Biologically active aromatic amines derived from carcinogenic polycyclic aromatic hydrocarbons: synthesis and mutagenicity of aminobenzo[a]pyrenes

The effects of functional groups on the sorption of naphthalene on microplastics

Ubiquitous microplastics were recognized as an ideal carrier for polycyclic aromatic hydrocarbons (PAHs) to spread in nature or transfer to biota, thus posing risks to human health. However, little is known about how the functional groups in PAH derivatives influence their sorption onto microplastics as compared to parent PAHs. This study investigated the sorption of naphthalene (NAP) and its derivatives onto polystyrene microspheres with (MP-COOH) or without (MP) surface modification of the carboxyl group. NAP derivatives with charged groups (e.g., -NH₂, -OH and -COOH) reached the sorption equilibrium more early than NAP and its derivative with an uncharged group (e.g., -CH₃), whereas their sorption capacities (K_d = 6.0-8.4 L/g for MP, K_d = 4.5-6.3 L/g for MP-COOH) were significantly lower than that of the latter (K_d = 11.6-12.0 L/g for MP, K_d = 9.4-10.0 L/g for MP-COOH). The inhibition of charged groups on sorption could be attributed to their facilitation on molecular polarity and, thus, their lower hydrophobicity (LogK_OW) since hydrophobicity was the crucial factor controlling the sorption of NAP and NAP derivatives onto both MP and MP-COOH. Computational modeling further showed that charged functional groups would weaken the π-π interaction or strengthen the repulsion between NAP or NAP derivative molecule and MP or MP-COOH molecule, which might also contribute to the suppressed sorption. This study provides new insight into the affinity of PAH derivatives with microplastics, which, therefore, improves our understanding of the environmental fates of microplastics and the organic pollutants.

Estimation of risk of developing bladder cancer among workers exposed to coal tar pitch volatiles in the primary aluminum industry

To confirm the relationship between exposure to coal tar pitch volatiles and bladder cancer among primary aluminum production workers, we carried out a case-control study among blue-collar workers who had worked more than 1 year between 1950-1979 in a major plant using mostly the Soderberg process in the Province of Québec. Cases of bladder cancer (ICD code 188) diagnosed between 1970-1979 (n = 69) were mostly included in a previously reported study. To these were added cases diagnosed between 1980-1988 (n = 69). Each case was matched to three controls on date of birth, date of hiring, and length of service at the company. Smoking habits were assessed from the medical records at the company. Benzene-soluble matter (BSM) and benzo(a)pyrene (BaP) were used as indicators of environmental exposure to coal tar pitch volatiles in the workplace. The estimated risk for current smokers was 2.63 (95% C.I. 1.29-5.37). Estimates of risk by occupational exposure were adjusted for smoking. Men who had worked in the Soderberg potrooms were at higher risk of developing the disease, the risk increasing with the time spent in these departments. Similarly, a strong association between risk and cumulative exposure to BSM or to BaP was observed. The risks associated with cumulative exposure to BSM (mg/m3-years) and to BaP (microgram/m3-years) were described with mathematical models. Using a linear model (1 + bx) and lagging 10 years before the diagnosis, BaP cumulative exposure was a better indicator of risk than BSM cumulative exposure. The risk for each year of exposure to BaP at a concentration of 1 microgram/m3 increased by 1.7% (0.8%-3.2%). Using the same model for BSM, a worker exposed to the current threshold limit value of 0.2 mg/m3 for 40 years will sustain a risk of 2.22 (1.56-3.48). Comparison of risks according to different periods of diagnosis (1970-1979 vs. 1980-1988) did not reveal any significant temporal changes on risk estimates.

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)

Separation of amino- and acetylamino-polycyclic aromatic hydrocarbons by reversed- and normal-phase high-performance liquid chromatography

In the field of chemical carcinogenesis, amino- and acetylamino-polycyclic aromatic hydrocarbons (PAHs) are among the most studied compounds. Many of these compounds have recently been detected in the environment. Presently, knowledge permitting predictions of the high-performance liquid chromatographic (HPLC) retention order of amino- and acetylamino-PAHs, particularly among their geometric isomers is lacking. In order to obtain a better understanding of the separation of these types of compounds, we have studied the separation of a series of structurally related amino- and acetylamino-PAHs derived from naphthalene, phenanthrene, anthracene, pyrene, benz[a]anthracene, benzo[a]pyrene, and benzo[e]pyrene by using reversed-phase and normal-phase HPLC columns of different types (monomeric, polymeric, and chiral stationary phase). The results indicate: (i) Pirkle-type chiral stationary phase columns and the Zorbax SIL column can efficiently separate both the amino-PAHs and acetylamino-PAHs; (ii) in general, there was no correlation between retention time and molecular size; (iii) when acetylamino-PAHs were separated on the monomeric Zorbax ODS column, the isomer with the acetylamino group located at the carbon position of higher electron density has a shorter retention time; and (iv) separation of the parent PAHs was better than that of the amino-PAHs and acetylamino-PAHs. Our results thus may provide useful information for the analysis of amino-PAHs, particularly for distinguishing the geometric isomers of environmental samples.

Metabolism of 1-, 3-, and 6-nitrobenzo[a]pyrene by intestinal microflora

The compounds 1-, 3-, and 6-nitrobenzo[a]pyrene (nitro-BaP) are environmental pollutants and have been shown to be potent bacterial mutagens. The anaerobic metabolism of these isomeric nitro-BaPs was investigated by the incubation of rat intestinal microflora with each isomer for 48 h. Aliquots were removed at several time intervals, extracted, fractionated by high-pressure liquid chromatography (HPLC), and the radioactivity determined. Metabolites were identified by comparison of their chromatographic, ultraviolet-visible absorption, and mass spectral properties with those of authentic standards. The order of the extent of nitroreduction for these isomers was 3-nitro-BaP greater than 6-nitro-BaP greater than 1-nitro-BaP. After 48 h of exposure, 84% of the added 3-nitro-BaP was present as 3-amino-BaP, 51% of the 6-nitro-BaP was metabolized to 6-amino-BaP, and 1-nitro-BaP was reduced to 1-amino-BaP (13%) and 1-nitro-BaP (4%). The order of the extent of microbial nitroreduction for these nitro-BaP isomers is different from the predictions based on electronic and steric hindrance effects. These results suggest that intestinal microflora nitroreductases exhibit a markedly high degree of substrate specificity toward nitro-BaPs that affects the extent of nitroreduction.

Microbial transformation of 6-nitrobenzo[a]pyrene

The fungal metabolism of the potent mutagenic and carcinogenic nitropolycyclic aromatic hydrocarbon (nitro-PAH) 6-nitrobenzo[a]pyrene (6-NO2-BaP) was investigated. Cunninghamella elegans was incubated with 6-NO2-BaP for periods ranging between 1 and 7 d, and the metabolites formed were separated by high-performance liquid chromatography and identified by their UV-visible absorption, mass, and 1H nuclear magnetic resonance spectra. The results of our study indicate that C. elegans metabolized 6-NO2-BaP to glucoside and sulfate conjugates of 1- and 3-hydroxy 6-NO2-BaP and suggests that glycosylation and sulfation reactions may represent detoxification pathways in the fungal metabolism of nitro-PAHs. Experiments using [G3H]-6-NO2-BaP indicated that C. elegans metabolized 62% of 6-NO2-BaP within 168 h. Our data also indicated that the nitro group at the C-6 position of benzo[a]pyrene blocked metabolism at the regions peri to the nitro substituent (C-7, C-8 positions) and enhanced metabolism at the C-1 and C-3 positions. The ability of the fungus C. elegans to metabolize 6-NO2-BaP to biologically inactive compounds may have practical applications in the detoxification of nitro-PAH-contaminated wastes.

Use of the human liver cell line Hep G2 in a modified Salmonella reversion assay

The human hepatoma cell line Hep G2 was used to activate promutagenic chemicals to mutagens in a modified Salmonella typhimurium reversion assay. Hep G2 cells mediated positive mutagenic responses in tester strain TA98 with 5 and 25 micrograms/plate of 2-aminofluorene, but these responses were consistently lower than those seen using primary rat hepatocytes. In addition, 3 and 6 X 10(6) Hep G2 cells per assay produced positive mutagenic responses with 2-aminoanthracene, benzidine, acetylbenzidine and aflatoxin B1, while benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, 3-methylcholanthrene, 4-aminobiphenyl and 4- and 11-aminobenzo[a]pyrene were nonmutagenic with Hep G2-cell activation. These results indicate that Hep G2 cells may be a useful intact cellular metabolizing system of human origin for predicting the genotoxicity of promutagenic agents, but that the use of Salmonella as a target cell may limit the classes of mutagens detected.

A quantitative comparison of a percentile rule with a 2-fold rule for assessing mutagenicity in the Ames assay

Using a model based on the bivariate normal density function, this paper compares the effectiveness of two commonly employed decision rules for assessing mutagenicity in the standard Ames Salmonella assay. The 2-fold method, which considers a compound significantly mutagenic if its mean number of revertants per plate at any dose is equal to or greater than twice the mean number of revertants per plate in the concurrent control, may be a poor indicator of significant mutagenesis. In the percentile method, the frequency of induced mutations for the test compound is tested against the 95th percentile of the accumulated historical data for the spontaneous mutation frequency. As judged by the higher probability of declaring a compound mutagenic that elevates the reversion rate above background, the percentile rule is more reliable than the 2-fold method.

In vitro mutagenicity of valepotriates

Valepotriates are epoxide-bearing triesters of the monoterpene alcohol 4,7-dimethylcyclopenta-(c)-pyrane isolated from the roots of several Valerianacae species. They are regarded as the main tranquilizing constituents of these drugs. Although the valepotriates valtrate/isovaltrate (VAL) and dihydrovaltrate (DH-VAL) showed a strong alkylating activity against the nucleophilic agent 4-(p-nitrobenzyl)-pyridine (NBP), they were not clearly mutagenic for the strains TA98, TA100, TA1535, and TA1537 of Salmonella typhimurium or for the strains WP2 and WP2 uvrA- of Escherichia coli in the absence of a metabolic activation system (S9-mix). However, the valepotriates were mutagenic for TA100, WP2 and WP2 uvrA- at concentrations up to about 1.0 mumole/plate when S9-mix was added to the test system. With more than 1 mumole/plate the valepotriates were toxic in the presence of a metabolic activation system for all strains tested. The mutagenicity of the valepotriates was inversely related to the protein content of the S9-mix used. The mutagenicity and toxicity of the valepotriates could be inhibited when the S9-mix was preincubated with the esterase inhibitor paraoxon (1 mM) for 5 min before the test compounds and bacteria were added. Therefore, bioactivation of the valepotriates by an enzymatic hydrolysis of their ester groups is considered. This could be proven by activating the valepotriates with purified esterase.

Mutagenic activation of 2-aminofluorene by fluorescent light

To determine the effect of artificially produced light on the direct mutagenicity of 2-aminofluorene, that arylamine was irradiated with either sun, cool-white, black, blue, or yellow fluorescent light or held in the dark prior to assaying for mutagenicity using Salmonella typhimurium strain TA98. The effectiveness of these exposures in potentiating the mutagenicity of 2-aminofluorene was sun greater than black greater than cool-white greater than blue greater than yellow approximately equal to dark. By varying the radiant flux densities produced by the lamps and using optical filters, wavelengths of light up to approximately 450 nm were found to be effective in the mutagenic potentiation. Studies using radical scavengers and oxygen modifiers indicated that the light-induced mutagenicity was dependent on oxygen and that singlet oxygen may be an effective activator of 2-aminofluorene. The mutagenicity of fluorene was not increased by exposure to light, while only sunlight potentiated the mutagenicity of 2-acetylaminofluorene. This result suggested the importance of the primary amine in the mutagenic activation of 2-aminofluorene by light. Light-activated 2-aminofluorene was less mutagenic in strains TA98NR and TA98/1,8-DNP6 than in TA98. This observation, combined with the dependence of the photoactivation on oxygen and amino-substitution, indicated that the light-enhanced mutagenicity was at least partially due to N-oxidized photoproducts. These studies indicate that the effect of light on environmental contaminants must be considered in assessing their genotoxic potential.

Computer analysis of toxicological data bases: mutagenicity of aromatic amines in Salmonella tester strains

There is an increasing problem in the world of toxicological evaluation in that, while test results of new compounds are appearing regularly, traditional methods of analysis of such data are cumbersome and slow. The new computer program CASE (computer automated structure evaluator) was designed to handle just such problems. It analyzes molecules and their associated biological activity on the basis of structural fragments found and identified by the program as being important for the activity based on statistical tests of significance. The program was used to examine mutagenicity in Salmonella typhimurium strains TA98 and TA100 (with S9 activation) of approximately 80-100 aromatic amines. The resulting structural features were then used in a predictive fashion to test the expected mutagenic properties of a smaller set of about 20 compounds.

Effect of bacterial concentration on reversions induced in Salmonella typhimurium TA1538 by N-hydroxy-2-acetylaminofluorene

Reversions induced by N-hydroxy-2-acetylaminofluorene (N-OH-AAF) were measured in the Salmonella/microsome quantitative plate assay using various concentrations of Salmonella typhimurium strain TA1538. The number of induced revertants increased with an increasing number of bacteria/plate, but the variation in reversion frequency was not as great as the variation in bacterial concentration. The effects of bacterial concentration on reversion fixation, phenotypic expression, selection of his+ revertants, and the interaction of the mutagen with bacterial DNA were examined. Suspension cultures of TA1538 were exposed to N-OH-AAF and various dilutions were prepared and assayed for revertants/10(8) bacteria. Reversion frequencies were very dependent on bacterial concentration between approximately 0.2 X 10(8) and 2 X 10(8) bacteria/plate. Revertants/10(8) TA1538 were reduced above about 2 X 10(8) bacteria/plate, indicating that culture conditions limited reversion fixation, expression and/or selection at these concentrations. The number of spontaneous revertants/plate determined both from bacteria exposed to dimethyl sulfoxide in suspension culture and in the Salmonella/microsome assay were not greatly affected by bacterial concentration. To study the effect of bacterial concentration on the interaction of the mutagen with bacterial DNA, various concentrations of TA1538 were exposed to the same dose of N-OH-AAF. Both revertants/10(8) TA1538 and DNA adducts varied inversely with bacterial concentration. The effect of bacterial concentration on both reversion fixation and/or expression and on mutagen binding to DNA may influence reversion frequencies in the Salmonella/microsome assay.