The inhibition of activated bovine coagulation factors X and VII by antithrombin III

Rethinking events in the haemostatic process: role of factor V and TFPI

Regulatory mechanisms responsible for limiting blood clot formation are critical for maintaining normal haemostasis. Dysregulation can lead to bleeding (e.g. haemophilia) or thrombosis. New findings showing that tissue factor pathway inhibitor-alpha (TFPIα) binds coagulation factor V(a) and inhibits prothrombinase assembly highlights that our understanding of the initiation of coagulation is evolving. Work over the past decade on the biochemistry of FV activation has laid the groundwork for deciphering the mechanistic bases that may underpin how TFPIα mediates these anticoagulant effects. Collectively, these new findings are re-shaping our thinking about how coagulation is initiated at the site of injury. These ideas could have important clinical implications and help identify new ways to bias the coagulation response for the treatment of haemophilia and other disorders of the haemostatic process.

New Fundamentals in Hemostasis

Hemostasis encompasses the tightly regulated processes of blood clotting, platelet activation, and vascular repair. After wounding, the hemostatic system engages a plethora of vascular and extravascular receptors that act in concert with blood components to seal off the damage inflicted to the vasculature and the surrounding tissue. The first important component that contributes to hemostasis is the coagulation system, while the second important component starts with platelet activation, which not only contributes to the hemostatic plug, but also accelerates the coagulation system. Eventually, coagulation and platelet activation are switched off by blood-borne inhibitors and proteolytic feedback loops. This review summarizes new concepts of activation of proteases that regulate coagulation and anticoagulation, to give rise to transient thrombin generation and fibrin clot formation. It further speculates on the (patho)physiological roles of intra- and extravascular receptors that operate in response to these proteases. Furthermore, this review provides a new framework for understanding how signaling and adhesive interactions between endothelial cells, leukocytes, and platelets can regulate thrombus formation and modulate the coagulation process. Now that the key molecular players of coagulation and platelet activation have become clear, and their complex interactions with the vessel wall have been mapped out, we can also better speculate on the causes of thrombosis-related angiopathies.

Demonstration of a threshold response in a proteolytic feedback system: control of the autoactivation of factor XII

Mathematical analysis of positive feedback loops in proteolytic systems has previously suggested that when the active enzymes are subject to inhibitory control these systems will exhibit threshold behavior. This is demonstrated in the present study, for the autolytic activation of factor XII in the presence of a contact activator and an irreversible inhibitor of factor XIIa. The threshold between the two system states - complete factor XII activation, or complete stability - is dependent on the kinetic balance between the catalytic rate of autoactivation and rate of enzyme (factor XIIa) inhibition. Activation of the system can be brought about by either increasing the catalytic rate (in this study, by using more potent contact-activation conditions), or by lowering the enzyme inhibition rate. Previous mathematical work predicted complete stability in a positive-feedback system that is below threshold, and this has been experimentally confirmed.

Helix D elongation and allosteric activation of antithrombin

Antithrombin requires allosteric activation by heparin for efficient inhibition of its target protease, factor Xa. A pentasaccharide sequence found in heparin activates antithrombin by inducing conformational changes that affect the reactive center of the inhibitor resulting in optimal recognition by factor Xa. The mechanism of transmission of the activating conformational change from the heparin-binding region to the reactive center loop remains unresolved. To investigate the role of helix D elongation in the allosteric activation of antithrombin, we substituted a proline residue for Lys(133). Heparin binding affinity was reduced by 25-fold for the proline variant compared with the control, and a significant decrease in the associated intrinsic fluorescence enhancement was also observed. Rapid kinetic studies revealed that the main reason for the reduced affinity for heparin was an increase in the rate of the reverse conformational change step. The pentasaccharide-accelerated rate of factor Xa inhibition for the proline variant was 10-fold lower than control, demonstrating that the proline variant cannot be fully activated toward factor Xa. We conclude that helix D elongation is critical for the full conversion of antithrombin to its high affinity, activated state, and we propose a mechanism to explain how helix D elongation is coupled to allosteric activation.

Antithrombin III prevents renal dysfunction and hypertension induced by enhanced intravascular coagulation in pregnant rats: pharmacological confirmation of the benefits of treatment with antithrombin III in preeclampsia

We tested the hypothesis that enhanced intravascular coagulation in pregnancy could produce clinical symptoms similar to those of preeclampsia, such as hypertension, proteinuria, and edema. Having confirmed this, we then examined whether the pathological changes caused by intravascular coagulation could be suppressed by administration of antithrombin III (AT III), an endogenous inhibitor active to thrombin and factor X a. Intravascular coagulation was induced in Wistar rats on day 16-20 of pregnancy by 1-h arterial infusion of tissue thromboplastin (TP) through the left ventricle of the heart. One hour after the end of the infusion period, organ blood flows were measured by the radioactive ((57)Co-labeled) microsphere method, and fibrin deposition in organs was measured by radiolabeling with [(125)I] fibrinogen injected before TP infusion. Infusion of TP produced fibrin deposition in the kidney, lung, and liver, but not in the myometrium and placenta, as well as an 80% decrease in renal blood flow (RBF), with oliguria and proteinuria. TP also caused an increase in blood pressure (BP) accompanied by an increase in plasma renin activity (PRA), both of which were suppressed by bilateral nephrectomy before TP infusion. The prophylactic administration of AT III concentrates (60 or 300 U/kg intravenously (i.v.), followed by infusion of 30 or 150 U/kg/2 h, respectively) prevented all pathological changes in a dose-dependent manner. AT III increased placental blood flow regardless of the state of coagulation. These findings suggest that intravascular coagulation plays a significant part in the pathophysiology of preeclampsia and that AT III concentrates may have therapeutic potential in the treatment of this condition.

Measurement of factor Xa-antithrombin III in plasma: relationship to prothrombin activation in vivo

The M(r) of the complexes formed when factor Xa reacts with antithrombin III (ATIII) in plasma were estimated by gel filtration and SDS-polyacrylamide electrophoresis. The predominant species of factor Xa-ATIII detected after plasma and plasma to which factor Xa had been added were gel filtered on Sephadex G-200 and Sepharose 4B had apparent M(r) > 200,000, in which factor Xa-ATIII was associated with vitronectin. Addition of factor Xa-ATIII to ATIII-depleted plasma also resulted in the formation of factor Xa-ATIII-vitronectin complexes with M(r) > 200,000. Using polyclonal antibodies to human factor Xa-ATIII and ATIII as the capture and detector antibodies, respectively, a sensitive and specific enzyme-linked immunosorbent assay was developed to quantify factor Xa-ATIII in plasma. The relationship between factor Xa-ATIII production and prothrombinase activity in vivo was investigated by quantifying factor Xa-ATIII and prothrombin fragment 1 + 2 endogenous to the plasmas of blood donors and patients with Hodgkin's and non-Hodgkin's lymphoma. Whereas the concentrations of prothrombin fragment 1 + 2 in the 84 normal plasmas increased with age, those of factor Xa-ATIII (mean +/- SD of 34.7 +/- 13.8 pM) did not, and no correlation existed between the concentrations of the two parameters in normal plasmas. In contrast, a highly significant correlation between the concentrations of these two parameters was found in the plasmas of the cancer patients which coincidentally also had higher concentrations of both factor Xa-ATIII and prothrombin fragment 1 + 2 than the normal plasmas. Thus, ATIII may differentially influence prothrombinase formation and activity in normal individuals and cancer patients.

Mechanism of potentiation of antithrombin III and heparin cofactor II inhibition by sulfated xylans

Kinetic analyses of antithrombin III (AT-III)-thrombin or heparin cofactor II (HC-II)-thrombin or AT-III-factor Xa interactions were carried out in the absence or in the presence of one of the sulfated xylans or unfractionated heparin or low molecular weight (LMW) heparin utilizing chromogenic substrates. These studies demonstrated that under pseudo first order conditions the inhibitions were proportional to the AT-III or HC-II concentrations used and the apparent second order rate constants determined from the slopes of the pseudo first order plots of log of thrombin or Xa remaining as a function of time were significantly elevated in presence of the sulfated compounds. On a molar basis oat spelts xylan sulfate was the most effective compound in accelerating the rate of thrombin-AT-III interaction followed by commercial heparin while the latter was most effective in accelerating the rate of thrombin-HC-II interaction. Heparin and LMW heparin were more effective in that order in accelerating the rate of Xa-AT-III interaction while oat spelts xylan sulfate, corn cob xylan sulfate, SP-54 were less effective than the heparins in that order. Studies were also conducted on the concentrations of the sulfated compounds required to inhibit by 50% the thrombin activity by AT-III or HC-II or that required to inhibit by 50% the factor Xa activity by AT-III. The results showed an inverse relationship between the increase in the rate of acceleration by the sulfated compound with the decrease in the amount required for 50% inhibition. SDS-polyacrylamide gel study of the reaction mixture containing thrombin, AT-III or HC-II along with heparin or oat spelts xylan sulfate showed that like heparin, oat spelts xylan sulfate potentiated the formation of thrombin-AT-III or thrombin-HC-II complexes which were stable in presence of denaturing or reducing agents. Chemical modification of arginine or lysine of AT-III significantly lowered its potentiation of thrombin or Xa inhibition by oat spelts xylan sulfate.

Effect of recombinant factor VIIa on the hemostatic defect in dogs with hemophilia A, hemophilia B, and von Willebrand disease

Recombinant factor VIIa (rF.VIIa) is a two-chain procoagulant enzyme (Mr, approximately 50,000) active only when complexed with tissue factor in the extrinsic clotting system. We administered human rF.VIIa to hemophilic and von Willebrand disease (vWD) dogs to determine its hemostatic effectiveness and survival in the circulation. Hemophilia A dogs lacking factor VIII demonstrated an immediate increase in plasma rF. VIIa and prompt stoppage of hemorrhage at bleeding time (BT) sites. In seven studies in two dogs, the range of dose of rF. VIIa was 50-220 micrograms/kg, with an apparent 7- to 11-fold increase in plasma factor VII and a mean recovery in plasma of 34%. The t1/2 was 2.8 +/- 0.5 hr. The BT was normalized except in an animal given the minimum dose. In four studies in two hemophilia B dogs lacking factor IX, BT was normalized. The elevation in plasma factor VII was by a factor of 8-30, with a mean recovery of rF.VIIa in plasma of 44%. In two studies in a homozygous vWD dog lacking von Willebrand factor, which is needed for platelet function, BT was not corrected even though large doses of rF. VIIa were given. The human rF. VIIa protein was immunogenic for dogs. These studies indicate that factor VIIa corrects the hemostatic defect in dogs with hemophilia A and B, diseases primarily of the intrinsic clotting system, but does not correct the hemostatic defect in vWD.

Regulation of factor VIIa activity in plasma: evidence that antithrombin III is the sole plasma protease inhibitor of human factor VIIa

The inhibition of human factor VIIa by antithrombin III and normal human plasma was studied in the presence and absence of heparin. In the absence of heparin, no apparent inhibition of factor VIIa was observed in either system. In the presence of heparin, factor VIIa activity was inhibited 50% by purified antithrombin III and plasma in 90 min and 75 min, respectively. No inhibition of factor VIIa was observed in heparinized plasma previously depleted of antithrombin III by immunoaffinity adsorption. Incubation of factor VIIa with antithrombin III-heparin or heparinized plasma resulted in the formation of a covalent complex with an apparent molecular weight of 100 kilodaltons. These data indicate that antithrombin III appears to be the sole plasma protease inhibitor of human factor VIIa, and the expression of its inhibitory activity against factor VIIa is absolutely dependent upon the presence of exogenous heparin.

Inhibition of tissue thromboplastin-mediated blood coagulation

Factors affecting the inhibition of tissue thromboplastin (TP)-mediated blood coagulation have been investigated. Human brain thromboplastin progressively loses procoagulant activity when incubated in the presence of defibrinated plasma and CaCl2. Inhibition is maximal at a CaCl2 concentration of 1.5 mM during incubation and involves the calcium dependent binding of a plasma component(s) to the TP-FVII complex, preventing the activation of FX. Chelation of calcium ions using EDTA releases active TP and FVII from the inhibited complex. No inhibition occurs during incubation of TP with Al (OH)3 adsorbed plasma and calcium ions unless a Factor VII concentrate (or purified FVII and FX) is also present. Incubation of TP with antithrombin III-deficient plasma and calcium ions also leads to inhibition. Moreover, purified AT III cannot substitute for adsorbed plasma in producing TP inhibition. The data are consistent with the presence in plasma of a potent AT III independent inhibitor of TP-mediated blood coagulation.

Measurement of the kinetics of inhibition of activated coagulation factor X in human plasma: the effect of plasma and inhibitor concentration

A method has been developed for detailed kinetic studies of the inhibition of factor Xa in human plasma. Radiolabeled enzyme is not required, and the method can be used at initial factor Xa levels of 1 nM. The method is discontinuous and based on the removal of samples into an amidolytic assay done in the presence of 1% Lubrol-PX detergent. This permits the study of inhibition in mixtures containing phospholipid, platelets, or thromboplastin. The method can be used at inhibition rates in excess of 1 min-1, and by suitable analysis can be used to estimate the contribution of inhibition by alpha 2-macroglobulin, which does not itself inhibit amidolytic activity. The method is at present limited to cases where thrombin is not generated in large excess. Factor Xa inhibition has been studied in citrated plasma as a function of total plasma concentration, and--by the use of antithrombin-depleted plasma--as a function of the antithrombin concentration of the plasma. In all situations inhibition is characterized by second-order behavior: (i) total inhibition rate is proportional to plasma concentration up to 95%, giving a maximum rate in the absence of calcium of 1 min-1; (ii) inhibition in depleted plasma reconstituted with antithrombin shows inhibition rate to remain linearly related to antithrombin concentration; and (iii) the estimated rate due to alpha 2-macroglobulin is proportional to plasma concentration. It is thus confirmed that, as in pure systems, inhibition of factor Xa in whole plasma is linearly related to the concentration of each class of inhibitor.

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.

Surface-governed molecular regulation of blood coagulation

Among extracellular biological processes the spatial control of blood clotting is a unique phenomenon. Localization in space has very important consequences in both normal and pathological conditions. Under physiological circumstances a clot is formed only in the vicinity of injury, albeit the prerequisites of coagulation are almost completely given in the whole circulation. The local character of blood clotting is secured by the following major conditions: The regulatory signal initiating coagulation-the damaged vascular wall-is itself a surface on which the majority of clotting reactions take place. The first enzyme, factor XII, of the intrinsic coagulation pathway is activated on the collagen fibers exposed in the damaged vascular wall, although the significance of this reaction in respect of the clotting process is ambiguous. On the membrane of platelets adhered to the damaged blood vessel is activated factor XI, too, which is a well-established participant of the intrinsic clotting process. The further consecutive reactions of coagulation are confined to the surface produced by injury, because the enzymes involved contain gamma-carboxyl-glutamyl side chains which are anchored through calcium bridges to the phospholipids of the platelet membrane. The last enzyme of the sequence is thrombin, which is released from the surface. The reactions taking place on the surface form an enzyme cascade, which amplifies the relatively weak triggering signal by several orders of magnitudes. Amplification is ensured not only by the enzyme-substrate relationship of the consecutive reaction partners, but also by spatial confinement, which endows the process with higher efficacy than could be expected on a statistical basis from reactions in solution. It contributes to the efficiency of enzyme cascade that the non-enzymatic regulatory proteins increase the activity of factors IXa and Xa, and thereby the overall process. While the partner of factor IXa, factor VIII, is captured from plasma, factor V, the partner of factor Xa, is derived from the platelets adhered to the damaged surface and orients the binding of factor Xa. The surface localization ensures the protection of the members of clotting system: In the activator complexes found on the surface, the spatial arrangement of clotting factors prevents the inactivation of factors by physiological inhibitors or by proteolytic enzymes and specific antibodies that appear in the circulation in pathological conditions. Platelet factor 4, derived from platelets, binds heparin and thereby markedly decreases the reactivity of antithrombin III, the physiological inhibitor of clotting factors. The above two circumstances are

Mechanism of inactivation of trypsin by antithrombin

General aspects of the mechanism of antithrombin action were elucidated by a comparison of the inactivation of trypsin by antithrombin with the inactivation of coagulation proteinases by the inhibitor. Bovine antithrombin and bovine trypsin were shown to form an inactive equimolar complex. A non-complexed, proteolytically modified form of antithrombin, electrophoretically identical with that formed in the reaction with coagulation proteinases, was also produced in the reaction with trypsin. In the absence of heparin, the inactivation of trypsin by antithrombin was 20 times faster than the inactivation of thrombin; the second-order rate constant was 1.5 x 10(5)m(-1).s(-1) at 25 degrees C and pH 7.4. However, the inhibition of thrombin was accelerated about 30 times more efficiently by small amounts of heparin than was trypsin inhibition. Dissociation of the antithrombin-trypsin complex at pH 7.4 followed first-order kinetics with a half-life for the complex of about 80h at 25 degrees C. The complex was rapidly and quantitatively dissociated at pH 11, resulting in the liberation of a modified two-chain form of the inhibitor, cleaved at the same Arg-Ser bond as in modified antithrombin released from complexes with thrombin, Factor Xa and Factor IXa. This supports the previous proposal that this bond is the active-site bond of antithrombin. Antisera specific for thrombin-modified antithrombin reacted with purified antithrombin-trypsin complex, indicating that the inhibitor was present in the complex in a form immunologically identical with thrombin-modified antithrombin. The results thus suggest a common mechanism, but different kinetics, for the inhibition of trypsin and coagulation proteinases by antithrombin.

Anticoagulant activity of artificial biomedical materials with co-immobilized antithrombin III and heparin

An approach to providing anticoagulant activity to biomedical materials was presented, applying an immobilization technique. Antithrombin III (AT III) inactivates the activated coagulation factors including Factor Xa and thrombin. Heparin stimulates the inactivation of Factor Xa and thrombin by AT III. Thus AT III and heparin were co-immobilized on Sepharose 4B, polyvinyl alcohol, polyhydroxy-ethyl methacrylate and silicone-coated nylon by the cyanogen bromide procedure. Those co-immobilized preparations, abbreviated as I-AT III . Hep, actively neutralized both Factor Xa and thrombin. The activity of I-AT III . Hep was much higher than immobilized heparin and/or immobilized AT III. I-AT III . Hep, like soluble AT III and heparin, instantaneously neutralized both thrombin and Factor Xa. When two enzymes, thrombin and Factor Xa, were present, I-AT III . Hep neutralized Factor Xa in preference to thrombin : The neutralization of thrombin was inhibited by the presence of Factor Xa, but neutralization of Factor Xa was independent of the presence of thrombin. The amount of Factor Xa neutralized was higher than that of thrombin.

Reaction of antithrombin with proteases. Evidence for a specific reaction with papain

Experiments were performed to determine if the sulfhydryl protease, papain (EC 3.4.22.2), reacts with the plasma protease inhibitor antithrombin (antithrombin III, heparin cofactor) on a specific manner analogous to the reaction of thrombin (EC 3.4.21.5) and other serine proteases with this inhibitor. The esterolytic activity of papain is blocked by the addition of antithrombin, but not by antithrombin-thrombin complex or by protein substrates such as bovine serum albumin. Likewise, in the presence of papain, antithrombin was unable to displace the active site dye proflavine from thrombin, or to inhibit thrombin-catalysed hydrolysis of an anilide substrate. The reaction of antithrombin and papain was not accelerated by low concentrations of heparin. Approximately stoichiometric amounts of heparin completely inhibited the reaction of papain with antithrombin. The mutual inhibition indicates that plasma antithrombin does react with papain but the reaction differs from the interaction with coagulation factors, particularly in the heparin effect.

[The importance of the plasmatic blood clotting system in the early stages of arterial thrombogenesis (author's transl)]

Apart from today's concept of arterial thrombogenesis, a mechanism of the early events in arterial thrombus formation is proposed, which also includes recent insight into the nature of the blood vessel wall and its interaction with the plasmatic blood clotting system. Observations have accumulated indicating that at the site of lesions of the interior vessel lining thromboplastic activity does appear to induce local thrombin formation. Under conditions of high arterial blood flow thrombin interacts with platelets culminating in the formation of a white thrombus. Based on these considerations, which in part may still be speculative, the possibility may evolve to develop a new highly specific prophylaxis and therapy of arterial thrombosis.