Induction Of Unscheduled DNA Synthesis Research Articles

SAR modeling of genotoxic phenomena: the consequence on predictive performance of deviation from a unity ratio of genotoxicants/non-genotoxicants

SAR approaches to the study of genotoxic phenomena are finding increased applications. However, the data being modeled are frequently not considered optimal due to the small size of the dataset and an uneven distribution of genotoxicants and non-genotoxicants in the dataset. The effects of such imbalances on the performance of one SAR approach were investigated with respect to the modeling of the induction of unscheduled DNA synthesis in rat hepotocytes and of sister chromatid exchanges and chromosomal aberrations in cultural CHO cells.The analyses indicate that if genotoxicants exceed non-genotoxicants, the performance of the SAR model can be improved if the dataset is supplemented with physiological chemicals which are assumed to be non-genotoxicants. On the other hand, if non-genotoxicants exceed genotoxicants, it was found that the predictive performance of the resulting SAR model is not improved by removal of genotoxicants from the dataset to achieve a ratio of genotoxicants/genotoxicants of unity.Overall, the present analyses did not result in the development of SAR models of greatly increased predictivity. Conceivably, for the particular datasets and SAR paradigm the limit of predictivity has been reached. The possibility of investigating the use of a “battery” of SAR paradigms should be considered.

Antigenotoxic, antimutagenic and ROS scavenging activities of a Rhoeo discolor ethanolic crude extract

Rhoeo discolor is a legendary plant used for treatment of superficial mycoses in Mexican traditional medicine. Despite its extended use, it is not known whether it has side-effects. An ethanolic crude extract from Rhoeo discolor was prepared, its mutagenic capacity was investigated by the Ames test, and its genotoxic activity in primary liver cell cultures using the unscheduled DNA synthesis assay. This extract was not mutagenic when tested with Salmonella typhimurium strains TA97, TA98 and TA100, and it did not elicit unscheduled DNA synthesis in hepatocyte cultures. In addition, we explored the antimutagenic and antigenotoxic activities of the extract and its ROS scavenger behaviour. Our results show that Rhoeo extract is antimutagenic for S. typhimurium strain TA102 pretreated with ROS-generating mutagen norfloxacin in the Ames test, and protects liver cell cultures against diethylnitrosamine induction of unscheduled DNA synthesis even at 1.9 ng per dish, which was the lowest dose tested. A free radical scavenging test was used in order to explore the antioxidant capacity of Rhoeo extract, as compared with three commercial well-known antioxidants quercetin, ascorbic acid and tocopherol. Rhoeo extract showed less radical scavenging effect than quercetin, but similar to that of α-tocopherol and more than ascorbic acid. It is important to note that this extract was neither mutagenic in S. typhimurium nor genotoxic in liver cell culture, even at concentrations as high as four- and 166-fold of those needed for maximal antimutagenic or chemoprotective activities, respectively.

Induction of unscheduled DNA synthesis in hairless mouse epidermis by 8-methoxypsoralen plus ultraviolet A (PUVA).

Induction of unscheduled DNA synthesis (UDS) by 8-methoxypsoralen (8-MOP) plus ultraviolet A (UV-A) (PUVA) was investigated in the epidermis of female hairless mice by means of an in vivo--in vitro assay using a liquid scintillation counting method. Groups of three to five 8-week-old female hairless mice had 8-MOP applied once onto two areas of the back after stripping of the stratum corneum with adhesive tape to enhance skin penetration, and were irradiated with UV-A. Skin samples were taken and cultured in a medium containing [3H]thymidine with or without hydroxyurea (HU) for 2 hr. DNA of the epidermis was extracted, and the incorporation of [3H]thymidine into DNA and the DNA content were determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS was judged in terms of the UDS index [(the ratio of DNA synthesis in the presence of HU to that in its absence) x 100]. In a time-course study, the UDS index was increased at 1, 2 and 24 hr after 1 x 10(5) J/m2 UV-A irradiation with 0.001% 8-MOP, reaching the maximum level at 24 hr. In a dose-response study, it was significantly increased at the dose of 1 x 10(5) J/m2 of UV-A at 24 hr with 0.001% 8-MOP, but showed no significant change at the doses of 0.5 x 10(5), 2 x 10(5) and 4 x 10(5) J/m2. In a further study on the effect of varying the dose of 8-MOP, the UDS index was significantly increased at 0.001 and 0.002% 8-MOP at 24 hr after 1 x 10(5) J/m2 UV-A irradiation, reaching the maximum level with 0.002% 8-MOP. The increase of the UDS index in these studies was less than 3-fold. These results show that PUVA causes a small induction of UDS, which might be due to slow DNA excision repair over a long period.

Induction of Unscheduled DNA Synthesis in Primary Human Urothelial Cells by the Mycotoxin Ochratoxin A

Ochratoxin A (OTA) is a widespread contaminant in human staple food. Exposure of humans to this mycotoxin is a matter of concern because OTA is a known rodent carcinogen. As the urothelium is one target tissue of this mycotoxin, primary cultured human urothelial cells (HUC) from adults and children were used to analyze the induction of unscheduled DNA synthesis (UDS) by OTA. HUC were isolated from the ureters or renal pelves of two nephrectomized adults and of two children with ureteropelvic junction stenosis and cultured under serum-free conditions. After a confluency of 70-80% was reached, cell proliferation was suppressed by arginine-deficient medium (ADM), and UDS was assessed autoradiographically by 3H-thymidine incorporation upon exposure to OTA (10-2000 nM), ethyl methanesulfonate (EMS, 5 mM, positive control), or dimethyl sulfoxide (DMSO, 0.2%, solvent control). In control cultures the level of UDS was low. Exposure to EMS resulted in an induction of UDS (2-to 5-fold compared to control), thus allowing the sensitive detection of repair resulting from induction of DNA lesions in all four specimens, and demonstrating that repair of EMS-induced DNA lesions can take place under the chosen culture conditions. In two HUC cultures derived from adults, a significant induction of UDS was observed in the concentration range of 50-500 nM OTA. The highest fraction of cells in repair (CIR) was found at 50 nM OTA for the HUC from the older male (50% CIR). The maximum response in the other specimens from the adult female and the 7-year-old boy were seen at OTA concentrations of 500 and 250 nM, respectively. In contrast to all other specimens, no significant induction of UDS by OTA was found in the HUC cultures derived from an infant's urothelium. Signs of cytotoxicity were observed above 500 nM OTA in all cultures. The varying susceptibility toward OTA observed in vitro may hint at varying predispositions of individuals in vivo.

Induction of unscheduled DNA synthesis in hairless mouse epidermis by ultraviolet light.

The activity of ultraviolet (UV) light to induce unscheduled DNA synthesis (UDS) was investigated in hairless mouse epidermis by means of an in vivo-in vitro assay using a liquid scintillation counting method. Groups of three to five 8-week-old female hairless mice were irradiated with UV-B or UV-A, then skin samples were taken and cultured individually in medium containing [3H]thymidine with or without hydroxyurea (HU) for 2 hr. DNA of the epidermis was extracted, and incorporation of [3H]thymidine and the DNA content were determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS was judged in terms of the UDS index [(the ratio of DNA synthesis in the presence of HU to that in its absence) x 100]. UV-B increased the UDS index 1 hr after irradiation of 500 J/m2, which corresponds to approximately 1 minimal erythema dose or 1 minimal edema dose, and showed a dose-dependent increase up to 17-fold in the UDS index at irradiation doses of 500 to 2,000 J/m2. In a time-course study, UV-B also increased replicative DNA synthesis (RDS) 48 hr after irradiation at 1,000 J/m2. On the other hand, UV-A did not increase the UDS index at irradiation doses of 2 x 10(5) to 8 x 10(5) J/m2. These results show that induction of UDS by UV irradiation depends on wavelength and an increase of RDS in the epidermis exposed to UV-B irradiation appears after induction of UDS.

Effect of aging on unscheduled DNA synthesis induction by UV-B irradiation in hairless mouse epidermis.

Age-related induction of unscheduled DNA synthesis (UDS) by ultraviolet B (UV-B) irradiation was investigated in the epidermis of female hairless mice by means of an in vivo--in vitro assay using a liquid scintillation counting method. Skin samples were taken and cultured in a medium containing [3H]thymidine with or without hydroxyurea (HU) for 2 hr. DNA of the epidermis was extracted, and the incorporation of [3H]thymidine into DNA and the DNA content were determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS by UV-B was judged in terms of the value of the UDS index calculated as a percentage of the respective unexposed control value taken as 100% [the UDS index is given by (the ratio of DNA synthesis in the presence of HU to that in its absence) x 100]. DNA synthesis both in the presence and absence of HU decreased with age [12 months old (M) < 8 M < 4 M < 2 M < 1 M)], concomitantly with a small but significant increase of UDS index. The decrease was high in the younger age groups and moderate in the older age groups. UV-B increased the UDS index approximately 14-, 12- and 9-fold at 1 hr after 1,000 J/m2 irradiation in 1 M, 2 M and 12 M mice, respectively, and these increases were partly reversed at 4 hr after irradiation. UV-B also increased the UDS index approximately 25-, 24- and 21-fold at 1 hr after 4,000 J/m2 irradiation for each age group. However, there was no statistically significant age-related difference in the magnitude of the UDS index after irradiation of UV-B. These results show that replicative DNA synthesis decreases with age, whereas DNA repair capacity after UV-B irradiation does not change with age under these conditions.

Induction of unscheduled DNA synthesis in hairless mouse epidermis by skin carcinogens.

Induction of unscheduled DNA synthesis (UDS) in hairless mouse epidermis by six chemicals was determined in an in vivo-in vitro assay by using a liquid scintillation counting method. Test chemicals were applied once onto two areas of the back of female hairless mice after stripping of the stratum corneum with adhesive tape to enhance skin penetration. After exposure, the skin samples were taken and cultured in a medium containing [3H]thymidine with or without hydroxyurea (HU, an inhibitor of replicative DNA synthesis). DNA of the epidermis was extracted, and incorporation of [3H]thymidine into DNA and the DNA content was determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS by chemicals was judged by calculation of the UDS index [(the ratio of DNA synthesis in the presence of HU to that in its absence) x 100]. A good correlation between UDS induction and organ specificity of carcinogens was observed. 4-Nitroquinoline 1-oxide, a skin carcinogen used as a positive control, induced a dose-dependent increase in the UDS index of approximately 12-fold at 2 hr after exposure, while 1,2-epoxydodecane, a non-skin carcinogen applied as a negative control, did not increase the UDS index. Four other skin carcinogens induced dose-dependent increases in the UDS index; N-methyl-N'-nitro-N-nitrosoguanidine and diepoxybutane at 2 hr after exposure, and 7,12-dimethylbenz[a]anthracene and benzo[a]pyrene at 24 hr after exposure. The results suggest that UDS is a good marker of the genotoxicity of skin carcinogens.

Differential sensitivities of spermatogonial and postspermatogonial cell stages of the mouse to induction of unscheduled DNA synthesis by ethyl- and methyl-nitrosourea,

An autoradiographic procedure was used to measure unscheduled DNA synthesis (UDS, DNA repair synthesis) in spermatogonial and postspermatogonial cell stages of mice after treatment with two doses of N-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU). Significant levels of UDS were measured in type A spermatogonia, meiotic spermatocytes, round spermatids, and early elongating spermatids but not in mature spermatids. The extent of UDS varied according to the germ cell stage and the dose. At equimolar concentrations, MNU was more efficient than ENU in eliciting a UDS response in all germ cells. After ENU treatment, type A spermatogonia showed the highest UDS response, while round and elongating spermatids showed the lowest. After MNU treatment, pachytene spermatocytes exhibited the highest UDS response while type A spermatogonia showed the lowest. The high UDS response of type A spermatogonia to ENU parallels the well-known high mutational sensitivity of spermatogonia to this chemical. Similarly, the high UDS response observed in meiotic spermatocytes and early spermatid stages after MNU treatment correlates with the high mutational sensitivity of postspermatogonial stages to MNU. Thus, the present results, like the specific locus mutation studies, indicate that ENU and MNU each has a unique effect on the spermatogenic cells. This effect is likely due to the different mechanism of action of ENU and MNU at the level of DNA and also to the physiological differences between different germ-cell stages. Teratogenesis Carcinog. Mutagen. 19:339-351, 1999. Published 1999 Wiley-Liss, Inc.

Differential sensitivities of spermatogonial and postspermatogonial cell stages of the mouse to induction of unscheduled DNA synthesis by ethyl‐ and methyl‐nitrosourea,

Differential sensitivities of spermatogonial and postspermatogonial cell stages of the mouse to induction of unscheduled DNA synthesis by ethyl‐ and methyl‐nitrosourea,

Effect of Some Cooked Food Mutagens on Unscheduled DNA Synthesis in Cultured Precision-cut Rat, Mouse and Human Liver Slices

Precision-cut liver slices were prepared from male Fischer 344 rats, female CDF 1 mice and humans (both male and female subjects). Liver slices were cultured for 24 hr in medium containing [ 3H]thymidine and either PhIP, IQ, MeIQ, MeIQ x, Glu-P-1 or Trp-P-1, and then processed for autoradiographic evaluation of unscheduled DNA synthesis (UDS). All six cooked food mutagens examined produced concentration-dependent increases in UDS in human liver slices. PhIP was the most potent compound examined, followed by MeIQ x, IQ and then MeIQ, Glu-P-1 and Trp-P-1. Significant increases in UDS were observed with PhIP, IQ and MeIQ x at concentrations as low as 5 μ m in the culture medium. The same rank order of potency was not apparent in either rat or mouse liver slices. In rat liver slices only MeIQ significantly induced UDS, although positive results were obtained with two other genotoxins, namely 2-acetylaminofluorene and aflatoxin B 1. Apart from MeIQ x, all the cooked food mutagens produced significant increases in UDS in mouse liver slices. This study demonstrates the usefulness of precision-cut liver slices to evaluate species differences in xenobiotic-induced genotoxicity. Both marked compound and species differences in induction of UDS were observed. The data provide further evidence that dietary cooked food mutagens are potential human carcinogens.

Accumulation of p53 induced by the adenovirus E1A protein requires regions involved in the stimulation of DNA synthesis.

It has been known for some time that expression of the 243-residue (243R) human adenovirus type 5 (Ad5) early region 1A (E1A) protein causes an increase in the level of the cellular tumor suppressor p53 and induction of p53-dependent apoptosis. Deletion of a portion of conserved region 1 (CR1) had been shown to prevent apoptosis, suggesting that binding of p300 and/or the pRB retinoblastoma tumor suppressor and related proteins might be implicated. To examine the mechanism of the E1A-induced accumulation of p53, cells were infected with viruses expressing E1A-243R containing various deletions which have well-characterized effects on p300 and pRB binding. It was found that in human HeLa cells and rodent cells, complex formation with p300 but not pRB was required for the rise in p53 levels. However, in other human cell lines, including MRC-5 cells, E1A proteins which were able to form complexes with either p300 or pRB induced a significant increase in p53 levels. Only E1A mutants defective in binding both classes of proteins were unable to stimulate p53 accumulation. This same pattern was also apparent in p53-null mouse cells coinfected by Ad5 mutants and an adenovirus vector expressing either wild-type or mutant human p53 under a cytomegalovirus promoter, indicating that the difference in importance of pRB binding may relate to differences between rodent and human p53 expression. The increase in p53 levels correlated well with the induction of apoptosis and, as shown previously, with the stimulation of cellular DNA synthesis. Thus, it is possible that the accumulation of p53 is induced by the induction of unscheduled DNA synthesis by E1A proteins and that increased levels of p53 then activate cell death pathways.

Comparative mutagenicity testing of ceftiofur sodium: III. Ceftiofur sodium is not an in vivo clastogen

Ceftiofur has been previously shown (Aaron et al., 1995a Mutation Res., this issue) to produce chromosome aberrations following extended treatment of Chinese hamster ovary cells in culture at high doses in the absence of S9 activation. The experiments described here address the relevance of this finding in vivo. Ceftiofur failed to induce aberrations in the bone marrow of CD-1 mice following intraperitoneal injection. Furthermore, ceftiofur did not induce micronucleated polychromatic erythrocytes in mouse bone marrow. In the experiments reported here no evidence of the induction of unscheduled DNA synthesis in the liver of rats treated in vivo was seen following oral exposure to high doses of the drug. However, the compound was foud to reach the bone marrow compartment in these experiments. Thus, the negative finding suggests that the in vitro finding of chromosome aberration induction by ceftiofur is not a cause of concern. This assertion is supported by mechanistic studies (Aaron et al., 1995b, Mutation Res., this issue) which show the cause of the in vitro findings to be most likely due cell cycle delay and by the observation that ceftiofur is rapidly metabolized in mammalian systems in vivo but not in the artificial conditions that occur in the in vitro experiments.

Enhanced cyto- and genotoxicity of tetracycline in Wilson disease fibroblasts

Tetracycline (TC) exerts DNA damaging properties which are accelerated in the presence of copper(II). Thereby, reactive oxygen species are generated. We investigated, if copper-accumulating cells show a higher sensitivity to TC compared to normal cells. Fibroblasts with an increased copper content were derived from patients of two genetic disorders, Wilson disease (WD) and Menkes disease (MD). Cytotoxic and genotoxic effects of TC were investigated in different human fibroblasts. The inhibition of cell growth by TC was measured in two normal fibroblast lines, fibroblast lines of two patients with WD and one patient with MD. While TC inhibited cell growth at similar concentrations in normal fibroblasts and the MD fibroblasts, the WD cells were much more sensitive. Furthermore, an increased inhibition of DNA synthesis and an enhnaced induction of unscheduled DNA synthesis (UDS) was found in WD cells after a TC-treatment compared to normal cells.

Induction of unscheduled DNA synthesis by skin carcinogens in hairless mouse epidermis

Induction of unscheduled DNA synthesis by skin carcinogens in hairless mouse epidermis

In vivo short-term assays for tumor initiation and promotion in the glandular stomach of Fischer rats

Here we summarize the data on 55 compounds tested in in vivo short-term assays for tumor-initiating and tumor-promoting activity in the glandular stomach of male Fischer (F344) rats. Most of the data has been previously published. Tumor-initiating activity was assayed by measuring the induction of unscheduled DNA synthesis (UDS) and DNA single strand scission; tumor-promoting activity was assayed by measuring the induction of ornithine decarboxylase (ODC) activity, increased replicative DNA synthesis (RDS), and of c- fos and c- myc oncogene expression. The compounds were orally administered. Twenty-nine compounds were tested for UDS. Eight were positive, including 5 glandular stomach carcinogens; 16 were negative, including 5 liver carcinogens; and 5 were equivocal. Twenty compounds were tested for DNA single strand scission. Twelve were positive, including 6 glandular stomach carcinogens; 7 negative, including 2 liver carcinogens; and 1 was equivocal. Thirty-two compounds were tested for RDS. Twenty-six were positive, including 8 glandular stomach carcinogens and 6 glandular stomach tumor-promoters; 4 were negative, including 3 liver carcinogens and a stomach irritant; and 2 were equivocal. Forty-five compounds were tested for ODC. Thirty-seven were positive, including 8 glandular stomach carcinogens and 6 glandular stomach tumor promoters; 7 were negative, including 3 liver carcinogens; and one was equivocal. All glandular stomach carcinogens and tumor-promoters examined were positive in both RDS and ODC. Two compounds were tested for c- fos and c- myc expression; one was a glandular stomach carcinogen and one was a glandular stomach tumor promoter, and both were positive. In addition, 2 compounds inhibited the increase in RDS induced by the tumor promoter NaCl, suggesting anti-tumor-promoter activity. Thus these assays are useful for assessing potential tumor-initiating and tumor-promoting activity in the rat glandular stomach.

Detection of polycyclic aromatic hydrocarbon exposure damage using different methods in laboratory animals.

Benzo[a]pyrene, benzo[b]fluoroanthene and dibenzo[a,h]anthracene dissolved in a 1:2 mixture of dimethylsulphoxide (DMSO) and water were administered to two groups of female mice, each group containing 15 mice. The doses were administered orally (via gavage) at the respective rates of 1 and 100 micrograms kg-1 body weight five times per week for a period of 9 weeks. The influence of the polycyclic aromatic hydrocarbons (PAHs) was determined using the following methods: determination of DNA-PAH adducts, of chromosome injuries (micronucleus test), of induction of repair using the unscheduled DNA synthesis (UDS) test, and by examination of the DNA structure after nucleoid sedimentation. All the methods investigated provided evidence of a significant effect resulting from exposure to PAHs on the parameters examined. Following chronic exposure to PAHs, the formation of DNA-PAH adducts and injury to the genetic material, as well as the appearance of micronuclei (micronucleus test), the induction of unscheduled DNA synthesis (UDS test) and mutation of the DNA structure (nucleoid sedimentation), were demonstrated. The described methods therefore provide a means for the detection of genetic damage caused by PAH exposure in humans.

Effects of dietary restriction on induction of unscheduled DNA synthesis (UDS) and replicative DNA synthesis (RDS) in rat liver

The effects of dietary restriction on the induction of unscheduled DNA synthesis (UDS) and replicative DNA synthesis (RDS) were studied in the hepatocytes of F344 rats exposed in vivo to dimethylnitrosamine (DMN) or CCl 4. The animals were given food ad libitum, a restricted amount of food (4 g/rat/overnight) or no food. Hepatocytes were isolated 2 h after oral administration of DMN at a dose of 5 mg/kg body weight and 48 h after oral administration of CCl 4 at a dose of 400 mg/kg body weight, and incubated for 4 h in Williams' medium E supplemented with either [ 3H]thymidine for UDS or 5-bromodeoxyuridine for RDS. UDS was determined by autoradiography and RDS was determined by the immunoenzymatic staining method. The background levels of UDS (net grains/nucleus) and RDS (cells in S phase) in control were −12.4 and 0.64% for ad libitum feeding, −6.8 and 0.04% for restricted feeding, and −8.1 and 0% for fasting. UDS induced by DMN and RDS induced by CCl 4 were 19.4 and 3.3% for ad libitum feeding, 34.5 and 10.4% for restricted feeding, and 47.8 and 15.1% for fasting. DMN demethylase activity in rat liver was also found to increase with dietary restriction. These results indicate that dietary restriction modulates the responses of UDS and RDS in the liver of rats.

Pathways for the mutagenesis of 1-nitropyrene and dinitropyrenes in the human hepatoma cell line HepG2.

The mutagenicity, metabolism, DNA adduction and induction of unscheduled DNA synthesis (UDS) of 1-nitropyrene and 1,8-dinitropyrene were investigated in the human hepatoma cell line HepG2. Previous results had demonstrated that 1-nitropyrene was both mutagenic at the hgprt locus and induced UDS in these cells. In the present study, we find that the dinitropyrenes, although highly mutagenic in Salmonella typhimurium, are not mutagenic and do not induce UDS in the HepG2. Although the rate of 1,8-dinitropyrene nitroreduction was less than that of 1-nitropyrene nitroreduction, this did not explain the lack of mutagenicity and UDS induction by the dinitropyrenes. Therefore, it is proposed that the arylhydroxylamine O-esterificase is not expressed in these cells. Since cytochrome P450-mediated C-oxidation is the predominant metabolic pathway in vivo, we sought to determine if an increase in the ratio of cytochrome P450-mediated C-oxidation over nitroreduction would result in increased or decreased DNA adducts in the HepG2. The administration of 2.5 microM 3-methylcholanthrene to the HepG2 increased the ratio of C-oxidation/nitroreduction from 2.8 +/- 1.9 to 50.4 +/- 46.1. This was accompanied by a decrease in the C8-guanyl adduct of 1-nitropyrene (via nitroreduction) from 18.7 +/- 7.0 to 4.8 +/- 1.7 fmoles/micrograms DNA, without any further increase in other 1-nitropyrene DNA adducts. These results suggest that the cytochrome P450-mediated metabolism of 1-nitropyrene to epoxides, phenols, and dihydrodiols is not an activation pathway in the HepG2 cells, and may explain the weak carcinogenicity of 1-nitropyrene in vivo, where cytochrome P450-mediated C-oxidation predominates.

Effects of sterigmatocystin and T-2 toxin on the induction of unscheduled DNA synthesis in primary cultures of human gastric epithelial cells.

Primary cultures of human gastric epithelial cells were tested for induction of unscheduled DNA synthesis (UDS) by sterigmatocystin (ST) and T-2 toxin. Autoradiographic results indicated that ST (10(-6)-10(-4)M) induced UDS in the presence of S9 activation system. The repair rates were 24-91% (net grains > or = 3) and 2-71% (net grains > or = 5). T-2 toxin did not induce UDS in this study.

Propane 2-nitronate is the major genotoxic form of 2-nitropropane

The mutagenicity of 2-nitropropane in Salmonella typhimurium (strain TA100) was proportional to the pH (range 6.1–9.1) of the medium used for pre-incubation of the agent and for incubation of the agent with the Salmonella. The mutagenicity correlated with an enhanced rate of tautomerisation to propane 2-nitronate at relatively high pH as measured by high performance liquid chromatography. Both the mutagenicity in Salmonella typhimurium (strains TA100 and TA102) and the rate of tautomerisation to the nitronate was lower with 2-deutero-2-nitropropane than with non-deuterated 2-nitropropane. Furthermore, 2-deutero-2-nitropropane was less potent in the induction of unscheduled DNA synthesis in rat hepatocytes over a 4-h period. Propane 2-nitronate therefore appears to be pivotal in the causation of the genetic toxicity of 2-nitropropane. The presence of hepatocytes enhanced nitronate production from 2-nitropropane suggesting a contribution from hepatic enzymes in the tautomerisation reaction.