Reaction of thrombins with human antithrombin III. I. Enzyme activity losses unrelated to the inhibitory reaction and their prevention

Thrombin: An Approach to Developing a Higher-Order Reference Material and Reference Measurement Procedure for Substance Identity, Amount, and Biological Activities

Thrombin, the proteolytic enzyme that catalyzes the transformation of soluble fibrinogen to the polymerized fibrin clot, participates in multiple reactions in blood coagulation in addition to the clotting reaction. Although reference materials have existed for many years, structural characterization and measurement of biological activity have never been sufficient to permit claims of clear metrological traceability for the thrombin preparations. Our current state-of-the-art methods for protein characterization and determination of the catalytic properties of thrombin now make it practical to develop and characterize a metrologically acceptable reference material and reference measurement procedure for thrombin. Specifically, α-thrombin, the biologically produced protease formed during prothrombin activation, is readily available and has been extensively characterized. Dependences of thrombin proteolytic and peptide hydrolytic activities on a variety of substrates, pH, specific ions, and temperature are established, although variability remains for the kinetic parameters that describe thrombin enzymatic action. The roles of specific areas on the surface of the thrombin molecule (exosites) in substrate recognition and catalytic efficiency are described and characterized. It is opportune to develop reference materials of high metrological order and technical feasibility. In this article, we review the properties of α-thrombin important for its preparation and suggest an approach suitable for producing a reference material and a reference measurement procedure that is sensitive to thrombin’s catalytic competency on a variety of substrates.

Blood compatibility of heparin-inspired, lactose containing, polyureas depends on the chemistry of the polymer backbone

Sulfated glycopolymers were synthesized from diisocyanates and lactose containing diamines. Blood compatibility assays indicated highly sulfated glycopolymers with methylene bis(4-cyclohexyl isocyanate) backbones result in prolonged clotting times.

Single fluorescence probes along the reactive center loop reveal site-specific changes during the latency transition of PAI-1

The serine protease inhibitor (serpin), plasminogen activator inhibitor-1 (PAI-1), is an important biomarker for cardiovascular disease and many cancers. It is therefore a desirable target for pharmaceutical intervention. However, to date, no PAI-1 inhibitor has successfully reached clinical trial, indicating the necessity to learn more about the mechanics of the serpin. Although its kinetics of inhibition have been extensively studied, less is known about the latency transition of PAI-1, in which the solvent-exposed reactive center loop (RCL) inserts into its central β-sheet, rendering the inhibitor inactive. This spontaneous transition is concomitant with a large translocation of the RCL, but no change in covalent structure. Here, we conjugated the fluorescent probe, NBD, to single positions along the RCL (P13-P5') to detect changes in solvent exposure that occur during the latency transition. The results support a mousetrap-like RCL-insertion that occurs with a half-life of 1-2 h in accordance with previous reports. Importantly, this study exposes unique transitions during latency that occur with a half-life of ∼5 and 25 min at the P5' and P8 RCL positions, respectively. We hypothesize that the process detected at P5' represents s1C detachment, while that at P8 results from a steric barrier to RCL insertion. Together, these findings provide new insights by characterizing multiple steps in the latency transition.

The N-terminal 1-16 peptide derived in vivo from protein seminal vesicle protein IV modulates alpha-thrombin activity: potential clinical implications

We have previously shown that seminal vesicle protein IV (SV-IV) and its 1-70 N-terminal fragment have anti-inflammatory activity and modulate anti-thrombin III (AT) activity. Moreover, mass spectrometry analysis of purified SV-IV has shown that the protein was found to be highly heterogeneous and 14% of the total SV-IV molecules are truncated forms, of particular interest the 1-16, 1-17, and 1-18 peptides. In this work we report experimental data which demonstrate that the 1-16 peptide (P1-16) possesses a marked effect on the AT activity by preventing the formation of the thrombin-AT complex. We found that the formation of thrombin-AT complex is markedly decreased in the presence of P1-16 used at equimolar concentration with thrombin as evaluated with SDS-PAGE. We also monitored the conformational changes of thrombin in the presence of different P1-16 concentrations, and calculated the K(d) of thrombin/P1-16 system by circular dichroism technique. The probable interaction sites of P1-16 with thrombin have been also evaluated by molecular graphics and computational analyses. These results have potential implications in the treatment of sterility and thrombotic diseases.

Resolution of Conformational Activation in the Kinetic Mechanism of Plasminogen Activation by Streptokinase

Streptokinase (SK) activates plasminogen (Pg) by specific binding and nonproteolytic expression of the Pg catalytic site, initiating Pg proteolysis to form the fibrinolytic proteinase, plasmin (Pm). The SK-induced conformational activation mechanism was investigated in quantitative kinetic and equilibrium binding studies. Progress curves of Pg activation by SK monitored by chromogenic substrate hydrolysis were parabolic, with initial rates (v(1)) that indicated no transient species and subsequent rate increases (v(2)). The v(1) dependence on SK concentration for [Glu]Pg and [Lys]Pg was hyperbolic with dissociation constants corresponding to those determined in fluorescence-based binding studies for the native Pg species, identifying v(1) as rapid SK binding and conformational activation. Comparison of [Glu]Pg and [Lys]Pg activation showed an approximately 12-fold higher affinity of SK for [Lys]Pg that was lysine-binding site dependent and no such dependence for [Glu]Pg. Stopped-flow kinetics of SK binding to fluorescently labeled Pg demonstrated at least two fast steps in the conformational activation pathway. Characterization of the specificity of the conformationally activated SK.[Lys]Pg* complex for tripeptide-p-nitroanilide substrates demonstrated 5-18- and 10-130-fold reduced specificity (k(cat)/K(m)) compared with SK.Pm and Pm, respectively, with differences in K(m) and k(cat) dependent on the P1 residue. The results support a kinetic mechanism in which SK binding and reversible conformational activation occur in a rapid equilibrium, multistep process.

Plasminogen activator inhibitor type 1 promotes the self-association of vitronectin into complexes exhibiting altered incorporation into the extracellular matrix

Serine proteinase inhibitors, including plasminogen activator inhibitor type 1 (PAI-1) and antithrombin, are key regulators of hemostatic processes such as thrombosis and wound healing. Much evidence suggests that PAI-1 can influence such processes, as well as pathological events like tumor metastasis, through its ability to directly regulate binding of blood platelets and cells to extracellular substrata. One way that PAI-1 influences these processes may be mediated through its binding to the plasma protein vitronectin. Binding to PAI-1 results in the incorporation of vitronectin into a higher order complex with a potential for multivalent interactions (Podor, T. J., Shaughnessy, S. G., Blackburn, M. N., and Peterson, C. B. (2000) J. Biol. Chem. 275, 25402-25410). In this study, evidence is provided to support this concept from studies on the effects of PAI-1-induced multimerization on the interactions of vitronectin with matrix components and cell surface receptors. By monitoring complex formation and stability over time using size-exclusion high performance liquid chromatography, a correlation is made between PAI-1-induced multimerization and enhanced cell/matrix binding properties of vitronectin. This evidence indicates that PAI-1 alters the adhesive functions of vitronectin by converting the protein via the higher order complex to a self-associated, multivalent species that is functionally distinct from the abundant monomeric form found in the circulation.

The pH dependence of serpin-proteinase complex dissociation reveals a mechanism of complex stabilization involving inactive and active conformational states of the proteinase which are perturbable by calcium

Serpin family protein proteinase inhibitors trap proteinases at the acyl-intermediate stage of cleavage of the serpin as a proteinase substrate by undergoing a dramatic conformational change, which is thought to distort the proteinase active site and slow deacylation. To investigate the extent to which proteinase catalytic function is defective in the serpin-proteinase complex, we compared the pH dependence of dissociation of several serpin-proteinase acyl-complexes with that of normal guanidinobenzoyl-proteinase acyl-intermediate complexes. Whereas the apparent rate constant for dissociation of guanidinobenzoyl-proteinase complexes (k(diss, app)) showed a pH dependence characteristic of His-57 catalysis of complex deacylation, the pH dependence of k(diss, app) for the serpin-proteinase complexes showed no evidence for His-57 involvement in complex deacylation and was instead characteristic of a hydroxide-mediated deacylation similar to that observed for the hydrolysis of tosylarginine methyl ester. Hydroxylamine enhanced the rate of serpin-proteinase complex dissociation but with a rate constant for nucleophilic attack on the acyl bond several orders of magnitude slower than that of hydroxide, implying limited accessibility of the acyl bond in the complex. The addition of 10-100 mm Ca(2+) ions stimulated up to 80-fold the dissociation rate constant of several serpin-trypsin complexes in a saturable manner at neutral pH and altered the pH dependence to a pattern characteristic of His-57-catalyzed complex deacylation. These results support a mechanism of kinetic stabilization of serpin-proteinase complexes wherein the complex is trapped as an acyl-intermediate by a serpin conformational change-induced inactivation of the proteinase catalytic function, but suggest that the inactive proteinase conformation in the complex is in equilibrium with an active proteinase conformation that can be stabilized by the preferential binding of an allosteric ligand such as Ca(2+).

Substrate-assisted catalysis of the PAR1 thrombin receptor. Enhancement of macromolecular association and cleavage

Platelet activation and aggregation are mediated by thrombin cleavage of the exodomain of the PAR1 receptor. The specificity of thrombin for PAR1 is enhanced by binding to a hirudin-like region (Hir) located in the receptor exodomain. Here, we examine the mechanism of thrombin-PAR1 recognition and cleavage by steady-state kinetic measurements using soluble PAR1 N-terminal exodomains. We determined that the primary role of the PAR1 Hir sequence is to reduce the kinetic barriers to formation of the docked thrombin-PAR1 complex rather than to form high affinity ground-state interactions. In addition, the exosite I-bound Hir motif facilitates the productive interaction of the PAR1 (38)LDPR/SFL(44) sequence with the active site of thrombin. This locking process is the most energetically unfavorable step of the overall reaction. The subsequent irreversible steps of peptide bond cleavage are rapid and allosterically enhanced by the presence of the docked Hir sequence. Furthermore, the C-terminal exodomain product of thrombin cleavage, corresponding to the activated receptor, binds tightly to thrombin. This would suggest that an additional role of the Hir sequence in the thrombin-activated receptor is to sequester thrombin to the platelet surface and modulate cleavage of other platelet receptors such as the PAR4 thrombin receptor, which lacks a functional Hir sequence.

Role of regulatory exosite I in binding of thrombin to human factor V, factor Va, factor Va subunits, and activation fragments

The blood coagulation proteinase, thrombin, converts factor V into factor Va through a multistep activation pathway that is regulated by interactions with thrombin exosites. Thrombin exosite interactions with human factor V and its activation products were quantitatively characterized in equilibrium binding studies based on fluorescence changes of thrombin covalently labeled with 2-anilinonaphthalene-6-sulfonic acid (ANS) linked to the catalytic site histidine residue by Nalpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl ([ANS]FPR-thrombin). Exosite I was shown to play a predominant role in the binding of factor V and factor Va from the effect of the exosite I-specific ligand, hirudin54-65, on the interactions. Factor V and factor Va bound to exosite I of [ANS]FPR-thrombin with similar dissociation constants of 3.4 +/- 1.3 and 1.1 +/- 0.4 microM and fluorescence enhancements of 182 +/- 41 and 127 +/- 17%, respectively. Native thrombin and labeled thrombin bound with similar affinity to factor Va. Among factor V activation products, the factor Va heavy chain was shown to contain the site of exosite I binding, whereas exosite I-independent, lower affinity interactions were observed for activation fragments E and C1, and no detectable binding was observed for the factor Va light chain. The results support the conclusion that the factor V activation pathway is initiated by exosite I-mediated binding of thrombin to a site in the heavy chain region of factor V that facilitates the initial cleavage at Arg709 to generate the heavy chain of factor Va. The results further suggest that binding of thrombin through exosite I to factor V activation intermediates may regulate their conversion to factor Va and that similar binding of thrombin to the factor Va produced may reflect a mode of interaction involved in the regulation of prothrombin activation.

Inactivation of thrombin by antithrombin is accompanied by inactivation of regulatory exosite I

Exosite I of the blood clotting proteinase, thrombin, mediates interactions of the enzyme with certain inhibitors, physiological substrates and regulatory proteins. Specific binding of a fluorescein-labeled derivative of the COOH-terminal dodecapeptide of hirudin ([5F] Hir54-65) to exosite I was used to probe changes in the function of the regulatory site accompanying inactivation of thrombin by its physiological serpin inhibitor, antithrombin. Fluorescence-monitored equilibrium binding studies showed that [5F]Hir54-65 and Hir54-65 bound to human alpha-thrombin with dissociation constants of 26 +/- 2 nM and 38 +/- 5 nM, respectively, while the affinity of the peptides for the stable thrombin-antithrombin complex was undetectable (>/=200-fold weaker). Kinetic studies showed that the loss of binding sites for [5F]Hir54-65 occurred with the same time-course as the loss of thrombin catalytic activity. Binding of [5F] Hir54-65 and Hir54-65 to thrombin was correlated quantitatively with partial inhibition of the rate of the thrombin-antithrombin reaction, maximally decreasing the bimolecular rate constants 1.7- and 2.1-fold, respectively. These results support a mechanism in which thrombin and the thrombin-Hir54-65 complex can associate with antithrombin and undergo formation of the covalent thrombin-antithrombin complex at modestly different rates, with inactivation of exosite I leading to dissociation of the peptide occurring subsequent to the rate-limiting inactivation of thrombin. This mechanism may function physiologically in localizing the activity of thrombin by allowing inactivation of thrombin that is bound in exosite I-mediated complexes with regulatory proteins, such as thrombomodulin and fibrin, without prior dissociation of these complexes. Concomitant with inactivation of thrombin, the thrombin-antithrombin complex may be irreversibly released due to exosite I inactivation.

The use of fluorescent probes to characterize conformational changes in the interaction between vitronectin and plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of tissue-type plasminogen activator and urokinase, is known to convert readily to a latent form by insertion of the reactive center loop into a central beta-sheet. Interaction with vitronectin stabilizes PAI-1 and decreases the rate of conversion to the latent form, but conformational effects of vitronectin on the reactive center loop of PAI-1 have not been documented. Mutant forms of PAI-1 were designed with a cysteine substitution at either position P1' or P9 of the reactive center loop. Labeling of the unique cysteine with a sulfhydryl-reactive fluorophore provides a probe that is sensitive to vitronectin binding. Results indicate that the scissile P1-P1' bond of PAI-1 is more solvent exposed upon interaction with vitronectin, whereas the N-terminal portion of the reactive loop does not experience a significant change in its environment. These results were complemented by labeling vitronectin with an arginine-specific coumarin probe which compromises heparin binding but does not interfere with PAI-1 binding to the protein. Dissociation constants of approximately 100 nM are calculated for the vitronectin/PAI-1 interaction from titrations using both fluorescent probes. Furthermore, experiments in which PAI-1 failed to compete with heparin for binding to vitronectin argue for separate binding sites for the two ligands on vitronectin.

Binding of fibrin monomer and heparin to thrombin in a ternary complex alters the environment of the thrombin catalytic site, reduces affinity for hirudin, and inhibits cleavage of fibrinogen

Interaction of the blood clotting proteinase, thrombin, with fibrin monomer and heparin to form a thrombin.fibrin monomer.heparin ternary complex is accompanied by a change in thrombin catalytic specificity. Equilibrium binding interactions in the assembly of the ternary complex were characterized quantitatively using thrombin labeled at the active site with a fluorescent probe and related to changes in thrombin specificity toward exosite I-dependent binding of hirudin and cleavage of fibrinogen. Changes in the active site environment accompanying binding of heparin or fibrin to thrombin in binary complexes were reported by fluorescence enhancements which contributed additively to the perturbation accompanying formation of the ternary complex. Quantitative analysis of the interactions supports a preferentially ordered path of ternary complex assembly, in which initial binding of heparin to thrombin facilitates binding of fibrin monomer with an approximately 40-fold increased affinity. Binding of fibrin monomer in the ternary complex decreased the affinity of native thrombin for hirudin by >100-fold and inhibited cleavage of fibrinogen, but this inhibition was overcome when fibrin(ogen)-fibrin interactions occurred. These results support a ternary complex model in which heparin binding through exosite II of thrombin facilitates fibrin monomer binding via exosite I, with accompanying changes in thrombin catalytic specificity resulting from perturbations in the active site and reduced accessibility of exosite I to hirudin and fibrinogen.

Analogs of human plasminogen that are labeled with fluorescence probes at the catalytic site of the zymogen. Preparation, characterization, and interaction with streptokinase

Fluorescent analogs of the proteinase zymogen, plasminogen (Pg), which are specifically inactivated and labeled at the catalytic site have been prepared and characterized as probes of the mechanisms of Pg activation. The active site induced non-proteolytically in Pg by streptokinase (SK) was inactivated stoichiometrically with the thioester peptide chloromethyl ketone. N alpha-[(acetylthio)acetyl]-(D-Phe)-Phe-Arg-CH2Cl; the thiol group generated subsequently on the incorporated inhibitor with NH2OH was quantitatively labeled with the fluorescence probe, 2-((4'-iodoacetamido)anilino)naphthalene-6-sulfonic acid; and the labeled Pg was separated from SK. Cleavage of labeled [Glu]Pg1 by urokinase-type plasminogen activator (uPA) was accompanied by a fluorescence enhancement (delta Fmax/Fo) of 2.0, and formation of 1% plasmin (Pm) activity. Comparison of labeled and native [Glu]Pg1 as uPA substrates showed that activation of labeled [Glu]Pg1 generated [Glu]Pm1 as the major product, while native [Glu]Pg1 was activated at a faster rate and produced [Lys]Pm1 because of concurrent proteolysis by plasmin. When a mixture of labeled and native Pg was activated, to include plasmin-feedback reactions, the zymogens were activated at equivalent rates. The lack of potential proteolytic activity of the Pg derivatives allowed their interactions with SK to be studied under equilibrium binding conditions. SK bound to labeled [Glu]Pg1, and [Lys]Pg1 with dissociation constants of 590 +/- 110 and 110 and 11 +/- 7 nM, and fluorescence enhancements of 3.1 +/- 0.1 and 1.6 +/- 0.1, respectively. Characterization of the interaction of SK with native [Glu]Pg1 by the use of labeled [Glu]Pg1 as a probe indicated a approximately 6-fold higher affinity of SK for the native Pg zymogen compared to the labeled Pg analog. Saturating levels of epsilon-aminocaproic acid reduced the affinity of SK for labeled [Glu]Pg1 by approximately 2-fold and lowered the fluorescence enhancement to 1.8 +/- 0.1, whereas the affinity of SK for labeled [Lys]Pg1 was reduced by approximately 98-fold with little effect on the enhancement. These results demonstrate that occupation of lysine binding sites modulates the affinity of SK for Pg and the changes in the environment of the catalytic site associated with SK-induced conformational activation. Together, these studies show that the labeled Pg derivatives behave as analogs of native Pg which report functionally significant changes in the environment of the catalytic site of the zymogen.

Role of the catalytic serine in the interactions of serine proteinases with protein inhibitors of the serpin family. Contribution of a covalent interaction to the binding energy of serpin-proteinase complexes

The contribution of a covalent bond to the stability of complexes of serine proteinases with inhibitors of the serpin family was evaluated by comparing the affinities of beta-trypsin and the catalytic serine-modified derivative, beta-anhydrotrypsin, for several serpin and non-serpin (Kunitz) inhibitors. Kinetic analyses showed that anhydrotrypsin had little or no ability to compete with trypsin for binding to alpha 1-proteinase inhibitor (alpha 1PI), plasminogen activator inhibitor 1 (PAI-1), antithrombin (AT), or AT-heparin complex when present at up to a 100-fold molar excess over trypsin. By contrast, equimolar levels of anhydrotrypsin blocked trypsin binding to non-serpin inhibitors. Equilibrium binding studies of inhibitor-enzyme interactions monitored by inhibitor displacement of the fluorescence probe, p-aminobenzamidine, from the enzyme active site, confirmed that the binding of serpins to anhydrotrypsin was undetectable in the case of alpha 1PI or AT (KI > 10(-5) M), of low affinity in the case of AT-heparin complex (KI 7-9 x 10(-6) M), and of moderate affinity in the case of PAI-1 (KI 2 x 10(-7) M). This contrasted with the stoichiometric high affinity binding of the serpins to trypsin as well as of the non-serpin inhibitors to both trypsin and anhydrotrypsin. Maximal KI values for serpin-trypsin interactions of 1 to 8 x 10(-11) M, obtained from kinetic analyses of association and dissociation rate constants, indicated that the affinity of serpins for trypsin was minimally 4 to 6 orders of magnitude greater than that of anhydrotrypsin. Anhydrotrypsin, unlike trypsin, failed to induce the characteristic fluorescence changes in a P9 Ser-->Cys PAI-1 variant labeled with a nitrobenzofuran fluorescent probe (NBD) which were shown previously to report the serpin conformational change associated with active enzyme binding. These results demonstrate that a covalent interaction involving the proteinase catalytic serine contributes a major fraction of the binding energy to serpin-trypsin interactions and is essential for inducing the serpin conformational change involved in the trapping of enzyme in stable complexes.

Mechanism of acceleration of antithrombin-proteinase reactions by low affinity heparin. Role of the antithrombin binding pentasaccharide in heparin rate enhancement

The role of the sequence-specific pentasaccharide region of high affinity heparin (HAH) in heparin acceleration of antithrombin-proteinase reactions was elucidated by determining the accelerating mechanism of low affinity heparin (LAH) lacking this sequence. LAH was shown to be free of HAH (< 0.001%) from the lack of exchange of added fluorescein-labeled HAH into LAH after separating the polysaccharides by antithrombin-agarose chromatography. Fluorescence titrations showed that LAH bound to antithrombin with a 1000-fold weaker affinity (KD 19 +/- 6 microM) and 5-6-fold smaller fluorescence enhancement (8 +/- 3%) than HAH. LAH accelerated the antithrombin-thrombin reaction with a bell-shaped dependence on heparin concentration resembling that of HAH, but with the bell-shaped curve shifted to approximately 100-fold higher polysaccharide concentrations and with a approximately 100-fold reduced maximal accelerating effect. Rapid kinetic studies indicated these differences arose from a reverse order of assembly of an intermediate heparin-thrombin-antithrombin ternary complex and diminished ability of LAH to bridge antithrombin and thrombin in this complex, as compared to HAH. By contrast, LAH and HAH both accelerated the antithrombin-factor Xa reaction with a simple saturable dependence on heparin or inhibitor concentrations which paralleled the formation of an antithrombin-heparin binary complex. The maximal accelerations of the two heparins in this case correlated with the inhibitor fluorescence enhancements induced by the polysaccharides, consistent with the accelerations arising from conformational activation of antithrombin. 1H NMR difference spectroscopy of antithrombin complexes with LAH and HAH and competitive binding studies were consistent with LAH accelerating activity being mediated by binding to the same site on the inhibitor as HAH. These results demonstrate that LAH accelerates antithrombin-proteinase reactions by bridging and conformational activation mechanisms similar to those of HAH, with the reduced magnitude of LAH accelerations resulting both from a decreased antithrombin affinity and the inability to induce a full activating conformational change in the inhibitor.

Retention of thrombin by polytetrafluoroethylene: influence on the adsorption of fibrinogen/fibrin

Beads of polytetrafluoroethylene were used to investigate adsorption of thrombin and the influence of the adsorbed protease on a subsequent deposition of fibrinogen. Adsorption of active thrombin was not detected by a specific fluorogenic substrate unless > 0.1 units/ml had been applied. Adsorption was considerably improved by albumin, which protected soluble thrombin from inactivation by hydrophobic surfaces. Retention of active thrombin was optimal at ca. 0.1% albumin and decreased at higher concentrations. After incubation with plasma, negligible thrombin activity was detected at the polytetrafluoroethylene beads by the fluorogenic substrate. However, repeated incubation with fresh plasma samples resulted in adsorbed activity rising with each step. This result suggested that thrombin activity should also accumulate at a polytetrafluoroethylene surface in vivo if fresh blood is permanently flowing past. Adsorbed thrombin improved the subsequent retention of fibrinogen, monitored by an antibody technique. Concomitantly, fibrinopeptides A, AP and AY were slowly released whilst fibrinopeptide B was not detectable before 24 h.

Catalytically competent human and bovine zeta-thrombin and chimeras generated from unfolded polypeptide chains

Human and bovine alpha-thrombin cleaved at the B-chain by chymotrypsin generates catalytically competent zeta-thrombins, which are comprised of two noncovalently linked fragments: a 36-(human) or 49-(bovine) residue A-chain linked by a disulfide to B-chain residues B1-148 (zeta 1-thrombin) and B-chain residues B149-259 (zeta 2-thrombin). Human and bovine D-Phe-Pro-Arg-CH2-zeta- and PhMeSO2-zeta-thrombins were prepared by reaction of the active-site histidine (H-B43) and serine (S-B205) with PPACK and PMSF, respectively. Unfolding and dissociation of the noncovalently linked polypeptide chains of either human or bovine D-Phe-Pro-Arg-CH2-zeta- and PhMeSO2-zeta-thrombins in 4.5 M guanidine-HCl and refolding upon 30-fold dilution in 50 mM sodium phosphate buffer pH 6.5, 750 mM NaCl, 0.1% PEG resulted in biphasic generation of catalytic activity. The slow phase was eliminated in the presence of the competitive inhibitor benzamidine-HCl. Unfolding and refolding mixtures of the appropriate inactive precursors generated the active chimeric thrombins bovine zeta 1-thrombin:human zeta 2-thrombin and human zeta 1-thrombin:bovine zeta 2-thrombin. Human zeta 1-thrombin and zeta 2-thrombin were isolated, and, upon recombining, the isolated fragments refolded to generate catalytically competent zeta-thrombin with an active-site content, specific activity toward Chromozym-TH, and a specificity constant (kcat/Km) for FPA release from fibrinogen that were all within 60% of those of native alpha-thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative evaluation of solution equilibrium binding interactions by affinity partitioning: application to specific and nonspecific protein-heparin interactions

A variation of the quantitative affinity chromatography (QAC) method of Winzor, Chaiken, and co-workers for the analysis of protein-ligand interactions has been developed and used to characterize sequence-specific and nonspecific protein-heparin interactions relevant to blood coagulation. The method allows quantitation of the binding of two components, A and B, from the competitive effect of one component, B, on the partitioning of the other component, A, between an immobilized acceptor phase and solution phase at equilibrium. Under the conditions employed, the differences in total A concentrations yielding an equivalent degree of saturation of the immobilized acceptor in the absence and presence of B defines the concentration of A bound to B in solution, thereby enabling conventional Scatchard or nonlinear least-squares analysis of the A-B equilibrium interaction. Like the QAC method, quantitation of the competitor interaction does not depend on the nature of the affinity matrix interaction, which need only be described empirically. The additional advantage of the difference method is that only the total rather than the free competitor ligand concentration need be known. The method requires that the partitioning component A be univalent, but allows for multivalency in the competitor, B, and can in principle be used to study binding interactions involving nonidentical, interacting, or nonspecific overlapping sites. Both the binding constant and the stoichiometry for the specific antithrombin-heparin interaction as well as the apparent binding constant for the nonspecific thrombin-heparin interaction at low thrombin binding densities obtained using this technique were in excellent agreement with values determined using spectroscopic probes.

Isolation of a catalytically competent phosphorylated tyrosine kinase from Rous sarcoma virus-induced rat tumor by immunoadsorption to and hapten elution from phosphotyrosine binding antibodies

A procedure has been developed for the isolation of a catalytically competent phosphorylated tyrosine kinase (RSV Y-kinase) from avian sarcoma virus-induced rat tumors. The procedure involves reaction of partially purified RSV Y-kinase with ATP to effect tyrosyl phosphorylation of catalytically competent RSV Y-kinase. Tyrosyl phosphorylated RSV Y-kinase was isolated from the heterogeneous reaction mixture by immunoadsorption on immobilized phosphotyrosyl binding antibodies and elution with the hapten p-nitrophenyl phosphate. Estimation of the phosphate content of the purified phosphorylated RSV Y-kinase indicated that 1-3 tyrosyl groups had been phosphorylated upon reaction with ATP. The specific activity toward histone 2B of the purified phosphorylated RSV Y-kinase was at least 30-fold greater than that estimated for the RSV Y-kinase prepared previously by immunoadsorption on immobilized antiserum from tumor bearing rabbits.

Isolation of human blood coagulation alpha-factor Xa by soybean trypsin inhibitor-sepharose chromatography and its active-site titration with fluorescein mono-p-guanidinobenzoate

A method based on active-site affinity chromatography on soybean trypsin inhibitor (SBTI)-Sepharose was developed for isolation of human factor Xa in primarily the undergraded alpha-form. The chromatography procedure separated factor Xa from factor X, the Russel's viper venom proteinase used to activate factor X, and traces of contaminating thrombin. alpha-Factor Xa was unstable at pH 7.6 and 25 degrees C, undergoing slow proteolytic degradation to functionally heterogeneous products as evidenced by the greater loss of coagulation assay activity compared to activity measured with a chromogenic substrate. The results of monitoring factor Xa degradation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were consistent with proteolysis of the light chain as a major component reaction occurring in parallel with slower proteolysis of the heavy chain. The decreased rates of these reactions at pH 6.0 enabled isolation and storage of factor Xa in greater than or equal to 88% alpha-form and minimized the heterogeneity due to proteolytic degradation. Characterization of the reaction of fluorescein mono-p-guanidinobenzoate (FMGB) with human and bovine factor Xa isolated by SBTI-Sepharose chromatography demonstrated its utility as a sensitive reagent for continuous fluorometric active-site titration. Analysis of the reaction kinetics as a function of FMGB and human factor Xa concentrations in G/2 0.3, pH 7.4, buffer at 25 degrees C indicated that the ratio of acylation to deacylation rate constants was greater than 200 and that the Km for FMGB was 0.06-0.11 microM, predicting pre-steady-state burst amplitudes of greater than or equal to 96-98% of the active-site concentration at FMGB concentrations greater than or equal to 5 microM. Human factor Xa active-site concentrations were consistent with 82-99% active preparations when compared with the protein concentrations determined from the 280-nm absorbance. Concentrations of human alpha-factor Xa as low as 20 nM could be measured with FMGB, indicating a sensitivity approximately 50 times greater than that measured by spectrophotometric active-site titration with p-nitophenyl p'-guanidinobenzoate.

Active site selective labeling of serine proteases with spectroscopic probes using thioester peptide chloromethyl ketones: demonstration of thrombin labeling using N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl

The feasibility of a new approach to incorporation of spectroscopic probes into the active sites of certain serine proteases has been demonstrated. The method is based on inactivation of a serine protease with a thioester derivative of a peptide chloromethyl ketone. The thiol group generated by reaction of the covalent enzyme-inhibitor complex with NH2OH provides a unique site for subsequent labeling with thiol-reactive probes. To evaluate the method, N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl was synthesized by reaction of the thrombin-specific tripeptide chloromethyl ketone with succinimidyl (acetylthio)acetate and purified by sulfopropyl-Sephadex and Sephadex G-10 chromatography. Reverse-phase high-performance liquid chromatography indicated that the product was 90 +/- 3% pure. The compound was quantitated by using 5,5'-dithiobis(2-nitrobenzoic acid) to measure the concentration of thiol produced in the presence of NH2OH. On this basis, titrations of the irreversible loss of human alpha-thrombin activity had end points of 1.1 +/- 0.1 mol of inhibitor/mol of active sites, indicating a 1:1 stoichiometry for inactivation. Incubation of N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-thrombin with 5-(iodoacetamido)fluorescein in the presence of NH2OH resulted in incorporation of 0.96 mol of the fluorescence probe/mol of active sites and the appearance of fluorescein fluorescence associated with the active site containing B-chain on sodium dodecyl sulfate-polyacrylamide gels. Fluorescence labeling of thrombin required reaction of the inhibitor at the active site as well as subsequent generation of the thiol group with NH2OH. It is concluded that active site selective labeling can be achieved by using this approach, which is likely to be applicable to other proteases, peptide chloromethyl ketones, and a wide variety of probes.