Glutathione transferase zeta-catalyzed biotransformation of dichloroacetic acid and other alpha-haloacids

Exposures to drinking water disinfection byproducts and kidney function in Chinese women

BACKGROUND

Disinfection byproducts (DBPs), the ubiquitous contaminants in drinking water, have been shown to impair renal function in experimental studies. However, epidemiological evidence is sparse.

OBJECTIVE

To investigate exposures to DBPs in associations with renal function among women.

METHODS

A total of 920 women from December 2018 to January 2020 were abstracted from the Tongji Reproductive and Environmental (TREE) Study, an ongoing cohort study in Wuhan, China. Urine samples were gathered at baseline recruitment and analyzed for dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) as biomarkers of DBP exposures. Serum uric acid (UA), creatinine, and estimated glomerular filtration rate (eGFR) were measured as indicators of renal function. Multivariate linear regression and restricted cubic spline (RCS) models were conducted to assess urinary DCAA and TCAA concentrations in associations with renal function indicators. Stratified analyses by age and body mass index (BMI) were also performed.

RESULTS

We found null evidence of urinary TCAA in associations with renal function indicators. However, elevated urinary DCAA tertiles were related to decreased eGFR (β = -1.78%, 95% CI: 3.21%, -0.36%, comparing the upper vs. lower tertile; P for trend = 0.01). This inverse association still existed when urinary DCAA concentration was treated as a continuous variable, and the dose-response relationship was linear based on the RCS model (P for overall association = 0.002 and P for non-linear associations = 0.44). In the stratified analyses, we found an association of urinary DCAA concentration with decreased UA level among women <30 years but an association with increased UA level among women ≥30 years (P for interaction = 0.04).

CONCLUSION

Urinary DCAA but not TCAA was associated with impaired renal function among women undergoing assisted reproductive technology.

Clinical physiology and pharmacology of GSTZ1/MAAI

Herein I summarize the physiological chemistry and pharmacology of the bifunctional enzyme glutathione transferase zeta 1 (GSTZ1)/ maleylacetoacetate isomerase (MAAI) relevant to human physiology, drug metabolism and disease. MAAI is integral to the catabolism of the amino acids phenylalanine and tyrosine. Genetic or pharmacological inhibition of MAAI can be pathological in animals. However, to date, no clinical disease consequences are unequivocally attributable to inborn errors of this enzyme. MAAI is identical to the zeta 1 family isoform of GST, which biotransforms the investigational drug dichloroacetate (DCA) to the endogenous compound glyoxylate. DCA is a mechanism-based inhibitor of GSTZ1 that significantly reduces its rate of metabolism and increases accumulation of potentially harmful tyrosine intermediates and of the heme precursor δ-aminolevulinic acid (δ-ALA). GSTZ1 is most abundant in rodent and human liver, with its concentration several fold higher in cytoplasm than in mitochondria. Its activity and protein expression are dependent on the age of the host and the intracellular level of chloride ions. Gene association studies have linked GSTZ1 or its protein product to various physiological traits and pathologies. Haplotype variations in GSTZ1 influence the rate of DCA metabolism, enabling a genotyping strategy to allow potentially safe, precision-based drug dosing in clinical trials.

Assessing the genotoxicity and carcinogenicity of 2-chloroethanol through structure activity relationships and in vitro testing approaches

The detection of 2-chloroethanol in foods generally follows an assumption that the pesticide ethylene oxide has been used at some stage in the supply chain. In this situation the Pesticide Residues in Food Regulation (EC) 396/2005 requires 2-chloroethanol to be assessed as if equivalent to ethylene oxide, which has been classified as a genotoxic carcinogen. This review investigated whether this is an appropriate risk assessment approach for 2-chloroethanol. This involved an assessment of existing genotoxicity and carcinogenicity data, application of Structure Activity Based Read Across for carcinogenicity assessment, biological reactivity in the ToxTracker assay and micronuclei formation in HepaRG cells. Although we identified there is an absence of a standard oral bioassay for 2-chloroethanol, carcinogenicity weight-of-evidence assessment along with data on relevant structural analogues do not show evidence for carcinogenicity for 2-chloroethanol. The absence of genotoxicity was demonstrated for 2-chloroethanol and suitable analogues. In contrast, ethylene oxide showed reactivity towards markers indicative of direct DNA damage which is consistent with what is known about its mode-of-action. These data facilitate the understanding of 2-chloroethanol and given that it is not a genotoxic carcinogen suggest it must be assessed relative to non-cancer endpoints and a health protective Reference Dose should be established on that basis.

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.

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.

Effects of Multiple Doses of Dichloroacetate on GSTZ1 Expression and Activity in Liver and Extrahepatic Tissues of Young and Adult Rats

Glutathione transferase zeta 1 (GSTZ1), expressed in liver and several extrahepatic tissues, catalyzes dechlorination of dichloroacetate (DCA) to glyoxylate. DCA inactivates GSTZ1, leading to autoinhibition of its metabolism. DCA is an investigational drug for treating several congenital and acquired disorders of mitochondrial energy metabolism, including cancer. The main adverse effect of DCA, reversible peripheral neuropathy, is more common in adults treated long-term than in children, who metabolize DCA more quickly after multiple doses. One dose of DCA to Sprague Dawley rats reduced GSTZ1 expression and activity more in liver than in extrahepatic tissues; however, the effects of multiple doses of DCA that mimic its therapeutic use have not been studied. Here, we examined the expression and activity of GSTZ1 in cytosol and mitochondria of liver, kidney, heart, and brain 24 hours after completion of 8-day oral dosing of 100 mg/kg per day sodium DCA to juvenile and adult Sprague Dawley rats. Activity was measured with DCA and with 1,2-epoxy-3-(4-nitrophenoxy)propane (EPNPP), reported to be a GSTZ1-selective substrate. In DCA-treated rats, liver retained higher expression and activity of GSTZ1 with DCA than other tissues, irrespective of rodent age. DCA-treated juvenile rats retained more GSTZ1 activity with DCA than adults. Consistent with this finding, there was less measurable DCA in tissues of juvenile than adult rats. DCA-treated rats retained activity with EPNPP, despite losing over 98% of GSTZ1 protein. These data provide insight into the differences between children and adults in DCA elimination under a therapeutic regimen and confirm that the liver contributes more to DCA metabolism than other tissues. SIGNIFICANCE STATEMENT: Dichloroacetate (DCA) is one of few drugs exhibiting higher clearance from children than adults, after repeated doses, for reasons that are unclear. We hypothesized that juveniles retain more glutathione transferase zeta 1 (GSTZ1) than adults in tissues after multiple DCA doses and found this was the case for liver and kidney, with rat as a model to assess GSTZ1 protein expression and activity with DCA. Although 1,2-epoxy-3-(4-nitrophenoxy)propane was reported to be a selective GSTZ1 substrate, its activity was not reduced in concert with GSTZ1 protein.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

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.

New Perspectives for Cancer Hazard Evaluation by the Report on Carcinogens: A Case Study Using Read-Across Methods in the Evaluation of Haloacetic Acids Found as Water Disinfection By-Products

BACKGROUND

Due to the large number of chemicals not yet tested for carcinogenicity but to which people are exposed, the limited number of human and animal cancer studies conducted each year, and the frequent need for a timely response, mechanistic data are playing an increasingly important role in carcinogen hazard identification.

OBJECTIVES

To provide a targeted approach to identify relevant mechanistic data in our cancer evaluation of haloacetic acids (HAAs), we used several approaches including systematic review, the 10 key characteristics of carcinogens (KCs), and read-across methods. Our objective in this commentary is to discuss the strengths, limitations, and challenges of these approaches in a cancer hazard assessment.

METHODS

A cancer hazard assessment for 13 HAAs found as water disinfection by-products was conducted. Literature searches for mechanistic studies focused on the KCs and individual HAAs. Studies were screened for relevance and categorized by KCs and other relevant data, including chemical properties, toxicokinetics, and biological effects other than KCs. Mechanistic data were organized using the KCs, and strength of evidence was evaluated; this information informed potential modes of action (MOAs) and read-across-like approaches. Three read-across options were considered: evaluating HAAs as a class, as subclass(es), or as individual HAAs (analog approach).

DISCUSSION

Because of data limitations and uncertainties, listing as a class or subclass(es) was ruled out, and an analog approach was used. Two brominated HAAs were identified as target (untested) chemicals based on their metabolism and similarity to source (tested) chemicals. In addition, four HAAs with animal cancer data had sufficient evidence for potential listing in the Report on Carcinogens (RoC). This is the first time that the KCs and other relevant data, in combination with read-across principles, were used to support a recommendation to list chemicals in the RoC that did not have animal cancer data. https://doi.org/10.1289/EHP5672.

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.

Genome-wide identification of the entire 90 glutathione S-transferase (GST) subfamily genes in four rotifer Brachionus species and transcriptional modulation in response to endocrine disrupting chemicals

Genome-wide identification of glutathione S-transferase (GST), a major phase II detoxification enzyme, was investigated in four different aquatic model rotifer species Brachionus koreanus, B. plicatilis, B. rotundiformis, and B. calyciflorus. GSTs are ubiquitous antioxidant enzymes that play versatile function including cellular detoxification, stress alleviation, and production of the radical conjugates. Among the four rotifers, B. rotundiformis was found with the least number of GST genes (total 19 GST genes), whereas the other three species shared 23 to 24 GST genes. Among the identified GST genes, belonging to the cytosolic GST superfamily, the expansion of GST sigma classes mainly occurs through tandem duplication, resulting in tandem-arrayed gene clusters on the chromosomes. Overall, the number of genes discovered in this study was highest in the sigma class, zeta, alpha, and omega in descending order. With integration of phylogenetic analysis and xenobiotic-mediated GST mRNA expression patterns along with previous enzymatic activities, the functional divergence among species-specific GST genes was clearly observed. This study covers full identification of GST classes in three marine rotifer and one fresh-water rotifer species and their important role in marine environmental ecotoxicology.

Age-Related Changes in Expression and Activity of Human Hepatic Mitochondrial Glutathione Transferase Zeta1

Glutathione transferase zeta1 (GSTZ1) catalyzes glutathione (GSH)-dependent dechlorination of dichloroacetate (DCA), an investigational drug with therapeutic potential in metabolic disorders and cancer. GSTZ1 is expressed in both hepatic cytosol and mitochondria. Here, we examined the ontogeny and characterized the properties of human mitochondrial GSTZ1. GSTZ1 expression and activity with DCA were determined in 103 human hepatic mitochondrial samples prepared from livers of donors aged 1 day to 84 years. DNA from each sample was genotyped for three common GSTZ1 functional single nucleotide polymorphisms. Expression of mitochondrial GSTZ1 protein increased in an age-dependent manner to a plateau after age 21 years. Activity with DCA correlated with expression, after taking into account the somewhat higher activity of samples that were homo- or heterozygous for GSTZ1A. In samples from livers with the GSTZ1C variant, apparent enzyme kinetic constants for DCA and GSH were similar for mitochondria and cytosol after correcting for the loss of GSH observed in mitochondrial incubations. In the presence of 38 mM chloride, mitochondrial GSTZ1 exhibited shorter half-lives of inactivation compared with the cytosolic enzyme (P = 0.017). GSTZ1 protein isolated from mitochondria was shown by mass spectrometry to be identical to cytosolic GSTZ1 protein in the covered primary protein sequence. In summary, we report age-related development in the expression and activity of human hepatic mitochondrial GSTZ1 does not have the same pattern as that reported for cytosolic GSTZ1. Some properties of cytosolic and mitochondrial GSTZ1 differed, but these were not related to differences in amino acid sequence or post-translationally modified residues.

Regulation of dichloroacetate biotransformation in rat liver and extrahepatic tissues by GSTZ1 expression and chloride concentration

Biotransformation of dichloroacetate (DCA) to glyoxylate by hepatic glutathione transferase zeta 1 (GSTZ1) is considered the principal determinant of the rate of plasma clearance of the drug. However, several other organismal and subcellular factors are also known to influence DCA metabolism. We utilized a female rat model to study these poorly understood processes. Rats aged 4 weeks (young) and 42-52 weeks (adult) were used to model children and adults, respectively. Hepatic chloride concentrations, which influence the rate of GSTZ1 inactivation by DCA, were lower in rat than in human tissues and rats did not show the age dependence previously seen in humans. We found GSTZ1 expression and activity in rat brain, heart, and kidney cell-free homogenates that were age-dependent. GSTZ1 expression in brain was higher in young rats than adult rats, whereas cardiac and renal GSTZ1 expression levels were higher in adult than young rats. GSTZ1 activity with DCA could not be measured accurately in kidney cell-free homogenates due to rapid depletion of glutathione by γ-glutamyl transpeptidase. Following oral administration of DCA, 100 mg/kg, to rats, GSTZ1 expression and activity were reduced in all rat tissues, but chloride concentrations were not affected. Together, these data extend our understanding of factors that determine the in vivo kinetics of DCA.

Haloacetic Acid Water Disinfection Byproducts Affect Pyruvate Dehydrogenase Activity and Disrupt Cellular Metabolism

The disinfection of drinking water has been a major public health achievement. However, haloacetic acids (HAAs), generated as byproducts of water disinfection, are cytotoxic, genotoxic, mutagenic, carcinogenic, and teratogenic. Previous studies of monoHAA-induced genotoxicity and cell stress demonstrated that the toxicity was due to inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), leading to disruption of cellular metabolism and energy homeostasis. DiHAAs and triHAAs are also produced during water disinfection, and whether they share mechanisms of action with monoHAAs is unknown. In this study, we evaluated the effects of mono-, di-, and tri-HAAs on cellular GAPDH enzyme kinetics, cellular ATP levels, and pyruvate dehydrogenase complex (PDC) activity. Here, treatments conducted in Chinese hamster ovary (CHO) cells revealed differences among mono-, di-, and triHAAs in their molecular targets. The monoHAAs, iodoacetic acid and bromoacetic acid, were the strongest inhibitors of GAPDH and greatly reduced cellular ATP levels. Chloroacetic acid, diHAAs, and triHAAs were weaker inhibitors of GAPDH and some increased the levels of cellular ATP. HAAs also affected PDC activity, with most HAAs activating PDC. The primary finding of this work is that mono- versus multi-HAAs address different molecular targets, and the results are generally consistent with a model in which monoHAAs activate the PDC through GAPDH inhibition-mediated disruption in cellular metabolites, including altering ATP-to-ADP and NADH-to-NAD ratios. The monoHAA-mediated reduction in cellular metabolites results in accelerated PDC activity by way of metabolite-ratio-dependent PDC regulation. DiHAAs and triHAAs are weaker inhibitors of GAPDH, but many also increase cellular ATP levels, and we suggest that they increase PDC activity by inhibiting pyruvate dehydrogenase kinase.

Model Informed Dose Optimization of Dichloroacetate for the Treatment of Congenital Lactic Acidosis in Children

Dichloroacetate (DCA) is an investigational drug used to treat congenital lactic acidosis and other mitochondrial disorders. Response to DCA therapy in young children may be suboptimal following body weight-based dosing. This is because of autoinhibition of its metabolism, age-dependent changes in pharmacokinetics, and polymorphisms in glutathione transferase zeta 1 (GSTZ1), its primary metabolizing enzyme. Our objective was to predict optimal DCA doses for the treatment of congenital lactic acidosis in children. Accordingly, a semimechanistic pharmacokinetic-enzyme turnover model was developed in a step-wise approach: (1) a population pharmacokinetic model for adults was developed; (2) the adult model was scaled to children using allometry and physiology-based scaling; and (3) the scaled model was externally qualified, updated with clinical data, and optimal doses were projected. A 2-compartment model accounting for saturable clearance and GSTZ1 enzyme turnover successfully characterized the DCA PK in adults and children. DCA-induced inactivation of GSTZ1 resulted in phenoconversion of all subjects into slow metabolizers after repeated dosing. However, rate and extent of inactivation was 2-fold higher in subjects without the wild-type EGT allelic variant of GSTZ1, resulting in further phenoconversion into ultraslow metabolizers after repeated DCA administration. Furthermore, DCA-induced GSTZ1 inactivation rate and extent was found to be 25- to 30-fold lower in children than in adults, potentially accounting for the observed age-dependent changes in PK. Finally, a 12.5 and 10.6 mg/kg twice-daily DCA dose was optimal in achieving the target steady-state trough concentrations (5-25 mg/L) for EGT carrier and EGT noncarrier children, respectively.

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.

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.

The influence of human GSTZ1 gene haplotype variations on GSTZ1 expression

BACKGROUND/OBJECTIVES

The zeta-1 family isoform of GST biotransforms the investigational drug dichloroacetate (DCA) and certain other halogenated carboxylic acids. Haplotype variability in GSTZ1 influences the kinetics and, possibly, the toxicity of DCA. DCA metabolism correlates with expression of the GSTZ1 protein, so it is important to document variables that affect expression. Following up on a limited previous study, we tested the hypothesis that a coding single nucleotide polymorphism (SNP), the lysine (K) amino acid (E32>K) in GSTZ1 haplotypes linked to a promoter region SNP results in lower hepatic expression of GSTZ1.

MATERIALS AND METHODS

The influence of K carrier and non-K carrier haplotypes on GSTZ1 expression was determined by analyzing 78 liver samples from individuals aged 7-84 years of various racial and ethnic backgrounds. GSTZ1 expression data were analyzed on the basis of the presence or absence of lysine 32.

RESULTS

GSTZ1 protein expression differed significantly between K carrier and non-K carrier haplotypes (P=0.001) in Whites, but not in African-Americans (P=0.277). We attribute this difference in GSTZ1 expression among K carrier haplotypes in Whites to the linkage disequilibrium between the K or A allele from the G>A SNP (rs7975), within the promoter G>A-1002 SNP (rs7160195) A allele. There is no linkage disequilibrium between these two polymorphisms in African-Americans.

CONCLUSION

We conclude that the lower expression of GSTZ1 in Whites who possess the K carrier haplotype results in lower enzymatic activity and slower metabolism of DCA, compared with those who possess the non-K carrier haplotype. These results further define safe, genetics-based dosing regimens for chronic DCA administration.

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.

Chloride and other anions inhibit dichloroacetate-induced inactivation of human liver GSTZ1 in a haplotype-dependent manner

The in vivo elimination rate of dichloroacetate (DCA), an investigational drug; is determined by the rate of its biotransformation to glyoxylate, catalyzed by glutathione transferase ζ1 (GSTZ1). DCA is a mechanism-based inactivator of GSTZ1, thus elimination of DCA is slowed with repeated dosing. We observed that chloride, a physiologically important anion, attenuated DCA-induced GSTZ1 inactivation in human liver cytosol in a concentration and GSTZ1 haplotype-dependent way. In the absence of chloride, incubation with 0.5mM DCA resulted in inactivation of GSTZ1 with a half-life of 0.4h (samples with the KRT haplotype) to 0.5h (EGT haplotype). At the hepatic physiological chloride concentration, 38mM, samples with the EGT haplotype retained more activity (80%) following a 2-h incubation with 0.5mM DCA than those possessing the KRT haplotype (55%). The chloride concentration that protected 50% of the GSTZ1 activity following 2-h incubation with 0.5mM DCA (EC50) was 15.0±3.1mM (mean±S.D., n=3) for EGT samples and 36.2±2.2mM for KRT samples. Bromide, iodide and sulfite also protected GSTZ1 from inactivation by DCA, however fluoride, sulfate, carbonate, acetate, cyanide did not. Protection by bromide varied by GSTZ1 haplotype: EC50 was 1.3±0.3mM for the EGT haplotype and 5.0±0.60mM for the KRT haplotype. The EC50 values for iodide and sulfite in liver cytosol samples with EGT haplotype were respectively 0.14±0.06mM and 9.6±1.1mM (mean±S.D., n=3). Because the in vivo half-life of DCA is determined by the fraction of active GSTZ1 in the liver, identifying factors that regulate GSTZ1 activity is important in determining appropriate DCA dosing in humans.

Preliminary X-ray crystallographic analysis of glutathione transferase zeta 1 (GSTZ1a-1a)

Glutathione transferase zeta 1 (GSTZ1-1) is a homodimeric enzyme found in the cytosol and mitochondrial matrix of the liver and other tissues. It catalyzes the glutathione-dependent isomerization of maleylacetoacetate to fumarylacetoacetate in the tyrosine catabolic pathway and can metabolize small halogenated carboxylic acids. GSTZ1a-1a crystals diffracted to a resolution of 3.1 Å and belonged to space group P1, with unit-cell parameters a = 42.0, b = 49.6, c = 54.6 Å, α = 82.9, β = 69.9, γ = 73.4°, with a calculated Matthews coefficient of 2.1 Å(3) Da(-1) assuming a dimer in the crystallographic asymmetric unit. Refinement of the structure is currently in progress.

Nephroprotective effect of date fruit extract against dichloroacetic acid exposure in adult rats

The aim of this study was to investigate the protective effects of aqueous date extract (ADE) on dichloroacetic acid (DCA)-induced nephrotoxicity. In vitro, total phenolic content estimated in the ADE were 417.71mg gallic acid equivalents/100g fresh weights (FW), while total flavonoid and tannins contents were 285.23 and 73.65mg catechin equivalents/100g FW, respectively. The ADE has strong scavenging activity. Ferulic, caffeic and p-coumaric acids are the major's compounds. Nephrotoxicity was induced in male Wistar rats by the administration of 0.5 and 2g/L DCA as drinking water. Some of these rats received also by gavage ADE (4mL/kg) before the administration of DCA. After two months of experiment, DCA administration caused elevated levels of renal MDA, significant depletion of GSH levels, altered the antioxidant enzyme activities and deteriorated the renal functions as assessed by the increased plasma urea, uric acid and creatinine levels compared to control rats. The treatment with the ADE significantly normalized the increased plasma levels of creatinine, urea and uric acid, reduced the elevated MDA levels, significantly normalized the antioxidant enzyme activities and GSH level and restored the altered kidney histology in rats treated with DCA. Therefore, it was speculated that ADE protects rats from kidney damage through its antioxidant capacity.

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.

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.

Cold sensitivity in rice (Oryza sativa L.) is strongly correlated with a naturally occurring I99V mutation in the multifunctional glutathione transferase isoenzyme GSTZ2

GSTZs [Zeta class GSTs (glutathione transferases)] are multifunctional enzymes that belong to a highly conserved subfamily of soluble GSTs found in species ranging from fungi and plants to animals. GSTZs are known to function as MAAIs [MAA (maleylacetoacetate) isomerases], which play a role in tyrosine catabolism by catalysing the isomerization of MAA to FAA (fumarylacetoacetate). As tyrosine metabolism in plants differs from animals, the significance of GSTZ/MAAI is unclear. In rice (Oryza sativa L.), a major QTL (quantitative trait locus) for seedling cold tolerance has been fine mapped to a region containing the genes OsGSTZ1 and OsGSTZ2. Sequencing of tolerant (ssp. japonica cv. M-202) and sensitive (ssp. indica cv. IR50) cultivars revealed two SNPs (single nucleotide polymorphisms) in OsGSTZ2 that result in amino acid differences (I99V and N184I). Recombinant OsGSTZ2 containing the Val99 residue found in IR50 had significantly reduced activity on MAA and DCA (dichloroacetic acid), but the Ile184 residue had no effect. The distribution of the SNP (c.295A>G) among various rice accessions indicates a significant association with chilling sensitivity in rice seedlings. The results of the present study show that naturally occurring OsGSTZ2 isoforms differ in their enzymatic properties, which may contribute to the differential response to chilling stress generally exhibited by the two major rice subspecies.

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.

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.

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.

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.

Evaluating glutathione S-transferase (GST) null genotypes (GSTT1 and GSTM1) as a potential biomarker of predisposition for developing leukopenia

Glutathione S-transferase (GST) enzymes protect cells against xenobiotics and oxidative stress products through an electrophilic conjugation process. We investigated the theta (GSTT1) and mu (GSTM1) null genotypes in a group of leukopenic subjects and normal subjects from Northeast Brazil, evaluating their use as biomarkers of susceptibility for developing leukopenia. In a sample-based case-control study, we analysed white blood cell (WBC) counts and GSTT1 and GSTM1 genotypes. A total of 278 subjects were analysed: 91 with leukopenia and 187 controls. GSTT1 null genotype conferred a 5.92-fold risk for occurrence of leukopenia [odds ratios (OR) = 5.92, CI(MLE): 1.64-26.72, P(MLE) = 0.002] and a 3.90-fold risk of neutropenia (OR = 3.90; CI(MLE): 1.05-13.66; P(MLE) = 0.02), while GSTM1 null genotype conferred a 1.78-fold risk for leukopenia (OR = 1.75; CI(MLE): 1.04-3.06, P(MLE) = 0.017) and no risk of neutropenia (OR = 1.71; CI(MLE): 0.88-3.35; P(MLE) = 0.06). The GSTT1, but not the GSTM1 null genotype, was found to be associated with leukopenia and neutropenia. More cellular and molecular studies are needed to evaluate the existence of genotype interactions, and to confirm the appropriateness of using the GSTT1 and/or GSTM1 null genotypes as biomarkers of susceptibility to white blood-cell deficiencies.

Applying Mode-of-Action and Pharmacokinetic Considerations in Contemporary Cancer Risk Assessments: An Example with Trichloroethylene

The guidelines for carcinogen risk assessment recently proposed by the U.S. Environmental Protection Agency (U.S. EPA) provide an increased opportunity for the consideration of pharmacokinetic and mechanistic data in the risk assessment process. However, the greater flexibility of the new guidelines can also make their actual implementation for a particular chemical highly problematic. To illuminate the process of performing a cancer risk assessment under the new guidelines, the rationale for a state-of-the-science risk assessment for trichloroethylene (TCE) is presented. For TCE, there is evidence of increased cell proliferation due to receptor interaction or cytotoxicity in every instance in which tumors are observed, and most tumors represent an increase in the incidence of a commonly observed, species-specific lesion. A physiologically based pharmacokinetic (PBPK) model was applied to estimate target tissue doses for the three principal animal tumors associated with TCE exposure: liver, lung, and kidney. The lowest points of departure (lower bound estimates of the exposure associated with 10% tumor incidence) for lifetime human exposure to TCE were obtained for mouse liver tumors, assuming a mode of action primarily involving the mitogenicity of the metabolite trichloroacetic acid (TCA). The associated linear unit risk estimates for mouse liver tumors are 1.5 x 10(-6) for lifetime exposure to 1 microg TCE per cubic meter in air and 0.4 x 10(-6) for lifetime exposure to 1 microg TCE per liter in drinking water. However, these risk estimates ignore the evidence that the human is likely to be much less responsive than the mouse to the carcinogenic effects of TCA in the liver and that the carcinogenic effects of TCE are unlikely to occur at low environmental exposures. Based on consideration of the most plausible carcinogenic modes of action of TCE, a margin-of-exposure (MOE) approach would appear to be more appropriate. Applying an MOE of 1000, environmental exposures below 66 microg TCE per cubic meter in air and 265 microg TCE per liter in drinking water are considered unlikely to present a carcinogenic hazard to human health.

Structure of bacterial glutathione-S-transferase maleyl pyruvate isomerase and implications for mechanism of isomerisation

Maleyl pyruvate isomerase (MPI) is a bacterial glutathione S-transferase (GST) from the pathway for degradation of naphthalene via gentisate that enables the bacterium Ralstonia to use polyaromatic hydrocarbons as a sole carbon source. Genome sequencing projects have revealed the presence of large numbers of GSTs in bacterial genomes, often located within gene clusters encoding the degradation of different aromatic compounds. This structure is therefore an example of this under-represented class of enzymes. Unlike many glutathione transferases, the reaction catalysed by MPI is an isomerisation of an aromatic ring breakdown product, and glutathione is a true cofactor rather than a substrate in the reaction. We have solved the structure of the enzyme in complex with dicarboxyethyl glutathione, an analogue of a proposed reaction intermediate, at a resolution of 1.3 A. The structure provides direct evidence that the glutathione thiolate attacks the substrate in the C2 position, with the terminal carboxylate buried at the base of the active site cleft. Our structures suggest that the C1-C2 bond remains fixed so when rotation occurs around the C2-C3 bond the atoms from C4 onwards actually move. We identified a conserved arginine that is likely to stabilize the enolate form of the substrate during the isomerisation. Arginines at either side of the active site cleft can interact with the end of the substrate/product and preferentially stabilise the product. MPI has significant sequence similarity to maleylacetoacetate isomerase (MAAI), which performs an analogous reaction in the catabolism of phenylalanine and tyrosine. The proposed mechanism therefore has relevance to the MAAIs. Significantly, whilst the overall sequence identity is 40% only one of the five residues from the Zeta motif in the active site is conserved. We re-examined the roles of the residues in the active site of both enzymes and the Zeta motif itself.

Quantitative evaluation of dichloroacetic acid kinetics in human--a physiologically based pharmacokinetic modeling investigation

Dichloroacetic acid is a common disinfection by-product in surface waters and is a probable minor metabolite of trichloroethylene. Dichloroacetic acid (DCA) liver carcinogenicity has been demonstrated in rodents but epidemiological evidence in humans is not available. High doses of DCA ( approximately 50mg/kg) are used clinically to treat metabolic acidosis. Biotransformation of DCA by glutathione transferase zeta (GSTzeta) in the liver is the major elimination pathway in humans. GSTzeta is also inactivated by DCA, leading to slower systemic clearance and nonlinear pharmacokinetics after multiple doses. A physiologically based pharmacokinetic (PBPK) model was developed to quantitatively describe DCA biotransformation and kinetics in humans administered DCA by intravenous infusion and oral ingestion. GSTzeta metabolism was described using a Michaelis-Menten equation coupled with rate constants to account for normal GSTzeta synthesis, degradation and irreversible covalent binding and inhibition by the glutathione-bound-DCA intermediate. With some departures between observation and model prediction, the human DCA PBPK model adequately predicted the DCA plasma kinetics over a 20,000-fold range in administered doses. Apparent inhibition of GSTzeta mediated metabolism of DCA was minimal for low doses of DCA (microg/kg day), but was significant for therapeutic doses of DCA. Plasma protein binding of DCA was assumed to be an important factor influencing the kinetics of low doses of DCA (microg/kg day). Polymorphisms of GSTzeta may help explain inter-individual variability in DCA plasma kinetics and warrants evaluation. In conclusion, using a previously published rodent DCA PBPK model (Keys, D.A., Schultz, I.R., Mahle, D.A., Fisher, J.W., 2004. A quantitative description of suicide inhibition of dichloroacetic acid in rats and mice. Toxicol. Sci. 82, 381-393) and this human DCA PBPK model, human equivalent doses (HEDs) were calculated for a 10% increase in mice hepatic liver cancer (2.1mg/kg day). The HEDs for the dosimetrics, area-under-the-concentration-curve (AUC) for total and free DCA in plasma, AUC of DCA in liver and amount of DCA metabolized per day were 0.02, 0.1, 0.1 and 1.0mg/kg day, respectively. Research on the mechanism of action of DCA and the relevance of mouse liver cancer is needed to better understand which dosimetric may be appropriate for extrapolation from animal studies to human.

The use of glutathione transferase-knockout mice as pharmacological and toxicological models

ADME/Tox studies are of increasing importance because of the necessity to eliminate poor drug candidates early in the development pipeline. The glutathione S-transferases (GSTs) are a family of phase II enzymes that have been shown to play a significant role in the disposition of a wide range of drugs and other xenobiotics. Several GST-knockout mice strains have been developed that can potentially be used in ADME/Tox studies. So far, mice have been generated with deficiencies of mGSTP1/2, mGSTA4-4, mGSTZ1-1, mGSTM1-1, mGSTO1-1 and mGSTS1-1, but studies of drug metabolism in these strains have been limited. As there are 21 recognised GST genes in mice there is potential for many more strains to be made. However, a review of the available data suggests that because of differences in the evolution of the GST gene family between rodents and humans, only some knockout strains can provide insights relevant to human drug metabolism. It is concluded that, of the strains generated so far, only those deficient in mGSTP1-1, mGSTA4-4, mGSTO1-1 and mGSTZ1-1 have direct human orthologues and can be considered as human models. In contrast, there may not be appropriate orthologues of some enzymes, such as hGSTM1-1, that are known to be of relevance in drug metabolism.

Quantitative oral dosing of water soluble and lipophilic contaminants in the Japanese medaka (Oryzias latipes)

Quantitative oral dosing in fish can be challenging, particularly with water soluble contaminants, which can leach into the aquarium water prior to ingestion. We applied a method of bioencapsulation using newly hatched brine shrimp (Artemia franciscana) nauplii to study the toxicokinetics of five chlorinated and brominated halogenated acetic acids (HAAs), which are drinking water disinfection by-products. These results are compared to those obtained in a previous study using a polybrominated diphenyl ether (PBDE-47), a highly lipophilic chemical. The HAAs and PBDE-47 were bioencapsulated using freshly hatched A. franciscana nauplii after incubation in concentrated solutions of the study chemicals for 18 h. Aliquots of the brine shrimp were quantitatively removed for chemical analysis and fed to individual fish that were able to consume 400-500 nauplii in less than 5 min. At select times after feeding, fish were euthanized and the HAA or PBDE-47 content determined. The absorption of HAAs was quantitatively similar to previous studies in rodents: rapid absorption with peak body levels occurring within 1-2 h, then rapidly declining with elimination half-life of 0.3-3 h depending on HAA. PBDE-47 was more slowly absorbed with peak levels occurring by 18 h and very slowly eliminated with an elimination half-life of 281 h.

Short-term exposures to dihaloacetic acids produce dysmorphogenesis in mouse conceptuses in vitro

The haloacetic acids (HAAs) are a family of xenobiotics found in tap water as a result of drinking water disinfection. Administration of HAAs to rats produces a variety of adverse effects, including developmental toxicity. The dysmorphogenic potencies of all nine bromo/chloro-acetic acids have been determined in rodent whole embryo culture using standard 26-h exposure. Since the half-lives of the HAAs in vivo are typically <8 h, the developmental effects of short-term exposures to dihaloacetates were evaluated. Gestation day 8 (3-6 somite pairs) CD-1 mouse conceptuses were exposed to 11,000 microM dichloroacetic acid (DCA), 300 microM dibromoacetic acid (DBA) or 300 microM bromochloroacetic acid (BCA) for culture periods of 1, 3, 6 or 26 h. Following 1, 3 or 6 h of exposure to HAAs, conceptuses were transferred to control medium to complete a 26-h culture period. The amounts of HAAs present in embryos after 1, 3 and 6h of exposure were determined. Increased incidences of dysmorphic embryos were produced by 6 or 26-h exposures to DCA; a 26-h exposure to DBA; or 3, 6 or 26-h exposures to BCA. The dysmorphology produced was dependent upon the length of exposure and chemical. The embryonic concentration of each HAA (104.5, 2.5 and 2.6 pmol/microg protein for DCA, DBA and BCA, respectively) was reached by 1h of exposure and did not change at the subsequent time points examined. The current studies demonstrate that BCA is more potent than DBA or DCA at disrupting embryogenesis since shorter exposures alter morphogenesis. Since the embryonic HAA concentrations were the same at the three time points measured, the time-dependence in dysmorphogenesis does not appear to be a simple function of increasing embryonic concentration of these chemicals. These studies demonstrate that for these dihaloacetic acids relatively high concentrations and long exposures are needed to alter rodent development in vitro.

(+/-)-2-Chloropropionic acid elevates reactive oxygen species formation in human neutrophil granulocytes

(+/-)-2-Chloropropionic acid (2-CPA) is a neurotoxic compound which kills cerebellar granule cells in vivo, and makes cerebellar granule cells in vitro produce reactive oxygen species (ROS). We have studied the effect of 2-CPA on ROS formation in human neutrophil granulocytes in vitro. We found an increased formation of ROS after 2-CPA exposure using three different methods; the fluorescent probe DCFH-DA and the chemiluminescent probes lucigenin and luminol. Four different inhibitors of ROS formation were tested on the cells in combination with 2-CPA to characterize the signalling pathways. The spin-trap s-PBN, the ERK1/2 inhibitor U0126 and the antioxidant Vitamin E inhibited the 2-CPA-induced ROS formation completely, while the mitochondrial transition permeability pore blocker cyclosporine A inhibited the ROS formation partly. We also found that 2-CPA induced an increased nitric oxide production in the cells by using the Griess reagent. The level of reduced glutathione, measured with the DTNB assay, was decreased after exposure to high concentrations of 2-CPA. Western blotting analysis showed that 2-CPA exposure led to an elevated phosphorylation of ERK MAP kinase. This phosphorylation was inhibited by U0126. Based on these experiments it seems like the mechanisms for 2-CPA induced toxicity involves ROS formation and is similar in neutrophil granulocytes as earlier shown in cerebellar granule cells. This also implies that 2-CPA may be immunotoxic.

Polymorphisms in the human glutathione transferase zeta promoter

OBJECTIVES

The zeta-class glutathione transferase GSTZ1-1 catalyses the glutathione-dependent isomerization of maleylacetoacetate to fumarylacetoacetate in the tyrosine catabolic pathway and the biotransformation of alpha-halo acids, including dichloroacetic acid (DCA). Genetic polymorphisms in the coding sequence of GSTZ1 result in significant changes in enzyme function, and deficiency of GSTZ1-1 in mice causes induction of a range of Phase-II enzymes. In this study, the potential for polymorphisms in regulatory sequences to alter gene transcription was investigated.

METHODS

A total of 10 single-nucleotide polymorphisms (SNP) were identified in African and Australian European subjects in a region extending 1.5-kb upstream of the GSTZ1 start of transcription. These SNPs formed at least 10 haplotypes and only two were shared between the two population samples. The effect of these SNPs on gene expression was evaluated by the transient expression of specific alleles fused to a luciferase reporter gene.

RESULTS

Of the 10 SNPs identified, only -1002 G>A and -289 C>T caused significant changes in promoter activity. The -1002 G>A SNP converts a v-Myb site to a S8 homeodomain (Prx2) site, and the -289 C>T SNP abolishes an Egr1 binding site.

CONCLUSION

These SNPs may alter GSTZ1 expression, which may alter the pharmacokinetics of DCA, which is used therapeutically for the treatment of lactic acidosis.

A GC–MS/MS method for the quantitative analysis of low levels of the tyrosine metabolites maleylacetone, succinylacetone, and the tyrosine metabolism inhibitor dichloroacetate in biological fluids and tissues

We developed a sensitive method to quantitate the tyrosine metabolites maleylacetone (MA) and succinylacetone (SA) and the tyrosine metabolism inhibitor dichloroacetate (DCA) in biological specimens. Accumulation of these metabolites may be responsible for the toxicity observed when exposed to DCA. Detection limits of previous methods are 200 ng/mL (1.2 pmol/microL) (MA) and 2.6 microg/mL (16.5 pmol/microL) (SA) but the metabolites are likely present in lower levels in biological specimens. To increase sensitivity, analytes were extracted from liver, urine, plasma and cultured nerve cells before and after dosing with DCA, derivatized to their pentafluorobenzyl esters, and analyzed via GC-MS/MS.

Is fluoroacetate-specific defluorinase a glutathione S-transferase?

Fluoroacetate-specific defluorinase (FSD) is a critical enzyme in the detoxication of fluoroacetate. This study investigated whether FSD can be classed as a glutathione S-transferase (GST) isoenzyme with a high specificity for fluoroacetate detoxication metabolism. The majority of FSD and GST activity, using 1-chloro-2,4-dinitrobenzene (CDNB) and 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) as GST substrates, in rat liver was cytosolic. GSTT1 specific substrate, EPNP caused a slight non-competitive inhibition of FSD activity. CDNB, a general substrate of GST isoenzyme, was a more potent non-competitive inhibitor of FSD activity. The fluoroacetate defluorination activity by GST isoenzymes was determined in this study. The results showed that the GSTZ1C had the highest fluoroacetate defluorination activity of the various GST isoenzymes studied, while GSTA2 had a limited activity toward fluoroacetate. The human GSTZ1C recombinant protein then was purified from a human GSTZ1C cDNA clone. Our experiments showed that GSTZ1C catalysed fluoroacetate defluorination. GSTZ1 shares many of the characteristics of FSD; however, it accounts only for 3% of the total cytosolic FSD activity. GSTZ1C based enzyme kinetic studies has low affinity for fluoroacetate. The evidence suggests that GSTZ1 may not be the major enzyme defluorinating fluoroacetate, but it does detoxify the fluoroacetate. To clarify the identity of enzymes responsible for fluoroacetate detoxication, further studies of the overall FSD activity are needed.

Effect of short-term drinking water exposure to dichloroacetate on its pharmacokinetics and oral bioavailability in human volunteers: a stable isotope study

Dichloroacetic acid (DCAA) is a by-product of drinking water disinfection, is a known rodent hepatocarcinogen, and is also used therapeutically to treat a variety of metabolic disorders in humans. We measured DCAA bioavailability in 16 human volunteers (eight men, eight women) after simultaneous administration of oral and iv DCAA doses. Volunteers consumed DCAA-free bottled water for 2 weeks to wash out background effects of DCAA. Subsequently, each subject consumed (12)C-DCAA (2 mg/kg) dissolved in 500 ml water over a period of 3 min. Five minutes after the start of the (12)C-DCAA consumption, (13)C-labeled DCAA (0.3 mg/kg) was administered iv over 20 s and plasma (12)C/(13)C-DCAA concentrations measured at predetermined time points over 4 h. Volunteers subsequently consumed for 14 consecutive days DCAA 0.02 microg/kg/day dissolved in 500 ml water to simulate a low-level chronic DCAA intake. Afterward, the (12)C/(13)C-DCAA administrations were repeated. Study end points were calculation of AUC(0-->infinity), apparent volume of distribution (V(ss)), total body clearance (Cl(b)), plasma elimination half-life (t((1/2),beta)), oral absorption rate (K(a)), and oral bioavailability. Oral bioavailability was estimated from dose-adjusted AUC ratios and by using a compartmental pharmacokinetic model after simultaneous fitting of oral and iv DCAA concentration-time profiles. DCAA bioavailability had large interindividual variation, ranging from 27 to 100%. In the absence of prior DCAA intake, there were no significant differences (p > 0.05) in any pharmacokinetic parameters between male and female volunteers, although there was a trend that women absorbed DCAA more rapidly (increased K(a)), and cleared DCAA more slowly (decreased Cl(b)), than men. Only women were affected by previous 14-day DCAA exposure, which increased the AUC(0-->infinity) for both oral and iv DCAA doses (p < 0.04 and p < 0.014, respectively) with a corresponding decrease in the Cl(b).

Activation of alkyl halides by glutathione transferases

Glutathione (GSH) transferases catalyze the conjugation of the tripeptide GSH with alkyl halides and related compounds. If a second leaving group is present, the substrate is at least a potential bis-electrophile and the initial conjugate may be susceptible to further attack by the sulfur atom. This process can yield potent electrophiles that modify DNA and are genotoxic. Much of the chemistry is understood in the context of the halide order and size of rings generated in reactive sulfonium ions. Similar chemistry has been demonstrated with the active site cysteine residue in the DNA repair protein O(6)-alkylguanine DNA-alkyltransferase.