Toxicity of peroxynitrite and related reactive nitrogen species toward Escherichia coli

The toxicity of peroxynitrite toward Escherichia coli (expressed as LD50, the concentration required to kill 50% of the bacteria) was found to be independent of bacterial cell densities over a wide experimental range, spanning 10(6)-10(10) colony-forming units/mL; the magnitude of LD50 was also pH-independent over the range pH 5.9-8.3. This highly unusual behavior can be quantitatively reproduced by a dynamical model in which (i) ONO2H is identified as the toxic form of the oxidant and (ii) the bulk of the added peroxynitrite decays to nitrate ion under these conditions. From the model, one estimates that 10(6)-10(7) ONO2H molecules are required to kill a bacterium, indicating a very high intrinsic toxicity (cf. HOCl, for which LD50 = 10(7)-10(8) molecules/cell of E. coli). Nearly complete protection was observed when bicarbonate ion was added to the buffer, even when concentrations of peroxynitrite exceeded 50 times the LD50 measured in the absence of bicarbonate. Consistent with previous reports, combinations of H2O2 and NO and, in weakly acidic media, H2O2 and NO2- were found to exhibit enhanced toxicities relative to the individual reactants. Protection by bicarbonate was utilized to assess the potential role of intermediary formation of ONO2H in bacterial killing in these systems. Approximately 25% protection by bicarbonate was observed for media containing H2O2 and NO2-, consistent with a minor contribution to killing by ONO2H under the experimental conditions. No protection was observed for media containing H2O2 and *NO in both anaerobic and aerobic environments, excluding extracellularly generated ONO2H as a participant in these bactericidal reactions.

Two pathways of the nitric oxide-induced cytotoxycal action

Nitric oxide is a diffusible messenger with multiple biological functions. We show here that NO-generating compound, S-nitrosoglutathione (GSNO) induces apoptosis in human chondrocytes and causes necrosis-like cell death in human epithelial CaOv cell line. Pretreatment of chondrocytes with low-dose GSNO or with gamma-interferon enhances their tolerance to the second high-concentration GSNO exposure. On the contrary, in CaOv cells low-dose GSNO pretreatment diminishes the resistance and increases cytolysis at the second GSNO exposure. We conclude that human chondrocytes possess specific and inducible mechanism preventing cell killing by nitric oxide.

Hypersensitivity of ataxia-telangiectasia fibroblasts to a nitric oxide donor

Ataxia-telangiectasia (A-T) is a human autosomal recessive disease characterised by immunodeficiency, extreme sensitivity to ionising radiation and progressive cerebellar ataxia. The defective gene has recently been cloned and is a member of the phosphatidylinositol 3-kinase family. We have investigated the possibility that the neurodegeneration in A-T might be induced by an endogenously formed mutagen causing radiation-like damage. Nitric oxide is known to be formed in the cerebellum and we present evidence that A-T fibroblasts are hypersensitive to killing by the nitric oxide donor S-nitrosoglutathione (GSNO), as are fibroblasts from a radiosensitive individual without ataxia. Killing was determined as loss of colony forming ability. GSNO induces dose-dependent DNA strand breakage, but to no greater extent in A-T fibroblasts. Breakdown of GSNO to nitrite and nitrate appears to occur to the same extent in both normal and A-T fibroblasts. Cell killing by GSNO appears to be associated in both types of cell with formation of nitrite, rather than nitrate, as the ultimate oxidation product of nitric oxide.

An evaluation of the roles of metabolic denitrosation and alpha-hydroxylation in the hepatotoxicity of N-Nitrosodimethylamine

N-Nitrosodimethylamine (NDMA) is a potent hepatotoxic agent in rats. NDMA causes cell death that does not correlate with known mechanisms of toxicity such as the production of oxidative stress or covalent binding to proteins. The following studies were designed to determine whether NDMA cytotoxicity is the result of metabolic denitrosation or alpha-hydroxylation of the nitrosamine. We determined the toxicity of various metabolites of NDMA in monolayer cultures of primary rat hepatocytes. NDMA was toxic at 0.1 mM in our cultures, but the metabolites formaldehyde, methanol, and methylamine were not toxic at this concentration. We used diazeniumdiolates that spontaneously release nitric oxide (NO) in aqueous media to deliver NO to hepatocytes in culture. The results show that, while NO released from diazeniumdiolates causes death in hepatocytes, the levels of NO produced during NDMA metabolism are insufficient to account for the toxicity of the nitrosamine. NDMA-d6, the fully deuteriated form of NDMA that undergoes approximately twice as much denitrosation in vivo as NDMA, was significantly less cytotoxic than NDMA. In contrast, N-nitroso(acetoxymethyl)methylamine (AcO-NDMA), a stable precursor of the methanediazonium ion, was found to cause toxicity equivalent of NDMA on a molar basis. Altogether, our results with methylamine, formaldehyde, methanol, the diazeniumdiolates, and NDMA-d6 indicate that NDMA toxicity is not the result of metabolic denitrosation, while the toxicity of AcO-NDMA provides strong strong evidence that the formation of the methanediazonium ion via alpha-hydroxylation leads to cell death.

Transport of reactive metabolites of procarcinogens between different liver cell types, as demonstrated by the single cell microgel electrophoresis assay

Procarcinogens have to be activated by specific cytochromes before showing adverse effects. Freshly isolated hepatocytes (parenchymal liver cells, PC) are characterized by a high content of such xenobiotic enzymes and are widely used to investigate chemically induced DNA damage. But in many cases liver tumors caused by indirect acting carcinogens can also originate from non-parenchymal liver cells (NPC). We used freshly isolated rat PC and NPC to demonstrate that only PC have activation capacity when treated in vitro with different genotoxic procarcinogens (N-nitrosodimethylamine, NDMA; vinyl chloride, VC). The alkaline single cell microgel electrophoresis assay was applied to measure the genotoxic activity of the activated compounds. In order to test the hypothesis that reactive metabolites can be transported from PC to NPC, we performed additional in vivo studies as well as studies in which PC were incubated together with NPC, only separated by a dialysis tube (in vitro coincubation). The results indicate that reactive metabolites of both NDMA and VC are stable enough to be transported intercellularly from PC to NPC.

N-nitroso compounds induce changes in carcinogen-metabolizing enzymes

The bioactivation of N-nitrosoamines and polycyclic aromatic hydrocarbons (PAH) is mediated by the mixed function oxidase system, which includes dimethylnitrosamine N-demethylase I (DMN-dI), arylhydrocarbon hydroxylase (AHH), cytochrome P-450, cytochrome b5 and NADPH-cytochrome c reductase of liver microsomes. The present study shows the influence of N-nitroso compounds on the activities of the above-mentioned enzymes. Single-dose treatment (20 mg/kg body weight) of male mice with ethylbutylnitrosamine, propylbutylnitrosamine, or dibutylnitrosamine: increased (1) the activity of DMN-dI by 108%, 104%, 51%, respectively; (2) the cytochrome P-450 content by 106%, 72%, 51%, respectively; (3) the activity of AHH by 95%, 106%, 80% respectively; (4) the cytochrome b5 content by 164%, 97%, 94% respectively; and (5) decreased the activity of NADPH-cytochrome c reductase by 55%, 50% and 45%, respectively. Methylpropylnitrosamine decreased the activity of DMN-dI by 44% and the P-450 content by 50%. Diphenylnitrosamine also decreased cytochrome P-450 by 54%, AHH activity by 64% but increased the activity of DMN-dI by 42%, the cytochrome b5 content by 159% and NADPH-cytochrome c reductase activity by 57%. It seems from this study that the activity of AHH is dependent on P-450 content but DMN-dI is not since the compounds that increased or decreased the activity of AHH had parallel effects on P-450 content. Also, the extent to which the altered activities of DMN-dI, P-450, AHH, cytochrome b5 and NADPH-cytochrome c reductase depends on the type of alkyl groups linked to the nitroso group.

Effects of low dose mixtures of four N-nitroso compounds on hepatic foci development in the rat

Potential synergism between four N-nitroso compounds (nitrosomorpholine, nitrosodimethylamine, nitrosodiethanolamine, nitroso-oxazolidine) in rat liver carcinogenesis was examined in the medium-term bioassay. Male F344 rats were initially given diethylnitrosamine (DEN, 200 mg/kg, ip) and beginning 2 weeks later received test chemicals for 6 weeks individually at a full or 1/4 dose of that proven to be carcinogenic individually or in combination. All animals were subjected to partial hepatectomy at week 3 and killed at week 8. Induction of immunohistochemically-demonstrated glutathione S-transferase placental form (GST-P) positive foci was evaluated. The numbers and size of GST-P positive foci were significantly higher than the control levels by the treatment with each nitrosamine at full (1/1) and one quarter doses (1/4), excepting nitrosodiethanolamine and by combination of the four chemicals at 1/4 and 1/16. Because the dose-response curves were considered non-linear for most nitrosamines, synergistic effects were not apparent for the 1/4 mixture. Interestingly, however, the values for rats treated with these four chemicals in combination at the 1/4 dose level were almost the same as the average of four individual treatments at the full dose, and those for the 1/16 dose mixture were almost the same as the average of 1/4 individual treatment groups. These results indicate that these nitrosamines worked additively, rather than synergistically, in rat liver carcinogenesis.

The mutagenicities of alkaloids and N-nitrosoguvacoline from betel quid

In Taiwan, betel quid is a natural masticatory, which is composed of fresh green areca fruit, Piper betle and slaked lime paste. Areca fruit contains some alkaloids, of which arecoline is the major one. N-Nitrosoguvacoline (NG), one of the N-nitrosation products of arecoline, is the only one N-nitrosamine found in Taiwanese betel quid chewing saliva. The mutagenic studies in Ames Salmonella microsome test showed that crude alkaloid extracts of areca fruit and arecoline were active in Salmonella typhimurium TA100, and NG was weakly active in TA98 and TA100. The activities in both arecoline and NG decreased further in the presence of rat liver S9 mix. Nitrite was significantly consumed during the N-nitrosation of arecoline and sodium nitrite at acidic condition (pH 3), whereas the formation of NG was favored at neutral condition (pH 7). Crude phenolic extracts of leaf and inflorescence of Piper betle inhibited the formation of NG by blocking the nitrite. However, a high amount of crude phenolic extracts of areca fruit enhanced the formation of NG.

Induction of microsomal enzymes in liver of rats treated with cyclohexanol

The S9 fraction obtained from rats orally pretreated for 3 days with cyclohexanol was able to activate the pro-mutagen N-nitrosodimethylamine (NDMA) into highly mutagenic metabolite(s) detected in the TA100 strain of Salmonella typhimurium. NDMA was not mutagenic when uninduced S9 was used as metabolic source but was approximately twice more mutagenic with cyclohexanol-induced S9 compared to ethanol-induced S9. Separation of microsomal proteins by sodium dodecylsulfate gel electrophoresis, displayed protein bands situated in the range of 50,000 to 52,000 molecular weight induced by both, ethanol and cyclohexanol. These results are evidence of the induction properties of cyclohexanol.

DNA damage induced by seven N-nitroso compounds in primary cultures of human and rat kidney cells

Seven N-nitroso compounds (NOC), known to induce kidney tumors in rats, were assayed for DNA-damaging activity in primary cultures of human and rat kidney cells. DNA fragmentation was measured by the alkaline elution technique. Positive responses were obtained in cells of both species with N-nitrosodimethylamine (32 mM), N-nitrosodiethylamine (32 mM), N-nitrosodi-n-propylamine (10 mM), N-ethyl-N-hydroxyethylnitrosamine (18 mM), and streptozotocin (1 mM). N-nitrosodiethanolamine and N-nitrosomorpholine were inactive at the highest concentration tested (32 mM). The responses of human kidney cells were qualitatively similar to those of rat kidney cells, but statistically significant differences between the two species in the DNA-damaging potencies were observed with N-ethyl-N-hydroxyethylnitrosamine and streptozotocin, both more genotoxic in rat cells. Taken as a whole, the results suggest on the one hand that the five active NOC might be carcinogenic for the kidney in humans, and on the other hand that the rat kidney cell/DNA damage assay is a valid model for predicting the genotoxic potential of NOC in human kidney cells.

Synergistic action of N-nitrosodialkylamines and near-UV in the induction of chromosome aberrations in Chinese hamster lung fibroblasts in vitro

N-Nitrosodialkylamines are promutagens and proclastogens, requiring metabolic activation for their actions. Previously, we showed that direct-acting bacterial mutagens can be formed from N-nitrosodialkylamines on exposure to near-UV. We have now found that N-nitrosodialkylamines with near-UV irradiation are clastogenic to Chinese hamster lung cells. When the cells in culture were irradiated with near-UV for 3 h in the presence of N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), N-nitrosopyrrolidine (NPYR), N-nitrosopiperidine (NPIP) or N-nitrosomorpholine (NMOR), and then further incubated for a total period of 24 h with the N-nitrosodialkylamines, chromosome aberrations were induced. Neither the N-nitrosodialkylamine nor near-UV alone were clastogenic. Severe clastogenicity (> 50% of cells examined showing aberrations) was observed for 0.5 mM NDEA, NPYR and NPIP. The order of the clastogenic activity was NDEA, NPYR > NPIP, NDMA > NMOR. This order differed from that of bacterial mutagenicity previously reported for these N-nitrosodialkylamines plus near-UV, in which NMOR gave the strongest activity. The chromosome aberrations induced by the NPYR and NDEA plus near-UV in CHL-cells were inhibited by superoxide dismutase, glutathione and L-cysteine. Dimethylsulfoxide or D-mannitol, scavengers of hydroxy radical, and L-histidine, a scavenger of single oxygen, were ineffective. These results suggest that superoxide formed by a synergistic action of an N-nitrosodialkylamine and near-UV is the cause of the chromosome aberrations observed, an assumption consistent with the known ability of superoxide to cleave DNA.

Genetic and redox determinants of nitric oxide cytotoxicity in a Salmonella typhimurium model

Paradoxically, nitric oxide (NO) has been found to exhibit cytotoxic, antiproliferative, or cytoprotective activity under different conditions. We have utilized Salmonella mutants deficient in antioxidant defenses or peptide transport to gain insights into NO actions. Comparison of three NO donor compounds reveals distinct and independent cellular responses associated with specific redox forms of NO. The peroxynitrite (OONO-) generator 3-morpholinosydnonimine hydrochloride mediates oxygen-dependent Salmonella killing, whereas S-nitrosoglutathione (GSNO) causes oxygen-independent cytostasis, and the NO. donor diethylenetriamine-nitric oxide adduct has no antibacterial activity. GSNO has the greatest activity for stationary cells, a characteristic relevant to latent or intracellular pathogens. Moreover, the cytostatic activity of GSNO may best correlate with antiproliferative or antimicrobial effects of NO, which are unassociated with overt cell injury. dpp mutants defective in active dipeptide transport are resistant to GSNO, implicating heterolytic NO+ transfer rather than homolytic NO. release in the mechanism of cytostasis. This transport system may provide a specific pathway for GSNO-mediated signaling in biological systems. The redox state and associated carrier molecules are critical determinants of NO activity.

Reproductive toxicity screen of ammonium dinitramide administered in the drinking water of Sprague-Dawley rats

The Department of Defense is currently considering replacing ammonium perchlorate with ammonium dinitramide (ADN), a class 1.1 explosive oxidizer to be used in solid rocket propellant mixtures and explosives. This study was intended to evaluate the potential of ADN to produce alterations in paternal fertility, maternal pregnancy and lactation, and growth and development of offspring. Male and female rats received drinking water containing 0.0, 0.2, 1.0, or 2.0 g ADN/liter throughout the study. Mating occurred following 14 days of treatment. All dams, one-half the males, and representative pups were maintained for a total of 90 days of treatment. No mortality occurred in parental animals during the study. Treatment with ADN resulted in no adverse effects on mating; 92-100% of the animals mated. No treatment-related effects were seen in parental animals clinically or histopathologically. Adverse treatment-related effects were noted in maternal and paternal fertility indices, gestational indices, and live birth indices in both the mid- and high-dose groups. Litter sizes in the mid- and high-dose groups were significantly smaller than those of the low-dose and control groups. Mean pup weights showed no statistically significant differences between ADN-treated pups and controls. Gross and histopathological examination of the animals failed to identify the cause for the decrease in litter production in the mid- and high-dose dams. This study indicates that ADN is a reproductive toxicant. The no-observable-effect level (NOEL) is 29 mg/kg/day, the median dose of the low level female rats.

Role of N-nitroso compounds (NOC) and N-nitrosation in etiology of gastric, esophageal, nasopharyngeal and bladder cancer and contribution to cancer of known exposures to NOC

The questions of whether and how N-nitroso compounds (NOC) may be inducing cancer in humans are discussed. The principal subjects covered include nitrite-derived alkylating agents that are not NOC, reasons for the wide tissue specificity of carcinogenesis by NOC, the acute toxicity of nitrosamines in humans, mechanisms of in vivo formation of NOC by chemical and bacterial nitrosation in the stomach and via nitric oxide (NO) formation during inflammation, studies on nitrite esters, use of the nitrosoproline test to follow human gastric nitrosation, correlations of nitrate in food and water with in vivo nitrosation and the inhibition of gastric nitrosation by vitamin C and polyphenols. Evidence that specific cancers are caused by NOC is reviewed for cancer of the stomach, esophagus, nasopharynx, urinary bladder in bilharzia and colon. I review the occurrence of nitrosamines in tobacco products, nitrite-cured meat (which might be linked with childhood leukemia and brain cancer) and other foods, and in drugs and industrial situations. Finally, I discuss clues from mutations in ras and p53 genes in human tumors about whether NOC are etiologic agents and draw some general conclusions.

Evaluation of DNA-damaging, clastogenic, and promoting activities of metoclopramide and procainamide in rats

The DNA-damaging and clastogenic activities of metoclopramide (MCA) and procainamide (PCA), two substituted benzamides not systematically tested for genotoxicity before clinical use, were investigated in rats given a single high oral dose (500 mg/kg) of these drugs. Neither MCA nor PCA induced DNA fragmentation in liver, kidney, gastric mucosa, spleen, and bone marrow, as detected by the alkaline elution technique. Moreover, neither drug increased the frequency of micronucleated hepatocytes and the frequency of micronucleated polychromatic erythrocytes in the bone marrow of partially hepatectomized rats. However, in rats initiated with N-nitrosodiethylamine and given water containing 0.125% MCA for 14 successive days a clear-cut and statistically significant increase in the number and size of liver gamma-glutamyltranspeptidase-positive foci and basophilic foci, which are consistent with potential promoting activity, was observed. Under the same experimental conditions the effect of PCA was markedly lower, only limited to a modest increase of the number and area of gamma-glutamyltranspeptidase-positive foci.

Sugar-linked dithiocarbamates as modulators of metabolic and genotoxic properties of N-nitroso compounds

A series of putative anticarcinogenic and antimutagenic compounds was synthesized on the basis of tetraethylthiuram disulfide (disulfiram) and its metabolite, diethyldithiocarbamate (DDTC). Diallyldithiocarbamate was synthesized in order to combine the anticarcinogenic properties of diallyl sulfide, a known inhibitor of chemical carcinogenesis from Allium species, and those of DDTC. Several sugar-linked dithiocarbamates (SDTCs) were prepared using glucose, cellobiose, and lactose as glycosyl donors and DDTC and diallyldithiocarbamate as acceptors. All the S--glycoside bonds of SDTCs were very stable under physiological conditions in vitro. At low nitrosamine concentrations, glucose-DDTC inhibited microsomal nitrosamine dealkylases in vitro. In vivo these enzymes were also inhibited 4 h after i.p. administration of glucose-DDTC or lactose-DDTC to rats (1.7 mmol/kg); after 24 h, the values had returned to control levels. Glucose-DDTC induced the activity of glutathione-related enzymes. Concomitant treatment of rats with glucose-DDTC and N-nitrosodiethylamine (NDEA) led to a depression of the oxidative metabolism of [14C]NDEA to 14CO2 but increased the elimination of unchanged [14C]NDEA in the urine. Furthermore, glucose-DDTC totally inhibited the formation of DNA single-strand breaks induced by NDEA. All these effects may contribute to possible antimutagenic and anticarcinogenic actions of the dithiocarbamates investigated.

N-nitroso-2-acetylaminofluorene: a direct-acting carcinogen inducing hepatocellular carcinoma in Sprague-Dawley rats

To compare the hepatotoxicity and hepatocarcinogenicity of N-nitroso-2-acetylaminofluorene (NO-AAF) and its parent compound, 2-acetylaminofluorene (AAF), male Sprague-Dawley rats were given intraperitoneal (i.p.) or subcutaneous (s.c.) injections of AAF or NO-AAF (60 mg/kg body weight/week) for ten months. In the AAF group, morphological changes were produced which involved gross distortions of the liver with multiple nodule formations. The rat livers in the NO-AAF group appeared to be smooth with a blunt-thick superior segment of the lateral lobe. The serum gamma-glutamyl transpeptidase activity in both the AAF group and the NO-AAF group was significantly elevated (P < 0.0005). The present study shows that i.p. and s.c. injections of NO-AAF resulted in a high incidence of well-differentiated hepatocellular carcinomas (HCC) (7/9 and 4/6, respectively), while poorly differentiated HCCs were induced by i.p. or s.c. administration of AAF (6/9 or 2/6, respectively). Subcutaneous lesions consisting of an inflammatory reaction and fibroadenoma formation were observed in the NO-AAF-treated rats, whereas no such skin lesions were detected in the AAF-treated animals. These results suggest that NO-AAF is a new direct-acting carcinogen which may be useful for investigating hepatocarcinogenesis.

O6-methylguanine-DNA adducts in rat lymphocytes after in vivo exposure to N-nitrosodimethylamine (NDMA)

A non-invasive approach in immunopathological risk assessment was applied for analysis of the in vivo formation of DNA adducts. DNA methylation was studied in peripheral blood lymphocytes (PBLs) collected from Sprague-Dawley rats exposed to a single dose (75 mg/kg b.w.) of N-nitrosodimethylamine (NDMA). Three different techniques were applied for characterization and quantification of DNA adducts: (i) colloidal gold ultraimmunocytochemical localization of O6-methylguanosine (O6-meG)-DNA adducts, using affinity-purified, polyclonal antibody directed against O6-meG, (ii) quantitative assay using enzyme-linked immunosorbent assay (ELISA), amplified by the avidin-biotin (AB) system, and (iii) high-performance liquid chromatography (HPLC). The O6-meG-immunoreactive sites in PBLs seem to be concentrated in the nucleus. However, significant immunolabelling was also noted in the cytoplasm of the in vivo NDMA-exposed PBLs. Control preparations showed no specific gold immunolabelling. The O6-meG-DNA adduct formation in PBLs and hepatocytes, at 2-24 h following the exposure to NDMA, was analogous for both types of cells. The data showed high correlation for the ELISA and HPLC analytical methods. The data suggest an efficient O6-metG-DNA repair mechanism in lymphocytes, possibly analogous to the enzymatic repair of DNA adducts in liver cells.

Genotoxic and cytotoxic effects of 4-aroyl-1-nitrosohydrazine-carboxamides on O6-alkylguanine-DNA alkyltransferase-positive and -negative human cell lines

Five different representatives (I-V) of a new class of bifunctional alkylating agents, the 4-aroyl-1-nitrosohydrazinecarboxamides ("nitrososemicarbazides"), were evaluated for their potential interaction with DNA and for their cytotoxic activity in vitro to O6-alkylguanine-DNA alkyltransferase-positive (Mer+) and -negative (Mer-) human cell lines. The HeLa MR cell line (Mer-) showed up to 20-fold higher sensitivity at IC50 (dose that inhibits colony formation by 50%) to agents I-V than did the HeLa S3 cell line (Mer+) in a colony-formation assay. These data were compared to those obtained by treatment of the two cell lines with carmustine, a currently used antitumor drug. In Mer+ cells comparable results to those with carmustine were obtained with compounds III, IV and V; in Mer- cells compounds I and II showed nearly the same effects as carmustine. Whether compounds I-V produce DNA strand breaks and/or DNA-protein cross-links was investigated using an alkaline filter elution technique. In this assay all compounds produced DNA single-strand breaks; no correlation could be detected between the strand breakage frequency and cytostatic, mutagenic and antitumor activity.

Mutagenicity of coal-dust and smokeless-tobacco extracts in Salmonella typhimurium strains with differing levels of O-acetyltransferase activities

Epidemiological studies have indicated an increased incidence of gastric neoplasia in coal miners. Because smokeless tobacco use is prevalent in the mining industry, nitrites or other components of these products may be etiologically associated with these gastric neoplasms. In this study both nitrosated and non-nitrosated coal-dust (from West Virginia and New Mexico) as well as smokeless-tobacco (snuff and chewing tobacco) extracts were examined for the presence of aromatic amines and nitroarenes by comparing the activities of these extracts in the pre-incubation variant of the Ames assay. Salmonella strains with differing O-acetyltransferase activities (TA98 and YG1024) were utilized in this investigation. The results of the examination of the coal-dust extracts indicated positive activity only in the nitrosated extracts. Both nitrosated extracts elicited an increased number of revertants (2-4-fold) on YG1024 without S9 in comparison to TA98, suggesting the presence of nitroarenes in these extracts. Additionally, the nitrosated West Virginia coal extract showed higher levels of activity on YG1024 with S9, indicating the possible presence of aromatic amines in this complex mixture. The non-nitrosated smokeless-tobacco extracts showed activity only on YG1024 in the presence of S9, with the highest amount of activity occurring in the snuff sample. Except for the chewing-tobacco extract on TA98 without S9, positive activity was found in both nitrosated tobacco extracts on YG1024 and TA98. As with the coal extracts, the presence of nitroarenes was inferred for these nitrosated materials. A comparative study of the non-nitrosated snuff extract across 5 tester strains with varying sensitivities to aromatic amines and nitroarenes (TA98NR, TA98/1,8-DNP6, TA98, YG1021 and YG1024) indicated that aromatic amines were a probable source of the mutagenic activity. The curing process and/or the addition of certain flavorants are potential sources of the mutagenic aromatic amines suggested to be present in the non-nitrosated snuff extract. These findings are consistent with an etiologic role supplementary to the nitroso compounds for mutagenic nitroarenes and aromatic amines in the development of gastric neoplasia in coal miners.

Report from working group on in vitro tests for chromosomal aberrations

The following summary represents a consensus of the working group except where noted. The items discussed are listed in the order in which they appear in the OECD guideline (473) for easy reference. Metabolic activation. S9 from animals induced either with Aroclor 1254 or with the combination of phenobarbital with beta-naphthoflavone is acceptable, and other systems could be used with suitable justification. Exposure concentrations. The upper limit of testing should be 10 mM (or 5 mg/ml where molecular weight is not known or mixtures are being tested), whichever is lower. Where this limit is inappropriate the investigator should give detailed justification of the choice of top concentration. Cytotoxicity should be measured not only in range-finding tests but also concurrently with the assay for chromosomal aberrations. Cytotoxicity should be assessed by measurements of cell growth such as cell counts or confluence estimation. Mitotic index data alone are not a sufficient measure of cytotoxicity, except in the case of blood cultures for which other methods are impractical. Cytotoxicity at the top dose should be greater than 50% of concurrent negative/solvent controls, if this can be achieved without exceeding a concentration limit of 10 mM or 5 mg/ml. There should be at least three concentrations scored for aberrations (each with and without S9), covering a toxicity range down to a concentration giving little or no cytotoxicity. This will usually mean that the concentrations scored will be quite closely spaced. It was not possible to reach a consensus on the issue of solubility limits. The group did not agree on whether (a) solubility rather than cytotoxicity should be the limiting factor, such that only one top dose with evident precipitate should be scored even if toxicity is not observed, or (b) several concentrations with evident precipitate should be scored for aberrations if this were necessary to obtain cytotoxicity. It was agreed that evidence of precipitation should be determined in the final culture medium. Controls. Concurrent positive controls are required but the working group thought it inappropriate to specify the control chemicals or the degree of response that should be obtained, leaving it up to the test laboratory to demonstrate that the system was working adequately based on historical data within the laboratory. It is not necessary to include both negative and solvent controls concurrently with the aberration test; solvent controls alone are acceptable provided that the laboratory has data to demonstrate that there is no effect of the solvent on baseline values. Preparation of cultures.(ABSTRACT TRUNCATED AT 400 WORDS)

Alkyltransferase transgenic mice: probes of chemical carcinogenesis

Transgenic mice expressing DNA-repair genes are an instructive model with which to study the protective role of DNA-repair pathways in both spontaneous and chemical carcinogenesis. Of particular interest in chemical carcinogenesis is the DNA-repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) which repairs O6-alkylguanine-DNA adducts. Transgenic mice carrying expression constructs for the alkyltransferase gene--either the human MGMT cDNA or the bacterial ada gene--express increased levels of alkyltransferase and have increased capacity to remove O6-methylguanine-DNA adducts. Protection from the DNA damaging effects of N-nitroso compounds occurs specifically in the cells and tissues in which the alkyltransferase transgene is expressed. For instance, mice carrying the PEPCKada construct have increased alkyltransferase in the liver and more rapid removal of O6methylguanine-DNA adducts. The protective effect is noted in hepatocytes, which express PEPCK-linked genes, not in nonparenchymal cells of the liver, which do not. Other tissues that express the transgene in the various models include the thymus, spleen, testes, muscle, stomach and brain. Mice expressing the human alkyltransferase in the thymus have a reduced incidence of thymic lymphomas following exposure to methyl nitrosourea (MNU), evidence of a role for this DNA-repair protein in protection from carcinogenesis due to N-nitroso compounds. Protection has also been observed in the induction of hepatic tumors by N-nitroso-dimethylamine (NDMA). These models will be used to identify whether overexpression of a single DNA-repair gene can block the carcinogenic process of N-nitroso compounds in many different tissues.

Strand scission in DNA induced by S-nitrosothiol with hydrogen peroxide

Strand breaks can be produced in pBluescript plasmid DNA by S-nitrosothiol and H2O2 in the presence of a metal chelator, diethylenetriaminepentaacetic acid. S-Nitrosothiols with a wide range of stability were found to be active as a DNA cleaver in the presence of H2O2. Strand breaks were temperature dependent, occurring more rapidly at higher temperature. Sodium azide and mannitol inhibited S-nitrosothiol/H2O2-induced strand breaks in DNA. Catalase inhibited damage to DNA in a concentration-dependent manner whereas both superoxide dismutase and a yeast protector protein did not prevent damage to DNA. It is suggested that observed strand breaks in DNA are mediated by hydroxyl radicals arising from the reaction between H2O2 and thiyl radicals generated by homolytic decomposition of S-nitrosothiol.

Experimental evidence for inhibition of N-nitroso compound formation as a factor in the negative correlation between vitamin C consumption and the incidence of certain cancers

Ascorbic acid (ASC) consumption is negatively correlated with the incidence of certain cancers. This is a review and update of the theory, which has recently been neglected, that this negative correlation is due to ASC inhibition of in vivo nitrosation. The review covers the older literature on ASC inhibition of carcinogenesis by nitrite administered with amines or amides and more recent studies on ASC inhibition of nitrosation by bacteria, nitrogen oxides, and activated macrophages; the role of oxygen in ASC inhibition of gastric nitrosation; ASC inhibition of N-nitrosoproline formation in subjects from areas with high incidences of certain cancers; dose and temporal relationships between ASC and in vivo nitrosation in humans; the role of substances other than ASC in the inhibition of nitrosation by vegetables and fruits; and the active secretion of ASC into the human stomach.