Characterization of the reaction of plasmin with alpha 2-macroglobulin: effect of antifibrinolytic agents

Frozen fresh blood plasma preserves the functionality of native human α2-macroglobulin

Human α₂-macroglobulin (hα₂M) is a large homotetrameric protein involved in the broad inhibition of endopeptidases. Following cleavage within a bait region, hα₂M undergoes stepwise transitions from its native, expanded, highly flexible, active conformation to an induced, compact, triggered conformation. As a consequence, the peptidase is entrapped by an irreversible Venus flytrap mechanism. Given the importance of hα₂M, biochemical studies galore over more than seven decades have attempted to ascertain its role, typically using authentic hα₂M purified from frozen and non-frozen fresh blood plasma, and even outdated plasma. However, hα₂M is sensitive once isolated and purified, and becomes heterogeneous during storage and/or freezing, raising concerns about the functional competence of frozen plasma-derived hα₂M. We therefore used a combination of native and sodium dodecylsulfate polyacrylamide gel electrophoresis, affinity and ion-exchange chromatography, multi-angle laser light scattering after size-exclusion chromatography, free cysteine quantification, and peptidase inhibition assays with endopeptidases of two catalytic classes and three protein substrates, to characterize the biochemical and biophysical properties of hα₂M purified ad hoc either from fresh plasma or frozen fresh plasma after thawing. We found no differences in the molecular or functional properties of the preparations, indicating that protective components in plasma maintain native hα₂M in a functionally competent state despite freezing.

Truncation of Huia versabilis Bowman-Birk inhibitor increases its selectivity, matriptase-1 inhibitory activity and proteolytic stability

A gradual truncation of the primary structure of frog skin-derived Huia versabilis Bowman-Birk peptidic inhibitor (HV-BBI) resulted in 18-times stronger inhibitor of matriptase-1 (peptide 6, K_i = 8 nm) in comparison to the full-length HV-BBI (K_i = 155 nm). Analogous increase in the inhibitory activity in correlation with the peptide length reduction was not observed in case of other serine proteases, bovine trypsin (K_i = 151 nm for peptide 6 and K_i = 120 nm for HV-BBI) and plasmin (K_i = 120 nm for peptide 6 and 82 nm for HV-BBI). Weaker binding affinity to these enzymes emphasized an inhibitory specificity of peptide 6. Molecular dynamic analysis revealed that the observed variations in the binding affinity of peptide 6 and HV-BBI with matriptase-1 are associated with the entropic differences of the unbound peptides. Moreover, several aspects explaining differences in the inhibition of matriptase-1 by peptide 6 (bearing the C-terminal amide group) and its two analogues, peptide 6∗ (having the C-terminal carboxyl group, K_i = 473 nm) and cyclic peptide 6∗∗ (K_i = 533 nm), both exhibiting more than 50-fold reduced inhibitory potency, were discovered. It was also shown that peptide 6 presented significantly higher resistance to proteolytic degradation in human serum than HV-BBI. Additional investigations revealed that, in contrast to some amphibian-derived inhibitors, HV-BBI and its truncated analogues do not possess bactericidal activity, thus they cannot be considered as bifunctional agents.

Recent advances on plasmin inhibitors for the treatment of fibrinolysis-related disorders

Growing evidence suggests that plasmin is involved in a number of physiological processes in addition to its key role in fibrin cleavage. Plasmin inhibition is critical in preventing adverse consequences arising from plasmin overactivity, e.g., blood loss that may follow cardiac surgery. Aprotinin was widely used as an antifibrinolytic drug before its discontinuation in 2008. Tranexamic acid and ε-aminocaproic acid, two small molecule plasmin inhibitors, are currently used in the clinic. Several molecules have been designed utilizing covalent, but reversible, chemistry relying on reactive cyclohexanones, nitrile warheads, and reactive aldehyde peptidomimetics. Other major classes of plasmin inhibitors include the cyclic peptidomimetics and polypeptides of the Kunitz and Kazal-type. Allosteric inhibitors of plasmin have also been designed including small molecule lysine analogs that bind to plasmin's kringle domain(s) and sulfated glycosaminoglycan mimetics that bind to plasmin's catalytic domain. Plasmin inhibitors have also been explored for resolving other disease states including cell metastasis, cell proliferation, angiogenesis, and embryo implantation. This review highlights functional and structural aspects of plasmin inhibitors with the goal of advancing their design.

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.

Blood inhibitory capacity toward exogenous plasmin

Stabilized, active plasmin is a novel thrombolytic for direct delivery to clots. Although it is known that protease inhibitors in plasma inhibit plasmin, the amount of plasmin that can be added to plasma/blood before free plasmin is observed is not clear. Determination of free plasmin activity in plasma using chromogenic substrates represents a challenge due to false-positive signals from plasmin entrapped by alpha2-macroglobulin. Size-exclusion chromatography was used to separate the plasmin-alpha2-macroglobulin complex from uninhibited, free plasmin. In this in-vitro study, exogenous plasmin is effectively inhibited up to 2.4 micromol/l after 5-min incubation with plasma at 37 degrees C. Initially, plasmin was consumed predominantly by alpha2-antiplasmin up to 1.2 micromol/l plasmin. Following exhaustion of alpha2-antiplasmin, plasmin was further consumed by alpha2-macroglobulin up to 2.4 micromol/l plasmin added to human plasma. Whole human blood was found to have an increased inhibitory capacity over that of plasma; free plasmin activity could be measured only above 3.8 micromol/l added plasmin. In conclusion, several mechanisms exist that control plasmin activity in human blood; in addition to alpha2-antiplasmin and alpha2-macroglobulin, blood cells contribute to the inhibition of exogenously administered plasmin. These in-vitro results indicate that doses of plasmin up to approximately 12 mg/kg in humans can be completely inactivated by blood.

Yield and aging of Cheddar cheeses manufactured from milks with different milk serum protein contents

Whey proteins in general and specifically beta-lactoglobulin, alpha-lactalbumin, and immunoglobulins have been thought to decrease proteolysis in cheeses manufactured from concentrated retentates from ultrafiltration. The proteins found in whey are called whey proteins and are called milk serum proteins (SP) when they are in milk. The experiment included 3 treatments; low milk SP (0.18%), control (0.52%), and high milk SP (0.63%), and was replicated 3 times. The standardized milk for cheese making of the low milk SP treatment contained more casein as a percentage of true protein and more calcium as a percentage of crude protein, whereas the nonprotein nitrogen and total calcium content was not different from the control and high SP treatments. The nonprotein nitrogen and total calcium content of the milks did not differ because of the process used to remove the milk SP from skim milk. The low milk SP milk contained less free fatty acids (FFA) than the control and high milk SP treatment; however, no differences in FFA content of the cheeses was detected. Approximately 40 to 45% of the FFA found in the milk before cheese making was lost into the whey during cheese making. Decreasing the milk SP content of milk by 65% and increasing the content by 21% did not significantly influence general Cheddar cheese composition. Higher fat recovery and cheese yield were detected in the low milk SP treatment cheeses. There was more proteolysis in the low milk SP cheese and this may be due to the lower concentration of undenatured beta-lactoglobulin, alpha-lactalbumin, and other high molecular weight SP retained in the cheeses made from milk with low milk SP content.

Interaction of a plasminogen activator proteinase, LV-PA with human α2-macroglobulin

Lachesis venom plasminogen activator (LV-PA) is a 33-kDa serine proteinase isolated from bushmaster (Lachesis muta muta) snake venom, which activates the fibrinolytic system in vitro. This study has examined the effect of the plasma proteinase inhibitor alpha2-macroglobulin (alpha2-M) towards LV-PA and compares it with the effect on tissue type plasminogen activator (t-PA). The proteolytic activity of LV-PA alone or previously incubated with human plasminogen (Plg) on the large molecular mass protein substrates, dimethylcasein (DMC) and fibrinogen (Fg) was completely inhibited by human alpha2-M. However, the synthetic peptides Tos-Gly-Pro-Lys-pNA and H-D-Pro-Phe-Arg-pNA (S-2302) were hydrolyzed with almost no reduction in rate. At pH 7.4 and 37 degrees C the proteinase (0.15 microM over 15 min) interacted with alpha2-M, and each mole of alpha2-M bound 2 mol of enzyme. Sodium dodecyl sulfate gel electrophoresis of reduced samples showed that the interaction of alpha2-M with either LV-PA or t-PA preincubated with Plg resulted in the formation of approximately 90 kDa fragments and high molecular mass complexes (Mr 180 kDa), generated by the incubation mixture (LV-PA or t-PA) and Plg. The data suggest that LV-PA is a direct-type PA and its fibrinolytic effect can be reduced by alpha2-M in vivo.

Three-dimensional structure of the human plasmin alpha2-macroglobulin complex

The three-dimensional reconstructions of the human plasmin alpha2-macroglobulin binary complex were computed from electron microscopy images of stain and frozen-hydrated specimens. The structures show excellent agreement and reveal a molecule with approximate dimensions of 170 (length) x 140 (width) x 140 A (depth). The asymmetric plasmin structure imparts significant asymmetry to the plasmin alpha2-macroglobulin complex not seen in the structures resulting from the reaction of alpha2-macroglobulin with methylamine or chymotrypsin. The structure shows, when combined with other studies, that the C-terminal catalytic domain of the rod-shaped plasmin molecule is entrapped inside of the alpha2-macroglobulin cavity, whereas its N-terminal kringle domains protrude outside one end between the two arm-like features of the transformed alpha2-macroglobulin structure. This arrangement ensures that the catalytic site of plasmin is prevented from degrading plasma proteins. The internalized C-terminal portion of the plasmin structure resides primarily on the major axis of alpha2-macroglobulin, suggesting that after the initial cleavage of the two bait domains and the thiol esters, the rod-shaped plasmin molecule enters the alpha2-macroglobulin cavity through the large openings afforded by the half-transformed structure. This mode of entrapment requires the untwisting and the separation of the two strands that constitute the alpha2-macroglobulin structure.

Inhibition of intracellular proteolytic processing of soluble proproteins by an engineered alpha 2-macroglobulin containing a furin recognition sequence in the bait region

The bait region of the general protease inhibitor alpha 2-macroglobulin (alpha 2M) was mutated by introducing a recognition sequence of furin. This did not interfere with folding, S-ester formation or tetramerization of the mutant recombinant alpha 2M (r alpha 2M). Mutant r alpha 2M inhibited furin in vitro, by a similar mechanism to that used by plasma alpha 2M to inhibit high-molecular-mass proteases. The mutant alpha 2M was intracellularly active in COS-1 cells in inhibiting the endogenous processing of the soluble substrates for furin (von Willebrand factor, transforming growth factor beta1 and a soluble form of the envelope glycoprotein gp160 from HIV-1) but not the membrane-bound form of gp160. The intracellular activity of mutant alpha 2M strongly indicated that alpha 2M attains its native conformation, and thus that the unusual internal S-ester is formed, before alpha 2M passes through the cleavage compartment(s). Our results show for the first time that modulation of the bait region of alpha 2M allows the creation of an inhibitor against membrane-bound proteases. It can be expected that the use of alpha 2M-bait mutants will become important as a technique for the study of various proteolytic processes and for the identification of the proteases involved.

Purification of pregnancy zone protein and its receptor binding domain from human plasma

A new significantly improved method for purification of pregnancy zone protein (PZP), alpha 2-macroglobulin (alpha 2M), and the C-terminal PZP receptor binding domain is presented. Several steps in an earlier procedure have been deleted, and modifications in the gradients in the DEAE step leave most of the contaminants bound to a DEAE-Sephacel gel. This procedure makes possible the rapid, simultaneous purification of both of these closely related unstable proteins in native form from human plasma, with no thiolester cleavage or formation of tetrameric PZP. The final preparations of both alpha 2M and PZP are pure as determined by nonreducing and reducing polyacrylamide gel electrophoresis following silver staining and no cross-contamination can be observed. The yield has been significantly improved and typically more than 500 mg PZP can be obtained from 1 liter pregnancy plasma. Furthermore, the stability of PZP at different temperatures on storage was studied. In liquid nitrogen PZP can be maintained in native dimeric form with intact thiolester for many years. The storage of native PZP with intact functional properties during and after purification is an obligatory prerequisite to elucidate the biological role of PZP. The receptor binding domain of PZP can be cleaved from the PZP-methylamine complex by papain and isolated from the other peptides by S-200 gel filtration. The cleavage site was determined and the C-terminal fragment was identified with several site-specific monoclonal antibodies against PZP.

Characterization of the antiplasmin activity of human thrombospondin-1 in solution

These studies demonstrate relatively rapid association of plasmin with thrombospondin and the effects of this interaction on plasmin activity towards D-Val-L-Leu-L-Lys p-nitroanilide hydrochloride (S-2251) and the proteinase inhibitors alpha 2-antiplasmin (alpha 2AP) and alpha 2-macroglobulin (alpha 2M). Binding of plasmin to thrombospondin reached an apparent reversible equilibrium within 3 min at 22 degrees C. The amidase activity of bound plasmin was inhibited. An analysis of S-2251 hydrolysis indicated that thrombospondin is a linear mixed-type plasmin inhibitor. The dissociation constant (KD) for the binding of plasmin to thrombospondin was 0.5 microM, assuming one plasmin binding site per thrombospondin homotrimer. Plasmin and miniplasmin slowly cleaved thrombospondin, yielding products which were comparable with those generated by other proteinases. Tranexamic acid inhibited the digestion of thrombospondin by plasmin and miniplasmin, suggesting an important role for the kringle-5 domain in this process. When plasmin was incubated first with thrombospondin and then with alpha 2AP, plasmin that was apparently bound to thrombospondin reacted with alpha 2AP at a decreased rate; however, within 20 min, all of the plasmin was recovered in complex with alpha 2AP. Similar results were obtained with alpha 2M. Transfer of plasmin from thrombospondin to alpha 2AP or alpha 2M probably required plasmin-thrombospondin-complex dissociation. A low level of reaction of alpha 2AP with thrombospondin-associated plasmin could not be ruled out. These results demonstrate that the activity of plasmin, when bound to thrombospondin, is greatly diminished or eliminated. The plasmin-thrombospondin complex, which is formed within 3 min, is fully reversible and the associated plasmin is in a latent form protected from proteinase inhibitors. Therefore, thrombospondin may regulate plasmin activity in a manner which is distinct from conventional proteinase inhibitors and other extracellular-matrix proteins.

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.

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)

Binding of transforming growth factor-beta 1 to methylamine-modified alpha 2-macroglobulin and to binary and ternary alpha 2-macroglobulin-proteinase complexes

The binding of 125I-labelled transforming growth factor-beta 1 (TGF-beta 1) to human alpha 2-macroglobulin (alpha 2M) was studied by native PAGE and autoradiography. TGF-beta 1 bound preferentially to alpha 2M-methylamine and minimally, if at all, to native alpha 2M. Preparations of alpha 2M-proteinase complex were generated by incubating a standard concentration of alpha 2M (0.4 microM) with different concentrations of trypsin, chymotrypsin or neutrophil elastase (0.04-2.0 microM). The 125I-TGF-beta 1-binding activity depended on the initial ratio of active proteinase to alpha 2M, or r value, used to form the alpha 2M-proteinase complex. With all three proteinases, r values of 2 or greater yielded preparations with unchanged or decreased TGF-beta 1-binding activity relative to native alpha 2M. By contrast, r values near 1 yielded preparations with significantly increased TGF-beta 1-binding activity. The results of [3H]thymidine-incorporation studies performed in mouse keratinocytes were consistent with the 125I-TGF-beta-binding experiments. alpha 2M-trypsin and alpha 2M-chymotrypsin prepared at an r value of 1.0 counteracted the activity of TGF-beta 1, whereas the equivalent complexes prepared at an r value of 3.0 had no effect. As determined by SDS/PAGE, 125I-TGF-beta 1 binding to alpha 2M-methylamine was at least 80% non-covalent. Reaction of alpha 2M-methylamine with iodoacetamide or 5,5'-dithiobis-(2-nitrobenzoic acid) decreased the percentage of covalent binding but had no effect on total binding. Neuraminidase treatment had no effect on the binding of 125I-TGF-beta 1 to alpha 2M-methylamine. Cleavage of the 'bait regions' in alpha 2M-methylamine by prolonged treatment with trypsin also had no effect. These studies suggest that TGF-beta 1 binding to alpha 2M is enhanced by conformational change in the proteinase inhibitor resulting from reaction with proteinase or amine. If both proteinase-binding sites in a single alpha 2M molecule are occupied, TGF-beta 1-binding activity is decreased or perhaps eliminated.

Soluble fibrin preparations inhibit the reaction of plasmin with alpha 2-macroglobulin. Comparison with alpha 2-antiplasmin and leupeptin

The kinetics of plasmin inhibition by alpha 2-antiplasmin (alpha 2AP), alpha 2-macroglobulin (alpha 2M) and leupeptin were studied in the presence of fibrin monomer (Fn) and CNBr fragments of fibrinogen (Fg-CNBr). Active plasmin was detected in continuous and discontinuous assays using the chromogenic substrate D-Val-L-Leu-L-Lys p-nitroanilide hydrochloride (S-2251). The two 'fibrin-like' preparations functioned as hyperbolic mixed-type inhibitors of S-2251 hydrolysis. The dissociation constants (KF) for the binding of plasmin to Fn and Fg-CNBr were 22 nM and 17 nM respectively. Fn and Fg-CNBr inhibited the reaction of plasmin with alpha 2AP: the extent of inhibition depended on the fibrin concentration. In the presence of 800 nM-Fn or 800 nM-Fg-CNBr, the experimental second-order rate constant (K"app.) was decreased from 2.4 x 10(7) M-1.s-1 to 1.2 x 10(6) and 5.3 x 10(5) M-1.s-1 respectively. The effect of Fn and Fg-CNBr on the rate of plasmin inhibition by alpha 2M was even greater. The k"app. value was decreased from 4.0 x 10(5) M-1.s-1 to 8.0 x 10(2) and 1.3 x 10(3) M-1.s-1 in the presence of 800 nM-Fn and -Fg-CNBr respectively. By contrast, the fibrin preparations caused only a small change in the rate of plasmin inhibition by leupeptin. The maximum change in k"app. was 3-fold. All plasmin inhibition curves were linear, suggesting that free and fibrin-bound forms of plasmin remained in equilibrium during the course of reaction with proteinase inhibitors. Fn and Fg-CNBr had no effect on the reaction of miniplasmin with S-2251, alpha 2AP or alpha 2M. When 125I-plasmin was incubated with Fg-CNBr and then allowed to react with a premixed solution of alpha 2AP and alpha 2M, the Fg-CNBr did not significantly change the percentage of plasmin bound to alpha 2AP. These experiments demonstrate that the reaction of plasmin with alpha 2M is inhibited by the non-covalent binding of plasmin to fibrin. We propose that plasmin bound to the surface of a clot is protected from inhibition by alpha 2M as well as by alpha 2AP.

Reaction of proteinases with alpha 2-macroglobulin: rapid-kinetic evidence for a conformational rearrangement of the initial alpha 2-macroglobulin-trypsin complex

The kinetics of the reaction of trypsin with alpha 2M were examined under pseudo-first-order conditions with excess inhibitor. Initial studies indicated that the fluorescent dye TNS is a suitable probe for monitoring the reaction over a wide concentration range of reactants. Titration experiments showed that the conformational changes associated with the binding of trypsin to alpha 2M result in an increased affinity of the inhibitor for TNS. Two distinct phases were observed when this dye was used to monitor the progress of the reaction. Approximately half of the fluorescence signal was generated during a rapid phase, with the remainder generated during a second, slower phase. The observed pseudo-first-order rate constant of the first phase varied linearly with the concentration of alpha 2M up to the highest concentration of inhibitor used, whereas the rate constant of the second phase was independent of alpha 2M concentration. The data fit a mechanism in which the association of trypsin with alpha 2M occurs in two consecutive, essentially irreversible steps, both leading to alterations in TNS fluorescence. The initial association occurs with a second-order rate constant of (1.0 +/- 0.1) X 10(7) M-1 s-1 and is followed by a slower, intramolecular conformational rearrangement of the initial complex with a rate constant of 1.4 +/- 0.2 s-1. The data are consistent with a previously proposed model for the reaction of proteinases with alpha 2M [Larsson et al. (1989) Biochemistry 28, 7636-7643].2+ this model, once an initial 1:1 alpha 2M-proteinase

Kinetics and mechanism of proteinase-binding of pregnancy zone protein (PZP). Appearance of sulfhydryl groups in reactions with proteinases

Proteinase binding by pregnancy zone protein (PZP), an alpha-macroglobulin involves bait region cleavages, association of dimeric-PZP into tetrameric and reaction of internal gamma-glutamyl-beta-cysteinyl thiol esters of PZP with proteinase side chains. The product is an equimolar enzyme-PZP(tetramer) covalently linked complex with four free sulfhydryl groups. The kinetics of the appearances of sulfhydryl groups during the reaction of PZP with chymotrypsin has been investigated using stopped-flow and conventional mixing techniques over a broad concentration range. Thiol ester cleavages followed double exponential decays corresponding with two steps. The faster one resulted in the appearance of three sulfhydryl groups with an observed rate constant, k(obs) = k1.1 + k1.2 delta E, dependent on the excess concentration of chymotrypsin, delta E, and k1.1 = 0.03 s-1 and k1.2 = 4 x 10(4) M-1 s-1. The last sulfhydryl group appeared in a slower step, with similar concentration dependence and k2.1 approximately 0.003 s-1 and k2.2 approximately 5 x 10(3) M-1s-1. Covalent binding of the enzyme apparently was simultaneous with the faster thiol ester cleavage step. Based on these and previous results a model of the reaction mechanism of the proteinase binding reaction of PZP is proposed. It consists of four major steps: (i) Bait region cleavage of PZP-dimers by the enzyme, (ii) fast association of enzyme-PZP(dimer) species with native PZP or with another enzyme-PZP(dimer) compound resulting in release of one of the associated enzyme molecules (iii) reaction of an average of three thiol esters of the enzyme-PZP(tetramer) intermediate with the associated internal enzyme molecule or with an external one. In this step one enzyme molecule becomes covalently linked to the PZP-(tetramer), three sulfhydryl groups appear and the enzymic activity of the bound enzyme molecule decreases to the level of that of the final complex. (iv) Hydrolysis of the last thiol ester and in the presence of excess enzyme, degradation of enzyme-PZP(tetramer) complexes and formation of fragments some of which are the size of PZP(dimer) with enzyme bound.

The mechanism of activation of plasminogen at the fibrin surface by tissue-type plasminogen activator in a plasma milieu in vitro. Role of alpha 2-antiplasmin

The mechanism of activation of human Glu-plasminogen by fibrin-bound tissue-type plasminogen activator (t-PA) in a plasma environment or in a reconstituted system was characterized. A heterogeneous system was used, allowing the setting of experimental conditions as close as possible to the physiological fibrin/plasma interphase, and permitting the separate analysis of the products present in each of the phases as a function of time. The generation of plasmin was monitored both by spectrophotometric analysis and by radioisotopic analysis with a plasmin-selective chromogenic substrate and radiolabelled Glu-plasminogen respectively. Plasmin(ogen)-derived products were identified by SDS/PAGE followed by autoradiography and/or immunoblotting. When the activation was performed in a plasma environment, the products identified on the fibrin surface were Glu-plasmin (90%) and Glu-plasminogen (10%), whereas in the soluble phase only complexes between Glu-plasmin and its fast-acting inhibitor were detected. Identical results were obtained with a reconstituted system comprising solid-phase fibrin, t-PA, Glu-plasminogen and and alpha 2-antiplasmin. In contrast, when alpha 2-antiplasmin was omitted from the solution, Lys-plasmin was progressively generated on to the fibrin surface (30%) and released to the soluble phase. In the presence of alpha 2-antiplasmin or in plasma, the amount of active plasmin generated on the fibrin surface was lower than in the absence of the inhibitor: in a representative experiment the initial velocity of plasmin generation was 2.8 x 10(-3), 2.0 x 10(-3) and 1.8 x 10(-3) (delta A405/min) for 200 nM-plasminogen, 200 nM-plasminogen plus 100 nM-alpha 2-antiplasmin and native plasma respectively. Our results indicate that in plasma or in a reconstituted purified system containing plasminogen and alpha 2-antiplasmin at a ratio similar to that found in plasma (1) the activation pathway of native Glu-plasminogen proceeds directly to the formation of Glu-plasmin, (2) Lys-plasminogen is not an intermediate of the reaction and therefore (3) Lys-plasmin is not the final active product. However, in the absence of the inhibitor, Lys-plasmin and probably Lys-plasminogen, which is more readily activated to plasmin than is Glu-plasminogen, are generated as well.

Regulation of plasmin, miniplasmin, and streptokinase-plasmin complex by alpha 2-antiplasmin, alpha 2-macroglobulin, and antithrombin III in the presence of heparin

The regulation of plasmin, miniplasmin, and streptokinase-plasmin complex (SkPm) was studied in vitro in the presence of unfractionated porcine intestinal heparin using purified plasma proteinase inhibitors. Heparin enhanced the reaction of antithrombin III (AT) with plasmin (up to 40-fold with 20 units/ml). The rate of plasmin inhibition by alpha 2-antiplasmin (alpha 2AP) and by alpha 2-macroglobulin (alpha 2M) was not changed by heparin (0.5-100 units/ml); the rank-order of plasmin-inhibitory activity remained alpha 2AP greater than alpha 2M greater than AT. The reaction of miniplasmin with AT was studied also. The second order rate constant was 9.2 x 10(2) M-1s-1 without heparin and 2.6 x 10(4) M-1s-1 in the presence of 20 units/ml heparin. Heparin did not affect the rank-order of miniplasmin-inhibitory activity; it remained alpha 2M greater than alpha 2AP greater than AT. While the reaction of AT with SkPm was negligible, heparin stimulated this reaction dramatically. The SkPm-inhibitory activity of alpha 2AP was not changed by heparin. When plasma concentrations of alpha 2AP (1.05 microM) and AT (4.76 microM) were compared, AT inhibited greater amounts of SkPm in the presence of more than 5 units/ml of heparin. The increased SkPm-inhibitory activity of AT in heparin did not result from SkPm dissociation, and heparin did not decrease the rapid rate of streptokinase association with plasmin. These studies demonstrate that heparin can affect the regulation of fibrinolysis at multiple levels of the enzyme cascade.

Reaction of proteinases with alpha 2-macroglobulin: evidence for alternate reaction pathways in the inhibition of trypsin

Titration experiments were employed to measure the binding stoichiometry of alpha 2M for trypsin at high and low concentrations of reactants. These titration experiments were performed by measuring the SBTI-resistant trypsin activity and by direct binding measurements using 125I-labeled trypsin. The binding stoichiometry displayed a marked dependence upon protein concentration. At high alpha 2M concentrations (micromolar), 2 mol of trypsin are bound/mol of inhibitor. However, at low alpha 2M concentrations (e.g., 0.5 nM), only 1.3 mol of trypsin were bound/mol of inhibitor. Sequential additions of subsaturating amounts of trypsin to a single aliquot of alpha 2M also resulted in a reduction in the final binding ratio. A model has been formulated to account for these observations. A key element of this model is the observation that purified 1:1 alpha 2M-proteinase complexes are not capable of binding a full mole of additional proteinase [Strickland et al. (1988) Biochemistry 27, 1458-1466]. The model predicts that once the 1:1 alpha 2M-proteinase complex forms, this species undergoes a time-dependent conformational rearrangement to yield a complex with greatly reduced proteinase binding ability. According to this model, the ability of alpha 2M to bind 2 mol of proteinase depends upon the association rate of the second enzyme molecule with the binary (1:1) complex, the enzyme concentration, and the rate of the conformational alteration that occurs once the initial complex forms. Modeling experiments suggest that the magnitude of the rate constant for this conformational change is in the order of 1-2 s-1.

Analysis of thiolester bond cleavage-dependent conformational changes in binary alpha 2-macroglobulin-proteinase complexes

The structures of the two proteinase-binding sites in human alpha 2-macroglobulin (alpha 2M) were probed by treatment of alpha 2M with the serine proteinases thrombin and plasmin. Each proteinase forms an equimolar complex with alpha 2M (a binary alpha 2M-proteinase complex) which results in the activation and cleavage of two internal thiolester bonds in alpha 2M. Binary alpha 2M-proteinase complexes demonstrated an incomplete conformational change as determined by nondenaturing polyacrylamide gel electrophoresis and incomplete receptor recognition site exposure as determined by in vivo plasma elimination studies. Treatment of binary alpha 2M-proteinase complexes with CH3NH2, trypsin, or elastase resulted in cleavage of an additional one or two thiolester bonds in alpha 2M and complete receptor recognition site exposure, demonstrating that a limited conformational change had occurred. Treatment of the alpha 2M-thrombin complex with elastase resulted in the incorporation of approximately 0.5 mol proteinase/mol alpha 2M and completion of the conformational change in the complex. Similar treatment of the alpha 2M-plasmin complex resulted in the incorporation of less than 0.1 mol proteinase/mol alpha 2M. Unlike the alpha 2M-thrombin complex, the alpha 2M-plasmin complex did not undergo a complete conformational change following treatment with CH3NH2 or trypsin. Incubation of this complex with elastase resulted in proteolysis of the kringle 1-4 region of the alpha 2M-bound plasmin heavy chain, and following this treatment the alpha 2M-plasmin complex underwent a complete conformational change. The results of this investigation demonstrate that binary alpha 2M-proteinase complexes retain a relatively intact proteinase-binding site. In the case of the alpha 2M-plasmin complex, however, the heavy chain of alpha 2M-bound plasmin protrudes from the proteinase-binding site and prevents a complete conformational change in the complex despite additional thiolester bond cleavage.