Increased urokinase and consumption of α2 -antiplasmin as an explanation for the loss of benefit of tranexamic acid after treatment delay

Essentials Delayed treatment with tranexamic acid results in loss of efficacy and poor outcomes. Increasing urokinase activity may account for adverse effects of late tranexamic acid treatment. Urokinase + tranexamic acid produces plasmin in plasma or blood and disrupts clotting. α2 -Antiplasmin consumption with ongoing fibrinolysis increases plasmin-induced coagulopathy. SUMMARY: Background Tranexamic acid (TXA) is an effective antifibrinolytic agent with a proven safety record. However, large clinical trials show TXA becomes ineffective or harmful if treatment is delayed beyond 3 h. The mechanism is unknown but urokinase plasminogen activator (uPA) has been implicated. Methods Inhibitory mechanisms of TXA were explored in a variety of clot lysis systems using plasma and whole blood. Lysis by tissue plasminogen activator (tPA), uPA and plasmin were investigated. Coagulopathy was investigated using ROTEM and activated partial thromboplastin time (APTT). Results IC50 values for antifibrinolytic activity of TXA varied from < 10 to > 1000 μmol L-1 depending on the system, but good fibrin protection was observed in the presence of tPA, uPA and plasmin. However, in plasma or blood, active plasmin was generated by TXA + uPA (but not tPA) and coagulopathy developed leading to no or poor clot formation. The extent of coagulopathy was sensitive to available α2 -antiplasmin. No clot formed with plasma containing 40% normal α2 -antiplasmin after short incubation with TXA + uPA. Adding purified α2 -antiplasmin progressively restored clotting. Plasmin could be inhibited by aprotinin, IC50 = 530 nmol L-1 , in plasma. Conclusions Tranexamic acid protects fibrin but stimulates uPA activity and slows inhibition of plasmin by α2 -antiplasmin. Plasmin proteolytic activity digests fibrinogen and disrupts coagulation, exacerbated when α2 -antiplasmin is consumed by ongoing fibrinolysis. Additional direct inhibition of plasmin by aprotinin may prevent development of coagulopathy and extend the useful time window of TXA treatment.

Measuring fibrinolysis: from research to routine diagnostic assays

Development and standardization of fibrinolysis methods have progressed more slowly than coagulation testing and routine high-throughput screening tests for fibrinolysis are still lacking. In laboratory research, a variety of approaches are available and are applied to understand the regulation of fibrinolysis and its contribution to the hemostatic balance. Fibrinolysis in normal blood is slow to develop. For practical purposes plasminogen activators can be added to clotting plasma, or euglobulin prepared to reduce endogenous inhibitors, but results are complicated by these manipulations. Observational studies to identify a 'fibrinolysis deficit' have concluded that excess fibrinolysis inhibitors, plasminogen activator inhibitor 1 (PAI-1) or thrombin-activatable fibrinolysis inhibitor (TAFI), zymogen or active enzyme, may be associated with an increased risk of thrombosis. However, results are not always consistent and problems of adequate standardization are evident with these inhibitors and also for measurement of fibrin degradation products (D-dimer). Few methods are available to investigate fibrinolysis under flow, or in whole blood, but viscoelastic methods (VMs) such as ROTEM and TEG do permit the contribution of cells, and importantly platelets, to be explored. VMs are used to diagnose clinical hyperfibrinolysis, which is associated with high mortality. There is a debate on the usefulness of VMs as a point-of-care test method, particularly in trauma. Despite the difficulties of many fibrinolysis methods, research on the fibrinolysis system, taking in wider interactions with hemostasis proteins, is progressing so that in future we may have more complete models and better diagnostic methods and therapeutics.

Basic mechanisms and regulation of fibrinolysis

Fibrinolysis appears in many diverse physiological situations, and the components of the system are well established, along with mechanistic details for the individual reactions and some high-resolution structures. Key questions in understanding the regulation of fibrinolysis surround mechanisms of initiation and propagation, the localization of fibrinolysis reactions to the fibrin clot, and the influence of fibrin structure and clot composition on thrombolysis. This review covers these key areas with a focus on recent developments on fibrin structure and binding, the effects of a variety of cell types, the consequences of histones and DNA released by neutrophils, and the influence of flow. A complete understanding of the regulation of fibrinolysis will come from the building of detailed mathematical models. Suitable models are at an early stage of development, but may improve as model clots increase in complexity to incorporate the components and interactions listed above.

Biosimilars: the process is the product. The example of recombinant streptokinase

BACKGROUND

Worldwide, streptokinase remains the most used thrombolytic agent for the treatment of myocardial infarction. Recombinant streptokinase, from E. coli, is increasingly used in developing countries as a biosimilar of native streptokinase; however, potency assignments relative to the WHO International Standard (IS) are highly variable with potentially dangerous consequences. A proportion of recombinant streptokinase appears to be incompletely processed, retaining the amino-terminal methionine engineered for intracellular expression.

OBJECTIVES

To investigate and quantify the impact of an amino-terminal methionine on streptokinase activity.

METHODS

Mature native streptokinase (rSK) was cloned and a novel variant constructed to include an amino-terminal methionine (rSK-Met) that is not susceptible to processing during expression. Potencies of rSK and rSK-Met were determined relative to the WHO IS using a chromogenic solution (European Pharmacopoeia) assay, and fibrin-based assays.

RESULTS

In the chromogenic solution assay there was no measurable difference between rSK and rSK-Met activities. In the fibrin-based methods, however, potency estimates for rSK-Met were greatly reduced compared with rSK, and fibrinolytic activity for rSK-Met was shown to increase over time with methionine aminopeptidase treatment. This apparent difference in activity and fibrin selectivity was consistent with potency estimates for several different batches of commercial recombinant streptokinase products also tested; consequently, different potencies would be assigned to therapeutic recombinant streptokinase products depending on the degree of amino-terminal methionine processing, and on the pharmacopoeial assay method used, affecting the dosage patients receive. This has serious health implications and provides an example of the danger in the unregulated clinical use of biosimilars.

Lytic and mechanical stability of clots composed of fibrin and blood vessel wall components

BACKGROUND

Proteases expressed in atherosclerotic plaque lesions generate collagen fragments, release glycosaminoglycans (chondroitin sulfate [CS] and dermatan sulfate [DS]) and expose extracellular matrix (ECM) proteins (e.g. decorin) at sites of fibrin formation.

OBJECTIVE

Here we address the effect of these vessel wall components on the lysis of fibrin by the tissue plasminogen activator (tPA)/plasminogen system and on the mechanical stability of clots.

METHODS AND RESULTS

MMP-8-digested collagen fragments, isolated CS, DS, glycosylated decorin and its core protein were used to prepare mixed matrices with fibrin (additives present at a 50-fold lower mass concentration than fibrinogen). Scanning electron microscopy (SEM) showed that the presence of ECM components resulted in a coarse fibrin structure, most pronounced for glycosylated decorin causing an increase in the median fiber diameter from 85 to 187 nm. Rheological measurements indicated that these structural alterations were coupled to decreased shear resistance (1.8-fold lower shear stress needed for gel/fluid transition of the clots containing glycosylated decorin) and rigidity (reduction of the storage modulus from 54.3 to 33.2 Pa). The lytic susceptibility of the modified fibrin structures was increased. The time to 50% lysis by plasmin was reduced approximately 2-fold for all investigated ECM components (apart from the core protein of decorin which produced a moderate reduction of the lysis time by 25%), whereas fibrin-dependent plasminogen activation by tPA was inhibited by up to 30%.

CONCLUSION

ECM components compromise the chemical and mechanical stability of fibrin as a result of changes in its ultrastructure.

Regulation of fibrinolysis by C-terminal lysines operates through plasminogen and plasmin but not tissue-type plasminogen activator

BACKGROUND

Binding of tissue-type plasminogen (Pgn) activator (t-PA) and Pgn to fibrin regulates plasmin generation, but there is no consistent, quantitative understanding of the individual contribution of t-PA finger and kringle 2 domains to the regulation of fibrinolysis. Kringle domains bind to lysines in fibrin, and this interaction can be studied by competition with lysine analogs and removal of C-terminal lysines by carboxypeptidase B (CPB).

METHODS

High-throughput, precise clot lysis assays incorporating the lysine analog tranexamic acid (TA) or CPB and genetically engineered variants of t-PA were performed. In particular, wild-type (WT) t-PA (F-G-K1-K2-P) and a domain-switched variant K1K1t-PA (F-G-K1-K1-P) that lacks kringle 2 but retains normal t-PA structure were compared to probe the importance of fibrin lysine binding by t-PA kringle 2.

RESULTS

WT t-PA showed higher rates of fibrinolysis than K1K1t-PA, but the inhibitory effects of TA or CPB were very similar for WT t-PA and the variant t-PA (< 10% difference). Urokinase plasminogen activator (u-PA)-catalyzed fibrinolysis was also inhibited by TA, even though Pgn activation could be stimulated. Fibrin treated with factor XIIIa (FXIIIa) generates crosslinked degradation products, but these did not affect the results obtained with WT t-PA and K1K1t-PA.

CONCLUSIONS

t-PA kringle 2 has a minor role in the initial interaction of t-PA and fibrin, but stimulation of fibrinolysis by C-terminal lysines (or inhibition by carboxypeptidases or TA) operates through Pgn and plasmin binding, not through t-PA. This is also true when fibrin is crosslinked by treatment with FXIIIa.

Hindered dissolution of fibrin formed under mechanical stress

BACKGROUND

Recent data indicate that stretching forces cause a dramatic decrease in clot volume accompanied by gross conformational changes of fibrin structure.

OBJECTIVE

The present study attempts to characterize the lytic susceptibility of fibrin exposed to mechanical stress as a model for fibrin structures observed in vivo.

METHODS AND RESULTS

The relevance of stretched fibrin models was substantiated by scanning electron microscopic (SEM) evaluation of human thrombi removed during surgery, where surface fibrin fibers were observed to be oriented in the direction of shear forces, whereas interior fibers formed a random spatial meshwork. These structural variations were modeled in vitro with fibrin exposed to adjustable mechanical stress. After two- and three-fold longitudinal stretching (2 × S, 3 × S) the median fiber diameter and pore area in SEM images of fibrin decreased two- to three-fold. Application of tissue plasminogen activator (tPA) to the surface of model clots, which contained plasminogen, resulted in plasmin generation which was measured in the fluid phase. After 30-min activation 12.6 ± 0.46 pmol mm(-2) plasmin was released from the non-stretched clot (NS), 5.5 ± 1.11 pmol mm(-2) from 2 × S and 2.3 ± 0.36 pmol mm(-2) from 3 × S clot and this hampered plasmin generation was accompanied by decreased release of fibrin degradation products from stretched fibrins. Confocal microscopic images showed that a green fluorescent protein-fusion variant of tPA accumulated in the superficial layer of NS, but not in stretched fibrin.

CONCLUSION

Mechanical stress confers proteolytic resistance to fibrin, which is a result of impaired plasminogen activation coupled to lower plasmin sensitivity of the denser fibrin network.

An international collaborative study to establish the WHO 1st international standard for alpha-1-antitrypsin

BACKGROUND AND OBJECTIVES

The aim was to establish the 1st International Standard (IS) for alpha-1-antitrypsin (AAT) to standardise potency assignment of therapeutic products, calibrated in moles and mg active AAT in line with product labelling practice. Assigning total protein and antigen values to the IS was also investigated.

MATERIALS AND METHODS

The active concentration of four candidate AAT preparations was determined in an international collaborative study by inhibition of trypsin (calibrated by active-site titration). Total protein and antigen content were determined for each candidate using local methods and in-house standards, and a common AAT preparation. The total protein content of the IS was also determined by amino acid analysis. Potency determination of recombinant and transgenic materials against the IS was investigated in a follow-up study.

RESULTS

Data analysis for potency determination indicated no statistical difference between any of the candidates, or between the results for recombinant and plasma-derived products. Total protein content of the IS determined by amino acid analysis was consistent with the potency value. The variability in the total protein and antigen results for the other candidates was reduced when the data were recalculated relative to the IS.

CONCLUSIONS

Candidate C (05/162) was established by the WHO Expert Committee on Biological Standardization (ECBS) in 2006 as the WHO 1st IS for AAT with a potency of 243 nmoles (12·4 mg) active inhibitor per ampoule. In 2008, ECBS approved the IS for potency determination of recombinant material and assigned a total protein and antigen value of 12·4 mg.

Fibrin binding and the regulation of plasminogen activators during thrombolytic therapy

First generation thrombolytics (streptokinase and urokinase) had no fibrin binding capabilities and caused systemic plasminogen activation with concomitant destruction of haemostatic proteins. A primary driving force behind the development of the second generation plasminogen activator tissue plasminogen activator (tPA or alteplase) was its ability to bind to fibrin and target thrombolysis. Although in vitro assays highlighted advantages of fibrin binding, clinical trials were disappointing, showing only small benefits in mortality with tPA versus streptokinase, but also with some increase in haemorrhagic stroke. Third generation thrombolytic agents (reteplase, tenecteplase and pamiteplase) are variants of tPA engineered to have improved structure/function, such as longer half life and resistance to inhibitors. However, clear therapeutic advantages of third generation thrombolytics in clinical trials have also been difficult to demonstrate. Although fibrin binding is critical in regulating the activity of tPA, it is not clear how important it is for thrombolytic treatment. Advances are needed in our understanding of the relationship between structure/binding and activity of PAs in vivo under normal conditions and when administered in pharmacological doses. Clearly the impact of fibrin structure and the other components in fibrin clots must also be considered. Ultimately these studies may lead to better engineered therapeutics or optimised mixtures of molecules. With a more detailed understanding of the regulation of plasminogen activation and fibrinolysis it might be possible to tailor thrombolytic therapy to different situations such as myocardial or cerebrovascular treatment or to the patient's age and sex and other characteristics.

Fibrinolysis in a lipid environment: modulation through release of free fatty acids

BACKGROUND

Thrombolysis is conventionally regarded as dissolution of the fibrin matrix of thrombi by plasmin, but the structure of clots in vivo includes additional constituents (proteins, phospholipids) that modulate their solubilization.

OBJECTIVE

We examined the presence of free fatty acids in thrombi and their effects on distinct stages of fibrinolysis (plasminogen activation, plasmin activity).

METHODS AND RESULTS

Using the fluorescent probe acrylodated intestinal fatty acid-binding protein, variable quantities (up to millimolar concentrations) of free fatty acids were demonstrated in surgically removed human thrombi. Oleic acid at relevant concentrations reversibly inhibits more than 90% of the amidolytic activity of plasmin on a synthetic substrate (Spectrozyme PL), but only partially inhibits its fibrinolytic activity measured using turbidimetry. Chromogenic assays detecting the generated plasmin activity show that plasminogen activation by tissue-type plasminogen activator (t-PA) is completely blocked by oleic acid in the fluid phase, but is accelerated on a fibrin matrix. A recombinant derivative of t-PA (reteplase) develops higher fibrin specificity in the presence of oleic acid, because both the inhibition of plasminogen activation in free solution and its enhancement on fibrin template are stronger than with wild-type t-PA.

CONCLUSION

Through the stimulation of plasminogen activation on a fibrin template and the inhibition of plasminogen activators and plasmin in the fluid phase, free fatty acids confine the action of fibrinolytic proteases to the site of clotting, where they partially oppose the thrombolytic barrier function of phospholipids.

The regulation by fibrinogen and fibrin of tissue plasminogen activator kinetics and inhibition by plasminogen activator inhibitor 1

BACKGROUND

Tissue plasminogen activator (tPA) is unusual in the coagulation and fibrinolysis cascades in that it is produced as an active single-chain enzyme (sctPA) rather than a zymogen. Two chain tPA (tctPA) is produced by plasmin but there are conflicting reports in the literature on the behaviour of sc- and tctPA and little work on inhibition by the specific inhibitor plasminogen activator inhibitor-1 (PAI-1) under physiological conditions.

OBJECTIVES

To perform a systematic study on the kinetics of sctPA and tctPA as plasminogen activators and targets for PAI-1.

METHODS

Detailed kinetic studies were performed in solution and in the presence of template stimulators, fibrinogen and fibrin, including native fibrin and partially digested fibrin. Numerical simulation techniques were utilized to cope with the challenges of investigating kinetics of activation and inhibition in the presence of fibrin(ogen).

RESULTS

Enzyme efficiency (k(cat)/K(m)) was higher for tctPA than sctPA in solution with chromogenic substrate (3-fold) and plasminogen (7-fold) but in the presence of templates, such as fibrinogen and native or cleaved fibrin, the difference disappeared. sctPA was more susceptible to PAI-1 in buffer solution and in the presence of fibrinogen; however, in the presence of fibrin, PAI-1 inhibited more slowly and there was no difference between sc and tctPA.

CONCLUSIONS

Fibrinogen and fibrin modulate the activity of tPA differently in regard to their activation of plasminogen and inhibition by PAI-1. Fibrinogen and fibrin stimulate tPA activity against plasminogen but fibrin protects tPA from PAI-1 to promote fibrinolysis.

Fibrinolysis at the fluid-solid interface of thrombi

Thrombolysis is conventionally regarded as dissolution of the fibrin matrix of thrombi by plasmin, a protease generated by plasminogen activators from its inactive precursor, plasminogen. Typically plasminogen activation occurs on the surface of the clot, where fibrin behaves as a cofactor in this process, and plasmin also initiates its proteolytic action at the fluid-solid interface. Although the basic reactions of the plasminogen/plasmin system in fluid phase are well characterized in terms of classical enzymology, they cannot explain completely the interfacial fibrinolytic events. Recently new methods have been introduced for quantitative evaluation of plasminogen activation on gel-phase fibrin and heterogenous-phase proteolysis, an overview of the new methodology is presented. Following formation of an interfacial lytic zone, fibrin dissolution proceeds through propagation of this zone to the core of the clot, which depends on diffusion and permeation phenomena affected by the composition of thrombi. Phospholipids (originating from platelets) form a diffusion barrier to the thrombolytic agents and also bind some of them; structural cellular proteins (namely myosin) interact with the fibrin fibers masking their cofactor and plasmin-cleavage sites. The contribution of these recent findings to our understanding of the limitations of current thrombolytic therapy is discussed. Finally, attention is focused on the termination of thrombus-associated proteolytic action in an environment abundant in proteinase inhibitors. Thus, combining together the interfacial events in the initiation, progress and termination of thrombolysis, a concept for modeling the thrombus as a temporary fibrinolytic compartment is presented.

Collaborative study to establish a new biological reference preparation for prekallikrein activator

An International Collaborative Study was organized to replace the current World Health Organization (WHO) International Standard (IS) for Prekallikrein Activator (PKA) and to establish a European Pharmacopoeia (Ph. Eur.) Biological Reference Preparation (BRP). The project was jointly organized by the European Directorate for the Quality of Medicines (EDQM) and the National Institute for Biological Standards and Control (NIBSC) to identify and calibrate suitable materials that could act as an IS and a Ph. Eur. BRP. The current IS for PKA (82/530) is popular and stocks are declining rapidly, therefore necessitating calibration of a replacement. A Ph. Eur. BRP is needed, as PKA control on the finished product is part of the Official Control Authority Batch Release (OCABR) of Human Albumin. The current IS, 82/530 is a 5 per cent albumin solution spiked with purified PKA. However, during planning stages it was decided that the replacement IS (and BRP) should be made from a 20 per cent albumin preparation containing a significant level of PKA as the current IS is used to measure PKA in albumin and high levels are more likely to be encountered in more concentrated 20 per cent solutions. A suitable material was sourced by the EDQM and filled into ampoules at NIBSC and vials by the EDQM. Both preparations were included in the collaborative study that involved 31 laboratories from 17 countries. Another important goal of this study was to investigate the influence of the prekallikrein substrate (PKS) on PKA determination in albumin solutions following earlier concerns that variability amongst PKS prepared in-house could significantly affect PKA determinations. Laboratories were requested to perform their routine assays following Ph. Eur. guidelines and recommendations on doses, replication and randomization were also provided to study participants. Participants were requested to use material A (the current IS, 82/530) to perform at least 4 assays to determine PKA levels in sample B (NIBSC ampouled material, candidate IS, 02/168), sample C (EDQM material in vials candidate Ph. Eur. BRP Batch 1), and sample D (an ampouled preparation of 2.5 per cent albumin containing a lower level of PKA). A commercial substrate was provided for participants to perform half the assays and the remaining assays were to be performed using the laboratories' in-house substrate (where available). Collation of participants' results showed that samples B and C had the same level of PKA of 29 IU/ampoule, the concentration anticipated from development studies. Importantly, there was no significant difference between the PKA level obtained using the commercial substrate provided and the laboratories' own in-house substrate. Previous observations on lyophilized preparations of PKA indicate that the enzyme is very stable. Detailed investigations conducted in this study show that the PKA in albumin used to make samples B and C is very stable and suitable for long-term storage as a reference material.

An international collaborative study to replace the 1st international standard for prekallikrein activator

BACKGROUND AND OBJECTIVES

The aim of this study was to replace the 1st International Standard (IS) for prekallikrein activator (PKA) (code 82/530) with a new IS and European Biological Reference Preparation (BRP). The new standards were freeze dried 20% albumin solution containing PKA, the same solution that is tested using these standards. Aspects of the methodology for PKA determination were also examined as part of this study.

MATERIALS AND METHODS

A batch of 20% albumin containing approximately 30 IU/ml was donated by a manufacturer of blood products and dispensed into ampoules at the National Institute for Biological Standards and Control (NIBSC) to create the candidate IS (02/168, sample B in the study) and at the European Directorate for the Quality of Medicines (EDQM) to create the candidate BRP (sample C in the study). The concentration of PKA in these preparations was determined in an international collaborative study involving 31 laboratories from 17 countries worldwide in comparison with the 1st IS for PKA (82/530) containing 85 IU of PKA per ampoule. Participants were requested to perform their own in-house method, based on the current Ph. Eur. monograph for determination of PKA in albumin solutions. Participants were provided with sufficient samples to perform two or three assays and were asked to use their local prekallikrein substrate (PKS) and also to use a commercial PKS provided as part of the study, in order to investigate the importance of the source of PKS on the final potency values.

RESULTS

Samples B and C emerged with identical PKA concentrations of 29 IU/ml, very close to the expected value. This figure was determined using a variety of statistical methods, with the participants' own calculated values and values calculated centrally at the EDQM using raw data. The value of 29 IU/ml was consistent and independent of the method of calculation, although interlaboratory variability was more sensitive to the statistical analysis method. There was no statistically significant difference in mean potencies when comparing results with the laboratories' own local substrate and the substrate provided for the study. All stability studies indicate that these lyophilized preparations of PKA in 20% albumin are extremely stable.

CONCLUSIONS

Samples B and C were established as the 2nd IS (code 02/168) and PKA activator in albumin BRP batch 1 (Y0000263), respectively, with a potency of 29 IU per ampoule. Results from this study indicate that testing for PKA in albumin may be less sensitive to the source of PKS than previously feared. The study highlights a number of methodological issues that may need revising in the Ph. Eur. general chapter 2.6.15.

Are we meeting the psychological needs of heterosexual men with HIV disease? A retrospective case controlled study of referrals to a psychological medicine unit in London, UK

This study set out to ascertain what proportion of HIV-positive heterosexual men cared for at a central London teaching hospital HIV medical unit, were referred to the Psychological Medicine Unit, and to compare those HIV-positive male heterosexual patients with age and sex matched HIV-positive gay male controls. Hospital and Psychological Medicine Unit databases were interrogated to identify relevant patients seen during the period between February 1992 and December 2002. Furthermore, 50 heterosexual patients, who had been referred to the Psychological Medicine Unit, were matched for age and date of referral, with one gay male HIV-positive control patient. Demographic and illness data was gathered for the subjects and controls. Data was collected on 50 subjects in each group. The main findings of the study were: (1) that heterosexual men with HIV are almost three times less likely to be referred for specialist mental health care than HIV-positive gay men; (2) that heterosexual men with HIV disease, who were referred to the Psychological Medicine Unit, were less likely to be from a white ethnic background compared to gay men; (3) were less likely to be given a diagnosis of a depressive illness; but (4) were more likely to have a substance misuse diagnosis. Gay male patients who are HIV-positive are more likely to experience difficulties with sexual dysfunction, and receive a formal psychiatric diagnosis. The implications of the findings are discussed.

A proposed reference method for plasminogen activators that enables calculation of enzyme activities in SI units

A method has been developed for accurately and precisely measuring the activity of a range of plasminogen activators (PAs) used as thrombolytic agents, including streptokinase, tissue plasminogen activator (tPA) and variants, and urokinase (uPA), both single and two chain forms. Plasminogen activation is monitored in a transparent, solid fibrin matrix but uses chromogenic substrate hydrolysis, rather than changes in fibrin, to quantitate the activity of PAs. The method has been tested in two recent international collaborative studies involving tPA and streptokinase where it has been shown to perform very well. Furthermore, the method is based on sound enzymological principles and once correction for the competitive inhibition of fibrin(ogen) is made, the generation of plasmin can be determined in molar terms and hence the activity of PAs can be expressed and compared in SI units (rate of increase in molar concentration of plasmin) as well as International Units. The assay is also arranged in such a way to reflect the behavior of PAs in vivo during thrombolytic therapy and it is shown that the specific activity of streptokinase and tPA in this system reflects plasmin generation capacity of these thrombolytics for doses given in infusions for treatment of myocardial infarction. The method would make a suitable reference method for PAs and provides a rigorous means of studying and modeling the enzymology of fibrinolysis and will be helpful in the rational design of third generation thrombolytic agents.

International collaborative study to establish the 3rd International Standard for Streptokinase

An international collaborative study was organized to calibrate a replacement for the current (2nd) International Standard (IS) for Streptokinase, stocks of which are almost exhausted. Two candidate preparations were assayed against the 2nd IS in a study involving 16 laboratories in 12 countries: preparation 88/824 (coded B), and preparation 00/464 (C and D, coded duplicates). Laboratories could use two methods provided, either a fibrin clot lysis assay or a solution chromogenic method, or an in-house method. Laboratories were encouraged to perform more than one method if possible. With the exception of one laboratory which gave outlying results for preparation 00/464, there was good agreement within and between laboratories and no significant differences between potencies using the different methods employed. This study demonstrates that a solution chromogenic assay is an acceptable format for potency determination of the streptokinase preparations in this study and fibrin is not necessary. It has now been agreed that a solution chromogenic plasminogen activation assay replace the current euglobulin reference method for streptokinase activity determination in the European Pharmacopoeia. Study participants, SSC of the International Society on Thrombosis and Haemostasis and the Expert Committee on Biological Standardization (ECBS) at the World Health Organization approved preparation 00/464 (C,D in the study) as the 3rd IS for Streptokinase with a potency of 1030 IU per ampoule.

Differences between neonates and adults in tissue-type-plasminogen activator (t-PA)-catalyzed plasminogen activation with various effectors and in carbohydrate sequences of fibrinogen chains

Our study investigates the effect of fetal and adult soluble fibrin (SF), fetal and adult fibrinogen Aalpha- and gamma-chains, as well as adult CNBr-fibrinogen fragments on tissue-type plasminogen activator (t-PA)-catalyzed plasminogen activation of both fetal and adult Glu-plasminogen types 1 and 2. In addition, we determined carbohydrate sequences of fetal and adult Bbeta- and gamma-chains by mass spectrometric analysis. In the absence of an effector, no substantial differences in the rate of plasmin formation could be seen between the fetal and adult plasminogen types. In the presence of an effector, both fetal Glu-plasminogen types revealed lower values for k(cat app) than the respective adult types. No differences could be seen in the values for K(m app). The resulting differences in catalytic efficiencies between the fetal and adult plasminogen types were much less than previously reported. No differences could be seen between fetal and adult effectors in stimulating t-PA-catalyzed plasminogen activation. Detailed analyses of the activation kinetics revealed a longer initial phase of slow plasmin formation of both fetal Glu-plasminogen types compared to their respective adult types, indicating a slower plasmin-induced modification of CNBr-fibrinogen fragments or SF by fetal plasmin. Mass spectrometric analysis of the N-glycans present on adult and fetal Bbeta- and gamma-fibrinogen chains showed the presence of a major monosialylated biantennary structure with lesser amounts of the disialylated form. In contrast to previous data, we conclude that catalytic efficiency of t-PA-catalyzed plasminogen activation in neonates is only slightly lower than in adults.

Characterization of cell-associated plasminogen activation catalyzed by urokinase-type plasminogen activator, but independent of urokinase receptor (uPAR, CD87)

The 55-kD urokinase (uPA) receptor (uPAR, CD87) is capable of binding uPA and may be involved in regulating cell-associated plasminogen activation and pericellular proteolysis. While investigating the relationship between uPAR levels and plasmin generation, we found that uPA-catalyzed plasminogen activation is stimulated by cells which do not express uPAR. This uPAR-independent mechanism appears to be at least as effective in vitro as uPAR-dependent stimulation, such that stimulation on the order of 30-fold was observed, resulting from improvements in both apparent kcat and apparent Km. The mechanism depends on simultaneous binding of both uPA and plasminogen to the cell and requires the presence of the amino-terminal fragment (ATF), available in single chain and two chain high-molecular-weight uPA, but not low-molecular-weight uPA. Stimulation was observed in all leukemic cell lines investigated at similar optimum concentrations of 10(6) to 10(7) cells/mL and may be more general. A mechanism is proposed whereby uPA can associate with binding sites on the cell surface of lower affinity, but higher capacity than uPAR, but these are sufficient to stimulate plasmin generation even at subphysiologic uPA concentrations. This mechanism is likely to operate under conditions commonly used for in vitro studies and may have some significance in vivo.

Regulation of tissue plasminogen activator activity by cells. Domains responsible for binding and mechanism of stimulation

A number of cell types have previously been shown to bind tissue plasminogen activator (tPA), which in some cases can remain active on the cell surface resulting in enhanced plasminogen activation kinetics. We have investigated several cultured cell lines, U937, THP1, K562, Molt4, and Nalm6 and shown that they bind both tPA and plasminogen and are able to act as promoters of plasminogen activation in kinetic assays. To understand what structural features of tPA are involved in cell surface interactions, we performed kinetic assays with a range of tPA domain deletion mutants consisting of full-length glycosylated and nonglycosylated tPA (F-G-K1-K2-P), DeltaFtPA (G-K1-K2-P), K2-P tPA (BM 06.022 or Reteplase), and protease domain (P). Deletion variants were made in Escherichia coli and were nonglycosylated. Plasminogen activation rates were compared with and without cells, over a range of cell densities at physiological tPA concentrations, and produced maximum levels of stimulation up to 80-fold with full-length, glycosylated tPA. Stimulation for nonglycosylated full-length tPA dropped to 45-60% of this value. Loss of N-terminal domains as in DeltaFtPA and K2P resulted in a further loss of stimulation to 15-30% of the full-length glycosylated value. The protease domain alone was stimulated at very low levels of up to 2-fold. Thus, a number of different sites are involved in cell interactions especially within finger and kringle domains, which is similar to the regulation of tPA activity by fibrin. A model was developed to explain the mechanism of stimulation and compared with actual data collected with varying cell, plasminogen, or tPA concentrations and different tPA variants. Experimental data and model predictions were generally in good agreement and suggest that stimulation is well explained by the concentration of reactants by cells.

Inhibition of thrombin generation by heparin and LMW heparins: a comparison of chromogenic and clotting methods

Inhibition of thrombin generation by heparin and low-molecular-weight (LMW) heparins is an important parameter which relates to their anticoagulant actions in vivo. Previous studies in our laboratory used a clotting assay for assessment of thrombin generation but other published studies have used a chromogenic method. We have therefore measured the inhibition of thrombin generation by unfractionated heparin (UFH) and LMW heparins by a modified chromogenic method using microtitre plates and compared the results with the clotting method. The degree of inhibition of thrombin generation when calculated from both peak thrombin concentrations and areas under the curve in the chromogenic assay was the same. When the activities of each heparin were expressed as EC80, i.e. concentrations required for 80% inhibition of thrombin generation, the EC80 were higher in the chromogenic assay than in the clotting system. However when the potencies of the LMW heparins were expressed as percentages of that of the UFH standard by parallel line analysis, the differences between clotting and chromogenic assay results were small and not statistically significant (P > 0.05). This study demonstrates the feasibility of measuring inhibition of thrombin generation by a modified chromogenic method using microtitre plates, and shows that the results with this method are similar to those obtained with the clotting method.

Fragmentation of therapeutic human immunoglobulin preparations

Some commercial batches of human therapeutic immunoglobulins (Ig) have been found to show evidence of molecular fragmentation when examined by molecular sizing methodologies including sodium dodecyl sulphate polyacrylamide gel electrophoresis [SDS-PAGE] and size exclusion high performance liquid chromatography (SE-HPLC). These batches all demonstrated impaired immunobiological activity (efficacy) as assessed by Fc function measured using a rubella haemolytic assay and as such are likely to be subpotent for therapeutic use. Fragmented Igs were characterized by the presence of at least three protein bands and peaks additional to monomeric IgG. Incubation of Igs with blood enzymes (plasmin and kallikrein) reproduced the fragmentation patterns observed for intrinsically degraded batches, suggesting that fragmentation occurred by contamination with these proteases from the source material (human blood) during manufacture. Intravenous Igs (IVIG) were found to be more susceptible to proteolysis than intramuscular Igs, probably as a consequence of the post-fractionation processing that some IVIGs receive which may induce molecular alterations, allowing enzyme access and fragmentation. Two of the products examined were found to be relatively resistant to proteolysis and both were formulated by processes that limit enzyme activity. These processes were inclusion of an enzyme inhibitor, alpha 2-macroglobulin, and formulation at acidic pH. Enzyme carry-over into the final product is a likely cause of Ig fragmentation, and reduction in levels of such contamination should lead to improvements in product stability and efficacy.

Studies on the mechanisms of action of aprotinin and tranexamic acid as plasmin inhibitors and antifibrinolytic agents

Both aprotinin and tranexamic acid are effective inhibitors of fibrinolysis in vitro and in vivo and both agents can act as plasmin inhibitors in purified systems, although there is some debate on their exact mechanism of action in vivo. The studies reported here using an in vitro clot lysis system designed to provide precise inhibition constants show that aprotinin remains a very potent inhibitor of plasmin even in the presence of fibrin with Ki = 2 nM. Plasmin-aprotinin interactions in solution are not affected by a number of kringle binding ligands, aminohexanoic acid, tranexamic acid or CNBr-fibrinogen fragments with Ki = 0.4 nM. The difference between these two Ki values is explained by competition for the plasmin active site between substrate (fibrin) and inhibitor (aprotinin). Inhibition of fibrinolysis by tranexamic acid is not readily analysed by a simple inhibition model which may be due to multiple overlapping ligand-kringle interactions or tranexamic-fibrin interactions. Experiments using combinations of aprotinin and tranexamic acid in the clot lysis system confirm the complementary nature of inhibitory mechanisms and suggest a slight synergism. These results support the idea that aprotinin inhibition of plasmin is a primary mode of action in vivo, and suggest that combination therapy of aprotinin with tranexamic acid might be more effective than either inhibitor alone.

Heparin binding affinity of normal and genetically modified antithrombin III measured using a monoclonal antibody to the heparin binding site of antithrombin III

The inhibitory activity of the plasma serine proteinase inhibitor antithrombin III (AT III) is enhanced about 1000-fold upon binding to heparin. We have determined the dissociation constants, Kd, of 48.8 nM for the heparin-AT III interaction, of 175 nM for the specific pentasaccharide-AT III interaction, and of 13 microM for the low-affinity heparin-AT III interaction, using a binding assay based on a monoclonal antibody (MAb) that recognizes an epitope at or close to the heparin binding site of AT III. The heparin binding affinities and proportions of normal and variant AT III in plasma from patients with mutations of AT III have been quantitated for the first time using the binding assay. Substitution mutations in three regions of AT III have been investigated: (i) mutations in the reactive site loop affecting Ala382, Arg393, and Ser394 have no discernible effect on heparin binding; (ii) mutations in the previously identified N-terminal heparin binding region, affecting Arg47, Leu99, and Arg129, produce variant AT III molecules with heparin affinities reduced 11-924-fold, the largest reduction being observed for the substitution mutation Arg47-Cys in Padua 2, which has an affinity of 65.6 microM; (iii) mutations in the hydrophobic regions around strand 1C of the C terminus, affecting Phe402, Ala404, Asn405, Pro407, and Pro429, have pleiotropic effects that include the production of reduced amounts of low-affinity AT III with dissociation constants from 6 to 43 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

An international collaborative study to investigate standardisation of hirudin potency

An international collaborative study has been carried out to investigate the reproducibility of hirudin assays in 13 laboratories using four recombinant hirudins and one natural, sulphated product. A simple assay procedure was proposed involving the titration of alpha-thrombin with inhibitor and measurement of residual activity using a chromogenic substrate. A standard alpha-thrombin preparation was supplied to ensure that this reagent was of uniform quality throughout the study. The method appeared to present no difficulties and laboratories reported similar potencies for the 5 hirudin samples, in line with expected values. This gave 200-222 Thrombin Inhibitory Units/ampoule (TIU/ampoule) of lyophilised hirudin, with geometric coefficient of variation (gcv) values ranging from 10.15-15.97%. This corresponds to specific activities of approximately 14,300-15,900 TIU/mg protein. This is close to the upper limit of previously reported values of specific activity. We conclude that the precision of this determination compared with the wider range of values in the literature (8,000-16,000 thrombin inhibitory units [TIU]/mg) results from the use of good quality standard alpha-thrombin by all laboratories. This study has important implications for hirudin standardisation.

In vivo significance of kinetic constants of tight binding reversible proteinase inhibitors

For a reaction between an enzyme and inhibitor, rate constants and equilibrium constants are both widely used as measures of inhibitor potency. Which one of these two parameters should be used depends on the mechanism of the reaction. Moreover, for biological studies it is not always the overall effectiveness of an inhibitor that is important, but also the rate at which it acts in conditions that may be encountered in vivo. In vitro investigations may be performed to determine the kinetic and thermodynamic constants in an enzyme inhibitor system and to investigate the influence of various other effector biomolecules. However, in translating these results into physiological conclusions, care must be taken that the in vitro experiments have been properly designed and data analysed correctly. This article is an attempt to indicate some of the problems that can be encountered in this process.

Characterization of recombinant C1 inhibitor P1 variants

Twelve human C1 inhibitor P1 variants were constructed by site-directed mutagenesis of the codon for arginine 444 and were expressed in COS-1 cells to analyze the functional properties. The ability to bind to target proteases, as well as potential substrate-like behavior, was investigated with radioimmunoassays. The P1-Lys variant retained binding capacity toward C1s, plasmin, and kallikrein. In addition, complex formation with C1s was detected for P1-Asn and P1-His. All other P1 substitutions resulted in C1 inhibitor variants that neither complexed with nor were inactivated by C1s, kallikrein, beta-factor XIIa, or plasmin. Electrophoretic studies confirmed that P1-Lys and P1-His can form sodium dodecyl sulfate-resistant complexes with C1s. In contrast, the C1s-P1-Asn complex dissociated upon addition of sodium dodecyl sulfate. Kinetic experiments by the method of progress curves generated association rate constants (kon) with C1s of 4.2 x 10(4) M-1 s-1 for recombinant wild-type C1 inhibitor and 1.7 x 10(4) M-1 s-1 for P1-Lys. For P1-Asn and P1-His, kon was decreased approximately 100-fold. The results from inhibition experiments were compatible with a model of reversible inhibition, although the observed dissociation rate for wild-type C1 inhibitor is too low (1-2 x 10(-6) s-1) to be physiologically relevant. The overall inhibition constant (Ki) was estimated to be 0.03 nM. With P1-Asn, reversible inhibition could be demonstrated directly upon dilution of preformed complexes; the observed dissociation rate constant was 3.2 x 10(-4) s-1; and Ki increased to approximately 380 nM. These findings are discussed in relation to inhibitor specificity and inhibition mechanism.

Studies on the mechanism of binding of serpins and serine proteases

The serpin family of inhibitors have an important role in the control of coagulation and fibrinolysis. For a full understanding of how these pathways operate in vivo and correct measurement of enzyme and inhibitor activity, in vitro knowledge of the mechanism of action of serpins is essential. Using alpha 2-antiplasmin as a model inhibitor we find, in contrast to most previous reports, a reversible mechanism: E + I in equilibrium with EI in equilibrium with EI', where complex formation is two stepped, but both steps are reversible. Our work with plasmin in the presence of 50 mM aminohexanoic acid shows that binding of alpha 2-antiplasmin is very tight (but reversible) with an overall Ki (Ki final) = 4.0 pM. With chymotrypsin (a model serine protease) Ki final = 100 pM, so as expected binding of alpha 2-antiplasmin is weaker with chymotrypsin. However, analysis of the individual rate constants shows that the difference in strength of binding is accounted for by the dissociation rate constant for the second step (k-2) = 1.9 x 10(-6) s-1 for plasmin and 1.1 x 10(-4) s-1 for chymotrypsin. Thus k-2, the rate constant previously ignored, explains the different affinities of alpha 2-antiplasmin for these two enzymes. Furthermore, this model of two (or more) step, reversible binding is accepted for protease inhibitors of other families. With one of these, aprotinin (a Kunitz inhibitor) with plasmin we also obtain a two-stage reversible mechanism with a Ki final = 200 pM and the strength of inhibition is also largely determined by k-2 = 3.5 x 10(-5) s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of plasmin activation of single chain urinary-type plasminogen activator (scu-PA) and demonstration of a high affinity interaction between scu-PA and plasminogen

The activation kinetics of single chain urinary-type plasminogen activator (scu-PA) by plasmin have been studied in detail. Nonstandard Michaelis-Menten kinetics were observed. To explain our results, we propose a model in which plasmin can exist in two conformations of lower activity (kcat/Km = 1.4 x 10(6) M-1 s-1) or higher activity (kcat/Km = 16.7 x 10(6) M-1 s-1) depending on whether a lysine binding site is occupied or free, respectively. These kinetic studies demonstrate that scu-PA interacts at this binding site (KD approximately 30 nM) and so is able to act as both a substrate and effector in this reaction. Binding was also demonstrated between scu-PA and Glu- or Lys-plasminogen at a high affinity site (KD approximately 65 nM), sensitive to the presence of lysine analogs. This suggests that scu-PA may be almost completely bound to plasminogen in plasma under normal physiological conditions and provides a possible explanation for the fibrin specificity of this activator, as discussed.

Fourier transform infrared studies of active-site-methylated rhodopsin. Implications for chromophore-protein interaction, transducin activation, and the reaction pathway

Fourier transform infrared studies of active-site-methylated rhodopsin (ASMR) show that, as compared to unmodified rhodopsin, the photoreaction is almost unchanged up to the formation of lumirhodopsin. Especially, the deviations are much smaller than those observed for the corresponding intermediates of 13-desmethyl-rhodopsin. In metarhodopsin-I, larger alterations are present with respect to the three internal carboxyl groups. Similar deviations have been observed in meta-I of 13-desmethyl-rhodopsin. This indicates that, in agreement with our previous investigations, these carboxyl groups are located in close proximity to the chromophore. Because this latter pigment is capable, when bleached, of activating transducin, our data provide support for the earlier conclusion that deprotonation of the Schiff base is a prerequisite for transducin activation. The positions of the C = C and C - C stretching modes of the retinal suggest that the redshift observed in ASMR and its photoproducts can be explained by an increased distance of the Schiff base from the counterion(s). It is further shown that the photoreaction does not stop at metarhodopsin-I, but that this intermediate directly decays to a metarhodopsin-III-like species.

Serpin-serine protease binding kinetics: alpha 2-antiplasmin as a model inhibitor

We have examined in detail the kinetics of binding of the serpin alpha 2-antiplasmin to the serine proteases alpha-chymotrypsin and plasmin. These represent model systems for serpin binding. We find, in contrast to earlier published results with alpha 2-antiplasmin and plasmin, that binding is reversible, and slow binding kinetics can be observed, under appropriate conditions. Binding follows a two-step process with both enzymes, with the formation of an initial loose complex which then proceeds to a tightly bound complex. In the absence of lysine and analogues, equilibrium between alpha 2-antiplasmin and plasmin is achieved rapidly, with an overall inhibition constant (Ki') of 0.3 pM. In the presence of tranexamic acid or 6-aminohexanoic acid, lysine analogues that mimic the effects of fibrin, plasmin binding kinetics are changed such that equilibrium is reached slowly following a lag phase after mixing of enzyme and inhibitor. The Ki' is also affected, rising to 2 pM in the presence of 6-aminohexanoic acid concentrations above 15 mM. Thus extrapolation to the in vivo situation indicates that complex formation in the presence of fibrin will be delayed, allowing a burst of enzyme activity following plasmin generation, but a tight, pseudoirreversible complex will result eventually. Chymotrypsin is more weakly inhibited by alpha 2-antiplasmin, exhibiting an overall Ki' of 0.1 nM, after two-stage complex formation. The inhibition constant for the initial loose complex (Ki) is very similar for both enzymes. The difference in binding strength between the two enzymes is accounted for by the dissociation rate constant of the second step of complex formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Recombinant chymotrypsin inhibitor 2: expression, kinetic analysis of inhibition with alpha-chymotrypsin and wild-type and mutant subtilisin BPN', and protein engineering to investigate inhibitory specificity and mechanism

The serine protease inhibitor chymotrypsin inhibitor 2 (CI2 or BSPI2) has been expressed in Escherichia coli with the pINIIIompA3 expression vector to produce 20-40 mg/L of culture. Recombinant CI2 purified from this system has been characterized and found to be identical with CI2 from barley. Slow-binding kinetics were observed for the interaction between CI2 and subtilisin BPN', with Ki = 2.9 x 10(-12) M. Analysis of slow-binding data indicates that binding of the inhibitor follows the simplest model of E + I = EI with no kinetically detectable intermediate steps or proteolytic cleavage of the reactive site bond in CI2 (Met-59-Glu-60). This, in agreement with crystallographic data, indicates that the enzyme-inhibitor adduct is the Michaelis complex, which is not chemically processed by the enzyme. Three mutant CI2 molecules with new P1 residues have also been examined with a range of serine proteases, including a mutant subtilisin. In agreement with earlier studies, we find the P1 amino acid an important determinant of specificity. CI2 Met----Lys-59 was found to be a temporary inhibitor of subtilisin BPN' but an effective inhibitor of subtilisin Carlsberg and subtilisin BPN'(Glu----Ser-156). The structural reasons for this are discussed in relation to mechanisms of inhibition of serine proteases.

The photoreaction of active-site-methylated bacteriorhodopsin: an investigation using static and time-resolved infrared difference spectroscopy

The photoreaction of active-site-methylated, permethylated bacteriorhodopsin has been investigated by static and time-resolved UV-vis and infrared difference spectroscopy. Additional information on the isomeric composition of the initial state and of photoproducts was obtained by retinal extraction and subsequent HPLC analysis. The data show that the dark-adapted state contains only all-trans-retinal. Prolonged illumination produces a metastable state which contains essentially only 9-cis-retinal and which decays back to the dark-adapted initial state within 8 h. The time-resolved infrared difference spectra clearly demonstrate that laser flash excitation produces an intermediate that has all the characteristics of the L intermediate. It is demonstrated that the methyl group at the Schiff base nitrogen introduces a steric hindrance with the protein which inhibits a photoreaction at 80 K, but which allows the generation of an L-like intermediate at room temperature and 173 K.

Photochemistry of monomethylated and permethylated bacteriorhodopsin

Methylation of the nonactive site lysines of bacteriorhodopsin to form permethylated bacteriorhodopsin does not interfere with the formation of the short wavelength intermediate M412 or light-induced proton release/uptake. The absorption spectrum is similar to that of the native bacteriorhodopsin. However, additional monomethylation of the active site lysine of bacteriorhodopsin causes a red shift of the absorption maximum from 568 nm in light-adapted bacteriorhodopsin [BR] to 630 nm. The photochemistry of active-site methylated BR does not proceed beyond the L-photointermediate. In particular, the photointermediate corresponding to M412 does not form, and there is no proton pumping. Moreover, there is no tyrosine deprotonation. Thus, the formation of an M-type photointermediate is required for proton pumping by BR.