Mutagenic activity of various chemicals in Salmonella strain TA100 and glutathione-deficient derivatives. On the role of glutathione in the detoxification or activation of mutagens inside bacterial cells

In vivo mutagenicity assessment of styrene in MutaMouse liver and lung

BACKGROUND

Styrene (CAS 100-42-5) is widely used as polystyrene and acrylonitrile-butadiene-styrene resin such as plastic, rubber, and paint. One of the primary uses of styrene is food utensils and containers, but a small amount of styrene transferred into food can be ingested by eating. Styrene is metabolized into styrene 7,8-oxide (SO). SO is mutagenic in bacteria and mouse lymphoma assays. It is clastogenic in cultured mammalian cells. However, styrene and SO are not clastogenic/aneugenic in rodents, and no rodent in vivo gene mutation studies were identified.

METHODS

To investigate the mutagenicity of orally administered styrene, we used the transgenic rodent gene mutation assay to perform an in vivo mutagenicity test (OECD TG488). The transgenic MutaMouse was given styrene orally at doses of 0 (corn oil; negative control), 75, 150, and 300 mg/kg/day for 28 days, and mutant frequencies (MFs) were determined using the lacZ assay in the liver and lung (five male mice/group).

RESULTS

There were no significant differences in the MFs of the liver and lung up to 300 mg/kg/day (close to maximum tolerable dose (MTD)), when one animal with extremely high MFs that were attributed to an incidental clonal mutation was omitted. Positive and negative controls produced the expected results.

CONCLUSIONS

These findings show that styrene is not mutagenic in the liver and lung of MutaMouse under this experimental condition.

Critical review of styrene genotoxicity focused on the mutagenicity/clastogenicity literature and using current organization of economic cooperation and development guidance

Styrene is an important high production volume chemical used to manufacture polymeric products. In 2018, International Agency for Research on Cancer classified styrene as probably carcinogenic to humans; National Toxicology Program lists styrene as reasonably anticipated to be a human carcinogen. The genotoxicity literature for styrene and its primary metabolite, styrene 7,8-oxide (SO), begins in the 1970s. Organization of Economic Cooperation and Development (OECD) recently updated most genotoxicity test guidelines, making substantial new recommendations for assay conduct and data evaluation for the standard mutagenicity/clastogenicity assays. Thus, a critical review of the in vitro and in vivo rodent mutagenicity/clastogenicity studies for styrene and SO, based on the latest OECD recommendations, is timely. This critical review considered whether a study was optimally designed, conducted, and interpreted and provides a critical assessment of the evidence for the mutagenicity/clastogenicity of styrene/SO. Information on the ability of styrene/SO to induce other types of genotoxicity endpoints is summarized but not critically reviewed. We conclude that when styrene is metabolized to SO, it can form DNA adducts, and positive in vitro mutagenicity/clastogenicity results can be obtained. SO is mutagenic in bacteria and the in vitro mouse lymphoma gene mutation assay. No rodent in vivo mutation studies were identified. SO is clastogenic in cultured mammalian cells. Although the in vitro assays gave positive responses, styrene/SO is not clastogenic/aneugenic in vivo in rodents. In addition to providing updated information for styrene, this review demonstrates the application of the new OECD guidelines for chemicals with large genetic toxicology databases where published results may or may not be reliable. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.

Screening and characterization of variant Theta-class glutathione transferases catalyzing the activation of ethylene dibromide to a mutagen

Ethylene dibromide (EDB) is a widespread environmental pollutant and mutagen/carcinogen. Certain Theta-class glutathione transferases (GSTs), enzymes that catalyze the reaction of reduced glutathione (GSH) with electrophiles, activate EDB to a mutagen. Previous studies have shown that human GST T1-1, but not rat GST T2-2, activates EDB. We have constructed an E. coli lacZ reversion mutagenicity assay system in which expression of recombinant GST supports activation of EDB to a mutagen. Hexa-histidine N-terminal tagging of GST T1-1 results in greatly enhanced expression of the recombinant enzyme and gives a lacZ strain that shows a mutagenic response to EDB at extremely low levels (approximately 1 ng EDB per plate). The hexa-histidine-tagged enzyme was purified in one step by Ni(2+)-affinity chromatography. We applied the lacZ mutagenicity assay to the rapid screening of a library of variant GST Theta enzymes. Sequence variants with altered catalytic activities were identified, purified, and characterized.

Applicability and limitation of OSARs for the toxicity of electrophilic chemicals

The appropriate selection and application of quantitative structure-activity relationships (QSARs) for the prediction of toxicity is based on the prior assignment of a chemical to its mode of toxic action. This classification is often derived from structural characteristics with the underlying assumption that chemically similar compounds have similar mechanisms of action, which is often but not necessarily the case. Instead of using structural characteristics for classification toward a mode of toxic action, we used Escherichia coli based bioanalytical assays to classify electrophilic chemicals. Analyzing a series of reactive organochlorines, epoxides, and compounds with an activated double bond, three subclasses of reactive toxicity were distinguished: "glutathione depletion-related toxicity", "DNA damage", and "unspecific reactivity". For both subsets of specifically reacting compounds a direct correlation between effects and chemical reactivity was found. Reaction rate constants with either glutathione or 2'-deoxyguanosine, which was used as a model for complex DNA, served well to set up preliminary QSARs for either glutathione depletion-related toxicity or toxicity based on DNA damage in the model organism E. coli. The applicability of QSARs for electrophilic chemicals based on mechanistically relevant reaction rate constants is a priori limited to a small subset of compounds with strictly identical mechanism of toxic action and similar metabolic rates. In contrast, the proposed bioanalytical assays not only allowed the experimental identification of molecular mechanisms underlying the observable toxicity but also their toxicity values are applicable to quantitatively predict toxic effects in higher organisms by linear correlation models, independent of the assigned mode of toxic action.

Characterization of glutathione amide reductase from Chromatium gracile. Identification of a novel thiol peroxidase (Prx/Grx) fueled by glutathione amide redox cycling

Among the Chromatiaceae, the glutathione derivative gamma-l-glutamyl-l-cysteinylglycine amide, or glutathione amide, was reported to be present in facultative aerobic as well as in strictly anaerobic species. The gene (garB) encoding the central enzyme in glutathione amide cycling, glutathione amide reductase (GAR), has been isolated from Chromatium gracile, and its genomic organization has been examined. The garB gene is immediately preceded by an open reading frame encoding a novel 27.5-kDa chimeric enzyme composed of one N-terminal peroxiredoxin-like domain followed by a glutaredoxin-like C terminus. The 27.5-kDa enzyme was established in vitro to be a glutathione amide-dependent peroxidase, being the first example of a prokaryotic low molecular mass thiol-dependent peroxidase. Amino acid sequence alignment of GAR with the functionally homologous glutathione and trypanothione reductases emphasizes the conservation of the catalytically important redox-active disulfide and of regions involved in binding the FAD prosthetic group and the substrates glutathione amide disulfide and NADH. By establishing Michaelis constants of 97 and 13.2 microm for glutathione amide disulfide and NADH, respectively (in contrast to K(m) values of 6.9 mm for glutathione disulfide and 1.98 mm for NADPH), the exclusive substrate specificities of GAR have been documented. Specificity for the amidated disulfide cofactor partly can be explained by the substitution of Arg-37, shown by x-ray crystallographic data of the human glutathione reductase to hydrogen-bond one of the glutathione glycyl carboxylates, by the negatively charged Glu-21. On the other hand, the preference for the unusual electron donor, to some extent, has to rely on the substitution of the basic residues Arg-218, His-219, and Arg-224, which have been shown to interact in the human enzyme with the NADPH 2'-phosphate group, by Leu-197, Glu-198, and Phe-203. We suggest GAR to be the newest member of the class I flavoprotein disulfide reductase family of oxidoreductases.

ogt alkyltransferase enhances dibromoalkane mutagenicity in excision repair-deficient Escherichia coli K-12

We examined the role of the O6-alkylguanine-DNA alkyltransferase encoded by ogt gene in the sensitivity of Escherichia coli to the mutagenic effects of the dibromoalkanes, dibromoethane and dibromomethane, by comparing responses in ogt- bacteria to those in their isogenic ogt+ parental counterparts. The effects of the uvrABC excision-repair system, the adaptive response, mucAB and umuDC mutagenic processing, and glutathione bioactivation on the differential responses of ogt- and ogt+ bacteria were also studied. Mutation induction was monitored by measuring the frequency of forward mutations to L-arabinose resistance. Induced mutations occurred only in excision repair-defective strains and were totally (with dibromomethane) or substantially (with dibromoethane) dependent on the alkyltransferase (ATase) encoded by the ogt gene. An increased mutagenic response to both dibromoalkanes was also seen in ogt- bacteria that overexpressed the ogt protein from a multicopy plasmid, indicating that the differences in mutability between ogt+ and ogt- bacteria were not dependent on the ogt- null allele carried by the defective strain. The ATase encoded by the constitutive ogt gene was more effective in promoting dibromoalkane mutagenicity than the ada ATase induced by exposure to low doses of a methylating agent. The mutagenicity promoted by the ogt ATase was dependent on both glutathione bioactivation and SOS mutagenic processing. To our knowledge, this paper presents for the first time evidence that DNA ATases, in particular the ATase encoded by the ogt gene, can increase the mutagenic effects of a DNA-damaging agent. The mechanism of this effect has yet to be established.

Glutathione deficiency does not elevate susceptibility of bacteria to the mutagenicity of chlorinated humic acids

1. Rat liver 9,000 g supernatant protected against the mutagenic effect of chlorinated hydrophilic macromolecular humic acids (CHMA) in Salmonella typhimurium strain TA100. 2. Protection against mutagenicity of CHMA was mediated by glutathione and was partially dependent on glutathione S-transferase activity. 3. In contrast to the above findings, CHMA showed lower mutagenicity in Salmonella typhimurium and Escherichia coli strains of bacteria that are deficient in glutathione compared to their mutagenicity in parental (glutathione-rich) bacterial strains. 4. Glutathione-deficient cells do not provide test systems with elevated sensitivity for the detection of mutagenic chlorinated humic substances.

The role of formaldehyde and S-chloromethylglutathione in the bacterial mutagenicity of methylene chloride

Methylene chloride was less mutagenic in Salmonella typhimurium TA100/NG-11 (glutathione-deficient) compared to TA100, indicating that glutathione is involved in the activation of methylene chloride to a mutagen in bacteria. In rodents, the pathway of methylene chloride metabolism utilizing glutathione produces formaldehyde via a postulated S-chloromethylglutathione conjugate (GSCH2Cl). Formaldehyde is known to cause DNA-protein cross-links, and GSCH2Cl may act as a monofunctional DNA alkylator by analogy with the glutathione conjugates of 1,2-dihaloalkanes. The lack of sensitivity of Salmonella TA100 towards formaldehyde (Schmid et al., Mutagenesis, 1 (1986) No. 6, 427-431) suggests that GSCH2Cl is responsible for methylene chloride mutagenicity in Salmonella. In Escherichia coli K12 (AB1157), formaldehyde was mutagenic only in the wild-type, a characteristic shared with cross-linking agents, whereas 1,2-dibromoethane (1,2-DBE) was more mutagenic in uvrA cells (AB1886). Methylene chloride, activated by S9 from mouse liver, was mutagenic only in wild-type cells, suggesting a mutagenic role for metabolically derived formaldehyde in E. coli. Mouse-liver S9 also enhanced the cell-killing effect of methylene chloride in the uvrA, and a recA/uvrA double mutant (AB2480) which is very sensitive to DNA damage. This pattern was consistent with formaldehyde damage. However, a mutagenic role in bacteria for the glutathione conjugate of methylene chloride cannot be ruled out by these E. coli experiments because S9 fractions did not increase 1,2-DBE mutagenicity, suggesting lack of cell wall penetration by this reactive species. Rat-liver S9 did not activate methylene chloride to a bacterial mutagen or enhance methylene chloride-induced cell-killing, which is consistent with the carcinogenicity difference between the species.

Acidification of Escherichia coli and Salmonella typhimurium cytoplasm reduces the mutagenic effect of N-methyl-N'-nitro-N-nitrosoguanidine

Preliminary acidification of the cytoplasm of E. coli cells growing at pH 6.9 by adding to the medium 50 mM of sodium acetate or propionate reduced the mutagenic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) to almost the spontaneous level. In experiments with S. typhimurium the protective effects of cytoplasm acidification against the mutagenic effect of MNNG was observed at pH 5.5 and was absent at a medium pH of 6.9. Alkalinization of the cytoplasm by adding 80 mM of methylamine to the growth medium at pH 8.1 did not modify the effect of MNNG on the cells of E. coli and S. typhimurium. Alkalinization of the cytoplasm of E. coli B/r and K12 was followed by a reduction of the intracellular non-protein SH group level by 25 and 50%, respectively. It is supposed that the protective effect of acidification may be due to a decline in the productivity of mutagenically more active intermediates of MNNG when the pH is reduced and the associated fall of the level of intracellular non-protein thiols occurs. The above situation may serve as a model for studying the effects of MNNG and other alkylating agents on cells differing in physiological status.

Metabolic activation of carcinogens

Most chemical carcinogens are not active in themselves but require bioactivation to electrophiles that bind covalently to DNA and often act by producing mutations. In recent years it has been realized that mutations can be important at many stages of carcinogenesis. A variety of different enzymes are involved in bioactivation reactions, which include oxidation, reduction, thiol conjugation, acetyl transfer, sulfur transfer, methyl transfer, glucuronosyl transfer, and epoxide hydrolysis. These processes often occur in concert with a single carcinogen. Humans vary considerably in activities of these enzymes and this variation may contribute to differences in risk.

Preventive action of thioethers towards in vitro DNA binding and mutagenesis in E. coli K12 by alkylating agents

Thioethers are effective scavengers of electrophilic metabolites derived from the hepatocarcinogen N-hydroxy-2-acetylaminofluorene (van den Goorbergh et al., 1987). In this study 2 of these thioethers, 4-(methylthio)benzoic acid (MTB) and its methylester, methyl 4-(methylthio)benzoate (MMTB), have been tested for their ability to prevent in vitro DNA binding and mutation induction in E. coli K12 by the direct alkylating agents ethylnitrosourea (ENU), methylnitrosourea (MNU), ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS). In addition to MTB and MMTB, the thioether L-methionine (Met), and the thiols glutathione (GSH) and L-cysteine (Cys) were included for reasons of comparison. MTB was able to (partially) prevent DNA binding and mutation induction by ENU. However, this thioether was ineffective with EMS. DNA binding and mutagenesis by EMS were (partially) prevented by GSH and Cys, while these thiols could not prevent DNA binding and mutation induction by ENU. MMTB was unable to prevent mutation induction by these ethylating agents. With the methylating agents, similar effects of MTB were observed: MTB effectively prevented mutation induction by MNU while it was much less effective towards MMS. GSH and Cys were comparably effective as antimutagenic agents towards both methylating agents. Met was unable to prevent either DNA binding or mutation induction by these agents. Taken together, the results show that aromatic thioethers are able to trap genotoxic electrophiles derived from the nitrosoureas ENU and MNU, and may therefore act as potential anticarcinogens towards these agents, which are only poorly detoxified by GSH.

Effects of iron chelators and glutathione depletion on the induction and repair of chromosomal aberrations by tert-butyl hydroperoxide in cultured Chinese hamster cells

The effects of iron chelators and glutathione (GSH) depletion on the induction of chromosomal aberrations by tert-butyl hydroperoxide (t-BuOOH) were investigated in cultured Chinese hamster V79 cells. t-BuOOH in a concentration range of 0.1-1.0 mM induced chromosomal structural aberrations, consisting mainly of chromatid gaps and breaks, in a dose-dependent fashion. The divalent iron chelator o-phenanthroline almost completely suppressed the formation of chromosomal aberrations while the trivalent chelator desferrioxamine was less effective. GSH depletion did not affect the formation of chromosomal aberrations and DNA single-strand breaks (ssb) by t-BuOOH. DNA ssb by 0.5 mM t-BuOOH were repaired within 60 min of treatment in both GSH-depleted (GSH-) and non-depleted (GSH+) cells. In contrast, chromosomal aberrations increased a little during the 60 min after treatment in both GSH- and GSH+ cells. The aberrations were then repaired in GSH+ cells but those in GSH- cells were maintained to a great extent during 20 h of post-treatment incubation. These results indicate that divalent iron mediates the induction of chromosomal aberrations by t-BuOOH. That t-BuOOH-induced chromosomal aberrations remain even after DNA ssb were repaired suggests involvement of other lesions than DNA ssb in the formation of chromosomal aberrations by the hydroperoxide.

Increased capacity for glutathione synthesis enhances resistance to radiation in Escherichia coli: a possible model for mammalian cell protection

A strain of Escherichia coli, enriched in its content of gamma-glutamylcysteine synthetase and glutathione synthetase activities by recombinant DNA techniques, is more resistant to the lethal effects of gamma-irradiation than is the corresponding wild strain. Although the gene-enriched strain has higher glutathione levels than the wild strain, the observed radioresistance appears to be associated with the increased capacity of the gene-enriched strain to synthesize glutathione when irradiated rather than to the cellular levels of glutathione per se. Thus, resistance was abolished in the presence of buthionine sulfoximine, a selective inactivator of gamma-glutamylcysteine synthetase that decreases glutathione synthesis but that does not act directly to lower cellular glutathione levels. Conclusions drawn from studies on this E. coli model system may have relevance to protection of mammalian cells by glutathione.

An analysis of the mutagenicity of 1,2-dibromoethane to Escherichia coli: influence of DNA repair activities and metabolic pathways

The mutagenicity of 1,2-dibromoethane (EDB) to Escherichia coli was reduced by the UV light-induced excision repair system but unaffected by the loss of a major apurinic/apyrimidinic site repair function. At high doses, 70-90% of the EDB-induced mutations were independent of SOS-mutagenic processing and approximately 50% were independent of glutathione conjugation. The SOS-independent mutations induced by EDB were unaffected by the enzymes that repair alkylation-induced DNA lesions. EDB-induced base substitutions were dominated by GC to AT and AT to GC transitions. These results suggest that EDB-induced premutagenic lesions have some, but not all, of the characteristics of simple alkyl lesions.

Biochemical characterization of glutathione-deficient mutants of Escherichia coli K12 and Salmonella strains TA1535 and TA100

Glutathione-deficient mutants of Escherichia coli K12/343/408 and Salmonella typhimurium TA1535 and TA100 were characterized biochemically by measuring the rate of formation of (14C)gamma-glutamylcysteine and (14C)glutathione in cell-free extracts of the strains. gamma-Glutamylcysteine synthetase activity was found to be absent in the NGR-2 mutant of E. coli and in the Salmonella mutants TA1535/NG-19, TA100/NG-57 and TA100/NG-11, while only low activities were found in the NGR-9 and NG-54 mutant of E. coli and Salmonella respectively. These results correspond with the decreased levels of glutathione found in these strains. Extracts of the parent strains have normal glutathione levels and show high gamma-glutamylcysteine synthetase activities. It is concluded that the present GSH-deficient strains of E. coli and Salmonella are gshA mutants, analogous to those previously described in E. coli. In addition, the present results show that the fluorometric method used for the determination of glutathione, employing o-phthalaldehyde as a reagent, is not specific for glutathione (at pH 8.0), but also sensitively reacts with gamma-glutamylcysteine.

Trenbolone growth promotant: covalent DNA binding in rat liver and in Salmonella typhimurium, and mutagenicity in the Ames test

DNA binding in vivo: [6,7-3H]beta-trenbolone (beta-TBOH) was administered p.o. and i.p. to rats. After 8 or 16 h, DNA was isolated from the livers and purified to constant specific radioactivity. Enzymatic digestion to deoxyribonucleotides and separation by HPLC revealed about 90% of the DNA radioactivity eluting in the form of possible TBOH-nucleotide adducts. The extent of this genotoxicity, expressed in units of the Covalent Binding Index, CBI = (mumol TBOH bound per mol nucleotide)/(mmol TBOH administered per kg body weight) spanned from 8 to 17, i.e. was in the range found with weak genotoxic carcinogens. Ames test: low doses of beta-TBOH increased the number of revertants in Salmonella strain TA100 reproducibly and in a dose-dependent manner. The mutagenic potency was 0.2 revertants per nmol after preincubation of the bacteria (20 min at 37 degrees C) with doses between 30 and 60 micrograms per plate (47 and 94 micrograms/ml preincubation mixture). Above this dose, the number of revertants decreased to control values, accompanied by a reduction in survival. The addition of rat liver S9 inhibited the mutagenicity. DNA binding in vitro: calf thymus DNA was incubated with tritiated beta-TBOH with and without rat liver S9. Highest DNA radioactivities were determined in the absence of the "activation" system. Addition of inactive S9 (without cofactors) reduced the DNA binding by a factor of up to 20. Intermediate results were found with active S9. DNA binding in Salmonella: beta-TBOH was irreversibly bound to DNA isolated from S. typhimurium TA100 after incubation of bacteria with [3H]beta-TBOH.

Biosynthesis and biotransformation of glutathione S-conjugates to toxic metabolites

The material presented in this review deals with the hypothesis that the nephrotoxicity of certain halogenated alkanes and alkenes is associated with hepatic biosynthesis of glutathione S-conjugates, which are further metabolized to the corresponding cysteine S-conjugates. Some glutathione or cysteine S-conjugates may be direct-acting nephrotoxins, but most cysteine S-conjugates require bioactivation by renal, pyridoxal phosphate-dependent enzymes, such as cysteine conjugate beta-lyase (beta-lyase). The biosynthesis of glutathione S-conjugates is catalyzed by both the cytosolic and the microsomal glutathione S-transferases, although the latter enzyme is a better catalyst for the reaction of haloalkenes with glutathione. When glutathione S-conjugate formation yields sulfur mustards, as occurs with vicinal-dihaloethanes, the S-conjugates are direct-acting toxins. In contrast, the S-conjugates formed from fluoro- and chloroalkenes yield S-alkyl- or S-vinyl glutathione conjugates, respectively, which are metabolized to the corresponding cysteine S-conjugates by gamma-glutamyltransferase and dipeptidases; inhibition of these enzymes blocks the toxicity of the glutathione S-conjugates. The cysteine S-conjugates must be metabolized by beta-lyase for the expression of toxicity; the beta-lyase inhibitor aminooxyacetic acid blocks the toxicity of cysteine S-conjugates, and the corresponding alpha-methyl cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates. Homocysteine S-conjugates are also potent cyto- and nephrotoxins. The high renal content of gamma-glutamyltransferase and the renal anion transport system are probably determinants of kidney tissue as a target site. Biochemical studies indicate that renal mitochondrial dysfunction is produced by the cysteine S-conjugates. Finally, some of the glutathione and cysteine conjugates are mutagenic in the Ames test, and reactive intermediates formed by the action of beta-lyase may contribute to the nephrocarcinogenicity of certain chloroalkenes.

Inhibitors of N-nitroso compounds-induced mutagenicity

N-Nitroso compounds are environmental mutagens that are present in the air, water, soil etc. or can be formed by nitrosation of various nitrosatable compounds. The present paper gives a survey of inhibitors of N-nitroso compounds-induced mutagenicity. Inhibitors covered include: thiols, metals, vitamins, phenolic acids, complex mixtures of plant, animal and human origin, organic solvents, inhibitors of mixed-function oxidases etc. Data on inhibitors that prevent the formation of N-nitroso compounds are not covered in this review.

1,2-Dibromo compounds. Their mutagenicity in Salmonella strains differing in glutathione content and their alkylating potential

The mutagenic activities of several structurally related dibromo compounds were compared in Salmonella strains sensitive to base substitution mutagenesis (TA1535 and/or TA100) and in the glutathione (GSH)-deficient derivative TA100/NG-57, using a preincubation procedure. The compounds tested were 1,2-dibromoethane (DBE), 1,2-dibromopropane (DBP), 1,2-dibromo-1-phenylethane (DBPE) and model compounds for the half-mustards resulting from their conjugation with GSH, i.e. the N-acetyl-S-2-bromoalkyl-L-cysteine methyl esters SBE, SBP, and SBPE, respectively. The alkylating potential of all compounds was assayed with the 4-(p-nitrobenzyl)pyridine (NBP) alkylation test. Five of the compounds showed a good correlation between relative mutagenic activity in TA100 and electrophilic reactivity in the NBP-test, the order of decreasing potency being SBE greater than SBP greater than DBPE greater than DBP. SBPE displayed the highest reactivity in the NBP-test, but was devoid of mutagenic activity. The mutagenic activity of DBE was substantially decreased in the GSH-deficient strain TA100/NG-57 and could be restored by pretreating the cells with GSH. None of the other chemicals showed different mutagenic activities in TA100 and TA100/NG-57. From the results it can be concluded that 2-bromothioethers possess higher alkylating activities than the 1,2-dibromo compounds. Methyl substitution has a deactivating effect on the mutagenic activity. The results with the phenyl-substituted analogue, DBPE, show that a higher alkylating activity does not always lead to a higher mutagenic activity.

Mutagenicity of halogenated and other substituted dinitrobenzenes in Salmonella typhimurium TA100 and derivatives deficient in glutathione (TA100/GSH-) and nitroreductase (TA100NR)

In a previous study, it was shown that 1-chloro-2,4-dinitrobenzene (CDNB) was less mutagenic in a glutathione (GSH)-deficient derivative of Salmonella typhimurium TA100 (TA100/GSH-) than in TA100 itself, suggesting that the mutagenicity of the compound is dependent on GSH, possibly mediated by the action of a bacterial nitroreductase(s) on the CDNB-GSH conjugate. In the present study a series of mutagenicity tests were performed to determine how CDNB could be activated after reaction with GSH. In liquid preincubation assays, strains TA100, TA100/GSH- and TA100NR, a nitroreductase-deficient derivative of TA100, were treated with CDNB and its fluoro and bromo analogues (FDNB and BDNB), further with its GSH conjugate (S-GSH-DNB) and possible metabolic products, such as S-cysteine-dinitrobenzene (S-Cys-DNB) and S-methyl-dinitrobenzene (S-methyl-DNB), and with 2 more analogues, O-methyl-dinitrobenzene (O-methyl-DNB) and dinitrobenzene (DNB). CDNB, FDNB and BDNB were found to be mutagenic in TA100 and TA100NR, while TA100/GSH- was much less sensitive to the mutagenic action of these halogenated dinitrobenzenes. DNB, O-methyl-DNB, S-methyl-DNB and S-Cys-DNB induced equal numbers of His+ revertants in TA100 and TA100/GSH-, but were not mutagenic in TA100NR. S-GSH-DNB showed no mutagenic activity in any of the 3 strains under the present experimental conditions. These results suggest that the halogenated aromatics may react with bacterial DNA and produce pre-mutagenic alterations according to 2 mechanisms: direct attack on the DNA through nucleophilic substitution (SN2) of the halogen atoms; activation through GSH conjugation and subsequent nitroreduction of the conjugate or its metabolic products to more reactive intermediates.

1,2-Dibromoethane causes rat hepatic DNA damage at low doses

Two oral doses of 1,2-dibromoethane (10-300 mumol/kg) were given to adult female rats 21 and 4 hours before sacrifice. Then hepatic DNA damage, ornithine decarboxylase, cytochrome P-450 content, glutathione content and serum alanine aminotransferase activity assays were performed. In addition, DNA damage was assessed in blood, bone marrow, kidney, spleen and thymus. Of the six organs studied, liver showed the largest amount of DNA damage. Doses at or above 10 mumol/kg EDB caused DNA damage as determined by the alkaline elution technique. Far greater doses (300 mumol/kg, 56.4 mg/kg) of EDB were required to cause other biochemical effects, such as increased activity of ornithine decarboxylase. Thus, the carcinogen EDB caused substantial DNA damage at doses far below those required to show other biochemical effects or frank liver toxicity. DNA damage occurred at a dose level 40-fold lower than that demonstrated in previous studies.

Mutagenicity of nitroxide-free radicals

Stable nitroxides were found to be mutagenic using Salmonella typhimurium tester strain TA 104, a strain chosen on the basis of its high sensitivity to oxidative damage. Nitroxide mutagenicity was dramatically increased in the presence of the superoxide radical generating system, xanthine oxidase/hypoxanthine, and it was suppressed in cells carrying the oxyR1 mutation, which causes induction of enzymes protecting against oxidative stress. As nitroxide-free radicals occur biologically, e.g., in the metabolism of aromatic amines, these radical-induced mutations could be a model for the carcinogenicity observed with these compounds.

Glutathione: a protective agent in Salmonella typhimurium and Escherichia coli as measured by mutagenicity and by growth delay assays

Cultures of some aerobically grown strains of Salmonella typhimurium and Escherichia coli contain up to 24 microM extracellular glutathione (GSH) [Owens RO, Hartman PE (1985): Environ Mutagen 7(Suppl 3): 47] in addition to having intracellular GSH concentrations in the millimolar range. The addition of 26 microM GSH to cultures of Salmonella typhimurium strain TA1534 partially protected the bacteria from the toxic effects causing growth delay by 54 microM N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). When MNNG was preincubated with equimolar GSH, the mutagenicity of the MNNG was neutralized. The addition of micromolar GSH to cultures of an Escherichia coli GSH- strain protected the cells from growth inhibition by micromolar concentrations of mercuric chloride, methyl mercuric chloride, silver nitrate, cisplatin, cadmium chloride, cadmium sulfate, and iodoacetamide. In the cases of mercuric chloride, cisplatin, MNNG, silver nitrate, and iodoacetamide, reaction products with GSH were detected by paper chromatography. In contrast to reduced GSH, micromolar concentrations of oxidized glutathione (GSSG) provided little or no protection and formed no detectable reaction products. Export of GSH by enteric bacteria may provide an important defense mechanism against exogenous toxic agents otherwise active in the micromolar range.