A Review of Methodologies for the Detection, Quantitation, and Localization of Free Cysteine in Recombinant Proteins: A Focus on Therapeutic Monoclonal Antibodies.

Free-cysteine residues in recombinant biotherapeutics such as monoclonal antibodies can arise from incorrect cellular processing of disulfide bonds during synthesis or by reduction of disulfide bonds during the harvest and purification stage of manufacture. Free cysteines can affect potency, induce aggregation, and decrease the stability of therapeutic proteins, and the levels and positions of free cysteines in proteins are closely monitored by both manufacturers and regulators to ensure safety and efficacy. This review summarizes the latest methodologies for the detection and quantification of free cysteines.

A proximity-based in silico approach to identify redox-labile disulfide bonds: The example of FVIII.

Allosteric disulfide bonds permit highly responsive, transient 'switch-like' properties that are ideal for processes like coagulation and inflammation that require rapid and localised responses to damage or injury. Haemophilia A (HA) is a rare bleeding disorder managed with exogenous coagulation factor(F) VIII products. FVIII has eight disulfide bonds and is known to be redox labile, but it is not known how reduction/oxidation affects the structure-function relationship, or its immunogenicity-a serious complication for 30% severe HA patients. Understanding how redox-mediated changes influence FVIII can inform molecular engineering strategies aimed at improving activity and stability, and reducing immunogenicity. FVIII is a challenging molecule to work with owing to its poor expression and instability so, in a proof-of-concept study, we used molecular dynamics (MD) to identify which disulfide bonds were most likely to be reduced and how this would affect structure/function; results were then experimentally verified. MD identified Cys1899-Cys1903 disulfide as the most likely to undergo reduction based on energy and proximity criteria. Further MD suggested this reduction led to a more open conformation. Here we present our findings and highlight the value of MD approaches.

The impact of thioredoxin reduction of allosteric disulfide bonds on the therapeutic potential of monoclonal antibodies.

Therapeutic mAbs are used to manage a wide range of cancers and autoimmune disorders. However, mAb-based treatments are not always successful, highlighting the need for a better understanding of the factors influencing mAb efficacy. Increased levels of oxidative stress associated with several diseases are counteracted by the activities of various oxidoreductase enzymes, such as thioredoxin (Trx), which also reduces allosteric disulfide bonds in proteins, including mAbs. Here, using an array of in vitro assays, we explored the functional effects of Trx-mediated reduction on the mechanisms of action of six therapeutic mAbs. We found that Trx reduces the interchain disulfide bonds of the mAbs, after which they remain intact but have altered function. In general, this reduction increased antigen-binding capacity, resulting in, for example, enhanced tumor necrosis factor (TNF) neutralization by two anti-TNF mAbs. Conversely, Trx reduction decreased the antiproliferative activity of an anti-tyrosine kinase-type cell-surface receptor HER2 mAb. In all of the mAbs, Fc receptor binding was abrogated by Trx activity, with significant loss in both complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity (ADCC) activity of the mAbs tested. We also confirmed that without alkylation, Trx-reduced interchain disulfide bonds reoxidize, and ADCC activity is restored. In summary, Trx-mediated reduction has a substantial impact on the functional effects of an mAb, including variable effects on antigen binding and Fc function, with the potential to significantly impact mAb efficacy in vivo.

A Proteomics Workflow for the Identification of Labile Disulphide Bonds at the Cell Surface.

Reduction of labile disulphide bonds on leukocyte cell surface proteins plays a regulatory role in immune cell activation. Here I describe a method for the fast, efficient, and unbiased purification of cell-surface proteins containing such labile disulphide bonds. Free thiols liberated from the reduction of labile disulphide bonds are labeled with biotin, purified, enriched, and subsequently identified using liquid chromatography coupled to tandem mass spectrometry. Both the proteins containing the labile disulphide bonds and the position of bonds within the protein are revealed, thus providing a valuable addition to the immunology or biochemistry toolkit.

The first World Health Organization International Standard for infliximab products: A step towards maintaining harmonized biological activity.

Due to the increase in the number of infliximab products, the need for global harmonization of the bioactivity of this monoclonal antibody was recognized by the World Health Organization (WHO). In response, the National Institute for Biological Standards and Control (NIBSC) developed the first international standard (IS) for infliximab, which targets tumour necrosis factor (TNF). Each ampoule is assigned values of 500 IU of TNF neutralizing activity and 500 IU of binding activity. Two preparations of infliximab were formulated and lyophilized at NIBSC prior to evaluation in a collaborative study for their suitability to serve as an IS for the in vitro biological activity of infliximab. The study involved participants using in vitro cell-based bioassays (TNF neutralization, antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity) and binding assays. The results of this study showed that the candidate preparation, coded 16/170, is suitable as an IS for infliximab bioactivity. This infliximab IS from NIBSC, is intended to support in vitro bioassay calibration and validation by defining international units of bioactivity. The proposed unitages, however, are not intended to revise product labelling or dosing requirements, as any decisions regarding this relies solely with the regulatory authorities. Furthermore, the infliximab IS is not intended for determining the specific activity of products, nor to serve any regulatory role in defining biosimilarity. We briefly discuss the future use of WHO international standards in supporting the global harmonisation of biosimilar infliximab products.

Reduction of leucocyte cell surface disulfide bonds during immune activation is dynamic as revealed by a quantitative proteomics workflow (SH-IQ).

Communication through cell surface receptors is crucial for maintaining immune homeostasis, coordinating the immune response and pathogen clearance. This is dependent on the interaction of cell surface receptors with their ligands and requires functionally active conformational states. Thus, immune cell function can be controlled by modulating the structure of either the receptor or the ligand. Reductive cleavage of labile disulfide bonds can mediate such an allosteric change, resulting in modulation of the immune system by a hitherto little studied mechanism. Identifying proteins with labile disulfide bonds and determining the extent of reduction is crucial in elucidating the functional result of reduction. We describe a mass spectrometry-based method-thiol identification and quantitation (SH-IQ)-to identify, quantify and monitor such reduction of labile disulfide bonds in primary cells during immune activation. These results provide the first insight into the extent and dynamics of labile disulfide bond reduction in leucocyte cell surface proteins upon immune activation. We show that this process is thiol oxidoreductase-dependent and mainly affects activatory (e.g. CD132, SLAMF1) and adhesion (CD44, ICAM1) molecules, suggesting a mechanism to prevent over-activation of the immune system and excessive accumulation of leucocytes at sites of inflammation.

Homodimerization of the Lymph Vessel Endothelial Receptor LYVE-1 through a Redox-labile Disulfide Is Critical for Hyaluronan Binding in Lymphatic Endothelium.

The lymphatic vessel endothelial receptor LYVE-1 is implicated in the uptake of hyaluronan (HA) and trafficking of leukocytes to draining lymph nodes. Yet LYVE-1 has only weak affinity for hyaluronan and depends on receptor clustering and higher order ligand organization for durable binding in lymphatic endothelium. An unusual feature of LYVE-1 not found in other HA receptors is the potential to form disulfide-linked homodimers. However, their influence on function has not been investigated. Here we show LYVE-1 homodimers are the predominant configuration in lymphatic endothelium in vitro and in vivo, and formation solely requires the unpaired cysteine residue Cys-201 within the membrane-proximal domain, yielding a 15-fold higher HA binding affinity and an ∼67-fold slower off-rate than the monomer. Moreover, we show non-dimerizing LYVE-1 mutants fail to bind HA even when expressed at high densities in lymphatic endothelial cells or artificially cross-linked with antibody. Consistent with these findings, small angle X-ray scattering (SAXS) indicates the Cys-201 interchain disulfide forms a hinge that maintains the homodimer in an "open scissors" conformation, likely allowing arrangement of the two HA binding domains for mutual engagement with ligand. Finally, we demonstrate the Cys-201 interchain disulfide is highly labile, and selective reduction with TCEP-HCl disrupts LYVE-1 homodimers, ablating HA binding. These findings reveal binding is dependent not just on clustering but also on the biochemical properties of LYVE-1 homodimers. They also mark LYVE-1 as the first Link protein superfamily member requiring covalent homodimerization for function and suggest the interchain disulfide acts as a redox switch in vivo.

Thioredoxin Inhibitors Attenuate Platelet Function and Thrombus Formation.

Thioredoxin (Trx) is an oxidoreductase with important physiological function. Imbalances in the NADPH/thioredoxin reductase/thioredoxin system are associated with a number of pathologies, particularly cancer, and a number of clinical trials for thioredoxin and thioredoxin reductase inhibitors have been carried out or are underway. Due to the emerging role and importance of oxidoreductases for haemostasis and the current interest in developing inhibitors for clinical use, we thought it pertinent to assess whether inhibition of the NADPH/thioredoxin reductase/thioredoxin system affects platelet function and thrombosis. We used small molecule inhibitors of Trx (PMX 464 and PX-12) to determine whether Trx activity influences platelet function, as well as an unbiased proteomics approach to identify potential Trx substrates on the surface of platelets that might contribute to platelet reactivity and function. Using LC-MS/MS we found that PMX 464 and PX-12 affected the oxidation state of thiols in a number of cell surface proteins. Key surface receptors for platelet adhesion and activation were affected, including the collagen receptor GPVI and the von Willebrand factor receptor, GPIb. To experimentally validate these findings we assessed platelet function in the presence of PMX 464, PX-12, and rutin (a selective inhibitor of the related protein disulphide isomerase). In agreement with the proteomics data, small molecule inhibitors of thioredoxin selectively inhibited GPVI-mediated platelet activation, and attenuated ristocetin-induced GPIb-vWF-mediated platelet agglutination, thus validating the findings of the proteomics study. These data reveal a novel role for thioredoxin in regulating platelet reactivity via proteins required for early platelet responses at sites of vessel injury (GPVI and GPIb). This work also highlights a potential opportunity for repurposing of PMX 464 and PX-12 as antiplatelet agents.

OX133, a monoclonal antibody recognizing protein-bound N-ethylmaleimide for the identification of reduced disulfide bonds in proteins.

In vivo, enzymatic reduction of some protein disulfide bonds, allosteric disulfide bonds, provides an important level of structural and functional regulation. The free cysteine residues generated can be labeled by maleimide reagents, including biotin derivatives, allowing the reduced protein to be detected or purified. During the screening of monoclonal antibodies for those specific for the reduced forms of proteins, we isolated OX133, a unique antibody that recognizes polypeptide resident, N-ethylmaleimide (NEM)-modified cysteine residues in a sequence-independent manner. OX133 offers an alternative to biotin-maleimide reagents for labeling reduced/alkylated antigens and capturing reduced/alkylated proteins with the advantage that NEM-modified proteins are more easily detected in mass spectrometry, and may be more easily recovered than is the case following capture with biotin based reagents.

CD44 Binding to Hyaluronic Acid Is Redox Regulated by a Labile Disulfide Bond in the Hyaluronic Acid Binding Site.

CD44 is the primary leukocyte cell surface receptor for hyaluronic acid (HA), a component of the extracellular matrix. Enzymatic post translational cleavage of labile disulfide bonds is a mechanism by which proteins are structurally regulated by imparting an allosteric change and altering activity. We have identified one such disulfide bond in CD44 formed by Cys77 and Cys97 that stabilises the HA binding groove. This bond is labile on the surface of leukocytes treated with chemical and enzymatic reducing agents. Analysis of CD44 crystal structures reveal the disulfide bond to be solvent accessible and in the-LH hook configuration characteristic of labile disulfide bonds. Kinetic trapping and binding experiments on CD44-Fc chimeric proteins show the bond is preferentially reduced over the other disulfide bonds in CD44 and reduction inhibits the CD44-HA interaction. Furthermore cells transfected with CD44 no longer adhere to HA coated surfaces after pre-treatment with reducing agents. The implications of CD44 redox regulation are discussed in the context of immune function, disease and therapeutic strategies.

Structures of CD6 and Its Ligand CD166 Give Insight into Their Interaction.

CD6 is a transmembrane protein with an extracellular region containing three scavenger receptor cysteine rich (SRCR) domains. The membrane proximal domain of CD6 binds the N-terminal immunoglobulin superfamily (IgSF) domain of another cell surface receptor, CD166, which also engages in homophilic interactions. CD6 expression is mainly restricted to T cells, and the interaction between CD6 and CD166 regulates T-cell activation. We have solved the X-ray crystal structures of the three SRCR domains of CD6 and two N-terminal domains of CD166. This first structure of consecutive SRCR domains reveals a nonlinear organization. We characterized the binding sites on CD6 and CD166 and showed that a SNP in CD6 causes glycosylation that hinders the CD6/CD166 interaction. Native mass spectrometry analysis showed that there is competition between the heterophilic and homophilic interactions. These data give insight into how interactions of consecutive SRCR domains are perturbed by SNPs and potential therapeutic reagents.

Heme enzymes. Neutron cryo-crystallography captures the protonation state of ferryl heme in a peroxidase.

Heme enzymes activate oxygen through formation of transient iron-oxo (ferryl) intermediates of the heme iron. A long-standing question has been the nature of the iron-oxygen bond and, in particular, the protonation state. We present neutron structures of the ferric derivative of cytochrome c peroxidase and its ferryl intermediate; these allow direct visualization of protonation states. We demonstrate that the ferryl heme is an Fe(IV)=O species and is not protonated. Comparison of the structures shows that the distal histidine becomes protonated on formation of the ferryl intermediate, which has implications for the understanding of O-O bond cleavage in heme enzymes. The structures highlight the advantages of neutron cryo-crystallography in probing reaction mechanisms and visualizing protonation states in enzyme intermediates.

Fine specificity and molecular competition in SLAM family receptor signalling.

SLAM family receptors regulate activation and inhibition in immunity through recruitment of activating and inhibitory SH2 domain containing proteins to immunoreceptor tyrosine based switch motifs (ITSMs). Binding of the adaptors, SAP and EAT-2 to ITSMs in the cytoplasmic regions of SLAM family receptors is important for activation. We analysed the fine specificity of SLAM family receptor phosphorylated ITSMs and the conserved tyrosine motif in EAT-2 for SH2 domain containing signalling proteins. Consistent with the literature describing dependence of CRACC (SLAMF7) on EAT-2, CRACC bound EAT-2 (KD = 0.003 µM) with approximately 2 orders of magnitude greater affinity than SAP (KD = 0.44 µM). RNA interference in cytotoxicity assays in NK92 cells showed dependence of CRACC on SAP in addition to EAT-2, indicating selectivity of SAP and EAT-2 may depend on the relative concentrations of the two adaptors. The concentration of SAP was four fold higher than EAT-2 in NK92 cells. Compared with SAP, the significance of EAT-2 recruitment and its downstream effectors are not well characterised. We identified PLCγ1 and PLCγ2 as principal binding partners for the EAT-2 tail. Both PLCγ1 and PLCγ2 are functionally important for cytotoxicity in NK92 cells through CD244 (SLAMF4), NTB-A (SLAMF6) and CRACC. Comparison of the specificity of SH2 domains from activating and inhibitory signalling mediators revealed a hierarchy of affinities for CD244 (SLAMF4) ITSMs. While binding of phosphatase SH2 domains to individual ITSMs of CD244 was weak compared with SAP or EAT-2, binding of tandem SH2 domains of SHP-2 to longer peptides containing tandem phosphorylated ITSMs in human CD244 increased the affinity ten fold. The concentration of the tyrosine phosphatase, SHP-2 was in the order of a magnitude higher than the adaptors, SAP and EAT-2. These data demonstrate a mechanism for direct recruitment of phosphatases in inhibitory signalling by ITSMs, while explaining competitive dominance of SAP and EAT-2.

Immunoregulation through membrane proteins modified by reducing conditions induced by immune reactions.

Selected disulfide bonds in membrane proteins are labile and are thus susceptible to changes in redox potential and/or the presence of thiol isomerase enzymes. Modification of these disulfide bonds can lead to conformational changes of the protein that in turn may alter protein activity and function. This occurs in the entry of several enveloped viruses into their host cells, e.g. HIV, hepatitis C virus and Newcastle disease virus. Labile disulfide bonds are also important in platelet activation, cytokine signalling and in a variety of diseases including cancer and arthritis. In this review we will concentrate on recent advances in understanding the conditions that lead to disulfide bond reduction in membrane proteins and their effects in regulating immune function.

Interleukin-2 signalling is modulated by a labile disulfide bond in the CD132 chain of its receptor.

Certain disulfide bonds present in leucocyte membrane proteins are labile and can be reduced in inflammation. This can cause structural changes that result in downstream functional effects, for example, in integrin activation. Recent studies have shown that a wide range of membrane proteins have labile disulfide bonds including CD132, the common gamma chain of the receptors for several cytokines including interleukin-2 and interleukin-4 (IL-2 and IL-4). The Cys(183)-Cys(232) disulfide bond in mouse CD132 is susceptible to reduction by enzymes such as thioredoxin (TRX), gamma interferon-inducible lysosomal thiolreductase and protein disulfide isomerase, which are commonly secreted during immune activation. The Cys(183)-Cys(232) disulfide bond is also reduced in an in vivo lipopolysaccharide (LPS)-induced acute model of inflammation. Conditions that lead to the reduction of the Cys(183)-Cys(232) disulfide bond in CD132 inhibit proliferation of an IL-2-dependent T cell clone and concomitant inhibition of the STAT-5 signalling pathway. The same reducing conditions had no effect on the proliferation of an IL-2-independent T cell clone, nor did they reduce disulfide bonds in IL-2 itself. We postulate that reduction of the Cys(183)-Cys(232) disulfide in CD132 inhibits IL-2 binding to the receptor complex. Published data show that the Cys(183)-Cys(232) disulfide bond is exposed at the surface of CD132 and in close contact with IL-2 and IL-4 in their respective receptor complexes. In addition, mutants in these Cys residues in human CD132 lead to immunodeficiency and loss of IL-2 binding. These results have wider implications for the regulation of cytokine receptors in general, as their activity can be modulated by a 'redox regulator' mechanism caused by the changes in the redox environment that occur during inflammation and activation of the immune system.

Crystal structure of guaiacol and phenol bound to a heme peroxidase.

Guaiacol is a universal substrate for all peroxidases, and its use in a simple colorimetric assay has wide applications. However, its exact binding location has never been defined. Here we report the crystal structures of guaiacol bound to cytochrome c peroxidase (CcP). A related structure with phenol bound is also presented. The CcP-guaiacol and CcP-phenol crystal structures show that both guaiacol and phenol bind at sites distinct from the cytochrome c binding site and from the δ-heme edge, which is known to be the binding site for other substrates. Although neither guaiacol nor phenol is seen bound at the δ-heme edge in the crystal structures, inhibition data and mutagenesis strongly suggest that the catalytic binding site for aromatic compounds is the δ-heme edge in CcP. The functional implications of these observations are discussed in terms of our existing understanding of substrate binding in peroxidases [Gumiero A et al. (2010) Arch Biochem Biophys 500, 13-20].

Proton delivery to ferryl heme in a heme peroxidase: enzymatic use of the Grotthuss mechanism.

We test the hypothesized pathway by which protons are passed from the substrate, ascorbate, to the ferryl oxygen in the heme enzyme ascorbate peroxidase (APX). The role of amino acid side chains and bound solvent is demonstrated. We investigated solvent kinetic isotope effects (SKIE) for the wild-type enzyme and several site-directed replacements of the key residues which form the proposed proton path. Kinetic constants for H(2)O(2)-dependent enzyme oxidation to Compound I, k(1), and subsequent reduction of Compound II, k(3), were determined in steady-state assays by variation of both H(2)O(2) and ascorbate concentrations. A high value of the SKIE for wild type APX ((D)k(3) = 4.9) as well as a clear nonlinear dependence on the deuterium composition of the solvent in proton inventory experiments suggest the simultaneous participation of several protons in the transition state for proton transfer. The full SKIE and the proton inventory data were modeled by applying Gross-Butler-Swain-Kresge theory to a proton path inferred from the known structure of APX. The model has been tested by constructing and determining the X-ray structures of the R38K and R38A variants and accounts for their observed SKIEs. This work confirms APX uses two arginine residues in the proton path. Thus, Arg38 and Arg172 have dual roles, both in the formation of the ferryl species and binding of ascorbate respectively and to facilitate proton transfer between the two.

Labile disulfide bonds are common at the leucocyte cell surface.

Redox conditions change in events such as immune and platelet activation, and during viral infection, but the biochemical consequences are not well characterized. There is evidence that some disulfide bonds in membrane proteins are labile while others that are probably structurally important are not exposed at the protein surface. We have developed a proteomic/mass spectrometry method to screen for and identify non-structural, redox-labile disulfide bonds in leucocyte cell-surface proteins. These labile disulfide bonds are common, with several classes of proteins being identified and around 30 membrane proteins regularly identified under different reducing conditions including using enzymes such as thioredoxin. The proteins identified include integrins, receptors, transporters and cell-cell recognition proteins. In many cases, at least one cysteine residue was identified by mass spectrometry as being modified by the reduction process. In some cases, functional changes are predicted (e.g. in integrins and cytokine receptors) but the scale of molecular changes in membrane proteins observed suggests that widespread effects are likely on many different types of proteins including enzymes, adhesion proteins and transporters. The results imply that membrane protein activity is being modulated by a 'redox regulator' mechanism.