Mechanism of the urokinase-catalyzed activation of human plasminogen

Plasminogen missense variants and their involvement in cardiovascular and inflammatory disease

Human plasminogen (PLG), the zymogen of the fibrinolytic protease, plasmin, is a polymorphic protein with two widely distributed codominant alleles, PLG/Asp453 and PLG/Asn453. About 15 other missense or non-synonymous single nucleotide polymorphisms (nsSNPs) of PLG show major, yet different, relative abundances in world populations. Although the existence of these relatively abundant allelic variants is generally acknowledged, they are often overlooked or assumed to be non-pathogenic. In fact, at least half of those major variants are classified as having conflicting pathogenicity, and it is unclear if they contribute to different molecular phenotypes. From those, PLG/K19E and PLG/A601T are examples of two relatively abundant PLG variants that have been associated with PLG deficiencies (PD), but their pathogenic mechanisms are unclear. On the other hand, approximately 50 rare and ultra-rare PLG missense variants have been reported to cause PD as homozygous or compound heterozygous variants, often leading to a debilitating disease known as ligneous conjunctivitis. The true abundance of PD-associated nsSNPs is unknown since they can remain undetected in heterozygous carriers. However, PD variants may also contribute to other diseases. Recently, the ultra-rare autosomal dominant PLG/K311E has been found to be causative of hereditary angioedema (HAE) with normal C1 inhibitor. Two other rare pathogenic PLG missense variants, PLG/R153G and PLG/V709E, appear to affect platelet function and lead to HAE, respectively. Herein, PLG missense variants that are abundant and/or clinically relevant due to association with disease are examined along with their world distribution. Proposed molecular mechanisms are discussed when known or can be reasonably assumed.

Measurement of Bradykinin Formation and Degradation in Blood Plasma: Relevance for Acquired Angioedema Associated With Angiotensin Converting Enzyme Inhibition and for Hereditary Angioedema Due to Factor XII or Plasminogen Gene Variants

Bradykinin (BK)-mediated angioedema (AE) states are rare acquired or hereditary conditions involving localized edema of the subcutaneous and submucosal tissues. Citrated plasma from healthy volunteers or patients with hereditary angioedema (HAE) with normal level of C1-inhibitor (C1-INH) was used to investigate pathways of BK formation and breakdown relevant to AE physiopathology. The half-life of BK (100 nM) added to normal plasma was 34 s, a value that was increased ~12-fold when the angiotensin converting enzyme (ACE) inhibitor enalaprilat (130 nM) was added (enzyme immunoassay measurements). The BK half-life was similarly increased ~5-fold following 2 daily oral doses of enalapril maleate in healthy volunteers, finding of possible relevance for the most common form of drug-associated AE. We also addressed the kinetics of immunoreactive BK (iBK) formation and decline, spontaneous or under three standardized stimuli: tissue kallikrein (KLK-1), the particulate material Kontact-APTT™ and tissue plasminogen activator (tPA). Relative to controls, iBK production was rapid (10–20 min) and very intense in response to tPA in plasma of female heterozygotes for variants in gene F12 coding for factor XII (FXII) (p.Thr328Lys, 9 patients; p.Thr328Arg, one). An increased response to Kontact-APTT™ and an early tPA-induced cleavage of anomalous FXII (immunoblots) were also observed. Biotechnological inhibitors showed that the early response to tPA was dependent on plasmin, FXIIa and plasma kallikrein. Results from post-menopausal and pre-menopausal women with HAE-FXII were indistinguishable. The iBK production profiles in seven patients with the plasminogen p.Lys330Glu variant (HAE-PLG) did not significantly differ from those of controls, except for an unexpected, rapid and lanadelumab-resistant potentiation of KLK-1 effect. This enzyme did not cleave plasminogen or factor XII, suggesting a possible idiosyncratic interaction of the plasminogen pathogenic variant with KLK-1 activity. KLK-1 abounds in salivary glands and human saliva, hypothetically correlating with the clinical presentation of HAE-PLG that includes the swelling of the tongue, lips and contiguous throat tissues. Samples from HAE patients with normal C1-INH levels and F12 gene did not produce excessive iBK in response to stimuli. The ex vivo approach provides physiopathological insight into AE states and supports the heterogeneous physiopathology of HAE with normal C1-INH.

Fibrinolysis: strategies to enhance the treatment of acute ischemic stroke

Stroke is a major cause of disability worldwide, and is the second leading cause of death after ischemic heart disease. Until recently, tissue-type plasminogen activator (t-PA) was the only treatment for acute ischemic stroke. If administered within 4.5 h of symptom onset, t-PA improves the outcome in stroke patients. Mechanical thrombectomy is now the preferred treatment for patients with acute ischemic stroke resulting from a large-artery occlusion in the anterior circulation. However, the widespread use of mechanical thrombectomy is limited by two factors. First, only ⁓ 10% of patients with acute ischemic stroke have a proximal large-artery occlusion in the anterior circulation and present early enough to undergo mechanical thrombectomy within 6 h; an additional 9-10% of patients presenting within the 6-24-h time window may also qualify for the procedure. Second, not all stroke centers have the resources or expertise to perform mechanical thrombectomy. Nonetheless, patients who present to hospitals where thrombectomy is not an option can receive intravenous t-PA, and those with qualifying anterior circulation strokes can then be transferred to tertiary stroke centers where thrombectomy is available. Therefore, despite the advances afforded by mechanical thrombectomy, there remains a need for treatments that improve the efficacy and safety of thrombolytic therapy. In this review, we discuss: (i) current treatment options for acute ischemic stroke; (ii) the mechanism of action of fibrinolytic agents; and (iii) potential strategies to manipulate the fibrinolytic system to promote endogenous fibrinolysis or to enhance the efficacy of fibrinolytic therapy.

Potential for Thrombolytic Therapy

The biochemical structure and interaction of the four components of the fibrinolytic enzyme system are described and the mechanisms for regulation and control are discussed.

The potential is defined for enhancement of thrombolysis at various levels by use of a variety of agents some of which have therapeutic potential in the human.

Exploitation of plasmin(ogen) by bacterial pathogens of veterinary significance

The plasminogen (Plg) system plays an important homeostatic role in the degradation of fibrin clots, extracellular matrices and tissue barriers important for cellular migration, as well as the promotion of neurotransmitter release. Plg circulates in plasma at physiologically high concentrations (150-200μg ml(-1)) as an inactive proenzyme. Proteins enriched in lysine and other positively charged residues (histidine and arginine) as well as glycosaminoglycans and gangliosides bind Plg. The binding interaction initiates a structural adjustment to the bound Plg that facilitates cleavage by proteases (plasminogen activators tPA and uPA) that activate Plg to the active serine protease plasmin. Both pathogenic and commensal bacteria capture Plg onto their cell surface and promote its conversion to plasmin. Many microbial Plg-binding proteins have been described underpinning the importance this process plays in how bacteria interact with their hosts. Bacteria exploit the proteolytic capabilities of plasmin by (i) targeting the mammalian fibrinolytic system and degrading fibrin clots, (ii) remodeling the extracellular matrix and generating bioactive cleavage fragments of the ECM that influence signaling pathways, (iii) activating matrix metalloproteinases that assist in the destruction of tissue barriers and promote microbial metastasis and (iv) destroying immune effector molecules. There has been little focus on the exploitation of the fibrinolytic system by veterinary pathogens. Here we describe several pathogens of veterinary significance that possess adhesins that bind plasmin(ogen) onto their cell surface and promote its activation to plasmin. Cumulative data suggests that these attributes provide pathogenic and commensal bacteria with a means to colonize and persist within the host environment.

Rapid binding of plasminogen to streptokinase in a catalytic complex reveals a three-step mechanism

Rapid kinetics demonstrate a three-step pathway of streptokinase (SK) binding to plasminogen (Pg), the zymogen of plasmin (Pm). Formation of a fluorescently silent encounter complex is followed by two conformational tightening steps reported by fluorescence quenches. Forward reactions were defined by time courses of biphasic quenching during complex formation between SK or its COOH-terminal Lys(414) deletion mutant (SKΔK414) and active site-labeled [Lys]Pg ([5-(acetamido)fluorescein]-D-Phe-Phe-Arg-[Lys]Pg ([5F]FFR-[Lys]Pg)) and by the SK dependences of the quench rates. Active site-blocked Pm rapidly displaced [5F]FFR-[Lys]Pg from the complex. The encounter and final SK ·[5F]FFR-[Lys]Pg complexes were weakened similarly by SK Lys(414) deletion and blocking of lysine-binding sites (LBSs) on Pg kringles with 6-aminohexanoic acid or benzamidine. Forward and reverse rates for both tightening steps were unaffected by 6-aminohexanoic acid, whereas benzamidine released constraints on the first conformational tightening. This indicated that binding of SK Lys(414) to Pg kringle 4 plays a role in recognition of Pg by SK. The substantially lower affinity of the final SK · Pg complex compared with SK · Pm is characterized by a ∼ 25-fold weaker encounter complex and ∼ 40-fold faster off-rates for the second conformational step. The results suggest that effective Pg encounter requires SK Lys(414) engagement and significant non-LBS interactions with the protease domain, whereas Pm binding additionally requires contributions of other lysines. This difference may be responsible for the lower affinity of the SK · Pg complex and the expression of a weaker "pro"-exosite for binding of a second Pg in the substrate mode compared with SK · Pm.

Decoy plasminogen receptor containing a selective Kunitz-inhibitory domain

Kunitz domain 1 (KD1) of tissue factor pathway inhibitor-2 in which P2' residue Leu17 (bovine pancreatic trypsin inhibitor numbering) is mutated to Arg selectively inhibits the active site of plasmin with ∼5-fold improved affinity. Thrombin cleavage (24 h extended incubation at a 1:50 enzyme-to-substrate ratio) of the KD1 mutant (Leu17Arg) yielded a smaller molecule containing the intact Kunitz domain with no detectable change in the active-site inhibitory function. The N-terminal sequencing and MALDI-TOF/ESI data revealed that the starting molecule has a C-terminal valine (KD1L17R-VT), whereas the smaller molecule has a C-terminal lysine (KD1L17R-KT). Because KD1L17R-KT has C-terminal lysine, we examined whether it could serve as a decoy receptor for plasminogen/plasmin. Such a molecule might inhibit plasminogen activation as well as the active site of generated plasmin. In surface plasmon resonance experiments, tissue plasminogen activator (tPA) and Glu-plasminogen bound to KD1L17R-KT (Kd ∼ 0.2 to 0.3 μM) but not to KD1L17R-VT. Furthermore, KD1L17R-KT inhibited tPA-induced plasma clot fibrinolysis more efficiently than KD1L17R-VT. Additionally, compared to ε-aminocaproic acid KD1L17R-KT was more effective in reducing blood loss in a mouse liver-laceration injury model, where the fibrinolytic system is activated. In further experiments, the micro(μ)-plasmin-KD1L17R-KT complex inhibited urokinase-induced plasminogen activation on phorbol-12-myristate-13-acetate-stimulated U937 monocyte-like cells, whereas the μ-plasmin-KD1L17R-VT complex failed to inhibit this process. In conclusion, KD1L17R-KT inhibits the active site of plasmin as well as acts as a decoy receptor for the kringle domain(s) of plasminogen/plasmin; hence, it limits both plasmin generation and activity. With its dual function, KD1L17R-KT could serve as a preferred agent for controlling plasminogen activation in pathological processes.

New insights into the role of Plg-RKT in macrophage recruitment

Plasminogen (PLG) is the zymogen of plasmin, the major enzyme that degrades fibrin clots. In addition to its binding and activation on fibrin clots, PLG also specifically interacts with cell surfaces where it is more efficiently activated by PLG activators, compared with the reaction in solution. This results in association of the broad-spectrum proteolytic activity of plasmin with cell surfaces that functions to promote cell migration. Here, we review emerging data establishing a role for PLG, plasminogen receptors and the newly discovered plasminogen receptor, Plg-RKT, in macrophage recruitment in the inflammatory response, and we address mechanisms by which the interplay between PLG and its receptors regulates inflammation.

Full time course kinetics of the streptokinase-plasminogen activation pathway

Our previously hypothesized mechanism for the pathway of plasminogen (Pg) activation by streptokinase (SK) was tested by the use of full time course kinetics. Three discontinuous chromogenic substrate initial rate assays were developed with different quenching conditions that enabled quantitation of the time courses of Pg depletion, plasmin (Pm) formation, transient formation of the conformationally activated SK·Pg* catalytic complex intermediate, formation of the SK·Pm catalytic complex, and the free concentrations of Pg, Pm, and SK. Analysis of full time courses of Pg activation by five concentrations of SK along with activity-based titrations of SK·Pg* and SK·Pm formation yielded rate and dissociation constants within 2-fold of those determined previously by continuous measurement of parabolic chromogenic substrate hydrolysis and fluorescence-based equilibrium binding. The results obtained with orthogonal assays provide independent support for a mechanism in which the conformationally activated SK·Pg* complex catalyzes an initial cycle of Pg proteolytic conversion to Pm that acts as a trigger. Higher affinity binding of the formed Pm to SK outcompetes Pg binding, terminating the trigger cycle and initiating the bullet catalytic cycle by the SK·Pm complex that converts the residual Pg into Pm. The new assays can be adapted to quantitate SK-Pg activation in the context of SK- or Pg-directed inhibitors, effectors, and SK allelic variants. To support this, we show for the first time with an assay specific for SK·Pg* that fibrinogen forms a ternary SK·Pg*·fibrinogen complex, which assembles with 200-fold enhanced SK·Pg* affinity, signaled by a perturbation of the SK·Pg* active site.

Severe bleeding tendency caused by a rare complication of excessive fibrinolysis with disseminated intravascular coagulation in a 51-year-old Japanese man with prostate cancer: a case report

Introduction

Disseminated intravascular coagulation causes thrombotic tendency leading to multiple organ failure and occurs in a wide variety of diseases including malignancy. Disseminated intravascular coagulation is a latent complication in people with prostate cancer.

Case presentation

A 51-year-old Japanese man with advanced castration-resistant prostate cancer was admitted to our hospital because of extensive purpura and severe anemia. Prolonged plasma coagulation time, hypofibrinogenemia and normal platelet count suggested that a decrease in fibrinogen induced a bleeding tendency causing purpura. However, elevated plasma levels of thrombin-antithrombin complex, fibrin and/or fibrinogen degradation products and D-dimers, with positive fibrin monomer test, manifested disseminated intravascular coagulation and subsequent fibrinolysis. Plasma levels of thrombin-antithrombin complex, fibrin and/or fibrinogen degradation products and D-dimers decreased after administration of low-molecular-weight heparin. However, low fibrinogen and α₂-antiplasmin levels were not improved and plasmin-antiplasmin complex did not decrease, which revealed excessive fibrinolysis complicated with disseminated intravascular coagulation. We suspected that prostate cancer cell-derived urokinase-type plasminogen activator caused excessive fibrinolysis. Administration of tranexamic acid for fibrinogenolysis was added together with high-dose anti-androgen therapy (fosfestrol) for prostate cancer. Thereafter, prostate-specific antigen and plasmin-antiplasmin complex decreased, followed by normalized fibrinogen and α₂-antiplasmin levels, and the patient eventually recovered from the bleeding tendency. Immunohistochemical staining of the biopsied prostate tissue exhibited that the prostate cancer cells produced tissue factor, the coagulation initiator, and urokinase-type plasminogen activator.

Conclusion

This patient with rare complications of disseminated intravascular coagulation and excessive fibrinolysis is a warning case of potential coagulation disorder onset in patients with prostate cancer. We propose that combined administration of tranexamic acid and low-molecular-weight heparin together with high-dose anti-androgen therapy is a useful therapeutic option for patients with this complicated coagulation disorder.

Monoclonal antibodies detect receptor-induced binding sites in Glu-plasminogen

When Glu-plasminogen binds to cells, its activation to plasmin is markedly enhanced compared with the reaction in solution, suggesting that Glu-plasminogen on cell surfaces adopts a conformation distinct from that in solution. However, direct evidence for such conformational changes has not been obtained. Therefore, we developed anti-plasminogen mAbs to test the hypothesis that Glu-plasminogen undergoes conformational changes on its interaction with cells. Six anti-plasminogen mAbs (recognizing 3 distinct epitopes) that preferentially recognized receptor-induced binding sites (RIBS) in Glu-plasminogen were obtained. The mAbs also preferentially recognized Glu-plasminogen bound to the C-terminal peptide of the plasminogen receptor, Plg-R(KT), and to fibrin, plasmin-treated fibrinogen, and Matrigel. We used trypsin proteolysis, immunoaffinity chromatography, and tandem mass spectrometry and identified Glu-plasminogen sequences containing epitopes recognized by the anti-plasminogen-RIBS mAbs: a linear epitope within a domain linking kringles 1 and 2; a nonlinear epitope contained within the kringle 5 domain and the latent protease domain; and a nonlinear epitope contained within the N-terminal peptide of Glu-plasminogen and the latent protease domain. Our results identify neoepitopes latent in soluble Glu-plasminogen that become available when Glu-plasminogen binds to cells and demonstrate that binding of Glu-plasminogen to cells induces a conformational change in Glu-plasminogen distinct from that of Lys-Pg.

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.

Small-molecule modulators of zymogen activation in the fibrinolytic and coagulation systems

The coagulation and fibrinolytic systems are central to the hemostatic mechanism, which works promptly on vascular injury and tissue damage. The rapid response is generated by specific molecular interactions between components in these systems. Thus, the regulation mechanism of the systems is programmed in each component, as exemplified by the elegant processes in zymogen activation. This review describes recently identified small molecules that modulate the activation of zymogens in the fibrinolytic and coagulation systems.

Skizzle is a novel plasminogen- and plasmin-binding protein from Streptococcus agalactiae that targets proteins of human fibrinolysis to promote plasmin generation

Skizzle (SkzL), secreted by Streptococcus agalactiae, has moderate sequence identity to streptokinase and staphylokinase, bacterial activators of human plasminogen (Pg). SkzL binds [Glu]Pg with low affinity (K(D) 3-16 mum) and [Lys]Pg and plasmin (Pm) with indistinguishable high affinity (K(D) 80 and 50 nm, respectively). Binding of SkzL to Pg and Pm is completely lysine-binding site-dependent, as shown by the effect of the lysine analog, 6-aminohexanoic acid. Deletion of the COOH-terminal SkzL Lys(415) residue reduces affinity for [Lys]Pg and active site-blocked Pm 30-fold, implicating Lys(415) in a lysine-binding site interaction with a Pg/Pm kringle. SkzL binding to active site fluorescein-labeled Pg/Pm analogs demonstrates distinct high and low affinity interactions. High affinity binding is mediated by Lys(415), whereas the source of low affinity binding is unknown. SkzL enhances the activation of [Glu]Pg by urokinase (uPA) approximately 20-fold, to a maximum rate indistinguishable from that for [Lys]Pg and [Glu]Pg activation in the presence of 6-aminohexanoic acid. SkzL binds preferentially to the partially extended beta-conformation of [Glu]Pg, which is in unfavorable equilibrium with the compact alpha-conformation, thereby converting [Glu]Pg to the fully extended gamma-conformation and accelerating the rate of its activation by uPA. SkzL enhances [Lys]Pg and [Glu]Pg activation by single-chain tissue-type Pg activator, approximately 42- and approximately 650-fold, respectively. SkzL increases the rate of plasma clot lysis by uPA and single-chain tissue-type Pg activator approximately 2-fold, confirming its cofactor activity in a physiological model system. The results suggest a role for SkzL in S. agalactiae pathogenesis through fibrinolytic enhancement.

Direct fibrinolytic agents: biochemical attributes, preclinical foundation and clinical potential

Direct fibrinolytics are proteolytic enzymes that degrade fibrin without requiring an intermediate step of plasminogen activation. This review summarizes the current information available for five such agents, namely, plasmin (the prototypical form), three derivatives of plasmin (mini-plasmin, micro-plasmin, and delta-plasmin), and alfimeprase, a recombinant variant of a snake venom alpha-fibrinogenase, fibrolase. Biochemical attributes of molecular size, fibrin binding and inhibitor neutralization are compared. Preclinical investigations that assess the potential for thrombolytic efficacy in vitro and in animal models of vascular occlusion and for hemostatic safety in animal models of bleeding are detailed. Clinical potential has been assessed in patients with peripheral arterial and graft occlusion, acute ischemic stroke, and access catheter and hemodialysis shunt occlusions. The direct fibrinolytic agents have impressive biochemical and preclinical foundations for ultimate clinical application. However, clinical trial results for micro-plasmin and alfimeprase have not measured up to their anticipated benefit. Plasmin has thus far shown encouraging hemostatic safety, but efficacy data await completion of clinical trials. Whether direct fibrinolytics will provide clinical superiority in major thrombotic disorders over currently utilized indirect fibrinolytics such as tissue plasminogen activator remains to be determined.

LRP1 regulates remodeling of the extracellular matrix by fibroblasts

Low density lipoprotein receptor-related protein (LRP1) is an endocytic receptor for diverse proteases, protease inhibitors, and other plasma membrane proteins, including the urokinase receptor (uPAR). LRP1 also functions in cell-signaling and regulates gene expression. The goal of this study was to determine whether LRP1 regulates remodeling of provisional extracellular matrix (ECM) by fibroblasts. To address this problem, we utilized an in vitro model in which type I collagen was reconstituted and overlaid with fibronectin. Either the collagen or fibronectin was fluorescently-labeled. ECM remodeling by fibroblasts deficient in LRP1, uPAR, or MT1-MMP was studied. MT1-MMP was required for efficient remodeling of the deep collagen layer but not involved in fibronectin remodeling. Instead, fibronectin was remodeled by a system that required urokinase-type plasminogen activator (uPA), uPAR, and exogenously-added plasminogen. LRP1 markedly inhibited fibronectin remodeling by regulating cell-surface uPAR and plasminogen activation. LRP1 also regulated remodeling of the deep collagen layer but not by controlling MT1-MMP. Instead, LRP1 deficiency or inhibition de-repressed a secondary pathway for collagen remodeling, which was active in MT1-MMP-deficient cells but not in uPAR-deficient cells. These results demonstrate that LRP1 regulates ECM remodeling principally by repressing pathways that require plasminogen activation by uPA in association with uPAR.

Plasminogen substrate recognition by the streptokinase-plasminogen catalytic complex is facilitated by Arg253, Lys256, and Lys257 in the streptokinase beta-domain and kringle 5 of the substrate

Streptokinase (SK) conformationally activates the central zymogen of the fibrinolytic system, plasminogen (Pg). The SK.Pg* catalytic complex binds Pg as a specific substrate and cleaves it into plasmin (Pm), which binds SK to form the SK.Pm complex that propagates Pm generation. Catalytic complex formation is dependent on lysine-binding site (LBS) interactions between a Pg/Pm kringle and the SK COOH-terminal Lys(414). Pg substrate recognition is also LBS-dependent, but the kringle and SK structural element(s) responsible have not been identified. SK mutants lacking Lys(414) with Ala substitutions of charged residues in the SK beta-domain 250-loop were evaluated in kinetic studies that resolved conformational and proteolytic Pg activation. Activation of [Lys]Pg and mini-Pg (containing only kringle 5 of Pg) by SK with Ala substitutions of Arg(253), Lys(256), and Lys(257) showed decreases in the bimolecular rate constant for Pm generation, with nearly total inhibition for the SK Lys(256)/Lys(257) double mutant. Binding of bovine Pg (BPg) to the SK.Pm complex containing fluorescently labeled Pm demonstrated LBS-dependent assembly of a SK.labeled Pm.BPg ternary complex, whereas BPg did not bind to the complex containing the SK Lys(256)/Lys(257) mutant. BPg was activated by SK.Pm with a K(m) indistinguishable from the K(D) for BPg binding to form the ternary complex, whereas the SK Lys(256)/Lys(257) mutant did not support BPg activation. We conclude that SK residues Arg(253), Lys(256), and Lys(257) mediate Pg substrate recognition through kringle 5 of the [Lys]Pg and mini-Pg substrates. A molecular model of the SK.kringle 5 complex identifies the putative interactions involved in LBS-dependent Pg substrate recognition.

Rapid-reaction kinetic characterization of the pathway of streptokinase-plasmin catalytic complex formation

Binding of the fibrinolytic proteinase plasmin (Pm) to streptokinase (SK) in a tight stoichiometric complex transforms Pm into a potent proteolytic activator of plasminogen. SK binding to the catalytic domain of Pm, with a dissociation constant of 12 pm, is assisted by SK Lys(414) binding to a Pm kringle, which accounts for a 11-20-fold affinity decrease when Pm lysine binding sites are blocked by 6-aminohexanoic acid (6-AHA) or benzamidine. The pathway of SK.Pm catalytic complex formation was characterized by stopped-flow kinetics of SK and the Lys(414) deletion mutant (SKDeltaK414) binding to Pm labeled at the active site with 5-fluorescein ([5F]FFR-Pm) and the reverse reactions by competitive displacement of [5F]FFR-Pm with active site-blocked Pm. The rate constants for the biexponential fluorescence quenching caused by SK and SKDeltaK414 binding to [5F]FFR-Pm were saturable as a function of SK concentration, reporting encounter complex affinities of 62-110 nm in the absence of lysine analogs and 4900-6500 and 1430-2200 nm in the presence of 6-AHA and benzamidine, respectively. The encounter complex with SKDeltaK414 was approximately 10-fold weaker in the absence of lysine analogs but indistinguishable from that of native SK in the presence of 6-AHA and benzamidine. The studies delineate for the first time the sequence of molecular events in the formation of the SK.Pm catalytic complex and its regulation by kringle ligands. Analysis of the forward and reverse reactions supports a binding mechanism in which SK Lys(414) binding to a Pm kringle accompanies near-diffusion-limited encounter complex formation followed by two slower, tightening conformational changes.

Stachybotrydial selectively enhances fibrin binding and activation of Glu-plasminogen

Stachybotrydial, a triprenyl phenol metabolite from a fungus, has a plasminogen modulator activity selective to Glu-plasminogen. Stachybotrydial enhanced fibrin binding and activation of Glu-plasminogen (2- to 4-fold enhancement at 60-120 microM) but not of Lys-plasminogen. Approximately 1.2-1.6 moles of [3H]stachybotrydial bound to Glu-plasminogen to exert such effects. The selective modulation of the Glu-plasminogen function by stachybotrydial may be related to alteration of its conformational status.

X-ray crystal structure of the fibrinolysis inhibitor α2-antiplasmin

The serpin α2-antiplasmin (SERPINF2) is the principal inhibitor of plasmin and inhibits fibrinolysis. Accordingly, α2-antiplasmin deficiency in humans results in uncontrolled fibrinolysis and a bleeding disorder. α2-antiplasmin is an unusual serpin, in that it contains extensive N- and C-terminal sequences flanking the serpin domain. The N-terminal sequence is crosslinked to fibrin by factor XIIIa, whereas the C-terminal region mediates the initial interaction with plasmin. To understand how this may happen, we have determined the 2.65Å X-ray crystal structure of an N-terminal truncated murine α2-antiplasmin. The structure reveals that part of the C-terminal sequence is tightly associated with the body of the serpin. This would be anticipated to position the flexible plasmin-binding portion of the C-terminus in close proximity to the serpin Reactive Center Loop where it may act as a template to accelerate serpin/protease interactions.

Resistance of porcine blood clots to lysis relates to poor activation of porcine plasminogen by tissue plasminogen activator

In-vitro experimentation was performed on porcine and human blood to determine their comparative responsiveness to a novel fibrinolytic inhibitor and thereby assess whether the pig is a suitable animal model for subsequent in-vivo testing of this inhibitor. Thromboelastography showed the clots formed from porcine whole blood to be highly resistant to tissue plasminogen activator (t-PA)-catalyzed lysis, and this communication offers the resistance of porcine plasminogen to activation by t-PA as an explanation. Porcine blood containing 100 and 1500 IU/ml added t-PA lysed very slowly, having LY30 values of 1.9 +/- 1.4 and 2.9 +/- 1.9%, respectively. In contrast, the LY30 values for the human clots containing 100 and 1500 IU/ml t-PA were 77.1 +/- 6.3 and 93.3 +/- 1.3%, respectively. Moreover, purified porcine plasminogen was activated very slowly by added t-PA in the presence of both human and porcine fibrin. Activation of plasminogen by the endogenous activators, as measured by the euglobulin clot lysis time, was greatly prolonged for the pig (22 +/- 3 h) compared with the human (3.5 +/- 1.5 h). These results suggest caution in using the pig as an experimental model when studying the effects of various agents on fibrinolysis.

Plasminogen and plasmin activity in patients with coronary artery disease

OBJECTIVE

While coronary artery disease (CAD) is associated with disturbances of the plasma fibrinolytic system, the nature of these disturbances is not fully defined. Fibrinolysis is regulated by plasmin, whose production is mediated by plasminogen activator conversion of plasminogen (Plg) to plasmin. The cascade is modulated by feedback loops that include Plg activator inhibitor 1 (PAI-1). Molecular interactions with Plg kringle domains play an important role in regulating plasmin production and its modulation of fibrinolysis. We hypothesized that interactions of tissue plasminogen activator (tPA) with Plg kringle domains regulates plasmin levels in patients with stable CAD.

METHODS

Plasma was collected from patients (n = 33) with an angiographically significant CAD and controls (n = 18) with angiographically established normal or minimally diseased arteries. Plasmin activity, tPA activity, and plasma levels of Plg, PAI-1, uPA, and tPA were determined.

RESULTS

CAD patients had 1.7-fold greater plasmin activity (P = 0.02) that correlated with 1.5-fold higher tPA activity when compared to controls. Epitope mapping of Plg domains showed Plg differences in epitope exposure between the two groups. Plasma from CAD patients had 50% less (P < 0.001) detectable kringle 4 and 48% less (P = 0.007) detectable kringles 1-3.

CONCLUSIONS

Based on detectable differences in Plg, we conclude that in patients with stable CAD, Plg complexed with tPA exists in a conformation that enables increased tPA activity and Plg conversion to plasmin.

Role of the streptokinase alpha-domain in the interactions of streptokinase with plasminogen and plasmin

The role of the streptokinase (SK) alpha-domain in plasminogen (Pg) and plasmin (Pm) interactions was investigated in quantitative binding studies employing active site fluorescein-labeled [Glu]Pg, [Lys]Pg, and [Lys]Pm, and the SK truncation mutants, SK-(55-414), SK-(70-414), and SK-(152-414). Lysine binding site (LBS)-dependent and -independent binding were resolved from the effects of the lysine analog, 6-aminohexanoic acid. The mutants bound indistinguishably, consistent with unfolding of the alpha-domain on deletion of SK-(1-54). The affinity of SK for [Glu]Pg was LBS-independent, and although [Lys]Pg affinity was enhanced 13-fold by LBS interactions, the LBS-independent free energy contributions were indistinguishable. alpha-Domain truncation reduced the affinity of SK for [Glu]Pg 2-7-fold and [Lys]Pg </=2-fold, but surprisingly, rendered both interactions near totally LBS-dependent. The LBS-independent affinity of SK for [Lys]Pm, 3000-fold higher compared with [Lys]Pg, was reduced dramatically by alpha-domain truncation. Thermodynamic analysis demonstrates that the SK alpha-domain contributes substantially to affinity for all Pg/Pm species solely through LBS-independent interactions, and that the higher affinity of SK for [Lys]Pm compared with [Lys]Pg involves all three SK domains. The residual affinity of the SK betagamma-fragment for all Pg/Pm species was increased by an enhanced contribution to complex stability from LBS-dependent interactions or free energy coupling between LBS-dependent and -independent interactions. Redistribution of the free energy contributions accompanying alpha-domain truncation demonstrates the interdependence of SK domains in stabilizing the SK-Pg/Pm complexes. The flexible segments connecting the SK alpha, beta, and gamma domains allow their rearrangement into a distinctly different bound conformation accompanying loss of the constraint imposed by interactions of the alpha-domain.

Coupling of Conformational and Proteolytic Activation in the Kinetic Mechanism of Plasminogen Activation by Streptokinase

Binding of streptokinase (SK) to plasminogen (Pg) induces conformational activation of the zymogen and initiates its proteolytic conversion to plasmin (Pm). The mechanism of coupling between conformational activation and Pm formation was investigated in kinetic studies. Parabolic time courses of Pg activation by SK monitored by chromogenic substrate hydrolysis had initial rates (v(1)) representing conformational activation and subsequent rates of activity increase (v(2)) corresponding to the rate of Pm generation determined by a specific discontinuous assay. The v(2) dependence on SK concentration for [Lys]Pg showed a maximum rate at a Pg to SK ratio of approximately 2:1, with inhibition at high SK concentrations. [Glu]Pg and [Lys]Pg activation showed similar kinetic behavior but much slower activation of [Glu]Pg, due to an approximately 12-fold lower affinity for SK and an approximately 20-fold lower k(cat)/K(m). Blocking lysine-binding sites on Pg inhibited SK.Pg* cleavage of [Lys]Pg to a rate comparable with that of [Glu]Pg, whereas [Glu]Pg activation was not significantly affected. The results support a kinetic mechanism in which SK activates Pg conformationally by rapid equilibrium formation of the SK.Pg* complex, followed by intermolecular cleavage of Pg to Pm by SK.Pg* and subsequent cleavage of Pg by SK.Pm. A unified model of SK-induced Pg activation suggests that generation of initial Pm by SK.Pg* acts as a self-limiting triggering mechanism to initiate production of one SK equivalent of SK.Pm, which then converts the remaining free Pg to Pm.

Resolution of Conformational Activation in the Kinetic Mechanism of Plasminogen Activation by Streptokinase

Streptokinase (SK) activates plasminogen (Pg) by specific binding and nonproteolytic expression of the Pg catalytic site, initiating Pg proteolysis to form the fibrinolytic proteinase, plasmin (Pm). The SK-induced conformational activation mechanism was investigated in quantitative kinetic and equilibrium binding studies. Progress curves of Pg activation by SK monitored by chromogenic substrate hydrolysis were parabolic, with initial rates (v(1)) that indicated no transient species and subsequent rate increases (v(2)). The v(1) dependence on SK concentration for [Glu]Pg and [Lys]Pg was hyperbolic with dissociation constants corresponding to those determined in fluorescence-based binding studies for the native Pg species, identifying v(1) as rapid SK binding and conformational activation. Comparison of [Glu]Pg and [Lys]Pg activation showed an approximately 12-fold higher affinity of SK for [Lys]Pg that was lysine-binding site dependent and no such dependence for [Glu]Pg. Stopped-flow kinetics of SK binding to fluorescently labeled Pg demonstrated at least two fast steps in the conformational activation pathway. Characterization of the specificity of the conformationally activated SK.[Lys]Pg* complex for tripeptide-p-nitroanilide substrates demonstrated 5-18- and 10-130-fold reduced specificity (k(cat)/K(m)) compared with SK.Pm and Pm, respectively, with differences in K(m) and k(cat) dependent on the P1 residue. The results support a kinetic mechanism in which SK binding and reversible conformational activation occur in a rapid equilibrium, multistep process.

Endogenous plasmin converts Glu-plasminogen to Lys-plasminogen on the monocytoid cell surface

Recently, we showed that localization of Glu-plasminogen on cell surfaces enhances its conversion to Lys-plasminogen by exogenous plasmin. This leads to stimulation of plasminogen activation because Lys-plasminogen is the preferred substrate on cell surfaces. Here, we show that Glu-plasminogen was converted to Lys-plasminogen on monocytoid cells in the absence of exogenous plasmin. Culture of cells under serum-free conditions did not affect this conversion, suggesting that the enzymatic activity was cell-derived. Therefore, we tested whether endogenous monocytoid plasminogen could provide a source of plasmin to convert cell-associated Glu-plasminogen to Lys-plasminogen because plasmin is the only enzyme known to effect this reaction. We used a recombinant human plasminogen mutant, [D(646)E]Pg, which can be cleaved by plasminogen activators, but cannot catalyze the generation of Lys-plasminogen. Upon incubation with either THP-1 or U937 monocytoid cells, 35 and 38%, respectively, of the cell-bound ligand was converted to Lys-[D(646)E]Pg. Trasylol, alpha2-antiplasmin, and an anticatalytic antiplasminogen monoclonal antibody decreased Lys-[D(646)E]Pg formation to < 5% on monocytoid cells, consistent with a plasmin-dependent mechanism. Plasminogen was detected in these cells by Northern blotting and RT-PCR. Our results suggest that plasmin converts cell-bound Glu-plasminogen to Lys-plasminogen and that this enzyme is produced by activation of monocytoid plasminogen by endogenous monocytoid plasminogen activators to enhance plasminogen activation on the monocytoid cell surface.

Plasmin system and microbial proteases in milk: characteristics, roles, and relationship

Proteolysis of milk proteins can be attributed to both native proteases and the proteases produced by psychrotrophic bacteria during storage of fresh raw milk. These proteases cause beneficial or detrimental changes, depending on the specific milk product. Plasmin, the major native protease in milk, is important for cheese ripening. Milk storage and cheese-making conditions can affect the level of plasmin in the casein and whey fractions of milk. A microbial protease from a psychrotrophic microorganism can indirectly increase plasmin levels in the casein curd. This relationship between the plasmin system and microbial proteases in milk provides a means to control levels of plasmin to benefit the quality of dairy products. This paper is a short review of both the plasmin system and microbial proteases, focusing on their characteristics and relationship and how the quality of dairy products is affected by their proteolysis of milk proteins.

Domain interactions between streptokinase and human plasminogen

Plasmin (Pm), the main fibrinolytic protease in the plasma, is derived from its zymogen plasminogen (Plg) by cleavage of a peptide bond at Arg(561)-Val(562). Streptokinase (SK), a widely used thrombolytic agent, is an efficient activator of human Plg. Both are multiple-domain proteins that form a tight 1:1 complex. The Plg moiety gains catalytic activity, without peptide bond cleavage, allowing the complex to activate other Plg molecules to Pm by conventional proteolysis. We report here studies on the interactions between individual domains of the two proteins and their roles in Plg activation. Individually, all three SK domains activated native Plg. While the SK alpha domain was the most active, its activity was uniquely dependent on the presence of Pm. The SK gamma domain also induced the formation of an active site in Plg(R561A), a mutant that resists proteolytic activation. The alpha and gamma domains together yielded synergistic activity, both in Plg activation and in Plg(R561A) active site formation. However, the synergistic activity of the latter was dependent on the correct N-terminal isoleucine in the alpha domain. Binding studies using surface plasmon resonance indicated that all three domains of SK interact with the Plg catalytic domain and that the beta domain additionally interacts with Plg kringle 5. These results suggest mechanistic steps in SK-mediated Plg activation. In the case of free Plg, complex formation is initiated by the rapid and obligatory interaction between the SK beta domain and Plg kringle 5. After binding of all SK domains to the catalytic domain of Plg, the SK alpha and gamma domains cooperatively induce the formation of an active site within the Plg moiety of the activator complex. Substrate Plg is then recognized by the activator complex through interactions predominately mediated by the SK alpha domain.

Conversion of Glu-plasminogen to Lys-plasminogen is necessary for optimal stimulation of plasminogen activation on the endothelial cell surface

When Glu-plasminogen is bound to cells, plasmin (Pm) formation by plasminogen (Pg) activators is markedly enhanced compared with the reaction in solution. It is not known whether the direct activation of Glu-Pg by Pg activators is promoted on the cell surface or whether plasminolytic conversion of Glu-Pg to the more readily activated Lys-Pg is necessary for enhanced Pm formation on the cell surface. To distinguish between these potential mechanisms, we tested whether Pm formation on the cell surface could be stimulated in the absence of conversion of Glu-Pg to Lys-Pg. Rates of activation of Glu-Pg, Lys-Pg, and a mutant Glu-Pg, [D646E]Glu-Pg, by either tissue Pg activator (t-PA) or urokinase (u-PA) were compared when these Pg forms were either bound to human umbilical vein endothelial cells (HUVEC) or in solution. ([D646E]Glu-Pg can be cleaved at the Arg(561)-Val(562) bond by Pg activators but does not possess Pm activity subsequent to this cleavage because of the mutation of Asp(646) of the serine protease catalytic triad.) Glu-Pg activation by t-PA was enhanced on HUVEC compared with the solution phase by 13-fold. In contrast, much less enhancement of Pg activation was observed with [D646E]Glu-Pg ( approximately 2-fold). Although the extent of activation of Lys-Pg on cells was similar to that of Glu-Pg, the cells afforded minimal enhancement of Lys-Pg activation compared with the solution phase (1.3-fold). Similar results were obtained when u-PA was used as activator. When Glu-Pg was bound to the cell in the presence of either t-PA or u-PA, conversion to Lys-Pg was observed, but conversion of ([D646E]Glu-Pg to ([D646E]Lys-Pg was not detected, consistent with the conversion of Glu-Pg to Lys-Pg being necessary for optimal enhancement of Pg activation on cell surfaces. Furthermore, we found that conversion of [D646E]Glu-Pg to [D646E]Lys-Pg by exogenous Pm was markedly enhanced ( approximately 20-fold) on the HUVEC surface, suggesting that the stimulation of the conversion of Glu-Pg to Lys-Pg is a key mechanism by which cells enhance Pg activation.

Streptokinase binds preferentially to the extended conformation of plasminogen through lysine binding site and catalytic domain interactions

Binding of streptokinase (SK) to plasminogen (Pg) activates the zymogen conformationally and initiates its conversion into the fibrinolytic proteinase, plasmin (Pm). Equilibrium binding studies of SK interactions with a homologous series of catalytic site-labeled fluorescent Pg and Pm analogues were performed to resolve the contributions of lysine binding site interactions, associated changes between extended and compact conformations of Pg, and activation of the proteinase domain to the affinity for SK. SK bound to fluorescein-labeled [Glu]Pg(1) and [Lys]Pg(1) with dissociation constants of 624 +/- 112 and 38 +/- 5 nM, respectively, whereas labeled [Lys]Pm(1) bound with a 57000-fold tighter dissociation constant of 11 +/- 2 pM. Saturation of lysine binding sites with 6-aminohexanoic acid had no effect on SK binding to labeled [Glu]Pg(1), but weakened binding to labeled [Lys]Pg(1) and [Lys]Pm(1) 31- and 20-fold, respectively. At low Cl(-) concentrations, where [Glu]Pg assumes the extended conformation without occupation of lysine binding sites, a 23-fold increase in the affinity of SK for labeled [Glu]Pg(1) was observed, which was quantitatively accounted for by expression of new lysine binding site interactions. The results support the conclusion that the SK affinity for the fluorescent Pg and Pm analogues is enhanced 13-16-fold by conversion of labeled [Glu]Pg to the extended conformation of the [Lys]Pg derivative as a result of lysine binding site interactions, and is enhanced 3100-3500-fold further by the increased affinity of SK for the activated proteinase domain. The results imply that binding of SK to [Glu]Pg results in transition of [Glu]Pg to an extended conformation in an early event in the SK activation mechanism.

Effects of a fucoidan on the activation of plasminogen by u-PA and t-PA

The effect of an anticoagulant fucoidan (C-I-H) from the brown seaweed Ecklonia kurome on the fibrinolytic system was studied in vitro using S-2251 as a substrate of plasmin. C-I-H enhanced the activation of Glu- and Lys-plasminogen by high molecular weight urokinase-type plasminogen activator (HMW u-PA) very effectively, but the activation by low molecular weight u-PA was hardly enhanced with C-I-H. C-I-H also potentiated moderately the activation by single- and two-chain tissue-type plasminogen activators (sct- and tct-PA). These effects of C-I-H were higher than those of heparin used. But C-I-H had no effect on the amidolytic activity of plasmin to S-2251. These results indicate that C-I-H promotes the generation of plasmin in the plasminogen activation by HMW u-PA and t-PA, but not the activity of generated plasmin. Kinetic analyses suggest that C-I-H enhances the HMW u-PA-mediated plasminogen activation by increasing the affinity of the activator for Glu- and Lys-plasminogen and by increasing the molecular activity of the activator. On the other hand, C-I-H had no effect on the affinity of tct-PA for both plasminogens. The catalytic efficiencies of HMW u-PA and tct-PA for the activation of both plasminogens were increased with C-I-H about 8- and 2-fold, respectively. The present results suggest that C-I-H has the fibrinolytic activity by stimulating the plasminogen activation by HMW u-PA and t-PA. The mechanism of the enhancement effect of C-I-H on the activation is presumed to be that C-I-H binds to plasminogen, thereby inducing a structural change of plasminogen susceptible to the action of plasminogen activators.

The kringle domains of human plasminogen

The mature form of the zymogen, human plasminogen (HPlg), contains 791 amino acids present in a single polypeptide chain. The fibrinolytic enzyme, human plasmin (HPlm), is formed from HPlg as a result of activator-catalysed cleavage of the Arg561-Val562 peptide bond in HPlg. The resulting HPlm contains a heavy chain of 561 amino acid residues, originating from the N-terminus of HPlg, doubly disulfide-linked to a light chain of 230 amino acid residues. This latter region, containing the C-terminus of HPlg, is homologous to serine proteases such as trypsin and elastase. The heavy chain of HPlm consists of five repeating triple-disulfide-linked peptide regions, c. 80 amino acid residues in length, termed kringles (K), that are responsible for interactions of HPlg and HPlm with substrates, inhibitors and regulators of HPlg activation. Important among the ligands of the kringles are positive activation effectors, typified by lysine and its analogues, and negative activation effectors, such as Cl-. The kringle domains of HPlg that participate in these binding interactions are K1, K4 and K5, and perhaps K2. These modules appear to function as independent domains. The amino acid residues important in these kringle/ligand binding interactions have been proposed by structural determinations, and their relative importance quantified by site-directed mutagenesis experimentation.

Enhancement of fibrin binding and activation of plasminogen by staplabin through induction of a conformational change in plasminogen

Staplabin (0.3-0.6 mM), a fungal triprenyl phenol, enhanced 3-fold the plasminogen activator-catalyzed activation of Glu-plasminogen and Lys-plasminogen as well as their binding to fibrin. Staplabin was not stimulatory to the amidolytic activity of plasmin and plasminogen activators. Even in the presence of epsilon-aminocaproic acid (EACA) and fibrinogen fragments, allosteric effectors for Glu-plasminogen, staplabin increased the activation of both forms of plasminogen. In size-exclusion chromatography of Glu-plasminogen and Lys-plasminogen, the molecular elution time, which varies as the conformation of a protein changes, was shortened by staplabin. These results suggest that staplabin causes plasminogens to be more susceptible to activation and fibrin binding by inducing a conformational change that is, at least in part, different from that induced by EACA and fibrinogen fragments.

Activation of Human Plasminogen by Staphylokinase. Direct Evidence That Preformed Plasmin Is Necessary for Activation to Occur

To directly determine whether the mechanism of activation of human plasminogen (HPg) by staphylokinase (Sak) required formation of an active complex of Sak and HPg, recombinant (r) variants of HPg were examined that allowed dissection of the steps involved in this activation. The rate of activation of wild-type (wt) r-HPg by equimolar levels of Sak was enhanced when small amounts of human plasmin (HPm) were included, suggesting that a Sak-HPm complex was a more effective plasminogen activator than a putative Sak-HPg complex. Incubation of equimolar Sak with a cleavage site resistant mutant of HPg (r-[R561 A]HPg) did not result in generation of amidolytic activity of the complex, in contrast to a similar experiment with streptokinase (SK) in place of Sak, where substantial amidolytic activity was generated. This result supplies evidence that an active complex of Sak and HPg does not form, as is the case with SK. Another mutant, r-[D646E]HPg, which, upon activation, would lead to a form of HPm defective in enzymatic activity, is also not converted to its two-chain form by Sak, but is converted to the inactive two-chain form by urokinase, a direct plasminogen activator, and by equimolar complexes of SK or Sak with wtr-HPm. This shows that the active site of HPm is the functional plasminogen activator entity in the Sak-HPm complex. These results show that the mechanism of activation of HPg by Sak proceeds in a distinctly different manner than the similar activation by SK. Although SK does not require the presence of HPm for this activation, a necessary condition for the activation by Sak is formation of a small amount of HPm generated via another activation pathway. These different mechanisms have significant implications in production of the fibrinolytic state by these two indirect bacterial plasminogen activators.

Activation of Human Plasminogen by Staphylokinase. Direct Evidence That Preformed Plasmin Is Necessary for Activation to Occur

To directly determine whether the mechanism of activation of human plasminogen (HPg) by staphylokinase (Sak) required formation of an active complex of Sak and HPg, recombinant (r) variants of HPg were examined that allowed dissection of the steps involved in this activation. The rate of activation of wild-type (wt) r-HPg by equimolar levels of Sak was enhanced when small amounts of human plasmin (HPm) were included, suggesting that a Sak-HPm complex was a more effective plasminogen activator than a putative Sak-HPg complex. Incubation of equimolar Sak with a cleavage site resistant mutant of HPg (r-[R561 A]HPg) did not result in generation of amidolytic activity of the complex, in contrast to a similar experiment with streptokinase (SK) in place of Sak, where substantial amidolytic activity was generated. This result supplies evidence that an active complex of Sak and HPg does not form, as is the case with SK. Another mutant, r-[D646E]HPg, which, upon activation, would lead to a form of HPm defective in enzymatic activity, is also not converted to its two-chain form by Sak, but is converted to the inactive two-chain form by urokinase, a direct plasminogen activator, and by equimolar complexes of SK or Sak with wtr-HPm. This shows that the active site of HPm is the functional plasminogen activator entity in the Sak-HPm complex. These results show that the mechanism of activation of HPg by Sak proceeds in a distinctly different manner than the similar activation by SK. Although SK does not require the presence of HPm for this activation, a necessary condition for the activation by Sak is formation of a small amount of HPm generated via another activation pathway. These different mechanisms have significant implications in production of the fibrinolytic state by these two indirect bacterial plasminogen activators.

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

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

Contributions of individual kringle domains toward maintenance of the chloride-induced tight conformation of human glutamic acid-1 plasminogen

The roles of each of the three omega-amino acid-binding kringles (K) of Glu1-Pg, viz., [K1Pg], [K4Pg], and [K5Pg], in engendering the Cl(-)-induced alteration to its tight (T) conformation and in effecting the epsilon-aminocaproic acid (EACA)-mediated change to the relaxed (R) protein conformation have been investigated by mutagenesis strategies wherein the omega-amino acid ligand-binding energies in the individual kringles in recombinant (r)-Glu1-Pg were greatly reduced. This was accomplished in the most conservative manner possible by altering a critical Asp residue in each relevant kringle to Asn. The particular mutations chosen were r-[D139N]Glu1-Pg, r-[D413N]Glu1-Pg, and r-[D518N]Glu1-Pg, in which a conserved Asp residue at a homologous sequence position in each of the three kringle domains is eliminated. These changes also lead to a great reduction of the EACA-binding strength of [K1Pg], [K4Pg], and [K5Pg], respectively. The s0(20,w) of wild-type (wt) r-Glu1-Pg in the presence of levels of Cl(-)-sufficient to fully occupy its binding sites on this protein was 5.9 S, a value reduced to 4.9 S as a result of addition of saturating concentrations of EACA to the Cl-/Glu1-Pg complex. Neither Cl- nor EACA substantially altered the s0(20,w) value of 5.2 S for r-[D139N]Glu1-Pg (4.8 S) or r-[D413N]Glu1-Pg (4.5 S). On the other hand, the s0(20,w) value of 5.2 S for r-[D518N]Glu1-Pg at saturating levels of Cl- is slightly reduced to 4.8 S upon addition of binding maximal concentrations of EACA.(ABSTRACT TRUNCATED AT 250 WORDS)

The activation-resistant conformation of recombinant human plasminogen is stabilized by basic residues in the amino-terminal hinge region

Fully activable recombinant human plasminogen (rPlg) was expressed in mammalian cells employing either recombinant vaccinia virus or stable lines coexpressing alpha 2-plasmin inhibitor. A panel of eight variants of rPlg was constructed, in which progressively up to 6 basic amino acid residues in the hinge region of rPlg between the NH2-terminal acidic domain ("proactivation peptide") and kringle 1 were substituted by neutral residues. Analysis of the cleavage rates of these variants by plasmin revealed that the peptide bond at Arg68 is most susceptible, followed by Lys62 and Lys77. A variant with all 6 basic residues substituted was cleaved at Lys20. Three of these variants, PlgB (R68A, R70A), PlgF (R68A, R70A, K77H, K78H), and PlgG (R61A, K62A, R68A, R70A, K77H, K78H), as well as rPlg, were analyzed in more detail. The conformation of these plasminogens was analyzed by monitoring the change in intrinsic fluorescence upon binding of lysine analogs. This revealed that rPlg exhibits the native tight Glu1-plasminogen conformation, whereas PlgB, PlgF, and Plg G display an open conformation similar to Lys78-plasminogen, leading to an increased affinity for lysine analogs. This allowed a direct study of the impact of the activation-resistant conformation on the properties of Glu1-plasminogen. The open conformation of rPlg variants leads to an increased rate of activation by urokinase-type plasminogen activator and streptokinase and increased binding to a fibrin clot. Fibrin clot lysis mediated by tissue-type plasminogen activator was accelerated for the variants as a result of a lower Km for tissue-type plasminogen activator-mediated plasminogen activation, resulting from the increased affinity of rPlg (variants) for intact fibrin. We conclude that the basic residues in the extremely plasmin susceptible hinge region of plasminogen are directly involved in maintaining the activation resistant Glu1-plasminogen conformation.

Conformations and stabilities of human Glu1- and Lys78-plasminogen and of the fragments mini- and microplasminogen, analysed by circular dichroism and differential scanning calorimetry

The conformations and stabilities of two forms of human plasminogen, Glu1-plasminogen (Glu1-HPg, Glu1-Asn791) and Lys78-plasminogen (Lys78-HPg, Lys78-Asn791), and two enzymatically derived plasminogen fragments, miniplasminogen (mini-HPg, Val443-Asn791) and microplasminogen (micro-HPg, Lys531-Asn791) were analysed by circular dichroism and differential scanning calorimetry. The two plasminogen forms differ by the lack of 77 N-terminal amino acids in Lys78-HPg in comparison to Glu1-HPg. Mini-HPg is composed of kringle 5 and the protease domain of HPg whereas micro-HPg is built from the protease domain of HPg and a stretch of about 15 amino acids from kringle 5. Differential scanning calorimetric measurements of Glu1-HPg and Lys78-HPg reveal seven thermal transitions for both plasminogen forms. The results obtained for Lys78-HPg largely agree with recently published data (Novokhatny, V. V., Kudinov, S. A. and Privalov, P. L. J. Mol. Biol. 1984, 179, 215). Three thermal transitions corresponding to kringle 5 and to two subdomains of the C-terminal protease region were identified for mini-HPg. In micro-HPg, the two thermal transitions of the protease region were found but one of the protease subdomains was modified and its stability was much higher than in any of the other studied proteins. According to the microcalorimetric data obtained for mini-HPg and micro-HPg, transitions 5 and 6 of Glu1-HPg and Lys78-HPg were reassigned to kringle 5 and to a subdomain of the protease region, respectively, in contrast to literature data.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of plasmin binding and urokinase activation of plasminogen bound to the Heymann nephritis autoantigen, gp330

Previously, we demonstrated that the Heymann nephritis autoantigen, gp330, can serve as a receptor site for plasminogen. This binding was not significantly inhibited by the lysine analogue epsilon-amino caproic acid (EACA), indicating that plasminogen binding was not just through lysine binding sites as suggested for other plasminogen binding sites. We now report that once plasminogen is bound to gp330, it can be converted to its active form of plasmin by urokinase. This conversion of plasminogen to plasmin proceeds at a faster rate when plasminogen is first prebound to gp330. Although there is a proportional increase in the Vmax of the urokinase-catalyzed reaction with increasing gp330 concentrations, no change in Km was observed. Once activated, plasmin remains bound to gp330 in an active state capable of cleaving the chromogenic tripeptide, S-2251. The binding of plasmin to gp330 did not significantly change its enzymatic activity; however, gp330 did have a stabilizing effect on plasmin activity at 37 degrees C. While bound to gp330, plasmin is protected from inactivation by its natural inhibitor alpha 2-antiplasmin. The binding of plasmin to gp330 as analyzed by ELISA was shown to be time dependent, reversible, saturable, and specific for gp330. Inhibition of binding of both plasminogen and plasmin to gp330 by benzamidine was similar, although EACA inhibited the binding of plasmin to gp330 slightly more than the binding of plasminogen to gp330. These results indicate that the binding of plasminogen to gp330 serves as an effective means of increasing the rate of plasmin production on the glomerular and tubular epithelial cell surface while protecting the active plasmin from natural inhibitors.

The binding of plasminogen fragments to cultured human umbilical vein endothelial cells

Glu-plasminogen, kringle 1-5, kringle 1-3, and miniplasminogen exhibited strong binding to human umbilical vein endothelial cells (HUVEC). On the other hand, no significant binding was obtained with microplasminogen and kringle 4. Kringle 1-5 and miniplasminogen, which both contained kringle 5, specifically inhibited the binding of plasminogen to HUVEC while kringle 1-3 did not. The results implied plasminogen molecule contained at least two binding sites, with which it interacted HUVEC. The stronger binding site was located in kringle 5 and the weaker one was in kringle 1-3. Kringle 4 and the active site domain exhibited no significant binding to HUVEC. The interaction of plasminogen with HUVEC is mainly through binding site on kringle 5.

The effect of the carboxy-terminal lysine of urokinase on the catalysis of plasminogen activation

When single-chain pro-UK is activated by plasmin or kallikrein, the Lys158-Ile159 bond is cleaved, leaving a C-terminal lysine on the A-chain (Lys-UK). Two-chain, high molecular weight urokinase (UK) purified from urine, however, has been shown to contain a phenylalanine residue as the C-terminal of the A-chain (Phe-UK). Since C-terminal lysine residues have a strong binding affinity for plasminogen that may promote its activation, we undertook kinetic studies comparing plasminogen activation by Lys- and Phe-UK. A two-stage method was employed in order to minimize factors known to interfere with plasminogen activation and plasmin determination. The Lys-UK was prepared by plasmin activation of pro-UK purified from human fetal kidney cell culture medium. The Phe-UK was prepared by carboxypeptidase B (CpB) treatment of Lys-UK. Removal of the C-terminal lysine of Lys-UK by CpB produced small but significant increases in the Michaelis constants for the activation of both Glu- and Lys-plasminogen. The apparent Michaelis constants for Glu-plasminogen activation by Lys- and Phe-UK were 3.7 microM +/- .36 microM and 5.9 microM +/- .70 microM, respectively and the Michaelis constants for Lys-plasminogen activation by Lys- and Phe-UK were 5.4 microM +/- .72 microM and 15.2 microM +/- 1.4 microM, respectively. The catalytic efficiency (kcat/Km) of Lys-UK was approximately 2-fold greater than that of Phe-UK for the activation of either Glu- or Lys-plasminogen. When the fibrinolytic activities of Lys- and Phe-UK were compared in a plasma milieu no significant differences were detected. In conclusion, the findings indicate that the C terminal lysine on the A-chain of UK significantly promotes the catalysis of plasminogen in a purified system. However, the higher catalytic efficiency of Lys-UK was not found to induce significant acceleration of clot lysis at pharmacological concentrations in plasma.

igh sialic acid content slows prourokinase turnover in rabbits

The clearance of natural and recombinant prourokinase (proUK) from the blood of rabbits was studied by means of a double-isotope method which allowed the differential removal of two distinct proUK species to be monitored when simultaneously administered to an individual animal. In initial experiments, proUK expressed in different cell lines contained between 0 and 2.5 molecules of sialic acid per molecule of protein. A slight trend toward slower clearance of proUK with higher sialic acid content was observed but rate differences were not statistically significant. Recombinant proUK produced in CHO cells grown in flow reactors, contained unusually high levels of sialic acid in excess of 3 moles/mole protein. Controlled exposure to immobilized neuraminidase was used to remove sialic acid from this protein in defined amounts. The clearance of the parent material was biphasic with average alpha and beta half-lives of 1.7 min and 16.7 min respectively. The AUC of the parent material was only slightly lowered upon removal of 30% of the original sialic acid. Species with 60% or 90% removal of sialate were much more rapidly cleared from the circulation respectively yielding AUCs equal to 56% and 41% of that observed with the parent material. Thus proUK containing 2.5-3.5 sialic acid molecules per molecule of protein turned over significantly more slowly in rabbits than did less sialylated proUK. The clearance rate was relatively insensitive to sialic acid content between 0 and 1.5 sialic acid residues per proUK molecule.

Partial purification and characterization of native plasminogen activators from bovine milk

At least four native plasminogen activators were detected in bovine milk, and two partially purified plasminogen activators were characterized. The plasminogen activators were dissociated from casein proteins by treatments with sulfuric acid and dimethylformamide. The plasminogen activators in the resulting fractions were partially purified with size exclusion, affinity, or metal chelate chromatographic techniques. Molecular weights of the two partially purified plasminogen activators were 47.2 and 30.5 kDa by gel electrophoresis. Size exclusion chromatography gave a molecular weight of 43.2 kDa for the first plasminogen activator. The isoelectric points of the two plasminogen activators were in the pH range 6.2 to 6.7. Because activity was not enhanced by the presence of fibrinogen fragments in a plasminogen activator assay mixture and decreased when human anti-urokinase Ig were added, at least some bovine milk native plasminogen activators appear to be urokinase-type plasminogen activators.

Immobilized urokinase column as part of a specific detection system for plasminogen species separated by high-performance affinity chromatography

Immobilized urokinase was used as part of a post-column reactor for the specific detection of human plasminogen species which were fractionated using a high-performance affinity column. After on-line activation of each peak, plasmin activity was measured by mixing the eluate with a specific fluorogenic substrate and the product was detected by a fluorescence monitor. This detection system gave linear calibration graphs for both purified plasminogens (0.1-50 micrograms) and plasminogens contained in plasma (25-100 microliters). Relative standard deviations for the determination of plasminogens in plasma were 6.1-6.6% (n = 12), showing good reproducibility. The detection limit was as low as 0.1 micrograms of plasminogen. Immobilized urokinase was very stable and no appreciable decrease in activity was found after 100 cycles of operation. In combination with an immobilized benzamidine column, this system made it possible to separate and detect Glu-plasminogen and Lys-plasminogen contained in human plasma samples as small as 100 microliters without any pretreatment.