Quinone-Thioether-Mediated Nephrotoxicity

DRUG METABOLISM REVIEWS, 27(1&2), 125-141 (1995)

QUINONE-THIOETHER-MEDIATED N EPH ROTOXlClTY * SERRINE S. LAU Division of Pharmacology and Toxicology College of Pharmacy University of Texas at Austin Austin, Texas 78712

I.

11.

111.

IV .

V.

VI .

VII.

INTRODUCTION ......................................................... 126

GLUTATHIONE AND QUINONE-MEDIATED TOXICITIES 126

QUINONE-THIOETHERS AND NEPHROTOXICITY ......... .127

SPECIES DIFFERENCES IN RESPONSE TO QUINONE- THIOETHERS ............................................................. 128

NEPHROCARCINOGENICITY OF QUINONE- THIOETHERS ............................................................. 130

MECHANISMS OF QUINONE-THIOETHER-MEDIATED NEPHROTOXICITY ...................................................... 132

SUMMARY ................................................................ 134

References ................................................................... 135

*Presented at the Second Arkansas Toxicology Symposium, honoring James R. Gillette, Ph.D., October 14-15, 1993, at Arkansas’ Excelsior Hotel, Little Rock, Arkansas.

125

Copyright 0 1995 by Marcel Dekker, Inc.

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I. INTRODUCTION

Glutathione (y-glutamylcysteinylglycine; GSH) is the predominant intra- cellular nonprotein sulfhydryl present in the cytosol of animal and plant cells, and plays an integral role in a large number of biologically important reactions [l-31. The numerous reactions that GSH participates in can be divided into those involving the y-glutamyl moiety of the tripeptide and those of the sulfhydryl moiety. Those reactions involving the latter function can be further divided into either oxidation-reduction reactions or nucleophilic reactions. In its reduced form GSH is a strong nucleophile and can react with electrophiles via a direct S n 2 reaction to form a thioether. These reactions may also be catalyzed by one or more of the cytosolic, microsomal, or mitochondria1 GSH S-transferase isoenzymes [4]. Most compounds that are conjugated with GSH are ultimately excreted in urine as the corresponding mercapturic acid, which are S-conjugates of N-acetyl- cysteine. Conjugation with GSH has usually been considered a detoxication reaction. The increased water solubility and the active secretion of organic acids by renal tubules greatly facilitates mercapturic acid excretion. However conjugation with GSH has now been implicated in the bio- activation of a variety of chemicals to mutagenic, carcinogenic and cytotoxic metabolites [5-71.

11. GLUTATHIONE AND QUINONE-MEDIATED TOXICITIES

The reactivity of quinones resides in their ability to undergo “redox cycling” and to create an oxidative stress [8] and/or to react directly with cellular nucleophiles such as protein and nonprotein sulfhydryls [9, 101. Although there are several studies on the addition of sulfur nucleophiles to quinones, little information is available on the biological consequences of these reactions. Recent evidence indicates that a variety of quinone- thioethers possess biological (re)activity [5-71. Thus, addition of GSH to quinones has little effect on their redox potentials and in some instances may even facilitate oxidation of the quinol [ll-131. Quinone-thioethers also interact with several enzymes that have either the quinone or GSH as their “usual” substrate or cosubstrate, respectively [ 14-16]. In addition to being potent nephrotoxicants (see below), quinone-thioethers have been implicated in the formation of cataracts [ 171, as potent ferrihemoglobin-form- ing agents [18], and as neurotoxicants [ 191. Quinone-thioethers have also been shown to bind to DNA [20]. This latter observation may have implications for hydroquinone (HQ)-mediated nephrocarcinogenicity (see below).

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QUINONE-THIOETHER-MEDIATED NEPHROTOXICITY I27

111. QUINONE-THIOETHERS AND NEPHROTOXICITY

4-Aminophenol is a metabolite of the analgesic acetaminophen and causes acute renal proximal tubular necrosis following administration to rats [21-231. Oxidative metabolism of 4-aminophenol to the quinoneimine and reaction with GSH gives rise to several isomeric multisubstituted conjugates [24]. Gartland et al. [25] demonstrated that either depletion of hepatic GSH by pretreatment of animals with buthionine sulfoximine or cannulation of the bile duct to decrease the delivery of hepatic metabolites to the kidney, afforded protection against 4-aminophenol nephrotoxicity. These data suggested a role for GSH conjugation in 4-aminophenol nephrotoxicity . Subsequently, Fowler et al. [26] investigated the toxicity of 4-amino-3-(GSyl)-phenol in male Fischer 344 rats and showed that the conjugate was capable of reproducing 4-minophenol nephrotoxicity at doses 3- to 4-fold lower than that of the parent aminophenol. Klos et al. [27] have also reported the identification of 4-amino-2-(GSyl)phenol, 4-amino-3-( GS y1)phenol , 4-amino-2,5-(diGSyl)phenol, and 4-amino-2,3,5 (or 6)-(triGSy1)phenol in the bile of Wistar rats following administration of 4-aminophenol (100 mg/kg, i.p.). The latter three conjugates were all capable of causing cytotoxicity when incubated with rat kidney cortical cells, and the toxicity could be prevented by inhibition of y-glutamyl transpeptidase (y-GT). y-GT catalyzes the first step in the metabolism of GSH and its S-conjugates. The toxicity of the aminophenol GSH conjugates can be reduced by inhibition of renal y-GT, indicating that the metabolism of these conjugates by yGT to their corresponding cystein-S-yl-glycine and/ or cystein-S-yl conjugates plays a major role in the observed toxicity.

The nephrotoxicity of bromobenzene in rats is probably mediated via its metabolism to 2-Br-(diGSyl)hydroquinone [28, 291. As little as 10 pmol/ kg of 2-Br-(diGSyl)hydroquinone is sufficient to cause glucosuria, enzymuria, and renal proximal tubular cell necrosis. The tissue selectiv- ity of 2-Br-(diGSyI)hydroquinone and 2,3,5-(triGSy1)hydroquinone appears to be a consequence of their targeting to renal proximal tubule cells by brush border y-GT. Thus, inhibition of y-GT by pretreatment of animals with AT- 125 protected them against both 2-Br-(diGSyl)hydroquinone- [29] and 2,3,5-(triGSy1)hydroquinone- [30] mediated nephrotoxicity . The uptake of 2-Br-(diGSyl)hydroquinone by freshly isolated kidney slices was also inhibited by AT-125 [31]. Probenecid, which inhibits organic anion transport in renal proximal tubules, did not protect against either 2-Br- (diGSy1)hydroquinone- or 2,3,5-(triGSyl)hydroquinone-mediated nephrotoxicity [29, 301.

The nephrotoxicity of GSH- and cysteine-conjugated halogenated alkanes and alkenes is dependent upon their metabolism by cysteine conjugate p- lyase. However, p-lyase does not appear to play a major role in either 2-

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Br-(diGSy1)hydroquinone- or 2,3,5-(triGSyl)hydroquinone-mediated nephrotoxicity. For example, pretreatment of rats with aminooxyacetic acid did not protect animals from 2-Br-(diGSyl)hydroquinone-mediated nephrotoxicity [29] and the nephrotoxicity of 6-Br-2,5-dihydroxy-thiophenol, a putative P-lyase-catalyzed metabolite of 2-Br-3-(GSyl)hydroquinone, was dependent upon the quinone, rather than the thiol function [32]. Studies on the relative toxicity of 2-Br-(di-cystein-S-yl)hydroquinone and 2-Br-(di- N-acetylcystein-S-y1)hydroquinone provided further evidence against a major role for P-lyase. Both 2-Br-(di-cystein-S-yl)hydroquinone and 2-Br-(di-N- acetylcystein-S-y1)hydroquinone caused renal proximal tubular necrosis in male rats. However, whereas the toxicity of the cysteine conjugate was not inhibited by pretreatment of animals with either aminooxyacetic acid or probenecid, which was consistent with findings obtained with 2-Br- (diGSyl)hydroquinone, inhibition of P-lyase and the organic anion trans- porter did protect against the toxicity of the mercapturate [33]. The data were consistent with earlier studies which indicated that metabolism of 2- Br-(diGSy1)hydroquinone to the mercapturate was a minor metabolic pathway. Mercapturic acid formation from quinone-GSH conjugates may also be limited by the ability of either the cysteinylglycine and/or cysteine conjugate to undergo an oxidative cyclization reaction that results in formation of a 1,4-benzothiazine. This reaction can therefore channel the products of the y-GT catalyzed metabolism of quinone-GSH conjugates away from the classic mercapturic acid pathway [34].

IV. SPECIES DIFFERENCES IN RESPONSE TO QUINONE-THIOETHERS

Species differences exist in response to quinone-thioether-mediated nephrotoxicity. For example, GSH conjugates of 1,4-benzoquinone are also nephrotoxic. The chemical reaction between 1,4-benzoquinone and GSH results in the formation of 2-(GSyl)HQ, 2.3-(diGSyl)HQ, 2,5-(diGSyl)HQ, 2,6-(diGSy1)HQ. 2,3,5-(triGSyl)HQ, and 2,3,5,6-(tetraGSy1)HQ [30]. 2- (GSyl)HQ, 2,5-(diGSyl)HQ, 2,6-(diGSyl)HQ, and 2,3,5-(triGSyl)HQ are formed in microsomal incubations of HQ in the presence of GSH [35-371 and the corresponding mercapturic acid, N-acetyl-S-(2,5-dihydroxyphenyl)- L-cysteine, has recently been identified as a urinary metabolite of benzene, phenol, and HQ in the rat [38]. We have also identified several HQ-GSH conjugates in the bile of rats treated with HQ in quantities sufficient to propose a role for such metabolites in HQ-mediated nephrotoxicity [37]. When administered to rats, these conjugates produce varying degrees of

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QUINONE-THIOETHER-MEDIATED NEPHROTOXICITY 129

renal proximal tubular necrosis. In particular, 2,3,5-(triGSy1)HQ is a potent renal proximal tubular toxicant in male Sprague-Dawley and Fischer 344 rats. Significant differences in susceptibility to nephrotoxicity were observed when 2.33-(triGSy1)HQ was injected into various species [39]. The most sensitive species was the rat, both Sprague-Dawley and Fischer 344 rats. In contrast, golden Syrian hamsters BALB/c and B6C3F, apr peared somewhat “resistant” to the effects of 2,3,5-(triGSyl)HQ. Guinea pigs were also susceptible to 2,3,5-(triGSy1)HQ nephrotoxicity but the dose required for these effects (200 pmol/kg) was 10-fold higher than that used in rats. The basis for these important species differences are not known. For example, do differences in renal anatomy and physiology contribute to the observed species differences in chemical-induced nephrotoxicity? Or are species differences in renal biochemistry responsible for the differences in susceptibility?

Quinone-thioethers represents a unique model with which to examine the mechanisms underlying the species and sex differences in the nephrotoxic response. As illustrated in Table 1, we have identified a species susceptible to benzoquinols and their thioethers (SD and Fischer 344 rats), a species that is nonresponsive to either benzoquinols or their thioethers (Syrian hamsters), and species responsive to benzoquinol only (BALB/c mice) or to benzoquinol thioether only (guinea pig).

Species differences in the specific activity of renal y-GT have been reported [40]. The relationship between renal y-GT and species susceptibility to 2-Br-(diGSyl)HQ and 2,3,5-(triGSy1)HQ nephrotoxicity was tliere- fore examined. Interestingly, although rats exhibited the highest specific activity of renal y-GT, and were the most sensitive species toward 2-Br- (diGSy1)HQ- and 2,3,5-(triGSyl)HQ-mediated nephrotoxicity , renal y-GT activity did not correlate with susceptibility in the other species examined [41]. Indeed, the guinea pig, which expressed the lowest activity of renal y-GT of the species, was the only other rodent found to be responsive toward 2-Br-(diGSyl)HQ and 2,3,5-(triGSyl)HQ, at the highest dose tested

TABLE 1

Species Differences in Quinone-Thioether-Mediated Nephrotoxicity

Benzoquinol Benzoquinol-thioether response response

Rats (SD and Fischer) + Guinea pigs -

BALB/c mice + Syrian hamsters -

+ + -

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(200 pmol/kg, intracardiac). Thus, factors other than y-GT activity probably play an important role in modulating species susceptibility to 2-Br- (diGSy1)HQ and 2,3,5-(triGSy1)HQ nephrotoxicity . Although the reason(s) for the interspecies variation in response to these quinone-thioethers is unclear at present, it seems possible that differences in both renal biochemistry, such as differences in the relative activities of cysteine conjugate N- acetyl transferase and N-deacetylase, and renal physiology [42], contribute to the observed results. Indeed, guinea pigs exhibited the highest renal cytosolic N-deacetylase activity and the lowest N-acetylase activity [39]. The ratios of N-deacetylation to N-acetylation in guinea pigs, BALB/c mice, B6C3F, mice, hamsters, Fischer F344 rats, and Sprague-Dawley rats were 4.57, 0.16, 0.14, 0.04, 0.03, and 0.02, respectively. Since quinol-cysteine conjugates appear to undergo oxidation more readily than the corresponding mercapturates, the balance of N-deacetylase and N- acetylase in the guinea pig may explain the susceptibility of this species to 2,3,5-(triGSyl)HQ nephrotoxicity [39].

V. NEPHROCARCINOGENICITY OF QUINONE-THIOETHERS

There are approximately 18,000 to 20,000 new cases of cancer of the kidney diagnosed each year in the United States [43, 441. This malignancy, which represents 2-3% of all cancers, ranks 1 lth in cancer incidence and results in 8,OOO fatalities annually in the United States [44]. About 85% of the kidney cancers diagnosed are renal cell cancers, which occur with a prevalence two to three times greater in men than in women [45]. In attempts to identify causal factors in the development of renal cell carcinoma, a number of environmental (occupational), hormonal, cellular, and genetic factors have been studied. Familial predisposition to this cancer exists in humans, but few animal models to study this exist [46]. Most causes of kidney cancer remain unknown and these malignancies consti- tute an important human health problem. A better understanding of the environmental and genetic factors involved in renal cancer, and the underlying molecular mechanisms of nephrotoxicity and carcinogenicity is clearly warranted. For example, nearly 400 chemicals have been evaluated for long-term toxicity and carcinogenicity by the National Cancer Institute and National Toxicology Program (NTP) [47-501. Complete 2-year studies in male and female Fischer 344 rats and B6C3F, mice have been completed on 375 compounds [47, 51, 521. In 358 long-term carcinogenesis studies, 26 chemicals induced kidney cancer. Twenty-three chemicals were positive in the male rat and eight in the female rat. This 3:l

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sex-related prevalence ratio compares directly with the relative incidences seen in male and female humans [44, 451. However, little is known of the mechanisms of chemical-induced renal neoplasia. A major class of chemicals identified as being capable of causing chronic renal injury are the aromatic amines. Quinone imine formation from these compounds is well established and DNA binding of the GSH conjugate of N-(4-ethoxy- pheny1)pbenzoquinone imine has been documented (20). In addition, the nephrotoxicity of 4-amino-3-S-glutathionylphenol has been reported (53). (see above)

HQ was determined by the NTP to exhibit carcinogenic activity in male F344/N rats, as shown by marked increases in tubular cell adenomas of the kidneys. In contrast, the evidence for nephrocarcinogenicity in female F344/N rats was somewhat equivocal and no evidence of nephrocarcinogenicity was found in male or female B6C3F1 mice [54]. These findings have been confirmed by Shibata et al. [55], although these workers did report a low incidence of tumors in male mice. Neither the mechanism of HQ-mediated nephrocarcinogenicity in male F344/N rats nor the basis for the species and sex differences is known. It is important to note, however, that no indications of hyaline droplet formation were seen, such as granular cast formation in the loop of Henle or mineralization in the renal papilla. Thus, a role for a-2u-globulin in HQ-mediated nephrotoxicity is unlikely. In addition, GSH conjugates of HQ have been shown to catalyze 8-hy- droxy-deoxyguanosine formation in calf thymus DNA [56]. Whether the rate of cell proliferation and regeneration plays an important role in the species differences in HQ-mediated nephrocarcinogenicity remains to be established. Our data with 2,3,5-(triGSy1)HQ are also consistent with the NTP observations. Whether HQ-GSH conjugates play a role in HQ-mediated nephrotoxicity and nephrocarcinogenicity is under investigation.

3-ferf-Butyl-4-hydroxyanisole (BHA) is a phenolic antioxidant widely used in foods [57]. The compound appears to lack genotoxicity, but dietary administration of BHA results in papiloma and carcinoma formation in the forestomach of rats, mice, and hamsters (58). Although BHA is also known to inhibit hepatocellular carcinomas, it enhances the development of preneoplastic and neoplastic lesions in rat kidney and urinary bladder [59]. Both BHA and BHA metabolites have been implicated in the toxicity of BHA in various organs [60]. Tajima et al. [61] have also recently reported the identification of several thioether metabolites of 3-tert-butyl- 4-hydroxyanisole in rat urine. In addition, incubation of 2-tea-butylhydro- quinone (TBHQ), a metabolite of 3-terf-butyl-4-hydroxyanisole in both humans and rats, with liver microsomes in the presence of GSH resulted in the formation of 2-rerf-butyl-5-(glutathion-S-yl)hydroquinone [5- (GSyl)TBHQ] and 2-fert-butyl-6-(glutathion-S-yl)hydroquinone [6-(GSy1)-

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TBHQ]. Recently, we identified 5-(GSyl)TBHQ, 6-(GSyl)TBHQ and 3,6- (di-GSy1)TBHQ as biliary metabolites of TBHQ [62]. The glutathione conjugates of TBHQ are also nephrotoxic, with 3,6-(di-GSyl)TBHQ being the most potent nephrotoxicant in Fischer-344 rats [62]. Since 3-ferf-butyl-4-hydroxyanisole increases the formation of preneoplastic and neoplastic foci in the kidney of rats [59], the role of thioethers in these processes warrants further investigation. A number of anticancer drugs of clinical and research interest also contain the quinone nucleus, and their therapeutic effectiveness is often limited by untoward side effects, particularly kidney failure. For example, renal cell tumors and dysplastic foci of renal tubular epithelium occur in rats given repeated low doses of adriamycin [63].

VI. MECHANISMS OF QUINONE-THIOETHER-MEDIATED NEPHROTOXICITY

The excretion of brush-border enzymes [(y-GT) and alkaline phosphatase (ALP)] into urine is a sensitive indicator of quinol-thioether-mediated nephrotoxicity. Following administration of 2-Br-(diGSyl)HQ (30 pmol/kg), approximately 90% of the total yGT excreted in urine appears during the first 4-6 h, during the first or second voiding of the bladder [64]. In- creased excretion of ALP, another BBM enzyme, also occurred rapidly. Thus, disruption of the brush-border membrane (BBM) occurs very early during 2-Br-(diGSyl)HQ-mediated renal proximal tubular cell injury. The cellular and molecular basis for the membrane shedding deserve further investigation. Significant elevations in blood urea nitrogen (BUN), an indicator of overt renal damage, only appeared 8 h after treatment with 2- Br-(diGSyl)HQ. Therefore, brush-border membrane damage takes place prior to overt signs of nephrotoxicity.

2-BrHQ has been shown to cause mitochondrial dysfunction in rabbit renal proximal tubule suspensions [65, 661; we therefore determined the effects of 2-Br-(diGSyl)HQ on mitochondria1 respiratory function. Mito- chondria isolated 2 h after 2-Br-(diGSyl)HQ administration exhibited a significant (20%) decrease in respiratory control ratios (RCR; State 3/State 4), a consequence of an increase in State 4 respiration. At later time points (8 h) State 4 respiration returned to control values, but the RCR remained significantly depressed due to decreases in State 3 respiration. However, the changes in respiratory function induced by quinone-thioethers observed at these early time points do not appear to be causally related to toxicity [64, 671.

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To correlate the biochemical changes with the onset of toxicity at the cellular level, we followed timedependent morphological alterations after in vivo administration of 2-Br-(diGSyl)HQ (30 pmol/kg). As early as 0.5 h there was extensive sloughing of the BBM which correlated with the rapid excretion of y-GT and ALP into urine. Also at this time, mitochondria appeared normal or slightly rounded but otherwise unaltered. An additional early target was the nucleus. At 0.5 h the pattern of injury consisted of margination of heterochromatin (dark chromatin) and reorga- nization of the endoplasmic reticulum into discrete aggregates. By 2 h the brush border was almost completely absent and the nucleus showed loss of chromatin staining consistent with severe karyolysis. The mitochondria appeared condensed at this time, consistent with a sublethal degree of injury. At 4 h the BBM had been completely lost. The nuclei displayed severe damage, with disruption of the nuclear membrane also evident at this time. The lack of an effect on mitochondria respiratory parameters after 2-Br-(diGSyl)HQ (see above) supports the interpretation that the changes observed in mitochondrial morphology do not adversely affect mitochondrial respiratory function. Although brush-border loss is commonly associated with proximal tubule injury, the early targeting of the nucleus represents an important and atypical finding. The finding that mitochondrial condensation is not followed by secondary high amplitude swelling prior to cell death and necrosis in this mode of chemical-induced nephrotoxicity represents a novel finding that to our knowledge has not been described with any other nephrotoxic agent [a].

There is a direct correlation between the extent of covalent binding of 2-Br-[I4C]-HQ to renal macromolecules and elevations in BUN levels in rats treated with 2-Br-[14C]-HQ [68]. Although the role of protein adducts in the development of toxicity remains a debatable issue, determination of covalently bound radiolabel to subcellular fractions serves as an indicator of the formation and localization of biologically reactive metabolites. More than 75% of the covalent binding derived from 2-Br-[I4C]-HQ in the kidney can be prevented by inhibition of y-GT, indicating that the major fraction of the bound material is derived from metabolites that require metabolism by y-GT (i.e., GSH conjugates). We therefore followed the time-dependent distribution of covalently bound material in different subcellular fractions after the in sifu generation of 2-Br-(diGSyl)-[ 14C]-HQ in rats treated with 2-Br-[I4C]-HQ (0.9 mmol/kg) [69]. After 0.5 h, the highest amount of covalently bound radioactivity was found in the cytosolic fraction, followed by the microsomal and plasma membrane fractions. Sub- sequently, the amount of covalently bound radiolabel measured in the cytosolic and microsomal fractions decreased, with a concomitant increase in radioactivity in the nuclear fraction. Coincident with the severe

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karyolysis observed at 4 h the nuclear fraction exhibited the highest level of radioactivity at this time point. Mitochondria accumulated the lowest amount of radioactivity. The results are consistent with the view that the reactive metabolite@) of 2-Br-(diGSyl)HQ is interacting with nuclear macromolecules.

The finding of extensive karyolysis after 2-Br-(diGSyl)HQ administration to rats prompted an in vitro investigation in renal epithelial cells (LLC- PK,) on the mechanism of the DNA damage. Exposure of LLC-PK, cells to 2-Br-(diGSyl)HQ caused concentration- and time-dependent DNA single- strand breaks as determined by alkaline elution [70], consistent with the view that DNA is an early target of 2-Br-(diGSyl)HQ-mediated cytotoxicity.

2-Br-(diGSyl)HQ also produced a time- and dose-dependent cytotoxicity as determined by the lyzosomal neutral red assay, and pretreatment of cells with the endonuclease inhibitor aurintricarboxylic acid had no effect on the cytotoxicity . In contrast, the specific ferric ion chelator defer- oxamine mesylate, and catalase. significantly alleviated the toxicity of 2- Br-(diGSy1)HQ [7 11. These results implicate the involvement of H20,, iron, and .OH in the cytotoxicity mediated by 2-Br-(diGSyl)HQ. DNA damage, as monitored by single-strand breaks, has also been reported in cultured human lung cells exposed to both HQ and H20, [72]. Experiments with catalase, dimethylthiourea, and o-phenanthroline suggested that .OH radicals, generated during iron-catalyzed H,O, reduction, were involved in the DNA damage [72]. In conclusion, both the electrophilic and redox properties of the y-GT catalyzed metabolites of 2-Br-(diGSyl)HQ appear to play important roles in the cytotoxicity. Moreover, the combined effects of quinone-thioether-mediated macromolecular alkylation and reactive oxygen species generation probably create a stress that the cells are unable to survive. Characterization of this unsuccessful stress response is under way.

VII. SUMMARY

It is clear that quinone-thioethers possess a variety of biological and toxicological activity [5]. The ubiquitous nature of quinones and the high concentrations of GSH within cells virtually guarantees that humans will be exposed to the potential adverse effects of the resulting quinone-thioethers. The generation of a biologically reactive intermediate is usually the initial and necessary step that eventually results in cell death, tissue necrosis, and/or tumor formation. The various mechanisms in which reactive intermediates interact with cellular constituents and trigger events that

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lead to cell death or cell transformation, are only now becoming unravelled. Knowledge of the disposition of quinone-thioethers will therefore be an important prerequiste to understanding their mechanism of ac- tion. Studies on the occurrence and biological and toxicological activity of quinone- thioethers therefore will be an important area for future research.

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Quinone-Thioether-Mediated Nephrotoxicity