of plasminogen are involved in its interaction with the cells and extracellular matrix. The plasminogen bound to corneal fibroblasts and their matrix was activated by t-PA secreted by the corneal fibroblasts themselves. The receptor was characterized by ligand blotting using biotinylated plasminogen. It was insoluble in water solution, but it could be solubilized in 1% SDS or 0.1% Triton X-100. The receptor was purified by affinity chromatography on biotinylated plasminogen coupled to a streptavidin-agarose gel.

Complement component C7 is a plasminogen-binding protein.

Ab deposition, whether by reaction with the specific Ag or by preformed immune complexes, is followed by activation and deposition of complement components. Tissue destruction is observed in the Ab- and complement-induced lesions. The proteolytic enzyme plasmin is thought to participate in the Ab- and complement-mediated organ pathology. Plasmin is generated from plasma-derived plasminogen by cell-derived plasminogen activators (PAs). Two types of PAs are known, urokinase-type PA (uPA) and tissue-type PA (tPA). We investigated whether the PA system and the complement system can interact to promote local plasmin generation. Among the terminal complement components C5b6, C7, C8, and C9, the nonenzymatic component C7 is a plasminogen-binding protein. Radioligand binding studies revealed that the isolated component, as well as C7 after its incorporation into the terminal complement complex C5b-9, can bind plasminogen. Binding was inhibited by the lysine analogues 6-aminohexanoic acid and tranexamic acid, implicating the lysine binding sites of plasminogen into the binding interaction. tPA-mediated plasminogen activation was enhanced in the presence of C7. Based on these findings, an interaction is proposed between the complement system and the plasminogen activator system; a mechanism that may focus plasmin activity to structures that have been tagged by Ab and complement deposition.

The role of an enolase-related molecule in plasminogen binding to cells.

The alpha isoform of enolase is a candidate plasminogen receptor on U937 monocytoid cells [Miles, L. A., Dahlberg, C. L., Plescia, J., Felez, J., Kato, K. & Plow, E. F. (1991) Biochemistry 30, 1682-1691]. In the present study, an enolase-related molecule was detected on the surfaces of peripheral blood monocytes and neutrophils by fluorescence-activated cell sorting. A mRNA transcript encoding a unique membrane form of an enolase-related molecule was not detected by Northern-blotting and primer-extension analyses, consistent with the cell-surface protein being authentic alpha-enolase. Both the alpha and beta isoforms of purified enolase, bound plasminogen with an affinity similar to that of the cell surface. Moreover, immunopurified alpha-enolase enhanced plasminogen activation by tissue plasminogen activator and blocked the binding of plasminogen to alpha 2-antiplasmin, mimicking functions arising from the association of plasminogen with cells. The interaction of the enolase isoforms with plasminogen was dependent upon recognition of the C-terminal lysyl residue of the enolases by the lysine-binding sites of plasminogen, as the interaction was blocked by (a) peptides with C-terminal lysine residues and (b) an antibody to the C-terminal aspect of enolase. A monoclonal antibody was developed, characterized and utilized to quantify the enolase molecules present on the surface of U937 cells. A substantial number of molecules, 1.8 x 10(6)/cell, was present, accounting for approximately 10% of the plasminogen-binding capacity of these cells. These studies clearly establish the role of enolase as a cell-surface plasminogen-binding site with profibrinolytic functions.

Biochemical properties of variant plasminogens associated with thrombophilia: Defective binding and activation of variant plasminogens on the surface of monocytoid U937 cells

Summary Plasma was obtained from 5 patients (K, Y, S, F, T) with a history of thrombotic disease, venous and arterial, and a diagnosis of dysplasminogenemia/hypoplasminogenemia was made. Glu-plasminogen was isolated from the plasma of these patients by affinity chromatography on L-lysine-substituted Sepharose. Plasminogen recovery by this method was quantitative for plasmas K, Y, and S, but F and T plasmas gave substantially lower yields, 33% and 59% of plasminogen antigen by nephelometry, respectively. These lowered recoveries correlated with a new ELISA which used a polyclonal antibody against kringles 1,2,3 (lysine-binding sites) of the plasminogen molecules. SDS-PAGE analysis identified a new higher molecular weight form of Glu-plasminogen, Glu(a). Binding of K, Y, and S plasminogens to U937 cells was analysed by flow cytometry using a fluorescence method and gave K d 's between 1 to 3μM, similar to normal Glu-plasminogen and with a B max between 1.7 and 3.5×10 6 binding sites/cell, compared to normal Glu-plasminogen with 3.6×10 6 sites per cell. Binding was inhibited by 10 mM lysine, showing that binding to the cell surface involved the lysine-binding sites of plasminogen. F and T plasminogens bound to the cell surface in the presence and in the absence of 10 mM lysine, showing that this binding was independent of the lysine-binding sites. The variant plasminogens had lower than normal levels of plasmin generation with HMW-urokinase and single chain urokinase on the cell surface, from 1 to 17% of normal. Variant plasminogens K, Y, and S activated single chain urokinase on the cell surface with levels ranging from 0 to 17% of normal, while variants F and T, with lysine-independent binding, activated single chain urokinase at 55 to 57% of normal levels. This study identified a new class of variant plasminogens with defective lysine-binding sites.

Plasminogen Adsorption to Sulfonated and Lysine Derivatized Model Silica Glass Materials

Silica glass and silylated silica glass have been used extensively as model surfaces for studying the interactions of biological systems with specific chemical functions. In this work, the adsorption of plasminogen to silylated silica glass materials has been investigated with the objective of providing data for the development of a profibrinolytic or clot lysing surface for blood-contacting applications. The adsorption of plasminogen from Tris buffer to silica glass, lysine modified silica glass, and its sulfonated silica glass precursor has been investigated. The isotherms (25°C) suggest high affinity, Langmuir type adsorption. Adsorption capacities are in the order sulfonated silica > lysinized silica > unmodified silica. Estimates of the apparent affinity constants based on the Langmuir equation indicate that the affinity of plasminogen is greater for the lysinized surface than for either the silica glass or sulfonated silica glass. Experiments on the adsorption of plasminogen to the sulfonated and lysinized silica glass in the presence of ϵ-amino caproic acid (EACA), known to interfere with plasminogen binding to lysine, were also carried out. Adsorption plateau values for the sulfonated surface with and without EACA in the buffer were the same. For the lysinized surface, adsorption was reduced in the presence of EACA. Desorption experiments against Tris buffer showed that for the lysinized and sulfonated surfaces, 70-80% of the protein was irreversibly bound, with slightly higher reversibility on the lysinized surface. It is concluded that the mechanisms of adsorption of plasminogen to the lysinized surface and sulfonated surface are different, and that the lysine binding sites of plasminogen may be utilized in its interactions with the lysinized surface.

Surface-associated activation of plasminogen on gram-positive bacteria. Effect of plasmin on the adherence of Staphylococcus aureus.

In this article we review a novel type of plasminogen activation on staphylococcal and streptococcal cells. The activation mechanism implies a specific binding of glu-plasminogen to bacterial surface via the lysine-binding sites of plasminogen. Association of plasminogen with bacterial surfaces greatly enhances the t-PA mediated activation which takes place only poorly in solution. The end product, surface-associated plasmin, is enzymatically active, protected against high molecular weight plasmin inhibitors and capable of converting itself from glu-plasmin to the lys-form. The modification is associated with an increased affinity of the bound lys-plasmin towards the binding molecules on bacterial surface. This novel way of retaining plasmin on the surface may be important for the bacteria to invade and penetrate surrounding tissues. Our data on the effect of plasmin on staphylococcal adherence indicate that plasmin is not very effective in cleaning bacteria from surfaces coated with extracellular matrix components, fibronectin and fibrinogen.

Binding and activation of plasminogen at the surface of Staphylococcus aureus

Untreated Staphylococcus aureus cells, strain Cowan I, specifically bound 125I-Glu-plasminogen. The binding was inhibited by both unlabeled Glu-plasminogen and Glu-plasmin. The Lys form of plasminogen, which lacks the 8-kDa amino-terminal activation peptide, was approximately 100-fold more effective than the Glu form in competing with the binding of 125I-labeled Glu-plasminogen. This suggests an increase in binding affinity upon removal of the activation peptide. Fibronectin, fibrinogen and IgG, plasma components known to bind to the staphylococcal surface, did not significantly interfere with the binding. The competing activity in plasma was abolished by specifically absorbing plasminogen from the plasma sample. L-Lysine and a fragment of plasminogen containing three of the first five protein attachment domains present in the molecule (kringle structures) also competed with plasminogen for binding suggesting that the lysine-binding sites of plasminogen were involved in its interaction with staphylococci. Scatchard analysis revealed high- and low-affinity binding sites. Kd and the number of high-affinity binding sites were 1.7 nM and 780 binding sites/bacterial cell, respectively. 125I-Glu-plasminogen bound to staphylococcal surface was converted to plasmin by tissue-type plasminogen activator. The conversion took place also in the presence of plasma. If the conversion was carried out in the absence of low-molecular-mass plasmin inhibitors such as aprotinin, the bound Glu-plasmin was further converted to Lys-plasmin. The surface-bound plasmin was enzymically active, as judged by digestion of the synthetic substrate, S-2251. The plasminogen conversion shown by the present experiments not only leads to the surface-bound plasmin but seems to considerably increase the affinity of plasmin for its binding site. This may represent a physiologically relevant method for a bacterial cell to retain surface-bound active plasmin which is also protected from its soluble plasma inhibitors. This novel mechanism for staphylococci to adopt surface-bound proteolytic activity, without the interference of plasma components, may have some role in the tissue penetration and invasion of microbes during infection.

Binding of plasminogen to Escherichia coli adhesion proteins

Immobilization of plasminogen via its lysine-binding sites is regarded as a prerequisite for its activation and function in fibrinolysis and pericellular proteolysis. In the present study, the interaction of plasminogen with fimbriae found on Escherichia coli strains causing invasive human infections was studied. Plasminogen displayed concentration-dependent and saturable binding to immobilized type 1 fimbriae and, several fold lower binding to P and S fimbriae. The binding to fimbriae was effectively inhibited by ε-aminocaproic acid indicating that it was mediated by the lysine-binding sites of plasminogen. Binding studies with mutated fimbriae and inhibition tests indicated that the interaction was not dependent on the lectin subunit of the fimbriae. These results indicate the existence of a novel type of host-microbe interaction which may be important in the invasion by bacteria of host tissues.

Thrombospondin Interaction With Plasminogen. Evidence for Binding to a Specific Region of the Kringle Structure of Plasminogen

Platelet thrombospondin interacts with plasminogen in a specific and saturable manner. Thrombospondin was found to specifically bind to plasminogen and the nonenzyme chain of plasmin. Preincubation of 125I-labeled thrombospondin with 30 mmol/L lysine was without effect in the binding of thrombospondin to immobilized plasminogen; preincubation of 125I-labeled plasminogen with 30 mmol/L lysine, on the other hand, significantly reduced the binding of plasminogen to immobilized thrombospondin, suggesting that the interaction of thrombospondin with plasminogen is not the direct result of the lysine binding sites of plasminogen. Arginine and benzamidine, ligands known to specifically bind to the kringle 5 domain of plasminogen, blocked the binding of thrombospondin to plasminogen. Limited elastase proteolysis of plasminogen and plasmin resulted in the generation of two distinct thrombospondin binding domains, one of which was retained on lysine-agarose. The isolation and amino-terminal analysis of these domains following elastase proteolysis of plasminogen identified them, respectively, as a domain containing kringle structures 4 and 5 and plasmin and the other domain consisting of kringle 5-plasmin. A 16-residue synthetic peptide, which represents the amino acids linking kringle 4 to kringle 5 (residues 435-450 of native plasminogen), was without effect in either binding to thrombospondin or blocking the binding of thrombospondin to plasminogen. Plasminogen, therefore, possesses a single thrombospondin interactive site that is independent of, but influenced by, the lysine binding site containing kringle structures and most likely is located within the kringle 5 domain.