Exploration of in vitro pro-drug activation and futile cycling by glutathione S-transferases: thiol ester hydrolysis and inhibitor maturation

Reversibility and Low Commitment to Forward Catalysis in the Conjugation of Lipid Alkenals by Glutathione Transferase A4-4

High concentrations of electrophilic lipid alkenals formed during oxidative stress are implicated in cytotoxicity and disease. However, low concentrations of alkenals are required to induce antioxidative stress responses. An established clearance pathway for lipid alkenals includes conjugation to glutathione (GSH) via Michael addition, which is catalyzed mainly by glutathione transferase isoform A4 (GSTA4-4). Based on the ability of GSTs to catalyze hydrolysis or retro-Michael addition of GSH conjugates, and the antioxidant function of low concentrations of lipid alkenals, we hypothesize that GSTA4-4 contributes a homeostatic role in lipid metabolism. Enzymatic kinetic parameters for retro-Michael addition with trans-2-Nonenal (NE) reveal the chemical competence of GSTA4-4 in this putative role. The forward GSTA4-4-catalyzed Michael addition occurs with the rapid exchange of the C2 proton of NE in D₂O as observed by NMR. The isotope exchange was completely dependent on the presence of GSH. The overall commitment to catalysis, or the ratio of first order k_cat,f for 'forward' Michael addition to the first order k_cat,ex for H/D exchange is remarkably low, approximately 3:1. This behavior is consistent with the possibility that GSTA4-4 is a regulatory enzyme that contributes to steady-state levels of lipid alkenals, rather than a strict 'one way' detoxication enzyme.

Mitochondrial toxicity of diclofenac and its metabolites via inhibition of oxidative phosphorylation (ATP synthesis) in rat liver mitochondria: Possible role in drug induced liver injury (DILI)

Diclofenac is a widely prescribed NSAID, which by itself and its reactive metabolites (Phase-I and Phase-II) may be involved in serious idiosyncratic hepatotoxicity. Mitochondrial injury is one of the mechanisms of drug induced liver injury (DILI). In the present work, an investigation of the inhibitory effects of diclofenac (Dic) and its phase I [4-hydroxy diclofenac (4'-OH-Dic) and 5-hydroxy diclofenac (5-OH-dic)] and Phase-II [diclofenac acyl glucuronide (DicGluA) and diclofenac glutathione thioester (DicSG)] metabolites, on ATP synthesis in rat liver mitochondria was carried out. A mechanism based inhibition of ATP synthesis is exerted by diclofenac and its metabolites. Phase-I metabolite (4'-OH-Dic) and Phase-II metabolites (DicGluA and DicSG) showed potent inhibition (2-5 fold) of ATP synthesis, where as 5-OH-Dic, one of the Phase-I metabolite, was a less potent inhibitor as compared to Dic. The calculated kinetic constants of mechanism based inhibition of ATP synthesis by Dic showed maximal rate of inactivation (Kinact) of 2.64 ± 0.15 min(-1) and half maximal rate of inactivation (KI) of 7.69 ± 2.48 μM with Kinact/KI ratio of 0.343 min(-1) μM(-1). Co-incubation of mitochondria with Dic and reduced GSH exhibited a protective effect on Dic mediated inhibition of ATP synthesis. Our data from this study strongly indicate that Dic as well as its metabolites could be involved in the hepato-toxic action through inhibition of ATP synthesis.

Characterization of the acyl-adenylate linked metabolite of mefenamic Acid

Mefenamic acid, (MFA), a carboxylic acid-containing nonsteroidal anti-inflammatory drug (NSAID), is metabolized into the chemically reactive conjugates MFA-1-O-acyl-glucuronide (MFA-1-O-G) and MFA-S-acyl-CoA (MFA-CoA), which are both implicated in the formation of MFA-S-acyl-glutathione (MFA-GSH) conjugates, protein-adduct formation, and thus the potential toxicity of the drug. However, current studies suggest that an additional acyl-linked metabolite may be implicated in the formation of MFA-GSH. In the present study, we investigated the ability of MFA to become bioactivated into the acyl-linked metabolite, mefenamyl-adenylate (MFA-AMP). In vitro incubations in rat hepatocytes with MFA (100 μM), followed by LC-MS/MS analyses of extracts, led to the detection of MFA-AMP. In these incubations, the initial rate of MFA-AMP formation was rapid, leveling off at a maximum concentration of 90.1 nM (20 s), while MFA-GSH formation increased linearly, reaching a concentration of 1.7 μM after 60 min of incubation. In comparison, MFA-CoA was undetectable in incubation extracts until the 4 min time point, achieving a concentration of 45.6 nM at the 60 min time point, and MFA-1-O-G formation was linear, attaining a concentration of 42.2 μM after 60 min of incubation. In vitro incubation in buffer with the model nucleophile glutathione (GSH) under physiological conditions showed MFA-AMP to be reactive toward GSH, but 11-fold less reactive than MFA-CoA, while MFA-1-O-G exhibited little reactivity. However, in the presence of glutathione-S-transferase (GST), MFA-AMP mediated formation of MFA-GSH increased 6-fold, while MFA-CoA mediated formation of MFA-GSH only increased 1.4-fold. Collectively, in addition to the MFA-1-O-G, these results demonstrate that mefenamic acid does become bioactivated by acyl-CoA synthetase enzyme(s) in vitro in rat hepatocytes into the reactive transacylating derivatives MFA-AMP and MFA-CoA, both of which contribute to the transacylation of GSH and may be involved in the formation of protein adducts and potentially elicit an idiosyncratic toxicity.

Development of pyrethroid-like fluorescent substrates for glutathione S-transferase

The availability of highly sensitive substrates is critical for the development of precise and rapid assays for detecting changes in glutathione S-transferase (GST) activity that are associated with GST-mediated metabolism of insecticides. In this study, six pyrethroid-like compounds were synthesized and characterized as substrates for insect and mammalian GSTs. All of the substrates were esters composed of the same alcohol moiety, 7-hydroxy-4-methylcoumarin, and acid moieties that structurally mimic some commonly used pyrethroid insecticides, including cypermethrin and cyhalothrin. CpGSTD1, a recombinant Delta class GST from the mosquito Culex pipiens pipiens, metabolized our pyrethroid-like substrates with both chemical and geometric preference (i.e., the cis-isomers were metabolized at 2- to 5-fold higher rates than the corresponding trans-isomers). A GST preparation from mouse liver also metabolized most of our pyrethroid-like substrates with both chemical and geometric preference but at 10- to 170-fold lower rates. CpGSTD1 and mouse GSTs metabolized 1-chloro-2,4-dinitrobenezene (CDNB), a general GST substrate, at more than 200-fold higher rates than our novel pyrethroid-like substrates. There was a 10-fold difference in the specificity constant (k(cat)/K(M) ratio) of CpGSTD1 for CDNB and those of CpGSTD1 for cis-DCVC and cis-TFMCVC, suggesting that cis-DCVC and cis-TFMCVC may be useful for the detection of GST-based metabolism of pyrethroids in mosquitoes.

Identifying and characterizing binding sites on the irreversible inhibition of human glutathione S-transferase P1-1 by S-thiocarbamoylation

Human glutathione S-transferase P1-1 (hGST P1-1) is involved in cell detoxification processes through the conjugation of its natural substrate, reduced glutathione (GSH), with xenobiotics. GSTs are known to be overexpressed in tumors, and naturally occurring isothiocyanates, such as benzyl isothiocyanate (BITC), are effective cancer chemopreventive compounds. To identify and characterize the potential inhibitory mechanisms of GST P1-1 induced by isothiocyanate conjugates, we studied the binding of GST P1-1 and some cysteine mutants to the BITC-SG conjugate as well as to the synthetic S-(N-benzylcarbamoylmethyl)glutathione conjugate (BC-SG). We report here the inactivation of GST P1-1 through the covalent modification of two Cys47 residues per dimer and one Cys101. The evidence has been compiled by isothermal titration calorimetry (ITC) and electrospray ionization mass spectrometry (ESI-MS). ITC experiments suggest that the BITC-SG conjugate generates adducts with Cys47 and Cys101 at physiological temperatures through a corresponding kinetic process, in which the BITC moiety is covalently bound to these enzyme cysteines through an S-thiocarbamoylation reaction. ESI-MS analysis of the BITC-SG incubated enzymes indicates that although the Cys47 in each subunit is covalently attached to the BITC ligand moiety, only one of the Cys101 residues in the dimer is so attached. A plausible mechanism is given for the emergence of inactivation through the kinetic processes with both cysteines. Likewise, our molecular docking simulations suggest that steric hindrance is the reason why only one Cys101 per dimer is covalently modified by BITC-SG. No covalent inactivation of GST P1-1 with the BC-SG inhibitor has been observed. The affinities and inhibitory potencies for both conjugates are high and very similar, but slightly lower for BC-SG. Thus, we conclude that the presence of the sulfur atom from the isothiocyanate moiety in BITC-SG is crucial for its irreversible inhibition of GST P1-1.

Dual negative precursor ion scan approach for rapid detection of glutathione conjugates using liquid chromatography/tandem mass spectrometry

Screening for conjugates formed by the tripeptide glutathione with new chemical entities is an essential step during the drug discovery process, as the formation of these conjugates serves as an indicator for the presence of reactive electrophilic intermediates. To increase the selectivity and throughput of this analysis, various mass spectral scan types have evolved over time. Historically, samples were analyzed under positive ionization conditions for the neutral loss of m/z 129 (loss of the pyroglutamic acid moiety from glutathione); however, more recently, negative precursor ion scanning for the loss of m/z 272 (deprotonated gamma-glutamyl-dehydroalanyl-glycine from glutathione) has emerged as a more selective tool. Further increasing the selectivity, we report on an extension of this methodology by incorporating a simultaneous dual negative precursor ion scan for two commonly observed ion fragments from glutathione conjugates, m/z 272 and 254 (the dehydrated form of m/z 272). This negative dual precursor ion scan methodology was first validated using substrates known to undergo metabolic bioactivation (diclofenac, carbamazepine, and 3-methyl indole) and has then been applied to the routine analysis of proprietary compounds undergoing active lead optimization. In comparison to alternative scan methodologies, the increased selectivity offered by this simultaneous dual precursor method results in a reduction in the generation of false positive results as well as reduced data interpretation time.

Interaction of the electrophilic ketoprofenyl-glucuronide and ketoprofenyl-coenzyme A conjugates with cytosolic glutathione S-transferases

Carboxylic acid-containing drugs are metabolized mainly through the formation of glucuronide and coenzyme A esters. These conjugates have been suspected to be responsible for the toxicity of several nonsteroidal anti-inflammatory drugs because of the reactivity of the electrophilic ester bond. In the present study we investigated the reactivity of ketoprofenyl-acylglucuronide (KPF-OG) and ketoprofenyl-acyl-coenzyme A (KPF-SCoA) toward cytosolic rat liver glutathione S-transferases (GST). We observed that KPF-SCoA, but not KPF-OG inhibited the conjugation of 1-chloro-2,4-dinitrobenzene and 4-nitroquinoline N-oxide catalyzed by both purified cytosolic rat liver GST and GST from FAO and H5-6 rat hepatoma cell lines. Photoaffinity labeling with KPF-SCoA suggested that the binding of this metabolite may overlap the binding site of 4-methylumbelliferone sulfate. Furthermore, high-performance liquid chromatography and mass spectrometry analysis showed that both hydrolysis and transacylation reactions were observed in the presence of GST and glutathione. The formation of ketoprofenyl-S-acyl-glutathione could be kinetically characterized (apparent K(m) = 196.0 +/- 70.6 microM). It is concluded that KPF-SCoA is both a GST inhibitor and a substrate of a GST-dependent transacylation reaction. The reactivity and inhibitory potency of thioester CoA derivatives toward GST may have potential implications on the reported in vivo toxicity of some carboxylic acid-containing drugs.

A promiscuous glutathione transferase transformed into a selective thiolester hydrolase

Human glutathione transferase A1-1 (hGST A1-1) can be reengineered by rational design into a catalyst for thiolester hydrolysis with a catalytic proficiency of 1.4 x 10(7) M(-1). The thiolester hydrolase, A216H that was obtained by the introduction of a single histidine residue at position 216 catalyzed the hydrolysis of a substrate termed GSB, a thiolester of glutathione and benzoic acid. Here we investigate the substrate requirements of this designed enzyme by screening a thiolester library. We found that only two thiolesters out of 18 were substrates for A216H. The A216H-catalyzed hydrolysis of GS-2 (thiolester of glutathione and naphthalenecarboxylic acid) exhibits a k(cat) of 0.0032 min(-1) and a KM of 41 microM. The previously reported catalysis of GSB has a k(cat) of 0.00078 min(-1) and KM of 5 microM. The k(cat) for A216H-catalyzed hydrolysis of GS-2 is thus 4.1 times higher than for GSB. The catalytic proficiency (k(cat)/KM)/k(uncat) for GS-2 is 3 x 10(6) M(-1). The promiscuous feature of the wt protein towards a range of different substrates has not been conserved in A216H but we have obtained a selective enzyme with high demands on the substrate.

The role of glutathione in cancer

Glutathione is an abundant natural tripeptide found within almost all cells. Glutathione is highly reactive and is often found conjugated to other molecules via its sulfhydryl moiety. It instils several vital roles within a cell including antioxidation, maintenance of the redox state, modulation of the immune response and detoxification of xenobiotics. With respect to cancer, glutathione metabolism is able to play both protective and pathogenic roles. It is crucial in the removal and detoxification of carcinogens, and alterations in this pathway, can have a profound effect on cell survival. However, by conferring resistance to a number of chemotherapeutic drugs, elevated levels of glutathione in tumour cells are able to protect such cells in bone marrow, breast, colon, larynx and lung cancers. Here we present a number of studies investigating the role of glutathione in promoting cancer, impeding chemotherapy, and the use of glutathione modulation to enhance anti-neoplastic therapy.

Incorporation of a single His residue by rational design enables thiol-ester hydrolysis by human glutathione transferase A1-1

A strategy for rational enzyme design is reported and illustrated by the engineering of a protein catalyst for thiol-ester hydrolysis. Five mutants of human glutathione (GSH; gamma-Glu-Cys-Gly) transferase A1-1 were designed in the search for a catalyst and to provide a set of proteins from which the reaction mechanism could be elucidated. The single mutant A216H catalyzed the hydrolysis of the S-benzoyl ester of GSH under turnover conditions with a k(cat)/K(M) of 156 M(-1) x min(-1), and a catalytic proficiency of >10(7) M(-1) when compared with the first-order rate constant of the uncatalyzed reaction. The wild-type enzyme did not hydrolyze the substrate, and thus, the introduction of a single histidine residue transformed the wild-type enzyme into a turnover system for thiol-ester hydrolysis. By kinetic analysis of single, double, and triple mutants, as well as from studies of reaction products, it was established that the enzyme A216H catalyzes the hydrolysis of the thiol-ester substrate by a mechanism that includes an acyl intermediate at the side chain of Y9. Kinetic measurements and the crystal structure of the A216H GSH complex provided compelling evidence that H216 acts as a general-base catalyst. The introduction of a single His residue into human GSH transferase A1-1 created an unprecedented enzymatic function, suggesting a strategy that may be of broad applicability in the design of new enzymes. The protein catalyst has the hallmarks of a native enzyme and is expected to catalyze various hydrolytic, as well as transesterification, reactions.