Secondary-site binding of Glu-plasmin, Lys-plasmin and miniplasmin to fibrin

Structural Biology and Protein Engineering of Thrombolytics

Myocardial infarction and ischemic stroke are the most frequent causes of death or disability worldwide. Due to their ability to dissolve blood clots, the thrombolytics are frequently used for their treatment. Improving the effectiveness of thrombolytics for clinical uses is of great interest. The knowledge of the multiple roles of the endogenous thrombolytics and the fibrinolytic system grows continuously. The effects of thrombolytics on the alteration of the nervous system and the regulation of the cell migration offer promising novel uses for treating neurodegenerative disorders or targeting cancer metastasis. However, secondary activities of thrombolytics may lead to life-threatening side-effects such as intracranial bleeding and neurotoxicity. Here we provide a structural biology perspective on various thrombolytic enzymes and their key properties: (i) effectiveness of clot lysis, (ii) affinity and specificity towards fibrin, (iii) biological half-life, (iv) mechanisms of activation/inhibition, and (v) risks of side effects. This information needs to be carefully considered while establishing protein engineering strategies aiming at the development of novel thrombolytics. Current trends and perspectives are discussed, including the screening for novel enzymes and small molecules, the enhancement of fibrin specificity by protein engineering, the suppression of interactions with native receptors, liposomal encapsulation and targeted release, the application of adjuvants, and the development of improved production systems.

Photonic activation of plasminogen induced by low dose UVB

Activation of plasminogen to its active form plasmin is essential for several key mechanisms, including the dissolution of blood clots. Activation occurs naturally via enzymatic proteolysis. We report that activation can be achieved with 280 nm light. A 2.6 fold increase in proteolytic activity was observed after 10 min illumination of human plasminogen. Irradiance levels used are in the same order of magnitude of the UVB solar irradiance. Activation is correlated with light induced disruption of disulphide bridges upon UVB excitation of the aromatic residues and with the formation of photochemical products, e.g. dityrosine and N-formylkynurenine. Most of the protein fold is maintained after 10 min illumination since no major changes are observed in the near-UV CD spectrum. Far-UV CD shows loss of secondary structure after illumination (33.4% signal loss at 206 nm). Thermal unfolding CD studies show that plasminogen retains a native like cooperative transition at ~70 ºC after UV-illumination. We propose that UVB activation of plasminogen occurs upon photo-cleavage of a functional allosteric disulphide bond, Cys737-Cys765, located in the catalytic domain and in van der Waals contact with Trp761 (4.3 Å). Such proximity makes its disruption very likely, which may occur upon electron transfer from excited Trp761. Reduction of Cys737-Cys765 will result in likely conformational changes in the catalytic site. Molecular dynamics simulations reveal that reduction of Cys737-Cys765 in plasminogen leads to an increase of the fluctuations of loop 760–765, the S1-entrance frame located close to the active site. These fluctuations affect the range of solvent exposure of the catalytic triad, particularly of Asp646 and Ser74, which acquire an exposure profile similar to the values in plasmin. The presented photonic mechanism of plasminogen activation has the potential to be used in clinical applications, possibly together with other enzymatic treatments for the elimination of blood clots.

Urinary angiostatin--a novel putative marker of renal pathology chronicity in lupus nephritis

There is a critical need to identify biomarkers for Systemic Lupus Erythematosus (SLE) which has a high prevalence of renal failure. When urine from patients with lupus nephritis was recently screened for the levels of ∼280 molecules using an exploratory array-based proteomic platform, elevated angiostatin levels were noted. Angiostatin is a bioactive fragment of plasminogen, and has been known to have modulatory function in angiogenesis and inflammation. The significant elevation in urinary angiostatin was next validated in an independent cohort of SLE patients (n = 100) using ELISA. Among patients with SLE, urine angiostatin was significantly increased in active SLE compared with inactive SLE, correlating well with the SLEDAI disease activity index and SLICC renal activity score (r = 0.66, p < 0.0001). ROC curve analysis further confirmed that urinary angiostatin had the capacity to discriminate patients with active SLE from those with inactive disease. Patients with Class IV lupus nephritis exhibited the highest levels of urinary angiostatin. Immunohistochemistry staining localized angiostatin expression to the renal tubular cells in these patients. Finally, when paired urine-kidney samples procured concurrently from patients with LN were next examined, urine angiostatin levels correlated strongly with the renal pathology chronicity index, but not with the activity index. Given that Class IV lupus nephritis and renal pathology chronicity changes forebode poor renal and patient survival, urinary angiostatin emerges as a novel noninvasive marker of renal disease in SLE. Longitudinal studies are in progress to further assess the disease-predictive potential of urinary angiostatin.

The plasmin-antiplasmin system: structural and functional aspects

The plasmin-antiplasmin system plays a key role in blood coagulation and fibrinolysis. Plasmin and α(2)-antiplasmin are primarily responsible for a controlled and regulated dissolution of the fibrin polymers into soluble fragments. However, besides plasmin(ogen) and α(2)-antiplasmin the system contains a series of specific activators and inhibitors. The main physiological activators of plasminogen are tissue-type plasminogen activator, which is mainly involved in the dissolution of the fibrin polymers by plasmin, and urokinase-type plasminogen activator, which is primarily responsible for the generation of plasmin activity in the intercellular space. Both activators are multidomain serine proteases. Besides the main physiological inhibitor α(2)-antiplasmin, the plasmin-antiplasmin system is also regulated by the general protease inhibitor α(2)-macroglobulin, a member of the protease inhibitor I39 family. The activity of the plasminogen activators is primarily regulated by the plasminogen activator inhibitors 1 and 2, members of the serine protease inhibitor superfamily.

Plasminogen alleles influence susceptibility to invasive aspergillosis

Invasive aspergillosis (IA) is a common and life-threatening infection in immunocompromised individuals. A number of environmental and epidemiologic risk factors for developing IA have been identified. However, genetic factors that affect risk for developing IA have not been clearly identified. We report that host genetic differences influence outcome following establishment of pulmonary aspergillosis in an exogenously immune suppressed mouse model. Computational haplotype-based genetic analysis indicated that genetic variation within the biologically plausible positional candidate gene plasminogen (Plg; Gene ID 18855) correlated with murine outcome. There was a single nonsynonymous coding change (Gly110Ser) where the minor allele was found in all of the susceptible strains, but not in the resistant strains. A nonsynonymous single nucleotide polymorphism (Asp472Asn) was also identified in the human homolog (PLG; Gene ID 5340). An association study within a cohort of 236 allogeneic hematopoietic stem cell transplant (HSCT) recipients revealed that alleles at this SNP significantly affected the risk of developing IA after HSCT. Furthermore, we demonstrated that plasminogen directly binds to Aspergillus fumigatus. We propose that genetic variation within the plasminogen pathway influences the pathogenesis of this invasive fungal infection.

X-ray crystallographic structure of the angiogenesis inhibitor, angiostatin, bound to a peptide from the group A streptococcal surface protein PAM

The crystal structure of the human Pg-derived angiogenesis inhibitor, angiostatin, complexed to VEK-30, a peptide from the group A streptococcal surface protein, PAM, was determined and refined to 2.3 A resolution. This is the first structure of angiostatin bound to a ligand and provides a model of the interaction between Pg and streptococcal-derived pathogenic proteins. VEK-30 contains a "through-space isostere" for C-terminal lysine, wherein Arg and Glu side chains, separated by one helical turn, bind within the bipolar angiostatin kringle 2 (K2) domain lysine-binding site. VEK-30 also makes several contacts with K2 residues that exist outside of the canonical LBS and are not conserved among the other Pg kringles, thus providing a molecular basis for the selectivity of VEK-30 for K2. The structure also shows that Pg kringle domains undergo significant structural rearrangement relative to one another and reveals dimerization between two molecules of angiostatin and VEK-30 related by crystallographic symmetry. This dimerization, which exists only in the crystal structure, is consistent with the parallel coiled-coil full-length PAM dimer expected from sequence similarities and homology modeling.

What the structure of angiostatin may tell us about its mechanism of action

Originally discovered in 1994 by Folkman and coworkers, angiostatin was identified through its antitumor effects in mice and later shown to be a potent inhibitor of angiogenesis. An internal fragment of plasminogen, angiostatin consists of kringle domains that are known to be lysine-binding. The crystal structure of angiostatin was the first multikringle domain-containing structure to be published. This review will focus on what is known about the structure of angiostatin and its implications in function from the current literature.

The fibrinolytic system in dissemination and matrix protein deposition during a mycobacterium infection

The fibrinolytic system is known to play an important role in the inflammatory response to bacterial infections. In the present study, relationships between protein components of the fibrinolytic system and infectivity by Mycobacterium avium were analyzed. Infections were initiated through noninvasive intratracheal administration of M. avium 724 in mice individually deficient for plasminogen, tissue-type plasminogen activator, urokinase-type plasminogen activator, and urokinase-type plasminogen activator receptor, along with wild-type control mice. There were no differences in lung colony counts among all mouse genotypes throughout a 10-week infection. However, in tissue-type plasminogen activator and plasminogen-deficient mice an earlier dissemination of M. avium to other organs was observed. Nevertheless, the M. avium growth rates in the liver, spleen, and lung did not differ between the various mouse populations throughout a 10-week infection. Histochemical and immunohistochemical analyses at 5 and 10 weeks after infection demonstrated that plasminogen-deficient mice, compared to wild-type mice, had enhanced fibrin and fibronectin deposition, as well as increased neutrophil infiltration within liver granulomas. These results suggest that plasmin(ogen) plays a role in the turnover of extracellular matrix proteins within granulomas and has a limited effect in the early dissemination of M. avium from lungs. Thus, plasmin(ogen) functions in limiting progressive fibrosis in the granuloma during a chronic mycobacterial infection.

Structure and binding determinants of the recombinant kringle-2 domain of human plasminogen to an internal peptide from a group A Streptococcal surface protein

The X-ray crystal structure of a complex of a modified recombinant kringle-2 domain of human plasminogen, K2Pg[C4G/E56D/L72Y] (mK2Pg), containing an upregulated lysine-binding site, bound to a functional 30 residue internal peptide (VEK-30) from an M-type protein of a group A Streptococcus surface protein, has been determined by molecular replacement methods using K4Pg as a model, and refined at 2.7 A resolution to a R-factor of 19.5 %. The X-ray crystal structure shows that VEK-30 exists as a nearly end-to-end alpha-helix in the complex with mK2Pg. The final structure also revealed that Arg17 and His18 of VEK-30 served as cationic loci for Asp54 and Asp56 of the consensus lysine-binding site of mK2Pg, while Glu20 of VEK-30 coordinates with Arg69 of the cationic binding site of mK2Pg. The hydrophobic ligand-binding pocket in mK2Pg, consisting primarily of Trp60 and Trp70, situated between the positive and negative centers of the lysine-binding site, is utilized in a novel manner in stabilizing the interaction with VEK-30 by forming a cation-pi-electron-mediated association with the positive side-chain of Arg17 of this peptide. Additional lysine-binding sites, as well as exosite electrostatic and hydrogen bonding interactions involving Glu9 and Lys14 of VEK-30, were observed in the structural model. The importance of these interactions were tested in solution by investigating the binding constants of synthetic variants of VEK-30 to mK2Pg, and it was found that, Lys14, Arg17, His18, and Glu20 of VEK-30 were the most critical amino acid binding determinants. With regard to the solution studies, circular dichroism analysis of the titration of VEK-30 with mK2Pg demonstrated that the peptidic alpha-helical structure increased substantially when bound to the kringle module, in agreement with the X-ray results. This investigation is the first to delineate structurally the mode of interaction of the lysine-binding site of a kringle with an internal pseudo-lysine residue of a peptide or protein that functionally interacts with a kringle module, and serves as a paradigm for this important class of interactions.

The effects of ligand binding on the backbone dynamics of the kringle 1 domain of human plasminogen

The internal motions of the backbone nitrogen atoms of the kringle 1 domain of human plasminogen (K1(Pg)) were examined in the absence and presence of the ligand, epsilon-aminocaproic acid. These dynamic properties were determined from (15)N NMR relaxation data in terms of the extended model-free parameters. The model of isotropic reorientation was found sufficient to account for overall molecular tumbling for both apo and EACA-bound K1(Pg). The global rotational correlation time (tau(m)) for apo-K1(Pg) was 5.87(+/-0.01) ns, while the tau(m) for ligand-bound K1(Pg) was 5.20(+/-0.01) ns, suggesting that perhaps some small degree of aggregation occurred in the apo form of the kringle module. Complexation of K1(Pg) with ligand mainly reduced those internal motions that occurred on a 100 ps to 5 ns time-scale. The magnitude of the chemical exchange was also attenuated upon ligand binding. These data are consistent with studies employing other approaches that suggest that the binding pocket is preformed in K1(Pg).

Nonfibrinolytic functions of plasminogen

Although the roles of plasminogen and plasmin in mediating blood clot dissolution are well known, the availability of mice deficient for components of the fibrinolytic system has allowed direct approaches to be made toward elucidating the role of these proteins in other diverse physiological and pathophysiological processes. A number of these studies have identified plasminogen as playing an important role in inflammation and other cell migratory processes. With the identification of receptors for plasminogen on a number of pathogens, and the ability to activate plasminogen through either endogenous production of plasminogen activators or utilization of host activators, mice deficient for components of the fibrinolytic system offer a unique approach toward further elucidating the importance of this system in pathogen infection and dissemination.

Enhancement through mutagenesis of the binding of the isolated kringle 2 domain of human plasminogen to omega-amino acid ligands and to an internal sequence of a Streptococcal surface protein

In the background of the recombinant K2 module of human plasminogen (K2(Pg)), a triple mutant, K2(Pg)[C4G/E56D/L72Y], was generated and expressed in Pichia pastoris cells in yields exceeding 100 mg/liter. The binding affinities of a series of lysine analogs, viz. 4-aminobutyric acid, 5-aminopentanoic acid, epsilon-aminocaproic acid, 7-aminoheptanoic acid, and t-4-aminomethylcyclohexane-1-carboxylic acid, to this mutant were measured and showed up to a 15-fold tighter interaction, as compared with wild-type K2(Pg) (K2(Pg)[C4G]). The variant, K2(Pg)[C4G/E56D], afforded up to a 4-fold increase in the binding affinity to these same ligands, whereas the K2(Pg)[C4G/L72Y] mutant decreased the same affinities up to 5-fold, as compared with K2(Pg)[C4G]. The thermal stability of K2(Pg)[C4G/E56D/L72Y] was increased by approximately 13 degrees C, as compared with K2(Pg)[C4G]. The functional consequence of up-regulating the lysine binding property of K2(Pg) was explored, as reflected by its ability to interact with an internal sequence of a plasminogen-binding protein (PAM) on the surface of group A streptococci. A 30-mer peptide of PAM, containing its K2(Pg)-specific binding region, was synthesized, and its binding to each mutant of K2(Pg) was assessed. Only a slight enhancement in peptide binding was observed for K2(Pg)[C4G/E56D], compared with K2(Pg)[C4G] (K(d) = 460 nM). A 5-fold decrease in binding affinity was observed for K2(Pg)[C4G/L72Y] (K(d) = 2200 nM). However, a 12-fold enhancement in binding to this peptide was observed for K2(Pg)[C4G/E56D/L72Y] (K(d) = 37 nM). Results of these PAM peptide binding studies parallel results of omega-amino acid binding to these K2(Pg) mutants, indicating that the high affinity PAM binding by plasminogen, mediated exclusively through K2(Pg), occurs through its lysine-binding site. This conclusion is supported by the 100-fold decrease in PAM peptide binding to K2(Pg)[C4G/E56D/L72Y] in the presence of 50 mM 6-aminohexanoic acid. Finally, a thermodynamic analysis of PAM peptide binding to each of these mutants reveals that the positions Asp(56) and Tyr(72) in the K2(Pg)[C4G/E56D/L72Y] mutant are synergistically coupled in terms of their contribution to the enhancement of PAM peptide binding.

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.

Reduction in stent and vascular graft thrombosis and enhancement of thrombolysis by recombinant Lys-plasminogen in nonhuman primates

BACKGROUND

To enhance thrombolytic responses without increasing hemorrhagic risks, the antithrombotic effects of recombinant Lys-plasminogen (r-LysPgn), a prothrombolytic plasminogen intermediate, were examined in baboon models of thrombus formation and dissolution.

METHODS AND RESULTS

The dose-response effects of r-LysPgn, alone or in combination with subthreshold dosing of tissue plasminogen activator (TPA), were measured with respect to the accumulation of (111)In-labeled platelets and (125)I-fibrin in thrombus forming on endovascular metallic stents or thrombogenic segments of vascular graft interposed in exteriorized long-term arteriovenous (AV) femoral shunts. Thrombolytic losses have also been determined for preformed, stable, (111)In-platelet- and (125)I-fibrin-labeled graft thrombus and corresponding propagated thrombotic tails, together with changes in blood tests of thrombosis, thrombolysis, and hemostasis. Bolus intravenous r-LysPgn in escalating doses (2, 4, or 8 mg/kg) increased circulating plasminogen levels in a dose-dependent manner, was removed by log-linear clearance with a T50 of 120 minutes, and reciprocally decreased the accumulating thrombus on metallic stents and segments of vascular graft (P<.001 in all cases for 8-mg/kg doses). r-LysPgn also impaired platelet aggregatory responses to physiological agonists in vitro but not ex vivo. Prethrombosis administration of low-dose r-LysPgn (2 mg/kg) greatly enhanced the lysis of radiolabeled nonoccluding thrombus by a subthreshold dose of TPA (0.1 mg/kg) compared with TPA-only controls (P=.03).

CONCLUSIONS

Elective bolus injections of r-LysPgn before stent deployment decrease the amount of thrombus formed without compromising hemostasis by facilitating endogenous TPA thrombolysis. r-LysPgn may provide effective and safe antithrombotic therapy for interventional vascular procedures.

Variants of tissue-type plasminogen activator that display extraordinary resistance to inhibition by the serpin plasminogen activator inhibitor type 1

Fibrinolysis is regulated in part by the interaction of tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor type 1 (PAI-1). Previous investigations suggest that three specific arginine residues, Arg-298, Arg-299, and Arg-304 of t-PA, play a critical role in this important regulatory interaction. Our earlier studies have demonstrated that conversion of any of these three residues to a glutamic acid residue reduced the rate of inhibition of t-PA by PAI-1 by factors varying from 58-64. In addition, we have reported that the second order rate constant for inhibition by PAI-1 of the variant t-PA/K296E,R298E,R299E is reduced by a factor of approximately 2800 compared with that of wild type t-PA. In this study, we have significantly extended our earlier observations by identifying t-PA variants that are substantially more resistant to inhibition by PAI-1 than any previously reported variants of t-PA or urokinase-type plasminogen activator. Single-chain t-PA/R275E,R298E, R299E,R304E, for example, is inhibited by PAI-1 approximately 120, 000 times less rapidly than single-chain, wild type t-PA. We also report the first direct comparison of the effects of charge reversal mutations of Arg-298, Arg-299, and/or Arg-304 on the properties of the single- and two-chain forms of t-PA. While these mutations confer extraordinary resistance to inhibition by PAI-1 to both forms of the enzyme, our observations reveal that the single-chain enzyme is affected to a greater extent than the two-chain enzyme. Two-chain, wild type t-PA is inhibited by PAI-1 approximately 1.4 times more rapidly than single-chain t-PA. The corresponding ratio increases to 7.6 or 6.7, respectively, for variants of t-PA containing the R298E, R299E or R298E,R299E,R304E mutations.

Glycosylation at Asn-289 facilitates the ligand-induced conformational changes of human Glu-plasminogen

Glu-plasminogen exists in two major glycoforms (I and II). Glycoform I contains carbohydrate chains linked to Asn-289 and Thr-346, whereas glycoform II is glycosylated only at Thr-346. Disparities in carbohydrate content lead to differences in the important functional properties of the zymogen, e.g. the kinetics of activation. The kinetics of the large ligand-induced conformational changes of each of the Glu-plasminogen glycoforms have been studied using stopped-flow fluorescence. The results are in accordance with a conformational change governed by positive co-operative binding at two weak lysine-binding sites. Additional glycosylation at Asn-289 in Glu-plasminogen I results in a two-fold increase in the overall dissociation constant of a ligand, trans-4-aminomethyl-cyclohexane carboxylic acid. This effect stems directly from the reaction step during which the conformational changes occur. This implies a higher population of Glu-plasminogen I in the open conformation even in the absence of ligands, and thus accounts for a higher rate of activation of Glu-plasminogen I, in comparison with Glu-plasminogen II.

Production and characterization of recombinant human plasminogen(S741C-fluorescein). A novel approach to study zymogen activation without generation of active protease

A variant of recombinant plasminogen with the plasmin active site serine (S741) replaced by cysteine was produced and labeled with fluorescein at this residue to provide the derivative Plg(S741C-fluorescein). Studies of cleavage, conformation, and fibrin-binding properties of the derivative showed it to be a good model substrate to study plasminogen activation. Both in solution and in a fully polymerized fibrin clot, cleavage of the single chain zymogen to the two-chain "plasmin" molecule was accompanied by a 50% quench of fluorescence intensity. This change allows facile, continuous monitoring of the kinetics of cleavage. Measurements of cleavage by single chain t-PA within intact, fully polymerized 3 microM fibrin yielded apparent kcat and Km values of (0.08 s-1, 0.52 microM) and (0.092 s-1, 0.098 microM) for [Glu1]- and [Lys78]Plg(S741C-fluorescein), respectively. These values are similar to those obtained by others with plasma plasminogen. The approach used here might generally be useful in simplifying the analysis of zymogen activation kinetics in cases where the product (protease) has a great influence on its own formation via positive or negative feedback loops.

Reaction of human alpha2-antiplasmin and plasmin stopped-flow fluorescence kinetics

The interaction of human plasmin with human alpha2-antiplasmin was measured in the presence and absence of lysine-binding ligands using the corresponding active site fluorescence changes. The stopped-flow method allows for direct determination of reliable values of the second order rate constant for the fast association step of plasmin and alpha2-antiplasmin in the absence of another interacting compound, e.g. a plasmin substrate. At pH 7.4, 25 degrees C, k1 = 2.2 x 10(7) M(-1)s(-1) was obtained. Substantial reductions in k1 were seen in the presence of trans-4-(aminomethyl)cyclohexane-1-carboxylic acid at concentrations corresponding to lysine-binding site interactions at kringle 4 of plasmin; at saturation the rate constant is reduced 20-fold, whereas the effect of saturation of kringle 1 is only a 2-fold reduction. It is thus found that the interaction of alpha2-antiplasmin with the lysine-binding site of kringle 1 is of little importance compared with that of kringle 4 in regulating the inhibition reaction of plasmin with alpha2-antiplasmin. Similar results were recently obtained for the bovine plasmin-bovine alpha2-antiplasmin reaction (Christensen et al. (1995) Biochem. J. 305, 97-102).

Recombinant lys-plasminogen given before, but not after, recombinant tissue-type plasminogen activator markedly improves coronary thrombolysis in dogs: relationship of thrombolytic efficacy with parameters of fibrinolysis

Recombinant tissue-type plasminogen activator (rt-PA) administration rapidly restores blood flow in thrombosed coronary arteries, but coronary arteries often reocclude after initial thrombolysis. This occurs because of the short half-life of rt-PA and rapid increase in plasminogen activator inhibitor (PAI-1) and alpha2-antiplasmin levels in plasma. We hypothesized that administration of lys-plasminogen, which binds to fibrin with 10 times greater affinity and results in a loose fibrin structure (as compared with native glu-plasminogen), before rt-PA would enhance the thrombolytic efficacy of rt-PA and modulate parameters of fibrinolysis. To examine this hypothesis, dogs with electrically induced stable thrombus in the left anterior descending coronary artery (LAD) were treated with saline (group A, n = 9) or lys-plasminogen (group B, 2 mg/kg, n = 5), followed 10 min later by rt-PA (1 mg/kg in 20 min). Four other dogs with occlusive LAD thrombus were first given rt-PA, followed by lys-plasminogen (2 mg/kg) 50 min later (group C). Lys-plasminogen given before rt-PA restored flow in all dogs in 14 +/- 4 min (vs. 22 +/- 9 min in group A, p < 0.05), continuing > 2 h (vs. 41 +/- 15 min in group A, p < 0.02). Lys-plasminogen given after rt-PA did not potentiate the effect of rt-PA. Plasma t-PA antigen concentrations were highest in group B dogs at 2 h after rt-PA infusion. PAI-1 and alpha2-antiplasmin plasma levels were suppressed in all dogs receiving lys-plasminogen whether it was given before or after rt-PA. Therefore, lys-plasminogen given before rt-PA markedly potentiates the effect of rt-PA and alters the parameters of fibrinolysis. In contrast, lys-plasminogen given after rt-PA does not influence the thrombolytic effect of rt-PA, whereas it suppresses PAI-1 and alpha2-antiplasmin levels in plasma. This study also suggests that binding of plasminogen to the clot is more important than the plasma levels of PAI-1 and alpha2-antiplasmin.

The importance of the hydrophobic components of the binding energies in the interaction of omega-amino acid ligands with isolated kringle polypeptide domains of human plasminogen

Three of the five kringle domains of human plasminogen (HPg), viz the first, fourth and fifth, exhibit significantly strong binding to omega-amino acids, such as epsilon-aminocaproic acid (EACA) and transaminomethylcyclohexane-1-carboxylic acid (AMCHA). In all cases, ligand stabilization is due to ion dipole attractions of its charged groups with polypeptide side chains, as well as hydrophobic clustering of the ligand methylene groups with appropriate hydrophobic residues within the kringle domain. In order to estimate the significance of the hydrophobic components of ligand stabilization, we have sought a more detailed description of these binding interactions. The standard thermodynamic binding parameters, delta G degrees, delta H degrees and delta S degrees, for association of EACA and AMCHA with isolated recombinant kringle regions of HPg have been determined at several temperatures to evaluate the changes in standard heat capacities (delta C degrees p) accompanying these interactions. In each case, the delta C degrees p values of binding were negative and in the range -36 to -91 cal mol -1 K -1, reflective of the importance of the hydrophobic components of the binding process and their probable effects on surrounding water structure.

Recombinant lys-plasminogen, but not glu-plasminogen, improves recombinant tissue-type plasminogen activator-induced coronary thrombolysis in dogs

OBJECTIVES

This study examined the modification of recombinant tissue-type plasminogen activator (rt-PA)-induced thrombolysis by recombinant lys-plasminogen.

BACKGROUND

Recombinant tissue-type plasminogen activator restores flow in the thrombosed coronary artery, but the artery often reoccludes. The rt-PA-induced thrombolysis is a result of activation of plasminogen bound to fibrin in the thrombus and results in generation of the fibrinolytic enzyme plasmin. Small amounts of lys-plasminogen are formed when rt-PA is used. Lys-plasminogen binds to fibrin with a 10-fold greater affinity than the predominant native glu-plasminogen, leading to a loose fibrin structure.

METHODS

Dogs with electrically induced occlusive intracoronary thrombus were treated with saline solution (n = 9), glu-plasminogen (2 mg/kg body weight, n = 5) or lys-plasminogen (2 mg/kg, n = 5), followed by infusion of rt-PA (1 mg/kg over 20 min) 10 min later.

RESULTS

Reperfusion rates were similar in all groups of dogs, but the time to reflow was lowest in dogs given lys-plasminogen compared with those given saline solution or glu-plasminogen before rt-PA (mean [+/- SE] 14 +/- 2 vs. 22 +/- 2 and 23 +/- 3 min, respectively, p < 0.05). None of the reperfused coronary arteries reoccluded in the lys-plasminogen plus rt-PA group, whereas 75% reoccluded in dogs given saline solution plus rt-PA, and 50% reoccluded in those given glu-plasminogen plus rt-PA. Accordingly, duration of reflow was greater in the lys-plasminogen plus rt-PA group (> 120 vs. 39 +/- 7 and 82 +/- 21 min, respectively, p < 0.05). Plasminogen activator inhibitor-1 activity decreased during rt-PA infusion and thereafter increased in all dogs, but less so in dogs given lys-plasminogen (p < 0.05 vs. those given saline solution before rt-PA).

CONCLUSIONS

Treatment with recombinant lys-plasminogen before rt-PA reduces time to reflow and sustains reflow after thrombolysis, whereas glu-plasminogen has no such effect.

Stopped-flow fluorescence kinetics of bovine alpha 2-antiplasmin inhibition of bovine midiplasmin

In the conversion of bovine plasminogen to bovine plasmin not only the expected urokinase-catalysed cleavage of Arg-557-Val-558, and the following autocatalytic cleavage separating the N-terminal peptide 1-77 from the heavy chain of plasmin, but also a cleavage at Arg-342-Met-343 between kringles 3 and 4 is seen. Here, kinetic studies of the interaction of bovine alpha 2-antiplasmin with bovine plasmin were performed on isolated bovine midiplasmin (lacking kringles 1-3) and on bovine plasmin containing all of the activation products from the bovine plasminogen. A series of experiments using stopped-flow fluorescence fast kinetics as well as conventional techniques suggests a reaction model in accordance with the one known for the human system. First, a tight complex (K1 in the nanomolar range) is formed in a fast reaction step; and second, a tightening of this complex occurs in a slow reaction step. The final complex is indeed so tight (Ki < or = pM), that the reaction for many practical purposes is legitimately considered irreversible. The stopped-flow method allows for the determination of reliable values of the second-order rate constant for the fast association step. At pH 7.4 and 25 degrees C, k+1 = 1.7 x 10(6) M-1 s-1 was obtained in the absence and k+1 = 0.9 x 10(6) M-1.s-1 in the presence of the kringles 1-3 domain of bovine plasmin. In contrast to this, substantial reductions of k+1 were seen in the presence of concentrations of 6-amino-hexanoic acid corresponding to lysine-binding-site interactions and far too low to be attributed to active-site interactions with the bovine plasmins (for each, Ki = 42 mM). All in all, the data indicated that the lysine-binding site(s) not of kringle 1, but of midiplasmin (those of kringles 4 and 5) are regulating the inhibition reaction.

1H-NMR assignments and secondary structure of human plasminogen kringle 1

The 1H-NMR spectrum of the kringle 1 domain of human plasminogen complexed with 6-aminohexanoic acid, an antifibrinolytic drug, has been assigned. Elements of secondary structure have been identified on the basis of sequential, medium and long-range dipolar interactions, back-bone amide spin-spin couplings (3JHN-H alpha) and 1H-2H exchange rates. The kringle contains scarcely any repetitive secondary structure: eight reverse turns and two short beta-sheets. These comprise 40% and 12% of the domain, respectively. No alpha-helix was found. An aromatic cluster formed by His31, Phe36, Trp62, Phe64, Tyr72 and Tyr74 is indicated by several inter-residue Overhauser connectivities. Contacts between the methyl groups of Leu46 and the side chains of Phe36, Trp62 and Trp25 are observed. A second hydrophobic cluster formed by Tyr9, Ile77 and Leu78 is also indicated. A comparison of secondary structure elements among plasminogen kringles 1 and 4 and tissue-type plasminogen activator kringle 2 suggests that there is variability in the position and number of reverse turns on going from one kringle to another; however, the beta-sheets are conserved among the homologs.

Inner clot diffusion and permeation during fibrinolysis

A model of fibrinolysis was developed using multicomponent convection-diffusion equations with homogeneous reaction and heterogeneous adsorption and reaction. Fibrin is the dissolving stationary phase and plasminogen, tissue plasminogen activator (tPA), urokinase (uPA), and plasmin are the soluble mobile species. The model is based on an accurate molecular description of the fibrin fiber and protofibril structure and contains no adjustable parameters and one phenomenological parameter estimated from experiment. The model can predict lysis fronts moving across fibrin clots (fine or coarse fibers) of various densities under different administration regimes using uPA and tPA. We predict that pressure-driven permeation is the major mode of transport that allows for kinetically significant thrombolysis during clinical situations. Without permeation, clot lysis would be severely diffusion limited and would require hundreds of minutes. Adsorption of tPA to fibrin under conditions of permeation was a nonequilibrium process that tended to front load clots with tPA. Protein engineering efforts to design optimal thrombolytics will likely be affected by the permeation processes that occur during thrombolysis.

The compact domain conformation of human Glu-plasminogen in solution

A complete understanding of the accelerating mechanisms of plasminogen activation and fibrinolysis necessarily requires structural information on the conformational forms of plasminogen. Given the absence of high-resolution structural data on plasminogen the use of lower resolution approaches has been adopted. Two such approaches have previously indicated a compact conformation of Glu-plasminogen (Tranqui, L., Prandini, M., and Chapel, A. (1979) Biol. Cellulaire, 34, 39-42; Bányai, L. and Patthy, L. (1985) Biochim. Biophys. Acta, 832, 224-227) whereas a third has suggested a fairly extended conformation (Mangel, W., Lin, B. and Ramakrishnan, V. (1990) Science, 248, 69-73). Native Glu-plasminogen has been investigated using small-angle X-ray scattering (SAXS) experiments. It is concluded that this molecule in solution is compact (radius of gyration, RG 3.05 +/- 0.02 nm and maximum intramolecular distance, Im 9.1 +/- 0.3 nm) and that the data are consistent with the right-handed spiral structure observed using electron microscopy by Tranqui et al. (1979). A spiral structure of native plasminogen would have important implications for the conformational response of plasminogen to fibrin and concomitant stimulation of plasminogen activation.

Antifibrinolytic activities of alpha-N-acetyl-L-lysine methyl ester, epsilon-aminocaproic acid, and tranexamic acid. Importance of kringle interactions and active site inhibition

alpha-N-acetyl-L-lysine methyl ester (NALME) is a lysine analogue that reportedly binds to low-affinity lysine binding sites in plasmin(ogen) and miniplasmin(ogen). In the studies presented here, we show that NALME has antifibrinolytic activity; however, unlike the therapeutic agents epsilon-amino-n-caproic acid (epsilon ACA) and tranexamic acid (TEA), the activity of NALME is based on inhibition of the plasmin active site. NALME (0.1-10 mM) significantly inhibited the amidase activity of plasmin, miniplasmin, and streptokinase-plasmin complex without affecting alpha-thrombin or tissue plasminogen activator. epsilon ACA and TEA (0.1-10 mM) did not affect the amidase activity of plasmin or miniplasmin. A kinetic analysis showed that NALME is a competitive inhibitor of D-Val-L-Lys-p-nitroanilide HCl (S-2251) hydrolysis by plasmin; NALME binding to plasmin completely prevented S-2251 binding. The Kl for the plasmin-NALME interaction was 0.4 mM. epsilon ACA and TEA inhibited fibrin monomer digestion by plasmin and miniplasmin without binding to the active site of either enzyme. This result suggests that epsilon ACA and TEA function as antifibrinolytics by disrupting the noncovalent association of fibrin monomer with a domain common to both plasmin and miniplasmin (probably kringle 5). NALME inhibited fibrin monomer digestion principally by decreasing amidase activity. NALME was the only lysine analogue that prevented fragment X formation; TEA and epsilon ACA primarily inhibited the formation of fragments Y and D. When plasmin was incubated simultaneously with alpha 2-antiplasmin and alpha 2-macroglobulin, epsilon ACA increased the fraction of plasmin reacting with alpha 2-macroglobulin; NALME had no effect on the plasmin distribution.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasminogen binding sites in normal human skin

Plasminogen is detected in the basal cell layer of the epidermis, keratinocytes can generate plasminogen activators and it is suggested that the generation of plasmin may facilitate keratinocyte division, migration and differentiation. In this study we have investigated the characteristics of plasminogen binding sites in normal human epidermis. It was found that 6-aminohexanoic acid and benzamidine displaced endogenous epidermal plasminogen from the basal layer suggesting that endogenous plasminogen binds initially via the kringle 5 aminohexyl (AH) site. Plasminogen binding sites in epidermis were further investigated by displacing endogenous plasminogen and incubating sections with exogenously added glu-plasminogen, lys-plasminogen and plasmin or the isolated plasminogen fragments kringles 1-3, kringle 4 and kringle 5L. The results suggest that the uptake of plasminogen involves primary interaction with the kringle 5AH site and a secondary interaction with lysine binding sites of kringles 1-3. Cell binding is not dependent upon additional reactions of the plasmin active centre.

Lipoprotein (a) inhibits the generation of transforming growth factor beta: an endogenous inhibitor of smooth muscle cell migration

Conditioned medium (CM) derived from co-cultures of bovine aortic endothelial cells (BAECs) and bovine smooth muscle cells (BSMCs) contains transforming growth factor-beta (TGF-beta) formed via a plasmin-dependent activation of latent TGF-beta (LTGF beta), which occurs in heterotypic but not in homotypic cultures (Sato, Y., and D. B. Rifkin. 1989. J. Cell Biol. 107: 1199-1205). The TGF-beta formed is able to block the migration of BSMCs or BAECs. We have found that the simultaneous addition to heterotypic culture medium of plasminogen and the atherogenic lipoprotein, lipoprotein (a) (Lp(a)), which contains plasminogen-like kringles, inhibits the activation of LTGF-beta in a dose-dependent manner. The inclusion of LDL in the culture medium did not show such an effect. Control experiments indicated that Lp(a) does not interfere with the basal level of cell migration, the activity of exogenous added TGF-beta, the release of LTGF-beta from cells, the activation of LTGF-beta either by plasmin or by transient acidification, or the activity of plasminogen activator. The addition of Lp(a) to the culture medium decreased the amount of plasmin found in BAECs/BSMCs cultures. Similar results were obtained using CM derived from cocultures of human umbilical vein endothelial cells and human foreskin fibroblasts. These results suggest that Lp(a) can inhibit the activation of LTGF-beta by competing with the binding of plasminogen to cell or matrix surfaces. Therefore, high plasma levels of Lp(a) might enhance smooth muscle cell migration by decreasing the levels of the migration inhibitor TGF-beta thus contributing to generation of the atheromatous lesions.

Characterization of an extremely large, ligand-induced conformational change in plasminogen

Native human plasminogen has a radius of gyration of 39 angstroms. Upon occupation of a weak lysine binding site, the radius of gyration increases to 56 angstroms, an extremely large ligand-induced conformational change. There are no intermediate conformational states between the closed and open form. The conformational chang is not accompanied by a change in secondary structure, hence the closed conformation is formed by interaction between domains that is abolished upon conversion to the open form. This reversible change in conformation, in which the shape of the protein changes from that best described by a prolate ellipsoid to a flexible structure best described by a Debye random coil, is physiologically relevant because a weak lysine binding site regulates the activation of plasminogen.

Mapping of the human plasmin domain recognized by the unique plasmin receptor of group A streptococci

A high-affinity surface receptor for human plasmin has been reported on certain group A streptococci. To map the region of the plasmin molecule that binds to the bacterial receptor, isolated domains of plasmin were tested for their ability to inhibit the binding of intact radiolabeled plasmin to receptor-positive bacteria. Complete inhibition of binding of labeled plasmin to bacteria by isolated heavy chains was achieved, but this inhibition was not as efficient on a molar basis when compared with that of unlabeled plasmin. By contrast, a conformationally altered form of native plasminogen was found to bind to bacteria and was as efficient a competitive inhibitor as intact plasmin was. The results of this study indicate that the selective binding of human plasmin to a group A streptococcus is dependent on structures present in the conformationally altered form of native plasminogen or plasmin that are not found on the native zymogen, the plasminogen with NH2-terminal glutamic acid.

Analysis of ligand binding to kringles 4 and 5 fragments from human plasminogen

The interaction of the isolated kringles 4 and 5 from human plasminogen with 6-aminohexanoic acid, pentylamine, pentanoic acid and arginine has been quantitatively characterized by scanning calorimetry and fluorescent spectroscopy. It has been found that the ligands with the positively charged group have a good binding ability while pentanoic acid in comparison with 6-aminohexanoic acid being devoid of amino group does not interact with the kringles under study. The positively charged group of the ligand is suggested to play a crucial role in ligand binding with the lysine-binding site.

Human fibrinogen

The structure and physical properties of human fibrinogen and fibrin are reviewed along with methods for the detection of products of their metabolism. Interactions of human fibrinogen with thrombin, factor XIII, plasminogen, glycoprotein IIb/IIIa, and other proteins are related to their relevance to thrombosis and hemostasis. To the extent information is available, the structural determinants of these interactions are delineated, and kinetic and thermodynamic parameters associated with the interactions are listed. Individual steps in the reaction pathway for the conversion of fibrinogen to cross-linked fibrin are characterized. The altered hemostatic properties of mutational variants of fibrinogen are related to their altered structure. The structures of the genes coding for the polypeptide chains of fibrinogen are discussed along with the current state of knowledge of the control and regulation of fibrinogen synthesis. Fibrinogen catabolism and fibrinolysis are also reviewed.

Quantitative analysis of fibrin-binding affinity of fibrinolytic components by frontal affinity chromatography

Binding affinity of fibrinolytic factors to insolubilized lysine and fibrin was quantitatively measured by frontal affinity chromatography using lysine-Toyopearl and fibrin-Sepharose column. The highest binding affinity was found with recombinant tissue-type plasminogen activator (t-PA), followed by lysyl-plasminogen and glutamyl-plasminogen (Glu-PLg) with intermediate affinity, but very low affinity by single chain UK-type plasminogen activator, high molecular weight UK and low molecular weight UK. At the coexistence of EACA, fibrin-binding affinity of Glu-PLg was greatly reduced, but those of UK's were substantially unchanged. It was concluded that high fibrin-binding affinity of t-PA and plasminogens were largely related to the lysine-binding affinity of these enzymes, but that of UK's would be related to the other binding affinity.

The interaction of streptokinase.plasminogen activator complex, tissue-type plasminogen activator, urokinase and their acylated derivatives with fibrin and cyanogen bromide digest of fibrinogen. Relationship to fibrinolytic potency in vitro

The effects of purified soluble fibrin and of fibrinogen fragments (fibrin mimic) on the activation of Lys-plasminogen (i.e. plasminogen residues 77-790) to plasmin by streptokinase.plasminogen activator complex and by tissue-type plasminogen activator were studied. Dissociation constants of both activators were estimated to lie in the range 90-160 nM (fibrin) and 16-60 nM (CNBr-cleavage fragments of fibrinogen). The kinetic mechanism for both types of activator comprised non-essential enzyme activation via a Rapid Equilibrium Ordered Bireactant sequence. In order to relate the fibrin affinity of plasminogen activators to their fibrinolytic potency, the rate of lysis of supported human plasma clots formed in the presence of unmodified or active-centre-acylated precursors of plasminogen activators was studied as a function of the concentration of enzyme derivative. The concentrations of unmodified enzyme giving 50% lysis/h in this assay were 0.9, 2.0 and 11.0 nM for tissue-type plasminogen activator, streptokinase.plasmin(ogen) and urokinase respectively. However, the potencies of active-centre-acylated derivatives of these enzymes suggested that acylated-tissue plasminogen activator and streptokinase.plasminogen complexes of comparable hydrolytic stability were of comparable potency. Both types of acyl-enzyme were significantly more potent than acyl-urokinases.

Effect of lectin-binding to fibrinogen D and E domains on coagulation and fibrinolysis

The effect on fibrinogen coagulation and fibrinolysis of the mannose-specific lectins concanavalin A, its acetyl derivative and Lens culinaris agglutinin was studied. Concanavalin A and acetyl-concanavalin A, which bind to the four carbohydrate chains of fibrinogen, and L. culinaris agglutinin, which only binds to the carbohydrate present in fibrinogen D domains, has the same effect on the coagulation rate: an inhibition at low lectin concentrations and an increase at high concentrations. On the other hand, L. culinaris agglutinin does not alter fibrin crosslinking while acetyl-concanavalin A produces a slight inhibition of both gamma-gamma and alpha-polymer formation. However, this effect is very small when compared with the clear inhibitory effect produced by concanavalin A. Concanavalin A and acetyl-concanavalin A have an inhibitory effect on the rate of fibrin clot lysis proportional to the lectin concentration. Nearly 100% inhibition was obtained when two lectin-binding sites were occupied by either concanavalin A or acetyl-concanavalin A. However, L. culinaris agglutinin has a clearly weaker effect and more than 50% inhibition was not observed. The comparative study of the effect of the three lectins on fibrinolysis as well as on the formation of fibrinogen aggregates suggests that the inhibitory effect of concanavalin A and acetyl-concanavalin A is primarily due to their binding to the carbohydrate chains of fibrinogen E domain.

Amino-acid sequence of human alpha 2-antiplasmin

The amino-acid sequence of human alpha 2-antiplasmin was determined by Edman degradation of peptides purified from CNBr, tryptic and chymotryptic digests. Of the total sequence of 452 amino acids of mature alpha 2-antiplasmin, as deduced from the cDNA sequence [Holmes et al. (1987) J. Biol. Chem. 262, 1659-1664], 444 residues were identified by amino-acid sequencing. Two differences were found between the peptide and cDNA analyses (Gly instead of Leu at position 10 and Gly instead of Ser at position 369). alpha 2-Antiplasmin contains two disulfide bridges (Cys64-Cys104 and Cys31-Cys113) and four glucosamine-based carbohydrate chains attached to Asn87, Asn256, Asn270 and Asn277. alpha 2-Antiplasmin is homologous with 12 other proteins belonging to the serine protease inhibitor (serpin) superfamily.