Thiol-disulfide interchange in the binding of bence jones proteins to alpha-antitrypsin, prealbumin, and albumin

Indirect effect of alpha-1-antitrypsin on endotoxin-induced IL-1β secretion from human PBMCs

Human alpha-1-antitrypsin (AAT) encoded by the SERPINA1 gene, is an acute phase glycoprotein that regulates inflammatory responses via both protease inhibitory and non-inhibitory activities. We previously reported that AAT controls ATP-induced IL-1β release from human mononuclear cells by stimulating the release of small bioactive molecules. In the current study, we aimed to elucidate the identity of these putative effectors released from human PBMCs in response to AAT, which may inhibit the LPS-induced release of IL-1β. We pre-incubated human PBMCs alone or with different preparations of AAT (4 mg/ml) for 30 min at 37°C, 5% CO₂, and collected cell supernatants filtered through centrifugal filters (cutoff 3 kDa) to eliminate AAT and other high molecular weight substances. Supernatants passed through the filters were used to culture PBMCs isolated from the autologous or a heterologous donors with or without adding LPS (1 μg/ml) for 6 h. Unexpectedly, supernatants from PBMCs pre-incubated with AAT (Zemaira^®), but not with other AAT preparations tested or with oxidized AAT (Zemaira^®), lowered the LPS-induced release of IL-1β by about 25%–60% without affecting IL1B mRNA. The reversed-phase liquid chromatography coupled with mass spectrometry did not confirm the hypothesis that small pro-resolving lipid mediators released from PBMCs after exposure to AAT (Zemaira^®) are responsible for lowering the LPS-induced IL-1β release. Distinctively from other AAT preparations, AAT (Zemaira^®) and supernatants from PBMCs pre-treated with this protein contained high levels of total thiols. In line, mass spectrometry analysis revealed that AAT (Zemaira^®) protein contains freer Cys232 than AAT (Prolastin^®). Our data show that a free Cys232 in AAT is required for controlling LPS-induced IL-1β release from human PBMCs. Further studies characterizing AAT preparations used to treat patients with inherited AAT deficiency remains of clinical importance.

Thiol-reactive compounds prevent nonspecific antibody binding in immunohistochemistry

Nonspecific antibody binding is the primary source of confounding background in immunohistochemistry (IHC). Based on observed patterns of background staining, and the known spontaneous reduction of immunoglobulin disulfide bonds in vivo and in vitro, we tested the hypothesis that nonspecific antibody binding in IHC is mediated by sulfhydryl interactions. Coincubation of primary antibodies with reduced glutathione (GSH), L-cysteine, iodoacetic acid, Ellman's reagent and other thiophilic reagents in pH 8 tris-EDTA (TE) buffer inhibited background staining. In contrast, oxidized glutathione (GSSG) exerted no effect. When empirically optimized, coincubation of GSH with primary antibodies significantly improved IHC signal:noise ratio. Tissue preincubation with mercaptans, soft and borderline metals, and other sulfhydryl-reactive reagents also inhibited background staining, but at the expense of target sensitivity. ELISA results confirmed direct binding between murine serum antibodies and GSH in a nonantigen-dependent manner. In summary, thiol-reactive compounds prevent nonspecific antibody binding in IHC. We propose a mechanism whereby nonspecific background resulting from formation of disulfide bridges and other sulfhydryl bonds between primary antibodies and tissue side groups is interrupted by prior exposure to thiol-reactive reagents such as GSH. These findings provide a molecular basis to improve the specificity of IHC and other immunoassays, and hold implications for antibody-based clinical diagnostics and therapeutics.

Fusion of insulin receptor ectodomains to immunoglobulin constant domains reproduces high-affinity insulin binding in vitro

A unique feature of the insulin receptor is that it is dimeric in the absence of ligand. Dimerization of two adjacent transmembrane domain (TMD) alpha helices has been shown to be critical in receptor kinase activation. Moreover, previous work has suggested that the TMD is involved in stabilizing the high-affinity binding site; soluble receptors expressed after simple truncation at the ectodomain-TMD junction have reduced affinity for insulin. To further examine this issue, we have replaced the TMD and intracellular domain of the soluble human insulin receptor (HIRs) with constant domains from immunoglobulin Fc and lambda subunits (HIRs-Fc and HIRs-lambda). Studies of receptor biosynthesis and binding characteristics were performed following transient transfection of receptor cDNAs into human embryonal kidney 293 cells. Each hybrid receptor was initially synthesized as a single chain proreceptor, followed by cleavage into alpha- and beta-Fc or beta-lambda subunits. The majority of secreted protein appeared in the cell medium as fully processed heterotetramer. Fc fragments released from HIRs-Fc by papain digestion and analyzed by nonreducing SDS-polyacrylamide gel electrophoresis were dimeric. Furthermore, dissociation constants for both chimeras were similar to those for the full-length holoreceptor (wild-type receptor, Kd1 = 200 pM and Kd2 = 2 nM; HIRs-Fc, Kd1 = 200 pM and Kd2 = 40 nM; and HIRs-lambda, Kd1 = 200 pM and Kd2 = 5 nM). These results extend previous observations that dimerization of the membrane-proximal ectodomain is necessary to maintain an intact high-affinity insulin-binding site.

Human alpha 1-proteinase inhibitor binds to extracellular matrix in vitro

alpha 1-Proteinase inhibitor (alpha 1-PI) is the major endogenous inhibitor of human leukocyte elastase (HLE). We have employed two different methods to quantitate the binding of alpha 1-PI to extracellular matrix (ECM), composed of 51% glycoproteins and proteoglycans, 37% types I and III collagen, and 12% elastin, derived from rat heart smooth muscle cells. alpha 1-PI is tightly bound to ECM via a saturable adsorption process; the bound protein fails to dissociate from the matrix after repeated washing. Binding of alpha 1-PI is unaffected by the prior removal of ECM glycoproteins with trypsin. Binding to ECM is not decreased in the presence of high salt but is decreased at low pH. A 40-fold excess of unlabeled alpha 1-PI displaces only 50% of [125I]alpha 1-PI prebound to ECM. A 30% decrease in the levels of alpha 1-PI bound to ECM is observed after DTT washes of ECM preincubated with alpha 1-PI or when alpha 1-PI is modified with iodoacetamide prior to incubation with ECM, implying that a fraction of bound alpha 1-PI is covalently linked to ECM via disulfide bond formation. Moreover, high molecular weight complexes between [125I]alpha 1-PI and ECM components can be visualized by SDS-PAGE under nonreducing conditions but disappear upon reduction. Approximately 50% of the total alpha 1-PI bound covalently or noncovalently to ECM retains the ability to inhibit HLE-mediated ECM proteolysis. alpha 1-PI-HLE complexes bound to ECM can be visualized by SDS-PAGE following the addition of HLE to ECM that was pretreated with [125I]alpha 1-PI. alpha 1-PI from normal plasma or serum also binds to ECM with retention of immunoreactivity and partial retention of inhibitory activity. However, ECM pretreated with alpha 1-PI-deficient serum retains no HLE-inhibitory activity.

Fragmentation and polymeric complexes of albumin in human urine

Analysis of urine proteins of some individuals with proteinuria by SDS-PAGE and silver staining revealed protein bands in urine which did not appear to be present in plasma. The bands migrated with apparent molecular weights of 260 000, 180 000, 110 000, 45 000, 40 000, 30 000, 24 000, 18 000 and 11 000. These bands were shown to be albumin polymer and fragments by using a polyclonal antibody to (a) immunoprecipitate radiolabelled urine proteins, and (b) identify bands blotted from SDS-PAGE gels onto nitrocellulose paper. The specificity of the polyclonal anti-albumin antibody was confirmed by using two mouse monoclonal antibodies raised against human albumin which, between them, recognized the same protein bands on nitrocellulose paper as did the polyclonal antibody. The results of these studies of albumin in human urine confirm that albumin exists as polymer and also show that albumin fragmentation occurs in urine. Fragmentation occurs by proteolysis of the albumin molecule both at sites within and outside disulfide loops. The predominant cleavage site appears to be approximately two-fifths of the distance from one end of the albumin molecule to produce disulfide-linked fragments of about 45 000 and 30 000 molecular weight.

Polymeric complexes and fragments of albumin in normal human plasma

Nitrocellulose blots of normal human plasma proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were examined for polymeric complexes and fragments of albumin using an immunoperoxidase-labelled mouse monoclonal anti-human albumin antibody. Under reducing conditions, no polymeric complexes were seen. Under non-reducing conditions, polymeric complexes were detected at the following molecular weights: 210 000, 168 000, 147 000, 132 000, and 110 000. These probably represent both homo- and heteropolymers of albumin. Fresh plasma samples were also analyzed by S-200 chromatography with the same results indicating that detection of polymeric complexes was not an artifact of the sodium dodecyl sulfate-polyacrylamide gel electrophoresis technique. In quantitative terms, polymeric complexes constituted 0.3-2.8% of the total albumin present. Fragments of albumin were also seen in normal human plasma with molecular weights of 45 000, 28 000 and 19 000. These fragments probably represent breakdown products of albumin in normal blood, and they constituted less than 2% of the total albumin present.

Existence of disulfide-bonded IgM-IgA, IgM-IgG and IgM-IgG fragment complexes in one patient

The existence of disulfide-bonded immunoglobulin (Ig) complexes of IgM-IgA, IgM-IgG, and IgM-IgG, in addition to a monoclonal IgM (kappa) paraprotein, free kappa chains and fragments of IgG (FIgG), were found in the plasma of a multiple gammopathy patient. This is the first report on the interclass disulfide-bonded Ig complexes. Upon exposure to a dissociating buffer containing 6 M urea, 0.1 M Tris-HCl, pH 8.0, a small portion of the IgM-IgA complexes and most of the IgM-IgG as well as the IgM-FIgG complexes dissociated, but the majority of the IgM-IgA and small amounts of other complexes remained intact. These intact complexes were not the result of antigen-antibody reactions and were apparently held together by disulfide bonds which could be broken into monomeric Ig basic units (H-L)2 and smaller components by mild reduction and alkylation.

Disulfide linking of albumin to the hinge region of immunoglobulin G in normal human serum

A protein-protein complex from human serum was isolated and characterized as a two-component system, containing albumin and IgG in a 1:1 mole ratio. The observations reported here suggest that the albumin-IgG complex may have formed through intermolecular disulfide bonds between albumin and IgG. Immunoelectrophoretic analysis of papain-digested fragments of albumin-IgG complex revealed that the combining sites for albumin may be located in the hinge region between the Fab and the Fc fragments of IgG.

The microheterogeneities of thyroxine-binding globulin and alpha 1 protease inhibitor (alpha 1-antitrypsin) are interrelated

Addition of 125I-thyroxine to serum allows autoradiography for thyroxine-binding globulin microheterogeneity to be carried out after isoelectric focusing has been performed to display (by protein stain) the heterogeneous bands of the alpha 1-antitrypsin (PI) system. Comparison of the protein stain for PI with the autoradiograph for thyroxine-binding globulin indicates that these two systems are interrelated with the major bands of the PI system corresponding to the bands on the autoradiograph. This correspondence holds for PI variants other than the common M type and in particular it holds for the deficient Z type in which the autoradiograph for thyroxine-binding globulin is strikingly different from normal. We conclude that the major cause of microheterogeneity of TBG is due to an association with the PI system under the conditions of isoelectric focusing as normally performed. Precipitation experiments with antisera to PI and TBG suggest that the complex between these biologically important globulins may occur under conditions other than isoelectric focusing, but further work will be needed to examine this possibility.

Studies on the disulfide region of alpha 1-protease inhibitor

The single disulfide bond of purified human alpha 1-protease inhibitor was reduced with dithiothreitol in the absence of denaturant and the resultant sulfhydryl groups were alkylated with iodoacetamide-1-C14. The product was found to be fully functional as an inhibitor of trypsin and elastase in esterolytic and proteolytic assays. The modified protein was also found to be nearly identical to native alpha 1-protease inhibitor when analyzed by immunological, electrophoretic, and spectral methods. The performic acid oxidized inhibitor, on the other hand, was devoid of any enzyme inhibitory activity. Analysis of the derivatized protein by amino acid analysis and by radioactive counting revealed only a single cysteine-containing peptide. The alkylated inhibitor was digested with cyanogen bromide and then trypsin, and subjected to two-dimensional peptide mapping. A single cysteine-containing peptide was recovered and shown to have the sequence Phe-Asn-Ile-Gln-His-Cys-Lys. A variety of experiments involving gel filtration or dialysis of reduced or oxidized alpha 1-protease inhibitor indicate that this Cys-peptide is covalently bound to either free cysteine or to glutathione via a disulfide bridge.

Use of 2-mercaptoethanol to facilitate detection and classification of IgM abnormalities by immunoelectrophoresis

IgM immunoglobulins in elevated serum concentrations have a tendency to polymerize and form aggregates. When subjected to immunoelectrophoreses these proteins may deposit at the point of origin. This can result in failure to detect an IgM abnormality or the masking of other serum protein abnormalities migrating near the area of application. This paper demonstrates the importance of using reducing agents such as 2-mercaptoethanol not only to detect and/or to confirm the monoclonal IgM gammopathies but also to unmask other protein abnormalities.

Properties of isolated human alpha1-antitrypsins of Pi types M, S and Z

1. alpha1-Antitrypsin contains a single thiol group partly blocked in native plasma and reactive after mild reduction. 2. Human alpha1-antitrypsins of Pi types F, M, S and Z have been isolated with native microheterogeneity using thiol-disulfide (SH-SS) interchange reactions utilizing the reactive thiol group. 3. The pI of the various microheterogeneous fractions are given for protein M. Stepwise desialylation of alpha1-antitrypsin indicates that the charge difference between the major fractions is one sialic acid residue between each. This is further supported by the pI changes obtained on substitution of the single thiol with positively or negatively charged compounds. 4. Desialyation of purified proteins from each Pi type converts the individual microheterogeneous fractions to one major fraction. The pI shift for the variants studied indicate a difference of plus or minus one or two charge units between protein M and the variants. 5. A difference of one sialic acid residue was obtained for proteins M and Z by the thiobarbituric assay, but stepwise removal of sialic acid with neuraminidase revealed almost identical stepwise change of pattern of both proteins indicating the same number of sialic acid residues. 6. Electrofocusing has been used to identify CNBr fragments from proteins M, S and Z. 7. An amino acid substitution has been found to be located in one of the eight CNBr fragments, glutamic acid in protein M is substituted by lysine in protein Z. 8. The average concentration of alpha1-antityprsin in plasma from healthy males was found to be 1.32 g/1.

Studies on the pre-alpha-lipoprotein of human serum. II. Evidence for the presence of an albumin-apo-A-I complex

By combined column affinity chromatography and preparative electrophoresis, a lipoprotein was isolated from the electrophoretically defined pre-alpha-region. The isolated fraction, designated Fraction II1, demonstrated one band after electrophoresis in agarose, in polyacrylamide, and in sodium dodecylsulphate containing polyacrylamid. Double diffusion experiments disclosed the presence of albumin and apolipoprotein-A-I within the fraction. After reduction with mercaptoethanol and subsequent electrophoresis in sodium dodecylsulphate containing polyacrylamide gel, two bands appeared. One of the bands had an electrophoretic mobility similar to albumin monomer (mol.wt 67,000), the other had the same electrophoretic mobility as apolipoprotein-A-I with a mol.wt of 28,400. It is suggested that Fraction II1 contains an albumin-apolipoprotein-A-I complex.

Studies on human alpha-fetoprotein. Isolation and characterization of monomeric and polymeric forms and amino-terminal sequence analysis

Human alpha-fetoprotein has been isolated from the serum and ascitic fluid of a patient with hepatoma by a combination of immunoadsorbent column chromatography and Sephadex G-150 gel filtration. Human alpha-fetoprotein is a sialylated glycoprotein with an estimated molecular weight of 67 500, composed of a single-chain polypeptide of approximately 580 amino acid residues and 3.6% carbohydrate. It is a negatively charged protein with an acid isoelectric point (pH 4.57). In addition to the monomeric form of alpha-fetoprotein, we have identified human alpha-fetoprotein polymers, including dimeric and trimeric forms, which dissociate to the monomer only upon exposure to disulfide-reducing reagents, implying that their formation is dependent upon intermolecular disulfide bonds. These polymers are found in human alpha-fetoprotein isolated by isoelectric focusing in both the major (pI 4.57) and minor (pI 5.2) alpha-fetoprotein fractions. The first 17 residues of the NH2-terminal amino acid sequence of the hepatoma-derived human alpha-fetoprotein have been identified. Fetal alpha-fetoprotein is indistinguishable from hepatoma alpha-fetoprotein by several criteria, including immunoelectrophoresis, acryalmide gel electrophoresis, and proclivity for dimerization.

Isolation and characterization of human plasma alpha 1-proteinase inhibitor and a conformational study of its interaction with proteinases

1. alpha 1-Proteinase inhibitor was isolated from human plasma by a five-step procedure. Isoelectric focusing showed that six components focused between pH4.85 and 4.95. 2. The mol.wt. of the inhibitor was 52000 by sedimentation equilibrium and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The amino acid and carbohydrate compositions of the inhibitor were also determined. 3. The far-u.v.c.d. (circular-dichroism) spectrum indicated that the inhibitor had about 36% alpha-helical content. 4. The loss of proteinase-inhibitory activity when the inhibitor was exposed to pH values less than 5.0 or greater than 10.5 was accompanied by small changes in the far-u.v.c.d. spectrum and large changes in the near-u.v.c.d. spectrum. The change at alkaline pH was associated with ionization of tyrosine residues. 5. Interaction of inhibitor with chymotrypsin caused perturbation of the c.d. spectrum and this was used to follow the interaction and show a 1:1 stoicheiometry. 6. C.d., electrophoresis and isoelectric focusing showed that the inhibitor-enzyme complex is degraded by free enzyme. 7. Parallel studies with trypsin indicated that it too forms a 1:1 complex with inhibitor and is degraded by excess of enzyme.

Spontaneous in vivo isomerization of bovine serum albumin as a determinant of its normal catabolism

By combination of isoelectric focusing and immunoelectrophoresis of fresh bovine plasma it is shown that 10% of the albumin in plasma has isoionic points equal to the intramolecular SS-interchanged isomers of bovine serum albumin (BSA). It is also shown that (a) albumin with the isoionic point of SS-interchanged BSA is produced in the cow from radioiodinated BSA depleted from SS-interchanged albumin before injection and (b) purified radiolabeled SS-interchanged BSA can be converted in vivo to albumin with the native isoionic point. On this basis, it is proposed that SS-interchanged albumin in vivo is in postsynthetic equilibrium with the "native" albumin conformation. The SS-interchanged isomers purified either from commerical BSA or from BSA submitted to SH-SS interchange was, after radioiodination with 125I, compared metabolically with "native" albumin labeled with 131I in the same calf. Both species of SS-interchanged albumins have fast initial turnover rates but obtain a normal rate of degradation after the reversion to native albumin. If the isomers formed in vivo have the same properties as the ones present in commercial BSA, at least 50% of the physiological degradation of albumin can be accounted for by the 6-7 times faster catabolic rates of these isomers.

Purification of alpha1-antitrypsin from plasma through thiol-disulfide interchange

1. Monomeric nu-chains were conjugated with CNBr-activated Sepharose 4B. The C-terminal cysteine of the conjugated nu-chain was converted to a mixed disulfide with 3-carboxy-4-nitro-benzenethiol (Nbs) and used to separate plasma proteins with reactive thiol groups. The plasma proteins, alpha1-antitrypsin and prealbumin have the greatest affinity for the interchange reaction with mixed disulfides. The disulfide link between alpha1-antitrypsin and nu-chain is sensitive to excess Nbs, and is selectively cleaved in the presence of 5,5'-dithiobis(2-nitrobenzoate) (Nbs2) which accepts the sulfhydryl group of alpha1-antitrypsin. 2. A Simple method developed for the isolation of human alpha1-antitrypsin was equally effective for the various inherited phenotypes and for alpha1-antitrypsin from the dog, baboon, and monkey, Glutathione-Sepharose was also used successfully, but the nu-chain conjugate yielded alpha1-antitrypsin less contaminated with mercaptalbumin and prealbumin. 3. The alpha1-antitrypsin is harvested from this procedure as a mixed disulfide with Nbs. The negative charge of Nbs at pH 8.1 causes an increased electrophoretic mobility of the alpha1-antitrypsin derivative. Mild reduction liberates Nbs and electrophoretic mobility of alpha1-antitrypsin returns to normal. The method described can increase the alpha1-antitrypsin content of a plasma fraction from 5% of the total protein to 95% within one day with a yield of about 50%. This purification procedure does not exert any detectable effect on microheterogeneity.