The subunit structure of thrombin-activated factor V. Isolation of activated factor V, separation of subunits, and reconstitution of biological activity

Involvement of thrombin anion-binding exosites 1 and 2 in the activation of factor V and factor VIII

The role of anion-binding exosites of thrombin in the activation of factor V and factor VIII was studied using thrombin Arg93 --> Ala, Arg97 --> Ala, and Arg101 --> Ala (thrombin RA), a recombinant exosite 2 defective mutant, and a synthetic N-acetylated dodecapeptide, Ac-Asn-Gly-Asp-Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr-O-SO4Leu (hirugen), which competitively inhibits binding of macromolecules to exosite 1. The catalytic efficiency of the activation of factor VIII or of the first step of factor V activation by thrombin RA was approximately 10% that of wild-type thrombin. The overall rate of conversion to factor Va was not influenced by the mutation. In contrast to factor V, the slow activation of factor VIII by thrombin RA was associated with a decreased rate of cleavage at all three proteolytic sites (Arg372, Arg740, and Arg1689). Hirugen inhibited factor V and factor VIII activation. These results indicate that both anion-binding exosites of thrombin are involved in the recognition of factor V and factor VIII.

Biochemical prototype for familial thrombosis. A study combining a functional protein C mutation and factor V Leiden

Resistance to activated protein C (APC) is associated with a single amino acid substitution in factor V (Arg506-->Gln, factor V Leiden) that results in delayed inactivation of the molecule by APC. The mutation is present in 20% of patients with a first episode of deep venous thrombosis. Arterial and venous thromboses are also associated with the type II protein C deficiency (protein CVermont). In protein CVermont, the substitution Glu20-->Ala alone (rPC gamma 20A) is responsible for the defective anticoagulant properties of PCVermont. It was recently established that a thrombotic episode occurred in 73% of family members who are heterozygous for both a functional protein C gene mutation and the factor V Leiden mutation. We evaluated the molecular defect that would accrue in the combined deficiency state of factor VR506Q/VaR506Q and rAPC gamma 20A using recombinant APC and natural purified factor VR506Q from patients homozygous for the Arg506-->Gln substitution. While wild-type recombinant APC (rAPC) slowly cleaves and inactivates factor VR506Q and factor VaR506Q, minimal cleavage of membrane-bound factor VR506Q and VaR506Q by rAPC gamma 20A at Arg306 and Arg679 occurs, and no loss in cofactor activity is observed. Our data demonstrate that rAPC gamma 20A cannot inactivate either factor VR506Q or factor VaR506Q at biologically relevant rates because of impaired cleavage at Arg306 and Arg679.(ABSTRACT TRUNCATED AT 250 WORDS)

Cofactors V and VIII after endotoxin administration to human volunteers

Coagulation factor V (FV) and factor VIII (FVIII) are usually decreased in septicemic DIC. Low doses of endotoxin administered to healthy volunteers stimulate activation of the fibrinolytic, contact and coagulation systems, but not clinical DIC. Following the administration of endotoxin (4 ng/kg) to normal volunteers (n = 15), we applied new assays for FV antigens using monoclonal antibodies to the activation peptide (C1) and to the light chain of FV. At 5 hours, FV coagulant activity was significantly decreased (64 +/- 9%), as was the FV light chain antigen (74 +/- 6%), without a change in factor V C1 antigen or total protein C. In contrast, FVIII coagulant activity was greater than preinfusion levels at 2-5 hours. The decrease in FV activity may be due to APC cleavage of FV heavy chain, but the loss of light chain antigen suggests that plasmin and/or calpain also contribute. APC may not be the only enzyme responsible for cofactor inactivation. FV is one of the most sensitive markers, even reflecting subclinical activation of coagulation.

Factor V is complexed with multimerin in resting platelet lysates and colocalizes with multimerin in platelet alpha-granules

Factor V stored in platelets is an important source of factor Va for the prothrombinase complex. Investigations of potential platelet factor Va-binding proteins, using factor Va light chain affinity chromatography, identified a disulfide-linked multimeric protein with a reduced mobility of 155 kDa in the column eluate. Immunodepletion and immunoblotting indicated that this protein was multimerin. Multimerin specifically bound factors V and Va and the isolated factor Va light chain, but not the heavy chain of factor Va. Factor V stored in platelets, but not plasma factor V, was found to be complexed with multimerin. Multimerin immunodepletion of resting platelet lysates was associated with the removal of factor V and the loss of factor V coagulant activity. Immunoelectron microscopic studies colocalized factor V with multimerin in the alpha-granules of resting platelets. With thrombin-induced platelet activation, we observed dissociation of factor Va-multimerin complexes, multimerin-independent membrane binding of factor Va, and prothrombinase activity that was not inhibitable by multimerin antibodies. This study indicates that platelet factor V is stored as a complex with multimerin and suggests a possible role for multimerin as a carrier protein for factor V stored in platelets.

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.

Contribution of residue 192 in factor Xa to enzyme specificity and function

Mutation of residue 192 (chymotrypsin numbering) from Glu to Gln in thrombin and activated protein C has been shown to dramatically alter substrate and inhibitor specificity, in large part by allowing these enzymes to accept substrates with acidic residues in the P3 and/or P3' positions. In factor Xa, residue 192 is already a Gln. We now compare the properties of a Q192E mutant of Gla-domainless factor X (GDFX). Kinetic analysis of prothrombin activation indicates similar affinity of factor Va for GDFXa and GDFXa Q192E (Kd(app) = 3.6 and 3.7 microM, respectively). Prothrombin activation rates are similar for both enzymes with factor Va, but are approximately 10-fold slower for the Q192E mutant without factor Va. This defect is in the activation of prethrombin 2 and is corrected by factor Va only in the presence of fragment 2. Without factor Va, fragment 2 has no influence on bovine prethrombin 2 activation by GDFXa, but fragment 2 enhances prethrombin 2 activation by the Q192E mutant at least 10-fold. These results indicate that the fragment 2 domain of prothrombin probably alters the conformation of the prethrombin 2 domain, selectively improving its presentation to GDFXa Q192E. With respect to inhibition, tissue factor pathway inhibitor and bovine pancreatic trypsin inhibitor are > or = 30 times poorer inhibitors of GDFXa Q192E than of GDFXa, but these enzymes are inhibited at comparable rates by antithrombin. These results indicate that Gln-192 in factor Xa is a key determinant of substrate/inhibitor specificity.

Factor V inhibitor associated with Sjögren's syndrome

We report an unusual case of a 74-year-old male who developed a serum autoantibody reactive with human coagulation factor V (FV) in an activated form, as demonstrated by coagulation studies and immunoblotting analysis. Despite marked prolongation of a prothrombin time and an activated partial thromboplastin time in this patient, the inhibitor was not associated with clinical bleeding but with multiple cerebral infarctions. The patient had suffered from Sjögren's syndrome with polyclonal hypergammaglobulinaemia. The patient's purified IgG, an immediately acting inhibitor to FV, reacted with a light chain of thrombin-activated FV (FVa) and inhibited the procoagulant activity of FVa without affecting the cleavage of FVa by activated protein C. The FV inhibitor may arise from activation of FV with consequent exposure of neoantigen during the activation of coagulation cascade in the patient with an autoimmune disorder for the background.

Thrombin-catalyzed activation of recombinant human factor V

Proteolytic activation of human factor V by thrombin results from the cleavage of three peptide bonds at Arg709, Arg1018, and Arg1545. In order to define the functional importance of these sites, mutants with isoleucine substitutions blocking thrombin cleavage at one, two, or all three activation sites were expressed in COS-7 cells. The wild type protein is activated approximately 10-fold by thrombin or Russell's viper venom (RVV-V). Thrombin cleavage at Arg709 alone did not result in an increase in procoagulant activity. Cleavage at both Arg709 and Arg1018 resulted in an approximately 3.4-fold increase in activity. Cleavage at these sites was required for rapid cleavage by thrombin at Arg1545, however, which resulted in maximal activation of the factor V molecule. In contrast, isolated cleavage at Arg1545 by RVV-V was sufficient for efficient and complete activation of factor V. The effect of isoleucine substitutions at one or both thrombin cleavage sites in a B-domain deletion mutant lacking amino acids 811-1491 was also investigated. The specific activity of all four mutants was approximately 30% compared to thrombin activated factor V, indicating that these isoleucine substitutions do not drastically alter the structure of the protein and that cleavage at these sites is not required for the expression of partial procoagulant activity.

The role of cleavage of the light chain at positions Arg1689 or Arg1721 in subunit interaction and activation of human blood coagulation factor VIII

The role of Factor VIII light chain cleavage in Factor VIII activation and subunit interaction was investigated. Purified Factor VIII was dissociated into its separate subunits, and the isolated light chain was cleaved by thrombin at position Arg1689 or by Factor Xa at position Arg1721. These Factor VIII light chain derivatives then were used for reconstitution with purified Factor VIII heavy chain to obtain heterodimers that were exclusively cleaved within the light chain. Intact and cleaved light chain could effectively be reassociated with heavy chain, with concomitant regain of Factor VIII cofactor function. The association rate constant of Factor Xa-cleaved light chain was found to be 3-fold lower than that of thrombin-cleaved or intact light chain, suggesting a role of the region Ser1690-Arg1721 in subunit assembly. Dissociation rate constants, however, were independent of Factor VIII light chain cleavage. Low ionic strength was observed to promote association but to destabilize the Factor VIII heterodimer. At high ionic strength, Factor VIII dissociation was extremely slow (kappa off approximately 10(-5) s-1) for all Factor VIII light chain derivatives, indicating that Factor VIII light chain cleavage is not related to Factor VIII dissociation. Furthermore, Factor VIII light chain cleavage does not affect enzyme-cofactor assembly, since the various light chain derivatives proved equally efficient in binding to Factor IXa (Kd approximately 15 nM). Studies in a purified Factor X-activating system demonstrated that thrombin and Factor Xa activate Factor VIII to the same extent. However, Factor Xa differed from thrombin in that it cleaved at Arg1721 rather than at Arg1689. Reassociated heterodimers of Factor VIII heavy chain and intact light chain did not promote Factor X activation. In contrast, heterodimers that contained cleaved light chain exhibited substantial Factor VIIIa activity. These data demonstrate that a single cleavage at either Arg1689 or Arg1721 converts the inactive Factor VIII heterodimer into an active cofactor of Factor IXa.

Coagulation factor Va is an actin filament binding and cross-linking protein

Bovine coagulation cofactor factor Va is shown to bind to filament of skeletal muscle actin with a dissociation constant of 40-50 nM in the presence of 50 mM NaCl. At saturation, approximately one molecule of factor Va was bound for every two actin molecules. The binding of factor Va to F-actin was inhibited by increasing ionic strength, being approximately 20-fold weaker at 150 mM NaCl. Addition of factor Va dramatically increased both the low-speed sedimentation and the low-shear viscosity of actin filament solutions, indicating that factor Va cross-linkis actin filaments. Factor Va bound to actin filaments saturated with myosin. The isolated 74-kilodalton light chain of factor Va displayed actin binding and cross-linking properties indistinguishable from those of intact factor Va. The procofactor factor V bound weakly to F-actin, indicating that proteolytic activation is required to uncover the actin binding sites within the light chain domain. Actin filaments had only a slight inhibitory effect on the prothombinase activity of the factor Va-factor Xa-phospholipid complex. Since high concentrations of actin filaments can be exposed to the circulation when cells are damaged, the interaction of factor Va with actin may be of physiological relevance.

Structure of membrane-bound human factor Va

Coagulation factor Va is an essential cofactor which combines with the serine protease factor Xa on a phospholipid surface to form the prothrombinase complex. In the present study, the structure of factor Va interacting with lipid surfaces containing phosphatidylserine was studied by electron microscopy. Two-dimensional crystals of factor Va were obtained on planar lipid films under quasi-physiological conditions. The two-dimensional projected structure of factor Va was calculated at a resolution of 2 nm, revealing dimers of factor Va arranged on the surface lattice with the symmetry of the plane group p2. Average unit cell dimensions are a = 14.4 nm, b = 8.8 nm, gamma = 107 degrees. Each factor Va molecule presents two distinct domains of protein density consisting of one small domain, of 3 nm in diameter, connected to a larger domain of about 6 nm x 4.5 nm. The projected structure of factor Va covers an area equivalent to about fifty phospholipid molecules. In addition, edge-on views of factor Va molecules bound to liposomes reveal a globular structure connected through a thin stem to the liposome surface. A three-dimensional model of membrane-bound factor Va is proposed.

Posttranslational sulfation of factor V is required for efficient thrombin cleavage and activation and for full procoagulant activity

Factor VIII and factor V function as cofactors in the blood coagulation cascade to accelerate the rate of activation of factor X and prothrombin, respectively. Both cofactors require proteolytic activation by either activated factor X or thrombin for functional activity. Human factor VIII and factor V expressed in mammalian cells are both modified by posttranslational sulfation of tyrosine residues. In the present study, the posttranslational addition of sulfate in factor V expressed in transfected Chinese hamster ovary (CHO) cells was demonstrated by [35S]sulfate incorporation into the thrombin-cleaved 94-kDa heavy chain and the 150-kDa activation peptide. The presence of tyrosine sulfate in recombinant factor V was confirmed by barium hydroxide hydrolysis and two-dimensional thin-layer electrophoresis. The importance of sulfation for factor V secretion and activity was evaluated by characterizing factor V expressed in Chinese hamster ovary cells grown in the presence of sodium chlorate, a potent inhibitor of posttranslational sulfation in intact cells. Increasing concentrations of sodium chlorate inhibited the incorporation of [35S]sulfate into factor V but did not inhibit the synthesis or secretion of factor V. However, the specific activity of factor V secreted in the presence of sodium chlorate was reduced 5-fold. The reduced activity was attributed to (1) slower cleavage and activation by thrombin and (2) a reduced intrinsic activity of factor Va. In contrast, sulfation of factor V did not affect the rate of activation mediated by factor Xa. These results show that sulfation of factor V is required for efficient thrombin activation but not for activation by factor Xa.(ABSTRACT TRUNCATED AT 250 WORDS)

Thrombin in inflammation and healing: relevance to rheumatoid arthritis

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Assembly of prothrombinase complex

The approaches described in this article have resulted in an increased understanding of the reaction steps involved in the stabilization and assembly of the prothrombinase complex. Because prothrombinase is considered an archetype for some of the other coagulation complexes, the quantitative information derived from these studies (Table I) provides the framework for future studies of prothrombinase and suggests experimental approaches for studies of the other analogous coagulation reactions.

Activation of bovine factor V by an activator purified from the venom of Naja naja oxiana

The crude venom of many elapid snakes appeared to contain proteins that activated blood coagulation factor V. The factor V activator present in the venom of Naja naja oxiana was purified to homogeneity by chromatography on a mono-S column. The activator was a single chain protein with an apparent mol. wt of 48,000, as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and by gel permeation chromatography on Sephacryl S200. Activation of bovine factor V by the purified venom activator was accompanied by proteolytic cleavage of factor V and resulted in the formation of two major polypeptide chains with mol. wts of about 90,000 and 77,000. The final product obtained was compared with thrombin-activated factor V for its ability to function as cofactor in factor Xa-catalysed prothrombin activation in the presence of negatively charged phospholipid vesicles (5 mole% phosphatidylserine/95 mole% phosphatidylcholine). The Km for prothrombin obtained at a saturating amount of venom-activated factor Va was nine-fold higher than with thrombin-activated factor V (0.83 microM vs 0.09 microM, respectively) whereas both factor Va molecules stimulated the Vmax of thrombin formation some 6000-fold. Both forms of factor Va promoted the binding factor Xa to negatively charged phospholipid vesicles. However, the apparent Kd for factor Xa was less favorable in the presence of venom-activated factor V (0.67 x 10(-9) M) than in the presence of thrombin-activated factor V (0.043 x 10(-9) M). Thrombin cleaved a peptide bond in the 77,000 mol. wt polypeptide chain of venom-activated factor V, which resulted in the formation of a normal factor Va light chain. This peptide bond cleavage was, however, not associated with a change of cofactor activity. Venom treatment of thrombin-activated factor V, on the other hand, did remove a small fragment (mol. wt approximately 4000) from the heavy chain of factor Va (94,000), yielding a molecule with reduced cofactor activity. The diminished cofactor activity of venom-activated factor V is, therefore, likely due to the fact that a small peptide fragment, involved in the interaction with prothrombin and factor Xa, is missing from the heavy chain of venom-activated factor V.

The effect of phospholipids, calcium ions and protein S on rate constants of human factor Va inactivation by activated human protein C

Rate constants for human factor Va inactivation by activated human protein C (APC) were determined in the absence and presence of Ca2+ ions, protein S and varying concentrations of phospholipid vesicles of different lipid composition. APC-catalyzed factor Va inactivation in free solution (in the presence of 2 mM Ca2+) was studied under first-order reaction conditions with respect to both APC and factor Va and was characterized by an apparent second-order rate constant of 6.1 x 10(5) M-1 s-1. Stimulation of APC-catalyzed factor Va inactivation by phospholipids was dependent on the concentration and composition of the phospholipid vesicles. Optimal acceleration (230-fold) of factor Va inactivation was observed with 10 microM phospholipid vesicles composed of 20 mol% dioleoylglycerophosphoserine (Ole2GroPSer) and 80 mol% dioleoylglycerophosphocholine (Ole2GroPCho). At higher vesicle concentrations and at higher molar fractions of Ole2GroPSer some inhibition of APC-catalyzed factor Va inactivation was observed. Membranes that contained anionic phospholipids other than phosphatidylserine also promoted factor Va inactivation. The ability of different anionic lipids to enhance factor Va inactivation increased in the order phosphatidylethanolamine less than oleic acid less than phosphatidic acid less than phosphatidylglycerol less than phosphatidylmethanol less than phosphatidylserine. APC-catalyzed factor Va inactivation in the presence of phospholipid vesicles could be saturated with respect to factor Va and the reaction obeyed Michaelis-Menten kinetics. Both the Km for factor Va and the Vmax of factor Va inactivation were a function of the phospholipid concentration. The Km increased from 1 nM at 2.5 microM phospholipid (Ole2GroPSer/Ole2GroPCho 20:80, mol/mol) to 65 nM at 250 microM phospholipid. The Vmax increased from 20 mol factor Va inactivated.min-1.mol APC-1 at 2.5 microM phospholipid to 62 mol factor Va inactivated.min-1.mol APC-1 at 10 microM phospholipid and remained constant at higher phospholipid concentrations. Protein S appeared to be a rather poor stimulator of APC-catalyzed factor Va inactivation. Protein-S-dependent rate enhancements were only observed in reaction mixtures that contained negatively charged phospholipid vesicles. Independent of the concentration and the lipid composition of the vesicles, protein S caused a twofold stimulation of APC-catalyzed factor Va inactivation. This suggests that, in the human system, enhancement of APC binding to phospholipid vesicles by protein S is of minor importance. Considering that protein S is a physiologically essential antithrombotic agent, it is likely that other factors or phenomena contribute to the in vivo antithrombotic action of protein S.

The association of factor VIII with von Willebrand factor

Factor VIII (FVIII) and von Willebrand factor (vWF) are plasma glycoproteins that circulate as a tightly associated complex. Because they tend to copurify during procedures designed to isolate the biologic activities associated with them, their identity as distinct entities became unequivocally established only during the past 10 years. Improved procedures for the isolation of FVIII, the deduction of the amino acid sequences of FVIII and vWF by using molecular cloning techniques and by direct sequencing, and the use of a variety of biophysical and immunochemical techniques have enhanced the understanding of the FVIII-vWF association. Each subunit of multimeric vWF potentially can bind a single heterodimeric FVIII molecule, although in vivo most of these binding sites are empty. The binding of FVIII to vWF is primarily, if not exclusively, mediated by the light chain of FVIII to the amino-terminal region of the vWF subunit. Cleavage of a fragment from the amino-terminal region of the FVIII light chain by thrombin results in rapid dissociation of the FVIII-vWF complex, a process that apparently is necessary for development of procoagulant activity. Whether this cleavage is needed for the activation of FVIII in the absence of vWF is controversial. The extracellular association of FVIII with vWF may be necessary for efficient secretion of FVIII from its cell of origin. The thermodynamics, kinetics, and nature of the molecular contacts involved in the interaction have not been studied. The association of FVIII with vWF prolongs the lifetime of FVIII in plasma. Whether the FVIII-vWF interaction has other functional roles, such as restricting the location of procoagulant activity, remains unknown.

Expression and characterization of recombinant human factor V and a mutant lacking a major portion of the connecting region

Human coagulation factor V is a protein cofactor that is an essential component of the prothrombinase complex. A full-length factor V cDNA has been subcloned into the mammalian expression vector pDX and used to transfect COS cells. Approximately 95 +/- 4% of the recombinant human factor V (rHFV) synthesized in COS cells is secreted into the culture medium. Forty-eight hours after transfection rHFV antigen levels in the conditioned medium were 70 +/- 15 ng/mL. Factor V activity determined by fibrometer assay increased approximately 5-fold from 0.027 +/- 0.012 to 0.124 +/- 0.044 unit/mL following activation by the factor V activating enzyme from Russell's viper venom (RVV-V). A chromogenic assay specific for factor Va indicated that recombinant factor V had 3.8 +/- 1.3% of the activity of the activated protein. The estimated specific activity of the recombinant factor Va was approximately 1800 +/- 500 units/mg, which is similar to the specific activity of purified plasma factor Va of 1700-2000 units/mg. Immunoprecipitation of [35S]methionine-labeled rHFV revealed a single high molecular mass component (approximately 330 kDa). Treatment of rHFV with thrombin or RVV-V resulted in the formation of proteolytic products that were similar to those seen with plasma factor V. We have also expressed a mutant, rHFV-des-B811-1441, that lacks a large portion of the highly glycosylated connecting region that is present in factor V. Immunoprecipitation of [35S]methionine-labeled rHFV-des-B811-1441 revealed a single-chain polypeptide with Mr approximately 230 kDa. This mutant constitutively expressed 38 +/- 7% of the activity of the RVV-V-activated protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of human factor V by factor Xa and thrombin

The activation of human factor V by factor Xa and thrombin was studied by functional assessment of cofactor activity and sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by either autoradiography of 125I-labeled factor V activation products or Western blot analyses of unlabeled factor V activation products. Cofactor activity was measured by the ability of the factor V/Va peptides to support the activation of prothrombin. The factor Xa catalyzed cleavage of factor V was observed to be time, phospholipid, and calcium ion dependent, yielding a cofactor with activity equal to that of thrombin-activated factor V (factor Va). The cleavage pattern differed markedly from the one observed in the bovine system. The factor Xa activated factor V subunits expressing cofactor activity were isolated and found to consist of peptides of Mr 220,000 and 105,000. Although thrombin cleaved the Mr 220,000 peptide to yield peptides previously shown to be products of thrombin activation, cofactor activity did not increase. N-Terminal sequence analysis confirmed that both factor Xa and thrombin cleave factor V at the same bond to generate the Mr 220,000 peptide. The factor Xa dependent functional assessment of 125I-labeled factor V coupled with densitometric analyses of the cleavage products indicated that the cofactor activity of factor Xa activated factor V closely paralleled the appearance of the Mr 220,000 peptide. This observation facilitated the study of the kinetics of factor V activation by allowing the activation of factor V to be monitored by the appearance of the Mr 220,000 peptide (factor Xa activation) or the Mr 105,000 peptide (thrombin activation). Factor Xa catalyzed activation of factor V obeyed Michaelis-Menten kinetics and was characterized by a Km of 10.4 nM, a kcat of 2.6 min-1, and a catalytic efficiency (kcat/Km) of 4.14 X 10(6) M-1 s-1. The thrombin-catalyzed activation of factor V was characterized by a Km of 71.7 nM, a kcat of 14.0 min-1, and a catalytic efficiency of 3.26 X 10(6) M-1 s-1. This indicates that factor Xa is as efficient an enzyme toward factor V as thrombin.

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.