Induction of stomach cancer in rats and mice by halogenated aliphatic fumigants

Formation of S-[2-(N6-Deoxyadenosinyl)ethyl]glutathione in DNA and Replication Past the Adduct by Translesion DNA Polymerases

1,2-Dibromoethane (DBE, ethylene dibromide) is a potent carcinogen due at least in part to its DNA cross-linking effects. DBE cross-links glutathione (GSH) to DNA, notably to sites on 2'-deoxyadenosine and 2'-deoxyguanosine ( Cmarik , J. L. , et al. ( 1991 ) J. Biol. Chem. 267 , 6672 - 6679 ). Adduction at the N6 position of 2'-deoxyadenosine (dA) had not been detected, but this is a site for the linkage of O⁶-alkylguanine DNA alkyltransferase ( Chowdhury , G. , et al. ( 2013 ) Angew. Chem. Int. Ed. 52 , 12879 - 12882 ). We identified and quantified a new adduct, S-[2-(N⁶-deoxyadenosinyl)ethyl]GSH, in calf thymus DNA using LC-MS/MS. Replication studies were performed in duplex oligonucleotides containing this adduct with human DNA polymerases (hPols) η, ι, and κ, as well as with Sulfolobus solfataricus Dpo4, Escherichia coli polymerase I Klenow fragment, and bacteriophage T7 polymerase. hPols η and ι, Dpo4, and Klenow fragment were able to bypass the adduct with only slight impedance; hPol η and ι showed increased misincorporation opposite the adduct compared to that of unmodified 2'-deoxyadenosine. LC-MS/MS analysis of full-length primer extension products by hPol η confirmed the incorporation of dC opposite S-[2-(N⁶-deoxyadenosinyl)ethyl]GSH and also showed the production of a -1 frameshift. These results reveal the significance of N⁶-dA GSH-DBE adducts in blocking replication, as well as producing mutations, by human translesion synthesis DNA polymerases.

Metabolic Activation of Xenobiotics: Ethylene Dibromide and Structural Analogs

A large number of compounds which can enter living organisms are relatively harmless as such, but are transformed by the body into reactive agents. The structure of such a compound is the factor determining its disposition in the organism. Its physicochemical characteristics determine the overall fate in terms of absorption, distribution and excretion, while the chemical structure is the decisive factor in its biotransformation. Whether formation of reactive intermediates occurs depends on what points of attack it has to offer to the different enzyme systems. The extent to which alkylation of cellular macromolecules by reactive intermediates occurs in turn depends on the balance of activating and detoxifying enzymes in the particular cell and on the reactivity of the intermediates towards critical targets in the cell macro-molecules.

Many chemicals undergo several concurrent metabolic pathways. The ratio between these pathways may be a decisive factor determining the extent of adverse effects caused by these chemicals. Small variations in structure may have a drastic effect on the activation and detoxification by competing enzyme systems.

These concepts are elucidated using the examples of ethy-lene dibromide and some structural analogs. Apart from the parent compound itself, two alkylating species may be responsible for the toxic effects of these compounds. Bromo-acetaldehyde is formed by oxidation, catalyzed by cyto-chrome P-450, followed by spontaneous loss of hydrogen bromide; S-2-bromoethylglutathione results from replacement of a bromine atom by glutathione, catalyzed by the glutathione transferases. The latter intermediate possesses a reactivity comparable to sulfur mustard. Results indicate that the reactive glutathione conjugate is responsible for the mutagenic and possibly also the carcinogenic effect of ethy-lene dibromide. However, in vivo, oxidation is quantitatively much more important as a primary process.

Nonproliferative and Proliferative Lesions of the Gastrointestinal Tract, Pancreas and Salivary Glands of the Rat and Mouse

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) project is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature and diagnostic criteria for nonproliferative and proliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature and diagnostic criteria for classifying lesions in the digestive system including the salivary glands and the exocrine pancreas of laboratory rats and mice. Most lesions are illustrated by color photomicrographs. The standardized nomenclature, the diagnostic criteria, and the photomicrographs are also available electronically on the Internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous and age related lesions as well as lesions induced by exposure to test items. Relevant infectious and parasitic lesions are included as well. A widely accepted and utilized international harmonization of nomenclature and diagnostic criteria for the digestive system will decrease misunderstandings among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists.

Secrecy and justice in the ongoing saga of DBCP litigation

Since the 1980s, banana workers from Central America and elsewhere have filed cases in the United States for sterility damages caused by exposure to the nematicide dibromochloropropane (DBCP) used during the 1960s and 1970s. These plaintiffs' efforts at holding fruit and chemical corporations accountable have been met with numerous obstacles. Many cases have been dismissed on the grounds that they would "more conveniently" be tried elsewhere, despite the fact that significant barriers exist to bringing such cases in many of these workers' home countries. Using this strategy, defendants including Dole Food, Chiquita, Dow and Shell Chemical have been mostly successful in avoiding any penalty for their part in exposing banana workers to DBCP without adequate protection or information. In fact, although a few cases have settled, the first DBCP case did not reach the trial stage until 2007. In that case, the damages awarded to the six Nicaraguan banana workers were $5 million, an amount later reduced to $2·3 million. In 2010, Dole successfully sought to dismiss not only that case, but other cases brought by Nicaraguan plaintiffs. The company claimed that there was evidence of widespread fraud among Nicaraguan plaintiffs, attorneys, and judges, as well as lawyers based in the US. However, many of those accused of fraud did not have a chance to respond to those allegations or cross-examine their accusers. In addition, allegations of fraudulent behavior on the part defendants suggest that the story is more complicated. Instead of dismissing these cases--a defacto victory for the defendant--US courts should move forward with deciding these cases on their own merits; leaving juries to determine the veracity of plaintiffs and defendants' claims.

Identification of EBP50 as a specific biomarker for carcinogens via the analysis of mouse lymphoma cellular proteome

To identify specific biomarkers generated upon exposure of L5178Y mouse lymphoma cells to carcinogens, 2-DE and MALDI-TOF MS analysis were conducted using the cellular proteome of L5178Y cells that had been treated with the known carcinogens, 1,2-dibromoethane and O-nitrotoluene and the noncarcinogens, emodin and D-mannitol. Eight protein spots that showed a greater than 1.5-fold increase or decrease in intensity following carcinogen treatment compared with treatment with noncarcinogens were selected. Of the identified proteins, we focused on the candidate biomarker ERM-binding phosphoprotein 50 (EBP50), the expression of which was specifically increased in response to treatment with the carcinogens. The expression level of EBP50 was determined by western analysis using polyclonal rabbit anti-EBP50 antibody. Further, the expression level of EBP50 was increased in cells treated with seven additional carcinogens, verifying that EBP50 could serve as a specific biomarker for carcinogens.

The bis-electrophile diepoxybutane cross-links DNA to human histones but does not result in enhanced mutagenesis in recombinant systems

1,2-Dibromoethane and 1,3-butadiene are cancer suspects present in the environment and have been used widely in industry. The mutagenic properties of 1,2-dibromoethane and the 1,3-butadiene oxidation product diepoxybutane are thought to be related to the bis-electrophilic character of these chemicals. The discovery that overexpression of O(6)-alkylguanine alkyltransferase (AGT) enhances bis-electrophile-induced mutagenesis prompted a search for other proteins that may act by a similar mechanism. A human liver screen for nuclear proteins that cross-link with DNA in the presence of 1,2-dibromoethane identified histones H2b and H3 as candidate proteins. Treatment of isolated histones H2b and H3 with diepoxybutane resulted in DNA-protein cross-links and produced protein adducts, and DNA-histone H2b cross-links were identified (immunochemically) in Escherichia coli cells expressing histone H2b. However, heterologous expression of histone H2b in E. coli failed to enhance bis-electrophile-induced mutagenesis. These results are similar to those found with the cross-link candidate glyceraldehyde 3-phosphate dehydrogenase (GAPDH) [ Loecken , E. M. and Guengerich , F. P. ( 2008 ) Chem. Res. Toxicol. 21 , 453 - 458 ], but in contrast to GAPDH, histone H2b bound DNA with even higher affinity than AGT. The extent of DNA cross-linking of isolated histone H2b was similar to that of AGT, suggesting that differences in postcross-linking events explain the difference in mutagenesis.

Analysis of DNA adducts formed in vivo in rats and mice from 1,2-dibromoethane, 1,2-dichloroethane, dibromomethane, and dichloromethane using HPLC/accelerator mass spectrometry and relevance to risk estimates

Dihaloalkanes are of toxicological interest because of their high-volume use in industry and their abilities to cause tumors in rodents, particularly dichloromethane and 1,2-dichloroethane. The brominated analogues are not used as extensively but are known to produce more toxicity in some systems. Rats and mice were treated i.p. with (14)C-dichloromethane, -dibromomethane, -1,2-dichloroethane, or -1,2-dibromoethane [5 mg (kg body weight)(-1)], and livers and kidneys were collected to rapidly isolate DNA. The DNA was digested using a procedure designed to minimize processing time, because some of the potential dihalomethane-derived DNA-glutathione (GSH) adducts are known to be unstable, and the HPLC fractions corresponding to major adduct standards were separated and analyzed for (14)C using accelerator mass spectrometry. The level of liver or kidney S-[2-(N(7)-guanyl)ethyl]GSH in rats treated with 1,2-dibromoethane was approximately 1 adduct/10(5) DNA bases; in male or female mice, the level was approximately one-half of this. The levels of 1,2-dichloroethane adducts were 10-50-fold lower. None of four known (in vitro) GSH-DNA adducts was detected at a level of >2/10(8) DNA bases from dibromomethane or dichloromethane. These results provide parameters for risk assessment of these compounds: DNA binding occurs with 1,2-dichloroethane but is considerably less than from 1,2-dibromoethane in vivo, and low exposure to dihalomethanes does not produce appreciable DNA adduct levels in rat or mouse liver and kidney of the doses used. The results may be used to address issues in human risk assessment.

Critical evaluation of the cancer risk of dibromochloropropane (DBCP)

Dibromochloropropane (1,2-dibromo-3-chloropropane, DBCP), a pesticide used widely for over 20 years to control nematodes on crops, turf and in nurseries, was banned by the United States Environmental Protection Agency (US EPA) in 1977 because of evidence of infertility in men and induction of a variety of tumors in laboratory animals. Despite the ban on the use of DBCP, this pesticide remains persistent in soil and continues to be detected as a groundwater contaminant in areas of past high use, in particular California's Central Valley. In this review, we present a critical evaluation of the available scientific literature on the potential for DBCP to affect cancer risk, including the results of animal cancer bioassays, human epidemiological studies and in vitro and in vivo genotoxicity studies. In addition, we provide updated information on DBCP chemistry and metabolism, production and past use, current regulations, its environmental fate, potential for human exposure and current remediation efforts. Results from long-term cancer bioassays in rodents show a statistically significant increase in the incidence of malignant and benign mammary gland tumors in female rats treated orally with DBCP compared to controls and some evidence of increased incidence of mammary fibroadenomas in DBCP low-dose treated female rats exposed by inhalation. Significantly increased incidence of tumors of the forestomach occurred in both sexes of rats and mice treated orally. Rats exposed to DBCP by inhalation showed significant increases in tumors of the tunica vaginalis in males; tumors of the pharynx and adrenal gland in females; and tumors of the tongue, nasal turbinate and nasal cavity in both sexes compared to controls. Male and female mice exposed to DBCP by inhalation experienced increased tumor incidence in the lungs and nasal cavity compared to controls. Significant increases in tumors of the lung and forestomach have also been reported in female mice treated by a dermal route. Although high mortality rates in both rat and mouse bioassays limited the ability to detect tumors late in life, the induction of a variety of tumors by multiple routes of exposure in two rodent species provides clear evidence of a DBCP tumorigenic response. In vitro, in vivo and human genotoxicity studies indicate that DBCP is capable of acting as a mutagen and clastogen. Few studies have been conducted to assess whether DBCP workplace or drinking water exposures affect cancer risk in humans. While case-control, cohort and ecological epidemiology studies have not found significant, positive associations between DBCP exposure and cancer in exposed populations, these studies have numerous limitations including small numbers of participants, a lack of control for confounding factors, lack of exposure information on DBCP and other chemicals and short follow-up times. Given the persistent nature of DBCP contamination in areas of past use, efforts should be made to continue remediation efforts and follow previously exposed populations for development of certain human cancers, including breast, ovarian, stomach, respiratory, oral and nasal cancers, among others.

S-(2-chloroethyl)glutathione-generated p53 mutation spectra are influenced by differential repair rates more than sites of initial dna damage

Several steps occur between the reaction of a chemical with DNA and a mutation, and each may influence the resulting mutation spectrum, i.e. nucleotides at which the mutations occur. The half-mustard S-(2-bro-moethyl)glutathione is the reactive conjugate implicated in ethylene dibromide-induced mutagenesis attributed to the glutathione-dependent pathway. A human p53-driven Ade reporter system in yeast was used to study the factors involved in producing mutations. The synthetic analog S-(2-chloroethyl)glutathione was used to produce DNA damage; the damage to the p53 exons was analyzed using a new fluorescence-based modification of ligation-mediated polymerase chain reaction and an automated sequencer. The mutation spectrum was strongly dominated by the G to A transition mutations seen in other organisms with S-(2-chloroethyl)glutathione or ethylene dibromide. The mutation spectrum clearly differed from the spontaneous spectrum or that derived from N-ethyl,N-nitrosourea. Distinct differences were seen between patterns of modification of p53 DNA exposed to the mutagen in vitro versus in vivo. In the four p53 exons in which mutants were analyzed, the major sites of mutation matched the sites with long half-lives of repair much better than the sites of initial damage. However, not all slowly repaired sites yielded mutations in part because of the lack of effect of mutations on phenotype. We conclude that the rate of DNA repair at individual nucleotides is a major factor in influencing the mutation spectra in this system. The results are consistent with a role of N(7)-guanyl adducts in mutagenesis.

Characterization of a mutagenic DNA adduct formed from 1,2-dibromoethane by O6-alkylguanine-DNA alkyltransferase

It has been proposed that the DNA repair protein O6-alkylguanine-DNA alkyltransferase increases the mutagenicity of 1,2-dibromoethane by reacting with it at its cysteine acceptor site to form a highly reactive half-mustard, which can then react with DNA (Liu, L., Pegg, A. E., Williams, K. M., and Guengerich, F. P. (2002) J. Biol. Chem. 277, 37920-37928). Incubation of Escherichia coli-expressed human alkyltransferase with 1,2-dibromoethane and single-stranded oligodeoxyribonucleotides led to the formation of covalent transferaseoligo complexes. The order of reaction determined was Gua>Thy>Cyt>Ade. Mass spectrometry analysis of the tryptic digest of the reaction product indicated that some of the adducts led to depurination with the release of the Gly136-Arg147 peptide cross-linked to a Gua at the N7 position, with the site of reaction being the active site Cys145 as established by chromatographic retention time and the fragmentation pattern determined by tandem mass spectrometry of a synthetic peptide adduct. The alkyltransferase-mediated mutations produced by 1,2-dibromoethane were predominantly Gua to Ade transitions but, in the spectrum of such rifampicin-resistant mutations in the RpoB gene, 20% were Gua to Thy transversions. The latter are likely to have arisen from the apurinic site generated from the Gua-N7 adduct. Support exists for an additional adduct/mutagenic pathway because evidence was obtained for DNA adducts other than at the Gua N7 atom and for mutations other than those attributable to depurination. Thus, chemical and biological evidence supports the existence of at least two alkyltransferase-dependent pathways for 1,2-dibromoethane-induced mutagenicity, one involving Gua N7-alkylation by alkyltransferase-S-CH2CH2Br and depurination, plus another as yet uncharacterized system(s).

Paradoxical enhancement of the toxicity of 1,2-dibromoethane by O6-alkylguanine-DNA alkyltransferase

The presence of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) paradoxically increases the mutagenicity and cytotoxicity of 1,2-dibromoethane (DBE) in Escherichia coli. This enhancement of genotoxicity did not occur when the inactive C145A mutant of human AGT (hAGT) was used. Also, hAGT did not enhance the genotoxicity of S-(2-haloethyl)glutathiones that mimic the reactive product of the reaction of DBE with glutathione, which is catalyzed by glutathione S-transferase. These experiments support a mechanism by which hAGT activates DBE. Studies in vitro showed a direct reaction between purified recombinant hAGT and DBE resulting in a loss of AGT repair activity and a formation of an hAGT-DBE conjugate at Cys(145). A 2-hydroxyethyl adduct was found by mass spectrometry to be present in the Gly(136)-Arg(147) peptide from tryptic digests of AGT reacted with DBE. Incubation of AGT with DBE and oligodeoxyribonucleotides led to the formation of covalent AGT-oligonucleotide complexes. These results indicate that DBE reacts at the active site of AGT to generate an S-(2-bromoethyl) intermediate, which forms a highly reactive half-mustard at Cys(145). In the presence of DNA, the DNA-binding function of AGT facilitates formation of DNA adducts. In the absence of DNA, the intermediate undergoes hydrolytic decomposition to form AGT-Cys(145)-SCH(2)CH(2)OH.

Activation or detoxification of mutagenic and carcinogenic compounds in transgenic Drosophila expressing human glutathione S-transferase

Sensitivity of transgenic Drosophila melanogaster with expression of a human gene encoding the glutathione S-transferase alpha subunit (GSTA1-1) to 1,2:5,6-dibenzanthracene (DBA) and 1,2-dichloroethane (DCE) was investigated in the somatic mutation and recombination test (SMART). We performed the same assay in control transgenic flies expressing the bacterial lacZ gene. Three types of transgenic Drosophila strains carrying GSTA1-1 were used: two transgenic strains homozygous for the second chromosome with a single-copy transgene insertion and one strain with two transgene insertions. Larvae carrying the lacZ gene were significantly more sensitive to genotoxic effects of DBA than those carrying three copies of the GSTA1-1 gene. The larvae with lacZ expression showed significantly lower sensitivity to DCE compared with those expressing GSTA1-1. Finally, a pretreatment with buthionine-sulphoximine (BSO) in experiment with DCE significantly decreased the frequency of mutation events in larvae with three GSTA1-1 copies in comparison with others.

Forging the links between metabolism and carcinogenesis

Metabolism plays important roles in chemical carcinogenesis, both good and bad. The process of carcinogen metabolism was first recognized in the first half of the twentieth century and developed extensively in the latter half. The activation of chemicals to reactive electrophiles that become covalently bound to DNA and protein was demonstrated by Miller and Miller [Cancer 47 (1981) 2327]. Today many of the DNA adducts formed by chemical carcinogens are known, and extensive information is available about pathways leading to the electrophilic intermediates. Some concepts about the stability and reactivity of electrophiles derived from carcinogens have changed over the years. Early work in the field demonstrated the ability of chemicals to modulate the metabolism of carcinogens, a phenomenon now described as enzyme induction. The cytochrome P450 enzymes play a prominent role in the metabolism of carcinogens, both in bioactivation and detoxication. The conjugating enzymes can also play both beneficial and detrimental roles. As an example of a case in which several enzymes affect the metabolism and carcinogenicity of a chemical, aflatoxin B1 (AFB1) research has revealed insight into the myriad of reaction chemistry that can occur even with a 1s half-life for a reactive electrophile. Further areas of investigation involve the consequences of enzyme variability in humans and include areas such as genomics, epidemiology, and chemoprevention.

A physiologically-based pharmacokinetic(PB-PK) model for ethylene dibromide: relevance of extrahepatic metabolism

A physiologically-based pharmacokinetic (PB-PK) model was developed for ethylene dibromide (1,2-dibromoethane, EDB) for rats and humans, partly based on previously published in vitro data (Ploemen et al., 1997). In the present study, this PB-PK model has been validated for the rat. In addition, new data were used for the human class ThetaGST T1-1. Validation experiments are described in order to test the predictive value of kinetics to describe "whole-body" metabolism. For the validation experiments, groups of cannulated rats were dosed orally or intravenously with different doses of EDB. Obtained blood concentration-time curves of EDB for all dosing groups were compared to model predictions. It appeared that metabolism, which previously was assumed to be restricted to the liver, was underestimated. Therefore, we extended the PB-PK model to include all the extrahepatic organs, in which the enzymes involved in EDB metabolism have been detected and quantified. With this extended model, the blood concentrations were much more accurately described compared to the predictions of the "liver-model". Therefore, extrahepatic metabolism was also included in the human model. The present study illustrates the potential application of in vitro metabolic parameters in risk assessment, as well as the use of PB-PK modelling as a tool to understand and predict in vivo data.

The toxicology of 1,2-dibromo-3-chloropropane (DBCP): a brief review

The accumulating data demonstrating the reproductive toxicity of 1, 2-dibromo-3-chloropropane (DBCP) are reviewed. The sentinel event was the discovery of infertility in male pesticide manufacturing workers. In spite of early evidence of testicular damage, first in laboratory animals and later in humans, DBCP has been widely used as a nematocide in the United States and is still used in other countries. The spermatogenic effects of DBCP are usually irreversible, and there is also evidence of toxicity to the female reproductive system. DBCP is also a CNS depressant, a liver and kidney toxin, and a skin, eye, and respiratory irritant, and is probably carcinogenic. Environmental contamination of air and water may be an additional source of exposure. Materials often used to protect workers from toxic chemicals are not being deployed for use by exposed workers, and in any case are relatively ineffective.

Analysis of DNA adduct, S-[2-(N7-guanyl)ethyl]glutathione, by liquid chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry

Sensitive and specific isotope dilution liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) methods were developed for the detection and quantitative analysis of S-[2-(N7-guanyl)ethyl]glutathione as a DNA adduct formed upon exposure of animals to carcinogenic 1,2-dihaloethanes. Separation and analysis were performed using microbore HPLC coupled in-line to an electrospray ionization triple quadrupole mass spectrometer. S-[2-(N7-guanyl)[2H4]-ethyl] glutathione was synthesized and used as internal standard. These methods provide structural confirmation of the adduct as well as quantitative analysis with the accuracy and precision necessary to measure biologically relevant levels in small tissue sample sizes (< 1 g). The sample detection limits in in vivo tissue extracts were 100 pg and 5 pg on-column for LC/MS and LC/MS/MS methods, respectively. Selected-ion monitoring mode was used to monitor the product ions of the doubly charged molecular ion. The application of these methods was demonstrated by measuring the DNA adduct levels in rat and fish samples after exposure to 1,2-dihaloethanes. The method has application in studies of DNA adduct formation as a biological marker of exposure to carcinogens and for environmental monitoring of 1,2-dihaloethanes.

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

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

Tea polyphenols as inhibitors of mutagenicity of major classes of carcinogens

Previous research suggested that the mutagenicity of some genotoxic carcinogens, mainly heterocyclic amines, was decreased by green or black tea extracts, or tea polyphenol fractions. Thus, it seemed important to test a variety of genotoxic carcinogens with distinct chemical structures and means of biochemical activation as regards modification of mutagenicity in appropriate strains of Salmonella typhimurium by 3 concentrations of polyphenols 60, 100, or B, standard commercial polyphenol preparations from green or black tea. Polyphenols sharply decreased the mutagenicity of a number of aryl- and heterocyclic amines, of aflatoxin B1, benzo[a]pyrene, 1,2-dibromoethane, and more selectively, of 2-nitropropane, all involving an induced rat liver S9 fraction. Good inhibition was found with 2 nitrosamines that required a hamster S9 fraction for biochemical activation. No effect was found with 1-nitropyrene, and with the direct-acting (no S9) 2-chloro-4-methyl-thiobutanoic acid. Thus, with some exceptions, polyphenols considerably decreased the mutagenicity of diverse types of carcinogens.

The involvement of cytochrome P4502E1 in 2-bromoethanol-induced hepatocyte cytotoxicity

The cytotoxicity of 2-bromoethanol towards hepatocytes isolated from rats was concentration-dependent (EC(50)100 mu M, 2 hr). Bromoacetaldehyde was more toxic (EC(50)60 mu M, 2 hr) and bromoacetic acid was less toxic (EC(50)150 mu M, 2 hr). Glutathione (GSH) depletion occurred before cytotoxicity ensued and GSH depleted hepatocytes were more susceptible to 2-bromoethanol. Lipid peroxidation increased steadily 1 hr after 2-bromoethanol addition and antioxidants, iron chelators or hypoxia prevented 2-bromoethanol induced lipid peroxidation and cell lysis. Alcohol dehydrogenase inhibitors, methyl pyrazole or dimethyl sulfoxide only partly prevented 2-bromoethanol induced GSH depletion, lipid peroxidation and cytotoxicity. However, cytochrome P4502E1 (CYP2E1) inhibitors/substrates were more effective at preventing 2-bromoethanol-induced GSH depletion, lipid peroxidation and cytotoxicity suggesting that 2-bromoethanol is mostly metabolically activated by CYP2E1. Also, hepatocytes isolated from CYP2E1 induced rats were more susceptible to 2-bromoethanol and hepatocytes isolated from rats pretreated with carbon disulfide to inactivate CYP2E1 were more resistant to 2-bromoethanol treatment. Formation of S-(formylmethyl)glutathione during 2-bromoethanol metabolism by microsomal mixed function oxidase in the presence of GSH was also prevented by cytochrome P4502E1 inhibitors/substrates or by Anti-Rat CYP2E1. Furthermore, aldehyde dehydrogenase inhibitors-cyanamide or chloral hydrate increased 2-bromoethanol dependent hepatocyte susceptibility. This suggests that 2-bromoethanol is preferably metabolised by CYP2E1 dependent monoxygenase to form 2-bromoacetaldehyde which causes cell lysis as a result of GSH depletion and lipid peroxidation.

Human protection against non-genotoxic carcinogens in the US without the Delaney Clause

Cancers of many types are major chronic diseases with a high fatality rate and a high cost to society. In the USA, the Delaney Clause was implemented in 1958 because the public believed that many cancers stem from food additives and food contaminants. In the intervening years, research has provided key information about the mechanisms of carcinogenesis and demonstrated that there are two major classes of carcinogens, genotoxic and non-genotoxic. Two case reports are presented, of sodium saccharin and ethylenebisdithiocarbamates that were banned based on the Delaney Clause in an unjustified manner, based on the underlying mechanisms not relevant for non-genotoxic carcinogens. Also, the causes of major cancers have been discovered. Most cancers are associated with lifestyle, specifically tobacco and excessive alcohol use, inappropriate nutritional traditions, and lack of exercise. These lifestyle components involve now known genotoxic carcinogens and importantly, non-genotoxic carcinogens. The effect of non-genotoxic carcinogens is highly dose dependent and also reversible upon lowering the dose below a threshold. Thus, it is quite possible to lower human cancer risk, and also the risk of related chronic diseases such as coronary heart disease, hypertension and stroke, adult on-set diabetes, by proper lifestyle adjustments. Clearly, the Delaney Clause plays no role in disease prevention.

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

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

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

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

Genotoxicity and carcinogenicity testing of 1,2-dibromopropane and 1,1,3-tribromopropane in comparison to 1,2-dibromo-3-chloropropane

The activities of 1,2-dibromopropane (DBP) and 1,1,3-tribromopropane (TBP) were studied in seven genotoxicity assays, (i) SOS-induction in E. coli, (ii) DNA repair in primary rat hepatocyte culture, (iii) the Salmonella/microsome assay, (iv) a host-mediated assay using Salmonella, (v) the somatic mutation and recombination assay in Drosophila melanogaster, (vi) HGPRT-mutagenesis assay in ARL 18 cells, and (vii) micronucleus formation assay in mouse polychromatophylic erythrocytes (PCE), forestomach (FS), glandular stomach (GS), duodenum (D), jejunum (J), cecum (C) and liver (L). The halopropanes were also tested for tumor formation in the fish Danio rerio. DBP was active in assays (ii), (v), (vii FS) and (vii L). TBP was positive in assays (ii) and (iii), strongly positive in (vii L) and borderline positive in (iv). However, neither DBP nor TBP induced tumors in fish, in contrast to the carcinogenic 1,2-dibromo-3-chloropropane. The genotoxicity and potential carcinogenicity of DBP and TBP in mammals is discussed.

Environmental and occupational pollutants are avoidable causes of breast cancer

For over three decades, evidence has accumulated relating avoidable exposures to environmental and occupational carcinogens to the escalating incidence of breast cancer in the United States and other major industrialized nations. This evidence has until very recently been totally ignored by the cancer establishment, the National Cancer Institute, and the American Cancer Society, despite expenditures of over $1 billion on breast cancer research. Recognition of these environmental and occupational risk factors should lead to the belated development of public health policies directed to the primary prevention of breast cancer. Their recognition should also lend urgency to the need for radical reforms in the priorities and leadership of the cancer establishment.

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.

In vitro metabolism and bioactivation of 1,2,3-trichloropropane

In vitro studies using rat and human hepatic microsomes have shown that the halogenated hydrocarbon 1,2,3-trichloropropane (TCP) is bioactivated to the direct acting mutagen 1,3-dichloroacetone (DCA). The presence of DCA in microsomal incubations was confirmed by gas chromatography-mass spectrometry. DCA formation was totally dependent on the presence of NADPH. The rate of DCA formation using rat and human microsomes was 0.268 +/- 0.029 and 0.026 +/- 0.006 nmol/min/mg protein +/- SE, respectively. When hepatic microsomes were isolated from rats pretreated with the cytochrome P-450 inducers, phenobarbital, and dexamethasone, 24- and 2.5-fold increases, respectively, in the rate of DCA production, were observed. Pretreatment with beta-naphthoflavone resulted in a 50% inhibition in DCA formation. The inhibitors of cytochromes P-450, SKF 525-A and 1-aminobenzotriazol, produced 85 and 70% inhibitions of DCA formation, respectively. When alcohol dehydrogenase and NADH were added to microsomal incubations, two TCP-related alcohols, 1,3-dichloro-2-propanol and 2,3-dichloropropanol, were formed. These alcohols are products of the initial microsomal metabolites, DCA and 2,3-dichloropropanal. [14C]TCP equivalents bound covalently to rat hepatic microsomal protein. This binding was increased 8-fold when hepatic microsomes from phenobarbital pretreated rats were used. The addition of either glutathione or N-acetylcysteine to the incubations completely inhibited this binding. In the presence of N-acetylcysteine, 1,3-(2-propanone)-bis-S-(N-acetylcysteine) (PDM) was the only N-acetylcysteine conjugate detected. It represented 87% of TCP microsomal metabolism. The formation of PDM implicates DCA as the major microsomal protein-binding metabolite of TCP. The formation of DCA, a direct-acting mutagen, may be responsible for the mutagenicity of TCP in systems using rat hepatic microsomes. Its role in the tumorigenicity and carcinogenicity of TCP remains to be established.

DNA damage and cell death induced by 1,2-dibromo-3-chloropropane (DBCP) and structural analogs in monolayer culture of rat hepatocytes: 3-aminobenzamide inhibits the toxicity of DBCP

1,2-Dibromo-3-chloropropane (DBCP) and a number of halogenated propane analogs induced DNA damage in rat hepatocytes in vitro measured by an automated alkaline elution method. Short-term (2 hrs) cytotoxic effects of DBCP were not observed until the DBCP concentration exceeded 1 mM. The short-term cytotoxicity of all the DBCP analogs occurred in the same concentration range. Significant membrane damage, measured as cell detachment, was observed after extended exposure to lower concentrations of DBCP (100 microM) for 20 hrs. The relative, delayed cytotoxic effect of DBCP and analogs correlated with their ability to cause DNA damage. In general, the halogenated propanes with more bromines relative to chlorines were the more potent compounds. Propane analogs lacking the third halogen had little cytotoxic activity. The addition of the proposed specific poly(ADP-ribosyl)transferase inhibitor 3-aminobenzamide (3-ABA) protected against DBCP-induced cytotoxic effects and NAD+ depletion. However, 3-ABA also reduced DBCP-induced DNA damage, DBCP metabolic loss, and the formation of water soluble and covalently bound DBCP metabolites. Thus, 3-ABA may block DBCP-induced cell death by decreasing the formation of reactive DBCP-metabolites.

Genetic toxicology of 1,2-dibromo-3-chloropropane (DBCP)

1,2-Dibromo-3-chloropropane (DBCP) is a nematocide, which has been used extensively as a soil fumigant in agriculture. Since sterility was found among male workers involved in the manufacture of DBCP, great concern has been focused on the genetic hazards of DBCP. DBCP gave positive results in many tests such as microbial, in vitro cytogenetics, and Drosophila studies. In mammalian test systems, DBCP caused chromosomal aberrations in the bone marrow cells and dominant-lethal mutations in germ cells in rats. In mice, there were no signs of DBCP-induced heritable mutation in germ cells, although point mutations were detected in somatic cells. The occurrence of Y-chromosomal non-disjunction was indicated in DBCP-exposed male workers by an increased number of sperm containing 2 Y-chromosomes.

Reactive intermediates and their toxicological significance

Many chemicals that cause toxicity do so via metabolism to biologically reactive metabolites. However, the nature of the interaction between such reactive metabolites and various cellular components, and the mechanism(s) by which these interactions eventually lead to cell death are poorly understood. The relative importance of macromolecular alkylation (covalent binding), lipid peroxidation, alterations in thiol, calcium and energy homeostasis are discussed with reference to specific toxicants. It is concluded that the cytotoxic effects of reactive metabolites are a consequence of simultaneous and/or sequential alterations in several cellular processes. Further studies are required to determine the relationship between these alterations and cell death.

Detection and mechanism of formation of the potent direct-acting mutagen 2-bromoacrolein from 1,2-dibromo-3-chloropropane

The nematocide 1,2-dibromo-3-chloropropane (DBCP) was converted to products which are mutagenic for Salmonella typhimurium TA 100 in the presence of rat liver microsomes, NADPH, and oxygen. Typical in vivo and in vitro inhibitors of cytochrome P-450 decreased DBCP mutagenicity in the presence of microsomes. Addition of glutathione to cytosolic preparations failed to bioactivate DBCP to mutagenic metabolites. Mutagenicity studies with selectively deuterated analogs showed that substitution of deuterium for hydrogen at C-1 or C-3 of DBCP modestly decreased mutagenicity, but that deuteration at both C-1 and C-3 markedly decreased mutagenicity. The formation rates of the potent direct-acting mutagen, 2-bromoacrolein (2-BA), in incubations of DBCP and its deuterated analogs with rat liver microsomes, correlated with the isotope effects on mutagenicity. Characterization of 2-BA was accomplished by gas chromatography-mass spectrometry using positive-ion chemical ionization. Mass spectral analysis of 2-BA formed from specifically deuterated analogs of DBCP indicated that initial oxidative dehalogenation at C-1 followed by a spontaneous beta-elimination reaction was the preferred pathway in the formation of 2-BA from DBCP. These results demonstrate that mutagenic metabolites of DBCP are formed by cytochrome P-450-mediated oxidative metabolism, and that 2-BA is a major mutagen formed.

The genetic toxicity of 1,2-dibromo-3-chloropropane, 1,2-dibromo-3-chloro-2-methylpropane, and 1,2,3-tribromo-2-methylpropane

1,2-Dibromo-3-chloro-2-methylpropane (DBCMP) and 1,2,3-tribromo-2-methylpropane (TBMP) are contaminants formed during the manufacture of bromobutyl rubber. These chemicals are structurally similar to 1,2-dibromo-3-chloropropane (DBCP), a known genotoxin and rodent carcinogen. The present study compared the genotoxic properties of DBCMP and TBMP to those of DBCP. In the Salmonella assay, DBCP was positive in strains TA-98, TA-100 and TA-1535 in the presence of exogenous activation; DBCP was weakly active in TA-1535 in the absence of activation. Neither DBCMP nor TBMP produced reproducible evidence of mutagenic activity in the Salmonella assay despite the use of several different variations of this test. In the mouse lymphoma gene mutation assay, DBCP and TBMP were positive in the presence and absence of activation, while DBCMP was positive only in the absence of activation. All three test compounds were active in the Syrian hamster embryo morphologic transformation assay. The results indicated that both DBCMP and TBMP exhibited some genotoxic activity as did DBCP. The presence of the methyl group on the 2-carbon position essentially eliminated the mutagenicity of DBCMP and TBMP in the Salmonella assay.

Toxicokinetics of 1,2-dibromo-3-chloropropane (DBCP) in the rat

The objective of this study was to determine the kinetics of absorption, distribution, and elimination of DBCP after intravenous (iv) administration in plasma, and after oral administration in water or corn oil, to conscious, fed, male Fischer 344 rats. Rats were prepared with an external jugular vein cannula and were dosed with 0.1, 1, or 10 mg/kg DBCP into the penile sinus or orally as a solution in water or in corn oil (1 mg/kg only). Blood was sampled at various times up to 12 hr, concentrations of DBCP were determined by gas chromatography, and data were evaluated by classical pharmacokinetic techniques. After oral administration in water, absorption of DBCP was rapid, and the distribution and elimination phase was biexponential. There did not appear to be any saturation of DBCP absorption, distribution, or elimination at the high oral or iv dose. After oral administration of DBCP in a corn oil vehicle, absorption was prolonged, suggesting retention of DBCP in the stomach; this could contribute to the toxic effects of DBCP on the forestomach when chronically administered in corn oil. The areas under the blood concentration/time curve were similar regardless of vehicle, suggesting that systemic toxicity might be independent of the vehicle.

A biochemical basis for 1,2-dibromo-3-chloropropane-induced male infertility: inhibition of sperm mitochondrial electron transport activity

A rapid decrease in male fertility in laboratory animals exposed to 1,2-dibromo-3-chloropropane (DBCP) has been suggested to be due, in part, to a postglycolytic inhibition of sperm carbohydrate metabolism. The present studies were performed to identify the specific site of DBCP-induced inhibition of intermediary metabolism. 14CO2 generation by epididymal sperm, isolated from Fischer 344 rats, was measured using radiolabeled tricarboxylic acid (TCA) cycle intermediates: acetyl CoA, citrate, alpha-ketoglutarate, and succinate. There was 0-28% inhibition of CO2 generation after addition of 0.5 mM DBCP and 81-98% inhibition with 3 mM DBCP, with all four substrates. The activities of alpha-ketoglutarate dehydrogenase, pyruvate dehydrogenase, malate dehydrogenase, and lactate dehydrogenase were not inhibited by DBCP. Since the DBCP-induced inhibition of metabolism of different substrates to CO2 was similar, and since DBCP did not inhibit enzyme activities of glycolysis or the TCA cycle, a common site of inhibition was suspected. In evaluations of mitochondrial electron transport chain activity, DBCP (3 mM) inhibited oxygen consumption resulting from metabolism of endogenous substrates plus alpha-ketoglutarate or malate by about 80%. When succinate, an FAD-dependent oxidation, was used as a substrate, oxygen consumption was not inhibited by DBCP. It is concluded that DBCP inhibits sperm carbohydrate metabolism at the NADH dehydrogenase step in the mitochondrial electron transport chain.

Cell proliferation in rat kidney induced by 1,2-dibromoethane

The effect of a single intragastric injection of 1,2-dibromoethane was investigated in kidneys of male Wistar rats. DNA synthesis as measured by the incorporation of tritiated thymidine was found to be approximately 5 times greater than that of controls 20-30 h after treatment. DNA synthesis was followed by a striking increase in the mitotic activity with a maximum at 30 h. The labeling and mitotic activities, after an initial increase, fell rapidly 48 h after treatment even though they were still higher than those of control animals. 1,2-Dibromoethane-induced cell proliferation is not a regenerative response because at the dose used in this study, no tubular necrosis was observed by histologic examination.

Induction of calcium efflux from isolated rat-liver mitochondria by 1,2-dibromoethane

Addition of 1,2-dibromoethane to rat-liver mitochondria induces a concentration-dependent depletion of mitochondrial glutathione. This event seems to be associated with the induction of Ca2+ release from mitochondria pre-loaded with a low pulse of Ca2+. The enhancement of the energy-dissipating process to reaccumulate the released Ca2+ ('Ca2+ cycling') results in a progressive drop of membrane potential. Addition of EGTA (ethyleneglycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid), when the membrane potential has reached the lowest level, restitutes it to a normal value. All these findings and the observation that Ca2+ release also occurs under non cycling conditions (e.g., in the presence of ruthenium red) suggest that 1,2-dibromoethane induces a Ca2+ efflux by activating a selective pathway which is sensitive to critical sulfhydryl groups.

The in vitro metabolism and bioactivation of 1,2-dibromoethane (ethylene dibromide) by human liver

The nematocide, grain fumigant, and gasoline additive 1,2-dibromoethane (DBE) is both a cellular and a genetic toxin that is metabolically activated in rats and mice by mixed function oxidases (MFO) as well as glutathione 5-transferases (GST). The purpose of this study was to determine whether DBE is similarly metabolized and bioactivated by human liver in vitro. Human liver microsomal and cytosolic metabolism of DBE was monitored by the production of aqueous-soluble metabolites from [14-C]-DBE. Reactive intermediates were detected as irreversibly bound adducts to protein or DNA. 1,2-Dibromoethane was metabolized by human liver cytosolic GST, microsomal GST, and microsomal MFO. Cytosolic GST activity (9 +/- 2 nmol/20 min/mg protein) was about four times greater than the other two activities. Only MFO activity resulted in adducts irreversibly bound to protein (1.5 +/- .4 nmol/20 min/mg protein) and was inhibited by the presence of glutathione. Both MFO and GST activity resulted in irreversibly bound adducts to DNA. Microsomal and cytosolic GST activity each produced about twice as many DNA adducts as microsomal MFO activity. These results suggest that human liver, like rat and mouse liver, metabolizes DBE to aqueous-soluble metabolites by both MFO and GST activity. Furthermore, each of these activities produces reactive metabolites that can irreversibly bind to cellular macromolecules.

A cytogenetic study of papaya workers exposed to ethylene dibromide

Ethylene dibromide (EDB) has been shown to be carcinogenic in animal studies and mutagenic in vitro. One cytogenetic study of workers exposed to low levels of EDB for short durations was negative. To test whether exposure to low levels of EDB over long periods caused cytogenetic changes, we have assessed the frequencies of sister-chromatid exchanges (SCE) and chromosomal aberrations (CA) in the peripheral blood lymphocytes of 60 men occupationally exposed to EDB. These men worked in papaya-packing plants where EDB was used to fumigate the fruit after harvest to kill fruit-fly larvae. 42 other men who worked at a nearby sugar mill served as controls. The average duration of exposure of the papaya workers was 5 years. 82 full shift personal breathing-zone air samples indicated that the papaya workers were exposed to a geometric mean of 88 ppb of EDB, as an 8-h time weighted average (TWA). Peaks up to 262 ppb were measured. The proposed OSHA 8-h TWA for EDB is 100 ppb, while NIOSH recommends 45 ppb. No differences in SCE levels were found between exposed and nonexposed workers. No differences were found in the total CA frequency between exposed and nonexposed workers. SCE levels were significantly increased in men who smoked cigarettes (p = 0.0001) and in men who smoked marijuana (p = 0.01). CA levels showed a significant increasing trend with age (p = 0.03).

Mutagenicity of 1,2-dibromo-3-chloropropane (DBCP) in the mouse spot test

The spot test with 1,2-dibromo-3-chloropropane (DBCP) was carried out using male PW and female C57BL/6 mice. DBCP induced recessive colour spots in offspring with a significantly high frequency of 2.9%, showing that this chemical is mutagenic for somatic cells of mice in vivo.

The role of glutathione in the toxicity of xenobiotic compounds: metabolic activation of 1,2-dibromoethane by glutathione

Unstable metabolites may arise during the metabolism of xenobiotic compounds with enzyme systems other than the cytochrome P-450 system. This depends on the enzyme system involved and the structure of the xenobiotic compound being metabolized. Normally detoxifying pathways may transform selected chemicals into toxic metabolites. In our laboratory we have demonstrated that DBE is metabolized by both cytochrome P-450 and GSH S-transferases. Although the cytochrome P-450 metabolite is reactive and will covalently bind to protein and nucleic acid to some extent, and the GSH S-transferase system conjugates it and under conditions of low DBE exposure is able to detoxify it. In contrast, GSH S-transferase catalyzes the direct conjugation of GSH with DBE. This can result in formation of a reactive intermediate that preferentially binds to nucleic acids and is responsible for the DNA damage observed following DBE exposure. The selective toxicity of this xenobiotic compound may be due to the preponderance of activating GSH conjugating enzymes in the extrahepatic organs. However, this difference alone does not appear sufficient to explain the selection of extrahepatic organs as sites of DBE-induced toxicity.

Comparison of two routes of chemical administration on the lung adenoma response in strain A/J mice

This study was undertaken to determine the ability of a series of 19 compounds representing different chemical classes of carcinogens to induce lung tumors in strain A/J mice after either ip or po administration. Aflatoxin B1, dibutylnitrosamine, 1,2-dimethylhydrazine, and methylnitrosourea induced a significant increase in the lung tumor response in both sexes after ip and po administration. Azaserine was active in both sexes only after ip administration. Benzene, 1,2-dibromoethane, and epichlorohydrin, following ip administration, produced significant increases in the tumor response in at least one sex. Aflatoxin B1, azaserine, benzene, 1,2-dibromoethane, dibutylnitrosamine, and epichlorohydrin were more active when given ip than after po administration. In contrast, dimethylhydrazine and methylnitrosourea were more active (in females only) when given po. 2-Acetylaminofluorene, azobenzene, chloroform, 1,4-dioxane, FANFT (N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide), lead subacetate, methylmethanesulfonate, beta-naphthylamine, beta-propiolactone, safrole, and 2,4,6-tri-chlorophenol did not induce lung tumors in strain A/J mice. These data confirm previous observations on the importance of the route of chemical administration on the lung tumor response in strain A mice, and on the inability of the lung tumor bioassay to detect certain liver and bladder carcinogens and unstable alkylating agents.

Pigeonpea as an important food source

Pigeonpea is an important source of proteins, carbohydrates, B-group vitamins, and certain minerals. India contributes over 90% of the pigeonpea production in the world where it is mostly consumed as dehusked splits or dhal. In African countries and Latin America, it is mainly consumed as canned peas. In this review, world production and distribution, genetic background, and biochemical and nutritional properties, storage and processing of pigeonpea are discussed. Future research needs to improve the utilization of pigeonpea as human food are also addressed.

In vivo cytogenetic studies on mice exposed to ethylene dibromide

The pesticide, ethylene dibromide (EDB), was evaluated with in vivo cytogenetic assays to determine its genotoxicity. CD1 male mice were exposed to EDB through intraperitoneal injections. Bone marrow cells isolated from femora were analyzed for sister-chromatid exchange (SCE), chromosome aberration and micronucleus formation. The results showed that only certain concentrations of EDB tested caused a slight but significant increase in SCEs and chromosome aberrations. However, these increases were not dose-related. No increase in the polychromatic erythrocytes with micronuclei was observed following EDB exposure. Also, EDB did not cause cell-cycle delay in comparison with controls. Thus, it appears that EDB is not an effective genotoxic agent in vivo in mice.

Initial and residual toxicity following acute exposure of developing male rats to dibromochloropropane

Male Fischer 344 rats were given a single, sc injection of 1,2-dibromo-3-chloropropane (DBCP) at 6 or 25 days of age. One group of treated animals was killed 1 to 3 days afterward to compare the dose and time relationships of the acute toxic response of neonatal and weanling male rats to DBCP and another group at approximate sexual maturity (approximately 120 days of age) to detect residual toxic effects resulting from acute exposure. The 6-day-old rats were more susceptible than the 25-day-old rats to the acute toxic effects of DBCP, as characterized by reduced 48-hr survival, renal dysfunction, and renal and hepatic necrosis over the dose range of 80 to 320 mg/kg. The lowest dose tested, 20 mg/kg, and all higher doses reduced subsequent body and gonadal weight gains, and caused hypospermatogenesis or seminiferous tubular atrophy in animals exposed at 6 days of age and killed at sexual maturity. Similar effects were observed in animals exposed at 25 days of age, except that doses of 160 mg/kg or greater were required to produce residual toxic effects. These data indicate enhanced susceptibility of neonatal male rats to the gonadotoxic effects of dibromochloropropane, including the possibility of apparent irreversible injury caused by acute exposure.

Carcinogenicity bioassays of bromoacetaldehyde and bromoethanol--potential metabolites of dibromoethane

1,2-Dibromoethane (DBE) and two of its potential metabolites, bromoethanol (BE) and bromoacetaldehyde (BA), were tested for carcinogenicity in male and female B6C3F1 mice using 30 animals of each sex per group. The carcinogen DBE was included in this assay as a positive control. The compounds were administered in distilled drinking water using equimolar concentrations, 4 mmol, of the chemicals. The dose chosen was based on subchronic bioassays of three months' duration. The chronic tests were continued for approximately 450 days in the case of DBE and approximately 560 days for both BE and BA. DBE induced squamous carcinomas of the forestomach in 22 females and 26 males and squamous papillomas of the esophagus in 3 females. BE induced squamous papillomas of the forestomach only in 10 females and 9 males. BA did not induce a significant incidence of tumors of the forestomach. Significant tumor incidences at other sites were not observed in any groups including the distilled water control group. Based on these findings, it is unlikely that BE or BA are activated carcinogenic intermediates of DBE.