Reversibility in glutathione-conjugate formation

Evaluation of possible cytotoxic, genotoxic and epigenotoxic effects of titanium dioxide nanoparticles and possible protective effect of melatonin

Nanoparticles (NPs) are particles of matter that are between 1 to 100 nm in diameter. They are suggested to cause toxic effects in both humans and environment thorough different mechanisms. However, their toxicity profile may be different from the parent material. Titanium dioxide (TiO2) NPs are widely used in cosmetic, pharmaceutical and food industries. As a white pigment, the use of TiO2 is used in food coloring, industrial paints, clothing and UV filters has increased tremendously in recent years. Melatonin, on the other hand, is a well-known antioxidant and may prevent oxidative stress caused by a variety of different substances, including NPs. In the current study, we aimed to comparatively investigate the effects of normal-sized TiO2 (220 nm) and nano-sized TiO2 (21 nm) on cytopathology, cytotoxicity, oxidative damage (lipid peroxidation, protein oxidation and glutathione), genotoxicity (8-hydroxydeoxyguanosine), apoptosis (caspase 3, 8 and 9) and epigenetic alterations (global DNA methylation, H3 acetylation) on 3T3 fibroblast cells. In addition, the possible protective effects of melatonin, which is known to have strong antioxidant effects, against the toxicity of TiO2 were also evaluated. Study groups were: a. the control group; b. melatonin group; c. TiO2 group; d. nano-sized TiO2 group; e. TiO2 + melatonin group and f. nano-sized TiO2 + melatonin group. We observed that both normal-sized and nano-sized TiO2 NPs showed significant toxic effects. However, TiO2 NPs caused higher DNA damage and global DNA methylation compared to normal-sized TiO2 whereas normal-sized TiO2 led to lower H3 acetylation vs. TiO2 NPs. Melatonin showed partial protective effect against the toxicity caused by TiO2 NPs.

Reversibility and Low Commitment to Forward Catalysis in the Conjugation of Lipid Alkenals by Glutathione Transferase A4-4

High concentrations of electrophilic lipid alkenals formed during oxidative stress are implicated in cytotoxicity and disease. However, low concentrations of alkenals are required to induce antioxidative stress responses. An established clearance pathway for lipid alkenals includes conjugation to glutathione (GSH) via Michael addition, which is catalyzed mainly by glutathione transferase isoform A4 (GSTA4-4). Based on the ability of GSTs to catalyze hydrolysis or retro-Michael addition of GSH conjugates, and the antioxidant function of low concentrations of lipid alkenals, we hypothesize that GSTA4-4 contributes a homeostatic role in lipid metabolism. Enzymatic kinetic parameters for retro-Michael addition with trans-2-Nonenal (NE) reveal the chemical competence of GSTA4-4 in this putative role. The forward GSTA4-4-catalyzed Michael addition occurs with the rapid exchange of the C2 proton of NE in D₂O as observed by NMR. The isotope exchange was completely dependent on the presence of GSH. The overall commitment to catalysis, or the ratio of first order k_cat,f for 'forward' Michael addition to the first order k_cat,ex for H/D exchange is remarkably low, approximately 3:1. This behavior is consistent with the possibility that GSTA4-4 is a regulatory enzyme that contributes to steady-state levels of lipid alkenals, rather than a strict 'one way' detoxication enzyme.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Brassica meal-derived allyl-isothiocyanate postharvest application: influence on strawberry nutraceutical and biochemical parameters

BACKGROUND

The antimicrobial activity of allyl-isothiocyanate (AITC) on plant pathogens is well known and has already been demonstrated in the strawberry with respect to Botritis cinerea fungal infection using postharvest biofumigation. In the present study, vapours of 0.08 mg L^-1 of Brassica meal-derived AITC were applied to strawberry to assess its effect on fruit nutraceutical and biochemical parameters after 2 days of storage at 20 °C and 90% relative humidity.

RESULTS

Allyl-isothiocyanate showed no detrimental effect on final strawberry quality, anti-oxidant properties or ascorbic acid content. By contrast, an increased amount of asparagine and a higher ascorbate and glutathione redox potential were registered in the fruit soon after treatment. A reversible glutathione depletion action of AITC was also observed. Finally, total AITC residues in treated strawberry were quantified and a relatively high amount of AITC-adducts was found in fruit tissues.

CONCLUSION

The findings of the present study not only confirm the high potentiality of biofumigation with respect to extending the shelf-life of fruit, but also provide some insight regarding the mechanisms of action of AITC at the cellular level as a possible elicitor of fruit protective responses. Nevertheless, the nature of the AITC-adducts formed in fruit tissues needs further attention to enable a health and safety assessment of the final fruit. © 2019 Society of Chemical Industry.

Metabolism of Glutathione S-Conjugates: Multiple Pathways

Many potentially toxic electrophilic xenobiotics and some endogenous compounds are detoxified by conversion to the corresponding glutathione S-conjugate, which is metabolized to the N-acetylcysteine S-conjugate (mercapturate) and excreted. Some mercapturate pathway components, however, are toxic. Bioactivation (toxification) may occur when the glutathione S-conjugate (or mercapturate) is converted to a cysteine S-conjugate that undergoes a β-lyase reaction. If the sulfhydryl-containing fragment produced in this reaction is reactive, toxicity may ensue. Some drugs and halogenated workplace/environmental contaminants are bioactivated by this mechanism. On the other hand, cysteine S-conjugate β-lyases occur in nature as a means of generating some biologically useful sulfhydryl-containing compounds.

Excretion pattern and dynamics of glutathione detoxification of microcystins in Sprague Dawley rat

The excretion route and dynamics of the glutathione (GSH) conjugate of microcystin-RR (MCRR), MCRR-GSH, were quantitatively studied in Sprague Dawley rat exposed with MCRR-GSH via liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). In the MCRR-GSH-treated rat, the average MCRR-Cysteine (MCRR-Cys)/MCRR-GSH ratio reached as high as 105.3, which indicated that the intermediate conjugate MCRR-GSH was rapidly converted to the product compound MCRR-Cys. Besides, MCRR was consistently detected in MCRR-GSH-treated rat, which suggested that MCRR can be dissociated from the MCRR-GSH conjugate and the reversibility of the MC-GSH conjugate. Results of total MC contents analysis in excrement showed that the total MC contents in urine were significantly higher than those in feces. The ratio of the total MC content in urine to feces was as high as 129.3, which demonstrates that the urine is the main route of excretion after MCRR-GSH-treatment. In urine, the MCRR-Cys concentration was 27.8-fold, 19.4-fold higher than MCRR-GSH and MCRR, respectively. Our results, for the first time, quantitatively found that MCRR-GSH was rapidly converted to MCRR-Cys after exposed to rat, and was excreted mainly through urine in the form of the MCRR-Cys conjugate. This study suggests that the GSH detoxification pathway of MCs could help to explain the greater sensitivity of mammals to MCs.

Rapid conversion and reversible conjugation of glutathione detoxification of microcystins in bighead carp (Aristichthys nobilis)

The glutathione and cysteine conjugates of microcystin (MC-GSH and MC-Cys, respectively) are two important metabolites in the detoxification of microcystins (MCs). Although studies have quantitated both conjugates, the reason why the amounts of MC-GSH are much lower than those of MC-Cys in various animal organs remains unknown. In this study, MC-RR-GSH and MC-RR-Cys were respectively i.p. injected into the cyanobacteria-eating bighead carp (Aristichthys nobilis), to explore the biotransformation and detoxification mechanisms of the two conjugates. The contents of MC-RR, MC-RR-GSH, MC-RR-Cys and MC-RR-N-acetyl-cysteine (MC-RR-Nac, the acetylation product of MC-RR-Cys) in the liver, kidney, intestine and blood of bighead carp in both groups were quantified via liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). In the MC-RR-GSH-treated group, the MC-RR-Cys content in the kidney increased 96.7-fold from 0.25 to 0.5h post-injection, demonstrating that MC-RR-GSH acts as a highly reactive intermediate and is rapidly converted to MC-RR-Cys. The presence of MC-RR in both MC-RR-GSH- and MC-RR-Cys-treated groups indicates, for the first time, that MC conjugation with the thiol of GSH/Cys is a reversible process in vivo. Total MC-RR concentrations dissociated from MC-RR-Cys were lower than those from MC-RR-GSH, suggesting that MC-RR-Cys is more capable of detoxifying MC-RR. MC-RR-Cys was the most effectively excreted form in both the kidney and intestine, as the ratios of MC-RR-Cys to MC-RR reached as high as 15.2, 2.9 in the MC-RR-GSH-treated group and 63.4, 19.1 in the MC-RR-Cys-treated group. Whereas MC-RR-Nac could not be found in all of the samples of the present study. Our results indicate that MC-RR-GSH was rapidly converted to MC-RR-Cys and then excreted, and that both glutathione and cysteine conjugates could release MC-RR. This study quantitatively proves the importance of the GSH detoxification pathway and furthers our understanding of the biochemical mechanism by which bighead carp are resistant to toxic cyanobacteria.

Use of cysteine-reactive small molecules in drug discovery for trypanosomal disease

INTRODUCTION

The roles of cysteine protease (CP) enzymes in the biochemistry and infectivity of the three trypanosomal parasitic infections, Chagas' disease, leishmaniasis and human African trypanosomiasis, which have been elucidated over the last three decades are summarized. Inhibitors of these enzymes, which act through trapping the active site cysteine with an electrophilic warhead, hold huge potential as therapeutic agents but the promise of these has yet to be realized in clinical studies. The article addresses aspects that ought to be considered in order to develop orally active CP inhibitors that are safe and effective therapies for trypanosomiasis.

AREAS COVERED

This article reviews learnings from CP research in the trypanosomal field and recent advances in developing cysteine protease inhibitors (CPIs) of human cathepsin K, a related enzyme. Considerations such as intra- and extracellular localization of the CPs, off-target activities against human cathepsin enzymes, basic versus neutral and potential pro-drug inhibitors are reviewed. A description of odanacatib, a cathepsin K inhibitor currently in late stage development, is made to illustrate the attributes of a clinically viable CPI.

EXPERT OPINION

The emerging role of CPs in a wide array of parasitic diseases is highlighted with the vision that CP inhibitors could become the 'β-lactams' of anti-parasitic treatments in the coming decades. New CPI research will see the optimization of intra- and extracellular enzyme targeting, reduction of off-target activities and better understanding of pharmacokinetic-pharmacodynamic interactions which will all lead to compounds with much improved efficacy and viability as clinical therapies.

Transthiocarbamoylation of proteins by thiolated isothiocyanates

Isothiocyanates, membrane-permeable electrophiles that form adducts with thiols, have been suggested to have important medical benefits. Here we shed light on isothiocyanate-thiol conjugates and studied their electrophilic potential transferring an isothiocyanate moiety to cellular proteins. When we examined the effect of sulfhydryl molecules on cellular response induced by 6-methylsulfinylhexyl isothiocyanate (6-HITC), an analog of sulforaphane isolated from broccoli, we observed significant induction of heme oxygenase-1 by 6-HITC even in the presence of N-acetyl-L-cysteine or glutathione (GSH). In addition, the authentic 6-HITC-β-mercaptoethanol (6-HITC-ME) conjugate markedly up-regulated the enzyme expression, suggesting the electrophilic potential of thiolated isothiocyanates. To gain a chemical insight into the cellular response induced by thiolated isothiocyanates, we studied the occurrence of transthiocarbamoylation of sulfhydryl molecules by 6-HITC-ME and observed that, upon incubation of 6-HITC-ME with GSH, a single product corresponding to the GSH conjugate of 6-HITC was generated. To test the functional ability of thiolated isothiocyanates to thiocarbamoylate proteins in living cells, we designed a novel probe, combining an isothiocyanate-reactive group and an alkyne functionality, and revealed that the transthiocarbamoylation of proteins occurred in the cells upon exposure to 6-HITC-ME. The target of thiocarbamoylation included heat shock protein 90 β (Hsp90β), a chaperone ATPase of the Hsp90 family implicated in protein maturation and targeting. To identify the sites of the Hsp90β modification, we utilized nano-LC/MALDI-TOF MS/MS and suggested that a thiol group on the peptide containing Cys-521 reacted with 6-HITC, resulting in a covalent adduct in a 6-HITC-treated recombinant Hsp90β in vitro. The site-selective binding to Cys-521 was supported by in silico modeling. Further study on the thiocarbamoylation of Hsp90β suggested that the formation of 6-HITC-Hsp90β conjugate might cause activation of heat shock factor-1, rapidly signaling a potential heat shock response. These data suggest that thiolated isothiocyanates are an active metabolite that could contribute to cellular responses through transthiocarbamoylation of cellular proteins.

Enzymes Involved in Processing Glutathione Conjugates

Many potentially toxic electrophiles react with glutathione to form glutathione S-conjugates in reactions catalyzed or enhanced by glutathione S-transferases. The glutathione S-conjugate is sequentially converted to the cysteinylglycine-, cysteine- and N-acetyl-cysteine S-conjugate (mercapturate). The mercapturate is generally more polar and water soluble than the parent electrophile and is readily excreted. Excretion of the mercapturate represents a detoxication mechanism. Some endogenous compounds, such as leukotrienes, prostaglandin (PG) A2, 15-deoxy-Δ^12,14-PGJ₂, and hydroxynonenal can also be metabolized to mercapturates and excreted. On occasion, however, formation of glutathione S- and cysteine S-conjugates are bioactivation events as the metabolites are mutagenic and/or cytotoxic. When the cysteine S-conjugate contains a strong electron-withdrawing group attached at the sulfur, it may be converted by cysteine S-conjugate β-lyases to pyruvate, ammonium and the original electrophile modified to contain an –SH group. If this modified electrophile is highly reactive then the enzymes of the mercapturate pathway together with the cysteine S-conjugate β-lyases constitute a bioactivation pathway. Some endogenous halogenated environmental contaminants and drugs are bioactivated by this mechanism. Recent studies suggest that coupling of enzymes of the mercapturate pathway to cysteine S-conjugate β-lyases may be more common in nature and more widespread in the metabolism of electrophilic xenobiotics than previously realized.

Absorption, distribution, metabolism and excretion of an inhalation dose of [14C] 4,4'-methylenediphenyl diisocyanate in the male rat

The received dose, tissue distribution, metabolism, routes and rates of excretion of [(14)C]-4, 4(')-methylenediphenyl diisocyanate (MDI) were investigated in the male rat following a 6-h inhalation exposure to [(14)C]-MDI at a target concentration of 2 mg m(-3). The mean dose received was equivalent to 0.078 mg MDI per animal, of this between 25 and 32% of radiolabelled material was available systemically. Radioactivity was distributed to all tissues examined with the highest proportions present in the respiratory and gastrointestinal tracts, suggesting that both oral ingestion and pulmonary absorption contributed to the systemic dose of [(14)C]-MDI derived material, with the oral ingestion and the majority of the internal dose resulting from ingestion of radiolabelled material by grooming the pelt after exposure. Radioactivity was excreted mainly via faeces (about 80% of the received dose). Excretion in bile and urine each accounted for less than 15% of the dose. MDI was extensively metabolized after uptake, with two routes of transformation evident; the proposed spontaneous formation of mixed molecular weight polyureas and the enzyme catalysed metabolism of systemically available MDI or MDI derivatives to give N-acetylated and N-acetylated hydroxylated products. No free MDA was detected in any of the biomatrices (urine, faeces, bile) investigated.

Exploration of in vitro pro-drug activation and futile cycling by glutathione S-transferases: thiol ester hydrolysis and inhibitor maturation

In addition to glutathione (GSH) conjugating activity, glutathione S-transferases (GSTs) catalyze "reverse" reactions, such as the hydrolysis of GSH thiol esters. Reverse reactions are of interest as potential tumor-directed pro-drug activation strategies and as mechanisms for tissue redistribution of carboxylate-containing drugs. However, the mechanism and specificity of GST-mediated GSH thiol ester hydrolysis are uncharacterized. Here, the GSH thiol esters of ethacrynic acid (E-SG) and several nonsteroidal antiinflammatory agents have been tested as substrates with human GSTs. The catalytic hydrolysis of these thiol esters appears to be a general property of GSTs. The hydrolysis of the thiol ester of E-SG was studied further with GSTA1-1 and GSTP1-1, as a model pro-drug with several possible fates for the hydrolysis products: competitive inhibition, covalent enzyme adduction, and sequential metabolism. In contrast to hydrolysis rates, significant isoform-dependent differences in the subsequent fate of the products ethacrynic acid and GSH were observed. At low [E-SG], only the GSTP1-1 efficiently catalyzed sequential metabolism, via a dissociative mechanism.

Toxicology of toluene diisocyanate

In studies on animals, toluene diisocyanate (TDI) was a contact and respiratory sensitizer, was not toxic by the oral or dermal routes, but was irritating, and toxic by inhalation. The respiratory tract was the target in acute, subchronic, and chronic exposure studies. Typically, at concentrations of above 0.1 ppm (parts per million), clinical signs of nasal irritation were evident, and histopathological investigations revealed rhinitis and epithelial hyperplasia of nasal passages. With increasing concentration, effects were more severe; affected the larynx, trachea, and lung; and, eventually, affected body weight and survival. The carcinogenicity of TDI to rats and mice was investigated. By inhalation, there was no treatment-related increase in tumor incidence in either species at the highest concentration tested (0.15 ppm). Effects of TDI were seen as rhinitis in nasal turbinates of both species, and as reduced body weight gain in mice. Through oral administration of TDI dissolved in corn oil to rats and mice (up to 120 mg/kg/day), increased incidence of a number of tumor types was seen. This route is of questionable relevance to occupational exposure. The dosing solutions were known to have degraded, and TDI would hydrolyze to diaminotoluene in the acidic stomach environment. Several in vitro tests for genotoxicity gave positive results, which can be ascribed to degradation of TDI by solvents. In properly conducted assays, in vivo TDI was negative for genotoxicity. In a two-generation reproduction study in rats, there were no effects on reproductive indices at the highest exposure concentration of 0.3 ppm TDI, which elicited toxicity in both generations. In a developmental toxicity study in rats, there was evidence of minimal fetotoxicity in the presence of maternal toxicity at 0.5 ppm, with no effects at 0.1 ppm. No treatment-related embryotoxicity or teratogenicity was observed.

Glutathione conjugates and their synthetic derivatives as inhibitors of glutathione-dependent enzymes involved in cancer and drug resistance

Alterations in levels of glutathione (GSH) and glutathione-dependent enzymes have been implicated in cancer and multidrug resistance of tumor cells. The activity of a number of these, the multidrug resistance-associated protein 1, glutathione S-transferase, DNA-dependent protein kinase, glyoxalase I, and gamma-glutamyl transpeptidase, can be inhibited by GSH-conjugates and synthetic analogs thereof. In this review we focus on the function of these enzymes and carriers in cancer and anti-cancer drug resistance, in relation to their inhibition by GSH-conjugate analogs.

Glutathione adducts of oxyeicosanoids

Glutathione (GSH) is a major cellular antioxidant, which can conjugate chemically reactive, electrophilic molecules and thus, prevent unwanted reactions with important cell constituents. A large number of electrophilic eicosanoids, in particular alpha/beta-unsaturated ketones, are synthesized during arachidonic acid oxidative metabolism which can participate in the Michael addition reaction with GSH catalyzed by the GSH-S-transferase (GST) family. The structures of these adducts have been determined primarily using mass spectrometry techniques in the past after degradation to volatile products, but more recently by electrospray ionization. GSH-adducts have been observed with molecules synthesized through the 5-lipoxygenase (LTB4, LTC4, and 5-oxo-ETE), 12-lipoxygenase (hepoxilin A3), 15-lipoxygenase (13-oxo-ODE), PGH synthase (PGA1, PGA2, PGD2, PGE2, and PGJ2), and cytochrome P450-epoxygenase (14,15-EET) pathways of arachidonic acid metabolism. It has also been demonstrated that these oxyeicosanoid GSH-adducts do not represent just inactivation products, but they can both retain (GSH-adduct of hepoxilin A3) or show novel biological activities (LTC4 and FOG7).

Glutathione conjugation as a bioactivation reaction

In general, glutathione conjugation is regarded as a detoxication reaction. However, depending on the properties of the substrate, bioactivation is also possible. Four types of activation reaction have been recognized: direct-acting compounds, conjugates that are activated through cysteine conjugate beta-lyase, conjugates that are activated through redox cycling and lastly conjugates that release the original reactive parent compound. The glutathione S-transferases have three connections with the formation of biactivated conjugates: they catalyze their formation in a number of cases, they are the earliest available target for covalent binding by these conjugates and lastly, the parent alkylating agents are regularly involved in the induction of the enzymes. Individual susceptibility for each of these agents is determined by individual transferase subunit composition and methods are becoming available to assess this susceptibility.

Efficient hepatic uptake and concentrative biliary excretion of a mercapturic acid

The role of the liver in the disposition of circulating mercapturic acids was examined in anesthetized rats and in the isolated perfused rat liver using S-2,4-dinitrophenyl-N-acetylcysteine (DNP-NAC) as the model compound. When DNP-NAC was infused into the jugular vein (150 or 600 nmol over 60 min) it was rapidly and nearly quantitatively excreted as DNP-NAC into bile (42-36% of the dose) and urine (48-62% of dose). Some minor metabolites were detected in bile (<4%), with the major metabolite coeluting on HPLC with the DNP conjugate of glutathione (DNP-SG). Isolated rat livers perfused single pass with 3 microM DNP-NAC removed 72 +/- 9% of this mercapturic acid from perfusate. This rapid DNP-NAC uptake was unaffected by sodium omission, or by L-cysteine, L-glutamate, L-cystine, or N-acetylated amino acids, but was decreased by inhibitors of hepatic sinusoidal organic anion transporters (oatp), indicating that DNP-NAC is a substrate for these transporters. The DNP-NAC removed from perfusate was promptly excreted into bile, eliciting a dose-dependent choleresis. DNP-NAC itself constituted approximately 75% of the total dose recovered in bile, reaching a concentration of 9 mM when livers were perfused in a recirculating mode with an initial DNP-NAC concentration of 250 microM. Other biliary metabolites included DNP-SG, DNP-cysteinylglycine, and DNP-cysteine. DNP-SG was likely formed by a spontaneous retro-Michael reaction between glutathione and DNP-NAC. Subsequent degradation of DNP-SG by biliary gamma-glutamyltranspeptidase and dipeptidase activities accounts for the cysteinylglycine and cysteine conjugates, respectively. These findings indicate the presence of efficient hepatic mechanisms for sinusoidal uptake and biliary excretion of circulating mercapturic acids in rat liver and demonstrate that the liver plays a role in their whole body elimination.

Comparative cytotoxicity of monocrotaline and its metabolites in cultured pulmonary artery endothelial cells

Metabolites of the pyrrolizidine alkaloid monocrotaline cause progressive development of pulmonary hypertension in rats. The putative reactive intermediate monocrotaline pyrrole (MCTP) has been shown to cause cytotoxicity, hypertrophy, decreased proliferation, and altered synthetic capability in cultured pulmonary endothelial cells. We compared effects of monocrotaline (MCT) at 60 micrograms/ml (0.185 mM) with previously identified metabolites, MCTP 10 micrograms/ml (0.031 mM) and glutathione-conjugated dihydropyrrolizine (GSH-DHP) 60 micrograms/ml (0.135 mM), in cultured bovine pulmonary artery endothelial cells (BPAECs). To determine whether endothelial metabolism might contribute to the mechanism of this toxicity, we used markers of cytotoxicity (LDH release), synthetic activity (PGI2 synthesis), hypertrophy (planimetry), cell density (cell count/area), and Evans blue albumin (EBA) transudation as a marker for loss of fluid barrier integrity. We found changes in all endothelial markers with MCTP only. MCTP caused increased LDH release by 48 hr, augmented PGI2 synthesis by 96 hr, and resulted in hypertrophy and decreased cell density by 48 hr that persisted at least 21 days. There was increased EBA transudation at 24 hr posttreatment. We concluded that, based on markers of endothelial damage, BPAECs showed no apparent ability to metabolize MCT to a reactive intermediate nor to further metabolize GSH-DHP to a toxic species. We also concluded that MCTP can cause a direct effect on fluid barrier integrity of endothelial cell monolayers in the absence of inflammation.

Forward and reverse catalysis and product sequestration by human glutathione S-transferases in the reaction of GSH with dietary aralkyl isothiocyanates

The reversible reaction of GSH with two dietary anticarcinogens, benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC), has been studied in the absence and presence of human glutathione S-transferases (GSTs). The spontaneous reaction at pH 7.4 and 37 degrees C yielded values for k2 of 17.9 and 6.0 M-1.s-1 for GSH conjugation of BITC and PEITC respectively (forward reaction), and k1 values of 6.9 x 10(-4) and 2.4 x 10(-4) s-1 for dissociation of the respective GSH conjugates, BITC-SG and PEITC-SG (reverse reaction). GSTs A1-1, A2-2, M1a-1a and P1-1 catalysed both the forward and reverse reactions with specific activities (mumol/min per mg at 30 microM isothiocyanate or GSH conjugate) ranging from 23.1 for the GSH conjugation of BITC by GST P1-1 to 0.03 for the dissociation of BITC-SG by GST A1-1. When present at similar concentration to substrates (12 microM), GSTs A1-1 and A2-2 but not GST M1a-1a shifted the equilibrium in favour of BITC-SG or PEITC-SG. Kinetic studies confirmed that GST A1-1 interacted selectively with the GSH conjugates in the micromolar range (Km 6.9 microM, Ki 4.3 microM), whereas GST M1a-1a interacted with BITC-SG and PEITC-SG with approx. 5-fold lower affinity. In conclusion, GSTs are true catalysts; at high intracellular concentration they also sequester GSH conjugates, promoting GSH conjugation, whereas trace extracellular GSTs promote dissociation of effluxed organic isothiocyanate-GSH conjugates.

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)