Glutathione Transferase: New Model for Glutathione Activation

Human GST P1-1 Redesigned for Enhanced Catalytic Activity with the Anticancer Prodrug Telcyta and Improved Thermostability

Protein engineering can be used to tailor enzymes for medical purposes, including antibody-directed enzyme prodrug therapy (ADEPT), which can act as a tumor-targeted alternative to conventional chemotherapy for cancer. In ADEPT, the antibody serves as a vector, delivering a drug-activating enzyme selectively to the tumor site. Glutathione transferases (GSTs) are a family of naturally occurring detoxication enzymes, and the finding that some of them are overexpressed in tumors has been exploited to develop GST-activated prodrugs. The prodrug Telcyta is activated by GST P1-1, which is the GST most commonly elevated in cancer cells, implying that tumors overexpressing GST P1-1 should be particularly vulnerable to Telcyta. Promising antitumor activity has been noted in clinical trials, but the wildtype enzyme has modest activity with Telcyta, and further functional improvement would enhance its usefulness for ADEPT. We utilized protein engineering to construct human GST P1-1 gene variants in the search for enzymes with enhanced activity with Telcyta. The variant Y109H displayed a 2.9-fold higher enzyme activity compared to the wild-type GST P1-1. However, increased catalytic potency was accompanied by decreased thermal stability of the Y109H enzyme, losing 99% of its activity in 8 min at 50 °C. Thermal stability was restored by four additional mutations simultaneously introduced without loss of the enhanced activity with Telcyta. The mutation Q85R was identified as an important contributor to the regained thermostability. These results represent a first step towards a functional ADEPT application for Telcyta.

Overexpression of Glutathione S-Transferases in Human Diseases: Drug Targets and Therapeutic Implications

Glutathione S-transferases (GSTs) are a major class of phase II metabolic enzymes. Besides their essential role in detoxification, GSTs also exert diverse biological activities in the occurrence and development of various diseases. In the past few decades, much research interest has been paid to exploring the mechanisms of GST overexpression in tumor drug resistance. Correspondingly, many GST inhibitors have been developed and applied, solely or in combination with chemotherapeutic drugs, for the treatment of multi-drug resistant tumors. Moreover, novel roles of GSTs in other diseases, such as pulmonary fibrosis and neurodegenerative diseases, have been recognized in recent years, although the exact regulatory mechanisms remain to be elucidated. This review, firstly summarizes the roles of GSTs and their overexpression in the above-mentioned diseases with emphasis on the modulation of cell signaling pathways and protein functions. Secondly, specific GST inhibitors currently in pre-clinical development and in clinical stages are inventoried. Lastly, applications of GST inhibitors in targeting cell signaling pathways and intracellular biological processes are discussed, and the potential for disease treatment is prospected. Taken together, this review is expected to provide new insights into the interconnection between GST overexpression and human diseases, which may assist future drug discovery targeting GSTs.

Acute toxicity, oxidative stress, and apoptosis due to short-term triclosan exposure and multi- and transgenerational effects on in vivo endpoints, antioxidant defense, and DNA damage response in the freshwater water flea Daphnia magna

In this study, we measured the acute toxicity of triclosan (TCS) in neonate and adult Daphnia magna water fleas. The median lethal concentrations were 184.689 and 349.511 μg/L, respectively. Oxidative stress induced by TCS was analyzed based on changes in reactive oxygen species (ROS) content and antioxidant enzymatic activities in D. magna. Based on these endpoints, TCS concentrations of 50 and 100 μg/L induced oxidative stress. However, several apoptosis-mediated proteins showed TCS-induced oxidative-stress damage in response to 25 μg/L, indicating that apoptotic proteins were the most sensitive mediators. We also evaluated the multi- and transgenerational effects of TCS on D. magna over three generations in terms of various in vivo endpoints, DNA damage responses, and biochemical reactions. The transgenerational group exposed to TCS exhibited greater negative impacts on antioxidant responses, DNA fragmentation status, and biological endpoints compared with the multigenerational exposure group, leading to decreased reproductive rates and higher ROS content. The transcriptional expression levels of glutathione S-transferase genes in the transgenerational exposure group were upregulated compared to those in the multigenerational group but were fully recovered in F2 offspring. Our findings provide an in-depth understanding of the adaptive effects of multigenerational exposure to TCS.

Biochemical and Structural Characterization of Chi-Class Glutathione Transferases: A Snapshot on the Glutathione Transferase Encoded by sll0067 Gene in the Cyanobacterium Synechocystis sp. Strain PCC 6803

Glutathione transferases (GSTs) constitute a widespread superfamily of enzymes notably involved in detoxification processes and/or in specialized metabolism. In the cyanobacterium Synechocsytis sp. PCC 6803, SynGSTC1, a chi-class GST (GSTC), is thought to participate in the detoxification process of methylglyoxal, a toxic by-product of cellular metabolism. A comparative genomic analysis showed that GSTCs were present in all orders of cyanobacteria with the exception of the basal order Gloeobacterales. These enzymes were also detected in some marine and freshwater noncyanobacterial bacteria, probably as a result of horizontal gene transfer events. GSTCs were shorter of about 30 residues compared to most cytosolic GSTs and had a well-conserved SRAS motif in the active site (¹⁰SRAS¹³ in SynGSTC1). The crystal structure of SynGSTC1 in complex with glutathione adopted the canonical GST fold with a very open active site because the α4 and α5 helices were exceptionally short. A transferred multipolar electron-density analysis allowed a fine description of the solved structure. Unexpectedly, Ser10 did not have an electrostatic influence on glutathione as usually observed in serinyl-GSTs. The S10A variant was only slightly less efficient than the wild-type and molecular dynamics simulations suggested that S10 was a stabilizer of the protein backbone rather than an anchor site for glutathione.

A novel microRNA IamiR-4-3p from water spinach (Ipomoea aquatica Forsk.) increased Cd uptake and translocation in Arabidopsis thaliana

MicroRNAs (miRNAs) play important roles in plant response to Cd stress. In our previous study, we observed significant differences in the expression levels of IamiR-4-3p between high-Cd and low-Cd cultivars of water spinach. The function of IamiR-4-3p was investigated by using wild type Arabidopsis (WT), Arabidopsis transfected with empty vector pCambia1302 (CK), and Arabidopsis transfected with IamiR-4-3p + vector pCambia1302 (p35S::miR-4-3p) in this study. In p35S::miR-4-3p Arabidopsis, the expression levels of GST3 and AWPM19-like were reduced by 20% and 24%. Under Cd treatment, higher root and shoot Cd concentrations were detected in the transgenic p35S::miR-4-3p Arabidopsis. MDA and H₂O₂ concentrations were positively correlated with the Cd concentrations in all Arabidopsis. The elevated GSH pool in p35S::miR-4-3p Arabidopsis should compensate for its restricted GST3 expression in response to Cd-induced oxidative stress. Lower F1 (cell wall) and higher F2 (organelle) and F3 (soluble fraction) Cd concentrations were observed along with the reduced ABA level in p35S::miR-4-3p Arabidopsis, which could induce a weakened apoplastic barrier and higher Cd accumulation and translocation in roots. It is suggested that IamiR-4-3p is able to reduce the expression levels of GST3 and AWPM19-like, resulting in higher Cd uptake and translocation in Arabidopsis.

A novel yeast strain Geotrichum sp. CS-67 capable of accumulating heavy metal ions

Bioremediation, which has several advantages over traditional methods, represents an alternative means of dealing with heavy metal pollution. We screened for microorganisms showing heavy metal tolerance in polluted mangrove soils. A novel yeast, Geotrichum sp. CS-67, was discovered and tested for tolerance of Cu²⁺, Zn²⁺, and Ni²⁺. Zn²⁺ was the most efficiently sequestered by Geotrichum sp. CS-67 followed by Ni²⁺ and Cu²⁺. Zn²⁺ and Ni²⁺ were actively taken up into the cell, while Cu²⁺ was adsorbed to the cell wall. We used RNA-Seq to show that a large number of genes involved in the physiological and biochemical processing of heavy metals were differentially expressed in this yeast when it was subjected to Zn²⁺ and Ni²⁺ stress. From this panel, we selected the SED1, GDI1 and ZRT1 genes for validation by qRT-PCR and discovered that, during Zn²⁺ and Ni²⁺ stress, SED1 and GDI1 were upregulated, while ZRT1 was downregulated, which was consistent with the RNA-Seq results and the biochemical function of these genes. In conclusion, the novel yeast Geotrichum sp. CS-67 has a marked ability to accumulate heavy metal ions, making it of great interest as a possible microbial agent for heavy metal pollution remediation in the future.

Rat liver glutathione S-transferase-catalyzed conjugation of glutathione to the endogenous epoxides of oleic acid and cholesterol

cis-9,10-Epoxy-octadecanoic acid (oleic acid epoxide, OAE) and 5α,6α-epoxy-cholesterol (ChE) are endogenous epoxides. Unlike other epoxides, the oxirane groups of OAE and ChE are relatively stable against nucleophiles. OAE lacks toxicity and mutagenicity, while ChE is considered harmful, mutagenic and cancerogenic to animals. In humans, ChE is associated with cancer. The metabolism of OAE and ChE includes hydrolysis by cytosolic and microsomal hydrolases to their diols and glutathione (GSH) conjugation by GSH S-transferases (GST) to form the GSH conjugates (R-SG; R, residue). The GST-catalyzed GSH conjugation of OAE and ChE is poorly investigated. This article reports on the GSH conjugation of OAE, its methyl ester (OAEMe) and of ChE by rat liver homogenate GST. The GSH conjugates of OAE, OAEMe and ChE, i.e., OAE-SG, OAEMe-SG and ChE-SG, respectively, were determined by pre-column derivatization with o-phthaldialdehyde (OPA)/2-mercaptoethanol, high-performance liquid chromatography (HPLC) and fluorescence detection. Complex biphasic kinetics were observed with substrate inhibition of GST activity by OAE, OAEMe and ChE, an optimum pH of about 8.3 for OAE, and no measurable chemical GSH conjugation, underlying the importance of GST for the biotransformation of these epoxides. The results confirm the substrate concentration-dependent kinetic mechanism of GST isoforms first reported by William B. Jakoby (J. Biol. Chem. 1974) for exogenous electrophiles including the epoxide 1,2-epoxy-3-(p-nitrophenoxy)propane and the organic nitrates. This mechanism allows for maximal GST activity that can be achieved under given concentrations of GSH, epoxides and other electrophiles.

Non-synonymous substitution of evolutionarily conserved residue in Tau class glutathione transferases alters structural and catalytic features

Plant-specific tau glutathione transferases (GSTs) are basically involved in catalysing γ-glutathione (GSH)-dependent conjugation reactions with pesticides and herbicides, which play an important role in the detoxification of pollutants. Given the lack of systematic biochemical and structural information on tau GSTs, the study of their mediated defence mechanisms against toxic compounds has been greatly hindered. Here, we reveal the importance of the Ile residue closely interacting with GSH for the structural stability and catalytic function of GST. Evolutionary conservation analysis indicated that the crucial G-site Ile55 in the SbGSTU6 was converted to Thr53 of SbGSTU7. The comparative biochemical data on SbGSTU6, SbGSTU7 and their mutants showed that the substitution of Ile by Thr caused significant decrease in the affinity and catalytic efficiency of the GSTs. The unfavourable structural flexibility and pKa distribution of the active cavity residues were also demonstrated. Crystallography studies and molecular dynamics simulations showed that the conversion resulted in the hydrogen bond recombination with GSH and conformational rearrangement of GST active cavity, in which the Ile residue was more conducive to the formation of enzyme substrate complexes. The extensive biochemical and structural data not only reveal the critical role of the conserved G-site Ile residue in catalysing GSH-conjugate reactions but also provide valuable resources for the development of GST engineering in analytical and agricultural biotechnology.

Copper Tolerance Mechanism of the Novel Marine Multi-Stress Tolerant Yeast Meyerozyma guilliermondii GXDK6 as Revealed by Integrated Omics Analysis

Various agricultural products used in food fermentation are polluted by heavy metals, especially copper, which seriously endangers human health. Methods to remove copper with microbial strategies have gained interests. A novel Meyerozyma guilliermondii GXDK6 could survive independently under high stress of copper (1400 ppm). The copper tolerance mechanism of GXDK6 was revealed by integrated omics in this work. Whole-genome analysis showed that nine genes (i.e., CCC2, CTR3, FRE2, GGT, GST, CAT, SOD2, PXMP4, and HSP82) were related to GXDK6 copper tolerance. Copper stress elevated glutathione metabolism-related gene expression, glutathione content, and glutathione sulfur transferase activity, suggesting enhanced copper conjugation and detoxification in cells. The inhibited copper uptake by Ctr3 and enhanced copper efflux by Ccc2 contributed to the decrease in intracellular copper concentration. The improved expression of antioxidant enzyme genes (PXMP4, SOD2, and CAT), accompanied by the enhanced activities of antioxidant enzymes (peroxidase, superoxide dismutase, and catalase), decreased copper-induced reactive oxygen species production, protein carbonylation, lipid peroxidation, and cell death. The metabolite D-mannose against harsh stress conditions was beneficial to improving copper tolerance. This study contributed to understanding the copper tolerance mechanism of M. guilliermondii and its application in removing copper during fermentation.

Structural and functional analysis of the inhibition of equine glutathione transferase A3-3 by organotin endocrine disrupting pollutants

Organotin compounds are highly toxic environmental pollutants with neurotoxic and endocrine-disrupting effects. They are potent inhibitors of glutathione transferases (GSTs), thus impeding their detoxication and antioxidant functions. Several GSTs, including equine GST A3-3 (EcaGST A3-3), exhibit steroid double-bond isomerase activity and are involved in the biosynthesis of testosterone and progesterone. We have performed enzyme kinetics analyses of the inhibition of EcaGST A3-3 by organotin compounds. We have also solved crystal structures of EcaGST A3-3 in complexes with glutathione, and with glutathione together with covalently bound triethyltin. Our structural data indicate that the tin atom forms strong bonds with a covalent character not only with the glutathione, but also with a tyrosyl residue of the enzyme itself, thereby preventing the release of the glutathione-organotin adduct and completely blocking the enzyme function. This work presents a structural basis for the general mechanism of GST inhibition by organotin compounds and contributes to the understanding of their neurotoxic and endocrine disrupting effects.

Biotransformation of organic nitrates by glutathione S-transferases and other enzymes: An appraisal of the pioneering work by William B. Jakoby

Organic nitrates (R-ONO₂; R, organic residue) such as nitroglycerin are used as drugs in part for more than a century. Their pharmacological use is associated with clinically relevant tolerance which is reportedly known since 1888. The underlying mechanisms of both, the mechanisms of action and the main pharmacological effect, which is vasodilatation and reduction of blood pressure, and the development of tolerance, which means increasing need of drug amount in sustained long-term therapy, are still incompletely understood. William B. Jakoby and associates were the first to report the biotransformation of organic nitrates, notably including nitroglycerin (i.e., glycerol trinitrate; GTN), by glutathione S-transferase (GST)-catalyzed conjugation of glutathione (GSH) to the nitrogen atom of one of the three nitrate groups of GTN to generate glutathione sulfenyl nitrite (glutathione thionitrate, S-nitroglutathione; GSNO₂). Jakoby's group was also the first to suggest that GSNO₂ reacts with a second GSH molecule to produce inorganic nitrite (ONO^-) and glutathione disulfide (GSSG) without the catalytic involvement of GST. This mechanism has been adopted by others to the biotransformation of GTN by mitochondrial aldehyde dehydrogenase (mtALDH-(CysSH)₂) which does not require GSH as a substrate. The main difference between these reactions is that mtALDH forms an internal thionitrate (mtALDH-(CysSH)-CysSNO₂) which releases inorganic nitrite upon intra-molecular reaction to form mtALDH disulfide (mtALDH-(CysS)₂). Subsequently, ONO^- and GSNO₂ are reduced by several proteins and enzymes to nitric oxide (NO) which is a very potent activator of soluble guanylyl cyclase to finally relax the smooth muscles thus dilating the vasculature. GSNO₂ is considered to rearrange to GSONO which undergoes further reactions including GSNO and GSSG formation. The present article is an appraisal of the pioneering work of William B. Jakoby in the area of the biotransformation of organic nitrates by GST. The two above mentioned enzymatic reactions are discussed in the context of tolerance development to organic nitrates, still a clinically relevant pharmacological concern.

Effects of temperature changes on life parameters, oxidative stress, and antioxidant defense system in the monogonont marine rotifer Brachionus plicatilis

Global warming is a big concern for all organisms and many efforts have been made to reveal the potential effects of temperature elevation on aquatic organisms. However, limited studies on molecular mechanistic approaches on physiological effects due to temperature changes are available. Here, we investigated the effects of temperature changes on life parameters (e.g., population growth [total number of rotifers], and lifespan), oxidative stress levels and antioxidant activities (e.g., glutathione S-transferase [GST], catalase [CAT], superoxide dismutase [SOD]) with expression levels in the monogonont marine rotifer Brachionus plicatilis. The changes in temperatures led to significant reduction (P < 0.05) in lifespan, possibly due to significant decrease (P < 0.05) in antioxidant activities, reducing the potential to cope with significant elevation in the temperature-induced oxidative stress in B. plicatilis. To further assess the actual induction and clearance of reactive oxygen species (ROS), N-acetyl-L-cysteine was used to examine whether the temperature-induced oxidative stress could be successfully scavenged. Furthermore, expression patterns of the antioxidant-related genes (GSTs, SODs, and CATs) were down- or upregulated (P < 0.05) in response to different temperatures in B. plicatilis. Overall, these findings indicate that ROS-mediated oxidative stress led to cellular damage and antioxidant defense system, resulting in deleterious effects on life parameters in rotifer.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Acid stress induces cross-protection for cadmium tolerance of multi-stress-tolerant Pichia kudriavzevii by regulating cadmium transport and antioxidant defense system

The toxicity of cadmium (Cd) is the major limitation to its removal using microorganisms. The Cd tolerance of Pichia kudriavzevii was obviously enhanced by acid stress based on multi-stress cross-protection. RNA-Seq showed that most differentially expressed genes (DEGs) in the Pentose phosphate pathway, Citrate cycle (TCA cycle), Glycolysis/Gluconeogenesis, Peroxisome and Glutathione metabolism were up-regulated by acid stress. The up-regulated expression of genes related to ATP synthesis (GOR1, ALD5, ADH4, ADH6, MDH2, IDH1, IDH2, and ATP19) and Cd transport (GSTY2, GTO2, GLO2, and YOR1), and the improvement of intracellular GSH level and GST activity, reduced the Cd toxicity towards P. kudriavzevii. Cd efflux by YOR1 played a key role in the decline of intracellular Cd level. Acid stress obviously improved the gene expression levels and activities of antioxidant enzymes (SOD, POD, and CAT), which inhibited the Cd-induced ROS outburst and oxidative damage of proteins and membrane lipids. In addition, the enhanced expression of HSP12 protected P. kudriavzevii from the damage of Cd stress. These results provide some important clues to reconstruct robust strains using for Cd removal in aquatic environments.

On the Inhibition Mechanism of Glutathione Transferase P1 by Piperlongumine. Insight From Theory

Piperlongumine (PL) is an anticancer compound whose activity is related to the inhibition of human glutathione transferase of pi class (GSTP1) overexpressed in cancerous tumors and implicated in the metabolism of electrophilic compounds. In the present work, the inhibition mechanism of hydrolyzed piperlongumine (hPL) has been investigated employing QM and QM/MM levels of theory. The potential energy surfaces (PESs) underline the contributions of Tyr residue close to G site in the catalytic pocket of the enzyme. The proposed mechanism occurs through a one-step process represented by the nucleophilic addition of the glutathione thiol to electrophilic species giving rise to the simultaneous C-S and H-C bonds formation. Both the used methods give barrier heights (19.8 and 21.5 kcal mol⁻¹ at QM/MM and QM, respectively) close to that experimentally measured for the C-S bond formations (23.8 kcal mol⁻¹).

Adverse effects of two pharmaceuticals acetaminophen and oxytetracycline on life cycle parameters, oxidative stress, and defensome system in the marine rotifer Brachionus rotundiformis

To investigate the adverse effect of two widely used pharmaceuticals, paracetamol (acetaminophen [APAP]) and oxytetracycline (OTC) on the marine rotifer Brachionus rotundiformis (B. rotundiformis), the animals were exposed to various environmentally-relevant concentrations. Up to date, acetaminophen and oxytetracycline have been considered as toxic, if used above threshold concentration, i.e. overdosed. However, this study demonstrated these two pharmaceuticals even at low concentration (i.e., μg/L scale) elicited oxidative stress through the generation of reactive oxygen species (ROS) along with the increased glutathione S-transferase activity, despite no-observed effect in in-vivo population growth. To validate the adverse effects of the two pharmaceuticals at relatively low concentrations, mRNA expression analysis was performed of the entire set of genes encoding 26 cytochrome P450s (CYPs) of phase I and 19 glutathione S-transferases (GSTs) of phase II of the rotifer B. rotundiformis. The mRNA expression analysis suggested specific genes CYP3045A2 and GSTσ1, GSTσ4, and GSTω1 take part in detoxification of APAP and OTC, resulting in no significant changes in the population growth and undetermined no observed effect concentration (NOEC) in the marine rotifer B. rotundiformis.

Cadmium detoxification induced by salt stress improves cadmium tolerance of multi-stress-tolerant Pichia kudriavzevii

Heavy metal tolerance of microorganisms is the basis of heavy metal removal by growing cells. In this study, a cross-protection effect generated by salt stress significantly enhanced the cadmium tolerance of multi-stress-tolerant Pichia kudriavzevii. Comparative transcriptome analysis using RNA-Seq linked with physiological and biochemical observation was used to elucidate the underlying mechanisms of the improved cadmium tolerance. The expression of cadmium transport related genes (GSTY2, GLR1, GLO2, YCF1 and YOR1), GSH content and GST activity were elevated by salt stress, suggesting enhanced cadmium conjugation and detoxification in yeast cells. The inhibited cadmium uptake by ZRT1 and enhanced cadmium efflux by YOR1 contributed to the decrease in the intracellular cadmium concentration. The improved expression of antioxidant enzyme genes (SOD1, SOD2, SOD6, CAT1 and PRXIID), along with the enhanced activities of antioxidant enzymes (SOD, CAT and POD) resulted in a decrease in cadmium-induced ROS production, protein carbonylation, lipid peroxidation and cell death. The abundant expression of heat shock protein genes (HSP12, HSP10 and SSC1) and genes related to trehalose synthesis (TPS1 and TSL1) induced by salt stress protected yeast cells against complex stress conditions, contributing to the improved cadmium tolerance. These findings will be useful to develop cadmium-tolerant yeasts for cadmium removal by growing cells.

Effects of bisphenol A and its analogs bisphenol F and S on life parameters, antioxidant system, and response of defensome in the marine rotifer Brachionus koreanus

To understand the adverse outcome in response to bisphenol A and its analogs bisphenol F and S (BPA, BPF, and BPS), we examined acute toxicity, life parameter, and defensome in the marine rotifer Brachionus koreanus. Among the bisphenol analogs, BPA showed the highest acute toxicity and then BPF and BPS, accordingly in the view of descending magnitude of toxicity. In life parameters including life span and reproduction, BPA, BPF, and BPS were found to cause adverse effect. Both intracellular ROS level and GST activity were significantly increased (P < 0.05) in response to each dosage of bisphenol analogs exposures. In response to bisphenol analogs, defensomes of phase I, II, and III detoxification mechanism demonstrated inverse relationship between the lipophilicity of bisphenol analogs and the expression patterns of defensomes. BPA and BPF were found to have significant modulation (P < 0.05) in the expression of cytochrome P450 (CYP) and GST genes. In phase III, BPS with comparatively lower lipophilicity demonstrated highly diversified expressional pattern, suggesting that BPS is likely caused less toxicity compared to BPA and BPF. In this study, via phase I, II, and III detoxification mechanism, bisphenol A and its analogs F and S demonstrated specific detoxification mechanism in rotifer.

Recent advances in protein engineering and biotechnological applications of glutathione transferases

Glutathione transferases (GSTs, EC 2.5.1.18) are a widespread family of enzymes that play a central role in the detoxification, metabolism, and transport or sequestration of endogenous or xenobiotic compounds. During the last two decades, delineation of the important structural and catalytic features of GSTs has laid the groundwork for engineering GSTs, involving both rational and random approaches, aiming to create new variants with new or altered properties. These approaches have expanded the usefulness of native GSTs, not only for understanding the fundamentals of molecular detoxification mechanisms, but also for the development medical, analytical, environmental, and agricultural applications. This review article attempts to summarize successful examples and current developments on GST engineering, highlighting in parallel the recent knowledge gained on their phylogenetic relationships, structural/catalytic features, and biotechnological applications.

Effects of triclosan (TCS) on fecundity, the antioxidant system, and oxidative stress-mediated gene expression in the copepod Tigriopus japonicus

Triclosan (TCS) is an antimicrobial agent that has been widely dispersed and detected in the marine environment. However, the effects of TCS in marine invertebrates are poorly understood. In this study, the effects of TCS on life cycle history (e.g. mortality and fecundity) along with cellular reactive oxygen species (ROS) levels, GSH content, antioxidant enzymatic activities, and mRNA expression levels of oxidative stress-mediated genes were measured in the copepod Tigriopus japonicus. The no observed effect concentration (NOEC) and median lethal concentration (LC50) of TCS in the adult stage were determined to be 300μg/L and 437.476μg/L, respectively, while in the nauplius stages the corresponding values were 20μg/L, and 51.76μg/L, respectively. Fecundity was significantly reduced (P<0.05) in response to TCS at 100μg/L. Concentration- and time-dependent analysis of ROS, GSH content (%), and antioxidant enzymatic activities (e.g. GST, GPx, and SOD) were significantly increased (P<0.05) in response to TCS exposure. Additionally, mRNA expression of detoxification (e.g., CYPs) and antioxidant (e.g., glutathione S-transferase-sigma isoforms, Cu/Zn superoxide dismutase, catalase) genes was modulated in response to TCS exposure at different concentrations over a 24h period. Our results revealed that TCS can induce reduced fecundity and oxidative stress with transcriptional regulation of oxidative stress-mediated genes with activation of the antioxidant system in the copepod T. japonicus.

Adverse effects of BDE-47 on life cycle parameters, antioxidant system, and activation of MAPK signaling pathway in the rotifer Brachionus koreanus

2,2',4,4'-Tetrabromodiphenyl ether (BDE-47) is widely dispersed endocrine disrupting chemicals (EDCs) in the aquatic ecosystem. Due to its devastating effect on marine organisms and insufficient database on toxicology, we investigated the adverse effects of BDE-47 on life parameters and antioxidant defense system following the reactive oxygen species (ROS) production in the monogonont rotifer Brachionus koreanus. In B. koreanus, the reduction in life cycle, fecundity, and population growth were observed in response to BDE-47. 50μg/L BDE-47 significantly reduced (P<0.05) life expectancy and net reproductive rate. In response to 10-50μg/L BDE-47 exposure, the oxidative stress was elicited via the generation of ROS, while the antioxidant related enzymes (e.g. glutathione S-transferase [GST] and glutathione reductase [GR]) have demonstrated significant activity levels (P<0.05) to further alleviate the oxidative stress in a concentration dependent manner. Furthermore, transcript profiles of antioxidant function (GST-A, -O, and -S1-S8)-related genes have shown the significant increase over 24h in response to BDE-47 (0, 10, 25, and 50μg/L). As for MAPK signaling pathway analysis, up-regulation of their activities was observed at 25μg/L BDE-47 but their activities have reduced at adult NOEC concentration of 50μg/L. This study provides a better understanding of the effects of BDE-47 on life parameters, molecular defense system, and activation of MAPK signaling pathway against generated oxidants in the rotifer.

RNA-seq analysis revealed ROS-mediated related genes involved in cadmium detoxification in the razor clam Sinonovacula constricta

The razor clam Sinonovacula constricta is a eukaryotic benthic intertidal bivalve species that is tolerant to different heavy metals, such as cadmium ion (Cd(2+)). However, the mechanism by which S. constricta responds to Cd(2+)-induced stress remains unclear. In this study, eight transcriptome libraries were constructed and sequenced using razor clams exposed to Cd(2+) for 12 and 48 h. A total of 18,330 unigenes with an average length of 500 bp were annotated. Among these 18,330 unigenes, 582 and 649 displayed differential expression profiles at 12 and 48 h in the gill, respectively. The corresponding differential unigenes in the hepatopancreas were 1056 and 382. Gene Ontology annotation revealed that these unigenes were highly pronounced in metabolic process, cellular process, binding, and catalytic activity. Notably, ROS production-related genes, such as heat shock proteins 32, metallothionein, and glutathione, were synchronously enriched in all experimental samples with induced expression profiles, which was also validated by qPCR. Our results highlighted the relation between immune regulation and Cd(2+)-induced stress in razor clam and provided new insights into the molecular mechanisms of heavy toxicology.

A dynamic Asp-Arg interaction is essential for catalysis in microsomal prostaglandin E2 synthase

Microsomal prostaglandin E2 synthase type 1 (mPGES-1) is responsible for the formation of the potent lipid mediator prostaglandin E2 under proinflammatory conditions, and this enzyme has received considerable attention as a drug target. Recently, a high-resolution crystal structure of human mPGES-1 was presented, with Ser-127 being proposed as the hydrogen-bond donor stabilizing thiolate anion formation within the cofactor, glutathione (GSH). We have combined site-directed mutagenesis and activity assays with a structural dynamics analysis to probe the functional roles of such putative catalytic residues. We found that Ser-127 is not required for activity, whereas an interaction between Arg-126 and Asp-49 is essential for catalysis. We postulate that both residues, in addition to a crystallographic water, serve critical roles within the enzymatic mechanism. After characterizing the size or charge conservative mutations Arg-126-Gln, Asp-49-Asn, and Arg-126-Lys, we inferred that a crystallographic water acts as a general base during GSH thiolate formation, stabilized by interaction with Arg-126, which is itself modulated by its respective interaction with Asp-49. We subsequently found hidden conformational ensembles within the crystal structure that correlate well with our biochemical data. The resulting contact signaling network connects Asp-49 to distal residues involved in GSH binding and is ligand dependent. Our work has broad implications for development of efficient mPGES-1 inhibitors, potential anti-inflammatory and anticancer agents.

Glutathione transferases in the bioactivation of azathioprine

The prodrug azathioprine is primarily used for maintaining remission in inflammatory bowel disease, but approximately 30% of the patients suffer adverse side effects. The prodrug is activated by glutathione conjugation and release of 6-mercaptopurine, a reaction most efficiently catalyzed by glutathione transferase (GST) A2-2. Among five genotypes of GST A2-2, the variant A2*E has threefold-fourfold higher catalytic efficiency with azathioprine, suggesting that the expression of A2*E could boost 6-mercaptopurine release and adverse side effects in treated patients. Structure-activity studies of the GST A2-2 variants and homologous alpha class GSTs were made to delineate the determinants of high catalytic efficiency compared to other alpha class GSTs. Engineered chimeras identified GST peptide segments of importance, and replacing the corresponding regions in low-activity GSTs by these short segments produced chimeras with higher azathioprine activity. By contrast, H-site mutagenesis led to decreased azathioprine activity when active-site positions 208 and 213 in these favored segments were mutagenized. Alternative substitutions indicated that hydrophobic residues were favored. A pertinent question is whether variant A2*E represents the highest azathioprine activity achievable within the GST structural framework. This issue was addressed by mutagenesis of H-site residues assumed to interact with the substrate based on molecular modeling. The mutants with notably enhanced activities had small or polar residues in the mutated positions. The most active mutant L107G/L108D/F222H displayed a 70-fold enhanced catalytic efficiency with azathioprine. The determination of its structure by X-ray crystallography showed an expanded H-site, suggesting improved accommodation of the transition state for catalysis.

Cytoprotective and regulatory functions of glutathione S-transferases in cancer cell proliferation and cell death

PURPOSE

Glutathione S-transferases (GSTs) family of enzymes is best known for their cytoprotective role and their involvement in the development of anticancer drug resistance. Recently, emergence of non-detoxifying properties of GSTs has provided them with significant biological importance. Addressing the complex interactions of GSTs with regulatory kinases will help in understanding its precise role in tumor pathophysiology and in designing GST-centered anticancer strategies.

METHODS

We reviewed all published literature addressing the detoxification and regulatory roles of GSTs in the altered biology of cancer and evaluating novel agents targeting GSTs for cancer therapy.

RESULTS

The role of GSTs, especially glutathione S-transferase P1 isoform in tumoral drug resistance, has been the cause of intense debate. GSTs have been demonstrated to interact with different protein partners and modulate signaling pathways that control cell proliferation, differentiation and apoptosis. These specific functions of GSTs could lead to the development of new therapeutic approaches and to the identification of some interesting candidates for preclinical and clinical development. This review focuses on the crucial role played by GSTs in the development of resistance to anticancer agents and the major findings regarding the different modes of action of GSTs to regulate cell signaling.

Computational evidence for the detoxifying mechanism of epsilon class glutathione transferase toward the insecticide DDT

A combined quantum mechanics/molecular mechanics (QM/MM) computation of the detoxifying mechanism of an epsilon class glutathione transferases (GSTs) toward organochlorine insecticide DDT, 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane, has been carried out. The exponential average barrier of the proton transfer mechanism is 15.2 kcal/mol, which is 27.6 kcal/mol lower than that of the GS-DDT conjugant mechanism. It suggests that the detoxifying reaction proceeds via a proton transfer mechanism where GSH acts as a cofactor rather than a conjugate. The study reveals that the protein environment has a strong effect on the reaction barrier. The experimentally proposed residues Arg112, Glu116 and Phe120 were found to have a strong influence on the detoxifying reaction. The influence of residues Pro13, Cys15, His53, Ile55, Glu67, Ser68, Phe115, and Leu119 was detected as well. It is worth noticing that Ile55 facilitates the detoxifying reaction most. On the basis of the structure of DDT, structure 2, (BrC6H4)2CHCCl3, is the best candidate among all the tested structures in resisting the detoxification of enzyme agGSTe2.

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.

Identification of glyceollin metabolites derived from conjugation with glutathione and glucuronic acid in male ZDSD rats by online liquid chromatography-electrospray ionization tandem mass spectrometry

Glyceollin-related metabolites produced in rats following oral glyceollin administration were screened in plasma, feces, and urine, and these metabolites were identified by precursor and product ion scanning using liquid chromatography coupled online with electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Precursor ion scanning in the negative ion (NI) mode was used to identify all glyceollin metabolites based on production of a diagnostic radical product ion (m/z 148) upon decomposition. Using this approach, precursor peaks of interest were found at m/z 474 and 531. Tandem mass spectra of these two peaks allowed us to characterize them as byproducts of glutathione conjugation. The peak at m/z 474 was identified as the deprotonated cysteinyl conjugate of glyceollins with an addition of an oxygen atom, whereas m/z 531 was identified as the deprotonated cysteinylglyceine glyceollin conjugate plus an oxygen. These results were confirmed by positive ion (PI) mode analyses. Mercapturic acid conjugates of glyceollins were also identified in NI mode. In addition, glucuronidation of glyceollins was observed, giving a peak at m/z 513 corresponding to the deprotonated conjugate. Production of glucuronic acid conjugates of glyceollins was confirmed in vitro in rat liver microsomes. Neither glutathione conjugation byproducts nor glucuronic acid conjugates of glyceollins have been previously reported.

Mechanism of glutathione transferase P1-1-catalyzed activation of the prodrug canfosfamide (TLK286, TELCYTA)

Canfosfamide (TLK286, TELCYTA) is a prodrug that upon activation by glutathione transferase P1-1 (GST P1-1) yields an anticancer alkylating agent and a glutathione derivative. The rationale underlying the use of TLK286 in chemotherapy is that tumor cells overexpressing GST P1-1 will be locally exposed to the released alkylating agent with limited collateral toxicity to the surrounding normal tissues. TLK286 has demonstrated clinical effects in phase II and III clinical trials for the treatment of malignancies, such as ovarian cancer, nonsmall cell lung cancer, and breast cancer, as a single agent and in combination with other chemotherapeutic agents. In spite of these promising results, the detailed mechanism of GST P1-1 activation of the prodrug has not been elucidated. Here, we propose a mechanism for the TLK286 activation by GST P1-1 on the basis of density functional theory (DFT) and on potential of mean force (PMF) calculations. A catalytic water molecule is instrumental to the activation by forming a network of intermolecular interactions between the active-site Tyr7 hydroxyl and the sulfone and COO(-) groups of TLK286. The results obtained are consistent with the available experimental kinetic data and provide an atomistic understanding of the TLK286 activation mechanism.

Environmental factors and unhealthy lifestyle influence oxidative stress in humans--an overview

Oxygen is the most essential molecule for life; since it is a strong oxidizing agent, it can aggravate the damage within the cell by a series of oxidative events including the generation of free radicals. Antioxidative agents are the only defense mechanism to neutralize these free radicals. Free radicals are not only generated internally in our body system but also trough external sources like environmental pollution, toxic metals, cigarette smoke, pesticides, etc., which add damage to our body system. Inhaling these toxic chemicals in the environment has become unavoidable in modern civilization. Antioxidants of plant origin with free radical scavenging properties could have great importance as therapeutic agents in several diseases caused by environmental pollution. This review summarizes the generation of reactive oxygen species and damage to cells by exposure to external factors, unhealthy lifestyle, and role of herbal plants in scavenging these reactive oxygen species.

Combined quantum mechanics/molecular mechanics (QM/MM) methods in computational enzymology

Computational enzymology is a rapidly maturing field that is increasingly integral to understanding mechanisms of enzyme-catalyzed reactions and their practical applications. Combined quantum mechanics/molecular mechanics (QM/MM) methods are important in this field. By treating the reacting species with a quantum mechanical method (i.e., a method that calculates the electronic structure of the active site) and including the enzyme environment with simpler molecular mechanical methods, enzyme reactions can be modeled. Here, we review QM/MM methods and their application to enzyme-catalyzed reactions to investigate fundamental and practical problems in enzymology. A range of QM/MM methods is available, from cheaper and more approximate methods, which can be used for molecular dynamics simulations, to highly accurate electronic structure methods. We discuss how modeling of reactions using such methods can provide detailed insight into enzyme mechanisms and illustrate this by reviewing some recent applications. We outline some practical considerations for such simulations. Further, we highlight applications that show how QM/MM methods can contribute to the practical development and application of enzymology, e.g., in the interpretation and prediction of the effects of mutagenesis and in drug and catalyst design.

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.

Anacardic acids from cashew nuts ameliorate lung damage induced by exposure to diesel exhaust particles in mice

Anacardic acids from cashew nut shell liquid, a Brazilian natural substance, have antimicrobial and antioxidant activities and modulate immune responses and angiogenesis. As inflammatory lung diseases have been correlated to environmental pollutants exposure and no reports addressing the effects of dietary supplementation with anacardic acids on lung inflammation in vivo have been evidenced, we investigated the effects of supplementation with anacardic acids in a model of diesel exhaust particle- (DEP-) induced lung inflammation. BALB/c mice received an intranasal instillation of 50 μg of DEP for 20 days. Ten days prior to DEP instillation, animals were pretreated orally with 50, 150, or 250 mg/kg of anacardic acids or vehicle (100 μL of cashew nut oil) for 30 days. The biomarkers of inflammatory and antioxidant responses in the alveolar parenchyma, bronchoalveolar lavage fluid (BALF), and pulmonary vessels were investigated. All doses of anacardic acids ameliorated antioxidant enzyme activities and decreased vascular adhesion molecule in vessels. Animals that received 50 mg/kg of anacardic acids showed decreased levels of neutrophils and tumor necrosis factor in the lungs and BALF, respectively. In summary, we demonstrated that AAs supplementation has a potential protective role on oxidative and inflammatory mechanisms in the lungs.

Glutathione transferases, regulators of cellular metabolism and physiology

BACKGROUND

The cytosolic glutathione transferases (GSTs) comprise a super family of proteins that can be categorized into multiple classes with a mixture of highly specific and overlapping functions.

SCOPE OF REVIEW

The review covers the genetics, structure and function of the human cytosolic GSTs with particular attention to their emerging roles in cellular metabolism.

MAJOR CONCLUSIONS

All the catalytically active GSTs contribute to the glutathione conjugation or glutathione dependant-biotransformation of xenobiotics and many catalyze glutathione peroxidase or thiol transferase reactions. GSTs also catalyze glutathione dependent isomerization reactions required for the synthesis of several prostaglandins and steroid hormones and the catabolism of tyrosine. An increasing body of work has implicated several GSTs in the regulation of cell signaling pathways mediated by stress-activated kinases like Jun N-terminal kinase. In addition, some members of the cytosolic GST family have been shown to form ion channels in intracellular membranes and to modulate ryanodine receptor Ca(2+) channels in skeletal and cardiac muscle.

GENERAL SIGNIFICANCE

In addition to their well established roles in the conjugation and biotransformation of xenobiotics, GSTs have emerged as significant regulators of pathways determining cell proliferation and survival and as regulators of ryanodine receptors that are essential for muscle function. This article is part of a Special Issue entitled Cellular functions of glutathione.

A sugar isomerization reaction established on various (βα)₈-barrel scaffolds is based on substrate-assisted catalysis

In the course of tryptophan biosynthesis, the isomerization of phosphoribosylanthranilate (PRA) is catalyzed by the (βα)₈-barrel enzyme TrpF. The reaction occurs via a general acid-base mechanism with an aspartate and a cysteine residue acting as acid and base, respectively. PRA isomerase activity could be established on two (βα)₈-barrel enzymes involved in histidine biosynthesis, namely HisA and HisF, and on a HisAF chimera, by introducing two aspartate-to-valine substitutions. We have analyzed the reaction mechanism underlying this engineered activity by measuring its pH dependence, solving the crystal structure of a HisF variant with bound product analogue, and applying molecular dynamics simulations and mixed quantum and molecular mechanics calculations. The results suggest that PRA is anchored by the C-terminal phosphate-binding sites of HisA, HisF and HisAF. As a consequence, a conserved aspartate residue, which is equivalent to Cys7 from TrpF, is properly positioned to act as catalytic base. However, no obvious catalytic acid corresponding to Asp126 from TrpF could be identified in the three proteins. Instead, this role appears to be carried out by the carboxylate group of the anthranilate moiety of PRA. Thus, the engineered PRA isomerization activity is based on a reaction mechanism including substrate-assisted catalysis and thus differs substantially from the naturally evolved reaction mechanism used by TrpF.

A glutathione transferase from Agrobacterium tumefaciens reveals a novel class of bacterial GST superfamily

In the present work, we report a novel class of glutathione transferases (GSTs) originated from the pathogenic soil bacterium Agrobacterium tumefaciens C58, with structural and catalytic properties not observed previously in prokaryotic and eukaryotic GST isoenzymes. A GST-like sequence from A. tumefaciens C58 (Atu3701) with low similarity to other characterized GST family of enzymes was identified. Phylogenetic analysis showed that it belongs to a distinct GST class not previously described and restricted only in soil bacteria, called the Eta class (H). This enzyme (designated as AtuGSTH1-1) was cloned and expressed in E. coli and its structural and catalytic properties were investigated. Functional analysis showed that AtuGSTH1-1 exhibits significant transferase activity against the common substrates aryl halides, as well as very high peroxidase activity towards organic hydroperoxides. The crystal structure of AtuGSTH1-1 was determined at 1.4 Å resolution in complex with S-(p-nitrobenzyl)-glutathione (Nb-GSH). Although AtuGSTH1-1 adopts the canonical GST fold, sequence and structural characteristics distinct from previously characterized GSTs were identified. The absence of the classic catalytic essential residues (Tyr, Ser, Cys) distinguishes AtuGSTH1-1 from all other cytosolic GSTs of known structure and function. Site-directed mutagenesis showed that instead of the classic catalytic residues, an Arg residue (Arg34), an electron-sharing network, and a bridge of a network of water molecules may form the basis of the catalytic mechanism. Comparative sequence analysis, structural information, and site-directed mutagenesis in combination with kinetic analysis showed that Phe22, Ser25, and Arg187 are additional important residues for the enzyme's catalytic efficiency and specificity.

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.

Crystal structures and kinetic studies of human Kappa class glutathione transferase provide insights into the catalytic mechanism

GSTs (glutathione transferases) are a family of enzymes that primarily catalyse nucleophilic addition of the thiol of GSH (reduced glutathione) to a variety of hydrophobic electrophiles in the cellular detoxification of cytotoxic and genotoxic compounds. GSTks (Kappa class GSTs) are a distinct class because of their unique cellular localization, function and structure. In the present paper we report the crystal structures of hGSTk (human GSTk) in apo-form and in complex with GTX (S-hexylglutathione) and steady-state kinetic studies, revealing insights into the catalytic mechanism of hGSTk and other GSTks. Substrate binding induces a conformational change of the active site from an ‘open’ conformation in the apo-form to a ‘closed’ conformation in the GTX-bound complex, facilitating formations of the G site (GSH-binding site) and the H site (hydrophobic substrate-binding site). The conserved Ser16 at the G site functions as the catalytic residue in the deprotonation of the thiol group and the conserved Asp69, Ser200, Asp201 and Arg202 form a network of interactions with γ-glutamyl carboxylate to stabilize the thiolate anion. The H site is a large hydrophobic pocket with conformational flexibility to allow the binding of different hydrophobic substrates. The kinetic mechanism of hGSTk conforms to a rapid equilibrium random sequential Bi Bi model.

Glutathione-related antioxidant defense system in elderly patients treated for hypertension

The purpose of this study was to analyze glutathione antioxidant defense system in elderly patients treated for hypertension. Studies were carried out in the blood collected from 18 hypertensive and 15 age- and sex-matched controls, all subjects age over 60. Hypertensives were on their usual antihypertensive treatment at the time of blood collection. The concentration of glutathione (GSH) in whole blood and activities of glutathione peroxidase (GPx-1), glutathione transferase (GST), and glutathione reductase (GR) in erythrocytes were measured. The data from patients and controls were compared using independent-samples t test. P value of 0.05 and less was considered statistically significant. We observed increased glutathione-related antioxidant defense in treated hypertensive elderly patients (HT) when compared with healthy controls (C). Mean GSH concentration was significantly higher in HT when compared with C: 3.1 ± 0.29 and 2.6 ± 0.25 mmol/L, respectively, P < 0.001. Mean activity of GR was significantly higher in HT group if compared with C: 83.4 ± 15.25 U/g Hb versus 64.2 ± 8.26 U/g Hb, respectively, P < 0.001. Mean activity of GST was significantly higher in HT group compared with C: 3.0 ± 0.60 mmol CDNB-GSH/mgHb/min and 2.6 ± 0.36 mmol CDNB-GSH/mgHb/min, respectively, P < 0.05. No difference in GPx activity was observed between two groups. These results show that glutathione-related antioxidant defense system was enhanced in elderly hypertensive patients treated for their conditions. This suggests important role of glutathione system in blood pressure regulation. Alterations in concentration and activity of antioxidants observed during antihypertensive medication are likely to be related to the effect of the treatment on NO bioavailability.

Cys-X scanning for expansion of active-site residues and modulation of catalytic functions in a glutathione transferase

We propose Cys-X scanning as a semisynthetic approach to engineer the functional properties of recombinant proteins. As in the case of Ala scanning, key residues in the primary structure are identified, and one of them is replaced by Cys via site-directed mutagenesis. The thiol of the residue introduced is subsequently modified by alternative chemical reagents to yield diverse Cys-X mutants of the protein. This chemical approach is orthogonal to Ala or Cys scanning and allows the expansion of the repertoire of amino acid side chains far beyond those present in natural proteins. In its present application, we have introduced Cys-X residues in human glutathione transferase (GST) M2-2, replacing Met-212 in the substrate-binding site. To achieve selectivity of the modifications, the Cys residues in the wild-type enzyme were replaced by Ala. A suite of simple substitutions resulted in a set of homologous Met derivatives ranging from normethionine to S-heptyl-cysteine. The chemical modifications were validated by HPLC and mass spectrometry. The derivatized mutant enzymes were assayed with alternative GST substrates representing diverse chemical reactions: aromatic substitution, epoxide opening, transnitrosylation, and addition to an ortho-quinone. The Cys substitutions had different effects on the alternative substrates and differentially enhanced or suppressed catalytic activities depending on both the Cys-X substitution and the substrate assayed. As a consequence, the enzyme specificity profile could be changed among the alternative substrates. The procedure lends itself to large-scale production of Cys-X modified protein variants.

The fungal glutathione S-transferase system. Evidence of new classes in the wood-degrading basidiomycete Phanerochaete chrysosporium

The recent release of several basidiomycete genome sequences allows an improvement of the classification of fungal glutathione S-transferases (GSTs). GSTs are well-known detoxification enzymes which can catalyze the conjugation of glutathione to non-polar compounds that contain an electrophilic carbon, nitrogen, or sulfur atom. Following this mechanism, they are able to metabolize drugs, pesticides, and many other xenobiotics and peroxides. A genomic and phylogenetic analysis of GST classes in various sequenced fungi--zygomycetes, ascomycetes, and basidiomycetes--revealed some particularities in GST distribution, in comparison with previous analyses with ascomycetes only. By focusing essentially on the wood-degrading basidiomycete Phanerochaete chrysosporium, this analysis highlighted a new fungal GST class named GTE, which is related to bacterial etherases, and two new subclasses of the omega class GSTs. Moreover, our phylogenetic analysis suggests a relationship between the saprophytic behavior of some fungi and the number and distribution of some GST isoforms within specific classes.