Mutagenicity of chloro-olefins in the Salmonella/mammalian microsome test. I. Allyl chloride mutagenicity re-examined

Contrary to findings published up to now, allyl chloride, a well known directly acting mutagen for Salmonella typhimurium, is efficiently activated by rat-liver homogenate (S9 mix) under non-standard mutagenicity testing conditions. Its indirect, S9-mediated mutagenic activity is greatly enhanced when longer than standard preincubation times are applied. The indirect mutagenicity of allyl chloride, thus revealed, greatly exceeds its direct mutagenic activity. Obviously, standard mutagenicity testing conditions cannot be regarded as reliable tools for the evaluation of the full genotoxic potential of allyl chloride and, possibly, of other related compounds.     

Studies on the mutagenicity of p-phenylenediamine in Salmonella typhimurium. Presence of PCB's in rat-liver microsomal fraction induced by Aroclor.

The mutagenicity of fresh solutions of p-phenylenediamine (PPD) and Aroclor 1254 was investigated. The histidine-requiring strains of Salmonella typhimurium were used in the absence and presence of uninduced and/or Aroclor-induced rat-liver homogenate. The presence of polychlorinated biphenyls (PCBs) was also examined by chromatographic methods in Aroclor-induced rat-liver homogenate. In the absence of metabolic activation, as well as in the presence of uninduced rat-liver homogenate, PPD was not mutagenic in the strains used. In the presence of Aroclor-induced S9 a twofold increase (or less) was observed in the number of revertant colonies over those of the controls in TA1538 and TA98. There was no increase in the number of revertant colonies over those of the controls when PPD was dissolved in NH4OH solution and the solution mixed with H2O2 before the addition of S9 mix. Aroclor 1254 was not mutagenic in TA1538 or TA98. However, the presence of PCBs in Aroclor-induced rat-liver homogenate (induced S9) was identified by gas-liquid chromatography (GLC), high-performance liquid chromatography (HPLC) and gas--liquid chromatography/mass spectrometry (GC/MS).     

Mutagenic effect of dichloromethane on Salmonella typhimurium.

The possible mutagenicity of the organic solvent dichloromethane was investigated with the mutation test as described by Ames et al. The compound was mutagenic in both tester strains used, namely TA98 and TA100. The administration of rat-liver homogenate did not appear to be essential though it slightly increased the number of mutations.     

Some factors determining the concentration of liver proteins for optimal mutagenicity of chemicals in the Salmonella/microsome assay.

In plate assays in the presence of S. typhimurium TA100 and various amounts of liver 9000 X g supernatant (S9) from either untreated, phenobarbitone- (PB) or Aroclor-treated rats, the S9 concentration required for optimal mutagenicity of aflatoxin B1 (AFB) depended both on the source of S9 and on the concentration of the test compound. In these assays, the water-soluble procarcinogen, dimethylnitrosamine (DMN) was mutagenic in S. typhimurium TA1530 only in the presence of a 35-fold higher concentration of liver S9 from PB-treated rats than that required for AFB, a lipophilic compound. In liquid assays, a biphasic relationship was observed in the mutagenicities in S. typhimurium TA100 of benzo[a]pyrene (BP) and AFB and the concentration of liver S9. For optimal mutagenesis of BP, the concentration of liver S9 from rats treated with methylcholanthrene (MC) was 4.4% (v/v); for AFB it was 2.2% (v/v) liver S9 from either Aroclor-treated or untreated rats. At higher concentrations of S9 the mutagenicity of BP and of AFB was related inversely to the amount of S9 per assay. The effect of Aroclor treatment on the microsomemediated mutagenicity of AFB was assay-dependent: in the liquid assay, AFB mutagenicity was decreased, whereas in the plate assay it did not change or was increased. As virtually no bacteria-bound microsomes were detected by electron microscopy, after the bacteria had been incubated in a medium containing 1-34% (v/v) MC-treated rat-liver S9, it is concluded that, in mutagenicity assays, mutagenic metabolites generated by microsomal enzymes from certain pro-carcinogens have to diffuse through the assay medium before reaching the bacteria. Thus the mutagenicity of BP was dependent on both the concentration of rat-liver microsomes and that of total cytosolic proteins and other soluble nucleophiles such as glutathione. At a concentration of 4.4% (v/v) liver S9, the mutagenicity of BP was about 3.6 times higher than in assays containing a 4-fold higher concentration of cytosolic fraction. Studies on the glutathione-dependent reduction of BP mutagenicity in plate assays has shown that, in the presence of liver S9 concentrations greater than that required for optimal mutagenicity, the reduction in mutagenicity was related directly to the concentration of liver S9. Thus, in the Salmonella/microsome assay, when the concentration of rat-liver S9 was increased over and above the amount required for the optimal mutagenicity of BP, the mutagenic metabolites of BP were inactivated (by being trapped with cytosolic nucleophiles and/or by enzymic conjugation with glutathione); this effect increased more rapidly than their rate of formation. The concentration of liver S9 for optimal mutagenicity of test compounds requiring activation catalyzed by mono-oxygenases seems, therefore, to be related to the departure from linearity of the relationship between the rate of formation of mutagenic metabolites and the concentration of liver S9.     

Toxic and mutagenic effects of formaldehyde in Salmonella typhimurium

Toxic and mutagenic activities of formaldehyde were studied in Salmonella typhimurium strain TM677, using forward mutation to 8-azaguanine (8-AG) resistance both in the absence and in the presence of Aroclor-induced rat-liver postmitochondrial supernatant (PMS). The results showed that formaldehyde was toxic and mutagenic to the bacteria in both systems, but toxicity and mutagenicity were reduced in the presence of PMS. The minimum concentration required to induce toxicity and mutagenicity was 0.17 mM in the absence of PMS and 0.33 mM in the presence of PMS.     

Mutagenicity of chloroolefins in the Salmonella/mammalian microsome test. III. Metabolic activation of the allylic chloropropenes allyl chloride, 1,3-dichloropropene, 2,3-dichloro-1-propene, 1,2,3-trichloropropene, 1,1,2,3-tetrachloro-2-propene and hexachloropropene by S9 mix via two different metabolic pathways

In the presence of S9 mix all allylic chloropropenes tested exert considerable indirect mutagenic activity which is most pronounced for 1,2,3-trichloropropene. Lower as well as higher chlorinated derivatives are clearly less mutagenic. Longer than standard incubation time (120 min instead of 20 min) at 37 degrees C always leads to an increase in mutagenic activity. An increase in concentration of rat-liver homogenate fraction (S9) in the metabolising system (S9 mix) enhances mutagenicity only for 1,3-dichloropropene, 2,3-dichloro-1-propene and for the cis isomer of 1,1,2,3-tetrachloro-2-propene. According to the effects of the enzyme inhibitors SKF525 1,1,1-trichloropropene-2,3-oxide and cyanamide the allylic chloropropenes fall into 3 groups distinguished by their mode of metabolic activation by S9 mix: (a) allyl chloride and 1,3-dichloropropene are hydrolysed to the corresponding allylic alcohols which can be oxidised to the respective acroleins (hydrolytic-oxidative pathway); (b) 2,3-dichloro-1-propene, 1,1,2,3-tetrachloro-2-propene and hexachloropropene are epoxidised in the C=C double bond, giving rise to reactive epoxides (epoxidative pathway); (c) only 1,2,3-trichloropropene is obviously activated by both these alternative metabolic pathways. Structural parameters like chloro-substitution of the central C atom of the C=C-C sequence and substituent-induced polarisation of the C=C double bond as well as cis/trans isomerism might be responsible for different substrate properties for the enzymes involved in allylic chloropropene metabolism, thus determining different degrees of activation by either one or both pathways.     

Fluroxene mutagenicity.

The commercially available volatile anesthetic fluroxene (2,2,2-trifluoroethyl vinyl ether) which contains the stabilizer N-phenyl-1-napthylamine, was tested for mutagenicity using four strains of S. typhimurium, TA1535, TA1537, TA98 and TA100, and one strain of E. coli, WP2. In addition, purified fluroxene; N-phenyl-1-napthylamine; trifluoroethanol, a major metabolite of fluoroxene; and urine from rats anesthetized with fluroxene were tested. Several procedures were utilized including exposure of bacteria to vapor in desiccators and in liquid suspension. Results indicate that fluroxene, but not its stabilizer, was mutagenic to strains TA1535, TA100 and WP2 only in liquid suspension and only in the presence of a rat-liver enzyme system. Trifluoroethanol and urine from fluroxene-treated rat were not mutagenic to any strain of bacteria. These findings indicate that fluroxene is a promutagen which requires preincubation before it is recognized. Further experiments were performed with enzymes prepared from mouse, hamster and human liver. Fluroxene was mutagenic only in the presence of enzymes prepared from Aroclor 1254 pretreated rodents. Since fluroxene was not mutagenic in the presence of enzymes prepared from three human livers, the significance of these findings to man are unclear.     

The Salmonella/mammalian microsome mutagenicity test: comparison of human and rat livers as activating systems

The mutagenicity of several test compounds was verified by the Salmonella/microsome mutagenicity test (Ames test), using both human liver and rat liver (untreated or pretreated with Aroclor 1254) S9 under identical experimental conditions. Aflatoxin B1, 3-methylcholanthrene, and cigarette-smoke condensate were less mutagenic in the presence of human-liver S9 than in the presence of rat-liver S9 (particularly after treatment with Aroclor 1254). The opposite was observed with 2-aminonanthracene and to a lesser degree with 2-aminofluorene; correlation studies indicate that the two compounds were activated by the same or by very similar enzymes, probably cytochrome P-450s. These results clearly indicate that human-liver S9, as an activating system, behaves differently than rat-liver S9; therefore, it may constitute a useful, additional tool for the study of mutagenicity and probably, carcinogenicity in man.     

Mutagenicity of N-nitrosopyrrolidine derivatives in Salmonella (Ames) and Escherichia coli K-12 (343/113) assays

The mutagenicity of nitrosopyrrolidine (NPYR) and its derivatives was determined by use of the Ames Salmonella assay. A clear specificity to revert the missense stain of TA1535 and a requirement for the phenobarbital-induced rat-liver activation system (S9 mix) were noted. 3,4-Dichloronitrosopyrrolidine was more mutagenic than NPYR, whereas 3-hydroxynitrosopyrrolidine was weakly mutagenic. The carcinogenic nitroso-3-pyrrolidine was not mutagenic under the test conditions. The noncarcinogenic derivatives (2,5-dimethylnitrosopyrrolidine, nitrosoproline and 4-hydroxynitrosoproline) were not mutagenic. Liquid preincubation assays were not any more effective than the pour-plate assays. Selected derivatives of NPYR were tested in the Escherichia coli K-12 (343/113) assay A specificity to revert the missense mutation at the arg locus and a dependence on phenobarbital-induced rat-liver S9 mix were noted with NPYR and its derivatives. 3,4-Dibromonitrosopyrrolidine, which was not mutagenic in Salmonella, was effective in E. coli, and the weakly carcinogenic NPRL was a weak mutagen resulting in a 2-fold enhancement in the E. coli arginine reversion assay.     

Bacterial mutagenicity of some methyl 2-cyanoacrylates and methyl 2-cyano-3-phenylacrylates

The mutagenic potential of three alkyl 2-cyanoacrylate adhesives, three commercial alkyl 2-cyanoacrylate adhesives and three methyl 2-cyano-3-phenylacrylates, was assessed using the Salmonella/microsome mutagenicity assay. Compounds were tested with and without Aroclor 1254-induced rat-liver homogenate (S9 mix). The methyl 2-cyanoacrylate adhesives were mutagenic in the standard plate test with S. typhimurium strain TA100 with and without S9 activation. Methyl 2-cyano-3-(2-bromophenyl)acrylate revealed a direct mutagenic action to S. typhimurium strain TA1535. The compounds most toxic towards the bacterium S. typhimurium, were the methyl 2-cyanoacrylate adhesives (greater than 500 micrograms/plate). All alkyl 2-cyanoacrylate adhesives were tested in a modified spot test for volatile compounds with tester strain TA100. Mutagenic and toxic effects were observed with the three methyl 2-cyanoacrylate adhesives. It can be concluded from the results that the bacterial toxicity and mutagenicity of methyl 2-cyanoacrylate adhesives may be due to the methyl 2-cyanoacrylate monomer.     

Structure-activity relationships of the N-methylcarbamate series in Salmonella typhimurium

Aromatic hydrocarbons of low molecular weight, hydroxy and N-methylcarbamate derivatives were tested for mutagenicity by the reversion of histidine-dependent Salmonella typhimurium TA98 and TA1535 in the presence of a rat-liver 9000 X g supernatant fraction. The presence of 2 or 3 aromatic rings resulted in a weak increase in revertants. Hydroxylation and carbamylation of aromatic rings increased the mutagenic activity of these aromatic compounds. In order to evaluate the structure-activity relationship, the specific molecular connectivity indices were calculated. A significant inverse relationship exists between mutagenicity and zero- and second-order specific molecular connectivity indices. Only compounds with second-order specific molecular connectivity indices lower than 0.300 increased mutagenic activity.     

The possible role of azoreduction in the bacterial mutagenicity of 4-dimethylaminoazobenzene (DAB) and 2 of its analogues (6BT and 5I)

The extent to which azoreductive cleavage contributes to the bacterial mutagenicity of 3 azo compounds has been investigated. The compounds studied were the rodent-liver carcinogens 4-dimethylaminoazobenzene (DAB) and 6-dimethylaminophenylazobenzthiazole (6BT), and the reported non-carcinogenic isostere 5-dimethylaminophenylazoindoline (5I). Although each of these compounds is mutagenic to Salmonella when evaluated using a pre-incubation protocol and in the presence of an induced rat-liver S9 mix, the constituent amines (cleavage products) were essentially inactive. It is therefore concluded that the mutagenic response reported for DAB, 6BT and 5I is related to metabolic activation of the intact molecules. In addition, the non-mutagenicity of 4'-phenyl-4-dimethylaminoazobenzene (4PhDAB) suggests that azoreductase activity is low in the Salmonella preincubation assay, at least as conducted in this laboratory. In the case of 4PhDAB, less than 1.4% azoreduction would yield sufficient quantities of the derived amine, 4-aminobiphenyl, for a positive mutagenic response to have been observed.     

Inhibitory effect of organic solvents on the mutagenicity of N-nitrosodialkylamines in Salmonella

The influence of organic solvents on the mutagenicity of 11 N-nitrosamines was examined in Salmonella typhimurium TA100 using the Ames's liquid incubation assay in the presence of rat-liver S9. The mutagenic activities of N-nitrosodimethylamine, N-nitrosodiethylamine, 6 oxidative derivatives of N-nitrosopropylamine and N-nitroso-2,6-dimethylmorpholine were considerably decreased by addition of dimethyl sulfoxide, dimethyl formamide, acetone, 95% ethanol or acetonitrile, which are recommended for use as solvents in the assay by Ames's group, to the incubation mixture. The mutagenic activities of N-nitrosodipropylamine and N-nitrosodibutylamine, which are barely soluble in water, were also suppressed by increasing concentrations of dimethyl sulfoxide. These organic solvents did not appear to exert their influence by desmutagenic and antimutagenic actions. In contrast, the recoveries of unmetabolized carcinogens from preincubation mixtures and from agar plates were significantly higher in the presence of organic solvents than in their absence. The results indicate that the inhibitory effect is a result of interference with the process of metabolic activation by liver S9.     

High mutagenicity of N-(alpha-acyloxy)alkyl-N-alkylnitrosamines in S. typhimurium: model compounds for metabolically activated N,N-dialkylnitrosamines.

A series of N,N-dialkylnitrosamines (alkyl means methyl, ethyl, n-propyl, n-butyl or tert-butyl group) mono-substituted at the alpha-carbon with an acetoxy group, were tested for their mutagenic action in Salmonella typhimurium TA1530 in the presence or absence of a rat-liver supernatant from 9000 X g. The presumed released of methyl, ethyl, n-butyl and n-propyl carbonium ions from the corresponding alpha-acetoxy derivatives, either by enzymic cleavage or by non-enzymic hydrolysis of the ester group, caused high mutagenicity in the bacteria. As has been demonstrated for certain alpha-acetoxy compounds, the mutagenicity of these compounds was inversely related to their half-lives in aqueous media. N-(Acetoxy)methyl-N-tert-butylnitrosamine and a beta-acetoxy derivative of N,N-diethylnitrosamine were not mutagenic either in the presence or in the absence of hydrolysing rat-liver enzymes. These results support the hypothesis that alpha-carbon hydroxylation is one mechanism involved in the metabolic activation of N,N-dialkylnitrosamines.     

Mutagenicity of tetrachloroethene in the Ames test--metabolic activation by conjugation with glutathione

The mutagenicity of tetrachloroethene (tetra) and its S conjugate, S-(1,2,2-trichlorovinyl)glutathione (TCVG) was investigated using a modified Ames preincubation assay. TCVG was a potent mutagen in presence of rat kidney particulate fractions containing high concentrations of gamma-glutamyl transpeptidase (GGT) and dipeptidases. Purified tetra was not mutagenic without exogenous metabolic activation or under conditions favoring oxidative metabolism. Preincubation of tetra with purified rat liver glutathione (GSH) S-transferases in presence of GSH and rat kidney fractions resulted in a time-dependent formation of TCVG as determined by (HPLC) analysis and in an unequivocal mutagenic response in the Ames test. Experiments with tetra in the isolated perfused rat liver demonstrated TCVG formation and its excretion with the bile; bile collected after the addition of tetra to the isolated perfused liver was unequivocally mutagenic in bacteria in the presence of kidney particulate fractions. The mutagenicity was reduced in all cases by the GGT inhibitor serine borate or the beta-lyase inhibitor aminooxyacetic acid. These results support the suggestion that cleavage of the GSH S conjugate formed from tetra by the enzymes of the mercapturic acid pathway and by beta-lyase may be involved in the nephrocarcinogenic effects of this haloalkene in rats.     

Mutagenic activation of 2-acetylaminofluorene by guinea-pig liver homogenates: essential involvement of cytochrome P-450 mixed-function oxidases

2-Acetylaminofluorene (AAF) was highly mutagenic to Salmonella typhimurium strain TA98, when activated by a liver post-mitochondrial supernatant fraction (S9 fraction) from guinea-pigs, in spite of the resistance of this species to AAF carcinogenesis and the low capacity of the liver of this species for N-hydroxylation of AAF. The mutagenicity was comparable to or higher than that resulting from activation by mouse- or rat-liver S9 fraction, and was not enchanced by treatment with cytochrome P-450 inducers, a combination of phenobarbital and 5,6-benzoflavone. In an attempt to understand this unexpected result we examined whether a cytochrome P-450 mixed-function oxidase system participated in the mutagenic activation of AAF by guinea-pig liver, as it does in the case of mouse liver. The mutagenic activation was: (1) completely dependent on the addition of a co-factor, NADPH, to the mutation assay system, (2) completely suppressed by antiserum against NADPH--cytochrome c reductase, and (3) sensitive to a cytochrome P-450 inhibitor, 7,8-benzoflavone. These results indicate that the cytochrome P-450 enzyme system is essentially involved even in the mutagenic activation of AAF by guinea-pig-liver S9 fraction. Based on both the present and other data, the mechanism of the mutagenic activation is discussed to explain the observed high mutagenic potential of AAF in the presence of guinea-pig-liver S9 fraction.     

Mutagenicity of bithionol sulfoxide and its metabolites in the salmonella/mammalian microsome test

The mutagenic effects of bithionol sulfoxide and its two major metabolites, bithionol and bithionol sulfone, on 4 Salmonella typhimurium strains (TA97, TA98, TA100 and TA102) were investigated. Bithionol sulfoxide was found to be mutagenic to TA98 and TA100. However, mutagenicity was abolished in the presence of rat-liver S9 fractions.     

Hepatocyte-mediated mutagenicity of mononitrobenzo[a]pyrenes in Salmonella typhimurium strains

The mononitro-substituted isomers of benzo[a]pyrene (B[a]P), 1-, 3- and 6-nitrobenzo[a]pyrene (NB[a]P), are environmental pollutants and are metabolized to mutagens in Salmonella by rat-liver homogenate postmitochondrial supernatant (S9) fractions. In this study, activation of these compounds to mutagens was investigated using the hepatocyte-mediated Salmonella mutagenicity assay. Hepatocytes from rats treated with Aroclor 1254 activated both 3-NB[a]P and 1-NB[a]P to mutagens, while 6-NB[a]P was not mutagenic. The positive mutagenicity responses were functions of both the chemical dose and the hepatocyte concentration. By using a nitroreductase-deficient strain (TA98NR) and a transesterificase-deficient strain (TA98/1,8-DNP6), it was verified that the direct-acting mutagenicities of 1- and 3-NB[a]P primarily were due to metabolic processes involving nitroreduction while the S9- and hepatocyte-mediated mutagenicity responses were also dependent on transesterification. When compared with the mutagenic responses produced with S9, the mutations induced by 1- and 3-NB[a]P in the presence of hepatocytes were relatively more dependent upon nitroreductase metabolism and less on transesterification. Thus, intact hepatocytes were capable of activating 1- and 3-NB[a]P to mutagenic metabolites and some of these metabolites appeared to be different from those produced by S9.     

Mutagenic potency in Salmonella typhimurium of organic extracts of soil samples originating from urban, suburban, agricultural, forest and natural areas

The purpose of the present work was to assess the mutagenic potency of soil samples presumably not contaminated by industrial wastes and discharges. A set of 51 soil samples was collected from areas considered as not contaminated by a known industrial activity: 11 urban samples (collected in cities), 15 suburban samples (collected in villages), 7 agricultural samples, and 18 forest or natural samples. Each soil sample was collected at the surface (0-5cm deep), dried, sieved (2mm), homogenized before organic extraction (dichloromethane/acetone 1/1 (v/v), 37 degrees C, 4h, soil/solvent ratio 1/2, m/v), solvent exchange to DMSO and sterilizing filtration. The micro-method adaptation of the standard bacterial mutagenicity test on Salmonella typhimurium strain TA98 was performed with and without a metabolic activation system (rat-liver homogenate S9), and thus detected the effect of pro-mutagens and direct mutagens, respectively. The use of a pre-incubation method increased the sensitivity of the assay. The results obtained showed a wide range of effect levels, from no effect to clear mutagenicity. In particular, the extract of all 11 urban soil samples demonstrated mutagenic activity, while the extracts of 10 of the 15 suburban samples showed mutagenicity. On the other hand, the extract of only one of the 7 agricultural samples studied induced mutations, and none of the 18 natural or forest-soil samples investigated produced mutagenic extracts. These findings seem to indicate the crucial influence of the diffuse pollution originating from different human activities on the mutagenic potency of urban soil samples. These findings make it possible to classify the soils according to their mutagenic potency. No clear correlation was found between the mutagenicity detected in soil extracts and the measured polycyclic aromatic hydrocarbon (PAH) content of the soils investigated.     

The metabolic activation of dichloromethane and chlorofluoromethane in a bacterial mutation assay using Salmonella typhimurium

The metabolic activation and mutagenicity of dichloromethane and chlorofluoromethane were investigated using rat liver fractions and Salmonella typhimurium strain TA100. Both dihalomethanes gave a mutagenic response without the addition of rat-liver fractions. This response has been shown to be due to bacterial metabolism of the test compounds by pathways believed to be similar to those known in the rat. When rat-liver post-mitochondrial supernatant was added to the mutagenicity assay, there was no significant increase in the mutagenicity of dichloromethane, whereas a 2-fold increase was observed for chlorofluoromethane under the same conditions. This increase was derived both from glutathione conjugation and cytochrome P450 oxidative dehydrochlorination. A significant increase in dichloromethane mutagenicity could only be achieved by increasing the concentration of post-mitochondrial supernatant. Under these conditions the increase in mutagenicity was derived solely from glutathione conjugation of dichloromethane. The difference in mutagenic response after the addition of rat-liver fractions can be explained by differences in the half lives of the reactive intermediates rather than a difference in overall metabolic rate between the two compounds.