Characterisation of a trisulphide derivative of biosynthetic human growth hormone produced in Escherichia coli

Impact of Trisulfide on the Structure and Function of Different Antibody Constructs

Trisulfide is a post-translational modification (PTM) commonly found in recombinant antibodies. It has been demonstrated that trisulfide had no impact on the bioactivity of mono-specific antibodies (MsAbs). However, the impact of trisulfide on multi-specific antibodies has not been evaluated. In this study, two mass spectrometric methods were developed for comprehensive trisulfide characterization. The non-reduced peptide mapping method combined with the unique electron activated dissociation (EAD) provided signature fragments for confident trisulfide identification as well as trisulfide quantitation at individual sites. A higher throughput method using Fab mass analysis was also developed and qualified to support routine monitoring of trisulfide during process development. Fab mass analysis features simpler sample preparation and shorter analysis time but provides comparable results to the non-reduced peptide mapping method. In this study, a bi-specific (BsAb) and a tri-specific antibody (TsAb) were compared side-by-side with a MsAb to evaluate the impact of trisulfide on the structure and function of multi-specific antibodies. Results indicated that trisulfide dominantly formed at similar locations across different antibody constructs and had no impact on the size heterogeneity, charge heterogeneity, or bioactivities of any assessed antibodies. Together with the in vitro stability under heat stress (25 °C and 40 °C for up to four weeks) and rapid conversion from trisulfide to disulfide during in vivo circulation, trisulfide could be categorized as a non-critical quality attribute (non-CQA) for antibody products.

Stabilities of Three Key Biological Trisulfides with Implications for Their Roles in the Release of Hydrogen Sulfide and Bioaccumulation of Sulfane Sulfur

Trisulfides and higher polysulfides are important in the body due to their function as key reservoirs of sulfane sulfur and their rapid reactions to release persulfides. Recent work has shown that persulfides act as powerful antioxidants and release hydrogen sulfide, an emerging gasotransmitter with numerous therapeutic effects. Despite the important role of polysulfides, there is a lack of understanding of their stabilities in aqueous systems. To investigate the reactivity of trisulfides and polysulfides, three key biologically important trisulfides were synthesized from cysteine, glutathione, and N-acetylcysteine, and the tetrasulfide of N-acetylcysteine was synthesized as a representative polysulfide. The stabilities of sulfides were monitored in buffered D2O using 1H NMR spectroscopy under a range of conditions including high temperatures and acidic and alkaline environments. The tri- and tetrasulfides degraded rapidly in the presence of primary and tertiary amines to the corresponding disulfide and elemental sulfur. The half-lives of N-acetylcysteine tri- and tetrasulfides in the presence of butylamine were 53 and 1.5 min, respectively. These results were important because they suggest that tri- and tetrasulfide linkages are short-lived species in vivo due to the abundance of amines in the body. Under basic conditions, cysteine and glutathione trisulfides were unstable due to the deprotonation of the ammonium group, exposing an amine; however, N-acetylcysteine trisulfide was stable at all pH values tested. Hydrogen sulfide release of each polysulfide in the presence of cysteine was quantified using a hydrogen sulfide-sensitive electrode and 1H NMR spectroscopy.

Optimization of expression, purification and secretion of functional recombinant human growth hormone in Escherichia coli using modified staphylococcal protein a signal peptide

Background

Human Growth Hormone (hGH) is a glycoprotein released from the pituitary gland. Due to the wide range of effects in humans, any disruption in hGH secretion could have serious consequences. This highlights the clinical importance of hGH production in the treatment of different diseases associated with a deficiency of this hormone. The production of recombinant mature hormone in suitable hosts and secretion of this therapeutic protein into the extracellular space can be considered as one of the best cost-effective approaches not only to obtain the active form of the protein but also endotoxin-free preparation. Since the natural growth hormone signal peptide is of eukaryotic origin and is not detectable by any of the Escherichia coli secretory systems, including Sec and Tat, and is therefore unable to secrete hGH in the prokaryotic systems, designing a new and efficient signal peptide is essential to direct hGh to the extracellular space.

Results

In this study, using a combination of the bioinformatics design and molecular genetics, the protein A signal peptide from Staphylococcus aureus was modified, redesigned and then fused to the mature hGH coding region. The recombinant hGH was then expressed in E. coli and successfully secreted to the medium through the Sec pathway. Secretion of the hGH into the medium was verified using SDS-PAGE and western blot analysis. Recombinant hGH was then expressed in E. coli and successfully secreted into cell culture medium via the Sec pathway. The secretion of hGH into the extracellular medium was confirmed by SDS-PAGE and Western blot analysis. Furthermore, the addition of glycine was shown to improve hGH secretion onto the culture medium. Equations for determining the optimal conditions were also determined. Functional hGH analysis using an ELISA-based method confirmed that the ratio of the active form of secreted hGH to the inactive form in the periplasm is higher than this ratio in the cytoplasm.

Conclusions

Since the native signal protein peptide of S. aureus protein A was not able to deliver hGH to the extracellular space, it was modified using bioinformatics tools and fused to the n-terminal region of hGh to show that the redesigned signal peptide was functional.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12896-021-00701-x.

Hydrogen Sulfide Oxidation by Sulfide Quinone Oxidoreductase

Hydrogen sulfide (H2 S) is an environmental toxin and a heritage of ancient microbial metabolism that has stimulated new interest following its discovery as a neuromodulator. While many physiological responses have been attributed to low H2 S levels, higher levels inhibit complex IV in the electron transport chain. To prevent respiratory poisoning, a dedicated set of enzymes that make up the mitochondrial sulfide oxidation pathway exists to clear H2 S. The committed step in this pathway is catalyzed by sulfide quinone oxidoreductase (SQOR), which couples sulfide oxidation to coenzyme Q10 reduction in the electron transport chain. The SQOR reaction prevents H2 S accumulation and generates highly reactive persulfide species as products; these can be further oxidized or can modify cysteine residues in proteins by persulfidation. Here, we review the kinetic and structural characteristics of human SQOR, and how its unconventional redox cofactor configuration and substrate promiscuity lead to sulfide clearance and potentially expand the signaling potential of H2 S. This dual role of SQOR makes it a promising target for H2 S-based therapeutics.

Trisulfide-Bearing PEG Brush Polymers Donate Hydrogen Sulfide and Ameliorate Cellular Oxidative Stress

A potential approach to combat cellular dysfunction is to manipulate cell communication and signaling pathways to restore physiological functions while protecting unaffected cells. For instance, delivering the signaling molecule H₂S to certain cells has been shown to restore cell viability and re-normalize cell behavior. We have previously demonstrated the ability to incorporate a trisulfide-based H₂S-donating moiety into linear polymers with good in vitro releasing profiles and demonstrated their potential for ameliorating oxidative stress. Herein, we report two novel series of brush polymers decorated with higher numbers of H₂S-releasing segments. These materials contain two trisulfide-based monomers co-polymerized with oligo(ethylene glycol methyl ether methacrylate) via reversible addition-fragmentation chain-transfer polymerization. The macromolecules were characterized to have a range of trisulfide densities with similar, well-defined molecular weight distribution, good H₂S-releasing profiles, and high cellular tolerance. Using an amperometric technique, the H₂S liberated and total sulfide release were found to depend on concentrations and chemical nature of triggering molecules (glutathione and cysteine) and, importantly, the position of reactive groups within the brush structure. Notably, when introduced to cells at well-tolerated doses, two macromolecular donors which have the same proportion as of the H₂S-donating monomer (30%) but differ in releasing moiety location show similar cellular H₂S-releasing kinetics. These donors can restore reactive oxygen species levels to baseline values, when polymer pretreated cells are exposed to exogenous oxidants (H₂O₂). Our work opens up a new aspect in preparing H₂S macromolecule donors and their application to arresting cellular oxidative cascades.

Dismantling and Rebuilding the Trisulfide Cofactor Demonstrates Its Essential Role in Human Sulfide Quinone Oxidoreductase

Sulfide quinone oxidoreductase (SQOR) catalyzes the first step in sulfide clearance, coupling H₂S oxidation to coenzyme Q reduction. Recent structures of human SQOR revealed a sulfur atom bridging the SQOR active site cysteines in a trisulfide configuration. Here, we assessed the importance of this cofactor using kinetic, crystallographic, and computational modeling approaches. Cyanolysis of SQOR proceeds via formation of an intense charge transfer complex that subsequently decays to eliminate thiocyanate. We captured a disulfanyl-methanimido thioate intermediate in the SQOR crystal structure, revealing how cyanolysis leads to reversible loss of SQOR activity that is restored in the presence of sulfide. Computational modeling and MD simulations revealed an ∼10⁵-fold rate enhancement for nucleophilic addition of sulfide into the trisulfide versus a disulfide cofactor. The cysteine trisulfide in SQOR is thus critical for activity and provides a significant catalytic advantage over a cysteine disulfide.

An overview of the bacterial SsrA system modulating intracellular protein levels and activities

In bacteria, the truncated forms of mRNAs, which usually lack a stop codon, are occasionally generated by premature termination of gene transcription and/or endo- or exonucleolytic cleavage events. Ribosomes proceeding on these molecules stall at the 3' end of the chain and are rescued by a widely distributed mechanism known as trans-translation, which includes two essential elements, ssrA RNA (a special RNA) and SmpB (a small protein). Through this mechanism, the polypeptides translated from truncated mRNAs are marked by a short peptide, known as SsrA tag, at their C-termini and directed to the specific endogenous proteases for C-terminal proteolysis. Based on the deep understanding of the SsrA tagging and degradation mechanisms, recently a series of SsrA-based genetic tools have been developed for gene regulation on the level of post-translation. They are successfully applied for controllable regulation of biological circuits in bacteria. In the present article, we systematically summarize the history, structural characteristics, and functional mechanisms of the SsrA tagging and degrading machineries, as well as their technical uses and limitations.Key Points• SsrA system plays an important role in ribosome rescue in bacteria.• SsrA-based genetic tools are useful for controlling protein levels and activities.

Thiyl Radical Reactions in the Chemical Degradation of Pharmaceutical Proteins

Free radical pathways play a major role in the degradation of protein pharmaceuticals. Inspired by biochemical reactions carried out by thiyl radicals in various enzymatic processes, this review focuses on the role of thiyl radicals in pharmaceutical protein degradation through hydrogen atom transfer, electron transfer, and addition reactions. These processes can lead to the epimerization of amino acids, as well as the formation of various cleavage products and cross-links. Examples are presented for human insulin, human and mouse growth hormone, and monoclonal antibodies.

Trisulfides over disulfides: highly selective synthetic strategies, anti-proliferative activities and sustained H2S release profiles

Highly selective synthesis of trisulfides over disulfides is demonstrated along with their potential as anti-proliferative agents and sustained donors of H₂S.

Monitoring of Deamidation and Lanthionine Formation in Recombinant Mugwort Allergen by Capillary Zone Electrophoresis (CZE)-UV and Transient Capillary Isotachophoresis-CZE-Electrospray Ionization-TOF-MS

The response to thermal stress is an important parameter relevant for characterizing the biological activity and long-term stability of recombinant proteins, which may show irreversible, pH dependent structural changes under these conditions. We selected the recombinant pollen allergen of mugwort ( Artemisia vulgaris) rArt v 3.0201 as a relevant model to study structural changes due to thermal and pH stress by means of capillary zone electrophoresis (CZE)-UV and capillary zone electrophoresis (CZE)-electrospray ionization (ESI)-TOF-MS. Therefore, this recombinant protein was exposed to 95 °C under acidic (pH 3.4) and slightly alkaline (pH 7.3) conditions for up to 120 min. CZE-UV data showed a continuous degradation of the allergen accompanied by the gradual formation of several reaction products. Characterization of novel allergen variants occurring at longer migration times was done via CZE-ESI-TOF-MS using in-capillary transient capillary isotachophoresis (tCITP) preconcentration to facilitate the identification of minor variants. MS data revealed various modifications of rArt v 3.0201 in response to heating. Variants with deamidations and sulfur-related modifications including both yield and loss of sulfur were identified at increased migration times. Desulfurization produced allergen variants with up to four lanthionines that replaced initial disulfide bonds. In addition, mass spectra revealed shifts in the charge state distribution which indicate concomitant conformational alterations. Moreover, several low-abundant oxidized variants were identified. With extended thermal stress, the portfolio of variants increased and progressively shifted toward rArt v 3.0201 with high lanthionine content. The kinetics of conversion and the complexity of variant composition were pH dependent and increased under alkaline conditions.

Synthesis of Unsymmetric Trisulfides from 9-Fluorenylmethyl Disulfides

A convenient method for the preparation of unsymmetrical trisulfides using 9-fluorenylmethyl (Fm) disulfide as the precursors is reported. This method gives the desired trisulfides in good yields under mild conditions.

3-Mercaptopyruvate sulfurtransferase produces potential redox regulators cysteine- and glutathione-persulfide (Cys-SSH and GSSH) together with signaling molecules H2S2, H2S3 and H2S

Cysteine-persulfide (Cys-SSH) is a cysteine whose sulfhydryl group is covalently bound to sulfur (sulfane sulfur). Cys-SSH and its glutathione (GSH) counterpart (GSSH) have been recognized as redox regulators, some of which were previously ascribed to cysteine and GSH. However, the production of Cys-SSH and GSSH is not well understood. Here, we show that 3-mercaptopyruvate sulfurtransferase (3MST) produces Cys-SSH and GSSH together with the potential signaling molecules hydrogen per- and tri-sulfide (H₂S₂ and H₂S₃). Cys-SSH and GSSH are produced in the brain of wild-type mice but not in those of 3MST-KO mice. The levels of total persulfurated species in the brain of 3MST-KO mice are less than 50% of that in the brain of wild-type mice. Purified recombinant 3MST and lysates of COS cells expressing 3MST showed that Cys-SSH and GSSH were produced in the presence of physiological concentrations of cysteine and glutathione, while those with longer sulfur chains, Cys-SS_nH and GSS_nH, were produced in the presence of lower than physiological concentrations of cysteine and glutathione. The present study provides new insights into the production and physiological roles of these persulfurated species as well as the therapeutic targets for diseases in which these molecules are involved.

Impact of S-sulfocysteine on fragments and trisulfide bond linkages in monoclonal antibodies

The quality of recombinant proteins such as monoclonal antibodies produced using Chinese hamster ovary cell-based mammalian systems is dependent on many factors, including cell line, process and cell culture media. Due to these factors, the generated product is heterogeneous and may have chemically-induced modifications or post-translational modifications that affect antibody stability, functionality and, in some cases, patient safety. This study demonstrates that S-sulfocysteine, a cysteine derivative, can increase the antibody specific productivity in different cell lines cultivated with different processes while minimizing trisulfide linkages in generated mAbs, mainly between heavy and light chain. The supplementation of a cell culture feed with S-sulfocysteine also proved to be useful to reduce the percentage of antibody fragments generated from the monoclonal antibody. Overall, this new component used in the upstream process allows a reduction of product heterogeneity.

A high-throughput hydrophilic interaction liquid chromatography coupled with a charged aerosol detector method to assess trisulfides in IgG1 monoclonal antibodies using tris(2-carboxyethyl)phosphine reaction products: Tris(2-carboxyethyl)phosphine-oxide and tris(2-carboxyethyl)phosphine-sulfide

A robust, high-throughput method using hydrophilic interaction liquid chromatography (HILIC) coupled with a charged aerosol detector (CAD) is reported as a novel approach for trisulfide quantitation in monoclonal antibodies (mAbs). The products of mAb reduction using tris(2-carboxyethyl)phosphine (TCEP) include a species (TCEP(S)) that is stoichiometrically produced from trisulfides. The TCEP reaction products are chromatographically separated, detected, and quantified by the HILICCAD method. The method was qualified to quantify trisulfides across a range of 1-40% (mol trisulfide/mol mAb). In all tested matrix components, assay linearity and intermediate precision were established with correlation coefficients (R(2))>0.99, and relative standard deviations (RSD)<10%. A method comparability study was performed using peptide mapping LC-MS as an orthogonal measurement. For the range of 1-40% trisulfides, the analysis demonstrates that, on average, HILICCAD reads between 0.95 and 1.10 times the value of LC-MS with 95% confidence. Applications of the HILICCAD method include trisulfide determination in purified mAbs to be used in the production of cysteine-linked antibody-drug conjugates, and in cell culture development studies to understand sources of, and strategies for control of, trisulfides.

Identification of H2S3 and H2S produced by 3-mercaptopyruvate sulfurtransferase in the brain

Hydrogen polysulfides (H2Sn) have a higher number of sulfane sulfur atoms than hydrogen sulfide (H2S), which has various physiological roles. We recently found H2Sn in the brain. H2Sn induced some responses previously attributed to H2S but with much greater potency than H2S. However, the number of sulfur atoms in H2Sn and its producing enzyme were unknown. Here, we detected H2S3 and H2S, which were produced from 3-mercaptopyruvate (3 MP) by 3-mercaptopyruvate sulfurtransferase (3MST), in the brain. High performance liquid chromatography with fluorescence detection (LC-FL) and tandem mass spectrometry (LC-MS/MS) analyses showed that H2S3 and H2S were produced from 3 MP in the brain cells of wild-type mice but not 3MST knockout (3MST-KO) mice. Purified recombinant 3MST and lysates of COS cells expressing 3MST produced H2S3 from 3 MP, while those expressing defective 3MST mutants did not. H2S3 was localized in the cytosol of cells. H2S3 was also produced from H2S by 3MST and rhodanese. H2S2 was identified as a minor H2Sn, and 3 MP did not affect the H2S5 level. The present study provides new insights into the physiology of H2S3 and H2S, as well as novel therapeutic targets for diseases in which these molecules are involved.

Gut microbiota facilitates dietary heme-induced epithelial hyperproliferation by opening the mucus barrier in colon

Colorectal cancer risk is associated with diets high in red meat. Heme, the pigment of red meat, induces cytotoxicity of colonic contents and elicits epithelial damage and compensatory hyperproliferation, leading to hyperplasia. Here we explore the possible causal role of the gut microbiota in heme-induced hyperproliferation. To this end, mice were fed a purified control or heme diet (0.5 μmol/g heme) with or without broad-spectrum antibiotics for 14 d. Heme-induced hyperproliferation was shown to depend on the presence of the gut microbiota, because hyperproliferation was completely eliminated by antibiotics, although heme-induced luminal cytotoxicity was sustained in these mice. Colon mucosa transcriptomics revealed that antibiotics block heme-induced differential expression of oncogenes, tumor suppressors, and cell turnover genes, implying that antibiotic treatment prevented the heme-dependent cytotoxic micelles to reach the epithelium. Our results indicate that this occurs because antibiotics reinforce the mucus barrier by eliminating sulfide-producing bacteria and mucin-degrading bacteria (e.g., Akkermansia). Sulfide potently reduces disulfide bonds and can drive mucin denaturation and microbial access to the mucus layer. This reduction results in formation of trisulfides that can be detected in vitro and in vivo. Therefore, trisulfides can serve as a novel marker of colonic mucolysis and thus as a proxy for mucus barrier reduction. In feces, antibiotics drastically decreased trisulfides but increased mucin polymers that can be lysed by sulfide. We conclude that the gut microbiota is required for heme-induced epithelial hyperproliferation and hyperplasia because of the capacity to reduce mucus barrier function.

Determination of disulfide linkages in antimicrobial peptides of the macin family by combination of top-down and bottom-up proteomics

Macins are a distinct class of antimicrobial peptides (AMPs) produced by leeches and Hydra. Their function depends strongly on their three-dimensional structure. In order to support structural elucidation of these AMPs, the knowledge and proper assignment of disulfide bonds formed in these cysteine-rich peptides is a prerequisite. In this report, we outline an analytical strategy, encompassing a combination of top-down MS based analytics and sequence-dependent enzyme cleavage under native conditions followed by high mass accuracy and high resolution MS/MS analysis by LTQ-Orbitrap MS to assign disulfide linkages of three members of the macin family, namely neuromacin, theromacin, and hydramacin-1. The results revealed that the eight cysteine residues conserved in all three macins form the same four disulfide bonds, i.e. [C1:C6], [C2:C5], [C3:C7], and [C4:C8]. Theromacin, which possess two additional cysteine residues, forms a fifth disulfide bond.

BIOLOGICAL SIGNIFICANCE

Beside the high biological significance which is based on the inherent dependence of biological activity on the structural features of antimicrobial peptides (which holds true for entirely every protein), the presented analytical strategy will be of wide interest, as it widens the available toolbox for the analysis of this important posttranslational modification.

Trisulfide modification impacts the reduction step in antibody-drug conjugation process

Antibody-drug conjugates (ADCs) utilizing cysteine-directed linker chemistry have cytotoxic drugs covalently bound to native heavy-heavy and heavy-light interchain disulfide bonds. The manufacture of these ADCs involves a reduction step followed by a conjugation step. When tris(2-carboxyethyl)phosphine (TCEP) is used as the reductant, the reaction stoichiometry predicts that for each molecule of TCEP added, one interchain disulfide should be reduced, generating two free thiols for drug linkage. In practice, the amount of TCEP required to achieve the desired drug-to-antibody ratio often exceeds the predicted, and is variable for different lots of monoclonal antibody starting material. We have identified the cause of this variability to be inconsistent levels of interchain trisulfide bonds in the monoclonal antibody. We propose that TCEP reacts with each trisulfide bond to form a thiophosphine and a disulfide bond, yielding no net antibody free thiols for conjugation. Antibodies with higher levels of trisulfide bonds require a greater TCEP:antibody molar ratio to achieve the targeted drug-to-antibody ratio.

The separation of recombinant human growth hormone variants by UHPLC

Reversed-phase ultra-high-performance liquid chromatography (RP-UHPLC) was used for the separation of recombinant human growth hormone (rhGH) variants. A bridged ethylene hybrid silica C18 column was used at 37°C. The composition and temperature of the mobile phase were optimized for the separation. An isocratic elution, with approximately 46% acetonitrile in 25 mM potassium borate buffer (pH 8.5), was found to give superior selectivity in comparison with commonly used mobile phases. The method separated eight rhGH variants: (i) di-oxy Met14/Met125 sulfoxide, (ii) Met125 sulfoxide, (iii) Met14 sulfoxide, (iv) mono-deamidated (Asn149 → Asp149 or Asn152 → Asp152), (v) di-deamidated (Asn149 → Asp149 and Asn152 → Asp152), (vi) clip (Thr142-Tyr143), (vii) desPhe1 and (viii) trisulfide (Cys182-SSS-Cys189) from each other and from the native rhGH. Characterization of the purified variants was conducted by liquid chromatography-mass spectrometry tryptic mapping. The novel mobile phase, in combination with the UHPLC system, generated a significantly higher resolution than previously reported reversed-phase LC methods, including pharmacopoeal methods, for analyzing rhGH.

Controlling trisulfide modification in recombinant monoclonal antibody produced in fed-batch cell culture

Molecular heterogeneity was detected in a recombinant monoclonal antibody (IgG1 mAb) due to the presence of a trisulfide linkage generated by the post-translational insertion of a sulfur atom into disulfide bonds at the heavy-heavy and heavy-light junctions. This molecular heterogeneity had no observable effect on antibody function. Nevertheless, to minimize the heterogeneity of the IgG1 mAb from run-to-run, an understanding of the impact of cell culture process conditions on trisulfide versus disulfide linkage formation was desirable. To investigate variables that might impact trisulfide formation, cell culture parameters were varied in bench-scale bioreactor studies. Trisulfide analysis of the samples from these runs revealed that the trisulfide content in the bond between heavy and light chains varied considerably from <1% to 39%. Optimizing the culture duration and feeding strategy resulted in more consistent trisulfide levels. Cysteine concentration in the feed medium had a direct correlation with the trisulfide level in the product. Systematic studies revealed that cysteine in the feed and the bioreactor media was contributing hydrogen sulfide which reacted with the IgG1 mAb in the supernatant leading to the insertion of sulfur atom and formation of a trisulfide bond. Cysteine feed strategies were developed to control the trisulfide modification in the recombinant monoclonal antibody.

Trisulfides in proteins

Trisulfides and other oligosulfides are widely distributed in the biological world. In plants, for example, garlic, trisulfides are associated with potentially beneficial properties. However, an extra neutral sulfur atom covalently bound between the two sulfur atoms of a pair of cysteines is not a common post-translational modification, and the number of proteins in which a trisulfide has been unambiguously identified is small. Nevertheless, we believe that its prevalence may be underestimated, particularly with the increasing evidence for significant pools of sulfides in living tissues and their possible roles in cellular metabolism. This review focuses on examples of proteins that are known to contain a trisulfide bridge, and gives an overview of the chemistry of trisulfide formation, and the methods by which it is detected in proteins.

Evidence for trisulfide bonds in a recombinant variant of a human IgG2 monoclonal antibody

The hinge region of human IgG2 contains four cysteine residues involved in disulfide linkages between the heavy chains, as well as the heavy and light chains. These linkages provide the fundamental framework of three distinct IgG2 disulfide isoforms recently described. Here, we detail another, disulfide-related post-translational modification in a recombinant variant of human IgG2. Heterogeneity associated with this antibody was separated into several fractions by anion-exchange chromatography (AEX), which is an important initial step that highlights the resolving power of surface charge-based HPLC techniques. Mass spectrometry of the intact antibody revealed weakly resolved discrete covalent additions of 25-35 Da in one of the two main AEX fractions. Digestion by endoproteinase Lys-C performed under nonreducing conditions, as well as tandem MS experiments, narrowed the modification to the peptide-containing disulfide-bridged hinge structure. High mass resolution and accuracy measurements of the peptide strongly suggested an addition of one or two S atoms. The modification could be eliminated by a mild reducing treatment of the intact antibody. Overall, these findings are consistent with the replacement of up to two disulfide bridges (S-S) with a like number of trisulfides (S-S-S) in the antibody hinge. The trisulfide modification is rather uncommon for proteins and its possible origins in the IgG2 variant are discussed.

Efficient on-column conversion of IgG1 trisulfide linkages to native disulfides in tandem with Protein A affinity chromatography

Protein trisulfide linkages are generated by the post-translational insertion of a sulfur atom into a disulfide bond. Molecular heterogeneity was detected in a recombinant IgG(1) monoclonal antibody (mAb) and attributed to the presence of a protein trisulfide moiety. The predominant site of trisulfide modification was the bond between the heavy and light chains. The trisulfide was eliminated during purification of the IgG(1) mAb via a cysteine wash step incorporated into Protein A affinity column chromatography. Analysis of the cysteine-treated mAb by electrophoresis and peptide mapping indicated that the trisulfide linkages were efficiently converted to intact disulfide bonds (13% trisulfide decreased consistently to 1% or less) without disulfide scrambling or an increase in free sulfhydryls. The on-column trisulfide conversion caused no change in protein folding detectable by hydrogen/deuterium exchange or differential scanning calorimetry. Consistent with this, binding of the mAb to its antigen in vitro was insensitive to the presence of the trisulfide modification and to its removal by the on-column cysteine treatment. Similar, high efficiency trisulfide conversion was achieved for a second IgG(1) mAb using the column wash strategy (at least 7% trisulfide decreased to 1% or less). Therefore, trisulfide/disulfide heterogeneity can be eliminated from IgG(1) molecules via a convenient and inexpensive procedure compatible with routine Protein A affinity capture.

Characterization of a covalent polysulfane bridge in copper-zinc superoxide dismutase

In the course of studies on human copper-zinc superoxide dismutase (SOD1), we observed a modified form of the protein whose mass was increased by 158 mass units. The covalent modification was characterized, and we established that it is a novel heptasulfane bridge connecting the two Cys111 residues in the SOD1 homodimer. The heptasulfane bridge was visualized directly in the crystal structure of a recombinant human mutant SOD1, H46R/H48Q, produced in yeast. The modification is reversible, with the bridge being cleaved by thiols, by cyanide, and by unfolding of the protein to expose the polysulfane. The polysulfane bridge can be introduced in vitro by incubation of purified SOD1 with elemental sulfur, even under anaerobic conditions and in the presence of a metal chelator. Because polysulfanes and polysulfides can catalyze the generation of reactive oxygen and sulfur species, the modification may endow SOD1 with a toxic gain of function.

Heterogeneity of commercial recombinant human growth hormone (r-hGH) preparations containing a thioether variant

The objective of the present study was to assess (I) the potential presence of a recently discovered thioether variant in commercially available recombinant human growth hormone (r-hGH) preparations, and (II) the impact of the thioether modification on the in-vivo bioactivity and the receptor binding kinetics. Samples were tested employing European (EP) and US Pharmacopeia (USP) Somatropin monograph and mass spectrometry methods. None of the international standards contained this variant. All products conformed to EP specifications but six out of eight lots contained the variant. An artificially enriched thioether sample exhibited a significantly reduced in vivo biopotency and altered receptor-binding properties compared with a control. The absence of the variant in the pituitary hGH standard, and the possibility to generate it artificially suggests that it is not naturally occurring and that it may arise from an uncontrolled manufacturing process. Controlled studies may be required to assess its clinical efficacy and safety. EP and USP methods may need to be adapted to reliably detect the presence of the variant.

Characterization of trisulfide modification in antibodies

Trisulfides are a posttranslational modification formed by the insertion of a sulfur atom into a disulfide bond. Although reports for trisulfides in proteins are limited, we find that they are a common modification in natural and recombinant antibodies of all immunoglobulin G (IgG) subtypes. Trisulfides were detected only in interchain linkages and were predominantly in the light-heavy linkages. Factors that lead to trisulfide formation and elimination and their impact on activity and stability were investigated. The peptide mapping methods developed for characterization and quantification of trisulfides should be applicable to any antibody and can be easily adapted for other types of proteins.

Characterization of alkali induced formation of lanthionine, trisulfides, and tetrasulfides from peptide disulfides using negative ion mass spectrometry

Peptide disulfides are unstable under alkaline conditions, resulting in the formation of products containing lanthionine and polysulfide linkages. Electrospray ionization mass spectrometry has been used to characterize major species obtained when cyclic and acyclic peptide disulfides are exposed to alkaline media. Studies on a model cyclic peptide disulfide (Boc-Cys-Pro-Leu-Cys-NHMe) and an acyclic peptide, oxidized glutathione, bis ((gamma)Glu - Cys - Gly - COOH), are described. Disulfide cleavage reactions are initiated by the abstraction of C(alpha)H or C(beta)H protons of Cys residues, with subsequent elimination of H(2)S or H(2)S(2). The buildup of reactive thiol species which act on intermediates containing dehydroalanine residues, rationalizes the formation of lanthionine and polysulfide products. In the case of the cyclic peptide disulfide, the formation of cyclic products is facilitated by the intramolecular nature of the Michael addition reaction of thiols to the dehydroalanine residue. Mass spectral evidence for the intermediate species is presented by using alkylation of thiol groups as a trapping method. Mass spectral fragmentation in the negative ion mode of the peptides derived from trisulfides and tetrasulfides results in elimination of S(2).

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

A trisulfide-linked glycoprotein

The first member of a novel class of chemoselective reagents, glycosyl methanedithiosulfonates, has been synthesized, identified and employed in the first examples of chemical, site-selective construction of a trisulfide-modified protein with complete conversion.

Characterization of Nα-acetyl methionyl human growth hormone formed during expression in Saccharomyces cerevisiae with liquid chromatography and mass spectrometry

We found a new variant of human growth hormone (hGH) from the recombinant hGH expression process in Saccharomyces cerevisiae. The variant was identified as N(alpha)-acetyl methionyl hGH which may be formed by N(alpha)-acetylation of met-hGH during the intracellular expression of hGH in S. cerevisiae. The variant was isolated from manufacturing process of LG Life Sciences' hGH product. The variant was subjected to trypsin digestion and RP-HPLC analysis, resulting in a delayed retention time and an increased mass (173 Da) of T1 tryptic peptide. The amino acid composition and amino acid sequence of the peptide showed the same result with T1 peptide of met-hGH except the N-terminal modification on methionine in the variant peptide. With collision induced dissociation (CID) experiments of the variant T1 tryptic peptide, we found the sequence and the a(1) fragment of N-terminal residue matched with those of acetyl-methionyl hGH. Within our production process, we produce the methionyl hGH first and then use the aminopeptidase to cut the N-terminal methionine. So the acetylation may inhibit the aminopeptidase to remove methionine and produces N(alpha)-acetyl methionyl hGH. And the biological activity of the variant was comparable to one of the unmodified hGH when tested by rat weight gain bioassay.

Structure determination of [Arg8]vasopressin methylenedithioether in dimethylsulfoxide using NMR

The structure of [Arg8]vasopressin methylenedithioether ([AVP]CH2) has been determined in dimethylsulfoxide-d6. Two-dimensional DQF-COSY and NOESY spectra were measured and used to derive angle and distance constraints for restrained molecular dynamics (MD) calculations. In the MD trajectory, two types of beta-turn structure were found in the region from Tyr2 to Asn5, suggesting an equilibrium between type-I and type-II' beta-turn structures. When Halpha chemical shifts were used as an additional constraint, the type-I turn was favoured. To validate this result, an independent energy minimization procedure was used, using differences between calculated and observed chemical shifts. The two approaches gave essentially identical results. It is therefore concluded that the type-I turn predominates in solution. Analysis of calculated chemical shift contributions suggests that the beta-turn structure found in AVP is well preserved in [AVP]CH2, although the pressin ring size is expanded.

Solid phase synthesis and biological activities of [Arg8]-vasopressin methylenedithioether

Solid phase synthesis of [Arg8]-vasopressin methylenedithioether, an analog of vasopressin which contains an extra methylene group between the two sulfur atoms of Cys1 and Cys6, is described. Methylene insertion occurred easily when the thiol free peptide on a solid support was treated with tetrabutylammonium fluoride in dichloromethane at room temperature for 3 h. The uterotonic in vitro, pressor, and antidiuretic activities of the compound were reduced in comparison to [Arg8]-vasopressin by one order of magnitude.

Major degradation pathway of thiuram in tap water processed by oxidation with sodium hypochlorite

Thiuram (3 microM), a fungicide, was incubated in deionized water by adding 0-100 mg/L free chlorine at 30 degrees C for 30 min, and the solution was analyzed by HPLC and IC. The byproducts were identified by LC/MS, EI-MS, infrared, and (13)C NMR spectra and a reduction technique using 2-mercaptoethanol. On the basis of these results, it was found that the oxidation of thiuram with sodium hypochlorite initially produced an intermediate dimethylthiocarbamoyl dimethylcarbamoyl disulfide, which was finally degraded to bis(dimethylcarbamoyl) disulfide, its trisulfide, and dimethylamine. Subsequently, it was suggested that monitoring of bis(dimethylcarbamoyl) disulfide, its trisulfide, and dimethylamine should be included for the management and control of thiuram in tap water processed by oxidation with sodium hypochlorite.

A labile sulfur in trisulfide affects cytochrome P-450 dependent lipid peroxidation in rat liver microsomes

The effects of trisulfide derivatives were studied on cytochrome P-450-dependent lipid peroxidation using rat liver microsomal systems. Cytochrome P-450-dependent lipid peroxidation was induced by carbon tetrachloride or tert-butylhydroperoxide and was evident by an increase in thiobarbituric acid-reactive substances (TBA-RS) and oxygen consumption. In these cytochrome P-450-dependent lipid peroxidation systems, pretreatment of microsome with trisulfide derivatives (cystine trisulfide and thiocyclam) significantly inhibited TBA-RS formation and oxygen consumption compared with disulfide or thiol analogs (cystine, nereistoxin, or cysteine). The labile sulfur contained in trisulfide disappeared during incubation with liver microsomes. In the CCl4-induced lipid peroxidation system, the cytochrome P-450 level and NAD(P)H-cytochrome P-450 reductase activity were significantly decreased by the addition of trisulfide derivatives. Therefore, in cytochrome P-450-dependent lipid peroxidation system, the potential effects of trisulfide appear to be mediated via enzyme inhibition. These results suggest that pretreatment of the trisulfide derivatives may affect the toxic function of exogenous xenobiotics or drugs, which are reduced by the liver enzyme cytochrome P-450 to radical species.

Quantifying biosynthetic human growth hormone in Escherichia coli with capillary electrophoresis under hydrophobic conditions

A method has been developed which is able to quantitate the content of precursor biosynthetic human growth hormone (Pre-bhGH) in the cytosol of E. coli cells containing the gene for human growth hormone (hGH). The method uses hydrophobic C18 coated capillaries with native biosynthetic human growth hormone (bhGH) as an internal standard. This allows for highly robust and precise determinations as well as the evaluation of the presence of deamidated forms in the cytosol samples. Furthermore, by modifying the running buffer with zwitterionic surfactants and an organic modifier, it is possible to detect a related form with a three sulfur atom Cys-Cys bridge (trisulfide Pre-bhGH). Thus, a strong tool for monitoring the effect of fermentation conditions on the biosynthesis of bhGH is obtained.

Identification and characterization of recombinant murine interleukin-6 with a C-terminal pentapeptide extension using capillary reversed phase HPLC-MS and edman degradation

We have identified a preparation of recombinant murine interleukin-6 (mIL-6) that, in addition to the anticipated product, also contained approximately equal amounts of mIL-6 with a C-terminal pentapeptide extension. The extension mutant was generated by readthrough of the stopcodon, and termination at a second in-frame stopcodon 12 base pairs 3' in the expression vector. Aliquots of the preparation were subjected to proteolytic digestion with Asp-N and Lys-C-endopeptidase. The resultant peptides were separated by reversed-phase capillary HPLC, and analysed using a combination of mass spectrometry and N-terminal sequence analysis. These data revealed a C-terminal pentapeptide (Gln-Gly-Ser-Val-Asp) extension, with the authentic stopcodon being translated as glutamine. The extension mutant was isolated by reversed-phase HPLC and shown to have similar mitogenic activity to mIL-6 on murine hybridoma 7TD1 cells.

Synthesis and pharmacology of novel analogues of oxytocin and deaminooxytocin: directed methods for the construction of disulfide and trisulfide bridges in peptides

Using as models the neurohypophyseal nonapeptide hormone oxytocin and its analogue deaminooxytocin, several directed routes to formation of sulfur-sulfur bridges have been developed and evaluated. The linear sequences (through common octapeptide-resin intermediates) were assembled smoothly on tris(alkoxy)benzylamide (PAL) poly(ethylene glycol)-polystyrene (PEG-PS) graft supports, using stepwise Fmoc solid-phase chemistry. Side-chain protection of beta-mercaptopropionic acid (Mpa) and/or cysteine (Cys) was provided by S-2,4,6-trimethoxybenzyl (Tmob), S-acetamidomethyl (Acm), and/or a series of sulfenyl thiocarbonate and carbamoylsulfenyl protecting/activating groups: S-(methoxycarbonyl)sulfenyl (Scm), S-(methoxycarbonyl)disulfanyl (Sscm), S-(N-methyl-N-phenylcarbamoyl)sulfenyl (Snm), and S-(N-methyl-N-phenylcarbamoyl)disulfanyl (Ssnm). Thiolytic displacement of S-Snm (preferred) or S-Scm provided intramolecular cyclized peptide disulfides, and homologation of the chemistry with S-Ssnm (again preferred) and S-Sscm provided the corresponding trisulfides along with smaller amounts of disulfides and tetrasulfides. These chemistries could be implemented both in solution and in solid-phase modes. Various parameters were studied systematically and optimized, and the novel trisulfides of oxytocin and deaminooxytocin were synthesized and purified to homogeneity. The trisulfide compounds were evaluated in three assays: uterotonic in vitro, uterotonic in vivo, and pressor tests, and they showed substantial potencies, ranging from 5% to 40% of the parent (disulfide) activities, as well as protracted actions. The affinities of the peptide trisulfides to uterine membrane receptors were only 3.3-3.6-fold lower than those of the parent disulfides. Possible explanations of the biological results are discussed.

Degradation pathways of salmon calcitonin in aqueous solution

Salmon calcitonin (sCT), a 32-amino-acid peptide, is the active component in many pharmaceuticals used for the management of bone diseases. The degradation pathways of sCT were determined, and the structures of the major degradation products were identified. Aqueous solutions of sCT at pH values of 3, 4, 5, and 6 were degraded, and the major degradation products were detected using reversed phase and size-exclusion high-performance liquid chromatography (HPLC). The degradation rate and pathways of sCT are strongly dependent on pH in the pH range between 3 and 6. The major degradation products were isolated by semipreparative HPLC and identified using a variety of spectroscopic and bioanalytical techniques. The results show that sCT can undergo hydrolyses resulting in cleavage of the 1-2 amide bond and deamidation of the Gln14 and Gln20 residues, sulfide exchange that leads to an unusual trisulfide derivative, and dimerization to reducible and nonreducible dimers. The mechanisms for the pathways can be rationalized from known degradation pathways of peptide and proteins.

Nuclear magnetic resonance studies of the C-terminal human growth hormone fragment I179-C182-[SS]-C189-P191 and the related trisulfide peptide I179-C182-[SSS]-C189-P191

The synthetic C-terminal hGH fragment I179-C182-[SS]-C189-P191 and the related trisulfide peptide I179-C182-[SSS]-C189-P191 have been studied using homonuclear 1H-NMR methods and distance geometry calculations. The 1H-NMR spectra of both the disulfide (diS) and the trisulfide (triS) were completely assigned. Amide proton exchange rates, NOEs and the temperature dependence of the NH chemical shifts indicate a hydrogen bond in triS between Val185 and Ser188 stabilizing a turn in this region. 3JH,H coupling constants and NOEs were measured and used as input for distance geometry calculations. For triS two families of structures with averaged pairwise backbone root mean square deviations for Cys182-Cys189 of 1.3-1.5 A were found, only one of which is compatible with experimental data. For diS only one family of structures was found, but with such a low structural definition (back bone rmsd > 2 A) that no interpretation into a consensus structure is useful. The generated structures were compared to the crystal structure of the terminal loop in hGH, complexed to its binding proteins. The resemblance was low between the solution structures of the tridecapeptides and the terminal hGH loop.

Confirmation by mass spectrometry of a trisulfide variant in methionyl human growth hormone biosynthesized in Escherichia coli

A sulfur-containing compound found in acid hydrolysates of proteins was identified 30 years ago as a trisulfide: bis-(2-amino-2-carboxyethyl) trisulfide (cysteine2S3). At that time, studies concerning the chemistry of sulfur-transferring enzyme systems suggested that cysteine2S3 also existed in biological systems. Two decades later, a cystine trisulfide structure was postulated in the regulator protein molecule for the activation of delta-aminolevulinate synthetase. Recently, a trisulfide bond was reported to occur in the minor loop disulfide at Cys182-Cys189 in human growth hormone. We have detected a trisulfide structure in methionyl human growth hormone in the major loop disulfide Cys53-Cys165. The development of mass spectral analyses of high molecular weight molecules, such as proteins, led to the eventual identification of the modification. A tandem mass spectral analysis on a Sciex electrospray instrument localized an addition of 32 Da to the Cys53-Cys165 fragment. Elemental composition was determined by accurate mass measurement obtained by peak matching to a synthetic peptide and established that an extra sulfur atom was involved.