Antithrombotic properties in rabbits of heparin and heparin fragments covalently coupled to human antithrombin III.

Clinical grade heparin is a very heterogeneous mucopolysaccharide, containing molecules with Mr ranging from 6,000 to 30,000 that have either a high affinity or a low affinity for antithrombin III (AT). In this study, the antithrombotic properties of intact high-affinity heparin (Mr = 15,000) and of two heparin fragments (h16, a 16-monosaccharide fragment, with Mr = 4,300, and h12, a 12-monosaccharide fragment, with Mr = 3,200) and of their functional covalent stoichiometric complexes with human AT were compared in a venous thrombosis stasis model in rabbits. Thrombosis was induced by injection of glass-activated human plasma and measured in a segment of the jugular vein that was isolated between two vascular clamps for 10 min. Injections of 55 micrograms/kg resulted in a clear antithrombotic effect for intact heparin, but not for the two fragments. Equivalent amounts (carbohydrate moiety) of covalent complexes of heparin or of both heparin fragments with human AT resulted in an antithrombotic effect lasting for 45-60 min. Injection of 110 micrograms/kg of heparin and of the heparin fragments yielded an antithrombotic effect, lasting 45-60 min; the corresponding amounts of covalent complexes caused an anti-thrombotic effect for 60-120 min. The free and conjugated fragments produced equal antithrombotic effects at equal plasma levels of anti-Factor Xa activity, but the specific antithrombotic activities of free and complexed intact heparin, on a molar basis, were 10-20-fold greater than those of the free and complexed heparin fragments. The plasma half-life of the covalent complexes of the heparin fragments with AT is, however, 10 times longer than that of the complex between intact heparin and AT and 30 times longer than that of free intact heparin. Covalent complexes between AT and heparin fragments could, therefore, be useful to maintain more stable levels of antithrombotic activity in plasma.     

Gabexate mesilate and antithrombin III for intraoperative anticoagulation in heparin pretreated patients.

Thirty patients scheduled for elective myocardial revascularization and having undergone preoperative heparin treatment have been admitted to this prospective, randomized study. The aim of the study was to test two different strategies for preserving circulating antithrombin III (AT-III) during cardiopulmonary bypass. Patients in the control group (group C, n = 10) were treated with a standard heparinization (300 IU/kg). Patients in group A (n = 10) received the same management plus two doses of purified antithrombin III (1000 IU each). Patients in group GA received 200 IU/kg heparin and a continuous infusion of heparin (100 IU/kg/h) and gabexate mesilate (2 mg/kg/h) plus the same dose of antithrombin III as group A. Both group A and group GA demonstrated a preservation of circulating AT-III when compared to group C; this effect was more pronounced in group GA. The total heparin dosage was less in group GA than in groups A and C. Purified AT-III administration is recommended in heparin pretreated patients; the addition of gabexate mesilate to this protocol decreases the heparin requirement and increases the AT-III preservation.     

Purification and characterization of hereditary abnormal antithrombin III with impaired thrombin binding

Investigations of a family predisposed to recurrent venous thromboses disclosed a hereditary antithrombin III deficiency. The reactive antithrombin III concentration in plasma was reduced approximately 50%, and the antigen concentration of the inhibitor was normal. Antithrombin III from two members of this family was purified by dextran sulfate precipitation, affinity chromatography on heparin-Sepharose, and ion-exchange chromatography on DEAE-Sephadex A-50. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and crossed immunoelectrophoresis showed that only approximately half of the purified antithrombin III was capable of forming a complex with thrombin. This corroborated the finding that approximately twice as much purified antithrombin III from these patients compared with antithrombin III from normal humans was needed for titration of a given amount of thrombin. The nonreactive as well as the reactive population of antithrombin III bound heparin with the same affinity as normal antithrombin III. This was shown by crossed immunoelectrophoresis using heparin in the first dimension, by the elution pattern during salt gradient elution of antithrombin III from heparin-Sepharose, and by heparin enhancement of intrinsic fluorescence. Kinetic studies in the absence and in the presence of heparin indicated that the fraction of antithrombin III that could inactivate thrombin was functionally normal. The affected family members appeared to be heterozygotes with two autosomal codominant alleles that encode a normal and an abnormal antithrombin III protein, respectively.     

Influence of heparin and hirudin on endothelial binding of antithrombin in experimental thrombinemia

OBJECTIVE: During the last decade, experimental and clinical evidence has accumulated that antithrombin (AT) exerts anti-inflammatory effects when given in high doses. Meanwhile, AT substitution has been shown to significantly increase prostacyclin release. However, the link between endothelial AT binding and anti-inflammatory AT effects remains to be established in vivo, although heparin has been shown to counteract anti-inflammatory AT effects. We hypothesized that the administration of heparin in endotoxin-challenged rats would decrease endothelial AT binding and systemic prostacyclin concentrations. DESIGN: Prospective, randomized, controlled experimental in vivo study. SETTING: Research laboratory of a university hospital. ANIMALS: Fifty-six Wistar rats. INTERVENTIONS: Baseline values of coagulation variables were measured in six animals. Forty of 50 Wistar rats in the study groups were given endotoxin (50 mg x kg(-1) iv) and were treated with saline (group LPS), AT (15 units x kg(-1) x hr(-1)) (LPS+AT), AT and heparin (80 IU x kg(-1) x hr(-1)), or AT and hirudin (0.12 mg x kg(-1) x hr(-1)); the other 10 received saline instead of endotoxin and were treated with AT alone. Before endotoxin application, a tracheostomy was performed, and venous and arterial catheters were inserted for blood sampling and infusion. MEASUREMENTS: Intravital endothelial AT binding was studied by using fluorescence isothiocyanate-marked antibodies during intravital microscopy of intestinal submucosal venules. Systemic prostacyclin, thrombin-AT complex, and fibrinogen concentrations were measured after 4 hrs. Intergroup differences were tested by Kruskal-Wallis analysis of variance on ranks. MAIN RESULTS: AT and AT + heparin were equally effective in inhibiting systemic procoagulant turnover as reflected by fibrinogen concentrations. Only the administration of AT + hirudin significantly prevented fibrinogen consumption (p < .05). In contrast with all other treatments, the administration of heparin significantly reduced intravital endothelial AT binding (p < .05). However, prostacyclin concentrations were similarly increased in all endotoxin-challenged study groups irrespective of the anticoagulatory treatment. CONCLUSIONS: There is evidence that heparin in contrast with hirudin prevents AT from being bound to the endothelial cell surface in this experimental model. Under low-dose AT substitution, systemic prostacyclin concentrations do not depend on whether heparin or hirudin is used for thrombin inhibition. These results support the view that heparin may counteract anti-inflammatory AT effects by keeping AT away from its endothelial binding sites; however, the results question the view that decreased endothelial prostacyclin release is solely responsible.     

Metabolism of antithrombin III (heparin cofactor) in man: effects of venous thrombosis and of heparin administration

The metabolism of human antithrombin III (heparin cofactor) was studied in four control subjects, in four subjects with peripheral obliterative arterial disease, in six patients with recent venous thrombosis and in one patient with clinically severe haemophilia A. The labelled antithrombin III has a high specific activity (5.75 units/mg) and displayed a single band on SDS-polyacrylamide gel electrophoresis. On Sephadex G-100 gel filtration the labelled material eluted in the same position as the antithrombin III activity in plasma. Crossed immunoelectrophoresis of a mixture of fresh plasma and labelled antithrombin III against a specific antiserum, revealed a single precipitin line in which radioactivity was concentrated. The changes in electrophoretic mobility of both the plasma antithrombin III and the labelled material following the addition of heparin to the mixture or following coagulation were identical. The purified antithrombin III behaved as a homogeneous protein in the turnover experiments. The plasma radioactivity data were approximated by a sum of two exponential terms and the metabolism of antithrombin III represented by a two compartment mammillary model. Results in the control subjects were as follows: plasma antithrombin III concentration 19.6 +/- 2.3 mg/100 ml; intravascular fraction 0.45 +/- 0.05; fractional catabolic rate 0.55 +/- 0.02 of the plasma pool per day; half-life of the plasma radioactivity 2.83 +/- 0.26 days. Circulating large molecular weight degradation products of labelled antithrombin III could not be detected by Sephadex G-100 gel filtration. No significant differences in these parameters were found in the patients with peripheral arterial insufficiency. The turnover rate of antithrombin III was normal in the patient with haemophilia A. In three patients with venous thrombosis not treated with heparin, the turnover of labelled antithrombin III was in the normal range. In three patients with venous thrombosis, treated with heparin, the plasma radioactivity half-life was significantly shortened (2.13 +/- 0.08 days) and the fractional catabolic rate increased (0.75 +/- 0.05) of the plasma pool per day). In one of these patients, the labelled antithrombin III had been incubated with an equimolar amount of heparin prior to injection. In this patient the plasma radioactivity half-life was in the same range as in the other two patients (2.15 days).     

Preservation of antithrombin III activity in stored whole blood

Antithrombin III (AT-III) is the major inhibitor of thrombin, Factor Xa, and other coagulation enzymes. Congenital and acquired deficiencies of AT-III are thought to contribute to thrombosis and disseminated intravascular coagulation. Because a recent report suggested reduced AT III in stored blood, we evaluated blood bank storage effects. Serial samples were taken from 6 units of whole blood drawn into citrate-phosphate-dextrose-adenine over 42 days, and assays for AT-III functional activity were performed on the same day. The values (mean +/- SD) were as follows: day 0,91.8 +/- 10.7 percent; day 2, 101.9 +/- 10.7 percent; day 8, 107.3 +/- 7.4 percent; day 15, 118.9 +/- 11.1 percent; day 22, 105.4 +/- 9.8 percent; day 35, 93.4 +/- 8.8 percent; and day 42, 97.4 +/- 7.5 percent. The rise from day 0 to day 15 was significant but presumably secondary to interassay variation because analysis of frozen aliquots showed no significant change when all samples from each unit were assayed in one batch. Immunoassay of AT-III also showed no change with storage. The results indicate AT-III retains functional activity in whole blood stored at 2 to 6 degrees C for 42 days, and AT-III replacement does not require fresh blood or fresh-frozen plasma. Low values may reflect individual donor differences or dilution of plasma by anticoagulant.     

Characterization of the biological activity of antithrombin III in patients with hepatic cirrhosis

A discrepancy was found in hepatic cirrhosis patients between two forms of biological activities, i.e. progressive antithrombin activity (prog AT) and heparin cofactor activity (heparin AT). A reduced level of heparin AT was obtained in comparison with that of prog AT, despite the fact that both activities correlated well in normal controls. The binding ability of AT III to heparin was observed by crossed immunoelectrophoresis, and showed a reduced second peak of AT III with a faster mobility. These results indicate the possible presence in hepatic cirrhosis of abnormal AT III with a qualitative defect in its binding ability to heparin due to impaired protein synthesis.     

Structure-activity relationships of heparin. Independence of heparin charge density and antithrombin-binding domains in thrombin inhibition by antithrombin and heparin cofactor II.

To better understand how heparin structure affects its activity the relationships between the functional domains for inhibitor binding and charge density were investigated to determine how these domains affect heparin-mediated thrombin inhibition by two different heparin-dependent protease inhibitors, antithrombin (AT) and heparin cofactor II (HC II). A series of heparins, fractionated systematically by charge density, was further fractionated on antithrombin agarose to isolate more homogeneous subfractions that were either inactive or highly active with respect to thrombin inhibition by AT. With AT, the activities of the AT-active subfractions increased sharply with heparin charge density, while those with little or no affinity for AT were virtually inactive. In contrast, with HC II inhibitor, the activities of the heparins depended only upon their charge densities and were independent of AT affinity. At any given charge density, the heparin before fractionation by AT affinity and the fractions that were highly active and inactive with AT were all equally active with HC II. The two inhibitors also differed in their reactivity with heparan sulfate and dermatan sulfate. A charge-density effect with the subfractions having similar high affinity for AT demonstrates that charge density represents a heparin functional domain that is independent of the AT-binding domain. The behavior of the AT-inactive heparins, being fully active with HC II, demonstrates the functional domain necessary for AT binding is not needed to produce HC II activity.     

A method of determining heparin in blood plasma based on its antithrombin activity]

The suggested method for measuring blood plasma heparin is based on heparin ability to enhance antithrombin activity of antithrombin III (AT-III), the major Xa and thrombin inhibitor. The method consists in measurement of blood plasma AT-III activity in the presence and absence of protamine sulfate that destroys the heparin--AT-III complex. Heparin content in U/ml is determined from the difference in the activities of heparin--AT-III complex and AT-III proper activity represented on the calibration curve. The method is sufficiently sensitive, it permits registration of heparin concentrations in a wide band (from 0.01 U/ml to 0.75 U/ml of plasma).     

Effects of N-deacetylation and N-desulfation of heparin on its anticoagulant activity and in vivo disposition

This investigation compares the anticoagulant activities and in vivo disposition characteristics of N-deacetylated heparin and N-desulfated heparin with those of standard heparin in an animal model. The N-deacetylated heparin retained 23% of the anticoagulant activity, 34% of the anti-Xa activity and 23% of the anti-IIa activity of standard heparin, whereas the N-desulfated heparin retained only 1.5% of the anticoagulant activity, 0.095% of the anti-Xa activity and 0.92% of the anti-IIa activity of the original heparin. After i.v. injections of 5 mg/kg of the modified heparins, both N-deacetylated and N-deacetylated heparin concentrations showed monoexponential declines with time. The elimination half-lives were similar, 11.7 +/- 3.8 and 13.5 +/- 5.2 min, respectively (N.S.). There were, however, significant differences in both total clearance (Cl) and apparent volume of distribution (Vd) for these heparins, both parameters being significantly larger for the N-deacetylated heparin (P = .0007). The Cl values were 4.15 +/- 1.11 and 0.58 +/- 0.26 ml/min/kg and the Vd values were 72.1 +/- 38.7 and 9.9 +/- 2.4 ml/kg for the N-deacetylated and N-desulfated heparins, respectively. We have reported previously that after a similar dose of standard heparin the elimination half-life was 69.0 +/- 13.1 min, Cl was 0.64 +/- 0.16 ml/min/kg and Vd was 62.6 +/- 16.7 ml/kg. These studies have thus demonstrated that selective N-deacetylation and N-desulfation of the glucosamine residues of heparin affect markedly both its anticoagulant activity and in vivo disposition characteristics. Although important for full anticoagulant activity of heparin, the N-acetyl groups are apparently not essential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Proposed heparin binding site in antithrombin based on arginine 47. A new variant Rouen-II, 47 Arg to Ser.

Antithrombin Rouen-II, a new inherited variant of antithrombin-III, was found in two members of a family with no definite history of thrombosis. The subjects had normal antigenic concentrations of antithrombin and normal progressive inhibitory activity. However, the variant had defective heparin and heparan sulfate cofactor activities, and was not activated by a synthetic pentasaccharide representing the minimum heparin sequence. The abnormal antithrombin was isolated using heparin-Sepharose chromatography, and on electrophoresis at pH 8.6 migrated more anodally than normal. Two-dimensional peptide mapping of tryptic and Staphylococcus aureus V8 protease digests was performed and the abnormal peptide was located by tryptophan staining. Amino acid sequence studies demonstrated a substitution of arginine at residue 47 by a serine. Evidence strongly suggests that arginine 47 is a prime heparin binding site in antithrombin and that it forms part of a proposed positively charged linear site (to which heparin binds) that stretches across the surface of the molecule from the A to the D helix.     

Bleeding tendency possibly related to increased plasma antithrombin III activity in patient treated with piroxicam

A patient with chronic rheumatoid arthritis under treatment with steroids was given piroxicam (Feldene) for relief of pain and disability. Overt cutaneous and occult gastrointestinal bleeding developed after two months of treatment. Bleeding subsided and laboratory values returned to normal upon discontinuation of the drug.     

Serum albumin levels anticipate antithrombin III activities before and after antithrombin III agent in critical patients with disseminated intravascular coagulation

Elevated thrombin-antithrombin complex (TAT) or decreased serum albumin levels suggest heightened vascular permeability in disseminated intravascular coagulation (DIC). In such a situation, plasma antithrombin III (AT-III) may decrease because of the leakage. We thus examined whether AT-III activity before and after administration of an AT-III agent changed depending on plasma TAT and/or serum albumin levels in 20 consecutive patients with DIC. We also analyzed the pharmacokinetics for AT-III using a two-compartment model. Serum albumin levels before AT-III administration correlated with preadministered and postadministered AT-III activity, but TAT levels did not. Regardless of TAT levels, AT-III trough activity on the third day increased significantly. In patients with albumin levels of 2.5 g/dL or less, AT-III trough levels on the third day were significantly lower than those with higher levels of albumin. The half-life of the distribution phase for AT-III agent in the patients was shortened to less than one third the value reported in congenital AT-III deficiency, suggesting increased vascular permeability in the acute state patients here. The distribution volume of the agent increased remarkably compared with the previous control. We report here for the first time that in critical patients with DIC, plasma AT-III levels before and after AT-III administration could be predicted by preadministered serum albumin levels, but not by TAT. These findings could be explained by the pharmacokinetic profile, increased vascular permeability and distribution volume, observed in critical patients.