Capillary affinity chromatography and affinity capillary electrophoresis of heparin binding proteins

A new approach for separation, capillary affinity chromatography, is introduced for studying the interaction of heparin with antithrombin III and secretory leukocyte proteinase inhibitor. Heparin is covalently immobilized on the surface of an etched capillary through a silane spacer. The proteins are injected into the heparinized capillary, bound to the heparin, washed with buffer, eluted with sodium chloride in the same buffer using a pressure injection mode and eluting protein detected by absorbance. The resulting affinity separation is similar to that obtained from traditional affinity chromatography. The quantity of loaded protein in capillary affinity chromatography is at the nanogram level, offering an improvement over the milligram levels required for standard affinity chromatographic methods.

Transgenically produced human antithrombin: structural and functional comparison to human plasma-derived antithrombin

Recombinant human antithrombin (rhAT) produced in transgenic goat milk was purified to greater than 99%. The specific activity of the rhAT was identical to human plasma-derived AT (phAT) in an in vitro thrombin inhibition assay. However, rhAT had a fourfold higher affinity for heparin than phAT. The rhAT was analyzed and compared with phAT by reverse phase high-performance liquid chromatography, circular dichroism, fluorophore-assisted carbohydrate electrophoresis (FACE), amino acid sequence, and liquid chromatography/mass spectrography peptide mapping. Based on these analyses, rhAT was determined to be structurally identical to phAT except for differences in glycosylation. Oligomannose structures were found on the Asn 155 site of the transgenic protein, whereas only complex structures were observed on the plasma protein. RhAT contained a GalNAc for galactose substitution on some N-linked oligosaccharides, as well as a high degree of fucosylation. RhAT was less sialylated than phAT and contained both N-acetylneuraminic and N-glycolylneuraminic acid. We postulate that the increase in affinity for heparin found with rhAT resulted from the presence of oligomannose-type structures on the Asn 155 glycosylation site and differences in sialylation.

Characterization and viral safety validation study of a pasteurized therapeutic concentrate of antithrombin III obtained through affinity chromatography

BACKGROUND AND OBJECTIVE

Antithrombin III (ATIII) concentrates are employed as therapy for congenital or acquired deficiencies. These concentrates are obtained from Cohn's fraction IV1. To improve yields, purity and safety, our group developed a procedure to obtain a pasteurized ATIII concentrate from the supernatant of Cohn's fraction II + III including a highly efficient heparin affinity chromatography purification and pasteurization as a viral inactivation step.

DESIGN AND METHODS

Three steps of the manufacturing procedure (Cohn's fraction II + III precipitation, affinity chromatography and pasteurization) were selected to examine their efficacy in inactivating and/or removing the selected viruses.

RESULTS

The industrial batches show a purity higher than 99% with approximately 95% native heparin binding ATIII. Only albumin and IgG could be detected at trace levels (0.07% and 0.16% of the total protein present, respectively). The specific activity of the product was approximately 6.65 IU/mg protein. Five viruses were spiked into the manufacturing starting materials and samples were collected at various points to determine the infection level of virus. The study showed a reduction factor (log 10) > or = 11.7 for HIV-1; > or = 8.1 for bovine herpes virus (analyzed as a model for herpes and hepatitis B viruses); > or = 8.1 for bovine diarrhea virus (model for hepatitis C and G) and > or = 6.0 for encephalomyocarditis virus (model for hepatitis A and other non-enveloped viruses).

INTERPRETATION AND CONCLUSIONS

No biochemical alterations of the ATIII were detected in the final product. A high viral elimination capacity of the production process was demonstrated. So far, more than 32 million units of ATIII have been transfused in the form of this therapeutic concentrate without any detected seroconversion.

The preparation of specific sorbents with polyclonal antibodies as a ligand for purification of human antithrombin III

Antithrombin III (AT III) is a serine protease inhibitor active against thrombin, factor X and factor VII. Major hematolytic abnormalities such as disseminated intravascular closing, coagulative vein inflammation, embolism in lungs or brain etc. frequently occur when the level of AT III is low. As a drug AT III is separated from blood preparations by bioselective sorption on sorbents containing heparine as a complementary ligand interacting with AT III molecules. The present paper describes the preparation procedures and the properties of sorbents with chemically bonded AT III antigen. The chromatographic ability of the prepared sorbents to separate AT III from human plasma are discussed in relation to the bonding procedure which was used for AT III antigen immobilization.

Five antithrombin variants, four associated with thrombosis

We have identified five mutations in antithrombin by direct sequencing of exons amplified using polymerase chain reaction. Four of these mutations are associated with thrombosis, three cause type I antithrombin deficiency and one has features of a type II deficiency. The fifth variant appears to have no functional consequences. The type I mutations are in exon 2, exon 3b and exon 4. The first of these is a nonsense mutation causing substitution of a Tyr-->stop at position -16 within the secretion signal sequence. The second is a missense mutation resulting in the substitution Cys-->Ser at position 247. This disrupts the disulphide bond with Cys 430 leaving a free cysteine residue and the C-terminus unconstrained. The third type I mutation is an in-frame deletion resulting in the loss of Ile 186. This is a highly conserved residue in the serpin superfamily and will predictably result in the disruption of the F-helix. The fourth mutation, in exon 3a, results in the substitution of Ser 162 by Asn. This residue is sited in the E-helix and the replacement of the buried side chain of serine by the larger asparagine side chain will predictably cause structural perturbation. The last example, Val 415-->Asp, was an incidental finding as a follow up investigation of a nephrotic patient. Although one other member of the family also had the mutation there was no linked history of thrombotic disease.

Commercial antithrombin concentrate contains inactive L-forms of antithrombin

The preparation of antithrombin concentrate for clinical use requires a viral inactivation step. In most commercial preparations this is achieved by heat pasteurisation. This process would be expected to alter the conformation of antithrombin from the active native species to an inactive latent (L-form) state (1, 2). To determine if this occurs during commercial preparation and to identify the proportion of the product in the inactive state, we examined the various antithrombin conformations within a therapeutic concentrate. The antithrombin concentrate was separated into five fractions by heparin-Sepharose chromatography. The fraction with the highest heparin affinity retained full activity, whereas the four fractions with reduced heparin affinity (approximately 40% of the total antithrombin) had lost their inhibitory function. These inactive antithrombins were intact, monomeric, thermostable and resistant to unfolding in 8 M urea. Moreover, the protein patterns on isoelectric focusing and non-denaturing-PAGE showed that there were at least two different L-forms with isoelectric points separate from the native active species. Our findings demonstrate that approximately 40% of the antithrombin preparation examined exists as inactive L-forms. The clinical significance of administering this altered material is uncertain.

End point attached heparin affinity matrix

Heparin is a highly sulfated long-chain glycosaminoglycan utilized extensively for its anticoagulation properties, which has found widespread use as a general affinity ligand. The polysaccharide is composed of repeating units of uronic acid and glucosamine with great variability in sequence within the chain. The high degree of sulfation imparts strong acidity to the molecule and it may bond with many compounds simply by ionic interaction. In addition, it has been established that heparin contains certain specific monosaccharide sequences which act as unique binding sites for some proteins. For utilization as a biospecific affinity ligand it has been reported that heparin may optimally be immobilized by a single point of attachment through a terminal sugar residue. This method of immobilization allows unrestricted access to sequences within the polysaccharide chain required for biospecific interaction. Heparin immobilized to beaded agarose by single point attachment through its terminal formyl moiety was prepared. Chromatographic performance characteristics were evaluated using thrombin and antithrombin III as model compounds and elution profiles are presented. Additionally, stability of attachment was directly compared to several other commercially available supports. In conclusion, this end-point attached affinity matrix demonstrates high capacity and good stability compared with that of other methods of preparation.

Hamster antithrombin III: purification, characterization and acute phase response

Antithrombin III was purified to homogeneity from hamster plasma by affinity chromatography on heparin-agrose, ion-exchange chromatography on Mono Q and size-exclusion chromatography on TSK G3000SWG column with 50% yield. The molecular mass of hamster antithrombin III was estimated at 62.5 kDa and the absorption coefficient (A280 nm 1%, 1 cm) at 6.48 (in 0.1 M sodium phosphate pH 7.0). Several isoforms of the inhibitor were detected with the pI in range of 4.95-5.25. The protein contains all residues characteristic for complex-type carbohydrate chains. The N-terminal amino acid sequence shows 84% of identity to mouse and 76% to human analogue. The hamster antithrombin III gives low immunological cross-reactivity with antibodies to human antithrombin III. Initiation of the acute phase response only slightly affected the plasma concentration of inhibitor (+/- 10% within 72-h period). The kinetic data suggest high efficiency in bovine and human thrombin inhibition. In summary, the study shows only similarities between hamster and other mammal antithrombins.

Isolation and characterization of the tsetse thrombin inhibitor: a potent antithrombotic peptide from the saliva of Glossina morsitans morsitans

A potent and specific inhibitor of the human coagulation protease thrombin was identified in salivary gland extracts of the tsetse fly, Glossina morsitans morsitans, an important vector of African trypanosomiasis. This low molecular weight peptide (MW = 3,530 Da as determined by laser desorption mass spectrometry) was purified using a combination of size-exclusion chromatography and reverse-phase, high-performance liquid chromatography, respectively. Amino terminal sequencing of the purified protein reveals no homology to any previously identified serine protease inhibitor or naturally occurring anticoagulant. The tsetse thrombin inhibitor (TTI) is a stoichiometric inhibitor of thrombin, with an apparent equilibrium dissociation inhibitory constant (Ki*) [corrected] of 584 x 10(-15)M. In addition, it is also a potent inhibitor of thrombin-induced platelet aggregation. Like other hematophagous arthropods, tsetse flies appear to have evolved a novel protease inhibitor capable of antagonizing host hemostasis and facilitating blood feeding.

Enzyme-linked immunosorbent assay for thrombin-antithrombin III complexes in horses

OBJECTIVES

To adapt and characterize a human ELISA kit to quantify thrombin-antithrombin III (TAT) complexes in horses, and to evaluate TAT as a marker for hypercoagulation in horses.

ANIMALS

29 clinically normal horses used as controls, and 4 ill horses used to evaluate assay for known causes of hypercoagulation.

PROCEDURE

A commercially available human sandwich-type ELISA kit with 2 antibodies against human thrombin and antithrombin III that bind selectively to their corresponding TAT antigenic sites was used. Equine TAT standards were made from purified equine thrombin and antithrombin III. Proteins diluted in a phosphate-buffered saline solution containing 0.1% Tween and 1 U of heparin/ ml were used to establish standard curves. Reference intervals for TAT concentration in citrated equine plasma, and intra- and interassay coefficients of variation were determined.

RESULTS

Mean +/- SD values were 3.95 +/- 1.93 micrograms/L, with median of 3.18 micrograms/L and range of 1.95 to 9.03 micrograms/ L. One horse with cecal perforation had TAT concentration of 174.30 micrograms/L, and a horse infused IV with endotoxin had TAT concentration of 62.98 micrograms/L 12 hours after infusion.

CONCLUSIONS

The data suggest that human TAT ELISA kits can be used to measure TAT concentration in citrated equine plasma, and that TAT is a marker for hypercoagulation in horses.

CLINICAL RELEVANCE

Assays for equine TAT many help to further characterize the hypercoagulable state in horses.

Iodine-mediated inactivation of lipid- and nonlipid-enveloped viruses in human antithrombin III concentrate

Human plasma-derived protein concentrates intended for clinical use must be treated for viral inactivation to ensure patient safety. This study explored the use of liquid iodine for inactivation of several lipid- and nonlipid-enveloped viruses in an antithrombin III (AT-III) concentrate. Iodine at levels of 0.01% to 0.02% caused between 43% and 94% loss of AT-III activity, as well as degradation of AT-III as shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis. However, addition of up to 0.1% human albumin protected the AT-III against both inactivation and fragmentation. At albumin levels sufficient to retain greater than 75% of AT-III activity, greater than 6 logs of sindbis, encephalomyocarditis, and vesicular stomatitis viruses, greater than 4 logs of pseudorabies, and greater than 3 logs of human immunodeficiency virus were inactivated. Except with sindbis virus, this represented complete inactivation of all the viruses spiked into the AT-III concentrate.

Antithrombin III concentrates--are they clinically useful?

Treatment with AT III concentrates is a good example for the discrepancy between the optimistic expectations based on theoretical considerations or animal experiments and the result of clinical studies. 15 years after the introduction into clinical practice, a benefit for patients treated with AT III concentrates has not been proven. In hereditary antithrombin III deficiency, randomized clinical trials are completely lacking and only few and small sized randomized studies were performed in patients with acquired AT III deficiency. In none of these trials, a significant clinical benefit with regard to reduction of morbidity or mortality was detectable. Based on the published data, one can state that AT III concentrates may be beneficial in some special clinical situations in patients with hereditary antithrombin III deficiency (such as delivery, acute serious thromboembolic complications and postoperative thromboprophylaxis). In acquired AT III deficiency, there is no proven indication for the use of AT III concentrates.

Two novel antithrombin variants, Asn187Asp and Asn187Lys, indicate a functional role for asparagine 187

Three unrelated families have been identified with mutations involving asparagine 187. Two of these families are asymptomatic and were identified during the screening of random blood donors, whilst the third has a history of recurrent thromboembolic disease. In two families the mutation (6460 AAC-->GAC) results in an asparagine to aspartate substitution and is associated with normal immunological levels of antithrombin but a reduction in functional activity. In the third family the mutation (6462 AAC-->AAA) results in an asparagine to lysine substitution at residue 187 and is associated with a parallel reduction in both immunological and functional antithrombin levels. Asparagine 187 is located in the middle of the F helix of antithrombin and forms the major link between the F helix and strand 3 of the A sheet. The F helix is seen to overlie the A sheet of the molecule and moves with strands 2 and 3 of this sheet as they open to allow entry of the reactive site loop to form strand 4. Substitutions of asparagine 187 are, therefore, likely to disrupt this sliding movement leading to a loss of inhibitory activity.

Rapid high-performance liquid chromatographic quantification of recombinant human antithrombin III during production and purification

For monitoring of recombinant human antithrombin III during cell culture processes and subsequent purification steps a rapid method for quantitative determination was developed. The need for the introduction of this rapid method came from the limited availability of a quantitative enzyme-linked immunosorbent assay (ELISA) and the very time-consuming ELISA procedure. The developed method is based on reversed-phase high-performance liquid chromatography using a C4 column. The separation by gradient elution using water and acetonitrile takes less than 20 min even when complex samples, such as serum containing cell culture samples, have to be analyzed. Automation and a high sample throughput are possible with this reliable method. If necessary, insulin, transferrin and albumin can also be quantified with minor changes of the elution profile.

Comparison of membrane adsorber (MA) based purification schemes for the down-stream processing of recombinant h-AT III

Several multistage chromatographic separation schemes based on Membrane Adsorbers as stationary phases were designed for the isolation of recombinant human Antithrombin III from supernatants of baby hamster kidney cell cultures. The Antithrombin III concentration of the culture supernatants varied between 6 micrograms/ml and 14 micrograms/ml. The culture media contained 10% foetal calf serum. The concomitant overall protein concentration ranged from 5.4 mg/ml to 6.2 mg/ml, with bovine serum albumin constituting approximately 60%. Strong cation and anion exchanger, Heparin-, and Cibacron Blue Membrane Adsorber were used. While Heparin-Membrane Adsorber were found to isolate and concentrate the Antithrombin III efficiently, the removal of the major bovine serum proteins (albumin, transferrin, immunoglobulins) required a multistage process. By using a sequence of ultrafiltration, diafiltration, Cibacron Blue, anion exchanger, and Heparin-Membrane Adsorber, an electrophoretically pure Antithrombin III could be obtained. Subsequent high sensitivity gel immunoelectrophoresis proved the isolated protein free of bovine IgG and bovine transferrin, while approximately 2% serum albumin could still be discerned. A reduction of the serum content (3%) allowed the isolation of high purity Antithrombin III (> 99.9%), however, the product's specific activity was halved. The Down-Stream-process was designed and optimised for a mobile phase flow rate of 2 ml/min (0.12 l/h). With the exception of the final Heparin affinity step, however, flow rates of up to 4.8 l/h could be used without adverse effect on the final purity, with a concomitant increase of the throughput. Batches of up to 10 1 cell culture supernatant were processed in an automated procedure.

A highly purified antithrombin III concentrate prepared from human plasma fraction IV-1 by affinity chromatography

We describe an improved method for large-scale purification of antithrombin III (AT-III) from human plasma involving heparin affinity chromatography of redissolved fraction IV-1 paste, viral inactivation by heating, followed by a second heparin affinity column. The characteristics of a new heparin affinity resin and the ability to extrapolate process behavior from small-scale (20 ml) to large-scale (40 liter) columns are described. This supports the use of the small-scale column for process optimization and validation studies in compliance with current regulatory requirements for biological products. The process has been characterized by analytical techniques including sodium dodecyl sulfate (SDS), reducing SDS, and nondenaturing polyacrylamide gel electrophoresis; laser desorption time-of-flight mass spectroscopy, and electrospray mass spectroscopy. These results demonstrate that greater than 95% of the protein in the final products is AT-III, which is greater than 95% active as defined by thrombin inhibition.

Immobilization of heparin on polyacrylamide derivatives

Heparin was coupled via its carboxyl group with a polyacrylamide derivative containing covalently bound amino groups using the carbodiimide reaction. Heparin immobilized in this way proved to be useful as an affinity carrier for the isolation of antithrombin III and heparin-binding proteins from boar seminal plasma.

Membrane chromatography for rapid purification of recombinant antithrombin III and monoclonal antibodies from cell culture supernatant

The task of purifying monoclonal antibodies (MAbs) and human recombinant antithrombin III (rATIII) from cell culture supernatant was carried out using two different approaches, both based on the use of membraneous matrices. The first approach employed a strongly acidic and a strongly basic membrane ion exchanger, which were evaluated for their ability to purify monoclonal antibodies and the human active recombinant antithrombin III from cell culture supernatant. Within minutes gram amounts of product could be purified in a high-flux system, specially developed for this purpose, achieving purities of 80% for MAbs and 75% for rATIII, respectively. The capacity of the acidic membrane ion exchanger for MAbs was found to be 1 mg/cm2 with recoveries up to 96% and that of the basic membrane ion exchanger for rATIII was 0.15 mg/cm2 with recoveries up to 91%. The second approach consisted of using heparin, a mucopolysaccharide with a strong affinity towards ATIII, coupled to amine-modified or epoxy-activated membranes by reductive amination, for the purification of rATIII. The ATIII binding capacities of the membranes were found to be 91 micrograms/cm2 for the amine-modified and 39 micrograms/cm2 for the epoxy-activated membrane, achieving purities of 75%. The coupling proved to be fairly stable over a period of 5 months and the membranes remained operable even after steam sterilization and treatment with sodium dodecyl sulphate. Final purification in both instances was carried out by gel filtration.

Isolation of plasma proteins from the clotting cascade by heparin affinity chromatography

The use of heparin affinity chromatography for the isolation of plasma proteins from the clotting cascade is described. The separation is carried out with heparin agarose and, in parallel operations, with different rigid gels on a polymer base. The quality of the separation and the reproducibility of the results were investigated and the stability of the materials at high pH was tested. The affinity supports were used for the isolation of antithrombin III from human plasma and for the separation of factor IX from factor X, after partial purification by anion-exchange chromatography. The isolation of antithrombin III from human plasma served as a model. The non-specific bindings were investigated, together with the resistance of the support when treated with 0.2 and 0.5 M sodium hydroxide. Heparin agarose has low non-specific bindings, but it cannot be exposed to high pH. The supports on a polymer base are resistant to high pH, up to 13.7. However, they may remain slightly hydrophobic, and the hydrophobicity of the matrix leads to an increase in non-specific bindings. When antithrombin III is isolated, the non-specific bindings result in contamination of the final product. The lack of resistance of the matrix at high pH causes a weaker binding of antithrombin III, and the product is eluted at lower and lower sodium chloride concentrations. The results can be indicative of the behaviour of the support in the separation of factor IX from factor X. High non-specific bindings will lead to contamination of the factor IX product and consequently to low specific activity. Insufficient resistance of the support at high pH will result in failure to separate the two clotting factors satisfactorily. The separation can be monitored by heparin high-performance membrane affinity chromatography (HPMAC). Contamination of the sample, which occur in sodium dodecyl sulphate-polyacrylamide gel electrophoresis are detected within minutes by fast heparin HPMAC.

Mutagenesis of thrombin selectively modulates inhibition by serpins heparin cofactor II and antithrombin III. Interaction with the anion-binding exosite determines heparin cofactor II specificity

Thrombin is a multifunctional serine protease that plays a critical role in hemostasis. Thrombin is inhibited by the serpins antithrombin III and heparin cofactor II in a reaction that is dramatically accelerated by glycosaminoglycans. The structural basis of the interaction with these inhibitors was investigated by introducing single amino acid substitutions into the anion-binding exosite (R68E, R70E) and unique insertion loops (K52E, K154A) of thrombin. The rate of inhibition of these recombinant thrombins by antithrombin III and heparin cofactor II was determined in the absence and presence of glycosaminoglycan. The second order rate constant (k2) for inhibition by antithrombin III without heparin was 3.7 x 10(5) M-1 min-1 for wild-type thrombin; rates for the mutant thrombins varied less than 2-fold. For inhibition by antithrombin III with heparin, the rate constant was 4.5 x 10(8) M-1 min-1 for wild-type thrombin with no significant differences between any of the recombinant thrombins. In contrast, the rate constant for inhibition by heparin cofactor II without glycosaminoglycan was 4.3 x 10(4) M-1 min-1 for wild-type thrombin; rates were 10-fold slower for thrombin K52E and 2- to 3-fold slower for thrombins R68E and R70E. The rate constants for inhibition of wild-type thrombin by HCII in the presence of heparin or dermatan sulfate were 9.2 x 10(8) M-1 min-1 and 9.0 x 10(8) M-1 min-1, respectively. Compared to wild-type thrombin, the rate of inhibition by HCII with glycosaminoglycan was 5- to 15-fold slower for thrombins K52E and R70E and 50- to over 100-fold slower for thrombin R68E. Thrombin K154A was inhibited by heparin cofactor II with rates similar to wild-type thrombin in all assays. These results suggest that heparin cofactor II interacts with residue Lys-52 in the proposed S1' subsite and with residues Arg-68 and Arg-70 in the anion-binding exosite of thrombin, and that these interactions contribute to the molecular basis of heparin cofactor II specificity for thrombin.

Ternary vitronectin-thrombin-antithrombin III complexes in human plasma. Detection and mode of association

Radiolabeled antithrombin III (ATIII) was incubated at 37 degrees C with purified vitronectin (VN) or fibrinogen-deficient plasma before thrombin was added to initiate complex formation. Incorporation of radiolabeled ATIII was detected using polyacrylamide gel electrophoresis (PAGE) and autoradiography. The PAGE conditions appeared to be crucial for the detection of VN.TAT complexes. In the absence of SDS, ternary complexes formed instantaneously, whereas in the presence of SDS, only 50% of the TAT was associated with VN after a 60-min incubation. Formation of ternary complexes could be confirmed by gel filtration of the plasma to which thrombin was added. Furthermore, TAT in patient plasmas (disseminated intravascular coagulation and sepsis) was found to bind to heparin-Sepharose, indicating that this endogenously formed TAT was also associated with VN. The amino-terminal region of VN and the thrombin moiety of the TAT complex were found to be responsible for their interaction, which was stabilized by disulfide bridges. These results indicate that in normal plasma all TAT is complexed with VN. This association alters the conformational state of plasma VN, which appears to be responsible for the clearance of thrombin complexes from the circulation.

Antithrombin III (AT3) polymorphism in the marsupial Monodelphis domestica: identification and genetics

Antithrombin III polymorphism was observed in the gray short-tailed opossum, Monodelphis domestica, by either one-dimensional polyacrylamide gel electrophoresis (PAGE; pH 7.9), two-dimensional PAGE (agarose, pH 5.4; 12% T, pH 7.9), or isoelectric focusing (pH 4.2-4.9) followed by immunoblotting with rabbit antiserum to human antithrombin III. Family studies demonstrated an inheritance of three codominant autosomal alleles, AT3A, AT3B, and AT3C, and a population study revealed frequencies of 0.70, 0.10, and 0.20, respectively.

Role of tryptophan 49 in the heparin cofactor activity of human antithrombin III

To probe the functional role of tryptophan 49 in human antithrombin III, a mutant antithrombin, W49K, has been expressed in baby hamster kidney cells. The mutation reduces the affinity for heparin pentasaccharide by 1.8 kcal mol-1 but does not alter the heparin enhancement of the rate of factor Xa inhibition. 1H NMR spectra of W49K antithrombin show that the structure of the protein and the mode of heparin binding appear to be unaltered by the mutation, although tryptophan 49 is perturbed by heparin binding. 19F NMR spectra of 6-fluorotryptophan-substituted antithrombin show that tryptophan 49 is in a solvent-exposed environment. The heparin-induced fluorescence enhancement of W49K antithrombin is significantly different from that of wild-type antithrombin. Pentasaccharide induces only a 24% enhancement of antithrombin fluorescence, while high affinity heparin induces an enhancement of 40%. The results indicate that tryptophan 49 is probably a heparin contact residue but can be mutated without altering the remaining heparin-antithrombin interactions or the heparin-induced conformational change and resultant activation toward Factor Xa. Hydrophobic as well as charge interactions are thus probably involved in the specificity of the antithrombin-heparin pentasaccharide interaction. The lower fluorescence enhancements suggest that the heparin-induced 40% fluorescence enhancement used as the hallmark of activating heparin species is not the best indicator of the structural change in antithrombin that results in enhancement of the rate of proteinase inhibition.

Antithrombin III concentrates

The general characteristics of antithrombin III (AT III) concentrates available in the United States are described in this article. The effectiveness of AT III concentrates in the prevention and treatment of thrombotic episodes in patients with hereditary AT III deficiency are summarized, and the use of this product in various conditions with acquired AT III deficiency are reported.

Molecular cloning and cell-free expression of mouse antithrombin III

The homology between antithrombin III (AT-III) of mouse, of man, and that of other species was investigated. Preliminary experiments showed that mouse AT-III inhibited human alpha-thrombin efficiently (second order rate constant [K2nd] 5.8 x 10(3) M-1 s-1) as compared to human AT-III (K2nd 6.7 x 10(3) M-1), but was not recognized on immunoblots by antibodies that recognized both human and rabbit AT-III. In order to compare AT-III from different species at the molecular level, a cDNA clone for murine AT-III was isolated from a lambda ZAP mouse liver cDNA library on the basis of hybridization to a rabbit AT-III cDNA probe. The 1509 bp murine AT-III cDNA consists of a 1398 bp open reading frame, preceded by a 15 bp 5' untranslated region, followed by a 75 bp 3' untranslated region. The deduced primary protein structure consists of a 32 amino acid signal sequence, with a mature portion of 433 residues. Mature murine AT-III is 89% identical to its human counterpart, 86% identical to bovine AT-III, and 82% identical to that of the rabbit. Constructs lacking the nucleotides encoding the signal sequence were engineered and expressed in a cell-free system. The resulting 47 kDa non-glycosylated translation product was capable of being cleaved by human alpha-thrombin, of forming SDS-stable complexes with the protease, and of binding to immobilized heparin. Isolation of the murine AT-III cDNA will make feasible molecularly defined experiments with murine AT-III in the mouse system.