Modular mutagenesis of exons 1, 2, and 8 of a glutathione S-transferase from the mu class. Mechanistic and structural consequences for chimeras of isoenzyme 3-3

GLUTATHIONE S-TRANSFERASE GENE POLYMORPHISM AND ISCHEMIC CEREBROVASCULAR DISEASE

Glutathione S-transferase polymorphisms (GST) were examined in 142 cases with ischemic cerebrovascular disease (ICVD) to explore whether the GST polymorphisms confer a risk to an individual to develop ICVD. Tobacco smoke is a major cause of both cancer and vascular disease. The subjects were therefore stratified with ICVD for smoking status, and then the authors examined whether polymorphisms in this detoxification enzyme gene, GST, influence risk of ICVD. The GST genotype was analyzed by the polymerase chain reaction. Neither GSTM1 nor GSTT1 genotypes in the ICVD group was significantly different from the control group (n=344), even in smokers. The authors attempted the combined analysis for GSTM1 and GSTT1 genotypes in ICVD for smoking status. No significant association was observed among the combined genotypes and ICVD. The observations do not confirm the effect of the GSTM1 and GSTT1 genotypes as a risk factor for ICVD, even in smokers. However, this approach provides a way of addressing the hypothesis that environmental genotoxins could play a role in the etiopathogenesis of ICVD.

Molecular Determinants of Xenobiotic Metabolism: QM/MM Simulation of the Conversion of 1-Chloro-2,4-dinitrobenzene Catalyzed by M1-1 Glutathione S-Transferase

Modeling methods allow the identification and analysis of determinants of reactivity and specificity in enzymes. The reaction between glutathione and 1-chloro-2,4-dinitrobenzene (CDNB) is widely used as a standard activity assay for glutathione S-transferases (GSTs). It is important to understand the causes of differences between catalytic GST isoenzymes and the effects of mutations and genetic polymorphisms. Quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations have been performed here to investigate the addition of the glutathione anion to CDNB in the wild-type M1-1 GST isoenzyme from rat and in three single point mutant (Tyr6Phe, Tyr115Phe, and Met108Ala) M1-1 GST enzymes. We have developed a specifically parameterized QM/MM method (AM1-SRP/CHARMM22) to model this reaction by fitting to experimental heats of formation and ionization potentials. Free energy profiles were obtained from molecular dynamics simulations of the reaction using umbrella sampling and weighted histogram analysis techniques. The reaction in solution has also been simulated and is compared to the enzymatic reaction. The free energies are in excellent agreement with experimental results. Overall the results of the present study show that QM/MM reaction pathway analysis provides detailed insight into the chemistry of GST and can be used to obtain mechanistic insight into the effects of specific mutations on this catalytic process.

Catalytic properties of glutathione-binding residues in a tau class glutathione transferase (PtGSTU1) from Pinus tabulaeformis

Glutathione transferases (GSTs) play important roles in stress tolerance and detoxification in plants. However, there is extremely little information on the molecular characteristics of GSTs in gymnosperms. In a previous study, we cloned a tau class GST (PtGSTU1) from a gymnosperm (Pinus tabulaeformis) for the first time. Based on the N-terminal amino acid sequence identity to the available crystal structures of plant tau GSTs, Ser13, Lys40, Ile54, Glu66 and Ser67 of PtGSTU1 were proposed as glutathione-binding (G-site) residues. The importance of Ser13 as a G-site residue was investigated previously. The functions of Lys40, Ile54, Glu66 and Ser67 of PtGSTU1 are examined in this study through site-directed mutagenesis. Enzyme assays and thermal stability measurements on the purified recombinant PtGSTU1 showed that substitution at each of these sites significantly affects the enzyme's substrate specificity and affinity for GSH, and these residues are essential for maintaining the stability of PtGSTU1. The results of protein expression and refolding analyses suggest that Ile54 is involved in the protein folding process. The findings demonstrate that the aforementioned residues are critical components of active sites that contribute to the enzyme's catalytic activity and structural stability.

Impact of domain interchange on conformational stability and equilibrium folding of chimeric class micro glutathione transferases

Rat micro class glutathione transferases M1-1 and M2-2 are homodimers that share a 78% sequence identity but display differences in stability. M1-1 is more stable at the secondary and tertiary structural levels, whereas its quaternary structure is less stable. Each subunit in these proteins consists of two structurally distinct domains with intersubunit contacts occurring between domain 1 of one subunit and domain 2 of the other subunit. The chimeric subunit variants M(12), which has domain 1 of M1 and domain 2 of M2, and its complement M(21), were used to investigate the conformational stability of the chimeric homodimers M(12)-(12) and M(21)-(21) to determine the contribution of each domain toward stability. Exchanging entire domains between class micro GSTs is accommodated by the GST fold. Urea-induced equilibrium unfolding data indicate that whereas the class micro equilibrium unfolding mechanism (i.e., N(2) <--> 2I <--> 2U) is not altered, domain exchanges impact significantly on the conformational stability of the native dimers and monomeric folding intermediates. Data for the wild-type and chimeric proteins indicate that the order of stability for the native dimer (N(2)) is M2-2 > M(12)-(12) M1-1 approximately M(21)-(21), and that the order of stability of the monomeric intermediate (I) is M1 > M2 approximately M(12) > M(21). Interactions involving Arg 77, which is topologically conserved in GSTs, appear to play an important role in the stability of both the native dimeric and folding monomeric structures.

Polymorphisms of glutathione S-transferase genes (GSTM1, GSTP1 and GSTT1) and breast cancer susceptibility

The glutathione S-transferase (GST) family of enzymes function in the body to detoxify carcinogenic compounds. Several genes that code for these enzymes are polymorphic, with particular genotypes previously shown to confer an increased cancer risk. In this study, we investigated the role of three GST genes (GSTM1, GSTP1 and GSTT1) in the development of sporadic breast cancer. Genotypes were determined in 129 breast cancer affected and 129 age and sex matched control individuals. Results did not support an involvement of these specific GST gene polymorphisms, either independently or in combination, in susceptibility to sporadic breast cancer in the tested Australian Caucasian population.

Structural determinants in domain II of human glutathione transferase M2-2 govern the characteristic activities with aminochrome, 2-cyano-1,3-dimethyl-1-nitrosoguanidine, and 1,2-dichloro-4-nitrobenzene

Two human Mu class glutathione transferases, hGST M1-1 and hGST M2-2, with high sequence identity (84%) exhibit a 100-fold difference in activities with the substrates aminochrome, 2-cyano-1,3-dimethyl-1-nitrosoguanidine (cyanoDMNG), and 1,2-dichloro-4-nitrobenzene (DCNB), hGST M2-2 being more efficient. A sequence alignment with the rat Mu class GST M3-3, an enzyme also showing high activities with aminochrome and DCNB, demonstrated an identical structural cluster of residues 164-168 in the alpha6-helices of rGST M3-3 and hGST M2-2, a motif unique among known sequences of human, rat, and mouse Mu class GSTs. A putative electrostatic network Arg107-Asp161-Arg165-Glu164(-Gln167) was identified based on the published three-dimensional structure of hGST M2-2. Corresponding variant residues of hGSTM1-1 (Leu165, Asp164, and Arg167) as well as the active site residue Ser209 were targeted for point mutations, introducing hGST M2-2 residues to the framework of hGST M1-1, to improve the activities with substrates characteristic of hGST M2-2. In addition, chimeric enzymes composed of hGST M1-1 and hGST M2-2 sequences were analyzed. The activity with 1-chloro-2,4-dinitrobenzene (CDNB) was retained in all mutant enzymes, proving that they were catalytically competent, but none of the point mutations improved the activities with hGST M2-2 characteristic substrates. The chimeric enzymes showed that the structural determinants of these activities reside in domain II and that residue Arg165 in hGST M2-2 appears to be important for the reactions with cyanoDMNG and DCNB. A mutant, which contained all the hGST M2-2 residues of the putative electrostatic network, was still lacking one order of magnitude of the activities with the characteristic substrates of wild-type hGST M2-2. It was concluded that a limited set of point mutations is not sufficient, but that indirect secondary structural affects also contribute to the hGST M2-2 characteristic activities with aminochrome, cyanoDMNG, and DCNB.

Evolution of differential substrate specificities in Mu class glutathione transferases probed by DNA shuffling

A library of variant enzymes was created by combined shuffling of the DNA encoding the human Mu class glutathione transferases GST M1-1 and GST M2-2. The parental GSTs are 84 % sequence identical at the protein level, but their specific activities with the substrates aminochrome and 2-cyano-1,3-dimethyl-1-nitrosoguanidine (cyanoDMNG) differ by more than 100-fold. Aminochrome is of particular interest as an oxidation product of dopamine and of possible significance in the etiology of Parkinson's disease, and cyanoDMNG is a model for genotoxic and potentially carcinogenic nitroso compounds. GST M2-2 has at least two orders of magnitude higher catalytic activity with both of the substrates than any of the other known GSTs, including GST M1-1. The DNA library of variant Mu class GST sequences contained "mosaic" structures composed of alternating segments of both parental sequences. All clones contained the 5'-end of a GST M1-1 clone optimized for high-level expression in Escherichia coli. The remainder of the sequences derived from segments of GST M2-2 and GST M1-1 DNA. All of the clones analyzed contained between two and seven distinct DNA segments. In addition, each clone contained an average of approximately one point mutation. None of the library clones analyzed was identical with either of the two parental structures. Variant GST sequences were expressed in E. coli, and their enzymatic activities with aminochrome, cyanoDMNG, and 1-chloro-2,4-dinitrobenzene (CDNB) were determined in bacterial lysates. Such screening of more than 70 clones demonstrated a continuous range of activities covering at least two orders of magnitude for each of the substrates. For a given clone, the activities with aminochrome and cyanoDMNG, in spite of their different chemistries, were clearly correlated, whereas no strong correlation was found with CDNB. This functional correlation suggests a common structural basis for the enzymatic mechanisms for conjugation of aminochrome and denitrosation of cyanoDMNG. From an evolutionary perspective, the results show that recombination of segments from homologous proteins gives rise to a large proportion of functionally competent proteins with a range of activities. The data support the proposal that natural evolution of protein functions may involve recombination of DNA segments followed by selection for advantageous functional properties of the resulting proteins. Clearly, the same approach can be utilized in the engineering of proteins displaying novel functions by in vitro evolution.

Solution structure of the carboxyl terminus of a human class Mu glutathione S-transferase: NMR assignment strategies in large proteins

Strategies to obtain the NMR assignments for the HN, N, CO, Calpha and Cbeta resonance frequencies for the human class mu glutathione-S-transferase GSTM2-2 are reported. These assignments were obtained with deuterated protein using a combination of scalar and dipolar connectivities and various specific labeling schemes. The large size of this protein (55 kDa, homodimer) necessitated the development of a novel pulse sequence and specific labeling strategies. These aided in the identification of residue type and were essential components in determining sequence specific assignments. These assignments were utilized in this study to characterize the structure and dynamics of the carboxy-terminal residues in the unliganded protein. Previous crystallographic studies of this enzyme in complex with glutathione suggested that this region may be disordered, and that this disorder may be essential for catalysis. Furthermore, in the related class alpha protein extensive changes in conformation of the C terminus are observed upon ligand binding. On the basis of the results presented here, the time-averaged conformation of the carboxyl terminus of unliganded GSTM2-2 is similar to that seen in the crystal structure. NOE patterns and 1H-15N heteronuclear nuclear Overhauser enhancements suggest that this region of the enzyme does not undergo motion on a rapid time scale.

Isolation and characterization of glutathione S-transferase isozymes from sorghum

Two glutathione S-transferase (GST) isozymes, A1/A1 and B1/B2, were purified from etiolated, O-1,3-dioxolan-2-yl-methyl-2,2,2, -trifluoro-4'-chloroacetophenone-oxime-treated sorghum (Sorghum bicolor L. Moench) shoots. GST A1/A1, a constitutively expressed homodimer, had a subunit molecular mass of 26 kD and an isoelectric point of 4.9. GST A1/A1 exhibited high activity with 1-chloro-2, 4, dinitrobenzene (CDNB) but low activity with the chloroacetanilide herbicide metolachlor. For GST A1/A1, the random, rapid-equilibrium bireactant kinetic model provided a good description of the kinetic data for the substrates CDNB and glutathione (GSH). GST B1/B2 was a heterodimer with subunit molecular masses of 26 kD (designated the B1 subunit) and 28 kD (designated the B2 subunit) and a native isoelectric point of 4.8. GST B1/B2 exhibited low activity with CDNB and high activity with metolachlor as the substrate. The kinetics of GST B1/B2 activity with GSH and metolachlor fit a model describing a multisite enzyme having two binding sites with different affinities for these substrates. Both GST A1/A1 and GST B1/B2 exhibited GSH-conjugating activity with ethacrynic acid and GSH peroxidase activity with cumene hydroperoxide, 9-hydroperoxy-trans-10, cis-12-octadecadienoic acid and 13-hydroperoxy-cis-9, trans-11-octadecadienoic acid. Both GST A1/A1 and GST B1/B2 are glycoproteins, as indicated by their binding of concanavalin A. Polyclonal antibodies raised against GST A1/A1 exhibited cross-reactivity with the B1 subunit of GST B1/B2. Comparisons of the N-terminal amino acid sequences of the GST A1, B1, and B2 subunits with other type I theta-GSTs indicated a high degree of homology with the maize GST I subunit and a sugarcane GST.

Mutagenesis of the active site of the human Theta-class glutathione transferase GSTT2-2: catalysis with different substrates involves different residues

The role of serine-11 in the catalytic mechanism of recombinant human GSTT2-2 was examined by site-directed mutagenesis. Amino acid sequence comparison of the Theta-class isoenzymes has identified a conserved serine residue in the N-terminal domain [Wilce, Board, Feil and Parker (1995) EMBO J. 14, 2133-2143]. This conserved serine has been implicated in the activation of the enzyme-bound glutathione [Board, Coggan and Parker (1995) Biochem. J. 311, 247-250]. Mutating the equivalent serine (residue 11) of GSTT2-2 to Ala, Thr or Tyr abolished the catalytic properties of GSTT2-2 with cumene hydroperoxide and ethacrynic acid as second substrate. However, with l-menaphthyl sulphate (MSu) as the second substrate, the specific activity of the S11A mutant was doubled, while the S11T mutant retained half the wild-type activity and the S11Y mutant was inactive. The role of Ser-11 in catalysis seems to vary with different second substrates. In the substitution reaction with MSu, GSTT2-2 activity appears to depend on the size of the Ser-11 replacement rather than the presence of a side-chain hydroxy group. In addition, the reaction rate appears to be a function of pH, and there is no non-enzymic reaction even at high pH. We demonstrated that a reaction between MSu and an alternative thiol such as L-cysteine or 2-mercaptoethanol can take place in the presence of S-methylglutathione and GSTT2-2. We propose that the catalytic activity of GSTT2-2 with MSu is preceded by a conformational or charge modification to the enzyme upon the binding of glutathione or S-methylglutathione. This is followed by the binding of MSu and the subsequent removal of the sulphate group, giving rise to the carbonium ion of l-methylnaphthelene as the electrophile that reacts with the nucleophilic species. The reaction mechanism of GSTT2-2 with MSu may represent a novel function of GSTT2-2 as a glutathione-dependent sulphatase.

Binding of 2-hydroxy-5-nitrobenzyl alcohol to rat alpha class glutathione S-transferases; evidence for binding at tryptophan 21

2-Hydroxy-5-nitrobenzyl alcohol (HNB) was prepared from dimethyl(2-hydroxyl-5-nitrobenzyl)sulfonium bromide (HNBB). HNB binds to glutathione S-transferases (GSTs) 1-2 and 2-2 with moderate affinity at a site separate from 1-anilino-8-naphthalenesulfonate (ANS). Intrinsic fluorescence due to Trp-21 is strongly quenched by HNB binding but there is no effect on catalytic activity. There appear to be two HNB binding sites per dimer in each GST isoenzyme. We suggest that HNB binds directly at Trp-21 of each subunit and that previously reported quenching of intrinsic fluorescence in these proteins upon ligand binding may be due to indirect structural effects rather than direct binding at this residue.

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.

The high activity of rat glutathione transferase 8-8 with alkene substrates is dependent on a glycine residue in the active site

Rat glutathione transferase (GST) 8-8 displays high catalytic activity with alpha, beta-unsaturated carbonyl compounds, including lipid peroxidation products such as 4-hydroxyalkenals. The catalytic efficiency of the related class Alpha GST 1-1 is substantially lower with the same substrates. Chimeric enzymes were prepared by replacing N-terminal subunit 8 segments of different lengths (6, 25, or 100 residues) with corresponding sequences from subunit 1 using recombinant DNA techniques. The chimeric subunit r1(25)r8, containing 25 amino acid residues from subunit 1, had the same low activity with alkenal substrates as that displayed by subunit 1. Mutation of Ala-12 into Gly in r1(25)r8 gave rise to the high alkenal activity characteristic of subunit 8, showing the importance of amino acid residue 12 for the activity. However, other structural determinants are also essential, as demonstrated by the corresponding Ala-12-->Gly mutation in subunit 1, which did not afford high alkenal activity. The results show that a single point mutation in a GST subunit may give rise to a 100-fold increase in catalytic efficiency with certain substrates. Introduction of such mutations may have contributed to the biological evolution of GST isoenzymes with altered substrate specificities and may also find use in the engineering of GSTs for novel functions.

Mammalian glutathione S-transferase: regulation of an enzyme system to achieve chemotherapeutic efficacy

The glutathione S-transferases are a family of Phase II detoxication enzymes that catalyze the conjugation of glutathione to a large variety of electrophilic compounds. In the 1990s, there have been many advances regarding the function of these enzymes in protecting a cell from the toxic effects of these electrophiles. The complexity of this enzyme family has been realized and much work has been performed to identify the specific roles played by individual isozymes in resistance to a variety of agents. Likewise, the determination of the crystal structure of these enzymes has allowed the identification of specific amino acid residues that are involved in the catalysis of important reactions. The important role that these enzymes play in carcinogenesis and in drug resistance has warranted an attempt to bring together these different subfields of glutathione S-transferase biology to investigate possible ways that this system could be regulated in therapeutically useful ways. In this report, we have reviewed the recent advances and ways in which this knowledge could be utilized in the advancement of the treatment of cancer.

Modification of glutathione S-transferase 3-3 mutants with 2-(S-glutathionyl)-3,5,6-trichloro-1,4-benzoquinone. Identification of the C-terminal tryptic fragment as part of the H-site and evidence that 2-(S-glutathionyl)-3,5,6-trichloro-1,4-benzoquinone is not specific for cysteine labelling

A triple mutant of rat liver glutathione S-transferase 3-3 that has all three cysteine residues replaced with serine (CallS) and a quadruple mutant with a Tyr-115 to phenylalanine substitution on CallS (CallSY115F) were reacted with 2-(S-glutathionyl)-3,5,6-trichloro-1,4-benzoquinone (GS-1,4-TCBQ). The modified proteins were analysed on a triple-quadrupole mass spectrometer equipped with an electrospray ionization source. At an enzyme: GS-1,4-TCBQ ratio of 1:10, the enzymes were modified at multiple sites. Covalent attachment of a single inhibitor on to the protein was achieved by lowering the enzyme: GS-1,4-TCBQ ratio to 1:1. Results from m.s. analyses suggest that the inhibitor on the CallSY115F mutant exists as a glutathionyl dichlorobenzoquinone derivative. The modifiers of the CallS mutants are glutathionyl monochlorobenzoquinone derivatives. Therefore, GS-1,4-TCBQ reacts at a single site on CallSY115F, but probably cross-links two regions on wild-type and CallS mutant. To confirm our observation, CallS was modified with 1-chloro2,4-dinitrobenzene, which specifically labels Tyr-115, before reacting with GS-1,4-TCBQ. The inhibitor formed a glutathionyl dichlorobenzoquinone adduct on the dinitrophenyl-CallS mutant. In addition, the benzoquinone derivative on the protein can be partially removed by 1-chloro-2,4-dinitrobenzene. Peptide mapping and sequencing analysis of the GS-1,4-TCBQ-modified CallS mutant revealed that the C-terminal 16-amino-acid fragment is labelled. Molecular modelling suggests the C(5) and C(6) on the benzoquinone ring of the inhibitor interact with the oxygen atoms of Tyr-115 and Ser-209 respectively.

Microbes, enzymes and genes involved in dichloromethane utilization

Dichloromethane (DCM) is efficiently utilized as a carbon and energy source by aerobic, Gram-negative, facultative methylotrophic bacteria. It also serves as a sole carbon and energy source for a nitrate-respiring Hyphomicrobium sp. and for a strictly anaerobic co-culture of a DCM-fermenting bacterium and an acetogen. The first step of DCM utilization by methylotrophs is catalyzed by DCM dehalogenase which, in a glutathione-dependent substitution reaction, forms inorganic chloride and S-chloromethyl glutathione. This unstable intermediate decomposes to glutathione, inorganic chloride and formaldehyde, a central metabolite of methylotrophic growth. Genetic studies on DCM utilization are beginning to shed some light on questions pertaining to the evolution of DCM dehalogenases and on the regulation of DCM dehalogenase expression. DCM dehalogenase belongs to the glutathione S-transferase supergene family. Analysis of the amino acid sequences of two bacterial DCM dehalogenases reveals 56% identity, and comparison of these sequences to those of glutathione S-transferases indicates a closer relationship to class Theta eukaryotic glutathione S-transferases than to a number of bacterial glutathione S-transferases whose sequences have recently become available. dcmA, the structural gene of the highly substrate-inducible DCM dehalogenase, is carried in most DCM utilizing methylotrophs on large plasmids. In Methylobacterium sp. DM4 its expression is governed by dcmR, a regulatory gene located upstream of dcmA, dcmR encodes a trans-acting factor which negatively controls DCM dehalogenase formation at the transcriptional level. Our working model thus assumes that the dcmR product is a repressor which, in the absence of DCM, binds to the promoter region of dcmA and thereby inhibits initiation of transcription.

Isolation and characterization of the Methylophilus sp. strain DM11 gene encoding dichloromethane dehalogenase/glutathione S-transferase

The restricted facultative methylotroph Methylophilus sp. strain DM11 utilizes dichloromethane as the sole carbon and energy source. It differs from other dichloromethane-utilizing methylotrophs by faster growth on this substrate and by possession of a group B dichloromethane dehalogenase catalyzing dechlorination at a fivefold-higher rate than the group A enzymes of slow-growing strains. We isolated dcmA, the structural gene of the strain DM11 dichloromethane dehalogenase, to elucidate its relationship to the previously characterized dcmA gene of Methylobacterium sp. strain DM4, which encodes a group A enzyme. Nucleotide sequence determination of dcmA from strain DM11 predicts a protein of 267 amino acids, corresponding to a molecular mass of 31,197 Da. The 5' terminus of in vivo dcmA transcripts was determined by primer extension to be 70 bp upstream of the translation initiation codon. It was preceded by a putative promoter sequence with high resemblance to the Escherichia coli sigma 70 consensus promoter sequence. dcmA and 130 bp of its upstream sequence were brought under control of the tac promoter and expressed in E. coli to approximately 20% of the total cellular protein by induction with isopropylthiogalactopyranoside (IPTG) and growth at 25 degrees C. Expression at 37 degrees C led to massive formation of inclusion bodies. Comparison of the strain DM11 and strain DM4 dichloromethane dehalogenase sequences revealed 59% identity at the DNA level and 56% identity at the protein level, thus indicating an ancient divergence of the two enzymes. Both dehalogenases are more closely related to eukaryotic class theta glutathione S-transferases than to a number of bacterial glutathione S-transferases.

Interaction of ebselen with glutathione S-transferase and papain in vitro

The interaction of ebselen(2-phenyl-1,2-benzisoselenazol-3(2H)-one) with rat liver cytosolic glutathione S-transferases (GSTs) and the plant cysteine protease, papain, was studied as cysteine residues are important for the activity of these enzymes. The capacity of GST 1-2 and 3-4 for ebselen binding is similar (1.5 mol ebselen/mol GST isozyme), while GST 2-2 and GST 7-7 bind 0.3 and more than 2.0 mol ebselen/mol GST isozyme, respectively. Ebselen does not bind to N-ethylmaleimide-treated GST, and its binding to GST is prevented by 5 mM thiols. Ebselen irreversibly inactivates the different GST isozymes with a second order rate constant ranging from 20 to 2250 M-1 sec-1 for the different subunits. GST inhibition by ebselen is partially restored by 5 mM thiols. Ebselen binds to untreated papain and to cysteine-treated papain at a ratio of about 0.1 and 0.75 mol ebselen/mol papain, respectively. Ebselen does not bind to N-ethylmaleimide-treated papain, and its binding to papain is interfered with by added thiols. Papain is inactivated by ebselen with a second order rate constant of 1800 M-1 sec-1 in the absence of thiols. However, in the presence of GSH, 2-mercaptoethanol or sodium borohydride, ebselen exerts an activating effect on papain. The binding of ebselen by a seleno-sulfide bond to cysteine residues of GSTs and papain leads to their inactivation.

X-ray crystal structures of cytosolic glutathione S-transferases. Implications for protein architecture, substrate recognition and catalytic function

Crystal structures of cytosolic glutathione S-transferases (EC 2.5.1.18), complexed with glutathione or its analogues, are reviewed. The atomic models define protein architectural relationships between the different gene classes in the superfamily, and reveal the molecular basis for substrate binding at the two adjacent subsites of the active site. Considerable progress has been made in understanding the mechanism whereby the thiol group of glutathione is destabilized (lowering its pKa) at the active site, a rate-enhancement strategy shared by the soluble glutathione S-transferases.

Sequence of the mRNA for a glutathione transferase Pi with a different substrate specificity in V79 Chinese hamster lung cells

The mRNA sequence for a glutathione transferase (GST) belonging to the Pi class has been determined. This was a first step towards elucidating, at the molecular level, why V79 Chinese hamster lung cells lack the capacity to conjugate the benzo[a]pyrene (BP) derivative BPDE, but nonetheless contain the GST pi gene, express GST pi mRNA and contain a protein that binds to antibodies directed against the human GST Pi enzyme. The sequencing strategy involved synthesis of a cDNA library, circularization of the GST pi cDNA for PCR amplification and subsequent DNA sequencing. The coding sequence for the GST Pi protein of V79 cells, designated CLOGSTP1, consisted of 627 bp coding for 209 amino acids (aa), corresponding to a 23-kDa protein. The cDNA sequence obtained demonstrated extensive homology to those from other species, especially rat and mouse, where this homology was 92 and 91%, respectively. Upon comparing the aa sequence predicted from CLOGSTP1 to those of rat, mouse, pig, cow and man, the most striking differences were found in aa positions 19, 39, 40, 110, 113 and 151. Consequently, the explanation for the lower capacity for GST Pi-catalyzed conjugation in V79 cells, as compared to other species, remains a matter of speculation, since none of these aa positions coincides with positions involved in the xenobiotic substrate-binding site of GST Pi from pig and human or of GST Mu from rat. The most likely candidates for causing the observed change in substrate specificity might be Lys110 and Glu113, which are the altered residues closest to this binding site and which might, thus, exclude BPDE as a substrate for the Chinese hamster enzyme.

Alpha,beta-unsaturated carbonyl compounds: inhibition of rat liver glutathione S-transferase isozymes and chemical reaction with reduced glutathione

Five different alpha,beta-unsaturated carbonyl compounds displayed different reactivities with regard to inhibition of alpha- and mu-class isozymes of rat liver glutathione S-transferases and the chemical reaction with glutathione. Only (E)-2-octenal and (E)-3-nonen-2-one exhibited significant levels of inhibition for each of the rat liver GST isozymes examined. The (E)-2-octenal was more effective as an inhibitor of the alpha-class of isozymes when compared to the mu-class, whereas the (E)-3-nonen-2-one showed a greater degree of inhibition of the mu-class of isozymes relative to the alpha-class. Isozyme 1-1 demonstrated the greatest degree of inhibition with (E)-2-octenal (IC50 = 5.89 microM) of all inhibitor/isozyme combinations. The Ki values for (E)-2-octenal and (E)-3-nonen-2-one toward selected alpha- and mu-class of rat liver glutathione S-transferase isozymes were determined and both of these compounds competitively inhibited all five of the rat liver glutathione S-transferase isozymes examined. The Ki values obtained for these two compounds were significantly different for each of the isozymes except for isozyme 4-4. With the alpha-class of rat liver GST isozymes, (E)-3-nonen-2-one showed a larger Ki value than (E)-2-octenal. Whereas, with the mu-class, (E)-2-octenal exhibited a larger Ki value than (E)-3-nonen-2-one. The rate constants of the forward reaction (k+1), as well as the equilibrium constants (Kd) were determined and the rate constants of the reverse reaction (k-1) were calculated.

Significance of an unusually low Km for glutathione in glutathione transferases of the alpha, mu and pi classes

1. Interactions of glutathione transferases (GST) of the alpha, mu and pi classes with glutathione (GSH) and glutathione conjugates (GS-X) are in contrast with those of a GST of the theta class (GST5-5). 2. GST 5-5 has a Km for GSH of approx. 5 mM. Thus Km/ambient [GSH] is approx. 1, within the range of Km/ambient [s] of glycolytic enzymes. GSTs of the alpha, mu and pi classes yield much lower values of Km for GSH (approx. 0.1 mM) hence Km/ambient [s] is significantly lower than those of most (non-GST) enzymes (p < 0.025). 3. GSTs of the alpha, mu and pi classes are sensitive to inhibition by GS-X (i.e. product) and GS-X analogues. GST 5-5 is not. 4. Rate enhancements up to 10(10), similar to an average enzyme (10(8)-10(12)), are seen in catalysis by GST 5-5, but not in catalysis by GSTs of alpha, mu and pi classes (> 10(7)). 5. Comparisons of primary structure indicate that theta class GSTs may have a decreased binding of the glu-alpha-amino- and gly-COO(-)-groups of GSH compared with GSTs of the other classes. 6. It is concluded that GSTs of alpha, mu and pi classes have evolved towards increased product binding at the expense of catalytic efficiency. Thus GSH is uniquely utilized both as a nucleophile and a 'tag' which can be used to bind and sequester product particularly during GSH-depletion. This interpretation unifies the catalytic and binding properties of these GSTs and alters their perceived role in detoxication.

Isozyme specificity of novel glutathione-S-transferase inhibitors

A systematically diversified set of peptide analogs of the reaction product of glutathione with an electrophilic substrate have been tested as isozyme-specific inhibitors of human glutathione-S-transferase (GST). The potency of the best of the inhibitors is in the 0.5 to 20 micromolar range, with kinetics indicative of competitive inhibition with glutathione at the active site. The specificity observed among three recombinant-derived GST isozymes at both low and high potency ranged from negligible to high (at least 20-fold over the next most sensitive isozyme). These results define a novel strategy for the design of drugs targeting cells with elevated levels of particular GST isozymes, such as tumor cells for which elevated levels of GST are believed to be an important cause of chemotherapeutic drug resistance.