Microsomal glutathione transferase: lipid-derived substrates and lipid dependence

Common structural features of MAPEG -- a widespread superfamily of membrane associated proteins with highly divergent functions in eicosanoid and glutathione metabolism

A novel superfamily designated MAPEG (Membrane Associated Proteins in Eicosanoid and Glutathione metabolism), including members of widespread origin with diversified biological functions is defined according to enzymatic activities, sequence motifs, and structural properties. Two of the members are crucial for leukotriene biosynthesis, and three are cytoprotective exhibiting glutathione S-transferase and peroxidase activities. Expression of the most recently recognized member is strongly induced by p53, and may therefore play a role in apoptosis or cancer development. In spite of the different biological functions, all six proteins demonstrate common structural characteristics typical of membrane proteins. In addition, homologues are identified in plants, fungi, and bacteria, demonstrating this superfamily to be generally occurring.

Leukotriene C4 is a tight-binding inhibitor of microsomal glutathione transferase-1. Effects of leukotriene pathway modifiers

Microsomal glutathione transferase-1 (MGST-1) is an abundant protein that catalyzes the conjugation of electrophilic compounds with glutathione, as well as the reduction of lipid hydroperoxides. Here we report that leukotriene C4 is a potent inhibitor of MGST-1. Leukotriene C4 was found to be a tight-binding inhibitor, with a Ki of 5.4 nM for the unactivated enzyme, and 9.2 nM for the N-ethylmaleimide activated enzyme. This is the first tight-binding inhibitor characterized for this enzyme. Leukotriene C4 was competitive with respect to glutathione and non-competitive toward the second substrate, CDNB. Analysis of stoichiometry supports binding of one molecule of inhibitor per homotrimer. Leukotrienes A4, D4, and E4 were much weaker inhibitors of the purified enzyme (by at least 3 orders of magnitude). Leukotriene C4 analogues, which have been developed as antagonists of leukotriene receptors, were found to display varying degrees of inhibition of MGST-1. In particular, the cysteinyl-leukotriene analogues SKF 104,353, ONO-1078, and BAYu9773 were strong inhibitors (IC50 values: 0.13, 3. 7, and 7.6 microM, respectively). In view of the partial structural similarity between MGST-1, leukotriene C4 synthase, and 5-lipoxygenase activating protein (FLAP), it was of interest that leukotriene C4 synthesis inhibitors (which antagonize FLAP) also displayed significant inhibition (e.g. IC50 for BAYx1005 was 58 microM). In contrast, selective 5-lipoxygenase inhibitors such as zileuton only marginally inhibited activity at high concentrations (500 microM). Our discovery that leukotriene C4 and drugs developed based on its structure are potent inhibitors of MGST-1 raises the possibility that MGST-1 influences the cellular processing of leukotrienes. These findings may also have implications for the effects and side-effects of drugs developed to manipulate leukotrienes.

Role of antioxidant defenses in the tolerance of severe dehydration by anurans. The case of the leopard frog Rana pipiens

Many anurans have excellent dehydration tolerance that allows endurance of the loss of up to 50-60% of total body water. One of the effects of severe dehydration is circulatory impairment due the reduced volume and increased viscosity of blood, which leads to organ hypoxia. The rehydration situation, therefore, involves a reoxygenation of tissues that may include elements of oxidative stress that resemble the injury in post-ischemic reperfusion of mammalian organs. The role of endogenous defenses against oxygen radicals in the tolerance of severe dehydration by leopard frogs, Rana pipiens, was investigated by monitoring the activities of antioxidant enzymes and glutathione levels (reduced GSH and oxidized GSSG) in leg muscle and liver of control, 50%-dehydrated, and fully rehydrated frogs. The maximal activities of muscle catalase and liver glutathione peroxidase, measured per mg soluble protein, increased significantly by 52 and 74%, respectively, after dehydration whereas muscle superoxide dismutase and glutathione reductase activities responded oppositely, decreasing by 32 and 35%, respectively. Enzyme activities returned to control levels after full rehydration. Hepatic GSH and GSSG increased early in the rehydration process (30% recovery of total body water), but returned to control levels after full recovery. A similar trend was observed for liver GSSG. The elevation of antioxidant defenses against peroxides during dehydration could provide protection against post-hypoxic oxyradical stress during rehydration. Indeed, analysis of one product of lipid peroxidation, thiobarbituric acid reactive substances, in frog tissues gave no indication of oxidative stress during the dehydration/rehydration cycle.

Parameters for the two-dimensional crystallization of the membrane protein microsomal glutathione transferase

Various crystallization parameters were investigated to obtain two-dimensional crystals of the detoxification enzyme microsomal glutathione transferase for structural analysis by electron crystallography. The protein was crystallized by reconstitution of the solubilized trimer into proteoliposomes. Crystallization occurs when minimal amounts of lipid in the range of three lipid molecules per protein trimer are added to the dialysate. Once crystals were obtained, the effect of several parameters on the crystallization was determined. The temperature and initial detergent concentration were found to be crucial parameters in influencing the size of the crystals, and conclusions could be drawn about the rate dependence of the crystallization process. Two highly ordered crystal forms, which are suitable for structural analysis by electron crystallography, were obtained under the two-dimensional crystallization conditions described here.

A highly active microsomal glutathione transferase from frog (Xenopus laevis) liver that is not activated by N-ethylmaleimide

Microsomal glutathione transferase has hitherto only been purified from mammalian species. N-ethylmaleimide and trypsin activation (discriminating features of this enzyme) has only been observed in microsomes from mammals. In this paper we describe the first isolation and characterization of a non-mammalian microsomal glutathione transferase from frog (Xenopus laevis) liver. This protein has a molecular weight similar to that of the mammalian enzyme (approximately 17 kDa), but cannot be activated by N-ethylmaleimide or trypsin. In fact the enzyme is rapidly inactivated by this sulfhydryl reagent and protease. It thus appears that N-ethylmaleimide activation is not an obligatory property of microsomal glutathione transferase. The frog liver microsomal glutathione transferase has one of the highest specific activities towards the second substrate 1-chloro-2,4-dinitrobenzene (CDNB) (200 mumol/min mg) obtained with any glutathione transferase and accounts for the high activity found in frog liver microsomes. The kcat/K(m) for glutathione and CDNB are 0.017 and 1.1 x 10(6) M-1 s-1, respectively. The enzyme also functions as a glutathione peroxidase (dilinoleoyl phosphatidylcholine hydroperoxide is reduced (5.2 mumol/min mg)). It is now evident that a highly active microsomal glutathione transferase, with a molecular weight similar to that of the mammalian enzymes also exists in a non-mammal species.

Phospholipid hydroperoxide glutathione peroxidase activity of human glutathione transferases

Human glutathione transferases (GSTs) from Alpha (A), Mu (M) and Theta (T) classes exhibited glutathione peroxidase activity towards phospholipid hydroperoxide. The specific activities are in the order: GST A1-1>GST T1-1>GST M1-1>GST A2-2>GST A4-4. Using a specific and sensitive HPLC method, specific activities towards the phospholipid hydroperoxide,1-palmitoyl-2-(13-hydroper oxy-cis-9, trans-11 -octadecadienoyl)-l-3-phosphatidylcholine (PLPC-OOH) were determined to be in the range of 0.8-20 nmol/min per mg of protein. Two human class Pi (P) enzymes (GST P1-1 with Ile or Val at position 105) displayed no activity towards the phospholipid hydroperoxide. Michaelis-Menten kinetics were followed only for glutathione, whereas there was a linear dependence of rate with PLPC-OOH concentration. Unlike the selenium-dependent phospholipid hydroperoxide glutathione peroxidase (Se-PHGPx), the presence of detergent inhibited the activity of GST A1-1 on PLPC-OOH. Also, in contrast with Se-PHGPx, only glutathione could act as the reducing agent for GST A1-1. A GST A1-1 mutant (Arg15Lys), which retains the positive charge between the GSH- and hydrophobic binding sites, exhibited a decreased kcat for PLPC-OOH but not for CDNB, suggesting that the correct topography of the GSH site is more critical for the phospholipid substrate. A Met208Ala mutation, which gives a modified hydrophobic site, decreased the kcat for CDNB and PLPC-OOH by comparable amounts. These results indicate that Alpha, Mu and Theta class human GSTs provide protection against accumulation of cellular phospholipid hydroperoxides.

Protein S-thiolation and redox regulation of membrane-bound glutathione transferase

Membrane-bound GST transferase (GSTm) occurs in hepatic microsomal and plasma membranes as well as in the outer mitochondrial membrane, and it is known to be activated by N-ethylmaleimide. We recently analysed the activation by GSSG in some detail. The approximately 5-fold stimulation is reversed upon reduction of GSSG by GSSG reductase. In steady-state experiments, the Kox value was determined to be 0.05, i.e. 20 times more GSSG than GSH produces half-maximal activation. Kox is independent of the total glutathione concentration, indicating that S-thiolation by mixed disulfide formation, rather than interchain or intrachain disulfide bridge formation, is responsible for activation. In Western blots, a 17.7 kDa band, in addition to the 17.3 kDa band, was detected upon treatment with GSSG or with GSH plus t-butyl hydroperoxide. We suggest that under oxidative stress, GSTm is activated through direct S-thiolation of the enzyme. Dethiolation occurs via thiol disulfide exchange governed by the cellular glutathione redox state.

Identification and characterization of a novel microsomal enzyme with glutathione-dependent transferase and peroxidase activities

5-Lipoxygenase activating protein (FLAP), leukotriene-C4 (LTC4) synthase, and microsomal glutathione S-transferase II (microsomal GST-II) are all members of a common gene family that may also include microsomal GST-I. The present work describes the identification and characterization of a novel member of this family termed microsomal glutathione S-transferase III (microsomal GST-III). The open reading frame encodes a 16.5-kDa protein with a calculated pI of 10.2. Microsomal GST-III has 36, 27, 22, and 20% amino acid identity to microsomal GST-II, LTC4 synthase, microsomal GST-I, and FLAP, respectively. Microsomal GST-III also has a similar hydrophobicity pattern to FLAP, LTC4 synthase, and microsomal GST-I. Fluorescent in situ hybridization mapped microsomal GST-III to chromosomal localization 1q23. Like microsomal GST-II, microsomal GST-III has a wide tissue distribution (at the mRNA level) and is predominantly expressed in human heart, skeletal muscle, and adrenal cortex, and it is also found in brain, placenta, liver, and kidney tissues. Expression of microsomal GST-III mRNA was also detected in several glandular tissues such as pancreas, thyroid, testis, and ovary. In contrast, microsomal GST-III mRNA expression was very low (if any) in lung, thymus, and peripheral blood leukocytes. Microsomal GST-III protein was expressed in a baculovirus insect cell system, and microsomes from Sf9 cells containing either microsomal GST-II or microsomal GST-III were both found to possess glutathione-dependent peroxidase activity as shown by their ability to reduce 5-HPETE to 5-HETE in the presence of reduced glutathione. The apparent Km of 5-HPETE was determined to be approximately 7 microM for microsomal GST-II and 21 microM for microsomal GST-III. Microsomal GST-III was also found to catalyze the production of LTC4 from LTA4 and reduced glutathione. Based on these catalytic activities it is proposed that this novel membrane protein is a member of the microsomal glutathione S-transferase super family, which also includes microsomal GST-I, LTC4 synthase, FLAP, and microsomal GST-II.

The 3.0 A projection structure of microsomal glutathione transferase as determined by electron crystallography of p 21212 two-dimensional crystals

Two-dimensional crystals of rat microsomal glutathione transferase were grown during dialysis of detergent-solubilized enzyme after addition of a small amount of phospholipid. The crystals had two-sided plane group symmetry p21212 with a calibrated unit cell size of a=91.90 A, b=90.83 A. Electron diffraction patterns were recorded showing significant reflections extending to 3.0 A. A combination of these structure factor amplitudes with phases from high-resolution images following image processing was used to calculate a projection map of the protein. The asymmetric unit of the structure consists of three microsomal glutathione transferase molecules. The local 3-fold axis at the center of the trimer is delineated by six parallel alpha-helices, two from each monomer. The two helices differ significantly in their respective projection structure. The inner helical core of the trimer is partly surrounded by elongated domains with extensions towards the helices and which contain resolved density maxima at a spacing of 4 to 5 A. A well-defined strong peak is localized close to the elongated domain and at a distance of about 9.5 A from two of the inner helices.

Glutathione transferase mimics: micellar catalysis of an enzymic reaction

Substances that mimic the enzyme action of glutathione transferases (which serve in detoxification) are described. These micellar catalysts enhance the reaction rate between thiols and activated halogenated nitroarenes as well as alpha,beta-unsaturated carbonyls. The nucleophilic aromatic substitution reaction is enhanced by the following surfactants in descending order: poly(dimethyldiallylammonium - co - dodecylmethyldiallylammonium) bromide (86/14) >>cetyltrimethylammonium bromide>zwittergent 3-16 (n-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulphonate)>zwittergent+ ++ 3-14 (n-tetradecyl-N,N-dimethyl - 3 - ammonio -1 - propanesulphonate) approximately N,N - dimethyl - laurylamine N-oxide>N,N-dimethyloctylamine N-oxide. The most efficient catalyst studied is a polymeric material that incorporates surfactant properties (n-dodecylmethyldiallylammonium bromide) and opens up possibilities for engineering sequences of reactions on a polymeric support. Michael addition to alpha,beta-unsaturated carbonyls is exemplified by a model substance, trans-4-phenylbut-3-en-2-one, and a toxic compound that is formed during oxidative stress, 4-hydroxy-2-undecenal. The latter compound is conjugated with the highest efficiency of those tested. Micellar catalysts can thus be viewed as simple models for the glutathione transferases highlighting the influence of a positive electrostatic field and a non-specific hydrophobic binding site, pertaining to two catalytic aspects, namely thiolate anion stabilization and solvent shielding.

Increased resistance to oxidative stress in transfected cultured cells overexpressing glutathione S-transferase mGSTA4-4

Peroxidation of unsaturated fatty acids in membrane phospholipids is one of the multiple cytotoxic effects of oxidative stress. Lipid peroxidation is significant because a single initiating event triggers a chain reaction, thus amplifying the initial stimulus. Many oxidative stress-related pathologies have been linked to lipid peroxidation. Mouse glutathione S-transferase mGSTA4-4 exhibits high glutathione conjugating activity with toxic products of lipid peroxidation, e.g., 4-hydroxynon-2-enal. In addition, mGSTA4-4 has glutathione peroxidase activity toward phospholipid hydroperoxides. On the basis of these catalytic properties, we have previously proposed that the enzyme may be physiologically important in alleviating the cytotoxic effects of lipid peroxidation. We have now experimentally confirmed this hypothesis by transfecting HepG2 cells with mGSTA4 cDNA, and demonstrating a protective effect of expressed mGSTA4-4 protein on cells exposed during plating to H2O2, organic hydroperoxides, and phosphatidylcholine hydroperoxide. As compared to cells transfected with insert-free vector, a larger proportion of mGSTA4-transfected cells was able to attach to the culture dish, and continued to divide in the presence of the above compounds. In addition to alleviating the cytotoxic effects of oxidative stress, mGSTA4-4 may interfere with the subtoxic but cytostatic signals generated by a low-level pro-oxidant state.

Role of glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase in the reduction of lysophospholipid hydroperoxides

1-linoleoyl lysophosphatidylcholine hydroperoxide is a substrate of GSH peroxidase (GPx) both purified from bovine erythrocytes and nonpurified from rat liver. The initial reaction rate for bovine erythrocyte GPx with 1-linoleoyl lysophosphatidylcholine hydroperoxide is about 76 and 95% of the reaction rate for hydrogen peroxide and linoleic acid hydroperoxide respectively. For rat liver GPx these initial reaction rates are about 66 and 75%, respectively. The rate constants for the reaction of GPx with 1-linoleoyl lysophosphatidylcholine hydroperoxide were calculated to be approximately 3 x 10(7) M-1s-1 and approximately 2 x 10(6) M-1s-1 for the bovine erythrocyte and the rat liver enzymes, respectively. By using kinetic models of lipid peroxidation we found by simulation that: (1) the main source of lysophospholipid hydroperoxides in vivo is the peroxidation of lysophospholipids, both in mitochondrial inner membranes and in endoplasmic reticulum; (2) a specialized enzyme able to reduce directly lysophospholipid hydroperoxides is important for the reduction of these hydroperoxides, because the detoxification of these species mediated by the action of acyl ester bond cleaving enzymes is not efficient; (3) the reduction through GPx predominates over phospholipid hydroperoxide glutathione peroxidase (PHGPx) in mitochondrial inner membranes and in the cytosolic phase of the endoplasmic reticulum; (4) in the luminal phase of endoplasmic reticulum PHGPx is predominant.

Acetaminophen-derived activation of liver microsomal glutathione S-transferase of rats

Effect of acetaminophen on glutathione (GSH) S-transferase and related drug metabolizing enzymes was studied in vivo. Rats were given acetaminophen (250 mg/kg, i.p.) 24 hr after the treatment with 3-methylcholanthrene (25 mg/kg, i.p.) and killed by decapitation at indicated times. Liver microsomal GSH S-transferase activity was increased to 331%, 193% and 158% of the control level at 3, 6 and 12 hr, respectively, after the administration of acetaminophen, while GSH content in the liver was markedly decreased at 3 and 6 hr. The increase in the transferase activity was not recovered by the treatment with dithiothreitol. Microsomal GSH peroxidase activity was significantly enhanced at 3 hr. Cytosolic GSH S-transferase and aniline hydroxylase in microsomes were gradually decreased with the increase in the time after administration of acetaminophen. Vmax values of both GSH S-transferase and GSH peroxidase activities in microsomes were increased at 3 hr. Two Km values were obtained for the peroxidase in the control, while only one was observed after the acetaminophen treatment. These results indicate that acetaminophen is converted via cytochrome P-450 to the reactive intermediate N-acetyl-p-benzoquinone imine, which binds to microsomal GSH S-transferase, resulting in the activation of the enzyme.

Identification and characterization of a novel human microsomal glutathione S-transferase with leukotriene C4 synthase activity and significant sequence identity to 5-lipoxygenase-activating protein and leukotriene C4 synthase

5-Lipoxygenase-activating protein (FLAP) and leukotriene C4 (LTC4) synthase, two proteins involved in leukotriene biosynthesis, have been demonstrated to be 31% identical at the amino acid level. We have recently identified and characterized a novel member of the FLAP/LTC4 synthase gene family termed microsomal glutathione S-transferase II (microsomal GST-II). The open reading frame encodes a 16.6-kDa protein with a calculated pI of 10.4. Microsomal GST-II has 33% amino acid identity to FLAP, 44% amino acid identity to LTC4 synthase, and 11% amino acid identity to the previously characterized human microsomal GST (microsomal GST-I). Microsomal GST-II also has a similar hydrophobicity pattern to FLAP, LTC4 synthase, and microsomal GST-I. Fluorescent in situ hybridization mapped microsomal GST-II to chromosomal localization 4q28-31. Microsomal GST-II has a wide tissue distribution (at the mRNA level) and was specifically expressed in human liver, spleen, skeletal muscle, heart, adrenals, pancreas, prostate, testis, fetal liver, and fetal spleen. In contrast, microsomal GST-II mRNA expression was very low (when present) in lung, brain, placenta, and bone marrow. This differs from FLAP mRNA, which was detected in lung, various organs of the immune system, and peripheral blood leukocytes, and LTC4 synthase mRNA, which could not be detected in any tissues by Northern blot analysis. Microsomal GST-II and LTC4 synthase were expressed in a baculovirus insect cell system, and microsomes from Sf9 cells containing microsomal GST-II or LTC4 synthase were both found to catalyze the production of LTC4 from LTA4 and reduced glutathione. Microsomal GST-II also catalyzed the formation of another product, displaying a conjugated triene UV absorption spectra with a maximum at 283 nm, suggesting less catalytic stereospecificity compared with LTC4 synthase. Also, the apparent Km for LTA4 was higher for microsomal GST-II (41 microM) than LTC4 synthase (7 microM). In addition, unlike LTC4 synthase, microsomal GST-II was able to catalyze the conjugation of 1-chloro-2, 4-dinitrobenzene with reduced glutathione. Therefore, it is proposed that this novel membrane protein is a member of the microsomal glutathione S-transferase family, also including LTC4 synthase, with significant sequence identities to both LTC4 synthase and FLAP.

Lipid peroxidation in mitochondrial inner membranes. I. An integrative kinetic model

An integrative mathematical model was developed to obtain an overall picture of lipid hydroperoxide metabolism in the mitochondrial inner membrane and surrounding matrix environment. The model explicitly considers an aqueous and a membrane phase, integrates a wide set of experimental data, and unsupported assumptions were minimized. The following biochemical processes were considered: the classic reactional scheme of lipid peroxidation; antioxidant and pro-oxidant effects of vitamin E; pro-oxidant effects of iron; action of phospholipase A2, glutathione-dependent peroxidases, glutathione reductase and superoxide dismutase; production of superoxide radicals by the mitochondrial respiratory chain; oxidative damage to proteins and DNA. Steady-state fluxes and concentrations as well as half-lives and mean displacements for the main metabolites were calculated. A picture of lipid hydroperoxide physiological metabolism in mitochondria in vivo showing the main pathways is presented. The main results are: (a) perhydroxyl radical is the main initiation agent of lipid peroxidation (with a flux of 10(-7)MS-1); (b) vitamin E efficiently inhibits lipid peroxidation keeping the amplification (kinetic chain length) of lipid peroxidation at low values (approximately equal to 10); (c) only a very minor fraction of lipid hydroperoxides escapes reduction via glutathione-dependent peroxidases; (d) oxidized glutathione is produced mainly from the reduction of hydrogen peroxide and not from the reduction of lipid hydroperoxides.