In vitro assessment of the effect of halogenated hydrocarbons: chloroform, dichloromethane, and dibromoethane on embryonic development of the rat

Halogenated hydrocarbons are widely used in industry, the laboratory, and in the home. In the present study three of these solvents--chloroform, dichloromethane, and dibromoethane--were examined for embryotoxic/teratogenic potential using rat embryo culture. The results showed that each of the solvents had a concentration-dependent embryotoxic effect on the developing rat embryo in vitro. The effect and no-effect concentrations (expressed in mumol/ml culture medium), respectively, for each of the halogenated hydrocarbons tested were: dibromoethane--0.33, < 0.18; chloroform--2.06, 1.05; dichloromethane--6.54, 3.46. The levels of chloroform and dichloromethane found to be embryotoxic in the present study were compared to reported blood levels attained following controlled human exposure. In the industrial situation, if the current exposure levels are adhered to, chloroform and dichloromethane appear to have little potential for reproductive toxicity in the human. Fatal or near fatal solvent levels would be required in the mother for the embryotoxic level to be reached. For dibromoethane, there are no reports following controlled human exposure presumably due to its carcinogenicity. In an attempt to elucidate the mechanism of embryotoxicity, histological studies were performed after exposure of rat embryos to an embryotoxic level of each of the halogenated hydrocarbons studied, for increasing time periods up to the standard 40-hour culture. Marked cell death in the neuroepithelium of the developing neural tube was a prominent feature in all embryos exposed to an embryotoxic level of these solvents for periods of 16 hours of longer.

Carcinogenic effects of 1,2-dibromoethane (ethylene dibromide; EDB) in Japanese medaka (Oryzias latipes)

The carcinogenicity of 1,2-dibromoethane (ethylene dibromide; EDB) was investigated in the Japanese medaka (Oryzias latipes), a small fish species. EDB was administered in water continuously for 97 days to a low concentration group, for 73 days to an intermediate concentration group, and intermittently for 24 h once each week over 97 days to a high concentration group. Medaka were 7 days old at the beginning of the tests. Mean measured EDB concentrations in the ambient water were 0.13 mg l-1, 6.20 mg l-1, and 18.58 mg l-1 in the low, intermediate, and high concentration groups, respectively. Two control groups, one inside and one outside the exposure apparatus, were used. Samples were examined histologically at 24, 36, and 58 weeks from the beginning of the tests. EDB was clearly carcinogenic to medaka in the intermediate and high concentration groups causing (1) hepatocellular adenomas and carcinomas, (2) cholangiomas, (3) chloangiocarcinomas, and (4) gall bladder papillary adenomas and adenocarcinomas. In separate studies, medaka exposed to 1.0 mg l-1 EDB for 2 to 5 weeks had elevated hepatic glutathione S-transferase activities, possibly indicating induction of a pathway that forms the reactive metabolite of EDB in mammals. SDS-PAGE of hepatic cytosolic fractions of EDB-exposed medaka showed a pronounced increase in a band at 26,000 Da, the expected position for GSH-S-transferase. Although little is known about EDB's mechanisms of action, medaka appear exceptionally sensitive to the carcinogenic effects of EDB and could serve as a model test species for studying similar compounds.

Reproductive toxins and alligator abnormalities at Lake Apopka, Florida

The alligator population at Lake Apopka in central Florida declined dramatically between 1980 and 1987. Endocrine-disrupting chemicals and specifically DDT metabolites have been implicated in the alligators' reproductive failure. The DDT metabolite hypothesis is based largely on the observation of elevated concentrations of p,p-DDE and p,p-DDD in alligator eggs obtained from Lake Apopka in 1984 and 1985. In the following commentary, we draw attention to two nematocides that are established reproductive toxins in humans, dibromochloropropane (DBCP) and ethylene dibromide (EDB), which could also have played a role in the reproductive failure observed in alligators from Lake Apopka in the early 1980s.

Bacterial and mammalian DNA alkyltransferases sensitize Escherichia coli to the lethal and mutagenic effects of dibromoalkanes

Here we confirm and extend our previous studies demonstrating that the mutagenic potency of 1,2-dibromoethane (DBE) and dibromomethane (DBM) is markedly enhanced (not prevented) in bacteria expressing the O6-alkylguanine-DNA alkyltransferase (ATase) encoded by the Escherichia coli ogt gene. We demonstrate that, in close parallel with mutagenesis, the Ogt ATase sensitizes the bacteria to the lethal effects of these carcinogens, suggesting that one or more of the potentially mutagenic lesions induced by DBE and DBM in the presence of Ogt has additional lethal capacity. We further demonstrate that the sensitization to both lethality and mutagenesis by DBE and DBM is a property shared by other DNA alkyltransferases. This objective was accomplished by quantifying the induction of mutations and lethal events in ogt- ada- E. coli expressing an exogenous bacterial or mammalian ATase from a multicopy plasmid. Mammalian recombinant ATases enhanced the lethal and mutagenic actions of DBE and suppressed the lack of sensitivity of the vector-transformed bacteria to DBM. In most cases the order of effectiveness of the ATases ranked: murine > human > Ogt > rat. Further comparisons included the full-length Ada ATase from E. coli and a truncated Ada version (T-ada) that retains the O6-methylguanine binding domain of the protein. The full-length Ada ATase was effective in enhancing the lethality but not the mutagenicity induced by DBE and DBM. The T-ada ATase provided less sensitization than Ada to lethality by DBE, but of the three bacterial ATases T-ada yielded the highest sensitization to mutagenesis by this compound. T-ada and Ada ATases were in general less effective than the mammalian versions, with the exception of the rat recombinant ATase. The effectiveness of the different mammalian and bacterial ATases in promoting the deleterious actions of dibromoalkanes was compared with the effectiveness of these proteins in suppressing the lethal and mutagenic effects induced by N-nitroso-N-methylurea. The ability to sensitize E. coli to the lethal and mutagenic effects of DBE and DBM seems restricted to DNA alkyltransferase, since overexpression of thioredoxin (Trx) or glutaredoxin (Grx1) in ogt- ada- cells showed no effect, in spite of the reported potential of bioactive dihaloethane-derived species to alkylate Trx.

Effect of decreased glutathione levels in hereditary glutathione synthetase deficiency on dibromoethane-induced genotoxicity in human fibroblasts

The genotoxic effect of dibromoethane is thought to be due to glutathione S-transferase mediated metabolism. The purpose of this study was to determine whether variations in endogenous glutathione in human cells could modify the genotoxicity of dibromoethane. Genotoxicity of dibromoethane, assessed by sister chromatid exchange, was examined in normal human skin fibroblasts and fibroblasts obtained from individuals with hereditary generalized glutathione synthetase deficiency. Cell proliferation was examined as a measure of dibromoethane toxicity. The number of sister chromatid exchanges induced by dibromoethane was significantly lower in the fibroblasts with glutathione synthetase deficiency compared to control cells. Inhibition of cell proliferation was similar in the glutathione-deficient and normal fibroblasts. In conclusion, low endogenous glutathione levels are protective against dibromoethane-induced genotoxicity in human fibroblasts.

The use of human in vitro metabolic parameters to explore the risk assessment of hazardous compounds: the case of ethylene dibromide

Ethylene dibromide (1,2-dibromoethane, EDB) is metabolized by two routes: a conjugative route catalyzed by glutathione S-transferases (GST) and an oxidative route catalyzed by cytochrome P450 (P450). The GST route is associated with carcinogenicity. An approach is presented to use human purified GST and P450 enzymes to explore the importance of these metabolic pathways for man in vivo. This strategy basically consists of four steps: (i) identification of the most important isoenzymes in vitro, (ii) scaling to rate per milligram cytosolic and microsomal protein, (iii) scaling to rate per gram liver, and (iv) incorporation of data in a physiologically based pharmacokinetic (PBPK) model. In the first step, several GST isoenzymes were shown to be active toward EDB and displayed pseudo-first-order kinetics, while the EDB oxidation was catalyzed by CYP2E1, 2A6, and 2B6, which all displayed saturable kinetics. In the second step, the predictions were in agreement with the measured activity in a batch of 21 human liver samples. In the third step, rat liver P450 and GST metabolism of EDB was predicted to be in the same range as human metabolism (expressed per gram). Interindividual differences in GST activity were modeled to determine "extreme cases." For the most active person, an approximately 1.5-fold increase of the amount of conjugative metabolites was predicted. Lastly, it was shown that the GST route, even at low concentrations, will always contribute significantly to total metabolism. In the fourth step, a PBPK model describing liver metabolism after inhalatory exposure to EDB was used. The saturation of the P450 route was predicted to occur faster in the rat than in man. The rat was predicted to have a higher turnover of EDB from both routes. Nevertheless, when all data are combined, it is crucial to recognize that the GST remains significantly active even at low EDB concentrations. The limitations and advantages of the presented strategy are discussed.

Review of experimental male-mediated behavioral and neurochemical disorders

Paternal exposures to exogenous agents have been reported to produce a variety of developmental defects in the offspring. In experimental animals, these effects include decreased litter size and weight, increased stillbirth and neonatal death, birth defects, tumors, and functional/behavioral abnormalities-some of these effects being transmitted to the second and third generations. This article reviews the exogenous agents that have reportedly caused behavioral or neurochemical alterations in offspring of experimental animals following paternal exposures, including advanced age, alcohols, cyclophosphamide, ethylene dibromide, lead, opiates, and a few miscellaneous chemicals. Based upon the consistency of effects in several of these agents in a variety of studies in experimental animals, the conclusion is that paternal exposures may contribute to the incidence of neurobehavioral disorders in humans.

DNA sequence analysis of methylene chloride-induced HPRT mutations in Chinese hamster ovary cells: comparison with the mutation spectrum obtained for 1,2-dibromoethane and formaldehyde

Glutathione-S-transferase-mediated metabolism of methylene chloride (MC) generates S-chloromethylglutathione, which has the potential to react with DNA, and formaldehyde, which is a known mutagen. MC-induced mutations in the HPRT gene of Chinese hamster ovary cells have been sequenced and compared with the mutations induced by 1, 2-dibromoethane (1,2-DEB), which is known to act through a glutathione conjugate, and formaldehyde. All three compounds induced primarily point mutations, with a small number of insertion and deletion events. The most common point mutations induced by MC were GC-->AT transitions (4/8), with two GC-->CG transversions and two AT-->TA transversions. This pattern of mutations showed greater similarity with 1,2-DBE, where the dominant point mutations were GC-->AT transitions (7/9), than formaldehyde, where all mutations were single base transversions and 5/6 occurred from AT base pairs. The mutation sequence results for MC suggest that S-chloromethylglutathione plays a major role in MC mutagenesis, with only a limited contribution from formaldehyde. The involvement of a glutathione (GSH) conjugate in MC mutagenicity would be analogous to the well-characterized pathway of activation of 1,2-DBE.

Mouse liver glutathione S-transferase mediated metabolism of methylene chloride to a mutagen in the CHO/HPRT assay

Although methylene chloride (MC) is readily detectable as a bacterial mutagen, published studies in mammalian cells have been inconclusive. We have previously shown (Graves et al., 1995) that glutathione S-transferase (GST)-mediated metabolism of MC by mouse liver cytosol (S100 fraction) causes DNA single-strand (ss) breaks in CHO cells. In this study, MC GST metabolites were shown to cause mutations at the HPRT locus of CHO cells. The mutagenicity of MC was enhanced by exposing the cells in suspension rather than as attached cultures. The MC GST metabolite formaldehyde was mutagenic in independent experiments, although the number of mutants induced was lower than with the MC. CHO HPRT mutations were also induced by the reference genotoxin 1,2-dibromoethane (1,2-DBE), which is activated to a mutagen by GST-mediated metabolism. Assay of DNA ss breaks and DNA-protein cross-links at mutagenic concentrations of MC, formaldehyde or 1,2-DBE, showed that all three compounds induced DNA ss breaks, but only formaldehyde induced significant DNA-protein cross-linking. These results suggest that whilst formaldehyde may play a role in MC mutagenesis, its weak mutagenicity and the absence of significant DNA-protein cross-linking after MC exposure, leads to the conclusion that the MC DNA damage and resulting mutations are induced by the glutathione conjugate of MC, S-chloromethylglutathione.

Enhancement of BALB/c 3T3 cells transformation by 1,2-dibromoethane promoting effect

Two of the most representative halogenated aliphatic hydrocarbons, 1,2-dibromoethane and 1,1,2,2-tetrachloroethane, were tested in the two-stage cell transformation model for analysing the promoting ability. Both of these compounds had previously been found to exert genotoxic effects, probably acting as moderate initiators. BALB/c 3T3 cells were initiated with subtransforming doses of N-methyl-N-nitro-N-nitrosoguanidine or 3-methylcholanthrene and then exposed to a chronic treatment with different non-transforming dosages of the two haloalkanes. 1,1,2,2-Tetrachloroethane did not exert any promoting activity in that system. By contrast, significant promoting effects by 1,2-dibromoethane were observed both in cells treated with N-methyl-N-nitro-N-nitrosoguanidine and in cells treated with 3-methylcholanthrene. Promotion of the transformation process initiated with 3-methylcholanthrene was detectable when confluent cells in the chemical-treated plates were replated in the level-II amplification test. This experimental procedure allowed cells to perform further rounds of replications and transformed foci to became detectable. Results gave evidence for a promoting role of 1,2-dibromoethane in multistep carcinogenesis, probably responsible for the higher oncogenic ability of this compound with respect to 1,1,2,2-tetrachloroethane.

A new Salmonella typhimurium NM5004 strain expressing rat glutathione S-transferase 5-5: use in detection of genotoxicity of dihaloalkanes using an SOS/umu test system

The Escherichia coli mu operon was subcloned into a pKK233-2 vector containing rat glutathione S-transferase (GST) 5-5 cDNA and the plasmid thus obtained was introduced into Salmonella typhimurium TA1535. The newly developed strain S.typhimurium NM5004, was found to have 52-fold greater GST activity than the original umu strain S.typhimurium TA1535/pSK1002. We compared sensitivities of these two tester strains, NM5004 and TA1535/pSK1002, for induction of umuC gene expression with several dihaloalkanes which are activated or inactivated by GST 5-5 activity. The induction of umuC gene expression by these chemicals was monitored by measuring the cellular beta-galactosidase activity produced by umuC"lacZ fusion gene in these two tester strains. Ethylene dibromide, 1-bromo-2-chloroethane, 1,2-dichloroethane, and methylene dichloride induced umuC gene expression more strongly in the NM5004 strain than the original strain. 4-Nitroquinoline 1-oxide and N-methyl-N'-nitro-N-nitrosoguanidine were found to induce umuC gene expression to similar extents in both strains. In the case of 1-nitropyrene and 2-nitrofluorene, however, NM5004 strain showed weaker umuC gene expression responses than the original TA1535/pSK1002 strain. 1,2-Epoxy-3-(4'-nitrophenoxy)propane, a known substrate for GST 5-5, was found to inhibit umuC induction caused by 1-bromo-2-chloroethane. These results indicate that this new tester NM5004 strain expressing a mammalian GST theta class enzyme may be useful for studies of environmental chemicals proposed to be activated or inactivated by GST activity.

Cytochrome P450 catalyzed metabolism of 1,2-dibromoethane in liver microsomes of differentially induced rats

The cytochrome P450 (P450) catalyzed oxidation of 1,2-dibromoethane (1,2-DBE) to 2-bromoacetaldehyde (2-BA) was measured in liver microsomes of both control and differentially induced rats. 2-BA formation was quantified by derivatization of 2-BA with adenosine (ADO), resulting in the formation of the highly fluorescent 1,N6-ethenoadenosine (epsilon-ADO), which was measured by HPLC. After microsomal incubation with 1,2DBE in the presence of ADO and removal of proteins by denaturation and centrifugation, derivatization by heating 4 h at 65 degrees C appeared necessary to ensure efficient formation of epsilon-ADO. Using this optimized derivatization method to quantitate 2-BA formation, the enzyme kinetics of the P450 catalyzed oxidation of 1,2-DBE to 2-BA were measured in liver microsomes prepared from untreated rats and rats pretreated with phenobarbital (PB), beta-naphtoflavone (beta NF) and pyrazole (PYR). P450 isoenzymes in PYR- and beta NF-induced microsomes showed linear enzyme kinetics while P450 isoenzymes in control and PB-induced microsomes showed non-linear enzyme kinetics. The apparent Vmax- and Km- values for the metabolism of 1,2-DBE to 2-BA were 2.5 nmol/min/mg protein and 144 microns for P450 isoenzymes in PYR-induced microsomes and 773 pmol/min/mg protein and 3.3 mM for P450 isoenzymes in beta NF-induced microsomes, respectively. Due to the non-linear enzyme kinetics of the P450 catalyzed oxidation of 1,2-DBE to 2-BA using control and PB-induced microsomes, no proper Vmax- and Km- values could be calculated. However, from Michaelis-Menten plots it was clear that the affinity of P450 isoenzymes for 1,2-DBE in control and PB-induced microsomes was in the same range when compared to beta NF-induced microsomes and thus much lower than the PYR-induced microsomes.

In vivo potentiation of 1,2-dibromoethane hepatotoxicity by ethanol through inactivation of glutathione-s-transferase

In the rat, a single ethanol (EtOH) pretreatment (2.5 g/kg b.w., per os) was able to strongly enhance the cytotoxicity of 1,2-dibromoethane (DBE)(87 mg/kg b.w., per os). The principal metabolic routes of DBE involve both oxidative and conjugative transformations. Microsomal cytochrome P450 content and dimethyl nitrosamine demethylase activity were not changed, while a significant loss of cytosolic total GSH-transferase was observed in rats killed 6 h after EtOH pretreatment. Pretreatment with methylpyrazole, an inhibitor of alcohol-dehydrogenase prevented the effects provoked by ethanol. The major EtOH metabolite, acetaldehyde. seemed thus to play a fundamental role in the mechanism responsible for the potentiation of DBE toxicity mediated by EtOH. To further support this hypothesis, disulfiram (75 mg/kg b.w.), an inhibitor of aldehyde dehydrogenase, was given i.p. to rats. When DBE was administered to disulfiram- and EtOH-pretreated rats, a marked increase of liver cytolysis was shown and cytosolic GSH-transferase activity was further inhibited if compared to that induced by EtOH treatment alone. The results are consistent with the hypothesis that EtOH given to rats increases DBE liver toxicity because its major metabolite, acetaldehyde, reduces the DBE conjugates to GSH transferase, with consequent shift of DBE metabolism to the oxidative route and accumulation of reactive oxidative intermediates no longer effectively conjugated with GSH.

Human glutathione S-transferase T1-1 enhances mutagenicity of 1,2-dibromoethane, dibromomethane and 1,2,3,4-diepoxybutane in Salmonella typhimurium

The rat theta class glutathione S-transferase (GST) 5-5 has been shown to affect the mutagenicity of halogenated alkanes and epoxides. In Salmonella typhimurium TA1535 expressing the rat GST5-5 the number of revertants was increased compared to the control strain by CH2Br2, ethylene dibromide (EDB) and 1,2,3,4-diepoxybutane (BDE); in contrast, mutagenicity of 1,2-epoxy-3-(4'-nitro-phenoxy)propane (EPNP) was reduced. S.typhimurium TA1535 cells were transformed with an expression plasmid carrying the cDNA of the human theta ortholog GST1-1 either in sense or antisense orientation, the latter being the control. These transformed bacteria were utilized for mutagenicity assays. Mutagenicity of EDB, BDE, CH2Br2, epibromohydrin and 1,3-dichloroacetone was higher in the S.typhimurium TA1535 expressing GSTT1-1 than in the control strain. The expression of active enzyme did not affect the mutagenicity of 1,2-epoxy-3-butene or propylene oxide. GSTT1-1 expression reduced the mutagenicity of EPNP. Glutathione S-transferase 5-5 and GSTT1-1 modulate genotoxicity of several industrially important chemicals in the same way. Polymorphism of the GSTT1 locus in humans may therefore cause differences in cancer susceptibility between the two phenotypes.

Acetaldehyde involvement in ethanol-induced potentiation of rat hepatocyte damage due to the carcinogen 1,2-dibromoethane

Previous experiments with hepatocytes isolated from ethanol-treated rats showed that alcohol potentiates the toxic action of 1,2-dibromoethane (DBE) by inhibiting its metabolism via glutathione-S-transferase. The aim of this study was to investigate whether acetaldehyde, the main product of ethanol metabolism, may be responsible for such inactivation. By pretreatment with 4-methylpyrazole, an inhibitor of acetaldehyde formation, the ethanol inactivation of glutathione transferase was actually prevented. As a consequence of this protective action, 4-methylpyrazole also prevented the high basal lipid peroxidation and the potentiated DBE toxicity observed in hepatocytes from ethanol-dosed animals. Finally, the inactivation of glutathione-S-transferase by concentrations of acetaldehyde likely to occur in the ethanol-intoxicated animal was confirmed in an in vitro model by direct aldehyde addition to hepatocyte suspensions.

Mutation spectrum of 2-chloroethyl methanesulfonate in Drosophila melanogaster premeiotic germ cells

The 2-chloroethyl methanesulfonate (2ClEMS)-induced alcohol dehydrogenase (Adh) null germline mutation frequency in treated Drosophila melanogaster second instar larval gonia was two orders of magnitude greater than the spontaneous mutation frequency. DNA sequence analysis of 83 Adh null mutations showed that 40 mutations of independent origin were at 23 sites in the Adh gene. The mutation spectrum contained only GC-->AT transitions with 35 mutations (87.5%) at the middle or 3' guanine. In addition, characteristics of glutathione (GSH)-mediated bioactivation were determined for 2ClEMS in vitro. Rates of GSH-mediated conjugation, catalyzed by purified rat liver glutathione-S-transferase (GST), and binding of [35S]GSH-mediated conjugation products to calf thymus DNA were determined for 2ClEMS, 1,2-dichloroethane (EDC) and 1,2-dibromoethane (EDB). The relative rates of GSH-mediated conjugation were the following: 5 mM EDB > 40 mM 2ClEMS > 40 mM EDC. A similar trend was observed for DNA binding of the [35S]GSH-mediated conjugation products when differences in mutagen concentration were considered: EDB > 2ClEMS > EDC. The ratios of DNA binding to GSH conjugation calculated for EDB, EDC and 2ClEMS were 6.8 x 10(-5), 9.3 x 10(-5) and 19.1 x 10(-5), respectively. A narrow range, less than a 3-fold difference, in the ratios of DNA binding to GSH conjugation indicates that the bioactivation of 2ClEMS is mediated by the same mechanism as EDB and EDC. Consequently, 2ClEMS, EDC and EDB may induce a specific mutation in premeiotic germ cells.

Immunotoxicologic effects of ethylene dibromide in the mouse and their modulation by the estrous cycle

Estrous cycle modulation of immunologic sensitivity to ethylene dibromide (EDB) was studied in addition to toxicologic end points. Female B6C3F1 mice were injected intragastrically with 31.25, 62.5, or 125 mg/kg EDB for 5 days a week for 12 weeks. Vaginal smears determined the estrous cycle. At 125 mg/kg there were decreases in hemoglobin and hematocrit and longer estrous cycles (5.5 vs 4.3 days, p = 0.006), and increases in cholesterol, triglycerides, total protein, and albumin. The negative dose response seen for T- and B-cell mitogenesis around metestrus was absent for mice near estrus. The high dose of EDB prolonged intervals between estrus, was immunotoxic and immunosuppressive.

Carcinogenicity of dietary dimethylnitrosomorpholine, N-methyl-N'-nitro-N-nitrosoguanidine, and dibromoethane in rainbow trout

Eighteen-mo feeding trials of rainbow trout were used to test the carcinogenicity of 5 chemicals in this species. A single exposure level was used for each substance. The doses and chemicals tested were 1,556 ppm 2,6-dimethylnitrosomorpholine (DMNM), 500 ppm N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 2,000 ppm 1,2-dibromoethane (DBE), 2,000 ppm 1,1-dichloroethylene (DCE), and 200 ppm cyclophosphamide (CP). Liver and/or glandular stomach neoplasms were produced by DMNM (liver and stomach), MNNG (stomach), and DBE (chiefly, stomach tumors). In addition, DMNM produced a low incidence of swimbladder papillomas and caused testicular atrophy in 50% of treated males. DCE and CP produced no neoplasms at the exposure levels used. No evidence of other chronic toxicity was seen for any of the 5 compounds.

Acute and long-term effects of nine chemicals on the Japanese medaka (Oryzias latipes)

Ninety-six-hour acute and 28-day larval survival and growth tests were conducted with nine organic chemicals, using the Japanese medaka (Oryzias latipes) as the test organism. The nine tested chemicals were allyl isothiocyanate, aniline, benzyl acetate, 4-chloroaniline, 2-chloroethanol, 2,4-diaminotoluene, 1,2-dibromoethane, 2,4-dichlorophenoxyacetic acid (2,4-D), and phenol. The derived 96-h LC50 values for medaka for all chemicals ranged from 0.077 mg/L for allyl isothiocyanate to 2,780 mg/L for 2,4-D. The chronic values for six of the nine chemicals tested ranged from 0.013 mg/L for allyl isothiocyanate to 42.5 mg/L for 2,4-D. Acute-to-chronic ratios for these six chemicals ranged from 1.4 for 2-chloroethanol to 70.9 for 2,4-D. Growth of medaka was significantly reduced in the lowest exposure concentration during 28-dy larval tests with aniline, 4-chloroaniline, and 2,4-diaminotoluene. The estimated maximum acceptable toxicant concentration was reported as less than the lowest exposure concentration of 4.6, 2.2 and 40.3 mg/L for tests with aniline, 4-chloroaniline and 2,4-diaminotoluene, respectively. Chronic values for 2-chloroethanol and medaka were 12.6 mg/L during an embryo-larval test and 22.1 mg/L during the 28-day larval test.

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.

Induction of delayed mutations by benzene and ethylene dibromide in Drosophila

Two carcinogens, ethylene dibromide and benzene, were used to induce delayed (germinal mosaic) sex-linked recessive lethal mutations in spermatozoa and spermatids of adult Drosophila males. Significant numbers of delayed mutations (in F3) were scored in absence of conventional (in F2) mutations. A large proportion of nonlethal F2 cultures carried delayed mutations, so much so that, in some cultures, all F2 females were carriers of mutations. The mechanism through which single strand damage to treated X chromosomes can result in such delayed lethals is discussed. These observations indicate that the delayed mutation test should be used for testing the mutagenicity of environmental compounds, especially carcinogens, which tested negative in the conventional sex-linked recessive lethal mutation test. The data will support the relationship between mutagenesis and carcinogenesis and, also will further enhance the sensitivity of the Drosophila mutation assay.

Role of kidney S9 in the mutagenic properties of 1,2-dibromoethane

The mutagenic properties of 1,2-dibromoethane (DBE) were studied in the Ames Salmonella typhimurium assay using the strains TA 1535 and TA 100. Kidney S9 fraction alone did not modify the direct mutagenic activity of DBE; but an addition of kidney S9 to liver S9 fraction yielded a higher mutagenic activity of DBE than with liver S9 fraction alone. Moreover, the addition of glutathione (GSH) to kidney S9 increased the mutagenic activity of DBE. Methimazole, a competitive inhibitor of the flavin-containing monooxygenase, reduced mutagenic activity suggesting that this enzyme may contribute to renal damage from DBE. No mutagens could be detected in the urine of rats treated with DBE.

Ethylene dibromide: evidence of systemic and immunologic toxicity without impairment of in vivo host defenses

Ethylene dibromide was administered intragastrically on 14 consecutive days to B6C3F1 female mice. Host resistance was not altered after challenge with B16F10 tumor cells, Listeria monocytogenes, influenza, or Herpes simplex viruses. In contrast, decreases were seen in relative thymus and spleen weights, red blood cells, hemoglobin, hematocrit, and in alveolar macrophage, natural killer cell, T-cell, and mixed lymphocyte culture responses. Increases occurred in relative kidney and liver weights, cholesterol, peripheral neutrophils, resident peritoneal exudate cells (with increased phagocytosis) and plaque-forming cells. There was little difference between the dose that caused immune modulation and that which produced significant toxicity.

Mutation spectra of 1,2-dibromoethane, 1,2-dichloroethane and 1-bromo-2-chloroethane in excision repair proficient and repair deficient strains of Drosophila melanogaster

DNA sequence changes produced by 1,2-dibromoethane (DBE), 1,2-dichloroethane (DCE) and 1-bromo-2-chloroethane (BCE) were analyzed using the vermilion locus of Drosophila melanogaster. Under excision repair proficient (exr+) conditions (mutagenized exr+ males mated with exr+ females) all mutants isolated from the first generation (F1) after DBE and DCE exposure represented DNA rearrangements (multi-locus deletions, small deletions with tandem repeats, duplicate insertions). By contrast, mutants expressing a vermilion phenotype only in the F2 (F1 mosaics) all carried single bp changes. When exr+ males, after exposure to DBE, were mated to excision repair deficient (exr-) mus 201 females 11 of 14 mutational events isolated from either F1 or F2 progeny were single bp changes. In general the mutation spectra for the three dihaloalkanes were similar to the spectrum obtained at the same locus for the direct-acting monofunctional agent methylmethanesulfonate (MMS). The data lend support to the conclusions that these 1,2-dihaloalkanes are genotoxic through modification at ring nitrogens in DNA, primarily at the N7 of guanine and, to a lesser extent, at the N1 of adenine. These N-adducts could be directly miscoding. However, more important for the mutagenic action of the chemicals seems to be the formation of non-coding lesions and/or misrepair.

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.