Effects of warfarin on vitamin K-dependent coagulation factors in Atlantic salmon and rainbow trout with special reference to factor X

Plasma factor X (FX) was severely depleted in Atlantic salmon and rainbow trout after 18 days of oral warfarin administration; this response was further reflected in prolonged prothrombin time. Thus, both species require vitamin K for the maintenance of normal haemostasis. FX activity in controls of rainbow trout was 3- to 5-fold that of salmon. The FX of both species show several of the features of human FX: it is activated by RVV, inhibited by SBTI, inactivated in the presence of heparin, and adsorubs to insoluble barium precipitates. Rainbow trout sustained warfarin poisoning better than did salmon, with mortalities occurring 8 days later in trout. After 18 days all remaining fish had developed a severe haemorrhagic anaemia with anisocytosis, poikilocytosis, leucopenia, and thrombocytopenia.

Activation of the coagulation mechanism on tumor necrosis factor-stimulated cultured endothelial cells and their extracellular matrix. The role of flow and factor IX/IXa

Infusion of tumor necrosis factor (TNF) into tumor-bearing mice led to intravascular clot formation with fibrin deposition in microvessels in the tumor bed in close association with the vessel wall, which could be prevented by active site-blocked factor IXa (IXai). This observation prompted us to examine the role of the intrinsic system in activation of the coagulation mechanism on TNF-stimulated human endothelial cell monolayers and endothelial-derived matrix during exposure to purified coagulation factors or flowing blood. Treatment of endothelial cells in intact monolayers with TNF induced expression of the procoagulant cofactor tissue factor (TF) in a dose-dependent manner, and after removal of the cells, TF was present in the matrix. TNF-treated endothelial cell monolayers exposed to blood anticoagulated with low molecular weight heparin induced activation of coagulation. Addition of IXai blocked the procoagulant response on TNF-treated endothelial cells, and consistent with this, the presence of factor IX/VIIIa enhanced endothelial TF/factor VII(a) factor X activation over a wide range of cytokine concentrations (0-600 pM). When TF-dependent factor X activation on endothelial cells was compared with preparations of subendothelium, the extracellular matrix was 10-20 times more effective. IXai blocked TF/factor VII(a) mediated activated coagulation on matrix, but only at lower concentration of TNF (less than 50 pM). Similarly, enhancement of factor Xa formation on matrix by factors IX/VIIIa was most evident at lower TNF concentrations. When anticoagulated whole blood flowing with a shear of 300 s-1 was exposed to matrices from TNF-treated endothelial cells, but not matrices from control cells, fibrinopeptide A (FPA) generation, fibrin deposition, and platelet aggregate formation were observed. FPA generation could be prevented by a blocking antibody to TF and by active site-blocked factor Xa (Xai) over a wide range of TNF concentrations (0-600 pM), whereas IXai only blocked FPA generation at lower TNF concentrations (less than 50 pM). Activation of coagulation on matrix from TNF-stimulated endothelial cells was dependent on the presence of platelets, indicating the important role of platelets in propagating the reactions leading to fibrin formation. These observations demonstrate the potential of cytokine-stimulated endothelium and their matrix to activate coagulation and suggest the importance of the intrinsic system in factor Xa formation on cellular surfaces.

Human plasma and recombinant factor VII. Characterization of O-glycosylations at serine residues 52 and 60 and effects of site-directed mutagenesis of serine 52 to alanine

Factor VII is a multidomain, vitamin K-dependent plasma glycoprotein that participates in the extrinsic pathway of blood coagulation. Earlier studies demonstrated a novel disaccharide (Xyl-Glc) or trisaccharide (Xyl2-Glc) O-glycosidically linked to serine 52 in human plasma factor VII (Nishimura, H., Kawabata, S., Kisiel, W., Hase, S., Ikenaka, T., Shimonishi, Y., and Iwanaga, S. (1989) J. Biol. Chem. 264, 20320-20325). In the present study, human plasma and recombinant factor VII were isolated and subjected to enzymatic fragmentation. Peptides comprising residues 48-62 of the first epidermal growth factor-like domain of each factor VII preparation were isolated for comparative analysis. Using a combined strategy of amino acid sequencing, carbohydrate and amino acid composition analysis, and mass spectrometry, three different glycan structures consisting of either glucose, glucose-xylose, or glucose-(xylose)2 were detected O-glycosidically linked to serine 52 in plasma and recombinant factor VII. Approximately equal amounts of the three glycan structures were observed in plasma factor VII, whereas in recombinant factor VII the glucose and the glucose-(xylose)2 structures predominated. In addition to the O-linked glycan structures observed at serine 52, a single fucose was found to be covalently linked at serine 60 in both human plasma and recombinant factor VII. Carbohydrate and mass spectrometry analyses indicated that the fucosylation of serine 60 was virtually quantitative. Metabolic labeling studies using [14C]fucose confirmed the presence of O-linked fucose at serine 60. In order to assess whether the carbohydrate moiety at serine 52 contributes to the biological activity of factor VII, we have constructed a site-specific mutant of recombinant factor VII in which serine 52 has been replaced with an alanine residue. Mutant factor VIIa exhibited approximately 60% of the coagulant activity of wild-type factor VIIa in a clotting assay. The amidolytic activity of mutant factor VIIa was indistinguishable from that observed for recombinant wild-type factor VIIa. In addition, the ability of mutant factor VIIa in complex with either purified relipidated tissue factor apoprotein or tissue factor on the surface of a human bladder carcinoma cell line (J82) to activate either factor X or factor IX was virtually identical to that observed for wild-type factor VIIa. These results indicate that the carbohydrate moiety O-glycosidically linked to serine 52 does not appear to be involved either in the interaction of factor VIIa with tissue factor, or the expression of its proteolytic activity toward factor X or factor IX following complex formation with tissue factor.(ABSTRACT TRUNCATED AT 400 WORDS)

Chromogenic substrate assay of extrinsic pathway inhibitor (EPI): levels in the normal population and relation to cholesterol

A two-stage chromogenic substrate assay was standardized to measure extrinsic pathway inhibitor (EPI) activity in plasma and serum samples. In the first stage, diluted plasma or serum (0-0.8%) was incubated with factor VIIa (25 pM), tissue thromboplastin (tissue factor, TF, 1% v/v) with excess binding sites for factor VIIa, and factor Xa (0.8 nM). In the second stage, excess factor X and chromogenic substrate were added as substrate for residual TF/factor VIIa catalytic activity. Heating the samples at 56 degrees C for 15 min before assay removed greater than 95% of the factor VII amidolytic activity of the samples, defibrinated the plasma, and produced only slight reduction of EPI activity. The coefficient of variation for the same sample assayed on different days was 8.7-10.6% and the intra-assay coefficient of variation was 5.0%. Addition of anti-EPI immunoglobulin to normal plasma completely abolished the EPI activity of the sample. EPI activity was stable in plasma samples stored at -20 degrees C, but in serum, some samples lost greater than 50% activity after 3 months at -70 degrees C. Median EPI activity of umbilical cord blood was 45% (range 33-93%). In a cohort of healthy blood donors (n = 176) EPI activity was significantly correlated with age; the regression line was y = 68% + 0.60x (r = 0.39). The approximated standard deviation for the regression line was 17.9% and the age-adjusted reference limits were determined. Equal levels were seen in males and females.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the epidermal growth factor-1 and hydrophobic stack domains of human factor IX in binding to endothelial cells

To determine the function and specificity in factor IX of the first epidermal growth factor (EGF)-like domain and the eight-amino acid hydrophobic stack encoded by exon C (residues 39-46), these domains were replaced by the corresponding polypeptide regions of factor X and chimeric proteins were produced in human embryo kidney cells. Both chimeras were activated by factor XIa at a rate similar to plasma factor IX and exhibited calcium-dependent fluorescence quenching similar to plasma factor IX. Both chimeras competed equally for binding to the endothelial cell receptor. Our findings make it unlikely that the first EGF-like domain or the hydrophobic stack of factor IX are responsible for the specific binding of factor IX to its endothelial cell receptor.

The effects of warfarin on HepG2 cells suggest that prothrombin and factor X interact differently with the vitamin K-dependent carboxylase in the secretory pathway

HepG2 cells have been shown to respond to warfarin by 1) enhanced vitamin K-dependent carboxylase activity; 2) enhanced intracellular concentration of the factor X clotting factor precursor and 3) enhanced vitamin K-dependent 14C-labelling of a 74 kDa microsomal protein which has been identified as the factor X precursor. There was no difference in any of these measured parameters whether the cells had been treated for 4 or 24 hours with warfarin. In contrast to the intracellular factor X concentration, the intracellular prothrombin precursor concentration was not affected by the drug which suggests there is a difference in the mechanism of processing of these two clotting factors by HepG2 cells. The data are consistent with the view that warfarin maintains its effect on the vitamin K-dependent carboxylation system in HepG2 cells for 24 hours and support the hypothesis that clotting factor X and prothrombin precursors interact differently with the vitamin K-dependent carboxylase.

Inhibition of recombinant human blood coagulation factor VIIa amidolytic and proteolytic activity by zinc ions

Although it is well established that calcium is an essential cofactor in blood coagulation, recent experimental evidence suggests that zinc may also play an important role in hemostasis. In the present study, we have examined the effect of zinc ions on the amidolytic and proteolytic activity of recombinant factor VIIa in the presence of physiological levels of calcium ions. The amidolytic activity of factor VIIa was inhibited half-maximally by 20 microM zinc. The amidolytic activity of a derivative of factor VIIa lacking the gamma-carboxyglutamic acid domain was also inhibited half-maximally by 20 microM zinc, suggesting that the mechanism of zinc inhibition of factor VIIa amidolytic activity did not involve its gamma-carboxyglutamic acid residues. The amidolytic activity of a complex of recombinant tissue factor and factor VIIa was inhibited half-maximally by 70 microM zinc. In contrast to the results obtained with factor VIIa, the amidolytic activities of other human vitamin K-dependent coagulation proteases including factor Xa, thrombin and activated protein C were not appreciably affected by 50-100 microM zinc. The proteolytic activation of factor X by a complex of factor VIIa and relipidated tissue factor apoprotein was inhibited half-maximally by 40 microM zinc, whereas activation of factor IX in this system was inhibited half-maximally by 70 microM zinc ions. Considerably higher levels of zinc (approximately 100 microM) were required to inhibit half-maximally the rate of factor X activation by a complex of factor VIIa and functional tissue factor on the surface of either a human bladder carcinoma cell line, J82, or stimulated human umbilical vein endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional difference between intrinsic and extrinsic coagulation pathways. Kinetics of factor X activation on human monocytes and alveolar macrophages

Activation of coagulation factor X via the intrinsic pathway requires the assembly of factors IXa and VIII on lipid membranes. It is known that the platelet expresses membrane sites for assembly of factors IXa/VIII and promotes efficient factor X activation. We now show that human blood monocytes, but not lymphocytes or polymorphonuclear leukocytes, also express appropriate sites for factors IXa/VIII assembly. The maximal rate of factor X activation by factors IXa (0.75 nM) and VIII (1 unit/ml) assembled on monocytes is similar to the maximal rate on platelets. This rate, adjusted per micromole of lipid phosphorus, is 1636 +/- 358 nM factor Xa/min on monocyte, and 1569 +/- 54 nM factor Xa/min on platelets. At physiologic concentrations of factors X and VIII, the activation rate increases with factor IXa concentration asymptotically approaching a maximum. Half-maximal rate is achieved with 1.0 +/- 0.16 nM factor IXa. Monocytes and macrophages, but not platelets, can express membrane tissue factor and thus promote simultaneous assembly of two distinct factor X-activating protease complexes. In these studies, blood monocytes and alveolar macrophages are used as membrane sources in kinetic experiments comparing factor X activation by intrinsic (factor IXa/VIII) versus extrinsic (factor VII/tissue factor) protease complexes. At plasma concentration of factors VIII and VII, apparent Km on the monocyte is 14.6 +/- 1.4 nM for intrinsic and 117.0 +/- 10.1 nM for extrinsic activation. The apparent Km on alveolar macrophages is 12.1 +/- 1.9 and 90.6 +/- 10.2 nM for intrinsic and extrinsic activation, respectively. Maximal rates on monocytes at saturating concentration of factors IXa, VIII, and VII are 48.0 +/- 11.2 nM factor Xa/min, for intrinsic activation, and 16.5 +/- 5.5 nM factor Xa/min, for extrinsic activation. These data show that the monocyte/macrophage is the only blood-derived cell type with membrane sites for both intrinsic and extrinsic pathway assembly. We have exploited this characteristic of the monocyte/macrophage membrane to demonstrate that factor X activation by the intrinsic pathway protease is more efficient than activation via the extrinsic pathway protease complex.

Coincidence of acquired factor-X deficiency and disseminated intravascular coagulation in patients with acute nonlymphoblastic leukemia

Systematic clotting studies were performed in 157 patients with de novo acute nonlymphoblastic leukemia (ANLL) prior to treatment. Sixteen patients had disseminated intravascular coagulation (DIC). Three of the patients with DIC (two with M3, one with M5 leukemia) had a marked isolated factor-X deficiency (factor X:C 21%, 33%, and 41%, respectively). Another four patients had a mild isolated factor-X deficiency (factor X:C 55%-68%). In these seven patients the remaining liver-synthesized clotting factors (factors II, VII, IX, V) as well as serum albumin and cholinesterase were within the normal range. Liver disease or vitamin-K deficiency could therefore be excluded. In none of the 141 patients without DIC was a marked isolated factor X deficiency observed; two patients had moderately reduced factor X:C levels but normal liver-synthesized proteins. Induction treatment led to the control of DIC with an almost parallel increase of fibrinogen and factor X up to normal in all patients with factor-X deficiency who achieved complete remission. In one patient, recurrence of leukemia was associated with reoccurrence of DIC and marked factor-X deficiency. We conclude that there is a coincidence of isolated factor-X deficiency and DIC in some patients with ANLL. In some patients, this factor-X deficiency may be severe enough to contribute to the bleeding tendency.

Factors affecting the interaction of tissue factor/factor VII with factor X in a heterogeneous tubular reactor

A novel reactor recently described for studying phospholipid-dependent blood coagulation reactions under flow conditions similar to those occurring in the vasculature has been further characterized. The reactor is a capillary whose inner wall is coated with a stable phospholipid bilayer (or two bilayers) containing tissue factor, a transmembrane protein that is required for the enzymatic activation of factor X by factor VIIa. Perfusion of the capillary at wall shear rates ranging from 25 s-1 to 1,200 s-1 with purified bovine factors X and VIIa led to steady state factor Xa levels at the outlet. Assay were performed using a chromogenic substrate, Spectrozyme TMFXa, or by using a radiometric technique. In the absence of Ca2+ or factor VIIa there was no product formation. No difference was noted in the levels of factor Xa achieved when non-activated factor VII was perfused. Once steady state was achieved further factor Xa production continued in the absence of factor VIIa implying a very strong association of factor VIIa with the tissue factor in the phospholipid membrane. In agreement with static vesicle-type studies the reactor was sensitive to wall tissue factor concentration, temperature and the presence of phosphatidylserine in the bilayer.

Changes in haemostasis after stopping the combined contraceptive pill: implications for major surgery

OBJECTIVE

To investigate the changes in haemostasis in the three months immediately after stopping the combined contraceptive pill.

DESIGN

Prospective randomised study.

SETTING

Family planning centre in London.

SUBJECTS

24 women aged 35-45 investigated before, during, and after six months' use of combined oral contraceptives containing 30 micrograms ethinyl oestradiol together with the progestogens desogestrel or gestodene.

MAIN OUTCOME MEASURES AND RESULTS

Blood samples were taken immediately before and after six months of oral contraceptive use and one, two, four, six, eight, and 12 weeks after the pill had been stopped. During the six months of oral contraceptive use the plasma concentration of factor X and fibrinogen increased and that of antithrombin III decreased. Between two and six weeks after stopping the pill a rebound phenomenon occurred with plasma concentrations of antithrombin III increasing (mean change from baseline at two weeks 0.06 IU/l and at six weeks 0.10 IU/l) and fibrinogen decreasing (0.26 g/l change at two weeks and 0.40 g/l at six weeks). Factor X concentrations fell gradually and the values at eight weeks were not significantly different from those found before the combined pill was started.

CONCLUSION

The combined pill should be stopped at least four weeks before major surgery, which carries the risk of postoperative thrombosis, to allow the potentially prothrombotic haemostatic changes that occur during its use to be corrected.

Comparative study of nine Bothrops snake venoms from adult female snakes and their offspring

Venoms of seven different Bothrops species and three subspecies (B. alternatus, B. cotiara, B. erythromelas, B. jararaca, B. jararacussu, B. moojeni, B. neuwiedi paranaensis. B.n. pauloensis and B.n. urutu) obtained from individual mothers and their young were investigated. Biometrics of snakes and protein content, toxicity (LD50), SDS-PAGE, proteolytic and clotting activities of venoms were estimated. Comparison of venoms from female snakes and their respective newborn offspring were variable in protein content, toxicity, fibrinolytic/amidolytic/thrombin-like activities and in venom yield in relation to snake length. B.n. paranaensis and B.n. pauloensis possessed the most toxic venoms. Caseinolytic activity of all venoms from female snakes and procoagulant activity of their offspring were consistently high. Venoms of B. erythromelas mother and offspring had no amidolytic activity and the highest levels of factor X and prothrombin activators without thrombin-like action. In contrast, the venom of newborn B. cotiara possessed the highest thrombin-like activity whereas a B. jararacussu adult female did not posses any procoagulant activity. An extremely high procoagulant activity of the venom of newborn Bothrops specimens was demonstrated.

Coupling of the adhesive receptor CD11b/CD18 to functional enhancement of effector macrophage tissue factor response

Initiation and regulation of localized selective proteolysis is an important effector property of cells of macrophage (Mo) lineage. Among such effector responses is the induced expression of tissue factor (TF) by cells of Mo lineage. In characterizing the regulation of the Mo responses that may influence the magnitude of the effector phase of the cellular immune response, we have identified a role for the cell surface adhesive receptor CD11b/CD18 (Mac-1, CR3) to amplify the induced TF response. Occupancy of CD11b/CD18 by MAb as surrogate ligands does not directly initiate a TF response. In contrast, after either T cell-derived cytokine or LPS as initial signals, engagement of CD11b/CD18 by MAb induces a two- to eight-fold functional enhancement of the TF response in murine and human Mo. This pathway of CD11b/CD18 enhancement of this Mo effector response was also confirmed with recognized ligands for CD11b/CD18 by exposure of Mo to immobilized fibrinogen. A quantitative increase of Mo surface expression of TF was validated by flow cytometry. We suggest that engagement of CD11b/CD18 by complementary ligands including adherence to extracellular matrix, and possibly in antigen-driven TH:Mo collaborative responses, results in the transduction of cellular signals that quantitatively enhance the expression of TF per se and thereby enhance the inflammatory component of Mo mediated response.

Fibrinogen deposition without thrombin generation in primary human breast cancer tissue

The occurrence and distribution of components of coagulation pathways in situ were determined using immunohistochemical techniques applied to 10 cases of primary carcinoma of the breast, normal breast tissue obtained from two patients undergoing reductive mammoplasty, and three patients with benign breast tumors. Tumor cells stained for factor X and thrombomodulin but not for tissue factor, factor V, factor VII, or factor XIII. Rare nonneoplastic duct epithelial cells stained for thrombomodulin, but these tissues did not otherwise stain for any of these antigens. Macrophages within the tumor stroma stained for tissue factor, factor VII, and factor XIII but not for factor V or factor X. These features of macrophages were the same in malignant and nonmalignant breast tissue. Fibrinogen was present in abundance throughout the connective tissue in breast cancer but not in nonmalignant tissues. By contrast, no staining was observed using fibrin-specific antibodies. These results suggest that an intact coagulation pathway does not exist in breast cancer tissue and that thrombin capable of transforming fibrinogen to fibrin is not generated in significant amounts in this tumor type. While fibrin is not a feature of the connective tissue stroma in breast cancer, it is conceivable that the abundant fibrinogen present in the tumor connective tissue (and factor XIII present in connective tissue macrophages) might contribute to the structural integrity of breast tumor tissues.

Procoagulant activities in venoms from central Asian snakes

The venoms from central Asian snakes (Echis carinatus, Echis multisquamatus, Vipera ursini, Vipera lebetina, Agkistrodon halys halys and Naja naja oxiana) contain several enzymes with amidolytic- and procoagulant activity. We have characterized the activities and the mol. wts of the venom enzymes that are able to convert a number of commercially available chromogenic substrates for activated coagulation factors. The chromogenic substrate cleavage patterns obtained for the crude venoms may be helpful tools in the further identification of venom fractions and venom enzymes with procoagulant activity. The crude venoms were also tested for their ability to clot fibrinogen, to lyse fibrin polymers and to activate the coagulation factors prothrombin, factor X and factor V. The products of venom-catalyzed coagulation factor activation were structurally characterized by SDS gel electrophoresis and were compared with activated coagulation factors that are generated under physiological conditions.

Degradation of human factor X by human polymorphonuclear leucocyte cathepsin G and elastase

Cathepsin G and elastase from human polymorphonuclear leucocytes were used in vitro to digest human factor X. Clotting assays showed that both proteinases affected a rapid loss in the coagulant activity of factor X. Calcium ions almost totally protected the coagulant activity of factor X against the action of cathepsin G but not elastase. Polyacrylamide gel electrophoresis (in nonreducing conditions and in the presence of SDS) indicated that the proteolytic action of cathepsin G led to the removal of a peptide of low molecular mass (pX) with the consequent formation of a single stable high molecular mass product (PX). SDS electrophoresis (under reducing conditions and in the presence of SDS) indicated that a pX was derived from the light chain of factor X. The proteolytic action of elastase led to the formation of numerous degradation products. Analysis of the products generated by the action of cathepsin G indicated that cathepsin G cleaved position Phe40:Trp41 in the light chain of factor X. In the presence of citrated plasma, cathepsin G but not elastase, was responsible for a loss in coagulant activity.

Comparative platelet binding and kinetic studies with normal and variant factor IXa molecules

We have recently shown that thrombin-stimulated human platelets have specific, saturable receptors for factor IXa, occupancy of which promotes factor X activation (Ahmad, S. S., Rawala-Sheikh, R., and Walsh, P. N. (1989) J. Biol. Chem. 264: 3244-3251, 20012-20016; Rawala-Sheikh, R., Ahmad, S. S., and Walsh, P. N. (1990) Biochemistry 29, 2606-2611). To study the structural requirements for factor IXa binding to platelets, equilibrium binding studies and kinetic studies of factor X activation were carried out with normal factor IXa and with two variant proteins: factor IXaAlabama (FIXaAL; Asp47----Gly substitution) and factor IXaChapel Hill (FIXaCH; Arg145----His substitution). In the absence of factors VIIIa and X, there were 331 binding sites/platelet for FIXaCH (Kdapp = 2.8 nM), and 540 sites/platelet for FIXaAL (Kdapp = 3.2 nM), compared with 540 sites/platelet (Kdapp = 2.3 nM) for normal factor IXa. The addition of factors VIIIa and X, both at saturating concentrations, had no effect on the number of binding sites for either normal or variant factor IXa, resulted in a decrease in the Kd for normal factor IXa to 0.67 nM, resulted in a suboptimal decrease in Kd for FIXaAL (1.4 nM), and had no effect on the Kd for FIXaCH. Kinetic studies of factor X activation at variable factor IXa concentration confirmed these values of Kd in the presence of factors VIIIa and X. Determination of rates of factor X activation at variable substrate concentrations yielded normal values of catalytic efficiency (kcat/Km) for the variant proteins, thereby indicating that the abnormally low rates of factor X activation obtained were a consequence of the low affinity binding of FIXaAL and FIXaCH to thrombin-activated platelets in the presence of factors VIIIa and X. These studies suggest that the presence of Asp47 and the cleavage of factor IX at Arg145-Ala146 are important structural features required for specific, high affinity factor IXa binding to platelets in the presence of factors VIIIa and X.

Calcium ion binding to human and bovine factor X

Human and bovine factor X contain 11 and 12 glutamyl residues, respectively, within the first 40 amino terminal residues that are post-translationally modified to gamma-carboxyglutamyl (Gla) residues. We have measured calcium ion binding to human factor X by equilibrium dialysis. This is the first examination of calcium ion binding to human factor X. We have also re-examined the equilibrium dialysis binding of calcium ions to bovine facor X in order to compare the two species. The data was analysed using a variety of models that allow for more than one class of binding site and for co-operativity among binding sites. Calcium ion binding to human factor X fits a model that had two classes of sites: one class with a single site that had an affinity of 0.1 mM and a second class with 19 equivalent, non-interacting sites with an average affinity of 3.5 mM. There was no evidence for co-operativity in calcium ion binding. Calcium ion binding to bovine factor X was best stimulated by a model that assumed one tight site, four co-operative sites, and 18 equivalent, non-interacting sites. To examine the co-operativity seen in calcium ion binding to bovine factor X, calcium ion binding to isolated Gla region (residues 1-44) and Gla-domainless factor X was measured by equilibrium dialysis. Calcium ion binding to Gla-domainless factor X was simulated by a model that had two classes of sites: one class with a single site that had an affinity of 0.25 mM, and a second class that had 15 sites with very low affinity sites (greater than 15 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

An endoprotease homologous to the blood clotting factor X as a determinant of viral tropism in chick embryo

Host cell proteases responsible for activation of viral fusion glycoproteins are an important determinant for spread and tropism of various animal viruses. Exemplifying such proteases for the first time, we isolated an endoprotease from chick embryo, that activates para- and orthomyxovirus fusion glycoproteins by cleaving their precursor proteins at a specific, single arginine site. The protease is a calcium dependent serine protease consisting of two subunits, the 33 kd catalytic chain and the 23 kd chain possibly required for Ca2+ binding, and was found to be highly homologous, if not identical, to the blood clotting factor X(FX), a member of the prothrombin family. Its high efficiency and specificity in cleavage reactions was attributable to the properties characteristic of FX. Its role in vivo was strongly supported by cleavage inhibition in ovo highly selective for this virus group with a specific peptide inhibitor against FX.

Coagulopathy after snake bite by Bothrops neuwiedi: case report and results of in vitro experiments

Coagulation studies were performed in a patient who had been bitten by a snake of the species Bothrops neuwiedi. The patient presented with hemorrhagic necrosis at the envenomization site and considerable bleeding from venous puncture sites. He developed a severe defibrination syndrome with a clottable fibrinogen level of approximately 0.1 g/l. Fibrinogen was not measurable by clotting time assay. Fibrin degradation products were greatly elevated. Treatment with antivenom caused an anaphylactic reaction within ten minutes and serum sickness after three days. In vitro experiments revealed that B. neuwiedi venom directly activates Factors II and X, but does not activate Factor XIII. In vivo consumption of Factor XIII after B. neuwiedi envenomization is ascribed to the action of Factor IIa. At low venom concentrations clotting is initiated by activation of prothrombin by the venom either directly or via Factor X activation. Treatment with heparin might be beneficial in coagulopathy secondary to snake bite by reducing circulating active thrombin. The venom contains thrombin-like proteases which cause slow clotting of fibrinogen, and plasmin-like components causing further proteolysis of fibrinogen and fibrin. Antivenom has no effect on the proteolytic action of the snake venom. The in vivo effects of antivenom are presumably caused by acceleration of the elimination of venom components from the circulation. Intravenous administration of antivenom caused normalization of blood coagulation parameters within 48 h.

Propeptide recognition by the vitamin K-dependent carboxylase in early processing of prothrombin and factor X

Precursors of vitamin K-dependent proteins are synthesized with a propeptide that is believed to target these proteins for gamma-carboxylation by the vitamin K-dependent carboxylase. In this study synthetic propeptides were used to investigate gamma-carboxylation of the prothrombin and factor X precursors in rat liver microsomes. The extent of prothrombin processing by the carboxylase was also investigated. Antisera raised against the human prothrombin and factor X propeptides only recognized precursors with the respective propeptide regions. The data demonstrate structural differences in the propeptide region of the prothrombin and the factor X carboxylase substrates which raises questions about the hypothesis of a common propeptide binding site on the carboxylase for all precursors of vitamin K-dependent proteins. The hypothesis of separate binding sites is supported by data which demonstrate differences in binding of the prothrombin and factor X precursors to membrane fragments from rough and smooth microsomes. gamma-Carboxylation of the prothrombin precursors in vitro was investigated with conformational specific antibodies raised against a portion of the Gla (gamma-carboxyglutamic acid) region extending from residue 15 to 24. The synthetic peptide used as antigen contains three of the ten potential Gla sites in prothrombin. It is shown that these antibodies do not recognize mature prothrombin but recognize the decarboxylated protein. It is also demonstrated that the epitope is Ca2(+)-dependent. The antibodies were used to assess gamma-carboxylation of the prothrombin precursor in membrane fragments from microsomal membranes. The results suggest that microsomal gamma-carboxylation does not involve Glu residues 16, 19 and 20 of the Gla region.

Analysis of the generation and inhibition of factor Xa. Area under generation curves is independent of enzyme generation rate

The activation of factor X in the presence of antithrombin has been studied in order to determine the parameters that control the area under the resulting factor Xa generation curve. Generation curves were analyzed using a model containing three parameters: the total generation of factor Xa, Emax; the rate of factor Xa generation, expressed as a first-order rate constant, kappa 1; and the rate of inhibition, expressed as another first-order rate constant, kappa 2. Using factor IXa-VIIIa to activate factor X, we found the area under the generation curve to be proportional to Emax, which was varied by varying the factor IXa concentration, and inversely proportional to kappa 2, which was varied by varying the antithrombin concentration. With this activator, however, kappa 1 varied in parallel with Emax, resulting in a correlation between integrated area and kappa 1. In order to determine whether Emax or kappa 1, or both, was a controlling parameter, similar activations were done with varying concentrations of the factor X-activating enzyme of Russell's viper venom. With this activator it was possible to vary Emax and kappa 1 independently, again at varying antithrombin concentrations. These results showed the integrated area to be proportional to Emax and inversely proportional to kappa 2, as before, but independent of the activation rate, kappa 1. In this system, therefore, the area under the factor Xa generation curve is controlled by the amount of factor Xa generated and its rate of inhibition but is independent of the rate of factor Xa generation.

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)

Hemophilia as a defect of the tissue factor pathway of blood coagulation: effect of factors VIII and IX on factor X activation in a continuous-flow reactor

The effect of factors VIII and IX on the ability of the tissue factor-factor VIIa complex to activate factor X was studied in a continuous-flow tubular enzyme reactor. Tissue factor immobilized in a phospholipid bilayer on the inner surface of the tube was exposed to a perfusate containing factors VIIa, VIII, IX, and X flowing at a shear rate of 57, 300, or 1130 sec-1. Factor Xa in the effluent was determined by chromogenic assay. The flux of factor Xa (moles formed per unit surface area per unit time) was strongly dependent on wall shear rate, increasing about 3-fold as wall shear rate increased from 57 to 1130 sec-1. The addition of factors VIII and IX at their respective plasma concentrations resulted in a further 2- to 3-fold increase. The direct activation of factor X by tissue factor-factor VIIa could be virtually eliminated by the lipoprotein-associated coagulation inhibitor; however, when factors VIII and IX were present at their approximate plasma concentrations, factor Xa production rates were enhanced 15- to 20-fold. These results suggest that the tissue factor pathway, mediated through factors VIII and IX, produces significant levels of factor Xa even in the presence of an inhibitor of the tissue factor-factor VIIa complex; moreover, the activation is dependent on local shear conditions. These findings are consistent both with a model of blood coagulation in which initiation of the system results from tissue factor and with the bleeding observed in hemophilia.

Effect of recombinant human FVIIA on warfarin-induced bleeding in rats

Recombinant human factor VIIa (rFVIIa) was given to warfarin-treated rats. One and two warfarin treatments reduced endogenous factor X levels to about 10% and less than 5%, respectively. The reduction of plasma levels of vitamin K-dependent coagulation factors was accompanied by a treatment-dependent prolongation of activated partial thromboplastin time (APTT) and prothrombin time (PT) as well as by increased bleeding time and blood loss in the rat tail bleeding test. rFVIIa 50 micrograms/kg and 250 micrograms/kg normalized PT and shortened APTT in rats given warfarin once. Bleeding was completely normalized by rFVIIa 250 micrograms/kg. In rats given warfarin twice rFVIIa 250 micrograms/kg shortened PT but had no effect on APTT, whereas the effect on bleeding was variable. The elimination half life of rFVIIa in rats was found to be 30-40 minutes. The study indicates that rFVIIa may be useful in patients with bleeding due to decreased levels of vitamin K-dependent coagulation factors.