Inactivation of factor Va by plasmin

Trauma-induced coagulopathy: What you need to know

ABSTRACT

Trauma-induced coagulopathy (TIC) is a global inflammatory state accompanied by coagulation derangements, acidemia, and hypothermia, which occurs after traumatic injury. It occurs in approximately 25% of severely injured patients, and its incidence is directly related to injury severity. The mechanism of TIC is multifaceted; proposed contributing factors include dysregulation of activated protein C, increased tPA, systemic endothelial activation, decreased fibrinogen, clotting factor consumption, and platelet dysfunction. Effects of TIC include systemic inflammation, coagulation derangements, acidemia, and hypothermia. Trauma-induced coagulopathy may be diagnosed by conventional coagulation tests including platelet count, Clauss assay, international normalized ratio, thrombin time, prothrombin time, and activated partial thromboplastin time; viscoelastic hemostatic assays such as thrombelastography and rotational thrombelastography; or a clinical scoring system known as the Trauma Induced Coagulopathy Clinical Score. Preventing TIC begins in the prehospital phase with early hemorrhage control, blood product resuscitation, and tranexamic acid therapy. Early administration of prothrombin complex concentrate is also being studied in the prehospital environment. The mainstays of TIC treatment include hemorrhage control, blood and component transfusions, and correction of abnormalities such as hypocalcemia, acidosis, and hypothermia.

LEVEL OF EVIDENCE

Therapeutic/Care Management; Level III.

Spray-dried plasma: A post-traumatic blood "bridge" for life-saving resuscitation

Massive bleeding remains a major source of morbidity and mortality worldwide. Recent studies have shed light on the pathophysiology of traumatic-induced coagulopathy and the central role of endotheliopathy. Transfusion therapy has changed dramatically in the last decade with use of red cells and plasma in a 1:1 ratio. The use of early transfusion increases the likelihood of a favorable outcome. Early intervention-preferably less than 60 min of injury-is a major factor in improved survival. Experience with dried plasma products-lyophilized or freeze-dried-in Europe and South Africa has demonstrated both safety and efficacy. Dry plasma products are not available in the United States but several products are in development. Spray-dried plasma contains clinically meaningful levels of coagulation activity and in vitro data suggest robust ability to generate thrombus. The decentralized, blood-center based manufacturing model of spray-dried plasma offers advantages for availability to meet routine and extraordinary demands.

ptFVa (Pseudonaja Textilis Venom-Derived Factor Va) Retains Structural Integrity Following Proteolysis by Activated Protein C

Supplemental Digital Content is available in the text.

Objective:

The Australian snake venom ptFV (Pseudonaja textilis venom-derived factor V) variant retains cofactor function despite APC (activated protein C)-dependent proteolysis. Here, we aimed to unravel the mechanistic principles by determining the role of the absent Arg306 cleavage site that is required for the inactivation of FVa (mammalian factor Va).

Approach and Results:

Our findings show that in contrast to human FVa, APC-catalyzed proteolysis of ptFVa at Arg306 and Lys507 does not abrogate ptFVa cofactor function. Remarkably, the structural integrity of APC-proteolyzed ptFVa is maintained indicating that stable noncovalent interactions prevent A2-domain dissociation. Using Molecular Dynamics simulations, we uncovered key regions located in the A1 and A2 domain that may be at the basis of this remarkable characteristic.

Conclusions:

Taken together, we report a completely novel role for uniquely adapted regions in ptFVa that prevent A2 domain dissociation. As such, these results challenge our current understanding by which strict regulatory mechanisms control FVa activity.

Thrombin and plasmin generation in patients with plasminogen or plasminogen activator inhibitor type 1 deficiency

INTRODUCTION

Deficiencies of plasminogen and plasminogen activator inhibitor type 1 (PAI-1) are rare disorders of fibrinolysis. Current laboratory assays for analysis of activity of plasminogen and PAI-1 do not provide an accurate correlation with clinical phenotype.

METHODS

The Nijmegen Hemostasis Assay (NHA) was used to simultaneously measure thrombin and plasmin generation in 5 patients with plasminogen deficiency (PLGD) and 10 patients with complete PAI-1 deficiency. Parameters analysed included: lag time ratio, thrombin peak time ratio, thrombin peak height, thrombin potential (AUC), fibrin lysis time, plasmin peak height and plasmin potential. Parameters were expressed as a percentage compared to a reference value of 53 healthy normal controls.

RESULTS

Patients with PLGD demonstrated a short lag time and thrombin peak time, with normal thrombin peak height but an increased AUC. Plasmin generation was able to be detected in only one (23% plasminogen activity) of the five PLGD patients. All ten PAI-1 deficient patients demonstrated a short lag and thrombin peak time, low thrombin peak height with normal AUC. Plasmin generation revealed an increased plasmin peak and plasmin potential; interestingly, there was a large variation between individual patients despite all patients having the same homozygous defect.

CONCLUSION

Patients with either PLGD or PAI-1 deficiency show distinct abnormalities in plasmin and thrombin generation in the NHA. The differences observed in the propagation phase of thrombin generation may be explained by plasmin generation. These results suggest that disorders of fibrinolysis also influence coagulation and a global assay measuring both activities may better correlate with clinical outcome.

Procoagulant and fibrinolytic activity after polytrauma in rat

The purpose of this study was to determine whether trauma-induced coagulopathy is due to changes in 1) thrombin activity, 2) plasmin activity, and/or 3) factors that stimulate or inhibit thrombin or plasmin. Sprague-Dawley rats were anesthetized with 1-2% isoflurane/100% oxygen, and their left femoral artery and vein were cannulated. Polytrauma included right femur fracture, and damage to the small intestines, the left and medial liver lobes, and right leg skeletal muscle. Rats were then bled 40% of blood volume. Plasma samples were taken before trauma, and at 30, 60, 120, and 240 min. Polytrauma and hemorrhage led to a significant fall in prothrombin levels. However, circulating thrombin activity did not change significantly over time. Antithrombin III and α2 macroglobulin fell significantly by 2 h, then rose by 4 h. Soluble thrombomodulin was significantly elevated over the 4 h. Circulating plasmin activity, plasminogen, and D-dimers were elevated for the entire 4 h. Tissue plasminogen activator (tPA) was elevated at 30 min, then decreased below baseline levels after 1 h. Plasminogen activator inhibitor-1 was significantly elevated at 2-4 h. Neither tissue factor pathway inhibitor nor thrombin activatable fibrinolysis inhibitor changed significantly over time. The levels of prothrombin and plasminogen were 30-100 times higher than their respective active enzymes. Polytrauma and hemorrhage in rats lead to a fibrinolytic coagulopathy, as demonstrated by an elevation in plasmin activity, D-dimers, and tPA. These results are consistent with the observed clinical benefit of tranexamic acid in trauma patients.

The pathogenesis of traumatic coagulopathy

Over the last 10 years, the management of major haemorrhage in trauma patients has changed radically. This is mainly due to the recognition that many patients who are bleeding when they come in to the emergency department have an established coagulopathy before the haemodilution effects of fluid resuscitation. This has led to the use of new terminology: acute traumatic coagulopathy, acute coagulopathy of trauma shock or trauma-induced coagulopathy. The recognition of acute traumatic coagulopathy is important, because we now understand that its presence is a prognostic indicator, as it is associated with poor clinical outcome. This has driven a change in clinical management, so that the previous approach of maintaining an adequate circulating volume and oxygen carrying capacity before, as a secondary event, dealing with coagulopathy, has changed to haemostatic resuscitation as early as possible. While there is as yet no universally accepted assay or definition, many experts use prolongation of the prothrombin time to indicate that there is, indeed, a coagulopathy. Hypoxia, acidosis and hypothermia and hormonal, immunological and cytokine production, alongside consumption and blood loss, and the dilutional effects of resuscitation may occur to varying extents depending on the type of tissue damaged, the type and extent of injury, predisposing to, or amplifying, activation of coagulation, platelets, fibrinolysis. These are discussed in detail within the article.

Fibrinolysis and the control of blood coagulation

Fibrin plays an essential role in hemostasis as both the primary product of the coagulation cascade and the ultimate substrate for fibrinolysis. Fibrinolysis efficiency is greatly influenced by clot structure, fibrinogen isoforms and polymorphisms, the rate of thrombin generation, the reactivity of thrombus-associated cells such as platelets, and the overall biochemical environment. Regulation of the fibrinolytic system, like that of the coagulation cascade, is accomplished by a wide array of cofactors, receptors, and inhibitors. Fibrinolytic activity can be generated either on the surface of a fibrin-containing thrombus, or on cells that express profibrinolytic receptors. In a widening spectrum of clinical disorders, acquired and congenital defects in fibrinolysis contribute to disease morbidity, and new assays of global fibrinolysis now have potential predictive value in multiple clinical settings. Here, we summarize the basic elements of the fibrinolytic system, points of interaction with the coagulation pathway, and some recent clinical advances.

Failure of recombinant factor VIIa to correct the coagulopathy in a case of severe postpartum hemorrhage

BACKGROUND

Postpartum hemorrhage (PPH)remains an important cause of maternal morbidity and mortality. Several published reports suggest that recombinant factor VIIa (rFVIIa) is effective in controlling bleeding in PPH. This study reports a case of severe PPH complicated by disseminated intravascular coagulation(DIC), in which early rFVIIa (44 mg/kg) administration not only failed to control the bleeding in vivo but also, surprisingly, failed to correct the patient's international normalized ratio (INR) in vitro. It was hypothesized that the failure of rFVIIa to correct the INR indicated a deficiency in a downstream coagulation factor(s). To investigate this, coagulation factor levels were measured in blood samples that had been drawn periodically during resuscitation in the operating room.

STUDY DESIGN AND METHODS

Clinical and laboratory data were extracted from the medical record.Plasma samples that had been obtained during resuscitation were frozen, and activity levels of the following factors were subsequently measured: fibrinogen, FII, FV, FVII, F IX, and FX.

RESULTS

After rFVIIa administration, the patient's INR remained elevated at 1.9, and bleeding continued. It was determined that at the time rFVIIa was administered, the patient's fibrinogen level was very low(60 mg/dL). INR normalization and control of bleeding was achieved only after the patient's fibrinogen level was restored to normal. FII, F IX, and FX remained at hemostatic levels throughout resuscitation.

CONCLUSIONS

In this case of severe PPH complicated by DIC, fibrinogen appears to have been limiting at the time rFVIIa was administered. It is suggested that fibrinogen levels should be corrected during PPH resuscitation before rFVIIa use is considered.

Identification of plasmin-interactive sites in the light chain of factor VIII responsible for proteolytic cleavage at Lys36

We have recently reported that plasmin likely associates with the factor VIII light chain to proteolyze at Lys36 within the A1 domain. In this study, we determined that the rate of plasmin-catalyzed inactivation on the forms of factor VIIIa containing A1-(1-336) and 1722A3C1C2, reflecting Lys36 cleavage, was reduced by approximately 60%, compared with those containing 1649A3C1C2 and 1690A3C1C2. SDS-PAGE analysis revealed that Lys36 cleavage of factor VIIIa with 1722A3C1C2 was markedly slower than those with 1649A3C1C2 and 1690A3C1C2. Surface plasmon resonance-based assays, using active site-modified anhydro-plasmin (Ah-plasmin) showed that 1722A3C1C2 bound to Ah-plasmin with an approximately 3-fold lower affinity than 1649A3C1C2 or 1690A3C1C2 (Kd, 176, 68.2, and 60.3 nM, respectively). Recombinant A3 bound to Ah-plasmin (Kd, 44.2 nM), whereas C2 failed to bind, confirming the presence of a plasmin-binding site within N terminus of A3. Furthermore, the Glu-Gly-Arg active site-modified factor IXa also blocked 1722A3C1C2 binding to Ah-plasmin by approximately 95%, supporting the presence of another plasmin-binding site overlapping the factor IXa-binding site in A3. In keeping with a major contribution of the lysine-binding sites in plasmin for interaction with the factor VIII light chain, analysis of the A3 sequence revealed two regions involving clustered lysine residues in 1690-1705 and 1804-1818. Two peptides based on these regions blocked 1649A3C1C2 binding to Ah-plasmin by approximately 60% and plasmin-catalyzed Lys36 cleavage of factor VIIIa with A1-(1-336) by approximately 80%. Our findings indicate that an extended surface, centered on residues 1690-1705 and 1804-1818 within the A3 domain, contributes to a unique plasmin-interactive site that promotes plasmin docking during cofactor inactivation by cleavage at Lys36.

Mechanisms of Plasmin-catalyzed Inactivation of Factor VIII

Plasmin not only functions as a key enzyme in the fibrinolytic system but also directly inactivates factor VIII and other clotting factors such as factor V. However, the mechanisms of plasmin-catalyzed factor VIII inactivation are poorly understood. In this study, levels of factor VIII activity increased approximately 2-fold within 3 min in the presence of plasmin, and subsequently decreased to undetectable levels within 45 min. This time-dependent reaction was not affected by von Willebrand factor and phospholipid. The rate constant of plasmin-catalyzed factor VIIIa inactivation was approximately 12- and approximately 3.7-fold greater than those mediated by factor Xa and activated protein C, respectively. SDS-PAGE analysis showed that plasmin cleaved the heavy chain of factor VIII into two terminal products, A1(37-336) and A2 subunits, by limited proteolysis at Lys(36), Arg(336), Arg(372), and Arg(740). The 80-kDa light chain was converted into a 67-kDa subunit by cleavage at Arg(1689) and Arg(1721), identical to the pattern induced by factor Xa. Plasmin-catalyzed cleavage at Arg(336) proceeded faster than that at Arg(372), in contrast to proteolysis by factor Xa. Furthermore, breakdown was faster than that in the presence of activated protein C, consistent with rapid inactivation of factor VIII. The cleavages at Arg(336) and Lys(36) occurred rapidly in the presence of A2 and A3-C1-C2 subunits, respectively. These results strongly indicated that cleavage at Arg(336) was a central mechanism of plasmin-catalyzed factor VIII inactivation. Furthermore, the cleavages at Arg(336) and Lys(36) appeared to be selectively regulated by the A2 and A3-C1-C2 domains, respectively, interacting with plasmin.

Coagulation factor V: a plethora of anticoagulant molecules

PURPOSE OF REVIEW

Thrombin is necessary for survival and is produced after activation of prothrombin by prothrombinase at the site of a vascular injury. While the enzyme component of prothrombinase alone, factor Xa, bound to a membrane surface can activate prothrombin, incorporation of the cofactor molecule, factor Va, into prothrombinase results in a five orders of magnitude increase in the catalytic efficiency of factor Xa that provides the physiologic pathway for thrombin generation. While the kinetic constants and the identity of peptide bonds cleaved in prothrombin to generate alpha-thrombin have been long established, the peptidyl portions of the factor Va molecule responsible for its interactions with factor Xa, prothrombin, and the lipid surface are still the subject of intense investigation. In this review, we summarize the current state of knowledge with respect to the interactions of the factor Va molecule with the various components of prothrombinase.

RECENT FINDINGS

Binding sites for factor Xa have been identified on both the heavy and light chains of factor Va. Two amino acid regions that interact with factor Xa have been delineated on the heavy chain of the cofactor. It has also been demonstrated that the carboxyl-terminal portion of the heavy chain of factor Va contains hirudin-like motifs and appears to be responsible for the interaction of factor Va with prothrombin. This region of the molecule is important for procofactor activation by thrombin as well as cofactor function. Finally, the membrane-binding site of factor Va is contributed by several elements of the light chain and involves both electrostatic and hydrophobic interactions.

SUMMARY

The absence or dysfunction of factor Va leads to hemorrhagic diseases while prolonged existence of the active cofactor species is associated with thrombosis. Thus, modulation of the incorporation of factor Va into prothrombinase in vivo by using synthetic peptides that have the potential to impair factor Va binding to any of the components of prothrombinase, will allow for control of the rate of thrombin generation at the site of vascular damage. As a consequence, a systematic definition of the regions of factor Va governing its incorporation within prothrombinase will provide the scaffold for the synthesis of potent anticoagulant molecules that could modulate thrombin formation and suppress excessive clotting in thrombotic individuals.

Structure-inhibitory activity relationship of plasmin and plasma kallikrein inhibitors

Based on the structure of Tra-Tyr(O-Pic)-octylamide, a portion of the octylamine was replaced with moieties bearing hydrophobic, basic or acidic groups. Replacement of the C-terminal residue with a moiety bearing a hydrophobic group gave the proper affinity of the inhibitor to both plasmin (PL) and plasma kallikrein (PK). While addition of a basic residue did not improve the affinity of the inhibitor, a carboxylic acid attached to the phenyl ring increased the PK selectivity of the inhibitor.

Mechanism of factor Va inactivation by plasmin. Loss of A2 and A3 domains from a Ca2+-dependent complex of fragments bound to phospholipid

The coagulation cofactor Va (FVa) is a noncovalent heterodimer consisting of a heavy chain (FVaH) and a light chain (FVaL). Previously, the fibrinolytic effector plasmin (Pn) has been shown to inhibit FVa function. To understand this mechanism, the fragmentation profile of human FVa by Pn and the noncovalent association of the derived fragments were determined in the presence of Ca(2+) using anionic phospholipid (aPL)-coated microtiter wells and large (1 microm) aPL micelles as affinity matrices. Following Pn inactivation of aPL-bound FVa, a total of 16 fragments were observed and their NH(2) termini sequenced. These had apparent molecular weights and starting residues as follows (single letter abbreviation is used): 50(L1766), 48(L1766), 43(Q1828), 40(Q1828), 30(S1546), 12(T1657), and 7(S1546) kDa from FVaL; and 65(A1), 50(A1), 45(A1), 34(S349), 30(L94), 30(M110), and 3 small <5(W457, W457, and K365) kDa from FVaH. Of these, 50(L1766), 48(1766), 43(Q1828), and 40(Q1828) spanning the C1/C2 domains, and 30(L94), but not the similar 30(M110), positioned within the A1 domain remained associated with aPL. These were detected antigenically during Pn- or tissue plasminogen activator-mediated lysis of fibrin clot formed in plasma. Chelation by EDTA dissociated the 30(L94)-kDa fragment, which was observed to associate with intact FVaL upon recalcification, indicating that the Leu-94 to Lys-109 region of the A1 domain plays a critical role in the FVaL and FVaH Ca(2+)-dependent association. By using domain-specific monoclonal antibodies and an assay for thrombin generation, loss of FVa prothrombinase function was coincident with proteolysis at sites in the A2 and A3 domains resulting in their dissociation. Inactivation of FV or FVa by Pn was independent of the thrombophilic R506Q mutation. These results identify the molecular composition of Pn-cleaved FVa that remains bound to membrane as largely A1-C1/C2 in the presence of Ca(2+) and suggest that Pn inhibits FVa by a process involving A2 and A3 domain dissociation.

The role of the membrane in the inactivation of factor va by plasmin. Amino acid region 307-348 of factor V plays a critical role in factor Va cofactor function

The mechanism of inactivation of bovine factor Va by plasmin was studied in the presence and absence of phospholipid vesicles (PCPS vesicles). Following 60-min incubation with plasmin (4 nm) membrane-bound factor Va (400 nm) is completely inactive, whereas in the absence of phospholipid vesicles following a 1-h incubation period, the cofactor retains 90% of its initial cofactor activity. Amino acid sequencing of the fragments deriving from cleavage of factor Va by plasmin demonstrated that while both chains of factor Va are cleaved by plasmin, only cleavage of the heavy chain correlates with inactivation of the cofactor. In the presence of a membrane surface the heavy chain of the bovine cofactor is first cleaved at Arg(348) to generate a fragment of M(r) 47,000 containing the NH(2)-terminal part of the cofactor (amino acid residues 1-348) and a M(r) 42,000 fragment (amino acid residues 349-713). This cleavage is associated with minimal loss in cofactor activity. Complete loss of activity of the membrane-bound cofactor coincides with three cleavages at the COOH-terminal portion of the M(r) 47,000 fragment: Lys(309), Lys(310), and Arg(313). These cleavages result in the release of the COOH terminus of the molecule and the production of a M(r) 40,000 fragment containing the NH(2)-terminal portion of the factor Va molecule. Factor Va was treated with plasmin in the absence of phospholipid vesicles followed by the addition of PCPS vesicles and activated protein C (APC). A rapid inactivation of the cofactor was observed as a result of cleavage of the M(r) 47,000 fragment at Arg(306) by APC and appearance of a M(r) 39,000 fragment. These data suggest a critical role of the amino acid sequence 307-348 of factor Va. A 42-amino acid peptide encompassing the region 307-348 of human factor Va (N42R) was found to be a good inhibitor of factor Va clotting activity with an IC(50) of approximately 1.3 microm. These data suggest that plasmin is a potent inactivator of factor Va and that region 307-348 of the cofactor plays a critical role in cofactor function and may be responsible for the interaction of the cofactor with factor Xa and/or prothrombin.

The action of Lonomia achelous caterpillars venom on human factor V

The bleeding syndrome produced by contact with the Lonomia achelous caterpillars is characterized by a decrease of fibrinogen, factor XIII, plasminogen, and factor V with normal platelets. In this study, we report the effect of crude hemolymph and some semipurified chromatographic fractions on human factor V. Incubation of factor V with crude hemolymph resulted in an increase in procoagulant activity, followed by a subsequent decline in factor V activity. Identical results were obtained with fraction I, whereas with fraction II there was only a decrease in activity reaching its minimum at 30 minutes. fraction III did not modify the activity of factor V. All concentrations of fraction I tested produced an initial rise and subsequent fall in activity. However, at lower relative concentrations of fraction I, more sustained increases in activity were observed. The activator and inactivator activities present in fraction I show differences in temperature and pH stability, susceptibility to different inhibitors, and in SDS/PAGE pattern. The factor V activator is a thermostable protein, with maximum activity at acid pH and is inhibited by o-phenantroline, EDTA, and EGTA, while the factor V inactivator is thermolabile, presents maximum activity at basic pH, precipitates at pH 5.0, and is completely inhibited by iodoacetic acid and TLCK. It is partially blocked by diisopropyl fluorophosphate, phenylmethylsulfonyl fluoride, and p-chloromercuribenzoic acid. These results suggest that the activator should be a metallo-proteinase, while the inactivator is a serine or cysteine proteinase with a serine, histidine, or cysteine residue in the active site.

Synthesis of positional-scanning libraries of fluorogenic peptide substrates to define the extended substrate specificity of plasmin and thrombin

We have developed a strategy for the synthesis of positional-scanning synthetic combinatorial libraries (PS-SCL) that does not depend on the identity of the P1 substituent. To demonstrate the strategy, we synthesized a tetrapeptide positional library in which the P1 amino acid is held constant as a lysine and the P4-P3-P2 positions are positionally randomized. The 6,859 members of the library were synthesized on solid support with an alkane sulfonamide linker, and then displaced from the solid support by condensation with a fluorogenic 7-amino-4-methylcoumarin-derivatized lysine. This library was used to determine the extended substrate specificities of two trypsin-like enzymes, plasmin and thrombin, which are involved in the blood coagulation pathway. The optimal P4 to P2 substrate specificity for plasmin was P4-Lys/Nle (norleucine)/Val/Ile/Phe, P3-Xaa, and P2-Tyr/Phe/Trp. This cleavage sequence has recently been identified in some of plasmin's physiological substrates. The optimal P4 to P2 extended substrate sequence determined for thrombin was P4-Nle/Leu/Ile/Phe/Val, P3-Xaa, and P2-Pro, a sequence found in many of the physiological substrates of thrombin. Single-substrate kinetic analysis of plasmin and thrombin was used to validate the substrate preferences resulting from the PS-SCL. By three-dimensional structural modeling of the substrates into the active sites of plasmin and thrombin, we identified potential determinants of the defined substrate specificity. This method is amenable to the incorporation of diverse substituents at the P1 position for exploring molecular recognition elements in proteolytic enzymes.

Plasmin converts factor X from coagulation zymogen to fibrinolysis cofactor

Known anticoagulant pathways have been shown to exclusively inhibit blood coagulation cofactors and enzymes. In the current work, we first investigated the possibility of a novel anticoagulant mechanism that functions at the level of zymogen inactivation. Utilizing both clotting and chromogenic assays, the fibrinolysis protease plasmin was found to irreversibly inhibit the pivotal function of factor X (FX) in coagulation. This was due to cleavage at several sites, the location of which were altered by association of FX with procoagulant phospholipid (proPL). The final products were approximately 28 and approximately 47 kDa for proPL-bound and unbound FX, respectively, which did not have analogues when activated FX (FXa) was cleaved instead. We next investigated whether the FX derivatives could interact with the plasmin precursor plasminogen, and we found that plasmin exposed a binding site only on proPL-bound FX. The highest apparent affinity was for the 28-kDa fragment, which was identified as the light subunit disulfide linked to a small fragment of the heavy subunit (Met-296 to approximately Lys-330). After cleavage by plasmin, proPL-bound FX furthermore was observed to accelerate plasmin generation by tissue plasminogen activator. Thus, a feedback mechanism localized by proPL is suggested in which plasmin simultaneously inhibits FX clotting function and converts proPL-bound FX into a fibrinolysis cofactor. These data also provide the first evidence for an anticoagulant mechanism aimed directly at the zymogen FX.

Relation of coagulation parameters to patency and recurrent ischemia in the Thrombolysis in Myocardial Infarction (TIMI) Phase II Trial

Current protocols for use of tissue-type plasminogen activator in acute myocardial infarction include heparin estimated by the activated partial thromboplastin time (aPTT). Recent reports indicate a risk of recurrent ischemic events with long aPTT values. Longer aPTT values in the Thrombolysis in Myocardial Infarction-II (TIMI II) Trial, obtained within the first 48 hours, were associated with patency at 18 to 48 hours and better left ventricular function at discharge (average 9.6 days), but also with emergency catheterizations within the first 48 hours and, weakly, with recurrent ischemia during the first 18 hours. A moderate decrease in fibrinogen, compared with a "small" decrease, was also associated with patency, but a "large" decrease was associated with hemorrhagic events. Patency was associated with higher fibrinogen values and higher plasminogen values at baseline. The aPTT results support frequent monitoring during the first 24 to 48 hours to ensure optimal clinical outcome. The coagulation factor results suggest that there may be an optimum window for fibrinogenolysis in this setting.

Thrombolytic therapy and proteolysis of factor V

OBJECTIVES

We sought to determine the extent of Factor V proteolysis during thrombolytic therapy.

BACKGROUND

Thrombin- or Factor Xa-activated Factor V is an essential cofactor in the prothrombinase complex. In purified systems, plasmin, the major product of thrombolytic therapy, is known to first activate then inactivate Factor V.

METHODS

We used quantitative gel electrophoresis and Western blotting to analyze the cleavages in plasma Factor V after thrombolytic therapy.

RESULTS

The addition of streptokinase to plasma resulted in the activation then inactivation of Factor V, confirming previous results using purified reagents. We also identified the Factor V fragments resulting from the action of thrombin and plasmin. After thrombolytic therapy, there was considerable Factor V cleavage. The cleavage patterns were consistent with the action of plasmin, with little evidence for the action of thrombin. In the Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries trial (n = 17), we observed an average 58% loss of intact Factor V at 6 h (range 1% to 91%). Samples from the Thrombolysis in Myocardial Infarction trial, Phase II (n = 12), collected on a shorter time scale, showed a loss of up to 99% at 50 min, with the loss of intact Factor V associated with the plasma concentration of plasminogen activator. Samples from patients with bleeding (n = 12) showed extensive Factor V cleavage.

CONCLUSIONS

Factor V cleavage in thrombolytic therapy is primarily plasmin mediated, rapid and often extensive. It is likely that transient increases, as well as longer term losses, of Factor V cofactor activity play a role in both ischemic and hemorrhagic events subsequent to thrombolytic therapy. The extensive loss of Factor V in some patients may affect the estimation of heparinization.

Cleavage requirements for activation of factor V by factor Xa

Coagulation factor V circulates in plasma as a single chain protein which expresses little procoagulant activity. After its activation by limited proteolysis by thrombin or factor Xa, factor Va functions as cofactor to factor Xa in the activation of prothrombin. Thrombin cleaves human factor V at Arg709, Arg1018 and Arg1545 and factor Va is formed by the heavy and light chains, which correspond to the N-terminal and C-terminal fragments, respectively. Factor Xa has been shown to cleave factor V at Arg1018 and at a second undefined position close to Arg709. The factor-Xa-mediated cleavage at Arg1018 has been proposed to be sufficient for expression of full factor Va activity. To study the activation of factor V by factor Xa, site-directed mutagenesis was used to convert Arg709 to Gln, Arg1018 to Ile, and Arg1545 to Gln. Constructs containing all possible combinations of native and mutated residues in these positions were expressed transiently in COS 1 cells. The various factor-V mutants were incubated with factor Xa or thrombin. The proteolytic cleavage pattern was analyzed by Western blotting, and the specific factor-Va activities determined in a prothrombinase assay. Control experiments using thrombin gave results which were in agreement with those on record, i.e. cleavages at both Arg709 and Arg1545 were required for expression of full factor-Va activity, whereas the cleavage at Arg1018 enhanced the rate of cleavage at Arg1545. Factor Xa was found to cleave factor V at all three thrombin cleavage sites, i.e. at Arg709, Arg1018 and Arg1545. An additional factor-Xa-cleavage site was found in the light chain region at Arg1765. Cleavage at Arg1018 by factor Xa was not sufficient for expression of full factor-Va activity. Full factor-Va activity was only obtained after cleavage at both Arg709 and Arg1545. The factor-Xa-mediated cleavage at Arg709 was kinetically favourable over that at Arg1545. Factor V which was mutated at all three sites (at positions 709, 1018 and 1545) was resistant to activation by thrombin. However, treatment with factor Xa yielded an increased factor-Va activity which was associated with the cleavage at Arg1765. Our study extends previously results on thrombin activation of factor V and elucidates the relative importance of the different cleavage sites for activation of factor V by factor Xa.

Autoproteolysis or plasmin-mediated cleavage of factor Xaalpha exposes a plasminogen binding site and inhibits coagulation

Blood coagulation factor Xa (FXa) has recently been shown to function as a plasminogen receptor in the presence of procoagulant phospholipid (phosphatidylserine; PS) and Ca2+. In the current work, the possible effect of autoproteolytic and plasmin-mediated cleavage of FXa on complex formation with plasminogen was investigated. 125I-plasminogen binding to derivatives of FXa electrotransferred to polyvinylidene difluoride revealed that the autoproteolytic conversion of FXaalpha to FXabeta was required for the expression of a plasminogen binding site. In the presence of PS and Ca2+, plasmin was shown to convert FXaalpha to a FXabeta-like species at least 3 orders of magnitude faster than the autoproteolytic mechanism. This also resulted in the exposure of a plasminogen binding site. Further processing by plasmin generated a fragment (33 kDa) due to cleavage at Gly331 in the FXa heavy chain. Production of this species enhanced apparent plasminogen binding compared with FXabeta and resulted in the loss of FXa amidolytic and clotting activity. In the absence of either PS or Ca2+, the plasmin-mediated fragmentation of FXaalpha was altered to include a FXabeta-like molecule and a species (40 kDa) with intact beta-heavy chain disulfide linked to a COOH-terminal fragment of the light chain starting at Tyr44. Neither of these products was observed to interact with plasminogen. The 40-kDa species had amidolytic activity comparable with FXaalpha but inhibited clotting activity. Cumulatively the data provide the first evidence for a functional difference between the FXa subforms and suggest a mechanism where autoproteolysis and plasmin-mediated cleavage modulate the function of FXaalpha from a procoagulant enzyme to a profibrinolytic plasminogen receptor.

Kinetics of blood coagulation factor Xaalpha autoproteolytic conversion to factor Xabeta. Effect on inhibition by antithrombin, prothrombinase assembly, and enzyme activity

Autoproteolysis of blood coagulation factor Xa (FXa) results in the excision of a 4-kDa fragment (beta-peptide) from the intact subform, factor Xaalpha (FXaalpha), to yield factor Xabeta (FXabeta). In the preceding paper, we showed that generation of FXabeta leads to expression of a plasminogen binding site. FXabeta may consequently participate in fibrinolysis; therefore, the timing of subform conversion compared with thrombin production is important. In the current study we evaluated the kinetics of FXabeta generation, which showed that autoproteolysis of FXaalpha followed a second order mechanism where FXaalpha and FXabeta behaved as identical enzymes. Rate constants of 9 and 172 M-1 s-1 were derived, respectively, in the absence and presence of FXaalpha binding to procoagulant phospholipid. Under identical conditions the latter is estimated to be 6 orders of magnitude slower than thrombin generation by prothrombinase. Since heparin binding and prothrombin recognition have been previously attributed to a region of FXaalpha proximal to the beta-peptide, functional comparisons were conducted using homogeneous and stabilized preparations of FXaalpha and FXabeta. Comparisons included 1) the recognition of small substrates; 2) the rate of interaction with antithrombin/heparin; 3) the assembly of prothrombinase; and 4) the activation of prothrombin by prothrombinase. Although the beta-peptide neighbors a probable functional region in FXaalpha, conversion to FXabeta was not observed to influence these functions. The data support a model where FXaalpha is predominantly responsible for thrombin generation and where slow conversion to FXabeta coordinates coagulation and the initiation of fibrinolysis at sites of prothrombinase assembly.

Prothrombinase components can accelerate tissue plasminogen activator-catalyzed plasminogen activation

The enzymatic and cofactor subunits of human prothrombinase, factor Xa (FXa) and factor Va (FVa), respectively, were evaluated as modulators of Glu- and Lys-plasminogen (Pg) activation by tissue plasminogen activator (tPA). The data revealed that both FXa and FVa could accelerate tPA activity by as much as 60-fold for Lys-Pg and > 150-fold for Glu-Pg. This function of FVa depended on pretreatment with plasmin (Pn), whereas the FXa fibrinolytic cofactor activity was endogenous. In the native state, FVa was observed to inhibit the acceleration of Pn generation by FXa. These effects were dependent on Ca2+ and procoagulant phospholipid. Interactions between plasminogen and prothrombinase components were quantified. The apparent Kd for binding to FXa was 35 nM. Strikingly, the affinity between FVa and Pg was increased by approximately 2 orders of magnitude when the FVa was Pn-pretreated (Kd = 0.1 microM). These data cumulatively suggest a mechanism by which Pn production is coordinated with coagulation and localized to sites where procoagulant phospholipid is exposed on a cell surface.

Assembly of plasmin-generating proteins on the surface of human endothelial cells

Traditionally, plasmin generation has been conceptualized as a process oriented on the surface of a fibrin-containing thrombus. Recent work, however, indicated that plasminogen and its activators, tissue plasminogen activator (t-PA) and urokinase, can assemble on the surface of cultured human umbilical vein endothelial cells (HUVECs). On binding to HUVECs, plasminogen is activated by t-PA approximately 12-fold more efficiently than fluid-phase plasminogen, and is converted to a plasmin-modified form, possibly unique to cell surfaces. In addition, t-PA interacts with HUVECs at two sites. The major binding site preserves its activity and represents a true (relative molecular weight 40,000) membrane-associated exoreceptor. The low-density lipoprotein (LDL)-like lipoprotein, lipoprotein(a), is highly associated with atherosclerosis, bears striking sequence homology to plasminogen, and competes with plasminogen for cell surface binding. In summary, functional assembly of plasminogen and t-PA may represent an important thromboregulatory system.

Normal haemostasis and its regulation

Regulation of normal haemostasis and blood flow involves complex interactions between plasma proteins and blood cells, including platelets, leukocytes and the endothelial lining of blood vessels. Thrombin acts as a pivot in the maintenance of the haemostatic balance; the vascular endothelial cell in particular limits the generation of thrombin by localisation of anticoagulant processes on its luminal membrane. The endothelial cell synthesises key molecules in this process and also binds exogenously derived molecules, as well as releasing proteins of the fibrinolysis cascade. The thromboresistance of the luminal surface is further regulated by lipoxygenase and cyclo-oxygenase metabolites of unsaturated fatty acids synthesised by the endothelial cell. In response to trauma, inflammatory reactions, normal wound healing and in association with a variety of disease states, the anticoagulant and fibrinolytic mechanisms are downregulated and the procoagulant and thrombotic mechanisms predominate with resultant generation of thrombin, fibrin clot formation and subsequent platelet adhesion and aggregation. Pro-inflammatory and prothrombotic cytokines downregulate the fibrinolytic and activated protein C pathways as well as inducing synthesis of specific procoagulant and prothrombotic mediators by platelets and leukocytes as well as endothelium.

Factor V: a prototype pro-cofactor for vitamin K-dependent enzyme complexes in blood clotting

The relative abundance of factor V, factor X and prothrombin has enabled detailed analyses of the prothrombinase complex. Determination of the primary structure for factor V has provided the basis for examination of structure-function relationships. The imminent in vitro expression of recombinant factor V will provide the opportunity for site-specific mutagenesis and a verification of these structure-function relationships. A comparison of the physical properties and primary structures for factors V and VIII has revealed extensive similarities in these two cofactor proteins. This observation indicates that a direct application of the technology developed for the analysis of prothrombinase will lead to an equal understanding of the factor Xase complex. Whether similar relationships exist for other blood coagulation enzyme complexes remains to be determined.

Elevated urokinase-type plasminogen activator level and bleeding in amyloidosis: case report and literature review

Hyperfibrinolytic states are reported to be a cause of bleeding in patients with amyloidosis. We reviewed the literature on excessive fibrinolysis in association with amyloidosis and report our findings from a patient with idiopathic amyloidosis who developed a bleeding diathesis. Coagulation laboratory studies indicated elevated plasminogen activator levels associated with a reduction of plasminogen and alpha 2-plasmin inhibitor (alpha 2-PI) levels. The level of tissue-type plasminogen activator (t-PA) inhibitor and t-PA antigen were normal. However, the patient did have a five- to sevenfold increase in amidolytic activity for the urokinase substrate pyro-Glu-Gly-Arg-pNA (S-2444). This case therefore represents a novel example of a hyperfibrinolytic state associated with amyloidosis caused by elevated urokinase-type plasminogen activator (u-PA). Epsilon-amino caproic acid (EACA) therapy resulted in an increase in alpha 2-PI and plasminogen levels and effectively reduced the blood loss. Hyperfibrinolytic states in amyloidosis have now been reported to be due to elevated t-PA and u-PA and depleted t-PA inhibitor.