Active-site tyrosyl residues are targets in the irreversible inhibition of a class Mu glutathione transferase by 2-(S-glutathionyl)-3,5,6-trichloro-1,4-benzoquinone

Activity-Based Probes for Isoenzyme- and Site-Specific Functional Characterization of Glutathione S-Transferases

Glutathione S-transferases (GSTs) comprise a diverse family of phase II drug metabolizing enzymes whose shared function is the conjugation of reduced glutathione (GSH) to endo- and xenobiotics. Although the conglomerate activity of these enzymes can be measured, the isoform-specific contribution to the metabolism of xenobiotics in complex biological samples has not been possible. We have developed two activity-based probes (ABPs) that characterize active GSTs in mammalian tissues. The GST active site is composed of a GSH binding "G site" and a substrate binding "H site". Therefore, we developed (1) a GSH-based photoaffinity probe (GSTABP-G) to target the "G site", and (2) an ABP designed to mimic a substrate molecule and have "H site" activity (GSTABP-H). The GSTABP-G features a photoreactive moiety for UV-induced covalent binding to GSTs and GSH-binding enzymes. The GSTABP-H is a derivative of a known mechanism-based GST inhibitor that binds within the active site and inhibits GST activity. Validation of probe targets and "G" and "H" site specificity was carried out using a series of competition experiments in the liver. Herein, we present robust tools for the characterization of enzyme- and active site-specific GST activity in mammalian model systems.

Quinone-induced inhibition of urease: Elucidation of its mechanisms by probing thiol groups of the enzyme

In this work we studied the reaction of four quinones, 1,4-benzoquinone (1,4-BQ), 2,5-dimethyl-1,4-benzoquinone (2,5-DM-1,4-BQ), tetrachloro-1,4-benzoquinone (TC-1,4-BQ) and 1,4-naphthoquinone (1,4-NQ) with jack bean urease in phosphate buffer, pH 7.8. The enzyme was allowed to react with different concentrations of the quinones during different incubation times in aerobic conditions. Upon incubation the samples had their residual activities assayed and their thiol content titrated. The titration carried out with use of 5,5'-di-thiobis(2-nitrobenzoic) acid was done to examine the involvement of urease thiol groups in the quinone-induced inhibition. The quinones under investigation showed two distinct patterns of behaviour, one by 1,4-BQ, 2,5-DM-1,4-BQ and TC-1,4-BQ, and the other by 1,4-NQ. The former consisted of a concentration-dependent inactivation of urease where the enzyme-inhibitor equilibrium was achieved in no longer than 10min, and of the residual activity of the enzyme being linearly correlated with the number of modified thiols in urease. We concluded that arylation of the thiols in urease by these quinones resulting in conformational changes in the enzyme molecule is responsible for the inhibition. The other pattern of behaviour observed for 1,4-NQ consisted of time- and concentration-dependent inactivation of urease with a nonlinear residual activity-modified thiols dependence. This suggests that in 1,4-NQ inhibition, in addition to the arylation of thiols, operative are other reactions, most likely oxidations of thiols provoked by 1,4-NQ-catalyzed redox cycling. In terms of the inhibitory strength, the quinones studied formed a series: 1,4-NQ approximately 2,5-DM-1,4-BQ<1,4-BQ<TC-1,4-BQ.

Delineation of xenobiotic substrate sites in rat glutathione S-transferase M1-1

Glutathione S-transferases catalyze the conjugation of glutathione with endogenous and exogenous xenobiotics. Hu and Colman (1995) proposed that there are two distinct substrate sites in rat GST M1-1, a 1-chloro-2,4-dintrobenzene (CDNB) substrate site located in the vicinity of tyrosine-115, and a monobromobimane (mBBr) substrate site. To determine whether the mBBr substrate site is distinguishable from the CDNB substrate site, we tested S-(hydroxyethyl)bimane, a nonreactive derivative of mBBr, for its ability to compete kinetically with the substrates. We find that S-(hydroxyethyl)bimane is a competitive inhibitor (K(I) = 0.36 microM) when mBBr is used as substrate, but not when CDNB is used as substrate, demonstrating that these two sites are distinct. Using site-directed mutagenesis, we have localized the mBBr substrate site to an area midway through alpha-helix 4 (residues 90-114) and have identified residues that are important in the enzymatic reaction. Substitution of alanine at positions along alpha-helix 4 reveals that mutations at positions 103, 104, and 109 exhibit a greater perturbation of the enzymatic reaction with mBBr than with CDNB as substrate. Various other substitutions at positions 103 and 104 reveal that a hydrophobic residue is necessary at each of these positions to maintain optimal affinity of the enzyme for mBBr and preserve the secondary structure of the enzyme. Substitutions at position 109 indicate that this residue is important in the enzyme's affinity for mBBr but has a minimal effect on Vmax. These results demonstrate that the promiscuity of rat GST M1-1 is in part due to at least two distinct substrate sites.

Inhibition of human glutathione S-transferase P1-1 by the flavonoid quercetin

In the present study, the inhibition of human glutathione S-transferase P1-1 (GSTP1-1) by the flavonoid quercetin has been investigated. The results show a time- and concentration-dependent inhibition of GSTP1-1 by quercetin. GSTP1-1 activity is completely inhibited upon 1 h incubation with 100 microM quercetin or 2 h incubation with 25 microM quercetin, whereas 1 and 10 microM quercetin inhibit GSTP1-1 activity to a significant extent reaching a maximum of 25 and 42% inhibition respectively after 2 h. Co-incubation with tyrosinase greatly enhances the rate of inactivation, whereas co-incubation with ascorbic acid or glutathione prevents this inhibition. Addition of glutathione upon complete inactivation of GSTP1-1 partially restores the activity. Inhibition studies with the GSTP1-1 mutants C47S, C101S and the double mutant C47S/C101S showed that cysteine 47 is the key residue in the interaction between quercetin and GSTP1-1. HPLC and LC-MS analysis of trypsin digested GSTP1-1 inhibited by quercetin did not show formation of a covalent bond between Cys 47 residue of the peptide fragment 45-54 and quercetin. It was demonstrated that the inability to detect the covalent quercetin-peptide adduct using LC-MS is due to the reversible nature of the adduct-formation in combination with rapid and preferential dimerization of the peptide fragment once liberated from the protein. Nevertheless, the results of the present study indicate that quinone-type oxidation products of quercetin likely act as specific active site inhibitors of GSTP1-1 by binding to cysteine 47.

Covalent binding of acrylonitrile to specific rat liver glutathione S-transferases in vivo

Acrylonitrile (AN) is an industrial vinyl monomer that is acutely toxic. When administered to rats, AN covalently binds to tissue proteins in a dose-dependent but nonlinear manner [Benz, F. W., Nerland, D. E., Li, J., and Corbett, D. (1997) Fundam. Appl. Toxicol. 36, 149-156]. The nonlinearity in covalent binding stems from the fact that AN rapidly depletes liver glutathione after which the covalent binding to tissue proteins increases disproportionately. The identity of the tissue proteins to which AN covalently binds is unknown. The experiments described here were conducted to begin to answer this question. Male Sprague-Dawley rats were injected subcutaneously with 115 mg/kg (2.2 mmol/kg) [2,3-(14)C]AN. Two hours later, the livers were removed, homogenized, and fractionated into subcellular components, and the radioactively labeled proteins were separated on SDS-PAGE. One set of labeled proteins was found to be glutathione S-transferase (GST). Specific labeling of the mu over the alpha class was observed. Separation of the GST subunits by HPLC followed by scintillation counting showed that AN was selective for subunit rGSTM1. Mass spectral analysis of tryptic digests of the GST subunits indicated that the site of labeling was cysteine 86. The reason for the high reactivity of cysteine 86 in rGSTM1 was hypothesized to be due to its potential interaction with histidine 84, which is unique in this subunit.

Glutathione conjugation as a bioactivation reaction

In general, glutathione conjugation is regarded as a detoxication reaction. However, depending on the properties of the substrate, bioactivation is also possible. Four types of activation reaction have been recognized: direct-acting compounds, conjugates that are activated through cysteine conjugate beta-lyase, conjugates that are activated through redox cycling and lastly conjugates that release the original reactive parent compound. The glutathione S-transferases have three connections with the formation of biactivated conjugates: they catalyze their formation in a number of cases, they are the earliest available target for covalent binding by these conjugates and lastly, the parent alkylating agents are regularly involved in the induction of the enzymes. Individual susceptibility for each of these agents is determined by individual transferase subunit composition and methods are becoming available to assess this susceptibility.

The role of biotransformation in dietary (anti)carcinogenesis

The fact that dietary compounds influence the susceptibility of human beings to cancer, is widely accepted. One of the possible mechanisms that is responsible for these (anti)carcinogenic effects is that dietary constituents may modulate biotransformation enzymes, thereby affecting the (anti)carcinogenic potential of other compounds. This ambiguous theme is the basis for the present paper. The possible effects of enzymatic bioactivation and detoxification of dietary constituents are discussed using two representative examples of phase I and phase II biotransformation enzymes i.e., cytochrome P450 and glutathione S-transferase. Furthermore, the impact of genetic polymorphisms of these two enzyme systems is considered. Although it is very difficult on the basis of the enzyme inducing or inhibiting properties of dietary compounds, especially to characterize them as anticarcinogenic, for certain constituents it is acknowledged that they have anticarcinogenic properties. As such, this provides for an important mechanistic substantiation of the established cancer chemopreventive effect of a diet rich in fruits and vegetables.

Affinity labeling of pig lung glutathione S-transferase pi by 4-(fluorosulfonyl)benzoic acid

The compound 4-(fluorosulfonyl)benzoic acid (4-FSB) functions as an affinity label of the dimeric pig lung pi class glutathione S-transferase yielding a completely inactive enzyme. Protection against inactivation is provided by glutathione-based ligands, suggesting that the reaction target is near or part of the glutathione binding site. Radioactive 4-FSB is incorporated to the extent of 1 mol per mole of enzyme subunit. Peptide mapping revealed that 4-FSB reacts with two tyrosine residues in the ratio 69% Tyr7 and 31% Tyr106. The ratio is not changed by the addition of ligands. The results suggest that only one of the tyrosine residues can be labeled in the active site of a given subunit; i.e., reactions with Tyr7 and Tyr106 are mutually exclusive. We propose that the difference in labeling of these tyrosine residues is related to their pKa values, with Tyr7 exhibiting the lower pKa. The modified enzyme no longer binds to a S-hexylglutathione-agarose affinity column, even when only one of the active sites contains 4-FSB; these results may reflect interaction between the subunits. We conclude that Tyr7 and Tyr106 of the pig lung class pi glutathione S-transferase are important for function and are located at or close to the substrate binding site of the enzyme.

GSTP1-1 stereospecifically catalyzes glutathione conjugation of ethacrynic acid

Using 1H NMR two diastereoisomers of the ethacrynic acid glutathione conjugate (EASG) as well as ethacrynic acid (EA) could be distinguished and quantified individually. Chemically prepared EASG consists of equal amounts of both diastereoisomers. GSTP1-1 stereospecifically catalyzes formation of one of the diastereoisomers (A). The GSTP1-1 mutant C47S and GSTA1-1 preferentially form the same diastereoisomer of EASG as GSTP1-1. Glutathione conjugation of EA by GSTA1-2 and GSTA2-2 is not stereoselective. When human melanoma cells, expressing GSTP1-1, were exposed to ethacrynic acid, diastereoisomer A was the principal conjugate formed, indicating that even at physiological pH the enzyme catalyzed reaction dominates over the chemical conjugation.

Modulation of glutathione conjugation in vivo: how to decrease glutathione conjugation in vivo or in intact cellular systems in vitro

Glutathione conjugation is involved in detoxification and toxification of a variety of electrophilic substrates. Thus it plays a major role in protection against reactive intermediates. At the same time this conjugation may cause resistance of tumor cells against certain cytostatics. In this review the methods available to decrease glutathione conjugation in vivo are discussed. So far the only in vivo active inhibitors of glutathione S-transferases are ethacrynic acid and a number of glutathione-derived structures; the latter seem very promising for further development. For (chronic) glutathione-depletion, buthionine sulfoximine is most effective, and surprisingly safe in clinical studies. Diethylmaleate can be used for acute depletion. Inhibition of glutathione transferases offers advantages over glutathione depletion as a method of decreasing glutathione conjugation since inhibition may be accomplished without changing the activities of other glutathione-dependent reactions in the cell. However, clinically safe, in vivo effective and isoenzyme-selective glutathione S-transferase inhibitors have not yet been developed.

Probing the active site of alpha-class rat liver glutathione S-transferases using affinity labeling by monobromobimane

Monobromobimane (mBBr) is a substrate of both mu- and alpha-class rat liver glutathione S-transferases, with Km values of 0.63 microM and 4.9 microM for the mu-class isozymes 3-3 and 4-4, respectively, and 26 microM for the alpha-class isozymes 1-1 and 2-2. In the absence of substrate glutathione, mBBr acts as an affinity label of the 1-1 as well as mu-class isozymes, but not of the alpha-class 2-2 isozyme. Incubation of rat liver isozyme 1-1 with mBBr at pH 7.5 and 25 degrees C results in a time-dependent inactivation of the enzyme but at a slower (threefold) rate than for reactions with the mu-class isozyme 3-3 and 4-4. The rate of inactivation of 1-1 isozyme by mBBr is not decreased but, rather, is slightly enhanced by S-methyl glutathione. In contrast, 17 beta-estradiol-3,17-disulfate (500 microM) gives a 12.5-fold decrease in the observed rate constant of inactivation by 4 mM mBBr. When incubated for 60 min with 4 mM mBBr, the 1-1 isozyme loses 60% of its activity and incorporates 1.7 mol reagent/mol subunit. Peptide analysis after thermolysin digestion indicates that mBBr modification is equally distributed between two cysteine residues at positions 17 and 111. Modification at these two sites is reduced equally in the presence of the added protectant, 17 beta-estradiol-3,17-disulfate, suggesting that Cys 17 and Cys 111 reside within or near the enzyme's steroid binding sites. In contrast to the 1-1 isozyme, the other alpha-class isozyme (2-2) is not inactivated by mBBr at concentrations as high as 15 mM. The different reaction kinetics and modification sites by mBBr suggest that distinct binding site structures are responsible for the characteristic substrate specificities of glutathione S-transferase isozymes.

Haloenol lactone is a new isozyme-selective and active site-directed inactivator of glutathione S-transferase

A haloenol lactone derivative has been synthesized and found to be an isozyme-selective and active site-directed inactivator of glutathione S-transferase (GST). Preincubation of the haloenol lactone (100 microM) with murine Alpha, Mu, or Pi GST isozyme (1.0 microM) at pH 6.5, 37 degrees C resulted in time-dependent loss of enzyme activity with highly selective inhibition of the Pi isozyme (t1/2, approximately 2 min). In a separate experiment, a 10-fold excess of the lactone was incubated with GST-Pi isozyme at 37 degrees C for 3 h, followed by dialysis against Nanopure water. GST activity lost upon incubation with the lactone could not be restored by exhaustive dialysis, and only 8% of enzyme activity for the modified GST remained relative to the control that was treated identically except the lactone was omitted from the incubation. Both control and modified GST were characterized using electrospray ionization mass spectrometry. No native GST (23,478 Da) was observed in the spectrum of modified GST. Instead, protein incubated with the lactone exhibited an increase in molecular mass of 230 Da relative to control GST. The lactone (100 microM) was incubated with GST Pi isozyme (1.0 microM) in the presence of the competitive inhibitor S-hexylglutathione (10 microM), which suppressed time-dependent inhibition of GST by the lactone. The results suggest that this haloenol lactone is an irreversible and active site-directed inhibitor of GST that appears to inhibit the enzyme through two consecutive steps of nucleophilic attack.

Enzyme kinetics and substrate selectivities of rat glutathione S-transferase isoenzymes towards a series of new 2-substituted 1-chloro-4-nitrobenzenes

1. Four different rat glutathione S-transferase (GST) isoenzymes, belonging to three different classes, were examined for their GSH conjugating capacity towards 11 2-substituted 1-chloro-4-nitrobenzene derivatives. Significant differences were found in their enzyme kinetic parameters Km, kcat and kcat/Km. 2. Substrates with bulky substituents on the ortho-position appeared to have high affinities (low Km's) for the active site of the GST-isoenzymes, suggesting that there is sufficient space in this area of the active site. A remarkably high Km (low affinity) was found for 2-chloro-5-nitropyridine towards all GST-isoenzymes examined. 3. GST 3-3 catalysed the reaction between GSH and the substrates most efficiently (high kcat) compared with the other GST-isoenzymes. Moreover, GST 3-3 showed clear substrate selectivities towards the substrates with a trifluoromethyl-, chlorine- and bromine-substituent. 1-Chloro-2,4-dinitrobenzene and 2-chloro-5-nitrobenzonitrile were most efficiently conjugated by all four GST-isoenzymes examined. 4. When the rate of the conjugation reactions was followed, a linear increase of formation of GS-conjugate could be seen for 2-chloro-5-nitrobenzonitrile during a much longer period of time than for 1-chloro-2,4-dinitrobenzene with all GST-isoenzymes examined. Therefore, it is suggested that 2-chloro-5-nitrobenzonitrile might be recommended as an alternative model substrate in GST-research.

Monobromobimane as an affinity label of the xenobiotic binding site of rat glutathione S-transferase 3-3

Monobromobimane (mBBr), besides being a substrate in the presence of glutathione, inactivates rat liver glutathione S-transferase 3-3 at pH 7.5 and 25 degrees C as assayed using 1-chloro-2,4-dinitrobenzene (CDNB). The rate of inactivation is enhanced about 5-fold by S-methylglutathione. Substrate analogs bromosulfophthalein and 2,4-dinitrophenol decrease the rate of inactivation at least 20-fold. Upon incubation for 60 min with 0.25 mM mBBr and S-methylglutathione, the enzyme loses 91% of its activity toward CDNB and incorporates 2.14 mol of reagent/mol of subunit, whereas incubation under the same conditions but with added protectant 2,4-dinitrophenol yields an enzyme that is catalytically active and contains only 0.89 mol of reagent/mol of subunit. mBBR-modified enzyme is fluorescent, and fluorescence energy transfer occurs between intrinsic tryptophan and covalently bound bimane in modified enzyme. Both Tyr115 and Cys114 are modified, but Tyr115 is the initial reaction target and its modification correlates with loss of activity toward CDNB. The fact that the activity toward mBBr is retained by the enzyme after modification suggests that rat isozyme 3-3 has two binding sites for mBBr.