Molecular basis for the susceptibility of fibrin-bound thrombin to inactivation by heparin cofactor ii in the presence of dermatan sulfate but not heparin

Although fibrin-bound thrombin is resistant to inactivation by heparin.antithrombin and heparin.heparin cofactor II complexes, indirect studies in plasma systems suggest that the dermatan sulfate.heparin cofactor II complex can inhibit fibrin-bound thrombin. Herein we demonstrate that fibrin monomer produces a 240-fold decrease in the heparin-catalyzed rate of thrombin inhibition by heparin cofactor II but reduces the dermatan sulfate-catalyzed rate only 3-fold. The protection of fibrin-bound thrombin from inhibition by heparin.heparin cofactor II reflects heparin-mediated bridging of thrombin to fibrin that results in the formation of a ternary heparin.thrombin.fibrin complex. This complex, formed as a result of three binary interactions (thrombin.fibrin, thrombin.heparin, and heparin.fibrin), limits accessibility of heparin-catalyzed inhibitors to thrombin and induces conformational changes at the active site of the enzyme. In contrast, dermatan sulfate binds to thrombin but does not bind to fibrin. Although a ternary dermatan sulfate. thrombin.fibrin complex forms, without dermatan sulfate-mediated bridging of thrombin to fibrin, only two binary interactions exist (thrombin.fibrin and thrombin. dermatan sulfate). Consequently, thrombin remains susceptible to inactivation by heparin cofactor II. This study explains why fibrin-bound thrombin is susceptible to inactivation by heparin cofactor II in the presence of dermatan sulfate but not heparin.     

RNA aptamers specific for bovine thrombin

Bovine thrombin is widely used in clinical wound healing after surgery. There is 85% homology between bovine thrombin and human thrombin, so most antibodies against bovine thrombin cross-react with human thrombin. Rare antibodies against bovine thrombin but not cross-reacting with human thrombin have been reported. RNA ligands (aptamers) have been used to bind to target molecules with sometimes higher specificity than antibodies. Here we report the isolation of aptamers specific for bovine thrombin by systematic evolution of ligands by exponential enrichment (SELEX) from an RNA pool containing a 25-nucleotide randomized region. After seven rounds of selection, two aptamers specific for bovine thrombin were identified with a K(d) of 164 and 240 nM, respectively. Significantly, these aptamers do not bind to human thrombin. Secondary structure prediction revealed potential stem-loop structures for these RNAs. Both RNA aptamers inhibit only bovine thrombin-catalyzed fibrin clot formation in vitro. Competition assay results suggested that the RNA aptamers might bind to the electropositive domain of bovine thrombin, that is, heparin-binding site, instead of fibrinogen-recognition exosite. The resulting bovine-specific thrombin inhibitor might be used in some clinical applications when bovine thrombin activity needs to be contained or in research where human and bovine thrombin need to be distinguished.     

Localization of the single-stranded DNA binding site in the thrombin anion-binding exosite.

Single-stranded DNA molecules containing a 15-nucleotide consensus sequence have been reported to inhibit thrombin activity. The mechanism of the inhibition was studied using a consensus 15-mer oligonucleotide and two recombinant mutant thrombins: the anion-binding exosite mutant thrombin R70E, and thrombin K154A, in which the mutation was located in a surface loop outside of the exosite. The consensus 15-mer oligonucleotide inhibited both fibrinogen-clotting and platelet-activation activities of plasma-derived thrombin, recombinant wild type thrombin, and mutant thrombin K154A in a sequence-specific and dose-dependent manner, whereas it did not inhibit either activity of mutant thrombin R70E. The 15-mer oligonucleotide also inhibited thrombomodulin-dependent protein C activation by plasma-derived thrombin. In competition equilibrium binding experiments, binding of 125I-labeled diisopropyl phosphoryl-thrombin to thrombomodulin was completely inhibited by the consensus 15-mer oligonucleotide with a Kd value of 2.68 +/- 0.16 nM. These results suggest that Arg-70 in the anion-binding exosite of thrombin is a key determinant for interaction with specific single-stranded DNA molecules, and that binding of single-stranded DNA molecules to the exosite prevents the interaction of thrombin with fibrinogen, the platelet thrombin receptor, and thrombomodulin.     

Binding and internalization of 125I thrombin in chick embryo fibroblasts: Possible role in mitogenesis

Human α thrombin acts as a mitogen for cultures of resting chick embryo fibroblasts (CEF) in serum free medium. The use of ¹²⁵I-labeled thrombin shows that thrombin specifically binds to CEF and that after a lag of approximately 30 to 60 minutes it can not be removed by subsequent exposure to trypsin. The entry of ¹²⁵I thrombin into the trypsin-insensitive domain is not inhibited to any great extent by excess unlabelled thrombin. The cell-associated thrombin retains its native molecular weight and its catalytic activity toward synthetic amide substrates. It appears to be located in the crude nuclear fraction of homogenized CEF cells. The association of thrombin with CEF is specific, since the non-mitogenic serine protease chymotrypsin is internalized to a much lesser extent than thrombin. The data are discussed in terms of a possible intracellular site for thrombin's mitogenic action.     

牛凝血酶等电点的初步测定

作为一种新型的速效局部止血药和工具酶,凝血酶在临床和生物学研究中的应用十分广泛,牛血浆是其重要的来源之一。等电点沉淀是提取牛凝血酶首要和关键的步骤,测定其等电点后,再用此法时将得到更纯的凝血酶粗制品。本实验的目的是采用载体两性电解质pH梯度等电聚焦电泳的方法,结合SDS-PAGE测定牛凝血酶的等电点。经双向电泳后,SDS聚丙烯酰胺凝胶中出现了4个清晰的斑点,分别测定它们的分子量和等电点, 其中一个斑点与牛凝血酶B链的分子量一致为32kDa,其等电点为5．19．     

Homologous desensitization of HEL cell thrombin receptors. Distinguishable roles for proteolysis and phosphorylation.

Loss of sensitivity to thrombin following an initial response is characteristic of a number of cell types, including platelets. It has recently been proposed that thrombin receptors resemble other G protein-coupled receptors, but that activation involves a novel mechanism in which thrombin cleaves the receptor, exposing a new N terminus that serves as the ligand for the receptor. Based upon this model, we have examined the mechanism of thrombin receptor desensitization by comparing the effects of thrombin with those of a peptide corresponding to the N-terminal sequence of the receptor following proteolysis by thrombin: SFLLRNPNDKYEPF or TRP42/55. Like thrombin, TRP42/55 stimulated pertussis toxin-sensitive inositol 1,4,5-trisphosphate formation, raised cytosolic Ca2+, and inhibited cAMP formation in the megakaryoblastic HEL cell line. Exposure to either thrombin or TRP42/55 desensitized the cells to both, but not to a third agonist, neuropeptide Y. The rate of recovery after desensitization depended upon the order of agonist addition. Resensitization of the cell to thrombin following a brief exposure to thrombin required up to 24 h and could be inhibited with cycloheximide. Resensitization to TRP42/55 after exposure to thrombin, or to thrombin after exposure to TRP42/55, on the other hand, was detectable within 30 min and could be inhibited by serine/threonine phosphatase inhibitors, but not by cycloheximide. Loss of responsiveness to thrombin and TRP42/55 was also observed following addition of the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA). However, while the protein kinase inhibitor staurosporine completely prevented the desensitization caused by TPA, it had only a limited effect on the desensitization caused by TRP42/55. These results demonstrate that the G protein-mediated effects of thrombin can be reproduced by a receptor-derived peptide and suggest that desensitization occurs by at least two mechanisms. The first, which is seen with thrombin, but not TRP42/55, involves proteolysis and requires protein synthesis for recovery. The second, which occurs with TRP42/55 and TPA, as well as with thrombin, involves phosphorylation, possibly of the receptor itself. Although protien kinase C is activated by thrombin and is presumably responsible for the desensitization caused by TPA, it does not appear to play a major role in receptor desensitization caused by thrombin and TRP42/55. This suggests that other kinases, such as those which inactivate adrenergic receptors and rhodopsin, are involved in the down-regulation of thrombin receptor function.     

Monocyte Chemoattractant Activity of Ser → Ala Active Site Mutant Recombinant α-Thrombin

α-Thrombin is chemotactic for human monocytes with optimal activity between 10-100 nM. The mechanism by which this response is mediated remains a point of controversy. The purpose of this study was to compare the chemotactic activity of proteolytically inactive thrombin (active site Ser¹⁹⁵ → Ala mutant or Phe-Pro-Arg-chloromethyl ketone-inactivated thrombin) to thrombin and the "tethered ligand" thrombin receptor agonist peptide SFLLRN (single-letter amino acid code). Monocyte chemotaxis was compared to an optimal concentration (10 nM, considered to be 100%) of formyl-Met-Leu-Phe (fMLP). Proteolytically inactive thrombin (38% of fMLP) had similar chemotactic activity to active thrombin (46% of fMLP) at a concentration of 100 nM. Chemotaxis to SFLLRN was comparable to that of a control hexapeptide (FSLNLR) which is not an agonist for the tethered ligand thrombin receptor. Cross-desensitization experiments showed that pretreatment of monocytes with either mutant or active thrombin reduced subsequent chemotaxis to both thrombin chemotaxins. Pretreatment with SFLLRN did not decrease subsequent chemotaxis to either form of thrombin. Calcium flux measurements showed that both active thrombin and SFLLRN induced a rapid increase in monocyte and platelet intracellular calcium concentration. However, there was no intracellular calcium change in response to mutant thrombin or FSLNLR. Likewise, active thrombin and SFLLRN induced a rapid net increase in polymerized actin, but mutant thrombin and FSLNLR did not. By contrast, both active and mutant thrombin induced a polarization of monotocyte morphology and actin distribution. This polarization has been associated with directed migration in many cell types. SFLLRN, however, induced a symmetrical increase in polymerized actin. These results suggest that measurements of intracellular calcium and polymerized actin are not perfect surrogate tests for true chemotactic activity. These results show that thrombin proteolysis is not required for monocyte chemotaxis and may be mediated by interaction with a binding site other than the tethered ligand thrombin receptor.     

Thrombin causes neurite retraction in neuronal cells through activation of cell surface receptors.

The mechanism by which thrombin induces neurite retraction was studied in NB2a mouse neuroblastoma cells. The rapid effect of thrombin (completed within minutes) appears to involve an interaction between its anion-binding exosite and the thrombin receptor. Structural alterations of this site increase the EC50 for thrombin-mediated retraction, and a hirudin C-terminal peptide that blocks this site inhibits the response. The thrombin effect was mimicked by a 14 amino acid peptide starting with Ser-42, at the proposed cleavage site of the human thrombin receptor. The protein kinase inhibitors staurosporine and H-7 blocked thrombin-induced retraction. It is therefore proposed that thrombin-mediated neurite retraction is caused by cleavage-induced activation of the thrombin receptor and involves stimulation of a protein kinase(s).     

The action of factor Xa, thrombin and trypsin on human factor II

The proteolytic action of human and bovine Factor Xa, bovine thrombin and bovine pancreatic trypsin Factor II at pH 7.5 and 25°C was monitored by sodium dodecylsulfate gel electrophoresis and thrombin assays. Purified human and bovine Factor Xa, and trypsin, were found to activate Factor II to thrombin. The conversion of Factor II to thrombin by either Factor Xa or trypsin was found to proceed through two thrombogenic intermediates. The reaction pathway appears to be sequential in that the Factor II (75 000 daltons) is first cleaved to a 55 000-dalton thrombogenic product (Intermediate 1) and a 25 000-dalton non-thrombogenic product (Fragment 1). Intermediate 1 is subsequently converted to an inactive 37 000-dalton thrombogenic protein (Intermediate 2) and a 16 000-dalton protein (Fragment 2). Intermediate 2 is finally converted to an active 37 000-dalton thrombin (α-thrombin). Purified bovine thrombin readily converted Factor II to Intermediate 1 and Fragment 1, but possessed little capacity to catalyze subsequent cleavages to produce active thrombin. The ability of thrombin to cleave Factor II was entirely obviated in the presence of hirudin. Under the conditions of the incubation, the maximum thrombin yield obtainable by Factor Xa or trypsin activation was 50% when compared to the two-stage potential thrombin.     

Participation of the hypophyseal-adrenal cortex system in thrombin clearance during immobilization stress]

The examination carried out with thrombin marked by 131J resulted in a considerable increase of the thrombin clearance rate in healty male rats during the stress (caused by an immobilization lasting 30 minutes) and in an increase of thrombin deposits in the liver. A further increase of thrombin clearance occurred by the combination of immobilization and administration of ACTH. Contrary to ACTH the thrombin clearance is not stimulated in healthy animals by hydrocortisone. Thrombin clearance and thrombin deposits in the liver are lowered in adrenalectomized rats. In these animals the administration of ACTH does not result in an increase of thrombin clearance. The rate of thrombin clearance is normalized in adrenalectomized animals after administering hydrocortisone without as well as under conditions of stress. In adrenalectomized animals having received hydrocortisone as well as in healthy animals the administration of ACTH will results in an increase of thrombin clearance. From these experiments the conclusion can be drawn that ACTH will increase the intensity of thrombin clearance in stress and that hydrocortisone plays a transmitting part here.     

ON THE NATURE OF FORCES OPERATING IN BLOOD CLOTTING : II. THE CLOTTING OF FIBRINOGEN AS A TWO-STEP REACTION

It is found that clotting of fibrinogen by thrombin does not occur on the acid side of the isoelectric point of the fibrinogen. At such pH values, however, a primary reaction between thrombin and fibrinogen takes place, leading to the formation of profibrin, a compound of thrombin and fibrinogen. At pH values at which clotting is possible, fibrinogen is negatively, thrombin positively charged, whereas profibrin has a pattern of positive and negative charges. The primary reaction, the formation of profibrin by combination of thrombin and fibrinogen, is inhibited by urea but not by neutral salts. The combination of thrombin with fibrinogen most probably takes place by hydrogen bonds. The second reaction, the polymerisation of profibrin to fibrin, is inhibited by neutral salts in the same way as complex or autocomplex coacervates. It is caused therefore by electrostatic attraction between the positive and the negative charges of the profibrin.     

Exosites 1 and 2 are essential for protection of fibrin-bound thrombin from heparin-catalyzed inhibition by antithrombin and heparin cofactor II

Assembly of ternary thrombin-heparin-fibrin complexes, formed when fibrin binds to exosite 1 on thrombin and fibrin-bound heparin binds to exosite 2, produces a 58- and 247-fold reduction in the heparin-catalyzed rate of thrombin inhibition by antithrombin and heparin cofactor II, respectively. The greater reduction for heparin cofactor II reflects its requirement for access to exosite 1 during the inhibitory process. Protection from inhibition by antithrombin and heparin cofactor II requires ligation of both exosites 1 and 2 because minimal protection is seen when exosite 1 variants (gamma-thrombin and thrombin Quick 1) or an exosite 2 variant (Arg93 --> Ala, Arg97 --> Ala, and Arg101 --> Ala thrombin) is substituted for thrombin. Likewise, the rate of thrombin inhibition by the heparin-independent inhibitor, alpha1-antitrypsin Met358 --> Arg, is decreased less than 2-fold in the presence of soluble fibrin and heparin. In contrast, thrombin is protected from inhibition by a covalent antithrombin-heparin complex, suggesting that access of heparin to exosite 2 of thrombin is hampered when ternary complex formation occurs. These results reveal the importance of exosites 1 and 2 of thrombin in assembly of the ternary complex and the subsequent protection of thrombin from inhibition by heparin-catalyzed inhibitors.     

Inactivation of human thrombin in the presence of human alpha1-proteinase inhibitor.

Both the clotting and esterase activities of thrombin are inhibited by alpha1-proteinase inhibitor (alpha1-antitrypsin). The inhibition is a time-and temperature-dependent reaction which is proportional to the molar ratio of thrombin to inhibitor. Both the active-site serine residue of thrombin and the reactive-site lysine residue of alpha1-proteinase inhibitor are involved. alpha1-Proteinase inhibitor forms a 1:1 complex with thrombin that is comparable with the complex formed with trypsin and other proteinases. Incubation of the inhibitor with excess of thrombin, however, results in inactivation of nearly all the enzyme, even though only as much complex is formed as alpha1-proteinase inhibitor present. A portion of the remaining thrombin apparently aggregates. These results suggest that the mechanism for inhibition of thrombin may not be exactly the same as for trypsin, which is inhibited only to the extent to which complex is formed.     

The effect of bovine thrombomodulin on the specificity of bovine thrombin

Bovine lung thrombomodulin is purified and used to investigate the basis of the change in substrate specificity of bovine thrombin when bound to thrombomodulin. Bovine thrombomodulin is a single polypeptide having an apparent molecular weight of 84,000 and associates with thrombin with high affinity and rapid equilibrium, to act as a potent cofactor for protein C activation and antagonist of reactions of thrombin with fibrinogen, heparin cofactor 2, and hirudin. Bovine thrombomodulin inhibits the clotting activity of thrombin with Kd less than 2.5 nM. Kinetic analysis of the effect of bovine thrombomodulin on fibrinopeptide A hydrolysis by thrombin indicates competitive inhibition with Kis = 0.5 nM. The active site of thrombin is little perturbed by thrombomodulin, as tosyl-Gly-Pro-Arg-p-nitroanilide hydrolysis and inhibition by antithrombin III are unaffected. Insensitivity of the reaction with antithrombin III is likewise observed with thrombin bound to thrombomodulin on intact endothelium. Antithrombin III-heparin, human heparin cofactor 2, and hirudin inhibit thrombin-thrombomodulin more slowly than thrombin. These effects may arise from a decrease in Ki of the inhibitors for thrombin-thrombomodulin or from changes in the active site not detected by tosyl-Gly-Pro-Arg-p-nitroanilide or antithrombin III. Bovine prothrombin fragment 2 inhibits thrombin clotting activity (Kd less than 7.5 microM) and acts as a competitive inhibitor of protein C activation (Kis = 2.1 microM). The data are consistent with a mechanism whereby thrombomodulin alters thrombin specificity by either binding to or allosterically altering a site on thrombin distinct from the catalytic center required for binding or steric accommodation of fibrinogen, prothrombin fragment 2, heparin cofactor 2, and hirudin.     

A thrombin receptor in resident rat peritoneal macrophages.

Resident rat peritoneal macrophages possess 6 x 10(2) high-affinity binding sites per cell for bovine thrombin with a Kd of 11 pM, and 7.5 x 10(4) low-affinity sites with a Kd of 5.8 nM. These binding sites are highly specific for thrombin. Half-maximal binding of 125I-labeled bovine thrombin is achieved after 1 min at 37 degrees C, and after 12 min at 4 degrees C. The reversibly bound fraction of the ligand dissociates according to a biexponential time course with the rate constants 0.27 and 0.06 min-1 at 4 degrees C. Part of the tracer remains cell-associated even after prolonged incubation, but all cell-associated radio-activity migrates as intact thrombin upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The bound thrombin is minimally endocytosed as judged by the resistance to pH 3 treatment, and the receptor does not mediate a quantitatively important degradation of the ligand. The binding is not dependent on the catalytic site of thrombin, since irreversibly inactivated thrombin also binds to the receptor. 125I-labeled thrombin covalently cross-linked to its receptor migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a Mr 160,000, corresponding to an approximate receptor size of Mr 120,000.     

Macromolecular specificity determinants on thrombin for fibrinogen and thrombomodulin

The endothelial cell surface membrane protein thrombomodulin binds thrombin with high affinity and acts as both a cofactor for protein C activation and an inhibitor of fibrinogen hydrolysis. We have previously shown that bovine thrombomodulin is a competitive inhibitor of fibrinogen binding to thrombin but has no effect on thrombin activity toward tripeptide substrates or antithrombin III. Hence, thrombomodulin and fibrinogen may share macromolecular specificity sites on thrombin which are distinct from the active site. In this investigation, we have studied the interaction of thrombin-thrombomodulin with fibrinogen and various thrombin derivatives. We show that fibrinogen is a competitive inhibitor of thrombomodulin binding to thrombin, with a Kis = 10 microM. Thrombin derivatives (bovine (pyridoxal phosphate)4-thrombin and human thrombin Quick I), which bind fibrinogen with much reduced affinity, are shown to also interact with thrombomodulin with greatly reduced affinity. These results are consistent with the hypothesis that thrombomodulin and fibrinogen share macromolecular specificity sites on thrombin.     

The thrombin high-affinity binding site on platelets is a negative regulator of thrombin-induced platelet activation. Structure-function studies using two mutant thrombins, Quick I and Quick II.

To elucidate the thrombin domains required for high-affinity binding and platelet activation, the platelet binding properties of thrombin and two mutant thrombins, thrombin Quick I and Quick II, were compared to their agonist effects in elevating intraplatelet [Ca2+]. In Quick I, a mutation within the fibrinogen binding groove results in decreased clotting and platelet aggregating activities, whereas in Quick II, a mutation in the primary substrate binding pocket abolishes both activities. Dysthrombin binding was decreased compared to thrombin. The fibrinogen binding groove appeared more important than the primary substrate pocket for high-affinity binding since Quick I showed drastically reduced, and Quick II only slightly reduced, binding affinity (Kd approximately 200 and approximately 10 nM, respectively). The deduced interaction of thrombin with its high-affinity binding site indicated that the thrombin catalytic site is directed toward the platelet surface and therefore, when bound, is proteolytically inactive. Quick I (0.5-5 nM) elicited intraplatelet [Ca2+] fluxes at concentrations where high-affinity binding was undetectable. Saturation of high-affinity binding sites with active-site-modified thrombin did not affect thrombin-induced (0.5 nM) or Quick I-induced (5 nM) responses. In contrast, addition of D-Phe-Pro-Arg chloromethyl ketone (FPRCK) subsequent to thrombin or Quick I stimulation of platelets abolished agonist-induced responses. Since Quick I was only 10-17% as effective as thrombin in increasing intraplatelet [Ca2+], our data support a model in which thrombin acts enzymatically on a platelet membrane "substrate", through an interaction mediated in part by the fibrinogen binding groove of thrombin. This conclusion is consistent with the inhibition observed with high concentrations (greater than 100 nM) of Quick II and FPRCK-modified thrombin (FPR-thrombin) in platelets stimulated with low concentrations of thrombin (less than 0.5 nM) or Quick I (less than 2 nM), consistent with inhibition by substrate depletion. In contrast, concentrations of FPR-thrombin or Quick II (less than 100 nM), which saturated predominantly the high-affinity binding sites, enhanced the platelet responses induced by thrombin (less than 0.5 nM). Thus, occupation of the high-affinity sites with inactive thrombin increased the concentration of active thrombin available for substrate interaction. Quick I-induced responses were not enhanced, consistent with its inability to interact with the high-affinity site. Since thrombin bound to the high-affinity site is proteolytically inactive, we hypothesize that the thrombin high-affinity binding site on platelets functions to alter thrombin activity and platelet activation.     

GpIbα Interacts Exclusively with Exosite II of Thrombin

Activation of platelets by the serine protease thrombin is a critical event in haemostasis. This process involves the binding of thrombin to glycoprotein Ibα (GpIbα) and cleavage of protease-activated receptors (PARs). The N-terminal extracellular domain of GpIbα contains an acidic peptide stretch that has been identified as the main thrombin binding site, and both anion binding exosites of thrombin have been implicated in GpIbα binding, but it remains unclear how they are involved. This issue is of critical importance for the mechanism of platelet activation by thrombin. If both exosites bind to GpIbα, thrombin could potentially act as a platelet adhesion molecule or receptor dimerisation trigger. Alternatively, if only a single site is involved, GpIbα may serve as a cofactor for PAR-1 activation by thrombin. To determine the involvement of thrombin's two exosites in GpIbα binding, we employed the complementary methods of mutational analysis, binding studies, X-ray crystallography and NMR spectroscopy. Our results indicate that the peptide corresponding to the C-terminal portion of GpIbα and the entire extracellular domain bind exclusively to thrombin's exosite II. The interaction of thrombin with GpIbα thus serves to recruit thrombin activity to the platelet surface while leaving exosite I free for PAR-1 recognition.     

The inactivation of single-chain urokinase-type plasminogen activator by thrombin on cultured human endothelial cells

Single-chain urokinase-type plasminogen activator (scu-PA) is cleaved by thrombin, resulting in an inactive molecule called thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T). There is no knowledge about cell-mediated inactivation of scu-PA. We have studied whether scu-PA bound to cultured human umbilical vein endothelial cells (HUVEC) could be inactivated by thrombin. High molecular weight scu-PA was bound to HUVEC and incubated with increasing amounts of thrombin for 30 min at 37 degrees C. Cell-bound urokinase-type plasminogen activator (u-PA) was released and levels of scu-PA, tcu-PA/T and active two-chain u-PA were measured using sensitive bioimmunoassays. Cell-bound scu-PA was efficiently inactivated by thrombin. Fifty percent inactivation of scu-PA occurred at about 0.2 nM thrombin. In the presence of monoclonal anti-urokinase receptor IgG, at least 50% of the binding of scu-PA to HUVEC was inhibited. The relative amount of tcu-PA/T that was generated by thrombin was not affected by the monoclonal antibody. These results indicated that scu-PA bound to HUVEC via the urokinase receptor can be inactivated by thrombin. The efficient inactivation of cell-bound scu-PA suggests that a cofactor for thrombin may be involved, like thrombomodulin or glycosaminoglycans. It is concluded that scu-PA bound to the urokinase receptor on a cell surface can be inactivated by thrombin, which may have profound effects on u-PA-mediated local fibrinolysis and extracellular proteolysis during processes in which thrombin is also involved.