Mutagenicity of halogenated aliphatic hydrocarbons in Salmonella typhimurium, Streptomyces coelicolor and Aspergillus nidulans

A Review on Mutagenicity Testing for Hazard Classification of Chemicals at Work: Focusing on in vivo Micronucleus Test for Allyl Chloride

Chemical mutagenicity is a major hazard that is important to workers' health. Despite the use of large amounts of allyl chloride, the available mutagenicity data for this chemical remains controversial. To clarify the mutagenicity of allyl chloride and because a micronucleus (MN) test had not yet been conducted, we screened for MN induction by using male ICR mice bone marrow cells. The test results indicated that this chemical is not mutagenic under the test conditions. In this paper, the regulatory test battery and several assay combinations used to determine the genotoxic potential of chemicals in the workplace have been described. Further application of these assays may prove useful in future development strategies of hazard evaluations of industrial chemicals. This study also should help to improve the testing of this chemical by commonly used mutagenicity testing methods and investigations on the underlying mechanisms and could be applicable for workers' health.

Potential carcinogenic hazards of non-regulated disinfection by-products: haloquinones, halo-cyclopentene and cyclohexene derivatives, N-halamines, halonitriles, and heterocyclic amines

Drinking water disinfectants react with natural organic material (NOM) present in source waters used for drinking water to produce a wide variety of by-products. Several hundred disinfections by-products (DBPs) have been identified, but none have been identified with sufficient carcinogenic potency to account for the cancer risks projected from epidemiological studies. In a search for DBPs that might fill this risk gap, the present study projected reactions of chlorine and chloramine that could occur with substructures present in NOM to produce novel by-products. A review of toxicological data on related compounds, supplemented by use of a quantitative structure toxicity relationship (QSTR) program TOPKAT®) identified chemicals with a high probability of being chronically toxic and/or carcinogenic among 489 established and novel DBPs. Classes of DBPs that were specifically examined were haloquinones (HQs), related halo-cyclopentene and cyclohexene (HCP&H) derivatives, halonitriles (HNs), organic N-chloramines (NCls), haloacetamides (HAMs), and nitrosamines (NAs). A review of toxicological data available for quinones suggested that HQs and HCP&H derivatives appeared likely to be of health concern and were predicted to have chronic lowest observed adverse effect levels (LOAELs) in the low μg/kg day range. Several HQs were predicted to be carcinogenic. Some have now been identified in drinking water. The broader class of HNs was explored by considering current toxicological data on haloacetonitriles and extending this to halopropionitriles. 2,2-dichloropropionitrile has been identified in drinking water at low concentrations, as well as the more widely recognized haloacetonitriles. The occurrence of HAMs has been previously documented. The very limited toxicological data on HAMs suggests that this class would have toxicological potencies similar to the dihaloacetic acids. Organic N-halamines are also known to be produced in drinking water treatment and have biological properties of concern, but no member has ever been characterized toxicologically beyond bacterial or in vitro studies of genotoxicity. The documented formation of several nitrosamines from secondary amines from both natural and industrial sources prompted exploration of the formation of additional nitrosamines. N-diphenylnitrosamine was identified in drinking waters. Of more interest, however, was the formation of phenazine (and subsequently N-chorophenazine) in a competing reaction. These are the first heterocyclic amines that have been identified as chlorination by-products. Consideration of the amounts detected of members of these by-product classes and their probable toxicological potency suggest a prioritization for obtaining more detailed toxicological data of HQs>HCP&H derivatives>NCls>HNs. Based upon a ubiquitous occurrence and virtual lack of in vivo toxicological data, NCls are the most difficult group to assign a priority as potential carcinogenic risks. This analysis indicates that research on the general problem of DBPs requires a more systematic approach than has been pursued in the past. Utilization of predictive chemical tools to guide further research can help bring resolution to the DBP issue by identifying likely DBPs with high toxicological potency.

Toxicity, biomarkers, genotoxicity, and carcinogenicity of trichloroethylene and its metabolites: a review

Trichloroethylene (TCE) is a prevalent occupational and environmental contaminant that has been reported to cause a variety of toxic effects. This article reviews toxicity, mutagenicity, and carcinogenicity caused by the exposure of TCE and its metabolites in the living system as well as on their (TCE and its metabolites) toxicity biomarkers.

Guanine N7-alkylation in mice in vivo by metrifonate - discussion of possible genotoxic risk in mammals

Following intraperitoneal administration to male mice (strain AB Jena/Halle) of 14CH3-labelled metrifonate, 22 Ci/mol, in dosages of 0.48, 0.40 and 0.065 mmol/kg, DNA from liver and kidneys was analysed for 14C in N-7 methylguanine (7-MeG). The extent of methylation in liver was found to be maximal at 6 hrs after injection in amounts of 6-8 and 0.8 mumol 7-MeG/mol guanine for the high and the low dose, corresponding to a covalent binding index CBI 4-5. The half-life of excretion of 7-MeG was 5 hrs for the high and 15 hrs for the low dose. The extent of methylation at 0-6 of guanine was estimated to be around 0.002-0.01 mumol 0-6 MeG/mol guanine. Data from references concerning methyl methanesulfonate and dimethyl sulfate are compared with those of metrifonate and the genotoxic response of methylating and non-methylating metabolites is discussed.

Mutagenicity of cytochrome P450 2E1 substrates in the Ames test with the metabolic competent S. typhimurium strain YG7108pin3ERb5

Poor metabolic competence of in vitro systems was proposed to be one of their major shortcomings accounting for false negative results in genotoxicity testing. For several "low molecular weight cancer suspects" this was specifically attributed to the lack of cytochrome P450 2E1 (CYP2E1) in conventional in vitro metabolising systems. One promising attempt to overcome this problem is the transfection of "methyltransferase-deficient"S. typhimurium strains with the plasmid pin3ERb5. This plasmid contains DNA encoding for a complete electron transport chain, comprising P450 reductase, cytochrome b5 and cytochrome P450 2E1. In order to answer the question if CYP2E1 substrates that yield negative or inconclusive results in the Ames test can be activated by metabolic competent bacterial strains, we used YG7108pin3ERb5 to investigate the following compounds: acetamide, acrylamide, acrylonitrile, allyl chloride, ethyl acrylate, ethyl carbamate, methyl-methacrylate, vinyl acetate, N-nitrosopyrrolidine, trichloroethylene and tetrachloroethylene. N-Nitrosodiethylamine served as a positive control. In addition to these known or proposed CYP2E1 substrates, we investigated the polycyclic aromatic hydrocarbon benzo[alpha]pyrene and the heterocyclic aromatic amines 2-aminofluorene and 2-aminoanthracene.

RESULTS

The extensive metabolic competence of the transformed strain is underlined by results showing strong mutagenicity between 10 and 500 micro g N-nitrosopyrrolidine per plate. Unexpectedly, 2-aminoanthracene was mutagenic at a concentration range between 25 and 250 micro g per plate using YG7108pin3ERb5. Moreover, we demonstrate for the first time a clear response of sufficiently characterised allyl chloride in the Ames test at a reasonably low concentration range between 300 and 1500 micro g per plate. We achieved similar results in the parent strain YG7108 with conventional metabolic activation. Without metabolic activation less pronounced mutagenicity occurred, suggesting a contribution of a direct alkylating effect. Propylene oxide is usually contained in allyl chloride as stabilizer at amounts up to 0.09%. Though YG7108 revealed to be very sensitive towards propylene oxide, allyl chloride dissolved in water was not mutagenic, showing that no water soluble compounds contribute to its mutagenicity. None of the remaining compounds showed mutagenic effects using YG7108pin3ERb5.

CONCLUSION

YG7108pin3ERb5 and its parent strain YG7108 are sensitive for compounds which are negative in conventional tester strains including N-nitrosodiethylamine, N-nitrosopyrrolidine, propylene oxide and allyl chloride.

Determination of chloral hydrate and its metabolites in blood plasma by capillary gas chromatography with electron capture detection

A sensitive, accurate, and reliable method is described for the quantitative determination of chloral hydrate (CH) and its metabolites in blood plasma of mice and rats. Metabolites of CH include trichloroacetic acid (TCA), trichloroethanol (TCE), and trichloroethanol glucuronide (TCE-Glu). This new method uses capillary gas chromatography with electron-capture detection (GC/ECD). Procedures for improving sample stability and quality assurance are also described that were not mentioned in previous literature. Rat or mouse plasma (50 microl) is acidified (or treated enzymatically for TCE-Glu determination) and extracted with peroxide free methyl t-butyl ether. Distilled diazomethane (CH(2)N(2)) is added to derivatize TCA to its methyl ester. Detection limits were estimated at 0.2 microg/ml for CH and TCE, and 0.1 microg/ml for TCA. Detector response to TCA and TCE were shown to be linear in the range of 3.125-200 microg/ml (r> or =0.9996). For CH, the response fits a second-order equation in this same range (r=0.99994)

Carcinogenicity of chloral hydrate administered in drinking water to the male F344/N rat and male B6C3F1 mouse

Male B6C3F1 mice and male F344/N rats were exposed to chloral hydrate (chloral) in the drinking water for 2 years. Rats: Measured chloral hydrate drinking water concentrations for the study were 0.12 g/L, 0.58 g/L, and 2.51 g/L chloral hydrate that yielded time-weighted mean daily doses (MDDs) of 7.4, 37.4, and 162.6 mg/kg per day. Water consumptions, survival, body weights, and organ weights were not altered in any of the chloral hydrate treatments. Life-time exposures to chloral hydrate failed to increase the prevalence (percentage of animals with a tumor) or the multiplicity (tumors/animal) of hepatocellular neoplasia. Chloral hydrate did not increase the prevalence of neoplasia at any other organ site. Mice: Measured chloral hydrate drinking water concentrations for the study were 0.12 g/L, 0.58 g/L, and 1.28 g/L that gave MDDs of 13.5, 65.0, and 146.6 mg/kg per day. Water consumptions, survival, body and organ weights, were not altered from the control values by any of the chloral hydrate treatments. Enhanced neoplasia was observed only in the liver. Prevalence and multiplicity of hepatocellular carcinoma (HC) were increased only for the high-dose group (84.4%; 0.72 HC/animal; p < or = 0.05). Values of 54.3%; 0.72 HC/animal and 59%; 1.03 HC/animal were observed for the 13.5- and 65.0-mg/kg per day treatment groups. Prevalence and multiplicity for the control group were 54.8%; 0.74 HC/animal. Hepatoadenoma (HA) prevalence and multiplicity were significantly increased (p < or = 0.05) at all chloral hydrate concentrations: 43.5%; 0.65 HA/animal, 51.3%; 0.95 HA/animal and 50%; 0.72 HA/animal at 13.5, 65.0, and 146.6 mg/kg per day chloral hydrate compared to 21.4%; 0.21 HA/animal in the untreated group. Altered foci of cells were evident in all doses tested in the mouse, but no significant differences were observed over the control values. Hepatocellular necrosis was minimal and did not exceed that seen in untreated rats and mice. Chloral hydrate exposure did not alter serum chemistry and hepatocyte proliferation in rats and mice or increase hepatic palmitoyl CoA oxidase in mice at any of the time periods monitored. It was concluded that chloral hydrate was carcinogenic (hepatocellular neoplasia) in the male mouse, but not in the rat, following a lifetime exposure in the drinking water. Based upon the increased HA and combined tumors at all chloral hydrate doses tested, a no observed adverse effect level was not determined.

Threshold-mediated mechanisms in mutagenesis: implications in the classification and regulation of chemical mutagens

Chemical mutagens are currently regulated and labelled on the basis of their hazardous properties defined in hazard classification schemes. The strength and type of experimental evidence is used as the only criterion for classification in categories which express different levels of concern for the possibility of adverse effects - notably transmissible genetic alterations - in humans. Differently from the classification of carcinogens, no consideration is given to potency, nor to the mechanism of action. The rationale of such hazard based classification is that the hazardous property of a chemical is an intrinsic feature, which is expressed independently of dosing. Changing of dose level results in a mere change in the probability to observe an adverse effect, but not in its potential occurrence. The lack of theoretical threshold underlying this approach can be envisaged, in principle, for stochastic processes such as DNA damage, which can be triggered by single molecular interactions. On the other hand, indirect mechanisms of genotoxicity, involving multiple interactions with non-DNA targets, are expected to show a threshold. At variance to DNA reactive agents, chemicals acting with threshold-mediated mechanism do change also qualitatively their toxic properties depending on the dose level. Possible problems arising in the application of hazard based schemes for the evaluation of chemicals with threshold-mediated mechanism of action are discussed, using the spindle poisons benzimidazole fungicides as an example.

Mutagenicity of three disinfection by-products: di- and trichloroacetic acid and chloral hydrate in L5178Y/TK +/- (-)3.7.2C mouse lymphoma cells

The disinfection of water, required to make it safe for human consumption, leads to the presence of halogenated organic compounds. Three of these carcinogenic 'disinfection by-products', dichloroacetic acid (DCA), trichloroacetic acid (TCA) and chloral hydrate (CH) have been widely evaluated for their potential toxicity. The mechanism(s) by which they exert their activity and the steps in the etiology of the cancers that they induce are important pieces of information that are required to develop valid biologically-based quantitative models for risk assessment. Determining whether these chemicals induce tumors by genotoxic or nongenotoxic mechanisms (or a combination of both) is key to this evaluation. We evaluated these three chemicals for their potential to induce micronuclei and aberrations as well as mutations in L5178Y/TK +/- (-)3.7.2C mouse lymphoma cells. TCA was mutagenic (only with S9 activation) and is one of the least potent mutagens that we have evaluated. Likewise, CH was a very weak mutagen. DCA was weakly mutagenic, with a potency (no. of induced mutants/microgram of chemical) similar to (but less than) ethylmethanesulfonate (EMS), a classic mutagen. When our information is combined with that from other studies, it seems reasonable to postulate that mutational events are involved in the etiology of the observed mouse liver tumors induced by DCA at drinking water doses of 0.5 to 3.5 g/l, and perhaps chloral hydrate at a drinking water dose of 1 g/l. The weight-of-evidence for TCA suggest that it is less likely to be a mutagenic carcinogen. However, given the fact that DCA is a weak mutagen in the present and all of the published studies, it seems unlikely that it would be mutagenic (or possibly carcinogenic) at the levels seen in finished drinking water.

Oxidative metabolism of 1-(2-chloroethyl)-3-alkyl-3- (methylcarbamoyl)triazenes: formation of chloroacetaldehyde and relevance to biological activity

(Methylcarbamoyl)triazenes have been shown to be effective cancer chemotherapeutic agents in a number of biological systems. Because of their chemical stability, it is likely that their activity in vivo is the result of a metabolic activation process. Previous studies have shown that 1-(2-chloroethyl)-3-methyl-3-(methylcarbamoyl)triazene (CMM) and 1-(2-chloroethyl)-3-benzyl-3-(methylcarbamoyl)triazene (CBzM) are metabolized by rat liver microsomes in the presence of NADPH to yield the ((hydroxymethyl)carbamoyl)triazene analogs of the parent compounds. The present studies show that both compounds are also oxidized at the chloroethyl substituent to yield chloroacetaldehyde and a substituted urea. In the case of CBzM metabolism, 47% of the metabolized parent compound was recovered as benzylmethylurea, 8% was recovered as benzylurea, and 26% was recovered as the ((hydroxymethyl)carbamoyl)-triazene and carbamoyltriazene metabolites. These results suggest that the chloroethyl group is the favored initial site of metabolism. In reaction mixtures containing initial concentrations of 300 microM CBzM, 78 microM chloroacetaldehyde was produced, as compared to 58 microM chloroacetaldehyde produced from the metabolism of 300 microM CMM. The formation of chloroacetaldehyde, a known mutagenic DNA alkylating agent, may explain the biological activity of these compounds.

Micronuclei induced in round spermatids of mice after stem-cell treatment with chloral hydrate: evaluations with centromeric DNA probes and kinetochore antibodies

The chromosomal effects of chloral hydrate (CH) on germ cells of male mice were investigated using two methods to detect and characterize spermatid micronuclei (SMN); (a) anti-kinetochore immunofluorescence (SMN-CREST) and (b) multicolor fluorescence in situ hybridization with DNA probes for centromeric DNA and repetitive sequences on chromosome X (SMN-FISH). B6C3F1 mice received single intraperitoneal (i.p.) injections of 82.7, 165.4, or 413.5 mg/kg and round spermatids were sampled at three time intervals representing cells treated in late meiosis, early meiosis, or as spermatogonial stem cells. No increases in the frequencies of SMN were detected for cells treated during meiosis using either SMN-CREST or SMN-FISH methods. After spermatogonial stem-cell treatment, however, elevated frequencies of SMN were detected by both methods. With SMN-FISH, dose trends were observed both in the frequencies of spermatids containing micronuclei and in the frequency of spermatids carrying centromeric label. These findings corroborate the recent report by Allen and colleagues [Allen JW et al.(1994): Mutat. Res. 323:81-88] that CH treatment of spermatogenic stem cells induced SMN. Furthermore, our findings suggest that chromosomal malsegregation or loss may occur in spermatids long after CH treatment of stem cells. Further studies are needed to understand the mechanism of action of the CH effect on stem cells and to determine whether similar effects are induced in human males treated with CH.

Substrate specificity of rat liver aldehyde dehydrogenase with chloroacetaldehydes

Chlorinated acetaldehydes have been the focus of research due to their role as reactive intermediates and their possible occurrence in chlorinated drinking water. This study investigated the in vitro substrate specificity of cytosolic and mitochondrial rat liver aldehyde dehydrogenase toward these compounds. Monochloroacetaldehyde was found to be extensively metabolized by these enzymes, to an even greater extent than the standard substrate propionaldehyde. Dichloroacetaldehyde was metabolized to a much lesser extent, and chloral hydrate is not metabolized by this enzyme family. The Km (mM) and Vmax (Vmax for propionaldehyde set to 100) values with the low Km cytosolic enzyme were monochloroacetaldehyde 0.046 and 582, and dichloroacetaldehyde 0.13 and 54.9, and those with the high Km cytosolic enzyme were dichloroacetaldehyde 0.35 and 23.4. The values with the low Km mitochondrial enzyme were monochloroacetaldehyde 0.057 and 462 and dichloroacetaldehyde 0.038 and 12.9, and those with the high Km mitochondrial enzyme were monocloroacetaldehyde 0.024 and 55.5 and dichloroacetaldehyde 0.29 and 3.44. These data suggest that aldehyde dehydrogenase plays a significant role in the metabolism of monochloroacetaldehyde and, to some extent, dichloroacetaldehyde. Some evidence also suggested that alcohol dehydrogenase plays a significant role in the metabolism of dichloroacetaldehyde and chloral hydrate.

A mechanism for the development of Clara cell lesions in the mouse lung after exposure to trichloroethylene

Female CD-1 mice exposed to trichloroethylene (6 h/day) at concentrations from 20-2000 ppm developed a highly specific lung lesion after a single exposure, characterised by vacuolation of the Clara cells, the number of cells affected increasing with increasing dose level. At the highest dose levels pyknosis of the Clara cells was apparent. After 5 days of repeated exposures the lesion had resolved but exposure of mice following a 2-day break resulted in recurrence of the lesion. The changes in mouse lung Clara cells were accompanied by a marked loss of cytochrome P-450 activities. No morphological changes were seen in the lungs of rats exposed to either 500 or 1000 ppm trichloroethylene. Isolated mouse lung Clara cells were shown to metabolize trichloroethylene to chloral, trichloroethanol and trichloroacetic acid. Chloral was the major metabolite. Trichloroethanol glucuronide was not detected. In comparative experiments using mouse hepatocytes the major metabolites were trichloroethanol and its glucuronide conjugate. The activity of UDP-glucuronosyltransferase was compared in mouse lung Clara cells and hepatocytes using two phenolic substrates and trichloroethanol. Hepatocytes readily formed glucuronides from all three substrates whereas Clara cells were only active with the two phenolic substrates. The three major metabolites of trichloroethylene, chloral, trichloroethanol and trichloroacetic acid were each dosed to mice and of these metabolites, only chloral had an effect on mouse lung causing a lesion (Clara cell) identical to that seen with trichloroethylene. It is proposed that the failure of Clara cells to conjugate trichloroethanol leads to an accumulation of chloral which results in cytotoxicity. The known genotoxicity of chloral suggests that this lesion may be related to the development of lung tumours in mice exposed to trichloroethylene by inhalation.

On the hepatotoxicity of 1,1,2,2-tetrachloroethane

Intoxication of male and female mice with a single dose (300 or 600 mg/kg) of 1,1,2,2-tetrachloroethane (TTCE) resulted in significant decreases in cytochrome P-450 (to 58-73% of the control) and NADPH-cytochrome (P-450) c-reductase (to 29-35% of the control) in hepatic microsomes. This was accompanied by an alteration of mixed function monooxygenases stemming from the marked reduction (to 20-64% of the control) of several oxidative activities to selected substrates towards different P-450 isozymes (classes IA1, IA2, IIB1, IIE1 and IIIA). As phase II markers, epoxide hydrolase (approximately 35% loss), UDP-glucuronosyl transferase (approximately 42% loss) and to a lesser extent glutathione S-transferase (approximately 17% loss) were all affected. Also, the activity of delta-aminolevulinic (ALA) synthetase was decreased (approximately 57% of the control). On the contrary, heme oxygenase activity was increased (up to 35%) at the maximal dose tested. The decrease of P-450-function may be explained in terms of an alteration in the rate of heme biosynthesis and degradation, provoking a loss of heme content (approximately 33%) as well as of the direct inactivation of both P-450 and reductase. Because of increasing evidence on the involvement of free radical intermediates in the case of toxicity of haloalkanes, electron spin resonance spectroscopy (ESR) spin-trapping in vivo techniques were used to characterize the possible free radical species involved in the observed liver damage. The results obtained with the spin-trap N-benzylidene-2-methylpropylamine N-oxide (phenyl t-butylnitrone, PBN) provide evidence for the formation and trapping of the CHCl2CHCl free radicals. The detection of conjugated diene signals by means of second-derivative spectrophotometry, have enabled us to show that in vivo lipid peroxidation may be one of the main mechanisms responsible for TTCE hepatotoxicity.

Fate of 2,2,2-trichloroacetaldehyde (chloral hydrate) produced during trichloroethylene oxidation by methanotrophs

Four different methanotrophs expressing soluble methane monooxygenase produced 2,2,2-trichloroacetaldehyde, or chloral hydrate, a controlled substance, during the oxidation of trichloroethylene. Chloral hydrate concentrations decreased in these cultures between 1 h and 24 h of incubation. Chloral hydrate was shown to be biologically transformed to trichloroethanol and trichloroacetic acid by Methylosinus trichosporium OB3b. At elevated pH and temperature, chloral hydrate readily decomposed and chloroform and formic acid were detected as products.

Genetic toxicology of 1,1,2-trichloroethylene

1,1,2-Trichloroethylene (TCE) is a widely used halogenated solvent, produced in hundreds of millions of kg each year for industrial purposes. Occupational and environmental exposure of human populations to TCE has been reported in industrialized areas. Long-term carcinogenicity studies in rodents demonstrate that exposure to high doses of TCE results in the induction of liver and lung tumors in the mouse, and tumors of the kidney and the testis in the rat. An indirect mechanism, based on the stimulation of liver peroxisome proliferation by TCE metabolites, was proposed to explain species differences in TCE hepatocarcinogenicity. Mutagenicity studies indicate that TCE is weakly active both in vitro, where liver microsomes produce electrophilic TCE metabolites, and also in vivo in mouse bone marrow, where high rates of micronuclei, but no structural chromosome aberrations, are found. Among TCE metabolites, trichloroacetic acid was reported to be carcinogenic to mouse liver. Furthermore, both trichloroacetic acid and chloral hydrate were found to be genotoxic in vivo, inducing structural and numerical chromosome abnormalities, respectively.

The covalent binding of 1,1,2,2-tetrachloroethane to macromolecules of rat and mouse organs

The in vivo interaction of the hepatocarcinogen 1,1,2,2-tetrachloroethane (1,1,2,2-TTCE) with DNA, RNA, and proteins of male Wistar rats and BALB/c mice was measured 22 hr after i.p. injection. Covalent binding index (CBI) to liver DNA was about 500 and was comparable to those of carcinogens classified as moderate initiators. It was higher than those of other chloroethanes, even than that of 1,2-dichloroethane (1,2-DCE), a symmetrically substituted haloethane whose genotoxicity has been widely demonstrated. In in vitro cell-free systems, 1,1,2,2-tetrachloroethane was bioactivated by mixed-function oxidase(s) and glutathione-S-transferase(s) (GSH-T) from microsomal and cytosolic fractions of rat and mouse liver and, to a lesser extent, of mouse lung. The in vitro activation led to formation of reactive species capable of binding to exogenous DNA and to the subcellular constituents of enzymatic fractions. These data, along with previous literature reports, provide sufficient evidence of 1,1,2,2-TTCE genotoxicity.

Mutagenicity of chloroolefins in the Salmonella/mammalian microsome test. III. Metabolic activation of the allylic chloropropenes allyl chloride, 1,3-dichloropropene, 2,3-dichloro-1-propene, 1,2,3-trichloropropene, 1,1,2,3-tetrachloro-2-propene and hexachloropropene by S9 mix via two different metabolic pathways

In the presence of S9 mix all allylic chloropropenes tested exert considerable indirect mutagenic activity which is most pronounced for 1,2,3-trichloropropene. Lower as well as higher chlorinated derivatives are clearly less mutagenic. Longer than standard incubation time (120 min instead of 20 min) at 37 degrees C always leads to an increase in mutagenic activity. An increase in concentration of rat-liver homogenate fraction (S9) in the metabolising system (S9 mix) enhances mutagenicity only for 1,3-dichloropropene, 2,3-dichloro-1-propene and for the cis isomer of 1,1,2,3-tetrachloro-2-propene. According to the effects of the enzyme inhibitors SKF525 1,1,1-trichloropropene-2,3-oxide and cyanamide the allylic chloropropenes fall into 3 groups distinguished by their mode of metabolic activation by S9 mix: (a) allyl chloride and 1,3-dichloropropene are hydrolysed to the corresponding allylic alcohols which can be oxidised to the respective acroleins (hydrolytic-oxidative pathway); (b) 2,3-dichloro-1-propene, 1,1,2,3-tetrachloro-2-propene and hexachloropropene are epoxidised in the C=C double bond, giving rise to reactive epoxides (epoxidative pathway); (c) only 1,2,3-trichloropropene is obviously activated by both these alternative metabolic pathways. Structural parameters like chloro-substitution of the central C atom of the C=C-C sequence and substituent-induced polarisation of the C=C double bond as well as cis/trans isomerism might be responsible for different substrate properties for the enzymes involved in allylic chloropropene metabolism, thus determining different degrees of activation by either one or both pathways.

Species differences in response to trichloroethylene. I. Pharmacokinetics in rats and mice

The elimination of radioactivity in two strains of rats and mice following a single po dose of trichloro[14C]ethylene at dose levels from 10 to 2000 mg/kg has shown a marked dose dependence in rats but not in mice. The metabolism of trichloroethylene in the mouse was linear over the range of doses used, whereas in the rat it became constant and independent of dose at 1000 mg/kg and above. At the 10-mg/kg dosage, both species metabolized trichloroethylene almost completely, 60% of the dose being excreted in urine with only 1 to 4% being eliminated unchanged in expired air in the first 24 hr. At 2000 mg/kg, 78% of the dose was eliminated unchanged in the rat, but only 14% in the mouse. Consequently at high dosages, the mouse was exposed to significantly higher concentrations of trichloroethylene metabolites than the rat. Blood level kinetics of trichloroethylene and its metabolites confirmed a faster rate of metabolism in the mouse than in the rat. Peak concentrations of the metabolites were reached within 2 hr of dosing in the mouse compared to 10 to 12 hr in the rat. The concentrations of both trichloroethanol (4X) and trichloroacetic acid (7X) were significantly higher in the mouse than in the rat. Whereas trichloroethanol was rapidly eliminated from blood, the higher concentrations of trichloroacetic acid were maintained for over 30 hr. The high blood quantities of trichloroethylene-derived trichloroacetic acid are known to induce hepatic peroxisome proliferation in mice but are insufficient to induce this response in rats. These data suggest that trichloroacetic acid blood amounts, peroxisome proliferation, and the link between peroxisomes and liver cancer are the basis of species difference in response to trichloroethylene.

Mutagenicity of trichloroethylene, trichloroethanol and chloral hydrate in Aspergillus nidulans

A trichloroethylene (TCE) sample, free of epoxides, has been assayed for its ability to induce gene mutations (methionine suppressors) and mitotic segregation in the mould Aspergillus nidulans. No increase in the spontaneous frequency of methionine suppressors was observed when conidia of a haploid strain were plated on selective medium and exposed to TCE vapours. A weak but statistically significant increase in methionine suppressors was detected, however, when conidia of cultures grown and conidiated in the presence of TCE vapours were plated onto selective media. Growing colonies of a heterozygous diploid strain were exposed to TCE vapours to investigate the induction of mitotic segregation. Scoring and phenotypic analysis of segregant sectors showed a significant increase in the frequency of haploids and 'non-disjunctional' diploids but not of cross-overs. Treatment of quiescent conidia in liquid medium was ineffective. Trichloroethanol and chloral hydrate, two main TCE metabolites in mammals, shared the ability to induce somatic segregation demonstrated by TCE vapours. On the grounds of these results the possible endogenous metabolic conversion of TCE into trichloroethanol and chloral hydrate is hypothesized.

Induction of somatic segregation by halogenated aliphatic hydrocarbons in Aspergillus nidulans

8 halogenated aliphatic hydrocarbons were assayed for their ability to induce somatic segregation in the mould Aspergillus nidulans. Induction of haploidization, mitotic non-disjunction and mitotic crossing-over was studied in heterozygous colonies exposed to the tested chemicals through the detection and phenotypic analysis of segregated sectors. The results obtained show that 1,2-dibromoethane induced all kinds of segregated sectors; 1,2-dichloroethane, allyl chloride, 2-chloroethanol, 2,2-dichloroethanol and 2,2-dichloroacetaldehyde significantly increased the frequency of haploid sectors and diploid non-disjunctional sectors; chloroform and 1,2-dichloropropane were ineffective.

alpha-chloroepoxides. 2. Mutagenicity of 1-chlorocyclohexene oxide and its thermal isomerization product, 2-chlorocyclohexanone

The pseudo-first-order hydrolysis rate constants in pH 7.4 buffer-acetone solution are reported for 1-chlorocyclohexene oxide and 2-chlorocyclohexanone at 0, 25 and 37 degrees C. The rate constants, in conjunction with product studies, demonstrate that the hydrolysis of 1-chlorocyclohexane oxide quantitatively affords 2-hydroxycyclohexanone and that there is no significant isomerization of 1-chlorocyclohexene oxide to 2-chlorocyclohexanone during the hydrolysis. Both 1-chlorocyclohexene oxide and 2-chlorocyclohexanone were reacted with 2-aminopyridine, a model for adenine, to yield the same product, N-(2'-pyridyl)-2-aminocyclohexanone. The mutagenicity of 1-chlorocyclohexene oxide and 2-chlorocyclohexanone in the Ames liquid incubation assay using TA100 shows 2-chlorocyclohexanone to be slightly more active than 1-chlorocyclohexene oxide, in spite of the finding that 1-chlorocyclohexene oxide is clearly a more reactive electrophile than 2-chlorocyclohexanone. These results are interpreted to indicate the important role that hydrolysis (detoxification) plays in the in vitro evaluation of the proposed ultimate electrophilic metabolites of chloroolefins.

Evaluation of 2 different genetic markers for the detection of frameshift and missense mutagens in A. nidulans

21 chemicals, known to induce missense and/or frameshift mutations directly, were assayed for their ability to forward mutate a haploid strain of A. nidulans. 2 genetic markers for forward mutations were used, namely 8-azaguanine resistance and induction of meth A1 suppressors. Missense mutagens were usually active when tested with the plate-incorporation technique, whereas frameshift agents were ineffective; some of these, on the other hand, turned out to be positive when tested with a liquid-test procedure. The 2 genetic markers used showed a similar sensitivity (with only 2 exceptions) in detecting the chemical mutagens assayed.

TOL plasmid pWW0 in constructed halobenzoate-degrading Pseudomonas strains: prevention of meta pathway

The hybrid pathway for chlorobenzoate metabolism was studied in WR211 and WR216, which were derived from Pseudomonas sp. B13 by acquisition of TOL plasmid pWW0 from Pseudomonas putida mt-2. Chlorobenzoates are utilized readily by these strains when meta cleavage of chlorocatechols is suppressed. When WR211 utilizes 3-chlorobenzoate (3CB), the expression of catechol 2,3-dioxygenase (C23O) and the catabolic activities for chloroaromatics via the ortho pathway coexist as a consequence of inactivation of the meta cleavage activity by 3-chlorocatechol. Utilization of 4-chlorobenzoate (4CB) by WR216 presupposes the suppression of C23O by a spontaneous mutation in the structural gene, so that 4-chlorocatechol is not misrouted into the meta pathway. Such C23O- mutants were also selected when WR211 was grown continuously on 3CB. Our data explain why the phenotypic characters 3CB+ and Mtol+ (m-toluate) are compatible, whereas 4CB+ and Mtol+ are incompatible.

Growth-mediated metabolic activation of promutagens in Aspergillus nidulans

7 procarcinogens belonging to different chemical classes (nitrosamines, hydrazoalkanes, oxazaphosphorines and aromatic amines) were tested in A. nidulans for the induction of point mutations with two genetic systems (8-AG resistance and induction of methionine suppressors). Dimethylnitrosamine, diethylnitrosamine, nitrosomorpholine, dimethylhydrazine, procarbazine and cyclophosphamide gave positive results with a good dose--effect relationship in the growth-mediated assay, whereas they gave negative or borderline positive results in the plate incorporation assay. 2-Aminoanthracene was completely negative with both experimental procedures. DMN, DEN and NM were also tested for their ability to induce somatic segregation: all were positive when assayed in the growth-mediated assay.

Carcinogenicity and mutagenicity of solvents. I. Glycidyl ethers, dioxane, nitroalkanes, dimethylformamide and allyl derivatives

The carcinogenicity and/or mutagenicity as well as structural features and relationships of the glycidylethers (principally phenyl-, butyl-, allyl-, and isopropyl-), dioxane, nitroalkanes (nitro methane, ethane and propane), dimethylformamide and allyl derivatives (chloride, alcohol and amine) were examined. Additionally, considerations of the production, use patterns, estimated populations at risk, TLV's and metabolism of the above agents were discussed.

Studies on the miscoding properties of 1,N6-ethenoadenine and 3,N4-ethenocytosine, DNA reaction products of vinyl chloride metabolites, during in vitro DNA synthesis

1,N6-Ethenoadenine (epsilon A) and 3,N4-ethenocytosine (epsilon C) are formed when electrophilic vinyl chloride (VC) metabolites, chloroethylene oxide (CEO) or chloroacetaldehyde (CAA) react with adenine and cytosine residues in DNA. They were assayed for their miscoding properties in an in vitro system using Escherichia coli DNA polymerase I and synthetic templates prepared by reaction of poly(dA) and poly(dC) with increasing concentrations of CEO or CAA. Following the introduction of etheno groups, an increasing inhibition of DNA synthesis was observed. dGMP was misincorporated on CAA- or CEO-treated poly(dA) templates and dTMP was misincorporated on CAA- or CEO-treated poly(dC) templates, suggesting that epsilon A and epsilon C may miscode. The error rates augmented with the extent of reaction of CEO or CAA with the templates. Base-pairing models are proposed for the epsilon A.G. and epsilon C.T pairs. The potentially miscoding properties of epsilon A and epsilon C may explain why metabolically-activated VC and its reactive metabolites specifically induce base-pair substitution mutations in Salmonella typhimurium. Promutagenic lesions may represent one of the initial steps in VC- or CEO-induced carcinogenesis.