Binding of nonsubstrate ligands to the glutathione S-transferases.

Fluorescence spectroscopy and inhibition kinetics were used to quantitate the affinity of nonsubstrate ligands for the rat liver glutathione S-transferases AA, A, B, and C in the presence of glutahione. The dissociation constants KD, for ligands such as bilirubin, indocyanine green, and hematin were determined by measuring the decrease in the intrinsic fluorescence of the proteins attendant on the addition of ligand. A second technique, used for compounds which absorb strongly at the excitation maxima of tryptophan, was to utilize 8-anilinonaphthalen sulfonate in the formation of protein complex fluorescing at a higher wavelength. The quenching of this complex allowed the determination of the dissociation constants for ligands such as 3,6-dibromosulfophthalein and cephalothin. These data indicate that all four proteins bind these ligands but do so with different affinities. The bilirubin-induced decrease in fluorescence was used to estimate the stoichiometry of binding as 1.2 mol of bilirubin bound/mol of transferase B and 0.7 mol/mol of transferase C. All of the ligands examine are inhibitors of catalytic activity, as tested in a standard assay with GSH and 1-chloro-2,4-dinitrobenzene as substrates. From these studies we conclude that these proteins have a broad specificity not only for their substrates, but for the binding of nonsubstrate ligands as well.     

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

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

Single amino acid changes outside the active site significantly affect activity of glutathione S-transferases

Glutathione S-transferases (GSTs: E.C. 2.5.1.18) are a multigene family of multifunctional dimeric proteins that play a central role in detoxication. Four allelic forms of the mosquito Anopheles dirus GST, adGST1-1, were cloned, expressed and characterized. The one or two amino acid changes in each allelic form was shown to confer different kinetic properties. Based on an available crystal structure, several of the residue changes were not in the putative substrate-binding pocket. Modeling showed that these insect Delta class GSTs also possess a hydrophobic surface pocket reported for Alpha, Mu and Pi class GSTs. The atom movement after replacement and minimization showed an average atom movement of about 0.1 A for the 0 to 25 A distance from the alpha carbon of the single replaced residue. This does not appear to be a significant movement in a static modeled protein structure. However, 200-500 atoms were involved with movements greater than 0.2 A. Dynamics simulations were performed to study the effects this phenomenon would exert on the accessible conformations. The data show that residues affecting nearby responsive regions of tertiary structure can modulate enzyme specificities, possibly through regulating attainable configurations of the protein.     

Indomethacin inhibition of glutathione S-transferases

Indomethacin inhibited rat liver glutathione S-transferases (EC 2.5.1.18). Its inhibition was non-competitive with respect to 3,4-dichloronitrobenzene with an apparent Ki of 5.3 X 10(-5) M and uncompetitive with respect to glutathione with an apparent Ki of 4.0 X 10(-5) M. 4-Chlorobenzoic acid and 5-methoxy-2-methylindole-3-acetic acid, two metabolites of indomethacin, were weak inhibitors of the enzymes. On the other hand, meclofenamic acid was a competitive inhibitor of the enzymes with an apparent Ki of 3.0 X 10(-4) M. Possible significance of these findings in arachidonic acid metabolism is discussed.     

Interaction of the mycotoxin penicillic acid with glutathione and rat liver glutathione S-transferases

The in vitro interaction of the mycotoxin penicillic acid (PA) with rat liver glutathione S-transferase (GST) was studied using reduced glutathione and 1-chloro-2,4-dinitrobenzene as substrates. The inhibition of the GST activity by PA in crude extracts was dose dependent. Each of the different GST isoenzymes was inhibited, albeit at different degrees. Kinetic studies never revealed competitive inhibition kinetics. The conjugation of PA with GSH occurred spontaneously; it was not enzymatically catalyzed by GST, indicating that an epoxide intermediate is not involved in conjugation. The direct binding of PA to GST provides an additional detoxication mechanism.     

Covalent labeling of opioid receptors with radioiodinated human beta-endorphin. Identification of binding site subunit

125I-beta-Endorphin (human) binds with high affinity, specificity, and saturability to rat brain and neuroblastoma X glioma hybrid cell (NG 108-15) membranes. Dissociation constants and binding capacities were obtained from Scatchard plots and are 2 nM and 0.62 pmol/mg of protein for rat whole brain and 6 nM and 0.8 pmol/mg of protein for NG 108-15 cells. Results from competition experiments also indicate that this ligand interacts with high affinity with both mu and delta opioid binding sites, with a slight preference for mu sites, while exhibiting low affinity at kappa sites. We have demonstrated that human 125I-beta-endorphin is a useful probe for the investigation of the subunit structure of opioid receptors. The specific cross-linking of this ligand has revealed the presence of four reproducible bands or areas after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography at 65, 53, 38, and 25 kDa. All labeled bands seem to be opioid receptor related since they are eliminated when binding is carried out in an excess of various opiates. The evidence we have obtained using rat whole brain (delta congruent to mu), rat thalamus (largely mu), bovine frontal cortex (delta:mu congruent to 2:1), and NG 108-15 cells (delta) demonstrates that different labeling patterns are obtained when mu and delta binding sites are cross-linked. The pattern obtained on sodium dodecyl sulfate-polyacrylamide gel electrophoresis from cross-linked mu sites contains a major (heavily labeled) component of 65 kDa and a minor component of 38 kDa, while patterns from delta sites contain a major labeled component of 53 kDa. This 53-kDa band appears clearly in extracts from NG 108-15 cells and bovine frontal cortex, while in rat whole brain a diffusely labeled region is present between 55 and 41 kDa. In addition, NG 108-15 cells also display a minor labeled component at 25 kDa. The relationship of the minor bands to the major bands is not clear.     

Tissue-specific expression of the rat glutathione S-transferases

Tissue-specific patterns of rat glutathione S-transferase expression have been demonstrated by in vitro translation of purified poly(A) RNAs and by protein purification. Poly(A) RNAs from six rat tissues including heart, kidney, liver, lung, spleen, and testis were used to program in vitro translation with the rabbit reticulocyte lysate system and [35S]methionine. The glutathione S-transferase subunits synthesized in vitro were purified from the translation products by affinity chromatography on S-hexylglutathione-linked Sepharose 6B columns. The affinity bound fractions were analyzed by Na dodecyl SO4-polyacrylamide gel electrophoresis and fluorography. A subunit of Mr = 22,000 detected in the in vitro translation products of poly(A) RNAs from heart, kidney, lung, spleen, and testis is missing from the translation products of liver poly(A) RNAs. This Mr = 22,000 subunit is present only in the anionic glutathione S-transferase fraction purified from rat heart, kidney, lung, spleen, and testis. Purified anionic glutathione S-transferase from rat liver does not contain this subunit. The relative specific activities toward a dozen different substrates also demonstrate the nonidentity between liver and kidney anionic glutathione S-transferases. In addition, among the glutathione S-transferase subunits expressed in the liver, some of them could not be detected in the other tissues investigated. Our results indicate that tissue-specific expression of rat glutathione S-transferases may occur pretranslationally.     

Cytosolic glutathione S-transferases of Oesophagostomum dentatum

Oesophagostomum dentatum stages were investigated for glutathione S-transferase (GST) expression at the protein and mRNA levels. GST activity was detected in all stages (infectious and parasitic stages including third- and fourth-stage larvae of different ages as well as males and females) and could be dose-dependently inhibited with sulfobromophthalein (SBP). Addition of SBP to in vitro larval cultures reversibly inhibited development from third- to fourth-stage larvae. Two glutathione-affinity purified proteins (23 and 25 kDa) were detected in lysates of exsheathed third-stage larvae by SDS-PAGE. PCR-primers were designed based on peptide sequences and conserved GST sequences of other nematodes for complete cDNA sequences (621 and 624 nt) of 2 isoforms, Od-GST1 and Od-GST2, with 72% nucleotide similarity and 75% for the deduced proteins. Genomic sequences consisted of 7 exons and 6 introns spanning 1296 bp for Od-GST1 and 1579 and 1606 bp for Od-GST2. Quantitative real-time-PCR revealed considerably elevated levels of Od-GST1 in the early parasitic stages and slightly reduced levels of Od-GST2 in male worms. Both Od-GSTs were most similar to GST of Ancylostoma caninum (nucleotides: 73 and 70%; amino acids: 80 and 73%). The first three exons (75 amino acids) corresponded to a synthetic prostaglandin D2 synthase (53% similarity). O. dentatum GSTs might be involved in intrinsic metabolic pathways which could play a role both in nematode physiology and in host-parasite interactions.     

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.     

Covalent labeling of immune cells

Inflammation is a common, fast, and innate response of the immune system to sterile or infectious tissue damage or autoimmune triggers. It aims at minimizing tissue destruction and maintaining organ function, hence is vital to life. Therefore, the immune system comprises the concerted action of a variety of different immune cells with specific tasks in the initiation, maintenance, and termination of inflammation. Visualizing their localization, trafficking, and interaction is of utmost importance to unravel the dynamics of inflammation in the living organism and requires tools for cell-specific labeling and imaging. Many concepts for covalent cell-type or protein-specific labeling have been developed, but only few have been implemented for labeling immune cells. Here, we review approaches that were already successful for fluorescent reporters and radioactive nuclides. We also provide a glimpse on emerging technologies that bear potential for immune cell labeling and imaging in vivo.     

The glutathione-binding site in glutathione S-transferases. Investigation of the cysteinyl, glycyl and gamma-glutamyl domains.

In hamster heart, the majority of the phosphatidylcholine is synthesized via the CDP-choline pathway, and the rate-limiting step of this pathway is catalysed by CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15). We have shown previously [Choy (1982) J. Biol. Chem. 257, 10928-10933] that, in the myopathic heart, the level of cardiac CTP was diminished during the development of the disease. In order to maintain the level of CDP-choline, and consequently the rate of phosphatidylcholine biosynthesis, cardiac cytidylyltransferase activity was increased. However, it was not clear if the same compensatory mechanism would occur when the cardiac CTP level was decreased rapidly. In this study, hypoxia of the hamster heart was produced by perfusion with buffer saturated with 95% N2. The heart was pulse-labelled with radioactive choline and then chased with non-radioactive choline for various periods under hypoxic conditions. There was a severe decrease in ATP and CTP levels within 60 min of hypoxic perfusion, with a corresponding fall in the rate of phosphatidylcholine biosynthesis. Analysis of the choline-containing metabolites revealed that the lowered ATP level did not affect the phosphorylation of choline to phosphocholine, but the lower CTP level resulted in the decreased conversion of phosphocholine to CDP-choline. Determination of enzyme activities revealed that hypoxic treatment resulted in the enhanced translocation of cytidylyltransferase from the cytosolic to the microsomal form. This enhanced translocation was probably caused by the accumulation of fatty acids in the heart during hypoxia. We postulate that the enhancement of translocation of the cytidylyltransferase to the microsomal form (a more active form) is a mechanism by which the heart can compensate for the decrease in CTP level during hypoxia in order to maintain phosphatidylcholine biosynthesis.     

Capturing of the free cysteine residue in the ligand-binding site by affinity labeling of the ORL1 nociceptin receptor

All of the δ, μ, and κ opioid receptors have a free thiol group of the Cys residue in the ligand-binding site, although its functional role is not yet known. In order to examine whether or not a similar Cys is also present in the ORL1 nociceptin receptor, we attempted to identify it by affinity labeling using a specific antagonist peptide. We first treated ORL1-expressing COS-7 cell membrane preparations with the thiol-alkylation reagent N-ethylmaleimide (NEM) to perform a binding assay using [³H]nociceptin as a tracer and nociceptin, an ORL1 agonist, or Ac-Arg-Tyr-Tyr-Arg-Ile-Lys-NH₂, a nociceptin/ORL1 antagonist, as a competitor. It was suggested that ORL1 has a free Cys in its ligand-binding site, since the NEM treatment reduced the population of ligand-binding sites. This was further confirmed by affinity labeling using Cys(Npys)-Arg-Tyr-Tyr-Arg-Ile-Lys-NH₂ with the SNpys group that can react with a free thiol group, resulting in the formation of a disulfide bond. This affinity labeling was approximately 23 times more specific than NEM alkylation. The results revealed that the ORL1 nociceptin receptor does contain a free Cys residue in the ligand-binding site.     

Role of yeast glutaredoxins as glutathione S-transferases

The yeast Saccharomyces cerevisiae contains two glutaredoxins, encoded by GRX1 and GRX2, that are required for resistance to reactive oxygen species. We recently reported that Grx1 is active as a glutathione peroxidase and can directly reduce hydroperoxides (Collinson, E. J., Wheeler, G. L., Garrido, E. O., Avery, A. M., Avery, S. V., and Grant, C. M. (2002) J. Biol. Chem. 277, 16712-16717). We now show that Grx2 is also a general hydroperoxidase, and kinetic data indicate that both enzymes have a similar pattern of activity, which is highest with hydrogen peroxide, followed by cumene hydroperoxide and tert-butyl hydroperoxide. Furthermore, both Grx1 and Grx2 are shown be active as glutathione S-transferases (GSTs), and their activity with model substrates such as 1-chloro-2,4-dinitrobenzene is similar to their activity with hydroperoxides. Analysis of the Grx1 active site residues shows that Cys-27, but not Cys-30, is required for both the peroxidase and transferase activities, indicating that these reactions proceed via a monothiol mechanism. Deletion analysis shows that Grx1 and Grx2 have an overlapping function with yeast GSTs, encoded by GTT1 and GTT2, and are responsible for the majority of cellular GST activity. In addition, multiple mutants lacking GRX1, GRX2, GTT1, and GTT2 show increased sensitivity to stress conditions, including exposure to xenobiotics, heat, and oxidants. In summary, glutaredoxins are multifunctional enzymes with oxidoreductase, peroxidase, and GST activity, and are therefore ideally suited to detoxify the wide range of xenobiotics and oxidants that can be generated during diverse stress conditions.     

Subunit composition of rat liver glutathione S-transferases

The plasmid pGTR112 contains partial coding sequences for one of the rat liver glutathione S-transferase subunits. We have used immobilized pGTR112 DNA to select for complementary and homologous liver poly(A)-RNAs under conditions of increasing stringency for hybridization. Each fraction of selected poly(A)-RNAs was assayed by in vitro translation followed by immunoprecipitation. A total of four distinct polypeptides precipitated by antiserum against rat liver glutathione S-transferases were resolved by NaDodSO₄ polyacrylamide gel electrophoresis. They are separated into two pairs according to the sequence homology of their poly(A)-RNAs with the pGTR112 DNA. Purified rat liver glutathione S-transferases can be resolved on gradient NaDodSO₄ polyacrylamide gels into four polypeptides. There should be ten isozymes of different binary combinations from four distinct subunits for the rat liver glutathione S-transferases.     

Acidic glutathione S-transferases of rat testis.

In most organs of the rat the predominant forms of glutathione S-transferase have alkaline (greater than 7.0) pI values. In contrast, in the cytosol from rat testes almost 50% of the transferase activity is due to isoenzymes with acidic (less than 7.0) pI values. We have purified three acidic forms of glutathione S-transferase from rat testis cytosol. One form accounted for more than 90% of the enzymic activity in the acidic fraction. This major form was a homodimer of a new subunit, termed Yt. This subunit had an electrophoretic mobility that was different from the subunits that form the alkaline transferases. In addition, functional and immunological studies were consistent with the unique nature of the Yt subunit. The two minor acidic enzymes of rat testis appeared to be heterodimers of the Yt subunit and a subunit with an electrophoretic mobility identical with that of the Yb subunit present in some alkaline enzymes.     

Expression of glutathione S-transferases in rat brains

The tissue-specific expression of glutathione S-transferases (GSTs) in rat brains has been studied by protein purification, in vitro translation of brain poly(A) RNAs, and RNA blot hybridization with cDNA clones of the Ya, Yb, and Yc subunit of rat liver GSTs. Four classes of GST subunits are expressed in rat brains at Mr 28,000 (Yc), Mr 27,000 (Yb), Mr 26,300, and Mr 25,000. The Mr 26,3000 species, or Y beta, has an electrophoretic mobility between that of Ya and Yb, similar to the liver Yn subunit(s) reported by Hayes (Hayes, J. D. (1984) Biochem. J. 224, 839-852). RNA blot hybridization of brain poly(A) RNAs with a liver Yb cDNA probe revealed two RNA species of approximately 1300 and approximately 1100 nucleotides. The band at approximately 1300 nucleotides was absent in liver poly(A) RNAs. The Mr 25,000 species, or Y delta, can be immunoprecipitated by antisera against rat heart and rat testis GSTs, but not by antiserum against rat liver GSTs. Therefore, the Y delta subunit may be related to the "Mr 22,000" subunit reported by Tu et al. (Tu, C.-P.D., Weiss, M.J., Li, N., and Reddy, C. C. (1983) J. Biol. Chem. 258, 4659-4662). The abundant liver GST subunits, Ya, are not expressed in rat brains as demonstrated by electrophoresis of purified brain GSTs and a lack of isomerase activity toward the Ya-specific substrate, delta 5-androstene-3,17-dione. This is apparently because of the absence of Ya mRNA expression prior to RNA processing. The data on the preferential expression of Yc subunits in rat brains, together with the differential phenobarbital inducibility of the Ya subunit(s) in rat liver reported by Pickett et al. (Pickett, C. B., Donohue, A. M., Lu, A. Y. H., and Hales, B. F. (1982) Arch. Biochem. Biophys. 215, 539-543), suggest that the Ya and Yc genes for rat GSTs are two functionally distinct gene families even though they share 68% DNA sequence homology. The expression of multiple GSTs in rat brains suggests that GSTs may be involved in physiological processes other than xenobiotics metabolism.     

Inhibition of purified glutathione S-transferases by indomethacin

Soluble rat liver glutathione S-transferases have been purified and a previously undescribed peak was observed. This peak contained glutathione S-transferase activity which was extensively inhibited by indomethacin. Glutathione conjugation of 1-chloro-2,4-dinitrobenzene by this isozyme, designated glutathione S-transferase VII, was inhibited 44 and 68% at indomethacin concentrations of 0.20 and 1.00 microM, respectively. The other six basic glutathione S-transferase isozymes were relatively unaffected by low concentrations of indomethacin. The pharmacological significance of this inhibition by indomethacin is largely dependent on the role of the glutathione S-transferase VII in leukotriene synthesis.     

Characterization of two Arabidopsis thaliana glutathione S-transferases

Glutathione S-transferases (GST) are multifunctional proteins encoded by a large gene family, divided on the basis of sequence identity into phi, tau, theta, zeta and lambda classes. The phi and tau classes are present only in plants. GSTs appear to be ubiquitous in plants and are involved in herbicide detoxification and stress response, but little is known about the precise role of GSTs in normal plant physiology and during biotic and abiotic stress response. Two cDNAs representing the two plant classes tau and phi, AtGSTF9 and AtGSTU26, were expressed in vitro and the corresponding proteins were analysed. Both GSTs were able to catalyse a glutathione conjugation to 1-chloro-2,4-dinitrobenzene (CDNB), but they were inactive as transferases towards p-nitrobenzylchloride (pNBC). AtGSTF9 showed activity towards benzyl isothiocyanate (BITC) and an activity as glutathione peroxidase with cumene hydroperoxide (CumHPO). AtGSTU26 was not active as glutathione peroxidase and towards BITC. RT-PCR analysis was used to evaluate the expression of the two genes in response to treatment with herbicides and safeners, chemicals, low and high temperature. Our results reveal that AtGSTU26 is induced by the chloroacetanilide herbicides alachlor and metolachlor and the safener benoxacor, and after exposure to low temperatures. In contrast, AtGSTF9 seems not to be influenced by the treatments employed.