Mechanism of activation of bovine factor VII. Products of cleavage by factor Xa

Structure of human factor VIIa-soluble tissue factor with calcium, magnesium and rubidium

Coagulation factor VIIa (FVIIa) consists of a γ-carboxyglutamic acid (GLA) domain, two epidermal growth factor-like (EGF) domains and a protease domain. FVIIa binds three Mg2+ ions and four Ca2+ ions in the GLA domain, one Ca2+ ion in the EGF1 domain and one Ca2+ ion in the protease domain. Further, FVIIa contains an Na+ site in the protease domain. Since Na+ and water share the same number of electrons, Na+ sites in proteins are difficult to distinguish from waters in X-ray structures. Here, to verify the Na+ site in FVIIa, the structure of the FVIIa-soluble tissue factor (TF) complex was solved at 1.8 Å resolution containing Mg2+, Ca2+ and Rb+ ions. In this structure, Rb+ replaced two Ca2+ sites in the GLA domain and occupied three non-metal sites in the protease domain. However, Rb+ was not detected at the expected Na+ site. In kinetic experiments, Na+ increased the amidolytic activity of FVIIa towards the synthetic substrate S-2288 (H-D-Ile-Pro-Arg-p-nitroanilide) by ∼20-fold; however, in the presence of Ca2+, Na+ had a negligible effect. Ca2+ increased the hydrolytic activity of FVIIa towards S-2288 by ∼60-fold in the absence of Na+ and by ∼82-fold in the presence of Na+. In molecular-dynamics simulations, Na+ stabilized the two Na+-binding loops (the 184-loop and 220-loop) and the TF-binding region spanning residues 163-180. Ca2+ stabilized the Ca2+-binding loop (the 70-loop) and Na+-binding loops but not the TF-binding region. Na+ and Ca2+ together stabilized both the Na+-binding and Ca2+-binding loops and the TF-binding region. Previously, Rb+ has been used to define the Na+ site in thrombin; however, it was unsuccessful in detecting the Na+ site in FVIIa. A conceivable explanation for this observation is provided.

Overexpression of factor VII ameliorates bleeding diathesis of factor VIII-deficient mice with inhibitors

INTRODUCTION

Factor VIII (FVIII) treatment for hemophilia A has difficulties in correcting bleeding diathesis in the presence of inhibitors.

MATERIALS AND METHODS

An adeno-associated virus type 8 (AAV8) vector containing the factor VII (FVII) gene or the activated factor VII (FVIIa) gene was used to investigate the therapeutic effect of FVII or FVIIa overexpression in FVIII-deficient mice with inhibitors.

RESULTS

Following repeated human FVIII injection, FVIII-deficient mice developed anti-human FVIII antibodies that cross-reacted with mouse FVIII. High transgene expression of murine FVII or murine FVIIa was achieved using the AAV8 vector and resulted in increased blood FVII activity greater than 800% of normal murine FVII levels in vector-injected FVIII-deficient mice. Thromboelastography analysis showed significant improvements in clotting time, clot formation time, α angle, and mean clot firmness in AAV8 vector-injected FVIII-deficient mice with inhibitors. Overexpression of FVIIa ameliorated the bleeding phenotype of FVIII-deficient mice with inhibitors and significantly increased the survival rate after tail clipping. In addition, overexpression of FVII increased the survival rate of FVIII-deficient mice with inhibitors after tail clipping though it was not as efficient as FVIIa overexpression.

CONCLUSIONS

These data suggest that FVII overexpression is an alternative strategy for the treatment of hemophilia A with inhibitors.

The influence of different glycosylation patterns on factor VII biological activity

In this study the bioactivity of three differently glycosylated blood coagulation factor VII (FVII) variants (human plasma FVII, recombinant human FVII produced in CHO and BHK cell cultures) were analyzed and compared. Surface plasmon resonance studies of FVII interaction with soluble and full length TF together with FVII autoactivation assays revealed that BHK-derived FVII has the highest bioactivity, while human plasma and CHO-derived FVII showed very similar bioactivity. The affinity of FVII variants to TF correlates with FVII autoactivation rates--the higher the affinity, the faster the autoactivation rate.

Seminal factor VII and factor VIIa: supporting evidence for the presence of an active tissue factor-dependent coagulation pathway in human semen

Human semen spontaneously coagulates into a semisolid mass and then wholly liquefies in a process that may have some similarity to that of normal blood. This well described phenomenon is referred to as coagulation and liquefaction of semen. Besides other active components of the haemostatic system, semen contains a significant amount of functional tissue factor (TF). However, TF needs factor (F)VII in order to exert it actions. In this study, we assessed human semen for the presence of FVII and FVIIa, and related their levels to conventional fertility parameters. Using a functional, one stage, clotting assay based upon the prolongation of the prothrombin clotting time, using the ACL 300R analyser and an Imubind FVIIa ELISA assay, FVII and FVIIa levels were measured in 97 semen specimens obtained from sub-fertile (sperm counts <20 x 10(6)/mL), normally fertile (sperm counts >or=20 x 10(6) but <60 x 10(6)/mL), fertile sperm donors (sperm counts >or=60 x 10(6)/mL), vasectomized subjects and in a pooled normal semen parameters group (categorization into groups was based on the World Health Organization guidelines on fertility criteria). In addition, conventional semen parameters were analysed on all semen samples. Both FVII and FVIIa were quantifiable in human semen. The mean levels of FVII and FVIIa were 4.4 IU/dL and 12 ng/mL respectively. Despite the observed variations of FVIIa levels in the studied groups they did not meet statistical significance when the groups were tested against each other. However, seminal FVIIa levels showed a significant positive association with semen liquefaction time, sperm motility and semen volume. The anti-sperm antibodies and sperm-agglutination groups were also associated with raised seminal FVIIa levels. We observed no significant relationship between FVIIa levels and total sperm concentration, sperm count per mL (sperm density), sperm progression and days of sexual abstinence. This study demonstrates that human semen contains appreciable amounts of FVII and FVIIa. It is possible to quantify these using commercially available assays. There also appears to be a direct correlation between the levels of these factors and certain seminal parameters. This finding reinforces the concept of an active clotting system in human semen, by establishing the missing link in the activation of a TF-dependent pathway.

Specific and efficient cleavage of fusion proteins by recombinant plum pox virus NIa protease

Site-specific proteases are the most popular kind of enzymes for removing the fusion tags from fused target proteins. Nuclear inclusion protein a (NIa) proteases obtained from the family Potyviridae have become promising due to their high activities and stringencies of sequences recognition. NIa proteases from tobacco etch virus (TEV) and tomato vein mottling virus (TVMV) have been shown to process recombinant proteins successfully in vitro. In this report, recombinant PPV (plum pox virus) NIa protease was employed to process fusion proteins with artificial cleavage site in vitro. Characteristics such as catalytic ability and affecting factors (salt, temperature, protease inhibitors, detergents, and denaturing reagents) were investigated. Recombinant PPV NIa protease expressed and purified from Escherichia coli demonstrated efficient and specific processing of recombinant GFP and SARS-CoV nucleocapsid protein, with site F (N V V V H Q▾A) for PPV NIa protease artificially inserted between the fusion tags and the target proteins. Its catalytic capability is similar to those of TVMV and TEV NIa protease. Recombinant PPV NIa protease reached its maximal proteolytic activity at approximately 30 °C. Salt concentration and only one of the tested protease inhibitors had minor influences on the proteolytic activity of PPV NIa protease. Recombinant PPV NIa protease was resistant to self-lysis for at least five days.

Co-crystal Structure and Inhibition of Factor Xa by PD0313052 Identifies Structurally Stabilized Active Site Residues of Factor Xa and Prothrombinase

The enzyme complex prothrombinase plays a pivotal role in fibrin clot development through the production of thrombin, making this enzyme complex an attractive target for therapeutic regulation. This study both functionally and structurally characterizes a potent, highly selective, active site directed inhibitor of human factor Xa and prothrombinase, PD0313052, and identifies structurally conserved residues in factor Xa and prothrombinase. Analyses of the association and dissociation of PD0313052 with human factor Xa identified a reversible, slow-onset mechanism of inhibition and a simple, single-step bimolecular association between factor Xa and PD0313052. This interaction was governed by association (k(on)) and dissociation (k(off)) rate constants of (1.0 +/- 0.1) x 10(7) M(-1) s(-1) and (1.9 +/- 0.5) x 10(-3) s(-1), respectively. The inhibition of human factor Xa by PD0313052 displayed significant tight-binding character described by a Ki* = 0.29 +/- 0.08 nM. Similar analyses of the inhibition of human prothrombinase by PD0313052 also identified a slow-onset mechanism with a Ki* = 0.17 +/- 0.03 nM and a k(on) and k(off) of (0.7 +/- 0.1) x 10(7) M(-1) s(-1) and (1.7 +/- 0.8) x 10(-3) s(-1), respectively. Crystals of factor Xa and PD0313052 demonstrated hydrogen bonding contacts within the S1-S4 pocket at residues Ser195, Asp189, Gly219, and Gly216, as well as interactions with aromatic residues within the S4 pocket. Overall, these data demonstrate that the inhibition of human factor Xa by PD0313052 occurs via a slow, tight-binding mechanism and indicate that active site residues of human factor Xa, including the catalytic Ser195, are effectively unaltered following assembly into prothrombinase.

Occurrence of cold activation of transfusion plasma during storage at +4 oC

BACKGROUND AND OBJECTIVES

The storage of transfusion plasma at +4 degrees C sometimes leads to the activation of several proteolytic systems. In this study the frequency of cold activation was investigated, as well as whether cold activation of plasma is an individually recurrent property of the donor.

MATERIALS AND METHODS

Plasma units prepared from whole blood obtained from 100 male donors were stored at +2 degrees to +5 degrees C, in bags for 28 days and in cryotubes for up to 42 days. Samples from plasma units, collected by apheresis from 100 male donors, were stored in cryotubes for up to 42 days. Cold activation was measured weekly as kallikrein-like activity of plasma. Samples from repeat apheresis plasma units from 32 donors were measured 12-20 months later. The effects of storage on the contact, coagulation and fibrinolytic systems were determined.

RESULTS

The cumulative frequency of cold-activated plasma units stored in bags was 5% on day 7 and 18% on day 28. After 42 days in cryotubes, 49% of the plasma units were cold activated. Large intraindividual differences in the onset-day of cold activation were observed in plasma samples of some donors. During cold activation, an increase in kallikrein-like activity was accompanied by a decrease in C1 esterase inhibitor activity and an increase in the concentrations of activated factor VII and fibrinopeptide A. The functional plasminogen level was unchanged, while a minor decrease in plasmin inhibitor activity was combined with a corresponding increase in the concentration of plasmin-plasmin inhibitor complex.

CONCLUSIONS

The cumulative frequency of cold-activated plasma units increased in a time-dependent manner during storage at +2 degrees to +5 degrees C for 42 days. The intraindividual onset-day of cold activation varied widely between plasma samples of some donors. Cold activation was associated with a high degree of activation of the contact and coagulation systems. The fibrinolytic system was scarcely affected.

An improved, reliable and practical kinetic assay for the detection of prekallikrein activator in blood products

An improved kinetic assay for prekallikrein activator (PKA), a potential vasodilator, has been developed to be used as an indicator for quality control during production of human albumin preparations. It consists of two reaction stages. In the first stage, PKA and prekallikrein are incubated at 37 degrees C for 45 min to allow the transformation into kallikrein. Kallikrein, a serine protease, catalyzes the splitting of p-nitroaniline (pNA) from its substrate H-D-Pro-Phe-Arg-pNA (S-2302). The rate at which pNA is released was measured spectrophotometrically at 405 nm. Prekallikrein, a substrate of PKA was purified by DEAE ion-exchange chromatography and the major potential variations in the assay were optimized; pH 8.0 and 150 mM sodium chloride were chosen to give a proper ionic strength. Reaction times in the range of 10 to 360 min provided linear dose-response curves. The concentration of prekallikrein was adjusted to fall between 1:1 and 1:3 dilutions to generate a linear standard calibration curve. Under the optimized conditions, reproducibility was checked. In a precision test, the coefficient of variation (CV) stayed within +/-4% and the dose-response curve showed a good correlation (r2=0.999). An accuracy test with an international standard of PKA afforded a mean recovery of 97.5%.

Residues Y179 and H101 of a hydrophobic patch of factor VII are involved in activation by factor Xa

We studied factor Xa activation of human factor VII in hopes of identifying factor VII residues, not adjacent to the cleavage site, involved in this interaction. We made eight factor VIIs with single mutations (N100A, H101A, D102Q, L144A, R147A, Y179A, D186A, and F256A) and two factor VIIs with multiple mutations [MM3 (L144A/R147A/D186A) and MM4 (N100A/H101A/Y179A/F256A)]. Residues in MM3 have previously been identified as affecting factor X activation, and the residues of MM4 are located at a hydrophobic patch of factor VII on the opposite side of the catalytic domain from those in MM3. Only H101A, Y179A, and MM4 were activated significantly more slowly than the wild type. Results of our kinetic analyses showed that the catalytic efficiency of factor Xa for activation of factor VII was 176- and 234-fold higher than that for H101A andY179A, respectively. All the mutants with measurable activity had affinities for tissue factor similar to those of the wild type. The activated hydrophobic patch residues, except N100A, which is adjacent to one of the catalytic residues, had normal activities toward both a small peptide substrate and factor X. The rest of the activated mutants (except D102Q with no activity) had reduced activities toward the small substrate (except R147A) and factor X. We conclude that factor VII activation by factor Xa and factor VIIa's catalytic interaction with factor X involve different regions in the catalytic domain, and residues H101 and Y179, part of an aromatic hydrophobic patch, are specifically involved in factor Xa activation of factor VII.

Surface-mediated control of blood coagulation: the role of binding site densities and platelet deposition

A mathematical model of the extrinsic or tissue factor (TF) pathway of blood coagulation is formulated and results from a computational study of its behavior are presented. The model takes into account plasma-phase and surface-bound enzymes and zymogens, coagulation inhibitors, and activated and unactivated platelets. It includes both plasma-phase and membrane-phase reactions, and accounts for chemical and cellular transport by flow and diffusion, albeit in a simplified manner by assuming the existence of a thin, well-mixed fluid layer, near the surface, whose thickness depends on flow. There are three main conclusions from these studies. (i) The model system responds in a threshold manner to changes in the availability of particular surface binding sites; an increase in TF binding sites, as would occur with vascular injury, changes the system's production of thrombin dramatically. (ii) The model suggests that platelets adhering to and covering the subendothelium, rather than chemical inhibitors, may play the dominant role in blocking the activity of the TF:VIIa enzyme complex. This, in turn, suggests that a role of the IXa-tenase pathway for activating factor X to Xa is to continue factor Xa production after platelets have covered the TF:VIIa complexes on the subendothelium. (iii) The model gives a kinetic explanation of the reduced thrombin production in hemophilias A and B.

The tissue factor region that interacts with factor Xa in the activation of factor VII

Tissue factor is the cell membrane-anchored cofactor for factor VIIa and triggers the coagulation reactions. The initial step is the conversion of factor VII to factor VIIa which, in vitro, is efficiently catalyzed by low concentrations of factor Xa. To identify the tissue factor region that interacts with the activator factor Xa during this process, we evaluated a panel of soluble tissue factor (1-219) mutants for their ability to support factor Xa-mediated activation of factor VII. The tissue factor residues identified as most important for this interaction (Tyr157, Lys159, Ser163, Gly164, Lys165, Lys166, and Tyr185) were identical to those found to be important for the interaction of substrate factor X with the tissue factor.factor VIIa complex. The residues form a continuous surface-exposed patch with an area of about 500 A(2), which appears to be located outside the tissue factor-factor VII contact zone. In agreement, the two monoclonal antibodies 5G6 and D3H44-F(ab')(2), whose epitopes overlap with this identified region, inhibited the rates of factor VII activation by 86% and 95%, respectively. These antibodies also strongly inhibited the conversion of (125)I-labeled factor VII when cell membrane-expressed, full-length tissue factor (1-263) was employed. Together the results suggest the usage of a common surface region of tissue factor in its dual role-as a cofactor for factor Xa-mediated factor VII activation and as a cofactor for factor VIIa-mediated factor X activation. The finding that factor Xa and factor X may engage in similar, if not identical, molecular interactions with tissue factor further indicates that factor Xa and factor X are similarly oriented toward their respective interaction partners in the ternary catalytic complexes.

Engineered recombinant factor VII Q217 variants with altered inhibitor specificities

Recombinant factor VII with residue 217 (chymotrypsinogen numbering system) converted to alanine (VIIQ217A), glutamic acid (VIIQ217E), or glycine (VIIQ217G) was characterized. In a prothrombin time assay, VIIQ217E demonstrated 100%, VIIQ217A 15%, and VIIQ217G <1% clotting activities relative to wild-type VII. Binding of VIIQ217A and VIIQ217G to TF was comparable to that of wild-type VII to TF. All the variants were readily activated by factor Xa. Autoactivation in the presence of TF was efficient with VIIQ217E, slow with VIIQ217A, but undetected with VIIQ217G. Relative to wild-type VII added at the same concentration, VIIQ217E had no effect on the PT of normal plasma, whereas VIIQ217A slightly and VIIQ217G dramatically prolonged the clotting time in a dose-dependent manner. Activation of macromolecular substrates paralleled this functional inhibition. The k(cat)/K(M) values for factor X activation in the presence of TF were 2.4 for VIIaQ217E as compared to 1.9 (M(-)(1) s(-)(1) x 10(7)) for wild-type VIIa, 1.57 for VIIaQ217A, and 0.05 with VIIaQ217G. In comparison to wild-type VIIa, VIIaQ217E cleaved the chromogenic substrate S2765 (Z-D-Arg-Gly-Arg-pNA) with 10-fold higher k(cat). Analysis of the interactions with the inhibitors TFPI and antithrombin III demonstrated that VIIaQ217A but not VIIaQ217E or VIIaQ217G was inhibited less efficiently by TFPI either in the presence or in the absence of factor Xa. In contrast, VIIaQ217A association with antithrombin III in the presence of heparin was the fastest among the variants with a second-order rate constant of 2.31 (x10(3) M(-)(1) min(-)(1)), as compared to 0.47 and 1.47 for VIIaQ217E and wild-type VIIa, respectively. Our results demonstrate that residue Q(217) is important in regulating substrate and, more importantly, inhibitor recognition by VIIa.

One Active C1r Subunit Is Sufficient for the Activity of the Complement C1 Complex: Stabilization of C1r in the Zymogen Form by Point Mutations

The binding of C1 (the first component of complement) to immune complexes leads to the autoactivation of C1r through the cleavage of the Arg463-Ile464 bond in the catalytic domain. Spontaneous activation of C1r (and C1) also occurs in the fluid phase, preventing the characterization of the zymogen form of C1r. To overcome this difficulty, the zymogen form of human C1r was stabilized by mutating the Arg in the Arg463-Ile464 bond to Gln. This mutant was designated as mutant QI. Recombinant C1r (wild type (wt) or mutant) was expressed in insect cells using serum-free medium in functionally pure form; therefore, the cell culture supernatant was suitable to reconstruct C1 for the hemolytic assay. Mutant QI was a stable, nonactivable zymogen and showed no hemolytic activity in reconstituted C1. However, this stable zymogen C1r mutant could form an active mixed dimer with the wt C1r, indicating that one active C1r subunit in the C1 complex is sufficient for the full activity of the entire complex. Our experiments also showed that the exchange of C1r monomers between the C1r dimers is completed in less than 16 h even at pH 7 and 4°C. Two other mutants were also constructed by changing Arg463 to Lys, or Ile464 to Phe, and were designated as mutants KI and RF, respectively. Although these substitutions did increase the stability of the proenzyme in the cell culture supernatant, the mutant proteins retained their ability to autoactivate, and both had a wt-like hemolytic activity.

Manipulation of the membrane binding site of vitamin K-dependent proteins: enhanced biological function of human factor VII

Recent studies suggested that modification of the membrane contact site of vitamin K-dependent proteins may enhance the membrane affinity and function of members of this protein family. The properties of a factor VII mutant, factor VII-Q10E32, relative to wild-type factor VII (VII, containing P10K32), have been compared. Membrane affinity of VII-Q10E32 was about 20-fold higher than that of wild-type factor VII. The rate of autoactivation VII-Q10E32 with soluble tissue factor was 100-fold faster than wild-type VII and its rate of activation by factor Xa was 30 times greater than that of wild-type factor VII. When combined with soluble tissue factor and phospholipid, activated factor VII-Q10E32 displayed increased activation of factor X. Its coagulant activity was enhanced in all types of plasma and with all sources of tissue factor tested. This difference in activity (maximum 50-fold) was greatest when coagulation conditions were minimal, such as limiting levels of tissue factor and/or phospholipid. Because of its enhanced activity, factor VII-Q10E32 and its derivatives may provide important reagents for research and may be more effective in treatment of bleeding and/or clotting disorders.

Evaluation of the initiation phase of blood coagulation using ultrasensitive assays for serine proteases

The initiation phase of enzyme generation in a reconstituted model of the tissue factor (TF) pathway to thrombin was evaluated. At 1.25 pM added TF, no thrombin generation was observed in the absence of factor V. The substitution of factor Va for factor V increased the rate of thrombin generation. Factor X activation during the initiation phase was not influenced by the absence of factor VIII or thrombin, leading to the conclusion that initially factor Xa is generated exclusively by the factor VIIa-TF complex. When thrombin was eliminated from the system, no contribution of the factor IXa-factor VIIIa complex to factor X activation was observed during the propagation phase. Similarly, factor V activation was also not observed in the absence of thrombin, indicating that thrombin is the only enzyme responsible for factor V and factor VIII activation. Only subnanomolar amounts of factor VII were activated when prothrombin activation was almost complete. In the absence of coagulation inhibitors, factor XI did not influence thrombin generation initiated by 1.25 pM factor VIIa-TF complex. The termination of factor XIa generation by added hirudin in the factor XI experiment indicates that factor XI activation occurs exclusively by thrombin.

The location of the active site of blood coagulation factor VIIa above the membrane surface and its reorientation upon association with tissue factor. A fluorescence energy transfer study

The topography of membrane-bound blood coagulation factor VIIa (fVIIa) was examined by positioning a fluorescein dye in the active site of fVIIa via a tripeptide tether to yield fluorescein-D-phenylalanyl-L-prolyl-L-arginyl-fVIIa (Fl-FPR-fVIIa). The location of the active-site probe relative to the membrane surface was determined, both in the presence and absence of tissue factor (TF), using fluorescence energy transfer between the fluorescein dye and octadecylrhodamine (OR) at the phospholipid vesicle surface. When Fl-FPR-fVIIa was titrated with phospholipid vesicles containing OR, the magnitude of OR-, calcium ion-, and phosphatidylserine-dependent fluorescence energy transfer revealed that the average distance of closest approach between fluorescein in the active site of fVIIa and OR at the vesicle surface is 82 A assuming a random orientation of donor and acceptor dyes (kappa2 = 2/3; the orientational uncertainty totals approximately 10%). The active site of fVIIa is therefore located far above the membrane surface, and the elongated fVIIa molecule must bind at one end to the membrane and project approximately perpendicularly out of the membrane. When Fl-FPR-fVIIa was titrated with vesicles that contained TF, the efficiency of energy transfer was increased by a TF-dependent translational and/or rotational movement of the fVIIa protease domain relative to the membrane surface. If this movement was solely translational, the height of the active site of fVIIa was lowered by an average of 6 A after binding to TF. The association of fVIIa with TF on the membrane surface therefore causes a significant reorientation of the active site relative to the membrane surface. This cofactor-dependent realignment of the active-site groove presumably facilitates and optimizes fVIIa cleavage of its membrane-bound substrates.

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.

Inhibitory properties of separate recombinant Kunitz-type-protease-inhibitor domains from tissue-factor-pathway inhibitor

Tissue-factor-pathway inhibitor (TFPI) is a multivalent inhibitor with three tandemly arranged Kunitz- type-protease-inhibitor (KPI) domains. Previous studies [Girard, Y. J., Warren, L. A., Novotny , W. F., Likert, K. M., Brown, S. G., Miletich, J. R & Broze, G. J. (1989) Nature 338, 518-520] by means of site-directed mutagenesis indicated that KPI domain 1 interacts with factor VIIa, that KPI domain 2 interacts with factor Xa, and that KPI domain 3 is apparently without inhibitory function. To elucidate the reaction mechanism of this complex inhibitor, we followed a different approach and studied the inhibitory properties of fragments of TFPI obtained by expression in yeast. Results obtained with TFPI-(1-161)-peptide and separate recombinant TFPI-KPI domains 1, 2 and 3 showed that KPI domain 1 inhibited factor VIIa/tissue factor (Ki = 250 nM), KPI domain 2 inhibited factor Xa (Ki = 90 nM), and that KPI domain 3 was without detectable inhibitory function. Studies with separate KPI domains also showed that KPI domain 2 was mainly responsible for inhibition of trypsin (Ki = 0.1 nM) and chymotrypsin (Ki = 0.75 nM), whereas KPI domain 1 inhibited plasmin (Ki = 26 nM) and cathepsin G (Ki = 200 nM). The structural basis for the interaction between serine proteases and KPI domains is discussed in terms of putative three-dimensional models of the proteins derived by comparative molecular-modelling methods. Studies of factor Xa inhibition by intact TFPI (Ki approximately 0.02 nM) suggested that regions other than the contact area of the KPI domain, interacted strongly with factor Xa. Secondary-site interactions were crucial for TFPI inhibition of factor Xa but was of little or no importance for its inhibition of trypsin.

The roles of factor VII's structural domains in tissue factor binding

Factor VIIa binds to tissue factor in one of the initial steps of blood clotting. In order to determine the role of the various domains of the factor VII molecule in this interaction, we made several chimeric factor VII proteins using recombinant DNA techniques. The molecules have factor IX domains substituted into factor VII and vice versa. The domains exchanged were the 4-carboxyglutamic acid plus aromatic stack domain (gla), the first epidermal growth factor-like domain (Egf-1), the second epidermal growth factor-like domain (Egf-2), and the catalytic domain. Using tissue factor-coated microtiter wells, competition binding studies with 125I-labeled factor VIIa indicated factor VIIa's Kd is 4.2 nM. Employing the same microtiter plate assay, koff and kon were determined and yielded a Kd of 1.5 nM. The results of competitive binding experiments and activation assays using chimeric proteins indicated the interaction between factor VIIa and tissue factor involves direct contact between tissue factor and factor VIIa's Egf-1 domain and catalytic domain. On the other hand, the gla and Egf-2 domains, while necessary for optimal binding, may merely impart structure to the rest of the molecule. However, either one or both of the latter domains might contribute a relatively small amount of energy to direct binding.

Factor VII coagulant activity and cholesterol changes in premenopausal women consuming a long-term cholesterol-lowering diet

We periodically obtained blood samples from mildly hypercholesterolemic, but otherwise healthy, premenopausal women who were recruited to participate in a study of a long-term, cholesterol-lowering diet. All meals were prepared and most meals were consumed in the study center dining facility. Tests performed on blood samples included fibrinogen, cholesterol, factor VII coagulant activity (VIIc), and other measures of factor VII. We found that when women switched from a typical American diet (37% fat, polyunsaturated fatty acid to saturated fatty acid [P/S] ratio 0.5, 300 mg cholesterol/d) to a diet lower in fat and cholesterol (American Heart Association phase 2 diet: 30% fat, P/S ratio of 1, 150 to 200 mg cholesterol/d) and maintained that diet for 20 weeks, their plasma cholesterol levels decreased by approximately 6% after 4 weeks and remained at that level until study termination. Likewise, VIIc decreased by approximately 11% while factor VII antigen, total factor VII activity, and fibrinogen concentration did not change appreciably from baseline values. Our results show that premenopausal women benefit from a diet lower in total and saturated fat by a reduction in blood cholesterol and VIIc. Extrapolation from data on men in the Northwick Park Heart Study indicates that the 11% decrease in VIIc activity would correspond to an approximately 30% decrease in risk of mortality from coronary heart disease.

Biochemistry of factor X

Factor X circulates as a serine protease which is converted to the active form at the point of convergence of the intrinsic and extrinsic coagulation pathways. Subsequently, the enzymatic species, factor Xa, is involved in macromolecular complex formation with its cofactor factor Va, a phospholipid surface and calcium to convert prothrombin into thrombin. The gene encoding factor X shares a number of structural and organisational features in common with the other vitamin K-dependent coagulation proteins, suggesting that they have evolved from a common ancestral gene. Each of the exons encoding these proteins can be considered as a module coding for a homologous domain in each protein. These structural domains in factor X are responsible for specific functional properties including gamma-carboxylase recognition, calcium binding, phospholipid surface interaction, as well as cofactor and substrate binding. Studies of recombinant proteins and proteolytic fragments continue to provide significant insight into structure-function relationships of the protein modules within factor X.

Molecular defects in CRM+ factor VII deficiencies: modelling of missense mutations in the catalytic domain of FVII

The molecular defects causing CRM+ factor VII deficiency were investigated in seven unrelated subjects and several members of their families. Four missense mutations located in the catalytic domain of factor VII were found. The previously reported 304Arg-->Gln substitution was present in the homozygous and heterozygous forms, with different polymorphic haplotypes, thus demonstrating that it is recurrent and frequent in the Italian population. The 310Cys-->Phe substitution was found in the homozygous form and in the compound heterozygous condition with the nonsense mutation 356Trp-->stop. Two missense mutations, 298Met-->Ile and 342Gly-->Arg, were found in the homozygous and in the heterozygous condition respectively. Molecular heterogeneity was further increased by finding of the 353Arg-->Gln polymorphism in the doubly heterozygous condition with the 304 and 342 mutations. Plausible explanations for loss of FVII function were found by inspecting a model of the serine protease domain of factor VIIa. Inefficient activation of the catalytic site is predicted for 298Met-->Ile. 342Gly-->Arg would directly distort the geometry of the 'oxyanion hole' preventing formation of a substrate enzyme intermediate. 310Cys-->Phe is predicted to have an adverse effect on tissue factor interaction. These mutations point to important regions of the factor VII molecule.

Analysis of the partial nucleotide sequences and deduced primary structures of the protease domains of mammalian blood coagulation factors VII and X

In order to obtain sequence data for the blood coagulation factor VII and factor X in several mammalian species, we amplified and sequenced the DNA segments of exon VIII from each gene by means of the polymerase chain reaction (PCR) method. The DNA segments from the following species were successfully amplified: factor VII from the rhesus monkey and dog, and factor X from the rhesus monkey, Syrian hamster and rat. In each factor, the nucleotide sequences and predicted primary structures of the protease domain showed a high degree of homology among species; amino acid identities of approximately 68%-92% and 80%-98% were demonstrated among species in factor VII and factor X, respectively. The locations of the active site residues and five Cys residues were evolutionarily conserved in both factors. Interestingly, the amino acids involved in the human genetic variants, both factor VII 304-Arg and factor X 326-Arg, were always conserved across species. The data presented here will be helpful for investigating human genetic variants of factor VII or X, and will provide considerable information for constructing in vitro site-specific mutants of these factors.

Dietary fat induces changes in factor VII coagulant activity through effects on plasma free stearic acid concentration

Previous studies have demonstrated activation of the contact system of coagulation and an increase in factor VII coagulant activity (VIIc) when citrated plasma is incubated in the presence of micellar stearate. The products of contact activation, factors XIIa and IXa, were responsible in this system for the activation of factor VII, thereby increasing factor VIIc. To obtain evidence that these in vitro interactions also operate in vivo, factor VIIc was examined in relation to plasma free fatty acid concentrations in five healthy individuals during the consumption of isocaloric high-saturated fat, high-unsaturated fat, and low-fat diets, each taken for 4 weeks in random order and separated by intervals of 12 weeks. For all but the final 3 days of each phase, subjects selected appropriate foods from prepared lists to meet the dietary requirements. Experimental diets of predetermined fat content and composition were fed on days 26 through 28 in each phase. Fat supplied on average 62% of energy in two of the experimental diets and less than 20% of energy in the third. On the final day of each dietary phase, the concentrations of the various free fatty acids and factor VIIc were measured before breakfast and at three 150-minute intervals thereafter. Plasma factor VIIc was, respectively, 6.5% and 13.1% of standard higher on the unsaturated and saturated fat diets than on the low-fat diet. Furthermore, the plasma concentration of stearic acid was strongly associated with factor VIIc (r = .58; P < .0001), and this relation remained significant (P = .003) after allowance for the plasma concentrations of palmitic, oleic, and linoleic acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular analysis of factor VII deficiency in Italy: a frequent mutation (FVII Lazio) in a repeated intronic region

Molecular defects and polymorphic haplotypes of coagulation factor VII gene were studied in eight unrelated Italian subjects with factor VII deficiency, seven having the factor VII- variant, one the factor VIIR variant. An intron 7 mutation, which alters the consensus donor splice site sequence, was found in six subjects. The presence of the founder effect is suggested by their common geographical origin (a mountain area in the Lazio region) and by the identical polymorphic haplotype underlying the mutation. A different mutation, also located in the 5' monomer of the repeated intron 7 sequence, was found in the heterozygous condition in a subject from Northern Italy. New polymorphic alleles were detected in the repeated intron 7 region in subjects from Eastern Africa. Two missense mutations in codon 97 (Gly-->Cys, Gly-->Ser), the first found in the compound heterozygous condition with the frequent intron 7 mutation, suggest the presence of a hot spot mutation site in the second epidermal growth factor domain. Two neutral dimorphisms at codon 333Ser and 115His were detected, the last in linkage disequilibrium with the 353Arg/Gln polymorphism, and showing differences in frequency in the FVII deficient and control subjects.

Accumulation of the recombinant factor VIIa in rat bone: importance of the Gla-domain and relevance to factor IX, another vitamin K-dependent clotting factor

The vitamin K-dependent clotting factors II, VII, IX, and X are proteins which undergo gamma-carboxylation of specific glutamic acid residues prior to secretion from the liver. These unique Ca2+ binding amino acids allow the interaction of the proteins with cell surface phospholipids, a function that is crucial for expression of full procoagulant activity of the proteins. The N-terminal region of the molecule contains the gamma-carboxylation sites and is termed the Gla-domain. A preliminary observation in rats suggested that mineralized bone accumulated activated recombinant FVII (rFVIIa: NovoSeven) as well as the non-activated, single chain rFVII. The present study investigated the role of the Gla-domain in the accumulation of rFVII in bone, as well as the influence of the activation state of FVII on this phenomenon. Rats were treated with 125I-labelled rFVII, rFVIIa, Gla-domainless rFVIIa, factor IX, iodide, or recombinant human growth hormone (rhGH). Following sacrifice, radioactivity was measured in mineralized bone, among other tissues. Following administration of 125I-radiolabelled rFVII, rFVIIa and factor IX, but not Gla-domainless rFVIIa, iodide or rhGH, extensive sequestration occurred in endochondrally as well as intramenbranously ossified bones. The results indicate that the proteins containing a Gla-domain, and only these, are sequestered in bone. Additionally, the normally occurring form of FVII in the circulation, the single-chain FVII, exhibited similar kinetics in rat bone and plasma, as the two-chain rFVIIa. The half-life of rFVII/rFVIIa in mineralized bone was between 3 and 4 days, implying that significant bone accumulation of the factor will take place at steady state.(ABSTRACT TRUNCATED AT 250 WORDS)

The dysfunction of coagulation factor VIIPadua results from substitution of arginine-304 by glutamine

This study addresses whether a mutation in the factor VIIPadua gene could explain the reduced activity of the inherited variant protein. All nine exons of the normal and Padua factor VII gene were amplified using the polymerase chain reaction, cloned into pUC19 and sequenced. A point mutation (G to A at nucleotide position 10828) was found which results in the substitution of a glutamine (CAG) for arginine (CGG) at amino acid position 304. This substitution creates a PvuII restriction site useful in screening for the defect and in demonstrating homozygosity. This substitution involves an arginine residue in the catalytic domain within a Leu*****Pro******Cys motif which occurs in conserved region 5 in up to 16 coagulation and other serine proteinases. On the basis of conformational homology among serine proteinases, it is suggested that the observed amino acid substitution in factor VIIPadua could cause structural changes affecting its activation and/or catalytic activity.

An overview of hemostasis

Hemostasis is a remarkable and a remarkably complex mechanism. It can maintain blood in a fluid state intravascularly but very quickly changes blood to a jellylike mass upon disruption of the vasculature. This review will give a synopsis of the 3 phases of hemostasis: platelet, vascular, and coagulation. Fibrinolysis and control mechanisms of hemostasis will also be covered. In addition, brief descriptions of the clinical and laboratory evaluation of patients and the diagnosis of bleeding disorders will be presented.

FVIIa derivatives obtained by autolytic and controlled cathepsin G mediated cleavage

The heavy chain of coagulation factor VII contains a serine esterase entity. A partial cleavage in the heavy chain occurs during purification and activation of the single-chain zymogen, presumably as a result of autolysis. Neutrophil cathepsin G initially generates a Gla-domainless FVIIa without coagulant activity. However, on extended exposure cleavage also occurs in the heavy chain, resulting in a complete loss of enzyme activity. Four cleavage sites on the heavy chain, two susceptible to trypsin-like autolysis and two susceptible to chymotrypsin-like cathepsin G-mediated catalysis have been identified. The hydrolysis of peptide bonds in the heavy chain might contribute to regulation of the coagulation process in vivo.

Characterization of a Ca (II)-independent monoclonal antibody that lacks reactivity with Gla-domainless human factor VII

Eight murine monoclonal antibodies to human factor VII have been produced and characterized. As tested by direct ELISA, all antibodies bound to immobilized factor VII and this interaction did not require Ca(II). On immunoblotting three antibodies reacted only with native factor VII and activated factor VII (rFVIIa), two with native factor VII, activated factor VII and with the light chain of reduced activated factor VII and three with reduced activated factor VIIa (light chain) only. When coupled to Sepharose five antibodies were capable of immunodepleting factor VII from normal plasma and inhibited coagulant activity with varied potencies. Among these antibodies, one (10C12.2) had peculiar characteristics, lacking reactivity with Gladomainless FVII and with the Ca(II)-dependent conformation. This antibody has been used to develop a two-site ELISA which appears to measure native factor VII but not poorly carboxylated, inactive forms of the molecule.

Interaction of thrombin with synthetic fluorescent substrate immobilized on polymer membrane

Synthetic fluorescent substrates for thrombin were immobilized on a polyurethane membrane. The interaction of thrombin with the immobilized substrates was investigated. Upon hydrolysis of immobilized substrates by thrombin, fluorescence was emitted, intensity increasing linearly with time. From a Lineweaver-Burk plot, kinetic parameters of the enzyme reaction on the membrane surface were determined. The dissociation constant (K) was increased and the catalytic constant (k) decreased markedly by immobilization. More reactive thrombin substrates in solution were also more reactive in the immobilized state. More thrombin was adsorbed and adsorbed thrombins were inactivated more strongly on the surface with more reactive substrates when immobilized. The immobilization of thrombin substrate markedly suppressed fibrin formation on the membrane surface and made the membrane non-thrombogenic.

Activation of human factor VII by the prothrombin activator from the venom of Oxyuranus scutellatus (Taipan snake)

The crude venom of Oxyuranus scutellatus (Taipan snake) was found to cleave single-chain human factor VII to yield a two-chain molecule indistinguishable from authentic factor VIIa by SDS-polyacrylamide gel electrophoresis. A protease that activates factor VII was purified from this venom by a combination of gel permeation and ion-exchange chromatography. Characterization of the venom factor VII activator revealed its apparent identity with the Oxyuranus scutellatus prothrombin activator. The purified venom prothrombin activator was observed to activate factor VII by limited proteolysis in a reaction that was greatly potentiated by calcium and phospholipids (75% phosphatidyl choline/25% phosphatidylserine). Treatment of the venom protease with 0.8 M NaSCN weakly inhibited its ability to activate factor VII indicating that, in contrast to prothrombin activation, the factor Va-like component of this oligomeric enzyme complex was not essential for the activation of factor VII.

In vitro non-thrombogenicity of a thrombin-substrate-immobilized polymer surface by the inhibition of thrombin activity

Derivatives of thrombin substrate were synthesized and immobilized on a poly(acrylic acid)-grafted polyurethane film. The carboxyl terminal of the thrombin substrate peptide should be blocked for a higher inhibitory effect of the thrombin activity. Immobilization of the thrombin substrate peptide enhanced adsorption and inactivation of thrombin on the polymer film to prolong the time for fibrin network formation, and suppressed adhesion and deformation of platelets on the film. Consequently, in vitro thrombus formation on the polymer film was strongly suppressed.

Proteolytic activation of human factors IX and X by recombinant human factor VIIa: effects of calcium, phospholipids, and tissue factor

Previous studies indicated that factor VIIa, in complex with tissue factor, readily activates either factor X or factor IX in the presence of calcium ions. In order to assess the relative physiological importance of the activation of factor IX versus the activation of factor X by recombinant factor VIIa, we have obtained steady-state kinetic parameters for the factor VIIa catalyzed activation of factor IX and factor X under a variety of cofactor conditions that include calcium alone, calcium and phospholipids, calcium, phospholipids, and tissue factor apoprotein, and calcium and cell-surface tissue factor. Calcium alone stimulated the activation of factors IX and X by factor VIIa maximally at 1 and 2.5 mM, respectively. In the presence of 25 microM phospholipids, maximal rates of factor IX and factor X activation were achieved at 2.5-5 mM calcium. With calcium alone, or with phospholipid and calcium, the initial rates of factor IX activation by factor VIIa were significantly higher than that observed for factor X. Kinetic studies revealed that the Km for the factor VIIa catalyzed activation of factor IX was essentially constant in the presence of 5 mM calcium and 1-500 microM phospholipid, whereas the Km for factor X activation varied with phospholipid concentration, reaching a minimum at 7-20 microM phospholipid. At all concentrations of added phospholipid, the kcat/Km ratio for the activation of factor IX by factor VIIa appeared to be considerably greater than that observed for the activation of factor X.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism by which recombinant factor VIIa shortens the aPTT: activation of factor X in the absence of tissue factor

Clinical trials have recently begun using high concentrations of activated recombinant factor VII (rFVIIa) for the treatment of hemophilic patients with inhibitors. Unexpectedly, the activated partial thromboplastin time (aPTT) was observed to be significantly shortened during infusion of the rFVIIa. To determine the mechanism for this shortening, the effect of rFVIIa on both the prothrombin time (PT) and the aPTT of normal and various factor-deficient plasmas was examined. rFVIIa shortened the PT of all plasmas tested except FX and FV deficient plasmas. rFVIIa also shortened the aPTT of all plasmas tested except FX and FV deficient plasmas. Since there is no added tissue factor (TF) in aPTT reagents, rFVIIa appeared to shorten the aPTT in the absence of TF. To investigate this possibility, the activity of rFVIIa in a purified system containing only FX, phospholipid vesicles (1:1 PS:PC), and calcium was examined. In this system, rFVIIa activated factor X in the absence of TF. If any component of the purified system was omitted, there was no detectable activation of FX. Thus it appears that calcium and phospholipids are required for the activation of FX by rFVIIa in the absence of TF. Increasing the concentration of rFVIIa increased the rate of FX activation, but the rate of activation was always much lower than that observed with even trace amounts of tissue factor. We conclude that high concentrations of rFVIIa, in the presence of calcium and phospholipid, can directly activate FX in the absence of TF and hence account for the shortening of the aPTT in inhibitor patients treated with rFVIIa.