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

Contraction of Blood Clots Is Impaired in Acute Ischemic Stroke

OBJECTIVE

Obstructive thrombi or thrombotic emboli are the pathogenic basis of ischemic stroke. In vitro blood clots and in vivo thrombi can undergo platelet-driven contraction (retraction), resulting in volume shrinkage. Clot contraction can potentially reduce vessel occlusion and improve blood flow past emboli or thrombi. The aim of this work was to examine a potential pathogenic role of clot contraction in ischemic stroke.

APPROACH AND RESULTS

We used a novel automated method that enabled us to quantify time of initiation and extent and rate of clot contraction in vitro. The main finding is that clot contraction from the blood of stroke patients was reduced compared with healthy subjects. Reduced clot contraction correlated with a lower platelet count and their dysfunction, higher levels of fibrinogen and hematocrit, leukocytosis, and other changes in blood composition that may affect platelet function and properties of blood clots. Platelets from stroke patents were spontaneously activated and displayed reduced responsiveness to additional stimulation. Clinical correlations with respect to severity and stroke pathogenesis suggest that the impaired clot contraction has the potential to be a pathogenic factor in ischemic stroke.

CONCLUSIONS

The changeable ability of clots and thrombi to shrink in volume may be a novel unappreciated mechanism that aggravates or alleviates the course and outcomes of ischemic stroke. The clinical importance of clot or thrombus transformations in vivo and the diagnostic and prognostic value of this blood test for clot contraction need further exploration.

Relationships among tumor burden, tumor size, and the changing concentrations of fibrin degradation products and fibrinolytic factors in the pleural effusions of rabbits with VX2 lung tumors

The VX2 tumor is derived from a papilloma virus-induced rabbit epithelial cell line. If VX2 tumor cells (trapped in a plasma clot) are introduced intravenously into NZW rabbits, the cells lodge in the lung capillary bed and produce tumors. Independently of the tumor burden (ie, the total tumor weight per rabbit), approximately 15% of rabbits with VX2 lung tumors accumulate an effusion in the interpleural space and this pleural effusion contains products of hemostasis. We hypothesized that these products were of intra-tumoral origin and that they changed in concentration as tumor burden increased. Interrelationships among lung-, tumor-weights, and pleural effusion volumes, and the concentrations of fibrinolytic factors, their catabolic products, and other proteins of pleural effusions were measured in rabbits with a wide range of tumor burdens. Positive correlations between tumor burden and total lung weight and between pleural effusion volume and net lung weight suggested that interstitial fluid from the stroma of tumors passed directly into the extravascular space of the lung(s) and into the interpleural space(s). Analyses of pleural effusions indicated that plasminogen-, alpha(2)-antiplasmin-, and plasminogen activator inhibitor-1-related proteins, urokinase-like- and tissue-plasminogen activator activities, and vascular endothelial growth factor increased in concentration up to a tumor burden of approximately 20-25 g. Plasmin activity and intact fibrinogen were absent. The concentration of fibrin(ogen) degradation products did not change significantly up to a tumor burden of approximately 25 g but increased substantially as tumor burdens exceeded 25 g. In conclusion, interstitial fluid from tumors enters the extravascular space of the host and may accumulate with fluid from non-tumor sources as a pleural effusion. The concentrations of fibrinolytic factors and their products in pleural effusions reflect the tumor burden of the rabbit. Conceivably, the components of a malignant effusion contain much information about the extent of tumor growth.

Fibrinogen catabolism within the procoagulant VX-2 tumor of rabbit lung in vivo: Effluxing fibrin(ogen) fragments contain antiangiogenic activity

Many types of solid tumors are known to be procoagulant environments. This is partly because a hyperpermeable vascular system within the tumor allows plasma hemostatic factors to accumulate in relatively high concentrations in the stroma, and many solid-tumor cells express tissue factor or a procoagulant factor. These circumstances appear to exist in the VX-2 lung tumor of the New Zealand White (NZW) rabbit, and they sustain a measurable turnover of stromal deposits of fibrin(ogen). We have measured the turnover of fibrinogen within tumors of the VX-2 tumor-burdened rabbit and analysed the catabolic products of fibrin(ogen) and the status of fibrinolysis in tumor-derived interpleural effusate. Using intravenously injected (125)I-labeled rabbit fibrinogen as a marker, we found that fibrinogen (approximate blood concentration 1740 microg/mL) passed from blood to VX-2 tumor stroma, saturating the tumor at a concentration of approximately 348 microg fibrinogen/g in approximately 12 hours. We measured fibrin(ogen) fragments, at a concentration of approximately 292 microg/mL, in interpleural effusates that we recovered from 13% of the VX-2-burdened rabbits. Unreduced fibrin(ogen) fragments consisted of 4 major components with a relative molecular mass of approximately 250,000 (assumed to be fragment X; approximately 9% of total fragments from densitometry of immunoblots), 200,000 (d-dimer; 41%), 110,000 (fragment D; 49%), and 50,000 to 55,000 (fragment E; 1%-2%) kD. Total fibrin(ogen) fragments immunopurified from effusates exhibited an antiangiogenic effect when subjected to a chick embryo chorioallantoic membrane procedure. Interpleural effusates were devoid of plasmin activity or active plasminogen activator inhibitor-1 but contained plasmin complexes and active urokinase-like plasminogen activator (uPA), alpha(2)-antiplasmin, and thrombin-activatable fibrinolysis inhibitor. We speculate that VX-2 cells release uPA to activate fibrinolysis within the tumor stroma. Catabolic products of hemostasis (eg, fibrinolytic fragments, angiostatin) flux from the stroma into the interpleural space, thereby providing a net antiangiogenic property to the effusate and ultimately to the lymphatic and circulatory systems.

Uptake of heparin cofactor II and antithrombin into the aorta wall after a deendothelializing injury in vivo: comparison with the behaviors of prothrombin and fibrinogen

The initiation of a denuding injury to the vascular endothelium rapidly leads to a deposition of platelets and fibrin at the site of injury. We have measured previously the responses of rabbit fibrinogen, prothrombin, and antithrombin to a deendothelializing balloon-catheter injury to the rabbit aorta in vivo. In this study, rabbit iodine 125-labeled HCII and iodine 125-labeled AT were coinjected intravenously into anesthetized rabbits 5 minutes before deendothelialization of the thoracic aorta. The rabbit was exsanguinated at 5 to 60 minutes after injury, the aorta was excised, and the accumulation of each radiolabeled protein in each layer of aorta wall was determined relative to the concentration of the respective native protein in circulating blood at exsanguination. The maximum flux rates into the aorta wall (i.e., platelet layer and intima-media) in the first minute after injury were calculated from the uptake data; approximately 2.8 molecules of AT accumulated for each HCII molecule. By comparison with previous measurements, the maximum flux rate of AT was similar to that of prothrombin. Further, the molar ratio of accumulated prothrombin/AT + HCII) in the aorta wall was 0.75. Detergent extracts of the injured aorta intima-media contained unreacted HCII and HCII complexes; the uninjured aorta contained only unreacted HCII. By contrast, high molecular weight AT complexes and unreacted AT were extracted from the uninjured, and in greater quantity from the injured, aorta wall. We conclude that, of the plasma antithrombins, AT accumulated more rapidly than HCII in vivo and appeared to be the more active inhibitor at the site of vascular injury. HCII may play a relatively minor role as an antithrombin and possibly only after injury.

Comparative metabolism and distribution of rabbit heparin cofactor II and rabbit antithrombin in rabbits

The metabolic characteristics of two rabbit plasma thrombin inhibitors, heparin cofactor II (HCII) and antithrombin (AT), have been compared in healthy young rabbits. Purified HCII and AT-alpha were differentially radiolabeled (125I, 131I) and injected intravenously; blood samples were taken at prescribed intervals over 7 days. From the plasma clearance curves of protein-bound radioactivities, fractional catabolic rates and compartmental distributions were calculated using a three-compartment model. The whole body fractional catabolic rate for HCII (jt, 0.43/day, equivalent to t1/2 = 1.61 days) was significantly faster than for AT (jt, 0.37/day; t1/2 = 1.89 days; P < 0.005). The fractional distribution of HCII in the intravascular compartment (Ap, 0.20) and in the extravascular compartment (Ac, 0.63) differed significantly from AT (Ap, 0.30; Ac, 0.56). From the catabolic data and blood concentrations, absolute quantities of HCII and AT catabolized by a 3-kg rabbit amounted to 12.8 and 19.9 mg/day, respectively, equivalent to a molar ratio, AT/HCII, of 1.7. The catabolic molar ratio was compared with the relative release rates of HCII and AT from perfused rabbit livers. Both proteins were released from the liver, the molar ratio in the perfusate rising to approximately 1.4 at 2.5 h. This report increases our understanding of the in vivo dynamics of these two proteins.

Antithrombin III-beta associates more readily than antithrombin III-alpha with uninjured and de-endothelialized aortic wall in vitro and in vivo

The properties of two isoforms, alpha and beta, of rabbit antithrombin III (ATIII) were compared in the presence of undamaged or de-endothelialized rabbit aortic wall. Similar quantities of ATIII-alpha and ATIII-beta bound to and rapidly saturated the endothelium in vitro, but the rate of transendothelial passage of ATIII-beta exceeded that of ATIII-alpha by 22%. Furthermore, ATIII-beta was adsorbed approximately twice as rapidly as ATIII-alpha by the subendothelium of the de-endothelialized aorta. Binding of both isoforms was decreased (ATIII-beta more than ATIII-alpha) by pretreating the subendothelial surface with heparitinase. Also, subendothelium-bound ATIII-beta was desorbed more readily than bound ATIII-alpha by thrombin. In vivo, the rate of uptake of iodine-131-labeled ATIII-beta from the circulation by the aortic wall and the major organs was 30-50% faster than that of iodine-125-labeled ATIII-alpha. In contrast, the uptake of 131I-ATIII-beta by the de-endothelialized aorta in vivo was three times faster than that of 125I-ATIII-alpha. By these criteria, ATIII-beta is the more active of the two isoforms. We surmise that plasma and, consequently, vessel wall levels of ATIII-beta may be vital for controlling thrombogenic events caused by injury to the vascular wall.

Comparative behavior of thrombin and an inactive derivative, FPR-thrombin, toward the rabbit vascular endothelium. Heparin liberates FPR-thrombin from the endothelium in vivo

Thrombin rapidly binds to and saturates rabbit aorta endothelium in vitro, a process that depends on pericellular glycosaminoglycans and that is inhibited by heparin. To characterize the initial adsorption of thrombin to the endothelium in vivo, an enzymatically inactive derivative, FPR-thrombin (i.e., thrombin inactivated by D-Phe-Pro-Arg-chloromethyl ketone), was prepared. The binding characteristics of thrombin and FPR-thrombin to heparin-Sepharose and to the endothelial surface of rabbit aorta segments in vitro were compared. From these experiments, we concluded that FPR-thrombin mimicked, qualitatively, the binding of thrombin to the endothelium. When injected intravenously, 125I-FPR-thrombin was removed rapidly from the rabbit circulation (T1/2, approximately 1.4 minutes) and simultaneously was adsorbed by the vascular endothelium, particularly in the lung. By injecting heparin (1,000 units/kg i.v.) before 125I-FPR-thrombin, adsorption by the aorta endothelium at 30 minutes after injection was reduced by 90%, and T1/2 was increased to approximately 3.4 minutes. Heparin, administered at various times after 125I-FPR-thrombin, liberated a significant proportion of 125I-FPR-thrombin from the endothelial surface into the plasma compartment as shown by a pronounced "spike" on the plasma curve, a concomitant loss of radioactivity from the lung and from the aorta endothelium, and analysis of the radioactive components of plasma taken before and after heparin injection. Thus, FPR-thrombin was cleared rapidly from the circulation, and endothelium-bound FPR-thrombin was released into the circulation by heparin.

Antithrombogenic heparin-bound polyurethanes

Many kinds of heparin-bound polyurethanes have been developed. Polyurethanes are a family of elastomers displaying better blood-compatibility than other polymeric materials. It is useful to modify this material by heparinization. Several approaches to heparinization have been devised: 1) a general method of heparinization, applicable to all polymeric materials, 2) a heparinization method specific to polyurethanes, and 3) the design of heparinizable polyurethane derivatives. These three approaches are first explained in detail. Then, the antithrombogenic mechanism of the heparinized polymers is discussed. Finally, the interactions of the heparinized polymers with blood coagulation factors, plasma proteins, and platelets are discussed.

Affinity chromatography of thrombin on modified polystyrene resins

Insoluble polystyrenes substituted with sulphonate and L-arginyl methyl ester (PAOM) present substituents mimicking the reactive binding site of antithrombin III. These materials have a specific affinity for thrombin. The binding of the enzyme is reversible and the eluted thrombin remains active. Consequently, these resins can be used as stationary phases in affinity liquid chromatography in order to purify thrombin with a high biological activity. The influence of different characteristics of such polymers (substitution ratio, average particle size, affinity constant, synthesis conditions) on the purification performance is studied. Human prothrombin complex concentrate is activated and applied onto the gel. A purified human thrombin of high specific activity is separated with a high recovery of biological activity of the enzyme.

Effect of NaC1 on inactivation of bovine thrombin by antithrombin III in the presence of low affinity-heparin or dextran sulfate

Heparin with low affinity (LA-heparin) to antithrombin III (AT III) enhanced the rate of inactivation of thrombin by AT III. The enhancement of the rate was saturable with AT III and was proportional to the LA-heparin concentration. Although the rate-enhancement in the presence of LA-heparin decreased with increase in NaC1 concentration, it was comparable with that in the presence of high affinity-heparin (HA-heparin) in the absence of NaC1. Inactivation of thrombin by AT III in the presence of dextran sulfate (DS) was also sensitive to NaC1 concentration. These findings indicate that free AT III is favorable for binding to the complexes of thrombin and highly sulfated polysaccharides having low affinities to AT III in the absence of NaC1.

Thrombin specificity. Requirement for apolar amino acids adjacent to the thrombin cleavage site of polypeptide substrate

alpha-Thrombin cleavage of 30 polypeptide hormones and their derivatives were analysed by quantitative amino-terminal analysis. The polypeptides included secretin, vasoactive intestinal polypeptide, cholecystokinin fragment, dynorphin A, somatostatins, gastrin-releasing peptide, calcitonins and human parathyroid hormone fragment. Most of them were selected mainly on the ground that they contain sequence structures homologous to the well known tripeptide substrates of alpha-thrombin. All selected polypeptides have one single major cleavage site and both Arg-Xaa and Lys-Xaa bonds were found to be selectively cleaved by alpha-thrombin. Under fixed conditions (1 nmol polypeptide/0.5 NIH unit alpha-thrombin in 20 microliters of 50 mM ammonium bicarbonate at 25 degrees C), the time required for 50% cleavage ranges from less than 1 min to longer than 24 h. Heparin invariably enhanced thrombin cleavage on all polypeptide analysed. The optimum cleavage site for alpha-thrombin has the structures of (a) P4-P3-Pro-Arg-P1'-P2', where P3 and P4 are hydrophobic amino acid and P1', P2' are nonacidic amino acids and (b) P2-Arg-P1', where P2 or P1' are Gly. The requirement for hydrophobic P3 and P4 was further demonstrated by the drastic decrease of thrombin cleavage rates in both gastrin-releasing peptide and calcitonins after chemical removal of hydrophobic P3 and P4 residues. The requirement for nonacidic P1' and P2' residues was demonstrated by the drastic increase of thrombin cleavage rates in both calcitonin and parathyroid hormone fragments, after specific chemical modification of acidic P1' and P2' residues. These findings confirm the importance of hydrophobic P2-P4 residues for thrombin specificity and provide new evidence to indicate that apolar P1' and P2' residues are also crucial for thrombin specificity. It is concluded that specific cleavage of polypeptides by alpha-thrombin can be reasonably predicted and that chemical modification can be a useful tool in enhancing thrombin cleavage.

Thrombin-reactive polypeptides of human blood. Some biochemical and immunological properties

Two polypeptides of 74 kDa and 55 kDa have been isolated from human platelets by immunoaffinity and lectin affinity chromatography and their effects on thrombin reactivity have been examined. These proteins in combination enhanced the aggregation of platelets by thrombin while aggregation induced by trypsin, collagen and adenosine diphosphate was not significantly affected. An enhancement in the action of thrombin on fibrinogen, N-benzoylarginine ethyl ester and H-D-phenylalanyl-L-pipecolyl-L-arginine-p-nitroanilide dihydrochloride was also observed in the presence of the platelet proteins. Under similar conditions, the proteins did not influence the esterolytic activity of trypsin or plasmin. Studies at different thrombin and protein concentrations showed maximum enhancement of enzyme reactivity when the ratio between the peptides and thrombin was optimal. In the presence of these proteins, the affinity of thrombin for N-benzoylarginine ethyl ester was about twofold higher than in the control. Two polypeptides with properties similar to those described above have also been isolated from human plasma. Antibodies to the above proteins isolated from either platelets or plasma were raised in rabbits. Intact platelets solubilized in Triton X-100 or plasma showed two precipitin lines in immunoelectrophoresis against both of the above antisera and a similar pattern was observed with the isolated polypeptides. The polypeptides did not interact in immunoelectrophoresis with antisera to whole serum, antithrombin, C4 binding protein or protein S. These 74-kDa and 55-kDa polypeptides contained radioactivity when radioiodinated platelets were used suggesting that they are located on the cell surface. Fresh plasma was analyzed by gel electrophoresis under nondenaturing and denaturing conditions and the proteins were transferred to nitrocellulose sheets. Staining with antibody to these thrombin-reactive proteins and 125I-protein A showed several reactive plasma proteins under nondenaturing conditions with the major band migrating in the albumin area. In plasma treated with sodium dodecyl sulfate, the 74-kDa and 55-kDa components were observed. A prominent 74-kDa band and a fainter 55-kDa component were again observed when platelets solubilized in sodium dodecyl sulfate were analysed by the above procedure. It is proposed that human platelets and plasma contain polypeptides which may directly modulate thrombin reactivity.

Catalytic and regulatory functions of N-bromosuccinimide-modified bovine thrombin

At pH 4.1, bovine thrombin reacts rapidly with N-bromo-succinimide to yield modified enzyme containing oxidized tryptophan residue. Both fibrinogen clotting activity and esterase activity are reduced considerably when three moles of tryptophan residues per mole of thrombin are oxidized, but the Michaelis constants for synthetic substrates are not appreciably altered. Reaction of NBS also results in a decrease in the affinity of thrombin for heparin. The dissociation constant for heparin-thrombin complex is increased by 2.6-fold due to the modification of one tryptophan residue. However, the magnitude of the increase in the dissociation constant remains the same for modified enzymes containing approximately two or three oxidized tryptophan residues. The rate constant for the inactivation of thrombin by antithrombin III is increased by 2.5-fold due to the modification of a single tryptophan residue. This increase in rate constant is not further amplified when more than one tryptophan residue is oxidized. In contrast, in the presence of heparin the rate of inactivation of modified and unmodified thrombins by antithrombin III are not significantly different. Thus, the heparin-sensitized inactivation of thrombin by antithrombin III is affected by the modification of one tryptophan residue. Spectrophotometric titrations of the phenolic hydroxyl groups suggest that the structural environments of tyrosyl groups for both unmodified and modified thrombin containing one oxidized tryptophan residue, are similar. The temperature for half loss of catalytic activity of control and NBS-modified thrombin, containing one oxidized tryptophan, are 52 and 51.5 degrees C respectively. It appears that the one tryptophan residue of thrombin is situated at or close to the binding site of heparin.

Effect of a pentosan polysulphate upon thrombin and factor Xa inactivation by antithrombin III

The kinetics of inhibition of human and bovine alpha-thrombin and human factor Xa by antithrombin III were examined under pseudo-first-order conditions as a function of the concentration of pentosan polysulphate [a fully sulphated (beta 1-4)-linked D-xylopyranose with a single laterally positioned 4-O-methyl-alpha-D-glucuronic acid]. Double-reciprocal plots of the observed first-order rate constant against concentration of pentosan polysulphate gave straight lines, intercepts on the axes giving values for maximum increase in second-order rate constant (by calculation) and apparent dissociation constant. These values were: for human alpha-thrombin 1.52 X 10(7) M-1 . min-1 and 3.6 microM respectively, for bovine alpha-thrombin 6.56 X 10(6) M-1 . min-1 and 0.16 microM and for factor Xa 6.86 X 106 M-1 . min-1 and 20 microM. In the presence of pentosan polysulphate the dissociation constant for the initial complex of antithrombin III and thrombin was shown to be reduced from approx. 2 X 10(-3) M to 61 X 10(-6) M without apparent change in the limiting rate constant of 750 min-1. An oligosaccharide (primarily 8-10 saccharide units) prepared from heparin and with high affinity for antithrombin III but low potency in the thrombin-antithrombin III interaction did not diminish the rate of interaction catalysed by pentosan polysulphate. The catalysis was shown to be due to a weak electrostatic interaction, since it was completely reversed by concentrations of NaCl greater than 0.3 M. It is concluded that the mechanism is independent of the heparin high-affinity binding site on antithrombin III and is probably due to binding of the high-charge-density polysaccharide to the proteinase. It is calculated that the acceleration in rate achieved, although lower than that of heparin, approaches that required to be of physiological significance and may be of importance in the anticoagulation role of antithrombin III at sites of high charge density which may occur in vivo.

The antiheparin effect of a heparinoid, pentosan polysulphate. Investigation of a mechanism

A pentosan polysulphate [a fully sulphated (1-4)-beta-D-xylopyranose with a single laterally positioned 4-O-methyl-alpha-D-glucuronic acid] has been shown to inhibit the anticoagulant activity of high-affinity heparin as observed in plasma and when using purified enzyme and inhibitor. The activity was shown to be concentration-dependent with an apparent Ki of approx. 2 microM. The antiheparin property was not shown by a number of other anionic carbohydrates when tested. The rate of thrombin inhibition at 0.33 microM-heparin was reduced from 7.1 X 10(8) M-1 X min-1 in the absence of pentosan polysulphate to 2.3 X 10(8) M-1 X min-1 at 2 microM-pentosan polysulphate and to 0.3 X 10(8)M-1 X min-1 at 20 microM. Using the random bireactant model of heparin action [Griffiths (1982) J. Biol. Chem. 257, 13899-13902] it was observed that the pentosan polysulphate had no effect on the Km for antithrombin III (150 nM) but increased the Km for thrombin from 25 nM to 450 nM. A reduction in the inhibition rate by 17.3-fold predicted by substitution of these values into the general two-substrate reaction-rate equation was confirmed experimentally.

Interaction of plasmin with endothelial cells

Interaction of human plasmin with a monolayer culture of mini-pig aortic endothelial cells was studied by using the 125I-labelled enzyme. The binding of plasmin was time- and concentration-dependent. Equilibrium between bound and free enzyme was obtained within 90s, and Scatchard analysis indicated a high- and a low-affinity population of binding sites of approx. 1.24 X 10(4) sites/cell having a Kd of 1.4 X 10(-9) M and 7.2 X 10(4) sites/cell with a Kd of 2 X 10(-8) M respectively. Plasmin, bound to cell, was spontaneously released within 2 min, suggesting a rapid equilibrium. Chemical modification of the enzyme with phenylmethanesulphonyl fluoride or pyridoxal 5'-phosphate revealed that neither the active centre nor the heparin-binding site of plasmin was involved in the interaction with the endothelial cell. In terms of endothelial-cell receptors, the binding sites of cells for plasmin and thrombin were different: the two enzymes did not compete with each other, and the pretreatment of cells with neuraminidase or chondroitin ABC lyase resulted in a 50% decrease of thrombin or plasmin binding respectively. Arachidonic acid incorporated into phospholipids of the cell was released by plasmin, but a change in the rate of prostacyclin formation was not measurable. The interaction of plasmin with endothelial cells seems to be specific in the fibrinolytic system, since plasminogen did not bind to these cells under similar conditions.

Modulatory effects on proteinase kinetics caused by association of both enzyme and substrate to heparin

Heparin is shown to produce modulatory effects on the amidolytic activity of trypsin, thrombin and plasmin with various synthetic peptide substrates. Simple Michaelis-Menten kinetics are observed in the absence of heparin. In its presence an enhancement effect is observed at low substrate concentrations, and an inhibitory effect is observed at high substrate concentrations. Other polyanions like dextran sulphate, phosvitin and inositol hexakisphosphate produces a similar effect. The modulatory effect of heparin is abolished when it binds cations. Co-binding of both substrate and enzyme to heparin seems to be a necessary requirement for the effect to occur. A model is proposed which can account semiquantitatively for the kinetics observed. It is suggested that the mechanism, which involves co-binding of substrate and enzyme in an competitive manner to a macromolecular structure, may be of primary importance as a regulatory mechanism in blood coagulation and fibrinolysis.

Calcium inhibits the heparin-catalyzed antithrombin III/thrombin reaction by decreasing the apparent binding affinity of heparin for thrombin

The present study has shown that calcium inhibits the heparin-catalyzed antithrombin III/thrombin reaction. The initial rate of thrombin (4.0 nM) inhibition by antithrombin III (200 nM) in the presence of heparin (2.5 ng/ml) decreased from 3.6 nM/min (in the absence of calcium) to 0.12 nM/min in the presence of 10 mM calcium. In the absence of heparin, the initial rate of thrombin inhibition by antithrombin III was not affected by calcium. The heparin-catalyzed antithrombin III/thrombin reaction is described by the general rate equation for a random-order, bireactant, enzyme-catalyzed reaction (M. J. Griffith (1982) J. Biol. Chem. 257, 13899-13902). As such, the reaction is saturable with respect to both thrombin and antithrombin III. The apparent kinetic parameters for the heparin-catalyzed antithrombin III/thrombin reaction were determined in the presence and absence of calcium. The apparent heparin/antithrombin III dissociation constant values were not measurably different in the presence of 0, 1.0, and 3.0 mM calcium. The apparent heparin/thrombin dissociation constant value increased from 7.0 nM, in the absence of calcium, to 10 and 30 nM in the presence of 1.0 and 3.0 mM calcium, respectively. The maximum reaction velocity, at saturation with respect to both proteins, was not affected by calcium. It is concluded that calcium binds to functional groups within the heparin molecule which are required for thrombin binding.

Identification of a heparin activated amidolytic enzyme in carp (Cyprinus carpio) plasma

A heparin activated amidolytic enzyme capable of cleaving synthetic thrombin sensitive chromogenic substrates was identified from the plasma of the carp (Cyprinus carpio). Activation was inhibited by KCl, protamine sulphate and human plasma. Heparin was not required for the continued action of the enzyme. Active enzyme was irreversibly inhibited by DFP. The pH and temperature optima was studied and the enzyme semi purified by Sephadex G100 superfine and Sephadex A50 chromatography. An approximate MW of 62,000 was found. Activity generated by as little as 0.002 units of heparin per ml carp plasma was detected. The trivial name carpamidase is proposed for the enzyme.

The interaction of heparin with human plasmin

1. The interaction of heparin with human plasmin was investigated measuring plasmin activity and enzyme inactivation in the presence of heparin. Hydrolysis of synthetic substrates (H-D-Val-Leu-Lys-pNA, H-D-Val-Phe-Lys-pNA and H-D-Pro-Phe-Lys-pNA) by plasmin was enhanced by heparin through an increase in kcat values. 2. This effect was the consequence of a change of Vmax since Km values were not altered in the presence of heparin. The polysaccharide also enhanced the rate of enzyme inactivation using TLCK as an active site blocking reagent. 3. Furthermore, heparin increased the heat sensitivity of plasmin, when synthetic substrate H-D-Val-Leu-Lys-pNA was used but it did not affect enzyme activity towards N-benzoyl-L-arginine-ethylester substrate. 4. The data show that microenvironmental conformation around the active center of plasmin is influenced by heparin.

On the mechanism of enhanced thromboresistance of polymeric materials in the presence of heparin

Polymeric materials with covalently immobilized heparin were shown to display enhanced thromboresistance in vitro and in vivo experiments. This property of heparin-containing polymers is due to the specific effect of immobilized heparin for every step of interaction of a polymer with blood. The presence of heparin substantially changes the character of adsorbed proteins on a polymer surface and the number of adhered platelets. Thromboresistance enhancement is largely carried out by the interaction of immobilized heparin with plasma proteins which is accompanied by the decrease in total blood coagulant activity, by the decrease in fibrinogen, prothrombin and thrombin concentrations, and by the supression of fibrinstabilizing factor activity. The free heparin content in blood is not changed. It was found that immobilized heparin forms complexes with fibrinogen, thrombin and plasmin that produce lytic action on unstabilized fibrin.

Specific binding of thrombin-antithrombin III complex to hepatocytes

Thrombin-antithrombin III complex binds selectively to isolated hepatocytes, whereas antithrombin III alone does not. The binding is time and concentration dependent at 37 degrees C: the apparent Km value is 0.8/microM. The rate of binding is approximately 1.6 X 10(5) molecules h-1 cell-1 at this concentration. At 4 degrees C there is no measurable interaction between the complex and the hepatocytes. The binding is also prevented by pretreatment of cells with trypsin. On the other hand, about 80% of the thrombin-antithrombin III complex bound to hepatocytes is releasable by trypsin digestion. NaF or carboxyatractyloside does not inhibit the process. The interaction of thrombin-antithrombin III complex with hepatocytes seems to be specific, since the complexes of antithrombin III with other proteinases, like trypsin or plasmin, are not bound at the concentrations used. Based on these data, a mechanism for the binding of the inactive complexed form of thrombin to hepatocytes is suggested.

On the interaction of heparin-containing polymers with plasma proteins and blood

In vitro exper iments showed that polymeric hydrogels containing covalently immobilized heparin affect the parameters of blood clotting system. This is not accounted for by the elution of heparin into the surrounding medium and this property of heparin-containing polymers is caused by specific interaction of immobilized heparin with plasma proteins. By the methods of coagulograms and thromboelastography it was revealed that the decrease of total blood anticoagulant activity takes place due to the decrease in blood of fibrinogen, prothrombin and thrombin content and the suppression of fibrin stabilizing factor activity. Free heparin content in blood was left unaffected. It was found that plasma proteins permeate the hydrophilic polymer matrix and interact with immobilized heparin. Binding strength for immobilized heparin is decreased in the array fibrinogen congruent to thrombin much greater than plasmin greater than serum albumin. Complexes of immobilized heparin with fibrinogen, thrombin and plasmin display lytic action towards unstabilized fibrin.

Heparin and the progressive anti-thrombin activity of alpha 2-macroglobulin

Interactions between alpha 2-macroglobulin (alpha 2M) and thrombin have been studied by spectroscopic, isotopic and electrophoretic methods in presence or in absence of heparin. It is shown that thrombin binds to alpha 2M in a 1:1 ratio. Fluorescamin labelled heparin of Mr 7 000 interacts with thrombin to form a 2:1 molar complex. This complex does not bind to alpha 2M and is unable to achieve any proteolytic cleavage of this protein. In contrast the interaction of alpha 2M with chymotrypsin is not significantly affected by the mucopolysaccharide. Moreover, heparin is unable to react with alpha 2M bound thrombin.

Kinetic analysis of the heparin-enhanced plasmin--antithrombin III reaction. Apparent catalytic role of heparin

Inactivation of plasmin by a 3-4-fold molar excess of antithrombin III follows pseudo-first-order kinetics and the apparent rate constants are proportional to the concentration of the inhibitor. Heparin accelerates the inactivation reaction without changing its pseudo-first-order character, and the apparent rate constants are also proportional to the concentration of the polysaccharide. Heparin results in a minimum 20-fold rate enhancement of the reaction between plasmin and antithrombin III when the concentrations of heparin and plasmin are approx. 0.5mum and 1mum respectively. Heparin at a molar concentration well below that of plasmin still accelerates the reaction: one molecule of the polysaccharide is able to facilitate the inactivation of about 100 molecules of plasmin. Heparin must bind to plasmin to accelerate the plasmin-antithrombin III reaction, since the modification of four to five lysine residues of the enzyme inhibits the rate-enhancement effect of heparin and the dissociation of heparin-plasmin complex decreases the inactivation rate of plasmin. Increasing the concentration of antithrombin III, at a constant amount of heparin, results in increase of the inactivation rate. By contrast, the effect of increasing the amount of plasmin in the presence of constant amount of heparin and antithrombin III is such that higher plasmin-to-heparin ratios are associated with lower rates of inactivation. It seems, therefore, that to obtain ;optimal' conditions for fast enzyme inactivation, the amount of heparin should be matched to plasmin rather than to antithrombin III. Arrhenius plots of the plasmin-antithrombin III reaction are linear both in the absence and presence of heparin, at concentrations of 1 or 2mug/ml, over a range of 26K. Under these experimental conditions, heparin increases activation entropy. The findings show that heparin seems to fulfil some criteria that are characteristic for biological catalysis: binding, reaction-rate enhancement (increasing activation entropy), recycling of heparin (effectiveness of non-stoichiometric amounts of the polysaccharide) and specificity.

Differential inhibition of serine proteinases by rabbit alpha 1-proteinase inhibitors F and S

Inhibition of six serine proteinases (bovine trypsin and chymotrypsin, equine leucocyte proteinases type 1 and 2A, porcine pancreatic elastase type III and rabbit plasmin) by rabbit alpha 1-proteinase inhibitors F and S was studied. In each case examined, the F form reacted more rapidly. The number of moles of an enzyme inhibited by one mole of alpha 1-proteinase inhibitor in a complete reaction (molar inhibitory capacity) ranged from 0.26 (leucocyte proteinase type 1) to 1.01 (trypsin). More significantly, however, the molar inhibitory capacities of both alpha 1-proteinase inhibitors differed for the same enzymes. The highest F/S inhibitory ratio was recorded with chymotrypsin (1.88), and the lowest with elastase (0.69). These differences in molar inhibitory capacities are likely to reflect the dual nature of the reaction between the inhibitor and a proteinase, that is, either complex formation or inactivation of alpha 1-proteinase inhibitor without enzyme inhibition. No evidence was obtained to suggest that differential reactivity and differential inhibitory capacity are interdependent. The observations are consistent with the view that rabbit alpha 1-proteinase inhibitors F and S are closely related yet functionally distinct proteins.

Clearance of thrombin from circulation in rabbits by high-affinity binding sites on endothelium. Possible role in the inactivation of thrombin by antithrombin III

The clearance of (125)I-thrombin and diisopropylphosphoryl-(125)I-thrombin (DIP-thrombin) from the circulation in rabbits was studied. When given either intraarterially or intravenously, DIP-thrombin, which is active-site blocked, was approximately 90% cleared from the circulation by 1 min, the time of earliest sampling, indicating a large first-pass effect. DIP-thrombin given intravenously is found predominantly in the lungs, whereas DIP-thrombin injected into the aortic arch is distributed diffusely in approximate proportion to the blood supply. Renal artery, femoral artery, ear artery, left atrium, and portal vein infusions demonstrate that kidney, muscle, ear, heart, and liver, respectively, can remove DIP-thrombin from the circulation. These data imply that the clearance of DIP-thrombin is not a function of a specific organ but of the vascular bed per se. The clearance of DIP-thrombin was reversible since injection of 0.5 mg of unlabeled DIP-thrombin 10 min after the injection of a tracer dose of DIP-(125)I-thrombin resulted in the rapid reappearance of the DIP-(125)I-thrombin into the circulation. In addition, the clearance of DIP-thrombin was saturable, i.e., clearance of DIP-(125)I-thrombin was inhibited by unlabeled DIP-thrombin in a dose-dependent fashion. In vivo Scatchard analysis of the saturation of the clearance process demonstrated that DIP-thrombin can be removed by binding to high-affinity binding sites, since dissociation constants (K(D)) of 10 and 13 nM were obtained for human and bovine DIP-thrombin, respectively. In contrast to DIP-thrombin, approximately 75% of the radioactivity associated with active thrombin remained in the circulation at 1 min. By 10 min 55% of (125)I-thrombin had been removed from the circulation, and essentially all of the radioactivity can be accounted for in the liver. Sodium dodecyl sulfate-polyacrylamide gel radioelectrophoresis of plasma samples taken after injection of (125)I-thrombin demonstrated that all of the active thrombin was converted to covalent thrombin-antithrombin III complex by the time of initial sampling (30 s). The in vitro conversion of (125)I-thrombin to thrombin-antithrombin III complex was considerably slower (50+/-5% conversion at 30 s). The simultaneous injection of excess unlabeled DIP-thrombin inhibited the rate of formation of (125)I-thrombin-antithrombin III complex formation in vivo (but not in vitro), which suggests that the binding of active thrombin to the high affinity binding sites is required for the rapid inactivation of thrombin in vivo. We propose that (a) thrombin in the circulation binds to active site-independent high-affinity binding sites on the endothelial cell surface; (b) the inactivation of thrombin by antithrombin III is faster in vivo than in vitro because the high-affinity binding sites, present in a high concentration in the microcirculation, catalyze the reaction; (c) thrombin-antithrombin III complexes are selectively removed by the liver.

Antithrombin III contribution to cholestasis differential diagnosis: its correlation with one stage prothrombin time and factor V

Levels of antithrombin III (AT III) measured by two different methods, Factor V (FV) and One Stage Prothrombin Time, have been studied in two groups of patients: 55 with chronic liver disease and 22 with extra hepatic cholestasis. Results show a statistically significant correlation between the decrease of AT III and FV in the first group, and the normality of these parameters (unrelated to One Stage Prothrombin Time) in the second group. Levels at AT III and FV correlate directly with the functional integrity of hepatic parenchyma, so they may help in the differential diagnosis of different types of cholestasis.

Heparinized styrene-butadiene-styrene elastomers

A heparinized high-strength elastomer has been developed which is potentially useful as a nonthrombogenic vascular prosthesis. A surface hydroxylated styrene-butadiene-styrene (SBS) block copolymer with at least 40% extent of reaction after glow-discharge cleaning was coated with a 20% acetylated polyvinyl alcohol/heparin mixture containing glutaraldehyde and magnesium chloride. After curing at 80 degrees C for 100 min, the polyvinyl alcohol, heparin, and hydroxylated SBS were covalently bound to each other by acetal bridges. The effects of the various substrate and coating parameters were optimized to achieve very strong adhesion between the coating layer and the surface hydroxylated SBS. Heparin was not leached from the surface of the new material using 3M saline at pH 7.4 despite a detection limit of 10(-5) micrograms heparin/cm2 min. Prolonged partial thromboplastin times of greater than 1200 sec were observed (control: PTT = 120 sec). Preliminary ex vivo testing using a simple arteriovenous shunt in the leg of a rabbit showed good thromboresistance. The heparinized SBS shunt chamber remained patent for more than two hours without desorption of heparin. It was concluded that surface hydroxylated SBS heparinized by acetal coupling owed its thromboresistance to the heparin covalently bound to the surface and not to a microenvironment of heparin in solution at the blood/material interface.