Spectroscopic and kinetic evidence for the thiolate anion of glutathione at the active site of glutathione S-transferase

New aspects of redox signaling mediated by supersulfides in health and disease

Oxygen molecules accept electrons from the respiratory chain in the mitochondria and are responsible for energy production in aerobic organisms. The reactive oxygen species formed via these oxygen reduction processes undergo complicated electron transfer reactions with other biological substances, which leads to alterations in their physiological functions and cause diverse biological and pathophysiological consequences (e.g., oxidative stress). Oxygen accounts for only a small proportion of the redox reactions in organisms, especially under aerobic or hypoxic conditions but not under anaerobic and hypoxic conditions. This article discusses a completely new concept of redox biology, which is governed by redox-active supersulfides, i.e., sulfur-catenated molecular species. These species are present in abundance in all organisms but remain largely unexplored in terms of redox biology and life science research. In fact, accumulating evidence shows that supersulfides have extensive redox chemical properties and that they can be readily ionized or radicalized to participate in energy metabolism, redox signaling, and oxidative stress responses in cells and in vivo. Thus, pharmacological intervention and medicinal modulation of supersulfide activities have been shown to benefit the regulation of disease pathogenesis as well as disease control.

Redox Regulation of Xenobiotics by Reactive Sulfur and Supersulfide Species

Significance: Routine exposure to xenobiotics is unavoidable during our lifetimes. Certain xenobiotics are hazardous to human health, and are metabolized in the body to render them less toxic. During this process, several detoxification enzymes cooperatively metabolize xenobiotics. Glutathione (GSH) conjugation plays an important role in the metabolism of electrophilic xenobiotics. Recent Advances: Recent advances in reactive sulfur and supersulfide (RSS) analyses showed that persulfides and polysulfides bound to low-molecular-weight thiols, such as GSH, and to protein thiols are abundant in both eukaryotes and prokaryotes. The highly nucleophilic nature of hydropersulfides and hydropolysulfides contributes to cell protection against oxidative stress and electrophilic stress. Critical Issues: In contrast to GSH conjugation to electrophiles that is aided by glutathione S-transferase (GST), persulfides and polysulfides can directly form conjugates with electrophiles without the catalytic actions of GST. The polysulfur bonds in the conjugates are further reduced by perthioanions and polythioanions derived from RSS to form sulfhydrated metabolites that are no longer electrophilic but rather nucleophilic, and differ from metabolites that are formed via GSH conjugation. Future Directions: In view of the abundance of RSS in cells and tissues, metabolism of xenobiotics that is mediated by RSS warrants additional investigations, such as studies of the impact of microbiota-derived RSS on xenobiotic metabolism. Metabolites formed from reactions between electrophiles and RSS may be potential biomarkers for monitoring exposure to electrophiles and for studying their metabolism by RSS. Antioxid. Redox Signal. 40, 679-690.

The catalytic role of glutathione transferases in heterologous anthocyanin biosynthesis

Anthocyanins are ubiquitous plant pigments used in a variety of technological applications. Yet, after over a century of research, the penultimate biosynthetic step to anthocyanidins attributed to the action of leucoanthocyanidin dioxygenase has never been efficiently reconstituted outside plants, preventing the construction of heterologous cell factories. Through biochemical and structural analysis, here we show that anthocyanin-related glutathione transferases, currently implicated only in anthocyanin transport, catalyse an essential dehydration of the leucoanthocyanidin dioxygenase product, flavan-3,3,4-triol, to generate cyanidin. Building on this knowledge, introduction of anthocyanin-related glutathione transferases into a heterologous biosynthetic pathway in baker's yeast results in >35-fold increased anthocyanin production. In addition to unravelling the long-elusive anthocyanin biosynthesis, our findings pave the way for the colourants' heterologous microbial production and could impact the breeding of industrial and ornamental plants.

The Multifaceted Role of Glutathione S-Transferases in Health and Disease

In humans, the cytosolic glutathione S-transferase (GST) family of proteins is encoded by 16 genes presented in seven different classes. GSTs exhibit remarkable structural similarity with some overlapping functionalities. As a primary function, GSTs play a putative role in Phase II metabolism by protecting living cells against a wide variety of toxic molecules by conjugating them with the tripeptide glutathione. This conjugation reaction is extended to forming redox sensitive post-translational modifications on proteins: S-glutathionylation. Apart from these catalytic functions, specific GSTs are involved in the regulation of stress-induced signaling pathways that govern cell proliferation and apoptosis. Recently, studies on the effects of GST genetic polymorphisms on COVID-19 disease development revealed that the individuals with higher numbers of risk-associated genotypes showed higher risk of COVID-19 prevalence and severity. Furthermore, overexpression of GSTs in many tumors is frequently associated with drug resistance phenotypes. These functional properties make these proteins promising targets for therapeutics, and a number of GST inhibitors have progressed in clinical trials for the treatment of cancer and other diseases.

Thiolated Mesoporous Silica Nanoparticles as an Immunoadjuvant to Enhance Efficacy of Intravesical Chemotherapy for Bladder Cancer

The characteristics of global prevalence and high recurrence of bladder cancer has led numerous efforts to develop new treatments. The spontaneous voiding and degradation of the chemodrug hamper the efficacy and effectiveness of intravesical chemotherapy following tumor resection. Herein, the externally thiolated hollow mesoporous silica nanoparticles (MSN-SH(E)) is fabricated to serve as a platform for improved bladder intravesical therapy. Enhanced mucoadhesive effect of the thiolated nanovector is confirmed with porcine bladder. The permeation-enhancing effect is also verified, and a fragmented distribution pattern of a tight junction protein, claudin-4, indicates the opening of tight junction. Moreover, MSN-SH(E)-associated reprogramming of M2 macrophages to M1-like phenotype is observed in vitro. The antitumor activity of the mitomycin C (MMC)-loaded nanovector (MMC@MSN-SH(E)) is more effective than that of MMC alone in both in vitro and in vivo. In addition, IHC staining is used to analyze IFN-γ, TGF-β1, and TNF-α. These observations substantiated the significance of MMC@MSN-SH(E) in promoting anticancer activity, holding the great potential for being used in intravesical therapy for non-muscle invasive bladder cancer (NMIBC) due to its mucoadhesivity, enhanced permeation, immunomodulation, and prolonged and very efficient drug exposure.

Enzymology of reactive intermediate protection: kinetic analysis and temperature dependence of the mesophilic membrane protein catalyst MGST1

Glutathione transferases (GSTs) are a class of phase II detoxifying enzymes catalysing the conjugation of glutathione (GSH) to endogenous and exogenous electrophilic molecules, with microsomal glutathione transferase 1 (MGST1) being one of its key members. MGST1 forms a homotrimer displaying third-of-the-sites-reactivity and up to 30-fold activation through modification of its Cys-49 residue. It has been shown that the steady-state behaviour of the enzyme at 5 °C can be accounted for by its pre-steady-state behaviour if the presence of a natively activated subpopulation (~ 10%) is assumed. Low temperature was used as the ligand-free enzyme is unstable at higher temperatures. Here, we overcame enzyme lability through stop-flow limited turnover analysis, whereby kinetic parameters at 30 °C were obtained. The acquired data are more physiologically relevant and enable confirmation of the previously established enzyme mechanism (at 5 °C), yielding parameters relevant for in vivo modelling. Interestingly, the kinetic parameter defining toxicant metabolism, k_cat /K_M , is strongly dependent on substrate reactivity (Hammett value 4.2), underscoring that glutathione transferases function as efficient and responsive interception catalysts. The temperature behaviour of the enzyme was also analysed. Both the K_M and K_D values decreased with increasing temperature, while the chemical step k₃ displayed modest temperature dependence (Q₁₀ : 1.1-1.2), mirrored in that of the nonenzymatic reaction (Q₁₀ : 1.1-1.7). Unusually high Q₁₀ values for GSH thiolate anion formation (k₂ : 3.9), k_cat (2.7-5.6) and k_cat /K_M (3.4-5.9) support that large structural transitions govern GSH binding and deprotonation, which limits steady-state catalysis.

Defining the S-Glutathionylation Proteome by Biochemical and Mass Spectrometric Approaches

Protein S-glutathionylation (SSG) is a reversible post-translational modification (PTM) featuring the conjugation of glutathione to a protein cysteine thiol. SSG can alter protein structure, activity, subcellular localization, and interaction with small molecules and other proteins. Thus, it plays a critical role in redox signaling and regulation in various physiological activities and pathological events. In this review, we summarize current biochemical and analytical approaches for characterizing SSG at both the proteome level and at individual protein levels. To illustrate the mechanism underlying SSG-mediated redox regulation, we highlight recent examples of functional and structural consequences of SSG modifications. Finally, we discuss the analytical challenges in characterizing SSG and the thiol PTM landscape, future directions for understanding of the role of SSG in redox signaling and regulation and its interplay with other PTMs, and the potential role of computational approaches to accelerate functional discovery.

Glutathione in Protein Redox Modulation through S-Glutathionylation and S-Nitrosylation

S-glutathionylation and S-nitrosylation are reversible post-translational modifications on the cysteine thiol groups of proteins, which occur in cells under physiological conditions and oxidative/nitrosative stress both spontaneously and enzymatically. They are important for the regulation of the functional activity of proteins and intracellular processes. Connecting link and “switch” functions between S-glutathionylation and S-nitrosylation may be performed by GSNO, the generation of which depends on the GSH content, the GSH/GSSG ratio, and the cellular redox state. An important role in the regulation of these processes is played by Trx family enzymes (Trx, Grx, PDI), the activity of which is determined by the cellular redox status and depends on the GSH/GSSG ratio. In this review, we analyze data concerning the role of GSH/GSSG in the modulation of S-glutathionylation and S-nitrosylation and their relationship for the maintenance of cell viability.

Development of Telintra as an Inhibitor of Glutathione S-Transferase P

Glutathione S-transferase P (GSTP) is a component of a complex series of pathways that provide cellular redox homeostasis. It is an abundant protein in certain tumors and is over-expressed in cancer drug resistance. It has diverse cellular functions that include, thiolase activities with small electrophilic agents or susceptible cysteine residues on the protein to mediate S-glutathionylation, and chaperone binding with select protein kinases. Preclinical and clinical testing of a nanomolar inhibitor of GSTP, TLK199 (Telintra; Ezatiostat) has indicated a role for the enzyme in hematopoiesis and utility for the drug in the treatment of patients with myelodysplastic syndrome.

Interplay among Oxidative Stress, Methylglyoxal Pathway and S-Glutathionylation

Reactive oxygen species (ROS) are produced constantly inside the cells as a consequence of nutrient catabolism. The balance between ROS production and elimination allows to maintain cell redox homeostasis and biological functions, avoiding the occurrence of oxidative distress causing irreversible oxidative damages. A fundamental player in this fine balance is reduced glutathione (GSH), required for the scavenging of ROS as well as of the reactive 2-oxoaldehydes methylglyoxal (MGO). MGO is a cytotoxic compound formed constitutively as byproduct of nutrient catabolism, and in particular of glycolysis, detoxified in a GSH-dependent manner by the glyoxalase pathway consisting in glyoxalase I and glyoxalase II reactions. A physiological increase in ROS production (oxidative eustress, OxeS) is promptly signaled by the decrease of cellular GSH/GSSG ratio which can induce the reversible S-glutathionylation of key proteins aimed at restoring the redox balance. An increase in MGO level also occurs under oxidative stress (OxS) conditions probably due to several events among which the decrease in GSH level and/or the bottleneck of glycolysis caused by the reversible S-glutathionylation and inhibition of glyceraldehyde-3-phosphate dehydrogenase. In the present review, it is shown how MGO can play a role as a stress signaling molecule in response to OxeS, contributing to the coordination of cell metabolism with gene expression by the glycation of specific proteins. Moreover, it is highlighted how the products of MGO metabolism, S-D-lactoylglutathione (SLG) and D-lactate, which can be taken up and metabolized by mitochondria, could play important roles in cell response to OxS, contributing to cytosol-mitochondria crosstalk, cytosolic and mitochondrial GSH pools, energy production, and the restoration of the GSH/GSSG ratio. The role for SLG and glyoxalase II in the regulation of protein function through S-glutathionylation under OxS conditions is also discussed. Overall, the data reported here stress the need for further studies aimed at understanding what role the evolutionary-conserved MGO formation and metabolism can play in cell signaling and response to OxS conditions, the aberration of which may importantly contribute to the pathogenesis of diseases associated to elevated OxS.

Mechanisms of promiscuity among drug metabolizing enzymes and drug transporters

Detoxication, or 'drug-metabolizing', enzymes and drug transporters exhibit remarkable substrate promiscuity and catalytic promiscuity. In contrast to substrate-specific enzymes that participate in defined metabolic pathways, individual detoxication enzymes must cope with substrates of vast structural diversity, including previously unencountered environmental toxins. Presumably, evolution selects for a balance of 'adequate' kcat /KM values for a wide range of substrates, rather than optimizing kcat /KM for any individual substrate. However, the structural, energetic, and metabolic properties that achieve this balance, and hence optimize detoxication, are not well understood. Two features of detoxication enzymes that are frequently cited as contributions to promiscuity include the exploitation of highly reactive versatile cofactors, or cosubstrates, and a high degree of flexibility within the protein structure. This review examines these intuitive mechanisms in detail and clarifies the contributions of the classic ligand binding models 'induced fit' (IF) and 'conformational selection' (CS) to substrate promiscuity. The available literature data for drug metabolizing enzymes and transporters suggest that IF is exploited by these promiscuous detoxication enzymes, as it is with substrate-specific enzymes, but the detoxication enzymes uniquely exploit 'IFs' to retain a wide range of substrates at their active sites. In contrast, whereas CS provides no catalytic advantage to substrate-specific enzymes, promiscuous enzymes may uniquely exploit it to recruit a wide range of substrates. The combination of CS and IF, for recruitment and retention of substrates, can potentially optimize the promiscuity of drug metabolizing enzymes and drug transporters.

Redox Signaling by Reactive Electrophiles and Oxidants

The concept of cell signaling in the context of nonenzyme-assisted protein modifications by reactive electrophilic and oxidative species, broadly known as redox signaling, is a uniquely complex topic that has been approached from numerous different and multidisciplinary angles. Our Review reflects on five aspects critical for understanding how nature harnesses these noncanonical post-translational modifications to coordinate distinct cellular activities: (1) specific players and their generation, (2) physicochemical properties, (3) mechanisms of action, (4) methods of interrogation, and (5) functional roles in health and disease. Emphasis is primarily placed on the latest progress in the field, but several aspects of classical work likely forgotten/lost are also recollected. For researchers with interests in getting into the field, our Review is anticipated to function as a primer. For the expert, we aim to stimulate thought and discussion about fundamentals of redox signaling mechanisms and nuances of specificity/selectivity and timing in this sophisticated yet fascinating arena at the crossroads of chemistry and biology.

Oxidative Stress in Methylmercury-Induced Cell Toxicity

Methylmercury (MeHg) is a hazardous environmental pollutant, which elicits significant toxicity in humans. The accumulation of MeHg through the daily consumption of large predatory fish poses potential health risks, and the central nervous system (CNS) is the primary target of toxicity. Despite well-described neurobehavioral effects (i.e., motor impairment), the mechanisms of MeHg-induced toxicity are not completely understood. However, several lines of evidence point out the oxidative stress as an important molecular mechanism in MeHg-induced intoxication. Indeed, MeHg is a soft electrophile that preferentially interacts with nucleophilic groups (mainly thiols and selenols) from proteins and low-molecular-weight molecules. Such interaction contributes to the occurrence of oxidative stress, which can produce damage by several interacting mechanisms, impairing the function of various molecules (i.e., proteins, lipids, and nucleic acids), potentially resulting in modulation of different cellular signal transduction pathways. This review summarizes the general aspects regarding the interaction between MeHg with regulators of the antioxidant response system that are rich in thiol and selenol groups such as glutathione (GSH), and the selenoenzymes thioredoxin reductase (TrxR) and glutathione peroxidase (Gpx). A particular attention is directed towards the role of the PI3K/Akt signaling pathway and the nuclear transcription factor NF-E2-related factor 2 (Nrf2) in MeHg-induced redox imbalance.

ROLE OF THIOLS IN OXIDATIVE STRESS

A well-regulated redox state is essential for normal physiological function and cellular metabolism. In most eukaryotic cells, protein cysteine thiols are most sensitive to fluctuations in the cellular redox state. Under normal physiological conditions, the cytosol has a highly reducing environment, which is due to high levels of reduced glutathione and complex system of redox enzymes that maintain glutathione in the reduced state. The reducing environment of the cytosol maintains most protein thiols in the reduced state; although some non-exposed cysteine could be present as disulfides. Upon physiological increase in cellular oxidants, such as due to growth factors, cytokines and thiol-disulfide exchange reactions, specific proteins could act as redox switches that regulate the conformation and activity of different proteins. This reversible post translational modification enables redox-sensitive dynamic changes in cell signaling and function. Physiological oxidative stress could lead to the formation of sulfenic acids, which are usually intermediate states of thiol oxidation that are converted to higher order oxidation states, intramolecular disulfides or mixed disulfides with glutathione. Such glutathiolation reactions have been found to regulate the function of several proteins involved in intracellular metabolism, signal transduction and cell structure. Excessive oxidative stress results in indiscriminate and irreversible oxidation of protein thiols, depletion of glutathione and cell death. Further elucidation of the relationship between changes in cell redox and thiol reactivity could provide a better understanding of how redox changes regulate cell function and how disruption of these relationships lead to tissue injury and dysfunction and the development of chronic diseases such as cancer and cardiovascular disease.

Dead-end complex, lipid interactions and catalytic mechanism of microsomal glutathione transferase 1, an electron crystallography and mutagenesis investigation

Microsomal glutathione transferase 1 (MGST1) is a detoxification enzyme belonging to the Membrane Associated Proteins in Eicosanoid and Glutathione Metabolism (MAPEG) superfamily. Here we have used electron crystallography of two-dimensional crystals in order to determine an atomic model of rat MGST1 in a lipid environment. The model comprises 123 of the 155 amino acid residues, two structured phospholipid molecules, two aliphatic chains and one glutathione (GSH) molecule. The functional unit is a homotrimer centered on the crystallographic three-fold axes of the unit cell. The GSH substrate binds in an extended conformation at the interface between two subunits of the trimer supported by new in vitro mutagenesis data. Mutation of Arginine 130 to alanine resulted in complete loss of activity consistent with a role for Arginine 130 in stabilizing the strongly nucleophilic GSH thiolate required for catalysis. Based on the new model and an electron diffraction data set from crystals soaked with trinitrobenzene, that forms a dead-end Meisenheimer complex with GSH, a difference map was calculated. The map reveals side chain movements opening a cavity that defines the second substrate site.

A possible S-glutathionylation of specific proteins by glyoxalase II: An in vitro and in silico study

Glyoxalase II, the second of 2 enzymes in the glyoxalase system, is a hydroxyacylglutathione hydrolase that catalyses the hydrolysis of S-d-lactoylglutathione to form d-lactic acid and glutathione, which is released from the active site. The tripeptide glutathione is the major sulfhydryl antioxidant and has been shown to control several functions, including S-glutathionylation of proteins. S-Glutathionylation is a way for the cells to store reduced glutathione during oxidative stress, or to protect protein thiol groups from irreversible oxidation, and few enzymes involved in protein S-glutathionylation have been found to date. In this work, the enzyme glyoxalase II and its substrate S-d-lactoylglutathione were incubated with malate dehydrogenase or with actin, resulting in a glutathionylation reaction. Glyoxalase II was also submitted to docking studies. Computational data presented a high propensity of the enzyme to interact with malate dehydrogenase or actin through its catalytic site and further in silico investigation showed a high folding stability of glyoxalase II toward its own reaction product glutathione both protonated and unprotonated. This study suggests that glyoxalase II, through a specific interaction of its catalytic site with target proteins, could be able to perform a rapid and specific protein S-glutathionylation using its natural substrate S-d-lactoylglutathione.

SIGNIFICANCE

This article reports for the first time a possible additional role of Glo2 that, after interacting with a target protein, is able to promote S-glutathionylation using its natural substrate SLG, a glutathione derived compound. In this perspective, Glo2 can play a new important regulatory role inS-glutathionylation, acquiring further significance in cellular post-translational modifications of proteins.

Protein Thiol Redox Signaling in Monocytes and Macrophages

SIGNIFICANCE

Monocyte and macrophage dysfunction plays a critical role in a wide range of inflammatory disease processes, including obesity, impaired wound healing diabetic complications, and atherosclerosis. Emerging evidence suggests that the earliest events in monocyte or macrophage dysregulation include elevated reactive oxygen species production, thiol modifications, and disruption of redox-sensitive signaling pathways. This review focuses on the current state of research in thiol redox signaling in monocytes and macrophages, including (i) the molecular mechanisms by which reversible protein-S-glutathionylation occurs, (ii) the identification of bona fide S-glutathionylated proteins that occur under physiological conditions, and (iii) how disruptions of thiol redox signaling affect monocyte and macrophage functions and contribute to atherosclerosis. Recent Advances: Recent advances in redox biochemistry and biology as well as redox proteomic techniques have led to the identification of many new thiol redox-regulated proteins and pathways. In addition, major advances have been made in expanding the list of S-glutathionylated proteins and assessing the role that protein-S-glutathionylation and S-glutathionylation-regulating enzymes play in monocyte and macrophage functions, including monocyte transmigration, macrophage polarization, foam cell formation, and macrophage cell death.

CRITICAL ISSUES

Protein-S-glutathionylation/deglutathionylation in monocytes and macrophages has emerged as a new and important signaling paradigm, which provides a molecular basis for the well-established relationship between metabolic disorders, oxidative stress, and cardiovascular diseases.

FUTURE DIRECTIONS

The identification of specific S-glutathionylated proteins as well as the mechanisms that control this post-translational protein modification in monocytes and macrophages will facilitate the development of new preventive and therapeutic strategies to combat atherosclerosis and other metabolic diseases. Antioxid. Redox Signal. 25, 816-835.

Evaluation of human and bovine serum albumin on oxidation characteristics by a photosensitization reaction under complete binding of talaporfin sodium

BACKGROUND

In order to investigate the therapeutic interaction of an extra-cellular photosensitization reaction, we evaluated the oxidation characteristics of human and bovine serum albumin by this reaction with talaporfin sodium under complete binding with albumin by spectroscopic analysis in a cell-free solution.

METHODS

The solution was composed of 20μg/ml talaporfin sodium and 2.1mg/ml human or bovine serum albumin. A 662nm laser light was used to irradiate the solution. Visible absorbance spectra of solutions were measured to obtain the oxidized and non-oxidized relative densities of albumin and talaporfin sodium before and after the photosensitization reaction. The defined oxidation path ratio of talaporfin sodium to albumin reflected the oxidation of the solution. Absorbance wavelengths at approximately 240 and 660nm were used to calculate normalized molecular densities of oxidized albumin and talaporfin sodium, respectively.

RESULTS AND CONCLUSIONS

The oxidation path ratio of talaporfin sodium to albumin when binding human serum albumin was approximately 1.8 times larger than that of bovine serum albumin during the photosensitization reaction from 1 to 50J/cm(2). We hypothesized that the oxidation path ratio results might have been caused by talaporfin sodium binding affinity or binding location difference between the two albumins, because the fluorescence lifetimes of talaporfin sodium bound to human and bovine serum albumin were 7.0 and 4.9ns, respectively. Therefore, the photodynamic therapeutic interaction might be stronger with human serum albumin than with bovine serum albumin in the case of extracellular photosensitization reaction.

Pleiotropic functions of glutathione S-transferase P

Glutathione S-transferase P (GSTP) is one member of the GST superfamily that is prevalently expressed in mammals. Known to possess catalytic activity through deprotonating glutathione allowing formation of thioether bonds with electrophilic substrates, more recent discoveries have broadened our understanding of the biological roles of this protein. In addition to catalytic detoxification, other properties so far ascribed to GSTP include chaperone functions, regulation of nitric oxide pathways, regulation of a variety of kinase signaling pathways, and participation in the forward reaction of protein S-glutathionylation. The expression of GSTP has been linked with cancer and other human pathologies and more recently even with drug addiction. With respect to human health, polymorphic variants of GSTP may determine individual susceptibility to oxidative stress and/or be critical in the design and development of drugs that have used redox pathways as a discovery platform.

Identification of glutathione conjugates of acetylene-containing positive allosteric modulators of metabotropic glutamate receptor subtype 5

A recent medicinal chemistry campaign to identify positive allosteric modulators (PAMs) of metabotropic glutamate receptor subtype 5 (mGluR5) led to the discovery of potent compounds featuring an oxazolidinone structural core flanked by biaryl acetylene and haloaryl moieties. However, biotransformation studies of some of these mGluR5 PAMs demonstrated the formation of glutathione (GSH) conjugates. The conjugates in question were formed independently of NADPH as the main products in liver microsomes and liver cytosol (rat and human) and exhibited masses that were 307 u greater than their respective substrates, indicating the involvement of a reductive step in the formation of these metabolites. To further characterize the relevant metabolic sequences, GSH conjugates of (4R,5R)-5-(3-fluorophenyl)-4-(5-(pyrazin-2-ylethynyl)pyridin-3-yl)oxazolidin-2-one and (4R,5R)-5-(4-fluorophenyl)-4-(6-((3-fluoropyridin-2-yl)ethynyl)pyridin-2-yl)oxazolidin-2-one were biosynthesized and isolated. Subsequent analysis by NMR showed that GSH had reacted with the acetylene carbon atoms of these mGluR5 PAMs, suggesting a conjugate addition mechanism and implicating cytosolic and microsomal GSH S-transferases (GSTs) in catalysis. Interestingly, five closely related mGluR5 PAMs were not similarly prone to the formation of GSH conjugates in vitro. These compounds also featured acetylenes, but were flanked by either phenyl or cyclohexyl rings, which indicated that the formation of GSH conjugates was influenced by proximal functional groups that modulated the electron density of the triple bond and/or differences in enzyme-substrate specificity. These results informed an ongoing drug-discovery effort to identify mGluR5 PAMs with drug-like properties and a low risk of reactivity with endogenous thiols.

Isomerization of Δ5-androstene-3,17-dione into Δ4-androstene-3,17-dione catalyzed by human glutathione transferase A3-3: a computational study identifies a dual role for glutathione

Glutathione transferases (GSTs) are important enzymes in the metabolism of electrophilic xenobiotic and endobiotic toxic compounds. In addition, human GST A3-3 also catalyzes the double bond isomerization of Δ5-androstene-3,17-dione (Δ(5)-AD) and Δ(5)-pregnene-3,20-dione (Δ(5)-PD), which are the immediate precursors of testosterone and progesterone. In fact, GST A3-3 is the most efficient human enzyme known to exist in the catalysis of these reactions. In this work, we have used density functional theory (DFT) calculations to propose a refined mechanism for the isomerization of Δ(5)-AD catalyzed by GST A3-3. In this mechanism the glutathione (GSH) thiol and Tyr9 catalyze the proton transfer from the Δ(5)-AD C4 atom to the Δ(5)-AD C6 atom, with a rate limiting activation energy of 15.8 kcal · mol(-1). GSH has a dual function, because it is also responsible for stabilizing the negative charge that is formed in the O3 atom of the enolate intermediate. The catalytic role of Tyr9 depends on significant conformational rearrangements of its side chain. Neither of these contributions to catalysis has been observed before. Residues Phe10, Leu111, Ala 208, and Ala 216 complete the list of the important catalytic residues. The mechanism detailed here is based on the GST A3-3:GSH:Δ(4)-AD crystal structure and is consistent with all available experimental data.

Causes and consequences of cysteine S-glutathionylation

Post-translational S-glutathionylation occurs through the reversible addition of a proximal donor of glutathione to thiolate anions of cysteines in target proteins, where the modification alters molecular mass, charge, and structure/function and/or prevents degradation from sulfhydryl overoxidation or proteolysis. Catalysis of both the forward (glutathione S-transferase P) and reverse (glutaredoxin) reactions creates a functional cycle that can also regulate certain protein functional clusters, including those involved in redox-dependent cell signaling events. For translational application, S-glutathionylated serum proteins may be useful as biomarkers in individuals (who may also have polymorphic expression of glutathione S-transferase P) exposed to agents that cause oxidative or nitrosative stress.

Proteolytically activated anti-bacterial hydrogel microspheres

Hydrogels are finding increased clinical utility as advances continue to exploit their favorable material properties. Hydrogels can be adapted for many applications, including surface coatings and drug delivery. Anti-infectious surfaces and delivery systems that actively destroy invading organisms are alternative ways to exploit the favorable material properties offered by hydrogels. Sterilization techniques are commonly employed to ensure the materials are non-infectious upon placement, but sterilization is not absolute and infections are still expected. Natural, anti-bacterial proteins have been discovered which have the potential to act as anti-infectious agents; however, the proteins are toxic and need localized release to have therapeutic efficacy without toxicity. In these studies, we explore the use of the glutathione s-transferase (GST) to anchor the bactericidal peptide, melittin, to the surface of poly(ethylene glycol) diacrylate (PEGDA) hydrogel microspheres. We show that therapeutic levels of protein can be anchored to the surface of the microspheres using the GST anchor. We compared the therapeutic efficacy of recombinant melittin released from PEGDA microspheres to melittin. We found that, when released by an activating enzyme, thrombin, recombinant melittin efficiently inhibits growth of the pathogenic bacterium Streptococcus pyogenes as effectively as melittin created by solid phase peptide synthesis. We conclude that a GST protein anchor can be used to immobilize functional protein to PEGDA microspheres and the protein will remain immobilized under physiological conditions until the protein is enzymatically released.

Glutathione S-transferases interact with AMP-activated protein kinase: evidence for S-glutathionylation and activation in vitro

AMP-activated protein kinase (AMPK) is a cellular and whole body energy sensor with manifold functions in regulating energy homeostasis, cell morphology and proliferation in health and disease. Here we apply multiple, complementary in vitro and in vivo interaction assays to identify several isoforms of glutathione S-transferase (GST) as direct AMPK binding partners: Pi-family member rat GSTP1 and Mu-family members rat GSTM1, as well as Schistosoma japonicum GST. GST/AMPK interaction is direct and involves the N-terminal domain of the AMPK β-subunit. Complex formation of the mammalian GSTP1 and -M1 with AMPK leads to their enzymatic activation and in turn facilitates glutathionylation and activation of AMPK in vitro. GST-facilitated S-glutathionylation of AMPK may be involved in rapid, full activation of the kinase under mildly oxidative physiological conditions.

Glutathione Transferase (GST)-Activated Prodrugs

Glutathione transferase (formerly GST) catalyzes the inactivation of various electrophile-producing anticancer agents via conjugation to the tripeptide glutathione. Moreover, several data link the overexpression of some GSTs, in particular GSTP1-1, to both natural and acquired resistance to various structurally unrelated anticancer drugs. Tumor overexpression of these proteins has provided a rationale for the search of GST inhibitors and GST activated cytotoxic prodrugs. In the present review we discuss the current structural and pharmacological knowledge of GST-activated cytotoxic compounds.

Glutathione-s-transferases as determinants of cell survival and death

SIGNIFICANCE

The family of glutathione S-transferases (GSTs) is part of a cellular Phase II detoxification program composed of multiple isozymes with functional human polymorphisms that have the capacity to influence individual response to drugs and environmental stresses. Catalytic activity is expressed through GST dimer-mediated thioether conjugate formation with resultant detoxification of a variety of small molecule electrophiles.

RECENT ADVANCES

More recent work indicates that in addition to the classic catalytic functions, specific GST isozymes have other characteristics that impact cell survival pathways in ways unrelated to detoxification. These characteristics include the following: regulation of mitogen-activated protein kinases; facilitation of the addition of glutathione to cysteine residues in certain proteins (S-glutathionylation); as a novel cellular partner of the human papilloma virus-16 E7 oncoprotein playing a pivotal role in preventing cell death in infected human cells; mitogenic influence in myeloproliferative pathways; participant in the process of cocaine addiction.

CRITICAL ISSUES

Some of these functions have provided a platform for targeting GST with novel small molecule therapeutics, particularly in cancer where evidence of clinical applications is emerging.

FUTURE DIRECTIONS

Our evolving understanding of the GST superfamily and their divergent expression patterns in individuals make them attractive candidates for translational studies in a variety of human pathologies. In addition, their role in regulating cell fate in signaling and cell death pathways has opened up a significant functional complexity that extends well beyond standard detoxification reactions.

Pre-steady-state kinetic characterization of thiolate anion formation in human leukotriene C₄ synthase

Human leukotriene C₄ synthase (hLTC4S) is an integral membrane protein that catalyzes the committed step in the biosynthesis of cysteinyl-leukotrienes, i.e., formation of leukotriene C₄ (LTC₄). This molecule, together with its metabolites LTD₄ and LTE₄, induces inflammatory responses, particularly in asthma, and thus, the enzyme is an attractive drug target. During the catalytic cycle, glutathione (GSH) is activated by hLTC4S that forms a nucleophilic thiolate anion that will attack LTA₄, presumably according to an S(N)2 reaction to form LTC₄. We observed that GSH thiolate anion formation is rapid and occurs at all three monomers of the homotrimer and is concomitant with stoichiometric release of protons to the medium. The pK(a) (5.9) for enzyme-bound GSH thiol and the rate of thiolate formation were determined (k(obs) = 200 s⁻¹). Taking advantage of a strong competitive inhibitor, glutathionesulfonic acid, shown here by crystallography to bind in the same location as GSH, we determined the overall dissociation constant (K(d((GS) = 14.3 μM). The release of the thiolate was assessed using a GSH release experiment (1.3 s⁻¹). Taken together, these data establish that thiolate anion formation in hLTC4S is not the rate-limiting step for the overall reaction of LTC₄ production (k(cat) = 26 s⁻¹), and compared to the related microsomal glutathione transferase 1, which displays very slow GSH thiolate anion formation and one-third of the sites reactivity, hLTC4S has evolved a different catalytic mechanism.

The role of glutathione S-transferase P in signaling pathways and S-glutathionylation in cancer

Glutathione S-transferase P is abundantly expressed in some mammalian tissues, particularly those associated with malignancies. While the enzyme can catalyze thioether bond formation between some electrophilic chemicals and GSH, novel nondetoxification functions are now ascribed to it. This review summarizes recent material that implicates GSTP in mediating S-glutathionylation of specific clusters of target proteins and in reactions that define a negative regulatory role in some kinase pathways through ligand or protein:protein interactions. It is becoming apparent that GSTP participates in the maintenance of cellular redox homeostasis through a number of convergent and divergent mechanisms. Moreover, drug platforms that have GSTP as a target have produced some interesting preclinical and clinical candidates.

Arginine 104 is a key catalytic residue in leukotriene C4 synthase

Human leukotriene C(4) synthase (hLTC(4)S) is an integral membrane enzyme that conjugates leukotriene (LT) A(4) with glutathione to form LTC(4), a precursor to the cysteinyl leukotrienes (LTC(4), LTD(4), and LTE(4)) that are involved in the pathogenesis of human bronchial asthma. From the crystal structure of hLTC(4)S, Arg-104 and Arg-31 have been implicated in the conjugation reaction. Here, we used site-directed mutagenesis, UV spectroscopy, and x-ray crystallography to examine the catalytic role of Arg-104 and Arg-31. Exchange of Arg-104 with Ala, Ser, Thr, or Lys abolished 94.3-99.9% of the specific activity against LTA(4). Steady-state kinetics of R104A and R104S revealed that the K(m) for GSH was not significantly affected. UV difference spectra of the binary enzyme-GSH complex indicated that GSH ionization depends on the presence of Arg-104 because no thiolate signal, with λ(max) at 239 nm, could be detected using R104A or R104S hLTC(4)S. Apparently, the interaction of Arg-104 with the thiol group of GSH reduces its pK(a) to allow formation of a thiolate anion and subsequent nucleophilic attack at C6 of LTA(4). On the other hand, exchange of Arg-31 with Ala or Glu reduced the catalytic activity of hLTC(4)S by 88 and 70%, respectively, without significantly affecting the k(cat)/K(m) values for GSH, and a crystal structure of R31Q hLTC(4)S (2.1 Å) revealed a Gln-31 side chain pointing away from the active site. We conclude that Arg-104 plays a critical role in the catalytic mechanism of hLTC(4)S, whereas a functional role of Arg-31 seems more elusive. Because Arg-104 is a conserved residue, our results pertain to other homologous membrane proteins and represent a structure-function paradigm probably common to all microsomal GSH transferases.

ERp16, an endoplasmic reticulum-resident thiol-disulfide oxidoreductase: biochemical properties and role in apoptosis induced by endoplasmic reticulum stress

We have characterized the properties and putative role of a mammalian thioredoxin-like protein, ERp16 (previously designated ERp18, ERp19, or hTLP19). The predicted amino acid sequence of the 172-residue human protein contains an NH(2)-terminal signal peptide, a thioredoxin-like domain with an active site motif (CGAC), and a COOH-terminal endoplasmic reticulum (ER) retention sequence (EDEL). Analyses indicated that the mature protein (comprising 146 residues) is generated by cleavage of the 26-residue signal peptide and is localized in the lumen of the ER. Biochemical experiments with the recombinant mature protein revealed it to be a thioldisulfide oxidoreductase. Its redox potential was about -165 mV; its active site cysteine residue Cys(66) was nucleophilic with a pK(a) value of approximately 6.6; it catalyzed the formation, reduction, and isomerization of disulfide bonds, with the unusual CGAC active site motif being responsible for these activities; and it existed as a dimer and underwent a redox-dependent conformational change. The observations that the redox potential of ERp16 (-165 mV) was within the range of that of the ER (-135 to -185 mV) and that ERp16 catalyzed disulfide isomerization of scrambled ribonuclease A suggest a role for ERp16 in protein disulfide isomerization in the ER. Expression of ERp16 in HeLa cells inhibited the induction of apoptosis by agents that elicit ER stress, including brefeldin A, tunicamycin, and dithiothreitol. In contrast, expression of a catalytically inactive mutant of ERp16 potentiated such apoptosis, as did depletion of ERp16 by RNA interference. Our results suggest that ERp16 mediates disulfide bond formation in the ER and plays an important role in cellular defense against prolonged ER stress.

Structural Basis for Detoxification and Oxidative Stress Protection in Membranes

Synthesis of mediators of fever, pain and inflammation as well as protection against reactive molecules and oxidative stress is a hallmark of the MAPEG superfamily (membrane associated proteins in eicosanoid and glutathione metabolism). The structure of a MAPEG member, rat microsomal glutathione transferase 1, at 3.2 A resolution, solved here in complex with glutathione by electron crystallography, defines the active site location and a cytosolic domain involved in enzyme activation. The glutathione binding site is found to be different from that of the canonical soluble glutathione transferases. The architecture of the homotrimer supports a catalytic mechanism involving subunit interactions and reveals both cytosolic and membraneous substrate entry sites, providing a rationale for the membrane location of the enzyme.

Oxidized and synchrotron cleaved structures of the disulfide redox center in the N-terminal domain of Salmonella typhimurium AhpF

The flavoprotein component (AhpF) of Salmonella typhimurium alkyl hydroperoxide reductase contains an N-terminal domain (NTD) with two contiguous thioredoxin folds but only one redox-active disulfide (within the sequence -Cys129-His-Asn-Cys132-). This active site is responsible for mediating the transfer of electrons from the thioredoxin reductase-like segment of AhpF to AhpC, the peroxiredoxin component of the two-protein peroxidase system. The previously reported crystal structure of AhpF possessed a reduced NTD active site, although fully oxidized protein was used for crystallization. To further investigate this active site, we crystallized an isolated recombinant NTD (rNTD); using diffraction data sets collected first at our in-house X-ray source and subsequently at a synchrotron, we showed that the active site disulfide bond (Cys129-Cys132) is oxidized in the native crystals but becomes reduced during synchrotron data collection. The NTD disulfide bond is apparently particularly sensitive to radiation cleavage compared with other protein disulfides. The two data sets provide the first view of an oxidized (disulfide) form of NTD and show that the changes in conformation upon reduction of the disulfide are localized and small. Furthermore, we report the apparent pKa of the active site thiol to be approximately 5.1, a relatively low pKa given its redox potential (approximately 265 mV) compared with most members of the thioredoxin family.

Insights into the Catalytic Mechanism of Glutathione S-Transferase: The Lesson from Schistosoma haematobium

Glutathione S-transferases (GSTs) are involved in detoxification of xenobiotic compounds and in the biosynthesis of important metabolites. All GSTs activate glutathione (GSH) to GS(-); in many GSTs, this is accomplished by a Tyr at H-bonding distance from the sulfur of GSH. The high-resolution structure of GST from Schistosoma haematobium revealed that the catalytic Tyr occupies two alternative positions, one external, involving a pi-cation interaction with the conserved Arg21, and the other inside the GSH binding site. The interaction with Arg21 lowers the pK(a) of the catalytic Tyr10, as required for catalysis. Examination of several other GST structures revealed the presence of an external pocket that may accommodate the catalytic Tyr, and suggested that the change in conformation and acidic properties of the catalytic Tyr may be shared by other GSTs. Arginine and two other residues of the external pocket constitute a conserved structural motif, clearly identified by sequence comparison.

Synthesis and activity of novel glutathione analogues containing an urethane backbone linkage

The new GSH analogues H-Glo(-Ser-Gly-OH)-OH (5), its O-benzyl derivative 4, and H-Glo(-Asp-Gly-OH)-OH (9), characterized by the replacement of central cysteine with either serine or aspartic acid, and containing an urethanic fragment as isosteric substitution of the scissile gamma-glutamylic junction, have been synthesized and characterized. Their ability to inhibit human GST P1-1 (hGST P1-1) in comparison with H-Glu(-Ser-Gly-OH)-OH and H-Glu(-Asp-Gly-OH)-OH, which are potent competitive inhibitors of rat GST 3-3 and 4-4, has been evaluated. In order to further investigate the effect of the isosteric substitution on the binding abilities of the new GSH analogues 4, 5 and 9, the previously reported cysteinyl-containing analogue H-Glo(-Cys-Gly-OH)-OH has been also evaluated as a co-substrate for hGSTP1-1.