Evidence that rat liver microsomal glutathione transferase is responsible for glutathione-dependent protection against lipid peroxidation

Structural organization of the murine microsomal glutathione S-transferase gene (MGST1) from the 129/SvJ strain: identification of the promoter region and a comprehensive examination of tissue expression

The structure and regulation of the murine microsomal glutathione transferase gene (MGST1) from the 129/SvJ strain is described and demonstrates considerable difference in nucleotide sequence and consequently in restriction enzyme sites as compared to other mouse strains. A comparison of the amino acid sequence for MGST1 revealed one difference in exon 2 between the 129/SvJ strain (arginine at position 5) and the sequence previously reported for the Balb/c strain (lysine). The promoter region immediately upstream of the dominant first exon is functional, transcriptionally responds to oxidative stress, and is highly homologous to the human region. Oxidative stress also induced the production of endogenous MGST1 mRNA. The tissue-specific expression of MGST1 mRNA was studied, and as anticipated, was indeed highest in liver. There was, however, marked mRNA expression in several tissues not previously studied including smooth muscle, epidymus, ovaries, and endocrine glands in which the expression of various peroxidases is also very high (salivary and thyroid). Overall, there was a good agreement between the mRNA content detected and previous reports of MGST1 activity with the exception of brain tissue.

Reactive intermediates and the dynamics of glutathione transferases

Reactive intermediates are a continuous burden in biology and several defense mechanisms have evolved. Here we focus on the functions of glutathione transferases (GSTs) with the aim to discuss the quantitative aspects of defense against reactive intermediates. Humans excrete approximately 0.1 mmol of thioether conjugates per day. As the amount of GST active sites in liver is approximately 0.5 mmol, it appears that glutathione transferase catalysts are present in tremendous excess. In fact, the known catalytic properties of GSTs reveal that the enzymes can empty the liver glutathione (GSH) pool in a matter of seconds when provided with a suitable substrate. However, based on the urinary output of conjugates (or derivatives thereof), individual GSTs turn over (i.e., catalyze a single reaction) only once every few days. Glutathione transferase overcapacity reflects the fact that there is a linear relation between GST enzyme amount and protection level (provided that GSH is not depleted). Put in a different perspective, a few reactive molecules will always escape conjugation and reach cellular targets. It is therefore not surprising that signaling systems sensing reactive intermediates have evolved resulting in the increase of GSH and GST levels. Precisely for this reason, more moderately reactive electrophiles (Michael acceptors) are receiving growing interest due to their anticarcinogenic properties. Another putative regulatory mechanism involves direct activation of microsomal GST1 by thiol-reactive electrophiles through cysteine 49. The toxicological significance of low levels of reactive intermediates are of interest also in drug development, and here we discuss the use of microsomal GST1 activation as a surrogate detection marker.

The ability of mineral dusts and fibres to initiate lipid peroxidation. Part II: relationship to different particle-induced pathological effects

Exposure to pathogenic mineral dusts and fibres is associated with pulmonary changes including fibrosis and cancer. Investigations into aetiological mechanisms of these diseases have identified modifications in specific macromolecules as well as changes in certain early processes, which have preceded fibrosis and cancer. Peroxidation of lipids is one such modification, which is observed following exposure to mineral dusts and fibres. Their ability to initiate lipid peroxidation and the parameters that determine this ability have recently been reviewed. Part II of this review examines the relationship between the capacity of mineral dusts and fibres to initiate lipid peroxidation and a number of pathological changes they produce. The oxidative modification of polyunsaturated fatty acids is a major contributor to membrane damage in cells and has been implicated in a great variety of pathological processes. In most pathological conditions where an induction of lipid peroxidation is observed it is assumed to be the consequence of disease, without further establishing if the induction of lipid peroxidation may have preceded or accompanied the disease. In the great majority of instances, however, despite the difficulty in proving this association, a causal relationship between lipid peroxidation and disease cannot be ruled out.

Cypermethrin-induced oxidative stress in rat brain and liver is prevented by vitamin E or allopurinol

Considering that the involvement of reactive oxygen species (ROS) has been implicated in the toxicity of various pesticides, this study was designed to investigate the possibility of oxidative stress induction by cypermethrin, a Type II pyrethroid. Either single (170 mg/kg) or repeated (75 mg/kg per day for 5 days) oral administration of cypermethrin was found to produce significant oxidative stress in cerebral and hepatic tissues of rats, as was evident by the elevation of the level of thiobarbituric acid reactive substances (TBARS) in both tissues, either 4 or 24 h after treatment. Much higher changes were observed in liver, increasing from a level of 60% at 4 h up to nearly 4 times the control at 24 h for single dose. Reduced levels (up to 20%) of total glutathione (total GSH), and elevation of conjugated dienes ( approximately 60% in liver by single dose at 4 h) also indicated the presence of an oxidative insult. Glutathione-S-transferase (GST) activity, however, did not differ from control values for any dose or at any time point in cerebral and hepatic tissues. Pretreatment of rats with allopurinol (100 mg/kg, ip) or Vitamin E (100 mg/kg per day, ig, for 3 days and a dose of 40 mg/kg on the 4th day) provided significant protection against the elevation of TBARS levels in cerebral and hepatic tissues, induced by single high dose of oral cypermethrin administration within 4 h. Thus, the results suggest that cypermethrin exposure of rats results in free radical-mediated tissue damage, as indicated by elevated cerebral and hepatic lipid peroxidation, which was prevented by allopurinol and Vitamin E.

Xenobiotic metabolism, oxidant stress and chronic pancreatitis. Focus on glutathione

Chronic pancreatitis, although relatively rare in the Western World, is common in certain tropical zones where staple crops such as cassava are rich in cyanogenic glycosides. This paper reviews the evidence for a cyanide connection, with reference to experimental studies using another plant nitrile, crambene; and then examines the hypothesis that chronic pancreatitis represents a manifestation of uncoordinated detoxification reactions between pancreatic cytochrome P450 mono-oxygenases and phase II conjugating enzymes, resulting in the irreversible consumption of glutathione in the acinar cell. The conclusion is that the central role of disrupted pancreatic glutathione status, as a result of 'xenobiotic stress', in the evolution of chronic pancreatitis cannot be overestimated. This position contrasts with that in acute pancreatitis, in which glutathione depletion has a pivotal role too, but occurs as a result of 'stress' from reactive oxygen species.

Structural organization of the microsomal glutathione S-transferase gene (MGST1) on chromosome 12p13.1-13.2. Identification of the correct promoter region and demonstration of transcriptional regulation in response to oxidative stress

The structure and regulation of the microsomal glutathione S-transferase gene (MGST1) are considerably more complex than originally perceived to be. The MGST1 gene has two alternative first exons and is located in the 12p13.1-13.2 region. Two other potential first exons were determined to be nonfunctional. The region between the functional first exons cannot direct transcription. Thus, one common promoter element directing transcription exists, and RNA splicing occurs such that only one of the first exons (containing only untranslated mRNA) is incorporated into each mRNA species with common downstream exons. MGST1 expression and regulation are therefore similar to those of other hepatic xenobiotic handling enzymes, which also produce mRNA species differing only in the 5'-untranslated regions to yield identical proteins. MGST1 was previously considered a "housekeeping" gene, as non-oxidant inducers had little effect on activity. However, the promoter region immediately upstream of the dominant first exon transcriptionally responds to oxidative stress. In this respect, MGST1 is similar to glutathione peroxidases that also transcriptionally respond to oxidative stress. The discovery that MGST1 utilizes alternative first exon splicing eliminates a problem with the first description of MGST1 cDNA in that it appeared that MGST1 expression was in violation of the ribosomal scanning model. The identification that the first exon originally noted is in fact a minor alternative first exon far downstream of the primary first exon eliminates this conundrum.

Dimerumic acid as an antioxidant of the mold, Monascus anka

We previously reported that the mold Monascus anka, traditionally used for fermentation of food, showed antioxidant and hepatoprotective actions against chemically induced liver injuries. In the present study, the antioxidant component of M. anka was isolated and identified. The antioxidant was elucidated to be dimerumic acid. DPPH (1,1-diphenyl-2-picrylhydrazyl) radical was significantly scavenged by the antioxidant whereas hydroxyl radical and superoxide anion were moderately scavenged. When the antioxidant (12 mg/kg) was given to mice prior to carbon tetrachloride (CCl(4), 20 microl/kg, ip) treatment, the CCl(4)-induced liver toxicity in mice seen in an elevation of serum aspartate aminotransferase and alanine aminotransferase activities was depressed, suggesting the hepatoprotective action of the antioxidant. The liver microsomal glutathione S-transferase activity, which is known to be activated by oxidative stress or active metabolites, was increased by CCl(4) treatment and the increase was also depressed by pretreatment with the mold antioxidant. Thus these data confirmed that the dimerumic acid isolated from M. anka is the potential antioxidant and protective against CCl(4)-induced liver injury.

Membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG). A widespread protein superfamily

The members of the MAPEG superfamily have been aligned and found to be distantly related, with a common pattern of hydropathy. Figure 2A shows the multiple sequence alignments of the human members and Figure 2B the corresponding superimposed hydropathy profiles. The alignment in Figure 2A demonstrates a total of six strictly conserved residues. The Arg-51 in LTC4 synthase has been suggested to function as proton donor for the opening of the LTA4 epoxide. This arginine is found in all but the FLAP sequences in accordance with the observation that FLAP has no known enzyme activity. Also the Tyr-93 in LTC4 synthase has been suggested to function as a base for the formation of the thiolate anion of glutathione. This tyrosine is not conserved in MGST1 or MGST1-L1. Table 1 summarizes some other properties of the individual human proteins. They are all of the same size, ranging from 147 to 161 amino acids. Only FLAP differs in that its isoelectric point is more neutral than that of the other, more basic proteins. The genes encoding these proteins all reside on different chromosomes (when known) (Table 1). In addition to the human proteins, MAPEG members have been identified in plants, fungi, and bacteria. It is clearly a challenge to elucidate their role in these different phyla in relation to their defined physiological functions in humans.

Glutathione and glutathione-dependent enzymes represent a co-ordinately regulated defence against oxidative stress

Increases in the intracellular levels of reactive oxygen species (ROS), frequently referred to as oxidative stress, represents a potentially toxic insult which if not counteracted will lead to membrane dysfunction, DNA damage and inactivation of proteins. Chronic oxidative stress has numerous pathological consequences including cancer, arthritis and neurodegenerative disease. Glutathione-associated metabolism is a major mechanism for cellular protection against agents which generate oxidative stress. It is becoming increasingly apparent that the glutathione tripeptide is central to a complex multifaceted detoxification system, where there is substantial inter-dependence between separate component members. Glutathione participates in detoxification at several different levels, and may scavenge free radicals, reduce peroxides or be conjugated with electrophilic compounds. Thus, glutathione provides the cell with multiple defences not only against ROS but also against their toxic products. This article discusses how glutathione biosynthesis, glutathione peroxidases, glutathione S-transferases and glutathione S-conjugate efflux pumps function in an integrated fashion to allow cellular adaption to oxidative stress. Co-ordination of this response is achieved, at least in part, through the antioxidant responsive element (ARE) which is found in the promoters of many of the genes that are inducible by oxidative and chemical stress. Transcriptional activation through this enhancer appears to be mediated by basic leucine zipper transcription factors such as Nrf and small Maf proteins. The nature of the intracellular sensor(s) for ROS and thiol-active chemicals which induce genes through the ARE is described. Gene activation through the ARE appears to account for the enhanced antioxidant and detoxification capacity of normal cells effected by many cancer chemopreventive agents. In certain instances it may also account for acquired resistance of tumours to cancer chemotherapeutic drugs. It is therefore clear that determining the mechanisms involved in regulation of ARE-driven gene expression has enormous medical implications.

Free-radical scavenging action of medicinal herbs from Ghana: Thonningia sanguinea on experimentally-induced liver injuries

The antioxidant action of medicinal herbs used in Ghana for treating various ailments was evaluated in vitro and in vivo. Five plants, Desmodium adscendens, Indigofera arrecta, Trema occidentalis, Caparis erythrocarpus, and Thonningia sanguinea were tested for their free radical scavenging action by their interaction with 1,1-diphenyl-2-picrylhydrazyl (DPPH). Of these five plants, only Thonningia sanguinea was found to scavenge the DPPH radical. Lipid peroxidation in liver microsomes induced by H2O2 was also inhibited by T. sanguinea. The hepatoprotective effect of T. sanguinea was studied on acute hepatitis induced in rats by a single dose of galactosamine (GalN, 400 mg/kg, IP) and in mice by carbon tetrachloride (CCl4, 25 microl/kg, IP). GalN induced hepatotoxicity in rats as evidenced by an increase in alanine aminotransferase (ALT) and glutathione (GSH) S-transferase activities in serum was significantly inhibited when T. sanguinea extract (5 ml/kg, IP) was given to rats 12 hr and 1 hr before GalN treatment. The activity of liver microsomal GSH S-transferase, which is known to be activated by oxidative stress, was increased by the GaIN treatment and this increase was blocked by T. sanguinea pretreatment. Similarly, T. sanguinea pretreatment also inhibited CCl4-induced hepatotoxicity in mice. These data indicate that T. sanguinea is a potent antioxidant and can offer protection against GalN- or CCl4-induced hepatotoxicity.

Distribution of microsomal glutathione transferase 1 in mammalian tissues. A predominant alternate first exon in human tissues

An extensive Northern blot analysis of microsomal glutathione transferase 1 in human and rat tissues was performed. When normalized against the glyceraldehyde-3-phosphate dehydrogenase or actin expression it was evident that the predominant expression occurs in liver and pancreas. An ontogenetic, as well as a functional, basis for the high levels in these two organs is possible. The relative expression levels in man ranged from: liver and pancreas (100%), to kidney, prostate, colon (30-40%), heart, brain, lung, testis, ovary, small intestine (10-20%), placenta, skeletal muscle, spleen, thymus and peripheral blood leucocytes (1-10%). Liver-enriched expression was detected in human fetal tissues with lung and kidney displaying lower levels (10-20%). No transcripts could be detected in fetal brain or heart. When comparing the expression levels between rat and man it is apparent that human extrahepatic mRNA levels are much higher relative to liver. Rat microsomal glutathione transferase mRNA expression ranges from 0.2 to 10% that of liver, with adrenal, uterus, ovary and stomach displaying the highest levels of the organs tested. Based on these observations, and the fact that the enzyme is encoded by a highly conserved single-copy gene, it is suggested that microsomal glutathione transferase 1 performs essential functions vital to most mammalian cell types. We suggest that protection against oxidative stress constitutes one such function. Human expressed sequence tag (EST) characterization yielded four alternate mRNA transcripts with different 5'-ends (four alternate noncoding exons 1). The predominant exon (based on the observed EST frequency) revealed a tissue distribution similar to that obtained using the reading frame as probe. Thus, it appears that one exon preferentially gives rise to mature mRNA in the human tissues examined. This exon is different from the one reported in the original cDNA characterized.

Screening of antioxidant action of various molds and protection of Monascus anka against experimentally induced liver injuries of rats

Antioxidant action of various molds, which are traditionally used for the production of foods or alcoholic beverages in Japan, was studied in vitro and in vivo. Antioxidant action was evaluated by scavenging stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH) and lipid peroxidation of rat liver microsomes. Among 40 molds, 16 species showed the DPPH scavenging action, and the molds that can scavenge the DPPH radical inhibited lipid peroxidation. The mold with the strongest action, Monascus anka, was chosen for the investigation of a protective action against liver injury of rats. When galactosamine (GalN, 400 mg/kg) or GalN plus lipopolysaccharide (LPS, 0.5 microg/kg) was given intraperitoneally to rats (Sprague-Dawley), aspartate aminotransferase (AST) and glutathione (GSH) S-transferase (GST) activities in serum were significantly increased. However, such hepatotoxicities seen in the increase in serum enzyme levels were depressed when the extract prepared from M. anka was given 1 and 15 h before the toxic insultant. Liver microsomal GST activity, which is known to be activated by oxidative stress, was increased by GalN or GaIN plus LPS treatment and the increase was also inhibited by pretreatment with the extract. Pathomorphological changes in the liver caused by GalN treatment also were prevented by the mold extract. These results indicate that the extract of M. anka has radical scavenging action and ameliorates chemically induced hepatotoxicity.

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.

A novel membrane-bound glutathione S-transferase functions in the stationary phase of the yeast Saccharomyces cerevisiae

The glutathione S-transferases (GSTs) represent a significant group of detoxification enzymes that play an important role in drug resistance in all eukaryotic species. In this paper we report an identification and characterization of the two Saccharomyces cerevisiae genes, GTT1 and GTT2 (glutathione transferase 1 and 2), coding for functional GST enzymes. Despite only limited similarity with GSTs from other organisms (approximately 50%), recombinant Gtt1p and Gtt2p exhibit GST activity with 1-chloro-2, 4-dinitrobenzene as a substrate. Both Gtt1p and Gtt2p are able to form homodimers, as determined by two hybrid assay. Subcellular fractionation demonstrated that Gtt1p associates with the endoplasmic reticulum. Expression of GTT1 is induced after diauxic shift and remains high throughout the stationary phase. Strains deleted for GTT1 and/or GTT2 are viable but exhibit increased sensitivity to heat shock in stationary phase and limited ability to grow at 39 degreesC.

Comparison of formation of D2/E2-isoprostanes and F2-isoprostanes in vitro and in vivo--effects of oxygen tension and glutathione

The isoprostanes (IsoPs) are bioactive prostaglandin-like compounds derived from the free-radical-catalyzed peroxidation of arachidonic acid in vitro and in vivo. IsoPs possessing either an F-type prostane ring (F2-IsoPs) or D/E-type prostane rings (D2/E2-IsoPs) are formed depending on whether IsoP endoperoxide intermediates undergo reduction or isomerization, respectively. Little, however, is known regarding factors influencing the formation of various classes of IsoPs, particularly D2/E2-IsoPs. Thus, studies were undertaken to examine the formation of D2/E2-IsoPs in relation to F2-Isops both in vitro and in vivo. In peroxidizing rat liver microsomes, the formation of D2/E2-IsoPs increased in a time- and oxygen-dependent manner and correlated with F2-IsoP generation and loss of precursor arachidonic acid, although the absolute amount of D2/E2-IsoPs formed exceeded by over 5-fold the levels of F2-IsoPs formed. Surprisingly, however, in liver tissue from rats exposed to an oxidant stress, levels of F2-IsoPs were up to 10-fold greater than those of D2/E2-IsoPs, suggesting that an endogenous process causes IsoP endoperoxide reduction in vivo. Addition of glutathione (GSH) to peroxidizing microsomes at concentrations from 0.01 to 5 mM increased the formation of F2-IsoPs at the expense of D2/E2-IsoPs. Boiling of microsomes did not alter the effect of GSH. Formation of D2/E2-IsoPs in liver tissue in vivo was greatly enhanced compared to F2-IsoPs in rats depleted of GSH. Thus, GSH modulates the formation of different classes of IsoPs in vitro and in vivo. Other thiols, including beta-mercaptoethanol, dithiothreitol, and cysteine, were able to substitute for GSH. These studies indicate that GSH promotes F2-IsoP formation and diminishes D2/E2-IsoP levels in vitro and in vivo by causing reduction of IsoP endoperoxides.

Modulation of glutathione S-transferase subunits A2, M1, and P1 expression by interleukin-1beta in rat hepatocytes in primary culture

The influence of various cytokines on the expression of glutathione S-transferases (GSTs) was investigated in rat hepatocytes in primary culture. Only treatment of hepatocytes with interleukin-1beta (IL-1) was effective, resulting in a marked decrease in GSTs. Steady-state mRNA levels of rGSTA2 and M1 were strongly down-regulated by IL-1 in a dose-dependent manner after a 24-h exposure while rGSTP1 mRNA level was increased by a 48-h treatment. Similar effects of IL-1 were observed at the protein level. The response to IL-1 appeared to be specific for each subunit within GST gene families. In addition, IL-1 strongly suppressed the induction of rGSTA2 by 3-methylcholanthrene, oltipraz (a synthetic derivative of 1, 2-dithiole-3-thione), and phenobarbital and that of rGSTM1 by oltipraz and phenobarbital, whereas it was ineffective on rGSTP1 induction by these compounds. Using in vitro nuclear run-on transcription assay and Northern blot analysis of alpha-amanitin-treated cells, IL-1-mediated rGSTM1 mRNA decrease was found to result from mRNA destabilization. These results provide the first demonstration that IL-1 regulates some major GST subunits in hepatocytes by a post-transcriptional mechanism.

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.

Glutathione-dependent factors and inhibition of rat liver microsomal lipid peroxidation

The effects of reduced glutathione (GSH) and glutathione disulfide (GSSG) on lipid peroxidation were investigated in rat liver microsomes containing deficient or adequate amounts of alpha-tocopherol (alpha-TH). Rates of formation of thiobarbituric acid reactive substances (TBARS) as well as rates of consumption of alpha-TH and O2 were decreased by GSH and were more pronounced in the NADPH-dependent assay system than in the ascorbate-dependent system. The GSH-dependent inhibition of lipid peroxidation was potentiated by GSSG in the NADPH-dependent assay system, but it had no effect in the nonenzymatic system. Diphenyliodonium chloride, an inhibitor of NADPH cytochrome P-450 reductase, completely prevented lipid peroxidation in the NADPH-dependent assay system whereas it had no effect on the ascorbate-dependent system. This is further evidenced by the fact that purified rat liver microsomal NADPH cytochrome P-450 reductase (EC 1.6.2.4) was inhibited approximately 24% and 52% by 5 mM GSH and 5 mM GSH + 2.5 mM GSSG, respectively. Glutathione disulfide alone had no effect on reductase activity. Similarly, other disulfides such as cystine, cystamine and lipoic acid were without effect on reductase activity. These results clearly delineate different mechanisms underlying the combined effects of GSH and GSSG on microsomal lipid peroxidation in rat liver. One mechanism involves recycling of microsomal alpha-TH by GSH during oxidative stress via a labile protein, ostensibly associated with "free radical reductase" activity. A second glutathione-dependent mechanism appears to be mediated through the inhibition of NADPH cytochrome P-450 reductase. The enhanced inhibition by GSH + GSSG of microsomal lipid peroxidation in the NADPH-dependent assay system suggests suppression of the initiation phase at the level of NADPH cytochrome P-450 reductase which is independent of microsomal alpha-TH.

Regulation of rat olfactory glutathione S-transferase expression. Investigation of sex differences, induction, and ontogenesis

The glutathione S-transferases (GSTs) of rat olfactory epithelium have been characterised with regard to sex differences, induction, and developmental regulation, and compared to those of the liver. Olfactory cytosolic GST activity with 1-chloro-2,4-dinitrobenzene (CDNB) as substrate was similar in both male and female animals, and there were no differences in subunit profile. Administration of trans-stilbene oxide and beta-naphthoflavone had no effect on olfactory GST activity with CDNB, although phenobarbitone treatment resulted in a small, but significant, increase in activity (130% compared to controls). HPLC analysis of subunit profiles indicated that all three agents induced olfactory subunit 1b and decreased subunit 6. The effect of age (3 to 84 days) on both cytosolic and microsomal CDNB activity was examined. In the liver, cytosolic activity was low at 3 days and climbed steadily to reach maximal levels around 28 days, but microsomal activity was relatively constant at all ages. Olfactory cytosolic activity was similar at all ages; microsomal activity was low until 21 days and then increased to reach a maximum at 56 days. Changes in individual cytosolic subunits were assessed by SDS-PAGE followed by immunoblotting. The significance of these results with regard to putative physiological roles for olfactory GSTs is discussed.

Oxidative stress in fish exposed to model xenobiotics. Oxidatively modified forms of Cu,Zn-superoxide dismutase as potential biomarkers

Fish (Sparus aurata) were intraperitoneally injected with model xenobiotics and several biomarkers of oxidative stress were analysed after 2 and 7 days exposure. The levels of soluble thiobarbituric acid reactive substances (TBARS) increased markedly in animals treated with polar xenobiotics, CuCl2 or paraquat; exposure to the apolar xenobiotics, dieldrin or malathion, enhanced significantly the microsomal TBARS while decreasing the microsomal glutathione transferase activity. The specific superoxide dismutase (SOD) activity increased in Cu(II)-injected animals but diminished in fish exposed to paraquat. After isoelectrofocusing separation and activity staining cell-free extracts of fish exposed to Cu(II), dieldrin or malathion displayed two new Cu,Zn-SOD isoforms of intermediate pI. An additional Mn-SOD was observed in dieldrin-injected fish, but only a faint new acidic isoform was observed in paraquat-injected animals. The new SOD bands were reproduced in vitro by incubation of cell-free extracts with systems generating superoxide anion or hydrogen peroxide and with a tert-butyl hydroperoxide/ADP-Fe system. Metallothionein induction was observed in Cu(II) or paraquat-exposed fish, but not in animals injected with apolar xenobiotics. So, the new SOD bands are possibly oxidized forms of this enzyme and can be considered as useful early biomarkers of oxidative stress due to transition metals or organic xenobiotics.

Heterologous expression of rat liver microsomal glutathione transferase in simian COS cells and Escherichia coli

The cDNA coding for rat liver microsomal glutathione transferase was subcloned into the mammalian expression vector pCMV-5 and the construct was transfected into, and transiently expressed in, simian COS cells. This resulted in high expression (0.7% of the microsomal protein). The activity towards 1-chloro-2,4-dinitrobenzene in microsomes was 15-30 nmol/min per mg, which increased upon N-ethylmaleimide treatment to 60-200 nmol/min per mg. Control and antisense-vector-treated cells displayed very low activity (3-6 nmol/min per mg). A DNA fragment coding for rat microsomal glutathione transferase was generated by PCR, cloned into the bacterial expression vector pSP19T7LT and transformed into Escherichia coli strain BL21 (DE3) (which contained the plasmid pLys SL). Isopropyl beta-D-thiogalactopyranoside (IPTG; 1 mM) induced the expression of significant amounts of enzymically active protein (4 mg/l of culture as measured by Western blots). The recombinant protein was purified and characterized and found to be indistinguishable from the rat liver enzyme with regard to enzymic activity, molecular mass and N-terminal amino acid sequence. Human liver cDNA was used to obtain the coding region of human microsomal glutathione transferase by PCR. This PCR product was cloned into pSP19T7LT, which, upon induction with IPTG, yielded significant amounts (9 mg/l of culture) of active enzyme in BL21 (DE3) cells. Thus, for the first time, it is now possible to express both human and rat microsomal glutathione transferase in an enzymically active form in Escherichia coli.

PHGPx and phospholipase A2/GPx: comparative importance on the reduction of hydroperoxides in rat liver mitochondria

The comparative importance of phospholipid hydroperoxide glutathione peroxidase (PHGPx) and of "classic" glutathione peroxidase (GPx) in the reduction of phospholipid hydroperoxides is unclear. Although GPx activity is 500-fold higher than that of PHGPx in rat liver, the reduction of phospholipid hydroperoxides by glutathione (GSH) through GPx may be strongly limited by a low PLA2 activity. We address this issue using a moderately detailed kinetic model of mitochondrial lipid peroxidation in rat liver. The model was based on published data and was subjected to validation as reported in the references. It is analysed by computer simulation and sensitivity analysis. Results suggest that in rat liver mitochondria PHGPx is responsible for almost all phospholipid hydroperoxide reduction. Under physiological conditions, the estimated flux of phospholipid hydroperoxides reduction through PHGPx is about four orders of magnitude higher than the estimated hydrolysis flux through PLA2. On the other hand, virtually all hydrogen peroxide is reduced through GPx. Therefore, a functional complementarity between PHGPx and GPx is suggested. Because the results are qualitatively robust to changes of several orders of magnitude in PLA2 and PHGPx levels, the conclusions may not be limited to mitochondria.

Ontogenesis of rat liver microsomal glutathione transferase

The ontogenesis of rat liver microsomal glutathione transferase was investigated by activity measurements and immunochemical methods. The activity rises from a very low level (3% of adults) at day 8 pre-partum to adult levels at days 50-150. Increases are associated with the neonatal and late-suckling clusters. Interestingly the capacity to become activated by N-ethylmaleimide is much lower in females early and late in life (days 35-100 and 300-550). After the initial increases (from 10% of adult levels at day 8 pre-partum), protein levels determined immunochemically remain constant throughout life with no apparent sex differences. The developmental pattern of microsomal glutathione transferase resembles those of other drug-metabolizing enzymes indicating that the function of the enzyme is required in adult life.

Microsomal glutathione transferase: lipid-derived substrates and lipid dependence

Rat liver microsomal glutathione transferase was found to display glutathione peroxidase activity toward a variety of oxidized lipids. 1-Linoleoyl-2-palmitoyl phosphatidylcholine hydroperoxide, 2-linoleoyl-1-palmitoyl phosphatidylcholine hydroperoxide, 2-linoleoyl-1-palmitoyl phosphatidylethanolamine hydroperoxide, and cholesteryl linoleate hydroperoxide all served as substrates (0.02, 0.04, 0.02, and 0.02 mumol/min mg, respectively). The phospholipid hydroperoxide glutathione peroxidase activity of the enzyme was found not to require detergent and increased when liposomes containing peroxidized phospholipid were fused with liposomes containing microsomal glutathione transferase. Methyl linoleate ozonide serves as a very efficient substrate for the microsomal glutathione transferase. The unactivated and N-ethylmaleimide-activated enzyme displayed specific activities of 0.74 and 5.9 mumol/min mg, respectively. Upon examination of a series of 4-hydroxyalk-2-enals it was found that the catalytic efficiency of the enzyme increases from the 4-hydroxyhept-2-enal up to the 4-hydroxytetradec-2-enal. The specific activities with the various 4-hydroxyalk-2-enals tested varied between 0.28 and 0.95 mumol/min mg. The phospholipid dependence of the microsomal glutathione transferase was examined in proteoliposomes formed by cholate dialysis. Phosphatidyl choline, phosphatidyl serine, phosphatidyl ethanolamine, and rat liver microsomal phospholipids could all be used successfully to reconstitute the enzyme. In conclusion, microsomal glutathione transferase can detoxify a number of lipid peroxidation products as well as a fatty acid ozonide. The results imply a protective role for the enzyme under conditions of oxidative stress.

Potential contribution of the glutathione S-transferase supergene family to resistance to oxidative stress

The glutathione S-transferase (GST) supergene family comprises gene families that encode isoenzymes that are widely expressed in mammalian tissue cytosols and membranes. Both cytosolic (particularly the isoenzymes encoded by the alpha, mu and theta gene families) and microsomal GST catalyse the conjugation of reduced glutathione (GSH) with a wide variety of electrophiles which include known carcinogens as well as various compounds that are products of oxidative stress including oxidised DNA and lipid. Indeed, several lines of evidence suggest certain of these isoenzymes play a pivotal role in protecting cells from the consequences of such stress. An assessment of the importance of these GST in humans is presently difficult however, because the number of alpha and theta class genes is not known and, the catalytic preferences of even identified isoforms is not always clear.

Status of hepatic glutathione-S-transferase(s) during Plasmodium berghei infection and chloroquine treatment in Mastomys natalensis

Plasmodium berghei infection in Mastomys natalensis impaired the hepatic mitochondrial, microsomal and cytosolic glutathione-S-transferase(s) activity with 1-chloro-2,4-dinitrobenzene as substrate. The enzyme activity was concomitantly decreased with rise in parasitaemia. The decreased enzyme activity due to infection was almost normalized with oral treatment of 16 mg (kg body wt)-1 of chloroquine for 4 days.

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C4 synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress. A description of the mechanisms of transcriptional and posttranscriptional regulation of GST isoenzymes is provided to allow identification of factors that may modulate resistance to specific noxious chemicals. The most abundant mammalian GST are the class alpha, mu, and pi enzymes and their regulation has been studied in detail. The biological control of these families is complex as they exhibit sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GST are regulated by a structurally diverse range of xenobiotics and, to date, at least 100 chemicals have been identified that induce GST; a significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be exposed regularly to such compounds. Many inducers, but not all, effect transcriptional activation of GST genes through either the antioxidant-responsive element (ARE), the xenobiotic-responsive element (XRE), the GST P enhancer 1(GPE), or the glucocorticoid-responsive element (GRE). Barbiturates may transcriptionally activate GST through a Barbie box element. The involvement of the Ah-receptor, Maf, Nrl, Jun, Fos, and NF-kappa B in GST induction is discussed. Many of the compounds that induce GST are themselves substrates for these enzymes, or are metabolized (by cytochrome P-450 monooxygenases) to compounds that can serve as GST substrates, suggesting that GST induction represents part of an adaptive response mechanism to chemical stress caused by electrophiles. It also appears probable that GST are regulated in vivo by reactive oxygen species (ROS), because not only are some of the most potent inducers capable of generating free radicals by redox-cycling, but H2O2 has been shown to induce GST in plant and mammalian cells: induction of GST by ROS would appear to represent an adaptive response as these enzymes detoxify some of the toxic carbonyl-, peroxide-, and epoxide-containing metabolites produced within the cell by oxidative stress. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the multidrug-resistant phenotype observed in these lesions. The majority of human tumors and human tumor cell lines express significant amounts of class pi GST. Cell lines selected in vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of class alpha and mu isoenzymes is also often observed. The mechanisms responsible for overexpression of GST include transcriptional activation, stabilization of either mRNA or protein, and gene amplification. In humans, marked interindividual differences exist in the expression of class alpha, mu, and theta GST. The molecular basis for the variation in class alpha GST is not known. (ABSTRACT TRUNCATED)

Activation of microsomal glutathione S-transferase in tert-butyl hydroperoxide-induced oxidative stress of isolated rat liver

The activation of microsomal glutathione S-transferase in oxidative stress was investigated by perfusing isolated rat liver with 1 mM tert-butyl hydroperoxide (t-BuOOH). When the isolated liver was perfused with t-BuOOH for 7 min and 10 min, microsomal, but not cytosolic, glutathione S-transferase activity was increased 1.3-fold and 1.7-fold, respectively, with a concomitant decrease in glutathione content. A dimer protein of microsomal glutathione S-transferase was also detected in the t-BuOOH-perfused liver. The increased microsomal glutathione S-transferase activity after perfusion with t-BuOOH was reversed by dithiothreitol, and the dimer protein of the transferase was also abolished. When the rats were pretreated with the antioxidant alpha-tocopherol or the iron chelator deferoxamine, the increases in microsomal glutathione S-transferase activity and lipid peroxidation caused by t-BuOOH perfusion of the isolated liver was prevented. Furthermore, the activation of microsomal GSH S-transferase by t-BuOOH in vitro was also inhibited by incubation of microsomes with alpha-tocopherol or deferoxamine. Thus it was confirmed that liver microsomal glutathione S-transferase is activated in the oxidative stress caused by t-BuOOH via thiol oxidation of the enzyme.

Purification of rat liver plasma membrane glutathione transferase

Glutathione transferases purified from plasma membrane and microsomal fractions from rat liver share similar enzymic properties. The activity of both proteins with 1-chloro-2,4-dinitrobenzene can be stimulated about 10-15-fold by N-ethylmaleimide. No activation is observed using p-nitrobenzylchloride as a substrate. The enzymes are immunologically related as indicated by Western-blot analysis using antibodies against the microsomal glutathione transferase or against a synthetic peptide corresponding to the amino acid positions 55-64 of microsomal glutathione transferase. Isolated plasma membrane and microsomal glutathione transferases possess the same amino-terminal amino acid sequence and digestion with different proteases results in identical fragment patterns as displayed by SDS/PAGE. These data suggest that plasma membrane and microsomal glutathione transferase are identical proteins.