An enzyme from rat liver catalysing conjugations with glutathione

Substrate profiling of glutathione S-transferase with engineered enzymes and matched glutathione analogues

The identification of specific substrates of glutathione S-transferases (GSTs) is important for understanding drug metabolism. A method termed bioorthogonal identification of GST substrates (BIGS) was developed, in which a reduced glutathione (GSH) analogue was developed for recognition by a rationally engineered GST to label the substrates of the corresponding native GST. A K44G-W40A-R41A mutant (GST-KWR) of the mu-class glutathione S-transferases GSTM1 was shown to be active with a clickable GSH analogue (GSH-R1) as the cosubstrate. The GSH-R1 conjugation products can react with an azido-based biotin probe for ready enrichment and MS identification. Proof-of-principle studies were carried to detect the products of GSH-R1 conjugation to 1-chloro-2,4-dinitrobenzene (CDNB) and dopamine quinone. The BIGS technology was then used to identify GSTM1 substrates in the Chinese herbal medicine Ganmaocongji.

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.

Targeting Glutathione S-transferase M4 in Ewing sarcoma

Ewing sarcoma is a malignant pediatric bone and soft tissue tumor. Although the 5-year survival rate of localized disease approaches 75%, the prognosis of metastatic and/or therapy-resistant disease remains dismal despite the wide use of aggressive therapeutic strategies. We previously reported that high expression of glutathione S-transferase M4 (GSTM4) in primary tumors correlates with poor patient outcomes. GSTM4 is required for oncogenic transformation and mediates resistance to chemotherapeutic drugs in Ewing sarcoma cells. Here, we performed RNA-sequencing analyses of Ewing sarcoma cells and combined our results with publicly available datasets to demonstrate that GSTM4 is a major GST specifically expressed in Ewing sarcoma. Pharmacological inhibition of GSTM4 activity using a pan GST inhibitor, 6-(7-nitro-2,1,3-benzoxadiazol-4-ylthio) hexanol (NBDHEX), significantly limited cellular proliferation and oncogenic transformation of Ewing sarcoma cells. Moreover, combined use of NBDHEX and etoposide synergistically increased cytotoxicity, suggesting a role for GSTM4 as an inhibitor of apoptosis. Mechanistic studies revealed that GSTM4 limits apoptosis owing to its ability to interact with Apoptosis Signal-regulating Kinase 1 (ASK1) and inhibit signaling via the c-Jun N-terminal Kinase axis. To exploit our observation that GSTM4 expression is specifically up-regulated in Ewing sarcoma, we tested the effect of a GSTM4-activated anti-cancer agent, O(2)-(2,4-dinitrophenyl) 1-[(4-ethoxycarbonyl)piperazin-1-yl]diazen-1-ium-1,2-diolate or JS-K, on tumor growth and survival. We found that JS-K robustly decreased Ewing sarcoma cell viability and xenograft tumor growth and improved overall survival of xenograft mice. Our data suggest that GSTM4 is a novel therapeutic target for the treatment of high GSTM4-expressing Ewing sarcoma. Strategies that combine standard chemotherapy with agents that inhibit GSTM4, that are activated by GSTM4, or that block GSTM4/ASK1 interactions, can potentially be more specific and/or efficacious than standard therapeutic approaches.

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.

Tributyltin induces oxidative stress and neuronal injury by inhibiting glutathione S-transferase in rat organotypic hippocampal slice cultures

Tributyltin (TBT) has been used as a heat stabilizer, agricultural pesticide and antifouling agents on ships, boats and fish-farming nets; however, the neurotoxicity of TBT has recently become a concern. TBT is suggested to stimulate the generation of reactive oxygen species (ROS) inside cells. The aim of this study was to determine the mechanism of neuronal oxidative injury induced by TBT using rat organotypic hippocampal slice cultures. The treatment of rat hippocampal slices with TBT induced ROS production, lipid peroxidation and cell death. Pretreatment with antioxidants such as superoxide dismutase, catalase or trolox, suppressed the above phenomena induced by TBT, indicating that TBT elicits oxidative stress in hippocampal slices, which causes neuronal cell death. TBT dose-dependently inhibited glutathione S-transferase (GST), but not glutathione peroxidase or glutathione reductase in the cytosol of rat hippocampus. The treatment of hippocampal slices with TBT decreased the GST activity. Pretreatment with reduced glutathione attenuated the reduction of GST activity and cell death induced by TBT, indicating that the decrease in GST activity by TBT is involved in hippocampal cell death. When hippocampal slices were treated with sulforaphane, the expression and activity of GST were increased. Notably, TBT-induced oxidative stress and cell death were significantly suppressed by pretreatment with sulforaphane. These results indicate that GST inhibition could contribute, at least in part, to the neuronal cell death induced by TBT in hippocampal slices. This study is the first report to show the link between neuronal oxidative injury and the GST inhibition elicited by TBT.

Five decades with glutathione and the GSTome

Uncle Folke inspired me to become a biochemist by demonstrating electrophoresis experiments on butterfly hemolymph in his kitchen. Glutathione became the subject for my undergraduate project in 1964 and has remained a focal point in my research owing to its multifarious roles in the cell. Since the 1960s, the multiple forms of glutathione transferase (GST), the GSTome, were isolated and characterized, some of which were discovered in our laboratory. Products of oxidative processes were found to be natural GST substrates. Examples of toxic compounds against which particular GSTs provide protection include 4-hydroxynonenal and ortho-quinones, with possible links to the etiology of Alzheimer and Parkinson diseases and other degenerative conditions. The role of thioltransferase and glutathione reductase in the cellular reduction of disulfides and other oxidized forms of thiols was clarified. Glyoxalase I catalyzes still another glutathione-dependent detoxication reaction. The unusual steady-state kinetics of this zinc-containing enzyme initiated model discrimination by regression analysis. Functional properties of the enzymes have been altered by stochastic mutations based on DNA shuffling and rationally tailored by structure-based redesign. We found it useful to represent promiscuous enzymes by vectors or points in multidimensional substrate-activity space and visualize them by multivariate analysis. Adopting the concept “molecular quasi-species,” we describe clusters of functionally related enzyme variants that may emerge in natural as well as directed evolution.

Dual enzyme activities assay by quantitative electrospray ionization quadrupole-time-of-flight mass spectrometry

A practical and rapid method based on electrospray ionization quadrupole-time of flight mass spectrometry (ESI-Q-ToF MS) was developed for detecting activities of both acetylcholinesterase IAChEI and glutathione S-transferase (GST). The simultaneous study of these two enzyme activities is significant for studying human bio-functions, especially for those who take in toxic compounds and have a risk of disease. Here, the enzyme activities were represented by the conversion of enzymatic substrates and determined by quantitatively analyzing enzymatic substrates. Different internal standards were used to quantify each enzymatic substrate and the good linearity of calibration curves demonstrated the feasibility of the internal standards. The Michaelis-Menten constants (Km) of both GST and AChE were measured by this method and were consistent with values previously reported. Furthermore, we applied this approach to detect GST and AChE activities of whole bloods from four deceased and healthy people. The variation in enzyme activity was in accord with information from gas chromatography mass spectrometry [GC/MS). The screening of AChE and GST provided reliable results and strong forensic evidence. This method offers an alternative choice for detecting enzyme activities and is anticipated to have wide applications in pharmaceutical research and prevention in toxic compounds.

Glutathione s-transferases in pediatric cancer

The glutathione S-transferases (GSTs) are a family of ubiquitously expressed polymorphic enzymes important for detoxifying endogenous and exogenous compounds. In addition to their classic activity of detoxification by conjugation of compounds with glutathione, many other functions are now found to be associated with GSTs. The associations between GST polymorphisms/functions and human disease susceptibility or treatment outcome, mostly in adults, have been extensively studied and reviewed. This mini review focuses on studies related to GST epidemiology and functions related to pediatric cancer. Opportunities to exploit GST in pediatric cancer therapy are also discussed.

Mechanisms of induction of cytosolic and microsomal glutathione transferase (GST) genes by xenobiotics and pro-inflammatory agents

Glutathione transferase (GST) isoezymes are encoded by three separate families of genes (designated cytosolic, microsomal and mitochondrial transferases), with distinct evolutionary origins, that provide mammalian species with protection against electrophiles and oxidative stressors in the environment. Members of the cytosolic class Alpha, Mu, Pi and Theta GST, and also certain microsomal transferases (MGST2 and MGST3), are up-regulated by a diverse spectrum of foreign compounds typified by phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, pregnenolone-16α-carbonitrile, 3-methylcholanthrene, 2,3,7,8-tetrachloro-dibenzo-p-dioxin, β-naphthoflavone, butylated hydroxyanisole, ethoxyquin, oltipraz, fumaric acid, sulforaphane, coumarin, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole, 12-O-tetradecanoylphorbol-13-acetate, dexamethasone and thiazolidinediones. Collectively, these compounds induce gene expression through the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), the aryl hydrocarbon receptor (AhR), NF-E2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor-γ (PPARγ) and CAATT/enhancer binding protein (C/EBP) β. The microsomal T family includes 5-lipoxygenase activating protein (FLAP), leukotriene C(4) synthase (LTC4S) and prostaglandin E(2) synthase (PGES-1), and these are up-regulated by tumour necrosis factor-α, lipopolysaccharide and transforming growth factor-β. Induction of genes encoding FLAP, LTC4S and PGES-1 is mediated by the transcription factors C/EBPα, C/EBPδ, C/EBPϵ, nuclear factor-κB and early growth response-1. In this article we have reviewed the literature describing the mechanisms by which cytosolic and microsomal GST are up-regulated by xenobiotics, drugs, cytokines and endotoxin. We discuss cross-talk between the different induction mechanisms, and have employed bioinformatics to identify cis-elements in the upstream regions of GST genes to which the various transcription factors mentioned above may be recruited.

Effects of four host plants on susceptibility of Spodoptera litura (Lepidoptera: Noctuidae) larvae to five insecticides and activities of detoxification esterases

BACKGROUND

The tobacco cutworm, Spodoptera litura (F.), is one of the most destructive polyphagous pests worldwide. The susceptibility of S. litura larvae reared on tobacco, Chinese cabbage, cowpea and sweet potato to phoxim, chlorfenapyr, methomyl, fenvalerate and emamectin benzoate under laboratory conditions was determined.

RESULTS

Spodoptera litura larvae reared on tobacco were most tolerant to all insecticides, whereas those that fed on sweet potato were most susceptible. When larvae were reared on each host plant for three generations, the susceptibilities to phoxim of larvae that fed on Chinese cabbage and cowpea were similar, whereas the susceptibility of larvae that fed on sweet potato decreased by the third generation, and on tobacco the susceptibility decreased in each consecutive generation. When nicotine was added to their diet for three consecutive generations, the tolerance of larvae to phoxim increased twofold, and to emamectin benzoate 3.1-fold, but the tolerance of larvae to fenvalerate and chlorfenapyr did not change. The acetylcholinesterase activities of the larvae that fed on sweet potato and cowpea were greater than the activities of those that fed on Chinese cabbage and tobacco. In contrast, the carboxylesterase activities of the larvae that fed on tobacco and Chinese cabbage were greater than the activities of those that fed on sweet potato and cowpea. The glutathione S-transferase activities of larvae were highest when they fed on tobacco, followed by Chinese cabbage and cowpea, and the lowest activities were observed when larvae fed on sweet potato.

CONCLUSION

Feeding on tobacco or with nicotine added to the diet, the larvae became more tolerant to insecticides, especially to phoxim and emamectin benzoate.

In vivo induction of phase II detoxifying enzymes, glutathione transferase and quinone reductase by citrus triterpenoids

Background

Several cell culture and animal studies demonstrated that citrus bioactive compounds have protective effects against certain types of cancer. Among several classes of citrus bioactive compounds, limonoids were reported to prevent different types of cancer. Furthermore, the structures of citrus limonoids were reported to influence the activity of phase II detoxifying enzymes. The purpose of the study was to evaluate how variations in the structures of citrus limonoids (namely nomilin, deacetyl nomilin, and isoobacunoic acid) and a mixture of limonoids would influence phase II enzyme activity in excised tissues from a mouse model.

Methods

In the current study, defatted sour orange seed powder was extracted with ethyl acetate and subjected to silica gel chromatography. The HPLC, NMR and mass spectra were used to elucidate the purity and structure of compounds. Female A/J mice were treated with three limonoids and a mixture in order to evaluate their effect on phase II enzymes in four different tissues. Assays for glutathione S-transferase and NAD(P)H: quinone reductase (QR) were used to evaluate induction of phase II enzymatic activity.

Results

The highest induction of GST against 1-chloro-2,4-dinitrobenzene (CDNB) was observed in stomach (whole), 58% by nomilin, followed by 25% isoobacunoic acid and 19% deacetyl nomilin. Deacetyl nomilin in intestine (small) as well as liver significantly reduced GST activity against CDNB. Additionally isoobacunoic acid and the limonoid mixture in liver demonstrated a significant reduction of GST activity against CDNB. Nomilin significantly induced GST activity against 4-nitroquinoline 1-oxide (4NQO), intestine (280%) and stomach (75%) while deacetyl nomilin showed significant induction only in intestine (73%). Induction of GST activity was also observed in intestine (93%) and stomach (45%) treated with the limonoid mixture. Finally, a significant induction of NAD(P)H: quinone reductase (QR) activity was observed by the limonoid mixture in stomach (200%). In addition, the deacetyl nomilin treatment group displayed an increase in QR activity in liver (183%) and intestine (22%).

Conclusion

The results of the present study suggests that, dietary intake of citrus limonoids may provide a protective effect against the onset of various cancers by inducing the activity of certain phase II detoxifying enzymes in specific organs.

Up-regulation of antioxidant and glyoxalase systems by exogenous glycinebetaine and proline in mung bean confer tolerance to cadmium stress

The present study investigates the possible mediatory role of exogenously applied glycinebetaine (betaine) and proline on reactive oxygen species (ROS) and methylglyoxal (MG) detoxification systems in mung bean seedlings subjected to cadmium (Cd) stress (1 mM CdCl2, 48 h). Cadmium stress caused a significant increase in glutathione (GSH) and glutathione disulfide (GSSG) content, while the ascorbate (AsA) content decreased significantly with a sharp increase in hydrogen peroxide (H2O2) and lipid peroxidation level (MDA). Ascorbate peroxidase (APX), glutathione S-transferase (GST), glutathione peroxidase (GPX), and glyoxalase I (Gly I) activities were increased in response to Cd stress, while the activities of catalase (CAT), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR) and glyoxalase II (Gly II) were sharply decreased. Exogenous application of 5 mM betaine or 5 mM proline resulted in an increase in GSH and AsA content, maintenance of a high GSH/GSSG ratio and increased the activities of APX, DHAR, MDHAR, GR, GST, GPX, CAT, Gly I and Gly II involved in ROS and MG detoxification system as compared to the control and mostly also Cd-stressed plants, with a concomitant decrease in GSSG content, H2O2 and lipid peroxidation level. These findings together with our earlier findings suggest that both betaine and proline provide a protective action against Cd-induced oxidative stress by reducing H2O2 and lipid peroxidation levels and by increasing the antioxidant defense and MG detoxification systems.

Metabolism of the herbicide 2,6-dichlorobenzonitrile in rabbits and rats

1. The metabolism of 2,6-dichlorobenzonitrile was studied in rabbits and rats. Oral administration caused an increased urinary excretion of glucuronides and ethereal sulphates. There was also an indication of mercapturic acid formation. 2,6-Dichloro-3-hydroxybenzonitrile and its 4-hydroxy analogue were identified as metabolites in the urine. A small amount of the unchanged substance was recovered from the faeces. 2. By using 2,6-dichlorobenzo[(14)C]nitrile the phenolic metabolites were determined quantitatively and some other possible metabolic routes were excluded. 3. Incubation of 2,6-dichlorobenzonitrile with enzyme preparations (papain and high-speed supernatant of rat-liver homogenate plus glutathione) gave no indications for a reaction with thiol compounds.

Expression and characterization of three new glutathione transferases, an epsilon (AcGSTE2-2), omega (AcGSTO1-1), and theta (AcGSTT1-1) from Anopheles cracens (Diptera: Culicidae), a major Thai malaria vector

Glutathione transferases (GSTs) (E.C.2.5.1.18) are multifunctional enzymes involved in the detoxification of many exogenous and endogenous compounds. This study aimed to characterize several new GSTs from Anopheles cracens, a major Thai malaria vector formerly known as Anopheles dirus. The three recombinant enzymes obtained were from the epsilon, theta and omega classes. They showed 80-93% identity to orthologous An. gambiae GSTs. AcGSTE2-2 possessed peroxidase activity that cannot be detected for the An. gambiae AgGSTE2-2. AcGSTT1-1 had high activity toward several substrates that are specific for mammalian theta class. The AcGSTO1-1 can use 1-chloro-2,4-dinitrobenzene, dichloroacetic acid, and hydroxyethyl disulfide substrates. The enzymes bound but did not metabolize the organophosphate temephos. The epsilon AcGSTE2-2 functioned as a peroxidase and DDT metabolizing enzyme. The theta AcGSTT1-1 functioned not only as peroxidase but also acted as a binding protein for organophosphates. The omega GST had thiol transferase activity suggesting a role in oxidative stress response.

Evidence for a role of exogenous glycinebetaine and proline in antioxidant defense and methylglyoxal detoxification systems in mung bean seedlings under salt stress

In mung bean seedlings, salt stress (300 mM NaCl) caused a significant increase in reduced glutathione (GSH) content within 24 h of treatment as compared to control whereas a slight increase was observed after 48 h of treatment. Highest oxidized glutathione (GSSG) content was observed after 48 h to treatment with a concomitant decrease in glutathione redox state. Glutathione peroxidase, glutathione S-transferase, and glyoxalase II enzyme activities were significantly elevated up to 48 h, whereas glutathione reductase and glyoxalase I activities were increased only up to 24 h and then gradually decreased. Application of 15 mM proline or 15 mM glycinebetaine resulted in an increase in GSH content, maintenance of a high glutathione redox state and higher activities of glutathione peroxidase, glutathione S-transferase, glutathione reductase, glyoxalase I and glyoxalase II enzymes involved in the ROS and methylglyoxal (MG) detoxification system for up to 48 h, compared to those of the control and mostly also salt stressed plants, with a simultaneous decrease in GSSG content, H2O2 and lipid peroxidation level. The present study suggests that both proline and glycinebetaine provide a protective action against saltinduced oxidative damage by reducing H2O2 and lipid peroxidation level and by enhancing antioxidant defense and MG detoxification systems.

Glutathione transferases from parasites: a biochemical view

The glutathione transferase (GST) system of parasites represents the main detoxification mechanism of hydrophobic and electrophilic compounds. Parasites lack the CYP450 activity, hence part of its function has been taken over by other enzymes including GSTs. Cytosolic GSTs (cGSTs) are found in this system and constitute a versatile and numerous group that in parasites display many peculiarities in contrast to mammalian cGSTs. This review summarizes aspects of the biochemistry of parasite cGSTs such as substrate specificities, inhibitor sensitivities, classification, kinetics and catalysis, as well as some aspects of their protective role.

Levels of insecticide resistance and resistance mechanisms in Aedes aegypti from some Latin American countries

Eight Latin American strains of Aedes aegypti were evaluated for resistance to 6 organophosphates (temephos, malathion, fenthion, pirimiphos-methyl, fenitrothion, and chlorpirifos) and 4 pyrethroids (deltamethrin, lambdacyhalothrin, betacypermethrin, and cyfluthrin) under laboratory conditions. In larval bioassays, temephos resistance was high (resistance ratio [RR50], > or =10X) in the majority of the strains, except for the Nicaragua and Venezuela strains, which showed moderate resistance (RR50, between 5 and 10X). The majority of the strains were susceptible to malathion, fenthion, and fenitrothion. However, resistance to pirimiphos-methyl ranged from moderate to high in most of the strains. Larvae from Havana City were resistant to 3 of the pyrethroids tested and moderately resistant to cyfluthrin. The Santiago de Cuba strain showed high resistance to deltamethrin and moderate resistance to the other pyrethroids (lambdacyhalothrin, betacypermethrin, and cyfluthrin). The rest of the strains were susceptible to pyrethroids, except for the Jamaica and Costa Rica strains, which showed moderate resistance to cyfluthrin, and Peru and Venezuela, which showed resistance to deltamethrin. Adult bioassays showed that all the strains were resistant to dichlorodiphenyl-trichloroethane and to the majority of pyrethroids evaluated. The use of the synergists S,S,S,-tributyl phosphorotrithioate and piperonil butoxide showed that esterase and monooxygenases played an important role in the temephos, pirimiphos-methyl, and chlorpirifos resistance in some strains. Biochemical tests showed high frequencies of esterase and glutathione-S-transferase activity; however, the frequency of altered acetylcholinesterase mechanism was low. The polyacrylamide electrophoresis gel detected the presence of a strong band called Est-A4. Insecticide resistance in Ae. aegypti is a serious problem facing control operations, and integrated control strategies are recommended to help prevent or delay the temephos resistance in larvae and pyrethroids resistance in adults.

The glutathione S-transferases: a group of multifunctional detoxification proteins

The physiological roles of the glutathione S-transferases, by whatever name, seem to result in detoxification. As is true of albumin, members of this group of proteins bind an enormous number of compounds that appear to have in common only hydrophobic topography; the binding of bilirubin is an example of a major function common to all higher species. If the ligand bears a sufficiently electrophilic center, it will be attacked by the nucleophile GSH; such compounds would be the substrates of the enzyme. And should such a ligand be extraordinarily reactive--as, for example, some of the epoxide carcinogens generated by the cytochrome P450-linked, mixed-function oxidases, or even 1-chloro-2,4-dinitrobenzene--then reaction may occur either with GSH or irreversibly with the transferase itself. By reason of the wide distribution and high intracellular concentration of these proteins, there appears to be sufficient enzyme for all three roles in detoxification.

The mode of action of organic carcinogens on cellular structures

Most genotoxic organic carcinogens require metabolic activation to exert their detrimental effects. The present review summarizes the mechanisms of how organic carcinogens are bioactivated into DNA-reactive descendants. Beginning with the history of discovery of some important human organic carcinogens, the text guides through the development of the knowledge on their molecular mode of action that has grown over the past decades. Some of the most important molecular mechanisms in chemical carcinogenesis, the role of the enzymes involved in bioactivation, the target gene structures of some ultimate carcinogenic metabolites, and implications for human cancer risk assessment are discussed.

Nomenclature for mammalian soluble glutathione transferases

The nomenclature for human soluble glutathione transferases (GSTs) is extended to include new members of the GST superfamily that have been discovered, sequenced, and shown to be expressed. The GST nomenclature is based on primary structure similarities and the division of GSTs into classes of more closely related sequences. The classes are designated by the names of the Greek letters: Alpha, Mu, Pi, etc., abbreviated in Roman capitals: A, M, P, and so on. (The Greek characters should not be used.) Class members are distinguished by Arabic numerals and the native dimeric protein structures are named according to their subunit composition (e.g., GST A1-2 is the enzyme composed of subunits 1 and 2 in the Alpha class). Soluble GSTs from other mammalian species can be classified in the same manner as the human enzymes, and this chapter presents the application of the nomenclature to the rat and mouse GSTs.

Peptide phage display for probing GST-protein interactions

Phage display is a powerful strategy for identifying protein-peptide interactions. Glutathione transferases (GSTs) play prominent roles in the cellular protection against oxidative stress by catalyzing detoxication reactions. In addition, GSTs seem to act in signaling pathways by means of interaction with other macromolecules such as protein kinases. This chapter describes how the technique of peptide phage display can be used to identify possible partners in GST-protein complexes.

Drug metabolism and pharmacogenetics: the British contribution to fields of international significance

The branch of pharmacology we now call 'drug metabolism', the consideration of the enzymes and procesess determining the disposition of drugs in the body, emerged in the 1840s on the continent of Europe, but British science made little or no contribution until the 1920s. From this point on, the development of the field through the 20th century was shaped to a very significant extent by a series of influential British workers, whose contributions were of global significance and who can now be seen as fathers of the subject. Since the 1950s, and gaining pace inexorably from the 1970s, the significance of drug metabolism to human therapeutics has been greatly added to by the emergence of pharmacogenetics, clinically important hereditary variation in response to drugs, which underpins the current emphasis on personalised medicine. This review examines the British contributions to both these fields through the lives of seven key contributors and attempts to place their work both in the context of its time and its lasting influence.

Pyrethroid insecticide-resistant strain of Aedes aegypti from Cuba induced by deltamethrin selection

A sample of Aedes aegypti L. from Santiago de Cuba with a high level of deltamethrin resistance (113.7 x at the 50% lethal concentration [LC50]), was subjected to deltamethrin selection to determine the capacity of this population to evolve higher resistance under intensive laboratory selection pressure, to characterize that resistance, to attempt to identify some of the mechanisms involved, and to use it as a reference strain for future molecular research. High resistance developed after 12 generations of selection (1,425 x). After selection for 12 generations with deltamethrin, the Santiago de Cuba colony (SAN-F12) showed little or no cross-resistance to the organophosphates evaluated, but high cross-resistance was observed for all the pyrethroids in larvae from this strain: lambdacyhalothrin (197.5 x), cypermethrin (45 x), and cyfluthrin (41.2 x). Adult bioassays reveal that a SAN-F12 strain was resistant to the pyrethroid and the organochlorine dichlorodiphenyltrichloroethane (DDT). Synergism tests implicated detoxifying esterase or glutathione S-transferase (GST) and monooxygenase in pyrethroid resistance. Biochemical tests reveal that acetylcholinesterase was not involved in deltamethrin resistance. The frequency of GST enzyme increased from 0.43 in Santiago de Cuba to 0.88 in SAN-F12. Esterase frequency increased from 0.12 in Santiago de Cuba to 0.63 in SAN-F6 and it diminished to 0.38 in SAN-F12. The polyacrylamide gel electrophoresis and inhibition study suggests the presence of elevated esterase activity not associated with pyrethroid resistance. The presence of both DDT and pyrethroid resistance in the SAN-F12 strain suggests the presence of a knockdown (Kdr)-type resistance mechanism, although the frequency of this mechanism was low. Resistance to deltamethrin could be associated with esterase or GST mechanisms, and more investigation is required. This information contributes to the improvement of resistance management strategies in the Cuban Ae. aegypti control program.

Intra-subunit residue interactions from the protein surface to the active site of glutathione S-transferase AdGSTD3-3 impact on structure and enzyme properties

Structural residues are one of the major factors that modulate the catalytic specificity as well as having a role in stability of the glutathione S-transferases (GST). To understand how residues remote from the active site can affect enzymatic properties, four mutants, His144Ala, Val147Leu, Val147Ala and Arg96Ala, were generated. The selected residues appear to be in a putative intra-subunit interaction pathway from the exterior Asp150 to the active site Arg66 of AdGSTD3-3. The analysis of the four mutants suggested that the interaction formed between Asp150 and His144 is required for the packing of the hydrophobic core in domain 2. Mutations of both Asp150 and His144 impacted upon enzymatic properties. Two Val147 mutants also showed contribution to packing and support of the N-capping box motif by demonstrating shorter half-lives. The planar guanidinium of Arg96 is in a stacked geometry with the face of the aromatic ring of Phe140 in a cation-pi interaction. The Arg96 also interacts with several other residues one of which, Asp100, is in the active site. These interactions restrict movement of the residues in this region and as the data demonstrates when Arg96 is changed have dramatic impact on stability and enzyme properties. These findings indicate the significance of the roles played by residue interactions which can cause conformational changes and thereby influence the catalytic activity and stability of an enzyme.

Nature and nurture - lessons from chemical carcinogenesis

The roles of genetic constitution versus environmental factors in cancer development have been a matter of debate even long before the discovery of 'oncogenes'. Evidence from epidemiological, occupational and migration studies has consistently pointed to environmental factors as the major contributing factors to cancer, so it seems reasonable to discuss the importance of chemical carcinogenesis in the present 'age of cancer genetics'.

Mosquito Glutathione Transferases

The glutathione transferases (glutathione S-transferases, GSTs) are a diverse family of enzymes involved in a wide range of biological processes, many of which involve the conjugation of the tripeptide glutathione to an electrophilic substrate. Relatively little is known about the endogenous substrates of mosquito GSTs, and most studies have focused on their role in insecticide metabolism, because elevated levels of GST activity have been associated with resistance to all the major classes of insecticides. In addition, there is growing interest in the role of this enzyme family in maintaining the redox status of the mosquito cell, particularly in relation to vectorial capacity. Most GSTs are cytosolic dimeric proteins, although a smaller class of microsomal GSTs exists in insects, mammals, and plants. Each GST subunit has a G site that binds glutathione and a substrate-binding site or H site. There are more than 30 GST genes in mosquitoes. Additional diversity is contributed by alternative splicing to produce GSTs with differing substrate specificities. In this review, we first discuss the diversity of insect GST enzymes and their mode of action before focusing on the various functions that have been attributed to specific mosquito GSTs.

Regulation of 4‐Hydroxynonenal Mediated Signaling By Glutathione S‐Transferases

4-Hydroxy-trans-2-nonenal (HNE) was initially considered to be merely a toxic end product of lipid peroxidation that contributed to oxidative stress-related pathogenesis. However, in recent years its physiological role as an important "signaling molecule" has been established. HNE can modulate various signaling pathways in a concentration-dependent manner. Glutathione S-transferases (GSTs) are major determinants of the intracellular concentration of HNE, because these enzymes account for the metabolism of most cellular HNE through its conjugation to glutathione. Evidence is emerging that GSTs are involved in the regulation of the HNE-mediated signaling processes. Against the backdrop of our current understanding on the formation, metabolism, and role of HNE in signaling processes, the physiological role of GSTs in regulation of HNE-mediated signaling processes is critically evaluated in this chapter. Available evidence strongly suggests that besides their well-established pharmacological role of detoxifying xenobiotics, GSTs also play an important physiological role in the regulation of cellular signaling processes.

Reduction and uptake of methylene blue by human erythrocytes

A thiazine dye reductase has been described in endothelial cells that reduces methylene blue (MB), allowing its uptake into cells. Because a different mechanism of MB uptake in human erythrocytes has been proposed, we measured MB uptake and reduction in this cell type. Oxidized MB (MB(+)) stimulated reduction of extracellular ferricyanide in a time- and concentration-dependent manner, reflecting extracellular reduction of the dye. Reduced MB was then taken up by the cells and partially oxidized to MB(+). Both forms were retained against a concentration gradient, and their redox cycling induced an oxidant stress in the cells. Whereas concentrations of MB(+) <5 microM selectively oxidized NAD(P)H, higher concentrations also oxidized both glutathione (GSH) and ascorbate, especially in the absence of d-glucose. MB(+)-stimulated ferricyanide reduction was inhibited by thiol reagents with different mechanisms of action. Phenylarsine oxide, which is selective for vicinal dithiols in proteins, inhibited MB(+)-dependent ferricyanide reduction more strongly than it decreased cell GSH and pentose phosphate cycle activity, and it did not affect cellular NADPH. Open erythrocyte ghost membranes facilitated saturable NAD(P)H oxidation by MB(+), which was abolished by pretreating ghosts with low concentrations of trypsin and phenylarsine oxide. These results show that erythrocytes sequentially reduce and take up MB(+), that both reduced and oxidized forms of the dye are concentrated in cells, and that the thiazine dye reductase activity initially responsible for MB(+) reduction may correspond to MB(+)-dependent NAD(P)H reductase activity in erythrocyte ghosts.

Non-active site residues Cys69 and Asp150 affected the enzymatic properties of glutathione S-transferase AdGSTD3-3

To elucidate how non-active site residues support the catalytic function, five selected residues of AdGSTD3-3 isoenzyme were changed to AdGSTD1-1 residues by means of site-directed mutagenesis. Analysis of the kinetic parameters indicated that Cys69Gln and Asp150Ser showed marked differences in Vmax and Km compared with the wild type enzyme. Both residues were characterized further by replacement with several amino acids. Both the Cys69 and Asp150 mutants showed differences with several GST substrates and inhibitors including affecting the interactions with pyrethroid insecticides. Cys69 and Asp150 mutants possessed a decreased half-life relative to the wild type enzyme. The Asp150 mutation appears to affect neighboring residues that support two important structural motifs, the N-capping box and the hydrophobic staple motif. The Cys69 mutants appeared to have subtle conformational changes near the active site residues resulting in different conformations and also directly affecting the active site region. The results show the importance of the cumulative effects of residues remote from the active site and demonstrate that minute changes in tertiary structure play a role in modulating enzyme activity.

Expression of drug-metabolizing enzymes in the pancreas of hamster, mouse, and rat, responding differently to the pancreatic carcinogenicity of BOP

BACKGROUND/METHODS

N-nitroso-bis(2-oxopropyl)amine (BOP) induces pancreatic ductal adenocarcinoma in Syrian golden hamsters, but not in rats or mice. To examine whether this difference is due to the diversity in the presence and distribution of enzymes involved in the metabolism of BOP, the cellular expression of nine cytochrome P-450 isozymes (CYP1A1, CYP1A2, CYP2B6, CYP2C8,9,19, CYP2D1, CYP2E1, CYP3A1, CYP3A2, and CYP3A4) and of three glutathione S-transferase isozymes (GST-pi, GST-alpha, and GST-mu) was investigated in the pancreas of hamsters, rats, and mice by immunohistochemistry.

RESULTS

We found a wide species variation in the presence and cellular localization of the enzymes and a lack of several enzymes, including GST-alpha in islets, CYP2B6, CYP2C8,9,19, CYP3A1 in acinar cells, and CYP3A4 in ductal cells, in the rat as compared with hamster and mouse.

CONCLUSION

Although the results could not clarify the reasons for the species differences in the pancreatic carcinogenicity of BOP, the presence of most of the cytochrome P-450 isozymes in pancreatic islets of all three species highlights the important role of the islets in drug metabolism.

Modulation of pumpkin glutathione S-transferases by aldehydes and related compounds

Induction of pumpkin (Cucurbita maxima Duch.) glutathione S-transferase (GST, EC 2.5.1.18) by aldehydes and related compounds was examined. All of the tested compounds induced pumpkin GST to different degrees, and it was found that (1) aldehydes induce GST directly and alcohols induce GST indirectly, and (2) alpha,beta-unsaturated aldehydes are the most effective inducers and their potency is related to the Michael acceptors reaction. The results of Western blot analysis showed that the patterns of induction of CmGSTU1, CmGSTU2 and CmGSTU3 were similar to the patterns of activity with the exception of alpha,beta-unsaturated carbonyl compounds. Among the three compounds, crotonaldehyde caused the highest activity induction (9.2-fold), but Western blot expression was the highest only for CmGSTU3. CmGSTF1 was almost non-responsive to all of the tested stresses. Results of induction studies suggested that efficient pumpkin GST inducers have distinctive chemical features. The in vitro activity of the enzyme was inhibited by ethacryanic acid, trans-2-hexenal, crotonaldehyde, and pentanal. Ethacryanic acid was found to be the most potent inhibitor with an apparent I(50) value of 6.90+/-2.06 micro M, while others were weak to moderate inhibitors. The results presented here indicate that plant GSTs might be involved in the detoxification of physiologically and environmentally hazardous aldehydes/alcohols.

Are islet cells the gatekeepers of the pancreas?

The pancreas is one of the body's most complex tissues composed of a mixture of endocrine and exocrine cell components. Although, islets comprise 1-2% of the pancreatic volume, there is some evidence that they control the function and the integrity of the pancreas and play the role of a gatekeeper. This review intends to highlight the importance of islet cells, not only for glucose metabolism, but also for their significant role in drug metabolism and diseases, especially in pancreatic cancer.

Alteration of glutathione S-transferase levels in Barrett's metaplasia compared to normal oesophageal epithelium

OBJECTIVE

Oesophageal cancer associated with the premalignant condition Barrett's oesophagus has increased in incidence over the last few years. Phase II detoxifying enzymes, including glutathione S-transferases (GSTs) protect the mucosa from carcinogens, which can cause oxidative damage to cells. Therefore, a reduction in these anti-oxidant enzymes can increase the risk of carcinogenesis. The aim of this study was to compare the extent of GST expression in normal oesophageal tissue, Barrett's oesophagus and oesophageal adenocarcinoma.

DESIGN

Antibodies raised against GST alpha, GST mu, GST pi and microsomal GST were used to identify expression of these proteins in tissue sections.

METHOD

Paraffin-embedded sections were stained using standard immunohistochemical techniques to demonstrate the pattern of expression of GST proteins in biopsy specimens. Twelve sections of Barrett's metaplasia and an equal number of specimens from normal oesophageal tissue were examined, together with sections from adenocarcinoma and normal gastric mucosa.

RESULTS

Expression of the GST enzymes appeared to be reduced in Barrett's tissue compared to normal oesophageal tissue. Nuclear staining featured in some of the normal tissue sections, but not in Barrett's tissue.

CONCLUSION

The reduction in GST expression suggested in Barrett's tissue is an interesting finding, as it is possible that reduced expression of these detoxifying enzymes may contribute to the risk of development of adenocarcinoma in Barrett's mucosa.

Cross-resistance to pyrethroid and organophosphorus insecticides induced by selection with temephos in Aedes aegypti (Diptera: Culicidae) from Cuba

A sample of Aedes aegypti (L.) from Santiago de Cuba, Cuba, with a high level of temephos resistance (19.58x at the 50% lethal concentration [LC50]), was subjected to temephos selection to evaluate the utility of this organophosphate insecticide for mosquito control. High resistance developed after six generations of selection (200.00x). Little or no cross-resistance was observed to the organophosphates, malathion and fenitrothion, but high cross-resistance was observed for the pyrethroid deltamethrin (337.5x) and the organophosphate fenthion (12.74x). Synergism tests implicated detoxifying esterases in temephos and fenthion resistance and deltamethrin resistance was associated with a cytochrome P450 monooxygenasa. Biochemical tests, polyacrylamide gel electrophoresis (PAGE), and inhibition studies confirmed the presence of elevated esterase activity associated with temephos resistance. Pyrethroid resistance could be associated with a glutathione-S-transferase mechanism but not the esterases. The cross-resistance to deltamethrin from temephos selection could limit the use of both insecticides for Ae. aegypti control.

Purification of a phi-type glutathione S-transferase from pumpkin flowers, and molecular cloning of its cDNA

A major species of glutathione S-transferase (GST), Pugf, was highly purified from pumpkin flowers. Two-dimensional electrophoresis of the purified enzyme gave two adjacent protein spots. The specific activity of the purified enzyme was 2.4 micromol min(-1) mg(-1) protein for 1-chloro-2,4-dinitrobenzene. This value is one to two orders of magnitude lower than that of pumpkin tau-type GSTs. The expression pattern of Pugf in healthy pumpkin plants and responses to various stresses were examined by western blotting. Pugf was found in high concentrations in petioles, stems, and roots as well as flowers, and was more abundant in expanding young organs than in fully expanded mature organs. Dehydration caused a slight increase in its concentration, but high and low temperatures, salty stress, and 2,4-dichlorophenoxyacetic acid seemed to have no effects. A cDNA encoding Pugf was cloned and sequenced. Sequence comparison with other plant GSTs suggested that it should be classified as a phi-type GST.