Eukaryotic translation elongation factor 1 gamma contains a glutathione transferase domain--study of a diverse, ancient protein superfamily using motif search and structural modeling

Using computer methods for multiple alignment, sequence motif search, and tertiary structure modeling, we show that eukaryotic translation elongation factor 1 gamma (EF1 gamma) contains an N-terminal domain related to class theta glutathione S-transferases (GST). GST-like proteins related to class theta comprise a large group including, in addition to typical GSTs and EF1 gamma, stress-induced proteins from bacteria and plants, bacterial reductive dehalogenases and beta-etherases, and several uncharacterized proteins. These proteins share 2 conserved sequence motifs with GSTs of other classes (alpha, mu, and pi). Tertiary structure modeling showed that in spite of the relatively low sequence similarity, the GST-related domain of EF1 gamma is likely to form a fold very similar to that in the known structures of class alpha, mu, and pi GSTs. One of the conserved motifs is implicated in glutathione binding, whereas the other motif probably is involved in maintaining the proper conformation of the GST domain. We predict that the GST-like domain in EF1 gamma is enzymatically active and that to exhibit GST activity, EF1 gamma has to form homodimers. The GST activity may be involved in the regulation of the assembly of multisubunit complexes containing EF1 and aminoacyl-tRNA synthetases by shifting the balance between glutathione, disulfide glutathione, thiol groups of cysteines, and protein disulfide bonds. The GST domain is a widespread, conserved enzymatic module that may be covalently or noncovalently complexed with other proteins. Regulation of protein assembly and folding may be 1 of the functions of GST.

A surface mutant (G82R) of a human alpha-glutathione S-transferase shows decreased thermal stability and a new mode of molecular association in the crystal

A chimeric enzyme (GST121) of the human alpha-glutathione S-transferases GST1-1 and GST2-2, which has improved catalytic efficiency and thermostability from its wild-type parent proteins, has been crystallized in a space group that is isomorphous with that reported for crystals of GST1-1. However, a single-site (G82R) mutant of GST121, which exhibits a significant reduction both in vitro and in vivo in protein thermostability, forms crystals that are not isomorphous with GST1-1. The mutant protein crystallizes in space group P2(1)2(1)2(1), with cell dimensions a = 49.5, b = 92.9, c = 115.9 A, and one dimer per asymmetric unit. Preliminary crystallographic results show that a mutation of the surface residue Gly 82 from a neutral to a charged residue causes new salt bridges to be formed among the GST dimers, suggesting that the G82R mutant might aggregate more readily than does GST121 in solution, resulting in a change of its solution properties.

A novel type of glutathione S-transferase in Onchocerca volvulus

Onchocerca volvulus is a pathogenic human filarial parasite which, like other helminth parasites, is capable of evading the host's immune responses by a variety of defense mechanisms which are likely to include the detoxification and repair mechanisms of the enzyme glutathione S-transferase (GST). In this study, we show that one of the previously described GSTs from O. volvulus appears to possess the characteristics of a secreted enzyme. When the complete O. volvulus GST1 (OvGST1) sequence presented here is compared with those of other GSTs, 50 additional residues at the N terminus are observed, the first 25 showing characteristics of a signal peptide. This is consistent with the N-terminal sequence data on the native mature enzyme which begins at amino acid 26, based on the deduced protein sequence from the cDNA. The native protein, without the signal peptide sequence, possesses a 24-amino-acid extension not present in other GSTs. The deduced amino acid sequence of the OvGST1 cDNA clone was shown to possess four potential N-glycosylation sites. Digestion of O. volvulus homogenate with endoglycosidase, followed by detection of OvGST1 with specific antibody, indicated that the enzyme possesses at least two N-linked oligosaccharide chains. Gel filtration of the Escherichia coli-produced recombinant OvGST1 showed that it is enzymatically active as a nonglycosylated dimer. OvGST1 is found in the media surrounding adult worms maintained in culture, indicating that, in vitro, this enzyme is released from the worm. The strongest immunostaining for OvGST1 was observed in the outer cellular covering of the adult worm body, the syncytial hypodermis, especially in the interchordal hypodermis, where the peripheral membrane forms a series of lamellae which run into the outer zone of the hypodermal cytoplasm.

Refined crystal structure of porcine class Pi glutathione S-transferase (pGST P1-1) at 2.1 A resolution

The crystal structure of class Pi glutathione S-transferase from porcine lung (pGST P1-1) in complex with glutathione sulphonate has been refined at 2.11 A resolution, to a crystallographic R-factor of 16.5% for 21, 165 unique reflections. The refined structure includes 3314 protein atoms, 46 inhibitor (glutathione sulphonate) atoms and 254 water molecules. The model shows good stereochemistry, with root-mean-square deviations from ideal bond lengths and bond angles of 0.011 A and 2.8 degrees, respectively. The estimated root-mean-square co-ordinate error is 0.2 A. The protein is a dimer assembled from identical subunits of 207 amino acid residues. The tertiary structure of the pGST P1 subunit is organized as two domains, the N-terminal domain (domain I, residues 1 to 74) and the larger C-terminal domain (domain II, residues 81 to 207). Glutathione sulphonate, a competitive inhibitor, binds to the G-site region (i.e. the glutathione-binding region) of the active site located on each subunit. Each G-site is, however, structurally dependent of the neighbouring subunit as structural elements forming a fully functional G-site are provided by both subunits, with domain I as the major supporting framework. A number of direct and water-mediated polar interactions are involved in sequestering the glutathione analogue at the G-site. The extended conformation assumed by the enzyme-bound inhibitor as well as the strategic interactions between inhibitor and protein, closely resemble those observed for the physiological substrate, reduced glutathione bound at the active site of class Mu glutathione S-transferase 3-3. Hydrogen bonding between the sulphonyl moiety of the inhibitor and the hydroxyl group of an evolutionary conserved tyrosine residue, Tyr7, provides the first direct structural evidence for a catalytic protein group in glutathione S-transferases that is involved in the activation of the substrate glutathione. The catalytic role for Tyr7 has subsequently been confirmed by mutagenesis and kinetic studies. Comparison of the known crystal structures for class Pi, class Mu and class Alpha isoenzymes, indicates that the cytosolic glutathione S-transferases share a common fold and that the structural features for catalysis are similar.

Molecular cloning and expression of human leukotriene-C4 synthase

Leukotriene-C4 synthase (LTC4S; EC 2.5.1.37) catalyzes the committed step in the biosynthesis of the peptidoleukotrienes, which are important in the pathogenesis of asthma. Antibodies were generated to a synthetic peptide based on the partial amino acid sequence previously reported for human LTC4S [Nicholson, D.W., Ali, A., Vaillancourt, J.P., Calaycay, J.R., Mumford, R.A., Zamboni, R.J. & Ford-Hutchinson, A. W. (1993) Proc. Natl. Acad. Sci. USA 90, 2015-2019] and specifically bound detergent-solubilized LTC4S obtained from THP-1 cells, confirming that the published sequence is associated with enzyme activity. Inosine-containing oligonucleotides based on the partial protein sequence were used to isolate a 679-bp cDNA for LTC4S from THP-1 cells. The cDNA contains an open reading frame that encodes a 150-amino acid protein (M(r) = 16,568) that has a calculated pI value of 11.1. The deduced protein sequence is composed predominantly of hydrophobic amino acids; hydropathy analysis predicts three transmembrane domains connected by two hydrophilic loops. Analysis of the deduced sequence identified two potential protein kinase C phosphorylation sites and a potential N-linked glycosylation site. The amino acid sequence for human LTC4S is unique and shows no homology to other glutathione S-transferases. LTC4S was found to be most similar to 5-lipoxygenase activating protein (31% identity, 53% similarity), another protein involved in leukotriene biosynthesis. Active enzyme was expressed in bacterial, insect, and mammalian cells as shown by the biosynthesis of LTC4 in incubation mixtures containing LTA4 and reduced glutathione. The cloning and expression of human LTC4S provide the basis for a better understanding of this key enzyme in peptidoleukotriene biosynthesis.

Cell cycle regulation of the p34cdc2/p33cdk2-activating kinase p40MO15

A key component of Cdc2/Cdk2-activating kinase (CAK) is p40MO15, a protein kinase subunit that phosphorylates the T161/T160 residues of p34cdc2/p33cdk2. The level and activity of p40MO15 were essentially constant during cleavage of fertilised Xenopus eggs and in growing mouse 3T3 cells, but serum starvation of these cells reduced both the level and activity of p40MO15. Although the level and activity of endogenous p40MO15 did not vary in the cell cycle, we found that bacterially expressed p40MO15 was activated more rapidly by M-phase cell extracts than by interphase cell extracts. Bacterially expressed p40MO15 was phosphorylated mainly on serine 170 (a p34cdc2 phosphorylation site) by mitotic cell extracts, but mutation of S170 to alanine did not affect the activation of p40MO15, whereas mutation of T176 (the equivalent site to T161/T160 in p34cdc2/p33cdk2) abolished the activation of P40MO15. These studies suggest that the level and activity of p40MO15 is probably not a major determinant of p34cdc2/p33cdk2 activity in the cell cycle, and that the activation of p40MO15 may require phosphorylation on T176.

Isolation and characterization of octopus hepatopancreatic glutathione S-transferase. Comparison of digestive gland enzyme with lens S-crystallin

Glutathione S-transferase from Octopus vulgaris hepatopancreas was purified to apparent homogeneity by single glutathione-Sepharose-4B affinity chromatography with overall yield 46% and purification 249-fold. The enzyme was a homodimer with subunit M(r) 24,000, which was smaller than that of the octopus lens S-crystallin (M(r) 27,000) with glutathione-S-transferase-like structure. Both proteins showed substrate specificities similar to alpha/pi-type isozyme of glutathione S-transferase. Under native conditions, both proteins exhibited multiple forms upon polyacrylamide gel electrophoresis or isoelectric focusing, albeit with distinct mobilities; however, only one kind of N-terminal amino acid sequence was determined for the multiple forms of each protein. The hepatopancreatic GST, with pI value 6.6-7.3, dissociated into two monomers in an acidic or alkaline environment. Two amino acid residues, with pKa values 5.69 +/- 0.14 and 9.03 +/- 0.11 were involved in the subunit interactions of the hepatopancreatic enzyme.

Oligonucleotide-directed nucleic acid scission by micrococcal nuclease

"Sequence-dictated" scission of DNA can be achieved by tethering a fusion protein composed of glutathione S-transferase and attenuated micrococcal nuclease (MN) to a targeting oligonucleotide using Cibacron blue (CB) F3G-A. Deoxyoligonucleotides derivatized with this dye bind to the fusion protein in gel mobility shift assays. This binding scheme was successfully used to achieve site-specific scission of a single-stranded DNA substrate after hybridization with a CB-derivatized complementary oligonucleotide. Although covalently cross-linked hybrids of MN and oligonucleotides have been successfully used in the past to target nucleolytic activity, this novel scheme opens new possibilities for targeting and probing both DNA and RNA sequences by allowing the addition of the nuclease subsequent to hybridization.

A rapid one-step method to purify baculovirus-expressed human estrogen receptor to be used in the analysis of the oxytocin promoter

We have produced a truncated form of the human estrogen receptor (hER) as a fusion protein with glutathione S-transferase (GST) in Spodoptera frugiperda (Sf) cells using the baculovirus expression vector (BEV) system. The protein is correctly produced and can be purified from crude whole-cell extracts by a single-step, batch-wise affinity-purification procedure. We show that this GST-hER fusion protein binds at its DNA-binding site specifically and in a hormone-inducible manner. Furthermore, we used the purified hER to analyze the complex estrogen response element (ERE) in the promoter of the oxytocin-encoding gene.

Photoaffinity labelling of the active site of the rat glutathione transferases 3-3 and 1-1 and human glutathione transferase A1-1

The glutathione transferases (GSTs) form a group of enzymes responsible for a wide range of molecular detoxications. The photoaffinity label S-(2-nitro-4-azidophenyl)glutathione was used to study the hydrophobic region of the active site of the rat liver GST 1-1 and 2-2 isoenzymes (class Alpha) as well as the rat class-Mu GST 3-3. Photoaffinity labelling was carried out using a version of S-(2-nitro-4-azidophenyl)glutathione tritiated in the arylazido ring. The labelling occurred with higher levels of radioisotope incorporation for the Mu than the Alpha families. Taking rat GST 3-3, 1.18 (+/- 0.05) mol of radiolabel from S-(2-nitro-4-azidophenyl)glutathione was incorporated per mol of dimeric enzyme, which could be blocked by the presence of the strong competitive inhibitor, S-tritylglutathione (Ki = 1.4 x 10(-7) M). Radiolabelling of the protein paralleled the loss of enzyme activity. Photoaffinity labelling by tritiated S-(2-nitro-4-azidophenyl)glutathione on a preparative scale (in the presence and absence of S-tritylglutathione) followed by tryptic digestion and purification of the labelled peptides indicated that GST 3-3 was specifically photolabelled; the labelled peptides were sequenced. Similarly, preparative photoaffinity labelling by S-(2-nitro-4-azidophenyl)glutathione of the rat liver 1-1 isoenzyme, the human GST A1-1 and the human-rat chimaeric GST, H1R1/1, was carried out with subsequent sequencing of radiolabelled h.p.l.c.-purified tryptic peptides. The results were interpreted by means of molecular-graphics analysis to locate photoaffinity-labelled peptides using the X-ray-crystallographic co-ordinates of rat GST 3-3 and human GST A1-1. The molecular-graphical analysis indicated that the labelled peptides are located within the immediate vicinity of the region occupied by S-substituted glutathione derivatives bound in the active-site cavity of the GSTs investigated.

Enzymatic characteristics of chimeric mYc/rYc1 glutathione S-transferases

Mice are resistant to aflatoxin carcinogenicity primarily due to expression of a glutathione S-transferase (mYc) with high catalytic activity toward aflatoxin B1-8,9-epoxide (AFBO). In contrast, rats are more sensitive to aflatoxin carcinogenicity due to the constitutive expression of a glutathione S-transferase with relatively low catalytic activity toward AFBO (rYc1). To identify the contribution of different regions of the mYc protein that confer high catalytic activity toward AFBO, six chimeric mYc/rYc1 GST enzymes were generated utilizing full and partial restriction enzyme digestions at two conserved StyI sites in the mYc and rYc1 complementary DNAs (between amino acid residues 56-57 and 142-143). Recombinant wild-type and chimeric glutathione S-transferases were bacterially expressed, affinity purified, and their catalytic activities measured toward AFBO, delta 5-androstene-3,17-dione, 1-chloro-2,4-dinitrobenzene, and ethacrynic acid. The set of chimeras displayed a wide range of catalytic activities toward the substrates assayed. The chimeras with the greatest activity toward AFBO were 1:56rat-57: 221mouse and 1:56mouse-57:142rat-143:221mouse, with AFBO conjugating activities 200 and 8 times greater than wild-type rYc1, respectively. These results demonstrate that the residues that confer high AFBO conjugation activity in mYc are located in the region spanning residues 57-221.

Engineering of a metal coordinating site into human glutathione transferase M1-1 based on immobilized metal ion affinity chromatography of homologous rat enzymes

Rat glutathione transferase (GST) 3-3 binds to Ni(II)-iminodiacetic acid (IDA)-agarose, whereas other GSTs that are abundant in rat liver do not bind to this immobilized metal ion affinity chromatography (IMAC) adsorbent. Rat GST 3-3 contains two superficially located amino acid residues, His84 and His85, that are suitably positioned for coordination to Ni(II)-IDA-agarose. This particular structural motif is lacking in GSTs that do not bind to the IMAC matrix. Creation of an equivalent His-His structure in the homologous human GST M1-1 by protein engineering afforded a mutant enzyme that displays affinity for Ni(II)-IDA-agarose, in contrast to the wild-type GST M1-1. The results identify a distinct site that is operational in IMAC and suggest an approach to the rational design of novel integral metal coordination sites in proteins.

Cloning of cDNAs from fetal rat liver encoding glutathione S-transferase Yc polypeptides. The Yc2 subunit is expressed in adult rat liver resistant to the hepatocarcinogen aflatoxin B1

Fetal rat liver possesses substantial levels of glutathione S-transferase (GST) activity toward aflatoxin B1-8,9-epoxide. The enzyme responsible for this activity is an Alpha-class GST heterodimer comprising Yc1 and Yc2 subunits. The cDNAs encoding these polypeptides have been cloned and shown to share approximately 91% identity over 920 base pairs, extending from nucleotide -23 to the AATAAA polyadenylation signal. GST Yc2Yc2 expressed in Escherichia coli was found to exhibit 150-fold greater activity toward aflatoxin B1-8,9-epoxide than GST Yc1Yc1. Comparison between the structures of Alpha-class GST suggests that tyrosine at residue 108 and/or aspartate at residue 208 is responsible for the high aflatoxin B1 detoxication capacity of Yc2. Immunoblotting and enzyme assays have shown that liver from adult female rats contains about 10-fold greater levels of Yc2 than is found in liver from adult male rats. This sex-specific expression of Yc2 in adult rat liver may contribute to the relative insensitivity of female rats to aflatoxin B1. Dietary administration of oltipraz, a synthetic antioxidant which protects against aflatoxin-hepatocarcinogenesis, serves as an inducer of GST Yc2.

Glutathione S-transferase M1 and its variants A and B as host factors of bladder cancer susceptibility: a case-control study

Glutathione S-transferase M1 (GSTM1) is a foreign compound-metabolizing enzyme with a heritable complete lack of activity in about 50% of Caucasians. GSTM1 deficiency may predispose individuals to urinary bladder cancer. Thus, a hospital-based case-control study was performed with 296 patients with bladder cancer and 400 controls, investigating this GSTM1 deficiency in relation to environmental risk factors and types of bladder cancer. Frequencies of the GSTM1 gene deletion (genotype, GSTM1*0/0) and of the allele variants A (mu) and B (psi) of the GSTM1-active trait were determined using an internal standard-controlled polymerase chain reaction technique. Moreover, in all patients GSTM1 expression was quantified in blood by an immunoassay. Of the cases, 59.1% had the GSTM1*0/0 genotype, in contrast to 50.7% of the controls (odds ratio, 1.40; 95% confidence limits, 1.02-1.92; P = 0.017). The odds ratio after adjustment for age and gender by logistic regression analysis was 1.54 (95% confidence limits, 1.12-2.13). Occupational risk was defined as previous employment in occupations with known increased bladder cancer risk, but the impact of GSTM1*0/0 was not significantly different in individuals with risk jobs versus those without. The greater proportion of the GSTM1-deficient individuals in the group with cancer was due to a lower frequency of carriers of GSTM1A. The odds ratio for the subgroup of individuals with the GSTM1B phenotype versus carriers of the GSTM1A phenotype in cases versus controls was 1.65 (95% confidence limits, 0.976-2.78; two-tailed Fisher's exact P = 0.057). Analysis of functional GSTM1 activity in a subset of 370 blood samples with the model substrate trans-stilbene oxide confirmed the genetic results and showed that 9 of 10 individuals with mu/psi heterodimers (genotype, GSTM1*A/B) had activities above the median of all genetically GSTM1-active individuals (24 pmol/min/1 x 10(6) lymphocytes; P < 0.01), indicating a gene dose relationship for GSTM1. GSTM1 expression in the urinary bladder endothelium detected by immunoassay and immunohistology corresponded to the genotype of the patients. It may be concluded from this study that the heritable GSTM1 deficiency is responsible for 17% (etiological fraction; 95% confidence limits, 2-30%) of bladder cancer cases.

A group of novel glutathione S-transferase isozymes showing high activity towards 4-hydroxy-2-nonenal are present in bovine ocular tissues

Recently, a mouse glutathione S-transferase (GST) isozyme, mGSTA4-4, which belongs to a distinct group of GSTs has been characterized in our laboratory. During the present studies, Western blot analyses of bovine ocular tissues using the antibodies raised against the recombinant mGSTA4-4 obtained by expression in Escherichia coli revealed that the orthologs of mGSTA4-4 were present in cornea, retina, iris-ciliary body and sclera, but absent in lens. These novel GST isozymes of bovine ocular tissues were purified by immunoaffinity chromatography using the antibodies against rec-mGSTA4-4 and were designated as bGST 5.8 (their pI value being 5.8). Amino acid sequences of CNBr fragments of bGST 5.8 from cornea, sclera, retina and iris-ciliary body showed high degree of primary structure homologies with the corresponding regions of mGSTA4-4 indicating these bovine GST isozymes were distinct from the alpha. mu and pi group GSTs and were the newest members of the group of GSTs to which mGSTA4-4 belongs. There were significant differences among the amino acid sequences of bGST 5.8 of cornea and iris-ciliary body and retina suggesting presence of at least two closely related genes at bGST 5.8 locus. bGST 5.8 isozymes showed high activity toward 4-HNE (four-to-five-fold higher than that towards 1-chloro-2,4-dinitrobenzene), expressed GSH-peroxidase activity towards fatty acid hydroperoxides and phospholipid hydroperoxides, and showed GSH-conjugating activity towards fatty acid epoxides suggesting that these isozymes may play an important role in protection mechanism against the endogenous toxicants formed during lipid peroxidation.

Class pi glutathione S-transferase: Meisenheimer complex formation

The enzyme-catalysed formation of the dead-end Meisenheimer complex, 1-(S-glutathionyl)-2,4,6-trinitrocyclohexadienate, between glutathione and 1,3,5-trinitrobenzene by two class pi glutathione S-transferases was studied under equilibrium conditions. The apparent formation constant of the complex at pH6.5, is 1.21 x 10(3) M-1 and 1.47 x 10(3) M-1 for isoenzyme pGSTP1-1 from porcine lung and hGSTP1-1 the human recombinant orthologue, respectively. These values are about 40- to 50-times larger than that determined for the nonenzymatic reaction in solution. Competitive inhibitors in the form of glutathione analogues that bind the G-site (glutathione sulphonate) or both the G-site and the H-site (S-hexylglutathione) regions of the active site markedly diminish complex formation. Comparison of kinetic data for glutathione S-transferase isoenzymes from the pi and mu gene classes suggests that the catalytic efficiencies for nucleophilic aromatic substitution reactions correspond with the ability of the enzyme's active site to stabilise the Meisenheimer complex. Formation of the red-coloured complex in orthorhombic crystals of pGSTP1-1 demonstrated that the crystallized protein retains its catalytically functional conformation in the crystal lattice.

Phosphorylation of 17 beta-hydroxysteroid dehydrogenase in BeWo choriocarcinoma cells

OBJECTIVE

The purpose of this study was to test whether 17 beta-hydroxysteroid dehydrogenase might exist in a phosphorylated form.

STUDY DESIGN

Phosphorylation of 17 beta-hydroxysteroid dehydrogenase was evaluated in BeWo choriocarcinoma cells. The phosphorylation of 17 beta-hydroxysteroid dehydrogenase expressed in Escherichia coli as a glutathione transferase fusion protein was also studied.

RESULTS

Human BeWo choriocarcinoma cells were metabolically labeled with phosphorus 32 orthophosphate. Immunoprecipitates were prepared with rabbit anti-17 beta-hydroxysteroid dehydrogenase antiserum from the labeled cells and separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A phosphorylated protein with a molecular size of 35 kd was obtained from anti-17 beta-hydroxysteroid dehydrogenase immunoprecipitates, which suggested that 17 beta-hydroxysteroid dehydrogenase was phosphorylated in BeWo cells. The predominant phosphoamino acid was phosphoserine. 17 beta-Hydroxysteroid dehydrogenase expressed in E. coli as a glutathione transferase fusion protein was a substrate of protein kinase A in vitro. Protein kinase A phosphorylated the recombinant 17 beta-hydroxysteroid dehydrogenase exclusively on serine. Incubation of BeWo cell lysates with bacterial alkaline phosphatase led to a decrease in the oxidative activity of 17 beta-hydroxysteroid dehydrogenase. Incubation of the alkaline phosphatase inhibitor levamisole with BeWo cell lysates resulted in a higher estradiol-to-estrone conversion rate, compared with cell lysates without any treatment.

CONCLUSION

Our data suggest that 17 beta-hydroxysteroid dehydrogenase may exist in phosphorylated forms and that phosphorylation may regulate the activity of 17 beta-hydroxysteroid dehydrogenase in vivo.

Novel procedure for structure refinement in homology modeling and its application to the human class Mu glutathione S-transferases

Glutathione S-transferases (GST) are a major class of phase II detoxifying enzymes that conjugate glutathione to electrophiles. Their involvement in the degradation of chemotherapeutic agents, which contributes to drug resistance, makes this family of enzymes potential targets for therapeutic agents. This study generates, by homology modeling, a 3-D structure of three GST human isozymes of the Mu class, M1b-1b, M2-2 and M3-3, using the Rat3-3 GST structure as a template. The high percentage of identity among these enzymes and the lack of insertions and deletions make the system ideally suited to the technique of homology modeling. A novel technique for the modeling of protein structures was applied. The structure of the template was used to generate a low-resolution crystallographic map in which the initial coordinates of the structure to be modeled were placed. The structure was then annealed within this envelope. In addition, a feedback-restrained molecular dynamics procedure was adopted to scale the template restraints during the simulations. Three independent validation procedures were applied. To assess the reliability of the methods, an identical series of simulation steps to those used in the refinement were applied to the template structure (self modeling). Further, a homology structure for the Rat3-3 template was generated, starting from the modeled M1b-1b structure (reverse modeling). To assess the reasonableness of the modeled structures, two recently developed methodologies to verify protein structures based on statistics of the nonbonded interactions were applied. Overall, the structures appear to be consistent.

Adducts of dienochlor miticide with glutathione, glutathione S-transferases, and hemoglobins

Dienochlor (Pentac) (C10Cl10) has been used for 30 years as a miticide with little knowledge of its mode of action or metabolic fate except that it is quickly degraded by rats. This study examines the reactions of dienochlor with GSH and proteins as models for its metabolism and interactions with tissues. Dienochlor reacts rapidly with 1.0 mM GSH in phosphate buffer (pH 7.4) at 37 degrees C (t1/2 approximately 11 min) as analyzed by UV/visible spectroscopy and HPLC, yielding a series of more than a dozen adducts. Octachlorofulvalene (C10Cl8), a candidate intermediate, also reacts to give the same apparent products (t1/2 < 0.2 min as above); however, its intermediacy in the dienochlor reaction was not established. Isolation and MS analyses characterized two isomeric C10H2Cl(SG)5 adducts and a C10H2(SG)6 derivative; these products react further in the presence of GSH to yield two even more polar adducts. Cysteine and N-acetylcysteine also react rapidly with dienochlor whereas GSSG and several non-thiol amino acids are much less reactive. Purified GSH S-transferases (GSTs) and hemoglobins, each from six species of mammals including humans, are extensively labeled in vitro by [14C]dienochlor to form adducts separable by gel electrophoresis and HPLC. [14C]Dienochlor readily derivatizes rat liver GSTs even in cytosol and in the presence of high GSH levels. The potency of dienochlor for inhibition of GST activity is maintained or enhanced upon conversion to GSH adducts.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of an alpha class glutathione S-transferase from melphalan-resistant Chinese hamster ovary cells and demonstration of its ability to catalyze melphalan-glutathione adduct formation

We have shown previously that a Chinese hamster ovary cell line (designated CHO-Chlr), generated by exposure to chlorambucil and demonstrating a greater than 20-fold collateral resistance to melphalan, showed increased expression of an alpha form of glutathione S-transferase (GST) associated with amplification of GST genes. Here, we demonstrate that GST purified from CHO-Chlr cells contains a form with a pI of 9, not present in CHO-K1 cells or Chinese hamster liver, which has the ability to accelerate the formation of glutathione-melphalan adducts. This result provides evidence that overexpression of the alpha class GST may be directly responsible for the development of resistance to bifunctional alkylating agents.

A comparison of the enzymatic and physicochemical properties of human glutathione transferase M4-4 and three other human Mu class enzymes

The multigene family of cytosolic glutathione S-transferases (GSTs) consists of four classes (Alpha, Mu, Pi, and Theta), all involved in the detoxication of reactive electrophiles. The human Mu class GSTs consist of at least four expressed isozyme subunits, GST M1, GST M2, GST M3, and GST M4, which have 70-90% amino acid sequence identity. The gene and cDNA sequences for GST M4 have been determined recently (K. E. Comstock, K. J. Johnson, D. Rifenbery, and W. D. Henner, J. Biol. Chem. (1993) 268, 16958-16965). Cloning of GST M4 cDNA into an Escherichia coli expression system permitted the production of the corresponding protein. The enzyme was purified and shown to have a relatively low specific activity with the standard GST substrate 1-chloro-2,4-dinitrobenzene (1.4 +/- 0.2 mumol min-1 mg-1 protein), but an activity equivalent to other Mu class enzymes with other tested substrates. The protein forms functional dimers composed of subunits with a M(r) of approximately 26,400. A detailed comparison of the activity with various substrates and inhibitors was performed between GST M4-4 and other human Mu class GSTs, GST M1a-1a, GST M2-2, and GST M3-3, produced in bacterial expression systems. Despite the high level of amino acid sequence identity, the enzymatic properties of these enzymes were quite different. Comparisons with the crystallographic structure of a homologous rat GST, GST 3-3, indicate that a number of the nonconserved amino acid residues can be assigned to the putative active site of GST M4-4. This suggests that diversification in the evolution of these genes has occurred primarily in the substrate binding regions to cope with an increasing variety of foreign compounds.

Regulation of the Yp subunit of glutathione S-transferase P in rat embryos and yolk sacs during organogenesis

Manipulation of the glutathione status of an embryo during organogenesis leads to abnormal development, as well as increasing the susceptibility of the embryo to insult by either xenobiotic or endogenous electrophiles. The glutathione S-transferases are a family of drug-metabolizing enzymes that catalyze the conjugation of reactive chemicals with glutathione, playing an important role in protecting cells against attack. The purpose of this study was to investigate the presence and regulation of one glutathione S-transferase, glutathione S-transferase P, a homodimer of the Yp subunit, in the conceptus during organogenesis. Northern blot analysis of the RNA isolated from rat embryos and their yolk sacs on days 10, 11 and 12 of gestation revealed a single Yp transcript. Steady-state concentrations of the Yp mRNA in embryos did not change with either gestational age or culture for 24 hr (day 11 in vitro) or 45 hr (day 12 in vitro). In contrast, concentrations of this transcript in yolk sac increased 3-fold from day 10 to 12 of gestation and a further 3-fold with culture (day 12 in vivo compared with in vitro). Transcription of the rat Yp subunit gene in cell lines is induced by treatment with phorbol esters. However, the addition of 12-O-tetradecanoylphorbol-13-acetate (TPA, 50 or 100 nM) to embryos in culture had no effect on the steady-state concentrations of the Yp transcript. Thus, the glutathione S-transferase Yp message is subject to tissue- and development-specific regulation in the conceptus during organogenesis. Moreover, culture of the embryos resulted in a further up-regulation of the steady-state concentrations of the Yp transcript in yolk sac. Western blot analysis demonstrated that a single immunoreactive Yp subunit band of 26 kDa was found in both embryos and yolk sacs. Neither age nor culture appeared to affect the concentrations of immunoreactive Yp subunit in the yolk sac. Thus, glutathione S-transferase Yp mRNA is translated in the conceptus during organogenesis. The apparent differences between the relative amounts of the message and immunoreactive protein in yolk sac suggest that this subunit may be subject to post-transcriptional as well as transcriptional regulation in this tissue. Immunohistochemical analysis of embryos cultured for 45 hr (day 12 in vitro) revealed that the Yp reaction product was localized over the hepatic primordia, mesonephric ducts, otocyst, yolk sac and ectoplacental cone.(ABSTRACT TRUNCATED AT 400 WORDS)

Crystal structure of human class mu glutathione transferase GSTM2-2. Effects of lattice packing on conformational heterogeneity

The structures of three crystal forms of the class mu human glutathione transferase GSTM2-2 have been determined. X-ray phase information was obtained independently from molecular replacement and from anomalous scattering by a single isomorphous derivative. One crystal form contains a single monomer in the asymmetric unit and has been refined to 1.85 A with an overall R factor of 22.6%. The second form contains a single dimer in the asymmetric unit and has been refined to 3.5 A with an R factor of 20.7%. The third form contains two dimers in the asymmetric unit and has been refined to 3.0 A with an R factor of 25.0%. Although all three crystal forms were grown from solutions that contained glutathione-dinitrobenzene, electron density can only be seen for the glutathione portion of the ligand. The first 202 residues in the seven crystallographically independent monomers of GSTM2-2 are essentially identical in structure. However, heterogeneity in the conformation of the side-chain of Tyr115 is observed in the different monomers. The tertiary structure of residues 1-202 is similar to that of the corresponding region in the class mu isoform of glutathione transferase from rat, GST3-3 (Ji et al. (1992), Biochemistry, 31, 10169-10184). However, significant differences in the conformation of the two enzymes have been observed in the region of the active site that binds hydrophobic substrates. These differences include a 2 A shift in the carboxy terminus of a helix, and significant heterogeneity in the conformation of the last 15 residues of the carboxy terminus. The conformation and degree of disorder of the last 15 residues correlates with the extent of protein-protein contacts within the unit cell.

Differential induction of glutathione S-transferase in rat aorta versus liver

Polycyclic aromatic hydrocarbons, cigarette smoke components that induce atherosclerosis in animals, require metabolic biotransformation to electrophilic intermediates to exhibit atherogenic effects. The formation of reactive metabolites depends on both rates of cytochrome P450-catalyzed oxidation and rates of detoxification through conjugation with glutathione. Thus, changes in the activity of glutathione S-transferase in vascular tissue could affect the risk of polycyclic aromatic hydrocarbon-induced atherogenesis. We compared the effects of several exogenous chemicals on levels of glutathione S-transferase in aorta and liver. Male Wistar rats were treated with 3-methylcholanthrene, a polycyclic aromatic hydrocarbon, phenobarbital and butylated hydroxytoluene, an antioxidant known to have anti-atherogenic properties. In control animals, glutathione S-transferase activity was about 20-fold greater in liver than in aorta. Subunit expression was tissue specific. GST-Yp, for example, was the most abundant subunit in aorta but was undetectable in liver. In contrast, GST-Ya was barely detectable in aorta but was abundant in liver. Each of the xenobiotics caused induction of glutathione S-transferase but the extent of induction was greater in liver than in aorta. Phenobarbital, for example, caused 300% induction in liver but only 70% induction in aorta. By western blot analysis, differences in amounts of enzyme subunits corresponded to changes in enzyme activity. Thus, exogenous chemicals differentially regulate levels of glutathione S-transferase in the aorta and liver.

Immunological comparison of cytosolic glutathione S-transferases between rat and two strains of houseflies

Five different antisera, which include three antisera raised against rat liver glutathione S-transferases (GST), one antiserum raised against human pi GST, and one antiserum raised against housefly GST1, were used to examine their cross-reactivity with different classes of GST subunits isolated from rat liver and the housefly. Two classes of rat liver GSTs, alpha and mu, were isolated from rat liver and two classes of housefly GSTs, GST1 and GST2, were isolated from both CSMA and Cornell-R strains. Antiserum against GST 3-3 was the most reactive antiserum and reacted not only with the mu class of GSTs but also with the GST1 class from both CSMA and Cornell-R strains. Antiserum against human pi GST and antiserum against housefly GST1 had weak immunological reactivity toward the GST1 class from both strains of housefly. Antiserum against GST 4-4 and antiserum against GST 1-1 had no immunological reactivity toward any class of GSTs from housefly. None of the five antisera had any immunological cross-reactivity toward subunit 2 of the alpha class of rat GST and the GST2 class of housefly GSTs from both strains.

Reversible immunoprecipitation using histidine- or glutathione S-transferase-tagged staphylococcal protein A

We have constructed, expressed, and purified hexahistidine- and glutathione S-transferase (GST)-tagged Staphylococcal protein A. The histidine-tagged protein A bound efficiently to iminodiacetic acid (IDA)-Sepharose loaded with Zn2+, and the GST-protein A was efficiently retained by glutathione-Sepharose. Both recombinant forms of protein A can be used in the normal way to harvest immune complexes with IgG. Both forms of protein A can be released from the Sepharose matrix by mild procedures. The his6-protein A:antibody:antigen complexes can be released from the matrix with EDTA, and immunoprecipitates bound to GST-protein A can be released either by elution with glutathione or by digestion with thrombin. We tested this method with immunoprecipitates of the p40MO15 protein kinase, and found that they retained their ability to phosphorylate p33cdk2 after elution from the affinity matrices.

Inhibition of human placenta glutathione transferase P1-1 by calvatic acid

The inhibition mechanism of the dimeric human placenta glutathione transferase (GST P1-1) by the antibiotic p-carboxyphenylazoxycyanide (calvatic acid) has been investigated at pH 7.0 and 30.0 degrees C. Experiments performed at different calvatic acid/GST P1-1 molar ratios indicate that one mole of calvatic acid inactivates one mole of the homodimeric enzyme molecule, containing two catalytically equivalent active sites. The apparent second order rate constant for GST P1-1 inactivation is 2.4 +/- 0.3 M-1 s-1. The recovery of all the 5,5'-dithio-bis(2-nitro-benzoic acid)-titratable thiol groups as well as the original catalytic activity of GST P1-1 after treatment of the inhibited enzyme with dithiothreitol indicates that two disulfide bridges per dimer, likely between Cys47 and Cys101, have been formed during the reaction with calvatic acid. To the best of the authors knowledge, calvatic acid represents a unique case of enzyme inhibitor acting also throughout its reaction product(s).

Molecular structure at 1.8 A of mouse liver class pi glutathione S-transferase complexed with S-(p-nitrobenzyl)glutathione and other inhibitors

The three-dimensional crystal structure of pi class glutathione S-transferase YfYf from mouse liver complexed with the inhibitor S-(p-nitrobenzyl)glutathione has been determined at 1.8 A resolution by X-ray diffraction. In addition two complexes with glutathione sulphonic acid and S-hexylglutathione have been determined at resolutions of 1.9 and 2.2 A, respectively. The high resolution of the S-(p-nitrobenzyl)glutathione complex allows a detailed analysis of the active site including the hydrophobic (H-) subsite. The nitrobenzyl moiety occupies a hydrophobic pocket with its aromatic ring sandwiched between Phe8 and the hydroxyl group of Tyr108. An insertion of two residues Gly41 and Leu42, with respect to the pig enzyme, splits helix alpha B into an alpha-helix and a 3(10) helix. Water bridges between carbonyl oxygen atoms of the alpha-helix at its C terminus and the amide NH groups of the 3(10) helix at its N terminus provide structural continuity between these two secondary elements. Tyr7 appears to be the only residue close to the sulphur atom of glutathione, while three conserved water molecules lie in the surrounding area in all complexes. The enzyme mechanism is discussed on the basis of the structural analysis.

A glutathione S-transferase (GST) isozyme from broccoli with significant sequence homology to the mammalian theta-class of GSTs

A novel glutathione S-transferase (GST) was purified from broccoli (Brassica oleracea var. italica). Partial amino-acid sequencing indicated that the protein shared significant homology with several different plant GSTs from maize, silene, Dianthus, Nicotiana and Triticum, but little homology to yeast (Issatchenkia) GST. One region of the polypeptide near the N-terminal also shared significant homology to a region of rat 5-5, rat 12-12 and human theta-GST (collectively referred to as the theta-GST-class) but little structural homology to the common mammalian cytosolic GSTs (alpha-, mu- or pi-classes). The broccoli GST was retained on a novel membrane based glutathione affinity matrix and displayed activity towards 1-chloro-2,4-dinitro-benzene (CDNB), a general GST substrate, as well as 4-nitrophenethyl bromide, a marker substrate for the theta-class of GSTs. The characteristics of the broccoli GST potentially define it as a member of the theta-class. This is consistent with the view that the theta-class may have arisen prior to the divergence of animals and plants while the mammalian mu-, pi- and alpha-classes evolved after the two kingdoms were established.

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.

Src homology 2 domains of protein tyrosine phosphatase are associated in vitro with both the insulin receptor and insulin receptor substrate-1 via different phosphotyrosine motifs

To clarify the role of protein tyrosine phosphatase containing Src homology 2 (SH2) regions on insulin signaling, we investigated the interactions among the insulin receptor, a pair of SH2 domains of SH-PTP2 coupled to glutathione-S-transferase (GST) and insulin receptor substrate-1 (IRS-1)-GST fusion protein (amino-portion, IRS-IN; carboxyl portion, IRS-1C). GST-SH2 protein of SH-PTP2 bound to the wild type insulin receptor, but not to that with a carboxyl-terminal mutation (Y/F2). Furthermore, even though Y/F2 receptors were used, the SH2 protein was also co-immunoprecipitated with IRS-IC, but not with IRS-IN. These results indicate that SH2 domains of SH-PTP2 can directly associate with the Y1322TXM motif on the carboxyl terminus of insulin receptors and also may bind to the carboxyl portion of IRS-1, possibly via the Y1172IDL motif in vitro.

Human glutathione S-transferases

1. Multiple forms of glutathione S-transferase (GST) isoenzymes present in human tissues are dimers of subunits belonging to three distinct gene families namely alpha, mu and pi. Only the subunits within each class hybridize to give active dimers. 2. These subunits are differentially expressed in a tissue-specific manner and the composition of glutathione S-transferases in various tissues differs significantly. 3. Minor GST subunits not belonging to these three classes are also present in some tissues. 4. An ortholog of rat GST 8-8 and mouse mGSTA4-4 is selectively expressed in some human tissues including bladder, brain, heart, liver, and pancreas. This isoenzyme designated as GST 5.8 expresses several fold higher activity towards 4-hydroxy-2,3-trans-nonenal as compared to the routinely used substrate 1-chloro-2,4-dinitrobenzene.

The GPQ-rich segment of Dictyostelium myosin IB contains an actin binding site

Myosin I has been implicated as the motor that drives protrusion of the leading edge of motile cells. This function requires a close association with the plasma membrane and the cytoskeleton. Association with the actin cytoskeleton is mediated by an ATP-dependent binding site in the motor-containing myosin head, as well as by a second, ATP-independent actin binding site. In myosin IC from Acanthamoeba, the ATP-independent actin binding site is located in the carboxy-terminal tail, in a domain composed of two segments. The first segment is basic and is referred to as the GPA-rich segment. The second is a highly conserved sequence called src homology region 3 (SH3), found in a variety of cytoskeletal-associated proteins. We have used bacterially-expressed fusion proteins containing portions of Dictyostelium myosin IB to determine if the tail of this myosin I isoform also binds to actin and to establish precisely where the actin binding site is located. We have determined that the carboxy-terminal portion of the tail of Dictyostelium myosin IB can bind to actin in an ATP-independent manner and that the actin binding site is contained within residues 922-1059, corresponding to the GPA-rich segment of Acanthamoeba myosin IC. We conclude that this region contains a specific actin binding site which may be responsible for the cytoskeletal association of this myosin I isoform.

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.

Functional and structural membrane topology of rat liver microsomal glutathione transferase

The membrane topology of rat liver microsomal glutathione transferase was investigated by comparing the tryptic cleavage products from intact and permeabilized microsomes. It was shown that lysine-4 of microsomal glutathione transferase is accessible at the luminal surface of the endoplasmic reticulum, whereas lysine-41 faces the cytosol. These positions are separated by a hydrophobic stretch of 25 amino acids (positions 11-35) which comprises the likely membrane-spanning region. Reaction of cysteine-49 of the microsomal glutathione transferase with the charged sulfhydryl reagent DTNB (2,2'-dithiobis(5-nitrobenzoic acid)) in intact microsomes further supports the cytosolic localization of this portion of the polypeptide chain. The role of two other potential membrane-spanning/associated segments in the C-terminal half of the polypeptide chain was examined by investigating the association of the protein to the membrane after trypsin cleavage at lysine-41. Activity measurements and Western blot analysis after washing with high concentrations of salt, as well as after phase separation in Triton X-114, indicate that this portion of the protein also binds to the membrane. It is also shown that cleavage of the purified protein at Lys-41 and subsequent separation of the fragments obtained yields a functional C-terminal polypeptide with the expected length for the product encompassing positions 42-154. The location of the active site of microsomal glutathione transferase was investigated using radiolabelled glutathione together with a second substrate. Since isolated rat liver microsomes do not take up glutathione or release the glutathione conjugate into the lumen, it can be concluded that the active site of rat liver microsomal glutathione transferase faces the cytosolic side of the endoplasmic reticulum.

A novel glutathione S-transferase isozyme similar to GST 8-8 of rat and mGSTA4-4 (GST 5.7) of mouse is selectively expressed in human tissues

A mouse glutathione S-transferase (GST) isozyme designated as GST 5.7 or mGSTA4-4 belongs to a distinct subclass of the alpha-class isozymes of GST. It is characterized by kinetic properties intermediate between the alpha- and pi-classes of GSTs. We have recently cloned and expressed this isozyme (rec-mGSTA4-4) in E. coli and have reported its complete primary sequence (Zimniak, P., et al. (1992) FEBS Lett., 313, 173-176). Using antibodies raised against the homogenous rec-mGSTA4-4 expressed in E. coli, we now demonstrate that an ortholog of this isozyme was selectively expressed in various human tissues. The human ortholog of mGST A4-4 purified from liver had a pI value of 5.8 and constituted approx. 1.7% of total GST protein of human liver. Similar to other alpha-class GSTs, the N-terminus of this isozyme (GST 5.8) was also blocked. CNBr digestion of the enzyme yielded two major fragments with M(r) values of 12 kDa and 6 kDa. The sequences of these two fragments showed identities in 16 out of 20 residues and 17 out of 20 residues with the corresponding sequences of its mouse ortholog (mGSTA4-4), and showed significant homologies with the rat and chicken orthologs, GST 8-8 and GST CL3. Human liver GST 5.8 showed more than an order of magnitude higher activity towards t-4-hydroxy-2-nonenal as compared to 1-chloro-2,4-dinitrobenzene. This isozyme also expressed glutathione-peroxidase activity towards fatty acid, as well as phospholipid hydroperoxides suggesting its role in protection mechanisms against the toxicants generated during lipid peroxidation. Western blot analysis of human tissues revealed that this GST isozyme was selectively expressed in human liver, pancreas, heart, brain and bladder tissues, but absent in lung, skeletal muscle, spleen and colon.

Vaccination of sheep against Fasciola hepatica with glutathione S-transferase. Identification and mapping of antibody epitopes on a three-dimensional model of the antigen

The glutathione S-transferases (FhGST) of the liver fluke Fasciola hepatica have been identified as novel vaccine candidates that protect sheep against a fluke infection. With the use of overlapping peptides covering the predicted amino acid sequences of four FhGST cDNAs, we have defined the linear epitopes recognized by polyclonal antibody from sheep vaccinated with FhGST. Dominant and minor epitopes were found to be present on all four of the sequences although some epitopes were shown to be specific to particular FhGST. A high percentage of the FhGST peptides were found to be antigenic although considerable variability in response to the peptides was observed among the animals. This analysis was extended to the IgG1 and IgG2 response at the peptide level. Based on the recently solved crystal structure of the rat mu-class GST 3-3, a three-dimensional model of one of the FhGST sequences was generated that allowed the predicted spatial localization of defined epitopes. Most epitopes were localized on regions of high flexibility and accessibility. A comparison of epitopes on FhGST with the B cell epitopes on Sm28, a 28-kDa GST from Schistosoma mansoni, has found few similarities. There was no correlation between an antibody response to linear peptide epitopes and the level of protection induced in sheep by vaccination with FhGST.

Purification of glutathione S-transferase fusion proteins as a non-degraded form by using a protease-negative E. coli strain, AD202

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Structure and function of the xenobiotic substrate binding site of a glutathione S-transferase as revealed by X-ray crystallographic analysis of product complexes with the diastereomers of 9-(S-glutathionyl)-10-hydroxy-9,10-dihydrophenanthrene

The three-dimensional structures of isoenzyme 3-3 of glutathione (GSH) transferase complexed with (9R,10R)- and (9S,10S)-9-(S-glutathionyl)-10-hydroxy-9,10-dihydrophenanthrene [(9R,10R)-2 and (9S,10S)-2], which are the products of the addition of GSH to phenanthrene 9,10-oxide, have been determined at resolutions of 1.9 and 1.8 A, respectively. The structures indicate that the xenobiotic substrate binding site is a hydrophobic cavity defined by the side chains of Y6, W7, V9, and L12 from domain I (the GSH binding domain) and I111, Y115, F208, and S209 in domain II of the protein. All of these residues are located in variable-sequence regions of the primary structure of class mu isoenzymes. Three of the eight residues (V9, I111, and S209) of isoenzyme 3-3 that are in direct van der Waals contact with the dihydrophenanthrenyl portion of the products are mutated (V9I, I111A, and S209A) in the related isoenzyme 4-4. These three residues are implicated in control of the stereoselectivity of the class mu isoenzymes. The hydroxyl group of Y115 is found to be hydrogen-bonded to the 10-hydroxyl group of (9S,10S)-2, a fact suggesting that this residue could act as an electrophile to stabilize the transition state for the addition of GSH to epoxides. The Y115F mutant isoenzyme 3-3 is about 100-fold less efficient than the native enzyme in catalyzing the addition of GSH to phenanthrene 9,10-oxide and about 50-fold less efficient in the Michael addition of GSH to 4-phenyl-3-buten-2-one. The side chain of Y115 is positioned so as to act as a general-acid catalytic group for two types of reactions that would benefit from electrophilic assistance. The results are consistent with the notion that domain II, which harbors most of the variability in primary structure, plays a crucial role in defining the substrate specificity of class mu isoenzymes.

Purification and biochemical characterization of a recombinant mouse seminal vesicle trypsin inhibitor produced in Escherichia coli

Escherichia coli cells were transformed with an expression vector constructed by inserting a DNA fragment encoding a Kazal-type trypsin inhibitor from mouse seminal vesicle into pGEX-2. The cloned cells were able to produce a high yield of a chimeric polypeptide made by fusing the trypsin inhibitor to glutathione S-transferase. The chimeric polypeptide could be purified through an affinity column of glutathione agarose beads. The purified protein could be digested with thrombin to release the recombinant trypsin inhibitor which could be further purified by HPLC of the thrombin digests on a reverse-phase C4 column. The recombinant trypsin inhibitor was homogeneous and showed trypsin inhibitor activity as strong as that of the naturally occurring trypsin inhibitor.

Photoaffinity labeling of Arabidopsis thaliana plasma membrane vesicles by 5-azido-[7-3H]indole-3-acetic acid: identification of a glutathione S-transferase

We used 5-azido-[7-3H]indole-3-acetic acid (5-azido-[7-3H]IAA), a photoaffinity analogue of the plant hormone indole-3-acetic acid (IAA), to search for auxin-binding proteins in Arabidopsis thaliana membranes. We identified an auxin-binding protein with a molecular mass of 24 kDa (Atpm24) in microsomes as well as in plasma membrane vesicles. Atpm24 was solubilized by 1% Triton X-100 and partially purified. A cDNA clone (Atpm24.1) corresponding to Atpm24 was isolated. The amino acid sequence predicted from the Atpm24.1 cDNA contains 212 amino acid residues with a relative molecular mass of 24,128 Da. Data base searches revealed that the predicted protein has homology to glutathione S-transferases (GSTs; EC 2.5.1.18). When Atpm24.1 was expressed in Escherichia coli, we found a high level of GST activity in the bacterial extracts. We have analyzed the substrate specificity of this protein and found that cumene hydroperoxide and trans-stilbene oxide but not trans-cinnamic acid or IAA-CoA were substrates. A role for this GST in physiological processes of plants is discussed.

Identification of a molecular species in porcine ovarian luteal glutathione S-transferase and its hormonal regulation by pituitary gonadotropins

Glutathione S-transferase (GST) was purified to a homogeneous state from the cytosol fraction of porcine corpora lutea. The present study showed that the final enzyme preparation consisted of a single molecular species of hepatic GSTA1-1, which has been identified from its enzymatic properties, amino acid composition, N-terminal amino acid sequences, and immunological reactivity. Furthermore, it is suggested that ovarian GST is involved in steroidogenesis, especially in the step of progesterone synthesis, from the following results. (1) The final enzyme preparation had a significant delta 5-3-ketosteroid isomerase activity, which acts in steroid synthesis. (2) GST activity in the cultured porcine granulosa cells was remarkably increased in the luteinizing process, which was induced by addition of pituitary gonadotropins such as follicle-stimulating hormone (FSH) and luteinizing hormone (LH) to the culture system. (3) Changes of GST activity induced by FSH and LH were closely associated with progesterone production. (4) GST was detected within luteinizing and theca interna cells, but not in granulosa cells, by immunohistochemical staining of ovary tissues using anti-ovarian luteal-GST antibodies.

Co-variation of glutathione transferase expression and cytostatic drug resistance in HeLa cells: establishment of class Mu glutathione transferase M3-3 as the dominating isoenzyme

Qualitative and quantitative analyses of glutathione, glutathione transferases (GSTs) and other glutathione-linked enzymes in HeLa cells have been made in order to study their significance in cellular resistance to electrophilic cytotoxic agents. The cytosolic concentrations of three GSTs, GST M1-1 (53 +/- 9 ng/mg of cytosolic protein), GST P1-1 (11 +/- 3 ng/mg) and GST A1-1 (1.1 +/- 0.4 ng/mg) were quantified by isoenzyme-specific enzyme-linked immunoassays. Electrophoretic analysis and immunoblotting demonstrated another component, GST M3-3, which was identified by amino acid sequence analysis. GST M3-3 was quantified (1550 +/- 250 ng/mg) by slot-blot immunoanalysis and was the most abundant GST in HeLa cells. An additional cytosolic 13 kDa protein with high affinity for immobilized glutathione or S-hexyglutathione was found to be identical with a macrophage migration-inhibitory factor, previously identified as a lymphokine. Cells grown in roller bottles (HR) rather than in ordinary culture flasks contain a significantly lower concentration of all the GSTs and were found to be more sensitive to the cytostatic agents doxorubicin (2.3-fold), cisplatin (1.7-fold) and melphalan (1.4-fold). The cytosolic concentrations of glutathione reductase and glyoxalase I were also lower in HR cells, whereas the total glutathione concentration was unchanged and the glutathione peroxidase activity was increased. The results indicate that GSTs contribute to the cellular resistance phenotype.