Glutathione transferase zeta catalyses the oxygenation of the carcinogen dichloroacetic acid to glyoxylic acid

Multi-Level System to Assess Toxicity in Water Distribution Plants

The toxicity of water samples from water distribution plants needs to be investigated further. Indeed, studies on the pro-oxidant effects driven by tap water are very limited. In this study, the water quality, pro-oxidant effects, and potential health risks driven by exposure to groundwater samples from two water plants (sites A and B) located in Northwestern Italy were investigated in a multi-level system. Physicochemical parameters and the absence of pathogens, cyanotoxins, and endocrine active substances indicated a good water quality for both sites. The 25 metals analyzed were found under the limit of quantification or compliant with the maximum limits set by national legislation. Water samples were concentrated by the solid-phase extraction system in order to assess the aquatic toxicity on Epithelioma papulosum cyprini (EPC) cell line. Levels of superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase, and glutathione reductase were evaluated through the Integrated Biomarkers Response (IBRv2) index. EPC cell line was found a sensible model for assessing the antioxidant responses driven by both water concentrates. A similar antioxidant response was shown by plots and IBRv2 suggesting a muted risk for the two sampling sites.

Glutathione S-transferases and their implications in the lung diseases asthma and chronic obstructive pulmonary disease: Early life susceptibility?

Our lungs are exposed daily to airborne pollutants, particulate matter, pathogens as well as lung allergens and irritants. Exposure to these substances can lead to inflammatory responses and may induce endogenous oxidant production, which can cause chronic inflammation, tissue damage and remodeling. Notably, the development of asthma and Chronic Obstructive Pulmonary Disease (COPD) is linked to the aforementioned irritants. Some inhaled foreign chemical compounds are rapidly absorbed and processed by phase I and II enzyme systems critical in the detoxification of xenobiotics including the glutathione-conjugating enzymes Glutathione S-transferases (GSTs). GSTs, and in particular genetic variants of GSTs that alter their activities, have been found to be implicated in the susceptibility to and progression of these lung diseases. Beyond their roles in phase II metabolism, evidence suggests that GSTs are also important mediators of normal lung growth. Therefore, the contribution of GSTs to the development of lung diseases in adults may already start in utero, and continues through infancy, childhood, and adult life. GSTs are also known to scavenge oxidants and affect signaling pathways by protein-protein interaction. Moreover, GSTs regulate reversible oxidative post-translational modifications of proteins, known as protein S-glutathionylation. Therefore, GSTs display an array of functions that impact the pathogenesis of asthma and COPD.

In this review we will provide an overview of the specific functions of each class of mammalian cytosolic GSTs. This is followed by a comprehensive analysis of their expression profiles in the lung in healthy subjects, as well as alterations that have been described in (epithelial cells of) asthmatics and COPD patients. Particular emphasis is placed on the emerging evidence of the regulatory properties of GSTs beyond detoxification and their contribution to (un)healthy lungs throughout life. By providing a more thorough understanding, tailored therapeutic strategies can be designed to affect specific functions of particular GSTs.

Anaerobic Microbial Metabolism of Dichloroacetate

Dichloroacetate (DCA) commonly occurs in the environment due to natural production and anthropogenic releases, but its fate under anoxic conditions is uncertain. Mixed culture RM comprising "Candidatus Dichloromethanomonas elyunquensis" strain RM utilizes DCA as an energy source, and the transient formation of formate, H2, and carbon monoxide (CO) was observed during growth. Only about half of the DCA was recovered as acetate, suggesting a fermentative catabolic route rather than a reductive dechlorination pathway. Sequencing of 16S rRNA gene amplicons and 16S rRNA gene-targeted quantitative real-time PCR (qPCR) implicated "Candidatus Dichloromethanomonas elyunquensis" strain RM in DCA degradation. An (S)-2-haloacid dehalogenase (HAD) encoded on the genome of strain RM was heterologously expressed, and the purified HAD demonstrated the cofactor-independent stoichiometric conversion of DCA to glyoxylate at a rate of 90 ± 4.6 nkat mg-1 protein. Differential protein expression analysis identified enzymes catalyzing the conversion of DCA to acetyl coenzyme A (acetyl-CoA) via glyoxylate as well as enzymes of the Wood-Ljungdahl pathway. Glyoxylate carboligase, which catalyzes the condensation of two molecules of glyoxylate to form tartronate semialdehyde, was highly abundant in DCA-grown cells. The physiological, biochemical, and proteogenomic data demonstrate the involvement of an HAD and the Wood-Ljungdahl pathway in the anaerobic fermentation of DCA, which has implications for DCA turnover in natural and engineered environments, as well as the metabolism of the cancer drug DCA by gut microbiota.IMPORTANCE Dichloroacetate (DCA) is ubiquitous in the environment due to natural formation via biological and abiotic chlorination processes and the turnover of chlorinated organic materials (e.g., humic substances). Additional sources include DCA usage as a chemical feedstock and cancer drug and its unintentional formation during drinking water disinfection by chlorination. Despite the ubiquitous presence of DCA, its fate under anoxic conditions has remained obscure. We discovered an anaerobic bacterium capable of metabolizing DCA, identified the enzyme responsible for DCA dehalogenation, and elucidated a novel DCA fermentation pathway. The findings have implications for the turnover of DCA and the carbon and electron flow in electron acceptor-depleted environments and the human gastrointestinal tract.

Novel ammonium dichloroacetates with enhanced herbicidal activity for weed control

Dichloroacetic acid (DCA) exhibits great potential as an herbicide (nontoxic, easily biodegradable), but its application in agriculture has scarcely been investigated. Since DCA readily undergoes photolysis when exposed to natural light or UV irradiation, there is a large activity loss in controlling weeds. To improve the activity of DCA, we proposed the transformation of DCA into an ionic salt form by using an herbicidal ionic liquids (HILs) strategy. Herein, fifteen novel ammonium dichloroacetates were designed and achieved for the first time. When compared to the anionic precursor DCA, three salts with longer alkyl chains ranging from dodecyl to hexadecyl chains were found to enhance not only the post emergence herbicidal activity but also the rates of activity against some broadleaf weeds under greenhouse conditions. The enhancement was due to the synergistic effect of structural factors, such as the surface activity, solubility and stability arising from their ionic nature. In addition, IL 13 possesses a low phytotoxicity to cotton plants with a favorable selectivity index above 2. This study will be useful for the design of new, high-performance herbicidal formulations.

Exposure of Rats to Multiple Oral Doses of Dichloroacetate Results in Upregulation of Hepatic Glutathione Transferases and NAD(P)H Dehydrogenase [Quinone] 1

Dichloroacetate (DCA) is an investigational drug that is used in the treatment of various congenital and acquired disorders of energy metabolism. Although DCA is generally well tolerated, some patients experience peripheral neuropathy, a side effect more common in adults than children. Repetitive DCA dosing causes downregulation of its metabolizing enzyme, glutathione transferase zeta 1 (GSTZ1), which is also critical in the detoxification of maleylacetoacetate and maleylacetone. GSTZ1 (-/-) knockout mice show upregulation of glutathione transferases (GSTs) and antioxidant enzymes as well as an increase in the ratio of oxidized glutathione (GSSG) to reduced glutathione (GSH), suggesting GSTZ1 deficiency causes oxidative stress. We hypothesized that DCA-mediated depletion of GSTZ1 causes oxidative stress and used the rat to examine induction of GSTs and antioxidant enzymes after repeated DCA exposure. We determined the expression of alpha, mu, pi, and omega class GSTs, NAD(P)H dehydrogenase [quinone] 1 (NQO1), gamma-glutamylcysteine ligase complex (GCLC), and glutathione synthetase (GSS). GSH and GSSG levels were measured by liquid chromatography-tandem mass spectrometry. Enzyme activity was measured in hepatic cytosol using 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene, and 2,6-dichloroindophenol as substrates. In comparison with acetate-treated controls, DCA dosing increased the relative expression of GSTA1/A2 irrespective of rodent age, whereas only adults displayed higher levels of GSTM1 and GSTO1. NQO1 expression and activity were higher in juveniles after DCA dosing. GSH concentrations were increased by DCA in adults, but the GSH:GSSG ratio was not changed. Levels of GCLC and GSS were higher and lower, respectively, in adults treated with DCA. We conclude that DCA-mediated depletion of GSTZ1 causes oxidative stress and promotes the induction of antioxidant enzymes that may vary between age groups. SIGNIFICANCE STATEMENT: Treatment with the investigational drug, dichloroacetate (DCA), results in loss of glutathione transferase zeta 1 (GSTZ1) and subsequent increases in body burden of the electrophilic tyrosine metabolites, maleylacetoacetate and maleylacetone. Loss of GSTZ1 in genetically modified mice is associated with induction of glutathione transferases and alteration of the ratio of oxidized to reduced glutathione. Therefore, we determined whether pharmacological depletion of GSTZ1 through repeat administration of DCA produced similar changes in the liver, which could affect responses to other drugs and toxicants.

The glutathione S-transferase genes in marine rotifers and copepods: Identification of GSTs and applications for ecotoxicological studies

Various xenobiotics are constantly being released and accumulated into the aquatic environments and consequently, the aquatic organisms are continuously being exposed to exogenous stressors. Among various xenobiotic detoxifying enzymes, Glutathione S-transferase (GST) is one of the major xenobiotic detoxifying enzyme which is widely distributed among living organisms and thus, understanding of the nature of GSTs is crucial. Previous studies have shown GST activity in response to various xenobiotics yet, full identification of GSTs in marine invertebrates is still limited. This review covers information on the importance of GSTs as a biomarker for emerging chemicals and their response to wide ranges of environmental pollutants as well as in-depth phylogenetic analysis of marine invertebrates, including recently identified GSTs belonging to rotifers (Brachionus spp.) and copepods (Tigriopus japonicus and Paracyclopina nana), with unique class-specific features of GSTs, as well as a new suggestion of GST evolutionary pathway.

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.

Dibromoacetic Acid Induced Hepatotoxicity in Mice through Oxidative Stress and Toll-Like Receptor 4 Signaling Pathway Activation

Dibromoacetic acid (DBA) is one of haloacetic acids, often as a by-product of disinfection in drinking water. DBA is a multiple-organ carcinogen in rodent animals, but little research on its hepatotoxicity has been conducted and its mechanism has not been elucidated. In this study, we found that DBA could induce obvious hepatotoxcity in Balb/c mice as indicated by histological changes, increasing serum level of alanine aminotransferase (ALT) and aspartate aminotransferase (AST), and accumulation of hepatic glycogen, after the mice were administered DBA at doses of 1.25, 5, and 20 mg/kg body weight for 28 days via oral gavage. In mechanism study, DBA induced oxidative stress as evidenced by increasing the level of malondialdehyde (MDA), reactive oxygen species (ROS) in the liver, advanced oxidative protein products (AOPPs) in the serum, and decreasing the level of glutathione (GSH) in the liver. DBA induced inflammation in the liver of the mice which is supported by increasing the production of tumor necrosis factor-α (TNF-α) and the mRNA levels of TNF-α, interleukin-6 (IL-6), interleukin-1β (IL-1β), and nuclear factor κB (NF-κB) in the liver. DBA also upregulated the protein levels of Toll-like receptor (TLR) 4, myeloid differentiation factor 88 (MyD88), tumor necrosis factor receptor-associated factor 6 (TRAF6), inhibitor of nuclear factor κB alpha (IκB-α), nuclear factor κB p65 (NF-κB p65), and the phosphoralation of P38 mitogen-activated protein kinase (P38MAPK) and c-Jun N-terminal kinase (JNK). Conclusion. DBA could induce hepatotoxicity in mice by oral exposure; the mechanism is related to oxidative stress, inflammation, and Toll-like receptor 4 signaling pathway activation.

GSTZ1 deficiency promotes hepatocellular carcinoma proliferation via activation of the KEAP1/NRF2 pathway

Background

Glutathione S-transferase zeta 1 (GSTZ1) is the penultimate enzyme in phenylalanine/tyrosine catabolism. GSTZ1 is dysregulated in cancers; however, its role in tumorigenesis and progression of hepatocellular carcinoma (HCC) is largely unknown. We aimed to assess the role of GSTZ1 in HCC and to reveal the underlying mechanisms, which may contribute to finding a potential therapeutic strategy against HCC.

Methods

We first analyzed GSTZ1 expression levels in paired human HCC and adjacent normal tissue specimens and the prognostic effect of GSTZ1 on HCC patients. Thereafter, we evaluated the role of GSTZ1 in aerobic glycolysis in HCC cells on the basis of the oxygen consumption rate (OCR) and extracellular acidification rate (ECAR). Furthermore, we assessed the effect of GSTZ1 on HCC proliferation, glutathione (GSH) concentration, levels of reactive oxygen species (ROS), and nuclear factor erythroid 2-related factor 2 (NRF2) signaling via gain- and loss- of GSTZ1 function in vitro. Moreover, we investigated the effect of GSTZ1 on diethylnitrosamine (DEN) and carbon tetrachloride (CCl₄) induced hepatocarcinogenesis in a mouse model of HCC.

Results

GSTZ1 was downregulated in HCC, thus indicating a poor prognosis. GSTZ1 deficiency significantly promoted hepatoma cell proliferation and aerobic glycolysis in HCC cells. Moreover, loss of GSTZ1 function depleted GSH, increased ROS levels, and enhanced lipid peroxidation, thus activating the NRF2-mediated antioxidant pathway. Furthermore, Gstz1 knockout in mice promoted DEN/CCl₄-induced hepatocarcinogenesis via activation of the NRF2 signaling pathway. Furthermore, the antioxidant agent N-acetylcysteine and NRF2 inhibitor brusatol effectively suppressed the growth of Gstz1-knockout HepG2 cells and HCC progression in Gstz1^−/− mice.

Conclusions

GSTZ1 serves as a tumor suppressor in HCC. GSH depletion caused by GSTZ1 deficiency elevates oxidative stress, thus constitutively activating the NRF2 antioxidant response pathway and accelerating HCC progression. Targeting the NRF2 signaling pathway may be a promising therapeutic approach for this subset of HCC.

The Importance of Gender-Related Anticancer Research on Mitochondrial Regulator Sodium Dichloroacetate in Preclinical Studies In Vivo

Sodium dichloroacetate (DCA) is an investigational medicinal product which has a potential anticancer preparation as a metabolic regulator in cancer cells’ mitochondria. Inhibition of pyruvate dehydrogenase kinases by DCA keeps the pyruvate dehydrogenase complex in the active form, resulting in decreased lactic acid in the tumor microenvironment. This literature review displays the preclinical research data on DCA’s effects on the cell pyruvate dehydrogenase deficiency, pyruvate mitochondrial oxidative phosphorylation, reactive oxygen species generation, and the Na⁺–K⁺–2Cl⁻ cotransporter expression regulation in relation to gender. It presents DCA pharmacokinetics and the hepatocarcinogenic effect, and the safety data covers the DCA monotherapy efficacy for various human cancer xenografts in vivo in male and female animals. Preclinical cancer researchers report the synergistic effects of DCA combined with different drugs on cancer by reversing resistance to chemotherapy and promoting cell apoptosis. Researchers note that female and male animals differ in the mechanisms of cancerogenesis but often ignore studying DCA’s effects in relation to gender. Preclinical gender-related differences in DCA pharmacology, pharmacological mechanisms, and the elucidation of treatment efficacy in gonad hormone dependency could be relevant for individualized therapy approaches so that gender-related differences in treatment response and safety can be proposed.

GSTZ1-1 Deficiency Activates NRF2/IGF1R Axis in HCC via Accumulation of Oncometabolite Succinylacetone

The IGF1R signaling is important in the malignant progression of cancer. However, overexpression of IGF1R has not been properly assessed in HCC. Here, we revealed that GSTZ1‐1, the enzyme in phenylalanine/tyrosine catabolism, is downregulated in HCC, and its expression was negatively correlated with IGF1R. Mechanistically, GSTZ1‐1 deficiency led to succinylacetone accumulation, alkylation modification of KEAP1, and NRF2 activation, thus promoting IGF1R transcription by recruiting SP1 to its promoter. Moreover, inhibition of IGF1R or NRF2 significantly inhibited tumor‐promoting effects of GSTZ1 knockout in vivo. These findings establish succinylacetone as an oncometabolite, and GSTZ1‐1 as an important tumor suppressor by inhibiting NRF2/IGF1R axis in HCC. Targeting NRF2 or IGF1R may be a promising treatment approach for this subset HCC.

Gender-Related Effect of Sodium Dichloroacetate on the Number of Hassall's Corpuscles and RNA NKCC1 Expression in Rat Thymus

The aim was to investigate the effect of dichloroacetate (DCA) on thymus weight, Hassall's corpuscle number (HCs), and NKCC1 RNA expression in Wistar rats aged 4–5 weeks. They were investigated in the controls and DCA-treated gonad-intact and castrated males and females. The treatment lasted 4 weeks with DCA 200 mg/kg/day. At the end of the experiment, rat thymus was weighted, and its lobe was taken for the expression of NKCC1 RNA determined by the PCR method and of Hassall's corpuscles by immunohistochemistry. DCA caused a thymus weight decrease in DCA-treated gonad-intact rats of both genders as compared with their controls (p < 0.05), and no such impact was found in castrated DCA-treated males and females. DCA caused an increase of the HCs in gonad-intact males (p < 0.05), and no such increase in the DCA-treated gonad-intact females was found. There was gender-related difference in the HCs when comparing DCA-treated gonad-intact males and females: males showed significantly higher HCs (p < 0.05); no gender-related differences were found in the castrated DCA-treated groups. The Slc12a2 gene RNA expression level was found to be significantly decreased only in gonad-intact and in castrated DCA-treated males. The authors discuss the gender-related DCA effects on the thymus.

The metabolite repair enzyme Nit1 is a dual-targeted amidase that disposes of damaged glutathione in Arabidopsis

The tripeptide glutathione (GSH) is implicated in various crucial physiological processes including redox buffering and protection against heavy metal toxicity. GSH is abundant in plants, with reported intracellular concentrations typically in the 1-10 mM range. Various aminotransferases can inadvertently transaminate the amino group of the γ-glutamyl moiety of GSH to produce deaminated glutathione (dGSH), a metabolite damage product. It was recently reported that an amidase known as Nit1 participates in dGSH breakdown in mammals and yeast. Plants have a hitherto uncharacterized homolog of the Nit1 amidase. We show that recombinant Arabidopsis Nit1 (At4g08790) has high and specific amidase activity towards dGSH. Ablating the Arabidopsis Nit1 gene causes a massive accumulation of dGSH and other marked changes to the metabolome. All plant Nit1 sequences examined had predicted plastidial targeting peptides with a potential second start codon whose use would eliminate the targeting peptide. In vitro transcription/translation assays show that both potential translation start codons in Arabidopsis Nit1 were used and confocal microscopy of Nit1-GFP fusions in plant cells confirmed both cytoplasmic and plastidial localization. Furthermore, we show that Arabidopsis enzymes present in leaf extracts convert GSH to dGSH at a rate of 2.8 pmol min^-1 mg^-1 in the presence of glyoxalate as an amino acceptor. Our data demonstrate that plants have a dGSH repair system that is directed to at least two cellular compartments via the use of alternative translation start sites.

Exposure Characterization of Haloacetic Acids in Humans for Exposure and Risk Assessment Applications: An Exploratory Study

Disinfected water is the major source of haloacetic acids (HAAs) in humans, but their inter- and intra-individual variability for exposure and risk assessment applications is under-researched. Thus, we measured HAAs in cross-sectional and longitudinal urine and water specimens from 17 individuals. Five regulated HAAs-mono, di, and trichloroacetic acid (MCAA, DCAA, and TCAA) and mono- and dibromoacetic acid (MBAA and DBAA)-and one unregulated HAA-bromochloroacetic acid (BCAA)-were measured. Urinary DCAA, MBAA, DBAA, and BCAA levels were always below the limits of detection (LOD). Measured levels and interindividual variability of urinary MCAA were higher than urinary TCAA. Longitudinal urinary specimens showed MCAA levels peaked in after-shower specimens, while TCAA levels remain unchanged. Correlation between urinary MCAA and TCAA was moderate but statistically significant. The prevalence of MCAA and TCAA in urine suggest they can be considered as biomarkers of HAA. Peak urinary MCAA in post-shower specimens suggest MCAA captures short-term exposure via dermal and/or inhalation, while urinary TCAA captures long-term exposure via ingestion. However, further research is warranted in a large pool of participants to test the reliability of MCAA as exposure biomarker.

Functional Role of Tyr12 in the Catalytic Activity of Novel Zeta-like Glutathione S-transferase from Acidovorax sp. KKS102

Glutathione S-transferases (GSTs) are a family of enzymes that function in the detoxification of variety of electrophilic substrates. In the present work, we report a novel zeta-like GST (designated as KKSG9) from the biphenyl/polychlorobiphenyl degrading organism Acidovorax sp. KKS102. KKSG9 possessed low sequence similarity but similar biochemical properties to zeta class GSTs. Functional analysis showed that the enzyme exhibits wider substrate specificity compared to most zeta class GSTs by reacting with 1-chloro-2,4-dinitrobenzene (CDNB), p-nitrobenzyl chloride (NBC), ethacrynic acid (EA), hydrogen peroxide, and cumene hydroperoxide. The enzyme also displayed dehalogenation function against dichloroacetate, permethrin, and dieldrin. The functional role of Tyr12 was also investigated by site-directed mutagenesis. The mutant (Y12C) displayed low catalytic activity and dehalogenation function against all the substrates when compared with the wild type. Kinetic analysis using NBC and GSH as substrates showed that the mutant (Y12C) displayed a higher affinity for NBC when compared with the wild type, however, no significant change in GSH affinity was observed. These findings suggest that the presence of tyrosine residue in the motif might represent an evolutionary trend toward improving the catalytic activity of the enzyme. The enzyme as well could be useful in the bioremediation of various types of organochlorine pollutants.

Developmental roles of tyrosine metabolism enzymes in the blood-sucking insect Rhodnius prolixus

The phenylalanine/tyrosine degradation pathway is frequently described as a catabolic pathway that funnels aromatic amino acids into citric acid cycle intermediates. Previously, we demonstrated that the accumulation of tyrosine generated during the hydrolysis of blood meal proteins in Rhodnius prolixus is potentially toxic, a harmful outcome that is prevented by the action of the first two enzymes in the tyrosine degradation pathway. In this work, we further evaluated the relevance of all other enzymes involved in phenylalanine/tyrosine metabolism in the physiology of this insect. The knockdown of most of these enzymes produced a wide spectrum of distinct phenotypes associated with reproduction, development and nymph survival, demonstrating a highly pleiotropic role of tyrosine metabolism. The phenotypes obtained for two of these enzymes, homogentisate dioxygenase and fumarylacetoacetase, have never before been described in any arthropod. To our knowledge, this report is the first comprehensive gene-silencing analysis of an amino acid metabolism pathway in insects. Amino acid metabolism is exceptionally important in haematophagous arthropods due to their particular feeding behaviour.

Hypersuccinylacetonaemia and normal liver function in maleylacetoacetate isomerase deficiency

BACKGROUND

A high level of succinylacetone (SA) in blood is a sensitive, specific newborn screening marker for hepatorenal tyrosinemia type 1 (HT1, MIM 276700) caused by deficiency of fumarylacetoacetate hydrolase (FAH). Newborns with HT1 are usually clinically asymptomatic but show liver dysfunction with coagulation abnormalities (prolonged prothrombin time and/or high international normalised ratio). Early treatment with nitisinone (NTBC) plus dietary restriction of tyrosine and phenylalanine prevents the complications of severe liver disease and neurological crises.

METHODS AND RESULTS

Six newborns referred for hypersuccinylacetonaemia but who had normal coagulation testing on initial evaluation had sequence variants in the GSTZ1 gene, encoding maleylacetoacetate isomerase (MAAI), the enzyme preceding FAH in tyrosine degradation. Initial plasma SA levels ranged from 233 to 1282 nmol/L, greater than normal (<24 nmol/L) but less than the initial values of patients with HT1 (16 944-74 377 nmol/L, n=15). Four individuals were homozygous for c.449C>T (p.Ala150Val). One was compound heterozygous for c.259C>T (p.Arg87Ter) and an intronic sequence variant. In one, a single heterozygous GSTZ1 sequence variant was identified, c.295G>A (p.Val99Met). Bacterial expression of p.Ala150Val and p.Val99Met revealed low MAAI activity. The six individuals with mild hypersuccinylacetonaemia (MHSA) were not treated with diet or nitisinone. Their clinical course has been normal for up to 13 years.

CONCLUSIONS

MHSA can be caused by sequence variants in GSTZ1. Such individuals have thus far remained asymptomatic despite receiving no specific treatment.

Therapeutic applications of dichloroacetate and the role of glutathione transferase zeta-1

Dichloroacetate (DCA) has several therapeutic applications based on its pharmacological property of inhibiting pyruvate dehydrogenase kinase. DCA has been used to treat inherited mitochondrial disorders that result in lactic acidosis, as well as pulmonary hypertension and several different solid tumors, the latter through its ability to reverse the Warburg effect in cancer cells and restore aerobic glycolysis. The main clinically limiting toxicity is reversible peripheral neuropathy. Although administration of high doses to rodents can result in liver cancer, there is no evidence that DCA is a human carcinogen. In all studied species, including humans, DCA has the interesting property of inhibiting its own metabolism upon repeat dosing, resulting in alteration of its pharmacokinetics. The first step in DCA metabolism is conversion to glyoxylate catalyzed by glutathione transferase zeta 1 (GSTZ1), for which DCA is a mechanism-based inactivator. The rate of GSTZ1 inactivation by DCA is influenced by age, GSTZ1 haplotype and cellular concentrations of chloride. The effect of DCA on its own metabolism complicates the selection of an effective dose with minimal side effects.

Pharmacogenetic considerations with dichloroacetate dosing

The investigational drug dichloroacetate (DCA) is a metabolic regulator that has been successfully used to treat acquired and congenital metabolic diseases and, recently, solid tumors. Its clinical use has revealed challenges in selecting appropriate doses. Chronic administration of DCA leads to inhibition of DCA metabolism and potential accumulation to levels that result in side effects. This is because conversion of DCA to glyoxylate is catalyzed by one enzyme, glutathione transferase zeta 1 (GSTZ1-1), which is inactivated by DCA. SNPs in the GSTZ1 gene result in expression of polymorphic variants of the enzyme that differ in activity and rates of inactivation by DCA under physiological conditions: these properties lead to considerable variation between people in the pharmacokinetics of DCA.

DCA promotes progression of neuroblastoma tumors in nude mice

Even in the presence of oxygen most cancer cells convert glucose to lactate via pyruvate instead of performing oxidative phosphorylation (aerobic glycolysis-Warburg effect). Thus, it has been considered to shift pyruvate - the metabolite of aerobic glycolysis - to acetylCoA by activation of pyruvate dehydrogenase (PDH). AcetylCoA will then be metabolized by oxidative phosphorylation. Therefore, the purpose of this study was to shift tumor cells from aerobic glycolysis to oxidative phosphorylation using dichloroacetate (DCA), an inhibitor of PDH-kinase. The effects of DCA were assayed in vitro in Neuro-2a (murine neuroblastoma), Kelly and SK-N-SH (human neuroblastoma) as well as SkBr3 (human breast carcinoma) cell lines. The effects of DCA on tumor development were investigated in vivo using NMRI nu/nu mice bearing subcutaneous Neuro-2a xenografts. For that purpose animals were treated continuously with DCA in the drinking water. Tumor volumes were monitored using caliper measurements and via [18F]-FDG-positron emission tomography. DCA treatment increased viability/proliferation in Neuro-2a and SkBr3 cells, but did not cause significant alterations of PDH activity. However, no significant effects of DCA could be observed in Kelly and SK-N-SH cells. Accordingly, in mice bearing Neuro-2a xenografts, DCA significantly increased tumor proliferation compared to mock-treated mice. Thus, we could demonstrate that DCA - an indicated inhibitor of tumor growth - efficiently promotes tumor growth in Neuro-2a cells in vitro and in vivo.

Characterization of glutathione-S-transferases in zebrafish (Danio rerio)

Glutathione-S-transferases (GSTs) are one of the key enzymes that mediate phase II of cellular detoxification. The aim of our study was a comprehensive characterization of GSTs in zebrafish (Danio rerio) as an important vertebrate model species frequently used in environmental research. A detailed phylogenetic analysis of GST superfamily revealed 27 zebrafish gst genes. Further insights into the orthology relationships between human and zebrafish GSTs/Gsts were obtained by the conserved synteny analysis. Expression of gst genes in six tissues (liver, kidney, gills, intestine, brain and gonads) of adult male and female zebrafish was determined using qRT-PCR. Functional characterization was performed on 9 cytosolic Gst enzymes after overexpression in E. coli and subsequent protein purification. Enzyme kinetics was measured for GSH and a series of model substrates. Our data revealed ubiquitously high expression of gstp, gstm (except in liver), gstr1, mgst3a and mgst3b, high expression of gsto2 in gills and ovaries, gsta in intestine and testes, gstt1a in liver, and gstz1 in liver, kidney and brain. All zebrafish Gsts catalyzed the conjugation of GSH to model GST substrates 1-chloro-2,4-dinitrobenzene (CDNB) and monochlorobimane (MCB), apart from Gsto2 and Gstz1 that catalyzed GSH conjugation to dehydroascorbate (DHA) and dichloroacetic acid (DCA), respectively. Affinity toward CDNB varied from 0.28 mM (Gstp2) to 3.69 mM (Gstm3), while affinity toward MCB was in the range of 5 μM (Gstt1a) to 250 μM (Gstp1). Affinity toward GSH varied from 0.27 mM (Gstz1) to 4.45 mM (Gstt1a). Turnover number for CDNB varied from 5.25s(-1) (Gstt1a) to 112s(-1) (Gstp2). Only Gst Pi enzymes utilized ethacrynic acid (ETA). We suggest that Gstp1, Gstp2, Gstt1a, Gstz1, Gstr1, Mgst3a and Mgst3b have important role in the biotransformation of xenobiotics, while Gst Alpha, Mu, Pi, Zeta and Rho classes are involved in the crucial physiological processes. In summary, this study provides the first comprehensive analysis of GST superfamily in zebrafish, presents new insight into distinct functions of individual Gsts, and offers methodological protocols that can be used for further verification of interaction of environmental contaminants with fish Gsts.

Functional classification and biochemical characterization of a novel rho class glutathione S-transferase in Synechocystis PCC 6803

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A novel class of glutathione S-transferase (GST) is reported.

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This GST catalyzes dichloroacetate (DCA) degradation and hydroperoxide reactions.

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Functionally this GST is similar to zeta and theta/alpha classes but structurally very different.

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In contrast to other bacterial GSTs, this GST exists as a monomer in solution.

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First report of DCA degradation by any bacterial GST and has potential biotechnological applications.

We report a novel class of glutathione S-transferase (GST) from the model cyanobacterium Synechocystis PCC 6803 (sll1545) which catalyzes the detoxification of the water pollutant dichloroacetate and also shows strong glutathione-dependent peroxidase activity representing the classical activities of zeta and theta/alpha class respectively. Interestingly, sll1545 has very low sequence and structural similarity with these classes. This is the first report of dichloroacetate degradation activity by any bacterial GST. Based on these results we classify sll1545 to a novel GST class, rho. The present data also indicate potential biotechnological and industrial applications of cyanobacterial GST in dichloroacetate-polluted areas.

Trichloroethylene biotransformation and its role in mutagenicity, carcinogenicity and target organ toxicity

Metabolism is critical for the mutagenicity, carcinogenicity, and other adverse health effects of trichloroethylene (TCE). Despite the relatively small size and simple chemical structure of TCE, its metabolism is quite complex, yielding multiple intermediates and end-products. Experimental animal and human data indicate that TCE metabolism occurs through two major pathways: cytochrome P450 (CYP)-dependent oxidation and glutathione (GSH) conjugation catalyzed by GSH S-transferases (GSTs). Herein we review recent data characterizing TCE processing and flux through these pathways. We describe the catalytic enzymes, their regulation and tissue localization, as well as the evidence for transport and inter-organ processing of metabolites. We address the chemical reactivity of TCE metabolites, highlighting data on mutagenicity of these end-products. Identification in urine of key metabolites, particularly trichloroacetate (TCA), dichloroacetate (DCA), trichloroethanol and its glucuronide (TCOH and TCOG), and N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (NAcDCVC), in exposed humans and other species (mostly rats and mice) demonstrates function of the two metabolic pathways in vivo. The CYP pathway primarily yields chemically stable end-products. However, the GST pathway conjugate S-(1,2-dichlorovinyl)glutathione (DCVG) is further processed to multiple highly reactive species that are known to be mutagenic, especially in kidney where in situ metabolism occurs. TCE metabolism is highly variable across sexes, species, tissues and individuals. Genetic polymorphisms in several of the key enzymes metabolizing TCE and its intermediates contribute to variability in metabolic profiles and rates. In all, the evidence characterizing the complex metabolism of TCE can inform predictions of adverse responses including mutagenesis, carcinogenesis, and acute and chronic organ-specific toxicity.

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.

Do Antioxidant Enzymes and Glutathione Play Roles in the Induction of Hepatic Oxidative Stress in Mice upon Subchronic Exposure to Mixtures of Dichloroacetate and Trichloroacetate?

Dichloroacetate (DCA) and trichloroacetate (TCA) are water chlorination byproducts, and their mixtures were previously found to induce additive to greater than additive effects on hepatic oxidative stress (OS) induction in mice after subchronic exposure. To investigate the roles of antioxidant enzymes and glutathione (GSH) in those effects, livers of B6C3F1 mice treated by gavage with 7.5, 15, or 30 mg DCA/kg/day, 12.5, 25, or 50 mg TCA/kg/day, and mixtures (Mix I, Mix II and Mix III) at DCA:TCA ratios corresponding to 7.5:12.5, 15:25 and 25:50 mg/kg/day, respectively, for 13 weeks. Livers were assayed for superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), as well as for GSH levels. In general, DCA suppressed SOD and GSH-Px activities and GSH levels but caused no changes in CAT activity; TCA increased SOD and CAT activities, suppressed GSH-Px activity, but did not change GSH levels; mixtures of DCA and TCA increased SOD and CAT activities and suppressed GSH-Px activity and GSH levels. In conclusion, antioxidant enzymes contribute to DCA-, TCA- and mixtures-induced OS, but not to changes from additive to greater than additive effects produced by different mixture compositions of the compounds. GSH on the hand may contribute to these changes.

Multiple glutathione S-transferase genes: identification and expression in oriental fruit fly, Bactrocera dorsalis

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis (Hendel), is widely distributed in Asia-Pacific regions, where it is a serious pest of a wide range of tropical and subtropical fruit and vegetable crops. In this study, 17 cDNA encoding glutathione S-transferases (GSTs) in B. dorsalis were sequenced and characterised.

RESULTS

Phylogenetic analysis revealed that 16 GSTs belonged to five different cytosolic classes, including four in delta, eight in epsilon, two in omega, one in theta, and one in zeta. The remaining GST (BdGSTu1) was unclassified. RT-qPCR assay showed that the relative expression levels of five GST genes were significantly higher in larval stages than in adulthood. Tissue-specific expression analysis found that BdGSTe3, BdGSTe9 and BdGSTd5 were expressed highly in the midgut, BdGSTe4, BdGSTe6, BdGSTd6 and BdGSTz2 were higher in the fat body, and six GSTs were higher in Malpighian tubules. RT-qPCR confirmed that the expressions of nine GST genes were increased by malathion exposure at various times and doses, while BdGSTe4, BdGSTe9 and BdGSTt1 were increased by β-cypermethrin exposure.

CONCLUSION

The increases in GST gene expression levels after malathion and β-cypermethrin exposure in B. dorsalis might increase the ability of this species to detoxify other insecticides and xenobiotics.

Characterization of catalytic activity and structure of selenocysteine-containing hGSTZ1c-1c based on site-directed mutagenesis and computational analysis

Human glutathione transferase zeta 1c-1c (hGSTZ1c-1c) is one of the glutathione transferase isoenzymes and considered to be a protein scaffold to imitate glutathione peroxidase (GPX) owing to the natural binding site of glutathione (GSH). In this report, several residues near GSH were mutated to selenocysteine (Sec) or cysteine (Cys) residues and the impacts of the substitutions on different activities were discussed. Mutations of Ser-14 or/and Ser-15 to Cys or Sec residues resulted in dramatic loss of catalytic activity of hGSTZ1c-1c with chlorofluoroacetic acid as substrate, which indicated the importance of the hydroxyl groups in Ser-14 and Ser-15. And subsequent study by molecular modeling suggested that Ser-15 was probably involved in catalysis, while Ser-14 may play a crucial role in binding and orientation of GSH and possibly had a synergistic effect with Ser-15 in catalysis. On the contrary, the result of converting Cys-16 to Ser indicated its trivial role in catalysis. The investigations of the selenocysteine-containing hGSTZ1c-1c (seleno-hGSTZ1c-1c) and the mutant S17C implied that the substitutions of multi-Sec for Cys residues at position 16, 137, and 205 could lead to subtle change in the structure of the protein molecule and concomitant change in catalytic activity as a direct result. This finding provides overwhelming evidence that the protein scaffold containing fewer cysteines should be chosen for imitating GPX using cysteine auxotrophic strain system to avoid unexpected structural changes.

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 preliminary characterization of the cytosolic glutathione transferase proteome from Drosophila melanogaster

The cytosolic GST (glutathione transferase) superfamily has been annotated in the Drosophila melanogaster genome database. Of 36 genes, four undergo alternative splicing to yield a total of 41 GST proteins. In the present study, we have obtained the 41 transcripts encoding proteins by RT (reverse transcription)-PCR using RNA template from Drosophila S2 cells, an embryonic cell line. This observation suggests that all of the annotated DmGSTs (D. melanogaster GSTs) in the proteome are expressed in the late embryonic stages of D. melanogaster. To avoid confusion in naming these numerous DmGSTs, we have designated them following the universal GST nomenclature as well as previous designations that fit within this classification. Furthermore, in the cell line, we identified an apparent processed pseudogene, gste8, in addition to two isoforms from the Delta class that have been published previously. Only approximately one-third of the expressed DmGSTs could be purified by conventional GSH affinity chromatography. The diverse kinetic properties as well as physiological substrate specificity of the DmGSTs are such that each individual enzyme displayed a unique character even compared with members from the same class.

In vitro toxicity and genotoxicity assessment of disinfection by-products, organic N-chloramines

Disinfection by-products (DBPs) are of concern to both water industries and health authorities. Although several classes of DBPs have been studied, and there are regulated safe levels in disinfected water for some, a large portion of DBPs are not characterized, and need further investigation. Organic N-chloramines are a group of DBPs, which can be formed during common disinfection processes such as chlorination and chloramination, but little is known in terms of their toxicological significance if consumed in drinking water. Only a few in vitro studies using bacterial assays have reported some genotoxic potential of organic N-chloramines, largely in the context of inflammatory processes in the body rather than exposure through drinking water. In this study, we investigated 16 organic N-chloramines produced by chlorination of model amino acids and amines. It was found that within the drinking water-relevant micromolar concentration range, four compounds were both cytotoxic and genotoxic to mammalian cells. A small reduction of cellular GSH was also observed in the treatment with these four compounds, but not of a magnitude to account for the cytotoxicity and genotoxicity. The results presented in this study demonstrate that some organic N-chloramines, at low concentrations that might be present in disinfected water, can be harmful to mammalian cells.

Prenatal and postnatal expression of glutathione transferase ζ 1 in human liver and the roles of haplotype and subject age in determining activity with dichloroacetate

Glutathione transferase ζ 1 (GSTZ1), also known as maleylacetoacetate isomerase, catalyzes the penultimate step of tyrosine catabolism and metabolizes several α-halocarboxylic acids, including dichloroacetic acid (DCA), an investigational drug used for lactic acidosis and, recently, solid tumors. Age-related differences have been suggested in DCA pharmacotoxicology, but no information is available on GSTZ1 ontogeny in humans. Here, we investigated the cytosolic GSTZ1 developmental expression pattern and the influence of haplotype on GSTZ1 activity with DCA by using human livers from donors between 10 weeks gestation and 74 years. GSTZ1 expression was very low in fetal livers (<2 pmol of GSTZ1/mg cytosol). The expression began to increase after birth in an age-dependent manner until age 7 years. GSTZ1 was then sustained at stable, yet variable, levels (median, 20.0 pmol/mg cytosol; range, 4.8-47.3 pmol/mg cytosol) until age 74 years. GSTZ1 activity with DCA was strongly associated with haplotype and expression level. Samples homozygous or heterozygous for GSTZ1A exhibited ∼3-fold higher DCA dechlorinating activity than samples carrying other alleles at a given level of expression. The correlations (r²) between activity and expression were 0.90 and 0.68, respectively, for GSTZ1A carriers (n = 11) and noncarriers (n = 61). GSTZ1 is expressed in mitochondria in addition to cytosol. The GSTZ1A allele exhibited similar effects in the mitochondrial fraction by conferring a higher activity with DCA. In summary, we report a neonatal onset and an age-related increase in GSTZ1 protein expression during human liver development. Haplotype influenced GSTZ1 activity with DCA but not protein expression.

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.

Kinetics of trihalogenated acetic acid metabolism and isoform specificity in liver microsomes

This study determined the metabolism of 3 drinking water disinfection by-products (halogenated acetic acids [HAAs]), bromodichloroacetic acid (BDCAA), chlorodibromoacetic acid (CDBAA), and tribromoacetic acid (TBAA), using rat, mouse, human liver microsomes, and recombinant P450. Metabolism proceeded by reductive debromination forming a di-HAA; the highest under nitrogen >>2% oxygen > atmospheric headspaces. V (max) for the loss of tri-HAA was 4 to 5 times higher under nitrogen than atmospheric headspace. Intrinsic metabolic clearance was TBAA>CDBAA>>BDCAA. At the high substrate concentrations, tri-HAA consumption rate was 2 to 3 times higher than the formation of di-HAA. Liberation of Br(-) from TBAA corresponded to the expected amount produced after DBAA formation, indicating retention of Br(-) by additional metabolite/metabolites. Subsequent experiments with CDBAA detected negligible formation of chlorodibromomethane (CDBM) and failed to account for the missing tri-HAA. Carbon monoxide and especially diphenyleneiodonium ([DPI] P450 reductase inhibitor) blocked CDBAA metabolism. Other chemical inhibitors were only partially able to block CDBAA metabolism. Most effective were inhibitors of CYP 2E1 and CYP 3A4. Immunoinhibition studies using human liver microsomes and anti-human CYP 2E1 antibodies were successful in reducing CDBAA metabolism. However, CDBAA metabolism in wild-type (WT) and CYP 2E1 knockout (KO) mouse liver microsomes was similar, suggesting significant interspecies differences in CYP isoform in tri-HAA metabolism. Additional assessment of CYP isoform involvement was complicated by the finding that recombinantly expressed rat and human P450 reductase was able to metabolize CDBAA, which may be a contributing factor in interspecies differences in tri-HAA metabolism.

Human polymorphisms in the glutathione transferase zeta 1/maleylacetoacetate isomerase gene influence the toxicokinetics of dichloroacetate

Dichloroacetate (DCA), a chemical relevant to environmental science and allopathic medicine, is dehalogenated by the bifunctional enzyme glutathione transferase zeta (GSTz1)/maleylacetoacetate isomerase (MAAI), the penultimate enzyme in the phenylalanine/tyrosine catabolic pathway. The authors postulated that polymorphisms in GSTz1/MAAI modify the toxicokinetics of DCA. GSTz1/MAAI haplotype significantly affected the kinetics and biotransformation of 1,2-¹³C-DCA when it was administered at either environmentally (µg/kg/d) or clinically (mg/kg/d) relevant doses. GSTz1/MAAI haplotype also influenced the urinary accumulation of potentially toxic tyrosine metabolites. Atomic modeling revealed that GSTz1/MAAI variants associated with the slowest rates of DCA metabolism induced structural changes in the enzyme homodimer, predicting protein instability or abnormal protein-protein interactions. Knowledge of the GSTz1/MAAI haplotype can be used prospectively to identify individuals at potential risk of DCA's adverse side effects from environmental or clinical exposure or who may exhibit aberrant amino acid metabolism in response to dietary protein.

Potential carcinogenic hazards of non-regulated disinfection by-products: haloquinones, halo-cyclopentene and cyclohexene derivatives, N-halamines, halonitriles, and heterocyclic amines

Drinking water disinfectants react with natural organic material (NOM) present in source waters used for drinking water to produce a wide variety of by-products. Several hundred disinfections by-products (DBPs) have been identified, but none have been identified with sufficient carcinogenic potency to account for the cancer risks projected from epidemiological studies. In a search for DBPs that might fill this risk gap, the present study projected reactions of chlorine and chloramine that could occur with substructures present in NOM to produce novel by-products. A review of toxicological data on related compounds, supplemented by use of a quantitative structure toxicity relationship (QSTR) program TOPKAT®) identified chemicals with a high probability of being chronically toxic and/or carcinogenic among 489 established and novel DBPs. Classes of DBPs that were specifically examined were haloquinones (HQs), related halo-cyclopentene and cyclohexene (HCP&H) derivatives, halonitriles (HNs), organic N-chloramines (NCls), haloacetamides (HAMs), and nitrosamines (NAs). A review of toxicological data available for quinones suggested that HQs and HCP&H derivatives appeared likely to be of health concern and were predicted to have chronic lowest observed adverse effect levels (LOAELs) in the low μg/kg day range. Several HQs were predicted to be carcinogenic. Some have now been identified in drinking water. The broader class of HNs was explored by considering current toxicological data on haloacetonitriles and extending this to halopropionitriles. 2,2-dichloropropionitrile has been identified in drinking water at low concentrations, as well as the more widely recognized haloacetonitriles. The occurrence of HAMs has been previously documented. The very limited toxicological data on HAMs suggests that this class would have toxicological potencies similar to the dihaloacetic acids. Organic N-halamines are also known to be produced in drinking water treatment and have biological properties of concern, but no member has ever been characterized toxicologically beyond bacterial or in vitro studies of genotoxicity. The documented formation of several nitrosamines from secondary amines from both natural and industrial sources prompted exploration of the formation of additional nitrosamines. N-diphenylnitrosamine was identified in drinking waters. Of more interest, however, was the formation of phenazine (and subsequently N-chorophenazine) in a competing reaction. These are the first heterocyclic amines that have been identified as chlorination by-products. Consideration of the amounts detected of members of these by-product classes and their probable toxicological potency suggest a prioritization for obtaining more detailed toxicological data of HQs>HCP&H derivatives>NCls>HNs. Based upon a ubiquitous occurrence and virtual lack of in vivo toxicological data, NCls are the most difficult group to assign a priority as potential carcinogenic risks. This analysis indicates that research on the general problem of DBPs requires a more systematic approach than has been pursued in the past. Utilization of predictive chemical tools to guide further research can help bring resolution to the DBP issue by identifying likely DBPs with high toxicological potency.

Multiple roles for plant glutathione transferases in xenobiotic detoxification

Discovered 40 years ago, plant glutathione transferases (GSTs) now have a well-established role in determining herbicide metabolism and selectivity in crops and weeds. Within the GST superfamily, the numerous and plant-specific phi (F) and tau (U) classes are largely responsible for catalyzing glutathione-dependent reactions with xenobiotics, notably conjugation leading to detoxification and, more rarely, bioactivating isomerizations. In total, the crystal structures of 10 plant GSTs have been solved and a highly conserved N-terminal glutathione binding domain and structurally diverse C-terminal hydrophobic domain identified, along with key coordinating residues. Unlike drug-detoxifying mammalian GSTs, plant enzymes utlilize a catalytic serine in place of a tyrosine residue. Both GSTFs and GSTUs undergo changes in structure during catalysis indicative of an induced fit mechanism on substrate binding, with an understanding of plant GST structure/function allowing these proteins to be engineered for novel functions in detoxification and ligand recognition. Several major crops produce alternative thiols, with GSTUs shown to use homoglutathione in preference to glutathione, in herbicide detoxification reactions in soybeans. Similarly, hydroxymethylglutathione is used, in addition to glutathione in detoxifying the herbicide fenoxaprop in wheat. Following GST action, plants are able to rapidly process glutathione conjugates by at least two distinct pathways, with the available evidence suggesting these function in an organ- and species-specific manner. Roles for GSTs in endogenous metabolism are less well defined, with the enzymes linked to a diverse range of functions, including signaling, counteracting oxidative stress, and detoxifying and transporting secondary metabolites.

Glutathione transferase zeta: discovery, polymorphic variants, catalysis, inactivation, and properties of Gstz1-/- mice

Glutathione transferase zeta (GSTZ1) is a member of the GST superfamily of proteins that catalyze the reaction of glutathione with endo- and xenobiotics. GSTZ1-1 was discovered by a bioinformatics strategy that searched the human-expressed sequence-tag database with a sequence that matched a putative plant GST. A sequence that was found was expressed and termed GSTZ1-1. In common with other GSTs, GSTZ1-1 showed some peroxidase activity, but lacked activity with most known GST substrates. GSTZ1-1 was also found to be identical with maleylacetoacetate isomerase, which catalyzes the penultimate step in the tyrosine-degradation pathway. Further studies showed that dichloroacetate (DCA) and a range of α-haloalkanoates and α,α-dihaloalkanoates were substrates. A subsequent search of the human-expressed sequence-tag database showed the presence of four polymorphic alleles: 1a, 1b, 1c, and 1d; GSTZ1c was the most common and was designated as the wild-type gene. DCA was shown to be a k(cat) inactivator of human, rat, and mouse GSTZ1-1; human GSTZ1-1 was more resistant to inactivation than mouse or rat GSTZ1-1. Proteomic analysis showed that hGSTZ1-1 was inactivated when Cys-16 was modified by glutathione and the carbon skeleton of DCA. The polymorphic variants of hGSTZ1-1 differ in their susceptibility to inactivation, with 1a-1a being more resistant to inactivation than the other variants. The targeted deletion of GSTZ1 yielded mice that were not phenotypically distinctive. Phenylalanine proved, however, to be toxic to Gstz1(-/-) mice, and these mice showed evidence of organ damage and leucopenia.

Genetic polymorphisms of glutathione S-transferase genes and susceptibility to colorectal cancer: a case-control study in an Indian population

BACKGROUND

Susceptibility to sporadic colorectal cancer is multifactorial and arises from interactive combinations of allelic variants in low-penetrance genes and relevant environmental risk factors. Genetic polymorphisms in metabolic enzymes as gene susceptibility factors may modify colorectal cancer risk. We evaluated the risk of colorectal cancer associated with respective or combined glutathione S-transferase (GST) polymorphisms and assessed the interactions between genes and environmental factors in a case-control study in an Indian population.

METHODS

The study included 59 colon and 243 rectal cancer cases, and 291 cancer-free healthy controls. GST genotypes were detected by multiplex PCR-based and PCR-RFLP methods. The risk of cancer associated with GST polymorphisms was estimated by calculation of odds ratios (ORs) and confidence intervals (95% CIs) using unconditional logistic regression.

RESULTS

The GSTM1 null genotype was found to be associated with a significantly increased rectal cancer risk (OR=1.55; 95% CI, 1.05-2.30), while the GSTT1 null genotype with a greater risk of colon cancer (OR=2.15; 95% CI, 1.04-4.32). A substantial increase of both colon (OR=10.81; 95% CI, 1.11-107.22) and rectal (OR=4.80; 95% CI, 0.94-35.91) cancer risk was shown for the combination of GSTM1 null, GSTT1 null and GSTP1 105Val allele. The combined GSTM1 null and GSTP1 114Val allele also revealed an increased risk for either colon cancer (OR=4.69; 95% CI, 0.84-23.87) or rectal cancer (OR=5.68; 95% CI, 1.79-22.16). Furthermore, the combination of GSTM1 null, GSTT1 null and GSTP1 114Val allele was found in 2 rectal cancer cases.

CONCLUSION

Our results suggest that co-exist of GSTM1 null, GSTT1 null and the variant GSTP1 105Val or 114Val allele may be predisposing risk factors for colorectal cancer in Indian population.

Dichloroacetate- and Trichloroacetate-Induced Modulation of Superoxide Dismutase, Catalase, and Glutathione Peroxidase Activities and Glutathione Level in the livers of Mice after Subacute and Subchronic exposure

Dichloroacetate (DCA) and trichloroacetate (TCA) were previously found to induce various levels of oxidative stress in the hepatic tissues of mice after subacute and subchronic exposure. The cells are known to have several protective mechansims against production of oxidative stress by different xenobiotics. To assess the roles of the antioxidant enzymes and glutathione (GSH) in DCA- and TCA-induced oxidative stress, groups of B6C3F1 mice were administered either DCA or TCA at doses of 7.7, 77, 154 and 410 mg/kg/day, by gavage for 4 weeks (4-W) and 13 weeks (13-W), and superoxide dismutase (SOD) catalase (CAT) and glutathione peroxidase (GSH-Px) activities, as well as GSH were determined in the hepatic tissues. DCA at doses ranging between 7.7-410, and 7.7-77 mg/kg/day, given for 4-W and 13-W, respectively, resulted in either suppression or no change in SOD, CAT and GSH-Px activities, but doses of 154-410 mg DCA/kg/day administered for 13-W were found to result in significant induction of the three enzyme activities. TCA administration on the other hand, resulted in increases in SOD and CAT activities, and suppression of GSH-Px activity in both periods. Except for the DCA doses of 77-154 mg/kg/day administered for 13-W that resulted in significant reduction in GSH levels, all other DCA, as well as TCA treatments produced no changes in GSH. Since these enzymes are involved in the detoxification of the reactive oxygen species (ROS), superoxide anion (SA) and H(2)O(2), it is concluded that SA is the main contributor to DCA-induced oxidative stress while both ROS contribute to that of TCA. The increases in the enzyme activities associated with 154-410 mg DCA/kg/day in the 13-W period suggest their role as protective mechanisms contributing to the survival of cells modified in response to those treatments.

Mitochondrion as a novel site of dichloroacetate biotransformation by glutathione transferase zeta 1

Dichloroacetate (DCA) is a potential environmental hazard and an investigational drug. Repeated doses of DCA result in reduced drug clearance, probably through inhibition of glutathione transferase ζ1 (GSTZ1), a cytosolic enzyme that converts DCA to glyoxylate. DCA is known to be taken up by mitochondria, where it inhibits pyruvate dehydrogenase kinase, its major pharmacodynamic target. We tested the hypothesis that the mitochondrion was also a site of DCA biotransformation. Immunoreactive GSTZ1 was detected in liver mitochondria from humans and rats, and its identity was confirmed by liquid chromatography/tandem mass spectrometry analysis of the tryptic peptides. Study of rat submitochondrial fractions revealed GSTZ1 to be localized in the mitochondrial matrix. The specific activity of GSTZ1-catalyzed dechlorination of DCA was 2.5- to 3-fold higher in cytosol than in whole mitochondria and was directly proportional to GSTZ1 protein expression in the two compartments. Rat mitochondrial GSTZ1 had a 2.5-fold higher (App)K(m) for glutathione than cytosolic GSTZ1, whereas the (App)K(m) values for DCA were identical. Rats administered DCA at a dose of 500 mg/kg/day for 8 weeks showed reduced hepatic GSTZ1 activity and expression of ∼10% of control levels in both cytosol and mitochondria. We conclude that the mitochondrion is a novel site of DCA biotransformation catalyzed by GSTZ1, an enzyme colocalized in cytosol and mitochondrial matrix.

Polymorphisms in GSTT1, GSTZ1, and CYP2E1, disinfection by-products, and risk of bladder cancer in Spain

BACKGROUND

Bladder cancer has been linked with long-term exposure to disinfection by-products (DBPs) in drinking water.

OBJECTIVES

In this study we investigated the combined influence of DBP exposure and polymorphisms in glutathione S-transferase (GSTT1, GSTZ1) and cytochrome P450 (CYP2E1) genes in the metabolic pathways of selected by-products on bladder cancer in a hospital-based case-control study in Spain.

METHODS

Average exposures to trihalomethanes (THMs; a surrogate for DBPs) from 15 years of age were estimated for each subject based on residential history and information on municipal water sources among 680 cases and 714 controls. We estimated effects of THMs and GSTT1, GSTZ1, and CYP2E1 polymorphisms on bladder cancer using adjusted logistic regression models with and without interaction terms.

RESULTS

THM exposure was positively associated with bladder cancer: adjusted odds ratios (ORs) and 95% confidence intervals (CIs) were 1.2 (0.8-1.8), 1.8 (1.1-2.9), and 1.8 (0.9-3.5) for THM quartiles 2, 3, and 4, respectively, relative to quartile 1. Associations between THMs and bladder cancer were stronger among subjects who were GSTT1 +/+ or +/- versus GSTT1 null (P(interaction) = 0.021), GSTZ1 rs1046428 CT/TT versus CC (P(interaction) = 0.018), or CYP2E1 rs2031920 CC versus CT/TT (P(interaction) = 0.035). Among the 195 cases and 192 controls with high-risk forms of GSTT1 and GSTZ1, the ORs for quartiles 2, 3, and 4 of THMs were 1.5 (0.7-3.5), 3.4 (1.4-8.2), and 5.9 (1.8-19.0), respectively.

CONCLUSIONS

Polymorphisms in key metabolizing enzymes modified DBP-associated bladder cancer risk. The consistency of these findings with experimental observations of GSTT1, GSTZ1, and CYP2E1 activity strengthens the hypothesis that DBPs cause bladder cancer and suggests possible mechanisms as well as the classes of compounds likely to be implicated.

Mechanisms of protein evolution and their application to protein engineering

Protein engineering holds great promise for the development of new biosensors, diagnostics, therapeutics, and agents for bioremediation. Despite some remarkable successes in experimental and computational protein design, engineered proteins rarely achieve the efficiency or specificity of natural enzymes. Current protein design methods utilize evolutionary concepts, including mutation, recombination, and selection, but the inability to fully recapitulate the success of natural evolution suggests that some evolutionary principles have not been fully exploited. One aspect of protein engineering that has received little attention is how to select the most promising proteins to serve as templates, or scaffolds, for engineering. Two evolutionary concepts that could provide a rational basis for template selection are the conservation of catalytic mechanisms and functional promiscuity. Knowledge of the catalytic motifs responsible for conserved aspects of catalysis in mechanistically diverse superfamilies could be used to identify promising templates for protein engineering. Second, protein evolution often proceeds through promiscuous intermediates, suggesting that templates which are naturally promiscuous for a target reaction could enhance protein engineering strategies. This review explores these ideas and alternative hypotheses concerning protein evolution and engineering. Future research will determine if application of these principles will lead to a protein engineering methodology governed by predictable rules for designing efficient, novel catalysts.

Physiologically based pharmacokinetic modeling of dibromoacetic acid in F344 rats

A novel physiologically based pharmacokinetic (PBPK) model structure, which includes submodels for the common metabolites (glyoxylate (GXA) and oxalate (OXA)) that may be involved in the toxicity or carcinogenicity of dibromoacetic acid (DBA), has been developed. Particular attention is paid to the representation of hepatic metabolism, which is the primary elimination mechanism. DBA-induced suicide inhibition is modeled by irreversible covalent binding of the intermediate metabolite alpha-halocarboxymethylglutathione (alphaH1) to the glutathione-S-transferase zeta (GSTzeta) enzyme. We also present data illustrating the presence of a secondary non-GSTzeta metabolic pathway for DBA, but not dichloroacetic acid (DCA), that produces GXA. The model is calibrated with plasma and urine concentration data from DBA exposures in female F344 rats through intravenous (IV), oral gavage, and drinking water routes. Sensitivity analysis is performed to confirm identifiability of estimated parameters. Finally, model validation is performed with data sets not used during calibration. Given the structural similarity of dihaloacetates (DHAs), we hypothesize that the PBPK model presented here has the capacity to describe the kinetics of any member or mixture of members of this class in any species with the alteration of chemical-and species-specific parameters.

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.

Trichloroethylene risk assessment: a review and commentary

Trichloroethylene (TCE) is a widespread environmental contaminant that is carcinogenic when given in high, chronic doses to certain strains of mice and rats. The capacity of TCE to cause cancer in humans is less clear. The current maximum contaminant level (MCL) of 5 ppb (microg/L) is based on an US Environment Protection Agency (USEPA) policy decision rather than the underlying science. In view of major advances in understanding the etiology and mechanisms of chemically induced cancer, USEPA began in the late 1990s to revise its guidelines for cancer risk assessment. TCE was chosen as the pilot chemical. The USEPA (2005) final guidelines emphasized a "weight-of-evidence" approach with consideration of dose-response relationships, modes of action, and metabolic/toxicokinetic processes. Where adequate data are available to support reversible binding of the carcinogenic moiety to biological receptors as the initiating event (i.e., a threshold exists), a nonlinear approach is to be used. Otherwise, the default assumption of a linear (i.e., nonthreshold) dose-response is utilized. When validated physiologically based pharmacokinetic (PBPK) models are available, they are to be used to predict internal dosimetry as the basis for species and dose extrapolations. The present article reviews pertinent literature and discusses areas where research may resolve some outstanding issues and facilitate the reassessment process. Key research needs are proposed, including role of dichloroacetic acid (DCA) in TCE-induced liver tumorigenesis in humans; extension of current PBPK models to predict target organ deposition of trichloroacetic acid (TCA) and DCA in humans ingesting TCE in drinking water; use of human hepatocytes to ascertain metabolic rate constants for use in PBPK models that incorporate variability in metabolism of TCE by potentially sensitive subpopulations; measurement of the efficiency of first-pass elimination of trace levels of TCE in drinking water; and assessment of exogenous factors' (e.g., alcohol, drugs) ability to alter metabolic activation and risks at such low-level exposure.

Characterization of the fluoroacetate detoxication enzymes of rat liver cytosol

Two forms of fluoroacetate-specific defluorinase (FSD) were purified from rat hepatic cytosol. The first form, FSD1 (molecular weight 38 kDa), contained 81% of the total cytosolic fluoroacetate defluorination activity and did not bind to the glutathione-affinity, orange A or mono P columns used in the purification procedures. The second form, FSD2 (molecular weight 27 kDa), contained only 13% of the fluoroacetate defluorination activity, had a pI = 7.8, and exhibited a high glutathione S-transferase (GST)-like activity towards dichloroacetic acid. The FSD1 proteins were identified from peptide mass data and best matched with rat sorbitol dehydrogenase (SDH) (short form), although pure sheep liver SDH enzyme did not possess defluorination activity when subsequently investigated. The FSD2 protein was identified from peptide mass data and best matched with the amino acid sequence of mouse and human Zeta 1 of glutathione S-transferase (GSTZ1) and showed a high GSTZ1 specific activity. This study suggests that the major FSD component (FSD1) represents a new and unique dehalogenating or dehydrogenating enzyme present in rat liver cytosol. The minor FSD component (FSD2) is due to the GSTZ1 present in rat liver cytosol. However, it is not yet clear that FSD1 is indeed SDH and FSD2 is indeed GSTZ1, due to sequence homology being less than 60 and 45%, respectively.