Human glutathione S-transferases. The Ha multigene family encodes products of different but overlapping substrate specificities

The human glutathione S-transferase cDNAs encoding subunits 1 and 2 contain intrinsic ribosome-binding sites in their 5'-untranslated regions for direct expression in Escherichia coli. We show that functional human GSH S-transferases 1-1 and 2-2 are synthesized from lambda gt11 cDNA clones lambda GTH1 and lambda GTH2 in phage lysates of E. coli Y1090, in lysogens of E. coli Y1089, and from the plasmid expression constructs in pKK223-3. The E. coli-expressed human GHS S-transferases 1-1 and 2-2 do not have blocked N termini in contrast to those directly purified from human livers. These two isozymes, with 11 amino acid substitutions between them, are similar in their Km values for GSH and 1-chloro-2,4-dinitrobenzene and Kcat values for this conjugation reaction. The human GSH S-transferase 2-2, however, is a more active GSH peroxidase than transferase 1-1 toward cumene hydroperoxide and t-butyl hydroperoxide. Our results indicate that different members of a GSH S-transferase gene family with limited amino acid substitutions have different with limited amino acid substitutions have different but overlapping substrate specificities. We propose that accumulation of single amino acid replacements may be an important mechanism for generating diversity in GSH S-transferases with various xenobiotic substrates. In situ chromosomal hybridization results show that the GSH transferase Ha genes are located in the region of 6p12.     

Interrelationship between cationic and anionic forms of glutathione S-transferases of bovine ocular lens.

Since the eye is constantly exposed to potentially damaging chemical compounds present in the atmosphere and vascular system, we investigated the physiological role of glutathione S-transferase (GSH S-transferase) in detoxification mechanisms operative in the ocular lens. We have purified an anionic and a cationic GSH S-transferase from the bovine lens to homogeneity through a combination of gel filtration, ion-exchange and affinity chromatography. The anionic (pI 5.6) and cationic (pI 7.4) S-transferases were found to have distinct kinetic parameters (apparent Km and Vmax. pH optimum and energy of activation). However, both species were demonstrated to have similar molecular weights and amino acid compositions. Double-immunodiffusion and immunotitration studies showed that both lens S-transferases were immunologically similar. The very close similarity in amino acid compositions and immunological properties strongly indicates that these two transferases either originate from the same gene or at least share common antigenic determinants and originate from similar genes. The bovine lens GSH S-transferases had no glutathione peroxidase activity with either t-butyl hydroperoxide or cumene hydroperoxide as substrate. However, the antibody raised against the homogeneous anionic glutathione S-transferase from the bovine lens was found to precipitate both glutathione S-transferase and glutathione peroxidase activities out of solution in the supernatant of a crude bovine liver homogenate.     

Interrelationship between anionic and cationic forms of glutathione S-transferases of human liver.

Human liver glutathione S-transferases (GSH S-transferases) were fractionated into cationic and anionic proteins. During fractionation with (NH4)2SO4 the anionic GSH S-transferases are concentrated in the 65%-saturated-(NH4)2SO4 fraction, whereas the cationic GSH S-transferases separate in the 80%-saturated-(NH4)2SO4 fraction. From the 65%-saturated-(NH4)2SO4 fraction two new anionic GSH S-transferases, omega and psi, were purified to homogeneity by using ion-exchange chromatography on DEAE-cellulose, Sephadex G-200 gel filtration, affinity chromatography on GSH bound to epoxy-activated Sepharose and isoelectric focusing. By a similar procedure, cationic GSH S-transferases were purified from the 80%-saturated-(NH4)2SO4 fraction. Isoelectric points of GSH S-transferases omega and psi are 4.6 and 5.4 respectively. GSH S-transferase omega is the major anionic GSH S-transferase of human liver, whereas GSH S-transferase psi is present only in traces. The subunit mol.wt. of GSH S-transferase omega is about 22500, whereas that of cationic GSH S-transferases is about 24500. Kinetic and structural properties as well as the amino acid composition of GSH S-transferase omega are described. The antibodies raised against cationic GSH S-transferases cross-react with GSH S-transferase omega. There are significant differences between the catalytic properties of GSH S-transferase omega and the cationic GSH S-transferases. GSH peroxidase II activity is displayed by all five cationic GSH S-transferases, whereas both anionic GSH S-transferases do not display this activity.     

Suppression of glutathione S-transferases in rat seminal vesicles or pituitary glands

We have studied the tissue-specific expression of GSH S-transferases in rat seminal vesicles and pituitary glands by in vitro translation and immunoprecipitation. The major GSH S-transferase subunit expressed in rat seminal vesicles belongs to the Yb mobility class whose expression diminishes when the rats are treated with pentobarbital. The pattern of GSH S-transferase expression in the pituitary gland is very similar to that of the rat brain with Yb size subunit(s) predominant. The Y beta size subunit is also expressed together with the Yc and Y delta subunits. The expression of GSH S-transferases was drastically reduced in pituitary gland poly(A) RNAs from diethylstilbestrol-treated, ovariectomized female rats. Xenobiotics such as phenobarbital, 3-methylcholanthrene, and trans-stilbene oxide induce rat liver GSH S-transferase activities, especially the Ya- and Yb-subunit containing isozymes. Induction of GSH S-transferases by a combination of the three xenobiotics is neither additive nor synergistic, however. Our results clearly demonstrate that GSH S-transferase expression in seminal vesicles and pituitary glands can be suppressed by phenobarbital and diethylstilbestrol, respectively. Our findings suggest that different GSH S-transferase isozymes respond differently to various xenobiotics. Both induction and suppression occur in rats treated with xenobiotics. This notion helps to explain the lack of additive or synergistic induction in rats treated with more than one xenobiotic.     

The role of human glutathione S-transferases hGSTA1-1 and hGSTA2-2 in protection against oxidative stress.

In order to elucidate the protective role of glutathione S-transferases (GSTs) against oxidative stress, we have investigated the kinetic properties of the human alpha-class GSTs, hGSTA1-1 and hGSTA2-2, toward physiologically relevant hydroperoxides and have studied the role of these enzymes in glutathione (GSH)-dependent reduction of these hydroperoxides in human liver. We have cloned hGSTA1-1 and hGSTA2-2 from a human lung cDNA library and expressed both in Escherichia coli. Both isozymes had remarkably high peroxidase activity toward fatty acid hydroperoxides, phospholipid hydroperoxides, and cumene hydroperoxide. In general, the activity of hGSTA2-2 was higher than that of hGSTA1-1 toward these substrates. For example, the catalytic efficiency (kcat/Km) of hGSTA1-1 for phosphatidylcholine (PC) hydroperoxide and phosphatidylethanolamine (PE) hydroperoxide was found to be 181.3 and 199.6 s-1 mM-1, respectively, while the catalytic efficiency of hGSTA2-2 for PC-hydroperoxide and PE-hydroperoxide was 317.5 and 353 s-1 mM-1, respectively. Immunotitration studies with human liver extracts showed that the antibodies against human alpha-class GSTs immunoprecipitated about 55 and 75% of glutathione peroxidase (GPx) activity of human liver toward PC-hydroperoxide and cumene hydroperoxide, respectively. GPx activity was not immunoprecipitated by the same antibodies from human erythrocyte hemolysates. These results show that the alpha-class GSTs contribute a major portion of GPx activity toward lipid hydroperoxides in human liver. Our results also suggest that GSTs may be involved in the reduction of 5-hydroperoxyeicosatetraenoic acid, an important intermediate in the 5-lipoxygenase pathway.     

Purification and characterization of the individual glutathione S-transferases from sheep liver

The glutathione S-transferases (EC 2.5.1.18) have been purified to electrophoretic homogeneity from 105,000g supernatant of sheep liver homogenate by employing a combination of gel filtration on Sephadex G-150 and affinity chromatography on S-hexylglutathione-linked Sepharose-6B columns. Approximately 70% of the original glutathione S-transferase activity toward 1-chloro-2,4-dinitrobenzene and glutathione peroxidase activity toward cumene hydroperoxide could be recovered by this purification method. Of particular importance in developing this procedure was the fact that the enzyme preparation obtained after affinity column chromatography represented all the isozymes of sheep liver glutathione S-transferases. Further purification by CM-cellulose and DEAE-cellulose column chromatography resolved the glutathione S-transferases into seven distinct cationic isozymes designated C-1, C-2, C-3, C-4, C-5, C-6, and C-7 and five overlapping anionic transferases designated A-1, A-2, A-3, A-4, and A-5, respectively, in the order of their elution from the ion-exchange columns. The sodium dodecyl sulfate SDS-gel electrophoretic data on subunit composition revealed that cationic enzymes are composed of two subunits with an identical Mr of 24,000 whereas a predominant subunit with Mr of 26,000 was observed in all anionic isozyme peaks except A-1. Cationic isozymes accounted for approximately 98% of the total peroxidase activity associated with the glutathione S-transferase whereas only A-1 of the anionic isozymes displayed some peroxidase activity. Isozyme C-4 was found to be the most abundant glutathione S-transferase in the sheep liver. Characterization of the individual transferases by their specificity toward a number of selected substrates, subunit composition, and isoelectric points showed some similarities to those patterns for human liver glutathione S-transferases.     

The relation between thromboxane and prostaglandin synthesis in human decidua tissue: a comparison of eicosanoid synthesis in minced tissue with that in a cell-free preparation

Radiotracer studies and radioimmunoassay measurements demonstrate that minced tissues of human decidua produce chiefly thromboxane B2 (TxB2) (70% of total eicosanoids) and small amounts of prostaglandin F2 alpha (PGF2 alpha) (13%) PGD2 (8%), 6-keto-PGF1 alpha (5%) and PGE2 (4%). Inhibition of thromboxane synthesis with a specific inhibitor (OKY-1581: sodium (E)-3-[4(-3-pyridylmethyl)-phenyl]-2-methyl propenoate) increased prostaglandin formation in general, with the main product being PGF2 alpha (38%), a nonenzymic derivative of PGH2. Crude particulate fractions prepared from the same tissue synthesized two major products from [3H]arachidonate, TxB2 and 6-keto-PGF1 alpha (54 and 30%, respectively) and some PGF2 alpha and PGE2 (8-8%). However, in the presence of reduced glutathione (GSH), PGE2 became the main product (81%) (TxB2, 15%; PGF2 alpha, 2%; and 6-keto-PGF1 alpha, 2%). Half-maximal stimulation of PGE2 synthesis occurred at 46 microM GSH. The GSH concentration of tissue samples was found to be 110 +/- 30 microM. We conclude that human first trimester decidua cells possess the key enzymes of prostaglandin and thromboxane synthesis. Apparently, the production of these compounds is controlled by a specific mechanism in the tissue, which keeps PGE and prostacyclin synthesis in a reversibly suppressed state, whereas the formation of thromboxane is relatively stimulated.     

Localization of a portion of the active site of two rat liver glutathione S-transferases using a photoaffinity label

The glutathione S-transferases are a family of dimeric enzymes that catalyze the reaction between GSH and a variety of electrophiles. Two closely related isozymes, referred to as YaYa and YcYc, were purified from rat liver. A radiolabeled azido derivative of glutathione (S-(p-azidophenacyl)[3H]glutathione) was prepared and used to label covalently the active site of the above two glutathione S-transferases. The noncovalently bound affinity label was a competitive inhibitor of glutathione S-transferase YaYa toward both 1-chloro-2,4-dinitrobenzene and GSH. The covalently labeled enzymes no longer bound to a GSH-affinity column, and covalent labeling was reduced in the presence of GSH and S-(dinitrophenyl)glutathione. These results suggest that the affinity label was binding at the active site. The covalently labeled enzymes were digested with trypsin, and the labeled peptides were purified by HPLC and then sequenced. A single-labeled peptide was identified in the tryptic digest of the YaYa isozyme, whereas two labeled peptides were present in the tryptic digest of YcYc. The Ya peptide sequence was identical with the published deduced sequence of amino acids between residues 212 and 218 and the sequences of the two peptides purified from Yc were identical with the deduced sequence of amino acids between 91 and 110 and 206 and 218. Hence, the Ya peptide and the smaller peptide purified from Yc came from the same region of the Ya and Yc subunits. This common region and a second region of the Yc subunit appear to form a portion of the active site of these two forms of glutathione S-transferase.     

Purification and cloning of a Delta class glutathione S-transferase displaying high peroxidase activity isolated from the German cockroach Blattella germanica

A highly active glutathione S-transferase was purified from adult German cockroaches, Blattella germanica. The purified enzyme appeared as a single band of 24 kDa by SDS/PAGE, and had a different electrophoretic mobility than, a previously identified Sigma class glutathione S-transferase (Bla g 5). Kinetic study of 1-chloro-2,4-dinitrobenzene conjugation revealed a high catalytic rate but common substrate-binding and cosubstrate-binding affinities, with V(max), k(cat), K(m) for 1-chloro-2,4-dinitrobenzene and K(m) for glutathione estimated to be 664 micromol x mg(-1) x min(-1), 545 s(-1), 0.33 mm and 0.76 mm, respectively. Interestingly, this enzyme possessed the highest activity for cumene hydroperoxide among insect glutathione S-transferases reported to date. Along with the ability to metabolize 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane and 4-hydroxynonenal, this glutathione S-transferase may play a role in defense against insecticides as well as oxidative stress. On the basis of the amino acid sequences obtained from Edman degradation and MS analyses, a 987-nucleotide cDNA clone encoding a glutathione S-transferase (BggstD1) was isolated. The longest ORF encoded a 24 614 Da protein consisting of 216 amino acid residues. The sequence had close similarities ( approximately 45-60%) to that of Delta class glutathione S-transferases, but had only 14% identity to Bla g 5. The putative amino acid sequence contained matching peptide fragments of the purified glutathione S-transferase. ELISA showed that BgGSTD1 bound to serum IgE obtained from patients with cockroach allergy, indicating that the protein may be a cockroach allergen.     

The role of selenium-dependent and selenium-independent glutathione peroxidases in the formation of prostaglandin F2 alpha

In recent years, growing evidence suggests that glutathione peroxidases (GSH-Pxs), both selenium-dependent GSH-Px (Se-GSH-Px) and selenium-independent GSH-Px (non-Se-GSH-Px) play an important role in the biosynthesis of prostaglandins and leukotrienes and in the regulation of key enzymes associated with the arachidonic acid cascade. The precise nature of their involvement in eicosanoid metabolism, however, is not yet completely understood. In the study reported here, we have systematically determined the catalytic efficiencies of Se-GSH-Px and non-Se-GSH-Px toward prostaglandin (PG) G2 (PGG2) and PGH2. Se-GSH-Px exhibited high catalytic activity for the reduction of PGG2 as indicated by Km and Vmax values of 12 microM and 78 mumol/min/mg, respectively, whereas PGH2 was found to be a poor substrate, an indication that Se-GSH-Px reduces the hydroperoxide moiety but not the endoperoxide moiety of PGG2. The kinetic constants of Se-GSH-Px toward PGG2 were comparable to those determined for such classical substrates as H2O2 and cumene hydroperoxide. In contrast to Se-GSH-Px, non-Se-GSH-Px associated with cationic isozyme II of glutathione S-transferases (GSTs) from sheep lung cytosol was very active in the conversion of PGH2 to PGF2 alpha with a Vmax of 960 nmol/min/mg and a Km of 77 microM. This study shows that PGF2 alpha formation by non-Se-GSH-Px occurred in a GSH-dependent reduction of either PGG2 or PGH2. When PGG2 was used as the substrate for non-Se-GSH-Px, a novel intermediate compound appeared and was later identified by several methods of structural analysis as 15-hydroperoxy PGF2 alpha. Thus, the reductive cleavage of the endoperoxide occurs faster than the 15-hydroperoxide reduction allowing 15-hydroperoxy PGF2 alpha to accumulate briefly. A study of GSTs from several different tissues and species indicated that the transformation of PG endoperoxides to PGF2 alpha is catalyzed specifically by GST isozymes, which contain Ya size subunits. This specificity of GST isozymes in PG biosynthesis, coupled with their tissue-specific expression, may be a mechanism by which the body modulates the type of PGs produced in these tissues. Also, these results suggest a possible interaction of Se-GSH-Px and non-Se-GSH-Px in the biosynthesis of PGF2 alpha.     

Novel glutathione conjugates formed from epoxyeicosatrienoic acids (EETs)

The catalysis of glutathione (GSH) conjugation to epoxyeicosatrienoic acids (EETs) by various purified isozymes of glutathione S-transferase was studied. A GSH conjugate of 14,15-EET was isolated by HPLC and TLC; this metabolite contained one molecule of EET and one molecule of GSH. Fast atom bombardment mass spectrometry of the isolated metabolite confirmed the structure as a GSH conjugate of 14,15-EET. Studies designed to determine the isozyme specificity of this reaction demonstrated that two isozymes, 3-3, and 5-5, efficiently catalyzed this conjugation reaction. The Km values for 14,15-EET were approximately 10 microM and the Vmax values ranged from 25 to 60 nmol conjugate formed min-1 mg-1 purified transferase 3-3 and 5-5. The 5,6-, 8,9-, and 11,12-EETs were also substrates for the reaction, albeit at lower rates. These results demonstrate that the EETs can serve as substrates for the cytosolic glutathione S-transferases.     

On the multiplicity of rat liver glutathione S-transferases

Rat liver glutathione S-transferases have been purified to apparent electrophoretic homogeneity by S-hexylglutathione-linked Sepharose 6B affinity chromatography and CM-cellulose column chromatography. At least 11 transferase activity peaks can be resolved including five Yb size homodimeric isozymes, two Yc size homodimeric isozymes, one Ya homodimeric isozyme, one Y alpha homodimeric isozyme, and two Ya-Yc heterodimeric isozymes. Distribution of the GSH peroxidase activity among the CM-cellulose column fractions suggests the existence of further multiplicity in this isozyme family. Substrate specificity patterns of the Yb subunit isozymes revealed a possibility that each of the five Yb-containing isozymes is composed of a different homodimeric Yb size subunit composition. Our findings on the increasing multiplicity of glutathione S-transferase isozymes are consistent with the notion that multiple isozymes of overlapping substrate specificities are required to detoxify a multitude of xenobiotics in addition to serving other important physiological functions.     

Glutathione peroxidase, glutathione reductase, glutathione S-transferase, and gamma-glutamyltranspeptidase activities in the human early pregnancy placenta

GSH peroxidase, GSSG reductase, GSH S-transferase, and gamma-glutamyltranspeptidase activities were measured in the supernatant of 13 human early pregnancy placenta homogenates. From measurements of GSH peroxidase activity with both H2O2 and cumene hydroperoxide as second substrate it was deduced that immature placenta contains only the Se-dependent form. All the specimens investigated exhibited GSSG reductase and gamma-glutamyltranspeptidase activities. GSH S-transferase activity was noted only using 1-chloro-2,4-dinitrobenzene as electrophilic substrate, while no detectable activity was found with 1,2-dichloro-4-nitrobenzene, 1,2-epoxy-3-(p-nitrophenoxy) propane, and p-nitrobenzylchloride. It is concluded that human placenta is equipped, from early pregnancy, with the enzymatic systems which are involved in GSH-mediated cellular detoxication and in preserving the integrity of the sulfhydryl status of the cells.     

Biochemical and immunological demonstration of prostaglandin D2, E2, and F2 alpha formation from prostaglandin H2 by various rat glutathione S-transferase isozymes

Glutathione S-transferase isozymes purified from normal rat liver (1-1, 1-2, 2-2, 3-3, 3-4, and 4-4), liver with hyperplastic nodules (7-7), brain (Yn1Yn1), and testis (Yn1Yn2) all had prostaglandin H2-converting activity. The prostaglandin H2 E-isomerase activity was high in 1-1 (1400 nmol/min/mg protein), 1-2 (1170), and 2-2 (420), moderate in 3-3, 3-4, 4-4, Yn1Yn1, and Yn1Yn2 (52-100), and weak but significant in 7-7 (33). The prostaglandin H2 D-isomerase activity was relatively high in 1-1 (170) and 1-2 (200), moderate in 2-2 (60) and Yn1Yn2 (43), and weak but marked in 3-3 (16), 4-4 (16), and 7-7 (14). The prostaglandin H2 F-reductase activity was remarkable in 1-1 (1250), 1-2 (920), and 2-2 (390), and weakly detected in 3-3 (24), 4-4 (28), and 7-7 (14). Glutathione was absolutely required for these prostaglandin H2-converting reactions, and its stoichiometric consumption was associated with F-reductase activity but not E- and D-isomerase activities. The Km values for glutathione and prostaglandin H2 were about 200 and 10-40 microM, respectively. By immunoabsorption analyses with various antibodies specific for each isozyme, we examined its contribution to the formation of prostaglandins D2, E2, and F2 alpha from prostaglandin H2 in 100,000g supernatants of rat liver, kidney, and testis. In the liver, about 90% of the F-reductase activity (9.8 nmol/min/mg protein) was shown to be catalyzed by the 1-2 group of isozymes. The E-isomerase activity (16.5) was catalyzed about 60 and 40% by the 1-2 and 3-4 groups, respectively; and the D-isomerase activity (3.7) was catalyzed by the 1-2 group (50%) and the 3-4 group and Yn1Yn2 (15-25%). In the kidney, the E-isomerase activity (9.4) was catalyzed by 1-1, 1-2 (40%), 2-2, 3-4 group, and 7-7 (10-20%). The F-reductase activity (3.3) was mostly catalyzed by the 1-2 group (75%). In the testis, the E-isomerase activity (3.9) was catalyzed by the 1-2 group (20-30%), the 3-4 group, and Yn1Yn2 (30-60%).     

Isozyme specificity of rat liver glutathione S-transferases in the formation of PGF2 alpha and PGE2 from PGH2

When prostaglandin H2 (PGH2) was incubated with a mixture of glutathione S-transferases (GSTs) obtained from S-hexylglutathione affinity chromatography, as much as 40% of it was transformed into a prostanoid whose Rf value corresponded to that of the standard PGF2 alpha. The reaction product was identified as PGF2 alpha by cochromatography with a standard on TLC and HPLC. The stereochemistry of the hydroxyl groups on C-9 and C-11 of the cyclopentane ring was confirmed by mass-spectral analysis of the butylboronate derivative of the reaction product. Neither PGE2 nor PGD2 could substitute for PGH2 in the reaction mixture, indicating that the mechanism of formation of PGF2 alpha is a direct two-electron reduction of the endoperoxide moiety and not through a reduction of the keto group on PGE2 or PGD2. Individual GST isozymes exhibited distinct differences in their catalytic rates of formation of PGF2 alpha from PGH2. Among various GSTs, isozyme IV, a homodimer of Ya size subunit showed the highest activity with a Vmax value of approximately 6000 nmol.min-1.mg-1. In general, the isozymes containing Ya and Yc subunits exhibited relatively high activity toward PGH2, indicating that it is the non-selenium-dependent glutathione peroxidase activity associated with the GSTs that might be responsible for the reduction of PGH2 to PGF2 alpha. Interestingly, isozyme IV also exhibited the highest PGE2 forming activity with a Vmax value of approximately 3000 nmol.min-1.mg-1 followed by isozyme I, a homodimer of Yb subunit, which had a Vmax value of 420 nmol.min-1.mg-1. Based on these results, it appears that the GSTs play an important role in the biosynthesis of classical PGs. Therefore, it is conceivable that the tissue-specific formation of PGF2 alpha and PGE2 might, in part, be due to the relative distribution of these enzyme activities in a given tissue. Our results have not only confirmed the previously published reports (E. Christ-Hazelhof et al. (1976) Biochim. Biophys. Acta 450, 450-461), but also have characterized the specificity of GST isozymes in the formation of PGF2 alpha.     

Purification and characterization of human muscle glutathione S-transferases: evidence that glutathione S-transferase zeta corresponds to a locus distinct from GST1, GST2, and GST3.

Human muscle glutathione S-transferase isozyme, GST zeta (pI 5.2) has been purified by three different methods using immunoaffinity chromatography, DEAE cellulose chromatography, and isoelectric focusing. GST zeta prepared by any of the three methods does not recognize antibodies raised against the alpha, mu, or pi class glutathione S-transferases of human tissues. GST zeta has a blocked N-terminus and its peptide fingerprints also indicate it to be distinct from the alpha, mu, or pi class isozymes. As compared to GSTs of alpha, mu, and pi classes, GST zeta displays higher activities toward t-stilbene oxide and Leukotriene A4 methyl ester. GST zeta also expresses GSH-peroxidase activity toward hydrogen peroxide. The Kms of GST zeta for CDNB and GSH were comparable to those reported for other human GSTs but its Vmax for CDNB, 7620 mol/mol/min, was found to be considerably higher than that reported for other human GSTs. The kinetics of inhibition of GST zeta by hematin, bile acids, and other inhibitors also indicate that it was distinct from the three classes of GST isozymes. These studies suggest that GST zeta corresponds to a locus distinct from GST1, GST2, and GST3 and probably corresponds to the GST4 locus as suggested previously by Laisney et al. (1984, Human Genet. 68, 221-227). The results of peptide fingerprints and kinetic analysis indicate that as compared to the pi and alpha class isozymes, GST zeta has more structural and functional similarities with the mu class isozymes. Besides GST zeta several other GST isozymes belonging to pi and mu class have also been characterized in muscle. The pi class GST isozymes of muscle have considerable charge heterogeneity among them despite identical N-terminal sequences.     

Residues 207, 216, and 221 and the catalytic activity of mGSTA1-1 and mGSTA2-2 toward benzo[a]pyrene-(7R,8S)-diol-(9S,10R)-epoxide

Murine class alpha glutathione S-transferase subunit types A2 (mGSTA2-2) and A1 (mGSTA1-1) have high catalytic efficiency for glutathione (GSH) conjugation of the ultimate carcinogenic metabolite of benzo[a]pyrene, (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene, [(+)-anti-BPDE]. Only 10 residues differ between the sequences of mGSTA1-1 and 2-2. However, the catalytic efficiency of mGSTA1-1 for GSH conjugation of (+)-anti-BPDE is >3-fold higher as compared with mGSTA2-2. The crystal structure of mGSTA1-1 in complex with the GSH conjugate of (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (GSBpd) reveals that R216 and I221 in the last helix play important roles in catalysis [Gu, Y., Singh, S. V., and Ji, X. (2000) Biochemistry 39, 12552-12557]. The crystal structure of mGSTA2-2 in complex with GSBpd has been determined, which reveals a different binding mode of GSBpd. Comparison of the two structures suggests that residues 207 and 221 are responsible for the different binding mode of GSBpd and therefore contribute to the distinct catalytic efficiency of the two isozymes.     

Metabolism of chlorambucil by rat liver microsomal glutathione S-transferase

Clinical efficacy of alkylating anticancer drugs, such as chlorambucil (4-[p-[bis [2-chloroethyl] amino] phenyl]-butanoic acid; CHB), is often limited by the emergence of drug resistant tumor cells. Increased glutathione (gamma-glutamylcysteinylglycine; GSH) conjugation (inactivation) of alkylating anticancer drugs due to overexpression of cytosolic glutathione S-transferase (GST) is believed to be an important mechanism in tumor cell resistance to alkylating agents. However, the potential involvement of microsomal GST in the establishment of acquired drug resistance (ADR) to CHB remains uncertain. In our experiments, a combination of lipid chromatography/electrospray ionization mass spectrometry (LC/ESI/MS) was employed for structural characterization of the resulting conjugates between CHB and GSH. The spontaneous reaction of 1mM CHB with 5 mM GSH at 37 degrees C in aqueous phosphate buffer for 1 h gave primarily the monoglutathionyl derivative, 4-[p-[N-2-chloroethyl, N-2-S-glutathionylethyl] amino]phenyl]-butanoic acid (CHBSG) and the diglutathionyl derivative, 4-[p-[2-S-glutathionylethyl] amino]phenyl]-butanoic acid (CHBSG2) with small amounts of the hydroxy-derivative, 4-[p-[N-2-S-glutathionylethyl, N-2-hydroxyethyl] amino]phenyl]-butanoic acid (CHBSGOH), 4-[p-[bis[2-hydroxyethyl] amino]phenyl]-butanoic acid (CHBOH2), 4-[p-[N-2-chloroethyl, N-2-S-hydroxyethyl]amino]phenyl]-butanoic acid (CHBOH). We demonstrated that rat liver microsomal GST presented a strong catalytic effect on these reactions as determined by the increase of CHBSG2, CHBSGOH and CHBSG and the decrease of CHB. We showed that microsomal GST was activated by CHB in a concentration and time dependent manner. Microsomal GST which was stimulated approximately two-fold with CHB had a stronger catalytic effect. Thus, microsomal GST may play a potential role in the metabolism of CHB in biological membranes, and in the development of ADR.     

A rapid equilibrium random sequential bi-bi mechanism for human placental glutathione S-transferase

Double-reciprocal plots of initial-rate data for the conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) and GSH by human placental GSH S-transferase pi were linear for both substrates. Computer modelling of the initial-rate data using nonlinear least-squares regression analysis favoured a rapid equilibrium random sequential bi-bi mechanism, over a steady-state random sequential mechanism or a steady-state or rapid equilibrium ordered mechanism. KGSH was calculated as 0.125 +/- 0.006 mM, KCDNB was 0.87 +/- 0.07 mM and alpha was 2.1 +/- 0.3 for the rapid equilibrium random model. The product, S-(2,4-dinitrophenyl)glutathione, was a competitive inhibitor with respect to GSH, and a mixed-type inhibitor toward CDNB (KP = 18 +/- 3 microM). The observed pattern of inhibition is consistent with a rapid equilibrium random mechanism, with a dead-end enzyme.CDNB.product complex, but inconsistent with the inhibition patterns of other bireactant mechanisms. Since rat liver GSH S-transferase 3-3 acts via a steady-state random sequential mechanism [1], while human placental GSH S-transferase and perhaps also rat liver GSH S-transferase 1-1 [2] exhibit rapid equilibrium random mechanisms, we conclude that the kinetic mechanism of the GSH S-transferases is isoenzyme-dependent.     

The purification and characterization of glutathione S-transferase from the hepatopancreas of the blue crab, Callinectes sapidus

High glutathione S-transferase activity was found in the cytosol of F-cells from the hepatopancreas of the blue crab (Callinectes sapidus). Purification of glutathione S-transferase from hepatopancreas extracts by Sephadex G-200, DEAE-Sephacel, and chromatofocusing resulted in the isolation of two isozymes with isoelectric points of 5.9 and 5.7, as determined by analytical isoelectric focusing. Using 1-chloro-2,4-dinitrobenzene as the substrate the specific activities of the two purified isozymes were 222 and 182 mumol/min/mg, respectively. There was no evidence for basic transferase isozymes. In addition to 1-chloro-2,4-dinitrobenzene the purified glutathione S-transferase isozymes showed activity with p-nitrophenyl acetate, p-nitrobenzyl chloride, bromosulfophthalein, and benzopyrene oxide. Thus, both substitution and addition reactions associated with vertebrate glutathione S-transferase were found in the crab transferases. There was no when ethacrynic acid, methyl iodide, trans-4-phenyl-3-buten-2-one, 1,2-epoxy-(p-nitrophenoxy)propane, cumene hydroperoxide, and t-butyl hydroperoxide were used as substrates. The lack of peroxidase activity is of interest since this activity is commonly found in vertebrate transferase isozymes. The two transferases had a dimeric Mr of 40,800 with similar amino acid compositions and similar kinetic parameters (Vmax, Km, and pH maxima) with 1-chloro-2,4-dinitrobenzene as substrate. The two transferases could be distinguished by their isoelectric points, molecular mass of the monomers (22,300 for GST 1 and 22,300 and 22,400 for GST 2), and different inhibitor mechanisms with hematin and bromosulfophthalein.