Microsomal glutathione S-transferase is the predominant leukotriene C4 binding site in cellular membranes

Location of Substrate Binding Sites within the Integral Membrane Protein Microsomal Glutathione Transferase-1†

Microsomal glutathione transferase-1 (MGST1) is a trimeric, membrane-bound enzyme with both glutathione (GSH) transferase and hydroperoxidase activities. As a member of the MAPEG superfamily, MGST1 aids in the detoxication of numerous xenobiotic substrates and in cellular protection from oxidative stress through the GSH-dependent reduction of phospholipid hydroperoxides. However, little is known about the location of the different substrate binding sites, including whether the transferase and peroxidase activities overlap structurally. Although molecular density attributed to GSH has been observed in the 3.2 A resolution electron crystallographic structure of MGST1, the electrophilic and phospholipid hydroperoxide substrate binding sites remain elusive. Amide H-D exchange kinetics and H-D ligand footprinting experiments indicate that GSH and hydrophobic substrates bind within similar, but distinct, regions of MGST1. Site-directed mutagenesis, guided by the H-D exchange results, demonstrates that specific residues within the GSH footprint effect transferase activity toward 1-chloro-2,4-dinitrobenzene. In addition, cytosolic residues surrounding the chemical stress sensor C49 but not modeled in the crystal structure appear to play an important role in the formation of the binding site for hydrophobic substrates. Although the fatty acid/phospholipid binding site structurally overlaps that for GSH, it does not appear to be localized to the same region as other hydrophobic substrates. Finally, H-D exchange mass spectrometry reveals a specific conformational transition that may mediate substrate binding and/or product release. Such structural changes in MGST1 are essential for activation of the enzyme and are important for its biological function.

Human microsomal prostaglandin E synthase 1: a member of the MAPEG protein superfamily

In this chapter, we briefly review the MAPEG superfamily (membrane associated proteins in eicosanoid and glutathione metabolism), a family of proteins in which all human members except one possess glutathione conjugating capacity. Recent findings regarding the biological functions of MAPEG proteins are highlighted. More extensively, the characterization of human microsomal prostaglandin E synthase 1 is presented, including results and applied methodology.

Pharmacological profile of MEN91507, a new CysLT(1) receptor antagonist

MEN91507 (8-[2-(E)-[4-[4-(4-fluorophenyl)butyloxy]phenyl]vinyl]-4-oxo-2-(5-1H-tetrazolyl)-4H-1-benzopyran sodium salt)) potently displaced [3H]leukotriene D(4) binding from guinea-pig lung and dimethylsulphoxide-differentiated U937 (dU937) cell membranes (K(i) 0.50 +/- 0.16 and 0.65 +/- 0.29 nM, respectively). On the other hand, MEN91507 did not display significant binding affinity for a series of receptors or channels. In functional studies on dU937 cells, MEN91507 behaved as insurmountable antagonist of leukotriene D(4)-induced calcium transients, with an apparent pK(B) of 10.25 +/- 0.15. In anaesthetized guinea-pigs, MEN91507 antagonized in a dose-dependent manner leukotriene D(4)-induced bronchoconstriction following i.v. or oral administration: the ED(50s) were 3.0 +/- 0.3 and 140 +/- 90 nmol/kg, respectively. The inhibition of leukotriene D(4)-induced bronchoconstriction by MEN91507 was long-lasting, since a dose of 0.6 micromol/kg produced 74% reduction of the response after 8 h from administration. Likewise, leukotriene D(4)-induced microvascular leakage was antagonized by MEN91507 either following i.v. or oral administration: a significant inhibitory effect was still evident at 16 h from oral administration of a dose of 6 micromol/kg. It is concluded that MEN91507 is a potent and selective antagonist of both guinea-pig and human CysLT(1) receptors; in addition, in vivo studies on guinea-pigs indicate that MEN91507 is an orally available and long-lasting antagonist of the bronchomotor and pro-inflammatory effects induced by leukotriene D(4) through the stimulation of CysLT(1) receptors.

[Pharmacological profile and clinical effects of montelukast sodium (Singulair chewable tablet), an antiasthmatic agent]

Montelukast (Singulair) is an antiasthmatic agent that has the chemical structure of a quinoline. Montelukast has a high affinity for the CysLT1 receptor and a potency that is not influenced by human serum protein. Montelukast antagonizes contractions of guinea-pig trachea induced by LTD4 in a competitive manner. Intravenous montelukast inhibited bronchoconstriction induced by LTD4 in guinea pigs. Oral montelukast inhibited increased airway resistance induced by antigen in squirrel monkeys. Montelukast also inhibited both inflammatory and immunologic responses induced by either LTD4 or antigen in guinea pigs and rats. Plasma concentrations of montelukast after oral administration of 10 mg in humans were shown to be over the effective level for at least 24 h. These lines of evidence support the effectiveness of a regimen of 10 mg/day for asthmatic symptoms in humans. In a number of clinical experiments, montelukast not only improved asthmatic symptoms and respiratory indices, but also inhibited airway inflammation and exercise-induced bronchoconstriction. These effects persisted during extended treatment. Montelukast produced an additive effect to basic therapy with an inhaled steroid. There were no differences in the incidence and magnitude of adverse effects between montelukast and placebo groups in clinical experiments. Montelukast is expected to serve as a first line of asthmatic therapy because of its consistent efficacy and good safety profile and it is associated with good compliance in patients because of its simple regimen of one 10 mg tablet/day.

Cysteinyl leukotriene receptors

The cysteinyl leukotrienes, leukotriene C4 (LTC4), leukotriene D4 (LTD4) and leukotriene E4 (LTE4), activate contractile and inflammatory processes via specific interaction with putative seven transmembrane-spanning receptors that couple to G proteins and subsequent intracellular signaling pathways. Pharmacological characterizations identified at least two subtypes of cysteinyl leukotriene (CysLT) receptor based on agonist and antagonist potency for biological responses. The rank potency of agonist activation for the CysLT1 receptor is LTD4 > LTC4 > LTE4 and for the CysLT2 receptor is LTC4 = LTD4 > LTE4. CysLT1 selective receptor antagonists are efficacious in the treatment of asthma. No selective CysLT2 receptor antagonists have been described. Molecular identification of the human and mouse CysLT1 and CysLT2 receptors has confirmed their structure as putative seven transmembrane domain G protein-coupled receptors and largely confirmed the previous pharmacological characterizations. The CysLT1 receptor is most highly expressed in spleen, peripheral blood leukocytes including eosinophils, and lung smooth muscle cells and interstitial lung macrophages. The CysLT2 receptor is most highly expressed in the heart, adrenal medulla, placenta and peripheral blood leukocytes. The molecular identification of the mouse CysLT1 and CysLT2 receptors show similar but not identical profiles to the orthologous human receptors.

Pharmacological differences among CysLT(1) receptor antagonists with respect to LTC(4) and LTD(4) in human lung parenchyma

We have previously reported, by means of equilibrium binding studies, the existence of two distinct binding sites with receptor characteristics for LTC(4) and LTD(4) in human lung parenchyma (HLP) membranes using S-decyl-glutathione (S-decyl-GSH) to inhibit LTC(4) binding to a number of non-receptor sites. Recently, we have been able to avoid the use of S-decyl-GSH in kinetic experiments and to characterize a distinctive pharmacological profile for the LTC(4) high affinity binding sites which do not correlates with the ability of both LTD(4) and LTC(4) to contract isolated HLP strips through the CysLT(1) receptor. Here, we report that the most advanced CysLT(1) receptor antagonists, some of which are already in clinical use, displayed a different behavior toward LTC(4) and LTD(4) in HLP. Equilibrium and kinetic binding studies demonstrated the following rank order of potency for (3)H-LTD(4) receptor (CysLT(1)): zafirlukast = montelukast > LM-1507 = LM-1484 = pranlukast. In addition, LM-1507, LM-1484, pranlukast and montelukast but not zafirlukast are able to interact also with the high affinity site for (3)H-LTC(4) (LM-1507 = LM-1484 > pranlukast; montelukast not detectable in the presence of S-decyl-GSH). In this respect, the behavior of the LM antagonists closely resembles that of pranlukast although LM-1507 and LM-1484 display a higher affinity for (3)H-LTC(4) sites. Montelukast has an intermediate behavior, inasmuch as its interaction with (3)H-LTC(4) sites can be revealed only in kinetic studies, while zafirlukast is totally unable to inhibit (3)H-LTC(4) binding. It might be, therefore, most relevant for a complete understanding of the clinical efficacy, besides their nominal potency, of the most advanced CysLT(1) receptor antagonists to consider their pharmacological differences with respect not only to LTD(4)/LTE(4), but also to LTC(4).

Functional characterization of cysteinyl leukotriene CysLT(2) receptor on human coronary artery smooth muscle cells

Cysteinyl leukotrienes (LTC(4), LTD(4), and LTE(4)) are a class of biologically active lipids that exert potent effects on the heart. To assess their roles, we investigated the distribution of their receptors, CysLT(1) and CysLT(2), in the cardiovascular system. CysLT(2) mRNA was detected at high levels in the human atrium and ventricle and at intermediate levels in the coronary artery, whereas CysLT(1) mRNA was barely detected. Further analysis by in situ hybridization revealed that CysLT(2) mRNA was expressed in myocytes, fibroblasts, and vascular smooth muscle cells, but not in endothelial cells. When human coronary smooth muscle cells were stimulated with LTC(4), the intracellular calcium concentration increased in a dose-dependent manner, and this action was partially inhibited by nicardipine. Additionally, these cells showed chemotactic responses to LTC(4). This is the first report on the physiological role of CysLT(2), and the findings suggest that CysLT(2) has biological significance in the cardiovascular system.

Leukotrienes as mediators of asthma

This review describes the aspects of leukotriene (LT) pharmacology and biology that are relevant to their important role in asthma. The biosynthesis and metabolism, including transcellular metabolism, of LTB4 and the cysteinyl-LTs (i.e. LTC4, LTD4 and LTE4) are described, and their transport is briefly outlined. The existence, distribution and pharmacological characterization of the receptors (BLT, CysLT1, CysLT2), as well as the transduction mechanisms triggered, are discussed in detail. We also describe their effects on airway smooth muscle tone, hyperresponsiveness and proliferation, on vascular tone and permeability, on mucus secretion, on neural fibers and inflammatory cell functions. Finally, the evidence supporting their role as asthma mediators is reviewed, including the effects of anti LT drugs (both biosynthesis inhibitors and receptor antagonists) in experimental and clinical asthma.

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.

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.

The projection structure of the membrane protein microsomal glutathione transferase at 3 A resolution as determined from two-dimensional hexagonal crystals

The formation of two-dimensional crystals of the membrane-bound enzyme microsomal glutathione transferase is sensitive to fractional changes in the lipid-to-protein ratio. Variation of this parameter results in crystal polymorphism. The projection structure of a p6 crystal form of the enzyme has been determined by the use of electron crystallography. The unit cell at 3 A resolution is comprised of two trimers. The hexagonal p6 and the orthorhombic p21212 crystal types have common elements in the packing arrangement which imply dominant crystal contacts. An overall structural similarity between the protein molecules in the two crystal forms is suggested by the projection maps. Furthermore, a comparison of the p6 and p21212 projection maps identifies additional corresponding protein densities which could not be assigned to the microsomal glutathione transferase trimer previously. Surprisingly, an ambiguity of the rotational orientation was found for trimers interspersed at certain positions within the crystal lattice.

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.

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

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

Identification and characterization of two cysteinyl-leukotriene high affinity binding sites with receptor characteristics in human lung parenchyma

We report the characterization of two distinct binding sites with receptor characteristics for leukotriene (LT)D4 and LTC4 in membranes from human lung parenchyma. The use of S-decyl-glutathione allowed us to characterize a previously unidentified high affinity binding site for LTC4. Computerized analysis of binding data revealed that each leukotriene interacts with two distinct classes of binding sites (Kd = 0.015 and 105 nM for LTC4 and 0.023 and 230 nM for LTD4) and that despite cross-reactivity, the two high affinity sites are different entities. LTD4 binding sites displayed features of G protein-coupled receptors, whereas LTC4 binding sites did not show any significant modulation by guanosine-5'-(beta, gamma-imido)triphosphate or stimulation of GTPase activity. The antagonists ICI 198,615 and SKF 104353 were unselective for the high and low affinity states of LTD4 receptor, whereas only SKF 104353 was able to recognize the two [3H]LTC4 binding sites although with different affinities. These data indicate that in human lung parenchyma, LTD4 and LTC4 recognize two different binding sites; these binding sites are different entities; and for LTD4, the two binding sites represent the interconvertible affinity states of a G protein-coupled receptor, whereas for LTC4, the high affinity site is likely to be a specific LTC4 receptor.

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

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

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

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

Leukotriene receptors

The current challenge in research on leukotriene receptors is to clone these molecules. Traditional protein purification approaches have not been successful in providing sequence information. Solubilization of cys-LT1 has been achieved but results in the dissociation of G-proteins and the loss of high affinity binding (Mong et al., 1986b; Mong and Sarau, 1990), while cys-LT2 activity cannot be monitored by other than functional assays and there have not been any purification attempts. Partial purification of B-LT has been reported but has not been continued to homogeneity (Sherman et al., 1992; Votta et al., 1990; Miki et al., 1990). Nor have attempts to clone these receptors through either homology screening or expression cloning been successful. The cloning of the prostanoid receptors, described in detail elsewhere in this volume, has shown that these receptors belong to a distinct family within the G-protein-coupled receptor superfamily. It is probable, therefore, that the leukotriene receptors will also belong to a separate group within this superfamily since phylogenic comparisons have shown that receptors displaying high affinity for structurally related ligands exist as discrete families. Recently, a human cDNA encoding an orphan FMLP-related receptor cloned from HL60 cells of myeloid lineage was identified as the receptor for another eicosanoid, lipoxin A (Fiore et al., 1994). FMLP has a similar profile of biological actions to LTB4. Moreover, LTD4 showed a high degree of cross-reactivity with this receptor with an affinity only 20-fold less that of lipoxin A, although LTB4 was inactive. It remains to be determined whether the leukotriene receptors will fall into this class of receptors. The cloning of the leukotriene receptors will allow identification of the different receptor types and subtypes and potentially splice variants. Evaluation of currently developed antagonists at these receptor types could also open the way for novel therapies for inflammatory conditions.

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)