Glutathione transferase zeta-catalyzed biotransformation of deuterated dihaloacetic acids

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.

Synthetic routes for a variety of halogenated (chiral) acetic acids from diethyl malonate

We focus on a synthetic route to synthesise chiral halogenated acetic acids with F, Cl, Br, and H/D isotopic substitution at the α-C-atom starting from diethyl malonate.

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.

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.

Human Glutathione Transferase Zeta

Zeta-class glutathione transferases (GSTZs) were recently discovered by a bioinformatics approach and the availability of human expressed sequence tag databases. Although GSTZ showed little activity with conventional GST substrates (1-chloro-2,4-dinitrobenzene; organic hydroperoxides), GSTZ was found to catalyze the oxygenation of dichloroacetic acid (DCA) to glyoxylic acid and the cis-trans isomerization of maleylacetoacetate to fumarylacetoacetate. Hence, GSTZ plays a critical role in the tyrosine degradation pathway and in alpha-haloacid metabolism. The GSTZ-catalyzed biotransformation of DCA is of particular interest, because DCA is used in the human clinical management of congenital lactic acidosis and because DCA is a common drinking water contaminant. Substrate selectivity studies showed that GSTZ catalyzes the glutathione-dependent biotransformation of a range of dihaloacetic acids along with fluoroacetic acid, 2-halopropanoic acids, and 2,2-dichloropropanoic acid. Human clinical studies showed that the elimination half-life of DCA increases with repeated doses of DCA; also, rats given DCA show low GSTZ activity with DCA as the substrate. DCA was found to be a mechanism-based inactivator of GSTZ, and proteomic studies showed that Cys-16 of human GSTZ1-1 is covalently modified by a reactive intermediate that contains glutathione and the carbon skeleton of DCA. Bioinformatics studies also showed the presence of at least four polymorphic variants of human GSTZ; these variants differ considerably in the rates of catalysis and in their susceptibility to inactivation by DCA. Finally, Gstz1(-/-) mouse strains have been developed; these mice fail to biotransform DCA or maleylacetone. Although the mice have no obvious phenotype, a high incidence of lethality is observed in young mice given phenylalanine in their drinking water. Gstz1(-/-) mice should prove useful in expanding the role of GSTZ in alpha-haloacid metabolism and in the tyrosine degradation pathway.

Mass spectral characterization of dichloroacetic acid-modified human glutathione transferase zeta

Glutathione transferase zeta (GSTZ1-1) is widely expressed in eukaryotic species, and four human allelic variants of hGSTZ1-1 have been described. GSTZ1-1 catalyzes the cis-trans isomerization of maleylacetoacetate to fumarylacetoacetate and the biotransformation of a range of alpha-haloalkanoic acids. GSTZ1-1-catalyzed biotransformation of fluorine-lacking alpha,alpha-dihaloalkanoic acids, including dichloroacetic acid (DCA), results in the mechanism-based inactivation and covalent modification of the enzyme. The objective of this study was to investigate further the DCA-induced inactivation of hGSTZ1c-1c and to explore the mechanism of inactivation by characterization of the sites and types of DCA-induced covalent modifications. The partition ratio for the DCA-induced, mechanism-based inactivation of hGSTZ1c-1c was (5.7 +/- 0.5) x 10(2), and the k(cat) for the biotransformation of DCA was 39 min(-)(1). Inactivation of hGSTZ1c-1c in vitro was limited at high enzyme concentrations and was inhibited by glyoxylate. The stoichiometry of DCA binding to hGSTZ1c-1c was approximately 0.5 mol of DCA/mol of enzyme monomer. A single DCA-derived adduct was observed and was assigned to cysteine-16 by a combination of matrix-assisted laser-desorption-ionization time-of-flight and electrospray-ionization quadrupole ion-trap mass spectrometry and by analysis of [1-(14)C]DCA binding to C16A hGSTZ1c-1c. The DCA-derived adduct contained both glutathione and the carbon skeleton of DCA, presumably in a dithioacetal linkage. Also, cysteine-16 formed a mixed disulfide bond with glutathione. These data support a mechanism of inactivation whereby glutathione displaces a chlorine atom from DCA, and cysteine-16 in the enzyme active site displaces the second chlorine atom to result in a covalently modified and inactivated enzyme. These findings explain the DCA-induced inactivation of GSTZ1-1 observed in humans and rats.

Stereospecific toxicokinetics of bromochloro- and chlorofluoroacetate: effect of GST-zeta depletion

The chloro- and bromohaloacetates are drinking water disinfection by-products and rodent carcinogens. Chloro-bromo dihaloacetates are also mechanism-based inhibitors of glutathione S-transferase-zeta (GSTZ1-1). We studied the stereospecific toxicokinetics and in vitro metabolism of two chiral dihaloacetates in male F344 rats: (-),(+)-bromochloroacetate (BCA) and racemic chlorofluoroacetate (CFA), a non-GST-zeta-inhibiting dihaloacetate. These experiments were repeated in animals that had previously been treated with dichloroacetate (DCA) to deplete GST-zeta activity. Results indicated that the elimination half-life of (-)-BCA was 0.07 compared to 0.40 h for (+)-BCA in naive rats. A comparable difference in elimination half-life was also observed for the CFA stereoisomers (0.79 vs 0.11 h). In GST-zeta-depleted rats, stereospecific elimination of (-),(+)-BCA was absent, with both stereoisomers having an elimination half-life of approximately 0.4 h. This finding was in contrast to results for CFA, which still maintained the same relative difference in elimination rate between its stereoisomers, although overall elimination was diminished in GST-zeta-depleted rats. Results of in vitro metabolism experiments indicated (-)-BCA was affected by modulating GST-zeta activity, with the intrinsic metabolic clearance decreasing from 2.81 to 0.15 ml h(-1) mg.protein(-1) (naive, GST-zeta depleted) compared with values for (+)-BCA (0.30 and 0.31 ml h(-1) mg.protein(-1)). Incubations with 350 microM diethyldithiocarbamate preferentially decreased (+)-BCA metabolism in naive and GST-zeta-depleted cytosol. These results indicate (+)-BCA is a poor substrate for GST-zeta and its metabolism is controlled by an additional GST isoenzyme.

Polymorphism- and species-dependent inactivation of glutathione transferase zeta by dichloroacetate

Glutathione transferase zeta catalyzes the glutathione-dependent oxidation or conjugation of a range of alpha-haloacids. Repeated administration of dichloroacetate to human subjects increases its plasma elimination half-life, and the activity of glutathione transferase zeta is decreased in rats given dichloroacetate. The objective of the studies presented here was to investigate the kinetics and mechanism of the dichloroacetate-induced decrease in glutathione transferase zeta activity. The rate constants (k(inact)) for the dichloroacetate-dependent inactivation of glutathione transferase zeta in liver cytosol are in the following order: rat > mouse > human; the half-maximal inhibitory concentration (K(inact)) of DCA did not differ among the species that were studied. In contrast to dichloroacetate, chlorofluoroacetate produced much less inactivation of mouse liver glutathione transferase zeta activity. Moreover, the addition of N-acetyl-L-cysteine or potassium cyanide did not fully block the dichloroacetate-induced inactivation of glutathione transferase zeta. The k(inact) values for the dichloroacetate-induced inactivation of four polymorphic variants of recombinant human glutathione transferase zeta (hGSTZ1-1) were in the following order: variant 1a-1a < 1b-1b approximately 1c-1c approximately 1d-1d. The dichloroacetate-induced inactivation of hGSTZ1-1 was irreversible. The binding of radioactivity from [1-(14)C]dichloroacetate and from [(35)S]glutathione to recombinant hGSTZ1c-1c was demonstrated, indicating covalent modification of the protein. These results show that dichloroacetate is a mechanism-based inactivator of glutathione transferase zeta and is biotransformed to electrophilic metabolites that covalently modify and, thereby, inactivate the enzyme.

Inactivation of glutathione transferase zeta by dichloroacetic acid and other fluorine-lacking alpha-haloalkanoic acids

Dichloroacetic acid (DCA) is a contaminant of chlorinated drinking water supplies, is carcinogenic in rats and mice, and is a therapeutic agent used for the treatment of congenital lactic acidosis. The biotransformation of DCA to glyoxylic acid is catalyzed by glutathione transferase zeta (GSTZ). Treatment of rats and human subjects with DCA increases its plasma elimination half-life and reduces the extent of DCA biotransformation in rat hepatic cytosol. In the investigation presented here, the kinetics of the DCA-induced inactivation of GSTZ, the turnover of GSTZ, and the susceptibility of GSTZ to inactivation by a panel of alpha-haloacids were studied. DCA rapidly inactivated GSTZ in both rat hepatic cytosol and intact Fischer 344 rats. The time course of inactivation in vivo was mirrored by a concomitant loss of immunoreactive GSTZ protein. The turnover of GSTZ in rat liver was 0.21 day(-1), which corresponded to a half-life of 3.3 days. The degree of GSTZ inactivation after daily administration of DCA could be predicted from the amount of inactivation after a single treatment. Other fluorine-lacking dihaloacetic acids also inactivated GSTZ, whereas alpha-monohaloacids and fluorine-containing dihaloacetic acids failed to inactivate GSTZ. These data show that the observed DCA-induced decrease in the level of DCA metabolism is caused by the inactivation of GSTZ.