Role of heparin in the inactivation of thrombin, factor Xa, and plasmin by antithrombin III

To characterize the mode of action of heparin, the kinetic behaviour of the inhibition of thrombin, factor Xa, and plasmin by antithrombin III was studied in the presence and absence of heparin. Following the concentration dependence of inactivation, in both cases a linear dependence of the apparent first-order inactivation rate constant on the antithrombin III concentration was found. These results are explained by enzyme-heparin interactions. Thus, heparin is believed to act as an activator of the enzymes that makes them more susceptible to antithrombin III.

Values of kinetic constants for the inactivation reaction of the several enzymes were determined. From these values it is concluded that heparin influences primarily the thrombin-antithrombin III interaction.     

Heparin chain-length dependence of factor Xa inhibition by antithrombin in plasma

Heparin anticoagulants function by enhancing the inhibition of coagulation proteases by the serpin antithrombin (AT). A direct evaluation of the specific anti-factor Xa (fXa) activity of therapeutic heparins in the physiologically relevant plasma-based clotting assays has not been feasible since thrombin, the final protease of the cascade, is the primary target for inhibition by AT in the presence of heparin. To circumvent this problem, we developed an assay in which the native AT in plasma was replaced with an AT mutant which exhibits identical affinity for heparin and near normal reactivity for fXa, but does not react with thrombin and other coagulation proteases in either the absence or presence of heparin. This assay was used to distinguish the anti-fXa activity of different molecular weight heparins from their anti-thrombin activity in clotting assays which were initiated by the triggers of either the extrinsic or intrinsic coagulation pathway. The results suggest that the acceleration of fXa inhibition by AT exhibits a marked heparin chain-length dependence, with fondaparinux (a pentasaccharide) having the lowest and unfractionated heparin having the highest effect. Interestingly, comparative studies revealed that the fondaparinux-catalyzed acceleration of thrombin inhibition by AT also contributes to the prolongation of the clotting time, possibly suggesting that the anticoagulant function of the therapeutic pentasaccharide is mediated though the inhibition of both fXa and thrombin.     

Inactivation of bovine and human thrombin and factor Xa by antithrombin III studied with amidolytic methods

Antithrombin III (At-III) purified from human plasma was incubated with thrombin (T), activated factor X (Xa) or mixtures of T and Xa. The amount of enzymes inactivated by the amount of At-III in one ml of normal plasma was determined. - At-III which had inactivated one enzyme, was incapable of inactivating the other. Inactivation of Xa and T by At-III was found to proceed like a second order reaction. The inactivation rate was highly dependent on the At-III concentration, the half-time values of the enzyme activities decreasing with increasing At-III concentration. With excess inhibitor, variations in the concentration of enzyme had little influence on the inactivation rate. Bovine Xa was more rapidly inactivated than bovine T. In contrast, human T was more rapidly inactivated than human Xa.     

Antithrombin-independent thrombin inhibitors, but not direct factor Xa inhibitors, enhance thrombin generation in plasma through inhibition of thrombin-thrombomodulin-protein C system

There is increasing concern that some anticoagulants can paradoxically increase thrombogenesis under certain circumstances. Previously, we demonstrated that at certain doses a direct thrombin inhibitor, melagatran, worsens the coagulation status induced by tissue factor (TF) injection in a rat model. We utilised an in vitro thrombin generation (TG) assay to determine if direct thrombin inhibitors could enhance TG in human plasma, and whether inhibition of the negative-feedback system [thrombin-thrombomodulin (TM)-protein C] contributed to the TG enhancement. TG in human plasma was assayed by means of the calibrated automated thrombography. In this assay, direct factor Xa (FXa) inhibitors such as edoxaban and antithrombin (AT)-dependent anticoagulants such as heparin did not increase, but simply suppressed TG. AT-independent thrombin inhibitors (melagatran, lepirudin, and active site blocked thrombin (IIai)) increased peak levels of TG (2.0, 1.6, and 2.2-fold, respectively) in the presence of 12 nM recombinant human soluble TM (rhsTM). Melagatran and lepirudin at higher concentrations began to suppress TG. In the absence of rhsTM, the enhancement of peak TG by melagatran decreased to 1.2-fold. Furthermore, in protein C-deficient plasma, AT-independent thrombin inhibitors failed to enhance TG. In addition, a human protein C neutralising antibody increased the peak height of TG in the presence of rhsTM. These results suggest that AT-independent thrombin inhibitors may activate thrombogenesis by suppression of the thrombin-induced negative-feedback system through inhibition of protein C activation. In contrast, direct FXa inhibitors are more useful than AT-independent thrombin inhibitors in terms of lower possibility of activation of the coagulation pathway.     

The inactivation of thrombin and plasmin by antithrombin III in the presence of sepharose-heparin.

The inhibition of thrombin and plasmin by antithrombin III was studied in the presence of heparin conjugated to Sepharose. When either enzyme was adsorbed to the heparin conjugate in quantities insufficient to occupy all the affinity sites, subsequent passage of antithrombin III through the column invariably produced complete inhibition as measured by the response to the chromogenic substrate S2160. However, when the loading sequence was reversed (i.e. adsorbing non-saturating quantities of antithrombin III before the enzyme), antithrombin III only insignificantly inhibited either enzyme over a 20-hour period. A second load of antithrombin III following the thrombin load resulted in complete inactivation. When thrombin was rendered incapable of binding to heparin by cyclohexanedione treatment and reacted with Sepharose-heparin to which antithrombin III was adsorbed, the protease inhibitor exhibited ‘progressive’ antithrombin activity. These studies may indicate that thrombin and plasmin possess higher affinities for heparin than for the antithrombin III-heparin complex. Furthermore antithrombin III reacts more readily with the enzyme-heparin complex than with heparin. If the relative affinities of heparin, thrombin and antithrombin III in plasma are similar to those observed under these affinity chromatographic conditions, then the present data are consistent with the view that heparin promotes the inactivation of thrombin and plasmin by augmenting their reaction with antithrombin III. An allosteric effect of heparin on thrombin is apparent from the 50% increase in esterase activity of thrombin on α-N-benzoyl-L-arginine ethyl ester which is observed in the presence of optimal concentrations of heparin.     

Plant inhibitors of serine proteinases: Hageman factor fragment, kallikreins, plasmin, thrombin, factor Xa, trypsin, and chymotrypsin

Numerous plants were tested for inhibitors of human Hageman factor fragment (HF_f), plasma kallikrein, urinary kallikrein, plasmin, thrombin, porcine pancreatic kallikrein, and bovine Factor X_a, trypsin, and chymotrypsin. Pumpkin seeds and iris bulbs contain trypsin inhibitors which specifically inhibit HF_f. Flower bulbs—especially those of tulip, lily, hyacinth, and calla—are hitherto unrecognized rich sources of inhibitors with different inhibitory spectrums and physicochemical properties.     

Low molecular weight heparin-catalyzed inactivation of factor Xa and thrombin by antithrombin III--effect of platelet factor 4.

Low molecular weight (LMW) heparin preparations have unknown distributions of ATIII-binding material, so mean molecular weights as such might bear little information on their anti-factor Xa and anti-thrombin activities, and on the neutralization of these activities by platelet factor 4 (PF4). These properties were investigated in pure systems with proteins of human origin. Pseudo-first order rate constants of inactivation of factor Xa and thrombin by antithrombin III were determined as function of heparin concentration, in the presence of 4.0 mM CaCl2. Despite a large variation in the mean molecular weights, the ratios of the anti-factor Xa over the anti-thrombin activities were essentially the same for the 4th International Standard for heparin (0.46), the 1st International Standard for LMW heparin (0.32), CY216 (0.42) and enoxaparin (0.50). The ultra LMW heparin CY222 had only a 2-times higher ratio (0.98). Analysis of CY216 subfractions, obtained by gel filtration, showed that the heparin molecules of the upper region of the molecular weight distribution are responsible for the anti-thrombin, but also to a large extent for the anti-factor Xa activities. The results indicate that depolymerization of unfractionated heparin does not result in an increased anti-factor Xa/anti-thrombin ratio, because in the presence of Ca(2+)-ions the rate constants of inactivation of factor Xa are lowered as compared to those of native heparin. PF4-dependent neutralization of anti-factor Xa and anti-thrombin activities of fixed concentrations of the LMW heparins was studied by measuring rate constants as function of PF4 concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of purified plasma factor VII by human plasmin, plasma kallikrein, and activated components of the human intrinsic blood coagulation system

Purified plasma factor VII was incubated with purified plasmin, plasma kallikrein, factor IX_a, factor XI_a, factor XII_f, or activated factor XII in a calcium-free milieu. Plasmin, factor IX_a, and glass—activated factor XII induced a considerable time- and temperature—dependent activation of factor VII. Plasma kallikrein gave weak activation of factor VII at 25°C and was inactive at 0°C. Kallikrein activated factor VII significantly through its effect as a plasminogen activator. The factor XI_a was inactive towards factor VII. The effects of plasmin on factor VII were fully inhibited by SBTI (0.1 mg/ml); factor IX_a and glass-activated factor XII were partially inhibited. The mechanism of the contact- and the cold-promoted activation of factor VII in human plasma is discussed.     

Reactivity of heparin with the human plasma heparin-binding proteins thrombin, antithrombin III, and apolipoproteins E and B-100

The heparin-binding properties of human plasma apolipoproteins B-100 and E (apoB-100 and E) of low density lipoproteins (LDL), thrombin, and antithrombin-III (AT-III) were investigated. A highly reactive heparin (HRH) to apoB-100 was isolated by chromatography of crude heparin on a column of LDL immobilized to Affi-Gel 10. This HRH showed a high, Ca²⁺-dependent precipitating activity towards LDL; 1 μg HRH uronic acid precipitated 50–70 μg LDL-protein. HRH was fractionated further by chromatography on a column of AT-III bound to concanavalin A-Sepharose. The unretained fraction of heparin (HRH₁)had a low affinity for AT-III. The bound heparin (HRH₂) had a high affinity for AT-III and precipitated LDL in the presence of Ca²⁺. To assess further their heparin-binding properties, the proteins were subjected to gradient-gel electrophoresis under denaturing conditions, transferred to nitrocellulose by electrophoresis, and then assayed for their ability to bind [¹²⁵I]-labeled HRH₂. Autoradiographic analysis showed that thrombin, apolipoproteins E and B-100, and the AT-III-thrombin covalent complex bound HRH₂. Denatured AT-III did not bind HRH₂, indicating that its heparin recognition site may depend on conformation.     

Inhibitory effects of TFPI variants on thrombin and factor Xa generation in fibrinogen-deficient human plasma

Using a fast kinetic centrifugal analyzer, the inhibitory effects of glycosylated and unglycosylated full-length and truncated forms of TFPI on protease generation were studied in fibrinogen-deficient human plasma after extrinsic (EA) or intrinsic (IA) activation of coagulation. When the assay system was supplemented with increasing amounts of the TFPI variants the generation of both thrombin and factor Xa was inhibited in a concentration-dependent manner. Clear differences in the effectiveness of the TFPI variants were found. After EA, the unglycosylated full-length TFPI was most effective followed by the glycosylated full-length form. The C-terminal truncated TFPI showed the lowest inhibitory activity in this system. However, its efficiency increased several fold when coagulation was activated via the intrinsic pathway. Comparing the IC₅₀ values after IA, the truncated TFPI was more effective than the unglycosylated full-length form and nearly as effective as the glycosylated full-length TFPI. After both EA and IA the thrombin generation inhibition by TFPI variants was more pronounced than the inhibition of factor Xa generation. The results show that chemical modifications of the TFPI structure can result in changes of TFPI's inhibitory properties to activated clotting factors leading to differences in protease generation inhibition.     

Effect of glycosaminoglycans and antithrombin III on uptake and inhibition of thrombin by the vascular wall

Thrombin binds to and is inactivated by the endothelium. The inactivation is potentiated by plasma. The present investigation was designed to clarify the role of vessel wall glycosaminoglycans (GAG) and plasma antithrombin III (AT III) in the inactivation and binding of thrombin by endothelium. Thrombin was shown to bind to vascular endothelium and artificial surfaces containing GAG:s. The binding could be inhibited on both types of surfaces by pretreating them with protamine. Thrombin bound to endothelium was rapidly inactivated in the presence of plasma but only slowly if the plasma was replaced by AT III, AT III-depleted plasma or a balanced salt solution. It is concluded that thrombin binds to vessel wall GAG:s and is inactivated by the endothelium. Potentiation of the inhibition of the endothelially bound thrombin by plasma is dependent upon presence of AT III but an additional plasma factor is also required.     

A purified antithrombin III--heparin complex as a potent inhibitor of thrombin in porcine endotoxin shock

Inhibition of activated clotting factors is an important therapeutic approach in disseminated intravascular coagulation (DIC). We examined the possible protective effect of a purified complex of human antithrombin III (AT III) and heparin in endotoxin-induced DIC in pigs. Two groups of endotoxemic pigs were studied. AT III-heparin group pigs (n = 8) were pretreated with a bolus injection of 500 units AT III-heparin complex, followed by a continuous infusion of 1000 units of the complex for 6 hours given simultaneously with the infusion of 10 micrograms/kgh of S. abortus equi endotoxin. Controls (n = 9) were given saline in addition to the continuous infusion of endotoxin. AT III activity, prothrombin and soluble fibrin in plasma were determined by chromogenic substrate methods. Fibrinogen was measured turbidimetrically. Human AT III antigen in the treated group was 64 +/- 3% at 2 hours and increased to 84 +/- 4% until the end of the experiment. AT III activity in the AT III-heparin group was elevated throughout the whole observation period (greater than 100%), whereas it was significantly lower in the controls. Prothrombin decreased similarly in both groups by approximately 35% until the end of the experiment. AT III-heparin treatment significantly attenuated the endotoxin-induced consumption of fibrinogen and completely prevented the increase in soluble fibrin in plasma. However, no significant effect of AT III-heparin was observed on endotoxin-induced mortality and dysfunction in pulmonary gas exchange. Therefore we conclude that the purified AT III-heparin complex inhibits thrombin effects and prevents development of DIC, but fails to significantly influence clinical outcome in endotoxin shock of the pig.