PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “ANTITHROMBIN”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13Linked to original sources
PubMed1998

[Coefficient of linear correlation between levels of fibrinogen, antithrombin III, thrombin-antithrombin complex and lipid fractions in women exposed chronically to carbon disulfide].

The authors examined 65 women, aged 22-58 years. They were derived from two industrial populations of different production profiles, and divided into three groups. Group I--the control group--was composed of 20 women, mean age 44.5 +/- 12.02 years, employed in the DANA Enterprise, Szczecin, not exposed to carbon disulfide; Group II--the group studied--comprised 22 women, mean age 42.5 +/- 4.95 years, employed in the WISKORD Enterprise, Szczecin, chronically exposed to carbon disulfide at the concentration of 9.36-23.4 mg/m2; and Group III--included 23 women, mean age 46.7 +/- 8.81 years, suffering from diabetes mellitus II. The study reveals that carbon disulfide exerts significant adverse effect on plasma lipid fractions, and induces changes in the coagulation system, creating the risk of ischaemic diseases.

Adult↗
PubMed1975

Antithrombin III: a backward glance o'er travel'd roads.

By devising and applying quantitative methods for the assay of thrombin and autoprothrombin C and by developing techniques for their purification, it was possible to obtain information about the function and properties of antithrombin. The inhibitor is a protein for which the initial purification steps consist of removing fibrinogen from plasma by heating to 56 degrees for 3 min, removing prothrombin complex by absorption on barium carbonate, absorbing the antithrombin on aluminum hydroxide, and eluting with phosphate buffer. Antithrombin is limited in its capacity to neutralize thrombin activity, and, under some conditions, the rate of inhibition was accelerated, but equivocal results were involved. Heparin cofactor was found to be essential for retarding the formation of thrombin, and, by inference, it is essential for retarding the formation of autoprothrombin C. Heparin cofactor and antithrombin III are the same. Thrombin absorbs on fibrin, and this has been referred to as the "antithrombin I effect." Interference with the thrombin-fibrinogen reaction by mixtures of antithrombin III and heparin is called the "antithrombin II henomenon." The acceleration of thrombin inactivation at the time thrombin forms is called the "antithrombin IV effect." It was discovered that antithrombin III neutralizes thrombin, as well as autoprothrombin C. The inhibitor and the enzyme form a mutual depletion system. To assay for antithrombin III, a standard quantity of thrombin (about 1,100U/ml) was reacted with antithrombin III for 2 hr. The percent thrombin inactivated was then measured. In random samples of human blood, a wide range of antithrombin III concentration was found. The inhibitor is relatively stable in plasma and serum. It is not changed in concentration when Dicumarol therapy is instituted. Ether extraction of plasma reduces antithrombin III activity. Seitz filtration of plasma did not remove activity. Under special conditions, antithrombin III enhances esterase activity of thrombin. Under special conditions, thrombin regenerates from the thrombin-antithrombin III complex. Antithrombin III neutralizes the activity of prethrombin-E and thrombin-E; consequently, an active histidine center found in the B1 chain of thrombin is not essential for the binding of antithrombin. Autoprothrombin II-A activity was neutralized by antithrombin III. Autoprothrombin C was found to be neutralized by antithrombin III; the amounts required varied with the molecular forms of autoprothrombin C. Thrombin and autoprothrombin C apparently occupy the same binding sites on antithrombin III. An equation was developed to account for all the known characteristics of antithrombin III functions. The kinetic aspects of thrombin neutralization were found to correspond exactly with those of autoprothrombin C. Antithrombin III is a high-capacity inhibitor of the two most powerful enzymes in blood coagulation.

Antithrombin III↗
PubMed1983

Heparin cofactor activities in a family with hereditary antithrombin III deficiency: evidence for a second heparin cofactor in human plasma.

Plasma levels of antithrombin-heparin cofactor, determined by heparin-dependent antithrombin assay, and antithrombin III antigen were measured in 22 members of a large kindred predisposed to venous thrombosis. While 11 members had reduced plasma levels of both antithrombin-heparin cofactor and antithrombin III antigen, the levels of antithrombin-heparin cofactor were always greater than the levels of antithrombin III antigen: 66% (+/- 7%) and 49% (+/- 5%) of normal plasma, respectively. Pooled normal plasma and plasma from one of the affected family members (60% antithrombin-heparin cofactor and 47% antithrombin III antigen) were fractionated by heparin-agarose affinity chromatography. Antithrombin-heparin cofactor, which eluted from heparin-agarose with buffer containing 0.4 M NaCl and did not cross-react with antibody specific for antithrombin III and did not inhibit factor Xa at an appreciable rate in the presence of heparin, was designated heparin cofactor A. Antithrombin-heparin cofactor, which eluted from heparin-agarose with buffer containing 2.0 M NaCl, was functionally and antigenically identified as antithrombin III. The concentrations of heparin cofactor A in normal and patient plasma were similar (4.5 x 10(-7) M), while the concentration of antithrombin III in patient plasma (8.0 x 10(-7) M) was only 50% of normal (1.6 x 10(-6) M). The functional properties of both heparin cofactor A and antithrombin III obtained from patient plasma were normal. From the results of the present study it would appear that the antithrombin-heparin cofactor concentrating measured in patient plasma reflects the combined concentrations of heparin cofactor A and antithrombin III. Since heparin cofactor A does not cross-react with antibody to antithrombin III, the concentration of antithrombin III antigen in patient plasma is thus lower than the concentration of antithrombin-heparin cofactor.

Antithrombin III↗
PubMed2006

Recombinant human antithrombin inhibits thrombin formation and interleukin 6 release in human endotoxemia.

We hypothesized that infusion of recombinant human antithrombin without concomitant heparin would have dose-dependent anticoagulant properties and potentially decrease endotoxin (lipopolysaccharide [LPS])-induced cytokine production. This was a randomized, double-blind, placebo-controlled study in parallel groups enrolling 30 healthy male volunteers. The active treatment groups received infusions of recombinant human antithrombin to increase antithrombin levels to 200% and 500% before infusion of 2 ng/kg endotoxin (LPS). Infusion of antithrombin dose-dependently decreased coagulation (P < .01 by repeated-measures ANOVA): peak levels of prothrombin fragment (1.8 nmol/L [95% confidence interval (CI), 1.3-2.3 nmol/L] in the 500% antithrombin group and 4.4 nmol/L [95% CI, 2.7-6.2 nmol/L] in the placebo group at 4 hours), thrombin antithrombin complexes (12 microg/L [95% CI, 8-16 microg/L] in the 500% antithrombin group and 34 microg/L [95% CI, 20-48 microg/L] in the placebo group at 4 hours), and D-dimer (0.2 microg/L [95% CI, 0.1-0.2 microg/L] in the 500% antithrombin group and 0.5 microg/L [95% CI, 0.4-0.7 microg/L] in the placebo group). Recombinant human antithrombin decreased peak interleukin-6 levels by 40% (222 pg/mL [95% CI, 148-295 pg/mL] and 216 pg/mL [95% CI, 112-320 pg/mL] in the 500% and 200% antithrombin groups, respectively, versus 357 pg/mL [95% CI, 241-474 pg/mL] in the placebo group; P < .001 by ANOVA). Finally, infusion of recombinant human antithrombin rapidly and transiently decreased neutrophil counts (by 19% [95% CI, 8%-30%] in the 500% antithrombin group versus 6% [95% CI, 1%-10%] in the placebo group, P = .002 by Kruskal-Wallis ANOVA) and monocyte counts (by 30% [95% CI, 16%-44%] in the 500% antithrombin group and 18% [95% CI, 9%-28%] in the 200% antithrombin group versus 8% [95% CI, 5%-20%] in the placebo group, P = .04) before LPS challenge, indicating that recombinant human antithrombin directly interacts with these leukocyte subsets. In summary, recombinant human antithrombin dose-dependently inhibited tissue factor-triggered coagulation. Effects on leukocytes and inhibition of interleukin-6 release seem to represent specific pharmacodynamic properties of recombinant human antithrombin.

Adult↗
PubMed2000

Measurement of antithrombin activity by thrombin-based and by factor Xa-based chromogenic substrate assays.

Functionally active antithrombin can be quantified by chromogenic substrate assays utilizing the heparin cofactor activity of antithrombin and the inhibition rates of thrombin or of activated factor X (FXa). Thrombin-based assays but not FXa-based assays may overestimate the antithrombin activity due to their sensitivity toward heparin cofactor II. We focused on the question whether an overestimation of antithrombin activity by thrombin-based assays involves the risk of misdiagnosing antithrombin-deficient individuals as being non-deficient. We determined antithrombin using two thrombin-based assays and one FXa-based assay in 27 plasma samples from patients with acquired antithrombin deficiency spiked with lepirudin, in antithrombin-deficient plasma and in mixtures of antithrombin-deficient plasma and normal plasma. We also measured antithrombin in healthy subjects, in patients with inherited and acquired antithrombin deficiency and in patients under high-dose heparin treatment. At therapeutic final concentrations of lepirudin, antithrombin activities were considerably overestimated by the thrombin-based assays but not by the FXa-based assay. The residual antithrombin activities in antithrombin-deficient plasma determined by the thrombin-based assays were markedly higher than the corresponding values obtained with the FXa-based assay. The thrombin-based assays also overestimated antithrombin activity in patients under high-dose heparin. However, the degree of overestimation in the range between 50 and 100 IU/dl was too low to misidentify individuals with inherited or acquired antithrombin deficiency as normal. We conclude that functionally active antithrombin can be reliably determined using FXa-based chromogenic substrate assays in all settings examined. Thrombin-based assays must not be used in patients under treatment with hirudin or other direct thrombin inhibitors.

Adult↗
PubMed1997

Preparative induction and characterization of L-antithrombin: a structural homologue of latent plasminogen activator inhibitor-1.

The inhibitory mechanism of the serpin family of serine protease inhibitors is characterized by a remarkable degree of conformational flexibility. Various conformational states have been elucidated by X-ray crystallography and indicate that the inhibitory loop, the central A-beta-sheet, and the outside edge of the C-beta-sheet are particularly mobile. However, no crystal structure of a serpin-enzyme complex is yet available, and the likely nature of the protease-complexed serpin remains for biochemical and biophysical researchers to examine. Here, we show that the biochemical induction of the latent state of antithrombin is slow relative to polymer formation, and infer that this may reflect structural features that are important for the regulation of the initial docking and subsequent locking of serpins with cognate proteases. L-Antithrombin was induced by incubation of native antithrombin at 60 degrees C for 10 h in the presence of citrate to prevent polymerization. L-Antithrombin was more stable to denaturation by both heat and urea than native antithrombin. Whereas native antithrombin formed binary complexes with synthetic peptide homologues of the inhibitory loop, biochemically induced L-antithrombin did not, indicating that the inhibitory loop of L-antithrombin is probably fully inserted into the A-beta-sheet as in the crystal structure. This was confirmed by limited proteolysis studies which demonstrated that the inhibitory loop of L-antithrombin could not be cleaved by five proteases which do cleave the loop of native antithrombin. The limited proteolysis studies also indicated that the "gate" region (residues 236-248) of the biochemically induced L-antithrombin was in a conformation substantially different from that of the native antithrombin. This again is similar to L-antithrombin in the crystal structure in which the gate has "opened" away from the body of the molecule by a rotation of 24 degrees to facilitate the relocation of strand 1C from its ordered position in the C-beta-sheet to a disordered surface loop. At 60 degrees C in the absence of citrate, antithrombin (and other serpins) rapidly polymerizes. In the presence of citrate, the formation of L-antithrombin is slow and increases with time, indicating that the inhibition of polymer formation by citrate allows the time necessary for the much slower formation of the L form. We therefore suggest that L-antithrombin formation is a two-step process: an initial rapid conformational change, probably including partial incorporation of the reactive loop into the A-sheet (as in the active molecule in the crystal structure) and displacement of s1C from the C-beta-sheet which supports polymer formation, and a much slower transition to complete loop insertion within the A-beta-sheet. It is likely that both the first rapid transitional step and the structural features that impose resistance to the second more extensive conformational change reflect the optimization of the unique inhibitory function in the serpins.

Amino Acid Sequence↗
PubMed1994

An overview of the mechanism of action of antithrombin and its inherited deficiency states.

Antithrombin is the most important physiological inhibitor of the various activated serine protease clotting factors, particularly thrombin. In vitro, the inhibition of thrombin by antithrombin is very slow; but greatly enhanced by heparin and related glycosaminoglycans. When a serine protease interacts with antithrombin, the two proteins form a covalent stable stoichiometric 1:1 complex that is rapidly removed from the circulation. The formation of this stable covalent complex involves the cleavage of the reactive centre of the inhibitor at arginine 393-serine 394 by the active site serine residue of the protease. This is followed by the formation of an ester linkage between the active site serine residue of the protease and the arginine 393 residue of the cleaved antithrombin molecule. The existence of an antithrombin deficiency state was first recognized in 1965, in a family some of whose members suffered from recurrent episodes of venous thrombosis. Subsequently, many kindreds with antithrombin deficiency have been described from diverse geographic locations. Moreover, the prevalence of antithrombin deficiency in the general population has been reported to vary from 1:500 to 1:5000. With the advent of recombinant DNA techniques, the definition of the molecular pathology of antithrombin deficiency has allowed the characterization of the specific mutation in more than 150 kindreds. Approximately 60 different mutations, resulting in either an absent or a pathological antithrombin gene product, have been reported. Inherited antithrombin deficiency is a well-recognized cause of predisposition to venous thrombosis and in a large type 2 antithrombin-deficient kindred with an alanine 382 threonine mutation (antithrombin-Hamilton), less than 20% of affected individuals were found to have objective evidence for past thrombotic events. In most of these, the initial thrombotic episode occurred when a predisposing factor was present (pregnancy, surgery, oral contraception, trauma, etc.). The incidence of thrombotic events in subjects with inherited antithrombin deficiency thus appears to be significantly lower than heretofore estimated; moreover, such events appear to occur predominantly in association with predisposing factors. Insights from studies of patients with inherited antithrombin deficiency could provide useful information in the management of those with acquired antithrombin deficiency.

Amino Acid Sequence↗
PubMed1991

A pilot study of antithrombin replacement in intensive care management: the effects on mortality, coagulation and renal function.

A prospective, randomized, controlled trial to examine the effects of antithrombin supplementation on mortality, coagulation and renal function has been carried out on 132 intensive care patients. Antithrombin activity was measured in all patients on admission to the intensive care unit (ICU). Patients with an antithrombin activity of less than 70% were randomized to either receive antithrombin replacement or to act as controls. Antithrombin activity was maintained above 70% in the treated patients throughout their stay on ICU. Ninety-three patients had an antithrombin activity of less than 70% and 35 received replacement therapy. Patients with antithrombin activity below 70% remained on the ICU significantly longer and had a significantly higher mortality rate than patients with antithrombin activity above 70%. Antithrombin supplementation neither reduced mortality nor shortened the intensive care stay. Fifty patients with reduced antithrombin activity remained on the ICU for at least 4 days, 25 received antithrombin and 25 acted as controls; coagulation parameters and renal function have been monitored in these patients. Fibrinogen concentration and platelet count were unaffected by antithrombin replacement. Antithrombin supplementation did not appear to reduce the incidence of impaired renal function in sepsis, trauma and postoperative patients. The creatinine clearance fell below 20 ml/min in eight patients in the no-treatment arm while by comparison only three patients in the treatment arm developed impaired renal function. Our study does not demonstrate a clear role for the use of antithrombin supplementation in intensive care, however the finding that antithrombin reduced renal impairment is encouraging and a larger study to confirm this finding is at present underway.

Adult↗
PubMed2002

Antithrombin, heparin, and heparan sulfate.

OBJECTIVES: To review the experimental and clinical evidence that antithrombin has multiple mechanisms for both its anticoagulant and anti-inflammatory properties. The interaction between antithrombin and specific polysulfated, acidic oligosaccharide moieties found on heparin and related proteoglycan molecules within the circulation and on endothelial surfaces will also be examined. DATA SOURCES: Review of the literature relating to antithrombin published during the past 25 yrs. DATA SUMMARY: Antithrombin is the most abundant endogenous anticoagulant circulating in human plasma. This serine protease inhibitor participates in the regulation of clotting in both physiologic and pathologic states. Reduced antithrombin activity in the early phases of sepsis contributes to a procoagulant state with excess activation of the innate immune response. Antithrombin binds to specific pentasaccharides expressed on heparin, glycosaminoglycans, and related proteoglycans within the circulation and along endothelial surfaces. The functions of neutrophils, monocytes, and endothelial cells are altered as a result of their interaction with antithrombin. These effects are mediated by the enzyme inhibitory action of antithrombin and its ability to function as a ligand for antithrombin receptors on cell surfaces. In addition, antithrombin exerts anti-inflammatory properties by both prostacyclin-dependent and prostacyclin-independent actions; heparin interferes with these anti-inflammatory properties. The role of antithrombin in sepsis, its therapeutic utility in severe sepsis, and its combination with heparin remain the subject of considerable debate. The results of a recent phase 3 clinical trials with high-dose antithrombin in sepsis suggested a beneficial effect in patients who did not concomitantly receive heparin, thereby generating new challenges in the understanding of interactions between antithrombin and heparin or heparin-like proteoglycans. CONCLUSIONS: Antithrombin has complex interactions with host coagulopathic and systemic inflammatory responses under physiologic conditions and in sepsis. The impact of these interactions in critically ill patients and the therapeutic implications of administration of antithrombin, and various doses and types of heparin in such patients, need further clarification.

Anticoagulants↗
PubMed1994

Defining the heparin-binding domain of antithrombin.

Antithrombin is a serine protease inhibitor that participates in the inactivation and removal from the circulation of thrombin and a variety of other procoagulant serine proteases. Antithrombin is also the major plasma cofactor of heparin which exerts its therapeutic effect primarily through its ability to substantially increase the rate of inactivation by antithrombin of the procoagulant serine proteases. Binding of heparin to antithrombin is thus believed to be a prerequisite for this rate enhancement effect. Heparin binding to antithrombin is mediated by a well-defined unique heparin pentasaccharide sequence. Interaction between this pentasaccharide sequence and antithrombin induces a conformational change in antithrombin, an alteration that appears to be sufficient to explain the enhanced ability of antithrombin to inhibit factor Xa and related serine proteases, but not thrombin. Heparin species with longer polysaccharide chains appear to be required in order to enhance the inhibition of thrombin by antithrombin. This may be because the enhancement of this reaction requires that heparin interacts simultaneously with both the antithrombin and the thrombin molecules. This review describes the interactions between heparin and antithrombin, focusing on the antithrombin residues which are involved in the binding of heparin. The role of the heparin-induced conformational change in enhancing serine protease inhibition by antithrombin is also explored. Then, based on available data, an hypothesis is proposed to explain the mechanisms by which heparin accelerates the rate of inactivation by antithrombin of the various serine proteases.

Animals↗
PubMed2004

Effect of long-term and high-dose antithrombin supplementation on coagulation and fibrinolysis in patients with severe sepsis.

OBJECTIVE: Sepsis is frequently associated with coagulatory activation, which may contribute to deteriorated organ function. Antithrombin is one important endogenous coagulation inhibitor that is therapeutically applied during sepsis. This study investigates the effect of 14-day antithrombin application on coagulatory variables. DESIGN: Prospective study. SETTING: Surgical intensive care unit of a university hospital. PATIENTS: Forty patients with severe sepsis. INTERVENTIONS: Patients with severe sepsis were randomly assigned to receive either conventional intensive care treatment (n = 20, controls) or antithrombin substitution that aimed at a plasma antithrombin activity > or =120% during a long-term (14-day) study period (n = 20, antithrombin). To allow comparative analysis of laboratory variables over time, all patients who did not survive the 14-day-period (five controls and six antithrombin patients) were prospectively excluded from the final evaluation. Their data were included in an intent-to-treat analysis. MEASUREMENTS AND MAIN RESULTS: Antithrombin supplementation normalized global coagulation tests and increased prothrombin activity as well as fibrinogen concentration, reflecting less coagulation factor consumption (percent change from baseline in prothrombin activity, p <.01 vs. controls at days 9, 11-14 of antithrombin vs. controls [unpaired Student's t-test]; fibrinogen concentration, p <.01 vs. controls at days 10, 11, 13, and 14 of antithrombin). Simultaneously, antithrombin reduced contact system activation as indicated by increasing prekallikrein activities over time (% change, p <.01 vs. controls at days 6, 9-14) and increased protein C activities when compared with controls (% change, p <.01 vs. controls at days 10-14). Most changes occurred from day 7 to day 14 of antithrombin supplementation. Antithrombin did not influence C1 esterase inhibitor, plasminogen, alpha2 antiplasmin, or platelet counts (p >.01). CONCLUSION: In this first study on long-term antithrombin therapy, antithrombin significantly reduced septic coagulatory response in patients with severe sepsis when given over 14 days.

Adult↗
PubMed1999

Formation of the antithrombin heterodimer in vivo and the onset of thrombosis.

Antithrombin is shown to undergo a slow spontaneous conversion to its inactive latent conformation with readily discernible amounts present in plasma on incubation at 37 degrees C for 72 hours. More rapid conversion occurs on incubation of isolated antithrombin at 41 degrees C or 50 degrees C, but the appearance on electrophoresis of free latent antithrombin is preceded by the formation, in reciprocal proportions, of a new slow band. This slow component is shown to be a heterodimer of active and latent antithrombin. It can be isolated as a single stable band either by incubation of antithrombin or by mixing equimolar proportions of active and latent antithrombin under the same conditions that give overnight crystallization of the active/latent antithrombin heterodimer. Similarly, equimolar addition of latent antithrombin to plasma results electrophoretically in a quantitative shift to the slower heterodimer mobility. Clinically, the presence of latent antithrombin is potentially deleterious, because its linkage to form the heterodimer results in inactivation of the otherwise normal molecule linked to the latent antithrombin. In the case of alpha-antithrombin, because the dimer readily dissociates, there is only a 11% additive loss of activity, but with beta-antithrombin the dimer appears more stable, with the additive loss of activity from the normal beta component being 21%, increasing to 33% on stabilization of the dimer with heparin. This linked and selective loss of activity of beta-antithrombin provides an explanation for the unexpected severity of thrombotic episodes in heterozygotes with conformationally unstable antithrombins.

Antithrombins↗
PubMed1985

Association of thrombin-antithrombin III complex with vitronectin in serum.

Purification of vitronectin by identical procedures from serum instead of plasma results in the coisolation of an additional protein component with mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of 82 kDa. We show that this component is the thrombin-antithrombin III complex based on the following evidence. Similar to a complex constructed using purified thrombin and antithrombin III, the 82-kDa component has a reduced molecular size of 69 kDa if it is not boiled prior to SDS-PAGE. Upon prolonged boiling in SDS it dissociates into 56- and 32-kDa components which co-migrate in SDS-PAGE with purified antithrombin III and thrombin, respectively. The 82- and 56-kDa components react with an antiserum against antithrombin III, and an antiserum prepared against the 82-kDa complex reacts with purified antithrombin III. Thrombin-antithrombin III complex, from either serum or recalcified clotted plasma, bound to vitronectin immobilized on Sepharose or plastic. However, purified antithrombin III which had not reacted with thrombin lacked affinity for vitronectin as did antithrombin III from citrated plasma. Purified antithrombin III acquired affinity for immobilized vitronectin if it was complexed with thrombin or was modified by radioiodination. Binding of vitronectin to antithrombin III coated on plastic was demonstrated using enzyme-linked immunosorbent assay. These results demonstrate that vitronectin binds thrombin-antithrombin III complexes through a cryptic site in antithrombin III which can be exposed when antithrombin III is radioiodinated, bound to plastic, or complexed with thrombin. Since vitronectin can interact with cells, the binding of vitronectin to the thrombin-antithrombin III complex may serve to facilitate the interaction of this complex with cell surfaces.

Antithrombin III↗
PubMed1989

Inactivation of human antithrombin by neutrophil elastase. Kinetics of the heparin-dependent reaction.

Human neutrophil elastase catalyzes the inactivation of antithrombin by a specific and limited proteinolytic cleavage. This inactivation reaction is greatly accelerated by an active anticoagulant heparin subfraction with high binding affinity for antithrombin. A potentially complex reaction mechanism is suggested by the binding of both neutrophil elastase and antithrombin to heparin. The in vitro kinetic behavior of this system was examined under two different conditions: 1) at a constant antithrombin concentration in which the active anticoagulant heparin was varied from catalytic to saturating levels; and 2) at a fixed, saturating heparin concentration and variable antithrombin levels. Under conditions of excess heparin, the inactivation could be continuously monitored by a decrease in the ultraviolet fluorescence emission of the inhibitor. A Km of approximately 1 microM for the heparin-antithrombin complex and a turnover number of approximately 200/min was estimated from these analyses. Maximum acceleratory effects of heparin on the inactivation of antithrombin occur at heparin concentrations significantly lower than those required to saturate antithrombin. The divergence in acceleratory effect and antithrombin binding contrasts with the anticoagulant functioning of heparin in promoting the formation of covalent antithrombin-enzyme complexes and is likely to derive from the fact that neutrophil elastase is not consumed in the inactivation reaction. A size dependence was observed for the heparin effect since an anticoagulantly active octasaccharide fragment of heparin, with avid antithrombin binding activity, was without effect on the inactivation of antithrombin by neutrophil elastase. Despite the completely nonfunctional nature of elastase-cleaved antithrombin and the altered physical properties of the inhibitor as indicated by fluorescence and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the inactivated inhibitor exhibited a circulating half-life in rabbits that was indistinguishable from native antithrombin. These results point to an unexpected and apparently contradictory function for heparin which may relate to the properties of the vascular endothelium in pathological situations.

Antithrombins↗
PubMed1986

On the interaction of rabbit antithrombin III with the luminal surface of the normal and deendothelialized rabbit thoracic aorta in vitro.

Pure rabbit antithrombin III was isotope labeled (with 125I or 3H) by two different methods; neither procedure caused a loss of antithrombin activity although both methods affected the affinity of the protein for Sepharose-heparin. From segments from freshly excised rabbit aorta, the uptake of isotope-labeled antithrombin III by the endothelium was rapid and saturable, although relatively small compared to the uptake of thrombin; binding of 3H-antithrombin III to the endothelium resembled that of 125I-antithrombin III. Transendothelial passage of antithrombin III into the subendothelial layers (intima-media) was slow and progressive. Endothelium binding was not affected by pretreating the vessel with either heparin, thrombin, or glycosaminoglycan-specific enzymes. Endothelium-bound antithrombin III was not selectively displaced by either heparin or thrombin. In contrast, endothelium-bound thrombin was rapidly dislodged by antithrombin III as a thrombin-antithrombin III complex. The surface of the deendothelialized aorta (ie, subjected to a balloon catheter) bound antithrombin III avidly. Pretreatment of the deendothelialized vessel with glycosaminoglycan-specific enzymes, particularly heparitinase, decreased intima-media binding by up to 80%. 125I-antithrombin III, when bound to the deendothelialized vessel surface, was actively displaced by either heparin, thrombin, or by unlabeled antithrombin III. The relatively poor binding of antithrombin III compared with that of thrombin by the endothelium in vitro supports an earlier proposal (Lollar P, Owen WG: J Clin Invest 66:1222-1230, 1980) that thrombin bound to high-affinity sites, possibly pericellular proteoglycan, of the endothelium is inactivated by plasma antithrombin III in vivo. Such a situation probably holds for large arteries at least.

Animals↗