The synthesis of sulfated dextran beads for isolation of human plasma coagulation factors II, IX, and X

The efficacy and safety of edoxaban versus warfarin in preventing clinical events in atrial fibrillation: A systematic review and meta-analysis

Atrial fibrillation (AF) is the most common type of arrhythmia. Warfarin reduces the incidence and mortality of strokes in patients with AF. Edoxaban reduces the bleeding risk in patients with AF. This study evaluates the efficacy and safety of edoxaban versus warfarin in preventing clinical events in patients with AF through a meta-analysis of randomized controlled trials (RCTs). RCTs were retrieved from medical literature databases. Risk ratios (RRs) and 95% confidence intervals (CIs) were calculated to compare the primary and safety endpoints. In total, five articles (10 trial comparisons) containing 24,836 patients were retrieved. Of these patients, 16,268 (65.5%) received edoxaban and 8,568 (34.5%) received warfarin. Compared with warfarin, edoxaban significantly reduced the incidence of cardiovascular death (CVD), major bleeding, and non-major bleeding (RR: 0.86, 95% CI: 0.80-0.93, I2 : 0.0%; RR: 0.65, 95% CI: 0.59-0.71, I2 : 75.6%; and RR: 0.80, 95% CI: 0.77-0.84, I2 : 79.3%, respectively). Edoxaban did not increase the incidence of stroke, systemic embolic events, myocardial infarction, and adverse events compared with warfarin (RR: 1.00, 95% CI: 0.90-1.11, I2 : 42.8%; RR: 1.00, 95% CI: 0.67-1.49, I2 : 0.0%; RR: 1.08, 95% CI: 0.93-1.27, I2 : 0.0%; RR: 1.00, 95% CI: 0.91-1.10, I2: 46.4%, respectively). This meta-analysis indicated that compared with warfarin, edoxaban can significantly reduce the incidence of CVD and major and non-major bleeding. The anticoagulant effect and safety of edoxaban may be better than those of warfarin.

The allosteric mechanism of activation of antithrombin as an inhibitor of factor IXa and factor Xa: heparin-independent full activation through mutations adjacent to helix D

Allosteric conformational changes in antithrombin induced by binding a specific heparin pentasaccharide result in very large increases in the rates of inhibition of factors IXa and Xa but not of thrombin. These are accompanied by CD, fluorescence, and NMR spectroscopic changes. X-ray structures show that heparin binding results in extension of helix D in the region 131-136 with coincident, and possibly coupled, expulsion of the hinge of the reactive center loop. To examine the importance of helix D extension, we have introduced strong helix-promoting mutations in the 131-136 region of antithrombin (YRKAQK to LEEAAE). The resulting variant has endogenous fluorescence indistinguishable from WT antithrombin yet, in the absence of heparin, shows massive enhancements in rates of inhibition of factors IXa and Xa (114- and 110-fold, respectively), but not of thrombin, together with changes in near- and far-UV CD and (1)H NMR spectra. Heparin binding gives only ∼3-4-fold further rate enhancement but increases tryptophan fluorescence by ∼23% without major additional CD or NMR changes. Variants with subsets of these mutations show intermediate activation in the absence of heparin, again with basal fluorescence similar to WT and large increases upon heparin binding. These findings suggest that in WT antithrombin there are two major complementary sources of conformational activation of antithrombin, probably involving altered contacts of side chains of Tyr-131 and Ala-134 with core hydrophobic residues, whereas the reactive center loop hinge expulsion plays only a minor additional role.

Prevalence and significance of anti-prothrombin (aPT) antibodies in patients with Lupus Anticoagulant (LA)

OBJECTIVE

Anti-prothrombin (aPT) antibodies have been found in Lupus Anticoagulant (LA) positive patients. Their prevalence and relative contribution to thromboembolic risk in LA-positive patients is not well defined. The aim of this study was to determine their presence and association with thromboembolic events in a large series of patients with confirmed LA.

METHODS

Plasma from LA-positive patients was collected at Thrombosis Centers and sent to a reference central laboratory for confirmation. Positive plasma was tested using home-made ELISA for the presence of aPT and anti-beta(2)GPI antibodies.

RESULTS

LA was confirmed in 231 patients. Sixty-one of 231 (26%, 95%CI 22-33) LA positive subjects were positive for IgG aPT and 62 (27%, 95% CI 21-33) were positive for IgM aPT antibodies. Clinical features of Antiphospholipid Syndrome (APS) were not associated with the presence of IgG aPT [43 APS in 61 (70%) positive and 109 APS in 170 (64%) negative IgG aPT subjects, p=ns] or IgM aPT. Rate of positivity of IgG and IgM a beta(2)GPI was significantly higher than that of IgG and IgM aPT. Clinical events accounting for APS occurred in 97 of 130 (75%) IgG a beta(2)GPI positive and in 55 of 101 (54%) IgG a beta(2)GPI negative patients (OR 2.4, 95% CI 1.4 to 4.3, p=0.002). No significant association with clinical events in patients positive for both IgG aPT and IgG a beta(2)GPI as compared to those positive for one or another test was found. When patients negative for both IgG aPT and IgG a beta(2)GPI (LA positive only) were compared with remaining patients, a significantly lower association with clinical events was found (OR=0.4, 95% CI: 0.2 to 0.7, p=0.004).

CONCLUSIONS

As compared to IgG a beta(2)GPI, the prevalence of IgG aPT in patients with LA is significantly lower and not associated with the clinical features of APS.

Expression and purification of recombinant human coagulation factor VII fused to a histidine tag using Gateway technology

BACKGROUND

Factor VII (FVII) is a plasma glycoprotein that participates in the coagulation process leading to the generation of fibrin. The aim of this study was to construct, express and purify recombinant FVII fused to a polyhistidine (his) tag using Gateway technology.

METHODS

To construct the entry clone, blunt-end FVII cDNA and subsequent polymerase chain reaction (PCR) product isolated from a HepG2 cell line was TOPO-cloned into a pENTR TOPO vector. To construct the expression clone, a LR recombination reaction was carried out between the entry clone and destination vector, pDEST26. Chinese hamster ovary (CHO) cells were transfected with 1 microg of DNA of PDEST26-FVII using the FuGENE HD transfection reagent. Two cell lines that permanently expressed recombinant FVII were established. The expression of recombinant FVII was confirmed by reverse transcriptase PCR and enzyme-linked immunosorbent assay. Culture medium containing his-tagged FVII was added to the nickel-nitrilotriacetic acid resin column and bound protein was eluted. The purified protein was detected by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analysis. The biological activity of the recombinant FVII was determined by a prothrombin time assay using FVII-depleted plasma.

RESULTS

The results showed that human recombinant FVII was successfully cloned and the accuracy of the nucleotide sequence of the gene and its frame in the vector were confirmed by DNA sequencing. Stable clones transfected with the construct expressed FVII mRNA and related protein but no expression was detected in the CHO cells containing an empty vector. A protein of about 52 KDa was detected in SDS-PAGE and was further confirmed by western blot analysis. A three-fold decrease in clotting time was observed using this recombinant FVII.

CONCLUSION

As far as we are aware, this is the first report of expression of recombinant FVII fused with a his-tag through Gateway technology. The next steps, including large scale expression, purification, activation and stabilisation, are underway.

Protein C inhibitor directly and potently inhibits activated hepatocyte growth factor activator

BACKGROUND

Hepatocyte growth factor (HGF) plays an important role in tissue repair and regeneration. HGF activator (HGFA), a factor XIIa-like serine protease, activates HGF precursor to HGF. The precursor of HGFA, proHGFA, is activated by thrombin generated at sites of tissue injury. It is known that protein C inhibitor (PCI), an inhibitor of activated protein C (APC), also inhibits thrombin-thrombomodulin (TM) complex.

OBJECTIVES

In the present study we evaluated the effect of PCI on thrombin-catalyzed proHGFA activation in the presence of TM, and on HGFA activity.

RESULTS

PCI did not inhibit thrombin-TM-mediated proHGFA activation, but it directly inhibited activated HGFA by forming an enzyme inhibitor complex. The second-order rate constants (m(-1) min(-1)) of the reaction between HGFA and PCI in the presence or absence of heparin (10 U mL(-1)) were 4.3 x 10(6) and 4.0 x 10(6), respectively. The inhibition of HGFA by PCI resulted in a significant decrease of HGFA-catalyzed activation of HGF precursor. Exogenous HGFA added to normal human plasma formed a complex with plasma PCI, and this complex formation was competitively inhibited by APC in the presence of heparin, but very weakly in the absence of heparin. We also demonstrated using recombinant R362A-PCI that Arg362 residue of PCI is important for HGFA inhibition by PCI as judged from the three-dimensional structures constructed using docking models of PCI and HGFA or APC.

CONCLUSION

These observations indicate that PCI is a potent inhibitor of activated HGFA, suggesting a novel function for PCI in the regulation of tissue repair and regeneration.

Approaches to selective peptidic inhibitors of factor Xa

Inhibitors of procoagulant enzymes, such as factor Xa (fXa) and thrombin, are important for treating thrombosis. Thrombin has complex pro- and anti-coagulant roles and thus fXa is thought to represent an ideal target. Discrete kcat and Km values for cleavage of a library of fluorescence-quenched substrates by fXa were determined. The results highlighted the low selectivity of fXa at its prime sites, and its poor efficiency compared with thrombin, creating a challenge for the design of fXa-specific peptidic inhibitors. We hypothesized that Km rather than kcat/Km values may be better indicators of inhibitor potential for a peptidic sequence, leading us to design peptide sequences for both fXa and thrombin in three forms: fluorescence-quenched substrates, standard alpha-peptides and peptides containing a beta-homoarginine at the cleavage site. Kinetic and competitive inhibition assays with both fXa and thrombin showed the fluorescence-quenched substrates to be the best inhibitors, while the inhibitory effect of the beta-homoarginine peptides varied for the two proteases. Importantly, fXa was inhibited to a much greater extent by the beta-peptides than the corresponding alpha-peptides, resulting in an increased selectivity for fXa inhibition over thrombin for those peptides containing a beta-amino acid at the cleavage site.

Specificity and reactive loop length requirements for crmA inhibition of serine proteases

The viral serpin, crmA, is distinguished by its small size and ability to inhibit both serine and cysteine proteases utilizing a reactive loop shorter than most other serpins. Here, we characterize the mechanism of crmA inhibition of serine proteases and probe the reactive loop length requirements for inhibition with two crmA reactive loop variants. P1 Arg crmA inhibited the trypsin-like proteases, thrombin, and factor Xa, with moderate efficiencies (approximately 10(2)-10(4) M(-1)sec(-1)), near equimolar inhibition stoichiometries, and formation of SDS-stable complexes which were resistant to dissociation (k(diss) approximately 10(-7) sec(-1)), consistent with a serpin-type inhibition mechanism. Trypsin was not inhibited, but efficiently cleaved the variant crmA as a substrate (k(cat)/K(M) of approximately 10(6) M(-1) sec(-1)). N-terminal sequencing confirmed that the P1 Arg-P1'Cys bond was the site of cleavage. Altering the placement of the Arg in a double mutant P1 Gly-P1'Arg crmA resulted in minimal ability to inhibit any of the trypsin family proteases. This variant was cleaved by the proteases approximately 10-fold less efficiently than P1 Arg crmA. Surprisingly, pancreatic elastase was rapidly inhibited by wild-type and P1 Arg crmAs (10(5)-10(6) M(-1)sec(-1)), although with elevated inhibition stoichiometries and higher rates of complex dissociation. N-terminal sequencing showed that elastase attacked the P1'Cys-P2'Ala bond, indicating that crmA can inhibit proteases using a reactive loop length similar to that used by other serpins, but with variations in this inhibition arising from different effective P2 residues. These results indicate that crmA inhibits serine proteases by the established serpin conformational trapping mechanism, but is unusual in inhibiting through either of two adjacent reactive sites.

Factor XII-dependent increases in thrombin activity induce carboxypeptidase-mediated attenuation of pharmacological fibrinolysis

Activation of the contact system in patients treated with fibrinolytic agents may be an important source of thrombin that activates thrombin-activated fibrinolysis inhibitor (TAFI) and attenuates fibrinolysis. Factor (F)XIIa in plasma increased 2-fold over 60 min in patients given either tissue plasminogen activator (t-PA) or streptokinase (SK). To determine whether FXIIa-mediated generation of thrombin and activated TAFI (TAFIa) attenuates fibrinolysis in vitro, plasma clots were incubated with SK (250 U mL-1) or t-PA (2.5 g mL-1) and the rate of lysis was measured. Plasma FXIIa impaired lysis judging from marked acceleration when 2.5 micro m corn trypsin inhibitor were added (lysis increased by 172 +/- 144% for SK and 40 +/- 31% for t-PA vs. no inhibitor, n = 16, P < 0.01). Moreover, inhibition of thrombin with hirudin and TAFIa with carboxypeptidase inhibitor accelerated lysis. We conclude that activation of FXII increases thrombin generation, which promotes TAFIa-mediated attenuation of fibrinolysis.

Localization of an Antithrombin Exosite That Promotes Rapid Inhibition of Factors Xa and IXa Dependent on Heparin Activation of the Serpin

We have previously shown that exosites in antithrombin outside the P6-P3' reactive loop region become available upon heparin activation to promote rapid inhibition of the target proteases, factor Xa and factor IXa. To identify these exosites, we prepared six antithrombin-alpha 1-proteinase inhibitor chimeras in which antithrombin residues 224-286 and 310-322 that circumscribe a region surrounding the reactive loop on the inhibitor surface were replaced in 10-16-residue segments with the homologous segments of alpha1-proteinase inhibitor. All chimeras bound heparin with a high affinity similar to wild-type, underwent heparin-induced fluorescence changes indicative of normal conformational activation, and were able to form SDS-stable complexes with thrombin, factor Xa, and factor IXa and inhibit these proteases with stoichiometries minimally altered from those of wild-type antithrombin. With only one exception, conformational activation of the chimeras with a heparin pentasaccharide resulted in normal approximately 100-300-fold enhancements in reactivity with factor Xa and factor IXa. The exception was the chimera in which residues 246-258 were replaced, corresponding to strand 3 of beta-sheet C, which showed little or no enhancement of its reactivity with these proteases following pentasaccharide activation. By contrast, all chimeras including the strand 3C chimera showed essentially wild-type reactivities with thrombin after pentasaccharide activation as well as normal full-length heparin enhancements in reactivity with all proteases due to heparin bridging. These findings suggest that antithrombin exosites responsible for enhancing the rates of factor Xa and factor IXa inhibition in the conformationally activated inhibitor lie in strand 3 of beta-sheet C of the serpin.

Prothrombin activation during carbon tetrachloride-induced acute liver injury in mice

Serine protease thrombin is a central factor for blood coagulation and is an integral part of inflammatory reactions. Prothrombin activation of the liver homogenates during carbon tetrachloride-induced liver injury was investigated. At 72 h after administration of carbon tetrachloride, the prothrombin activation activity reached the maximal value, and then decreased to the basal level at 168 h. The alanine transaminase (ALT) activity, an index of tissue injury, was dramatically elevated at 36 h after treatment, and almost recovered to normal levels at 72 h. Histochemical analysis revealed that the proliferation of hepatocytes and remarkable phagocytosis by macrophages was observed at 72 h, in contrast to severe tissue damage at 36 h. Finally, we showed that prothrombinase activity, composed of factor Xa and factor Va, was involved in the injury-associated prothrombin activation. Taken together, these results strongly suggest that prothrombin activation by the prothrombinase complex occurred following tissue destruction in carbon tetrachloride-induced liver injury.

Mutations in autolytic loop-2 and at Asp554 of human prothrombin that enhance protein C activation by meizothrombin

Thrombin acts on many protein substrates during the hemostatic process. Its specificity for these substrates is modulated through interactions at regions remote from the active site of the thrombin molecule, designated exosites. Exosite interactions can be with the substrate, cofactors such as thrombomodulin, or fragments from prothrombin. The relative activity of alpha-thrombin for fibrinogen is 10 times greater than that for protein C. However, the relative activity of meizothrombin for protein C is 14 times greater than that for fibrinogen. Modulation of thrombin specificity is linked to its Na(+)-binding site and residues in autolytic loop-2 that interact with the Na(+)-binding site. Recombinant prothrombins that yield recombinant meizothrombin (rMT) and rMT des-fragment 1 (rMT(desF1)) enable comparisons of the effects of mutations at the Na(+)-binding residue (Asp(554)) and deletion of loop-2 (Glu(466)-Thr(469)) on the relative activity of meizothrombin for several substrates. Hydrolysis of t-butoxycarbonyl-VPR-p-nitroanilide by alpha-thrombin, recombinant alpha-thrombin, or rMT(desF1) was almost identical, but that by rMT was only 40% of that by alpha-thrombin. Clotting of fibrinogen by rMT and rMT(desF1) was 12-16% of that by alpha-thrombin, as already known. Strikingly, however, although meizothrombins modified by substitution of Asp(554) with either Ala or Leu or by deletion of loop-2 had 6-8 and <1%, respectively, of the clotting activity of alpha-thrombin, the activity of these meizothrombins for protein C was increased to >10 times that of alpha-thrombin. It is proposed that interactions within thrombin that involve autolytic loop-2 and the Na(+)-binding site primarily enhance thrombin action on fibrinogen, but impair thrombin action on protein C.

gamma-Carboxyglutamic acid content of hepatocellular carcinoma-associated des-gamma-carboxy prothrombin

Serum des-gamma-carboxy prothrombin (DCP) is a useful marker for the diagnosis of hepatocellular carcinoma (HCC), but the exact mechanism of its synthesis and its structural properties in liver diseases are unknown. DCP is measured by the monoclonal antibody MU-3. The purpose of this study was to examine the epitope of MU-3 and to characterize the differences in DCP between HCC and benign liver diseases. The epitope of MU-3 was examined by ELISA using prothrombin Gla domain polypeptides and was determined to be amino acid residues 17-27 of the prothrombin Gla domain, which has four gamma-carboxyglutamic acid residues (Gla) at positions 19, 20, 25 and 26. Peptides having a glutamic acid residue (Glu) at these positions reacted strongly to MU-3 but lost reactivity when Glu 19 or 20 was changed to Gla. In the order of gamma-carboxylation, MU-3 reacted strongly to DCP containing 0-1 Gla, weakly to 2-4 Gla and not at all to DCP containing more than five Gla. After adsorbing normal prothrombin with barium carbonate, DCP reaction to MU-3 was measured by determining the amount of DCP that was adsorbed by MU-3-coated beads. The proportion of DCP reacting to MU-3 in HCC was 41.0-76.8%, whereas in patients with benign liver diseases, only 0-42.1% reacted to MU-3. These results indicate that DCP variants preferentially synthesized in HCC have less than four Gla, which are restricted to positions 16, 25, 26 and 29, whereas DCP variants in benign liver diseases have more than five Gla.

Directed glycosylation of human coagulation factor X at residue 333. Insight into factor Va-dependent prothrombin catalysis

Based on homology, amino acids 326-336 (143-154 in chymotrypsin numbering) of factor X (fX) comprise a flexible surface loop, which is susceptible to self-proteolysis and influences substrate catalysis. To investigate the role of this autolysis loop in fX function, a recombinant variant with a new site for asparagine-linked glycosylation has been produced by changing glutamine 333 to asparagine. Q333N fX is activated normally by factor VIIa and tissue factor, factors IXa and VIIIa, and Russell's viper venom. Proteolysis of the loop is prevented by the mutation. Reactivity of the free enzyme toward substrates and inhibitors is attenuated 4-20-fold; relative to wild type fXa, Spectrozyme Xa(TM) hydrolysis is 25%, inhibition by antithrombin III and the tissue factor pathway inhibitor is approximately 20%, and prothrombin activation in the absence of the cofactor Va is only 5%. Surprisingly, activities of the variant and wild type enzymes are equivalent when part of the prothrombinase complex. N-Glycanase cleaves the new oligosaccharide from Q333N fXa leaving aspartic acid. Q333D fXa is approximately 1.6-fold more reactive with Spectrozyme Xa(TM), antithrombin III and tissue factor pathway inhibitor, and prothrombin than its glycosylated counterpart, Q333N fXa, but still quite abnormal relative to wild type fXa. Like Q333N fXa, Q333D fXa is fully functional as part of the prothrombinase complex. We conclude that Gln-333 is geographically close to a site of proteolytic degradation but not to activator, cofactor, or membrane binding sites. Mutation of Gln-333 impairs catalytic function, but given normal prothrombin activation by the complexed enzyme, the importance of Gln-333 for catalysis is not manifest in the prothrombinase assembly, suggesting a conformational change in complexed fXa.

Purification and characterization of ostrich prothrombin

The work focused on the penultimate enzyme, prothrombin, in the coagulation cascade. Prothrombin was purified and characterized from ostrich plasma. The results obtained contribute to a better understanding of blood coagulation in the ostrich and the evolution of prothrombin and the coagulation cascade. Prothrombin was purified from ostrich plasma by barium chloride precipitation, ammonium sulfate fractionation, and DEAE-cellulose and Cu(2+)-chelate Sepharose chromatography. Ostrich prothrombin exhibited a M(r) of 72,800 and a pI of 6.9 using SDS-PAGE and PAG-isoelectrofocusing, respectively. The N-terminal sequence of ostrich prothrombin showed 78 and 87% identity with human and bovine, respectively. The cDNA was isolated from ostrich liver and the predicted amino acid sequence compared with those from other species. Ostrich prothrombin shares sequence identity with chicken (84%), human (60%), bovine (59%), rat (60%), mouse (59%) and hagfish (50%) prothrombin, suggesting a common function of prothrombin in these vertebrates. Amino acid sequence identities indicate that the thrombin beta-chain (62%) and the propeptide-Gla (75%) domains are the regions most constrained for the common functions of vertebrate prothrombins. Ostrich prothrombin, therefore, shows similarity in structure to other vertebrate prothrombins.

Selective boron-containing thrombin inhibitors--X-ray analysis reveals surprising binding mode

Based on the structural comparison of the S1 pocket in different trypsin-like serine proteases, a series of Boc-D-trimethylsilylalanine-proline-boro-X pinanediol derivatives, with boro-X being different amino boronic acids, have been synthesized as inhibitors of thrombin. Among the novel compounds, a number of derivatives were synthesized which appeared to have side-chain variants too big to fit into the S1 pocket. Nevertheless, these compounds inhibited thrombin in the nM range. The X-ray structure of one of these inhibitors bound to the active side of thrombin reveals that a new binding mode is responsible for these surprising results.

Release of active tissue factor by human arterial smooth muscle cells

Tissue factor (TF), the initiator of coagulation, is thought to function predominantly at the cell surface. Recent data have suggested that active TF is present extracellularly in atherosclerotic plaques, the arterial wall, and the blood. This study was conducted to determine whether smooth muscle cells (SMCs), a major source of arterial TF, could generate extracellular TF. Active TF accumulated in the medium of cultured human SMCs, representing approximately 10% of that measured in the underlying cells at 24 hours. Platelet-derived growth factor, phorbol ester, and tumor necrosis factor-alpha caused approximately 3-fold increases in TF activity in the medium. Release of TF into the medium was dependent on the presence of the TF transmembrane domain but not the cytoplasmic domain. Antibodies to TF precipitated most of the activity from the culture medium, whereas antibodies to the beta(1)-integrin subunit precipitated approximately 33% of the activity. Treatment with detergent or phosphatidylserine:phosphatidylcholine did not increase activity, suggesting that all TF released by SMCs was in the appropriate lipid milieu and not encrypted. Western blotting showed that the medium contained full-length TF protein. Fluorescent cytometry showed that extracellular TF was present largely in particles < or =200 nm, which had a density of 1.10 g/mL. We hypothesize that active extracellular TF found in the injured arterial wall and atherosclerotic plaques derives, in part, from SMC microparticles.

Serine 380 (P14) --> glutamate mutation activates antithrombin as an inhibitor of factor Xa

Heparin regulates the inhibitory activity of antithrombin. It has been proposed that residues P15 and P14 are expelled from beta-sheet A of antithrombin by heparin binding, permitting better interaction of the reactive center loop with factor Xa. We have made a P14 antithrombin variant (S380E) to create an activated inhibitory form of antithrombin in which P14 is already expelled from beta-sheet A. S380E antithrombin fluorescence is enhanced 35 +/- 5% compared with control antithrombin. There is minimal further increase in antithrombin fluorescence upon heparin binding. The variant has a 5 degrees C lower T(m) than control antithrombin. The variant is an inhibitor of proteinases and has a nearly 200-fold increased basal rate of inhibition of factor Xa, after correction for an increased stoichiometry of inhibition. This is comparable to that of antithrombin activated by high affinity heparin pentasaccharide. Full-length high affinity heparin causes only a 7-fold additional increase in rate and a large increase in stoichiometry of inhibition. In contrast, the basal rate of inhibition of thrombin is similar to that of control antithrombin but is increased 300-fold by heparin. These findings suggest that the native state of the S380E variant exists in a loop-expelled conformation that is consequently highly reactive toward factor Xa.

Critical role of the linker region between helix D and strand 2A in heparin activation of antithrombin

The binding of pentasaccharide heparin to antithrombin induces a conformational change that is transmitted to the reactive center loop and increases the rate of inhibition of factor Xa by approximately 300-fold. The mechanism of such transmission is not known. To test the role of residues 134-137, which link helix D to beta-sheet A, in this signal transduction, we created variant antithrombins in which we removed amino acids 134-137 stepwise and cumulatively. Although the deletions did not compromise the fundamental ability of antithrombin to bind to heparin or to inhibit target proteinases thrombin and factor Xa, they did largely decouple conformational changes in the heparin-binding site from conformational activation of the reactive center loop. Because the variant with only Ala(134) removed was as compromised as variants with larger deletions, yet the variant with Ser(137) removed was normal, we concluded that the length of the linker is less important than the precise interrelationship between residues in this region and other residues involved in conformational activation of antithrombin.

Design, synthesis and biological evaluation of selective boron-containing thrombin inhibitors

Based on the structural comparison of the S-1 pocket in different trypsin-like serine proteases, a series of Boc-D-trimethylsilylalanine-proline-boro-X pinanediol derivatives, with boro-X being different amino boronic acids, have been synthesised as inhibitors of thrombin. The influence of hydrogen donor/acceptor properties of different residues in the P-1 side chain of these inhibitors on the selectivity profile has been investigated. This study confirmed the structure-based working hypothesis: The hydrophobic/hydrophilic character of amino acid residues 190 and 213 in the neighbourhood of Asp 189 in the S-1 pocket of thrombin (Ala/Val), trypsin (Ser/Val) and plasmin (Ser/Thr) define the specificity for the interaction with different P-1 residues of the inhibitors. Many of the synthesised compounds demonstrate potent antithrombin activity with Boc-D-trimethylsilylalanine-proline-boro-methoxypropylglycine++ + pinanediol (9) being the most selective thrombin inhibitor of this series.

CA-1 method, a novel assay for quantification of normal prothrombin using a Ca2+ -dependent prothrombin activator, carinactivase-1

We established a novel prothrombin assay, designated CA-1 method, for quantification of normal prothrombin in application of a Ca2+ -dependent prothrombin activator, carinactivase-1 (CA-1), found in the venom of Echis carinatus leucogaster. On microplate, thrombin converted from normal prothrombin in plasma sample by CA-1 cleaves a thrombin specific chromogenic substrate, t-butoxy-Val-Pro-Arg-p-nitroanilide and liberates p-nitroaniline. Then, the normal prothrombin level is decided by measuring the velocity of p-nitroaniline liberation. Normal prothrombin levels in plasma from warfarin-treated individuals were highly correlated with coagulant activities assayed by both prothrombin time and thrombotest. CA-1 method is not only a rapid and highly sensitive chromogenic microplate assay for quantification of normal prothrombin in the range of 10-200 ng/100 microl in plasma samples but also suitable for analyses of many samples in a short time. In addition, normal prothrombin levels obtained by CA-1 method are not inhibited by EDTA and heparin, which reduce prothrombin time and thrombotest activities. CA-1 method is a novel assay for monitoring coagulant activity in warfarin-treated individuals.

Cooperation between VEGF and TNF-alpha is necessary for exposure of active tissue factor on the surface of human endothelial cells

This study was undertaken to characterize tissue factor (TF) induction, localization, and functional activity in cultured human umbilical vein endothelial cells (HUVECs) exposed to recombinant vascular endothelial growth factor (rVEGF) and recombinant tumor necrosis factor-alpha (rTNF-alpha). rVEGF (1 nmol/L) and rTNF-alpha (500 U/mL) synergistically increased TF mRNA, protein, and total activity, as measured in cell lysates. To examine surface TF expression, living cells were treated with antibody to TF and examined microscopically. Almost no staining was seen in control cells or cells treated with a single agent. In contrast, cells treated with both agonists showed intense membrane staining with surface patches, appearing as buds by confocal microscopy. To determine surface TF activity, studies were performed using a parallel-plate flow chamber, which allows detection of factor Xa generation on living cells. rVEGF and rTNF-alpha induced little surface TF activity (0.032+/-0.008 and 0.014+/-0.008 fmol/cm2, respectively). In combination, they significantly increased TF expression on the cell surface (0.429+/-0.094 fmol/cm2, P<0.05). These data indicate that the synergistic effect of rVEGF and rTNF-alpha is necessary to generate functional TF on the surface of endothelial cells. The requirement for multiple agonists to expose active TF may serve to protect endothelial cells from acting as a procoagulant surface, even under conditions of cell perturbation.

Deconvolution of the fluorescence emission spectrum of human antithrombin and identification of the tryptophan residues that are responsive to heparin binding

Heparin causes an allosterically transmitted conformational change in the reactive center loop of antithrombin and a 40% enhancement of tryptophan fluorescence. We have expressed four human antithrombins containing single Trp --> Phe mutations and determined that the fluorescence of antithrombin is a linear combination of the four tryptophans. The contributions to the spectrum of native antithrombin at 340 nm were 8% for Trp-49, 10% for Trp-189, 19% for Trp-225, and 63% for Trp-307. Trp-225 and Trp-307 accounted for the majority of the heparin-induced fluorescence enhancement, contributing 37 and 36%, respectively. Trp-49 and Trp-225 underwent spectral shifts of 15 nm to blue and 5 nm to red, respectively, in the antithrombin-heparin complex. The blue shift for Trp-49 is consistent with partial burial by contact with heparin, whereas the red shift for Trp-225 and large enhancement probably result from increased solvent access upon heparin-induced displacement of the contact residue Ser-380. The enhancement for Trp-307 may result from the heparin-induced movement of helix H seen in the crystal structure. The time-resolved fluorescence properties of individual tryptophans of wild-type antithrombin were also determined using the four variants and showed that Trp-225 and Trp-307 experienced the largest change in lifetime upon heparin binding, providing support for the steady-state fluorescence deconvolution.

Prolonged activation of prothrombin on the vascular wall after arterial injury

This study was designed to characterize the relative roles of bound Xa/Va and thrombin activity in vascular wall procoagulant activity after balloon-induced injury and the extent to which intravenous aspirin and heparin attenuate procoagulant activity associated with the vascular wall. Abdominal aortic injury was induced in rabbits by overinflation and multiple passages ofa 4F embolectomy catheter. Rabbits were killed 15 minutes or 4, 8, 24, 48, 72, 96, or 120 hours after injury. Aortic segments were incubated ex vivo to define bound procoagulant activity. Thrombin activity bound to the aorta was detected by 4 hours after injury and was most marked over the first 24 hours, as estimated by increases in concentration of fibrinopeptide A during incubation of segments with recalcified barium-adsorbed plasma or activity against the thrombin-synthetic substrate S-2238. Based on comparison with purified human thrombin incubated under the same conditions, a maximum of 0.04 to 0.1 nmol/L per square centimeter of thrombin activity was associated with the vascular wall during the first 24 hours and remained detectable for 72 hours. In contrast, bound Xa/Va complex activity to injured segments was detected within 15 minutes and induced activation of prothrombin added to recalcified barium-adsorbed plasma incubated with injured segments for 96 hours. Aspirin (15 mg/kg) administered 30 minutes before injury attenuated 111In-platelet deposition at 4 hours by 67%, with an associated decrease in bound Xa/Va and thrombin activity at 15 minutes and 4 hours. However, intravenous heparin did not attenuate bound Xa/Va activity at 15 minutes or thrombin activity at 15 minutes and 4 hours. Platelet-dependent bound Xa/Va activity occurs rapidly after arterial injury and may promote thrombin elaboration for up to 96 hours. Bound thrombin activity and de novo thrombin elaboration on the vascular wall may play an important role in the progression of thrombosis and vascular wall remodeling.

Tissue factor expression in human arterial smooth muscle cells. TF is present in three cellular pools after growth factor stimulation

Tissue factor (TF) is a transmembrane glycoprotein that initiates the coagulation cascade. Because of the potential role of TF in mediating arterial thrombosis, we have examined its expression in human aortic and coronary artery smooth muscle cells (SMC). TF mRNA and protein were induced in SMC by a variety of growth agonists. Exposure to PDGF AA or BB for 30 min provided all of the necessary signals for induction of TF mRNA and protein. This result was consistent with nuclear runoff analyses, demonstrating that PDGF-induced TF transcription occurred within 30 min. A newly developed assay involving binding of digoxigenin-labeled FVIIa (DigVIIa) and digoxigenin-labeled Factor X (DigX) was used to localize cellular TF. By light and confocal microscopy, prominent TF staining was seen in the perinuclear cytoplasm beginning 2 h after agonist treatment and persisting for 10-12 h. Surface TF activity, measured on SMC monolayers under flow conditions, increased transiently, peaking 4-6 h after agonist stimulation and returning to baseline within 16 h. Peak surface TF activity was only approximately 20% of total TF activity measured in cell lysates. Surface TF-blocking experiments demonstrated that the remaining TF was found as encrypted surface TF, and also in an intracellular pool. The relatively short-lived surface expression of TF may be critical for limiting the thrombotic potential of intact SMC exposed to growth factor stimulation. In contrast, the encrypted surface and intracellular pools may provide a rich source of TF under conditions associated with SMC damage, such as during atherosclerotic plaque rupture or balloon arterial injury.

Characterization of membrane-associated prothrombin activator in normal and injured murine tissues

Thrombin is a multifunctional enzyme involved in coagulation, cell modulation and inflammation. We recently reported a novel membrane-associated prothrombin activator, abbreviated as MAPA, found in cultured fibroblasts and glial cell lines. In this study, we examined the physiological role of this enzyme. MAPA-like activity was detected in the liver, kidney, lung and heart but not in the spleen or brain in normal mice. To examine whether MAPA participates in biological reactions, hepatic and renal injury were induced by administration of CCl4 and HgCl2, respectively. MAPA-like activity was specifically increased in the injured tissues: the activity was elevated by about 100-fold in 48 h in the liver and increased by about 5-fold in 12 h in the kidney. Their enzymatic properties were the same as those of MAPA in 8C feline kidney fibroblast cells. Phospholipids are required for activation of prothrombin by MAPA obtained from both 8C cells and tissues. These results suggest that MAPA activates prothrombin on the cell surface in injured tissue and participates in inflammation and regeneration associated with tissue injury.

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.

Role of arginine 132 and lysine 133 in heparin binding to and activation of antithrombin

The binding of heparin to antithrombin greatly accelerates the rate of inhibition of the target proteinases thrombin and factor Xa. Acceleration of the rate of inhibition of factor Xa involves a conformational change in antithrombin that is translated from the heparin binding site to the reactive center loop. A mechanism has been proposed for generation and propagation of the conformational change in which the binding of the negatively charged heparin reduces ionic repulsions between positively charged residues on and adjacent to the D-helix in the heparin binding site of antithrombin (van Boeckel, C. A. A., Grootenhuis, P. D. J., and Visser, A. (1994) Nature Struct. Biol. 1, 423-425). This charge neutralization is proposed to elongate the D-helix and initiate the conformational change which is then translated to the reactive center loop. Several basic residues, including arginine 132 and lysine 133, were predicted to be important both in heparin binding and in this mechanism of heparin activation. To test both the helix extension mechanism and the role of these two residues in heparin binding and factor Xa inhibition, we individually changed arginine 132 and lysine 133 to uncharged methionine by site-directed mutagenesis. The Kd values for binding of R132M and K133M variants to the high affinity pentasaccharide were weakened only 2.3- and 4.5-fold respectively, suggesting a location for R132 and K133 peripheral to the main pentasaccharide binding site. However, the Kd values for long chain high affinity heparin were weakened at least 17-fold for both R132M and K133M, indicating involvement of each residue in binding extended chain heparin species. These reductions in affinity were ionic strength-dependent. The rates of inhibition of factor Xa and thrombin by each variant, however, were indistinguishable from those of control antithrombin, and the accelerations of the rate of inhibition produced by heparin were normal. We conclude that neither arginine 132 nor lysine 133 plays an important role in the binding of heparin pentasaccharide or in the mechanism of heparin activation, suggesting that D-helix extension through charge neutralization is not the mechanism for transmission of conformational change from the heparin binding site to the reactive center region. Arginine 132 and lysine 133 do, however, play a role in tight binding of longer chain heparin species through ionic interactions.

Factor XSt. Louis II. Identification of a glycine substitution at residue 7 and characterization of the recombinant protein

A molecular defect in factor X (fX) results from a point mutation that causes glycine substitution for gamma-carboxylated glutamic acid at position 7. The variant (fXSt. Louis II) and wild type (fXWT) proteins were produced in a mammalian expression system and characterized. fXSt. Louis II has <1% and approximately 3% of normal clotting activity in modified prothrombin time and partial thromboplastin time assays, respectively. The rate of activation of fXSt. Louis II by factor VIIa and tissue factor is undetectable under conditions that result in complete activation of fXWT; activation by factors VIIIa and IXa is approximately 30% of normal activation. The X-activating protein from Russell's viper venom activates fXSt. Louis II completely but at a reduced rate. Thrombin generation on phoshopolipid vesicles or activated platelets is approximately 30% or approximately 5%, respectively. Membrane-dependent autolysis is markedly reduced for fXSt. Louis II. In reactions that are not surface-dependent, fXSt. Louis II is nearly identical to that of fXWT. The rate of inhibition by antithrombin is indistiguishable, as is the rate of thrombin formation in the absence of phospholipid, with or without factor Va.

Sustained inhibition of whole-blood clot procoagulant activity by inhibition of thrombus-associated factor Xa

Progression of arterial thrombosis partly depends on thrombus-associated thrombin and activated factor X (Xa) activity. However, whether Xa or thrombin is the most appropriate target for inhibition of recurrent thrombosis is unknown. This study was designed to determine whether inhibition of Xa results in more sustained attenuation of thrombus-associated precoagulant activity than does inhibition of thrombin. Clots prepared ex vivo from human whole blood and pathological arterial thrombi from patients were preincubated in citrated plasma containing no inhibitor, 0.5 to 1 U/mL heparin, 0.5 to 1 mumol/L hirudin, 5 to 10 mumol/L tick anticoagulant peptide (TAP), 0.15 to 3 mumol/L tissue factor pathway inhibitor (TFPI), or a combination of 1 mumol/L hirudin and 10 mumol/L TAP for 2 hours. After preincubation the clots were removed from first-stage plasma, extensively washed in phosphate-buffered saline, and added to nonanticoagulated whole blood. Clots preincubated in plasma without inhibitors induced marked activation of the coagulation system in whole blood, as characterized by greater increases in the concentration of fibrinopeptide A (FPA) over 7 minutes than in blood without added clots (1522 +/- 568 compared with 117 +/- 170 ng/mL, P < .01). Preincubation of clots with heparin or hirudin did not attenuate the increases in FPA in wholeblood. In contrast, compared with incubation without an inhibitor, preincubation of clots with TAP or TFPI markedly attenuated the increases in FPA when clots were added to whole blood (551 +/- 316 and 508 +/- 208 ng/mL, respectively, P < .01). Similar results were obtained with arterial thrombi from patients. Inhibition of Xa but not of thrombin results in sustained attenuation of thrombus-associated procoagulant activity. Uninhibited thrombus-associated Xa activity may account for the increases in thrombin activity that are commonly observed in clinical trials after discontinuation of thrombin inhibitors.

Magnesium(II) is a crucial constituent of the blood coagulation cascade. Potentiation of coagulant activities of factor IX by Mg2+ ions

We recently showed that not only Ca2+ ions but also Mg2+ ions play a crucial role in stabilizing the native conformation of coagulation factor IX. We here report that Mg2+ ions at physiological concentrations greatly augment the biological activities of factor IX. In clotting assays with dialyzed plasma, addition of Mg2+ ions enhanced the apparent coagulant activity of factor IXa, while that of factor Xa was scarcely affected. Activation of factor X by factor IXa in the presence of factor VIIIa, phospholipids, and Ca2+ ions was accelerated by Mg2+ ions. It appeared that the cation increased the affinity between factor IXa and factor VIIIa, thereby increasing the apparent catalytic efficacy of the enzyme. We also evaluated the effect of Mg2+ ions in the coagulation pathway initiated by tissue factor and found that activation of factor IX by factor VIIa*tissue factor was accelerated by the cation. Consequently, clotting of normal plasma induced by factor VIIa*tissue factor was shortened by the cation, while no such effect was observed in plasma deficient in factor IX or VIII. These results indicate that the previously unrecognized plasma component, Mg2+ ions, plays crucial roles in blood coagulation and, moreover, that contributions of factors IX and VIII in the coagulation cascade have been seriously underestimated in previous investigations.

Isolation and characterization of carinactivase, a novel prothrombin activator in Echis carinatus venom with a unique catalytic mechanism

The venom of the viper Echis carinatus contains a metalloprotease, ecarin, that is a potent prothrombin activator. We here show that the venom is also rich in another prothrombin activator, which does not belong to any known category of prothrombin activators. The novel enzyme, designated carinactivase-1 (CA-1), consists of two subunits held together non-covalently but very tightly. One subunit is a 62-kDa polypeptide that has metalloprotease activity and is homologous to the single-chain enzyme ecarin; the other subunit of 25 kDa consists of two disulfide-linked polypeptides of 17 and 14 kDa, and this subunit resembles the anticoagulant in the habu snake venom, IX/X-bp, that specifically binds the Gla domains of coagulation factors IX and X in a Ca2+-dependent fashion. The activation of prothrombin by CA-1 requires Ca2+ ions at millimolar concentrations and in the absence of Ca2+ ions this enzyme is virtually inactive. By contrast, activation by ecarin is completely independent of Ca2+ ions. CA-1, unlike ecarin, does not activate prothrombin derivatives, in which binding of Ca2+ ions has been perturbed, namely prethrombin-1 and acarboxyprothrombin. Furthermore, the isolated catalytic subunit, although its activity is greatly reduced as compared to that of the holoenzyme, no longer requires Ca2+ ions for the activation of prothrombin. Reconstitution with the non-catalytic 25-kDa subunit restores high level activity and the dependence on Ca2+ ions. Finally, prothrombin activation by CA-1 is inhibited by prothrombin fragment 1, and the isolated non-catalytic subunit is capable of binding fragment 1 in the presence of Ca2+ ions. From these observations, we postulate the following unique mechanism for the activation of prothrombin by CA-1. The enzyme primarily recognizes the Ca2+-bound conformation of the Gla domain in prothrombin via the 25-kDa regulatory subunit, and the subsequent conversion of prothrombin to active thrombin is catalyzed by the 62-kDa catalytic subunit.

Molecular mechanism of the dysfunction of protein S(Tokushima) (Lys155-->Glu) for the regulation of the blood coagulation system

The congenital abnormal protein S(Tokushima) has Glu substituted for Lys155 in the second epidermal growth factor domain of the protein S molecule (Hayashi T., Nishioka J., Shigekiyo, T. Saito, S. and Suzuki, K. (1994) Blood 83, 683-690). To elucidate the molecular mechanism of the dysfunction of the protein S(Tokushima), a comparative evaluation between the molecular interaction of the abnormal protein S and that of normal protein S with other clotting factors was carried out using recombinant normal protein S (rPSN) and protein S(Tokushima) (rPST) expressed in baby hamster kidney cells. While rPSN and plasma protein S exhibited cofactor activity for activated protein C (APC), rPST did not show this property. rPSN and rPST bound equally to phospholipids and C4b-binding protein fixed on microplate wells. APC bound to rPSN but not to rPST in an assay using immobilized monoclonal anti-protein S antibody. On the other hand, rPSN and plasma protein S inhibited the activity of prothrombinase complex composed of factor Xa and thrombin-stimulated platelets, whereas rPST lacked this inhibitory effect. Assessment of the mechanism by which rPST lacks inhibitory activity on the platelet-prothrombinase complex was also performed. Factor Xa bound to rPSN but not to rPST. Binding to rPSN to biotinylated factor Va in solution phase did not differ significantly from that of rPST. Binding of prothrombin to factor Va in solution phase was not inhibited either by rPSN or rPST. Binding of 4-amidinophenylmethanesulfonyl-factor Xa to factor Va in solution phase increased in the presence of rPSN but not in that of rPST. These findings suggest that the dysfunction of protein S(Tokushima) occurs because it fails to interact with APC and factor Xa. This molecular interaction is required for the expression of the APC cofactor activity and for the inhibition of the prothrombinase complex activity.

Species differences in anticoagulant and anti-Xa activity of DX-9065a, a highly selective factor Xa inhibitor

Activation of the blood coagulation cascade results in the formation of factor Xa (FXa) in either its intrinsic or extrinsic pathways, which in turn converts prothrombin to thrombin. We recently described the synthesis and characterization of DX-9065a, a highly selective FXa inhibitor, as an orally active anticoagulant agent (1). Although DX-9065a potently inhibited human FXa (2), a much larger dosage was required to inhibit thrombus formation in a rat thrombosis model (3): The plasma concentration of DX-9065a after administration of a dose which reduced thrombus formation by 50% in rats was in the microM range, or 40 times higher than the Ki value for FXa. From this, several assumptions arise, namely that an unknown mechanism other than FXa inhibition contributes to the antithrombotic effect of DX-9065a, or that the efficacy of DX-9065a differs among species. However, as the anticoagulant effect of DX-9065a was closely consistent with anti-Xa activity in plasma and as DX-9065a inhibited only FXa activity (2), the former assumption could be disregarded. To clarify the latter, we examined the inhibitory effect of DX-9065a on FXa from several animal species and its anticoagulant effect ex vivo in these species. Further, we also compared the anticoagulant effect of DX-9065a in plasma from each species with that of NAPAP, a benzamidine-derived direct thrombin inhibitor (4).

Chromatographic techniques for the isolation and purification of lipoproteins

Various modes of chromatography are available for lipoprotein separation. Gel permeation and affinity chromatography are used for preparative purposes and to separate lipoproteins according to size and apolipoprotein content, respectively. Development of rigid supports for gel permeation has led to large improvements in speed and resolution. Reversed-phase high-performance liquid chromatography (HPLC) of apolipoproteins offers the best performance in terms of speed and resolution of structural variants. Due to its high speed and superior resolving power, the recently developed technique of capillary electrophoresis should emerge as an important method for lipoprotein analysis.

Characterization of the binding of factor Xa to fibrinogen/fibrin derivatives and localization of the factor Xa binding site on fibrinogen

The binding of human factor Xa to fibrinogen and its derivatives was characterized. Factor Xa bound to immobilized fibrin with a concentration at half-maximal binding (C50) of 100 nM. The 4-carboxyglutamic acid (Gla) domain of factor Xa is important in factor Xa binding to fibrin monomer, based on the following observations; the binding requires Ca2+; Gla-domain-lacking factor Xa could not bind to fibrin; factor Xa binding was significantly reduced by prior treatment of factor Xa with factor IX/factor-X-binding protein from the venom of Trimeresurus flavoviridis which specifically binds to the Gla domain of human factors IX and X. Factor Xa also bound to fibrinogen, fibrinogen degradation products (FDP)-D and FDP-E, with a similar affinity (C50 = 75-131 nM). In a solution-phase equilibrated binding assay, approximately 0.76 mol factor Xa bound to 1 mol fibrinogen with a dissociation constant of 180 nM. The binding of 125I-labeled factor Xa to the fibrin monomer was inhibited markedly by unlabeled factor Xa, but only slightly by thrombin, suggesting that the binding site of factor Xa on fibrin monomer differs from that of thrombin. We localized the binding site of factor Xa on fibrinogen: factor Xa bound strongly to the A alpha chain, but weakly to the B beta and gamma chains of fibrinogen. The A alpha chain was then digested with lysyl endopeptidase and separated by reverse-phase HPLC. Among resulting peptides, factor Xa bound specifically to a peptide corresponding to residues Asp82-Lys123 of the A alpha chain. This factor-Xa-binding site is located in the boundary between the central E domain and the terminal D domain of fibrinogen and is apparently distinct from the reported thrombin-binding site.

Improved virus safety and purity of a chromatographically produced factor IX concentrate by nanofiltration

A virus removal system based on tangential flow filtration was introduced into a Factor IX production process. Beside the intended virus reduction potency of filter membranes, an additional purification effect could be achieved. This purification effect was evaluated in detail by means of sodium dodecyl sulphate-polyacrylamide gel electrophoresis and size-exclusion HPLC. High-molecular-mass impurities were retained by the membrane, thus increasing the specific activity of the product.

Regulation of the tertiary structure and function of coagulation factor IX by magnesium (II) ions

The indispensable role of Ca2+ ions in the maintenance of the functional tertiary structures of vitamin K-dependent coagulation factors has been definitively established but the participation of Mg2+ ions, another alkaline-earth metal that is present abundantly in blood plasma, in such a process is not yet understood. We show here that the Ca(2+)-stabilized conformation of coagulation factor IX undergoes a further conformational change upon binding of Mg2+ ions using three independent structural probes. The probes we used were (i) IX/X-bp, a snake venom anticoagulant that recognizes the Gla domains in coagulation factors IX and X, (ii) conformation-specific polyclonal antibodies against bovine factor IX, and (iii) monoclonal antibodies against the Gla domain of human factor IX. The binding of all these probes had an absolute requirement for Ca2+ ions, and Mg2+ ions alone were ineffective. However, when added together with Ca2+ ions, Mg2+ ions at physiological concentrations greatly augmented the binding of these probes to factor IX; the required concentration of Ca2+ ions was much reduced, and the affinity of each probe for factor IX was increased even in the presence of an excess of Ca2+ ions. These results suggest the presence of a Mg(2+)-specific binding site that does not interact with Ca2+ ions in factor IX. Furthermore, Mg2+ ions potentiated the susceptibility of factor IX to activation by factor XIa, concomitant with their effect on the conformation. Similarly, the required Ca2+ concentration was reduced by Mg2+ ions, and the rate of conversion to factor IXa was increased by Mg2+ ions in the presence of an excess of Ca2+ ions. At a saturating concentration of Ca2+ ions (5 mM), addition of 1 mM Mg2+ reduced the apparent Km value for factor IX from 0.31 to 0.18 microM, and in the presence of a physiological concentration of Ca2+ ions (1 mM), the reduction in Km by Mg2+ ions was far more striking (from 0.91 to 0.24 microM). The apparent Vmax values were hardly affected by Mg2+ ions. Our present data reveal a hitherto novel physiological role of the Mg2+ ions in plasma. Not only Ca2+ ions but also Mg2+ ions are important regulators of the stabilization of the native conformation of factor IX as well as of its efficient activation.

Identification of the oligosaccharide structures of human coagulation factor X activation peptide at each glycosylation site

Human blood coagulation factor X has two N-linked oligosaccharides at Asn39 and Asn49 residues and two O-linked oligosaccharides at Thr17 and Thr29 residues in the region of the factor X activation peptide (XAP) which is cleaved off during its activation by factor IXa. We determined the structure of oligosaccharides in the XAP region of human factor X. Four glycopeptides each containing a glycosylation site were isolated by digestion of XAP with endoproteinase Asp-N followed by reversed-phase HPLC. N-linked oligosaccharides released from the glycopeptides by glycoamidase A digestion were derivatized with 2-aminopyridine. Pyridylamino(PA)-oligosaccharides were separated by HPLC into neutral and sialyl oligosaccharides using an anion-exchange column. Structures of oligosaccharides and their contents at each glycosylation site were determined by a two-dimensional sugar mapping method. The contents of the neutral oligosaccharides at Asn39 and Asn49 residues were 32.5% and 30.0%, respectively. Six neutral and twelve monosialyl oligosaccharides isolated from both N-linked glycosylation sites showed similar elution profiles composed of bi-, tri- and tetra-antennary complex type oligosaccharides. The predominant component in neutral oligosaccharides was biantennary without a fucose residue. Two major monosialyl oligosaccharides were also biantennary without fucose and with a Neu5Ac alpha 2-->6 residue. In addition, the structures of O-linked oligosaccharides at Thr17 and Thr29 residues were suggested to be disialylated Gal beta 3GalNAc sequences by their component analyses.

A functional factor X deficiency

A functional factor X deficiency is described which caused pronounced reduction in the in vitro activation of the extrinsic system while marginally affecting the in vitro activation of the intrinsic pathway. All studies were normal with the exception of a prolonged PT, an elevated factor X antigen, and low factor X activity. Western blot analysis revealed the presence of two factor X species. The abnormal molecule was of higher molecular weight. Interestingly, there was no bleeding associated with this deficiency. The biochemical basis of this defect is currently under investigation.

A transient entanglement coupling mechanism for DNA separation by capillary electrophoresis in ultradilute polymer solutions

Using capillary electrophoresis, large DNA molecules (2.0-23.1 kbp) may be rapidly separated in ultradilute polymer solutions (< 0.002% w/w) under a high-voltage, steady field (265 V/cm). At this polymer concentration, the separation mechanism appears to be significantly different from that postulated to occur in crosslinked gels. Based on experimental results obtained with DNA restriction fragments and with negatively charged latex microspheres, we conclude that the Ogston and reptation models typically used to describe gel electrophoresis are not appropriate for DNA separations in such dilute polymer solutions. Electrophoresis experiments employing solutions of both small and large hydroxyethyl cellulose polymers highlight the importance of polymer length and concentration for the optimum resolution of DNA fragments varying in size from 72 bp to 23.1 kbp. A transient entanglement coupling mechanism for DNA separation in dilute polymer solutions is developed, which suggests that there is no a priori upper size limit to DNA that can be separated by capillary electrophoresis in a constant field.

Carbohydrate isoforms of antithrombin variant N135Q with different heparin affinities

We have changed one of the carbohydrate-bearing asparagine residues of human antithrombin to glutamine by site-directed mutagenesis and expressed the variant antithrombin, N135Q, in baby hamster kidney cells. Two isoforms were secreted, both of which had higher affinity for heparin than human plasma alpha antithrombin. Both forms had normal inhibitory activity toward factor Xa and showed normal heparin acceleration of proteinase inhibition. The mutation resulted in a higher production of the very high affinity form from about 30% to 60% of the total secreted antithrombin. This form should be the most useful for comparison of the effects of other mutations on heparin binding and proteinase inhibition.

Different effects of two methods of low-density lipoprotein apheresis on the coagulation and fibrinolytic systems

OBJECTIVE

Immunoadsorption (IMA) and dextran sulfate adsorption (DSA) are two methods for selective extracorporeal elimination of low-density lipoproteins which are known as LDL apheresis. Their influence on haemostasis until now is widely unknown.

DESIGN

The effects of both LDL apheresis procedures on the coagulation and fibrinolytic systems were compared amongst five patients treated with IMA and four patients who received a DSA therapy.

SUBJECTS

All patients with severe heterozygous familial hypercholesterolaemia were participants in a long-term LDL apheresis programme with treatments every 1-2 weeks.

INTERVENTION

Combined anticoagulation with heparin and citrate in IMA, and also heparin exclusively in DSA were used for the extracorporeal circulation.

MEASURES

Blood samples were taken immediately before and after a single LDL apheresis and five times during the weekly interval until the next therapy.

RESULTS

DSA had a significantly greater effect on standard clotting tests than IMA at the end of plasma therapy despite identical dosages of heparin. DSA caused a considerable reduction of the coagulation factors V, VIII:C, vWF:Ag, XI, XII, and prekallikrein by 48-99% at the end of apheresis treatment whereas only factor VIII:C showed a marked decrease of 72% after IMA. All abnormalities of the global coagulation tests and of most clotting factors were restored 1 day after treatment in both procedures followed by a moderate rebound phenomenon of single coagulation factors during the next few days in IMA-treated patients.

CONCLUSION

DSA exerts a more profound effect on the coagulation system than IMA by a substantial co-elimination of various clotting factors in addition to the desired removal of atherogenic lipoproteins.

Importance of factor Xa in determining the procoagulant activity of whole-blood clots

The binding of thrombin to fibrin is thought to be an important mechanism by which thrombi exhibit procoagulant activity; however, the extent to which other procoagulants are associated with thrombi has not been previously defined. This study was designed to determine whether clotting factors other than thrombin are bound to whole-blood clots and can thereby contribute to significant procoagulant activity. Clots formed in vitro from human blood exhibited minimal thrombin activity when incubated in plasma depleted of vitamin K-dependent factors by barium-citrate adsorption, as indicated by increases in the concentration of fibrinopeptide A (FPA), a marker of fibrin formation, to 72 nM after 30 min. Incubation of clots in barium-absorbed plasma repleted with 0.9 microM human prothrombin under the same conditions resulted in marked increases in the concentration of FPA (> 1,000 nM) and clotting by 30 min. The increases in FPA were attributable to activation of the added prothrombin by clot-associated Factor Xa, judging from concomitant increases in the concentration of prothrombin fragment 1.2. Similar results were obtained with thrombi induced in the axillary arteries of dogs by vascular injury and incubated with plasma in vitro. Activation of prothrombin was inhibited in a dose-dependent manner by tick anticoagulant peptide, a direct inhibitor of Factor Xa, at concentrations of 0.5-5.0 microM. Clot-associated Factor Xa activity was resistant to inhibition by anti-thrombin III, judging from the lack of inhibition of prothrombin activation during incubation of clots in plasma containing heparin pentasaccharide, an anti-thrombin III-mediated inhibitor of Factor Xa. Thus, the activity of Factor Xa appears to be an important determinant of the procoagulant activity of whole-blood clots and arterial thrombi, and is resistant to inhibition by anti-thrombin III-dependent inhibitors.

Effects of annexin V on the activity of the anticoagulant proteins C and S

The effects of annexin V on the anticoagulant activity of activated protein C (APC) and protein S were examined. Although annexin V did not influence the amidolytic potential of APC, it inhibited both APC and protein S function in a factor Va inactivation assay. Competition experiments demonstrated that annexin V inhibits protein S binding to phospholipid vesicles in a dose-dependent manner. These results demonstrate that annexin V effectively interferes with the anticoagulant arm of the hemostatic system.