Binding site of alpha 2-plasmin inhibitor to plasminogen

Plasmin Inhibitor in Health and Diabetes: Role of the Protein as a Therapeutic Target

The vascular obstructive thrombus is composed of a mesh of fibrin fibers with blood cells trapped in these networks. Enhanced fibrin clot formation and/or suppression of fibrinolysis are associated with an increased risk of vascular occlusive events. Inhibitors of coagulation factors and activators of plasminogen have been clinically used to limit fibrin network formation and enhance lysis. While these agents are effective at reducing vascular occlusion, they carry a significant risk of bleeding complications. Fibrin clot lysis, essential for normal hemostasis, is controlled by several factors including the incorporation of antifibrinolytic proteins into the clot. Plasmin inhibitor (PI), a key antifibrinolytic protein, is cross-linked into fibrin networks with higher concentrations of PI documented in fibrin clots and plasma from high vascular risk individuals. This review is focused on exploring PI as a target for the prevention and treatment of vascular occlusive disease. We first discuss the relationship between the PI structure and antifibrinolytic activity, followed by describing the function of the protein in normal physiology and its role in pathological vascular thrombosis. Subsequently, we describe in detail the potential use of PI as a therapeutic target, including the array of methods employed for the modulation of protein activity. Effective and safe inhibition of PI may prove to be an alternative and specific way to reduce vascular thrombotic events while keeping bleeding risk to a minimum. Key Points Plasmin inhibitor (PI) is a key protein that inhibits fibrinolysis and stabilizes the fibrin network.This review is focused on discussing mechanistic pathways for PI action, role of the molecule in disease states, and potential use as a therapeutic target.

A new ELISA method for the measurement of total α2-plasmin inhibitor level in human body fluids

The ever-increasing research efforts to develop new antithrombotic therapies have led to the reassessment of the role of alpha-2-plasmin inhibitor (α₂-PI) in pathological conditions. In particular, experimental stroke studies have suggested correlation between increased free α2-PI level and mortality. However there are only a small number of well-characterized and specific assays available for the measurements of free α₂-PI. In plasma α₂-PI undergoes both N- and/or C-terminal cleavages resulting four isoforms with modified susceptibility to FXIII catalyzed cross-linking to fibrin and/or loss of plasmin(ogen) binding. Present paper describes a new sandwich ELISA method for the determination of free total α₂-PI in plasma and other body fluids. A newly generated biotinylated monoclonal antibody recognizes and captures all the four N- and/or C-terminally modified isoforms of α₂-PI while HRPO-labeled polyclonal anti-α₂-PI antibody detects the captured antigen. Performing the 2-step assay in streptavidin-coated microplate can be completed within three hours. The assay is well reproducible, total (within laboratory) imprecision in the normal, pathological and very low ranges were 7.4%, 9.1% and < 19%, respectively. When examining the plasma samples of 197 healthy volunteers, 100 acute ischemic stroke patients and 102 patients with venous thrombosis, strong correlation was observed between total α2-PI antigen levels and α2-PI activity for each group. Using the assay a reference interval of 45-86 mg/L was established for total α₂-PI mass concentration in the plasma. α₂-PI levels were also measured in cerebrospinal fluid samples of 47 individuals the median value and range was 132 (36-379) μg/L. In conclusion, our ELISA enables accurate and fast measurement of total free α₂-PI in human body fluids.

Natural heterogeneity of α2-antiplasmin: functional and clinical consequences

Human α2-antiplasmin (α2AP, also called α2-plasmin inhibitor) is the main physiological inhibitor of the fibrinolytic enzyme plasmin. α2AP inhibits plasmin on the fibrin clot or in the circulation by forming plasmin-antiplasmin complexes. Severely reduced α2AP levels in hereditary α2AP deficiency may lead to bleeding symptoms, whereas increased α2AP levels have been associated with increased thrombotic risk. α2AP is a very heterogeneous protein. In the circulation, α2AP undergoes both amino terminal (N-terminal) and carboxyl terminal (C-terminal) proteolytic modifications that significantly modify its activities. About 70% of α2AP is cleaved at the N terminus by antiplasmin-cleaving enzyme (or soluble fibroblast activation protein), resulting in a 12-amino-acid residue shorter form. The glutamine residue that serves as a substrate for activated factor XIII becomes more efficient after removal of the N terminus, leading to faster crosslinking of α2AP to fibrin and consequently prolonged clot lysis. In approximately 35% of circulating α2AP, the C terminus is absent. This C terminus contains the binding site for plasmin(ogen), the key component necessary for the rapid and efficient inhibitory mechanism of α2AP. Without its C terminus, α2AP can no longer bind to the lysine binding sites of plasmin(ogen) and is only a kinetically slow plasmin inhibitor. Thus, proteolytic modifications of the N and C termini of α2AP constitute major regulatory mechanisms for the inhibitory function of the protein and may therefore have clinical consequences. This review presents recent findings regarding the main aspects of the natural heterogeneity of α2AP with particular focus on the functional and possible clinical implications.

Proteolytic and genetic variation of the alpha-2-antiplasmin C-terminus in myocardial infarction

Alpha-2-antiplasmin (α2AP) undergoes both N- and C-terminal cleavages, which significantly modify its activities. Compared with other Ser protease inhibitors (serpins), α2AP contains an ~50-residue-extended C-terminus, which binds plasmin(ogen). We developed 2 new ELISAs to measure the antigen levels of free total α2AP and free C-terminally intact α2AP to investigate whether α2AP antigen levels or α2AP C-terminal cleavage were associated with myocardial infarction (MI) in 320 male MI survivors and 169 age-matched controls. Patients had 15.2% reduced total α2AP antigen levels compared with controls (93.8 vs 110.6 U/dL, P < .001), with a 10.1-fold (95% confidence interval [CI]: 5.5-18.9) increased MI risk for levels in the 1st quartile compared with the 4th quartile. The percentage of C-terminal cleavage did not differ between patients and controls (38.7% and 38.1%, respectively, P = .44). In addition, all individuals were genotyped for the polymorphism Arg407Lys, which is located near the start of the extended C-terminus. Arg407Lys was not associated with α2AP C-terminal cleavage, total α2AP antigen levels, or MI risk (odds ratios compared with Arg/Arg: Arg/Lys 0.74, 95% CI: 0.50-1.10; Lys/Lys 0.77, 95% CI: 0.31-1.92). Our data show that levels of free full-length α2AP were decreased in MI, that the percentage of C-terminally cleaved α2AP was unaltered, and that Arg407Lys did not influence α2AP levels or MI risk.

The interaction between plasminogen and antiplasmin variants as studied by surface plasmon resonance

The interaction between immobilized plasminogen or an elastase-degradation product from plasminogen, constituting "kringles" 1-3 and different purified variants of antiplasmin has been studied by surface plasmon resonance utilising a BIAcore. The antiplasmin variants studied are wild-type, K429E, K436E, E443G, D444G, K452E and K452T. It is shown that the two mutants K452T and K452E react in quite a similar way as wt-antiplasmin, suggesting that Lys452 is not involved in the lysine-binding site interaction between plasminogen and antiplasmin. On the other hand, the mutant K436E displays a much lower k(a). The affinity between plasminogen or the fragment constituting "kringles" 1-3 and K436E were also much lower than with wt-antiplasmin. Thus, also the data obtained with surface plasmon resonance show that Lys436 indeed is very important in the lysine-binding site mediated interaction between plasminogen and antiplasmin.

A novel plasma proteinase potentiates alpha2-antiplasmin inhibition of fibrin digestion

Human alpha2-antiplasmin (alpha2AP), also known as alpha2-plasmin inhibitor, is the major inhibitor of the proteolytic enzyme plasmin that digests fibrin. There are 2 N-terminal forms of alpha2AP that circulate in human plasma: a 464-residue protein with Met as the N-terminus, Met-alpha2AP, and a 452-residue version with Asn as the N-terminus, Asn-alpha2AP. We have discovered and purified a proteinase from human plasma that cleaves the Pro12-Asn13 bond of Met-alpha2AP to yield Asn-alpha2AP and have named it antiplasmin-cleaving enzyme (APCE). APCE is similar in primary structure and catalytic properties to membrane-bound fibroblast activation protein/seprase for which a physiologic substrate has not been clearly defined. We found that Asn-alpha2AP becomes cross-linked to fibrin by activated factor XIII approximately 13 times faster than native Met-alpha2AP during clot formation and that clot lysis rates are slowed in direct proportion to the ratio of Asn-alpha2AP to Met-alpha2AP in human plasma. We conclude that APCE cleaves Met-alpha2AP to the derivative Asn-alpha2AP, which is more efficiently incorporated into fibrin and consequently makes it strikingly resistant to plasmin digestion. APCE may represent a new target for pharmacologic inhibition, since less generation and incorporation of Asn-alpha2AP could result in a more rapid removal of fibrin by plasmin during atherogenesis, thrombosis, and inflammatory states.

Identification of amino acids in antiplasmin involved in its noncovalent 'lysine-binding-site'-dependent interaction with plasmin

The lysine-binding-site-mediated interaction between plasmin and antiplasmin is of great importance for the fast rate of this reaction. It also plays an important part in regulating the fibrinolytic enzyme system. To identify structures important for its noncovalent interaction with plasmin, we constructed seven single-site mutants of antiplasmin by modifying charged amino acids in the C-terminal part of the molecule. All the variants were expressed in the Drosophila S2 cell system, purified, and shown to form stable complexes with plasmin. A kinetic evaluation revealed that two mutants of the C-terminal lysine (K452E or K452T) did not differ significantly from wild-type antiplasmin in their reactions with plasmin, in either the presence or absence of 6-aminohexanoic acid, suggesting that this C-terminal lysine is not important for this reaction. On the other hand, modification of Lys436 to Glu decreased the reaction rate about fivefold compared with wild-type. In addition, in the presence of 6-aminohexanoic acid, only a small decrease in the reaction rate was observed, suggesting that Lys436 is important for the lysine-binding-site-mediated interaction between plasmin and antiplasmin. Results from computerized molecular modelling of the C-terminal 40 amino acids support our experimental data.

A splicing donor site point mutation in intron 6 of the plasmin inhibitor (alpha2 antiplasmin) gene with heterozygous deficiency and a bleeding tendency

A new case of familial plasmin inhibitor (alpha2 antiplasmin) deficiency is reported. The bleeding symptoms are moderate, happening after surgery or trauma or consisting of abnormal uterine bleeding induced by hormone replacement therapy. It is easily corrected with tranexamic acid. Gene sequencing makes it possible to find a splicing donor site mutation of intron 6, leading to exon 6 skipping. Neither a shortened variant nor an abnormal plasmin interaction was found in plasma by immunoblotting, and fibrin binding is unaffected. The mutation is heterozygous, associated with an intermediate decrease of both antiplasmin activity and antigen levels, and was found in four other family members out of five tested. It is different from the five mutations previously reported. At the time of diagnosis, the patient was living in Artas, France, allowing the defect to be named plasmin inhibitor (alpha2 antiplasmin) Artas.

Function of the 90-loop (Thr90-Glu100) region of staphylokinase in plasminogen activation probed through site-directed mutagenesis and loop deletion

Staphylokinsae (SAK) forms a bimolecular complex with human plasmin(ogen) and changes its substrate specificity by exposing new exosites that enhances accession of substrate plasminogen (PG) to the plasmin (Pm) active site. Protein modelling studies indicated the crucial role of a loop in SAK (SAK 90-loop; Thr(90)-Glu(100)) for the docking of the substrate PG to the SAK-Pm complex. Function of SAK 90-loop was studied by site-directed mutagenesis and loop deletion. Deletion of nine amino acid residues (Tyr(92)-Glu(100)) from the SAK 90-loop, resulted in approximately 60% reduction in the PG activation, but it retained the ability to generate an active site within the complex of loop mutant of SAK (SAKDelta90) and Pm. The preformed activator complex of SAKDelta90 with Pm, however, displayed a 50-60% reduction in substrate PG activation that remained unaffected in the presence of kringle domains (K1+K2+K3+K4) of PG, whereas PG activation by SAK-Pm complex displayed approximately 50% reduction in the presence of kringles, suggesting the involvement of the kringle domains in modulating the PG activation by native SAK but not by SAKDelta90. Lysine residues (Lys(94), Lys(96), Lys(97) and Lys(98)) of the SAK 90-loop were individually mutated into alanine and, among these four SAK loop mutants, SAK(K97A) and SAK(K98A) exhibited specific activities about one-third and one-quarter respectively of the native SAK. The kinetic parameters of PG activation of their 1:1 complex with Pm indicated that the K(m) values of PG towards the activator complex of these two SAK mutants were 4-6-fold higher, suggesting the decreased accessibility of the substrate PG to the activator complex formed by these SAK mutants. These results demonstrated the involvement of the Lys(97) and Lys(98) residues of the SAK 90-loop in assisting the interaction with substrate PG. These interactions of SAK-Pm activator complex via the SAK 90-loop may provide additional anchorage site(s) to the substrate PG that, in turn, may promote the overall process of SAK-mediated PG activation.

Effect of a synthetic carboxy-terminal peptide of alpha(2)-antiplasmin on urokinase-induced fibrinolysis

alpha(2)-Antiplasmin (alpha(2)AP) interferes with the binding of plasminogen to fibrin because lysine residues in its carboxy-terminal region compete with those in fibrin, presumably the same way that free lysine or epsilon-aminocaproic acid (EACA) inhibits plasminogen binding to fibrin. While this overall process causes an inhibition of fibrinolysis, the converse was observed with a 26-residue synthetic peptide (AP26) corresponding to the carboxy-terminal region of alpha(2)AP. The AP26 peptide, in fact, accelerated urokinase-induced lysis of (1) fully crosslinked fibrin with complete gamma-dimer and alpha-polymer formation; (2) partially crosslinked fibrin that had undergone only gamma-dimerization; and (3) noncrosslinked fibrin. The AP26 peptide also inhibited factor XIIIa-catalyzed crosslinking of fibrin alpha-chains, and this also accelerated lysis of fibrin. EACA had no effect. In the presence of noncrosslinked fibrin, AP26 promoted plasminogen activation by urokinase and fibrinolysis. EACA only slightly increased the rate of plasminogen activation, and as expected, it inhibited fibrinolysis. Since AP26 peptide enhanced the lysis of partially crosslinked and noncrosslinked fibrin, our results indicate that inhibition of factor XIIIa-catalyzed alpha-polymer formation by AP26, although associated with accelerated fibrinolysis, is not the primary mechanism. Instead, our data support the conclusion that AP26 enhances the conversion of plasminogen to plasmin approximately 5-fold, probably by inducing a conformational change in plasminogen structure just as occurs with low concentrations of lysine or EACA. At higher concentrations, however, AP26 apparently does not approach the avidity or affinity of lysine or EACA for the kringle structures of plasminogen or plasmin so that their binding to fibrin is blocked. Whether AP26 alone, or as part of another molecule, could have potential for enhancing thrombolysis will require further study.

Inactivation of the serpin alpha(2)-antiplasmin by stromelysin-1

Matrix metalloproteinase-3 (MMP-3 or stromelysin-1) hydrolyzes the Met(374)-Ser(375) (P3-P2), Glu(416)-Leu(417) and Ser(432)-Leu(433) peptide bonds in human alpha(2)-antiplasmin (alpha(2)-AP), the main physiological plasmin inhibitor. Cleavage is completely abolished in the presence of the MMP inhibitors EDTA or 1,10-phenanthroline. At enzyme/substrate ratio of 1:10 at 37 degrees C, alpha(2)-AP protein cleavage occurs with a half-life of 8 min, and is associated with rapid loss of inhibitory activity towards plasmin with a half-life of 5 min. alpha(2)-AP cleaved by MMP-3 does no longer form a stable complex with plasmin, as shown by SDS-PAGE, and does no longer interact with plasminogen, as shown by crossed immunoelectrophoresis with plasminogen added to the gel. These data are compatible with the removal of a COOH-terminal fragment containing the reactive site peptide bond and the plasmin(ogen)-binding site. In addition, MMP-3 cleaves the Pro(19)-Leu(20) peptide bond in alpha(2)-AP, thereby removing the fibrin-binding site from the inhibitor. A dysfunctional alpha(2)-AP variant (Ala-alpha(2)-AP or alpha(2)-AP Enschede), with an alanine insertion in the reactive site sequence converting it from a plasmin inhibitor into a substrate, was also efficiently cleaved by MMP-3 (half-life of 13 min at 37 degrees C and enzyme/substrate ratio of 1:10). Cleavage and inactivation of alpha(2)-AP by MMP-3 may constitute a mechanism favoring local plasmin-mediated proteolysis.

Participation of the Dengue virus in the fibrinolytic process

To date, the phatophysiology of hemorrhagic dengue is still unknown and hypotheses which aim to explain the unfortunate cases of the disease (hemorrhagic fever/shock syndrome) are based on epidemiological data and favor the notion of the participation of heterotypic non-neutralizing antibodies during the course of secondary infection (immunologic status of the host). However, cases of hemorrhagic dengue have been reported during the course of primary infections. We propose that the dengue virus, specifically the envelope glycoprotein can participate directly in the installation of the hemorrhagic phenomenon by means of the binding and activation of plasminogen (PLG) as condition previous to the development of the fibrinolytic process. Based on this hypothesis, we evaluated the biological activity of some viral isolates proceeding from hemorrhagic and from dengue fever cases in an in vitro model of fibrinolysis. Dengue isolates were capable of activating PLG. The plasmin generated specifically degraded the fibrin/fibrinogen molecule. This catalytic process can be prevented by the presence of the specific plasmin inhibitor, alpha-2-antiplasmin, for virus isolates from dengue fever, but not for isolates associated with dengue hemorrhagic disease, favoring the exacerbation of the fibrinolytic activity. This new approach allows us to suggest the importance of viral factors in the dengue hemorrhagic fever.

A novel missense mutation in the human plasmin inhibitor (alpha2-antiplasmin) gene associated with a bleeding tendency

Heterozygosity for a G --> A mutation converting Val384(GTG) to Met(ATG) associated with plasmin inhibitor (alpha2-antiplasmin) deficiency was identified in three family members with bleeding tendency. The proband had traumatic breast haematoma and per-/postoperative bleeds. An affected daughter required a blood transfusion after a normal delivery and a son had prolonged bleeding after tooth extraction. The plasma plasmin inhibitor activities of the affected family members were reduced to 49-66% of normal. The antigenic concentrations determined by electroimmunoassay were reduced to 57-68% of normal. Crossed immunoelectrophoresis of plasma from the proband showed a normal pattern. The amino acid Val384 is located eight residues C-terminal (P8') of the P1 residue (Arg376) in the reactive site. The P8' residues of bovine and mouse plasmin inhibitor are both Val. Among other serpins the P8' residue is unconserved. The mutation was not present in the non-affected family member or 30 blood donors. In addition to the Val384Met mutation two new polymorphisms Ala-26(GCG)/Val(GTG) and Arg407(AGG)/Lys(AAG) and one previously reported polymorphism Arg6(CGG)/Trp(TGG) were identified in the plasmin inhibitor gene of the family. The allelic frequencies among 30 blood donors with normal values of plasma plasmin inhibitor (functional) were 0.84/0.16 for C/T in codon -26, 0.81/0.19 for C/T in codon 6 and 0.83/0.17 for G/A in codon 407.

2-Antiplasmin Gene Deficiency in Mice Is Associated With Enhanced Fibrinolytic Potential Without Overt Bleeding

2-antiplasmin (2-AP) is the main physiologic plasmin inhibitor in mammalian plasma. Inactivation of the murine 2-AP gene was achieved by replacing, through homologous recombination in embryonic stem cells, a 7-kb genomic sequence encoding the entire murine protein (exon 2 through part of exon 10, including the stop codon) with theneomycin resistance expression cassette. Germline transmission of the mutated allele was confirmed by Southern blot analysis. Mendelian inheritance of the inactivated 2-AP allele was observed, and homozygous deficient (2-AP−/−) mice displayed normal fertility, viability, and development. Reverse transcription-polymerase chain reaction confirmed the absence of 2-AP mRNA in kidney and liver from 2-AP−/− mice, in contrast to wild-type (2-AP+/+) mice. Immunologic and functional 2-AP levels were undetectable in plasma of 2-AP−/− mice, and were about half of wild-type in heterozygous littermates (2-AP+/−). Other hemostasis parameters, including plasminogen activator inhibitor-1, plasminogen, fibrinogen, hemoglobin, hematocrit, and blood cell counts were comparable for 2-AP+/+, 2-AP+/−, and 2-AP−/− mice. After amputation of tail or toe tips, bleeding stopped spontaneously in 2-AP+/+, as well as in 2-AP+/− and 2-AP−/− mice. Spontaneous lysis after 4 hours of intravenously injected 125I-fibrin–labeled plasma clots was significantly higher in 2-AP−/− than in 2-AP+/+ mice when injecting clots prepared from 2-AP+/+ plasma (78% ± 5% v 46% ± 9%; mean ± SEM, n = 6 to 7; P = .02) or from 2-AP−/−plasma (81% ± 5% v 46% ± 5%; mean ± SEM, n = 5; P = .008). Four to 8 hours after endotoxin injection, fibrin deposition in the kidneys was significantly reduced in 2-AP−/− mice, as compared with 2-AP+/+ mice (P ≤ .005). Thus, 2-AP−/− mice develop and reproduce normally; they have an enhanced endogenous fibrinolytic capacity without overt bleeding.

2-Antiplasmin Gene Deficiency in Mice Is Associated With Enhanced Fibrinolytic Potential Without Overt Bleeding

2-antiplasmin (2-AP) is the main physiologic plasmin inhibitor in mammalian plasma. Inactivation of the murine 2-AP gene was achieved by replacing, through homologous recombination in embryonic stem cells, a 7-kb genomic sequence encoding the entire murine protein (exon 2 through part of exon 10, including the stop codon) with theneomycin resistance expression cassette. Germline transmission of the mutated allele was confirmed by Southern blot analysis. Mendelian inheritance of the inactivated 2-AP allele was observed, and homozygous deficient (2-AP−/−) mice displayed normal fertility, viability, and development. Reverse transcription-polymerase chain reaction confirmed the absence of 2-AP mRNA in kidney and liver from 2-AP−/− mice, in contrast to wild-type (2-AP+/+) mice. Immunologic and functional 2-AP levels were undetectable in plasma of 2-AP−/− mice, and were about half of wild-type in heterozygous littermates (2-AP+/−). Other hemostasis parameters, including plasminogen activator inhibitor-1, plasminogen, fibrinogen, hemoglobin, hematocrit, and blood cell counts were comparable for 2-AP+/+, 2-AP+/−, and 2-AP−/− mice. After amputation of tail or toe tips, bleeding stopped spontaneously in 2-AP+/+, as well as in 2-AP+/− and 2-AP−/− mice. Spontaneous lysis after 4 hours of intravenously injected 125I-fibrin–labeled plasma clots was significantly higher in 2-AP−/− than in 2-AP+/+ mice when injecting clots prepared from 2-AP+/+ plasma (78% ± 5% v 46% ± 9%; mean ± SEM, n = 6 to 7; P = .02) or from 2-AP−/−plasma (81% ± 5% v 46% ± 5%; mean ± SEM, n = 5; P = .008). Four to 8 hours after endotoxin injection, fibrin deposition in the kidneys was significantly reduced in 2-AP−/− mice, as compared with 2-AP+/+ mice (P ≤ .005). Thus, 2-AP−/− mice develop and reproduce normally; they have an enhanced endogenous fibrinolytic capacity without overt bleeding.

Site-directed mutagenesis of streptococcal plasmin receptor protein (Plr) identifies the C-terminal Lys334 as essential for plasmin binding, but mutation of the plr gene does not reduce plasmin binding to group A streptococci

Plasmin(ogen) binding is a common property of many pathogenic bacteria including group A streptococci. Previous analysis of a putative plasmin receptor protein, Plr, from the group A streptococcal strain 64/14 revealed that it is a glyceraldehyde-3-phosphate dehydrogenase and that the plr gene is present on the chromosome as a single copy. This study continues the functional characterization of Plr as a plasmin receptor. Attempts at insertional inactivation of the plr gene suggested that this single-copy gene may be essential for cell viability. Therefore, an alternative strategy was applied to manipulate this gene in vivo. Site-directed mutagenesis of Plr revealed that a C-terminal lysyl residue is required for wild-type levels of plasmin binding. Mutated Plr proteins expressed in Escherichia coli demonstrated reduced plasmin-binding activity yet retained glyceraldehyde-3-phosphate dehydrogenase activity. A novel integration vector was constructed to precisely replace the wild-type copy of the plr gene with these mutations. Isogenic streptococcal strains expressing altered Plr bound equivalent amounts of plasmin as wild-type streptococci. These data suggest that Plr does not function as a unique plasmin receptor, and underscore the need to identify other plasmin-binding structures on group A streptococci and to assess the importance of the plasminogen system in pathogenesis by inactivation of plasminogen activators and the use of appropriate animal models.

Structure and ligand binding determinants of the recombinant kringle 5 domain of human plasminogen

The X-ray crystal structure of the recombinant (r) kringle 5 domain of human plasminogen (K5HPg) has been solved by molecular replacement methods using K1HPg as a model and refined at 1.7 A resolution to an R factor of 16.6%. The asymmetric unit of K5HPg is composed of two molecules related by a noncrystallographic 2-fold rotation axis approximately parallel to the z-direction. The lysine binding site (LBS) is defined by the regions His33-Thr37, Pro54-Val58, Pro61-Tyr64, and Leu71-Tyr74 and is occupied in the apo-form by water molecules. A unique feature of the LBS of apo-K5HPg is the substitution by Leu71 for the basic amino acid, arginine, that in other kringle polypeptides forms the donor cationic center for the carboxylate group of omega-amino acid ligands. While wild-type (wt) r-K5HPg interacted weakly with these types of ligands, replacement by site-directed mutagenesis of Leu71 by arginine led to substantially increased affinity of the ligands for the LBS of K5HPg. As a result, binding of omega-amino acids to this mutant kringle (r-K5HPg[L71R]) was restored to levels displayed by the companion much stronger affinity HPg kringles, K1HPg and K4HPg. Correspondingly, alkylamine binding to r-K5HPg[L71R] was considerably attenuated from that shown by wtr-K5HPg. Thus, employing a rational design strategy based on the crystal structure of K5HPg, successful remodeling of the LBS has been accomplished, and has resulted in the conversion of a weak ligand binding kringle to one that possesses an affinity for omega-amino acids that is similar to K1HPg and K4HPg.

Role of tryptophan-63 of the kringle 2 domain of tissue-type plasminogen activator in its thermal stability, folding, and ligand binding properties

Conservative (F and Y) and radical (H and S) mutations have been engineered at a rigidly conserved aromatic residue, W63, of the isolated recombinant kringle 2 domain of tissue-type plasminogen activator (r-K2tPA), an amino acid residue predicted from the X-ray crystal structure to be important in the ligand binding properties of this isolated protein domain. The variants were expressed in Pichia pastoris cells. The binding constants of epsilon-aminocaproic acid (EACA), 7-aminoheptanoic acid (7-AHpA), and trans-(aminomethyl)cyclohexanecarboxylic acid (AMCHA) to each of these mutant polypeptides were determined by titrations of the alterations in intrinsic fluorescence of the variant kringles with the ligands. As compared to wild-type r-K2tPA, increases in the Kd (dissociation) values of approximately 15-fold and 20-200-fold were found for the W63F and W63Y mutants, respectively, toward these three ligands. Neither the W63H nor the W63S variant interacted with these same ligands. Differential scanning calorimetric analyses were also performed on each of the peptides to determine whether the alterations affected the conformational stability of wtr-K2tPA. The data demonstrated that all of these mutants were thermally destabilized, possessing temperatures of maximum heat capacity (Tm) values that were 12-20 degrees C lower than that of wtr-K2tPA. Addition of EACA resulted in increases (approximately 12 degrees C) in the Tm values of r-[W63F]-K2tPA and r-[W63Y]K2tPA, a result showing that EACA stabilized the native conformations adopted by these kringle domains. As expected from its greatly diminished binding to r-[W63H]K2tPA and r-[W63S]-K2tPA, high concentrations of EACA had little effect on the Tm of thermal denaturation of these latter mutants. 1H-NMR analysis of the two aromatic mutant kringles was employed to assess their overall comparative folding properties. The high upfield chemical shifts (-0.98 ppm) of the CH3(delta') protons of L47, a major signal of proper kringle folding, were slightly lowered to -0.83 to -0.86 ppm in the cases of all of the mutants. This is due to alterations in the W25-L47 side-chain spatial orientations, possibly the result of slight conformational alterations that affect the distance relationships of these two amino acid side chains. Assignments of nearly all of the protons of the aromatic residues in the W63F and W63Y mutants were accomplished, and few additional differences from their wild-type counterpart were noted. Reactivities of the mutants against four different monoclonal antibodies directed to wtr-K2tPA revealed the possibility that some small local conformational alterations might have resulted from the residues that have replaced the W63. We conclude that W63 possesses an important direct role in the ligand binding properties of r-K2tPA. This residue also contributes significantly to the stability of the native conformation of this kringle domain and perhaps to maintenance of local conformations.

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).

Mechanisms of physiological fibrinolysis

The fibrinolytic system comprises an inactive proenzyme, plasminogen, that is converted by plasminogen activators to the active enzyme, plasmin, which degrades fibrin. Two immunologically distinct plasminogen activators (PA) have been identified: tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). t-PA mediated plasminogen activation is mainly involved in the dissolution of fibrin in the circulation, whereas u-PA mediated plasminogen activation mainly plays a role in pericellular proteolysis. Plasminogen activation is regulated by specific molecular interactions between its main components, such as binding of plasminogen and t-PA to fibrin, or to specific cellular receptors resulting in enhanced plasminogen activation, inhibition of t-PA and u-PA by plasminogen activator inhibitors (PAI) and inhibition of plasmin by alpha 2-antiplasmin. Controlled synthesis and release of PAs and PAIs primarily from endothelial cells also contributes to the regulation of physiological fibrinolysis. The lysine binding sites situated in the kringle structures of plasminogen play a crucial role in the regulation of fibrinolysis by modulating its binding to fibrin and to cell surfaces, and by controlling the inhibition rate of plasmin by alpha 2-antiplasmin.

The apolipoprotein B3304-3317 peptide as an inhibitor of the lipoprotein (a):apolipoprotein B-containing lipoprotein interaction

Lipoprotein (a) [Lp(a)] is a risk factor for coronary artery disease. It is characterized by apolipoprotein (a) [apo(a)] disulphide linked to apolipoprotein B (apoB), by Cys4057 of apo(a) and possibly Cys3734 of apoB. We call this the covalent apo(a):apoB-Lp interaction, to distinguish it from the non-covalent Lp(a):apoB-Lp interaction, mediated by the proline-binding kringle-4-like domain(s) of Lp(a). The Lp(a):apoB-Lp interaction was inhibited by an apoB peptide spanning residues 3304-3317. This peptide was found by a computerized search for sites on apoB similar to the plasminogen's kringle-4-binding site of alpha 2-antiplasmin. It probably constitutes part of the Lp(a)-binding site on apoB because: (1) it corresponds to the alpha 2-antiplasmin minimum binding domain for plasminogen's kringle-4; (2) the competitive nature of inhibition [KI = (1.5 +/- 0.7) x 10(-4) M, n = 5] suggested that it and apoB-Lp bound to Lp(a) by the same mechanism at the same site; and (3) it specifically bound Lp(a) and not apoB-Lp, and the bound Lp(a) was dissociated by inhibitors of the Lp(a):apoB-Lp interaction, 6-aminohexanoic acid and L-proline. Inhibition was independent of its proline residue, suggesting that proline in the context of a peptide is not a ligand for the kringle(s) which mediated the binding of Lp(a) to apoB-Lp.

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.

Lipoprotein (a) regulates plasmin generation and inhibition

The relationship between lipoprotein (a) (Lp(a)) and atherosclerosis has been appreciated for a number of years. Only in recent years, however, has the structural relationship of Lp(a) to plasminogen resulted in studies of the effect of this lipoprotein on fibrinolysis. Lp(a) inhibits activation of plasminogen by tissue-type (t-PA) and urinary-type (u-PA) plasminogen activators. These inhibitory reactions are surface-dependent. When Lp(a) binds to fibrin, fibrinogen, heparin or cells it blocks activation of plasminogen by t-PA. u-PA-mediated activation of plasminogen is blocked on surfaces including heparin and chondroitin sulfate. Lp(a) also favors inhibition of plasmin by alpha 2-antiplasmin (alpha 2-AP). The ability of Lp(a) to compete with plasmin for fibrin binding displaces plasmin into solution where alpha 2-AP rapidly inhibits this proteinase. These effects are all antifibrinolytic. Lp(a) also exhibits one profibrinolytic effect, since it blocks inhibition of t-PA by plasminogen activator type 1 in the presence of fibrinogen or heparin. Thus, Lp(a) modulates most of the reactions involved in plasmin generation and inhibition. Its overall effect will depend primarily on the concentrations of Lp(a), PAI-1 and t-PA in vivo.

Study of polynucleotide conformation by resolution-enhanced ultraviolet spectroscopy poly(rC) and poly(dC)

Self-deconvolution and the fourth derivative of ultraviolet absorption spectra have been used to study stacked single-stranded and double-helix structures of different cytosine-containing polynucleotides for the first time. These compounds were studied under different solution conditions (pH and organic solvents) and at low temperatures. The red shift of the lower band (B2u band plus possibly some n-->pi* transition) of the absorption spectra in the cytosine-containing polynucleotides and the appearance of new peaks in the deconvoluted and derivative spectra in the 280-310 nm region are attributed mainly to cytosine-cytosine stacking interactions. In particular, the fourth-derivative peaks at wavelengths higher than 290 nm can be associated to coupling of electronic transitions of cytosine bases. The nature of the electronic transitions producing the absorption bands which are resolved in the aforementioned fourth-derivative peaks is discussed. It is concluded that the resolution-enhancement techniques used in this work, i.e. self-deconvolution and fourth derivative, complement each other and are useful methods to study structural changes of single-stranded and double-stranded polynucleotides allowing, at the same time, more information to be obtained about specific stacking interactions than classical absorption spectrophotometry.

The plasminogen-plasmin system in malignancy

The study of the plasminogen-plasmin system has, in the past, contributed much to the understanding of fibrinolysis and thrombolysis. Attention is now focused on the role of the components of this system in many biologic functions. Findings of uPA, its receptor and its inhibitor in many tumor tissues and tumor cell lines, strongly implicate their involvement in tumor invasion, tumor cell proliferation and metastasis. The characteristics of the plasminogen activators, the uPA receptor and the plasminogen activator inhibitors as well as their expression and regulation in tumors and tumor cell lines are reviewed.

Lipoprotein (a) promotes plasmin inhibition by alpha 2-antiplasmin

Plasmin inhibition by alpha 2-antiplasmin (alpha 2AP) is regulated by the vascular components fibrin(ogen) fragments, plasminogen and lipoprotein (a). Kinetic analysis demonstrates that CNBr-derived fibrinogen fragments completely protect plasmin from alpha 2AP. Plasminogen and 6-aminohexanoic acid decrease the rate of inhibition by 5- and 10-fold respectively. These studies show that CNBr-derived fibrinogen fragments and 6-aminohexanoic acid bind plasmin kringle(s) with binding constants of 2 micrograms/ml and 120 microM respectively, and that plasminogen binds to alpha 2AP with an affinity of 0.5 nM. The unmodulated inhibition is not effected by the presence of lipoprotein (a), but in the presence of protective CNBr-derived fibrinogen fragments the rate of inhibition is increased by the presence of the lipoprotein. The kinetics demonstrate that lipoprotein (a) binds to CNBr-derived fibrinogen fragments with an affinity of 4 nM, displacing plasmin from the protective surface. In addition, tissue-type plasminogen activator and trypsin inhibition by alpha 2AP is not slowed by the presence of CNBr-derived fibrinogen fragments or plasminogen (Pg), respectively. These kinetics suggest that the initial reversible interaction between plasmin and alpha 2AP is mediated by binding of the inhibitor to the kringle 1 domain of plasmin, with a reversible inhibition constant (Ki) of 5.0 x 10(-10) M. Under conditions where this kringle-inhibitor interaction is blocked, the reversible inhibition still occurs between the plasmin and alpha 2AP, but the initial Ki is increased to 5.0 x 10(-9) M. These data suggest that, in the circulation, plasmin inhibition by alpha 2AP may be down-regulated by fibrin, fibrin(ogen) fragments and Pg, but up-regulated by lipoprotein (a) in the presence of fibrin or fibrin(ogen) fragments. The lipoprotein (a)-mediated promotion of plasmin inhibition may provide an additional mechanism by which the lipoprotein impairs fibrinolysis and promotes atherosclerosis.

Distribution of plasminogen and plasmin in fractions of bovine milk

The relative amounts of immunoreactive plasminogen and active plasmin in different fractions of bovine milk were examined. Raw milk was centrifuged to separate skim, cream, and a somatic cell pellet. Skim milk was centrifuged to separate milk serum and casein micelles. Milk fat globule membranes were isolated from the cream fraction of bovine milk. Proteins from somatic cells were isolated following sonication of the cells. Western blot analysis showed the presence of several forms of plasminogen in bovine milk. The predominant forms of plasminogen identified following electrophoresis under nonreducing conditions were proteins with approximate molecular weights of 88,000, 152,000, and 160,000. The predominant forms of plasminogen identified after electrophoresis under reducing conditions were two proteins with approximate molecular weights of 88,000 and 50,000. The highest amount (82% of the total plasminogen), as determined by an ELISA, was associated with the casein fraction. Lower plasminogen concentrations were associated with the serum, cream fractions, and milk fat globule membranes. The SDS-PAGE of the cream and milk fat globule membranes indicated that some casein was present in both fractions. Thus, the low plasminogen concentrations in these fractions may be associated with the caseins there. No immunoreactive plasminogen was present in the somatic cells. Active plasmin was present in the same milk fractions in which plasminogen was detected: casein, serum, and cream.

The N-terminal domain of antithrombin-III is essential for heparin binding and complex-formation with, but not cleavage by, alpha-thrombin

Normal and mutant forms of human antithrombin-III (AT-III) were synthesized in a cell-free system in order to identify putative functional domains required for heparin binding and complex-formation with alpha-thrombin. Heparin-Sepharose chromatography resulted in the elution of approx. 70% of cell-free-derived normal AT-III-(1-432)-polypeptide as a peak between 0.2 M- and 0.7 M-NaCl. The cell-free-derived normal AT-III also reacted with alpha-thrombin. Approx. 15% of this AT-III formed covalent complexes with alpha-thrombin in 2 min. Unfractionated heparin accelerated the rate of formation of such complexes. Two truncated forms of AT-III (amino acid residues 219-432 and 251-432), containing only the putative thrombin-binding domain, were synthesized independently in this cell-free system. These truncated AT-III polypeptides did not bind heparin and were unable to form stable covalent complexes with alpha-thrombin. However, both of these AT-III polypeptides were cleaved by alpha-thrombin, presumably at the reactive centre Arg-393-Ser-394. The formation of the disulphide bond between Cys-247 and Cys-430 in AT-III-(219-432)-polypeptide had no effect on the results obtained. Mutations in full-length AT-III at Cys-430 had no effect on the ability of AT-III to bind heparin. There was, however, a slight decrease in the formation of stable inhibitory complexes with alpha-thrombin. A cell-free-derived AT-III mutant, devoid of amino acid residues 41-49, which comprise heparin-binding region 1 of AT-III, had slightly decreased heparin binding compared with cell-free-derived normal AT-III-(1-432)-polypeptide. This mutant AT-III polypeptide was unable, however, to form a stable complex with alpha-thrombin. We conclude therefore that the N-terminal domain of AT-III is essential for both heparin binding and complex-formation with alpha-thrombin, but not for the cleavage of AT-III at its reactive centre by alpha-thrombin.

C1 inhibitor: different mechanisms of reaction with complement component C1 and C1s

Inactivation of human complement subcomponent C1-s by its regulator C1 inhibitor at physiological ionic strength proceeded at a 3-fold higher rate when C1-s was in the physiological C1- complex with subcomponents C1q and C1-r rather than as purified subunit. When the C1- complex was disassembled by chelation of calcium, the C1-s subcomponent was inactivated by C1 inhibitor at rates similar to those for the purified proteinase. Increasing ionic strength had little effect on the reaction of purified C1-s with C1 inhibitor but greatly diminished the rate of reaction of intact C1-. Addition of heparin accelerated the inactivation of purified C1-s by C1 inhibitor up to 25-fold but increased the inactivation of intact C1- only about 5-fold. These differences in the inactivation of C1-s by C1 inhibitor, depending on whether the proteinase is free or complexed with other subcomponents of C1-, suggest different mechanisms of reaction. Occurrence of subcomponent C1-s in a macromolecular complex with C1q and C1-r, thus, appears to be critical not only for directing its physiological activation but also its inactivation.

Plasmin's peptide-binding specificity: characterization of ligand sites in alpha 2-antiplasmin

The basis for specific binding of plasmin to alpha 2-antiplasmin (AP) was analyzed by preparing overlapping synthetic peptides of 11, 17, 18, 19, 26, 33, and 40 amino acid residues corresponding to the carboxy-terminal sequence of AP. Affinities of the peptides for plasmin were estimated by competitive inhibition of the association of AP with plasmin. Dissociation constants with increasing peptide length were: 200, 54, 19, 18, 9.8, 4.7, and 2.8 microM, respectively. Peptides blocked binding sites on plasmin, not the catalytic site, as evidenced by lack of effect on the hydrolysis of chromogenic substrates. Substituting arginine for lysine at the carboxy-terminus or the 17th residue from the carboxy-terminus decreased the affinity of peptides for plasmin 9-fold and 5-fold, respectively, implicating these lysine residues of AP as major ligand sites for plasmin. Several stepwise increases in affinity of peptides for plasmin as peptide length increased up to 40 residues suggest contributions by additional sites, possibly other lysine residues. A potential plasmin binding site in fibrin, analogous to that in AP, is identified by affinity for plasmin of synthetic peptides corresponding to part of the alpha-chain ending with residue 207. To explain these data, we propose that plasmin recognizes physiological ligands by binding two or more lysine residues which are optimally presented to favor simultaneous interaction with separate lysine-binding site in plasmin.

Alpha 2-antiplasmin's carboxy-terminal lysine residue is a major site of interaction with plasmin

alpha 2-Antiplasmin (AP) inhibits plasmin in a two-step reaction in which AP reversibly binds to lysine-binding sites of plasmin and, then, more slowly complexes covalently with the enzyme's active site. Here, we show that the C-terminal lysine residue of AP has a key role in binding of the inhibitor to plasmin. A synthetic peptide corresponding to the C-terminal 26 amino acid residues of AP blocked association of AP with plasmin, but this activity of the peptide was lost when its C-terminal lysine residue was removed with carboxypeptidase B. The essential role of this lysine residue was shown more directly by treating AP with carboxypeptidase B and observing that AP lost its ability to inhibit plasmin rapidly.