Mutagenicity and DNA-modifying activity of 2-nitropropane

Hazard characterization of carcinogenicity, mutagenicity, and reproductive toxicity for short chain primary nitroalkanes

Nitroalkanes are organic aliphatic hydrocarbon compounds with a nitro moiety that are commonly used as solvents or intermediates to synthesize a variety of organic compounds due to their inherent reactivity. In June 2020, a harmonized classification and labeling (CLH) proposal was submitted to the European Chemicals Agency (ECHA) for the following harmonized carcinogenicity, mutagenicity, and reproductive toxicity ("CMR") classifications for nitromethane (NM), nitroethane (NE), and 1-nitropropane (1-NP): NM Carc. 1B and Repr. 1B; NE Repr. 1B; and 1-NP Repr. 2. In this assessment, a weight of evidence (WoE) evaluation of studies on animal carcinogenicity and reproductive and developmental toxicity, genotoxicity, and mode of action for these three nitroalkanes was performed to critically assess the relevance of the proposed CMR classifications. Overall, the WoE indicates that NM, NE, and 1-NP are not carcinogenic, genotoxic, nor selective reproductive or developmental toxicants. Based on our analysis, classifying NM, NE, and 1-NP as Category 2 reproductive toxicants is most appropriate. Furthermore, not classifying NE and 1-NP with respect to their carcinogenicity is appropriate based on the available studies for this endpoint coupled with negative results in genotoxicity studies, metabolism data, and in silico predictions. We determined that the classification for NM of Carc. 1B is not appropriate, based on the fact that rat mammary and harderian tumors are likely not relevant to humans and lung and liver tumors reported in mice were equivocal in their dose-response and statistical significance.

Toxicity and metabolism of nitroalkanes and substituted nitroalkanes

A series of low molecular weight nitro-containing compounds has recently been discovered to have a variety of biological activities including the reduction of anaerobic methane production in ruminant animals and activity against economically important human pathogens, including Salmonella sp. and shigella-toxin producing Escherichia coli . Although some of these nitrocompounds, nitroethane and 2-nitropropane, for example, have been industrial chemicals and synthetic intermediates for years, others such as carboxymethyl nitro-amino acid analogues are new to science and have not been previously described. The purpose of this paper is to review the toxicological profiles, especially as related to events occurring during metabolism and biotransformation, which contribute to toxicological end points of established nitroaliphatic compounds. It is hoped that by summarizing existing knowledge, an understanding of the activities and toxicological profiles of newly established nitrocompounds might be anticipated or adverse events associated with their use might be avoided.

Concordance between Results of Medium-term Liver Carcinogenesis Bioassays and Long-term Findings for Carcinogenic 2-Nitropropane and Non-carcinogenic1-Nitropropane in F344 Rats

This study was conducted to determine the concordance of results for a pair of structural isomers, 2-nitropropane (2-NP) and 1-nitropropane (1-NP), using the rat medium-term liver carcinogenesis bioassay (Ito test) and previously published long-term carcinogenicity tests. Male F344 rats were given a single intraperitoneal injection of DEN (200 mg/kg b.w.) to initiate hepatocarcinogenesis. After 2 weeks, they received per os 0, 0.8, 4 or 20 mg/kg/day of 2-NP or 1-NP six times a week and were subjected to two-thirds partial hepatectomy at week 3. Non-initiated groups receiving 0 or 20 mg/kg/day were also included. The animals were sacrificed for quantitative analysis of GST-P-positive foci at week 8. With the highest dose of 2-NP, significantly increased numbers and areas of GST-P-positive foci were demonstrated as compared with the respective control but were not noted with 1-NP. In the non-DEN-initiated groups, many small GST-P-positive foci of less than 0.2 mm in diameter were also induced in the rats treated with 2-NP at 20 mg/kg/day but were lacking with 1-NP. These results strongly support that 2-NP is a complete hepatocarcinogen with a potent initiation activity, whereas 1-NP is not.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Mechanism of metal-mediated DNA damage induced by metabolites of carcinogenic 2-nitropropane

2-Nitropropane (2-NP), a widely used industrial solvent, is carcinogenic to rats. To clarify the mechanism of carcinogenesis by 2-NP, we investigated DNA damage by 2-NP metabolites, N-isopropylhydroxylamine (IPHA) and hydroxylamine-O-sulfonic acid (HAS), using 32P-5'-end-labelled DNA fragments obtained from genes that are relevant to human cancer. In the presence of Fe(III) EDTA, both IPHA and HAS caused DNA damage at every nucleotide position without marked site preference. The damage was inhibited by free hydroxyl radical (-*OH) scavengers, catalase and deferoxamine mesilate, an iron chelating agent. These results suggest that the DNA damage was caused by -*OH generated via H(2)O(2) by both IPHA and HAS. In contrast, in the presence of Cu(II), IPHA frequently caused DNA damage at thymine. The Cu(II)-mediated DNA damage caused by IPHA was inhibited by catalase, methional and bathocuproine, a Cu(I)-specific chelator, suggesting the involvement of H(2)O(2) and Cu(I). These results suggest that the DNA damage induced by IPHA in the presence of Cu(II) was caused by a reactive oxygen species like the Cu(I)-hydroperoxo complex. On the other hand, HAS most frequently induced DNA damage at 5'-TG-3', 5'-GG-3' and 5'-GGG-3' sequences. Catalase and methional only partly inhibited the Cu(II)-mediated DNA damage caused by HAS, suggesting that the reactive oxygen species and another reactive species participate in this process. Formation of 8-oxodG by IPHA or HAS increased in the presence of metal ions. This study suggests that metal-mediated DNA damage caused by 2-NP metabolites plays an important role in the mutagenicity and the carcinogenicity of 2-NP.

Site-specifically located 8-amino-2'-deoxyguanosine: thermodynamic stability and mutagenic properties in Escherichia coli

2-Nitropropane (2-NP), an important industrial solvent and a component of cigarette smoke, is mutagenic in bacteria and carcinogenic in rats. 8-Amino-2'-deoxyguanosine (8-amino-dG) is one of the types of DNA damage found in liver, the target organ in 2-NP-treated rats. To investigate the thermodynamic properties of 8-amino-dG opposite each of the four DNA bases, we have synthesized an 11mer, d(CCATCG*CTACC), in which G* represents the modified base. By annealing a complementary DNA strand to this modified 11mer, four sets of duplexes were generated each containing one of the four DNA bases opposite the lesion. Circular dichroism studies indicated that 8-amino-dG did not alter the global helical properties of natural right-handed B-DNA. The thermal stability of each duplex was examined by UV melting measurements and compared with its unmodified counterpart. For the unmodified 11mer, the relative stability of the complementary DNA bases opposite G was in the order C > T > G > A, as determined from their -DeltaG degrees values. The free energy change of each modified duplex was lower than its unmodified counterpart, except for the G*:G pair that exhibited a higher melting transition and a larger -DeltaG degrees than the G:G duplex. Nevertheless, the stability of the modified 11mer duplex also followed the order C > T > G > A when placed opposite 8-amino-dG. To explore if 8-amino-dG opposite another 8-amino-dG has any advantage in base pairing, a G*:G* duplex was evaluated, which showed that the stability of this duplex was similar to the G*:G duplex. Mutagenesis of 8-amino-dG in this sequence context was studied in Escherichia coli, which showed that the lesion is weakly mutagenic (mutation frequency approximately 10(-3)) but still can induce a variety of targeted and semi-targeted mutations.

Mutagenic properties of the 8-amino-2'-deoxyguanosine DNA adduct in mammalian cells

The DNA adduct 8-amino-2'-deoxyguanosine (8-amino-dG) is found in liver DNA of rats treated with the hepatocarcinogen 2-nitropropane. Site-specifically modified oligodeoxynucleotides were used to explore the mutagenic potential of 8-amino-dG in simian kidney (COS-7) cells. Oligodeoxynucleotides (5'-TCCTCCTX1G2CCTCTC and 5'-TCCTCCTG1X2CCTCTC, X = dG or 8-amino-dG) with the lesion positioned at codon 60 or 61 of the non-coding strand of the human c-Ha- ras1 gene were inserted into single-stranded phagemid vectors and transfected into COS-7 cells. The progeny plasmid obtained was used to transform Escherichia coli DH10B. Transformants were analyzed by oligodeoxynucleotide hybridization and DNA sequencing to establish the mutation frequency and spectrum produced by the modified base. The correct base, dCMP, was incorporated preferentially opposite 8-amino-dG at X1and X2. When 8-amino-dG was at X1, targeted GNH2-->T transversions were detected, along with smaller numbers of GNH2-->A transitions and GNH2-->C transversions. When the adduct was at X2, only GNH2-->T transversions were observed. The frequencies of targeted mutation at X1and X2were 2.7 and 1.7%, respectively. Mutation frequency and mutagenic spectrum were sequence context dependent. In addition, non-targeted G-->T transversions, accompanied by some G-->A transitions, were detected 5' to 8-amino-dG when the lesion was at X2. We conclude that 8-amino-dG is a mutagenic lesion, generating G-->T and G-->C transversions and G-->A transitions in mammalian cells.

Sulfotransferase-mediated genotoxicity of propane 2-nitronate in cultured ovine seminal vesicle cells

2-Nitropropane (2-NP) is a well-known genotoxin and carcinogen in rat liver. Several metabolic pathways, particularly cytochrome P450-, peroxidase- and sulfotransferase-dependent ones, have been suggested to lead to the formation of DNA-reactive species from 2-NP. Because rat liver cells express most types of xenobiotic-metabolizing enzymes, the role of specific pathways in the metabolic activation of 2-NP is difficult to assess in these cells. We have therefore investigated the genotoxicity of 2-NP and its anionic form, propane 2-nitronate (P2N), in cultured ovine seminal vesicle (OSV) cells. OSV cells lack cytochrome P450-dependent monooxygenase activity, but express prostaglandin-H-synthase (PHS) and, as we found out, phenol sulfotransferase. The induction of DNA repair synthesis and specific DNA modifications served as indicators for the genotoxicity of 2-NP and P2N. Both forms strongly induced repair, P2N being more active than 2-NP. The secondary nitroalkanes nitrocyclopentane and nitrocyclohexane also induced repair, whereas 1-nitropropane and the reduction product of 2-NP, acetone oxime, did not. P2N also elicited the formation of the characteristic DNA modifications 'DX1' and 8-aminodeoxyguanosine and increased the level of 8-oxodeoxyguanosine residues in the DNA. Pretreatment of OSV cells with indomethacin, an inhibitor of PHS, affected neither the induction of repair nor the formation of the DNA modifications, and P2N was not a reducing substrate for the PHS-peroxidase activity. In contrast, the sulfotransferase inhibitor pentachlorophenol strongly reduced genotoxicity. The results show that cytochrome P450-dependent monooxygenases are not required for the metabolic conversion of secondary nitroalkanes or their nitronates into DNA-damaging products, nor is PHS involved in the metabolic activation. Instead, the data corroborate an essential role of sulfotransferase(s) in the genotoxicity and carcinogenicity of secondary nitroalkanes. Moreover, it is demonstrated for the first time that these compounds can be genotoxic in cells other than hepatocytes or hepatoma cells. This implies that in species other than the rat, organs other than the liver can be targets for the genotoxicity, and possibly carcinogenicity, of secondary nitroalkanes.

Denitrification of the genotoxicant 2-nitropropane: relationship to its mechanism of toxicity

1. The stabilities of the industrial chemical and constituent of cigarette smoke 2-nitropropane (2-NP) and its aci tautomer propane 2-nitronate (P2N) towards hepatic enzymes and proteins such as serum proteins and oxyhaemoglobin were investigated in vitro in biological (hepatocytes and subcellular liver fractions) and model systems (serum proteins, oxyhaemoglobin, methylene blue). 2. Denitrification of 2-NP, P2N and 2-deutero 2-nitropropane (2H-NP) occurred in murine hepatocytes significantly faster than in rat cells. For 2-NP the rates were 1271 +/- 167 versus 820 +/- 125 pmol nitrite x min-1 x 10(6) cells-1. 3. A similar observation was made in microsomes, where 2-NP denitrification was 1460 +/- 110 (mouse) versus 480 +/- 80 pmol nitrite x min-1 x mg protein-1 (rat). 4. The major NO2(-)-forming activity was found to be localized in the microsomal fraction. 5. Conversion of 2-NP into P2N, either chemically or enzymatically, was a prerequisite for rapid denitrification. 6. Serum proteins and oxyhaemoglobin proved to be capable of denitrifying P2N (198 +/- 24 pmol nitrite x min-1 x mg protein-1 and 7.1 +/- 1.0 nmol nitrite x min-1 x nmol HbO2(-1) respectively), but were much less active towards 2-NP (24 +/- 2 pmol nitrite x min-1 x mg protein-1 and none respectively). 7. Methylene blue decomposed 2-NP and P2N at rates of 11 +/- 3 and 192 +/- 4 pmol nitrite x min-1 x nmol, methylene blue-1 respectively. The dye also enhanced NO2- formation from P2N and 2-NP in the presence of hepatocytes or serum proteins, with a concomitant enhancement of both 2-NP and P2N toxicity. 8. The results presented report species differences in the denitrification rate of 2-NP and highlight the crucial nitro-aci tautomerism of 2-NP as a pivotal determinant of 2-NP toxicity.

Catalysis of nitro-aci tautomerism of the genotoxicant 2-nitropropane by cytosol from rodent and human liver

2-Nitropropane (2-NP) is a genotoxicant and hepatocarcinogen in rodents. Conversion to propane 2-nitronate (P2N), the anion of the tautomeric aci form of 2-NP, seems to be a pivotal part of the mechanism by which 2-NP causes its toxicity. We tested the hypothesis that the tautomeric equilibrium is influenced by enzymes in the liver, the target organ of 2-NP toxicity. Rat or mouse hepatocytes were incubated with 2-NP, P2N or the 2-NP isotopomer 2-deutero 2-nitropropane (2H-2-NP), which equilibrates with P2N much more slowly than 2-NP. Tautomers were analyzed by HPLC. The rates of conversion of 2-NP to P2N expressed as nmol P2N x (10(6) cells/ml)-1 x min-1 were 4.0 and 4.2 in the presence of hepatocytes from rats or mice, respectively, and 2.6 in the absence of cells. Production of 2-NP to P2N expressed as nmol 2-NP x (10(6) cells/ml)-1 x min-1 was increased from 6.1 in the absence of cells to 11.9 or 9.9 in the presence of hepatocytes from rats or mice, respectively. The rate of formation of P2N from 2H-2-NP as compared to 2-NP was characterised by a primary isotope effect of 3.4 and 3.8 in hepatocytes from rats and mice, respectively, contrasting with a value of 9.6 measured in medium omitting cells. When 2-NP was incubated with subfractions of rodent or human liver homogenate, production of P2N by cytosol was between 7.3 (mouse liver) and 28.1 times (human liver) higher than that observed in microsomes. Similarly generation of 2-NP from P2N by cytosol exceeded that in microsomes by a factor of two. Tautomerism in heat-activated cytosol, mitochondria or microsomes was not different from that in buffer only. The results suggest that the nitro-aci tautomerism of secondary nitroalkanes is catalysed by a hepatic enzyme which resides predominantly in the cytosol and may thus contribute to the generation of the toxic species via which 2-NP exerts its toxicity.

DNA modification and repair by 2-nitropropane is extensive in hepatocytes of rats compared to those of humans and mice

Hepatocytes from rats, mice or humans were exposed to either 2-nitropropane or propane 2-nitronate and the extent of unscheduled DNA synthesis (UDS) was measured. At a concentration of 2-nitropropane (0.1 mM) that produced a marked induction of UDS in rat hepatocytes, a negative or minimal response was seen in hepatocytes from mice and humans. A 10-fold higher concentration of 2-nitropropane was required in mouse hepatocytes for an equivalent UDS response to that seen in rat-liver cells. All three species showed UDS after exposure of hepatocytes to propane 2-nitronate at 0.1 mM although the effect in human cells was variable. In rat hepatocytes the induction of UDS by 2-nitropropane was not inhibited by the antioxidant dimethyl sulphoxide (1%). In these cells, 2-nitropropane produced an electrochemically active modified deoxynucleoside, similar to that reported to be formed in livers of rats which had received 2-nitropropane in vivo. A smaller but significant production of this altered deoxynucleoside was found in mouse hepatocytes but not in human hepatocytes treated identically. The level of 8-oxo-7,8-dihydrode-oxyguanosine was not increased in hepatocytes from any of the three species exposed to 2-NP under the same conditions. It is suggested that the UDS caused by 2-nitropropane may be in response to damage other than that produced by oxygen radicals.

Evaluation of secondary nitroalkanes, their nitronates, primary nitroalkanes, nitrocarbinols, and other aliphatic nitro compounds in the Ames Salmonella assay

The secondary nitroalkanes 2-nitropropane, 2-nitrobutane, 3-nitropentane and nitrocyclopentane, as well as their anionic forms (nitronates); the primary nitroalkanes 1-nitropropane, 1-nitrobutane, and 1-nitropentane and their respective nitronates; the nitrocarbinols 2-nitro-1-propanol, 2-nitro-1-butanol, 3-nitro-2-butanol, and 3-nitro-2-pentanol and their respective nitronates; 2-methyl-2-nitropropane, and 2-nitroso-2-nitropropane were tested in the Ames Salmonella assay using strains TA98, TA100 and TA102. Nitronates of the secondary nitroalkanes 2-nitropropane, 2-nitrobutane, 3-nitropentane, and nitrocyclopentane were significantly mutagenic in Salmonella strains TA100 and TA102 at 10-80 mumoles/plate, but the parent compounds were mutagenic at only a single dose level or were not mutagenic at all in the same dose range. The primary nitroalkanes and the nitrocarbinols were not mutagenic, or only marginally so, at the concentrations tested. The nitronates of the primary nitroalkanes and the nitrocarbinols reprotonated too rapidly under the conditions of the assay for adequate evaluation of mutagenicity. 2-Methyl-2-nitropropane was not mutagenic in strains TA100 and TA102; 2-nitroso-2-nitropropane was also not mutagenic in strains TA100 and TA102, but induced an equivocal mutagenic response in TA98. The positive Salmonella mutation data for the nitronates of the secondary nitroalkanes studied correlate very well with the very slow rate of reprotonation of secondary nitroalkane nitronates at pH 7.7 (Conaway et al. (1991) Cancer Res., 51, 3143), and provide further evidence that nitronates of secondary nitroalkanes, rather than the neutral parent forms with which they may be in equilibrium, are the more proximate mutagenic species.

Relationship of hepatocarcinogenicity and hepatocellular proliferation induced by mutagenic noncarcinogens vs carcinogens. II. 1- vs 2-nitropropane

2-Nitropropane (2-NP) is mutagenic in a number of short-term mutagenicity assays in vitro and in vivo, and is a potent hepatocarcinogen in rats. A structural isomer, 1-nitropropane (1-NP), is mutagenic in V79 cells and can induce unscheduled DNA synthesis in rat hepatocytes, yet did not induce tumors in rats following chronic exposure. We examined the correlation of cell proliferation and hepatocarcinogenesis induced by this mutagenic noncarcinogen-carcinogen pair in a rat liver proliferation model. Rats were exposed to gavage doses of 0.5, 1, or 2 mmol/kg of 1-NP or 2-NP daily for 10 days; the highest two dose groups were similar to the doses used in the carcinogenesis bioassay. Cell proliferation was quantitated by incorporation of bromodeoxyuridine, detected immunohistochemically, into newly synthesized DNA. Animals exposed to the vehicle exhibited a labeling index (LI) of approximately 1.9% and animals exposed to CCL4 had a LI of approximately 30%. Rats exposed to the hepatocarcinogen 2-NP exhibited a dose-related increase in LI to 6.3 and 11% at the 1 and 2 mmol/kg doses, respectively, and no increase above control at the 0.5 mmol/kg exposure level. Animals exposed to the noncarcinogenic isomer 1-NP showed no statistically significant increase in LI above controls at any dose level tested. Serum chemistries were consistent with mild to moderate decreases in hepatocellular function, cholestasis, and necrosis following 2-NP exposure, but only minimal effects were observed, probably due to slight dehydration resulting from 1-NP exposure. These data indicate a positive association between increased cell proliferation and hepatocarcinogenesis induced by these two nitropropane isomers.

Micronucleus test in bone marrow of mice treated with 1-nitropropane, 2-nitropropane and cisplatin

Micronucleus tests were carried out in bone marrow of mice treated with 1-nitropropane, 2-nitropropane and cisplatin. For 1-nitropropane and 2-nitropropane the results were negative. With cisplatin a dose-dependent increase in the number of polychromatic erythrocytes with micronuclei was observed. The lowest positive dose was 0.1 mg/kg (P less than 0.001, Mann-Whitney-Wilcoxon test). The hepatocarcinogen 2-nitropropane showed clastogenic activity in human lymphocytes in vitro in the presence of S9 (Bauchinger et al., 1987). The negative results in bone marrow suggest that short-lived genotoxic metabolites may be formed in the liver but do not reach the bone marrow.

Enhanced mutagenicity of 2-nitropropane nitronate with respect to 2-nitropropane--possible involvement of free radical species

The mutagenicities of the neutral and the anionic (nitronate) forms of 2-nitropropane (2-NP) were compared in S. typhimurium strains TA98, TA100 and TA102. The latter is a special strain sensitive to compounds producing oxidative damage to DNA at thymine-adenine base pair loci. Neutral 2-NP was not mutagenic in TA98, and produced significant mutagenic response in TA100 and TA102 only at levels of 55 mumoles/plate. In contrast, 2-NP nitronate was significantly mutagenic in TA98 at 14 mumoles/plate and, in strains TA100 and TA102, at approximately 4 mumoles/plate. Inclusion of S9 slightly increased the mutagenicity of 2-NP nitronate in TA102, but decreased it in TA100. The mutagenic response in TA102, but not in TA100, was inhibited by DMSO, a scavenger of hydroxyl radicals, in a dose-dependent manner. When 2-NP nitronate was incubated with thymidine and horseradish peroxidase-H2O2, the formation of 2-NP dimer (2,3-dimethyl-2,3-dinitrobutane), a condensation product of 2-NP free radicals, was observed. This was accompanied by 2-NP nitronate-dependent oxidation of thymidine to thymidine glycol, thymine glycol, 5-hydroxymethyldeoxyuridine and thymine, indicating that hydroxyl radicals and/or other reactive oxygen species capable of causing thymidine damage are also formed in this reaction. As a working hypothesis, we suggest that the genotoxicity of 2-NP nitronate may be due to the generation of DNA-damaging reactive forms of oxygen or 2-NP free radicals.

Analysis of cytogenetic effect in human lymphocytes induced by metabolically activated 2-nitropropane

Chromosome analyses were carried out in human lymphocytes treated in vitro with 2-nitropropane (2-NP) in the presence and absence of the mammalian metabolic activation system, S9 mix. Without S9 mix, only the frequency of gaps was significantly increased at 80 mM 2-NP as compared to controls. With S9 mix, the incidences of gaps and chromatid-type aberrations were significantly increased at 60 mM and 80 mM. Sister-chromatid exchanges (SCE) have been induced at concentrations as low as 7.5 mM. The present findings demonstrate that in human lymphocytes, 2-NP requires metabolic activation to express clastogenicity and SCEs.