Effect of the orientation of nitro substituent on the bacterial mutagenicity of dinitrobenzo[e]pyrenes

UVA photoirradiation of nitro-polycyclic aromatic hydrocarbons-induction of reactive oxygen species and formation of lipid peroxides

Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) are a class of genotoxic environmental contaminants. We have long been interested in determining the mechanisms by which nitro-PAHs induce genotoxicity. Although the metabolic activation of nitro-PAHs leading to toxicological activities has been well studied, the photo-induced activation of nitro-PAHs has seldom been reported. In this paper, we report photo-induced lipid peroxidation by 19 nitro-PAHs. The results indicated that all but two of the nitro-PAHs can induce lipid peroxidation. Mechanistic studies suggest that lipid peroxidation by nitro-PAHs is mediated by free radicals generated in the reaction. There was no structural correlation between the nitro-PAHs and their ability to induce lipid peroxidation upon UVA irradiation, or between the HOMO-LUMO gap and the ability to cause lipid peroxidation. Most of the nitro-PAHs are less potent in terms of causing lipid peroxidation than their parent PAHs. The lack of correlation is attributed to the complex photophysics and photochemistry of the nitro-PAHs and the yield of reactive oxygen species (ROS) and other factors.

Phototoxicity and environmental transformation of polycyclic aromatic hydrocarbons (PAHs)-light-induced reactive oxygen species, lipid peroxidation, and DNA damage

Polycyclic aromatic hydrocarbons (PAHs) are a class of mutagenic and tumorigenic environmental contaminants. Although the mechanisms by which PAHs induce cancer in experimental animals have been extensively studied and the metabolic activation pathways have been determined, the environmental fate of PAHs and the phototoxicity exerted by PAHs, as well as their photoreaction products formed in the environment, have received much less attention. In this review, the formation of oxygenated PAHs, PAH quinones, nitro-PAHs, and halogenated PAHs from photoreaction of environmental PAHs are addressed. Upon light irradiation, PAHs and all PAH photoreaction products can absorb light energy to reach photo-excited states, which react with molecular oxygen, medium, and coexisting chemicals to produce reactive oxygen species (ROS) and other reactive intermediates, such as oxygenated PAHs and free radicals. These intermediates, including ROS, induce lipid peroxidation, and DNA damage including DNA strand breakage, oxidation to 8-oxo-2'-deoxyguanosine, and DNA-adducts. Since these toxicological endpoints are associated with age-related diseases, including cancer, environmental PAHs concomitantly exposed to sunlight may potentially promote human skin damage, leading to ageing and skin cancers. Thus, we suggest that (i) in addition to the widely recognized metabolic pathways, more attention must be paid to photoreaction as an important activation pathway for PAHs, (ii) risk assessment of environmental PAHs should take into consideration the complex photochemical reactions leading to mixtures of products that are also phototoxic; and (iii) the study of structure-toxicity relationships should be expanded to cover the complex photoreactions and extrinsic factors that affect phototoxicity endpoints.

Mutagenic activities and physicochemical properties of selected nitrobenzanthrones

Mutagenic activity of nine nitro derivatives of benzanthrone, namely 1-nitro-, 2-nitro-, 3-nitro-, 9-nitro-, 11-nitro-, 1,9-dinitro-, 3,9-dinitro-, 3,11-dinitro- and 3,9,11-trinitrobenzanthrone were tested with Salmonella strains TA98, TA100, YG1021 and YG1024 in both the presence and absence of an S9 mix. Each compound exhibited mutagenic activity with all the strains. Among these nine isomers, 3-nitrobenzantrone exhibited the most mutagenic activity with all the strains without the S9 mix. The mutagenic activities of the dinitro and trinitro derivatives of benzanthrone were lower than that of the 3-nitro derivative; this is evident from the mutagenic activity of nitrated polyaromatic hydrocarbons (PAH), which is generally enhanced with an increase in nitration. The physicochemical properties of nitrated benzanthrone (reduction potential, hydrophobicity and orientation of nitro groups to the aromatic ring) demonstrated that mononitrated benzanthrone exhibits a lower reduction potential than mononitroPAHs such as 1-nitropyrene and 3-nitrofluoranthene, but was almost equivalent to that of dinitroPAH. Moreover, the mutagenic activity of mononitrobenzanthrones clearly depend on the reduction potential of each compound; however, this tendency was not observed in polynitrobenzanthrones, probably because the reduction of the nitro groups to amino groups of polynitrated benzanthrone might be predominant without a sufficient formation of corresponding hydroxyamines. These results suggest that aromatic compounds that contain keto groups, when nitrated, may act as potentially powerful direct-acting mutagens.

Structural activity relationship between Salmonella-mutagenicity and nitro-orientation of nitroazaphenanthrenes

Nitroazaphenanthrenes (NAphs) and their N-oxides (NAphOs) were synthesized as derivatives with nitrogen atoms in the 1, 4, and 9 positions of phenanthrene rings, and as nitrated derivatives substituted at the 1, 2, 3, 4, 5, 6, 7, and 8 positions of phenanthrene rings. To determine the structure activity relationship of these derivatives, all 19 isomers were bioassayed with Salmonella tester strains. NAphs substituted at the 4, 6, 7 and 8 positions were mutagenic for TA98, and 1-, 2-, and 3-N-9-AphOs, 6-N-1-AphO and 6-N-4-AphO were mutagenic for TA98 and TA100 without the S9 mix, while 5-N-1-AphO and 5-N-9-AphO were non- or weakly mutagenic. Nitrated derivatives, 6-N-4-Aph, 6-N-9-Aph, 6-N-1-AphO, and 6-N-4-AphO, were powerful mutagens for TA98 and TA100. Mutagenicity was enhanced by mutant strains producing nitroreductase, such as YG1021 and 1026, and by those producing O-acetyltransferase, such as YG1024 and 1029. Nitro derivatives substituted at positions 4 and 5 in the phenanthrene rings were perpendicular, while those at positions 2, 3, 6 and 7 were coplanar to the phenanthrene rings. NAphs substituted at the 1 and 8 positions were noncoplanar due to steric hindrance of the aromatic proton at the peri position. On the other hand, 1,5- and 1,8-dinitro-4-azaphenanthrenes showed high mutagenicity for strains TA98 and TA100 in the absence of the S9 mix, and were strongly enhanced by nitroreductase and O-acetyltransferase, over-producing mutants. Therefore, it was found that the mutagenic potency of NAphs and NAphOs was closely associated with the chemical properties and orientation of nitro substitution of aromatic rings.

Mutagenicity of sulfoscanate: a comparative study

The mutagenic activity of sulfoscanate (SSC) (4-isothiocyanate-4'-nitrodiphenyl sulphide) has been compared with that of the following reported drugs: (a) nitroscanate (NSC) (4-isothiocyanate-4'-nitrodiphenyl ether) which is a veterinary anthelmintic drug and (b) amoscanate (ASC) (4-isothiocyanate-4'-nitrodiphenyl amine) which is effective against schistosomes. SSC has been found to be a very potent mutagen towards TA98 and TA100 inducing 26.0 and 475.5revertants/nmole, respectively. NSC was found to induce mutations at a rate of 11.1 and 21.5revertants/nmole in TA98 and TA100, respectively. ASC was found to be non-mutagenic as such, but the urine of animals given the drug displayed mutagenicity. When SSC was tested in TA98/1,8-DNP(6), deficient in O-acetyltransferase, the activity decreased to 10.0revertants/nmole. However, in case of NSC the mutagenic activity was reduced to 0.24revertants/nmole, indicating the importance of O-acetyltransferase in generating N-acetoxyarylamine. In TA98NR, deficient in nitroreductase, the mutagenicity of SSC and NSC was totally absent. The positional isomers of SSC, 4-isothiocyanate-3'-nitro- and 4-isothiocyanate-2'-nitrodiphenyl sulphide, were found to be non-mutagenic in both TA98 and TA100. Our comparison of the mutagenic activity of SSC, NSC and ASC indicates that the pattern of activity is SSC>NSC>ASC.

Effect of various alkyl and unsaturated substituents on the mutagenicity of some nitrophenyl thioethers

A variety of nitro-substituted phenyl alkyl/aryl thioethers and nitroso-substituted phenyl alkyl/aryl thioethers have been synthesized and tested for their mutagenicity towards Salmonella typhimurium strain TA100, TA98, TA98NR and TA98/1,8-DNP(6) in the absence of S9 mix. The relative order of mutagenicity in TA98 and TA100 among p-nitrophenyl thioethers having alkyl or aryl substituents is allyl>phenyl>benzyl>butyl>propyl>ethyl>methyl. Compounds having an alkyl chain C(6) to C(12) were found to be non-mutagenic. Among the various positional isomers (ortho, meta and para) of nitro-substituted diphenyl thioethers only the compounds having the -NO(2) function at the para position is mutagenic, whereas compounds having a -NO(2) function at ortho and meta are non-mutagenic. However, the reduced intermediate, ortho-nitroso derivative was found to be mutagenic in all the four strains but the meta-nitroso derivative was found to be non-mutagenic. All mutagens were found to be non-mutagenic when tested in nitroreductase deficient strain TA98NR, whereas their nitroso intermediates are found to be mutagenic. A substantial fall in the mutagenic activity is observed when some mutagens are tested in O-acetyltransferase deficient strain TA98/1,8-DNP(6).

Structure, tumorigenicity, microsomal metabolism, and DNA binding of 7-Nitrodibenz[a,h]anthracene

It has been previously proposed that a nitropolycyclic aromatic hydrocarbon (nitro-PAH) with its nitro functional group perpendicular or nearly perpendicular to the aromatic moiety exhibits lower tumorigenicity than the corresponding parent aromatic hydrocarbon. We also hypothesized that reduction of the nitro group is not involved, or contributed less significantly in the metabolic activation of this class of nitro-PAHs. To verify this hypothesis, we selected 7-nitrodibenz[a,h]anthracene (7-NDB[a,h]A) for study. The X-ray crystallographic structure of 7-NDB[a,h]A was determined and indicated that the dihedral angle between the nitro functional group and the aromatic dibenz[a,h]anthracenyl moiety was 80.6 degrees, indicating the nitro group preferentially adopts a nearly perpendicular orientation. The tumorigenicity of 7-NDB[a,h]A and dibenz[a,h]anthracene (DB[a,h]A) was determined in the male B6C3F1 neonatal mouse. Mice were administered ip injections of 1/7, 2/7, and 4/7 of the total dose of 7-NDB[a,h]A (400 nmol in 35 microL of DMSO per mouse) within 24 h of birth and at 8 and 15 days of age, respectively, and sacrificed at 12 months of age. DB[a,h]A induced 78 and 96% hepatocellular adenomas and carcinomas, respectively. However, 7-NDB[a,h]A induced only 50 and 8% hepatocellular adenomas and carcinomas compared with the 8 and 4% hepatocellular adenomas and carcinomas induced by the solvent vehicle, DMSO. Aerobic metabolism of 7-NDB[a,h]A by liver microsomes of 15-day old male B6C3F1 neonatal mice resulted in trans-3,4-dihydroxy-3, 4-dihydro-7-nitrodibenz[a,h]anthracene (7-NDB[a,h]A trans-3, 4-dihydrodiol) and trans-10,11-dihydroxy-10, 11-dihydro-7-nitrodibenz[a,h]anthracene (7-NDB[a,h]A trans-10, 11-dihydrodiol) as predominant metabolites. Under anaerobic conditions, 7-NDB[a,h]A was not metabolized (nitroreduced). The DNA adduct levels in liver and lung tissues of male B6C3F1 mice treated with 7-NDB[a,h]A and sacrificed 24 h and 6 days after final dosing were determined by 32P-postlabeling/TLC. In all cases, the DNA adducts derived from 7-NDB[a,h]A trans-3,4-dihydrodiol and 7-NDB[a, h]A trans-10,11-dihydrodiol were formed. These results suggest that both of the metabolites, 7-NDB[a,h]A trans-3,4-dihydrodiol and 7-NDB[a,h]A trans-10,11-dihydrodiol, are involved in the metabolic activation of 7-NDB[a,h]A, leading to tumor induction in the neonatal mouse. Thus, our results described in this paper support our hypotheses that a nitro-PAH with a perpendicular nitro orientation exhibits lower tumorigenicity than the corresponding parent PAH and that nitroreduction contributes less significantly in the metabolic activation.

Mutagenicity of nitrophenanthrene derivatives for Salmonella typhimurium: effects of nitroreductase and acetyltransferase

To determine the mutagenicity of nitrophenanthrenes, three mononitrophenanthrenes (NPhs), 11 dinitrophenanthrenes (diNPhs) and eight trinitrophenanthrenes (tiNPhs) were synthesized, and their mutagenicity was investigated by using Salmonella typhimurium his- strains TA98, TA100, and TA98NR, nitroreductase-deficient, and TA98/1,8-DNP6, O-acetyltransferase-deficient mutants, and strains YG1021 and YG1026, nitroreductase-overproducing mutants of TA98 and TA100, respectively, and strains YG1024 and YG1029, O-acetyltransferase-overproducing mutants of TA98 and TA100, respectively. 1-, 3- and 9-NPhis induced 329, 620 and 438 revertants per nmol in strain TA100, respectively, and 4,839, 11,309 and 16728 revertants per nmol, respectively, in strain YG1029. Mutagenicity of 1,6-, 2,6-, 2,9-, 2,10-, 3,5-, 3,6- and 3,10-diNPh was elevated in strains YG1021, YG1024, YG1026 and YG1029. Among these derivatives, 1,6-, 2,6-, 3,6- and 3,10-diNPhs were more mutagenic in strains YG1024 and YG1029 than YG1021 and YG1026, and they showed a structure-activity relationship between mutagenicity and NO2-substitution. Nitro derivatives substituted at the 3 and 6 positions of their chemical structure strongly mutated both strains YG1024 and YG1029, whereas those substituted at the 9 and 10 positions showed weak mutagenicity. In addition, nitro substituents at positions 4 and 5 were perpendicular while those on positions 2,3,6 and 7 were nearly coplanar to the aromatic ring. Furthermore, 2,6,9-, 3,6,9- and 1,6,9-trinitrophenanthrenes (triNPhs) were mutagenic for strain TA100, and their mutagenicity was more enhanced in YG1024 and YG1029 than in YG1021 and YG1026. Of the eight triNPhs all except 1,5,10-triNP were mutagenic in TA98 and TA100, and their mutagenicity was more enhanced in YG1024 and YG1029 than in YG1021 and YG1026. These results suggest that these compounds are mutagens that are activated by O-acetyltransferase esterification following nitroreductase. The nitrated derivatives substituted at the 2(7) and 3(6) positions of the phenanthrene ring were highly mutagenic. The relationship between chemical structure and the mutagenicity of NPh derivatives is discussed.

Mutagenicity of nitrobenzyl derivatives: potential bioreductive anticancer agents

Ortho-, meta- and para-nitrobenzyl bromides, alcohols, ethers and esters were synthesised and tested for their mutagenicity toward Salmonella typhimurium TA100, TA100NR (nitroreductase deficient) and TA98 in absence of S9 mix and in TA100 with S9 mix. Compounds of the ortho- and meta-series were non mutagenic with and without S9 mix. Except for the alcohol and ether, the compounds of the para-series were mutagenic in TA100 with activity sequence propionate > butyrate > benzoate > acetate > bromide and this specific activity was reduced considerably by S9 mix. The Ames Salmonella test system does not seem to be an appropriate model to evaluate mutagenicity of o-nitrobenzyls. However, further work is in progress to test all the compounds for mutagenicity in mammalian system.

Metabolism of 7-nitrobenz[a]anthracene by intestinal microflora

Pure cultures of anaerobic intestinal bacteria and mixed fecal microflora from human, rat, mouse, and pig were screened for the ability to metabolize 7-nitrobenz[a]anthracene (7-NO2BA). Based on analysis by high-performance liquid chromatography (HPLC) and by ultraviolet (UV), mass, and nuclear magnetic resonance (NMR) spectral techniques, the compounds were identified as 7-aminobenz[a]anthracene (7-NH2BA) and benz[a]anthracene 7,12-dione (dione). Identification of 7-NH2BA as a metabolite of 7-NO2BA indicates that the anaerobic intestinal bacteria are capable of reducing 7-NO2BA to potentially bioactive intermediates. The reductive capacities of the mixed intestinal microflora were generally greater than those of pure cultures. Thus, metabolism of 7-NO2BA in the intestinal tract may be underestimated if pure cultures are used as the sole method for evaluating the potential hazard.

DNA adducts and carcinogenicity of nitro-polycyclic aromatic hydrocarbons

We have been interested in the structure-activity relationships of nitro-polycyclic aromatic hydrocarbons (nitro-PAHs), and have focused on the correlation of structural and electronic features with biological activities, including mutagenicity and tumorigenicity. In our studies, we have emphasized 1-, 2-, 3-, and 6-nitrobenzo[a]pyrenes (nitro-B[a]Ps) and related compounds, all of which are derived from the potent carcinogen benzo[a]pyrene. While 1-, 2-, and 3-nitro-B[a]P are potent mutagens in Salmonella, 6-nitro-B[a]P is a weak mutagen. In vitro metabolism of 1- and 3-nitro-B[a]P has been found to generate multiple pathways for mutagenic activation. The formation of the corresponding trans-7,8-dihydrodiols and 7,8,9,10-tetrahydrotetrols suggests that 1- and 3-nitro-B[a]P trans-7,8-diol-9,10-epoxides are ultimate metabolites of the parent nitro-B[a]Ps. We have isolated a DNA adduct from the reaction between 3-nitro-B[a]P trans-7,8-diol-anti9,10-epoxide and calf thymus DNA, and identified it as 10-(deoxyguanosin-N2-yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydro-3-ni tro-B[a]P . The same adduct was identified from in vitro metabolism of [3H]3-nitro-B[a]P by rat liver microsomes in the presence of calf thymus DNA. A DNA adduct of 3-nitro-B[a]P formed from reaction of N-hydroxy-3-amino-B[a]P, prepared in situ with calf thymus DNA was also isolated. This adduct was identified as 6-(deoxyguanosin-N2-yl)-3-amino-B[a]P. The same adduct was obtained from incubating DNA with 3-nitro-B[a]P in the presence of the mammalian nitroeductase, xanthine oxidase, and hypoxanthine.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutagenicity of nitro-azabenzo[a]pyrene and its related compounds

The mutagenicity of nitrated benzo[a]pyrene (BP) and the related compounds, 1- and 3-nitrobenzo[a]pyrene (NBP), 1- and 3-nitro-6-cyanobenzo[a]pyrene (N-6-CBP), 1- and 3-nitro-6-azabenzo[a]-pyrene (N-6-ABP), 1- and 3-nitro-6-azabenzo[a]-pyrene-N-oxide (N-6-ABPO) and 1,6- and 3,6-dinitrobenzo[a]-pyrene (DNBP), was investigated. The mutagenic activities of 3-N-6-CBP and 3-N-6-ABP were 117 and 76 times, respectively, that of 3-NBP. In addition, 3,6-DNBP was more mutagenic than 1,6-DNBP. It is suggested that the mutagenic activation differs with the position of NO2 substitution in the chemical structure. A nitro derivative with NO2 substitution at the 3 position of the aromatic ring of BP was more mutagenic than that with the substitution at the 1 or 6 position. The reducibility of DNBPs was then determined by detecting 1- or 3-amino-6-nitrobenzo[a]pyrene (A-6-NBP), a metabolite of DNBP; 3,6- and 1,6-DNBP were reduced to 3- and 1-A-6-NBP at frequencies of 958 +/- 26 and 79 +/- 8, respectively, pmole per mg of protein, when the compound was incubated anaerobically with rat liver S9 mix at 37 degrees C for 15 min. NO2 substituted at the 3 position of the aromatic ring of BP was readily reduced by a microsome enzyme to form an amino derivative. The result suggests that these compounds have a structure-activity relationship between mutagenicity and NO2 substitution of BP.

Detection of 3,6-dinitrobenzo[a]pyrene in airborne particulates

3,6-Dinitrobenzo[a]pyrene, a new mutagen, was detected in airborne particulates collected in Santiago (Chile). The quantity of the compound in the airborne particulates was very small, accounting for 0.01 micrograms/g of total particulates (0.002 ng/m3 of air) at the lowest concentration. It was found that 3,6-dinitrobenzo[a]pyrene is readily decomposed by UV irradiation at 312 nm. The decomposed product was identified as 3-nitrobenzo[a]pyrene-6-quinone by means of mass spectrometry and proton nuclear magnetic resonance analysis. The mutagenicity of 3,6-dinitrobenzo[a]pyrene was 137,000 revertants/nmole for Salmonella typhimurium strain TA98, less than that for strain TA98/1,8-DNP6, an acetyltransferase-deficient mutant, and more than that for strain YG1024, an acetyltransferase-rich mutant.

A comparative study of mutagenic and SOS-inducing activity of biphenyls, phenanthrenequinones and fluorenones

A total of 23 chemicals--biphenyls, phenanthrenequinones and fluorenones--were tested for mutagenicity towards Salmonella typhimurium strains TA1538, TA1535 and TA98. SOS-inducing activity of the same chemicals was studied in terms of the SOS-inducing potency in Escherichia coli PQ37, using an automated instrument controlled by a dedicated computer program for the SOS Chromotest. Of the 23 chemicals studied 14 induced His+ revertants in S. typhimurium TA1538 hisD305 (-1 frameshift); none induced His+ reversions in TA1535 (base-pair substitution). The mutagenicity of the chemicals in S. typhimurium TA98 (pKM 101) was lower than in TA1538. There was a close correlation between mutagenicity and SOS-inducing activity of fluorenones and phenanthrenequinones. None of the biphenyls tested induced SOS response and this property does not depend upon the mutagenic activity of the chemicals. SOS Chromotest is particularly valid in detecting chemicals which give rise to base-pair substitutions through SOS induction. If positive results are obtained, the Salmonella assay may be omitted. However, this test cannot replace the Ames test especially for the primary screening of mutagenicity of chemicals with unknown structure.