Protein-protein coupling reactions and the applications of protein conjugates

Chemical and Enzymatic Methods for Post-Translational Protein-Protein Conjugation

Fusion proteins play an essential role in the biosciences but suffer from several key limitations, including the requirement for N-to-C terminal ligation, incompatibility of constituent domains, incorrect folding, and loss of biological activity. This perspective focuses on chemical and enzymatic approaches for the post-translational generation of well-defined protein-protein conjugates, which overcome some of the limitations faced by traditional fusion techniques. Methods discussed range from chemical modification of nucleophilic canonical amino acid residues to incorporation of unnatural amino acid residues and a range of enzymatic methods, including sortase-mediated ligation. Through summarizing the progress in this rapidly growing field, the key successes and challenges associated with using chemical and enzymatic approaches are highlighted and areas requiring further development are discussed.

Development of an efficient and simple method for conjugation of laccase to immunoglobulin and its characterization by enzyme immunoassay

A new laccase-conjugated antibody is developed, containing laccase as an enzyme marker, obtained from culture supernatant of the basidiomycetous fungi Pleurotus ostreatus. The efficacy of the laccase-conjugated antibody was demonstrated in indirect and direct enzyme immunoassay after using periodate and glutaraldehyde conjugation methods. The assay based on laccase-conjugated antibody is potent to detect antigens when compared with peroxidase conjugated antibody.

Quantitation of epsilon-amino group using amino acids as reference standards by trinitrobenzene sulfonic acid. A simple spectrophotometric method for the estimation of hapten to carrier protein ratio

A simple, sensitive and direct spectrophotometric method is presented for the determination of epsilon-amino groups of L-lysine present in carrier proteins, using the free amino acids L-lysine and L-glutamic acid as reference standards, and 2,4,6-trinitrobenzene 1-sulfonic acid (TNBS) reagent. The amount of epsilon-amino group present in the carrier protein after coupling with hapten is directly quantitated using the standard curve generated by the difference in absorbance observed with L-lysine and L-glutamic acid after their reaction with TNBS reagent. Spectral analysis of L-lysine and L-glutamic acid (27.36 nmol) derivatives of TNBS at 335 nm showed that TNP-L-lysine had twice the absorbance of TNP-L-glutamic acid, since TNBS reagent interacts equally with the alpha-amino and epsilon-amino groups present in L-lysine and the alpha-amino group of L-glutamic acid, respectively. The relationship between absorbance and concentration of epsilon-amino groups (up to 16 micrograms/ml) was found to be linear. The number of epsilon-amino groups of lysine present in carrier proteins such as BSA, HSA, thyroglobulin and the enzyme, horseradish peroxidase, were analyzed by the present method and were found to be similar to the reported values. Various carrier protein-hapten conjugates (protein-mycotoxin/vitamin/steroid hormone conjugates) were made and analyzed by the method developed in order to determine their mole to mole ratio.

The solid phase attachment of fungal hyphae in an ELISA to screen for antifungal antibodies

A method to immobilize fungal hyphae onto the wells of 96-well microplates for use in an in-direct ELISA to screen for antifungal antibodies in sera and cell culture supernatants is described. The hyphae from three genera (Penicillium, Eurotium and Fusarium) were successfully attached by overnight drying onto wells precoated with poly-L-lysine and glutaraldehyde. Microscopy revealed that the hyphae remained attached to the wells throughout the ELISA and antiserum titrations showed that the attached hyphae were uniformly coated and remained reactive. Background absorbances were low and the plates could be stored at -20 degrees C without loss of reactivity.

Establishment of a sensitive radioimmunoassay for serum thymic factor with a special reference to extraction procedures

A radioimmunoassay (RIA) for serum thymic factor (facteur thymique serique) was established. This assay could detect less than 0.5 pg of serum thymic factor without the interference by other thymic hormones, including thymopoietin II and thymosin alpha 1. Both zinc-free and zinc-bound serum thymic factor showed a similar standard curve, indicating that this RIA could detect both forms of serum thymic factor in the same way. We observed that two procedures were required prior to the measurement in order to measure serum thymic factor content in plasmas; the first, the extraction procedure to remove substances interfering with the assay nonspecifically; the second, the storage of the sample with ethylenediamine tetraacetic acid disodium salt to inhibit the proteases in the plasma to degrade serum thymic factor. By employing this assay system, we observed that cord blood plasmas contained significantly higher level of serum thymic factor than did adult blood plasmas.

Functionally-stabilized proteins — A review

The maintenance or stabilization of protein or enzyme function is of vital importance in Biotechnology. Investigations of thermophilic organisms, studies of denaturation and the use of enzymes in organic solvents have each contributed to an understanding of protein stability. Enzymes can reliably and reproducibly be stabilized by variety of means including immobilization, use of additives, chemical modification in solution and protein engineering. Examples of each of these are discussed. With these recent advances it appears that a rational strategy for achieving a particular stabilized enzyme or protein may be within reach.

A dye-photosensitized reaction that generates stable protein-protein crosslinks

Irradiation of fibrinogen with visible light for 30 s in the presence of 1-1000 microM fluorescein was found to crosslink fibrinogen both inter- and intramolecularly. Optimum crosslinking was achieved at dye concentrations of around 100 microM and the amount of crosslinking was shown to increase with pH. Crosslinking was inhibited in the presence of 50 microM tryptophan or tyrosine and enhanced in the presence of 5 mM histidine. Twice as much crosslinking was found to take place under anaerobic conditions. These observations are consistent with a dye-photosensitized reaction following the hydrogen abstraction pathway. The subunits of eukaryotic ribosomes and those of phosphorylase a were also crosslinked by the method described.

The use of N-[beta-(4-diazophenyl)ethyl]maleimide as a coupling agent in the preparation of enzyme-antibody conjugates

The present study was undertaken to develop a novel method for the enzyme labeling of antibodies. Goat anti-rabbit IgG was used as a prototype and coupled to beta-D-galactosidase (Gal) with a new heterobifunctional cross-linking agent N-[beta-(4-diazophenyl)ethyl]maleimide (DPEM). The antibody was first azo-coupled with DPEM to introduce the maleimide groups into the molecule; excess reagent was removed by gel filtration and then the activated antibodies were crosslinked to the thiol groups of Gal. The conjugates were purified by DEAE-cellulose column chromatography, being completely separated from non-reacted antibodies but remaining mixed with free Gal, and showing approximately 30% enzyme activity bound to antibody. This method is simple, reproducible and so mild that almost full enzyme and antibody activity can be retained. The conjugates were used as a label in an immunoassay and were able to detect first antibody at concentrations as low as 2 ng/tube. Furthermore, the present method was compared with the method using N-(gamma-maleimidobutyryloxy)succinimide (GMBS), and it was found that both conjugates produced comparable results.

The use of N-[beta-(4-diazophenyl)ethyl]maleimide as a heterobifunctional agent in developing enzyme immunoassay for neurotensin

A heterobifunctional crosslinking agent N-[beta-(4-diazophenyl)ethyl]maleimide (DPEM) was newly synthesized and characterized to possess the maleimide group with a stability greater than that previously reported for N-(4-diazophenyl)maleimide. Using the peptide hormone neurotensin (NT) as a model hapten, DPEM was used in the conjugation reaction with bovine serum albumin (BSA) and with beta-D-galactosidase (beta-Gal) in developing an enzyme immunoassay (EIA) for NT. The NT-DPEM-BSA conjugate elicited anti-NT antibodies in rabbits and the NT-beta-Gal conjugate behaved as an enzyme marker of NT in the EIA. The EIA developed double antibody was reproducible and sensitive in detecting NT at concentrations as low as 30 fmol per tube. The specificity of anti-NT serum seems to be primarily toward the carboxy-terminal region of NT, showing cross-reactions with such NT fragments as NT2-13, NT8-13, and NT1-8 for 120, 22, and less than 0.1%, respectively. The utility of this assay was also demonstrated by measuring the NT immunoreactivity in several rat organs. DPEM could be useful for developing EIAs for other peptide hormones (even those which contain neither a free amino group nor a free carboxyl group), using the imidazole, phenolic, or indole group(s) of amino acids as a binding site for carrier proteins.

Leukocyte interferon conjugated to beta-D-galactosidase for receptor studies: a preliminary report

In this report, we describe a simple procedure for labeling leukocyte interferon (IFN) with the enzyme beta-D-galactosidase (beta-gal), through a low-molecular-weight linking agent. The labeling procedure did not appear to alter the properties of the enzyme, the antiviral activity of the recombinant DNA-derived human clone A leukocyte IFN (rIFN alpha A), or the ability of the IFN to bind to Daudi cells. The binding of the putative IFN-enzyme complex to Daudi cells could be blocked by excess unlabeled IFN or by monoclonal antibody to IFN receptors. Our preliminary results indicate that enzyme labeling of IFN can result in a biologically active agent that has potential for use in the quantitation of circulating IFN and specific receptor proteins. In addition, this labeling procedure may be potentially useful with other immunomodulatory proteins.

Comparison of three conjugation procedures for the formation of tracers for use in enzyme immunoassays

One of the requirements for enzyme immunoassay is the formation of a labelled component of the assay system which can be either the ligand or the binder. Three methods for conjugating alkaline phosphatase to human placental lactogen were investigated, involving water-soluble carbodiimide, glutaraldehyde, and the periodate oxidation technique. Of the 3, the periodate oxidation technique proved superior giving a conjugate with only slight changes in the Michaelis constant (Km) and maximum velocity (Vmax) which could be used for an enzyme immunoassay for human placental lactogen in human plasma.

The influence of formalin and paraffin embedding on the immunohistochemical reaction of monoclonal antibodies applied to female breast tissue

Monoclonal antibodies, raised against purified human milk fat globule membranes, were applied to human tissue and the corresponding antigenic determinants detected by an immunoperoxidase method. In human breast tissue, the presence of antigenic determinants was characterized by considerable heterogeneity although it is confined to epithelial cells and cannot be detected in non-epithelial areas. The effect of fixation and paraffin embedding on the reaction pattern was evaluated on tissue from 25 primary breast carcinomas. For each tumor, both frozen, acetone fixed and fixed, paraffin embedded tissue was available. Four of the eight applied antibodies could detect fewer positive cells in formalin-fixed tissue as compared to frozen, acetone-fixed tissue from the same tumor. The positive peroxidase reaction was better preserved after fixation in Lillie's solution. Treatment of formalin-fixed tissue with pronase did not give a better effect. Antibodies with preserved positive reaction in formalin-fixed breast tissue were applied to a range of epithelial and non-epithelial human malignant tumors of varying origin. A heterogeneous positive reaction could be detected in the majority of tumors derived from epithelial cells, but not in non-epithelial tumors.

Rapid preparation and purification of alkaline phosphatase and 125I-labeled antibody by minicolumn gel centrifugation chromatography

Gel centrifugation chromatography was used for the preparation, purification and concentration of enzyme and 125I-labeled antibody. Low-molecular-weight reactants were rapidly and efficiently removed from either 125I-labeled antibody or enzyme-antibody conjugates by centrifugation of the reaction mixture through a minicolumn of Sephadex. Further purification of both radiolabeled and enzyme labeled antibodies were possible by the application and elution of each labeled antibody through a minicolumn of Protein A Sepharose. Minicolumns were constructed from readily available inexpensive components and allowed the rapid preparation and purification of antibody of high specific enzyme, and radioactivity. No difference between the 125I, or enzyme labeled antibody detection limit could be detected by a direct solid-phase immunoassay.

Rapid micromethod for preparation of enzyme-antibody conjugates

A rapid method is described for labeling antibody with alkaline phosphatase by one-step glutaraldehyde linkage. The method involves the centrifugation of a small volume of an enzyme and antibody mixture through a minicolumn packed with hydrated Sephadex. This procedure rapidly removes ammonium sulfate and glutaraldehyde from the enzyme-antibody mixture and results in the efficient recovery of conjugated antibody without significant dilution.

Molecular forms and hydrodynamic properties of acetylcholine receptor from electric tissue

We have studied purified acetylcholine receptor proteins from Electrophorus electricus and Torpedo marmorata which function in both binding and reconstitution experiments. The molecular properties of these receptor-channel complexes were analyzed under non-denaturing conditions by polyacrylamide gradient gel electrophoresis, ultracentrifugation sedimentation studies and laser light scattering. The purified receptor proteins exist in different interconvertible forms depending on both the type and concentration of detergent present, and the presence or absence of an intersubunit disulfide bridge. Receptor purified in the absence of sulfhydryl-blocking agents exists in two monomeric and two dimeric forms at very low detergent concentrations (0.01-0.05% Tween 80). At intermediate detergent concentrations (0.4% Triton X-100) one monomeric and one dimeric form are present. Only the monomeric form remains at high levels of detergent (2% Triton X-100). This form has a sedimentation coefficient of 9.29 S, as measured by ultracentrifugation using Schlieren optics. If receptor is purified in the presence of sulfhydryl-blocking agents, conversion of dimers into monomers by high concentrations of detergent does not occur. Disulfide-reducing agents convert dimers into monomers independent of whether the receptor's free sulfhydryl groups are blocked or not. These findings imply that dimer formation is primarily due to hydrophobic interactions between monomers. When these interactions are reduced by high levels of detergent the intersubunit disulfide bridge is dissociated at the expense of newly formed interasubunit ones, as long as the sulfhydryl group(s) have remained unblocked. Monomers and dimers bind alpha-cobratoxin with the same affinity and kinetics. Dimers of disulfide-linked monomers are not required for the reconstitution of a functional ion-translocation system. The presence of EDTA during the purification of receptor proteins (as recommended by many groups to inhibit proteolysis) adversely influences the activity of the receptor in channel gating. These observations are discussed in terms of the requirements for a purification procedure yielding receptor preparations unaffected by proteolysis and functioning in both binding and ion translocation. The molecular weight of the receptor monomer was determined by several independent techniques and yielded values in the range of 250-300 x 10(3). With a translational diffusion constant D20, w = 2.95 x 10(-7) cm2 s-1 and a sedimentation coefficient s20, w = 9.29 S, the frictional coefficient ratio f/f0 = 1.51 was calculated for the receptor monomer from Torpedo marmorata. This indicates a considerable asymmetry in the shape of the detergent-solubilized receptor.

Novel preparation method of immunogen for hydrophobic hapten, enzyme immunoassay for daunomycin and adriamycin

The present study was undertaken to develop a novel method of preparing a hydrophobic hapten as immunogen. The anticancer drug, daunomycin (DM) was used as prototype and coupled to mercaptosuccinylated bovine serum albumin (MS, BSA) with N-(gamma-maleimidobutyryloxy)succinimide (GMBS). Injection of rabbits with a conjugate (DM-GMBS-MS.BSA) which contained 7.3 DM per BSA molecule produced good levels of anti-DM antibody which was detected by the reaction of diluted antiserum with beta-D-galactosidase-labeled DM. beta-D-galactosidase-labeled DM was used to develop a double antibody enzyme immunoassay for DM and for the DM homologue adriamycin (AM) which reproducibly detected as little as 1 pmole of either drug. A variety of commonly used other anticancer drugs were tested and had little reactivity in this immunoassay. These studies indicate that the anti-DM serum produced is highly specific.

Conjugation of enzymes to immunoglobulins using dimaleimides

A method is described for coupling enzymes to immunoglobulins using sulphydryl derivatives of the proteins and a dimaleimide which is relatively water-soluble. Parameters affecting the performance of the conjugates have been examined including level of sulphydryl incorporation, ratio of enzyme/immunoglobulin and nature of dimaleimide used. Peroxidase-immunoglobulin conjugates made by the dimaleimide method have been compared with those made by the periodate oxidation method and found to have a superior performance. Immunoglobulin has been linked to peroxidase (horseradish peroxidase, EC 1.11.1.7), glucose oxidase from Aspergillus niger, (EC 1.1.3.4), penicillinase from Bacillus cereus beta-lactamase I (EC 3.5.2.6), and beta-galactosidase from Escherichia coli (EC 3.2.1.23).

Enzyme immunoassay: a review

In the last few years, the use of enzyme labels in immunoassays has been investigated. The aim of this review is to evaluate critically the role of such labels in clinical biochemistry. Special attention has been given to the problems involved in preparing enzyme labels and the ways in which such labels can be used in a variety of heterogeneous and homogeneous assay systems.

A method for enhancing the stability of thiol-containing enzymes immobilised on nylon

The limited stability of immobilised enzymes is a major factor restricting their use in clinical chemistry. The immobilisation procedure described here affords a simple method of enhancing the stability of certain enzymes immobilised on nylon. Studies with alcohol dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase have indicated that a significant increase in stability can be obtained using this immobilisation procedure as compared with conventional procedures. As anticipated the immobilisation procedure was unsatisfactory for oxidases such as glucose oxidase.