Conformation-specific monoclonal antibodies directed against the calcium-stabilized structure of human prothrombin

Circulating Des-gamma-carboxy prothrombin is not associated with cardiovascular calcification or stiffness: The Multi-Ethnic Study of Atherosclerosis (MESA)

BACKGROUND AND AIMS

Vitamin K-dependent protein (VKDP) activity may have a role in preventing cardiovascular calcification, but has not previously been studied in large, generally healthy populations.

METHODS

Using an elevated ankle-brachial index (ABI) as a measure of medial vascular calcification, we performed a case-cohort analysis within the Multi-Ethnic Study of Atherosclerosis, measuring Des-gamma-carboxy prothrombin (DCP) to estimate VKDP activity. In secondary analyses of the weighted subcohort, we examined the cross-sectional associations between DCP and prevalent vascular calcification of the coronary vessels, aortic and mitral valves, and aortic wall, and with vascular stiffness.

RESULTS

In adjusted analysis, cases (n = 104) had 0.21 ng/ml (-0.94-0.52) lower DCP concentrations than the subcohort (n = 613). Furthermore, amongst the 717 participants in the weighted cohort, VKDP activity was not associated with coronary artery, mitral valve, aortic valve or aortic wall calcification, nor was it associated with vascular stiffness.

CONCLUSIONS

Our negative results do not support a role of circulating VKDP activity in cardiovascular calcification in community-dwelling adults.

Quantifying vitamin K-dependent holoprotein compaction caused by differential γ-carboxylation using high-pressure size exclusion chromatography

This study uses high-pressure size exclusion chromatography (HPSEC) to quantify divalent metal ion (X(2+))-induced compaction found in vitamin K-dependent (VKD) proteins. Multiple X(2+) binding sites formed by the presence of up to 12 γ-carboxyglutamic acid (Gla) residues are present in plasma-derived FIX (pd-FIX) and recombinant FIX (r-FIX). Analytical ultracentrifugation (AUC) was used to calibrate the Stokes radius (R) measured by HPSEC. A compaction of pd-FIX caused by the filling of Ca(2+) and Mg(2+) binding sites resulted in a 5 to 6% decrease in radius of hydration as observed by HPSEC. The filling of Ca(2+) sites resulted in greater compaction than for Mg(2+) alone where this effect was additive or greater when both ions were present at physiological levels. Less X(2+)-induced compaction was observed in r-FIX with lower Gla content populations, which enabled the separation of biologically active r-FIX species from inactive ones by HPSEC. HPSEC was sensitive to R changes of approximately 0.01nm that enabled the detection of FIX compaction that was likely cooperative in nature between lower avidity X(2+) sites of the Gla domain and higher avidity X(2+) sites of the epidermal growth factor 1 (EGF1)-like domain.

Crystal Structure of the Calcium-stabilized Human Factor IX Gla Domain Bound to a Conformation-specific Anti-factor IX Antibody

The binding of Factor IX to membranes during blood coagulation is mediated by the N-terminal gamma-carboxyglutamic acid-rich (Gla) domain, a membrane-anchoring domain found on vitamin K-dependent blood coagulation and regulatory proteins. Conformation-specific anti-Factor IX antibodies are directed at the calcium-stabilized Gla domain and interfere with Factor IX-membrane interaction. One such antibody, 10C12, recognizes the calcium-stabilized form of the Gla domain of Factor IX. We prepared the fully carboxylated Gla domain of Factor IX by solid phase peptide synthesis and crystallized Factor IX-(1-47) in complex with Fab fragments of the 10C12 antibody. The overall structure of the Gla domain in the Factor IX-(1-47)-antibody complex at 2.2 A is similar to the structure of the Factor IX Gla domain in the presence of calcium ions as determined by NMR spectroscopy (Freedman, S. J., Furie, B. C., Furie, B., and Baleja, J. D. (1995) Biochemistry 34, 12126-12137) and by x-ray crystallography (Shikamoto, Y., Morita, T., Fujimoto, Z., and Mizuno, H. (2003) J. Biol. Chem. 278, 24090-24094). The complex structure shows that the complementarity determining region loops of the 10C12 antibody form a hydrophobic pocket to accommodate the hydrophobic patch of the Gla domain consisting of Leu-6, Phe-9, and Val-10. Polar interactions also play an important role in the antibody-antigen recognition. Furthermore, the calcium coordination network of the Factor IX Gla domain is different than in Gla domain structures of other vitamin K-dependent proteins. We conclude that this antibody is directed at the membrane binding site in the omega loop of Factor IX and blocks Factor IX function by inhibiting its interaction with membranes.

Identification and purification of vitamin K-dependent proteins and peptides with monoclonal antibodies specific for gamma -carboxyglutamyl (Gla) residues

Novel monoclonal antibodies that specifically recognize gamma-carboxyglutamyl (Gla) residues in proteins and peptides have been produced. As demonstrated by Western blot and time-resolved immunofluorescence assays the antibodies are pan-specific for most or all of the Gla-containing proteins tested (factors VII, IX, and X, prothrombin, protein C, protein S, growth arrest-specific protein 6, bone Gla protein, conantokin G from a cone snail, and factor Xa-like proteins from snake venom). Only the Gla-containing light chain of the two-chain proteins was bound. Decarboxylation destroyed the epitope(s) on prothrombin fragment 1, and Ca(2+) strongly inhibited binding to prothrombin. In Western blot, immunofluorescence, and surface plasmon resonance assays the antibodies bound peptides conjugated to bovine serum albumin that contained either a single Gla or a tandem pair of Gla residues. Binding was maintained when the sequence surrounding the Gla residue(s) was altered. Replacement of Gla with glutamic acid resulted in a complete loss of the epitope. The utility of the antibodies was demonstrated in immunochemical methods for detecting Gla-containing proteins and in the immunopurification of a factor Xa-like protein from tiger snake venom. The amino acid sequences of the Gla domain and portions of the heavy chain of the snake protein were determined.

Improving the binding affinity of an antibody using molecular modeling and site-directed mutagenesis

Activated Factor X releases F1.2, a 271-amino acid peptide, from the amino terminus of prothrombin during blood coagulation. A nine-amino acid peptide, C9 (DSDRAIEGR), corresponding to the carboxyl terminus of F1.2 was synthesized and used to produce a monoclonal antibody, TA1 (K(D)) 1.22 x 10(-6) M). To model the TA1 antibody, we entered the sequence information of the cloned TA1 Fv into the antibody modeling program, ABM, which combines homology methods, conformational search procedures, and energy screening and has proved to be a reliable and reproducible antibody modeling method. Using a novel protein fusion procedure, we expressed the C9 peptide fused to the carboxyl terminus of the PENI repressor protein from Bacillus licheniformis in Escherichia coli. We constructed fusion proteins containing alanine substitutions for each amino acid in the C9 epitope. Binding studies, using the C9 alanine mutants and TA1, and spatial constraints predicted by the modeled TA1 binding cleft enabled us to establish a plausible conformation for C9 complexed with TA1. Furthermore, based on binding results of conservative amino acid substitutions in C9 and mutations in the antibody, we were able to refine the complex model and identify antibody mutations that would improve binding affinity.

Localization of a metal-dependent epitope to the amino terminal residues 33-40 of human factor IX

Metal binding sites within the Gla domain of vitamin K-dependent coagulation factors have been divided into nonspecific metal sites and calcium-specific sites. We demonstrate here that five residues within the Gla domain of factor IX are responsible for the reactivity with the metal-dependent factor IX monoclonal antibody, A-7. First we demonstrate that modifying any one of three residues within this site in factor IX abolishes the binding of A-7. To confirm the specificity of the antibody, the Gla domain of factor VII was changed at residues 32, 33, 34, 38 and 39 to the homologous residues of human factor IX. These changes were sufficient to generate a factor VII Gla domain with an A-7 binding site of the same affinity as that in factor IX. The site identified is one of the two major surfaces of the Gla domain and may represent the metal-dependent binding site.

Monoclonal antibodies specific for mercuric ions

Monoclonal antibodies (mAbs) that react with soluble mercuric ions have been produced by injection of BALB/c mice with a hapten-carrier complex designed to maximize exposure of the metal to the immune system. Three hybridomas producing antibodies that reacted with bovine serum albumin (BSA)-glutathione-HgCl, but not with BSA-glutathione, were isolated from the spleen of a mouse given multiple injections with glutathione-HgCl conjugated to keyhole limpet hemocyanin. Stable subclones were established from two of these antibodies, designated mAb 4A10 and mAb 1F10. The binding of both antibodies to immobilized BSA-glutathione-HgCl was inhibited by soluble HgCl2, and dissociation constants for mercuric chloride binding were 2.3 and 3.7 nM for mAbs 4A10 and 1F10, respectively. Both antibodies bound mercuric acetate with similar affinities, demonstrating that the antibodies were capable of binding to mercuric ions in the presence of a different counterion than the one used in the immunogen. Reactions were not observed with other metal cations by either antibody. These data demonstrate the successful induction of antibodies that react very specifically with mercuric ions in solution regardless of the presence of a carrier.

Detection of mercuric ions in water by ELISA with a mercury-specific antibody

An immunoassay that detects mercuric ions in water at concentrations of 0.5 ppb and above is described. The assay utilizes a monoclonal antibody that binds specifically to mercuric ions immobilized in wells of microtiter plates. Within the range of 0.5-10 ppb mercury, the absorbance in the enzyme-linked immunosorbent assay (ELISA) is linear to the log of the mercuric ion concentration. The quantitation of mercury by ELISA correlates closely with results from cold-vapor atomic absorption. Other divalent metal cations do not interfere with the assay, although there is interference in the presence of 1 mM chloride ions. The optimum pH for mercury detection is 7.0, although 2 ppb mercury can be detected over a wide pH range. The assay is as sensitive as cold-vapor atomic absorption for mercury detection and can be performed with only 100 microliters of sample.

Discrimination between rat thiostatin (T-kininogen) and one of its cystatin-like inhibitory fragments by a monoclonal antibody, and localization of the epitope

A monoclonal antibody (mAb D3) raised against rat thiostatin (T-kininogen) strongly interacted with a fragment, identified as cystatin-like domain 3, which inhibits cysteine proteinases but did not recognize intact, native thiostatin. The antigen-antibody reaction requires cleavage of the single peptide chain of thiostatin in its inter-domain 2-3 region. This mAb can also differentiate between the two molecular varieties of thiostatin, reacting only with immobilized domain 3 from T1 thiostatin, which differs from the T2 variety by only 10 out of 125 residues. mAb D3 did not react with an N-terminally truncated domain 3 of T1 thiostatin prepared by submaxillary gland kallikrein k10 proteolysis. This suggests that the epitope, or an essential part of it, is located on a stretch of 12 residues at the N-terminal of the T1 thiostatin domain 3. This sequence in T1 thiostatin differs from that in T2 thiostatin by four amino acids, two of which are arginyl residues in T1. Chemical modification of these residues located at positions 246 and 250 decreased the reactivity of T1 domain 3 towards the antibody, suggesting that at least one of them is a critical residue of the epitope. Arginine 246 is part of a small disulfide loop between cysteines 245 and 248 which is also necessary for antibody recognition. This antibody does not change the inhibitory properties of purified domain 3 towards papain or rat liver cathepsin L, indicating that the N-terminal part of domain 3 is not involved in inhibition. mAb D3 was used to demonstrate the presence of inhibitory thiostatin fragments in ascites fluid but not in plasma from normal or turpentine-injected rats.

Anti-human factor IX monoclonal antibodies specific for calcium ion-induced conformations

Four monoclonal antibodies have been produced, which are specific for the Ca2+ or Sr2(+)-induced conformation of human factor IX. Certain, but not all, gamma-carboxy-glutamic acid residues in factor IX are involved in the epitope expression together with the conformation stabilized by the adjacent region of Gla-domain and a disulfide bridge. All the antibodies interfered with the binding of factor IX to phospholipids and inhibited the procoagulant activity of factors IX and IXa beta.

Antibody affinity measurements

The use of antibodies in immunoaffinity separations represents one of the most specific methods for purifying substances of biological interest. Since the binding affinity of antibody greatly influences its behavior in such separations, it is often important to know the value of the antibody affinity expressed as an equilibrium constant K. The present review discusses the equations used in the quantitative analysis of antigen/antibody interactions and describes currently used experimental methods for measuring K values. Advantages and shortcomings of the solution phase and solid phase approaches used for measuring antibody affinity are discussed.

An anti-plasminogen antibody preparation that inhibits the activation of human plasminogen but enhances the activator activity of the B-chain-streptokinase complex

An anti-Glu-plasminogen (GLU-PLG) polyclonal antibody antiserum was prepared in the goat, and specific IgG anti-GLU-PLG antibodies were purified by affinity chromatography using a sepharose-GLU-PLG column. In chromogenic assay studies, the anti-GLU-PLG antibody preparation was found to be an effective inhibitor of the activation of PLG, and it produced different inhibition curves with four different PLG activators. 80% inhibition of streptokinase (SK) activation of GLU-PLG occurred with an anti-GLU-PLG antibody/GLU-PLG molar ratio of 1:1, whereas at this ratio only 28% and 36% inhibition of the plasmin-streptokinase (PLN-SK) and the B-chain-streptokinase (B-SK) complexes occurred. At a 1:1 molar ratio of antibody to PLN, no inhibition of PLN activity occurred. When the anti-GLU-PLG antibody preparation was incubated with each PLG activator, an enhancement in the activator activity of the B-SK complex, but not the other activators was observed with mini-plasminogen (MINI-PLG). Enhancement occurred at a molar ratio of 1:1 and reached a peak of 97% enhancement at a molar ratio of 10:1. Enhanced activator activity of the B-SK complex of 49% occurred at a molar ratio of 1:1 when GLU-PLG was the substrate. At higher molar ratios inhibition of activator activity on GLU-PLG was observed, but not on MINI-PLG. These results indicate that there are multiple activator binding sites on PLG, that these four activators all bind differently to GLU-PLG and MINI-PLG, that some antibody populations that are specific for the PLN B-chain can stimulate activator activity, while other antibody populations that are specific for either the PLN A-chain or the B-chain are inhibitory.

Production of monoclonal antibodies with specificity for different epitopes on the human epidermal growth factor molecule

Four different monoclonal antibodies (MAbs) to human epidermal growth factor (hEGF) were obtained from mouse hybridoma cell lines. They were highly specific for hEGF and did not cross-react appreciably with mouse EGF. The immunoglobulin class of these MAbs was IgG1 or IgG2a. The apparent Kd values for hEGF of these MAbs ranged from 10(-10) to 10(-9) M. These MAbs all inhibited DNA synthesis in BALB/c3T3 cells stimulated by hEGF and 125I-labeled hEGF binding to the receptor of the cells. Reduction of hEGF with dithiothreitol decreased its reactivity with these MAbs, suggesting that they recognized the conformation of the hEGF molecule maintained by the three disulfide bonds. By competitive binding experiments with hEGF, these four MAbs were classified into three groups: two binding to an A domain of hEGF, one to a B domain, and one to a C domain and preventing the bindings of the other MAbs at the A and B domains. When a methionyl residue of hEGF was removed by its treatment with CNBr, the immunological reactivity of the C domain, but not the A or B domain, was destroyed in association with a decrease in the biological activity of the growth factor. These MAbs should be useful in studies on the structure-function relationship of hEGF.

A new technique for mapping epitope specificities of monoclonal antibodies using quasi-elastic light scattering spectroscopy

Quasi-elastic light scattering (QLS) was used to determine relative epitope specificities of a group of anti-bovine serum albumin (BSA) monoclonal antibodies (MAb). QLS is a non-invasive technique which can determine the mean size and size distributions of macromolecular scatterers by analysis of the fluctuations in the intensity of laser light scattering. When two MAbs are mixed together with antigen, QLS detects the complex formation which results from the Ag-Ab reaction, and can easily distinguish between the large complexes formed by interaction of non-competitive MAbs and the smaller complexes formed by competitive MAbs. In this report, the competitive or non-competitive behavior of six anti-BSA MAbs were assessed by radioimmunoassay (RIA) and QLS analysis. The results obtained by QLS analysis confirmed the RIA findings indicating that the six MAbs examined can be categorized into three distinct, non-interacting groups. QLS analysis represents a simple, and extremely rapid technique for epitope mapping studies.

Inhibition of human vitamin-K-dependent protein-S-cofactor activity by a monoclonal antibody specific for a Ca2+-dependent epitope

Protein S is an anticoagulant vitamin-K-dependent plasma protein functioning as a cofactor to activated protein C in the degradation of factors Va and VIIIa. A murine monoclonal antibody, HPS 7, specific for a calcium-stabilized epitope in human protein S, is described. The epitope was available in intact protein S, both in its free form and when protein S was bound to C4b-binding protein. It disappeared upon reduction of disulfide bridges and also after thrombin of chymotrypsin cleavage of protein S. Thrombin cleaves protein S close to the calcium-binding region containing gamma-carboxyglutamic acid (Gla). The cleaved protein still contains the Gla region, linked by a disulfide bridge, but it has a lower affinity for calcium and no protein C cofactor activity. The thrombin-mediated cleavage of protein S could be inhibited by HPS 7. The Ka for the interaction between protein S and the monoclonal was estimated to be approximately 0.7 X 10(8) M-1. Half-maximal binding between HPS 7 and protein S was observed at a calcium concentration of 0.50 mM, indicating that saturation of the Gla region with calcium was required for the interaction. The recently reported Gla-independent high-affinity calcium binding did not induce the epitope. The calcium-dependent binding of protein S to phospholipid vesicles as well as the protein C cofactor activity was inhibited by HPS 7. The data suggests that the epitope for HPS 7 is located in the Gla region of protein S or in the closely positioned thrombin-sensitive region.

Warfarin and the biochemistry of the vitamin K dependent proteins

We have reviewed the biochemistry of the normal and warfarinized vitamin K-dependent coagulation proteins, both individually and as members of their respective macromolecular enzyme complexes. Much has been learned in the past 10-15 years about the molecular mechanisms of coagulation. However, we have only scratched the surface and are beset by a number of interesting challenges. Clearly, we must begin to dissect the other vitamin K-dependent macromolecular enzyme complexes as has been done for prothrombinase. Prothrombin differs in fundamental ways from the other vitamin K-dependent proteins and we can anticipate significant differences as well as similarities in the other complexes. The presence of potentially functional domains in the non-catalytic portions of the vitamin K-dependent proteins and their similarities within this family of proteins should prove a fruitful area of future research. Finally, the evaluation of warfarinized vitamin K-dependent proteins, both in fundamental studies with purified systems and using uniquely engineered immunoassays, should shed light on basic molecular mechanisms and modes of monitoring warfarin therapy.

Conformational effects of ligand binding on the beta 2 subunit of Escherichia coli tryptophan synthase analyzed with monoclonal antibodies

Five monoclonal antibodies recognizing five different epitopes of the native beta 2 subunit of Escherichia coli tryptophan synthase (EC 4.1.2.20) were used to analyze the conformational changes occurring upon ligand binding or chemical modifications of the enzyme. For this purpose, the affinities of each antibody for the different forms of the enzyme were determined by using an enzyme-linked immunosorbent assay which allows measurement of the dissociation constant of antigen-antibody equilibrium in solution. The fixation of the coenzyme pyridoxal 5'-phosphate and the substrate L-serine modifies the affinity constants of most of the antibodies for the enzyme, thus showing the existence of extended conformational rearrangements of the protein. The association of the alpha subunit with the beta 2 subunit, which brings about an increase of the tryptophan synthase activity and abolishes the serine deaminase activity of beta 2, is accompanied by an important conformational change of the N-terminal domain of beta 2 (F1) since none of the anti-F1 monoclonal antibodies can bind to alpha 2 beta 2. Similarly, chemical modifications of beta 2 which are known to produce significant effects on the enzymatic activities of beta 2 result in changes of the affinities of the monoclonal antibodies which can be interpreted as the acquisition of different conformational states of the enzyme.

Characterization of monoclonal antibodies against human protein C specific for the calcium ion-induced conformation or for the activation peptide region

Three monoclonal antibodies have been produced that are specific for the activation peptide region in human protein C. These antibodies inhibited the activation of protein C by thrombin and by the thrombin-thrombomodulin complex. A fourth monoclonal antibody specifically recognized the Ca2+-stabilized conformation in protein C. This antibody bound both intact protein C and protein C from which the gamma-carboxyglutamic acid-containing region had been removed by limited proteolysis. These results indicate that this antibody recognizes the conformation in protein C stabilized by Ca2+ bound to the single binding site that is independent of gamma-carboxyglutamic acid.

Intrinsic and extrinsic factors in protein antigenic structure

Recent advances in the preparation of synthetic peptide vaccines and the use of synthetic peptides as probes of antigenic structure and function have led to renewed interest in the prediction of antigenic sites recognized by antibodies and T cells. This review focuses on antibodies. Features intrinsic to the antigen, such as hydrophilicity and mobility, may be useful in the selection of amino acid sequences of the native protein that will elicit antibodies cross-reacting with peptides, or sequences which, as peptides, will be more likely to elicit antibodies cross-reactive with the native protein. Structural mobility may also contribute to protein-protein interactions in general. However, the entire accessible surface of a protein is likely to be detectable by a large enough panel of antibodies. Which of these antibodies are made in any individual depends on factors extrinsic to the antigen molecule, host factors such as self-tolerance, immune response genes, idiotype networks, and the immunoglobulin structural gene repertoire.

Calcium-dependent changes in properties of human prothrombin: a study using high-performance size-exclusion chromatography and gel-permeation chromatography

High-performance size-exclusion chromatography using a TSK 3000 SW column and aqueous gel filtration with Sephacryl S-200 SF have been used to characterize the effects of calcium ions on the hydrodynamic properties of human prothrombin and prethrombin 1. The results suggest that the effective hydrodynamic radius of prothrombin is less in the presence than in the absence of calcium ions. In addition, when using the TSK-3000 SW column, Ca2+-dependent formation of a hydrophobic site in the fragment 1 region of prothrombin results in an apparent further decrease in hydrodynamic radius.

Effect of divalent cations on the limited proteolysis of prothrombin by thrombin

The inhibitory influence of divalent cations on the ability of bovine alpha-thrombin to hydrolyze prothrombin showed the trend Mn2+ much greater than Ca2+ greater than or equal to Mg2+ greater than Sr2+ much greater than Ba2+. This effect was not due to an inhibition of thrombin's catalytic activity as measured by hydrolysis of a specific synthetic substrate, H-D-Phe-pipecolyl-Arg-p-nitroanilide (D-PhePipArgNA). The presence of divalent cations did not inhibit thrombic proteolysis of gamma-carboxyglutamic acid (Gla)-domainless prothrombin. Prothrombin and Gla-domainless prothrombin were used as competitive inhibitors in the thrombic hydrolysis of D-PhePipArgNA. The apparent Ki value calculated for prothrombin was 18 microM. When either Ca2+ or Mn2+ were present, there was no inhibition. The apparent Ki value determined for Gla-domainless prothrombin was 28 microM in either the absence or presence of Ca2+. Addition of divalent cations to prothrombin, but not to Gla-domainless prothrombin, resulted in an altered protein conformation as measured by high-performance size-exclusion chromatography and ultraviolet difference spectroscopy. These results suggest that a conformational change secondary to the interaction of divalent cations with the Gla-containing domain of prothrombin is required for cation-dependent inhibition of thrombin hydrolysis.

Immunoaffinity purification of factor IX (Christmas factor) by using conformation-specific antibodies directed against the factor IX-metal complex

Factor IX is a vitamin K-dependent blood clotting zymogen that is functionally defective or absent in patients with hemophilia B. A method of immunoaffinity chromatography has been developed for a one-step high yield purification of factor IX directly from plasma. The technique utilizes conformation-specific antibodies that bind solely to the metal-stabilized factor IX conformer, but not to the conformer of factor IX found in the absence of metal ions. Anti-factor IX-Ca(II) antibodies were immobilized on an agarose matrix. Human plasma in the presence of 7.5 mM MgCl2 was applied to the antibody-agarose column. The factor IX that binds to these antibodies was specifically eluted by metal chelation with EDTA. This immunopurification resulted in a 10,000-fold one-step purification of the fully functional zymogen. Purified factor IX yielded a single band upon gel electrophoresis in Na-DodSO4 and had a specific activity of 120-150 units/mg. The purified factor IX was separated from other vitamin K-dependent blood clotting proteins and hepatitis virus; no activated factor IX was detected. This method has application for the large scale purification of factor IX for the treatment of hemophilia B.

Monoclonal antibodies directed against human alpha-thrombin and the thrombin-antithrombin III complex

Human alpha-thrombin was poorly immunogenic in Balb/c mice. Nevertheless, following fusion of spleen cells from a responding mouse with NS-1 cells, 8 mouse monoclonal antibodies against alpha-thrombin were isolated, and 6 were characterised. Five of these were isotype IgG2a, and one was IgG1. One, EST 1, bound thrombin only minimally, and was directed against a neoantigen on the thrombin-ATIII (T-AT) complex. This antibody also recognised a site on prothrombin, though with much lower affinity. Its binding was markedly temperature-dependent, indicating a requirement for molecular mobility. A second antibody, EST 4, would not bind the T-AT complex. It inhibited both the clotting and amidase activities of thrombin, and modification of the active site histidine, but not the active site serine, reduced the affinity constant of binding to EST 4. This antibody appears to be directed against an epitope in the vicinity of the enzyme active site. The epitopes for EST 1 and EST 4 were both remote from those of the other monoclonal antibodies, EST 2, 6, 7 and 8. These four competed with each other for binding to thrombin, and all inhibited clotting but not amidase activity. Thrombin binding was not affected by modification of the active site, though formation of the T-AT complex reduced the affinity of binding to EST 6 and EST 8. These monoclonals recognise epitopes in the region of the fibrinogen binding site.

A gel electrophoresis method for epitope mapping studies with monoclonal antibodies

The linear polyacrylamide gradient gel electrophoresis method of Lambin and Fine (1979, Anal. Biochem. 98, 160-168) has been adapted for estimation of the molecular weights of antigen-antibody complexes, thereby providing information useful in epitope mapping studies with monoclonal antibodies. The method has been applied to mapping of the epitopes recognized by four different monoclonal antibodies raised against rat brain hexokinase (1984, Finney et al., J. Biol. Chem., 259, 8232-8237). The results obtained with the gel electrophoresis method were in agreement with epitope mapping studies conducted by the molecular sieve high-performance liquid chromatographic method of Crawford et al., (1982, Proc. Natl. Acad. Sci. USA 79, 7031-7035). Epitope mapping by the gel electrophoretic method offers several advantages in terms of speed, convenience, and economy when compared with alternative procedures that have been described.