Expression of mammalian glutathione S-transferase 5-5 in Salmonella typhimurium TA1535 leads to base-pair mutations upon exposure to dihalomethanes

Dihalomethanes can produce liver tumors in mice but not in rats, and concern exists about the risk of these compounds to humans. Glutathione (GSH) conjugation of dihalomethanes has been considered to be a critical event in the bioactivation process, and risk assessment is based upon this premise; however, there is little experimental support for this view or information about the basis of genotoxicity. A plasmid vector containing rat GSH S-transferase 5-5 was transfected into the Salmonella typhimurium tester strain TA1535, which then produced active enzyme. The transfected bacteria produced base-pair revertants in the presence of ethylene dihalides or dihalomethanes, in the order CH2Br2 > CH2BrCl > CH2Cl2. However, revertants were not seen when cells were exposed to GSH, CH2Br2, and an amount of purified GSH S-transferase 5-5 (20-fold excess in amount of that expressed within the cells). HCHO, which is an end product of the reaction of GSH with dihalomethanes, also did not produce mutations. S-(1-Acetoxymethyl)GSH was prepared as an analog of the putative S-(1-halomethyl)GSH reactive intermediates. This analog did not produce revertants, consistent with the view that activation of dihalomethanes must occur within the bacteria to cause genetic damage, presenting a model to be considered in studies with mammalian cells. S-(1-Acetoxymethyl)GSH reacted with 2'-deoxyguanosine to yield a major adduct, identified as S-[1-(N2-deoxyguanosinyl)methyl]GSH. Demonstration of the activation of dihalomethanes by this mammalian GSH S-transferase theta class enzyme should be of use in evaluating the risk of these chemicals, particularly in light of reports of the polymorphic expression of a similar activity in humans.

Human glutathione S-transferase-expressing Salmonella typhimurium tester strains to study the activation/detoxification of mutagenic compounds: studies with halogenated compounds, aromatic amines and aflatoxin B1

We have developed Salmonella typhimurium strains expressing human glutathione S-transferases (GSTs) to establish the role of these enzymes in chemical activation and deactivation. Alpha and pi class GSTs, GSTA1-1 and GSTP1-1, were expressed in Salmonella TA100 using a regulatable tac promoter expression system. The ability of these GST to modulate the mutagenicity of a range of mutagens including ethylene dibromide, ethylene dichloride and methylene dichloride was then investigated. Ethylene dibromide, ethylene dichloride and methylene dichloride were directly mutagenic in the control TA100 strain. The mutagenicity of ethylene dibromide and ethylene dichloride was increased in cells expressing GSTA1-1, but not in cells expressing GSTP1-1. In contrast, methylene dichloride mutagenicity was unaffected by the presence of either GST. The mutagenicity of 2-aminofluorene, was not altered by the presence of either GST isozyme, while that of N-hydroxy-2-acetylaminofluorene was slightly reduced with both isozymes. The mutagenicity of aflatoxin B1 (AFB1) was marginally decreased in strains expressing GSTP1-1. When GSTA1-1 expression was maximally induced, however, a more pronounced reduction was observed suggesting a role for GSTA1-1 in AFB1 deactivation. The tester strains described here should be valuable in establishing the specificity of human GST isozymes towards chemical toxins and carcinogens, especially for compounds whose reactive intermediates are short lived.

Molecular mechanisms of dibromoalkane cytotoxicity in isolated rat hepatocytes

The cytotoxicity of dibromoalkanes to isolated hepatocytes was proportional to the dibromoalkane concentration and increasing chain length of the dibromoalkane (C2-C6). The rapid hepatocyte glutathione (GSH) depletion which occurred upon addition of the dibromoalkanes was also dependent on the concentration and chain length of the dibromoalkane. When added to hepatocytes, dibromoalkanes also caused a loss in protein sulfhydryl groups. After a lag period, lipid peroxidation occurred before the onset of cytotoxicity. Antioxidants or removing the oxygen from the medium markedly delayed dibromoalkane cytotoxicity. Bromoaldehydic metabolites formed by cytochrome P450-dependent mixed-function oxidases were probably responsible for lipid peroxidation as deuterated 1,2-dibromoethane (d4-DBE) induced less lipid peroxidation and was less cytotoxic even though GSH was depleted as rapidly and as effectively. Hepatocytes were also more resistant to dibromoalkanes if cytochrome P450 isoenzymes were inactivated with SKF 525A or methyl pyrazole. Furthermore, hepatocyte susceptibility to dibromoalkanes was increased markedly if aldehyde dehydrogenase was inactivated with disulfiram, cyanamide or chloral hydrate. Cytochrome P450-induced hepatocytes isolated from pyrazole-, phenobarbital- or 3-methylcholanthrene-pretreated rats were also more susceptible to dibromoalkanes. These results suggest that dibromoalkane-induced cell lysis is due to lipid peroxidation as well as cytochrome P450-dependent formation of toxic bromoaldehydic metabolites which can bind with cellular macromolecules. Dibromoethane GSH conjugates also contribute to DBE cytotoxicity as depleting hepatocyte GSH beforehand increased hepatocyte resistance to DBE but not other dibromoalkanes.

Mutagenicity of peroxyacetyl nitrate (PAN) in vivo: tests for somatic mutations and chromosomal aberrations

A series of experiments was conducted in which Chinese hamsters inhaled PAN, an ubiquitous pollutant that is present in the atmosphere at concentrations that are as high as, or higher than, other known genotoxic agents. The animals were exposed to PAN in air at concentrations of approximately 3 ppm for up to 1 month and then examined for somatic mutations and chromosomal aberrations. Mutations were assayed by measuring the frequency of thioguanine-resistant lung fibroblasts (isolated de novo and cultured). Chromosomal aberrations were assayed by measuring the frequency of micronuclei in either the bone marrow (polychromatic erythrocytes) or the lungs (binucleate lung fibroblasts cultured in the presence of cytochalasin B). The results for the test animals were compared to those from animals exposed similarly, but without PAN. Although in each experiment the mutation frequencies for the test animals were higher than the corresponding controls, the mutation frequencies were not significantly different from the concurrent negative controls (P > .05) or the historical controls, except for experiment C. In experiment C, there was a significant regression of mutation frequency versus dose (P < 0.001) if all of the historical controls for pooled animals are included at zero dose. No reproducible evidence of chromosomal breakage was found in either lung or bone marrow. Thus, although PAN has been found to be a bacterial mutagen, we did not find statistically significant evidence of mutagenicity in vivo. The toxicity of PAN limited the exposure concentration that could be used. When all of the PAN data were used, the best estimate of the mutagenic potency proved to be comparable to that of ethylene dibromide, a carcinogenic atmospheric pollutant.

Rat hepatic glutathione S-transferase-mediated embryotoxic bioactivation of ethylene dibromide

The embryotoxic effects of ethylene dibromide (EDB) bioactivation, mediated by purified rat liver glutathione S-transferases (GST), were investigated using rat embryos in culture. Significant EDB metabolism was observed with rat liver GST purified by affinity chromatography (specific activity of 188 +/- 11.3 nmol/min/mg protein). The reaction was enzymatic in nature and the conjugation rate was proportional to the concentration of EDB (up to 0.75 mM) and the enzyme present in the reaction medium. EDB activation by 100 units (1 unit = 1 nmol of glutathione consumed per min) of purified rat liver GST caused a significant reduction in general development as measured by crown-rump length, yolk sac diameter, somite number, and the composite score for different morphological parameters (Brown and Fabro methodology). Structures most significantly affected were the central nervous and olfactory systems as well as the yolk sac circulation and allantois. The results of this study clearly indicate that under in vitro conditions, bioactivation of EDB by GST can lead to embryotoxicity.

Bioactivation of halogenated hydrocarbons by rabbit pulmonary cells

1,1-Dichloro-2,2-bis (4'-chlorophenyl)ethane (DDD), 1,2-dibromoethane (DBE) and trichloroethylene are three halogenated hydrocarbons that selectively bind to pulmonary epithelial cells and that may be pneumotoxic. The susceptibility of pulmonary cells and the mechanisms of cytotoxicity of these compounds were evaluated using enriched subpopulations of isolated rabbit lung cells incubated with DDD, DBE, and trichloroethylene. These chlorinated and brominated hydrocarbons were studied to evaluate their ability to induce selective pneumotoxicity by their bioactivation in three cell types, i.e. Clara cells, alveolar type II cells, and alveolar macrophages. Evidence of cytochrome P-450 bioactivation was assessed by utilizing the suicide inhibitor, 1-aminobenzotriazole (ABT) to ameliorate cytotoxicity. DDD, DBE and trichloroethylene were cytotoxic to Clara cells, type II cells and alveolar macrophages and the order of cell susceptibility to DDD was Clara > type II > macrophages. DBE and trichloroethylene were nonselectively cytotoxic. ABT reduced the cytotoxic effects of DDD and DBE in Clara cells. These studies indicated that all three compounds were toxic to isolated lung cells and that bioactivation of DDD and DBE in rabbit Clara cells to a cytotoxic intermediate was mediated, at least in part, by cytochrome P-450 oxidation.

Ethylene dibromide: negative results with the mouse dominant lethal assay and the electrophoretic specific-locus test

Ethylene dibromide (1,2-dibromoethane; EDB) was tested for the induction of dominant lethal and electrophoretically-detectable specific-locus mutations in the germ cells of DBA/2J male mice. Males were treated with a single intraperitoneal injection of 100 mg/kg EDB and mated to two C57BL/6J females. In the dominant lethal assay, matings were carried out to measure the effect of EDB on meiotic and postmeiotic stages; germ cells representing spermatogonial stem cells were analyzed in the electrophoretic specific-locus test. Neither of these germ cell tests produced any evidence that EDB is a germ cell mutagen. It appears from these data and those reported in the literature that EDB, a genotoxic carcinogen that affects male fertility in some mammalian species, is not mutagenic in the germ cells of the male mouse.

In vivo studies on halogen compound interactions. IV. Interaction among different halogen derivatives with and without synergistic action on liver toxicity

The liver toxicity of several halogen compound mixtures have been tested. The compounds were selected on the basis of their metabolic pathways: carbon tetrachloride (CT) and trichlorobromomethane (TCBM) undergo a dehalogenation via P450-dependent enzyme system, 1,2-dichloroethane (DCE) and 1,2-dibromoethane (DBE) are mainly conjugated with the cytosolic glutathione (GSH) by means of the GSH-S-transferase. The mixture TCBM+DBE shows a more than additive action on lipid peroxidation and liver necrosis. TCBM, like CT, reduces the hepatic level of GSH-S-transferase, increasing the amount of DBE available for cytochrome P450-dependent metabolism, with the production of toxic metabolites. Thus, the behavior of the mixture TCBM+DBE is very similar to that of the mixture CT+DBE, previously reported. Mixtures composed of CT+TCBM and DCE+DBE do not show any synergistic effect on liver toxicity. The results allow one to conclude that the toxicity of mixtures of halogen compounds can be partly predicted on the basis of their metabolic pathways. When the metabolism is quite different, a synergistic toxicity can occur if one pathway interferes with a detoxification mechanism of the other compound. If the two metabolisms are very similar they produce, at most, an additive toxicity.

Micronucleated reticulocyte induction by ethylating agents in mice

Six model ethylating agents were tested for clastogenic potency by means of a new technique of the micronucleus assay with mouse peripheral blood cells using acridine orange (AO)-coated slides, to evaluate the test. The alkylating agents were: N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-ethyl-N-nitrosourea (ENU), diethylsulfate (DES), ethyl methanesulfonate (EMS), epichlorohydrin (ECH) and ethylene dibromide (EDB). The animals were given a single intraperitoneal injection of the following doses of the chemicals: ENNG and ENU, 25, 50 and 100 mg/kg; EMS and DES, 100, 200 and 400 mg/kg body weight. For EDB and ECH, the doses were 50, 100 and 200 mg/kg, given twice, 24 h apart. Before and after the injection, blood samples were taken from the tails at 24-h intervals up to 72 h and preparations were made on AO-coated slides. For each dose group, 4 animals were used and 1000 reticulocytes were examined per slide for the presence of micronuclei. At the optimum induction time of 48 h, ENU induced micronucleated reticulocytes (MNRETs) at all 3 doses. ENNG and EMS induced MNRETs significantly at 2 dose levels each and DES only at the highest dose. ECH and EDB failed to induce MNRETs. On the basis of the dose of chemical needed to double the spontaneous frequency, the order of clastogenic potency was ENU greater than ENNG greater than EMS greater than DES. The results obtained compared favorably with those from other in vivo methods. The present technique proves to be simple, flexible and relatively sensitive. Shifts in the optimum induction peak in individual animals and by some chemicals can be picked up easily which is important when testing weak mutagens and chemicals with an unknown mechanism of action.

A novel model to assess developmental toxicity of dihaloalkanes in humans: Bioactivation of 1,2-dibromoethane by the isozymes of human fetal liver glutathione S-transferase

Glutathione S-transferase (GST) isozymes from human fetal liver (16-18 weeks gestation) were purified by affinity chromatography followed by ion-exchange high performance liquid chromatography (HPLC). The purified isozymes were used to investigate toxicity of 1,2-dibromoethane(EDB) in an in vitro model of rat embryos in culture as passive targets. At least five isozymes of GST were found in the human fetal liver. Two anionic forms [pI values 5.5 (P-2) and 4.5 (P-3)] and one basic form [pI value 8.7 (P-6)] were clearly separated. The presence of two near-neutral forms was also identified. All the isozymes of the human fetal liver GSTs tested metabolized EDB (specific activities were 2.1, 7.0, and 2.0 mumol of GSH consumed/min/mg protein for P-2, P-3, and P-6 isozymes, respectively). Covalent binding of EDB to DNA and protein was 144% and 212% higher, respectively, with the P-3 anionic isozyme when compared to the P-6 basic isozyme of GST. No covalent binding to either protein or DNA was observed with the P-2 isozyme. EDB bioactivation by the GST isozyme P-3 (15 units; 1 unit = 1 nmol of GSH consumed/min) resulted in toxicity to cultured rat embryos. Significant reductions of crown rump length, yolk sac diameter, and the composite score of morphological parameters (Brown and Fabro method) were observed. The central nervous system, optic and olfactory systems, and the hind limb were most significantly affected. The results of this investigation suggest that EDB may be classified as a suspected developmental toxicant in humans.

Prevention of 1,2-dibromo-3-chloropropane (DBCP)-induced kidney necrosis and testicular atrophy by 3-aminobenzamide

The poly(ADP-ribosyl)transferase inhibitor, 3-aminobenzamide (3-ABA), reduced morphological evidence of 1,2-dibromo-3-chloropropane (DBCP)-induced DNA damage determined by alkaline elution. The DBCP plasma, kidney, and testis tissue doses determined between 1 and 8 hr after a single intraperitoneal injection were somewhat higher with than without 3-ABA pretreatment. Furthermore, the amount of DBCP metabolites covalently bound to macromolecules was reduced to about 20-30 percent of control, indicating that 3-ABA may have an effect on the formation/detoxication of reactive DBCP metabolites. Inhibitors of replicative DNA synthesis such as hydroxyurea or stimulation of DNA replication by nephrectomy did not affect the cytotoxicity, neither did inhibitors of DNA repair such as beta-cytosine arabinoside and beta-lapachone.

Transforming activity of ethylene dibromide in BALB/c 3T3 cells

Ethylene dibromide was capable of inducing in vitro transformation of BALB/c 3T3 cells either in the presence or in the absence of exogenous metabolic activation (S9-mix). This transforming effect was evidenced by the induction of a higher number of transformed foci as compared to the controls performed with untreated cells or solvent vehicle-treated cells. In the absence of exogenous activation, all assayed doses (ranging from 23.4 micrograms/ml to 187.9 micrograms/ml) exerted transforming activity. Number of foci obtained in EDB-treated plates antransformation frequency of the target cells were higher than those detected in the transformation test performed in the presence of S9-mix.

The effects of ethylene dibromide on semen quality and fertility in the rabbit: evaluation of a model for human seminal characteristics

Mature (12 months old) male New Zealand White rabbits (8-10/group) were dosed subcutaneously with ethylene dibromide (EDB) in corn oil (untreated and vehicle controls, 15, 30, or 45 mg/kg body wt/day for 5 days). Weekly semen samples (for 6 weeks preexposure, during treatment, and 12 weeks postdosing [pd]) were analyzed for sperm concentration, number, morphology, viability, and motion parameters (velocity, linearity, beat cross-frequency, amplitude of lateral head displacement (ALH), and circularity), and semen pH, osmolality, volume, fructose, citric acid, carnitine, protein, and acid phosphatase (AP). Male fertility was assessed preexposure and at 4 and 12 weeks pd by artificial insemination of three females/male/time point with one million motile sperm. The percentage pregnant females, litter size, fetal body weights, and structural development were assessed. In the 45 mg/kg dose group of males there was 30% mortality and liver damage in 43% of the survivors as evidenced by increased levels of serum enzymes. Also in this group, EDB produced significant decreases in sperm velocity, percentage motility, and ALH (up to 25% at various times pd). There were also dose-related decreases in semen pH (up to 2%) and total ejaculate volume (up to 23%, 15 and 30 mg/kg dose groups only). AP activities were significantly elevated (up to 116%) 2 weeks pd in the 45 mg/kg dose group. All other semen parameters evaluated were unaffected. Male fertility and fetal structural development were also unaffected. Of the seven semen parameters perturbed by EDB in humans (Schrader et al., 1988), four were also affected in the rabbit (sperm velocity, percentage motility, pH, and volume), whereas sperm number, viability, and morphology were not. Thus, some of the male reproductive effects of EDB in the human have been modelled in the rabbit, although the rabbit appears not to be as sensitive, since semen parameters were affected only at doses close to the LD50 (55 mg/kg). The present study (together with other published data) suggests that the rabbit appears to be a potential model for male reproductive toxicity in humans, warranting further evaluation.

Inhalation exposure in Drosophila mutagenesis assays: experiments with aliphatic halogenated hydrocarbons, with emphasis on the genetic activity profile of 1,2-dichloroethane

A series of mutation experiments was carried out with Drosophila melanogaster using inhalation exposure. 1,2-Dichloroethane (DCE) and 1,2-dibromoethane (DBE) were active in the sex-linked recessive lethal assay (SLRLT), whereas dichloromethane, dibromomethane, 1,2-dichloropropane and 1,3-dichloropropane were not. Compared to DBE, DCE is a less potent mutagen in the SLRL system. For both compounds, there is no evidence of a clear-cut dose-rate effect. DCE and dichloromethane were also investigated in the somatic mutation and recombination test (SMART), with results similar to those from the SLRLT. For DCE the genetic activity profile was further analyzed by carrying out a sex-chromosome loss assay and a complementation analysis of a series of induced recessive lethal mutations. A review of the use of inhalation in mutagenicity assays with Drosophila shows that this route of exposure is an effective one. Especially with chronic exposure times, rather low exposure concentrations can be detected. With compounds of intermediate volatility inhalation is not superior to other modes of administration; nor is it likely to be sensitive enough for in situ monitoring.

In vivo studies on halogen compound interactions. II. Effects of carbon tetrachloride plus 1,2-dibromoethane on relative liver weight and hepatic steatosis

A mixture of carbon tetrachloride (CCl4) and 1,2-dibromoethane (DBE) results in a significant increase in relative liver weight (RLW) when compared with the effect of each substance individually. At the same time, the liver triglyceride levels of rats treated with the mixture are lower than in those treated with CCl4 alone. This increase in RLW is not caused by simple tissue oedema. Lipoprotein secretion and the concentration of circulating non esterified fatty acids (NEFA) are not involved in the DBE-induced reduction of steatosis provoked by CCl4. The synthesis of triglycerides is significantly depressed in animals treated with the mixture. These findings provide further evidence of the previously observed enhancement of liver toxicity caused by the simultaneous presence of both agents, since the apparent protection against steatosis is due to greater damage to the liver cell, which is no longer able to counter the steatogenic action of CCl4 through a sufficient synthesis of triglycerides.

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.

Toxic exposure to ethylene dibromide and mercuric chloride: effects on laboratory-reared octopuses

The effects of acute and chronic exposure to either ethylene dibromide (EDB) or mercuric chloride (MC) were studied in laboratory-reared Octopus joubini, O. maya and O. bimaculoides. The advantages of using octopuses were that the responses were immediate, highly visible and sensitive. All species demonstrated signs of toxicity to acute and chronic exposure to EDB and to MC. A dosage-sensitive relationship for the loss and subsequent recovery of locomotor response and of chromatophore expansion was found for each species after acute exposure. For each species the LC50 for chronic exposure occurred within 12 hr at 100 mg/l for EDB and within 3 hr at 1,000 mg/l for MC. This study demonstrated the potential usefulness of laboratory-reared octopuses in evaluating the toxicity of marine environmental pollutants.

Toxicity of 1,2-dibromoethane in primary hepatocyte monolayer cultures: lack of dependence on oxygen concentration

Hepatic 1,2-dibromoethane (DBE) metabolism proceeds via two pathways: oxidation by cytochrome P-450 and direct conjugation with the ubiquitous tripeptide glutathione (GSH) via the GSH S-transferases. The toxicity of DBE in monolayers of hepatocytes was assessed to establish whether the toxicity of this compound is increased under conditions of reductive metabolism at low oxygen concentrations. Our previous studies with t-butyl hydroperoxide and the calcium ionophore A23187 suggested that hypoxia would exacerbate toxicity that was mediated through lipid peroxidation or loss of calcium homeostasis. Monolayers of hepatocytes were exposed for 2 hr to 0, 14, 140, 1400, or 14,000 ppm of DBE in an atmosphere of either 1, 2, or 20% oxygen. Toxicity was measured by leakage of aspartate aminotransferase (AST) and trypan blue exclusion. The time course of the development of cytotoxicity was examined by assaying cell death both immediately following a 2-hr exposure and 24 hr later. The LC50 of DBE vapor was found to be approximately 14,000 ppm when assayed immediately after exposure but only 140 ppm when assayed 24 hr after exposure. The similarity of the percentages of DBE-induced cell death after incubations at 1, 2, and 20% oxygen demonstrates that the toxicity of DBE is oxygen-independent. We conclude that while DBE is highly toxic to rat hepatocytes, hypoxia does not appear to contribute to the toxicity of DBE, even under conditions of low oxygen concentrations. This result is in direct contrast to a previous report where we showed that the toxicity of halothane is potentiated under hypoxic conditions.

In vivo studies on halogen compound interactions. I. Effects of carbon tetrachloride plus 1,2-dibromoethane on liver necrosis

Investigation of the hepatotoxicity of a mixture of two halogen compounds--carbon tetrachloride (CT) and 1,2-dibromoethane (DBE) used in association, especially in agriculture--is reported. Their simultaneous administration in the rat potentiates the necrosis provoked by exposure to CT alone. This damage can be totally prevented by prior treatment with vitamin E, which raises the liver antioxidant level. Determination of the TBA-reacting substances released from the liver homogenates of animals treated with one or both substances also corroborates the importance of lipid peroxidation in the pathogenesis of this potentiation. Furthermore, no significant difference between poisoning by CT and CT + DBE is noted when liver GSH values are measured under the same experimental conditions. This finding rules out the possibility that potentiation is only the simply sum of the damage caused by the two compounds (lipid peroxidation due to a radical initiator for CT, depletion of GSH for DBE). It is thus something more than a mere combination of two mechanisms of action and further investigation is required to unravel its more complex pathogenesis.

Forestomach lesions induced by butylated hydroxyanisole and ethylene dibromide: a scientific and regulatory perspective

Selected pathology lesions from 9 studies, 5 with butylated hydroxyanisole (BHA) and 4 with ethylene dibromide (EDB) are reviewed and their relative importance in regulatory evaluation is discussed. When Fischer 344 (F344) rats were fed BHA at 0.5% and 2.0% of the diet for 2 years, an increased number of rats of both sexes had epithelial hyperplasia of the forestomach at both treatment levels, compared to controls. At the 2.0% level, an increased number of rats had forestomach papilloma or forestomach squamous cell carcinoma. In a second study, in which F344 rats were fed BHA at 1.0% and 2.0% of the diet for 2 years, increased numbers of rats in both treatment groups were reported to have hyperplasia or papilloma of the forestomach. At the 2.0% level, increased numbers of rats developed squamous cell carcinoma of the forestomach. More Syrian golden hamsters fed BHA at 1.0% and 2.0% of the diet for 2 years reportedly had hyperplasia, papilloma or squamous cell carcinoma of the forestomach than did nontreated animals. Ingestion of BHA at 0.5% and 1.0% of the diet by B6C3F1 mice for 2 years was reported to produce an increase of animals with hyperplasia or papilloma of the forestomach at both dosage levels, compared to nontreated mice. When beagle dogs were fed BHA at 1.0% and 1.3% of the diet for 180 days, no lesions/tumors of the distal esophagus or stomach were identified at gross necropsy or by light or electron microscopy. When EDB was administered by gavage to Osborne-Mendel rats and B6C3F1 mice under conditions of the National Toxicology Bioassay Program, more rats and mice, both male and female, developed squamous cell carcinoma of the forestomach than did nontreated groups. EDB administered via inhalation to F344 rats and B6C3F1 mice did not cause squamous cell carcinoma of the forestomach; however, other neoplasms occurred which were considered to be treatment-related. Information gleaned from the BHA and EDB studies with multiple animal species facilitated regulatory decision-making regarding the potential toxicity/carcinogenicity of these compounds to man.

Activation of dihaloalkanes by glutathione conjugation and formation of DNA adducts

Ethylene dibromide (1,2-dibromoethane, EDB) can be activated to electrophilic species by either oxidative metabolism or conjugation with glutathione. Although conjugation is generally a route of detoxication, in this case it leads to genetic damage. The major DNA adduct has been identified as S-[2-(N7-guanyl)ethyl]glutathione, which is believed to arise via half-mustard and episulfonium ion intermediates. The adduct has a half-life of about 70 to 100 hr and does not appear to migrate to other DNA sites. Glutathione-dependent DNA damage by EDB was also demonstrated in human hepatocyte preparations. The possible relevance of this DNA adduct to genetic damage is discussed.

Effects of time-variant exposure on toxic substance response

Sources of time-variant exposure to toxic substances are identified and examined for their effects on the estimation of response. It is shown that only time-averaged target tissue concentrations are required to obtain rigorous risk estimates from the one-hit and multihit models. In contrast, detailed concentration histories need to be retained throughout analyses involving two-event models with intermediate-stage clonal growth advantage (clonal two-stage) and multistage models. Cumulative incidence ratios, based on the exact to time-averaged treatment of concentration time dependencies, are evaluated for substances whose toxic responses exhibit moderate (arsenic) and strong (ethylene dibromide) dependence on time of actual exposure. These ratios reveal that time-averaged dose approximations may lead to several orders of magnitude error in both the multistage and clonal two-stage models if exposure periods are short, and that 3.4-fold (arsenic) and 8-fold (ethylene dibromide) errors still exist even when an actual two-thirds lifetime exposure is averaged over a full lifetime. Finally, the effects of time-variant exposure on risk estimation due to migration and birth-death in an epidemiological setting are examined. A residence time distribution calculation shows that, if these effects are ignored for a population orally exposed to arsenic and characterized by an out-migration rate in excess of 5%/yr, response errors will exceed an order of magnitude.

Tissue binding of 1,2-dibromoethane in the cynomolgus monkey (Macaca fascicularis)

Autoradiography at different levels of resolution was used to study the tissue-binding of 1,2-dibromo[14C]ethane (DBE) in the cynomolgus monkey (Macaca fascicularis) in vivo (i.p. injection) and in vitro. The results show that DBE is metabolized to products which become bound to the tissues, preferentially in the liver and the kidney tubules. A distinct binding of radioactivity was also found in the adrenal zona reticularis. The binding of radioactivity in the surface epithelia of the respiratory and upper alimentary tract was not as high and striking as that previously observed in rodents. The results show that the sites of tissue-binding of DBE in the cynomolgus monkey correspond to the sites of tissue lesion observed in humans poisoned with DBE.