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

Tracing genetic diversity captures the molecular basis of misfolding disease

Genetic variation in human populations can result in the misfolding and aggregation of proteins, giving rise to systemic and neurodegenerative diseases that require management by proteostasis. Here, we define the role of GRP94, the endoplasmic reticulum Hsp90 chaperone paralog, in managing alpha-1-antitrypsin deficiency on a residue-by-residue basis using Gaussian process regression-based machine learning to profile the spatial covariance relationships that dictate protein folding arising from sequence variants in the population. Covariance analysis suggests a role for the ATPase activity of GRP94 in controlling the N- to C-terminal cooperative folding of alpha-1-antitrypsin responsible for the correction of liver aggregation and lung-disease phenotypes of alpha-1-antitrypsin deficiency. Gaussian process-based spatial covariance profiling provides a standard model built on covariant principles to evaluate the role of proteostasis components in guiding information flow from genome to proteome in response to genetic variation, potentially allowing us to intervene in the onset and progression of complex multi-system human diseases.

Co-translational formation of disulfides guides folding of the SARS-CoV-2 receptor binding domain

Many secreted proteins, including viral proteins, contain multiple disulfide bonds. How disulfide formation is coupled to protein folding in the cell remains poorly understood at the molecular level. Here, we combine experiment and simulation to address this question as it pertains to the SARS-CoV-2 receptor binding domain (RBD). We show that the RBD can only refold reversibly if its native disulfides are present before folding. But in their absence, the RBD spontaneously misfolds into a nonnative, molten-globule-like state that is structurally incompatible with complete disulfide formation and that is highly prone to aggregation. Thus, the RBD native structure represents a metastable state on the protein's energy landscape with reduced disulfides, indicating that nonequilibrium mechanisms are needed to ensure native disulfides form before folding. Our atomistic simulations suggest that this may be achieved via co-translational folding during RBD secretion into the endoplasmic reticulum. Namely, at intermediate translation lengths, native disulfide pairs are predicted to come together with high probability, and thus, under suitable kinetic conditions, this process may lock the protein into its native state and circumvent highly aggregation-prone nonnative intermediates. This detailed molecular picture of the RBD folding landscape may shed light on SARS-CoV-2 pathology and molecular constraints governing SARS-CoV-2 evolution.

Capturing the conversion of the pathogenic alpha-1-antitrypsin fold by ATF6 enhanced proteostasis

Genetic variation in alpha-1 antitrypsin (AAT) causes AAT deficiency (AATD) through liver aggregation-associated gain-of-toxic pathology and/or insufficient AAT activity in the lung manifesting as chronic obstructive pulmonary disease (COPD). Here, we utilize 71 AATD-associated variants as input through Gaussian process (GP)-based machine learning to study the correction of AAT folding and function at a residue-by-residue level by pharmacological activation of the ATF6 arm of the unfolded protein response (UPR). We show that ATF6 activators increase AAT neutrophil elastase (NE) inhibitory activity, while reducing polymer accumulation for the majority of AATD variants, including the prominent Z variant. GP-based profiling of the residue-by-residue response to ATF6 activators captures an unexpected role of the "gate" area in managing AAT-specific activity. Our work establishes a new spatial covariant (SCV) understanding of the convertible state of the protein fold in response to genetic perturbation and active environmental management by proteostasis enhancement for precision medicine.

Cotranslational formation of disulfides guides folding of the SARS COV-2 receptor binding domain

Many secreted proteins contain multiple disulfide bonds. How disulfide formation is coupled to protein folding in the cell remains poorly understood at the molecular level. Here, we combine experiment and simulation to address this question as it pertains to the SARS-CoV-2 receptor binding domain (RBD). We show that, whereas RBD can refold reversibly when its disulfides are intact, their disruption causes misfolding into a nonnative molten-globule state that is highly prone to aggregation and disulfide scrambling. Thus, non-equilibrium mechanisms are needed to ensure disulfides form prior to folding in vivo. Our simulations suggest that co-translational folding may accomplish this, as native disulfide pairs are predicted to form with high probability at intermediate lengths, ultimately committing the RBD to its metastable native state and circumventing nonnative intermediates. This detailed molecular picture of the RBD folding landscape may shed light on SARS-CoV-2 pathology and molecular constraints governing SARS-CoV-2 evolution.

Structural alteration in hypochlorous acid modified antithrombin indicates generation of neo-epitopes

Increased tendency of cancer patients to develop venous thromboembolism (VTE) is associated with high rates of mortality. Elevation of procoagulant proteins and down regulation of naturally occurring coagulation inhibitors appears to form the basis of high risk of VTE in malignancy. A reduced level of anticoagulant protein like antithrombin (AT) will influence both coagulation and angiogenesis, as its cleaved and latent conformations show potent antiangiogenic activity. We show a concentration dependent perturbation in the secondary and tertiary structures of AT conformers exposed to hypochlorous acid (HOCl). Modulated under a very narrow concentration range of HOCl, native AT undergoes oligomerization, aggregation and fragmentation based on spectroscopic, SDS and native-PAGE studies. Factor Xa inhibition assay demonstrated a progressive decrease in inhibition activity of AT on modification by HOCl. Bis-ANS result showed that hydrophobic patches were more exposed in the case of HOCl-modified AT when assessed fluorometrically. Dosage of HOCl-modified AT in experimental animals induced high titer antibodies showing more specificity towards modified forms in comparison to unmodified forms. Auto-antibodies isolated from cancer patients also showed enhanced binding with HOCl-modified AT in comparison to native counterpart. Compared to normal AT, structurally and functionally altered conformation of HOCl-modified AT showed increased immunogenic sensitivity. HOCl modified AT can contribute to prothrombotic and angiogenic environment during cancer progression/development.

Leveraging Population Genomics for Individualized Correction of the Hallmarks of Alpha-1 Antitrypsin Deficiency

Deep medicine is rapidly moving towards a high-definition approach for therapeutic management of the patient as an individual given the rapid progress of genome sequencing technologies and machine learning algorithms. While considered a monogenic disease, alpha-1 antitrypsin (AAT) deficiency (AATD) patients present with complex and variable phenotypes we refer to as the "hallmarks of AATD" that involve distinct molecular mechanisms in the liver, plasma and lung tissues, likely due to both coding and non-coding variation as well as genetic and environmental modifiers in different individuals. Herein, we briefly review the current therapeutic strategies for the management of AATD. To embrace genetic diversity in the management of AATD, we provide an overview of the disease phenotypes of AATD patients harboring different AAT variants. Linking genotypic diversity to phenotypic diversity illustrates the potential for sequence-specific regions of AAT protein fold design to play very different roles during nascent synthesis in the liver and/or function in post-liver plasma and lung environments. We illustrate how to manage diversity with recently developed machine learning (ML) approaches that bridge sequence-to-function-to-structure knowledge gaps based on the principle of spatial covariance (SCV). SCV relationships provide a deep understanding of the genotype to phenotype transformation initiated by AAT variation in the population to address the role of genetic and environmental modifiers in the individual. Embracing the complexity of AATD in the population is critical for risk management and therapeutic intervention to generate a high definition medicine approach for the patient.

Multi-step conformational transitions in heat-treated protein therapeutics can be monitored in real time with temperature-controlled electrospray ionization mass spectrometry

Heat-induced conformational transitions are frequently used to probe the free energy landscapes of proteins. However, the extraction of information from thermal denaturation profiles pertaining to non-native protein conformations remains challenging due to their transient nature and significant conformational heterogeneity. Previously we developed a temperature-controlled electrospray ionization (ESI) source that allowed unfolding and association of biopolymers to be monitored by mass spectrometry (MS) in real time as a function of temperature. The scope of this technique is now extended to systems that undergo multi-step denaturation upon heat stress, as well as relatively small-scale conformational changes that are precursors to protein aggregation. The behavior of two therapeutic proteins (human antithrombin and an IgG1 monoclonal antibody) under heat-stress conditions is monitored in real time, providing evidence that relatively small-scale conformational changes in each system lead to protein oligomerization, followed by aggregation. Temperature-controlled ESI MS is particularly useful for the studies of heat-stressed multi-domain proteins such as IgG, where it allows distinct transitions to be observed. The ability of native temperature-controlled ESI MS to monitor both the conformational changes and oligomerization/degradation with high selectivity complements the classic calorimetric methods, lending itself as a powerful experimental tool for the thermostability studies of protein therapeutics.

An Asymmetric Runaway Domain Swap Antithrombin Dimer as a Key Intermediate for Polymerization Revealed by Hydrogen/Deuterium-Exchange Mass Spectrometry

Antithrombin deficiency is associated with increased risk of venous thrombosis. In certain families, this condition is caused by pathogenic polymerization of mutated antithrombin in the blood. To facilitate future development of pharmaceuticals against antithrombin polymerization, an improved understanding of the polymerogenic intermediates is crucial. However, X-ray crystallography of these intermediates is severely hampered by the difficulty in obtaining well-diffracting crystals of transient and heterogeneous noncovalent protein assemblies. Furthermore, their large size prohibits structural analysis by NMR spectroscopy. Here, we show how hydrogen/deuterium-exchange mass spectrometry (HDX-MS) provides detailed insight into the structural dynamics of each subunit in a polymerization-competent antithrombin dimer. Upon deuteration, this dimer surprisingly yields bimodal isotope distributions for the majority of peptides, demonstrating an asymmetric configuration of the two subunits. The data reveal that one subunit is very dynamic, potentially intrinsically disordered, whereas the other is considerably less dynamic. The local subunit-specific deuterium uptake of this polymerization-competent dimer strongly supports a β4A-β5A β-hairpin runaway domain swap mechanism for antithrombin polymerization. HDX-MS thus holds exceptional promise as an enabling analytical technique in the efforts toward future pharmacological intervention with protein polymerization and associated diseases.

Characterization of conformers and dimers of antithrombin by capillary electrophoresis-quadrupole-time-of-flight mass spectrometry

Antithrombin (AT) is a plasma glycoprotein which possesses anticoagulant and anti-inflammatory properties. AT exhibits various forms, among which are native, latent and heterodimeric ones. We studied the potential of capillary electrophoresis-mass spectrometry (CE-MS) using a sheath liquid interface, electrospray ionization (ESI), and a quadrupole-time-of-flight (Q-TOF) mass spectrometer to separate and quantify the different AT forms. For CE separation, a neutral polyvinyl alcohol (PVA) coated capillary was employed. The protein conformation was preserved by using a background electrolyte (BGE) at physiological pH. A sheath liquid of isopropanol-water 50:50 (v/v) with 14 mM ammonium acetate delivered at a flow rate of 120 μL h^-1 resulted in optimal signal intensities. Each AT form exhibited a specific mass spectrum, allowing unambiguous distinction. Several co-injection experiments proved that latent AT had a higher electrophoretic mobility (μ_ep) than native AT, and that these conformers could associate to form a heterodimer during the CE analysis. The developed CE-MS method enabled the detection and quantitation of latent and heterodimeric forms in a commercial AT preparation stored at room temperature for three weeks.

Cellular folding pathway of a metastable serpin

Although proteins generally fold to their thermodynamically most stable state, some metastable proteins populate higher free energy states. Conformational changes from metastable higher free energy states to lower free energy states with greater stability can then generate the work required to perform physiologically important functions. However, how metastable proteins fold to these higher free energy states in the cell and avoid more stable but inactive conformations is poorly understood. The serpin family of metastable protease inhibitors uses large conformational changes that are downhill in free energy to inhibit target proteases by pulling apart the protease active site. The serpin antithrombin III (ATIII) targets thrombin and other proteases involved in blood coagulation, and ATIII misfolding can thus lead to thrombosis and other diseases. ATIII has three disulfide bonds, two near the N terminus and one near the C terminus. Our studies of ATIII in-cell folding reveal a surprising, biased order of disulfide bond formation, with early formation of the C-terminal disulfide, before formation of the N-terminal disulfides, critical for folding to the active, metastable state. Early folding of the predominantly β-sheet ATIII domain in this two-domain protein constrains the reactive center loop (RCL), which contains the protease-binding site, ensuring that the RCL remains accessible. N-linked glycans and carbohydrate-binding molecular chaperones contribute to the efficient folding and secretion of functional ATIII. The inability of a number of disease-associated ATIII variants to navigate the folding reaction helps to explain their disease phenotypes.

Three new alpha1-antitrypsin deficiency variants help to define a C-terminal region regulating conformational change and polymerization

Alpha1-antitrypsin (AAT) deficiency is a hereditary disorder associated with reduced AAT plasma levels, predisposing adults to pulmonary emphysema. The most common genetic AAT variants found in patients are the mildly deficient S and the severely deficient Z alleles, but several other pathogenic rare alleles have been reported. While the plasma AAT deficiency is a common trait of the disease, only a few AAT variants, including the prototypic Z AAT and some rare variants, form cytotoxic polymers in the endoplasmic reticulum of hepatocytes and predispose to liver disease. Here we report the identification of three new rare AAT variants associated to reduced plasma levels and characterize their molecular behaviour in cellular models. The variants, called Mpisa (Lys259Ile), Etaurisano (Lys368Glu) and Yorzinuovi (Pro391His), showed reduced secretion compared to control M AAT, and accumulated to different extents in the cells as ordered polymeric structures resembling those formed by the Z variant. Structural analysis of the mutations showed that they may facilitate polymerization both by loosening ‘latch’ interactions constraining the AAT reactive loop and through effects on core packing. In conclusion, the new AAT deficiency variants, besides increasing the risk of lung disease, may predispose to liver disease, particularly if associated with the common Z variant. The new mutations cluster structurally, thus defining a region of the AAT molecule critical for regulating its conformational state.

Molecular mechanisms of antithrombin-heparin regulation of blood clotting proteinases. A paradigm for understanding proteinase regulation by serpin family protein proteinase inhibitors

Serpin family protein proteinase inhibitors regulate the activity of serine and cysteine proteinases by a novel conformational trapping mechanism that may itself be regulated by cofactors to provide a finely-tuned time and location-dependent control of proteinase activity. The serpin, antithrombin, together with its cofactors, heparin and heparan sulfate, perform a critical anticoagulant function by preventing the activation of blood clotting proteinases except when needed at the site of a vascular injury. Here, we review the detailed molecular understanding of this regulatory mechanism that has emerged from numerous X-ray crystal structures of antithrombin and its complexes with heparin and target proteinases together with mutagenesis and functional studies of heparin-antithrombin-proteinase interactions in solution. Like other serpins, antithrombin achieves specificity for its target blood clotting proteinases by presenting recognition determinants in an exposed reactive center loop as well as in exosites outside the loop. Antithrombin reactivity is repressed in the absence of its activator because of unfavorable interactions that diminish the favorable RCL and exosite interactions with proteinases. Binding of a specific heparin or heparan sulfate pentasaccharide to antithrombin induces allosteric activating changes that mitigate the unfavorable interactions and promote template bridging of the serpin and proteinase. Antithrombin has thus evolved a sophisticated means of regulating the activity of blood clotting proteinases in a time and location-dependent manner that exploits the multiple conformational states of the serpin and their differential stabilization by glycosaminoglycan cofactors.

Conformational pathology of the serpins: themes, variations, and therapeutic strategies

Point mutations cause members of the serine protease inhibitor (serpin) superfamily to undergo a novel conformational transition, forming ordered polymers. These polymers characterize a group of diseases termed the serpinopathies. The formation of polymers underlies the retention of alpha(1)-antitrypsin within hepatocytes and of neuroserpin within neurons to cause cirrhosis and dementia, respectively. Point mutations of antithrombin, C1 inhibitor, alpha(1)-antichymotrypsin, and heparin cofactor II cause a similar conformational transition, resulting in a plasma deficiency that is associated with thrombosis, angioedema, and emphysema. Polymers of serpins can also form in extracellular tissues where they activate inflammatory cascades. This is best described for the Z variant of alpha(1)-antitrypsin in which the proinflammatory properties of polymers provide an explanation for both progressive emphysema and the selective advantage of this mutant allele. Therapeutic strategies are now being developed to block the aberrant conformational transitions and so treat the serpinopathies.

Functional unfolding of alpha1-antitrypsin probed by hydrogen-deuterium exchange coupled with mass spectrometry

The native state of alpha(1)-antitrypsin (alpha(1)AT), a member of the serine protease inhibitor (serpin) family, is considered a kinetically trapped folding intermediate that converts to a more stable form upon complex formation with a target protease. Although previous structural and mutational studies of alpha(1)AT revealed the structural basis of the native strain and the kinetic trap, the mechanism of how the native molecule overcomes the kinetic barrier to reach the final stable conformation during complex formation remains unknown. We hypothesized that during complex formation, a substantial portion of the molecule undergoes unfolding, which we dubbed functional unfolding. Hydrogen-deuterium exchange coupled with ESI-MS was used to analyze this serpin in three forms: native, complexing, and complexed with bovine beta-trypsin. Comparing the deuterium content at the corresponding regions of these three samples, we probed the unfolding of alpha(1)AT during complex formation. A substantial portion of the alpha(1)AT molecule unfolded transiently during complex formation, including not only the regions expected from previous structural studies, such as the reactive site loop, helix F, and the following loop, but also regions not predicted previously, such as helix A, strand 6 of beta-sheet B, and the N terminus. Such unfolding of the native interactions may elevate the free energy level of the kinetically trapped native serpin sufficiently to cross the transition state during complex formation. In the current study, we provide evidence that protein unfolding has to accompany functional execution of the protein molecule.

Antiangiogenic forms of antithrombin specifically bind to the anticoagulant heparin sequence

A specific pentasaccharide sequence of heparin binds with high affinity to native antithrombin and induces a conformational change in the inhibitor by a previously described two-step interaction mechanism. In this work, the interactions of heparin with the antiangiogenic latent and cleaved antithrombin forms were studied. Binding of heparin to these antithrombin forms was specific for the same pentasaccharide sequence as native antithrombin. Rapid kinetic studies demonstrated that this pentasaccharide induced a conformational change also in latent and cleaved antithrombin. The binding affinities of these antithrombin forms for the pentasaccharide, as compared to native antithrombin, were approximately 30-fold lower due to two to three fewer ionic interactions, resulting in less stable conformationally altered states. Affinities of latent and cleaved antithrombin for longer heparin chains, containing the pentasaccharide sequence, were 2-fold lower than for the pentasaccharide itself. This contrasts the interaction with native antithrombin and demonstrates that residues flanking the pentasaccharide sequence of heparin are repelled by the latent and cleaved forms. These findings contribute to delineating the mechanism by which heparin or heparan sulfate mediates antiangiogenic activity of antithrombin.

Characterization of the conformational alterations, reduced anticoagulant activity, and enhanced antiangiogenic activity of prelatent antithrombin

A conformationally altered prelatent form of antithrombin that possesses both anticoagulant and antiangiogenic activities is produced during the conversion of native to latent antithrombin (Larsson, H., Akerud, P., Nordling, K., Raub-Segall, E., Claesson-Welsh, L., and Björk, I. (2001) J. Biol. Chem. 276, 11996-12002). Here, we show that the previously characterized prelatent antithrombin is a mixture of native antithrombin and a modified, true prelatent antithrombin that are resolvable by heparin-agarose chromatography. Kinetic analyses revealed that prelatent antithrombin is an intermediate in the conversion of native to latent antithrombin whose formation is favored by stabilizing anions of the Hofmeister series. Purified prelatent antithrombin had reduced anticoagulant function compared with native antithrombin, due to a reduced heparin affinity and consequent impaired ability of heparin to either bridge prelatent antithrombin and coagulation proteases in a ternary complex or to induce full conformational activation of the serpin. Significantly, prelatent antithrombin possessed an antiangiogenic activity more potent than that of latent antithrombin, based on the relative abilities of the two forms to inhibit endothelial cell growth. The prelatent form was conformationally altered from native antithrombin as judged from an attenuation of tryptophan fluorescence changes following heparin activation and a reduced thermal stability. The alterations are consistent with the limited structural changes involving strand 1C observed in a prelatent form of plasminogen activator inhibitor-1 (Dupont, D. M., Blouse, G. E., Hansen, M., Mathiasen, L., Kjelgaard, S., Jensen, J. K., Christensen, A., Gils, A., Declerck, P. J., Andreasen, P. A., and Wind, T. (2006) J. Biol. Chem. 281, 36071-36081), since the (1)H NMR spectrum, electrophoretic mobility, and proteolytic susceptibility of prelatent antithrombin most resemble those of native rather than those of latent antithrombin. Together, these results demonstrate that limited conformational alterations of antithrombin that modestly reduce anticoagulant activity are sufficient to generate antiangiogenic activity.

An immunochemical method for quantitative determination of latent antithrombin, the reactive center loop-inserted uncleaved form of antithrombin

Antithrombin (AT) is a serine protease inhibitor that has thrombin, factors IXa and Xa as target proteases. In addition to active native AT, two other forms have been identified in plasma: the reactive center loop inserted cleaved and latent, uncleaved forms. Both have been shown to be present in normal human blood. Latent AT forms a dimer with native AT in vitro, thus inactivating the native form. Here we describe a mouse monoclonal antibody, 8C8, that is specific for latent AT. The affinity of 8C8 was found to be 500-fold higher for latent than for native AT and 5000-fold higher for latent than for cleaved AT. A sandwich assay was developed to measure the concentration of latent AT in plasma, which was found to be approximately 4.8 mg L(-1) in healthy individuals. The K(D) of the interaction between native and latent AT was found to be 51 mum, i.e. far above the plasma concentration of both native and latent AT, indicating a negligible complex formation in blood.

Structures of importance for the stability of antiplasmin as studied by site-directed mutagenesis

Human antiplasmin, a fast-acting inhibitor of plasmin in plasma, belongs to the serpin super-family of proteins. Like other members of this family, antiplasmin has a scissile peptide bond exposed within a reactive centre loop, typically present at the surface of the molecule. Antiplasmin is stable at neutral pH, but at acidic pH or at elevated temperatures it rapidly becomes inactivated. Data regarding "native" antiplasmin have demonstrated that both polymerization processes and formation of latent molecules are important in this respect. In this work we used site-directed mutagenesis to produce 11 single-site mutants (mainly within Abeta-sheet, Bbeta-sheet and reactive centre loop), which were expressed in Drosophila S2 cells, purified and characterized. Five of the 11 mutants were found to have a deviating stability at decreased pH. Glu346Thr was the only mutant with a lesser stability as compared to wt-antiplasmin, but the other 4 were more stable. The most stable mutant, His341Thr, was 7-fold more stable at pH 4.9 as compared to wt-antiplasmin. The wt-antiplasmin had a much more pronounced tendency to polymerize at decreased pH, as compared to "native" antiplasmin. However, many of the mutants clearly rather formed latent molecules, as judged both from PAGE-analysis at non-denaturing condition and reactivation experiments.

The cleaved and latent forms of antithrombin are normal constituents of blood plasma: a quantitative method to measure cleaved antithrombin

Antithrombin (AT), a member of the serine protease inhibitor family, is the key regulator of thrombin activity in vivo. Thrombin inhibition is accomplished by the formation of covalent thrombin-AT (TAT) complex. The rate of inhibition is accelerated by heparin, which also leads to the formation of a substantial amount of cleaved AT. We produced a murine monoclonal antibody (mAb) (M9) that is specific for the two forms of AT, in which the reactive center loop is inserted into beta-sheet A, i.e. cleaved and latent AT. The antibody has no measurable affinity for native AT. Using M9 as a catcher antibody in conjunction with a mAb (M27) that does not bind latent AT, we developed a sandwich assay that measures cleaved AT without interference from latent and native AT. The concentration in healthy subjects was determined to be 1.3 mg L(-1) (range: 1.0-1.9), which was about 100-fold lower than the plasma concentration of native AT and 1000-fold higher than the concentration of the TAT complex. The cleaved AT concentration is higher than what would be expected from the rate of formation of cleaved AT in vitro in conjunction with TAT complex formation in the presence of heparin. The concentration of cleaved AT did not correlate with the TAT concentration in plasma from patients with venous thrombosis.

The heparin-binding site of antithrombin is crucial for antiangiogenic activity

The heparin-binding site of antithrombin is shown here to play a crucial role in mediating the antiangiogenic activity of conformationally altered cleaved and latent forms of the serpin. Blocking the heparin-binding site of cleaved or latent antithrombin by complexation with a high-affinity heparin pentasaccharide abolished the serpin's ability to inhibit proliferation, migration, capillary-like tube formation, basic fibroblast growth factor (bFGF) signaling, and perlecan gene expression in bFGF-stimulated human umbilical vein endothelial cells. Mutation of key heparin binding residues, when combined with modifications of Asn-linked carbohydrate chains near the heparin-binding site, also could abrogate the anti-proliferative activity of the cleaved serpin. Surprisingly, mutation of Lys114, which blocks anticoagulant activation of antithrombin by heparin, caused the native protein to acquire antiproliferative activity without the need for conformational change. Together, these results indicate that the heparin-binding site of antithrombin is of crucial importance for mediating the serpin's antiangiogenic activity and that heparin activation of native antithrombin constitutes an antiangiogenic switch that is responsible for turning off the antiangiogenic activity of the native serpin.

Purification and characterization of recombinant human antithrombin containing the prelatent form in Chinese hamster ovary cells

Antithrombin (AT) is a serine proteinase inhibitor and a major regulator of the blood coagulation cascade. AT in human plasma has two isoforms, a predominant alpha-isoform and a minor beta-isoform; the latter lacks N-glycosylation at Asn 135 and has a higher heparin affinity. From the difference in its folding states, the AT molecule can be separated into three forms: a native form, a denatured and inactive form known as the latent form, and a partially denatured form called the prelatent form. In this study, we purified and characterized recombinant human AT (rAT) containing the prelatent form produced by Chinese hamster ovary (CHO) cells. When rAT was purified at physiological pH, its specific activity was lower than that of plasma-derived human AT (pAT). The latent and prelatent forms were detected in rAT by using hydrophobic interaction chromatography analysis. However, when rAT was purified at alkaline pH, the prelatent form was reversibly folded to the native form and the inhibitory activity of rAT increased to a value similar to that of pAT. Highly purified rAT was analyzed and compared with pAT by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, circular dichroism spectroscopy, amino acid composition, N-terminal sequence, monosaccharide composition, peptide mapping, and heparin-binding affinity. From these analyses, rAT was found to be structurally identical to pAT, except for carbohydrate side-chains. rAT in CHO cells had a high beta-isoform content and it caused a higher heparin affinity than by pAT and also pH-dependent reversible inhibitory activity.

Inactivation of antiplasmin at low pH: evidence for the formation of latent molecules

Several serine proteinase inhibitors (serpins) are metastable proteins which under certain conditions may undergo conformational changes resulting in the insertion of the reactive centre loop into the so-called Abeta-sheet and hence forming latent molecules. Here we have studied the inactivation of antiplasmin as a function of pH and temperature with time. At decreased pH (4.9-5.8) and at room temperature, antiplasmin activity decreased following first-order kinetics. Analysis by polyacrylamide gel electrophoresis under non-denaturing conditions demonstrated that only minor amounts of polymerized material formed after extensive incubation (4 days) at room temperature. However, on incubation at elevated temperatures (45 or 55 degrees C), a rapid formation of polymerized material was observed. We also demonstrated that antiplasmin inactivated by treatment at pH approximately 5 at room temperature spontaneously slowly regained some activity if incubated in a buffer of neutral pH. Furthermore, by treatment with 4 M guanidinium chloride for about 30 min, followed by dialysis against a neutral phosphate buffer, considerable activity (almost 40%) was regained. Thus, we conclude that antiplasmin, at least partially, at lower temperatures is transformed into a latent form, which could be reactivated, in a similar manner as PAI-1. At increased temperature, however, polymerization seems to be the predominant reason for inactivation.

Latent S49P neuroserpin forms polymers in the dementia familial encephalopathy with neuroserpin inclusion bodies

The serpinopathies result from conformational transitions in members of the serine proteinase inhibitor superfamily with aberrant tissue deposition or loss of function. They are typified by mutants of neuroserpin that are retained within the endoplasmic reticulum of neurons as ordered polymers in association with dementia. We show here that the S49P mutant of neuroserpin that causes the dementia familial encephalopathy with neuroserpin inclusion bodies (FENIB) forms a latent species in vitro and in vivo in addition to the formation of polymers. Latent neuroserpin is thermostable and inactive as a proteinase inhibitor, but activity can be restored by refolding. Strikingly, latent S49P neuroserpin is unlike any other latent serine proteinase inhibitor (serpin) in that it spontaneously forms polymers under physiological conditions. These data provide an alternative method for the inactivation of mutant neuroserpin as a proteinase inhibitor in FENIB and demonstrate a second pathway for the formation of intracellular polymers in association with disease.

The conversion of active to latent plasminogen activator inhibitor-1 is an energetically silent event

PAI-1 is a proteinase inhibitor, which plays a key role in the regulation of fibrinolysis. It belongs to the serpins, a family of proteins that behave either as proteinase inhibitors or proteinase substrates, both reactions involving limited proteolysis of the reactive center loop and insertion of part of this loop into beta-sheet A. Titration calorimetry shows that the inhibition of tissue-type plasminogen and pancreatic trypsin are exothermic reactions with DeltaH = -20.3, and -22.5 kcal.mol(-1), respectively. The Pseudomonas aeruginosa elastase-catalyzed reactive center loop cleavage and inactivation of the inhibitor is also exothermic (DeltaH = -38.9 kcal.mol(-1)). The bacterial elastase also hydrolyses peptide-bound PAI-1 in which acetyl-TVASSSTA, the octapeptide corresponding to the P(14)-P(7) sequence of the reactive center loop is inserted into beta-sheet A of the serpin with DeltaH = -4.0 kcal.mol(-1). In contrast, DeltaH = 0 for the spontaneous conversion of the metastable active PAI-1 molecule into its thermodynamically stable inactive (latent) conformer although this conversion also involves loop/sheet insertion. We conclude that the active to latent transition of PAI-1 is an entirely entropy-driven phenomenon.

Neuroserpin Portland (Ser52Arg) is trapped as an inactive intermediate that rapidly forms polymers: implications for the epilepsy seen in the dementia FENIB

The dementia familial encephalopathy with neuroserpin inclusion bodies (FENIB) is caused by point mutations in the neuroserpin gene. We have shown a correlation between the predicted effect of the mutation and the number of intracerebral inclusions, and an inverse relationship with the age of onset of disease. Our previous work has shown that the intraneuronal inclusions in FENIB result from the sequential interaction between the reactive centre loop of one neuroserpin molecule with beta-sheet A of the next. We show here that neuroserpin Portland (Ser52Arg), which causes a severe form of FENIB, also forms loop-sheet polymers but at a faster rate, in keeping with the more severe clinical phenotype. The Portland mutant has a normal unfolding transition in urea and a normal melting temperature but is inactive as a proteinase inhibitor. This results in part from the reactive loop being in a less accessible conformation to bind to the target enzyme, tissue plasminogen activator. These results, with those of the CD analysis, are in keeping with the reactive centre loop of neuroserpin Portland being partially inserted into beta-sheet A to adopt a conformation similar to an intermediate on the polymerization pathway. Our data provide an explanation for the number of inclusions and the severity of dementia in FENIB associated with neuroserpin Portland. Moreover the inactivity of the mutant may result in uncontrolled activity of tissue plasminogen activator, and so explain the epileptic seizures seen in individuals with more severe forms of the disease.

Latent antithrombin does not affect physiological angiogenesis: an in vivo study on vascularization of grafted ovarian follicles

Latent antithrombin (L-AT), a heat-denatured form of native antithrombin (AT), is a potent inhibitor of pathological tumor angiogenesis. In the present study, we have investigated whether L-AT has comparable antiangiogenic effects on physiological angiogenesis of ovarian tissue. For this purpose, preovulatory follicles of Syrian golden hamsters were mechanically isolated and transplanted into dorsal skinfold chambers chronically implanted in L-AT- or AT-treated hamsters. Non-treated animals served as controls. Over 14 days after transplantation neovascularization of the follicular grafts was assessed in vivo by quantitative analysis of the newly developed microvascular network, its microvessel density, the diameter of the microvessels, their red blood cell velocity and volumetric blood flow as well as leukocyte-endothelial cell interaction using fluorescence microscopic techniques. In each group, all of the grafted follicles were able to induce angiogenesis. At day 3 after transplantation, sinusoidal sacculations and capillary sprouts could be observed, finally developing complete glomerulum-like microvascular networks within 5 to 7 days. Overall revascularization of grafted follicles did not differ between the groups studied. Interestingly, follicular grafts in L-AT- and AT-treated hamsters presented with higher values of microvessel diameters and volumetric blood flow, when compared to non-treated controls, which may be best interpreted as a reactive response to an increased release of vasoactive mediators. In conclusion, the present study demonstrates, that L-AT has no adverse effects on physiological angiogenesis of freely transplanted ovarian follicles. Thus, L-AT may be an effective drug in tumor therapy, which blocks tumor growth by selective suppression of tumor vascularization without affecting new vessel formation in the female reproductive system.

Pasteurization of antithrombin without generation of the prelatent form of antithrombin

Human antithrombin (AT) is the major inhibitor of blood coagulation and has also been shown to exert anti-inflammatory and anti-angiogenic effects. Pasteurization of pharmaceutical AT products is usually performed at 60 degrees C for 10h in the presence of sodium citrate as stabilizer, sometimes in combination with sucrose. These stabilizers significantly decrease the aggregation and denaturation of AT, but during the pasteurization, a small amount of latent AT (LAT), a partially denatured form, is usually generated, as is an equal amount of another latent form of AT, the so-called prelatent AT (PLAT). The LAT formed during pasteurization has a rather low affinity to heparin and is easily removed by using a second heparin affinity chromatography step in the production process. This is in contrast to the PLAT, which has a slightly lower affinity to heparin than does native AT, which makes it hard to remove. Hence, four commercial products of pasteurized AT were previously shown to contain about 4% of PLAT. In the present work, an alternative pasteurization method is presented, where 2M ammonium sulfate and 50% sucrose are used as stabilizers. During this pasteurization, no, or trace amounts ( < 0.5%), of PLAT may be generated with no formation of aggregates. Moreover, the pasteurized AT has the same specific thrombin-inhibiting activity when compared to incubation in the presence of citrate and sucrose. Heparin affinity high-performance liquid chromatography was used for the determination of PLAT, LAT, and AT.

Separation of latent, prelatent, and native forms of human antithrombin by heparin affinity high-performance liquid chromatography

Latent antithrombin (LAT) is a partially denatured form of human antithrombin (AT). LAT does not inhibit clotting of the blood, but has previously been shown to inhibit angiogenesis and carcinogenesis. Another probably partially denatured form is the so-called prelatent AT (P-LAT), described by Larsson et al. [J. Biol. Chem. 276 (2001) 11996]. In the present work, an analytical heparin affinity chromatography method is described that separates an AT form, which is formed during the pasteurization process and which we believe to be identical to the previously described P-LAT, from native AT and LAT. Non-pasteurized AT was shown to contain no P-LAT, while four, heat-treated commercial AT products all contained P-LAT (1-6%, mean=4%). P-LAT has a slightly lower affinity to heparin than does native AT, but exhibits a much stronger heparin affinity when compared to LAT. P-LAT and native AT were shown to have very similar thrombin inhibiting activity, while LAT lacks such activity.

Detection of conformational transformation of antithrombin in blood with crossed immunoelectrophoresis: new application for a classical method

The structural flexibility of antithrombin is essential for its molecular trapping mechanism but also makes it vulnerable to even minor changes affecting its conformational stability, which influences hemostasis significantly. The conformational transformation of this serpin has been poorly investigated in biologic samples because available immunologic methods hardly differentiate between different conformations of this protein. Crossed immunoelectrophoresis (CIE) in presence of heparin has been classically used to identify mutant antithrombins with low heparin affinity. We demonstrate that this method also separates native and relaxed antithrombin, permitting the analysis of conformational variations of this potent anticoagulant with just a few microliters of plasma. However, CIE does not distinguish between antithrombin conformations with reduced heparin affinity: latent, cleaved, thrombin-antithrombin complexes, or heparin-binding mutants. Therefore, clinical interpretation of CIE results should be examined with caution. Using this and other methods, and evaluating the functional activity of antithrombin, we analyzed the conformational transformation of antithrombin in biologic samples. We confirmed its transformation to the latent configuration by incubating it at 50 degrees C. This conformational change also occurs at 37 degrees C, supporting the idea that this process is involved in the senescence of antithrombin. However, fresh plasma contains only traces of latent antithrombin, suggesting that this conformation is rapidly cleared in vivo. Finally, small increases in temperature (to 40 degrees C) resulted in a faster conformational transformation of antithrombin. Fever has been suggested to have key structural, functional, and clinical consequences in patients with conformational mutations in antithrombin. Our results support a role for small changes in temperature in nonmutated antithrombin, suggesting that fever is a general risk factor for thrombosis.

Antiangiogenic antithrombin down-regulates the expression of the proangiogenic heparan sulfate proteoglycan, perlecan, in endothelial cells

Antithrombin, a key serpin family regulator of blood coagulation proteases, is transformed into a potent antiangiogenic factor by limited proteolysis or mild heating. Here, we show by cDNA microarray, semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR), Northern blotting, and immunoblotting analyses that the expression of the proangiogenic heparan sulfate proteoglycan (HSPG), perlecan, but not other HSPGs, is dramatically down-regulated in human umbilical vein endothelial cells (HUVECs) treated with antiangiogenic cleaved and latent forms of antithrombin but not with the native form. Down-regulation of perlecan expression by cleaved and latent antithrombins was observed in both basic fibroblast growth factor (bFGF)-stimulated and unstimulated cells, whereas the antiangiogenic antithrombins inhibited the proliferation of only bFGF-stimulated HUVECs by arresting cells at the G(1) cell cycle phase. The importance of perlecan expression levels in mediating the antiproliferative effect of the antiangiogenic antithrombins was suggested by the finding that transforming growth factor-beta 1, a potent stimulator of perlecan expression in endothelial cells, blocked the down-regulation of perlecan expression and antiproliferative activity of cleaved antithrombin on endothelial cells. The previously established key role of perlecan in mediating bFGF stimulation of endothelial cell proliferation and angiogenesis suggests that a primary mechanism by which antiangiogenic antithrombins exert their effects is through the down-regulation of perlecan expression.

Probing plasma clearance of the thrombin-antithrombin complex with a monoclonal antibody against the putative serpin-enzyme complex receptor-binding site

A high-affinity monoclonal antibody (M27), raised against the human thrombin-antithrombin complex, has been identified and characterized. The epitope recognized by M27 was located to the linear sequence FIREVP (residues 411-416), located in the C-terminal cleavage peptide of antithrombin. This region overlaps, by two residues, the putative binding site of antithrombin for the serpin-enzyme complex receptor. Studies in rats and with HepG2 cells in culture indicated that the Fab fragment of M27 does not block binding and uptake of the thrombin-antithrombin complex, suggesting that this region does not play a major role in the recognition and clearance of the thrombin-antithrombin complex. M27 blocked the ability of antithrombin to inhibit thrombin as well as antithrombin cleavage, both in the presence and absence of heparin.

Deletion of P1 arginine in a novel antithrombin variant (antithrombin London) abolishes inhibitory activity but enhances heparin affinity and is associated with early onset thrombosis

A novel variant of antithrombin, the major serpin inhibitor of coagulation proteases, has been identified in a patient with early onset thrombosis and abnormal plasma antithrombin activity. Sequencing of the antithrombin genes of the patient revealed that one of the two alleles was abnormal due to an in-frame deletion of the codon for the P1 arginine residue. The abnormal antithrombin was separated from the normal inhibitor by complexing the latter with thrombin followed by heparin-agarose affinity chromatography. The purified variant, antithrombin London, was completely inactive as a thrombin or factor Xa inhibitor even after heparin activation. Surprisingly, the variant bound heparin with a K(D) reflecting an approximately 10-fold greater affinity than the normal inhibitor. Stopped-flow kinetic analysis showed that this was almost entirely due to a more favorable conformational activation of the variant than the normal inhibitor, as reflected by a decreased rate constant for reversal of the activation. Consistent with its higher than normal heparin affinity, the inactive antithrombin variant was a potent competitive antagonist of the heparin-catalyzed reaction of normal antithrombin with thrombin but did not affect the uncatalyzed reaction. These results suggest that deletion of the antithrombin P1 residue partially activates the serpin by inducing strain in the reactive center loop, which destabilizes the native loop-buried state and favors the activated loop-exposed state with high heparin affinity. The unusually severe thrombosis associated with the heterozygous mutation may be explained by the ability of antithrombin London to bind endogenous heparan sulfate or heparin molecules with high affinity and to thereby block activation of the normal inhibitor.

Separation between the alpha and beta forms of human antithrombin by hydroxyapatite high-performance liquid chromatography

Human antithrombin (AT) inhibits several proteases in the coagulation system, including thrombin and factor Xa, and thus, plays an important role in the regulation of blood coagulation. The predominant form of AT in plasma is ATalpha, which contains four glycosylated asparagine residues, and the minor form is ATbeta, which lacks the Asn-135 glycosylation. In this study, hydroxyapatite high-performance liquid chromatography, using a segmented sodium phosphate gradient, was utilized for the high-resolution separation of ATalpha and ATbeta. The detection limit (signal-to-noise ratio of 3) for ATbeta was 30 microg/mL, corresponding to 0.5% of the injected concentration of AT. Two analyzed commercial AT products both contained about 2% ATbeta. This method is suitable for the determination of ATbeta in pure samples of native AT.

Purification of a modified form of bovine antithrombin III as an HIV-1 CD8+ T-cell antiviral factor

CD8(+) T-cells secrete soluble factor(s) capable of inhibiting both R5- and X4-tropic strains of human immunodeficiency virus type 1 (HIV-1). CCR5 chemokine ligands, released from activated CD8(+) T-cells, contribute to the antiviral activity of these cells. These CC-chemokines, however, do not account for all CD8(+) T-cell antiviral factor(s) (CAF) released from these cells, particularly because the elusive CAF can inhibit the replication of X4 HIV-1 strains that use CXCR4 and not CCR5 as a coreceptor. Here we demonstrate that activated CD8(+) T-cells of HIV-1-seropositive individuals modify serum bovine antithrombin III into an HIV-1 inhibitory factor capable of suppressing the replication of X4 HIV-1. These data indicate that antithrombin III may play a role in the progression of HIV-1 disease.

Preparative conversion of native human antithrombin to the latent form

Human native antithrombin (AT) can be converted to a partially denaturated form of AT, known as latent AT (L-AT). This latent form of AT has been shown to exhibit strong antiangiogenic activity and also to suppress tumor growth in mice models. In the present work, a method is presented which induces the conversion of native AT to L-AT, using incubation at 60 degrees C, for 16 h, with 0.9 M ammonium sulfate, in 5mM Hepes buffer, pH 7.4, giving a recovery of more than 70%. L-AT was determined by integration of the low heparin affinity peak when analyzed by the affinity chromatography method. Native polyacrylamide gel electrophoresis was used to show that the preparation contained no aggregates. Hydrophobic interaction chromatography was also used for the separation of AT and L-AT.

Separation of active and inactive forms of human antithrombin by heparin affinity chromatography

During the manufacturing of an antithrombin preparation, it is necessary to define all steps that may damage or alter the target molecule, and thus decrease the biological activity of the inhibitor in blood coagulation. Pasteurization, commonly used procedure for viral inactivation of plasma derived antithrombin concentrates, was shown to partially alter the conformation of the active native antithrombin to an inactive latent form. To study intensively the different forms of inactive antithrombin that are formed upon heat treatment, human alpha-antithrombin, human beta-antithrombin and an equimolar mixture of the two isoforms were incubated at 60 degrees C for 15 h in the presence of citrate as stabilizing agent. Using two subsequent heparin affinity chromatography steps, three different inactive fractions were separated. By comparison of the heparin binding capacities, isoelectric points and unfolding characteristics of these inactive forms, the alpha-latent and beta-latent antithrombin isoforms could be identified. It was also shown that additional inactive forms such as proteinase cleaved and/or oxidized forms of antithrombin are formed during the heat treatment process. In four commercially available antithrombin preparations, all produced by pasteurization, the amount of inactive protein varied between 0.5% and 9.5%.

The serine protease inhibitor antithrombin III inhibits LPS-mediated NF-kappaB activation by TLR-4

In Drosophila, the Toll family of proteins mediates the innate immune response. Toll is activated by Spaetzle, which is generated in response to pathogens via a serine protease cascade. We wished to investigate if lipopolysaccharides (LPS) might activate Toll-like receptor (TLR) 4 via a serine protease in humans. The serpin antithrombin III (ATIII) and the thrombin inhibitor hirudin both inhibited nuclear factor (NF)-kappaB activation by LPS and Lipid A. ATIII and hirudin were also able to inhibit LPS-induced NF-kappaB activation in cells stably transfected with TLR4. These results suggest that LPS may activate a mammalian serine protease, which generates a product required for TLR4 signalling.

High affinity binding of receptor-associated protein to heparin and low density lipoprotein receptor-related protein requires similar basic amino acid sequence motifs

The 39-kDa receptor-associated protein (RAP) is a specialized chaperone for members of the low density lipoprotein receptor gene family, which also binds heparin. Previous studies have identified a triplicate repeat sequence within RAP that appears to exhibit differential functions. Here we generated a series of truncated and site-directed RAP mutants in order to define the sites within RAP that are important for interacting with heparin and low density lipoprotein receptor-related protein (LRP). We found that high affinity binding of RAP to heparin is mediated by the carboxyl-terminal repeat of RAP, whereas both the carboxyl-terminal repeat and a combination of amino and central repeats exhibit high affinity binding to LRP. Several motifs were found to mediate the binding of RAP to heparin, and each contained a cluster of basic amino acids; among them, an intact R(282)VSR(285)SR(287)EK(289) motif is required for high affinity binding of RAP to heparin, whereas two other motifs, R(203)LR(205)R(206) and R(314)ISR(317)AR(319), also contribute to this interaction. We also found that intact motifs of both R(203)LR(205)R(206) and R(282)VSR(285)SR(287)EK(289) are required for high affinity binding of RAP to LRP, with the third motif, R(314)ISR(317)AR(319), contributing little to RAP-LRP interaction. We conclude that electrostatic interactions likely contribute significantly in the binding of RAP to both heparin and LRP and that high affinity interaction with both heparin and LRP appears to require mostly overlapping sequence motifs within RAP.

A novel anti-angiogenic form of antithrombin with retained proteinase binding ability and heparin affinity

Latent antithrombin, an inactive antithrombin form with low heparin affinity, has previously been shown to efficiently inhibit angiogenesis and tumor growth. We now show that heat treatment similar to that used for preparation of latent antithrombin also transforms antithrombin to another form, which we denote prelatent, with potent anti-angiogenic and anti-tumor activity but with retained proteinase- and heparin-binding properties. The ability of prelatent antithrombin to inhibit angiogenesis is presumably due to a limited conformational change, which may partially resemble that in latent antithrombin. Such a change is evidenced by a different cleavage pattern of prelatent than of native antithrombin by nontarget proteinases. Prelatent antithrombin exerts its anti-angiogenic effect by a similar mechanism as latent antithrombin, i.e. by inhibiting focal adhesion formation and focal adhesion kinase activity, thereby leading to decreased proliferation of endothelial cells. The proteinase inhibitory fractions in commercial antithrombin preparations, which have been heat treated during production, also have anti-angiogenic activity, comparable with that of the prelatent antithrombin form.

Polymerization of plasminogen activator inhibitor-1

The activity of the serine proteinase inhibitor (serpin) plasminogen activator inhibitor-1 (PAI-1) is controlled by the intramolecular incorporation of the reactive loop into beta-sheet A with the generation of an inactive latent species. Other members of the serpin superfamily can be pathologically inactivated by intermolecular linkage between the reactive loop of one molecule and beta-sheet A of a second to form chains of polymers associated with diverse diseases. It has long been believed that PAI-1 is unique among active serpins in that it does not form polymers. We show here that recombinant native and latent PAI-1 spontaneously form polymers in vitro at low pH although with distinctly different electrophoretic patterns of polymerization. The polymers of both the native and latent species differ from the typical loop-A-sheet polymers of other serpins in that they readily dissociate back to their original monomeric form. The findings with PAI-1 are compatible with different mechanisms of linkage, each involving beta-strand addition of the reactive loop to s7A in native PAI-1 and to s1C in latent PAI-1. Glycosylated native and latent PAI-1 can also form polymers under similar conditions, which may be of in vivo importance in the low pH environment of the platelet.

Separation of native and latent forms of human antithrombin by hydrophobic interaction high-performance liquid chromatography

Hydrophobic interaction high-performance liquid chromatography (HIC-HPLC) was utilized for the separation of native human antithrombin (AT) and a partially denaturated form of AT, known as the latent form (L-AT). The AT used in this study is commercially available (Atenativ, Pharmacia & Upjohn, Sweden) and contains albumin as the main stabilizer. The AT was reconstituted and heat treated in order to generate L-AT. This latent form of AT has been shown to exhibit a strong antiangiogenic activity and also to suppress tumor growth. The HPLC system included a TSK Phenyl 5PW column and a segmented gradient, 4.5-0 mol/L sodium chloride. Antithrombin was eluted at about 13 min, and L-AT, at 30 min, corresponding to about 4.2 and 1.6 mol/L sodium chloride, respectively. A reference sample gave 42% L-AT when analyzed by the HIC method and 41% L-AT when analyzed by the heparin affinity chromatography method. The resolution between AT and L-AT was higher with the HIC method than with the heparin affinity method. Incubation of Atenativ at 45 degrees C for 15 h gave about 18% L-AT and was shown by native polyacrylamide gel electrophoresis to contain only monomeric AT. A good resolution between AT and L-AT, but not between albumin and L-AT, was also achieved by a linear gradient of 2-0 mol/L ammonium sulfate, in 25 mmol/L Tris/HCl, pH 8.0.

The citrate ion increases the conformational stability of alpha(1)-antitrypsin

Sodium citrate has previously been shown to convert native alpha(1)-antitrypsin into the inactive latent state and cause alpha(1)-antitrypsin to polymerize via the C-sheet pathway instead of the more common A-sheet pathway. In order to begin to understand these dramatic effects, we have examined the influence of low concentrations of sodium citrate upon the structure, stability and function of alpha(1)-antitrypsin. In 0.5 M citrate, the midpoint of guanidine hydrochloride-induced unfolding was increased by 1.8 M and the rate of heat inactivation was decreased approximately 30-fold compared with Tris or phosphate buffer. alpha(1)-Antitrypsin was fully active in the presence of a range of citrate concentrations (0. 1-0.5 M), forming a stable 1:1 complex with chymotrypsin. The association rate constant between alpha(1)-antitrypsin and chymotrypsin was decreased with increasing citrate concentration. Fluorescence and circular dichroism spectroscopy demonstrated no significant changes in the tertiary structure due to the presence of citrate. However, the insertion rate of exogenous reactive-center loop peptide increased with increasing citrate concentration, indicating some structural changes in the A beta-sheet region. Taken together, these data suggest that in the presence of 0.5 M citrate alpha(1)-antitrypsin adopts a highly stable but active conformation.

Role of arginine 129 in heparin binding and activation of antithrombin

The contribution of Arg(129) of the serpin, antithrombin, to the mechanism of allosteric activation of the protein by heparin was determined from the effect of mutating this residue to either His or Gln. R129H and R129Q antithrombins bound pentasaccharide and full-length heparins containing the antithrombin recognition sequence with similar large reductions in affinity ranging from 400- to 2500-fold relative to the control serpin, corresponding to a loss of 28-35% of the binding free energy. The salt dependence of pentasaccharide binding showed that the binding defect of the mutant serpin resulted from the loss of approximately 2 ionic interactions, suggesting that Arg(129) binds the pentasaccharide cooperatively with other residues. Rapid kinetic studies showed that the mutation minimally affected the initial low affinity binding of heparin to antithrombin, but greatly affected the subsequent conformational activation of the serpin leading to high affinity heparin binding, although not enough to disfavor activation. Consistent with these findings, the mutant antithrombin was normally activated by heparin for accelerated inhibition of factor Xa and thrombin. These results support an important role for Arg(129) in an induced-fit mechanism of heparin activation of antithrombin wherein conformational activation of the serpin positions Arg(129) and other residues for cooperative interactions with the heparin pentasaccharide so as to lock the serpin in the activated state.

Distinct physical and structural properties of the ovine uterine serpin

Experiments were performed to examine the relationship between the structure and function of ovine uterine serpin (OvUS). Limited proteolytic digestion of OvUS caused cleavage of the 55-57 kDa OvUS to a 42 kDa product nearly identical in molecular weight to a naturally-occurring breakdown product of OvUS. N-terminal amino acid sequencing and MALDI-MS revealed that, unlike other serpins, OvUS was preferentially cleaved at about 70 amino acids upstream of the putative reactive center loop. Analysis of the partially-digested protein by gel filtration chromatography suggested that the C-terminal fragment of the protein was still associated under nondenaturing conditions. Partial digestion of OvUS had no effect on the protein's secondary structure, thermal stability, ability to bind lymphocytes or pepsin, or inhibitory activity towards pepsin or mitogen-induced lymphocyte proliferation. In contrast, mild denaturation of OvUS with 0.5 M guanidine HCl increased thermal stability. Unlike for other serpins, the increase in thermal stability was lost upon removal of the denaturant. Incubation of OvUS with 100 fold molar excess of a peptide corresponding to the putative P(14)-P(2) region of the RCL for 24 h at 37 degrees C to induce binary complex formation had no effect on its secondary structure and did not alter the biological activity of the protein. Synthetic peptides corresponding to the putative P(14)-P(2) region and the P(7)-P(15') region of the RCL were not inhibitory to pepsin activity or lymphocyte proliferation. Taken together, these results indicate that the conformation of OvUS is distinct from the prototypical serpin because conditions that lead to the large-scale conformational change in other serpins such as antithrombin III and alpha(1)-antitrypsin do not cause similar changes in OvUS. Moreover, the putative RCL does not seem to contain the activity required to inhibit lymphocyte proliferation or pepsin activity.