Erythrocyte-derived microparticles supporting activated protein C-mediated regulation of blood coagulation

Elevated levels of erythrocyte-derived microparticles are present in the circulation in medical conditions affecting the red blood cells. Erythrocyte-derived microparticles expose phosphatidylserine thus providing a suitable surface for procoagulant reactions leading to thrombin formation via the tenase and prothrombinase complexes. Patients with elevated levels of circulating erythrocyte-derived microparticles have increased thrombin generation in vivo. The aim of the present study was to investigate whether erythrocyte-derived microparticles are able to support the anticoagulant reactions of the protein C system. Erythrocyte-derived microparticles were isolated using ultracentrifugation after incubation of freshly prepared erythrocytes with the ionophore A23187 or from outdated erythrocyte concentrates, the different microparticles preparations yielding similar results. According to flow cytometry analysis, the microparticles exposed phoshatidylserine and bound lactadherin, annexin V, and protein S, which is a cofactor to activated protein C. The microparticles were able to assemble the tenase and prothrombinase complexes and to stimulate the formation of thrombin in plasma-based thrombin generation assay both in presence and absence of added tissue factor. The addition of activated protein C in the thrombin generation assay inhibited thrombin generation in a dose-dependent fashion. The anticoagulant effect of activated protein C in the thrombin generation assay was inhibited by a monoclonal antibody that prevents binding of protein S to microparticles and also attenuated by anti-TFPI antibodies. In the presence of erythrocyte-derived microparticles, activated protein C inhibited tenase and prothrombinase by degrading the cofactors FVIIIa and FVa, respectively. Protein S stimulated the Arg306-cleavage in FVa, whereas efficient inhibition of FVIIIa depended on the synergistic cofactor activity of protein S and FV. In summary, the erythrocyte-derived microparticle surface is suitable for the anticoagulant reactions of the protein C system, which may be important to balance the initiation and propagation of coagulation in vivo.

An engineered factor Va prevents bleeding induced by anticoagulant wt activated protein C

OBJECTIVE

An increased risk of bleeding is observed in patients receiving activated protein C (APC), which may be a limiting factor for the application of novel APC therapies. Since APC's therapeutic effects often require its cytoprotective activities on cells but not APC's anticoagulant activities, an agent that specifically antagonizes APC's anticoagulant effects but not its cytoprotective effects could provide an effective means to control concerns for risk of bleeding. We hypothesized that superFVa, an engineered activated FVa-variant that restores hemostasis in hemophilia could reduce APC-induced bleeding.

APPROACH AND RESULTS

SuperFVa was engineered with mutations of the APC cleavage sites (Arg506/306/679Gln) and a disulfide bond (Cys609-Cys1691) between the A2 and A3 domains, which augment its biological activity and cause high resistance to APC. SuperFVa normalized APC-prolonged clotting times and restored APC-suppressed thrombin generation in human and murine plasma at concentrations where wild-type (wt) FVa did not show effects. Following intravenous injection of APC into BALB/c mice, addition to whole blood ex vivo of superFVa but not wt-FVa significantly normalized whole blood clotting. Blood loss following tail clip or liver laceration was significantly reduced when superFVa was administered intravenously to BALB/c mice prior to intravenous APC-treatment. Furthermore, superFVa abolished mortality (∼50%) associated with excessive bleeding following liver laceration in mice treated with APC.

CONCLUSIONS

Our results provide proof of concept that superFVa is effective in preventing APC-induced bleeding and may provide therapeutic benefits as a prohemostatic agent in various situations where bleeding is a serious risk.

Factor Xa-driven thrombin generation in plasma: dependency on the aminophospholipid density of membranes and inhibition by phospholipid-binding proteins

Phosphatidylserine (PS) externalization of activated platelets plays a pivotal role in haemostasis and thrombosis. In the present study we have explored the relationship between the PS density of membranes and the rate of thrombin generation in plasma. Factor (F)Xa-initiated thrombin generation was measured in platelet-free plasma (PFP) containing either phospholipid vesicles of varying PS-content or non-stimulated platelets (reconstituted PRP). The duration of the initiation phase of FXa-driven thrombin generation decreased dramatically with increasing PS density. Concomitantly, the maximal rate of thrombin generation during the propagation phase (maxR) increased non-linearly, with the steepest incline between 5 and 10 mol% PS. Titration of FVa into plasma containing 2 mol% PS increased maxR proportionally and diminished the lag phase. In contrast, platelet-dependent thrombin generation was not influenced by addition of FVa. With increasing platelet concentration, the duration of the initiation phase drastically decreased, and maxR increased proportionally. At a physiologically relevant platelet concentration, maxR corresponded with the maxR found with 2 microM of 10 mol% PS. Annexin A5 (AnxA5) and lactadherin appeared to be powerful inhibitors of in-situ thrombin generation under all conditions examined, with AnxA5 being three- to four-fold more potent than lactadherin. In conclusion, maximal thrombin generation in plasma requires membranes with a density of 10-20 mol% PS. Our data further indicate that thrombin formed in situ induces externalization of PS to approx 10 mol% in a substantial platelet subpopulation.

The protein kinase C inhibitor RO318220 potentiates thrombin-stimulated platelet-supported prothrombinase activity

Prothrombinase activity was tested on thrombin- and SFLLRN-activated platelets treated with RO318220, a potent inhibitor of protein kinase C. RO318220 completely inhibited platelet dense and alpha-granule secretion at a concentration of 20 microM but had no effect on prothrombinase activity in the presence of excess factor Va (20 nM). This indicates that protein kinase C activity and agonist-initiated secretion are not necessary for the development of a procoagulant surface. Treatment with 75 to 150 microM RO318220 potentiated platelet-supported thrombin generation up to 280% of control platelets with no change in Kd appFXa. Treated with increasing concentrations of RO318220, an increasing proportion of thrombin-stimulated platelets bound annexin V with decreasing binding sites per platelet. A lower mean forward scatter (FSC-H) of platelets treated with RO318220 suggested platelet vesiculation as a result of RO318220 treatment; however, 100 microM calpeptin pretreatment eliminated the decrease in FSC-H without affecting either the increase in platelets positive for annexin V binding, the decrease in binding sites per platelet, or the 3-fold increase in prothrombinase activity. Thus, RO318220 appears to increase prothrombinase activity by increasing platelet responsiveness to thrombin rather than by inducing platelet vesiculation. This suggests that RO318220 inhibits a signaling molecule within a negative regulatory pathway that governs platelet procoagulant surface changes.

Factor Va increases the affinity of factor Xa for prothrombin: a binding study using a novel photoactivable thiol-specific fluorescent probe

The multiprotein complex of factor Xa, factor Va, and prothrombin efficiently generates the blood-clotting agent, thrombin. Here, the formation of the factor Xa*prothrombin complex and the effects of factor Va on this complex were examined using a photoactivable thiol-specific fluorescent probe (LWB), which was synthesized and incorporated into the active site of factor Xa. The use of fluorescent LWB illustrated that factor Xa has an increased affinity for prothrombin in the presence of factor Va. Further exposure of these components to UV light resulted in a specific photocrosslinking of LWB-factor Xa to prothrombin, suggesting a physical association between these proteins. These data demonstrate that LWB can successfully function both as a spectroscopic probe and as a photocrosslinking reagent for studying protein-protein interactions.

Probing the molecular basis of factor Xa specificity by mutagenesis of the serpin, antithrombin

The molecular basis of the substrate and inhibitor specificity of factor Xa, the serine proteinase of the prothrombinase complex, was investigated by constructing two mutants of human antithrombin (HAT) in which the reactive site loop of the serpin from the P4-P4' site was replaced with the corresponding residues of the two factor Xa cleavage sites in prothrombin (HAT/Proth-1 and HAT/Proth-2). These mutants together with prethrombin-2, the smallest zymogen form of thrombin containing only the second factor Xa cleavage site, were expressed in mammalian cells, purified to homogeneity and characterized in kinetic reactions with factor Xa in both the absence and presence of cofactors; factor Va, high affinity heparin and pentasaccharide fragment of heparin. HAT/Proth-1 inactivated factor Xa approximately 3-4-fold better than HAT/Proth-2 in either the absence or presence of heparin cofactors. In the absence of a cofactor, factor Xa reacted with the HAT/Proth-2 and prethrombin-2 with similar second-order rate constants (approximately 2-3x10(2) M(-1)s(-1)). Pentasaccharide catalyzed the inactivation rate of factor Xa by the HAT mutants 300-500-fold. A similar 10(4)-10(5)-fold enhancement in the reactivity of factor Xa with prethrombin-2 and the HAT mutants was observed in the presence of the cofactors Va and heparin, respectively. Factor Va did not influence the reactivity of factor Xa with either one of the HAT mutants. These results suggest that (1) in the absence of a cofactor, the P4-P4' residues of HAT and prethrombin-2 primarily determine the specificity reactions with factor Xa, (2) factor Va binding to factor Xa is not associated with allosteric changes in the catalytic pocket of enzyme that would involve interactions with the P4-P4' binding sites, and (3) similar to allosteric activation of HAT by heparin, a role for factor Va in the prothrombinase complex may involve rearrangement of the residues surrounding the scissile bond of the substrate to facilitate its optimal docking into the catalytic pocket of factor Xa.

Contribution of platelet-derived factor Va to thrombin generation on immobilized collagen- and fibrinogen-adherent platelets

Adhesion of platelets to immobilized collagen induces the expression of anionic phospholipids, e.g. phosphatidylserine (PS), in the outer leaflet of the plasma membrane of these platelets. In contrast, of the platelets that adhere to immobilized fibrinogen only a small sub-population representing 10 +/- 3% of the total population of the fibrinogen-adherent platelets has exposed PS as probed by annexin V binding. Although the presence of PS is thought to be critical for thrombin generation at the platelet surface, no information is available about the effect of this differential PS exposure on the ability of adherent platelets to support thrombin generation. Perfusion of the fibrinogen- or collagen-adherent platelets with solutions containing factor Xa and prothrombin resulted in thrombin generation that i) increased linear during the first perfusion minutes, ii) was about two-fold faster at collagen-adherent than at fibrinogen-adherent platelets and iii) was for more than 98% restricted to the surface of the adherent platelets. It appeared that the lower thrombin generating capacity of fibrinogen-adherent platelets is not due to a lower overall surface density of PS, but is caused by lower amounts of platelet-bound factor Va. Firstly, in both cases thrombin generation could be completely attenuated with antibodies against human factor Va, and secondly, in the presence of an excess of exogenous plasma-derived factor Va similar initial rates of thrombin formation were measured for collagen- and fibrinogen-adherent platelets. Our findings suggest a unique role for immobilized collagen in maintaining haemostasis.

Platelet factor V New York: a defect in factor V distinct from that in factor V Quebec resulting in impaired prothrombinase generation

Studies were performed on a patient with a longstanding bleeding disorder whose major defects were impaired platelet prothrombinase activity in the absence of added factor Va, and a platelet factor V value that was either decreased or at the lower limit of normal when assayed on multiple occasions. In contrast, plasma factor V values were consistently normal. Unlike Scott Syndrome, in which platelet prothrombinase activity is decreased in both the presence and absence of added factor V, her platelets appeared to utilize added factor Va normally in supporting the generation of prothrombinase activity. These findings suggest an intrinsic defect in platelet factor V as the basis of her platelet prothrombinase defect. This defect appears to be different than that described in the Quebec platelet disorder (factor V Quebec). Immunoblot analyses of washed platelet lysates demonstrated a pattern of variably sized factor V molecules that was entirely similar to that observed in normal platelets, and both the heavy and light chains of her factor V after thrombin cleavage were of the same size as that observed in normal platelets. In addition, her platelet multimerin was normal and immunoblot analysis excluded the type of generalized granular protein defect and pathological proteolysis that has been suggested to explain the factor V defect in the Quebec platelet disorder. The findings in this patient thus suggest a new type of platelet factor V defect as the basis for the impaired capacity of her activated platelets to support prothrombinase generation. The findings further support an important role for platelet factor V in hemostasis.

Reversible modulation of human factor Xa activity with phosphonate esters: media effects

Enantiomers of 4-nitrophenyl 4-X-phenacyl methylphosphonate esters (X = H, PMN; CH3 and CH3O) inactivate human factor Xa with rate constants 8-86 M(-1)s(-1) at pH 6.75 in 0.025 M Hepes buffer, 0.15 M NaCl and 2 mM CaCl2 at 7.0+/-0.1 degrees C. The stereoselectivity of the inactivation of factor Xa is 2-10 and favors the levorotatory enantiomers. The pH-dependence of inactivation of factor Xa by (-)-PMN is sigmoidal and consistent with the participation of a catalytic residue with a pKa of 6.2+/-0.1. Factor Xa reactivates from its phosphonyl adducts through a self-catalyzed intramolecular reaction, which is much influenced by the presence of phospholipids. The rate of reactivation in the absence of phospholipids is not pH dependent at pH <9, but it increases very much at pH >9. In the presence of phospholipids, the pH dependence of the rate constant for reactivation is sigmoidal in the pH 6.5-10.3 range and levels off at pH >9 indicating that the enzyme catalyzes its reactivation. The kinetic pKa for the recovery of factor Xa from its adducts with the PMNs is in the range of 6.7-8.1 and is consistent with the participation of the catalytic His57 in the reactivation process.

A kinetic assay to determine prothrombin binding to membranes

Activation of prothrombin by multisquamase, the prothrombin activator from the venom of Echis multisquamatus (Central Asian sand viper), is inhibited by membranes containing negatively charged anionic phospholipids. This inhibition appears to be due to the fact that the venom activator cannot activate membrane-bound prothrombin. Initial steady state rates of prothrombin activation by multisquamase in the presence of phospholipids appeared to depend on the fraction unbound prothrombin only and this phenomenon was used to quantitate binding of prothrombin to membranes of varying phospholipid composition. In this method, the initial rate of prothrombin activation by multisquamase is measured in the absence (total prothrombin) and in the presence of a procoagulant surface (rate depending only on free prothrombin) and from the difference in activation rates the amount of membrane-bound prothrombin is calculated. The validity of the method was established by determination of the binding parameters for prothrombin binding to 100 microM phospholipid vesicles composed of 20 mole% phosphatidylserine and 80 mole% phosphatidylcholine. The binding parameters obtained were Kd=0.84 microM and n=0.021 micromoles prothrombin bound per micromole phospholipid which is in agreement with literature. Due to the nature of the measurement the method is especially suitable to quantitate binding of prothrombin at concentrations as low as 5 nM prothrombin.

Prothrombin kringle 1 domain interacts with factor Va during the assembly of prothrombinase complex

The kringle 2 domain of prothrombin has been shown to interact with factor Va during the activation of prothrombin by the prothrombinase complex composed of factor Xa, factor Va, negatively charged phospholipids and Ca2+ ions. However, contradictory results have been reported about the role of the kringle 1 domain of prothrombin during the assembly of the prothrombinase complex. In an attempt to clarify the role of the kringle 1 domain of prothrombin, its effect on the activation of prothrombin by the prothrombinase complex and its direct binding to human factor Va were assessed. Comparative evaluation with the effects caused by other prothrombin structural components [a fragment 1 (gamma-carboxyglutamic acid and kringle 1 domains), a kringle 2 domain and a catalytic protease domain] was also performed. In the presence of factor Va, each kringle 1 and kringle 2 fragment significantly inhibited the factor Xa-catalysed prothrombin activation in the absence of phospholipids. However, in the absence of both factor Va and phospholipids, kringle 2 fragment, but not kringle 1 fragment, inhibited prothrombin activation. Evaluation of the molecular interaction of the kringle domains with factor Va in assays with solid-phase phospholipid vesicles showed that each kringle 1 and kringle 2 fragment inhibited the prothrombinase complex activity. Assessment of the direct binding of prothrombin and each kringle domain of prothrombin with factor Va by fluorescence polarization showed that prothrombin, kringle 1 and kringle 2 fragments bind directly to factor Va with dissociation constants of 1.9+/-0.1, 2.3+/-0.1 and 2.0+/-0.4 microM (means+/-S.D.) respectively. These findings suggest that both kringle 1 and 2 domains of prothrombin interact with factor Va during the assembly of the prothrombinase complex.

Selective inhibition of the prothrombinase complex: factor Va alters macromolecular recognition of a tick anticoagulant peptide mutant by factor Xa

The prothrombinase complex assembles through reversible interactions between the protease, factor Xa, the cofactor, factor Va, and acidic phospholipid membranes in the presence of calcium ions. Changes in macromolecular recognition by factor Xa which may result from its interaction with factor Va in the prothrombinase complex have been probed using a recombinant derivative of tick anticoagulant peptide where Arg3 has been replaced with Ala (R3A-TAP). In contrast to the wild type inhibitor, R3A-TAP was a weak competitive inhibitor of factor Xa (Ki = 794 nM). The inhibition of the prothrombinase complex by R3A-TAP was characterized by slow, tight-binding kinetics with an increased affinity of approximately 4000-fold (Ki* = 0.195 nM) relative to that of solution-phase factor Xa. Stopped-flow measurements using p-aminobenzamidine (PAB) demonstrated that the reaction between solution-phase factor Xa and R3A-TAP could be adequately described by a single reversible step with rate constants that were consistent with equilibrium binding measurements. The rate-limiting bimolecular combination of R3A-TAP and factor Xa was competitive with PAB binding of the protease. In contrast, the reaction of R3A-TAP with prothrombinase measured using PAB yielded biphasic stopped-flow traces, indicating a multistep pathway for the reaction of the inhibitor with the enzyme complex. The kinetic measurements were consistent with the initial formation of a ternary complex between R3A-TAP, prothrombinase, and PAB followed by two unimolecular steps which lead to PAB dissociation from the enzyme. In this case, prior occupation of the active site by PAB had no effect on the bimolecular reaction between R3A-TAP and prothrombinase. Thus, the interaction of factor Xa with factor Va on the membrane surface alters recognition of R3A-TAP by the protease, leading to changes in the thermodynamics as well as in the observed kinetic mechanism for the reaction. Therefore, a single amino acid substitution in TAP reveals large changes in macromolecular recognition by factor Xa as a consequence of its interaction with the cofactor within the prothrombinase complex.

Binding sites for blood coagulation factor Xa and protein S involving residues 493-506 in factor Va

Inactivation due to cleavage of Factor Va (FVa) at Arg 506 by activated protein C (APC) helps to downregulate blood coagulation. To identify potential functional roles of amino acids near Arg 506, synthetic overlapping pentadecapeptides comprising FVa heavy chain residues 481-525 were tested for their ability to inhibit prothrombin activation by prothrombinase complexes [Factor Xa (FXa):FVa:phospholipids:Ca2+]. The most potent inhibition was observed for peptide VP493 (residues 493-506), with 50% inhibition at 2.5 microM. VP493 also inhibited FXa in plasma in FXa-1-stage clotting assays by 50% at 3 microM. When the C-terminal carboxamide group of VP493 was replaced by a carboxyl group, most prothrombinase inhibitory activity was lost. VP493 preincubated with FXa inhibited prothrombinase with a pattern of mixed inhibition. Homologous peptides from Factor VIII sequences did not inhibit prothrombinase. Affinity-purified antibodies to VP493 inhibited prothrombinase activity and prolonged FXa-1-stage clotting times. VP493 also blocked the ability of protein S to inhibit prothrombinase independently of APC. Immobilized VP493 bound specifically with similar affinity to both FXa and protein S (Kd approximately 40 nM), but did not measurably bind prothrombin or APC. These studies suggest that FVa residues 493-506 contribute to binding sites for both FXa and protein S, providing a rationale for the ability of protein S to negate the protective effect of FXa toward APC cleavage of FVa. Possible loss of this FVa binding site for FXa due to cleavage at Arg 506 by APC may help explain why this cleavage causes 40% decrease in FVa activity and facilitates inactivation of FVa.

Kinetics of human factor VII activation

In this study the activation of human factor VII by a variety of potential activators in the presence and absence of mixed phospholipid vesicles [25% phosphatidylserine (PS), 75% phosphatidylcholine (PC)] is evaluated. At the plasma concentration of factor VII, 10 nM, the activation rate of the zymogen by 0.05 nM factor Xa is anionic phospholipid (PCPS) dependent and achieves a maximum value of 18 pM/s at 5-20 microM PCPS; further increases in the levels of PCPS decrease the activation rate of factor VII. The maximum activation rate of factor VII (10 nM) by the factor VIIa-tissue factor complex (0.1 nM), 0.76 pM/s, is achieved at 200 microM PCPS. No detectable activation of 10 nM factor VII is observed under similar conditions when either thrombin (0.1 nM) or factor IXa (0.1 nM) is used as an activator. Factor VIIa (10 nM) and factor XIa (1 nM) are not observed to activate factor VII at detectable rates. The observed Michaelis-Menten constants (KM) for factor VII activation in the presence of PCPS at optimal concentrations vary from 1.2 microM for factor Xa to 3.2 microM for the factor VIIa-tissue factor complex. The highest catalytic constant (kcat) value (15.2 s-1) is observed for factor Xa-PCPS. The factor VIIa-tissue factor complex, factor IXa, and thrombin kcat values are 1.4, 0.32, and 0.061 s-1, respectively. Tissue factor does not increase the factor VII activation rate by factor Xa, factor IXa, or thrombin. Factor VIIIa in the presence of PCPS has no effect on factor VII activation by factor IXa. In contrast, factor Va decreases the factor VII activation rate by factor Xa, reaching saturation at concentrations consistent with complete prothrombinase complex formation. The formed prothrombinase complex activates factor VII at approximately 30% the rate of factor Xa bound to phospholipids. These data allow us to conclude that the predominant physiological factor VII activator is, most likely, membrane-bound factor Xa.

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.

Structural requirements for tissue factor pathway inhibitor interactions with factor Xa and heparin

Tissue factor pathway inhibitor (TFPI) is a multivalent Kunitz-type protease inhibitor, which inhibits factor Xa directly and in a factor Xa dependent manner inhibits the factor VIIa/tissue factor catalytic complex. Altered forms of recombinant TFPI (rTFPI) were tested for their ability to inhibit human factor Xa and bovine gamma-carboxyglutamate (Gla)-domainless factor Xa in the presence and absence of calcium ions, heparin, phospholipids, and factor Va. Sequential deletions of the positively charged C-terminus of TFPI produces proteins that have decreasing inhibitory activity against factor Xa as well as decreasing affinity for heparin-agarose. Deletion of the C-terminus distal to Leu181, which eliminates the third Kunitz-type domain, results in the loss of heparin-agarose binding at physiological ionic strength. Furthermore, the entire C-terminal polypeptide, including at least a portion of the third Kunitz-type domain, appears to be involved in heparin binding. Residues 230-241 probably form an alpha helix in which Lys231 and Arg237 within the Kunitz domain and Lys240 and Lys241 could provide a positively charged surface epitope capable of binding heparin. Heparin and Ca2+ together, but not individually, enhance the rate of factor Xa inhibition by full-length TFPI. The effect of heparin is concentration dependent and biphasic (maximal between 0.1 and 1.0 unit/ml) suggesting that the acceleration of factor Xa inhibition occurs at least in part through a 'template' mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic procoagulant surface induced by hypercholesterolaemia on rabbit aortic endothelium

The effect of hyperlipidaemia on endothelial cell haemostatic properties was examined using ex vivo studies on aortic segments obtained from fat-fed Chinchilla rabbits, mounted in a template device which exposed the luminal surface. Exposure of arterial endothelium to lipids resulted in marked enhancement of externally exposed anionic phospholipids, detected using either fluorescence microscopy with the probe merocyanine 540 or by binding of 125I-polymyxin B and 125I-Annexin V. Consistent with the known procoagulant properties of anionic phospholipid, following the lipid and cholesterol-rich diet intake, intact endothelial cells demonstrated enhanced binding of radioiodinated factors IX/IXa and Xa, and enhanced factor IXa/VIII-dependent factor X activation and factor Xa-factor Va-mediated prothrombin activation. Both factor Xa and thrombin formation were blocked, in large part, by polymyxin B, suggesting dependence of the reaction on anionic phospholipids. Consistent with these results, evidence of increased activation of the coagulation mechanism in vivo was observed in hyperlipidaemic animals, as assessed by a three-fold increase in levels of circulating antithrombin-protease complexes, compared with normolipidaemic controls.

The influence of factor Va on the active site of factor Xa

The interaction of factor Xa with factor Va on the membrane surface results in a 3,000-fold increase in the kcat for the activation of prothrombin catalyzed by factor Xa. The reaction between the transition state irreversible inhibitor dansylglutamyl-glycyl-arginyl chloromethyl ketone (DEGRck) and factor Xa was characterized and employed to evaluate changes in the active site of factor Xa resulting from its interaction with factor Va, which may account for the increased catalytic efficiency of the enzyme complex. Inhibition studies indicated a two-step inhibition reaction involving a reversible binding step (Ki = 1.13 microM) and an irreversible alkylation step (ki = 0.65 s-1). The interaction between factors Va and Xa in solution or on membranes resulted in a small decrease in the overall second-order rate constant (ki/Ki) for the inhibition reaction. The incorporation of DEGRck into the active site of factor Xa results in a large change in the fluorescence intensity of the dansyl moiety. The fluorescence change was employed to study the reaction between enzyme and inhibitor directly by stopped-flow fluorescence measurements. The fluorescence traces were biphasic, indicating that the association of DEGRck with factor Xa and the subsequent covalent modification of the active site could be resolved because of differences in fluorescence intensities of the intermediate and product. This interpretation was verified by rapid chemical quench experiments. The reaction between DEGRck and factor Xa was characterized by a second-order association rate constant of 8.38 +/- 0.28 x 10(5) M-1.s-1 and an intrinsic rate constant for the alkylation step of 1.67 +/- 0.25 s-1. The rate constant for the alkylation step was unchanged in the presence of factor Va and membranes, whereas the association rate constant was modestly decreased by approximately 50%. The decrease in the association rate constant did not result from the partitioning of factor Xa to the membrane surface and could therefore be ascribed to an effect of factor Va on the protease. The data suggest that the interaction between factors Va and Xa on the membrane surface does not detectably alter the catalytic residues but may result in changes in the binding or accessibility of substrate to extended macromolecular recognition sites on the protease.

Protein S enhances C4b binding protein interaction with neutrophils

Protein S, an inhibitor of coagulation, interacts reversibly with C4b binding protein (C4bBP). The physiologic role of this complex formation remains unknown. In this study we examined the possibility that protein S would facilitate C4bBP binding to the surface of neutrophils where, in turn, this complex could help protect the cell from complement-mediated damage at the site of inflammation. Neutrophils bind approximately 60,000 protein S molecules per cell (kd = 140 nmol/L) in a Ca(2+)-dependent fashion. C4bBP also binds to neutrophils (23,000 sites per cell at physiologic C4bBP concentration), but this binding is not Ca2+ dependent. Protein S approximately doubled C4bBP binding, but protein S only influenced C4bBP binding in the presence of Ca2+. Protein S binding to neutrophils decreased approximately twofold in the presence of saturating concentrations of C4bBP. Neutrophil activation had only minor effects on the affinity and number of sites for protein S or the protein S-C4bBP complex. These results indicate that the protein S-C4bBP complex binds to the neutrophil surface where it can presumably modulate complement assembly on the cell surface.

Factor Xa interacts with two sites on monocytes with different functional activities

Studies were performed to elucidate the functional significance of factor Xa interactions at the monocyte membrane in the presence and absence of factor Va, with respect to prothrombin and factor IX cleavage. Factor Xa-catalyzed prothrombin activation at the monocyte surface was absolutely dependent on the addition of factor Va, indicating that thrombin was generated solely by a membrane-bound complex of factors Va and Xa. In contrast, in the absence of added factor Va, factor Xa bound to monocytes catalyzed the cleavage of factor IX to the nonenzymatic intermediate factor IX alpha through a reaction that was dependent on both monocyte and factor Xa concentration. At limiting factor Xa concentration, added factor Va inhibited the factor Xa-catalyzed cleavage of factor IX, suggesting that a monocyte-bound complex of factors Va and Xa did not recognize factor IX as a substrate. These combined data suggest that factor Xa interacts with the monocyte through two sites which can be distinguished by their requirement for added factor Va and their expression of different functional activities. Both functional sites could be distinguished also by their differential susceptibility to inhibition by a monoclonal antibody directed against the light chain of factor Va (alpha-HFV1). At the monocyte surface, the factor Va/Xa-catalyzed activation of prothrombin was maximally inhibited with 0.25 mumol/L alpha-HFV1, whereas 1.0 mumol/L alpha-HFV1 was required to effect 50% inhibition of the factor Xa-catalyzed cleavage of factor IX. The ability of factor Va to modulate factor Xa substrate specificity was investigated further. Factor Xa bound to thrombin-activated platelets either through platelet-released factor Va or added factor Va did not cleave factor IX. Consistent with this result, a plasma concentration of factor IX had no effect on thrombin generation catalyzed by a platelet-bound complex of factors Va and Xa. In marked contrast, factor Xa bound to phospholipid vesicles either independently or in complex with factor Va catalyzed factor IX cleavage with equal efficiency. These combined data indicate that factor Va bound to cell surfaces modulates factor Xa substrate specificity, whereas no discriminatory effect is conferred by factor Va bound to phospholipid vesicles. Thus, by providing two distinct sites at its membrane surface, the monocyte modulates factor Xa binding and the functional activity expressed by the bound enzyme, depending on the availability of factor Va.

Equilibrium binding of thrombin to recombinant human thrombomodulin: effect of hirudin, fibrinogen, factor Va, and peptide analogues

Thrombomodulin is an endothelial cell surface receptor for thrombin that acts as a physiological anticoagulant. The properties of recombinant human thrombomodulin were studied in COS-7, CHO, CV-1, and K562 cell lines. Thrombomodulin was expressed on the cell surface as shown by the acquisition of thrombin-dependent protein C activation. Like native thrombomodulin, recombinant thrombomodulin contained N-linked oligosaccharides, had Mr approximately 100,000, and was inhibited or immunoprecipitated by anti-thrombomodulin antibodies. Binding studies demonstrated that nonrecombinant thrombomodulin expressed by A549 carcinoma cells and recombinant thrombomodulin expressed by CV-1 and K562 cells had similar Kd's for thrombin of 1.3 nM, 3.3 nM, and 4.7 nM, respectively. The Kd for DIP-thrombin binding to recombinant thrombomodulin on CV-1(18A) cells was identical with that of thrombin. Increasing concentrations of hirudin or fibrinogen progressively inhibited the binding of 125I-DIP-thrombin, while factor Va did not inhibit binding. Three synthetic peptides were tested for ability to inhibit DIP-thrombin binding. Both the hirudin peptide Hir53-64 and the thrombomodulin fifth-EGF-domain peptide Tm426-444 displaced DIP-thrombin from thrombomodulin, but the factor V peptide FacV30-43 which is similar in composition and charge to Hir53-64 showed no binding inhibition. The data exclude the significant formation of a ternary complex consisting of thrombin, thrombomodulin, and hirudin. These studies are consistent with a model in which thrombomodulin, hirudin, and fibrinogen compete for binding to DIP-thrombin at the same site.

Binding of human factors X and Xa to HepG2 and J82 human tumor cell lines. Evidence that factor Xa binds to tumor cells independent of factor Va

Previous studies demonstrated that several cultured human tumor cell lines potentiate the conversion of prothrombin to thrombin by factor Xa and calcium in the absence of exogenous factor Va. In the present study, the specific binding of radioiodinated preparations of human factor X and factor Xa to a human hepatocellular carcinoma cell line (HepG2) that constitutively synthesizes a factor V/Va molecule, and a human bladder carcinoma cell line (J82) that does not synthesize factor V/Va, was examined. Radioiodinated factor Xa bound specifically to J82 and HepG2 cells, whereas no significant specific binding of 125I-factor X to either cell was observed. The binding isotherm of 125I-factor Xa to each tumor cell line exhibited a hyperbolic profile, and Scatchard analysis demonstrated a single class of binding site for factor Xa on each cell surface with Kd values of 1.66 +/- 0.39 and 1.64 +/- 0.52 nM and 566,000 +/- 71,000 and 28,000 +/- 6,000 binding sites/cell for HepG2 and J82 cells, respectively. Thrombin formation by cell-bound factor Xa was hyperbolic and saturable at 5 nM factor Xa on each cell line. Hanes-Woolf plots of the prothrombin activation data indicated that half-maximal rates of thrombin formation occurred at factor Xa concentrations of 1.50 +/- 0.43 nM and 1.42 +/- 0.48 nM on HepG2 and J82 cells, respectively. Pretreatment of J82 cells with polyclonal anti-human factor V IgG had no measurable effect on either the binding of 125I-factor Xa or prothrombin activation. However, pretreatment of HepG2 cells with anti-human factor V IgG inhibited prothrombin activation in a dose-dependent manner, but did not inhibit the binding of factor Xa to this cell. When both cell lines were preincubated with exogenous human factor Va, the binding of factor Xa to either HepG2 or J82 cells was marginally affected. These data indicate that HepG2 and J82 cells have cell surface factor Xa binding sites proximal to, but independent of, cell surface factor Va of either exogenous or endogenous origin.

The active site of membrane-bound meizothrombin. A fluorescence determination of its distance from the phospholipid surface and its conformational sensitivity to calcium and factor Va

The distance between the phospholipid surface and the active site of membrane-bound meizothrombin, a derivative of prothrombin, was determined directly using fluorescence energy transfer. The active site of prothrombin was exposed after a single cleavage by Echis carinatus protease in the presence of [5-(dimethylamino)-1-naphthalenesulfonyl]glutamylglycylarginyl+ ++ (DEGR) chloromethyl ketone to yield DEGR-meizothrombin and thereby minimize secondary proteolysis. When DEGR-meizothrombin was titrated with 80% phosphatidylcholine, 20% phosphatidylserine vesicles containing octadecylrhodamine, singlet-singlet energy transfer was observed between the donor dyes in the active sites of the membrane-bound proteins and the acceptor dyes at the outer surface of the phospholipid bilayer. This energy transfer required both Ca2+ and phosphatidylserine. Assuming k2 = 2/3, the dependence of the efficiency of energy transfer upon the acceptor density showed that the distance of closest approach between the active site probe and the bilayer surface was 71 +/- 2 A. In the presence of factor Va, the distance was 67 +/- 3 A. These direct measurements show that the active site of meizothrombin is located far above the membrane surface. Also, association of factor Va with meizothrombin on the phospholipid surface appears to cause a slight movement of the meizothrombin protease domain toward the membrane surface. The environment of the dansyl dye covalently attached to the active site of meizothrombin was particularly sensitive to the presence of calcium: addition of Ca2+ ions to metal-free DEGR-meizothrombin reduced the dansyl fluorescence lifetime from 11.7 to 9.0 ns and the dansyl emission intensity by 24%. Hence, the conformation of the active site changed when Ca2+ ions bound to meizothrombin. Since the intensity change was half-maximal at 0.2 mM and was also elicited by the binding of Mg2+ ions, this spectral change correlates with the calcium-dependent conformational change previously observed in fragment 1. We conclude, therefore, that the binding of Ca2+ ions to meizothrombin and, by extension, perhaps to prothrombin, elicits a conformational change that extends beyond the fragment 1 domains into the distant (cf. above) active site or protease domain. The association of factor Va with membrane-bound DEGR-meizothrombin increased both the dansyl emission intensity (by 7%) and polarization. This intensity change and the factor-Va dependent change in energy transfer indicate that the cofactor of the prothrombinase complex functions to modulate the conformation and orientation of both the substrate and the enzyme of the complex.