An enzyme from rat liver catalysing conjugations with glutathione

Doxorubicin resistance in human melanoma cells: MDR-1 and glutathione S-transferase pi gene expression

Cellular drug resistance is believed to involve P-glycoprotein-related drug efflux as well as xenobiotic detoxification. In the present study, we analyzed five human melanoma cell lines with 1- to 6-fold doxorubicin resistance for doxorubicin retention and MDR-1 and GST pi gene expression. All the cell lines had high doxorubicin retention, and efflux blockers such as trifluoperazine and verapamil did not have a major effect on drug retention or cytotoxicity. Even though all the cell lines carried the MDR-1 and GST pi genes, gene amplification was not associated with drug resistance. Both laser flow cytometry and immunoperoxidase staining showed high expression of C-219 reactive P-glycoprotein in some of the resistant cells which was not accompanied by either high drug efflux or sensitivity to doxorubicin efflux blockers.

Loss of suppression of GSH synthesis at low cell density in primary cultures of rat hepatocytes

Primary cultures of adult rat hepatocytes shift into the growth phase when plated at low density (LD). We used this model to examine changes in glutathione (GSH) metabolism, since cells undergoing active growth may be more susceptible to environmental toxins. When primary cultures of adult rat hepatocytes were plated on collagen or Matrigel-precoated dishes, cell number and GSH varied inversely. This density effect on cell GSH occurred as early as 2 h after plating, when the media contained 1 mM methionine, but was delayed until 20 h if the media contained only 0.5 mM cystine. The density effect on GSH synthesis occurred in the absence of serum, hormones, changes in cell volume, GSH efflux, ATP levels, and uptake of methionine or cystine and was blocked by cycloheximide or actinomycin D. When methionine was available, the cellular cysteine level was 65% higher at LD than at high density (HD). gamma-Glutamylcysteine synthetase (GCS) activity was 64% higher at LD than at HD. GSH synthetase activity was unaffected by density. Both the increase in cellular cysteine levels and GCS activity were blocked by cycloheximide and actinomycin D. When cells were cocultured using cluster plates and Transwell inserts for 4 h, cell GSH of HD cells was unaffected by the density of cocultured cells; however, LD cells exhibited significantly lower GSH and GCS activity when cocultured with HD cells than when cocultured with LD cells. Cysteine levels were elevated in the LD cells regardless of the density of cocultured cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of vitamin A on hexachlorocyclohexane (HCH) toxicity in the rat

1. Technical hexachlorocyclohexane (HCH) depleted hepatic stores of vitamin A in male albino rats to cause secondary vitamin A deficiency. 2. Toxicity of HCH in rats is augmented by dietary vitamin A-deficiency as evidenced by growth retardation, organ hypertrophies and alterations in the serum and liver levels of the marker enzymes of toxicity. 3. Supplementation of dietary vitamin A to the rats either in adequate (2000 IU/kg diet) or in an excess but not hypervitaminotic level (10(5) IU/kg diet) resulted in significant protection against the toxicity of HCH. 4. The activities of the hepatic xenobiotic metabolizing enzymes were generally low (with the exception of glutathione S-transferase) in the vitamin A-deficient rats compared to those of the vitamin A supplemented diet groups. 5. The results indicated that dietary vitamin A influences the response of male albino rats to HCH toxicity possibly by modulating the activities of hepatic xenobiotic metabolizing enzymes.

Insulin and glucocorticoid dependence of hepatic gamma-glutamylcysteine synthetase and glutathione synthesis in the rat. Studies in cultured hepatocytes and in vivo

We reported that glucagon and phenylephrine decrease hepatocyte GSH by inhibiting gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in GSH synthesis (Lu, S.C., J. Kuhlenkamp, C. Garcia-Ruiz, and N. Kaplowitz. 1991. J. Clin. Invest. 88:260-269). In contrast, we have found that insulin (In, 1 microgram/ml) and hydrocortisone (HC, 50 nM) increased GSH of cultured hepatocytes up to 50-70% (earliest significant change at 6 h) with either methionine or cystine alone as the sole sulfur amino acid in the medium. The effect of In occurred independent of glucose concentration in the medium. Changes in steady-state cellular cysteine levels, cell volume, GSH efflux, or expression of gamma-glutamyl transpeptidase were excluded as possible mechanisms. Both hormones are known to induce cystine/glutamate transport, but this was excluded as the predominant mechanism since the induction in cystine uptake required a lag period of greater than 6 h, and the increase in cell GSH still occurred when cystine uptake was blocked. Assay of GSH synthesis in extracts of detergent-treated cells revealed that In and HC increased the activity of GCS by 45-65% (earliest significant change at 4 h) but not GSH synthetase. In and HC treatment increased the Vmax of GCS by 31-43% with no change in Km. Both the hormone-mediated increase in cell GSH and GCS activity were blocked with either cycloheximide or actinomycin D. Finally, when studied in vivo, streptozotocin-treated diabetic and adrenalectomized rats exhibited lower hepatic GSH levels and GCS activities than respective controls. Both of these abnormalities were prevented with hormone replacement. Thus, both in vitro and in vivo, In and glucocorticoids are required for normal expression of GCS.

Increase in rat brain glutathione following intracerebroventricular administration of gamma-glutamylcysteine

The effects of intracerebroventricularly (i.c.v.) administered gamma-glutamylcysteine (gamma-GC) and glutathione (GSH) monoethyl ester, subcutaneously (s.c.) injected L-2-oxo-4-thiazolidinecarboxylic acid (OTC) and intraperitoneally (i.p.) administered cysteine on the concentration of GSH in rat brain were investigated. The brain content of GSH, cysteine and gamma-GC was determined by HPLC with electrochemical detection (gold/mercury electrode) using N-acetylcysteine as internal standard. A dose-dependent increase in the GSH concentration (145-170% of controls) was found in the substantia nigra (SN) and in the rest of the brain stem after injection of gamma-GC, whereas no significant alterations in GSH were observed in the striatum and in the cerebral cortex. High levels of gamma-GC could be detected in the brain tissue after the administration, and the concentration of cysteine did also increase markedly after gamma-GC injection in all brain regions assessed. I.c.v. administration of L-buthionine sulfoximine (L-BSO) reduced the brain concentration of GSH by 50-70% within 24 hr. Injection of gamma-GC 24 hr after L-BSO resulted in an increase in GSH up to control values within 1-3 hr in the SN and the rest of the brain stem, whereas only a slight increase in GSH was observed in the striatum and the cerebral cortex. The concentration of GSH in the striatum and SN did not change after i.p. injection of cysteine, but a slight increase in the GSH concentration in the limbic region was observed. GSH monoethyl ester (i.c.v.) and OTC (s.c.) did not produce any significant increase in the GSH concentration in the brain. When the GSH concentration had been reduced by administration of L-BSO (i.c.v.; 24 hr) subsequent injection of GSH monoethyl ester led to a slight increase in the striatal and limbic GSH levels. These data show that, of the drugs studied, gamma-GC was the most effective in increasing brain GSH. It could thus serve as a valuable tool in future studies regarding metabolism and function of GSH in the brain. The observed difference in the effects of gamma-GC in different brain regions indicate that the brain tissue is not homogeneous with regard to GSH synthesizing capacity.

Effects of reproductive tract glutathione enhancement and depletion on ethyl methanesulfonate-induced dominant lethal mutations in Sprague-Dawley rats

The effects of altering glutathione (GSH) levels in the male reproductive tract have been studied in an attempt to establish a link between chemical-induced perturbations in glutathione and susceptibility of spermatozoa to chemical insult. Tissue GSH levels were enhanced by a treatment regimen of N-acetylcysteine (NAC) (250 mg/kg, 4 treatments at 2 h intervals). With this treatment, GSH levels in liver, testis, caput epididymis, and cauda epididymis were elevated to 126%, 110%, 178%, and 136% of control values. Sexually mature male rats were then treated with NAC and challenged with a dose of EMS (100 mg/kg) to determine if enhanced tissue GSH would protect against EMS-induced dominant lethal mutations. Pretreatment with NAC significantly decreased the post-implantation loss from 7.05 +/- 0.57 with EMS alone to 5.28 +/- 0.47. Conversely, a dominant lethal assay was conducted using different doses of phorone pretreatment to determine the relative contribution of hepatic versus reproductive tract GSH in protecting against EMS-induced dominant lethal resorptions. Doses of 100 mg/kg and 250 mg/kg phorone significantly lowered both hepatic and reproductive tract GSH while 25 mg/kg lowered only hepatic GSH. These three dose levels were used as pretreatments in a dominant lethal study followed by a challenge administration of EMS (50 mg/kg), which is a threshold dose of EMS for producing dominant lethal mutations. Comparison against controls demonstrated a significant potentiation of fetal resorptions in all groups receiving phorone pretreatment, including the 25 mg/kg pretreatment group which only lowered hepatic GSH prior to EMS challenge. The results of these experiments indicate that GSH reserves in the male reproductive tract are insufficient to protect developing spermatozoa from damage by alkylating agents in the absence of hepatic GSH.

Glutathione transferases and cancer

The glutathione transferases, a family of multifunctional proteins, catalyze the glutathione conjugation reaction with electrophilic compounds biotransformed from xenobiotics, including carcinogens. In preneoplastic cells as well as neoplastic cells, specific molecular forms of glutathione transferase are known to be expressed and have been known to participate in the mechanisms of their resistance to drugs. In this article, following a brief description of recently identified molecular forms, we review new findings regarding the respective molecular forms involved in carcinogenesis and anticancer drug resistance, with particular emphasis on Pi class forms in preneoplastic tissues. The rat Pi class form, GST-P (GST 7-7), is strongly expressed not only in hepatic foci and hepatomas, but also in initiated cells that occur at the very early stages of chemical hepatocarcinogenesis, and is regarded as one of the most reliable markers for preneoplastic lesions in the rat liver. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-responsive element-like sequences have been identified in upstream regions of the GST-P gene, and oncogene products c-jun and c-fos are suggested to activate the gene. The Pi-class forms possess unique enzymatic properties, including broad substrate specificity, glutathione peroxidase activity toward lipid hydroperoxides, low sensitivity to organic anion inhibitors, and high sensitivity to active oxygen species. The possible functions of Pi class glutathione transferases in neoplastic tissues and drug-resistant cells are discussed.

t-butylated hydroxytoluene enhances intracellular levels of glutathione and related enzymes of rat lens in vitro organ culture

Studies were undertaken to investigate the effect of t-butylated hydroxytoluene (BHT) on reduced glutathione (GSH) levels and related enzymes in rat ocular tissues. GSH levels were significantly enhanced when 1 microM BHT was included in the medium of rat lens cultures. BHT had a dose-dependent effect on GSH levels of lenses in cultures. Inclusion of 10 microM BHT in the culture medium resulted in a twofold increase in GSH levels of the lens within 24 hr. Increased gamma-glutamylcysteine synthetase activity concomitant with the increased amount of [35S]methionine incorporation in GSH strongly suggested that BHT caused enhanced levels of GSH in lenses by increasing de novo biosynthesis. A significant increase was also observed in glutathione S-transferase (GST) levels of lenses in culture containing BHT in the medium. Present studies also demonstrated that rat lens expresses only the mu and pi class GST isoenzymes and both these classes of isoenzymes were elevated by BHT. Oral administration of BHT to rats also resulted in enhanced in vivo levels of GSH in lens, retina and cornea. In addition, a significant in vivo increase in the levels of GST, GSH-peroxidase, GSH-reductase, gamma-glutamylcysteine synthetase, and glucose 6-phosphate dehydrogenase was observed in the lens, retina, and cornea of BHT-fed rats.

Purification and characterization of glutathione S-transferases from rat pancreas

Glutathione S-transferases (GSTs) of rat pancreas have been characterized and their interrelationship with fatty acid ethyl ester synthase (FAEES) has been studied. Seven GST isozymes with pI values of 9.2, 8.15, 7.8, 7.0, 6.3, 5.9 and 5.4 have been isolated and designated as rat pancreas GST suffixed by their pI values. Structural, immunological and kinetic properties of these isozymes indicated that GST 9.2 belonged to the alpha class, GST 7.8, 7.0, 6.3 and 5.9 belonged to the mu class, whereas GST 8.15 and 5.4 belong to pi class. The N-terminal sequences and pI values of the mu class isozymes suggested that rat GST subunits 3, 4 and 6 may be expressed in pancreas. N-Terminal sequences of both the pi class isozymes, GST 8.15 and 5.4, were similar to that of GST-P, but there were significant differences in the substrate specificities of these two enzymes. Results of peptide finger print studies also indicated minor structural differences between these two isozymes. None of the GST isozymes of rat pancreas expressed FAEES activity. Rat pancreas had a significant amount of FAEES activity, but it segregated independently during the purification of GST indicating that these two activities are expressed by different proteins and are not related as suggested previously.

Characterization of a novel glutathione S-transferase isoenzyme from mouse lung and liver having structural similarity to rat glutathione S-transferase 8-8

In mouse lung, glutathione S-transferase (GST, EC 2.5.1.18) isoenzymes belonging to the three major known classes, Alpha, Mu and Pi, have been previously characterized, along with an isoenzyme (pI 5.7) that could not be identified with the Alpha, Mu or Pi classes of GSTs. In the present studies we have demonstrated that this isoenzyme is also expressed in liver. Its structural, kinetic, and immunological properties have been determined and compared with those of the three classes of GSTs. GST 5.7 has a subunit molecular mass of 23 kDa, which is intermediate between that of the previously characterized Alpha (25 kDa) and Pi (22.5 kDa) class GST subunits of mouse lung. Comparison of peptide maps of GST 5.7 with those representative of Alpha, Mu and Pi class GST isoenzymes of mouse lung showed that it had a distinct peptide fragmentation pattern. Kinetic and immunological properties of GST 5.7 were also distinct from other mouse GST isoenzymes belonging to the Alpha, Mu or Pi classes. N-Terminal amino-acid-sequence analysis of a 6 kDa fragment generated by CNBr digestion of mouse lung GST 5.7 revealed a 15-residue sequence that was distinct from sequences of known Alpha, Mu and Pi class mouse GSTs. The sequence, however, matched with the sequence of rat GST 8-8 between amino acid residues 106 and 120 with a 73% identity. The 6 kDa and 12 kDa fragments generated by CNBr digestion of mouse liver GST 5.7 also gave sequences which matched with those of rat GST 8-8 between positions 106 and 120 and 167 and 186, with a high degree of identity. These studies suggest that mouse GST 5.7 structurally corresponds to rat GST 8-8 and belongs to the Alpha class.

Rat liver canalicular membrane vesicles contain an ATP-dependent bile acid transport system

The secretion of bile by the liver is primarily determined by the ability of the hepatocyte to transport bile acids into the bile canaliculus. A carrier-mediated process for the transport of taurocholate, the major bile acid in humans and rats, was previously demonstrated in canalicular membrane vesicles from rat liver. This process is driven by an outside-positive membrane potential that is, however, insufficient to explain the large bile acid concentration gradient between the hepatocyte and bile. In this study, we describe an ATP-dependent transport system for taurocholate in inside-out canalicular membrane vesicles from rat liver. The transport system is saturable, temperature-dependent, osmotically sensitive, specifically requires ATP, and does not function in sinusoidal membrane vesicles and right side-out canalicular membrane vesicles. Transport was inhibited by other bile acids but not by substrates for the previously demonstrated ATP-dependent canalicular transport systems for organic cations or nonbile acid organic anions. Defects in ATP-dependent canalicular transport of bile acids may contribute to reduced bile secretion (cholestasis) in various developmental, inheritable, and acquired disorders.

Hormone-mediated down-regulation of hepatic glutathione synthesis in the rat

Our present work characterized the role of hormone-mediated signal transduction pathways in regulating hepatic reduced glutathione (GSH) synthesis. Cholera toxin, dibutyryl cAMP (DBcAMP), and glucagon inhibited GSH synthesis in cultured hepatocytes by 25-43%. Cellular cAMP levels exhibited a lower threshold for stimulation of the GSH efflux than inhibition of its synthesis. The effect of DBcAMP was independent of the type of sulfur amino acid precursor and cellular ATP levels and unassociated with increased GSH mixed disulfide formation or altered GSH/oxidized glutathione ratio. In liver cytosols, addition of DBcAMP and cAMP-dependent protein kinase (A-kinase) inhibited GSH synthesis from substrates (cysteine, ATP, glutamate, and glycine) by approximately 20% which was prevented by the A-kinase inhibitor. However, if only substrates of the second step in GSH synthesis were used (gamma-glutamylcysteine, glycine, and ATP), DBcAMP and A-kinase exerted no inhibitory effect. Phenylephrine, vasopressin, and phorbol ester also inhibited GSH synthesis in cultured cells by approximately 20%, and depleted cell GSH independent of the type of sulfur amino acid precursor. Cellular cysteine level was unchanged despite the significant fall in GSH after glucagon or phenylephrine treatment. Pretreatment with either staurosporine, C-kinase inhibitor, or calmidazolium, a calmodulin inhibitor, partially prevented but, together, completely prevented the inhibitory effect of phenylephrine. The same combination had no effect on the inhibitory effect of glucagon. The effects of hormones were confirmed in both the intact perfused liver and after in vivo administration. Thus, two classes of hormones acting through distinct signal transduction pathways may down-regulate hepatic GSH synthesis by phosphorylation of gamma-glutamylcysteine synthetase.

Cell age dependent decay of human erythrocytes glutathione S-transferase

Human red blood cells contain both glutathione S-transferase sigma (GST sigma) and glutathione S-transferase rho (GST rho). While the first isozyme does not change in red blood cell fractions of different mean density (age), GST rho, the main isozyme, shows a pronounced cell age dependent decay. Ion-exchange chromatographic experiments show that GST rho consists of only one isozymic form in young erythrocytes but is present in two components, with different electric charge, in mature and old cells. The "secondary" GST rho isozyme is more heat stable than the "primary" GST rho isozyme with the result that the total GST activity shows an apparent increase in heat stability during cell aging due to the formation of "secondary" isozymes. The kinetic properties and specificity of this enzyme do not show appreciable modifications during cell ageing. The data reported in this paper suggest that red blood cell aging is associated with a reduced detoxifying ability due to GST rho decay.

Glutathione S-transferase activity of leukemic cells as a prognostic factor for response to chemotherapy in acute leukemias

This paper presents an analysis of glutathione S-transferase (GST) activity of leukemic cells in 30 patients with acute leukemias and its predictive value for therapy. Blast cells were isolated from peripheral blood or bone marrow before induction therapy using Ficoll density gradient. GST activity was measured according to the spectrophotometric assay based on the use of 1-chloro-2,4-dinitrobenzene as a substrate. The results did not show any significant differences between activities of the enzyme within the different leukemia types according to the French-American-British (FAB) classification. The patients who achieved complete remission demonstrated the lowest value of enzyme activity. The highest enzyme activity was observed in those patients who achieved partial remission and the non-responsive patients presented a GST value within the median of these two groups. Two categories of patients were represented within the non-responsive treatment group. One was resistant to the conventional therapy and in the other death was caused by infectious or hemorrhagic complications. The mean GST activity in these two groups of patients differ greatly. These results suggest that low GST activity of leukemic cells could be a favourable prognostic factor whereas high GST values could help to find out the group of patients who should be further analysed prior to induction therapy.

The presence of glutathione in primary neuronal and astroglial cultures from rat cerebral cortex and brain stem

The concentration of the tripeptide glutathione (GSH) was measured in primary cultures of neurons and astroglial cells from rat cerebral cortex and brain stem. The concentration of GSH was found to be approximately 20 nmol/mg protein in the neuronal culture from the cerebral cortex and ca. 40 nmol/mg protein in the neuronal brain stem cultures. A GSH concentration of approximately 20 nmol/mg was observed in the astrocyte cultures from both brain regions. The possibility to increase the GSH concentration was tested by incubating the cultures in the presence of the GSH precursor gamma-glutamylcysteine (gamma-GC). In the cultured astrocytes gamma-GC produced a dose-dependent increase in GSH. A similar increase was observed in the neuronal cultures, but this effect failed to reach statistical significance.

Glutathione deficiency produced by inhibition of its synthesis, and its reversal; applications in research and therapy

Glutathione, which is synthesized within cells, is a component of a pathway that uses NADPH to provide cells with their reducing milieu. This is essential for (a) maintenance of the thiols of proteins (and other compounds) and of antioxidants (e.g. ascorbate, alpha-tocopherol), (b) reduction of ribonucleotides to form the deoxyribonucleotide precursors of DNA, and (c) protection against oxidative damage, free radical damage, and other types of toxicity. Glutathione interacts with a wide variety of drugs. Despite its many and varied cellular functions, it is possible to achieve therapeutically useful modulations of glutathione metabolism. This article emphasizes an approach in which the synthesis of glutathione is selectively inhibited in vivo leading to glutathione deficiency. This is achieved through use of transition-state inactivators of gamma-glutamylcysteine synthetase, the enzyme that catalyzes the first and rate-limiting step of glutathione synthesis. The effects of marked glutathione deficiency, thus produced in the absence of applied stress, include cellular damage associated with severe mitochondrial degeneration in a number of tissues. Such glutathione deficiency is not prevented or reversed by giving glutathione. The cellular utilization of GSH involves its extracellular degradation, uptake of products, and intracellular synthesis of GSH. This is a normal pathway by which cysteine moieties are taken up by cells. Glutathione deficiency induced by inhibition of its synthesis may be prevented or reversed by administration of glutathione esters which, in contrast to glutathione, are readily transported into cells and hydrolyzed to form glutathione intracellularly. Research derived from this model has led to several potentially useful therapeutic approaches, one of which is currently in clinical trial. Thus, certain tumors, including those that exhibit resistance to several drugs and to radiation, are sensitized to these modalities by selective inhibition of glutathione synthesis. An alternative interpretation is suggested which is based on the concept that some resistant tumors have high capacity for glutathione synthesis and that such increased capacity may be as significant or more significant in promoting the resistance of some tumors than the cellular levels of glutathione. Therapeutic approaches are proposed in which normal cells may be selectively protected against toxic antitumor agents and radiation by cysteine- and glutathione-delivery compounds. Current studies suggest that research on other modulations of glutathione metabolism and transport would be of interest.

The role of glutathione-dependent enzymes in drug resistance

Glutathione and glutathione-dependent enzymes are ubiquitously distributed through nature. These enzyme systems appear to have evolved to protect cells from toxic and mutagenic environmental chemicals. There is now unequivocal evidence demonstrating that these enzymes play a role in chemical resistance in a variety of phylogeny including, bacteria, plants and insects. There is also increasing circumstantial, as well as genetic evidence which indicates that these enzymes are also a determinant in the sensitivity of tumor cells to anticancer drugs, particularly alkylating agents and those drugs whose toxic effects are mediated by free radicals. In this review some of the experimental data which leads to these conclusions is discussed.

The action of the glutathione transferase substrate, 1-chloro-2,4-dinitrobenzene on synaptosomal glutathione content and the release of hydrogen peroxide

We studied the action of the glutathione transferase substrate, 1-chloro-2,4-dinitrobenzene (CDNB) on the synaptosomal production of H2O2. We found that CDNB (30-40 microM) readily depletes the cytosolic glutathione but is almost without effect on the mitochondrial fraction. The depletion of the cytosolic glutathione induced by CDNB affords the detection in the extracellular space of H2O2 produced intrasynaptosomally upon increasing the cytosolic Ca2+ concentration that is otherwise destroyed by glutathione peroxidase. Higher concentrations of CDNB induce a H2O2 production which is not related to the glutathione content. This H2O2 is of mitochondrial origin and requires that NAD be reduced. The primary product of the mitochondrial CD-NB-dependent oxygen reduction is at least in part the superoxide anion.

Resolution and kinetic characterization of glutathione S-transferases from human jejunal mucosa

Cytosolic glutathione S-transferases were purified from human jejunal mucosa by affinity chromatography on S-hexylglutathione-Sepharose 4B. Chromatofocusing in the pH range 7-4 yielded peaks with apparent pI's of 7.2 (peak 1), 5.2 (peak 2), and 4.4 (peak 3). Each enzymatic fraction was shown to have a homodimeric structure, with subunit mass of 24.9 +/- 0.5 (P1), 27.9 +/- 0.9 (P2), and 23.4 +/- 0.8 (P3) kDa, as determined by SDS-PAGE. The substrate specificity of each peak was tested using discriminating substrates for basic, near-neutral, and acidic GSTs. With cumene hydroperoxide, the diagnostic substrate for the alpha (basic) class of GSTs, P1 showed 8- to 36-fold higher activity than P2 and P3. Ethacrynic acid, the selective substrate for the acidic enzyme (pi), gave highest activity with P3. The inhibitory potentials of sulfobromophthalein, cibacron blue, tributyltin acetate, triphenyltin chloride, and bromphenol blue were also tested. A qualitative resemblance between P1 and alpha, and P3 and pi GSTs was noted. The substrate specificity and inhibiton parameters of P2 corresponded most closely to those of mu-GST. The relative abundances of P1, P2, and P3 (based on CDNB-conjugating activity) were 35, 5, and 60%, respectively.

Glutathione S-transferase and P-glycoprotein in multidrug resistant Chinese hamster cells

Glutathione S-transferases (GSTs) have been reported to be elevated in some forms of hepatic carcinogenesis, in multidrug resistant (MDR) cells exhibiting elevated P-glycoprotein, and in cells resistant to alkylating agents independent of the MDR phenotype. The reported elevation of GST in association with the MDR phenotype and the overexpression of P-glycoprotein along with induction of GST in hepatic carcinogenesis suggest a correlation in the two mechanisms of cellular detoxification. To evaluate this hypothesis we examined the expression of GSTs in an MDR Chinese hamster fibroblast cell line overexpressing P-glycoprotein. We were unable to demonstrate concordant elevation of GST in these MDR cells. We conclude that GST expression is independent of P-glycoprotein expression in MDR Chinese hamster fibroblasts. The overexpression of GSTs in certain cells may provide an alternative mechanism for the development of drug resistance, either in association with or independent of P-glycoprotein overexpression, but is not essential for the MDR phenotype.

Free radicals, antioxidant enzymes, and carcinogenesis

Free radicals are found to be involved in both initiation and promotion of multistage carcinogenesis. These highly reactive compounds can act as initiators and/or promoters, cause DNA damage, activate procarcinogens, and alter the cellular antioxidant defense system. Antioxidants, the free radical scavengers, however, are shown to be anticarcinogens. They function as the inhibitors at both initiation and promotion/transformation stage of carcinogenesis and protect cells against oxidative damage. Altered antioxidant enzymes were observed during carcinogenesis or in tumors. When compared to their appropriate normal cell counterparts, tumor cells are always low in manganese superoxide dismutase activity, usually low in copper and zinc superoxide dismutase activity and almost always low in catalase activity. Glutathione peroxidase and glutathione reductase activities are highly variable. In contrast, glutathione S-transferase 7-7 is increased in many tumor cells and in chemically induced preneoplastic rat hepatocyte nodules. Increased glucose-6-phosphate dehydrogenase activity is also found in many tumors. Comprehensive data on free radicals, antioxidant enzymes, and carcinogenesis are reviewed. The role of antioxidant enzymes in carcinogenesis is discussed.

Genetic heterogeneity of the human glutathione transferases: a complex of gene families

The glutathione transferases (GSTs) are involved in the metabolism of a wide range of compounds of both exogenous and endogenous origin. There is evidence that deficiency of GST may increase sensitivity to certain environmentally derived carcinogens. In contrast, elevated expression has been implicated in resistance to therapeutic drugs. The GSTs are the products of several gene families. This review summarizes the present knowledge of the genetic interrelationships between the various isoenzymes, their deficiencies and the physical locations of their genes.

Potentiation of ethyl methanesulfonate-induced germ cell mutagenesis and depression of glutathione in male reproductive tissues by 1,2-dibromoethane

EDB significantly depressed GSH in caput and cauda epididymis, but not in testis, 2 hours following injection. This depression was dose-related. EDB enhanced EMS-induced dominant lethal mutations at mating weeks 2 and 3 (of 6). At mating week 2 the fetal death rate was increased two-fold, while at week 3, the fetal death rate had increased to nearly three-fold greater than the EMS-only controls. Enhancement of fetal death rate was confined to postimplantation loss. As with EMS alone, the EDB potentiation of EMS-induced mutations was limited to postmeiotic stages of spermatogenesis. EDB also enhanced alkylation of rat spermatozoa by labeled EMS. Depression of GSH in reproductive tissues is correlated with a potentiation of dominant lethal mutations, as well as an increase in the binding of EMS to sperm heads.

ATP dependent primary active transport of xenobiotic-glutathione conjugates by human erythrocyte membrane

We have demonstrated the presence of a dinitrophenyl glutathione (Dnp-SG) stimulated ATPase in human erythrocyte membranes. This ATPase mediates the active transport of glutathione-xenobiotic conjugate such as Dnp-SG from erythrocytes into the plasma. It is suggested that this transport system is distinct from the system which actively transports oxidized glutathione from the erythrocytes.

Glutathione S-transferases and glutathione peroxidases in doxorubicin-resistant murine leukemic P388 cells

Energy-dependent rapid drug efflux is believed to be a major factor in cellular resistance to doxorubicin (DOX). However, several recent studies have demonstrated that cellular DOX retention alone does not always correlate with its cytotoxicity and suggest that mechanisms other than rapid drug efflux may also be important. In the present study, we have compared glutathione (GSH) S-transferase (GST), selenium-dependent GSH peroxidase and selenium-independent GSH peroxidase II activities in DOX-sensitive (P388/S) and resistant (P388/R) mouse leukemic cells. The GST activity towards 1-chloro-2,4-dinitrobenzene (CDNB) and ethacrynic acid (EA) was markedly higher in P388/R cells compared to P388/S cells. Purification of GST by GSH-affinity chromatography from an equal number of P388/S and P388/R cells revealed an increased amount of GST protein in P388/R cells. Immunological studies indicated that alpha and pi type GST isoenzymes were 1.27- and 2.2-fold higher, respectively, in P388/R cells compared to P388/S cells. Selenium-dependent GSH peroxidase activity was similar in both the cell lines, whereas selenium-independent GSH peroxidase II activity was approximately 1.36-fold higher in P388/R cells compared to P388/S cells. These results suggest that increased GSH peroxidase II activity in P388/R cells may contribute to cellular DOX resistance by enhancing free radical detoxification in this cell line.

Cytosolic glutathione transferases from rat liver. Primary structure of class alpha glutathione transferase 8-8 and characterization of low-abundance class Mu glutathione transferases

Six GSH transferases with neutral/acidic isoelectric points were purified from the cytosol fraction of rat liver. Four transferases are class Mu enzymes related to the previously characterized GSH transferases 3-3, 4-4 and 6-6, as judged by structural and enzymic properties. Two additional GSH transferases are distinguished by high specific activities with 4-hydroxyalk-2-enals, toxic products of lipid peroxidation. The most abundant of these two enzymes, GSH transferase 8-8, a class Alpha enzyme, has earlier been identified in rat lung and kidney. The amino acid sequence of subunit 8 was determined and showed a typical class Alpha GSH transferase structure including an N-acetylated N-terminal methionine residue.

Glutathione-dependent dechlorination of 1,6-dichloro-1,6-dideoxyfructose

The metabolism of 14C- and 36Cl-labelled 1,6-dichloro-1,6-dideoxyfructose (DCF) was studied in the isolated perfused rat liver system. Dechlorination of DCF occurred in the liver and erythrocytes and was GSH-dependent. The GSH conjugate formed was identified by 13C and 1H n.m.r. as the 6-chlorofructos-1-yl-SG conjugate. It is proposed that the GS- anion attacks the low steady-state concentration of the reactive keto form of DCF and that the conjugate formed cyclizes to the dominant beta-anomer. 6-Chlorofructos-1-yl-SG conjugate of hepatic origin is excreted into bile, whereas that produced in erythrocytes does not enter the liver.

Effects of chronic ethanol feeding on rat hepatocytic glutathione. Relationship of cytosolic glutathione to efflux and mitochondrial sequestration

Chronic ethanol feeding to rats increases the sinusoidal component of hepatic glutathione (GSH) efflux, despite a lower steady-state GSH pool size. In the present studies, no increase of biliary GSH efflux in vivo was found in chronic ethanol-fed cells. Studies were performed on ethanol-fed and pair-fed cells to identify the kinetic parameters of cellular GSH concentration-dependent efflux. The relationship between cytosolic GSH and the rate of efflux was modeled by the Hill equation, revealing a similar Vmax, 0.22 +/- 0.013 vs. 0.20 +/- 0.014 nmol/min per 10(6) cells for ethanol-fed and pair-fed cells, respectively, whereas the Km was significantly decreased (25.3 +/- 2.3 vs. 33.5 +/- 1.4 nmol/10(6) cells) in ethanol-fed cells. The difference in Km was larger when the data were corrected for the increased water content in ethanol-fed cells. We found a direct correlation between mitochondria and cytosolic GSH, revealing that mitochondria from ethanol-fed cells have less GSH at all cytosolic GSH values. The rate of resynthesis in depleted ethanol-fed cells in the presence of methionine and serine was similar to control cells and gamma-glutamylcysteine synthetase remained unaffected by chronic ethanol. However, the reaccumulation of mitochondrial GSH as the cytosolic pool increased was impaired in the ethanol cells. The earliest time change in GSH regulation was a 50% decrease in the mitochondrial GSH at 2 wk.

The glutathione S-transferases: an update

Over the last 15 years, we have passed through an initial period in which multiple forms of GST in various organs and different species were identified and characterized. The focus of current research is to define the role of the numerous isozymes in cell function, to ascertain the relationship between structure and function of different isozymes and to determine how the expression of GST is regulated in different tissues. During these studies, it is expected that new roles for the GST will be proposed, and this family of multifunctional proteins will continue to hold the interest of numerous investigators for many years.

Glutathione S-transferase of bovine iris and ciliary body: characterization of isoenzymes

Five forms of glutathione (GSH) S-transferase (GST) having catalytic activities towards a variety of xenobiotics were present in bovine iris-ciliary body. In contrast to that in lens, cornea, and retina, GST isoenzymes belonging to all the three classes (alpha, mu and pi) were present in iris as well as in the ciliary body. GST isoenzymes of iris-ciliary body had pI values of 8.7, 7.4, 7.0, 6.6, and 6.0. GST 8.7 and GST 7.4 were apparent homodimers of 27,000 and 22,500 Mr subunits, respectively. GST 8.7 cross-reacted only with antibodies raised against the alpha class GST of human liver and GST 7.4 cross-reacted with the antibodies raised against GST pi of human placenta. GST 7.0 and 6.6 were heterodimers of Mr 26,500 and 25,000 subunits and both these subunits cross-reacted with the antibodies raised against the mu class human GST. Iris-ciliary body contained both, GSH peroxidase I and GSH peroxidase II activities and in this respect also, they differ from lens, cornea, and retina each of which have only one of these two activities. The presence of several GST isoenzymes belonging to all the three major classes and both GSH peroxidase I and II activities in iris-ciliary body may be important for the detoxification of oxidants and xenobiotics in order to prevent their infiltration in aqueous humor.

Administration of L-2-oxothiazolidine-4-carboxylate increases glutathione levels in rat brain

Sterile L-2-oxothiazolidine-4-carboxylate, given as a neutral 100 mmol solution at a dose of 8 mmol/kg subcutaneously, caused an increase in total glutathione concentration in the brain of treated rats. This finding could be important in understanding the role of glutathione in the central nervous system.

Properties of the microsomal and cytosolic glutathione transferases involved in hexachloro-1:3-butadiene conjugation

Hexachloro-1,3-butadiene (HCBD) is a substrate for the hepatic microsomal glutathione transferases and is metabolised at higher rates by these enzymes than their cytosolic counterparts. Conjugation reactions catalysed by the microsomal and cytosolic transferases have been studied and characterized using this substrate and 1-chloro-2,4-dinitrobenzene (CDNB). In rat liver microsomes the Km values for HCBD and CDNB were 0.91 and 0.012 mM and in cytosol 0.51 and 0.10 mM respectively. Vmax values for HCBD were 1.39 and 0.35 nmol conjugate formed/min/mg protein for microsomes and cytosol respectively. In microsomal systems HCBD was a potent competitive inhibitor of the metabolism of CDNB with a Ki value of approximately 10 microM. However, CDNB did not inhibit HCBD metabolism significantly. These data suggest that more than one microsomal enzyme is involved in HCBD metabolism. The microsomal membrane could be solubilized without significant inhibition of HCBD activity; however, some detergents did inhibit the conjugation reaction. Activity was also lost on treatment of microsomal membranes with trypsin indicating the enzyme is localized on the cytoplasmic surface of the endoplasmic reticulum. Pretreatment of the rats with Aroclor 1254, 3-methylcholanthrene or phenobarbital did not change the microsomal conjugation of HCBD or CDNB with glutathione. Of seven species investigated, a human liver sample showed the highest ratio of microsomal to cytosolic glutathione transferase activity for HCBD (in microsomes 40-fold higher specific activity than in cytosol). Glutathione conjugation appears to play a critical role in the toxicity and carcinogenicity of some halogenated hydrocarbons. These data substantiate the potentially important role for the microsomal glutathione transferase in catalysing these reactions.

Multiple regulatory elements and phorbol 12-O-tetradecanoate 13-acetate responsiveness of the rat placental glutathione transferase gene

We have analyzed the cis-acting regulatory DNA elements of the placental rat glutathione S-alkyltransferase (GST-P) gene. Various regions of the 5' flanking sequence were fused with a bacterial chloramphenicol acetyltransferase gene. The transcriptional activity of each construct was determined by the transient expression assay after introduction into a hepatoma cell line. Multiple regulatory elements were identified. Two enhancing elements were located 2.5 and 2.2 kilobases upstream from the transcription start site and designated GST-P enhancers I and II (GPEI and GPEII, respectively). A consensus sequence of the phorbol 12-O-tetradecanoate 13-acetate responsive elements was present in the GPEI and at position -61. GPEII contained two of the simian virus 40 and one of the polyoma enhancer core-like sequences. A silencing element was also found 400 base pairs upstream from the cap site. In accordance with the above observation, endogenous GST-P gene was found to be stimulated when the rat fibroblast line 3Y1 was treated with phorbol 12-O-tetradecanoate 13-acetate. Phorbol 12-O-tetradecanoate 13-acetate enhanced the expression of the transfected GST-P gene to a much higher degree in HeLa cells than in the hepatoma cells, which constitutively expressed the endogenous GST-P. The results are discussed in terms of the specific derepression of GST-P gene during hepatocarcinogenesis in the rat.

Differential expression of alpha, mu and pi classes of isozymes of glutathione S-transferase in bovine lens, cornea, and retina

Isozyme characterization of glutathione S-transferase (GST) isolated from bovine ocular tissue was undertaken. Two isozymes of lens, GST 7.4 and GST 5.6, were isolated and found to be homodimers of a Mr 23,500 subunit. Amino acid sequence analysis of a 20-residue region of the amino terminus was identical for both isozymes and was identical to GST psi and GST mu of human liver. Antibodies raised against GST psi cross-reacted with both lens isozymes. Although lens GST 5.6 and GST 7.4 demonstrated chemical and immunological relatedness, they were distinctly different as evidenced by their pI and comparative peptide fingerprint. A corneal isozyme, GST 7.2, was also isolated and established to be a homodimer of Mr 24,500 subunits. Sequence analysis of the amino-terminal region indicated it to be about 67% identical with the GST pi isozyme of human placenta. Antibodies raised against GST pi cross-reacted with cornea GST 7.2. Another corneal isozyme, GST 8.7, was found to be homodimer of Mr 27,000 subunits. Sequence analysis revealed it to have a blocked amino-terminus. GST 8.7 immunologically cross-reacted with the antibodies raised against cationic isozymes of human liver indicating it to be of the alpha class. Two isozymes of retina, GST 6.8 and GST 6.3, were isolated and identified to be heterodimers of subunits of Mr 23,500 and 24,500. Amino-terminal sequence analysis gave identical results for both retina GST 6.8 and GST 6.3. The sequence analysis of the Mr 23,500 subunit was identical to that obtained for lens GSTs. Similarly, sequence analysis of the Mr 24,500 subunit was identical to that obtained for the cornea GST 7.2 isozyme. Both the retina isozymes cross-reacted with antibodies raised against human GST psi as well as GST pi. The results of these studies indicated that all three major classes of GST isozymes were expressed in bovine eye but the GST genes were differentially expressed in lens, cornea, and retina. In lens only the mu class of GST was expressed, whereas cornea expressed alpha and pi classes and retina expressed mu and pi classes of GST isozymes.

Inhibition of human glutathione S-transferases by bile acids

Glutathione S-transferase (GST) isoenzymes isolated from various human tissues are differentially inhibited by bile acids. Trihydroxy bile acid (lithocholate) was found to be more inhibitory to all the human GST isoenzymes tested in this study, as compared to the monohydroxy (cholate) and dihydroxy (chenodeoxycholate) bile acids. Among the three major classes of GST, mu class isoenzymes are generally inhibited to a greater extent than the alpha and pi class isoenzymes. The results of this study also indicate that differential inhibition of GST by various bile acids may be used to distinguish closely related GST isoenzymes within the mu class of GST isoenzyme. Likewise, the pi class or the anionic isoenzymes of human kidney, placenta, and erythrocytes can be distinguished using bile acid inhibition studies. These studies also provide further support for tissue-specific expression of GST isoenzymes in humans.