Bispecific antibody mimicking factor VIII

There are some issues in the current factor (F)VIII replacement therapy for severe hemophilia A. One is mental and physical burden for the multiple intravenous infusions, and the other is difficulty in the hemostatic treatment for the patients with FVIII inhibitor. The development of novel drug with fully hemostatic effect, simply procedure, and long-acting reaction has been expected. Recently, FVIIIa-mimicking humanized recombinant bispecific antibody (ACE910) against FIXa and FX was developed. In the non-human clinical study, primate model of acquired hemophilia A demonstrated that the ACE910 was effective on both on-going and spontaneous bleedings. A phase I clinical study was conducted in healthy adults by single subcutaneous infusion of ACE910, followed by the patients' part study, Japanese patients with severe hemophilia A without or with inhibitor were treated with once-weekly subcutaneous injection of ACE910 at three dose levels for 12 successive weeks. There was no significant adverse event related to ACE910 in the clinical and laboratorial findings, and t1/2 of ACE910 was ∼30 days. The median annual bleeding rates were reduced very markedly dose-dependently, independently of inhibitor. Furthermore, among the patients with dose escalation, bleeding rate was decreased as ACE910 dose was increased. In conclusion, ACE910 would have a number of promising features: its high subcutaneous bioavailability and long half-life make the patients possible to be injected subcutaneously with a once-a-week or less frequency. In addition, ACE910 would provide the bleeding prophylactic efficacy, independently of inhibitor.

[New hemophilia treatment employing a bispecific antibody to factors IXa and X]

Unmet needs of current hemophilia A treatment include the requirement for frequent intravenous infusions, inhibitor development, and containment of high medical costs. In order to overcome these issues, we produced FVIII which mimics a bispecific antibody against FIXa/FX. ACE910 demonstrated hemostatic effects on both ongoing and spontaneous joint bleeding in the primate acquired hemophilia A model. Recently, a phase 1 study for PK, PD, and the safety of ACE910 was initiated. The t1/2 was approximately 30 days. There were no severe ACE910 related adverse events. Furthermore, bleeding was remarkably decreased by weekly subcutaneous administration in patients with severe hemophilia A, regardless of whether an inhibitor was used. ACE910 has the remarkable advantages of prophylactic efficacy which can be achieved by convenient subcutaneous administrations at a markedly reduced frequency.

Manipulating adenovirus hexon hypervariable loops dictates immune neutralisation and coagulation factor X-dependent cell interaction in vitro and in vivo

Adenoviruses are common pathogens, mostly targeting ocular, gastrointestinal and respiratory cells, but in some cases infection disseminates, presenting in severe clinical outcomes. Upon dissemination and contact with blood, coagulation factor X (FX) interacts directly with the adenovirus type 5 (Ad5) hexon. FX can act as a bridge to bind heparan sulphate proteoglycans, leading to substantial Ad5 hepatocyte uptake. FX “coating” also protects the virus from host IgM and complement-mediated neutralisation. However, the contribution of FX in determining Ad liver transduction whilst simultaneously shielding the virus from immune attack remains unclear. In this study, we demonstrate that the FX protection mechanism is not conserved amongst Ad types, and identify the hexon hypervariable regions (HVR) of Ad5 as the capsid proteins targeted by this host defense pathway. Using genetic and pharmacological approaches, we manipulate Ad5 HVR interactions to interrogate the interplay between viral cell transduction and immune neutralisation. We show that FX and inhibitory serum components can co-compete and virus neutralisation is influenced by both the location and extent of modifications to the Ad5 HVRs. We engineered Ad5-derived HVRs into the rare, native non FX-binding Ad26 to create Ad26.HVR5C. This enabled the virus to interact with FX at high affinity, as quantified by surface plasmon resonance, FX-mediated cell binding and transduction assays. Concomitantly, Ad26.HVR5C was also sensitised to immune attack in the absence of FX, a direct consequence of the engineered HVRs from Ad5. In both immune competent and deficient animals, Ad26.HVR5C hepatic gene transfer was mediated by FX following intravenous delivery. This study gives mechanistic insight into the pivotal role of the Ad5 HVRs in conferring sensitivity to virus neutralisation by IgM and classical complement-mediated attack. Furthermore, through this gain-of-function approach we demonstrate the dual functionality of FX in protecting Ad26.HVR5C against innate immune factors whilst determining liver targeting.

Adenoviruses are mostly considered self-limiting pathogens associated with respiratory, gastrointestinal and ocular infections; however, in immunocompromised subjects disseminated Ad infection can occur with life-threatening consequences. Many human Ads are capable of binding to coagulation factor X (FX). Following intravenous administration in animal models, FX binds directly to the major Ad capsid protein, the hexon, which subsequently results in virus accumulation in the liver. FX coating Ad5 also acts to shield against immune neutralisation via natural IgM antibodies and the classical complement system. Here we show that FX protection is not a conserved mechanism amongst Ads and identify the Ad5 hexon hypervariable regions (HVR) as the capsid proteins targeted by this host defense pathway. Furthermore, we show that genetic inclusion of Ad5 HVRs onto a native non-FX binder Ad26 to be sufficient to confer sensitivity to immune attack in vitro and in vivo. Using intravenous administration, we determine the significance of FX binding to the Ad5-derived HVRs with respect to defending the virus from neutralisation whilst mediating virus tropism. Our study gives new insight into the role of the viral HVRs and of FX at the interface between virus and host defense mechanisms.

Anti-factor IXa/X bispecific antibody (ACE910): hemostatic potency against ongoing bleeds in a hemophilia A model and the possibility of routine supplementation

BACKGROUND

We previously reported that a humanized anti-factor IXa/X bispecific antibody, hBS23, mimics the function of FVIII even in the presence of FVIII inhibitors, and has preventive hemostatic activity against bleeding in an animal model of acquired hemophilia A. After further molecular engineering of hBS23, we recently identified an improved humanized bispecific antibody, ACE910, for clinical investigation.

OBJECTIVES

To elucidate the in vivo hemostatic potency of ACE910 by examining its effect against ongoing bleeds, and to determine its pharmacokinetic parameters for discussion of its potency for prophylactic use.

METHODS

A nonhuman primate model of acquired hemophilia A was established by injecting anti-primate FVIII neutralizing antibody. When bleeds emerged following an artificial bleed-inducing procedure, either ACE910 or recombinant porcine FVIII (rpoFVIII) was intravenously administered. rpoFVIII was additionally administered twice daily on the following 2 days. Bleeding symptoms were monitored for 3 days. A pharmacokinetic study and multiple-dosing simulations of ACE910 were also performed.

RESULTS

A single bolus of 1 or 3 mg kg⁻¹ ACE910 showed hemostatic activity comparable to that of 10 U kg⁻¹ (twice daily) rpoFVIII against ongoing bleeds. The determined ACE910 pharmacokinetic parameters included a long half-life (3 weeks) and high subcutaneous bioavailability (nearly 100%). The simulation results based on pharmacokinetic parameters indicated that the above hemostatic level could be maintained with once-weekly subcutaneous administration of ACE910, suggesting the possibility of more effective prophylaxis.

CONCLUSIONS

ACE910 may offer an alternative on-demand treatment option for patients with hemophilia A, as well as user-friendly and aggressive routine supplementation.

Treatment of acquired factor X inhibitor by plasma exchange with concomitant intravenous immunoglobulin and corticosteroids

A patient with spontaneous hemorrhage from multiple body sites was found to have markedly prolonged international normalized ratio (INR) and activated partial thromboplastin times (aPTT) with incomplete correction of aPTT on mixing studies using normal plasma. The cause of this severe hemorrhage was due to a specific factor X inhibitor. No underlying or associated diseases were found. Initial treatment with fresh frozen plasma, vitamin K, and recombinant activated factor VII (rFVIIa) was unsuccessful. However, therapy utilizing plasma exchange with concomitant intravenous immunoglobulin and corticosteroids resulted in a rapid and sustained normalization of factor X levels with a clinical hemostatic response.

Antibodies against the activated coagulation factor X (FXa) in the antiphospholipid syndrome that interfere with the FXa inactivation by antithrombin

Antiphospholipid Ab have been shown to promote thrombosis and fetal loss in the antiphospholipid syndrome (APS). Previously, we found IgG anti-thrombin Ab in some APS patients that could interfere with inactivation of thrombin by antithrombin (AT). Considering that activated coagulation factor X (FXa) is homologous to thrombin in the catalytic domains and is also regulated primarily by AT, we hypothesized that some thrombin-reactive Ab may bind to FXa and interfere with AT inactivation of FXa. To test these hypotheses, we studied reactivity of eight patient-derived monoclonal IgG antiphospholipid Ab with FXa and the presence of IgG anti-FXa Ab in APS patients and investigated the effects of FXa-reactive mAb on AT inactivation of FXa. The results revealed that six of six thrombin-reactive IgG mAb bound to FXa and that the levels of plasma IgG anti-FXa Ab in 38 APS patients were significantly higher than those in 30 normal controls (p < 0.001). When the mean plus 3 SDs of the 30 normal controls was used as the cutoff, 5 of 38 APS patients (13.2%) had IgG anti-FXa Ab. Importantly, three of six FXa-reactive mAb significantly inhibited AT inactivation of FXa. Combined, these results indicate that anti-FXa Ab may contribute to thrombosis by interfering with the anticoagulant function of AT on FXa in some APS patients.

Molecular characterization of two novel mutations causing factor X deficiency in a Chinese pedigree

Factor X (FX) deficiency is a rare bleeding disorder inherited as an autosomal recessive trait. In this study, we investigated the molecular basis of FX deficiency in a Chinese pedigree. The proposita showed a markedly prolonged activated partial thromboplastin time and a mild prolongation of prothrombin time. The levels of FX antigen and FX activity were 58.6% and 2.5%, respectively. Molecular analysis revealed that the proposita was compound heterozygous for two novel mutations: IVS1 + 1G > A and G1185A (Arg347His). The aberrant transcripts from the IVS1 + 1G > A mutant allele were not detected by analyzing the splicing pattern of ectopic transcripts in leukocytes of the patient with nested polymerase chain reaction after reverse transcription. We thus hypothesize that the mRNA molecules originating from the IVS1 + 1G > A mutation were rapidly destroyed in vivo. Site-directed mutagenesis of FX cDNA was used to introduce FXG1185A mutation, and wild-type as well as mutant FX proteins were expressed by transient transfection in HEK 293 cells. Normal FX antigen levels both in the conditioned media of cells expressing the mutant and in cell lysates were detected by an enzyme-linked immunoadsorbent assay. Evaluation of wild-type and mutant coagulant activity demonstrated that the FX molecules carrying the Arg347His mutation have dramatically decreased activity.

Extrahepatic synthesis of factor VII in human atherosclerotic vessels

OBJECTIVE

Coagulation is initiated by the interaction of tissue factor (TF) with plasma coagulation factors VII (FVII) and X (FX). TF is highly expressed in atherosclerotic lesions, but little is known about the synthesis of FX or FVII outside of the liver. Previous studies suggested that macrophages synthesize FVII. We therefore hypothesized that macrophages within atherosclerotic lesions may produce FVII, leading to partial activation of the coagulation cascade.

METHODS AND RESULTS

Immunohistochemistry was performed using antibodies against FVII, FX, and TF on normal and atherosclerotic vessels. In atherosclerotic lesions, FVII immunostaining was colocalized with TF in macrophages and spindle-shaped smooth muscle cells. FVII mRNA was also detected in these cells using in situ hybridization, suggesting the local synthesis of FVII in atherosclerosis. Reverse transcriptase-polymerase chain reaction confirmed the presence of FVII mRNA in normal and atherosclerotic vessels as well as smooth muscle cells, fibroblasts, and keratinocytes in vitro.

CONCLUSIONS

The localization of FVII synthesis outside the liver may be indicative of other cellular functions for this coagulation protein. The observed coexpression of TF and FVII may contribute to autocrine signaling via thrombin-independent mechanisms and may represent a novel mechanism contributing to growth in the setting of vascular disease.

The N-terminal epidermal growth factor-like domain of coagulation factor IX. Probing its functions in the activation of factor IX and factor X with a monoclonal antibody

The absence or reduced activity of coagulation factor IX (FIX) causes the severe bleeding disorder hemophilia B. FIX contains an N-terminal Gla domain followed by two epidermal growth factor-like (EGF) domains and a serine protease domain. In this study, the epitope of monoclonal antibody AW, which is directed against the C-terminal part of the first EGF domain in human FIX, was defined, and the antibody was used to study interactions between the EGF domain of FIX and other coagulation proteins. Antibody AW completely blocks activation of FIX by activated factor XI, but activation by activated factor FVII-tissue factor is inhibited only slightly. The antibody also causes a marginal reduction in the apparent k(cat) for factor X both in the presence and absence of activated factor VIII. Based on these results, we produced a preliminary model of the structure of the activated factor IX-activated factor VIII-AW complex on the surface of phospholipid. The model suggests that in the Xase complex, EGF1 of activated factor IX is not involved in direct binding to activated factor VIII. Studies of the interaction of antibody AW with a mutated FIX molecule (R94D) also suggest that the Glu(78)-Arg(94) salt bridge is not important for maintaining the structure of FIX.

Differences between human and rabbit coagulation factor X-implications for in vivo models of thrombosis

The activation of factor X (fX) to factor Xa (fXa) marks the penultimate step in the coagulation cascade and modulating fXa activity may be effective for antithrombotic therapy. Even though fXa inhibitors are screened using in vitro inhibition of human fXa (HfXa) while subsequent evaluation uses in vivo rabbit models, there is limited knowledge of species differences between the coagulation proteins. When comparing amino acid sequences for the human (HfX) and rabbit (RafX) protein, differences are found in the activation peptide and active site regions. In order to study the relative functional characteristics of HfX and RafX, we asked (1) whether fX from the two species is immunologically related, (2) whether the two proteins are activated to fXa in a similar manner, (3) whether HfXa and rabbit factor Xa (RafXa) have similar catalytic activities toward tripeptide substrates. To answer (1), we expressed RafX-glutathione S-transferase (RafX-GST) fusion protein in bacteria and purified the protein for use as an antigen. The resulting monoclonal antibodies were suitable for affinity purification of plasma RafX and for effective anticoagulation in rabbit plasma clotting assays. We found two antibodies (mAb 214 and mAb 290) that anticoagulated rabbit plasma in a dose responsive manner but did not cross-react with human plasma. At a concentration of 500 nM, mAb 214 attained a two-fold extension of rabbit plasma activated partial thromboplastin time (aPTT). To answer (2), we purified plasma RafX and compared the activation of HfX and RafX with Russell's viper venom (RVV-X). Under equivalent reaction conditions, conversion was 30% slower for the rabbit protein. To answer (3), amidolytic activity of HfXa and RafXa were assayed by cleavage of three para-nitroanilide (pNA) substrates (S2222 [Bz-Ile-Glu(gamma-OR)-Gly-Arg-pNA.HCl], S2765 [Z-D-Arg-Gly-Arg-pNA.HCl] and Spectrozyme Xa [MeO-CO-D-CHG-Gly-Arg-pNA.AcOH]). Michaelis constants (K(m)) for the rabbit protein were 187, 72 and 69 microM, respectively, and for the human analog, 255, 63 and 135 microM, respectively. Comparing the extent of substrate turnover (V(max)) for HfXa and RafXa, the latter was shown to cleave all three substrates at a reduced rate. Based on these observations, it can be speculated that the relative antithrombotic potency of active site directed fXa inhibitors might be different between the two species. Predicted human therapeutic doses derived from in vivo results in rabbit models should therefore take species variation into consideration.

A new factor X defect (factor X Padua 3): a compound heterozygous between true deficiency (Gly(380)-->Arg) and an abnormality (Ser(334)-->Pro)

We report a novel mutation in Factor X (FX) gene which results in a phenotype without any bleeding tendency. The proband has been found to be a compound heterozygote between a novel FX true deficiency (Gly(380)-->Arg) and a previously reported dysfunctional mutation Ser(334)-->Pro (FX Marsiglia). Prothrombin time (PT) and partial thromboplastin time (PTT) were moderately prolonged and were fully corrected by the addition of normal serum. Her FX activity level varied between 8% and 19% of normal according to the method used whereas the FX antigen level was 40% of the normal control value. All the exons and intron/exon junctions of the FX gene were studied using a combined approach of polymerase chain reaction and conformation sensitive gel electrophoresis. A transversion G to A in exon 8 resulting in the replacement of Gly380 by Arg was found in the proband, in the father and in a proband's brother, whereas heterozygous FX Marsiglia was present in the proband's mother and her sister. Gly380 is strictly linked to Ser379, a component of the catalytic triad. The substitution of Gly for Arg causes the introduction of a large charged amino acid which could affect the catalytic function of FX leading to secretion problem, accounting for the cross-reactive material (CRM) negative phenotype.

Modulation of the binding of matrix Gla protein (MGP) to bone morphogenetic protein-2 (BMP-2)

Matrix Gla protein (MGP) is an inhibitor of calcification of the arterial wall but the mechanism of inhibition has not been resolved. Since chondrogenesis has been identified in calcified arteries from MPG null mice, we hypothesized that locally produced MGP might inhibit calcification by neutralizing the known effect of bone morphogenetic proteins (BMPs) as promotors of chondrogenesis and bone formation. As the first step to test this hypothesis, we demonstrate that MGP is a binding protein for 125I-BMP-2. Optimal binding is dependent on metals which suggests that the metal binding Gla region in MGP is involved. MGP is shown to undergo a Ca++ induced conformational change despite the presence of the gamma-carboxylase binding site being part of the mature protein sequence. The data propose that MGP matures earlier in the secretory pathway than other vitamin K-dependent proteins. Antibodies were used in an attempt to identify MGP in bovine serum. Conformational specific MGP antibodies were shown to also recognize the Gla region in prothrombin and factor X but did not identify MGP in serum. This finding is supported by electrophoresis data which demonstrate the absence of MGP among Ba-citrate absorbed vitamin K-dependent serum proteins. We conclude that MGP does not exist in normal bovine serum.

The impact of Glu102Lys on the factor X function in a patient with a doubly homozygous factor X deficiency (Gla14Lys and Glu102Lys)

Two homozygous point mutations were found in a patient with factor X (FX) deficiency; One results in substitution of Lys for Gla+ 14 and the second causes a Lys substitution for Glu102. The proposita has a severely reduced FX coagulant activity in the extrinsic (<1% of normal) and in the intrinsic (30% of normal) system of coagulation and after activation with Russel's viper venom (18% of normal). The FX antigen is reduced in this patient to 20% of normal. The substitution of Lys for Glu102 in FX deficiency has been reported previously in a heterozygous state in conjunction with a Lys for Gla+14 substitution and with a Pro for Ser334 substitution. The contribution of the Lys for Glu102 substitution in the observed combined FX defect in these patients was unclear. The mutation causing the Glu102Lys substitution was introduced by site directed mutagenesis into a wild-type FX cDNA, and recombinant protein was expressed in HEK 293 cells. Compared to the wild-type FX cDNA, the mutant construct had a 67% activity upon activation in the extrinsic system, 93% activity upon activation in the intrinsic system and 72% after activation with RVV. The data presented show that the substitution of Lys for Glu102 results in a minor functional defect of the FX molecule.

[The young hemophiliac, inhibitors and immune tolerance]

The occurrence of inhibitor in a hemophilic patient is the greatest therapeutic complication in 1999. The inhibitor incidence is higher in hemophilia A (20-30%) than in hemophilia B (3%). At the moment, the best management is immune tolerance induction. This consists in frequent infusion of antihemophilic factor (every day or every other day). The risk of inhibitor development is higher in a young child than in an adult. Consequently, a venous access device is essential for this treatment although not devoid of complications in the young boy. Moreover, the probability of inhibitor disappearance is higher in a child (recent inhibitor) than in an adult ("old" inhibitor).

Artificial antibodies for affinity chromatography of homologous proteins: application to blood clotting proteins

A method to readily isolate antibodies that bind to only one member of a family of homologous proteins is described. A library of different single chain antibody fragments can be displayed on the surface of a bacteriophage vector. Individual antibodies from this library recognizing a particular protein from a family of homologous proteins can be readily isolated by a two-step affinity screening process. In the first step antibodies which bind specifically to the undesired proteins or to homologous regions of the proteins are removed. In the second step, those antibodies specifically recognizing the desired protein are then isolated. Using this procedure and starting with a naive antibody library, a single chain antibody fragment specific to the blood clotting protein, Protein C, which did not recognize either of the homologous proteins, Factor IX or Factor X, was isolated. Similarly an antibody specific to Factor IX, but not Factor X or Protein C, was also isolated. The isolated antibodies can be readily produced, purified, and affixed to sepharose beads for affinity chromatography of the blood clotting factors. One of the key advantages to this procedure over conventional monoclonal antibody isolation is that the antibodies are isolated and produced in vitro so a broad range of related proteins, toxins, viruses, or other products can be targeted.

Acquired inhibitors to factors V and X after exposure to topical thrombin: interference with monitoring of low molecular weight heparin and warfarin

Repeated surgical exposure to topical bovine thrombin is known to be associated with the development of antibodies to bovine and human thrombin and factor V. This is demonstrated by abnormalities of in vitro coagulation assays and, rarely, postoperative bleeding. We describe a 4-year-old child in whom an antibody to bovine factor X developed after cardiac surgery; this antibody interfered with the heparin anti-Xa assay, thereby complicating the monitoring of heparin therapy.

Anti-beta2-glycoprotein I (beta2GPI) monoclonal antibodies with lupus anticoagulant-like activity enhance the beta2GPI binding to phospholipids

beta2-Glycoprotein I (beta2GPI), a plasma glycoprotein with phospholipid-binding property, is known to be the actual target antigen for autoimmune type anticardiolipin antibodies (aCLs). Certain groups of aCLs (anti-beta2GPI antibodies) exert lupus anticoagulant (LA) activity and perturb the function of vascular endothelial cells. This investigation aimed at highlighting some insights into the molecular basis by which aCLs exert their biological effects by using anti-beta2GPI mAbs with well-characterized epitopes from mice and from patients with antiphospholipid syndrome. Anti-beta2GPI mAbs directed against the third domain (Cof-20 and Cof-22) and fourth domain (Cof-21, EY1C8, and EY2C9) of beta2GPI inhibited the thrombin generation induced by Russell's viper venom in diluted plasma and that induced by the prothrombinase complex reconstituted with purified clotting factors. This anticoagulant activity was abrogated in the presence of an excess amount of phospholipids, thus resembling the LA activity. In stark contrast, anti-beta2GPI mAbs directed against the fifth domain and the carboxy-terminal region of the fourth domain showed no LA-like activity. These findings suggest that the LA activity of anti-beta2GPI antibodies depends on their epitope specificity. Experiments carried out to clarify the mechanism of the LA activity showed that anti-beta2GPI mAbs with LA-like activity, but not those without this effect, enhance the beta2GPI binding to phospholipids. In addition, the F(ab')2 fragment, but not the Fab' fragment, of the anti-beta2GPI mAbs was found to enhance the LA activity and the beta2GPI binding to phospholipids, suggesting that anti-beta2GPI antibodies induce formation of multiple complexes of beta2GPI on the surface of phospholipids because of their bivalent property. This clustering of beta2GPI molecules induced by anti-beta2GPI antibodies, probably because of their multivalent property and epitope specificity, might hinder the lateral mobility and activation of clotting factors on the surface of phospholipids and thus exert LA activity. Clustering of beta2GPI molecules may also explain the molecular mechanism by which anti-beta2GPI antibodies alter the function of leukocytes and endothelial cells. The well-documented heterogeneous LA activity of aCLs (anti-beta2GPI antibodies) may also be explained by their epitope specificity.

Transient hemorrhagic diathesis associated with an inhibitor of prothrombin with lupus anticoagulant in a 1 1/2-year-old girl: report of a case and review of the literature

Acquired inhibitors of coagulation causing bleeding manifestations are rare in children, particularly without an associated underlying disorder such as autoimmune disease. We describe an otherwise healthy 1 1/2-year-old girl who had extensive spontaneous bruising and prolonged bleeding from venipuncture sites. Prothrombin time (PT) and activated partial thromboplastin time (APTT) were prolonged, with evidence of an immediate-acting inhibitor. Thrombin clotting time, fibrinogen, and platelets were normal. Biologic assay of factors II, V, VII, and X were all low, with increasing values at higher dilutions. However, by immunoassay and/or chromogenic assays, only factor II was reduced. An antibody which failed to neutralize prothrombin activity in vitro was detected against radiolabeled prothrombin. Coagulation studies normalized in parallel with clinical recovery and disappearance of the antibody. This case demonstrates acute hypoprothrombinemia-lupus anticoagulant syndrome as a rare presentation of bleeding diathesis in a healthy young child.

Inhibition of factor Xa-mediated procoagulant activity of human lung fibroblasts and pleural mesothelial cells

Extravascular fibrin deposition characterizes diverse forms of lung and pleural injury. Fibrin formation in these compartments is locally potentiated by the assembly and expression of the prothrombinase procoagulant complex (factors Xa, Va and II) at the surface of human lung fibroblasts and pleural mesothelial cells. We sought to identify structural domains on factor Xa that mediate expression of prothrombinase activity by these cells. In order to accomplish this objective, we used panels of monoclonal antibodies (MoAbs) to factor X to block prothrombinase assembly and function on the surface of cultured human lung fibroblasts and pleural mesothelial cells. Of 30 factor X MoAbs that recognized native factors X and Xa, 10 completely inhibited factor Xa function (prothrombin activation), and five others neutralized Xa function without affecting cell-binding, presumably by blocking the prothrombin binding site. Western blots showed that these inhibitory MoAbs reacted with the Xa heavy-chain. One MoAb that recognized the factor Xa light-chain blocked prothrombin activation at the factor Va binding site. Our results indicate that prothrombinase activity at the surface of lung parachymal or pleural cells can be blocked by MoAbs that interact with either the heavy- or light-chain of factors X. Antibodies that neutralize cell surface-expressed prothrombin activation offer a potential means to arrest pericellular fibrin formation in the lung and pleural space.

Factors XWenatchee I and II: compound heterozygosity involving two variant proteins

Variant factor X in an individual with a mild bleeding tendency was suspected based on deficient procoagulant activity (10-20% of normal) and antigen (30-35% of normal) levels of plasma factor X. Heteroduplex analysis of factor X gene exons indicated heterozygosity for mutations in both exons 6 and 4, confirmed by direct sequencing of the amplified exons. Substitution of C by T at nucleotide position 13,984 (Arg-139 to Cys) was found in the factor X gene exon 6 of the propositus. This mutation creates a BsmI site and the patient tested heterozygous for the BsmI cleavage involved, as did one of his two daughters. In addition, exon 4 was found to have the normal A and a novel C (Asn-57 to Thr) at nucleotide position 9338. The exon 4 mutation creates a BsaJI site, detectable after amplification mismatch to remove an existing BsaJI site. Both the patient and the second of his two daughters were heterozygous for this cleavage. The two variant proteins are called factors XWenatchee I (Arg-139 to Cys) and II (Asn-57 to Thr). A mixed variant isolate derived from the plasma of the propositus exhibited heavy/light chains of normal size, as well as an apparent single-chain molecule not dissociable by reducing agent. A single-chain molecule would be predicted for form I, if the mutation blocks processing cleavages that normally remove a tripeptide interposed between the heavy and light chains. A Western blot of partially purified factor X from the daughter who inherited the form I defect revealed a component migrating the same as the putative single-chain species. Based upon the factor X activity vs. antigen ratios for the propositus and both daughters, both forms I and II are probably dysfunctional molecules.

Neutralization of factor X activity by factor X-specific monoclonal antibodies

Factor X, a vitamin K-dependent protein, is the plasma zymogen for the active serine protease factor Xa. Factor Xa is the proteolytic enzyme for prothrombinase, the multi-protein membrane complex that catalyses the cleavage of prothrombin to thrombin. A panel of 10 monoclonal antibodies (identified by their corresponding clone numbers: 1, 2, 3, 5, 7, 26, 27, 54, 73, and 79) to factor X were produced by immunizing mice with purified factor X. All of the antibodies bound both human factor X and factor Xa in a solid-phase ELISA and binding of the antibodies was not affected by removal of Ca2+ with EDTA. In immunoblot analysis, antibody alpha HFX-54 bound to the light chain and antibodies alpha HFX-1, -5, -7, and -26 bound to the heavy chain of reduced factor X. Antibodies alpha BFX-2b, alpha HFX-27, -54, and -73 prolonged both the factor X-dependent clotting time and activated partial thromboplastin time (APTT) of normal plasma while antibody alpha HFX-1 only prolonged the APTT. None of the antibodies significantly inhibited factor X activation by purified Russell's viper venom factor X activator. In prothrombin activation assays using purified factor Xa, factor Va, prothrombin, Ca2+ and phospholipid vesicles, seven of the antibodies (alpha HFX-1, -3, -26, -27, -54, -73 and alpha BFX2b) showed some inhibition of thrombin generation ranging from 18 to 60% of the control. The decrease in factor X plasma clotting activity was most likely due to inhibition of factor Xa activity in prothrombinase, although some antibody-dependent inhibition of factor X activation may contribute to the observed inhibition of plasma clotting. Prothrombinase activity on platelets was inhibited in an identical manner by the monoclonal antibodies. When prothrombin was activated in the absence of factor Va, only antibody alpha BFX-2b inhibited activation. Calcium-independent determinants on both the heavy chain (determinants 1 and 26) and light chain (determinant 54) of factor X may play a role in prothrombin activation by prothrombinase. Other epitopes (antibodies alpha HFX-3, -27, -73) appeared to be influenced by association of factor Xa with factor Va. Topographic regions on factor X important for factor X activation and factor Xa function may be identified by the use of these monoclonal antibodies.

Heparin therapy in Russell's viper bite victims with disseminated intravascular coagulation: a controlled trial

A controlled clinical trial of low dose heparin was carried out in confirmed cases of Russell's viper bite. Twenty patients with systemic envenoming were included in the study. They were randomized to receive low dose heparin in an initial dose of 50 units/kg body weight intravenously immediately after antivenom followed by a continuous infusion of 10 unit 3 kg/hour in isotonic saline for 24 hours, or antivenom alone. Response to treatment was assessed clinically as well as by serial measurements of coagulation factors and biochemical values. No significant difference was observed in the outcome among two groups, the recovery rate from the clotting defect being similar in both. The mean serum creatinine values of the two groups were also not statistically different. The results indicated that there is no beneficial effect of adding heparin to the standard treatment by antivenom.

Processing and trafficking of clotting factor X in the secretory pathway. Effects of warfarin

Clotting factor X undergoes several post-translational processing steps in the liver before the protein appears in blood as the mature two-chain zymogen. In this study we have followed the factor X precursor through the secretory pathway in rat liver in order to identify the site for proteolytic processing of the precursor into a two-chain form and the site for warfarin inhibition of precursor trafficking within the pathway. Isolated rat liver Golgi apparatus was shown to harbour two single-chains of factor X precursors of 70 and 74 kDa and the heavy (50 kDa) and light chains of factor X. It was demonstrated that the two-chain factor X form was produced from a late processing form of the factor X precursor, which indicated that the site for proteolytic conversion to a two-chain form was in the trans-Golgi compartment. The 70 and 74 kDa single-chain precursors and also the light chain of the two-chain form were shown to contain the factor X propeptide which is normally removed before the coagulation factor appears in blood. The data demonstrate that intra-chain cleavage of a single chain factor X precursor in the trans-Golgi compartment can precede release of the propeptide. Warfarin was shown to affect trafficking of the factor X precursor between the endoplasmic reticulum (ER) and the Golgi apparatus. The data suggest a link between vitamin K-dependent gamma-carboxylation of the precursor and its exit from the ER. Warfarin administration resulted in accumulation of factor X precursors associated with the ER membrane. These precursors appear to be stabilized from intracellular degradation while in the ER. In contrast to the large increase in the factor X precursor concentration in the ER membrane, there was no change in the prothrombin precursor concentration as a result of warfarin action on the liver. However, intracellular turnover of the microsomal prothrombin precursor pool in warfarin-treated rats resulted in a pool of less negatively charged proteins, indicating ongoing protein synthesis but inhibition of gamma-carboxylation. The data are consistent with previous findings [Wallin & Martin (1988) J. Biol. Chem. 263, 9994-10001] suggesting that prothrombin and factor X are processed differently by the vitamin K-dependent carboxylase in the ER membrane.

Characterisation of monoclonal antibodies to human factor X/Xa. Initial observations with a quantitative ELISA procedure

Monoclonal antibodies to human plasma factor X (FX) and factor Xa (FXa) have been developed using several modifications of previously described techniques. These include the use of footpad immunisation with a suspension of free and nitrocellulose-bound antigen with subsequent fusion of popliteal lymph node cells. From a panel of 17 reactive hybridomas to FX, 3 were selected for further characterisation. An additional hybridoma reactive to FXa but not FX was also selected. Two monoclonal antibodies designated FX52 and FX64 were specific for FX with no reactivity to FXa, while antibody FXa24 was specific for FXa. Another FX/FXa95 reacted with both FX and FXa. All selected antibodies were of the IgG isotype and reacted both with native antigen and antigen transferred to nitrocellulose by Western blotting. Initial observations suggest that Mab FX52 may be used to quantitate FX levels in plasma.

Local thrombin synthesis and fibrin formation in an in vitro thrombosis model result in platelet recruitment and thrombus stabilization on collagen in heparinized blood

The role of the local synthesis of thrombin in platelet recruitment and thrombus stabilization in heparinized blood was examined in vitro. Mural thrombosis was visualized and measured in a thin, rectangular, collagen-coated capillary under controlled rheological conditions by using fluorescence digital videomicroscopy and fluorescence microphotometry. Thrombin activity was inhibited in heparinized blood by the synthetic competitive inhibitor, D-phenylalanyl-L-prolyl-L-arginyl chloromethylketone (FPRCH2Cl), resulting in a marked reduction in the rate of platelet accumulation on collagen surfaces, indicating a role for thrombin in platelet recruitment. Similar although lesser effects were observed by reducing thrombin synthesis with antibodies to factors II and X. To decouple the role of thrombin in platelet recruitment by direct stimulation of platelet activity from its role in thrombus stabilization via fibrin formation, thrombosis was measured in heparinized blood treated with the tetrapeptide glycyl-prolyl-arginyl-proline, which inhibits fibrin monomer assembly into fibrin. The ultimate level but not the initial rate of platelet accumulation was reduced markedly, indicating a role for fibrin in thrombus stabilization against hemodynamic forces. Scanning electron micrographs demonstrated fibrin stands in the heparinized control samples but not in the heparinized samples with glycyl-prolyl-arginyl-proline. These results demonstrate a role for the local action of thrombin synthesized on the surfaces of thrombi even under conditions when the thrombin exerts no bulk effect, such as under heparin anticoagulation. Furthermore, this role appears to be a result of both platelet recruitment and thrombus stabilization.

[Contraception and pregnancy in systemic lupus erythematosus]

In 1990 conflicting views are still being held concerning oral contraception and risks associated with pregnancy in women with systemic lupus erythematosus. As regards oral contraceptives, all authors agree that oestrogen-progestin combination pills are harmful, but the best alternative hormonal contraception remains to be determined. Protein-based "micropills" seem to be harmless; cyproterone acetate appears to be without side-effects, and its usefulness in preventing recurrences is being evaluated. As regards pregnancy, it would be wise not to contemplate having a child until 6 months have elapsed since the onset of remission. Blood pressure, platelet count and serum creatinine and uric acid levels must be closely monitored. Two types of antibodies may be present in the mother and are known to be responsible for foetal complications. These are antibodies to phospholipids (antiprothrombinase, anticardiolipin, antibodies responsible for dissociated treponema serology), which expose to spontaneous abortion or intrauterine death, and the antibody to SS-A (or anti-Ro), which exposes to foetal cardiomyopathy and congenital atrioventricular block.

Conformational changes in an epitope localized to the NH2-terminal region of protein C. Evidence for interaction of protein C domains

Murine monoclonal antibodies, developed following immunization with human protein C, were characterized for their ability to bind antigen in the presence of either CaCl2 or excess EDTA. Three stable clones were obtained which produced antibodies that bound to protein C only in the presence of EDTA. All three antibodies bound to the light chain of protein C on immunoblots and also bound to the homologous proteins factor X and prothrombin in solid-phase radioimmunoassays. One antibody, 7D7B10 was purified and studied further. The binding of 7D7B10 to human protein C was characterized by a KD of 1.4 nM. In competition studies, it was found that the relative affinity of the antibody for protein C was 20-40-fold higher than for prothrombin, fragment 1 of prothrombin, or factor X. In contrast, 7D7B10 was unable to bind to factor IX or bovine protein C. The effect of varying Ca2+ concentration on the interaction of the antibody with protein C was complex. Low concentrations of Ca2+ enhanced the formation of the protein C-antibody complex with half-maximal effect occurring at approximately 60 microM metal ion. However, higher concentrations of Ca2+ completely inhibited 7D7B10 binding to protein C with a K0.5 of 1.1 mM. Furthermore, millimolar concentrations of Mn2+, Ba2+, or Mg2+ also completely abolished antibody binding to protein C. The location of the epitope was delineated by immunoblotting and peptide studies and found to be present in the NH2-terminal 15 residues of protein C. Although residues corresponding to positions 10-13 of human protein C were necessary for maximal binding of the antibody, they were not sufficient. No evidence could be found for involvement of the epitope in metal binding per se. Therefore, the effect of Ca2+ on antibody binding is thought to be due to metal-dependent conformational changes in protein C. It seems likely that Ca2+ occupation of a high affinity site, shown by others to be located in the epidermal growth factor-like domain, causes a conformational change in the NH2-terminal region of protein C which is favorable for antibody interaction, whereas Ca2+ binding to the low affinity site(s), known to be present in the gamma-carboxyglutamic acid domain, causes an unfavorable conformational change.

Evidence for a common metal ion-dependent transition in the 4-carboxyglutamic acid domains of several vitamin K-dependent proteins

The murine monoclonal antibody H-11 binds a conserved epitope found at the amino terminal of the vitamin K-dependent blood proteins prothrombin, factors VII and X, and protein C. The sequence of polypeptide recognized by antibody H-11 contains 2 residues of gamma-carboxyglutamic acid, and binding of the antibody is inhibited by divalent metal ions. By using a solid-phase immunoassay with 125I-labeled antibody and immobilized vitamin K-dependent protein, binding of the antibody to the vitamin K-dependent proteins was inhibited by increasing concentrations of calcium, manganese, and magnesium ion. The transition midpoints for antibody binding were in the millimolar concentration range and were different for each metal ion. In general, the transition midpoints were lowest for manganese ion, intermediate for calcium ion, and highest for magnesium ion. Antibody H-11 bound specifically to a synthetic peptide corresponding to residues 1-12 of human prothrombin that was synthesized as the gamma-carboxyglutamic acid-containing derivative. Binding of the antibody to the peptide was not inhibited by calcium ion. These data suggest that inhibition of antibody H-11 binding by divalent metal ions is not due simply to neutralization of negative charge by Ca2+. This transition which is conserved in vitamin K-dependent proteins containing the H-11 antigenic site is likely due to a structural transition of the amino-terminal polypeptide possibly from a random (accessible) to ordered (inaccessible) structure.

An inhibitory monoclonal antibody to factor X that blocks prothrombin activation but not prothrombinase enzyme assembly

A monoclonal antibody (designated alpha BFX-2b) prepared against bovine factor X inhibited factor X activity in human, bovine, porcine, rabbit, and canine plasma. In assays using purified prothrombinase components, factor Xa, factor Va, phospholipid vesicles, and calcium ion with the fluorescent active site thrombin inhibitor dansylarginyl-N-(3-ethyl-1,5-pentanediyl)amide, the antibody inhibited the conversion of prothrombin to thrombin. Antibody alpha BFX-2b also blocked prothrombinase cleavage of the macromolecular substrates prethrombin 1 and prethrombin 2 but did not inhibit factor Xa hydrolysis of the synthetic substrate benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide. The antibody also prevented the inactivation of factor Xa by antithrombin III but did not prevent the inactivation by soybean trypsin inhibitor. Antibody alpha BFX-2b bound factor Xa with a stoichiometry of 1:1 and an apparent dissociation constant of 9.0 x 10(-11) mol/L as estimated from its inhibition of prothrombinase activity. Antibody alpha BFX-2b did not prevent binding of factor Xa to factor Va-phospholipid as measured by using fluorescence polarization or high-pressure liquid gel chromatography with the fluorescent Factor Xa analogue dansyl-glutamyl-glycyl-arginyl-Xa. Immunoblotting of factor X following electrophoresis on sodium dodecyl sulphate-polyacrylamide gels and transfer to nitrocellulose indicated that the antigenic determinant recognized by antibody alpha BFX-2b was found on the heavy chain of factors X and Xa. From these observations it can be concluded that antibody alpha BFX-2b recognizes a highly conserved epitope on the factor X heavy chain that is remote from the topographic sites required for prothrombinase complex assembly and substrate hydrolysis but may be located at or near a portion of the macromolecular substrate binding site.

Factor X Roma: a congenital factor X variant defective at different degrees in the intrinsic and the extrinsic activation

A factor X molecular variant was identified in a 13-year-old girl affected by a bleeding tendency. Factor X antigen levels and activation by Russel's viper venom (tested both by clotting and amidolytic assays) were normal. Factor X crossed immunoelectrophoresis was found to be identical to that of the control plasma. Factor X functional activity was low (3% of the normal) if tested by PTT-derived assays, whereas it was found at intermediate levels (about 30-50% of the normal) if measured by prothrombin time-derived assays. The defect in the extrinsic activation was more clearly disclosed using as activating agent thromboplastin from ox brain. The factor X of the patient was completely adsorbed by aluminum hydroxide. The parents of the propositus (first degree cousins) showed factor X functional levels compatible with a condition of heterozygosity for the abnormality. This factor X molecular variant appears different from the other ones so far described and was named 'Factor X Roma'.

Enzyme-linked coagulation assay. III. Sensitive immunoassays for clotting factors II, VII, and X

The solid-phase clotting assay utilizing fibrinogen coated on the wells of a microtiter plate and peroxidase-fibrinogen in solution as a substrate for thrombin (enzyme-linked coagulation assay, ELCA) has been modified for use as an immunoassay. Direct inhibition of factors II, VII, and X by polyclonal (rabbit) antibodies and of factor X by monoclonal antibodies has been demonstrated at high dilution of these antibodies and detection of the specific factors using ELCA. Using plates coated with a second antibody (goat anti-mouse IgG) as well as fibrinogen, monoclonal antibodies to factors X and VII were measured by binding the active factor to the plate and detection of the bound factor using ELCA. The assay was very sensitive, permitting the detection of as little as 0.2 ng/ml (30 pg/assay) of monoclonal antibody, or less than 0.4 ng/ml (60 pg/assay) of factor Xa. When plates were coated with monoclonal antibody to factor X and fibrinogen, the assay permitted the identification of distinct epitope specificities for two monoclonal antibodies to factor X by distinct competition of the monoclonal antibodies added in the solution phase for binding of factor Xa to the plate. This assay could be applied generally for immunoassay of clotting factors, and could have application in general as an immunoassay amplification system.

[Course of the manifestations associated with the antiprothrombinase circulating anticoagulant]

Fifteen patients (9 females and 6 males) with the "lupus type" circulating anticoagulant have been studied. The underlying disease was an auto-immune disorder in 11 cases and a malignant hemopathy in 4 cases. The manifestations frequently associated with the lupus inhibitor, such as thrombosis, thrombocytopenia and false-positive VDRL test were analysed. Hemorrhagic syndrome occurred only when thrombocytopenia or acquired abnormality of Willebrand's factor was present. Thrombotic events (8 cases) were frequent. Deep venous thrombosis was complicated with pulmonary embolism in 4 patients. Platelet abnormality, decreased fibrinolytic response or acquired Willebrand's syndrome were found in all patients with a thrombotic event. These different manifestations followed diverging courses in some patients with persistent thrombocytopenia although the anticoagulant had disappeared in 3 cases, negativation of the false-positive VDRL test while the anticoagulant remained unchanged in 1 case, occurrence of a thromboembolic episode although the anticoagulant had disappeared in 1 case.

[Detection of autoantibodies to blood coagulation factors by the passive hemagglutination reaction and by an immunoenzyme (ELISA) method]

Plasma of healthy persons and intact animals contains antibodies to blood coagulation factors IIa and Xa of autologous origin demonstrated by passive hemagglutination and ELISA. During fibrin formation, autoantibodies are bound and thus are not detectable in the serum. The importance of the facts described for hemostasis control is discussed.

Measurement of human activated factor X-antithrombin complex by an enzyme-linked differential-antibody immunosorbent assay

An enzyme-linked immunoabsorbent assay (ELISA) has been developed for the measurement of the complex of human antithrombin and Factor Xa. Rabbit anti-human Factor X antibodies are adsorbed to ELISA plates, and samples containing Xa-antithrombin complex are added. This is followed by the addition of F(ab')2 fragments of rabbit antibodies against human antithrombin, previously labeled with alkaline phosphatase, and subsequent measurement of the bound labeled antibody by hydrolysis of p-nitrophenylphosphate. The minimum level of complex detectable in a sample is ca. 0.1 nM. The assay has been used to follow the generation of Xa-antithrombin complex in kinetic situations by the addition of 1 microM Ile-Glu-Gly-Arg-chloro- methylketone to the ELISA sampling buffer, and it has also been used in plasma systems, where a 20-fold reduction in the sensitivity of the assay is observed. This reduction was shown to be entirely caused by the plasma Factor X. The assay has been used to follow generation of the Xa-antithrombin complex in defibrinated plasma upon activation of the clotting system with the Factor X-activating protein of Russell's Viper venom, and has been compared with the total generation of Factor Xa, measured by a radiopeptide assay of Factor X activation in the same mixtures.

Functional characterization of single-chain factor X from rat liver

14C-Labeled single-chain factor X prepared by vitamin K-dependent carboxylation in vitro was partially purified by adsorption to BaSO4 and chromatography on DEAE-Sephacel. Known activators of factor X were analyzed for their effect on the single-chain molecule. 14C-Labeled factor X antigens were recovered immunochemically from incubation mixtures and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Incubation with trypsin resulted in the generation of factor Xa clotting activity, and the 14C-labeled product migrated after reduction with an apparent molecular weight of 22,500 +/- 1500 (mean +/- 1 SD). The light chain produced by factor Xa was similar to that produced by trypsin (Mr 24,500 +/- 1500; mean +/- 1 SD). Incubation of single-chain factor X with factor VII and thromboplastin, factor IXa, or the factor X activating enzyme from Russell's viper venom gave a reducible product with a light chain of higher apparent molecular weight (Mr 37,000-38,000). Incubation with factor VII and thromboplastin also resulted in the generation of factor Xa clotting activity. Incubation of single-chain factor X with platelets resulted in the binding of about 20% of the 14C. The bound 14C-labeled factor X antigen released by freezing and thawing in the presence of EDTA was reduced to give a 14C-labeled polypeptide with Mr 31,000. Walker 256 tumor cells bound about 30% of the 14C. The bound material, after reduction, gave a 14C-labeled polypeptide with Mr 23,000.

Antithrombin III Trento. A 'new' congenital AT III abnormality with a peculiar crossed-immunoelectrophoretic pattern in the absence of heparin

A family with a new congenital abnormality of antithrombin III (AT III) is presented. 5 members, all females, were affected. The proposita has had several thrombotic manifestations. The other patients, so far, are asymptomatic. Antithrombin activities were all decreased regardless of the method used (chromogenic or clotting) and regardless of the presence or absence of heparin in the assay system. AT III antigen, on the contrary, was normal in all patients regardless of the method used (electroimmunoassay, radial immunodiffusion or Laser nephelometer). The crossed immunoelectrophoresis without heparin showed in plasma the presence of an abnormal peak which was more anodal than the normal counterpart. The same pattern was seen in serum. In the heparin-modified cross-immunoelectrophoresis a normal pattern was seen in plasma and an abnormal one in serum. In the latter the anodal peak was in fact larger than the normal counterpart. Chromatographic studies using Heparin-Sepharose column failed to show changes in heparin affinity, and indicated that both the normal and the abnormal proposed to describe this abnormality. These studies further emphasize the great heterogeneity of AT III defects. This is the first AT III abnormality to show an abnormal crossed-immunoelectrophoresis in the absence of heparin.

Effect of coumadin-induced coagulopoietin plasma on vitamin K-dependent carboxylation of liver microsomes

Coumadin-treated rabbits have a humoral substance(s) (coagulopoietin) which is capable of elevating vitamin K-dependent coagulation factors when injected into recipient rabbits (Karpatkin & Karpatkin, 1973). Biologic levels of coagulation factors II, V, VII and X; immunologic levels of factors II and X; and vitamin K-dependent liver microsomal carboxylase activity were measured in recipient rabbits receiving coumadin-induced coagulopoietin plasma. Factor II biologic activity increased 3.5-fold compared to the increase in immunologic activity. Factor X biologic activity increased 1.7-fold compared to the increase in immunologic activity. This indicates an increase in specific activity of factors II and X. Coumadin-induced coagulopoietin plasma had no effect on vitamin K-dependent liver microsomal carboxylase activity in vitro. However, livers obtained from recipient animals treated with coumadin-induced coagulopoietin plasma enhanced their carboxylase activity (compared to control animals) 2.4-fold employing endogenous microsomal precursor for carboxylation, and 6.2-fold employing synthetic substrate, phe-leu-glu-glu-val. Thus, coumadin-induced coagulopoietin plasma enhances the biologic activity of vitamin K-dependent coagulation factors II, VII, and X as well as the ex vivo vitamin K-dependent carboxylase activity of liver microsomes.

A new family with classical factor X deficiency as demonstrated by electroimmunoassay

A new family with classical factor X deficiency is described. The proposita is a 6 year old girl who presented with occasional epistaxis and a hematoma after an intramuscular injection. The main laboratory features consisted in a prolongation of partial thromboplastin, prothrombin and Stypven clotting times corrected by the addition of normal serum. Factor X activity varied between 3 and 6%. Factor X amydolytic activity was 15% of normal. Electroimmunoassay failed to show the presence of factor X antigen. No inhibitor was found in the proposita plasma. Parents and other family members showed intermediate levels of factor X activity and antigen and were considered to be heterozygotes. No consanguineity was found in the family.

Immunohistochemical identification of factor X-like antigen in the A cells of the normal human pancreas

An indirect immunofluorescence technique using two rabbit anti-human factor X sera was used to investigate the presence of factor X in various human tissues. Of all tissues studied only the cytoplasm of some islet cells of the six human pancreases illustrated specific reactivity for factor X-like material. No reaction was seen in the exocrine pancreas. Cross-absorption studies with factor X-deficient plasma, glucagon or alpha-1-antitrypsin did not modify the pattern. On the contrary, cross-absorption with factor X Friuli or with normal plasma eliminated the positivity. No factor X-like material was seen in rat or guinea pig pancreas. The double-immunofluorescence technique demonstrated that the reactivity was localized only in the cytoplasm of the A cells.

Structural transitions in bovine factor X associated with metal binding and zymogen activation. Studies using conformation-specific antibodies

Conformation-specific antibodies against distinct regions of Factor X were employed to locate antigenic determinants which are altered during zymogen activation or by metal binding. Anti-Factor X antibodies, raised in rabbits against Factor X, were purified by affinity chromatography using Factor X covalently bound to Sepharose. Quantitative equilibrium and kinetic measurements of precipitation of Factor X and Factor Xa by antibodies indicated differences in the antigenic structure of the zymogen and the enzyme form of factor X. The factor X antibodies were further fractionated by sequential immunoabsorption using fragments of Factor X and Factor Xa. With conformation-specific antibodies directed against the heavy chain and the light chain of Factor X, zymogen activation was shown to involve a structural transition in the heavy chain but not the light chain. Antibodies directed against the activation peptide domain 1-51 of the heavy chain, the trypsin-like region of the heavy chain 52-290, and the substrate-binding site suggest a generalized conformational transition in the heavy chain. Antibodies were isolated which are specific for the Factor X:Ca(II) complex and bind to Factor X only in the presence of metal ions. Subfractions were directed against either the heavy chain or the light chain, indicating that both the heavy chain and the light chain of Factor X undergo a metal-induced conformational transition. Half-maximal antibody-factor X interaction was observed at 0.13 mM CaCl2 for the light chain and 0.7 mM CaCl2 for the heavy chain. These results indicate that zymogen activation is limited to structural changes in the heavy chain, but metal binding is associated with changes in the structure of both the heavy and light chains. Metal-dependent binding of Factor X to the platelet Factor Xa receptor after activation may involve surfaces of the heavy as well as the light chains.

The inhibitor of prothrombin conversion in plasma of patients on oral anticoagulant treatment

Pooled plasma of patients under stable oral anticoagulation has been analysed with respect to the presence of the vitamin-K dependent factors (factors II, VII, IX and X). Of all factors 1.5-2 times more antigen than procoagulant activity was present. The concentration of factors II, X (measured spectrophotometrically) and VI is about 0.25 U/ml while factor IX is slightly higher. Coagulation assays of factor X always gave lower values than the spectrophotometric assay. This discrepancy was not influenced by the removal of either factor II-, factor VII- or factor IX antigen. However, when the factor X antigen was replaced by normal factor X, all factor X assay gave identical results, indicating that PIVKA X is responsible for these discrepancies. Using the technic of the Thrombotest-dilution curve it was shown that PIVKA X is the factor that causes the abnormal prolongation of ox-brain prothrombin time in these plasmas.