The single-chain form of tissue-type plasminogen activator has catalytic activity: studies with a mutant enzyme that lacks the cleavage site

Activity-based protein profiling reveals active serine proteases that drive malignancy of human ovarian clear cell carcinoma

Ovarian clear cell carcinoma (OCCC) is an understudied poor prognosis subtype of ovarian cancer lacking in effective targeted therapies. Efforts to define molecular drivers of OCCC malignancy may lead to new therapeutic targets and approaches. Among potential targets are secreted proteases, enzymes which in many cancers serve as key drivers of malignant progression. Here, we found that inhibitors of trypsin-like serine proteases suppressed malignant phenotypes of OCCC cell lines. To identify the proteases responsible for malignancy in OCCC, we employed activity-based protein profiling to directly analyze enzyme activity. We developed an activity-based probe featuring an arginine diphenylphosphonate warhead to detect active serine proteases of trypsin-like specificity and a biotin handle to facilitate affinity purification of labeled proteases. Using this probe, we identified active trypsin-like serine proteases within the complex proteomes secreted by OCCC cell lines, including two proteases in common, tissue plasminogen activator and urokinase-type plasminogen activator. Further interrogation of these proteases showed that both were involved in cancer cell invasion and proliferation of OCCC cells and were also detected in in vivo models of OCCC. We conclude the detection of tissue plasminogen activator and urokinase-type plasminogen activator as catalytically active proteases and significant drivers of the malignant phenotype may point to these enzymes as targets for new therapeutic strategies in OCCC. Our activity-based probe and profiling methodology will also serve as a valuable tool for detection of active trypsin-like serine proteases in models of other cancers and other diseases.

A Novel and Potent Thrombolytic Fusion Protein Consisting of Anti-Insoluble Fibrin Antibody and Mutated Urokinase

Tissue plasminogen activator (tPA) is used clinically because it has a higher binding specificity for insoluble fibrin (IF) than urokinase (UK), but even pro-tPA has catalytic activity against substrates other than IF. UK has the advantage that it is specifically activated on IF; however, it binds IF weakly. Previously, we established a monoclonal antibody (mAb) that recognizes a pit structure formed only in IF. Here, we developed a new mAb against the pit, 1101, that does not affect coagulation or fibrinolysis, and prepared a fusion protein of UK with humanized 1101 Fab to transport UK selectively to IF. In IF-containing lesions, UK is cleaved by plasmin at two sites, Lys158/Ile159 and Lys135/Lys136. Cleavage of the former leads to activation of UK; however, because activated UK is linked by S-S bonds before and after cleavage, it is not released from the fusion. Cleavage at the latter site causes UK to leave the fusion protein; hence, we mutated Lys135/Lys136 to Gly135/Gly136 to prevent release of UK. This engineered UK-antibody fusion, AMU1114, significantly decreased the reduction of plasma plasminogen levels in vivo relative to UK. In a photochemically induced mouse model of thrombus, the vascular patency rate was 0% (0/10) in the control, 50% (5/10) in the tPA treatment group, and 90% (9/10) in the AMU1114 treatment group. Although no death was observed 1 hour after administration of each thrombolytic agent, some mice died within 24 hours in all treatment groups, including control. These data indicate the need for further basic studies of AMU1114.

Therapeutics targeting the fibrinolytic system

The function of the fibrinolytic system was first identified to dissolve fibrin to maintain vascular patency. Connections between the fibrinolytic system and many other physiological and pathological processes have been well established. Dysregulation of the fibrinolytic system is closely associated with multiple pathological conditions, including thrombosis, inflammation, cancer progression, and neuropathies. Thus, molecules in the fibrinolytic system are potent therapeutic and diagnostic targets. This review summarizes the currently used agents targeting this system and the development of novel therapeutic strategies in experimental studies. Future directions for the development of modulators of the fibrinolytic system are also discussed.

The fibrinolytic system was originally identified to dissolve blood clots, and is shown to have important roles in other pathological processes, including cancer progression, inflammation, and thrombosis. Molecules or therapeutics targeting fibrinolytic system have been successfully used in the clinical treatments of cancer and thrombotic diseases. The clinical studies and experimental models targeting fibrinolytic system are reviewed by Haili Lin at Sanming First Hosipital, Mingdong Huang at Fuzhou University in China, and Peng Xu at A*STAR in Singapore to demonstrate fibrinolytic system as novel therapeutic targets. As an example, the inhibition of fibrinolytic system protein can be used to suppress cancer prolifieration and metastasis. This review also discusses the potential therapeutic effects of inhibitiors of fibrinolytic system on inflammatory disorders.

Blood coagulation dissected

Hemostasis is the physiological control of bleeding and is initiated by subendothelial exposure. Platelets form the primary vascular seal in three stages (localization, stimulation and aggregation), which are triggered by specific interactions between platelet surface receptors and constituents of the subendothelial matrix. As a secondary hemostatic plug, fibrin clot formation is initiated and feedback-amplified to advance the seal and stabilize platelet aggregates comprising the primary plug. Once blood leakage has been halted, the fibrinolytic pathway is initiated to dissolve the clot and restore normal blood flow. Constitutive and induced anticoagulant and antifibrinolytic pathways create a physiological balance between too much and too little clot production. Hemostatic imbalance is a major burden to global healthcare, resulting in thrombosis or hemorrhage.

Optimized tPA: A non-neurotoxic fibrinolytic agent for the drainage of intracerebral hemorrhages

Intracerebral hemorrhage (ICH) is the most severe form of stroke. Catheter-delivered thrombolysis with recombinant tissue-type plasminogen activator (rtPA) for the drainage of ICH is currently under evaluation in a phase III clinical trial (MISTIE III). However, in a pig model of ICH, in situ fibrinolysis with rtPA was reported to increase peri-lesional edema by promoting N-methyl-D-aspartate (NMDA)-dependent excitotoxicity. In the present study, we engineered a non-neurotoxic tPA variant, OptPA, and investigated its safety and efficacy for in situ fibrinolysis in a rat model of ICH. Magnetic resonance imaging analyses of hematoma and edema volumes, behavioral tasks and histological analyses were performed to measure the effects of treatments. In vitro, OptPA was equally fibrinolytic as rtPA without promoting NMDA-dependent neurotoxicity. In vivo, in situ fibrinolysis using OptPA reduced hematoma volume, like rtPA, but it also reduced the evolution of peri-hematomal neuronal death and subsequent edema progression. Overall, this preclinical study demonstrates beneficial effects of OptPA compared to rtPA for the drainage of ICH.

SmSP2: A serine protease secreted by the blood fluke pathogen Schistosoma mansoni with anti-hemostatic properties

BACKGROUND

Serine proteases are important virulence factors for many pathogens. Recently, we discovered a group of trypsin-like serine proteases with domain organization unique to flatworm parasites and containing a thrombospondin type 1 repeat (TSR-1). These proteases are recognized as antigens during host infection and may prove useful as anthelminthic vaccines, however their molecular characteristics are under-studied. Here, we characterize the structural and proteolytic attributes of serine protease 2 (SmSP2) from Schistosoma mansoni, one of the major species responsible for the tropical infectious disease, schistosomiasis.

METHODOLOGY/PRINCIPAL FINDINGS

SmSP2 comprises three domains: a histidine stretch, TSR-1 and a serine protease domain. The cleavage specificity of recombinant SmSP2 was determined using positional scanning and multiplex combinatorial libraries and the determinants of specificity were identified with 3D homology models, demonstrating a trypsin-like endopeptidase mode of action. SmSP2 displayed restricted proteolysis on protein substrates. It activated tissue plasminogen activator and plasminogen as key components of the fibrinolytic system, and released the vasoregulatory peptide, kinin, from kininogen. SmSP2 was detected in the surface tegument, esophageal glands and reproductive organs of the adult parasite by immunofluorescence microscopy, and in the excretory/secretory products by immunoblotting.

CONCLUSIONS/SIGNIFICANCE

The data suggest that SmSP2 is secreted, functions at the host-parasite interface and contributes to the survival of the parasite by manipulating host vasodilatation and fibrinolysis. SmSP2 may be, therefore, a potential target for anti-schistosomal therapy.

Neuroserpin Differentiates Between Forms of Tissue Type Plasminogen Activator via pH Dependent Deacylation

Tissue-type plasminogen activator (t-PA), initially characterized for its critical role in fibrinolysis, also has key functions in both physiologic and pathologic processes in the CNS. Neuroserpin (NSP) is a t-PA specific serine protease inhibitor (serpin) found almost exclusively in the CNS that regulates t-PA's proteolytic activity and protects against t-PA mediated seizure propagation and blood-brain barrier disruption. This report demonstrates that NSP inhibition of t-PA varies profoundly as a function of pH within the biologically relevant pH range for the CNS, and reflects the stability, rather than the formation of NSP: t-PA acyl-enzyme complexes. Moreover, NSP differentiates between the zymogen-like single chain form (single chain t-PA, sct-PA) and the mature protease form (two chain t-PA, tct-PA) of t-PA, demonstrating different pH profiles for protease inhibition, different pH ranges over which catalytic deacylation occurs, and different pH dependent profiles of deacylation rates for each form of t-PA. NSP's pH dependent inhibition of t-PA is not accounted for by differential acylation, and is specific for the NSP-t-PA serpin-protease pair. These results demonstrate a novel mechanism for the differential regulation of the two forms of t-PA in the CNS, and suggest a potential specific regulatory role for CNS pH in controlling t-PA proteolytic activity.

Impacts of tissue-type plasminogen activator (tPA) on neuronal survival

Tissue-type plasminogen activator (tPA) a serine protease is constituted of five functional domains through which it interacts with different substrates, binding proteins, and receptors. In the last years, great interest has been given to the clinical relevance of targeting tPA in different diseases of the central nervous system, in particular stroke. Among its reported functions in the central nervous system, tPA displays both neurotrophic and neurotoxic effects. How can the protease mediate such opposite functions remain unclear but several hypotheses have been proposed. These include an influence of the degree of maturity and/or the type of neurons, of the level of tPA, of its origin (endogenous or exogenous) or of its form (single chain tPA versus two chain tPA). In this review, we will provide a synthetic snapshot of our current knowledge regarding the natural history of tPA and discuss how it sustains its pleiotropic functions with focus on excitotoxic/ischemic neuronal death and neuronal survival.

Single-chain factor XII exhibits activity when complexed to polyphosphate

BACKGROUND

The mechanism underpinning factor XII autoactivation was originally characterized with non-physiological surfaces, such as dextran sulfate (DS), ellagic acid, and kaolin. Several 'natural' anionic activating surfaces, such as platelet polyphosphate (polyP), have now been identified.

OBJECTIVE

To analyze the autoactivation of FXII by polyP of a similar length to that found in platelets (polyP70 ).

METHODS AND RESULTS

PolyP70 showed similar efficacy to DS in stimulating autoactivation of FXII, as detected with amidolytic substrate. Western blotting revealed different forms of FXII with the two activating surfaces: two-chain αFXIIa was formed with DS, whereas single-chain FXII (scFXII; 80 kDa) was formed with polyP70 . Dissociation of scFXII from polyP70 abrogated amidolytic activity, suggesting reversible exposure of the active site. Activity of scFXII-polyP70 was enhanced by Zn(2+) and was sensitive to NaCl concentration. A bell-shaped concentration response to polyP70 was evident, as is typical of surface-mediated reactions. Reaction of scFXII-polyP70 with various concentrations of S2302 generated a sigmoidal curve, in contrast to a hyperbolic curve for αFXIIa, from which a Hill coefficient of 3.67 was derived, indicative of positive cooperative binding. scFXII-polyP70 was more sensitive to inhibition by H-d-Pro-Phe-Arg-chloromethylketone and corn trypsin inhibitor than αFXIIa, but inhibition profiles for C1-inhibitor were similar. Active scFXII-polyP70 was also able to cleave its physiological targets FXI and prekallikrein to their active forms.

CONCLUSIONS

Autoactivation of FXII by polyP, of the size found in platelets, proceeds via an active single-chain intermediate. scFXII-polyP70 shows activity towards physiological substrates, and may represent the primary event in initiating contact activation in vivo.

Detection of active matriptase using a biotinylated chloromethyl ketone peptide

Matriptase is a member of the family of type II transmembrane serine proteases that is essential for development and maintenance of several epithelial tissues. Matriptase is synthesized as a single-chain zymogen precursor that is processed into a two-chain disulfide-linked form dependent on its own catalytic activity leading to the hypothesis that matriptase functions at the pinnacle of several protease induced signal cascades. Matriptase is usually found in either its zymogen form or in a complex with its cognate inhibitor hepatocyte growth factor activator inhibitor 1 (HAI-1), whereas the active non-inhibited form has been difficult to detect. In this study, we have developed an assay to detect enzymatically active non-inhibitor-complexed matriptase by using a biotinylated peptide substrate-based chloromethyl ketone (CMK) inhibitor. Covalently CMK peptide-bound matriptase is detected by streptavidin pull-down and subsequent analysis by Western blotting. This study presents a novel assay for detection of enzymatically active matriptase in living human and murine cells. The assay can be applied to a variety of cell systems and species.

Fibrils colocalize caspase-3 with procaspase-3 to foster maturation

Most proteases are expressed as inactive precursors, or zymogens, that become activated by limited proteolysis. We previously identified a small molecule, termed 1541, that dramatically promotes the maturation of the zymogen, procaspase-3, to its mature form, caspase-3. Surprisingly, compound 1541 self-assembles into nanofibrils, and localization of procaspase-3 to the fibrils promotes activation. Here, we interrogate the biochemical mechanism of procaspase-3 activation on 1541 fibrils in addition to proteogenic amyloid-β(1-40) fibrils. In contrast to previous reports, we find no evidence that procaspase-3 alone is capable of self-activation, consistent with its fate-determining role in executing apoptosis. In fact, mature caspase-3 is >10(7)-fold more active than procaspase-3, making this proenzyme a remarkably inactive zymogen. However, we also show that fibril-induced colocalization of trace amounts of caspase-3 or other initiator proteases with procaspase-3 dramatically stimulates maturation of the proenzyme in vitro. Thus, similar to known cellular signaling complexes, these synthetic or natural fibrils can serve as platforms to concentrate procaspase-3 for trans-activation by upstream proteases.

The optimal activity of a pseudozymogen form of recombinant matriptase under the mildly acidic pH and low ionic strength conditions

Matriptase is a transmembrane serine protease that is strongly expressed in epithelial cells. The single-chain zymogen of matriptase is considered to have inherent activity, leading to its own activation (i.e. conversion to the disulphide-linked-two-chain form by cleavage after Thr-Lys-Gln-Ala-Arg614). Also, there is growing evidence that the activation of zymogen occurs at the cell surface and in relation to the acidification and lowering of ionic strength within cell-surface microenvironments. The present study aimed to provide evidence for the involvement of zymogen activity in its activation in physiologically relevant cellular contexts. For this purpose, the activity of a pseudozymogen form of recombinant matriptase (HL-matriptase zymogen) was examined using acetyl-l-Lys-l-Thr-l-Lys-l-Gln-l-Leu-l-Arg-4-methyl-coumaryl-7-amide as a substrate. HL-matriptase zymogen exhibited optimal activity toward the substrate pH approximately 6.0. The substrate hydrolysis at the pH value was hardly detected when NaCl was present at a concentration of 145 mM. In a buffer of pH 6.0 containing 5 mM NaCl, the activity of HL-matriptase zymogen was only approximately 30-times lower than that of the respective two-chain form. These findings suggest that the in vivo activation of matriptase zymogen occurs via a mechanism involving the zymogen activity.

Regulation of intrapleural fibrinolysis by urokinase-alpha-macroglobulin complexes in tetracycline-induced pleural injury in rabbits

The proenzyme single-chain urokinase plasminogen activator (scuPA) more effectively resolved intrapleural loculations in rabbits with tetracycline (TCN)-induced loculation than a range of clinical doses of two-chain uPA (Abbokinase) and demonstrated a trend toward greater efficacy than single-chain tPA (Activase) (Idell S et al., Exp Lung Res 33: 419, 2007.). scuPA more slowly generates durable intrapleural fibrinolytic activity than Abbokinase or Activase, but the interactions of these agents with inhibitors in pleural fluids (PFs) have been poorly understood. PFs from rabbits with TCN-induced pleural injury treated with intrapleural scuPA, its inactive Ser195Ala mutant, Abbokinase, Activase, or vehicle, were analyzed to define the mechanism by which scuPA induces durable fibrinolysis. uPA activity was elevated in PFs of animals treated with scuPA, correlated with the ability to clear pleural loculations, and resisted (70-80%) inhibition by PAI-1. Alpha-macroglobulin (alphaM) but not urokinase receptor complexes immunoprecipitated from PFs of scuPA-treated rabbits retained uPA activity that resists PAI-1 and activates plasminogen. Conversely, little plasminogen activating or enzymatic activity resistant to PAI-1 was detectable in PFs of rabbits treated with Abbokinase or Activase. Consistent with these findings, PAI-1 interacts with scuPA much slower than with Activase or Abbokinase in vitro. An equilibrium between active and inactive scuPA (k(on) = 4.3 h(-1)) limits the rate of its inactivation by PAI-1, favoring formation of complexes with alphaM. These observations define a newly recognized mechanism that promotes durable intrapleural fibrinolysis via formation of alphaM/uPA complexes. These complexes promote uPA-mediated plasminogen activation in scuPA-treated rabbits with TCN-induced pleural injury.

Disulfide locked variants of factor VIIa with a restricted beta-strand conformation have enhanced enzymatic activity

Proteolytic processing of zymogen Factor VII to Factor VIIa (FVIIa) is necessary but not sufficient for maximal proteolytic activity, which requires an additional allosteric influence induced upon binding to its cofactor tissue factor (TF). A key conformational change affecting the zymogenicity of FVIIa involves a unique three-residue shift in the position of beta-strand B2 in their zymogen and protease forms. By selectively introducing new disulfide bonds, we locked the conformation of these strands into an active TF*FVIIa-like state. FVIIa mutants designated 136:160, 137:159, 138:160, and 139:157, reflecting the position of the new disulfide bond (chymotypsinogen numbering), were expressed and purified by TF affinity chromatography. Mass spectrometric analysis of tryptic peptides from the FVIIa mutants confirmed the new disulfide bond formation. Kinetic analysis of amidolytic activity revealed that all FVIIa variants alone had increased specific activity compared to wild type, the largest being for variants 136:160 and 138:160 with substrate S-2765, having 670- and 330-fold increases, respectively. Notably, FVIIa disulfide-locked variants no longer required TF as a cofactor for maximal activity in amidolytic assays. In the presence of soluble TF, activity was enhanced 20- and 12-fold for variants 136:160 and 138:160, respectively, compared to wild type. With relipidated TF, mutants 136:160 and 137:159 also had an approximate threefold increase in their V(max)/K(m) values for FX activation but no significant improvement in TF-dependent clotting assays. Thus, while large rate enhancements were obtained for amidolytic substrates binding at the active site, macro-molecular substrates that bind to FVIIa exosites entail more complex catalytic requirements.

Structural and functional basis of the serine protease-like hepatocyte growth factor beta-chain in Met binding and signaling

Hepatocyte growth factor (HGF), a plasminogen-related growth factor, is the ligand for Met, a receptor tyrosine kinase implicated in development, tissue regeneration, and invasive tumor growth. HGF acquires signaling activity only upon proteolytic cleavage of single-chain HGF into its alpha/beta heterodimer, similar to zymogen activation of structurally related serine proteases. Although both chains are required for activation, only the alpha-chain binds Met with high affinity. Recently, we reported that the protease-like HGF beta-chain binds to Met with low affinity (Stamos, J., Lazarus, R. A., Yao, X., Kirchhofer, D., and Wiesmann, C. (2004) EMBO J. 23, 2325-2335). Here we demonstrate that the zymogen-like form of HGF beta also binds Met, albeit with 14-fold lower affinity than the protease-like form, suggesting optimal interactions result from conformational changes upon cleavage of the single-chain form. Extensive mutagenesis of the HGF beta region corresponding to the active site and activation domain of serine proteases showed that 17 of the 38 purified two-chain HGF mutants resulted in impaired cell migration or Met phosphorylation but no loss in Met binding. However, reduced biological activities were well correlated with reduced Met binding of corresponding mutants of HGF beta itself in assays eliminating dominant alpha-chain binding contributions. Moreover, the crystal structure of HGF beta determined at 2.53 A resolution provides a structural context for the mutagenesis data. The functional Met binding site is centered on the "active site region" including "triad" residues Gln(534) [c57], Asp(578) [c102], and Tyr(673) [c195] and neighboring "activation domain" residues Val(692), Pro(693), Gly(694), Arg(695), and Gly(696) [c214-c219]. Together they define a region that bears remarkable resemblance to substrate processing regions of serine proteases. Models of HGF-dependent Met receptor activation are discussed.

Acyl-enzyme complexes between tissue-type plasminogen activator and neuroserpin are short-lived in vitro

The serine protease tissue-type plasminogen activator (t-PA) initiates the fibrinolytic protease cascade and plays a significant role in motor learning, memory, and neuronal cell death induced by excitotoxin and ischemia. In the fibrinolytic system, the serpin PAI-1 negatively regulates the enzymatic activity of both single-chain and two-chain t-PA (sct-PA and tct-PA). In the central nervous system, neuroserpin (NSP) is a serpin thought to regulate t-PA enzymatic activity. We report that although both sct-PA and tct-PA rapidly form acyl-enzyme complexes with NSP in vitro, the interactions are short-lived, rapidly progressing to complete cleavage of NSP and regeneration of fully active enzyme. All NSP molecules appear to transit through the detectable acyl-enzyme intermediate and progress to completion of cleavage; no subpopulation that functions as a pure substrate was detected. Likewise, all molecules were reactive, with no evidence of a latent subpopulation. The interactions between NSP and t-PA were distinct from those between plasmin and NSP, wherein the same peptide bond was cleaved but there was no evidence of a detectable plasmin-NSP acyl-enzyme complex. The interactions between t-PA and NSP contrast with the formation of long-lived, physiologically irreversible acyl-enzyme complexes between t-PA and PAI-1, suggesting that the physiologic effect of t-PA-NSP interactions may be more complex than previously thought.

Dual cell seeding and the use of zymogen tissue plasminogen activator to improve cell retention on polytetrafluoroethylene grafts

OBJECTIVE

The purpose of this study was to enhance the retention of seeded endothelial cells (EC) on prosthetic vascular grafts. Dual-layer EC and smooth muscle cell (SMC) seeding and gene transfer of a zymogen tissue plasminogen activator gene (tPA) into seeded EC were studied.

METHODS

Polytetrafluoroethylene (PTFE) grafts were precoated with fibronectin, seeded with SMC followed by EC a day later, and then, 24 hours later, exposed to an in vitro flow system for 1 hour. Cell retention rates were determined for grafts seeded with EC only, a dual layer of EC on top of SMC, EC transduced with wild-type tPA, and EC transduced with zymogen tPA.

RESULTS

Seeding efficiency of PTFE pretreated with fibronectin was 260 +/- 8 cell/mm(2). After exposure to flow, only 39% +/- 14% of the EC were retained when EC were seeded alone, whereas 73% +/- 22% of EC remained on grafts when EC were seeded on top of SMC (P <.001, n = 10). The enzyme activity of a mutant zymogen tPA in absence of fibrin was 14 +/- 1 IU/mL, which is 3.6-fold lower than that in the presence of fibrin (50 +/- 19 IU/mL), whereas fibrin has no effect on the wild-type tPA activity. EC expressing a high level of wild-type tPA had a lower retention rate (37%) when compared with normal EC (45%). EC expressing the mutant zymogen tPA had an improved retention rate (54%, P =.001, n = 10) in absence of fibrin, whereas its retention rate was reduced to 43% when the cells were exposed to fibrin.

CONCLUSION

SMC seeded between EC and PTFE improves EC retention in vitro. Transduction of zymogen tPA increases thrombolytic ability of seeded cells with less adverse impact on cell retention than wild-type tPA.

The energetic cost of induced fit catalysis: Crystal structures of trypsinogen mutants with enhanced activity and inhibitor affinity

The contribution of induced fit to enzyme specificity has been much debated, although with little experimental data. Here we probe the effect of induced fit on enzyme specificity using the trypsin(ogen) system. BPTI is known to induce trypsinogen to assume a trypsinlike conformation. Correlations are observed between BPTI affinity and the values of k(cat)/K(m) for the hydrolysis of two substrates by eight trypsin(ogen) variants. The slope of both correlations is -1.8. The crystal structures of the BPTI complexes of four variant trypsinogens were also solved. Three of these enzymes, K15A, DeltaI16V17/D194N, and DeltaI16V17/Q156K trypsinogen, are 10- to 100-fold more active than trypsinogen. The fourth variant, DeltaI16V17 trypsinogen, is the lone outlier in the correlations; its activity is lower than expected based on its affinity for BPTI. The S1 site and oxyanion hole, formed by segments 184A-194 and 216-223, are trypsinlike in all of the enzymes. These structural and kinetic data confirm that BPTI induces an active conformation in the trypsin(ogen) variants. Thus, changes in BPTI affinity monitor changes in the energetic cost of inducing a trypsinlike conformation. Although the S1 site and oxyanion hole are similar in all four variants, the N-terminal and autolysis loop (residues 142-152) segments have different interactions for each variant. These results indicate that zymogen activity is controlled by a simple conformational equilibrium between active and inactive conformations, and that the autolysis loop and N-terminal segments control this equilibrium. Together, these data illustrate that induced fit does not generally contribute to enzyme specificity.

Zymogen activation in the streptokinase-plasminogen complex. Ile1 is required for the formation of a functional active site

Plasminogen (Plgn) is usually activated by proteolysis of the Arg561-Val562 bond. The amino group of Val562 forms a salt-bridge with Asp740, which triggers a conformational change producing the active protease plasmin (Pm). In contrast, streptokinase (SK) binds to Plgn to produce an initial inactive complex (SK.Plgn) which subsequently rearranges to an active complex (SK.Plgn*) although the Arg561-Val562 bond remains intact. Therefore another residue must substitute for the amino group of Val562 and provide a counterion for Asp740 in this active complex. Two candidates for this counterion have been suggested: Ile1 of streptokinase and Lys698 of Plgn. We have investigated the reaction of SK mutants and variants of the protease domain of microplasminogen (muPlgn) in order to determine if either of these residues is the counterion. The mutation of Ile1 of SK decreases the activity of SK.Plgn* by 100-fold (Ile1Val) to >/= 104-fold (Ile1-->Ala, Gly, Trp or Lys). None of these mutations perturb the binding affinity of SK, which suggests that Ile1 is not required for formation of SK.Plgn but is necessary for SK.Plgn*. The substitution of Lys698 of muPlgn decreases the activity of SK.Plgn* by only 10-60-fold. In contrast with the Ile1 substitutions, the Lys698 mutations also decreased the dissociation constant of the SK complex by 15-50-fold. These observations suggest that Lys698 is involved in formation of the initial SK.Plgn complex. These results support the hypothesis that Ile1 provides the counterion for Asp740.

Distinct contributions of residue 192 to the specificity of coagulation and fibrinolytic serine proteases

Archetypal members of the chymotrypsin family of serine proteases, such as trypsin, chymotrypsin, and elastase, exhibit relatively broad substrate specificity. However, the successful development of efficient proteolytic cascades, such as the blood coagulation and fibrinolytic systems, required the evolution of proteases that displayed restricted specificity. Tissue-type plasminogen activator (t-PA), for example, possesses exquisitely stringent substrate specificity, and the molecular basis of this important biochemical property of t-PA remains obscure. Previous investigations of related serine proteases, which participate in the blood coagulation cascade, have focused attention on the residue that occupies position 192 (chymotrypsin numbering system), which plays a pivotal role in determining both the inhibitor and substrate specificity of these enzymes. Consequently, we created and characterized the kinetic properties of new variants of t-PA that contained point mutations at position 192. These studies demonstrated that, unlike in coagulation serine proteases, Gln-192 does not contribute significantly to the substrate or inhibitor specificity of t-PA in physiologically relevant reactions. Replacement of Gln-192 with a glutamic acid residue did, however, decrease the catalytic efficiency of mature, two-chain t-PA toward plasminogen in the absence of a fibrin co-factor.

Activating a zymogen without proteolytic processing: mutation of Lys15 and Asn194 activates trypsinogen

The zymogen and mature enzyme forms of trypsin-like serine proteases exhibit a wide range of activities. The prototypical trypsinogen-trypsin system is an example of a minimally active zymogen and a maximally active mature protease. The present work identifies several features of trypsinogen which govern its activity. Our results indicate that rat trypsin is 10(8)-fold more active than rat trypsinogen. Rat trypsinogen appears to be less active than bovine trypsinogen. His40 is believed to be an important determinant of zymogen activity. We are unable to verify this role for His40 in trypsinogen since the mutation of His40 to Phe appears to change the trypsin-substrate interface. Deletion of the N-terminal Ile16 from trypsin is expected to produce a trypsinogen-like protein since the Ile16-Asp194 salt bridge cannot form. Such mutants have higher activity and BPTI affinity than trypsinogen, which indicates that the activation peptide stabilizes the inactive trypsinogen conformation. The mutation of Lys15 to Ala increases the BPTI affinity and activity of trypsinogen to an even greater extent; thus, removal of Lys15 can account for the effect of the loss of the activation peptide. These results suggest that Lys15 is an important determinant of zymogen activity. The mutation of Asp194 to Asn also increases the BPTI affinity and activity of trypsinogen. This result suggests that in addition to stabilizing the active conformation of trypsin via the Ile16-Asp194 salt bridge, Asp194 also maintains the inactive conformation of trypsinogen. A correlation exists between the values of kcat/Km and BPTI affinity of mutant trypsinogens and trypsins. However, the slope of this correlation is 0.64, which indicates that different "active" conformations are involved in BPTI binding and substrate hydrolysis. DeltaI16V17 trypsinogen is the lone outlier; its BPTI affinity is higher than would be expected based on the value of kcat/Km. We show that the rate of BPTI association is slower for DeltaI16V17 trypsinogen than for a mutant trypsinogen with a similar BPTI affinity. This observation suggests that BPTI binds to an "active" trypsinogen conformation that is not kinetically accessible to substrates.

The axonally secreted serine proteinase inhibitor, neuroserpin, inhibits plasminogen activators and plasmin but not thrombin

Neuroserpin is an axonally secreted serine proteinase inhibitor that is expressed in neurons during embryogenesis and in the adult nervous system. To identify target proteinases, we used a eucaryotic expression system based on the mouse myeloma cell line J558L and vectors including a promoter from an Ig-kappa-variable region, an Ig-kappa enhancer, and the exon encoding the Ig-kappa constant region (C kappa) and produced recombinant neuroserpin as a wild-type protein or as a fusion protein with C kappa. We investigated the capability of recombinant neuroserpin to form SDS-stable complexes with, and to reduce the amidolytic activity of, a variety of serine proteinases in vitro. Consistent with its primary structure at the reactive site, neuroserpin exhibited inhibitory activity against trypsin-like proteinases. Although neuroserpin bound and inactivated plasminogen activators and plasmin, no interaction was observed with thrombin. A reactive site mutant of neuroserpin neither formed complexes with nor inhibited the amidolytic activity of any of the tested proteinases. Kinetic analysis of the inhibitory activity revealed neuroserpin to be a slow binding inhibitor of plasminogen activators and plasmin. Thus, we postulate that neuroserpin could represent a regulatory element of extracellular proteolytic events in the nervous system mediated by plasminogen activators or plasmin.

Identification of a hydrophobic exosite on tissue type plasminogen activator that modulates specificity for plasminogen

A wide variety of important biological processes, including both the formation and dissolution of blood clots, depend on specific cleavage of individual target proteins by serine proteases. For example, tissue type plasminogen activator (t-PA), a trypsin-like enzyme that catalyzes the rate-limiting step of the endogenous fibrinolytic cascade, has only one known substrate in vivo, a single peptide bond (Arg561-Val562) in the proenzyme plasminogen. We have previously suggested that the specificity of t-PA for plasminogen is mediated in part by direct protein-protein interactions between the protease domain of t-PA and plasminogen that are distinct from those occurring within t-PA's active site. We demonstrate in this study that residues 420-423 of t-PA, which form a fully solvent-exposed, hydrophobic region of a surface loop mapping near one edge of the active site of t-PA, form, or are essential for the integrity of, an important, secondary site of interaction between t-PA and plasminogen that significantly modulates the rate of plasminogen activation in the absence, but not the presence, of fibrin. Identification of this secondary site of interaction between t-PA and plasminogen provides new insight into molecular details of the evolution of stringent substrate specificity by t-PA and suggests a novel strategy to enhance the fibrin dependence of plasminogen activation by t-PA. While the activity of wild type t-PA is stimulated by fibrin by a factor of approximately 650, the activity of two variants characterized in this study, t-PA/R275E,P422G and t-PA/R275E,P422E, is stimulated by a factor of approximately 39,000 or 61,000, respectively. It is therefore possible that, compared with wild type t-PA, the two variants would display enhanced "clot selectivity" in vivo due to reduced activity in the circulation but full activity at a site of fibrin deposition.

Converting tissue type plasminogen activator into a zymogen. Important role of Lys156

In stark contrast to most other members of the chymotrypsin family of serine proteases, tissue type plasminogen activator (t-PA) is not synthesized and secreted as a true zymogen. Instead, single-chain t-PA exhibits very significant catalytic activity. Consequently, the zymogenicity, or ratio of the catalytic efficiencies of the mature, two-chain enzyme and the single-chain precursor, is only 3-9 for t-PA. Both we and others have previously proposed that Lys156 may contribute directly to this exceptional property of t-PA by forming interactions that selectively stabilize the active conformation of the single-chain enzyme. To test this hypothesis we created variants of t-PA in which Lys156 was replaced by a tyrosine residue. As predicted, the K156Y mutation selectively suppressed the activity of the single-chain enzyme and thereby substantially enhanced the enzyme's zymogenicity. In addition, however, this mutation produced a very dramatic increase in the ability of single-chain t-PA to discriminate among distinct fibrin co-factors. Compared with wild type t-PA, one of the variants characterized in this study, t-PA/R15E,K156Y, possessed substantially enhanced response to and selectivity among fibrin co-factors, resistance to inhibition by plasminogen activator inhibitor type 1, and significantly increased zymogenicity. The combination of these properties, and the maintenance of full activity in the presence of fibrin, suggest that the R15E,K156Y mutations may extend the therapeutic range of t-PA.

Converting tissue-type plasminogen activator into a zymogen

In striking contrast to most other members of the chymotrypsin family of serine proteases, tissue-type plasminogen activator (t-PA) is not synthesized and secreted as a true zymogen. The zymogenicity, or ratio of the catalytic efficiencies of the mature, two-chain enzyme and the single-chain precursor, is only 5-10 for t-PA. This exceptional property of t-PA, however, is not shared by urokinase (u-PA), a plasminogen activator that is very closely related to t-PA. The molecular basis of this important functional distinction between these two intimately related serine proteases has not been previously investigated. Based on observation of the recently described structures of the protease domains of two-chain t-PA and u-PA, and molecular modeling of the corresponding single-chain enzymes, we propose that the presence or absence of an acidic residue at position 144 (chymotrypsin numbering system) is the primary determinant of the distinct zymogenicities of the two enzymes. Consistent with this hypothesis, mutation of histidine 144 of t-PA to an acidic residue, as in u-PA, selectively suppressed the activity of single-chain t-PA and thereby significantly enhanced the enzyme's zymogenicity. A variant of t-PA containing an aspartate residue at position 144, for example, exhibited a zymogenicity of 150, compared to a value of 9 for wild type t-PA and 250 for u-PA.

Analogs of human plasminogen that are labeled with fluorescence probes at the catalytic site of the zymogen. Preparation, characterization, and interaction with streptokinase

Fluorescent analogs of the proteinase zymogen, plasminogen (Pg), which are specifically inactivated and labeled at the catalytic site have been prepared and characterized as probes of the mechanisms of Pg activation. The active site induced non-proteolytically in Pg by streptokinase (SK) was inactivated stoichiometrically with the thioester peptide chloromethyl ketone. N alpha-[(acetylthio)acetyl]-(D-Phe)-Phe-Arg-CH2Cl; the thiol group generated subsequently on the incorporated inhibitor with NH2OH was quantitatively labeled with the fluorescence probe, 2-((4'-iodoacetamido)anilino)naphthalene-6-sulfonic acid; and the labeled Pg was separated from SK. Cleavage of labeled [Glu]Pg1 by urokinase-type plasminogen activator (uPA) was accompanied by a fluorescence enhancement (delta Fmax/Fo) of 2.0, and formation of 1% plasmin (Pm) activity. Comparison of labeled and native [Glu]Pg1 as uPA substrates showed that activation of labeled [Glu]Pg1 generated [Glu]Pm1 as the major product, while native [Glu]Pg1 was activated at a faster rate and produced [Lys]Pm1 because of concurrent proteolysis by plasmin. When a mixture of labeled and native Pg was activated, to include plasmin-feedback reactions, the zymogens were activated at equivalent rates. The lack of potential proteolytic activity of the Pg derivatives allowed their interactions with SK to be studied under equilibrium binding conditions. SK bound to labeled [Glu]Pg1, and [Lys]Pg1 with dissociation constants of 590 +/- 110 and 110 and 11 +/- 7 nM, and fluorescence enhancements of 3.1 +/- 0.1 and 1.6 +/- 0.1, respectively. Characterization of the interaction of SK with native [Glu]Pg1 by the use of labeled [Glu]Pg1 as a probe indicated a approximately 6-fold higher affinity of SK for the native Pg zymogen compared to the labeled Pg analog. Saturating levels of epsilon-aminocaproic acid reduced the affinity of SK for labeled [Glu]Pg1 by approximately 2-fold and lowered the fluorescence enhancement to 1.8 +/- 0.1, whereas the affinity of SK for labeled [Lys]Pg1 was reduced by approximately 98-fold with little effect on the enhancement. These results demonstrate that occupation of lysine binding sites modulates the affinity of SK for Pg and the changes in the environment of the catalytic site associated with SK-induced conformational activation. Together, these studies show that the labeled Pg derivatives behave as analogs of native Pg which report functionally significant changes in the environment of the catalytic site of the zymogen.

Structural features mediating fibrin selectivity of vampire bat plasminogen activators

The distinguishing characteristic of vampire bat (Desmodus rotundus) salivary plasminogen activators (DSPAs) is their strict requirement for fibrin as a cofactor. DSPAs consist of structural modules known from urokinase (u-PA) and tissue-type plasminogen activator (t-PA) such as finger (F), epidermal growth factor (E), kringle (K), and protease (P), combining to four genetically and biochemically distinct isoenzymes, exhibiting the formulas FEKP (DSPA alpha 1 and alpha 2) and EKP and KP (DSPA beta and DSPA gamma). Only DSPA alpha 1 and alpha 2 bind to fibrin. All DSPAs are single-chain molecules, displaying substantial amidolytic activity. In a plasminogen activation assay, all four DSPAs are almost inactive in the absence of fibrin but strongly stimulated by fibrin addition. The catalytic efficiency (kcat/Km) of DSPA alpha 1 increases 10(5)-fold, whereas the corresponding value of t-PA is only 550. The ratio of the bimolecular rate constants of plasminogen activation in the presence of fibrin versus fibrinogen (fibrin selectivity) of DSPA alpha 1, alpha 2, beta, gamma, and t-PA was found to be 13,000, 6500, 250, 90, and 72, respectively. Whereas all DSPAs are therefore more fibrin dependent and fibrin selective than t-PA, the extent depends on the respective presence of the various domains. The introduction of a plasmin-sensitive cleavage site in a position akin to the one in t-PA partially obliterates fibrin cofactor requirement. Fibrin dependence and fibrin selectivity of DSPAs are accordingly mediated by fibrin binding, which involves the F domain, as yet undefined determinants within the K and P domains, and by the absence of a plasmin-sensitive activation site. These findings transcend the current understanding of fibrin-mediated stimulation of plasminogen activation: in addition to fibrin binding, specific protein-protein interactions come into play, which stabilize the enzyme in its active conformation.

Variants of tissue-type plasminogen activator with substantially enhanced response and selectivity toward fibrin co-factors

Unlike most proteases, tissue-type plasminogen activator (t-PA) is not synthesized as an inactive precursor or zymogen. Instead, the single-chain "proenzyme" form of t-PA possesses very significant catalytic activity. Recent investigations of the molecular basis of the unusually high enzymatic activity of single-chain t-PA have focused attention upon Asp-194, a residue that is invariant among chymotrypsin-like enzymes. The critical role of this residue in securing the active conformation of mature chymotrypsin-like enzymes has been discussed extensively. Subsequent work, however, has indicated that this conserved residue can also form interactions that dramatically influence the catalytic activity of serine protease zymogens. While Asp-194 forms interactions that suppress the activity of the zymogen chymotrypsinogen, it may, by contrast, directly promote the catalytically active conformation of single-chain t-PA. To test the hypothesis that Asp-194 promotes the activity of both single- and two-chain t-PA and therefore plays opposing roles in single-chain t-PA and chymotrypsinogen, and also to examine whether this invariant residue plays an essential role in the stimulation of t-PA by fibrin, we used site-directed mutagenesis to construct the following variants of t-PA: t-PA/D194E, t-PA/D194N, t-PA/R15E,D194E, and t-PA/R15E,D194N. In the absence of fibrin, the activity of enzymes carrying a mutation at position 194 was reduced by factors of 1000-2000 compared to wild type t-PA. Similar reductions of activity were observed for both single- and two-chain variants, suggesting an important role for Asp-194 in both forms of the enzyme. The mutated enzymes, however, displayed a dramatically enhanced response to fibrin monomers. While the activity of wild type t-PA was stimulated by fibrin monomers by a factor of 960, the corresponding stimulation factor for the mutated enzymes varied from 498,000-1,050,000.

Variants of tissue-type plasminogen activator which display substantially enhanced stimulation by fibrin

Unlike most proteases, tissue-type plasminogen activator (t-PA) is secreted from cells as an active, single chain "proenzyme" whose catalytic efficiency is comparable with that of the corresponding mature, two-chain enzyme. We have previously suggested that the absence of the "zymogen triad" (Asp194-His40-Ser32; chymotrypsin numbering) contributes to this unusually high enzymatic activity of single chain t-PA. Consistent with this prediction, the single chain form of a variant of t-PA containing the zymogen triad displayed dramatically reduced activity toward synthetic substrates. Activation cleavage of this variant, however, resulted in a mature, two-chain enzyme with full catalytic activity. To further examine the functional significance of the zymogen triad, we used site-specific mutagenesis to construct a variant of t-PA, t-PA/R275E,A292S,F305H, that contained this triad but could not be converted into its two-chain form by plasmin. Characterization of this variant demonstrated that the presence of the zymogen triad specifically suppressed plasminogen activation by single chain t-PA in the absence of fibrin. In addition, these studies indicated that, like wild type t-PA, zymogen activation of this variant could be accomplished by binding to the co-factor fibrin. The combination of full activity in the presence of fibrin and reduced activity in its absence resulted in novel variants of t-PA that displayed dramatically enhanced stimulation by fibrin. While the presence of fibrin increased the catalytic efficiency of t-PA toward plasminogen by a factor of approximately 520, this stimulation factor increased to 130,000 for t-PA/R275E,A292S,F305H. Plasmin-resistant, zymogen-like variants of t-PA, therefore, may represent thrombolytic enzymes with enhanced "clot selectivity."

Substrate specificity of tissue type plasminogen activator. Characterization of the fibrin independent specificity of t-PA for plasminogen

Tissue-type plasminogen activator (t-PA) is a remarkably specific protease: the only known substrate of this enzyme in vivo is a single peptide bond (Arg560-Val561) within the proenzyme plasminogen. Part of the substrate specificity of t-PA is due to a ternary interaction between fibrin, t-PA and plasminogen which reduces the Km of t-PA for plasminogen by a factor of 440. However, even in the absence of fibrin, t-PA continues to hydrolyze plasminogen more rapidly than does trypsin, a homologous serine protease. We have measured the extent of the specificity of t-PA for plasminogen by assaying t-PA and trypsin toward substrates modeled after the peptide sequence in plasminogen surrounding Arg560-Val561. Surprisingly, t-PA hydrolyzes these substrates with kcat/Km values which are 28,000-210,000-fold lower than those obtained using trypsin. Both the high activity toward plasminogen and the low activity toward peptides are also exhibited by the isolated protease domain. This suggests that the protease domain, in spite of its high homology to the nonspecific enzyme trypsin, is inherently specific for recognition of one or more structural features displayed by native plasminogen.

Dual effect of synthetic plasmin substrates on plasminogen activation

The effect of plasmin substrates D-valyl-L-leucyl-lysine-p-nitroanilide (S-2251) and H-D-norleucyl-hexahydrotyrosyl-lysine-p-nitro-anilide (Spectrozyme-PL) on the rate of activation of native human plasminogen in physiological salt solution is studied. Plasminogen activation by two-chain urokinase-type plasminogen activator (urokinase), two-chain tissue-type plasminogen activator (tc-tPA) or trypsin, but not by single chain tPA (sc-tPA) is increased 5- to 10-fold by both substrates, as determined by electrophoretic and spectrophotometric kinetic analysis. The amidolytic activity of sc-tPA, on the other hand, is inhibited by the plasmin substrates in a non-competitive manner (K1 of 6.4 . 10(-4) M for S-2251 and 2.9 . 10(-4) M for Spectrozyme-PL), whereas urokinase and tc-tPA activities are not affected. It is concluded that plasmin substrates containing a lysine residue have a general capacity to enhance plasminogen activation presumably by inducing a conformational change in the native zymogen in a manner similar to 6-aminohexanoate, while the same substrates are inhibitory both on the amidolytic activity of sc-tPA and the activation of native and des1-77-plasminogen by sc-tPA.

Converting tissue plasminogen activator to a zymogen: a regulatory triad of Asp-His-Ser

Unlike most serine proteases of the chymotrypsin family, tissue-type plasminogen activator (tPA) is secreted from cells as an active, single-chain enzyme with a catalytic efficiency only slightly lower than that of the proteolytically cleaved form. A zymogenic mutant of tPA has been engineered that displays a reduction in catalytic efficiency by a factor of 141 in the single-chain form while retaining full activity in the cleaved form. The residues introduced in the mutant, serine 292 and histidine 305, are proposed to form a hydrogen-bonded network with aspartate 477, similar to the aspartate 194-histidine 40-serine 32 network found to stabilize the zymogen chymotrypsinogen.

Physical characterization of recombinant tissue plasminogen activator

Electron microscopic and physical-chemical properties of one- and two-chain tissue plasminogen activator (t-PA) were studied. The molecular weight of one-chain t-PA obtained by both sedimentation equilibrium and SDS-PAGE was estimated to be about 65,000, while both chains in the reduced two-chain form were in the range of 35,000-40,000. Sedimentation coefficients were identical for both forms of t-PA (S(0)20,w = 4.12). The two forms of t-PA were indistinguishable by electron microscopic analysis, which confirmed the sedimentation results, and showed that they were ellipsoidal and relatively compact. The major and minor axes were approx. 13 nm and approx. 10 nm and f/f0 was 1.36. The individual domains of t-PA are relatively small and are folded within the molecule, so that the overall appearance is globular.

Tissue and urokinase plasminogen activators in bone tissue and their regulation by parathyroid hormone

The identification of the plasminogen activator (PA) types present in bone and the regulation of their activity by parathyroid hormone (PTH) were investigated in cultures of fetal mouse calvariae with the use of either a chromogenic substrate or a zymographic assay. PA was detected essentially in the tissue extracts of the explanted bones, with only 1-2% of the total activity released in the surrounding culture media. From their electrophoretic behavior compared to PAs of other mouse tissues and from their response to a specific antibody raised against the tissue type PA (tPA), two major molecular species, of 70 and 48 kD were identified as tPA and urokinase (uPA), respectively, a third minor species of 105 kD being likely to correspond to complexes between tPA and an inhibitor; the culture fluids, moreover, contained enzymatically active degradation products of uPA of 42 and 29 kD. The PA activity of the bone extracts was only minimally affected by the addition of fibrinogen fragments to the chromogenic assays. PTH induced bone resorption and stimulated in parallel the accumulation of PA in the tissue; other bone-resorbing agents, 1,25-dihydroxyvitamin D3 and prostaglandin E2, had similar effects. Densitometric scanning of the zymograms of the bone extracts indicated that PTH stimulated only the production of tPA and had no effect on that of uPA. However, PTH also enhanced the release of uPA (both the 48 kD and the 29 kD forms) from the bones into the media. Although inhibiting bone resorption, calcitonin had no effect on the PTH-induced accumulation of PA in bone or on the release of tPA, but it prevented the PTH-induced accumulation of 29 kD uPA in the culture fluids. Thus these studies support the view that tPA and possibly also uPA may have a role in the physiology of bone; the nature of this role remains to be elucidated, however.

Effects of intact fibrin and partially plasmin-degraded fibrin on kinetic properties of one-chain tissue-type plasminogen activator

A comparative kinetic analysis of the enzymatic activities of one-chain and two-chain tissue-type plasminogen activator (t-PA) demonstrates that two-chain t-PA catalyzes the hydrolysis of the peptide substrate D-Val-Leu-Arg-pNA about 4-fold more effectively than one-chain t-PA. The difference is accounted for almost entirely by a corresponding difference is the kcat values of the enzymes, whereas the Km values are similar. The amidolytic activity of two-chain t-PA is not enhanced by intact or partially plasmin-degraded fibrin. In contrast, the activity of one-chain t-PA is stimulated up to 3.7-fold by intact fibrin and up to 4.7-fold by plasmin-degraded fibrin (fibrin X-fragment). The stimulatory effects are realized via increases in the kcat values. It appears thus that in the presence of fibrin the intrinsically inferior catalytic properties of one-chain t-PA become similar to the properties of two-chain t-PA. The dependency of the activity of one-chain t-PA on the concentration of fibrin monomer is consistent with a single association site of both proteins and an association constant of Kass = 6.25 x 10(6) l/mol. Stimulation of one-chain t-PA by plasmin-degraded fibrin is more complex and appears to involve two different binding sites with association constants of Kass = 0.67 x 10(9) l/mol and Kass = 3.85 x 10(6) l/mol, respectively. The stimulatory effects of fibrin and partially plasmin-degraded fibrin on one-chain t-PA are suppressed by epsilon-aminocaproic acid and by a monoclonal antibody directed against the lysine binding site of t-PA. The latter findings support the notion that fibrin activation of one-chain t-PA is mediated by the lysine binding site on kringel domains of the enzyme.

Amino acid residues that affect interaction of tissue-type plasminogen activator with plasminogen activator inhibitor 1

Fibrinolysis is regulated in part by the interaction between tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor 1 (PAI-1, a serine protease inhibitor of the serpin family). It is known from our earlier work that deletion of a loop of amino acids (residues 296-302) from the serine protease domain of t-PA suppresses the interaction between the two proteins without altering the reactivity of t-PA towards its substrate, plasminogen. To define more precisely the role of individual residues within this loop, we have used site-directed mutagenesis to replace Lys-296, Arg-298, and Arg-299 with negatively charged glutamic residues. Replacement of all three positively charged amino acids generates a variant of t-PA that associates inefficiently with PAI-1 and is highly resistant to inhibition by the serpin. Two t-PAs with point mutations (Arg-298----Glu and Arg-299----Glu) are partially resistant to inhibition by PAI-1 and associate with the serpin at intermediate rates. Other point mutations (Lys-296----Glu, His-297----Glu, and Pro-301----Gly) do not detectably affect the interaction of t-PA with PAI-1. None of these substitutions has a significant effect on the rate of catalysis by t-PA or on the affinity of the enzyme for its substrate, plasminogen. On the basis of these results, we propose a model in which positively charged residues located in a surface loop near the active site of t-PA form ionic bonds with complementary negatively charged residues C-terminal to the reactive center of PAI-1.

Quenching of the amidolytic activity of one-chain tissue-type plasminogen activator by mutation of lysine-416

In contrast to most other serine proteases, tissue-type plasminogen activator (t-PA) possesses enzymatic activity as the one-chain zymogen form. The hypothesis that lysine residues 277 or 416 may be involved in stabilization of an active conformation of one-chain t-PA via salt-bridge formation with aspartic acid residue 477 was tested by site-directed mutagenesis. Four recombinant t-PA mutants were constructed. The amidolytic activities of these analogues were compared to that of authentic t-PA. Substitution of arginine-275 provided an analogue [( R275G]t-PA) resistant to plasmin cleavage. The amidolytic activity of [R275G]t-PA was comparable to that of authentic one-chain t-PA, and so was the activity of [R275L,K277L]t-PA, in which additional substitution of lysine residue 277 was carried out. This suggested that its presence was nonessential for obtaining one-chain t-PA activity. In contrast, substitution of lysine residue 416 to obtain [K416S]t-PA and [K416S,H417T]t-PA resulted in substantial quenching of amidolytic one-chain activity. As expected, the amidolytic activities of the two-chain forms were less affected by the substitution. Involvement of lysine residue 416 in one-chain t-PA activity was also indicated by decreased activities of [K416S]t-PA and [K416S,H417T]t-PA with plasminogen as the substrate. The one-chain activity of the lysine residue 416 substitution analogues was partially restored in the presence of fibrin. This could indicate that strong ligands such as fibrin might provide an alternative stabilization of the active conformation of one-chain t-PA.

Synthesis, purification, and characterization of an Arg152----Glu site-directed mutant of recombinant human blood clotting factor VII

Coagulation factor VII circulates in blood as a single-chain zymogen of a serine protease and is converted to its activated two-chain form, factor VIIa, by cleavage of an internal peptide bond located at Arg152-Ile153. Previous studies using serine protease active-site inhibitors suggest that zymogen factor VII may possess sufficient proteolytic activity to initiate the extrinsic pathway of blood coagulation. In order to assess the putative intrinsic proteolytic activity of single-chain factor VII, we have constructed a site-specific mutant of recombinant human factor VII in which arginine-152 has been replaced with a glutamic acid residue. Mutant factor VII was purified in a single step from culture supernatants of baby hamster kidney cells transfected with a plasmid containing the sequence for Arg152----Glu factor VII using a calcium-dependent, murine anti-factor VII monoclonal antibody column. Purified mutant factor VII was indistinguishable from plasma-derived or recombinant wild-type factor VII by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and migrated as a single band with an apparent molecular weight of 50,000. The average specific activity of several mutant factor VII preparations was 0.00025 unit/micrograms, or 0.01% of that observed for recombinant wild-type factor VII preparations. The clotting activity of mutant factor VII was, however, completely inhibited following incubation with dansyl-Glu-Gly-Arg chloromethyl ketone, suggesting that the apparent clotting activity of mutant factor VII was due to a contaminating serine protease.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the one-chain to two-chain conversion in tissue plasminogen activator: characterization of mutations at position 275

Tissue plasminogen activator (t-PA) is homologous to other serine proteases and contains an apparent activation cleavage site at arginine 275. It has been demonstrated that this arginine-275 can be replaced with either glutamic acid (Tate, K. M., Higgins, D. L., Holmes, W. E., Winkler, M. E., Heyneker, H. L., and Vehar, G. A. Biochemistry 26, 338-343, 1987) or glycine (Peterson, L. C., Johannessen, M., Foster, D., Kumar, A., and Mulvihill, E. Biochim. Biophys. Acta 952, 245-254, 1988; Boose, J. A., Kuismanen, E., Gerard, R., Sambrook, J. and Gething, M.-J. Biochemistry 28, 635-643, 1989) so that the product of the plasminogen activation reaction, plasmin, can no longer hydrolyze the one-chain form of t-PA to the two-chain form. These "one-chain" t-PA variants had diminished activity, compared to wild-type t-PA, in the absence of a cofactor, but in the presence of the fibrin(ogen) cofactor the two variants had activity similar to wild-type t-PA. In order to compare the effects of all possible substitutions, t-PA variants with each of the other nineteen amino acids besides arginine at position 275 were produced by site-directed mutagenesis. All were recovered from cell culture supernatants completely in the one-chain form, except for R275 (wild-type) and R275K, which were partially converted to the two-chain form. These latter two species could be completely converted to the two-chain form by plasmin. In addition, these two forms showed significantly more plasminogen activating activity in the absence of a fibrin(ogen) cofactor, compared to the other 18 variants. In the presence of a cofactor, all of the t-PA mutants had plasminogen activating activity equivalent to wild-type t-PA, except for R275C. The R275C t-PA had comparatively less clot lysis and fibrin binding activity as well. Presumably the new cysteine in this variant was involved in a mixed disulfide or caused misfolding of the molecule resulting in decreased activity. The difference in the plasminogen activating activity of one- and two-chain forms of t-PA was investigated by determining the apparent Michaelis constants and the apparent turnover numbers for R275E t-PA, which remains in the one-chain form throughout the assay, and two-chain R275 t-PA. The kinetic constants were measured in both the presence and the absence of plasmin-digested fibrinogen.(ABSTRACT TRUNCATED AT 400 WORDS)