Characterization of the binding of urokinase-type plasminogen activator (u-PA) to plasminogen, to plasminogen-activator inhibitor-1 and to the u-PA receptor

Number and brightness image analysis reveals ATF-induced dimerization kinetics of uPAR in the cell membrane

We studied the molecular forms of the GPI-anchored urokinase plasminogen activator receptor (uPAR-mEGFP) in the human embryo kidney (HEK293) cell membrane and demonstrated that the binding of the amino-terminal fragment (ATF) of urokinase plasminogen activator is sufficient to induce the dimerization of the receptor. We followed the association kinetics and determined precisely the dimeric stoichiometry of uPAR-mEGFP complexes by applying number and brightness (N&B) image analysis. N&B is a novel fluctuation-based approach for measuring the molecular brightness of fluorophores in an image time sequence in live cells. Because N&B is very sensitive to long-term temporal fluctuations and photobleaching, we have introduced a filtering protocol that corrects for these important sources of error. Critical experimental parameters in N&B analysis are illustrated and analyzed by simulation studies. Control experiments are based on mEGFP-GPI, mEGFP-mEGFP-GPI, and mCherry-GPI, expressed in HEK293. This work provides a first direct demonstration of the dimerization of uPAR in live cells. We also provide the first methodological guide on N&B to discern minor changes in molecular composition such as those due to dimerization events, which are involved in fundamental cell signaling mechanisms.

Skizzle is a novel plasminogen- and plasmin-binding protein from Streptococcus agalactiae that targets proteins of human fibrinolysis to promote plasmin generation

Skizzle (SkzL), secreted by Streptococcus agalactiae, has moderate sequence identity to streptokinase and staphylokinase, bacterial activators of human plasminogen (Pg). SkzL binds [Glu]Pg with low affinity (K(D) 3-16 mum) and [Lys]Pg and plasmin (Pm) with indistinguishable high affinity (K(D) 80 and 50 nm, respectively). Binding of SkzL to Pg and Pm is completely lysine-binding site-dependent, as shown by the effect of the lysine analog, 6-aminohexanoic acid. Deletion of the COOH-terminal SkzL Lys(415) residue reduces affinity for [Lys]Pg and active site-blocked Pm 30-fold, implicating Lys(415) in a lysine-binding site interaction with a Pg/Pm kringle. SkzL binding to active site fluorescein-labeled Pg/Pm analogs demonstrates distinct high and low affinity interactions. High affinity binding is mediated by Lys(415), whereas the source of low affinity binding is unknown. SkzL enhances the activation of [Glu]Pg by urokinase (uPA) approximately 20-fold, to a maximum rate indistinguishable from that for [Lys]Pg and [Glu]Pg activation in the presence of 6-aminohexanoic acid. SkzL binds preferentially to the partially extended beta-conformation of [Glu]Pg, which is in unfavorable equilibrium with the compact alpha-conformation, thereby converting [Glu]Pg to the fully extended gamma-conformation and accelerating the rate of its activation by uPA. SkzL enhances [Lys]Pg and [Glu]Pg activation by single-chain tissue-type Pg activator, approximately 42- and approximately 650-fold, respectively. SkzL increases the rate of plasma clot lysis by uPA and single-chain tissue-type Pg activator approximately 2-fold, confirming its cofactor activity in a physiological model system. The results suggest a role for SkzL in S. agalactiae pathogenesis through fibrinolytic enhancement.

Urokinase plasminogen activator system as a potential target for cancer therapy

Proteolysis of extracellular matrix (ECM) and basement membrane is an essential mechanism used by cancer cells for their invasion and metastasis. The ECM proteinases are divided into three groups: metalloproteinases, cysteine proteinases and serine proteinases. The urokinase plasminogen activator (uPA) system is one of the serine proteinase systems involved in ECM degradation. Members of this system, including uPA and its receptor (uPAR), are overexpressed in several malignant tumors. This system plays a major role in adhesion, migration, invasion and metastasis of cancer cells, thus making it an important target for anticancer drug therapy. Several strategies, including the use of antisense oligodeoxynucleotides, ribozymes, DNAzyme, RNAi, uPA inhibitors, soluble uPAR, catalytically inactive uPA fragments, synthetic peptides and synthetic hybrids are under study, as they interfere with the expression and/or activity of uPA or uPAR in tumor cells. Herein, we discuss the various pharmaceutical strategies under investigation to combat the uPA activity in cancer.

Regulation of the single-chain urokinase-urokinase receptor complex activity by plasminogen and fibrin: novel mechanism of fibrin specificity

Activation of plasminogen by urokinase plasminogen activator (uPA) plays important roles in several physiologic and pathologic conditions. Cells secrete uPA as a single-chain molecule (scuPA). scuPA can be activated by proteolytic cleavage to a 2-chain enzyme (tcuPA). scuPA is also activated when it binds to its receptor (uPAR). The mechanism by which the enzymatic activity of the scuPA/suPAR complex is regulated is only partially understood. We now report that the plasminogen activator activity of the scuPA/suPAR complex is inhibited by Glu- and Lys-plasminogen, but not by mini-plasminogen. In contrast, neither Glunor Lys-plasminogen inhibits the activation of plasminogen by 2-chain uPA. Inhibition of scuPA/suPAR activity was evident at a Glu-plasminogen concentration of approximately 100 nM, and at physiologic plasma concentrations inhibition was nearly complete. A plasminogen fragment containing kringles 1-3 inhibited the enzymatic activity of scuPA/suPAR with an inhibition constant (Ki) equal to 1.9 microM, increased the Michaelis constant (Km) of scuPA/suPAR from 18 nM to 49 nM, and decreased the catalytic constant (Kcat) approximately 3-fold from 0.035 sec(-1) to 0.011 sec(-1). Inhibition of scuPA/suPAR by plasminogen was completely abolished in the presence of fibrin clots. These studies provide insight into the regulation of uPA-mediated plasminogen activation and identify a novel mechanism for its fibrin specificity.

Urokinase plasminogen activator system: a multifunctional role in tumor progression and metastasis

The urokinase plasminogen activator (uPA) system is central to a spectrum of biologic processes including fibrinoloysis, inflammation, atherosclerotic plaque formation, matrix remodeling during wound healing, tumor invasion, angiogenesis, and metastasis. Binding of uPA with its receptor (uPAR) initiates a proteolytic cascade that results in the conversion of plasminogen to plasmin. Plasmin through its own proteolytic function degrades a range of extracellular basement membrane components and activates others such as the metalloproteinases. Independent of catalytic activity, uPAR also is involved in cell signaling, interactions with integrins, cell motility, adhesion and invasion, and angiogenesis. Over expression of uPA or uPAR is a feature of malignancy and is correlated with tumor progression and metastasis. In contrast, inhibition of expression of these components leads to a reduction in the invasive and metastatic capacity of many tumors. Strategies that target uPA or its receptor with the aim of disrupting the interaction between the two or the ligand independent actions of uPAR include antisense technology, monoclonal antibodies, cytotoxic antibiotics, and synthetic inhibitors of uPA. Targeted therapy is a goal of future cancer treatment and the uPA system is a likely candidate for manipulation.

The kringle stabilizes urokinase binding to the urokinase receptor

The structural basis of the interaction between single-chain urokinase-type plasminogen activator (scuPA) and its receptor (uPAR) is incompletely defined. Several observations indicated the kringle facilitates the binding of uPA to uPAR. A scuPA variant lacking the kringle (Delta K-scuPA) bound to soluble uPAR (suPAR) with the similar "on-rate" but with a faster "off-rate" than wild-type (WT)-scuPA. Binding of Delta K-scuPA, but not WT-scuPA, to suPAR was comparably inhibited by its growth factor domain (GFD) and amino-terminal fragment (ATF). ATF and WT-scuPA, but not GFD, scuPA lacking the GFD (Delta GFD-scuPA), or Delta K-scuPA reconstituted the isolated domains of uPAR. ATF completely inhibited the enzymatic activity of WT-scuPA-suPAR unlike comparable concentrations of GFD. Variants containing mutations that alter the charge, length, or flexibility of linker sequence (residues 43-49) between the GFD and the kringle displayed a lower affinity for uPAR, were unable to reconstitute uPAR domains, and their binding to uPAR was inhibited by GFD in the same manner as Delta K-scuPA. A scuPA variant in which the charged amino acids in the heparin binding site (HBS) in the kringle domain were mutated to alanines behaved like Delta K-scuPA, indicating that that the structure of the kringle as well as its interaction with the GFD govern receptor binding. These data demonstrate an important role for the kringle in stabilizing the binding of scuPA to uPAR.

The molecular basis of skeletal metastases

Metastasis is the culmination of numerous highly regulated sequences of steps that results in the proliferation and migration of cells from the primary site to a distant location. The biologic consequence of skeletal metastasis is focal bone sclerosis or osteolysis that leads to pain, pathologic fracture, and biochemical derangement. The difficulty in determining a point of control for clinical application has been because of the numerous systems, substrates, ligands, receptors, factors, and pathways that exist. These may be grouped into functional mechanisms identifiable by their relevance to the metastatic process. These include cell-cell or cell-matrix adhesion, invasion and migration, interactions with endothelial cells, growth factor regulation, proteolysis, and stimulation of differentiated osteoblast and osteoclast function. The challenge for cancer therapy will be to identify means to prevent metastasis or reduce its effect once it occurred. This review examines recent advances in the study of molecular processes of metastasis, which have identified potential sites and substrates for targeting with novel therapies and agents.

The pro-urokinase plasminogen-activation system in the presence of serpin-type inhibitors and the urokinase receptor: rescue of activity through reciprocal pro-enzyme activation

The reciprocal pro-enzyme activation system of plasmin, urokinase-type plasminogen activator (uPA) and their respective zymogens is a potent mechanism in the generation of extracellular proteolytic activity. Plasminogen activator inhibitor type 1 (PAI-1) acts as a negative regulator. This system is complicated by a poorly understood intrinsic reactivity of the uPA pro-enzyme (pro-uPA) before proteolytic activation, directed against both plasminogen and PAI-1. We have studied the integrated activation mechanism under the repression of PAI-1 in a purified system. A covalent reaction between pro-uPA and PAI-1 was positively demonstrated but the reaction of PAI-1 with two-chain uPA was found to be at least 1000-fold faster. However, in spite of this very fast inhibition, two-chain uPA still became the dominant plasminogen activator when plasminogen was incubated with pro-uPA and PAI-1. The activity pattern observed under these conditions revealed an initial lag phase, followed by a continuous generation of minute amounts of active two-chain uPA, this uPA having a short lifetime before inhibition but still succeeding to generate new plasmin activity, thus preventing a complete inactivation of the feedback system. This property of the activation system was retained even in the simultaneous presence of PAI-1 and alpha(2)-antiplasmin. Addition of soluble uPA receptor to the system did not change the role of pro-uPA and the same pattern was observed when pro-uPA was bound to the uPA receptor on U937 cells. The present mechanism maintains the system at standby level and may be triggered to increased activity without the need for an external initiating event.

Biospecific interaction analysis: a tool for drug discovery and development

The recent development of surface plasmon resonance (SPR)-based biosensor technologies for biospecific interaction analysis (BIA) enables the monitoring of a variety of molecular reactions in real-time. The biomolecular interactions occur at the surface of a flow cell of a sensor chip between a ligand immobilized on the surface and an injected analyte. SPR-based BIA offers many advantages over most of the other methodologies available for the study of biomolecular interactions, including full automation, no requirement for labeling, and the availability of a large variety of activated sensor chips that allow immobilization of DNA, RNA, proteins, peptides and cells. The assay is rapid and requires only small quantitities of both ligand and analyte in order to obtain informative results. In addition, the sensor chip can be re-used many times, leading to low running costs. Aside from the analysis of all possible combinations of peptide, protein, DNA and RNA interactions, this technology can also be used for screening of monoclonal antibodies and epitope mapping, analysis of interactions between low molecular weight compounds and proteins or nucleic acids, interactions between cells and ligands, and real-time monitoring of gene expression. Applications of SPR-based BIA in medicine include the molecular diagnosis of viral infections and genetic diseases caused by point mutations. Future perspectives include the combinations of SPR-based BIA with mass spectrometry, the use of biosensors in proteomics, and the application of this technology to design and develop efficient drug delivery systems.

Critical role of glutamic acid 202 in the enzymatic activity of stromelysin-1 (MMP-3)

To test the hypothesis that Glu202, adjacent to the His201 residue that participates in the coordination of Zn(2+) in matrix metalloproteinase-3 (MMP-3 or stromelysin-1), plays a role in its enzymatic activity it was substituted with Ala, Lys or Asp by site-specific mutagenesis. Wild-type proMMP-3, proMMP-3(E202A), proMMP-3(E202K) and proMMP-3(E202D) were expressed in Escherichia coli and purified to apparent homogeneity. Whereas 33-kDa wild-type proMMP-3 (consisting of the propeptide and catalytic domains) was quantitatively converted to 24-kDa active MMP-3 by treatment with p-aminophenyl-mercuric acetate (APMA), proMMP-3(E202A) and proMMP-3 (E202K) were fully resistant to APMA and proMMP-3 (E202D) was quantitatively converted into a 14-kDa species. In contrast, treatment with plasmin quantitatively converted the wild-type and the three mutant proMMP-3 moieties into the corresponding 24-kDa MMP-3 moieties. Biospecific interaction analysis revealed comparable affinity for binding to plasminogen of wild-type and mutant proMMP-3 (K(a) of 2.6-6.3 x 10(6) M(-1)) or MMP-3 (K(a) of 33-58 x 10(6) M(-1)) moieties. The affinity for binding to single-chain urokinase-type plasminogen activator (scu-PA) was also similar for wild-type and mutant proMMP-3 (K(a) of 5.0-6.9 x 10(6) M(-1)) or MMP-3 (K(a) of 37-72 x 10(6) M(-1)) moieties. However, MMP-3(E202A) and MMP-3(E202K) did not hydrolyze plasminogen whereas MMP-3(E202D) showed an activity of 20--30% of wild-type MMP-3. All three mutants were inactive towards scu-PA under conditions where this was quantitatively cleaved by wild-type MMP-3. Furthermore, MMP-3(E202A) and MMP-3(E202K) were inactive toward a fluorogenic substrate and MMP-3 (E202D) displayed about 15% of the activity of wild-type MMP-3. Taken together, these data suggest that Glu202 plays a crucial role in the enzymatic activity of MMP-3.

Inactivation of plasminogen activator inhibitor-1 by specific proteolysis with stromelysin-1 (MMP-3)

Matrix metalloproteinase-3 (MMP-3 or stromelysin-1) specifically hydrolyzes the Ser(337)-Ser(338) (P10-P9) and Val(341)-Ile(342) (P6-P5) peptide bonds in human plasminogen activator inhibitor-1 (PAI-1). Cleavage is completely abolished in the presence of the metal chelators EDTA or 1,10-phenanthroline. A stabilized active PAI-1 variant was also cleaved by MMP-3. At an enzyme/substrate ratio of 1/10 at 37 degrees C, PAI-1 protein cleavage occurred with half-lives of 27 or 14 min for active or stable PAI-1 and was associated with rapid loss of inhibitory activity toward tissue-type plasminogen activator with half-lives of 15 or 13 min, respectively. A substrate-like variant of PAI-1, lacking inhibitory activity but with exposed reactive site loop, was cleaved with a half-life of 23 min, whereas latent PAI-1 in which a major part of the reactive site loop is inserted into the molecule, was resistant to cleavage. Biospecific interaction analysis indicated comparable binding of active, stable, and substrate PAI-1 to both proMMP-3 and MMP-3 (K(A) of 12-22 x 10(6) m(-1)), whereas binding of latent PAI-1 occurred with lower affinity (1.7-2.3 x 10(6) m(-1)). Stable PAI-1 bound to vitronectin was cleaved and inactivated by MMP-3 in a manner comparable with that of free PAI-1; however, the cleaved protein did not bind to vitronectin. Cleavage and inactivation of PAI-1 by MMP-3 may thus constitute a mechanism decreasing the antiproteolytic activity of PAI-1 and impairing the potential inhibitory effect of vitronectin-bound PAI-1 on cell adhesion and/or migration.

Prostromelysin-1 (proMMP-3) stimulates plasminogen activation by tissue-type plasminogen activator

Matrix metalloproteinase-3 (MMP-3 or stromelysin-1) specifically binds to tissue-type plasminogen activator (t-PA), without however, hydrolyzing the protein. Binding affinity to proMMP-3 is similar to single chain t-PA, two chain t-PA and active site mutagenized t-PA (Ka of 6.3 x 106 to 8.0 x 106 M-1), but is reduced for t-PA lacking the finger and growth factor domains (Ka of 2.0 x 106 M-1). Activation of native Glu-plasminogen by t-PA in the presence of proMMP-3 obeys Michaelis-Menten kinetics; at saturating concentrations of proMMP-3, the catalytic efficiency of two chain t-PA is enhanced 20-fold (kcat/Km of 7.9 x 10-3 vs. 4.1 x 10-4 microM-1.s-1). This is mainly the result of an enhanced affinity of t-PA for its substrate (Km of 1.6 microM vs. 89 microM in the absence of proMMP-3), whereas the kcat is less affected (kcat of 1.3 x 10-2 vs. 3.6 x 10-2 s-1). Activation of Lys-plasminogen by two chain t-PA is stimulated about 13-fold at a saturating concentration of proMMP-3, whereas that of miniplasminogen is virtually unaffected (1.4-fold). Plasminogen activation by single chain t-PA is stimulated about ninefold by proMMP-3, whereas that by the mutant lacking finger and growth factor domains is stimulated only threefold. Biospecific interaction analysis revealed binding of Lys-plasminogen to proMMP-3 with 18-fold higher affinity (Ka of 22 x 106 M-1) and of miniplasminogen with fivefold lower affinity (Ka of 0.26 x 106 M-1) as compared to Glu-plasminogen (Ka of 1.2 x 106 M-1). Plasminogen and t-PA appear to bind to different sites on proMMP-3. These data are compatible with a model in which both plasminogen and t-PA bind to proMMP-3, resulting in a cyclic ternary complex in which t-PA has an enhanced affinity for plasminogen, which may be in a Lys-plasminogen-like conformation. Maximal binding and stimulation require the N-terminal finger and growth factor domains of t-PA and the N-terminal kringle domains of plasminogen.

Glycosylation profile of a recombinant urokinase-type plasminogen activator receptor expressed in Chinese hamster ovary cells

Association of urokinase-type plasminogen activator (uPA) to cells via binding to its specific cellular receptor (uPAR) augments the potential of these cells to support plasminogen activation, a process that has been implicated in the degradation of extracellular matrix proteins during cell migration and tissue remodeling. The uPA receptor is a glycolipid-anchored membrane protein belonging to the Ly-6/uPAR superfamily and is the only multidomain member identified so far. We have now purified the three individual domains of a recombinant soluble uPAR variant, expressed in Chinese hamster ovary cells, after limited proteolysis using chymotrypsin and pepsin. The glycosylation patterns of these domains have been determined by matrix assisted laser desorption ionization and electrospray ionization mass spectrometry. Of the five potential attachment sites for asparagine-linked carbohydrate in uPAR only four are utilized, as the tryptic peptide derived from domain III containing Asn233 was quantitatively recovered without carbohydrate. The remaining four attachment sites were shown to exhibit site-specific microheterogeneity of the asparagine-linked carbohydrate. The glycosylation on Asn52 (domain I) and Asn172 (domain II) is dominated by the smaller biantennary complex-type oligosaccharides, while Asn162 (domain II) and Asn200 (domain III) predominantly carry tri- and tetraantennary complex-type oligosaccharides. The carbohydrate moiety on Asn52 in uPAR domain I could be selectively removed by N-glycanase treatment under nondenaturing conditions. This susceptibility was abrogated when uPAR participitated in a bimolecular complex with pro-uPA or smaller receptor binding derivatives thereof, demonstrating the proximity of the ligand-binding site to this particular carbohydrate moiety. uPAR preparations devoid of carbohydrate on domain I exhibited altered binding kinetics toward uPA (a 4-6-fold increase in Kd) as assessed by real time biomolecular interaction analysis.

Regulation of single chain urokinase binding, internalization, and degradation by a plasminogen activator inhibitor 1-derived peptide

The internalization and degradation of cell-associated urokinase type plasminogen activator (uPA) through the alpha2-macroglobulin receptor/low density lipoprotein-related receptor (alpha2MR/LRP) represent important steps in the control of plasmin formation. Complexes between two chain urokinase (tcuPA) and plasminogen activator type 1 are degraded rapidly whereas single chain urokinase (scuPA) is not, suggesting that alpha2MR/LRP requires specific epitopes in the serpin for effective function. We report an alternative mechanism that may contribute to this process. The binding of scuPA to LM-TK- cells that lack the uPA receptor was stimulated by the hexapeptide EEIIMD, corresponding to amino acids 350-355 of plasminogen activator type 1, which contacts the sequence RHRGGS, corresponding to amino acids 179-184 in uPA. EEIIMD increased the Bmax of scuPA binding 4-fold with the half-maximal effect achieved at a peptide concentration of 50 microM. Stimulation was dependent on the charge on the COOH-terminal amino acid but not on the NH2 terminus of the peptide. EEIIMD also stimulated the internalization and degradation of scuPA. Both the binding and internalization of scuPA in the presence of EEIIMD were blocked by recombinant, 39-kDa alpha2MR/LRP-associated protein as well as by an anti-alpha2MR/LRP antibody. EEIIMD also stimulated the binding of scuPA to purified alpha2MR/LRP. EEIIMD had no effect on the binding of tcuPA or of complexes between scuPA and its receptor. These results suggest that EEIIMD regulates the binding of scuPA with alpha2MR/LRP. These findings also suggest a potential mechanism by which scuPA can be cleared which is independent of activation by plasmin or binding to uPA receptor.

Plasminogen activator inhibitor type 1 in cancer: therapeutic and prognostic implications

Degradation of the extracellular matrix plays a crucial role in cancer invasion. This degradation is accomplished by the concerted action of several enzyme systems, including generation of the serine protease plasmin by the urokinase pathway of plasminogen activation, different types of collagenases and other metalloproteinases, and other extracellular enzymes. The degradative enzymes are involved also in tissue remodelling under non-malignant conditions, and the main difference appears to be that mechanisms which regulates these processes under normal conditions are defective in cancer. Specific inhibitors have been identified for most of the proteolytic enzymes, e.g. plasminogen activator inhibitors (PAI's) and tissue inhibitors of metalloproteinases (TIMP's). It has been contemplated that these inhibitors counteracted the proteolytic activity of the enzymes, thereby inhibiting extracellular tissue degradation which in turn should prevent tumor cell invasion. This review focuses on plasminogen inhibitor type 1 (PAI-1). It is described that PAI-1 is not produced by the epithelial cancer cell but by the stromal cells in the tumors, suggesting a concerted action between stroma and tumor cells in the processes controlling proteolysis in cancer. The specific localization of PAI-1 to the tumor stroma and in many cases to areas surrounding the tumor vessels has lead us to suggest that PAI-1 serves to protect the tumor stroma from the ongoing uPA-mediated proteolysis. This hypothesis is supported by recent clinical data showing increased levels of PAI-1 in metastases as compared to the primary tumor as well as data demonstrating that high levels of PAI-1 in tumor extracts from breast, lung, gastric and ovarian cancer is associated with a shorter overall survival.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue-type plasminogen activator (tPA) interacts with urokinase-type plasminogen activator (uPA) via tPA's lysine binding site. An explanation of the poor fibrin affinity of recombinant tPA/uPA chimeric molecules

Differential scanning calorimetry was used to study the domain structure and intramolecular interactions of tPA/uPA chimeras. A high temperature transition centered near 90 degrees C was observed upon melting of the tPA/uPA chimera (amino acids 1-274 of tPA and 138-411 of uPA) and its variant lacking the finger and epidermal growth factor-like modules (residues 1-3 and 87-274 of tPA and 138-411 of uPA). Since neither of the two parent plasminogen activators display such a stable structure, one may suggest that a new stabilizing intramolecular interaction occurs in the chimeras. We found that occupation of the lysine binding site of tPA by a lysine or arginine side chain from the urokinase moiety is responsible for the high temperature transition as well as for the failure of the chimeras to exhibit the expected fibrin binding properties. All uPA species, single- and two-chain high molecular weight uPA (Pro-Uk and HMW-Uk) and two-chain low molecular weight uPA (LMW-Uk), interact intermolecularly with tPA and its kringle-containing derivatives. This intermolecular interaction was strongly inhibited by epsilon-aminocaproic acid indicating that the lysine binding site of tPA is involved. The binding of uPA with the fluorescein-labeled A-chain of tPA, registered by changes in fluorescence anisotropy, was estimated to have a Kd range of 1-7 microM. The interaction of tPA with uPA determined by solid-phase assays appeared to be tighter, with a Kd range of 50-300 nM. Two synthetic peptides, with and without carboxyl-terminal lysine, corresponding to urokinase residues 144-158 and 144-157, were approximately 100-fold more potent than epsilon-aminocaproic acid with respect to inhibition of the tPA-uPA interaction, indicating that the tPA binding site on urokinase is located within this sequence, close to the activation site Lys158-Ile159. The discovered intermolecular interaction may be related to the reported synergistic effect of simultaneous administration of these two plasminogen activators.

Structural domains of streptokinase involved in the interaction with plasminogen

Two fragments of recombinant streptokinase, comprising amino acids Val143-Lys293 (17-kDa rSK) or Val143-Lys386 (26-kDa rSK), were cloned and expressed in Escherichia coli, purified to homogeneity and their interactions with plasmin(ogen) were evaluated. Both 17-kDa rSK and 26-kDa rSK bound to plasminogen with a 1:1 stoichiometry and with affinity constants of 3.0 x 10(8) M-1 and 12 x 10(8) M-1, respectively, as compared to 6.3 x 10(8) M-1 for the binding of intact recombinant streptokinase to plasminogen. Binding of 17-kDa rSK to plasminogen-Sepharose was displaced by addition of increasing concentrations of recombinant streptokinase, whereas bound recombinant streptokinase was not displayed by 17-kDa rSK. In equimolar mixtures of plasminogen and 26-kDa rSK, the appearance of amidolytic activity as monitored with a chromogenic substrate, was significantly delayed compared to the equimolar mixture with recombinant streptokinase (60% of the maximal activity after 30 min, compared to maximum activity within < or = 2 min). In contrast, no amidolytic activity was generated in equimolar mixtures of plasminogen and 17-kDa rSK. Plasminogen was rapidly activated by catalytic amounts (1:100 molar ratio) of recombinant streptokinase (60-70% within 10-15 min), whereas only 4% of the plasminogen was activated within 60 min with 26-kDa rSK, and no plasmin was generated with 17-kDa rSK. Complexes of plasmin with 17-kDa rSK or with 26-kDa rSK were very rapidly inhibited by alpha 2-antiplasmin (apparent second-order inhibition rate constant of approximately 2 x 10(7) M-1 s-1), whereas the complex with recombinant streptokinase was resistant to inhibition. With 26-kDa rSK, inhibition by alpha 2-antiplasmin resulted in dissociation of the complexes and recycling of functionally active 26-kDa rSK to other plasminogen molecules; 17-kDa rSK, in contrast, remained associated with the plasmin-alpha 2-antiplasmin complex. These findings suggest that different regions of the streptokinase molecule are involved in binding to plasminogen, in active-site exposure, and in impairment of the inhibition of plasmin by alpha 2-antiplasmin. Thus, the 17-kDa region spanning Val143-Lys293 in streptokinase mediates its binding to plasminogen but does not induce activation. Furthermore, this region does not interfere with the inhibition of the complex with plasmin by alpha 2-antiplasmin.