Metabolic activation of haloalkanes and tests in vitro for mutagenicity

The development of environmentally acceptable fluorocarbons

Chlorofluorocarbons (CFCs) were introduced in the 1930s as the safe replacements for the toxic and flammable refrigerants being used at that time. Subsequently, hydrochlorofluorocarbons (HCFCs) were also developed. In addition to refrigerant applications, they were used as foam blowing agents, as solvents and as propellants for many aerosols. In the 1970s and 1980s, concern developed about their environmental impact, specifically on stratospheric ozone depletion. Industry began to consider acceptable replacements. In 1987, many of the governments of the world came together and drafted the Montreal Protocol, calling upon Industry to initially phase out production of the CFCs and later HCFCs. Within 4 months of the signing of the Montreal Protocol, the 15 global major producers joined together to form the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS), which sponsored research into environmental effects and the Program for Alternative Fluorocarbons toxicity Testing, PAFT), which examined the toxicology of potential replacements for the CFCs and HCFCs. Nine replacements were identified by companies and, through this international cooperation; toxicology programs were designed, conducted, and evaluated without duplication of effort and testing; consequently these new products were introduced within less than 10 years. Indeed the Montreal Protocol has been recognized as the most appropriate international treaty to phase-down HFCs. In 2016 the Kigali Amendment to the Montreal Protocol set out a phase-down schedule for the consumption and production of HFCs. In order to reduce the consumption and emissions of high GWP HFCs. Recently lower GWP HFCs and very low GWP HFOs (hydrofluoroolefins and HCFOs (hydrochlorofluoroolefins) have been introduced into a range of applications. Summaries of the toxicology profiles of some of the original CFCs and HCFCs, the replacements and the new post-PAFT replacements are described. The chemicals in this review include CFC-11, CFC-12, CFC-113, CFC-114, HCFC 22, HCFC-123, HCFC-124, HCFC-141b, HCFC-142b, HCF-32, HFC-125, HFC-134a, HFC-143a, HFC-152a, HFC-245ea, HFC-245fa, HFO-1234yf, HFO-1234ze, and HCFO-1233zd.

Genotoxicity test system based on p53R2 gene expression in human cells: examination with 80 chemicals

p53R2, which encodes a subunit of ribonucleotide reductase, is activated by DNA damage induced by gamma-ray and ultraviolet irradiation, and also by genotoxic chemicals such as adriamycin. For the purpose of constructing an easy-operating genotoxicity test system using human cell lines, we developed a p53R2-dependent luciferase reporter gene assay, and demonstrated dose-dependent luminescence caused by adriamycin in two human cell lines that express wild-type p53, MCF-7 and HepG2. The performance of this assay system was evaluated with 80 chemicals including those known in the Ames test as genotoxic or non-genotoxic. When the luciferase activity of cells treated with the test sample was over 200% to that of control cells in a dose-dependent increasing manner, the sample was judged positive as a genotoxic chemical. Forty of 43 Ames-positive chemicals induced luciferase activity in this assay system. Eight Ames-negative chemicals also induced luciferase activity. These eight chemicals are genotoxic in other in vitro test systems using mammalian cells. It is suggested that this assay system can be applied to rapid screening of chemicals for potential human genotoxicity.

Mutagenicity of carbon tetrachloride and chloroform in Salmonella typhimurium TA98, TA100, TA1535, and TA1537, and Escherichia coli WP2uvrA/pKM101 and WP2/pKM101, using a gas exposure method

The volatile solvents carbon tetrachloride and chloroform are carcinogens that are often reported as nonmutagenic in bacterial mutagenicity assays. In this study, we evaluated the mutagenicity of these compounds in Salmonella typhimurium TA98, TA100, TA1535, and TA1537, and Escherichia coli WP2uvrA/pKM101 and WP2/pKM101, with and without S9 mix, using a gas exposure method. Tests were also conducted with a glutathione-supplemented S9 mix. Carbon tetrachloride was mutagenic in TA98 without S9 mix, and in WP2/pKM101 and WP2uvrA/pKM101 with and without S9 mix; carbon tetrachloride was not mutagenic in TA100, TA1535 or TA1537. Chloroform was mutagenic in WP2/pKM101, but only in the presence of glutathione-supplemented S9 mix. Chloroform was not mutagenic in TA98, TA100, TA1535, TA1537, or WP2uvrA/pKM101 with or without S9 mix, and was not mutagenic in TA98, TA100, TA1535, TA1537, or WP2uvrA/pKM101 in the presence of glutathione-supplemented S9 mix. The data indicate that carbon tetrachloride and chloroform are bacterial mutagens when adequate exposure conditions are employed and suggest that a genotoxic mode of action could contribute to the carcinogenicity of these compounds.

Reaction of trichloroethylene and trichloroethylene oxide with cytochrome P450 enzymes: inactivation and sites of modification

Trichloroethylene (TCE) has been shown to be toxic to experimental animals and humans. TCE oxide is a reactive electrophile formed during TCE oxidation and rearranges to acylating intermediates [Cai, H., and Guengerich, F. P. (1999) J. Am. Chem. Soc. 121, 11656-11663], which may be related to the toxicity. Mice treated with TCE have been reported to contain N(6)-dichloroacetylLys residues in P450 2E1, as detected by immunochemical methods. TCE can be oxidized by both P450 2E1 and (rat) 2B1. In this work, direct reaction of TCE oxide with either human P450 2E1, P450 2B1, or NADPH-P450 reductase was shown to lead to enzyme inactivation, and no recovery of the activity of either enzyme occurred, consistent with the view of inactivation reactions with Lys groups and not hydroxyls or Cys. Furthermore, Lys adducts were detected in the reaction of TCE oxide with both P450 2E1 and NADPH-P450 reductase, with a larger amount of N(6)-formylLys observed compared to N(6)-dichloroacetylLys in both cases. Inactivation of P450 2E1 during NADPH-dependent TCE oxidation was not observed, compared to control experiments. However, inactivation of P450 2B1 during NADPH-dependent TCE oxidation was detected. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of tryptic peptides indicated that the direct reaction of TCE oxide with human P450 2E1 resulted in the modification of peptides containing Lys87 (AVKEALLDYK), Lys251 (VKEHHQSLDPNCPR), and Lys487 (YKLCVIPR), with either a formyl or dichloroacetyl group attached. Lys87 and Lys487 of human P450 2E1 appear to be modified during the oxidation of TCE, using the same approach. The results are considered in the context of comparison of species and P450s.

Reaction of trichloroethylene oxide with proteins and dna: instability of adducts and modulation of functions

Trichloroethylene (TCE) shows several types of toxicities, some of which may be the result of bioactivation. Oxidation by P450s yields the electrophile TCE oxide. We previously analyzed N(6)-acyllysine adducts formed from the reaction of TCE oxide with proteins [Cai, H., and Guengerich, F. P. (2000) Chem. Res. Toxicol. 13, 327-335]; however, we had been unable to measure ester adducts under the prolonged conditions of proteolysis and derivatization. Protein amino acid adducts were directly observed by mass spectrometry during the reaction of TCE oxide with the model polypeptides insulin and adrenocorticotropic hormone (ACTH, residues 1-24). The majority (80%) of the protein adducts were unstable under physiological conditions and had a collective t(1/2) of approximately 1 h, suggesting that they are ester type adducts formed from reactions of Cys, Ser, Tyr, or Thr residues with intermediates formed in TCE oxide hydrolysis. Synthetic O-acetyl-L-Ser and O-acetyl-L-Tyr had half-lives of 1 h and 10 min at pH 8.0, respectively, similar to the stabilities of the protein adducts. The effects of TCE oxide adduct formation on catalytic activities were examined with five model enzymes. No recovery of catalytic activity was observed during the reaction of TCE oxide with two model enzymes for which the literature suggests roles of a Lys, rabbit muscle aldolase and glucose-6-phosphate dehydrogenase. However, in the cases of papain (essential Cys residue in the active site), alpha-chymotrypsin (critical Ser residue), and D-amino acid oxidase (essential Cys and Tyr residues), time-dependent recoveries of enzyme activity were observed following reaction with TCE oxide or either of two model nucleophiles (dichloroacetyl chloride and acetic formic anhydride), paralleling the kinetics of removal of adducts from insulin and ACTH. Formation of adducts ( approximately 2%) was detected in the direct reaction of TCE oxide with 2'-deoxyguanosine, but not with the other three nucleosides found in DNA. During the reaction of TCE oxide with a synthetic 8-mer oligonucleotide, formation of adducts was observed by mass spectrometry. However, the adducts had a t(1/2) of 30 min at pH 8.5. These results indicate the transient nature of the adducts formed from the reaction of TCE oxide with macromolecules and their biological effects.

Mechanisms for the hepatoprotective action of kolaviron: studies on hepatic enzymes, microsomal lipids and lipid peroxidation in carbontetrachloride-treated rats

The present work examines the protective mechanisms of a biflavonoid fraction of an extract from Garcinia kola seeds, kolaviron, in rats treated with carbontetrachloride (CCl(4)). CCl(4)administered at a dose of 1.2 g kg(-1), three times a week for 2 weeks, significantly depressed the activities of microsomal aniline hydroxylase, aminopyrine N -demethylase, ethoxyresorufin O -demethylase and p -nitroanisole O -demethylase. Kolaviron (200 mg kg(-1)), administered for 14 days consecutively, inhibited (P<0.001) the CCl(4)mediated decrease in the activities of these enzymes by 60, 65, 55, and 63%, respectively. Kolaviron reduced the CCl(4)increase in the cholesterol/phospholipid ratio. Similarly, kolaviron attenuated the toxic onslaught imposed by CCl(4)on 5'nucleotidase, glucose 6-phosphatase (microsomal marker enzymes) and malondialdehyde formation by 41, 54 and 77%, respectively. Kolaviron elicited 168% and 234% increases in the activity of UDP-glucuronosyl transferase and glutathione S -transferase. Simultaneous administration of kolaviron with CCl(4)modulated the effect of CCl(4)on the activities of these enzymes. On the basis of the above data, it can be postulated that kolaviron exerts its protective action against carcinogen-induced liver damage, first, by acting as an in vivo natural antioxidant and, second, by enhancement of drug-detoxifying enzymes.

Acylation of protein lysines by trichloroethylene oxide

Stable lysine adducts were formed in proteins following reaction with trichloroethylene (TCE) oxide, the major reactive compound generated by the metabolism of TCE. The order of formation of these adducts is N(6)-formyllysine > N(6)-(dichloroacetyl)lysine >> N(6)-glyoxyllysine, with the ratio being influenced by the particular protein. Protein lysine adducts were also analyzed following the enzymatic oxidation of TCE with several different cytochrome P450 (P450) enzyme systems. The ratio of formyl/dichloroacetyl lysine adducts was influenced by the enzyme system that was used. Chloral and TCE oxide formation was more extensive with rat liver microsomes isolated from phenobarbital-treated rats than with rat microsomes in which P450 2E1 was induced by treatment with isoniazid or in human P450 2E1 systems. Glutathione (GSH) and GSH transferase had inhibitory effects on the reaction of TCE oxide with albumin, with formylation being atteunated much more than the formation of dichloroacetyllysine. GSH is likely to react with the reactive acyl chloride intermediates formed from TCE oxide hydrolysis, instead of direct reaction with TCE oxide, as judged by the lack of an effect of GSH on the rate of decomposition of TCE oxide. Studies with the model enzymes aldolase and glucose-6-phosphate dehydrogenase, both known to have sensitive lysine groups, indicate that TCE oxide has effects similar to known acylating agents that form the same adducts; concentrations of TCE oxide (or the model acylating agents) in the low-millimolar range were needed for inhibition. The characterization of TCE-derived protein adducts can be used as a basis for consideration of the exposure and risk of TCE to humans. Human P450 2E1 was less likely to oxidize TCE to form TCE oxide and protein lysine adducts than rat P450 2B1, and the difference is rationalized in terms of the influence of the protein on chloride migration in an enzyme reaction intermediate.

Effects of four trihalomethanes on DNA strand breaks, renal hyaline droplet formation and serum testosterone in male F-344 rats

All four possible trihalomethanes (THMs) containing bromine and chlorine, as well as perchloroethylene (PCE), were evaluated for their ability to produce DNA strand breaks, alpha 2u-globulin rich renal deposits, and testosterone changes in male F-344 rats. Rats received daily equimolar doses (0.75 or 1.5 mmol/kg) of THMs or PCE (1000 mg/kg) in 4% Emulphor vehicle by oral gavage for 7 days. No significant DNA strand breaks were produced by any THM or PCE treatment. PCE treatment produced increased hyaline droplet formation in renal tubules. However, all THM treatments reduced or eliminated the appearance of renal hyaline droplets. All four THM treatments also produced a decrease in serum testosterone concentrations on day 7, which might account for decreased hyaline droplet formation. No significant increase in cell proliferation, measured by [3H]thymidine incorporation in vivo, appeared in this 1-week study.

Validation of the SOS/umu test using test results of 486 chemicals and comparison with the Ames test and carcinogenicity data

The present study gives a comprehensive update of all umu genotoxicity assay results published so far. The available data of 486 chemicals investigated with the umu test are compared with the Ames test (274 compounds) as well as rodent carcinogenicity data (179 compounds). On the whole, there is good agreement between the umu test and the Ames test results, with a concordance of about 90%. The umu test was able to detect 86% of the Ames mutagens, while the Ames test (using at least 5 strains) detected 97% of the umu positive compounds. The elimination of TA102 from the set of Ames tester strains reduced the percentage of detectable umu genotoxins from 97 to 86%. The agreement between carcinogenesis and umu response was 65%, which is comparable to earlier studies concerning rodent carcinogenesis and Salmonella mutagenesis. The present compilation of umu results provides a database that can be used for the comparison of the SOS-inducing activity of chemicals and their mutagenicity, respectively, carcinogenicity. The results presented here clearly demonstrate that a chemical which induces the expression of the umu operon can be regarded a rodent carcinogen with a high degree of certainty (93%).

Field bean protease inhibitor mitigates the sister-chromatid exchanges induced by bromoform and depresses the spontaneous sister-chromatid exchange frequency of human lymphocytes in vitro

The mutagenicity of a trihalomethane-bromoform (CHBr3)-was assessed by the in vitro sister-chromatid exchange (SCE) assay using human peripheral blood lymphocytes. CHBr3 was found to induce SCEs significantly in a dose-dependent manner. When the cells were exposed to 600 ng CHBr3/ml of the medium, the SCE/cell mean reached a value as high as 18.78 +/- 0.17. Beyond this concentration. CHBr3 proved to be cytotoxic. A protease inhibitor (PI), purified appreciably by affinity chromatography from fieldbean (FB), was able to suppress significantly in a dose-dependent way the high SCE frequencies induced by this specific concentration of CHBr3 (600 ng/ml). Addition of 600 micrograms of FBPI/ml of the medium brought down the CHBr3-induced high SCEs to near (8.80 +/- 0.15) base line or control value (8.45 +/- 0.21). A study of the effect of FBPI on the normal low SCE frequencies in these cells indicated that the FBPI has the intrinsic property to suppress in a dose-dependent manner these SCEs in the lymphocytes. This functional property of FBPI, which is related to its protease inhibitory activity and which is destroyed when it is inactivated by autoclaving, makes it an effective antimutagenic/chemopreventive agent.

UDS-test with freon 11 (R-11)

Trichlorofluoromethane, in concentrations of 80, 400, 2 000, 10 000 and 50 000 ppm, was administered to rats of both sexes (Sprague Dawley) by inhalation exposure.2 000 ppm in air (= 11 200 mg/m(3)) amount to twice the MAK value of 1 000 ppm. At exposure times of 4 hours this corresponds to the MAK value defined for an 8 hour workday.Unscheduled DNA synthesis (UDS) was measured in single-cell suspensions of hepatocytes, pulmonary epithelial cells and lymphocytes of the spleen, respectively.In the pulmonary cells concentrations of 2 000, 10 000 and 50 000 ppm of freon 11 lead to a significantly increased mean silver grain count compared to a negative control group.In spleen and liver cells increasing concentrations of R 11 tend to increase the incorporation of thymidine into the DNA of the cells. These changes of the extent of unscheduled DNA synthesis can, however, not be statistically verified.

Effect of eugenol on drug-metabolizing enzymes of carbon tetrachloride-intoxicated rat liver

The chemoprotection extended by eugenol against carbon tetrachloride (CCl4) intoxication was established by studies on drug-metabolizing phase I and phase II enzymes. An overall decrease in drug-metabolizing enzymes, namely NADPH-cytochrome c reductase, NADH-cytochrome reductase, coumarin hydroxylase, 7-ethoxy coumarin-O-deethylase, UDP-glucuronyltransferase and glutathione-S-transferase, was observed with CCl4 intoxication, with a subsequent decrease in cytochrome P450 and cytochrome b5 content. CCl4 caused a significant decrease in microsomal phospholipids and the marker enzymes glucose-6-phosphatase and 5'-nucleotidase, and an increase in thiobarbituric acid reactive substances (TBARS). Simultaneous administration of eugenol with CCl4 inhibited the accumulation of TBARS and the decrease in the microsomal phospholipids and marker enzymes. Further, the chemical onslaught imposed by CCl4 on the drug-metabolizing system was removed successfully by eugenol. Eugenol appears to act as an in vivo antioxidant and as a better inducer of phase II enzymes than phase I enzymes. It is therefore suggested that eugenol could be an interesting basic structure for drug design.

Interaction of trichloromethyl free radicals with thymine in a model system: a mass spectrometric study

In previous studies from our laboratory we found that the CCl4 reactive metabolites produced during enzymatic in vitro or in vivo CCl4 biotransformation covalently bind to DNA. Further, chemically produced.CCl3 produce many adducts of unknown structure with the four DNA bases when the reaction proceeds in model systems. In the present work, we describe our attempt to elucidate by GLC/MS the structures of the adducts resulting when chemically generated.CCl3 interact with thymine. The following reaction products were identified: (i) 5-hydroxymethyl uracil; (ii) thymineglycol; (iii) 5-trichloroethyl uracil (tentative) and (iv) two isomeric 5,6-monochloro monohydroxy adducts of thymine (tentative). Reaction products found do not involve thymine positions directly participating in base-pairing processes. However, alterations in thymine structure reported if they occurred in DNA from livers of CCl4 poisoned animals, might potentially have biological significance that remains to be established.

Cytosine attack by free radicals arising from bromotrichloromethane in the presence of benzoyl peroxide catalyst: a mass spectrometric study

We and others previously reported that CCl4 reactive metabolites are able to covalently bind to liver DNA either in vivo or in vitro. However, no demonstration of the structure of resulting adducts is available in literature. That information would be of relevance, for CCl4 exhibits null or contradictory mutagenic properties and is currently considered a non-genotoxic carcinogen. In the present study we report the nature of the reaction products formed when the putative CCl4 metabolites, .CCl3 and CCl3O2. attack cytosine in a purely chemical system where they were generated from CCl3Br in a benzoyl peroxide catalyzed reaction. Reaction products formed and identified were a) under nitrogen (.CCl3 present)--5-bromo cytosine and cytosine-5-carboxylic acid; b) under air (CCl3O2. present)--5-bromo cytosine, 5-chloro cytosine, 5-hydroxy cytosine, 6-hydroxy cytosine (tentative), chloro hydroxy uracil, 5,6-dihydroxy uracil, and chloro trichloromethyl cytosine. Results from present experiments suggest that if these reaction products were also produced in vivo during either CCl4 or CCl3Br poisoning and they were not repaired in due time prior to replication, they would lead to mutagenic events. Studies directed to obtain evidence for their in vivo formation are in course in our laboratory.

Biotransformation of carbon tetrachloride and lipid peroxidation promotion by liver nuclear preparations from different animal species

Liver nuclear preparations from male Syrian Golden hamster (SG); C3H mice and Sprague-Dawley (SD) rats were able to biotransform CCl4 to CHCl3. That ability was not NADPH dependent and proceeded to an equal extent under N2 or air. Studies in more detail with C3H mice preparations revealed that only one of the processes was of an enzymatic nature and that it was inhibited by 1 mM EDTA. There was a correlation between liver nuclear ability to biotransform CCl4 to CHCl3 in the species tested and their liver carcinogenic response to CCl4. That correlation was not observed when biotransformation was studied using liver slices instead of liver nuclei. Liver nuclear preparations from the 3 species were able to promote a lipid peroxidation (LP) process in the presence of CCl4. The process was fully NADPH dependent in the case of SG and SD preparations but not in C3H mice. Study of the process in detail in the case of C3H mice shows that in that case LP was heat and EDTA sensitive, particularly in the absence of NADPH. There was no correlation between the intensity of CCl4 promoted LP either in liver nuclear or liver slices preparations in the 3 species tested and their carcino genic response to CCl4. Results might suggest that LP does not determine or rate limit the process of cancer development by CCl4 but do not exclude its participation in a given stage of the overall process.

Carcinogenic and mutagenic potential of several fluorocarbons

A series of chlorofluorocarbons (CFC) have been evaluated for carcinogenic potential in two comprehensive toxicity studies. The first of these studies involved an assessment of their potential carcinogenicity using in vitro short-term predictive tests followed by a limited gavage validation assay in rats. The second study was a more conventional inhalation study of CFC22 employing rats and mice coupled with an assessment of in vivo genotoxicity of the material. The current paper briefly reviews these two studies and assesses the overall genotoxicity profile of CFC22 in terms of risk to man. It is concluded that CFCs are not biologically inert, but that the series contains bacterial mutagens, cell-transforming agents, and rodent carcinogens, and for this series of compounds at least, prokaryotic mutation does not accurately predict carcinogenic potential. In addition it is concluded that CFC22 does not represent a carcinogenic or mutagenic threat to man.

SOS-inducing activity of chemical carcinogens and mutagens in Salmonella typhimurium TA1535/pSK1002: examination with 151 chemicals

The umu test system is a newly developed method to evaluate genotoxic activities of a wide variety of environmental carcinogens and mutagens (Oda et al., 1985). In the present study, we further examined the abilities of 151 chemicals to induce umu gene expression in Salmonella typhimurium TA1535/pSK1002. Among the chemicals examined, 72 compounds induced umu gene expression, which could be defined on a basis of increased beta-galactosidase activity by 2-fold over the background level. The potent genotoxic compounds without metabolic activation were adriamycin, bleomycin, daunorubicin, 1,3-dinitropyrene, 1,6-dinitropyrene, 1,8-dinitropyrene, N-ethyl-N'-nitro-N-nitrosoguanidine, furylfuramide, methyl methanesulfonate, N-methyl-N'-nitro-N-nitrosoguanidine, mitomycin C, 1-nitropyrene and 4-nitroquino-line-1-oxide. In the presence of S9, aflatoxin B1, 2-aminoanthracene, Glu-P-1, IQ, MeIQ, MeIQx, Trp-P-1 and Trp-P-2 also induced umu gene expression markedly. Several chemicals such as 2-acetylaminofluorene, 9-aminoacridine, azobenzene, benzanthracene, benzidine, diethyl nitrosamine, 1-nitronaphthalene, paraquat, potassium dichromate and sodium nitrite were weakly genotoxic and the induction by these compounds could be detected only when the incubation time was prolonged from 2 h to 5 h. Data are also presented that some of the chemicals such as dimethyl sulfoxide, m-dioxan, 5-fluorouracil and paraquat, which have been reported to be non-mutagenic in Ames/Salmonella assay, were found to be active in inducing umu gene expression, while the known mutagenic compounds including acrylonitrile, 4,4'-dinitrobiphenyl, furfural, methylene chloride, 1-naphthylamine, sodium azide, o-tolidine and o-toluidine were non-genotoxic in the present assay system.

Carcinogenicity of by-products of disinfection in mouse and rat liver

By-products of disinfection were tested for initiating and/or promoting activity in rat liver by using the rat liver foci bioassay. The assay uses an increased incidence of gamma-glutamyltranspeptidase-positive foci (GGT foci) as an indicator of carcinogenicity. The by-products of disinfection, including chloramine, halogenated humic acids, halogenated ethanes, halogenated acetonitriles, halogenated methanes, halogenated ethylenes, and N-Cl-piperidine, did not initiate GGT foci, which would indicate that they are not capable of initiating carcinogenesis. Chloroform and halogenated benzenes were tested in this assay for their ability to promote the occurrence of GGT foci and tumors initiated by diethylnitrosamine (DENA). Chloroform (1800 ppm in the drinking water) either had no effect or inhibited the occurrence of GGT foci when administered subsequent to a single dose of DENA. However, when the chloroform was administered in drinking water concurrently with weekly doses of DENA, it enhanced the formation of liver tumors. Of 20 halogenated benzenes tested, only 1,2,4,5-tetrachlorobenzene and hexachlorobenzene promoted the occurrence of DENA-initiated GGT foci. Thus in rat liver, the tested by-products of drinking water disinfection did not demonstrate tumor-initiating activity, although a few appeared to possess tumor-promoting activity. Chloroform was also tested for tumor-promoting activity in 15-day-old Swiss mice initiated with ethylnitrosourea (ENU). At weaning they started to receive either 1800 ppm chloroform or 500 ppm sodium phenobarbital (the positive control for tumor promotion) in their drinking water. The mice continued to receive either chloroform or phenobarbital until 51 weeks of age and were sacrificed at 52 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Genotoxicity and carcinogenicity of fluorocarbons: assessment by short-term in vitro tests and chronic exposure in rats

Two short-term in vitro tests for mutagenicity (Salmonella reverse mutation and BHK21 cell transformation) were conducted on a series of fluorocarbons. Some of these materials (FC22, FC31, FC142b, FC143, and FC143a) were found to be positive in one or both of the tests and could therefore be considered as being potentially carcinogenic to animals. Such activity was not anticipated for what were previously considered inert materials and in consequence several examples of these fluorocarbons, which represented different combinations of short-term test results, were tested for carcinogenicity in limited in vivo bioassays. In these studies, rats were dosed for 1 year by gavage 5 days a week with either FC22, FC31, FC133a, FC134a, or FC143a dissolved in a corn-oil at a single dosage of 300 mg/kg body weight. The animals were then observed until week 125 with detailed necropsy at termination. The study revealed that FC31 was a potent carcinogen (to the rat stomach), a result which reflected the short-term test predictions, but FC133a, which gave a negative response in both the in vitro assays, induced a high incidence of reproductive tract tumors. The weak bacterial mutagens FC22 and FC143a did not induce tumors in this study, and the nonmutagenic FC134a was without overt carcinogenic activity. It is concluded that, while recognizing the limitations of the in vivo component of this study, the short-term tests were only partially successful in identifying potential carcinogens for this series of chemicals. Fluorocarbon 31 was a potent carcinogen which was first identified by bacterial mutation and cell transformation, whereas the equally potent carcinogen FC133a was not so identified. The lack of genotoxic activity with this particular compound leads us to believe that the carcinogenic activity may be due to mechanisms other than those which involve direct DNA interactions.

Trihalomethanes induce sister chromatid exchanges in human lymphocytes in vitro and mouse bone marrow cells in vivo

The four major trihalomethanes (THMs) found in chlorinated drinking water (CHCl3, CHCl2Br, CHClBr2, and CHBr3) have been investigated for their ability to induce sister chromatid exchanges (SCEs) and cell-cycle delays in human lymphocytes in vitro and to induce SCEs in mouse bone marrow cells in vivo. Each THM tested caused dose-dependent increases in SCE frequency and delays in the cell cycle. The THMs differed greatly in their ability to induce these cytological effects in vitro, with CHBr3 being the most active compound and CHCl3 the least active compound, whereas they were not markedly different in their ability to induce SCEs in vivo.

Mechanistic considerations for carcinogenic risk estimation: chloroform

Chloroform has been reported to induce cancer in rodents after chronic administration of high doses by gavage. However, the interpretation of these findings is hampered by a lack of knowledge concerning the relative roles of genetic and nongenetic mechanisms in these bioassays. The present studies were carried out in male B6C3F1 mice in order to investigate the potential of chloroform to induce genetic damage and/or organ toxicity at the sites where tumors have been observed in the various bioassays. These studies revealed that carcinogenic doses of chloroform produced severe necrosis at the sites where tumors later developed. This was demonstrated by light microscopy as well as by determination of the cellular regeneration index following administration of 3H-thymidine. Noncarcinogenic doses of chloroform failed to induce these responses. In contrast, studies of DNA alkylation and DNA repair in vivo failed to give any indication that chloroform had produced the type of genetic alterations associated with known genotoxic chemicals. These data suggest that the primary mechanism of chloroform-induced carcinogenesis is nongenetic in nature. If the same mechanism predominates in man, there should be little to no carcinogenic risk associated with exposure to noncytotoxic levels of chloroform.

The effect of dose and vehicle on early tissue damage and regenerative activity after chloroform administration to mice

The relationship between the acute toxicity of orally-administered chloroform and its long-term tumorigenic potential was studied in male mice of the CFLP outbred Swiss albino mouse strain. A single dose of approximately 18 mg CHCl3/kg had no detectable acute toxic effect on the liver or kidneys and did not stimulate regenerative activity, whereas both toxicity and subsequent tissue regeneration were observed with single doses of about 60 mg/kg or higher. The severity of the toxic effects and regenerative changes was greater when corn oil was used as a vehicle for chloroform than when the vehicle was a toothpaste base. In earlier long-term studies in mice of the same strain, kidney tumours occurred in males given 60 mg/kg/day throughout life but not in mice given 17 mg/kg/day. The tumour response was greater when the 60-mg/kg/day dose was given in an oily vehicle than when it was given in toothpaste. The findings are consistent with the hypothesis that early acute toxic change and subsequent repair are a sine qua non for tumorigenesis in the kidney and liver in response to chloroform.

Bacterial mutagenicity studies on chloroform in vitro

Chloroform was tested for mutagenicity in the Salmonella/microsome assay using five strains of Salmonella typhimurium. In view of previous reports describing the development of liver and kidney tumours in some experiments involving long-term administration of chloroform to rats and mice, the mutagenicity tests were carried out in the absence of any S-9 microsomal-enzyme preparation and in the presence of S-9 microsomal-enzyme preparations derived from (a) livers and (b) kidneys of rats and mice previously exposed to the microsomal-enzyme inducer Aroclor 1254. No evidence of potential mutagenicity was observed under any of the test conditions. To determine whether the findings might have been influenced by the volatility of the chloroform, the test organisms were exposed to chloroform vapour, but again chloroform gave no indication of potential mutagenicity. Taken in conjunction with already published data from mutagenicity studies with chloroform, it appears unlikely that the tumours observed in some long-term rodent studies are attributable to a genotoxic action of the compound.

Covalent binding of chloroform metabolites to nuclear proteins - no evidence for binding to nucleic acids

No evidence was found for covalent binding of CHCl3 metabolites to male Sprague-Dawley rats or strain A/J male mice liver DNA or RNA under different experimental conditions including phenobarbital or 3-methyl-cholanthrene or diethylmaleate treatments or multiple CHCl3 injections. In addition, no binding to DNA was observed when DNA was exposed to CHCl3 in incubation mixtures containing microsomes and NADPH or when tissue slices were incubated with CHCl3. In contrast, nuclear histone and non-histone proteins bound to CHCl3 metabolites.

Cytochrome P-450 mediated genetic activity and cytotoxicity of seven halogenated aliphatic hydrocarbons in Saccharomyces cerevisiae

Cells of Saccharomyces cerevisiae, harvested from log-phase cultures, contain cytochrome P-450 and are capable of metabolizing promutagens to genetically active products. The activities of 7 halogenated aliphatic hydrocarbons in the yeast system have been investigated. All of the compounds tested (methylene chloride, halothane, chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene and s-tetrachloroethane) induced mitotic gene convertants and recombinants and, to a lesser extent, gene revertants when incubated with log-phase cells of the yeast strain D7. An examination of the difference spectra observed upon the addition of carbon tetrachloride, halothane and trichloroethylene to whole-cell or microsomal suspensions of yeast suggested that cytochrome P-450 mediated the metabolism of the hydrocarbons tested to cytotoxic and genetically active compounds.

In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

The covalent binding of chemical carcinogens to DNA of mammalian organs is expressed per unit dose, and a 'Covalent-Binding Index', CBI, is defined. CBI for various carcinogens span over 6 orders of magnitude. A similar range is observed for the carcinogenic potency in long-term bioassays on carcinogenicity. For the assessment of a risk from exposure to a carcinogen, the total DNA dmaage can be estimated if the actual dose is also accounted for. A detailed description is given for planning and performing a DNA-binding assay. A complete literature survey on DNA binding in vivo (83 compounds) is given with a calculation of CBI, where possible, 153 compounds are listed where a covalent binding to any biological macromolecule has been shown in vivo or in vitro. Recent, so far unpublished findings with aflatoxin M1, macromolecule-bound aflatoxin B1, diethylstilbestrol, and 1,2-epithiobutyronitrile are included. A comparison of CBI for rat-liver DNA with hepatocarcinogenic potency reveals a surprisingly good quantitative correlation. Refinements for a DNA-binding assay are proposed. Possibilites and limitations in the use of DNA binding in chemical carcinogenesis are discussed extensively.

Biotransformation of chloroform by rat and human liver microsomes; in vitro effect on some enzyme activities and mechanism of irreversible binding to macromolecules

The effects of chloroform on some rat microsomal enzyme activities were studied in vitro. Maximum inhibition of oxygen consumption, NADPH oxidase and NADPH-cytochrome c reductase was observed at 0.5 mM chloroform; prior metabolization of CHCl3 by microsomal monooxygenases increased inhibition by about 50% at 0.2-0.5 mM chloroform. Higher concentrations produced a paradoxical reversal of inhibition, whereas p-nitroanisole demethylase was steadily inhibited by about 50% up to 10 mM chloroform. Irreversible binding of 14CHCl3 was confirmed to depend on chloroform metabolization by monooxygenases. The increased irreversible binding due to phenobarbital induction is accompanied by a diminished affinity towards chloroform as shown by increased KM of irreversible binding, and a higher spectral dissociation constant KS. Aminoacids with nucleophilic functions (histidine, cysteine) partially prevented the irreversible binding of chloroform metabolites to microsomes; non-volatile radioactive derivatives were recovered in trichloracetic acid supernatants when microsomes were incubated with cysteine, but not with histidine. Phosgene has been demonstrated as a biological metabolite of chloroform: its possible reactions with nucleophilic groups of macromolecules, water and added aminoacids partly explain these experimental data. Similar results were obtained with human microsomes, showing that chloroform hepatotoxicity in man could involve the same mechanisms.

Sensitive gas chromatographic methods for the determination of vinyl epoxide synthetase activity using trichloroethylene as a model substrate

Two specific and very sensitive methods for the determination of vinyl epoxide synthetase activity in liver microsomes are described. Trichloroethylene, which is used as a substrate, is converted into trichloroethylene oxide by a hepatic epoxide synthetase. Chloral hydrate, the final rearrangement product of trichloroethylene oxide, is determined by electron-capture gas chromatography, either after derivatization with pentafluorophenylhydrazine or after its conversion into chloroform under alkaline conditions. The kinetic parameters of the epoxidation reaction were determined on rat hepatic microsomal suspensions.

A gas chromatographic method for the evaluation of the vinyl epoxidase activity

Two alternative specific and very sensitive methods for determination of vinyl epoxide synthetase activity in liver microsomes are described. Trichloroethylene, which is used as a substrate, is converted into trichloroethylene oxide by a hepatic epoxide synthetase. Chloral hydrate, the final rearrangement product of trichloroethylene oxide, is evaluated by electron capture gas chromatography, either after derivatization with pentafluorophenyl-hydrazine or after its conversion into chloroform under alcaline conditions. The kinetic parameters of the epoxidative reaction have been determined on rat hepatic microsomal suspensions.

Mutagenicity and chromosomal aberrations as an analytical tool for in vitro detection of mammalian enzyme-mediated formation of reactive metabolites

1. Incubation of trichloroethylene, 1,1-dichloroethylene, vinylchloride, tetra-chlorocyclopentadiene, the nitroso derivatives of the pesticides Carbaryl, Prometryn, and Dodin in the presence of metabolically active mouse liver microsomes and bacteria as target cells were mutagenic, whereas tetrachloroethylene, 1,2 cis- and transdichloroethylene, hexachlorocyclopentadiene, carbontetrachloride, chloroform, halothane, trichlorofluoromethane and styrene were not activated to mutagenic species. 2. In a similar in vitro test system using freshly isolated human lymphocytes as target cells dimethylnitrosamine induced chromosomal aberrations. 3. It is concluded from the experiments that submammalian or mammalian in vitro cell systems with metabolically active liver microsomes are not only suitable to screen for chemical mutagens but to demonstrate formation of reactive intermediates, which are short lived and cannot be detected by chemical procedure.

Irreversible binding of 14C-labelled trichloroethylene to mice liver constituents in vivo and in vitro

1. 14C-labelled trichloroethylene was injected i.p. into male mice (10 mumole/g of b.w.). The radioactivity irreversibly bound to hepatic protein reached highest levels after 6 h: 2 nmole/mg in cytosol protein, 4.4 nmole/mg in mitochondrial protein, and 7.6 nmole/mg in microsomal protein. 2. The commercial trichloroethylene contained radioactive impurities binding to proteins without metabolic activation. Purification by various extractions removed 60-70% of those materials. In aerobic incubates of mice hepatic microsomes and NADPH the covalent binding rate of the purified trichloroethylene was 1.4 nmole/mg protein in 60 min. The activity of rat liver microsomes was approximately 40% less. Covalent binding increased 2-fold with microsomes of mice pretreated with phenobarbital.