N-nitrosodimethylamine Treatment Research Articles

Combined Toxic Effects and Mechanisms of Chloroacetic Acid and N-Nitrosodimethylamine on Submerged Macrophytes

Disinfection by-products (DBPs) such as chloroacetic acids (CAAs) and N-Nitrosodimethylamine (NDMA) are prevalent pollutants in surface waters, particularly with the increasing use of chlorine-based disinfectants. The entry of these DBPs into water bodies may increase accordingly, posing ecological risks to aquatic life. To assess the toxic effects of CAAs and NDMA on submerged macrophytes, Vallisneria natans was exposed to different concentrations of CAAs (1.0, 10.0, and 100.0 μg L−1) and NDMA (0.1, 1.0, and 10.0 μg L−1). A RI value of <1 indicates that simultaneous exposure to CAAs and NDMA can produce an antagonistic effect. Both CAAs and NDMA adversely affect the photosynthetic system of plants. In the NDMA treatment group, chlorophyll a content decreases with increasing concentration, accounting for 96.03%, 60.80%, and 58.67% of the CT group, respectively. Additionally, it effectively triggers the plant’s antioxidant response, with significant increases in SOD, POD, and GSH levels. Among these, the combined treatment group AN2 (10 + 1 μg L−1) showed the most significant change in SOD activity, reaching 3.57 times that of the CT group. Ultrastructural changes also revealed stress responses in leaf cells and damage to organelles. Furthermore, metabolomics provided insights into the metabolic responses induced by CAAs or NDMA in V. natans leaves, where the composition and metabolism of lipids, fatty acids, cofactors and vitamins, amino acids, nucleotides, and some antioxidants were regulated, affecting plant growth. This study provides preliminary information for the ecological risk assessment of submerged plants by complex contamination with the disinfection by-products CAA and NDMA.

N-nitrosodimethylamine exposure to zebrafish embryos/larvae causes cardiac and spinal developmental toxicity

N-nitrosodimethylamine (NDMA), one of the new nitrogen-containing disinfection by-products, is potentially cytotoxic, genotoxic, and carcinogenic. Its potential toxicological effects have attracted a wide range of attention, but the mechanism is still not sufficiently understood. To better understand the toxicological mechanisms of NDMA, zebrafish embryos were exposed to NDMA from 3 h post-fertilization (hpf) to 120hpf. Mortality and malformation were significantly increased, and hatching rate, heart rate, and swimming behavior were decreased in the exposure groups. The result indicated that NDMA exposure causes cardiac and spinal developmental toxicity. mRNA levels of genes involved in the apoptotic pathway, including p53, bax, and bcl-2 were significantly affected by NDMA exposure. Moreover, the genes associated with spinal and cardiac development (myh6, myh7, nkx2.5, eph, bmp2b, bmp4, bmp9, run2a, and run2b) were significantly downregulated after treatment with NDMA. Wnt and TGF-β signaling pathways, crucial for the development of diverse tissues and organs in the embryo and the establishment of the larval spine, were also significantly disturbed by NDMA treatment. In summary, the disinfection by-product, NDMA, exhibits spinal and cardiac developmental toxicity in zebrafish embryos, providing helpful information for comprehensive analyses and a better understanding the mechanism of its toxicity.

Neurotoxicity of N-Nitrosodimethylamine (NDMA) in Swiss albino mice and neuroprotection by nanocurcumin

Nitrosamine is a highly toxic compound which is released in various industries like rubber, cosmetics, pesticides etc. NDMA is formed in the processed food where sodium nitrite is used as preservative. It can also be found in alcoholic beverages and vegetables. Human’s exposure to NDMA is unavoidable. Mutagenic and hepatotoxic effects of NDMA has been reported but neurotoxicity data is not well established. In this study-four groups of Swiss albino mice were taken; Group I received the vehicle i.e. distilled water, group II received NDMA at the dose of 0.58 mg/kg body weight (1/25th of LD50) and group III received NDMA (0.58 mg/kg b.w.) + Nanocurcumin (20 mg/kg body weight) and group IV received only Nanocurcumin (20 mg/kg body weight). Various parameters of neuromuscular strength and neurotransmitters like Serotonin, Dopamine and Acetylcholinesterase were significantly decreased by NDMA treatment. Nanocurcumin administration along with NDMA didn’t cause any toxicity. In NDMA treated group’s antioxidant enzymes viz. SOD, catalase and glutathione peroxidase were significantly reduced whereas nanocurcumin showed protective effects as the nanocurcumin administration with NDMA showed near normal levels of the antioxidant enzymes. Cerebral cortexshowed pia mater encephalic, intracellular and extracellular vacuoles, irregular, pyramidal cells and broadening vessel peripheral clearance was also seen in NDMA treatment group, which were normal in the control and nanocurcumin treated group. These findings proved that NDMA caused neurotoxicity at the tested dose level and nanocurcumin have neuroprotective potential against NDMA treatment.

Biological treatment of N-nitrosodimethylamine (NDMA) and N-nitrodimethylamine (NTDMA) in a field-scale fluidized bed bioreactor

The ex situ treatment of N-nitrosodimethylamine (NDMA) and N-nitrodimethylamine (NTDMA) in groundwater was evaluated in a field-scale fluidized bed bioreactor (FBR). Both of these compounds, which originally entered groundwater at the test site from the use of liquid rocket propellant, are suspected human carcinogens. The objective of this research was to examine the application of a novel field-scale propane-fed fluidized bed bioreactor as an alternative to ultraviolet irradiation (UV) for treating NDMA and NTDMA to low part-per-trillion (ng/L) concentrations. Previous laboratory studies have shown that the bacterium Rhodococcus ruber ENV425 can biodegrade NDMA and NTDMA during growth on propane as a primary substrate and that the strain can effectively reduce NDMA concentrations in propane-fed bench-scale bioreactors of different design. R. ruber ENV425 was used as a seed culture for the FBR, which operated at a fluidization flow of ∼19 L-per-min (LPM) and received propane, oxygen, and inorganic nutrients in the feed. The reactor effectively treated ∼1 μg/L of influent NDMA to effluent concentrations of less than 10 ng/L at a hydraulic residence time (HRT) of only 10 min. At a 20 min HRT, the FBR reduced NDMA to <4.2 ng/L in the effluent, which was the discharge limit at the test site where the study was conducted. Similarly, NTDMA was consistently treated in the FBR from ∼0.5 μg/L to <10 ng/L at an HRT of 10 min or longer. Based on these removal rates, the average NDMA and NTDMA elimination capacities achieved were 2.1 mg NDMA treated/m3 of expanded bed/hr of operation and 1.1 mg NTDMA treated/m3 of expanded bed/hr of operation, respectively. The FBR system was highly resilient to upsets including power outages. Treatment of NDMA, but not NTDMA, was marginally affected when trace co-contaminants including trichloroethene (TCE) and trichlorofluoromethane (Freon 11) were initially added to feed groundwater, but performance recovered over a few weeks in the continued presence of these compounds. Strain ENV425 appeared to be replaced by native propanotrophs over time based on qPCR analysis, but contaminant treatment was not diminished. The results suggest that a FBR can be a viable alternative to UV treatment for removing NDMA from groundwater.

Efficient photoreductive decomposition of N-nitrosodimethylamine by UV/iodide process

N-nitrosodimethylamine (NDMA) has aroused extensive concern as a disinfection byproduct due to its high toxicity and elevated concentration levels in water sources. This study investigates the photoreductive decomposition of NDMA by UV/iodide process. The results showed that this process is an effective strategy for the treatment of NDMA with 99.2% NDMA removed within 10min. The depletion of NDMA by UV/iodide process obeyed pseudo-first-order kinetics with a rate constant (k1) of 0.60±0.03min−1. Hydrated electrons (eaq−) generated by the UV irradiation of iodide were proven to play a critical role. Dimethylamine (DMA) and nitrite (NO2−) were formed as the main intermediate products, which completely converted to formate (HCOO−), ammonium (NH4+) and nitrogen (N2). Therefore, not only the high efficiencies in NDMA destruction, but the elimination of toxic intermediates make UV/iodide process advantageous. A photoreduction mechanism was proposed: NDMA initially absorbed photons to a photoexcited state, and underwent a cleavage of NNO bond under the attack of eaq−. The solution pH had little impact on NDMA removal. However, alkaline conditions were more favorable for the elimination of DMA and NO2−, thus effectively reducing the secondary pollution.

Antinephrotoxic efficacy of Operculina turpethum and its isolated Stigma-5,22dien-3-O-b-D-glucopyranoside against N-Nitrosodimethylamine induced renal carcinogenesis in male mice

Objectives: Human exposure to nitrosamines can result from the formation of N ? nitroso compounds either in food during storage or preparation or in vivo , usually in the stomach. N-Nitrosodimethylamine (NDMA) is one of the main N-nitroso compounds which is commonly found in drinking water and is a potent carcinogen. The therapeutic effect of ethanolic root extract of Operculina turpethum was studied for its possible anti-cancerous potential induced by N- Nitrosodimethylamine in male albino mice as the in vivo model for the study. Methods: Renal malondialdehyde (MDA), Superoxide dismutase (SOD), catalase (CAT), glutathione content (GSH), aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), protein, cholesterol, urea and creatinine content were measured as oxidant/antioxidant markers. Electron Microscopy investigations of the renal tissue were also performed. One-way ANOVA test was used for comparisons of parameters in study groups. Results: Renal antioxidant defense systems, such as superoxide dismutase, catalase, glutathione peroxidase activities and reduced glutathione level, depleted by NDMA were restored to normal by the treatment. Oral administration of Operculina turpethum ethanolic extract, both crude and isolated groups recovered the enzyme activities and caused significant increase in serum protein, the treatment significantly reduced the elevated serum creatinine and urea levels (p>0.05), (p<0.01), (p<0.001). The ultrastructural electron microscopical analysis showed a decrease in cellular degradation comparing to the intoxicated mice. Conclusion: These findings prove the potential of Operculina turpethum as an antioxidant therapy to counteract mitochondrial and post-mitochondrial oxidative stress generated in kidney upon NDMA treatment thereby acting against renal toxicity by a carcinogen.

Increase of exostosin 1 in plasma as a potential biomarker for opisthorchiasis-associated cholangiocarcinoma

A proteomic-based approach was used to search for potential markers in the plasma of hamsters in which cholangiocarcinoma (CCA) was induced by Opisthorchis viverrini infection and N-nitrosodimethylamine treatment. The plasma proteins of CCA-induced hamsters were resolved by 1-D PAGE, digested by trypsin, and analyzed by LC-MS/MS. From the criteria of protein ID scores >15 and an overexpression of at least three times across all time points, 37 proteins were selected. These overexpressed proteins largely consisted of signal transduction, structural, transport, and transcriptional proteins in the order. Among the most frequently upregulated proteins, exostosin 1 (EXT1) was selected for further validation. By western blot analysis, the EXT1 expression level in the plasma of hamster CCA was significantly higher than that of controls at 1 month and thereafter. Immunohistochemistry revealed that EXT1 was expressed at vascular walls and fibroblasts at 21 days (before tumor onset) and at 2 months (early CCA) posttreatment. Its expression was also observed in bile duct cancer cells during tumor progression at 6 months posttreatment. In the human CCA tissue microarray, EXT1 immunoreactivity was found not only in vascular walls and fibroblasts but also in bile duct cancer cells and was positive in 89.7 % (61/68) of the cases. By ELISA and immunoblotting, plasma EXT1 level was significantly higher in human CCA compared to healthy controls. In conclusion, these results suggest that increased expression of EXT1 level in the plasma might be involved in CCA genesis and might be a potential biomarker of CCA.

Ex situ treatment of N-nitrosodimethylamine (NDMA) in groundwater using a fluidized bed reactor

N-nitrosodimethylamine (NDMA) is a suspected human carcinogen that has traditionally been treated in water using ultraviolet irradiation (UV). The objective of this research was to examine the application of a laboratory-scale fluidized bed reactor (FBR) as an alternative technology for treating NDMA to part-per-trillion (ng/L) concentrations in groundwater. Previous studies have shown that the bacterium Rhodococcus ruber ENV425 is capable of cometabolizing NDMA during growth on propane as a primary substrate in batch culture (Fournier et al., 2009) and in a bench-scale membrane bioreactor (Hatzinger et al., 2011) to low ng/L concentrations. R. ruber ENV425 was inoculated into the FBR during this study. With a hydraulic residence time (HRT) of 20 min, the FBR was found to be an effective means to treat 10–20 μg/L of NDMA to effluent concentrations less than 100 ng/L. When the HRT was increased to 30 min and oxygen and propane addition rates were optimized, the FBR system demonstrated treatment of the NDMA to effluent concentrations of less than 10 ng/L. Short-term shutdowns and the presence of trichloroethene (TCE) at 6 μg/L as a co-contaminant had minimal effect on the treatment of NDMA in the FBR. The data suggest that the FBR technology can be a viable alternative to UV for removing NDMA from groundwater.

Aerobic treatment of N-nitrosodimethylamine in a propane-fed membrane bioreactor

N-Nitrosodimethylamine (NDMA) is a suspected human carcinogen that has recently been detected in wastewater, groundwater and drinking water. Treatment of this compound to low part-per-trillion (ng/L) concentrations is required to mitigate cancer risk. Current treatment generally entails UV irradiation, which while effective, is also expensive. The objective of this research was to explore potential bioremediation strategies as alternatives for treating NDMA to ng/L concentrations. Batch studies revealed that the propanotroph Rhodococcus ruber ENV425 was capable of metabolizing NDMA from 8 μg/L to <2 ng/L after growth on propane, and that the strain produced metabolites that do not pose a significant risk at the concentrations generated ( Fournier et al., 2009). A laboratory-scale membrane bioreactor (MBR) was subsequently constructed to evaluate the potential for long-term ex situ treatment of NDMA. The MBR was seeded with ENV425 and received propane as the primary growth substrate and oxygen as an electron acceptor. At an average influent NDMA concentration of 7.4 μg/L and a 28.5 h hydraulic residence time, the reactor effluent concentration was 3.0 ± 2.3 ng/L (>99.95% removal) over more than 70 days of operation. The addition of trichloroethene (TCE) to the reactor resulted in a significant increase in effluent NDMA concentrations, most likely due to cell toxicity from TCE-epoxide produced during its cometabolic oxidation by ENV425. The data suggest that an MBR system can be a viable treatment option for NDMA in groundwater provided that high concentrations of TCE are not present.

Alterations of gene expression of RB pathway in Opisthorchis viverrini infection-induced cholangiocarcinoma

Opisthorchiasis has the significant relationship with the high prevalence of cholangiocarcinoma (CCA; a bile duct cancer) in the endemic areas in Southeast Asia. To reveal the molecular mechanism of the tumorigenesis induced by Opisthorchis viverrini infection, the present study investigated the kinetic expression of RB pathway genes, including RB1, p16(INK4), cyclin D1, and CDK4, during the development of opisthorchiasis-associated CCA in hamster model. The results of quantitative real-time polymerase chain reaction indicated that the expressions of RB1 and p16(INK4) were down-regulated during the development of CCA induced by infection plus N-nitrosodimethylamine treatment in a time-dependent manner. On the other hand, the expressions of cyclin D1 and CDK4 were up-regulated. The expression kinetics was corresponding to the pathological progression of the opisthorchiasis-associated CCA, revealed by histopathological observation. Moreover, the analysis of the expression of these genes in human opisthorchiasis-associated CCA cases showed the decreased expression of RB1 and p16(INK4) in 50% and 82.7% cases and overexpression of cyclin D1 and CDK4 in half cases, respectively. The results suggested that RB pathway is likely involved in the tumorigenesis of opisthorchiasis-induced CCA and proposed the potential application of some of these genes as biomarkers in predispose and molecular therapy of the parasite-associated cancer.

About mechanisms of formation of multinuclear hepatocytes during toxic action of N-nitrosodimethylamine on rats

The processes of hepatocyte multinucleation were studied in rats exposed to N-nitrosodimethylamine (NDMA). Using the immunohistochemical reaction to γ-tubulin, it was established that the number of cells containing three or more centrosomes increased 48 h after the NDMA injection. The formation of additional centrosomes in hepatocytes was shown to be based on the oxidative stress induced by NDMA metabolism with the participation of the cytochrome P450 superfamily. The administration of NDMA led to a sharp increase in the cytochrome P450 content in liver, especially 24 and 48 h (3.3 and 2.8 times, respectively) after the NDMA injection. The immunohistochemical reaction for cytochrome P4502E1 revealed an intensive staining of the cytoplasm of centrilobular hepatocytes 24 and 48 h after the NDMA action. In the same time period, a 1.1-2.0-fold increase occurs in the concentration of malonic dialdehyde (MDA) (a derivative of lipid peroxidation) and a 1.1-1.3-fold decrease in catalase activity (an enzyme of the cell antioxidative system). At a later time (72–120 h) after the NDMA action, the number of cells with three or more centrosomes, the intensity of cell cytoplasmic staining for cytochrome P450 2E1, and the concentrations of P450 and MDA in the liver decreased, whereas catalase activity increased. After 48 h of NDMA treatment, the incorporation of binuclear hepatocytes with various 3H-thymidines into nuclei occured, which indicates asynchronous DNA synthesis. The immunohistochemical reaction for pKi-67, nuclear protein that is a marker of cell proliferation, has established that the asynchronicity of nuclear proliferative activity in binuclear cells is not only characteristic of the S phase, but also of other cell cycle phases, including G1, G2, and M. Thus, the main mechanisms of hepatocyte multinucleation under the influence of NDMA are as follows: (1) increased hyperamplification of centrosomes as a consequence of oxidative stress and (2) asynchronous DNA synthesis in nuclei of binuclear hepatocytes with subsequent asynchronous acytokinetic mitosis.

N‐Nitrosodimethylamine: A Disinfectant Byproduct and Its Occurrence in Wastewater

This paper will provide wastewater treatment utility professionals with a comprehensive synthesis of information pertinent to N-nitrosodimethylamine (NDMA) so that plant operators can make informed and cost-effective decisions regarding appropriate management techniques. A suspect disinfection byproduct, NDMA is a potential carcinogen and is presently under scrutiny from the U.S. Environmental Protection Agency because it poses a threat to groundwaters from reclaimed wastewaters. Recognizing that the current state of knowledge pertaining to the occurrence and treatment of NDMA from wastewater treatment is in its infancy, the information presented in this paper is timely and will help utility professionals develop confidence toward controlling NDMA during wastewater treatment. Given the increased probability of the formation of NDMA using current wastewater treatment technologies and also in the complex matrices of the wastewaters subjected to UV treatment, the investigation of occurrence pathways and means of suppression of NDMA formation before and after treatment needs to be investigated. This paper also summarizes strategies to minimize exposure such as modifying treatment or instituting waste and agricultural management practices that minimize inorganic and organic nitrogen discharges to wastewaters.

NDMA Treatment by Sequential GAC Adsorption and Fenton-Driven Destruction

N-nitrosodimethylamine (NDMA) is a highly toxic environmental contaminant that was first detected in groundwater tainted by rocket fuel manufacturing wastes. NDMA is also a by-product of certain industrial processes including the chlorination of treated water and wastewater. Water treatment by carbon adsorption is costly because NDMA partitions only sparingly to carbon and frequent carbon replacement or regeneration is required. If activated carbon could be regenerated cheaply and quickly in place, NDMA adsorption on carbon, an easily implemented technology, could become attractive. In this study, the feasibility of adsorbing NDMA onto carbon followed by in-place carbon regeneration using Fenton's reagent was assessed. Batch and column tests indicated that the concentration of sorbed NDMA can be lowered to nondetectable levels in hours using reasonable hydrogen peroxide and iron concentrations. Three-log destruction of sorbed NDMA loaded to 1.04 mg NDMA/g carbon was achieved in approximately 12 h. Results...

Site-Specific Induction of Metallothionein in N-Nitrosodimethylamine-Treated Rat Liver.

The effects of dietary protein and sulfur-containing amino acids on oxidative responses against N-nitrosodimethylamine (NDMA) treatment were investigated in rat liver, focusing on the mechanism of metallothionein (MT) induction. Rats were fed a 10% soybean protein isolate (SPI) diet or SPI supplemented with 0.3% L-methionine (SPI-Met) for 3 weeks after weaning. Rats fed on SPI showed lower glutathione (GSH) levels and higher MT levels in the liver than those fed on SPI-Met. After treatment with NDMA (20 mg/kg, i.p.), MT levels were elevated significantly in both groups, whereas GSH levels were only elevated in the SPI group. Pathologically, necrosis and hemorrhage were seen in the livers of NDMA-treated rats of both dietary groups, suggesting oxidative damage caused by NDMA treatment. Immunological histochemistry using anti-MT antibody showed that most of the MT was distributed in the cytosol region of all the rat groups. The ratios of the MT-positive cells were correlated to MT levels that were chemically analyzed. It was notable that the localization of MT staining in the hepatic lobules was altered after NDMA injection. The MT staining was principally observed around the central vein before injection, whereas it was more markedly observed at the peripheral region after NDMA treatment. These results indicate that the synthesis of hepatic MT basically occurs around the central vein with oxygen stress caused by insufficient oxygen supplement and/or stimulated oxygen consumption. NDMA could be oxidatively metabolized at the peripheral region with sufficient supplemental oxygen from the artery, resulting in oxidative stress and stimulation of MT synthesis there. Thus, distribution of tissue MT might vary depending on the site of oxidative stress.

Effects of N-nitrosodimethylamine (NDMA) on the oxidative status of rat liver.

To investigate oxidative effects of N-nitrosodimethylamine (NDMA) on the liver, rats were challenged by the reagent with a dose range of 10 to 40 mg/kg. With lower dose levels, protective responses were prominent, such as elevation of the hepatic glutathione and metallothionein (MT) levels. Increased activities were also evident of gamma-glutamylcysteine synthetase, glucose-6-phosphate dehydrogenase (G6PD), and malic enzyme. In the high dose range, however, toxic responses, such as increases in lipid peroxide levels in liver and serum, and glutamic-oxaloacetic transaminase (GOT), glutamic-pyruvic transaminase (GPT), and ketone bodies in serum became marked. Some of the protective responses became less marked at the highest dose. Catalase and glutathione peroxidase activities in the liver were also inhibited by NDMA treatment. On the other hand, when NDMA was injected as a series of doses (10 mg/kg on four separate occasions), the effects were less marked, and the hepatic levels of MT and lipid peroxide remained unchanged even after the 4th injection. Only the increase in G6PD activity was more marked after four times repeated injection than after a single injection. These results suggest that oxidative and hepatotoxic effects of NDMA are more moderate when given in repeated doses than in a single dose. In contrast to the liver, elevation of MT levels was the only detectable change in the kidney.

Protective effects of diallyl sulfide on N-nitrosodimethylamine-induced immunosuppression in mice

Diallyl sulfide (DAS), a flavor component of garlic that has been used as a food additive, exerts chemopreventive effects at several organ sites in rodents after administration of chemical carcinogens possibly by inhibiting carcinogen activation via cytochrome P450-mediated oxidative metabolism. In this study, we investigated the protective effect of DAS on the N-nitrosodimethylamine (NDMA)-induced immunosuppression of humoral and cellular responses in BALB/c mice and the possible mechanisms involved in this protection. We observed that oral administration of DAS prior to NDMA treatment for 14 consecutive days blocked the NDMA-induced suppression of the antibody response to a T-cell-dependent antigen, sheep erythrocytes, and the lymphoproliferative response to the T-cell and the B-cell mitogens in dose-dependent manners. Treatment of mice with DAS resulted in a significant decrease of cytochrome P450 2E1-dependent p-nitrophenol hydroxylase and NDMA demethylase activities. The results show that the protective effects of DAS against the NDMA-induced immunotoxicity may, at least in part, be due to its ability to block bioactivation of NDMA mainly by the inhibition of cytochrome P450 2E1.

Analysis of tissue-specific lacZ mutations induced by N-nitrosodibenzylamine in transgenic mice.

N-Nitrosodibenzylamine (NDBzA) is a contaminant found frequently in rubber baby bottle nipples and pacifiers. To evaluate more fully the mutagenic potential and analyse the molecular nature of possible mutations induced in vivo, we have studied the mutagenicity of NDBzA in vivo using the MutaMouse system. NDBzA, suspended in olive oil, was administered orally once to male mice at different doses (0, 30, 100, 425 and 750 mg/kg) and the mice were killed 30 and 90 days after treatment. As a positive control, and to compare relative mutagenicity, N-nitrosodimethylamine (NDMA) was also administered to animals in the same experiment at doses of 0, 2, 6 and 10 mg/kg. Mutant frequencies were increased in both 30 and 90 day liver samples, but not in bone marrow, after both NDBzA and NDMA treatment. However, NDBzA was >100 times less mutagenic than NDMA. A total of 81 mutants obtained from liver samples of treated animals (750 mg/kg) were characterized by DNA sequencing. While spontaneous mutations in transgenic mice have been characterized previously by a preponderance of G:C-->A:T transitions, mainly at 5'-CpG-3' dinucleotide sites, the predominant type of NDBzA-induced mutation in this study was transversion, mainly G:C-->T:A changes. The molecular characteristics of mutations induced by NDBzA indicate that they may arise from specific unidentified DNA adducts and benzylation appears to be the primary mechanism involved in formation of these DNA adducts.

DNA single-strand breaks and toxicity induced by 4-(methyl-nitrosamino)-1-(3- pyridyl)-1-butanone or N-nitrosodimethylamine in hamster and rat liver.

Syrian golden hamsters and F344 rats display contrasting susceptibilities to hepatocarcinogenesis induced by the tobacco-specific nitrosamine 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosodimethylamine (NDMA). In this study, the time courses of DNA single-strand breaks (SSB) and toxicity induced by NNK and NDMA in hamster and rat liver were compared. DNA SSB reached a maximum 12 h after carcinogen treatment, partially correlating with previous reports on time courses of DNA methylation. The persistence of DNA SSB up to 2-3 weeks after NNK or NDMA treatment reflects a deficient repair of some DNA lesions. No significant species differences in the kinetics of DNA SSB induction and repair were observed following NNK or NDMA (0.39 mmol/kg) treatment. However, NNK induced slightly more DNA SSB than NDMA in both species. This could reflect the formation of intermediates with more DNA-damaging capacity or inhibiting DNA processes. A significant hepatotoxic effect of NNK, evaluated by plasma markers of liver injury, was observed in rats and hamsters 12-24 h post-treatment. In contrast, NDMA induced earlier (< 12 h) enzyme elevations. Maximum hepatotoxic effects were observed 24 h (NNK-treated hamsters and rats, NDMA-treated rats) or 2 weeks (NDMA-treated hamsters) after carcinogen administration. Three weeks after treatment, hepatotoxicity of NNK and NDMA was still detected in hamsters, but not in rats. These results suggest that the toxic effects of NNK and NDMA initiate a regenerative process that occurs faster in rat than in hamster liver. Since hepatocarcinogenesis occurs in NNK- but not in NDMA-treated rats, promutagenic lesions generated from NNK might be fixed preferentially during cell proliferation.