Protective effects of sodium selenite on killing and mutation by N-methyl-N''-nitro-N-nitrosoguanidine in E. coli.

Sodium selenite was found to protect Escherichia coli cells against killing and mutagenic effects of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG). Such protective effects were not observed when cells were treated with N-methyl-N-nitrosourea (MNU). The protection by sodium selenite was not controlled by the ada gene, which is responsible for the repair of alkylated damage in DNA. A reduction of the amount of glutathione was found when cells were treated with sodium selenite, and glutathione is known to be involved in the methylation of DNA by MNNG, not by MNU. Reduced methylation by MNNG due to the reduction of the amount of glutathione caused by abundant sodium selenite was suggested to be the mechanism of protection.     

Aphidicolin allows a rapid and simple evaluation of DNA-repair synthesis in damaged human cells

The decrease in microbial mutagenicity of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU) was compared in an animal mediation with rats and in direct incubation with human as well as rat blood and blood components. The mutagenic activity was assayed by reverse mutation from streptomycin (SM) dependence to non-dependence in Escherichia coli, strain Sd-B (TC). The mutagenic response curves of both MNNG and MNU were approximately linear and parallel at non-cytotoxic concentrations. However, the mutagenic capabilities of MNNG were estimated to be 10-fold more potent than those of MNU. The mutagenic activity in blood and liver preparations from rats killed immediately after intravenous injection of MNNG, 50 mg/kg, was negative. Results with MNU, 100 mg/kg, were positive in both cases.For the detection of mutagenicity, blood was diluted 50 times for the final testing mixture (1 ml) to avoid bactericidal effects of the blood itself. When a larger amount of liver preparation was used in the tests, and diluted 8 times, mutagenic activity was still detected 15 min after injection of MNU, 80 mg/kg. Comparisons of the diminished rate of mutagenicity between MNNG and MNU during certain periods of incubation with blood indicated that MNNG was inactivated much more rapidly than MNU with both human and rat blood. Plasma showed a moderate inactivating effect on both MNNG and MNU. Red blood cells inactivated MNNG at a remarkably rapid rate similar to that of whole blood, but was less effective on MNU. In further experiments with red- cell components, the cell contents inactivated both MNNG and MNU at rates similar to those with red cells, but cell membrane had absolutely no effect in decreasing the mutagenicity in either MNNG or MNU.     

Cross-resistance studies with V79 Chinese hamster cells adapted to the mutagenic or clastogenic effect of N-methyl-N′-nitro-N-nitrosoguanidine

When V79 cells are exposed to a single low dose of MNNG or MNU they acquire resistance to the mutagenic or to the clastogenic effect of the agents. Here the effect of MNNG pretreatment on mutagenesis (6-thioguanine resistance) and aberration formation in cells challenged with various mutagens/clastogens is reported. MNNG-adapted cells were resistant to the mutagenic effects of MNU and, to a lower extent, of EMS. No mutagenic adaptation was observed when MNNG-pretreated cells were challenged with MMS, ENU, MMC or UV.

Cells pretreated with a dose of MNNG which makes them resistant to the clastogenic effect of this compound were also resistant to the clastogenic activity of other methylating agents (MNU, MMS), but not so with respect to ethylating agents (EMS, ENU). Cycloheximide abolished the aberration-reducing effect of pretreatment. However, when given before the challenge dose of MNNG, MNU or MMS, it drastically enhanced the aberration frequency in both pretreated and non-pretreated cells. No significant enhancement of aberration frequency by cycloheximide was found for ethylating agents.

The results indicate that clastogenic adaptation is due to inducible cellular functions. It is concluded that mutagenic and clastogenic adaptation are probably caused by different adaptive repair pathways.     

Cytotoxicity and mutagenicity of alkylating agents in cultured mammalian cells (CHO/HGPRT system): mutagen treatment in the presence or absence of serum

Cytotoxicity and mutation induction at the hypoxanthine-guanine phosphoribosyl transferase locus in Chinese hamster ovary cells (CHO/HGPRT system) were measured for a range of concentrations of 6 alkylating agents [methyl and ethyl methanesulfonate (MMS, EMS), N-methyl- and N-ethyl-N'-nitro-N-nitrosoguanidine (MNNG, ENNG), and methyl- and ethyl-nitrosourea (MNU, ENU)] to determine the effect of the presence or absence of serum during the time of mutagen treatment. Cultures were treated with the mutagens for 5 h, a time period which results in no growth inhibition in the absence of serum, to estimate the potential decrease in effective mutagen dose to the cells which might result from reactivity with the serum proteins. With all 6 agents, identical results were found for cytotoxicity and for mutagenicity regardless of the presence or absence of serum during treatment. This finding demonstrates that the use of serum in cell-culture medium does not present any problems in apparent dosimetry studies, at least with these alkylating agents.     

The influence of selenium and caffeine on chemical carcinogenesis in rats,mutagenesis in bacteria,and unscheduled DNA synthesis in human lymphocytes

The influence of sodium selenite (Na₂SeO₃) and caffeine on chemical carcinogenesis induced in rats by diethylnitrosamine (DEN), N-nitrosomorpholine (NM), andN-methyl-N-nitro-N-nitrosoguanidine (MNNG) was investigated. A dose-dependent inhibitory effect of Na₂SeO₃ (l–10 ppm) on hepatocarcinogenesis induced by DEN was demonstrated. Na₂SeO₃ also increased the latency period for stomach tumor formation in rats treated with MNNG. Combined treatment of rats with Na₂SeO₃ plus vitamin C added to the diet resulted in a slight inhibition of NM-induced liver carcinogenesis. Supplementation of diet with Na₂SeO₃ plus butylated hydroxytoluene, vitamin C, and vitamin E did not reveal any additive inhibitory effect compared to the inhibitory effect of Na₂SeO₃ given alone. Caffeine (600 rag/L) reduced the number of liver tumors induced in rats by DEN. Preliminary experiments have indicated that combined treatment of rats with selenium and caffeine could result in more effective inhibition of DEN-induced liver carcinogenesis. Further experiments are being conducted to study the influence of selenium and caffeine on mutagenic activity of 1-methyl-l-nitrosourea (MNU) inSalmonella typhimurium TA 1535. The pretreatment of bacteria cells with Na₂SeO₃ (3–10 p.g/mL) increased the mutagenic response of bacteria to MNU. A synergistic stimulation of mutagenic activity of MNU was observed in bacteria pretreated simultaneously with Na₂SeO₃ and caffeine. In addition the influence of Na₂SeO₃ on UDS induced by DEN in human lymphocytes was investigated. The trace element inhibited the UDS up to 82%. The possible role of potentiation by NazSeO3 of the cell killing effect of DEN in inhibition of liver carcinogenesis was discussed.     

Modulation of the mutagenic response in prokaryotes.

Short-term tests investigating genetic end-points in prokaryotes have been extensively used worldwide not only for risk assessment purposes but also for evaluating the modulation of the mutagenic response. In spite of some intrinsic limitations, such as the lack of cell compartmentalization or the need for an exogenous metabolic system working extracellularly, experimental systems in bacteria can provide useful preliminary indications and some information on the mechanisms involved. In the large majority of studies the putative modulator is mixed with a known mutagen and then assayed in target bacteria, with suitable controls. However, under natural conditions exposure of target cells to modulators may either precede, co-exist with, or follow exposure to mutagens. Therefore, a variety of methodological variations, involving pre-treatment, co-treatment, or post-treatment of bacteria with the putative modulator, have been designed. Application of these procedures showed that the effects of modulators can be completely upset, from inhibition to enhancement, or vice versa, by changing the experimental conditions. Use of methodological variations may provide more complete information on the spectrum of possible effects in bacteria as well as a better insight into modulation mechanisms. Several examples illustrating the flexibility of the Salmonella test in this field of research are available. On the other hand, the widespread use of these relatively simple techniques, yet requiring skillfulness and experience, may lead to some misuse or oversimplifications. A rather common inadequacy is to use excessive amounts of test mutagens, or to express the results in terms of revertants/survivors, rather than revertants/plate. In fact, in the Salmonella test the number of revertants is rather unrelated to the initial number of plated bacteria, provided a normal background lawn of bacterial growth is formed. Thus, a 50% killing of bacteria will not appreciably influence the number of revertants/plate, but expressed as revertants/survivors the effect will look twice as large.     

Comparison of toxicity and mutagenicity of methylnitrosourea, methylnitronitrosoguanidine and ICR-191 among human lymphoblast lines

The toxic and mutagenic effects of the alkylating agents methylnitrosourea (MNU) and methylnitronitrosoguanidine (MNNG) and of the frameshift mutagen, ICR-191 were compared among 3 human diploid lymphoblast lines, MIT-2, WI-L2 and GM 130. The MIT-2 and WI-L2 lines were both sensitive to the toxic and mutagenic effects of all 3 agents tested. The WI-L2 line was more sensitive to the toxic effects of MNU and MNNG than the MIT-2 line, while it was somewhat less sensitive to the mutagenic effects of these alkylating agents. The GM 130 line was strikingly resistant to both the toxic and mutagenic effects of the alkylating agents. The order of sensitivity to the toxic effect of ICR-191 was MIT-2 > WI-L2 > GM 130, while the order of sensitivity to the mutagenic effects of this frameshift mutagen was GM 130 > MIT-2 > WI-L2. These results point to the importance of accounting possible variations in mutability among individuals when extrapolating from any single mutagenicity assay for human risk assessment.     

Crude tea extracts decrease the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine in vitro and in intragastric tract of rats

The effects of tea extracts and their ingredients, catechins and L-ascorbic acid (AsA), on the mutagenicity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were examined in vitro and in the stomachs of rats using E. coli WP2 and S. typhimurium TA100. The extracts of green tea and black tea leaves decreased the mutagenic activity of MNNG to E. coli WP2 in vitro in a desmutagenic manner. Catechins such as (-)-epigallocatechin from green tea leaves and the low-molecular-weight tannin fraction isolated from black tea extract with HP-20 resin also exhibited inhibitory effects against the mutagenic activity of MNNG. A desmutagenic effect of AsA on MNNG-induced mutagenicity was observed depending on the dose, though it was complicated. The effects were also demonstrated in the stomachs of rats by assaying the bacterial mutagenic in vitro; the tea extracts previously given orally to rats reduced the mutagenic activity of MNNG remarkably, though simultaneous administration showed less effect. The effectiveness of tea extracts for the decrease of MNNG-induced mutagenesis in vitro and in vivo suggests that the habitual drinking of tea may reduce the tumor-initiating potency of MNNG-type nitrosoureido compounds if they are formed in the stomach.     

Gene expression in E. coli after treatment with streptozotocin

Gene induction by the methylating agents streptozotocin (STZ), N-methyl-N-nitrosourea (MNU), and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was evaluated in E. coli fusion mutants. These mutants have fusions of the lac operon to genes induced by treatment with sublethal levels of alkylating agents and were previously selected from random insertions of the Mu-dl (Apr lac) phage by screening for induction of beta-galactosidase activity in the presence of methyl methanesulfonate or MNNG. The results demonstrate that STZ differs from MNNG and MNU in failing to induce aidC expression. Further, expression of aidC after exposure to MNU and MNNG occurs only in nonaerated cultures; aeration blocks the induction. Induction of aidD, alkA, aidB, and sfiA expression occurs with all 3 agents although at markedly lower concentrations of MNNG and STZ compared to MNU. alkA and to a lesser extent aidD mutants of E. coli strains were more sensitive to these agents, while no differences were evident between wild-type and aidB or aidC fusion mutants.     

Additive coclastogenicity of sodium selenite and caffeine in CHO cells treated withN-methyl-N′-Nitro-N-Nitrosoguanidine

The clastogenic effect ofN-methyl-N′-nitro-N-nitrosoguanidine (MNNG) in Chinese hamster ovary (CHO) cells and its modulation by Na₂SeO₃ and caffeine were studied by metaphase analysis of chromosome aberrations (CA) as well as by measuring the formation and repair of single-strand (ss) DNA breaks employing hydroxylapatite chromatography. Treatment of CHO cells with MNNG (1.25 or 2.5 × 10-5M) for 3 h caused CA in 11 and 19% of metaphases scored, respectively. Pretreatment of cells with Na₂SeO₃ (1–5 μg/mL) or caffeine (0.2–2.0 mg/mL) for 2 h resulted in a 2–3.5-fold increase of CA frequency. Addition of both modulators during the mutagen exposure tended to cause a slight inhibition of clastogenic activity of MNNG (1.25 × 10⁻⁵ M) or had no effect on CA number when MNNG was used at a concentration of 2.5 × 10−5M. Posttreatment of CHO cells with Na₂SeO₃ for 20 h after MNNG was ineffective in influencing the number of metaphases with CA, whereas, at these conditions, caffeine enhanced up to 6-7-fold the clastogenic activity of MNNG. Addition of both modulators during the whole experiment, 2 h pretreatment included, resulted in a further significant increase of CA frequency up to the total pulverization of chromosomes in all metaphases scored. The coclastogenic effect of caffeine was greater in this case. The enhancement of chromosome-damaging activity of MNNG by selenite and caffeine was better expressed when this carcinogen was applied at the higher concentration used. An additive coclastogenic effect was observed in CHO cells treated simultaneously with Na₂SeO₃ and caffeine plus MNNG. In addition, the treatment of CHO cells with MNNG (5 × 10⁻⁶ M) caused a rapid increase of ssDNA breaks number reaching maximal values after 30–45 min. However, up to 50–60% of MNNG-induced ssDNA breaks were repaired during the first 60–150 min after the mutagen exposure. The 2 h pretreatment of CHO cells with Na₂SeO₃ (2 μg/mL) or the addition of this trace element after MNNG had no effect on formation and repair of MNNG-induced ssDNA breaks. The coclastogenic effect of Na₂SeO₃ in CHO cells treated with MNNG was not directly linked to the induction and disappearance of ssDNA breaks measured by hydroxylapatite chromatography.     

Differential sensitivity of DNA replication and repair in permeable Escherichia coli exposed to various monofunctional methylating or ethylating agents.

Escherichia coli cells made permeable to deoxynucleoside triphosphates by brief treatment with toluene (permeablized) were used to measure the effect of the following chemical alkylating agents on either DNA replication or DNA repair synthesis: methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS), N-methyl-N-nitrosourea (MNU), N-ethyl-N-nitrosourea (ENU), N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) and N-ethyl-N′-nitro-N-nitrosoguanidine (ENNG). Replication of DNA in this pseudo-in vivo system was completely inhibited 10–15 min after exposure to MMS at concentrations of 5 mM or higher or to MNU or MNNG at concentrations of 1 mM or higher. The ethyl derivatives of the alkylating agents were less inhibitory than their corresponding methyl derivatives, and inhibition of DNA replication occurred in the following order: EMS < ENNG < ENU. Maximum inhibition of DNA replication by all of the alkylating agents tested except EMS occurred at a concentration of 20 mM or lower. The extent of replication in cells exposed to EMS continued to decrease with concentrations of EMS up to 100 mM (the highest concentration tested).The experiments in which the inhibition of DNA replication by MMS, MNU, or MNNG was measured were repeated under similar assay conditions except that a density label was included and the DNA was banded in CsCl gradients. The bulk of the newly synthesized DNA from the untreated cells was found to be of the replicative (semi-conservative) type. The amount of replicative DNA decreased with increasing concentration of methylating agent in a manner similar to that observed in the incorporation experiments.Polymerase I (Pol I)-directed DNA repair synthesis induced by X-irradiation of permeablized cells was assayed under conditions that blocked the activity of DNA polymerases II and III. Exposure of cells to MNNG or ENNG at a concentration of 20 mM resulted in reductions in Pol I activity of 40 and 30%, respectively, compared with untreated controls. ENU was slightly inhibitory to Pol I activity, while MMS, EMS, and MNU all caused some enhancement of Pol I activity.These data show that DNA replication in a pseudo-in vivo bacterial system is particularly sensitive to the actions of known chemical mutagens, whereas DNA repair carried out by the Pol I repair enzyme is much less sensitive and in some cases apparently unaffected by such treatment. Possible mechanisms for this differential effect on DNA metabolism and its correlation with current theories of chemically induced mutagenesis and carcinogenesis are discussed.     

Cell killing by various monofunctional alkylating agents in Chinese hamster ovary cells

Cell killing by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), N-methyl-N-nitrosourea (MNU), N-ethyl-N-nitrosourea (ENU), and methyl methanesulfonate (MMS) was measured in Chinese hamster ovary (CHO) cells using the colony-formation assay. Cell killing by these agents was determined in exponentially growing asynchronous cells, in synchronous cells as a function of cell-cycle position and in nondividing cells. Distinct differences in the cytotoxic effect of the 4 alkylating agents were found in respect to dose-response, cell cycle phase-sensitivity and growth state. MNNG and MNU showed the same biphasic dose-survival relationship in exponentially growing cells, with an initial steep decline followed by a shallow component. The shallow component disappeared in growth-arrested cells. MNNG and MNU differed, however, in the cell-cycle age response. No cell-cycle phase difference was seen with MNNG, whereas cells in G1 seemed more sensitive to MNU than cells in S phase. MMS and ENU both showed shouldered dose-response curves for exponentially growing asynchronous cells, and the same cell-cycle pattern for synchronous cultures with cells in early S phase being the most sensitive. However, survival of nondividing cells versus dividing cells was reduced much more by MMS than by ENU. Caffeine, which interferes with the regulation of DNA synthesis and is known to modify cell killing by DNA-damaging agents, enhanced cell killing by all agents. It is concluded that there must be a number of factors which contribute to cell killing by monofunctional alkylating agents, and that besides alkylation of DNA reaction with other cellular macromolecules should be considered.     

Two types of antimutagenic effects of gallic and tannic acids towards N-nitroso-compounds-induced mutagenicity in the amesSalmonella Assay

The frequency ofhis ⁺ revertants induced by N-methyl-N-nitrosourea (MNU) and N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) in the strain TA100 ofSalmonella typhimurium was decreased by gallic and tannic acid. In weak buffer solutions, the inhibition effects of gallic acid towards MNU and MNNG mutagenicity was caused primarily by a decrease of pH in the incubation mixtures. At adjusted pH (pH 5.0 and 6.5), the antimutagenic effects are largely the result of an interaction between MNU or MNNG with phenolic acids outside the cells.     

Mechanisms of mutagenicity and toxicity of sodium selenite (Na2SeO3) in Salmonella typhimurium

The mechanisms of selenite toxicity and mutagenicity in S. typhimurium have been characterized. In contrast to previous reports, selenite toxicity was shown not to involve nonspecific incorporation into protein via the sulfur metabolic pathways. Selenite toxicity was, however, shown to involve its ability to act as an oxidizing agent, primarily through reactions with sulfhydryls. Strains which lack glutathione (GSH) are more sensitive to killing by sulfhydryl reagents. The selenite sensitivity of such a mutant was a biphasic phenomenon. The mutant was much more sensitive than a strain which contained GSH at lower selenite concentrations whereas, at higher concentrations, the mutant was much more resistant to selenite. The mechanism of selenite toxicity at lower concentrations in this mutant thus appeared to involve damage to intracellular sulfhydryls. The sensitization to higher doses of selenite by GSH could be explained by the generation of toxic oxygen species. The in vitro reactions of selenite with both cysteine and GSH readily produced H2O2 and O2-. A S. typhimurium strain which overproduces superoxide dismutase (SOD) and catalase was more resistant to high concentrations of selenite, but not killing by the lower doses. Pretreatment of cells with a nonlethal dose of selenite induced the synthesis of proteins which protected the cells from killing by H2O2 or high doses of selenite. Selenite was also a mutagen in the tester strain TA104, in which a number of other oxidizing agents have also been found to be mutagens. These results were consistent with a model in which the reactions of selenite and intracellular thiols with concomitant production of active oxygen species are the primary causal agents of selenite mutagenicity and toxicity in S. typhimurium.     

Modulation of diethylnitrosamine carcinogenesis in rat liver and oesophagus

A series of 16 experiments, using a total of 2,000 BD₆ rats, was designed in order to assess the ability of 8 individual agents or their combinations to modulate the liver and oesophageal carcinogenesis induced by multiple doses of diethylnitrosamine (DEN). Of the antioxidants tested, sodium selenite, ascorbic acid, and butylated hydroxytoluence generally exhibited protective effects on both types of tumors. In contrast, retinoic acid behaved as a promoter of DEN hepatocarcinogenesis, but this effect could be eliminated by its combination with either selenite or butylated hydroxytoluene. Caffeine and theophyline, when individually assayed, were devoid of significant protective effects, and the later methylxanthine stimulated oesophageal tumorigenesis when administered afer exposure to the carcinogen. Caffeine tended to decrease tje multiplicityof tumors and potentiated the inhibitory effect of selenite in the liver. Irrespective of combination with caffeine, treatment with phwnobarbital before each DEN injection tended to reduce the multiplicity of both liver and oesophageal tumors. On the other hand, the metabolic inhibitoe diethyldithiocarbamate, given after each DEN injection, dramatically enhancedd the incidence and multiplicity of oesophageal tumors. Thus, on the whole, modulation of DEN carcinogenesis varied depending on test agents, their conbinations, dosages, treatment schedules, and target organ.     

In vitro anti-mutagenic effect of magnolol against direct and indirect mutagens

Magnolol, a component of the bark of Magnolia obovata, has been reported to possess various biological activities, such as anti-carcinogenicity, anti-promotion activity and anti-oxidative activity. These findings suggest potential for this compound in cancer chemoprevention. Interestingly, there have been no reports to date on the potential anti-mutagenic activity of magnolol, involving inhibition of initiation processes of the primary stage of carcinogenicity. In this study, anti-mutagenic activity of magnolol against mutagenicity induced by direct mutagens [1-nitropyrene (1-NP), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG)] and indirect mutagens [2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-aminodipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2), benzo(a)pyrene (B(a)P), 2-aminoanthracene (2-AA) and 7,12-dimethylbenz[a]anthracene (DMBA)] were investigated using the bacterial mutagenicity test (Ames test). Results show that magnolol strongly inhibits mutagenicity induced by indirect mutagens, but does not affect direct mutagens. To elucidate the mechanism of this effect against indirect mutagens, effect of magnolol on CYP1A1- and CYP1A2-related enzyme activities of ethoxyresorufin-O-deethylase (EROD) and methoxyresorufin-O-demethylase (MROD) were investigated. Magnolol strongly and competitively suppressed these enzyme activities, suggesting it inhibited mutation induced by indirect mutagens through suppression of CYP1A1 and CYP1A2 activity.     

Antimutagenicity of propionic acid bacteria.

The antimutagenic effect of dialysed cell extracts of 4 strains of propionic acid bacteria was examined against the mutagenicity of sodium azide in the TA1535 tester strain of Salmonella typhimurium using the Ames test. It was noted that dialysates of 2 strains of Propionibacterium shermanii, P. pentosaceum and P. acnes, significantly reduced sodium azide-induced revertants. The dialysate of propionic acid cocci did not show an antimutagenic effect. The inhibitory activity was enhanced if the mutagen and extract were coincubated for 20 min prior to performing the mutagenicity assay. Antimutagenicity of dialysates from P. shermanii VKM-103 against MNNG and 9-aminoacridine was shown in S. typhimurium strains TA1535 and TA97. The antimutagenic activity was found in the protein fraction of the cell extract of P. shermanii. The proteins of the dialysate of P. shermanii were separated using a Toyopearl gel column into 3 main peaks according to their molecular weights. The antimutagenic activity towards sodium azide was found in the second and the third peaks. We suggest that dialysates of the cells of propionic acid bacteria contain several kinds of antimutagenic substances with different molecular weights.     

Synergic activity of selenium and probiotic bacterium<Emphasis Type="Italic">Enterococcus faecium</Emphasis> M-74 against selected mutagens in<Emphasis Type="Italic">Salmonella</Emphasis> assay

Concentrated extracts of MRS (De Man-Rogosa-Sharpe) media in which probiotic bacterium Enterococcus faecium strain M-74 was grown exerted different antimutagenic activity against ofloxacin-, N-methyl, N'-nitro-N-nitrosoguanidine- and sodium 5-nitro-2-furylacrylate-induced mutagenicity in Salmonella typhimurium assay depending on the presence (+Se) or absence of disodium selenite pentahydrate (-Se). The antimutagenicity of MRS(+Se) extract was higher than that of MRS(-Se) extract. Selenium enhanced also the antimutagenic effect of both live and killed cells of E. faecium M-74, respectively. The live bacteria decreased the mutagenicity of selected substances more than killed cells. Synergic activity of selenium with the bacterium was also manifested.     

Coenzymatic Activity of Epinephrine for Vitamin B2-Aldehyde Forming Enzyme and Its Physiological Significance

The effects of artificial food dyes on the mutagenicity of carcinogenic mutagens were examined using the Salmonella/microsome system. Indigocarmine (IC), an indigoid dye widely used for coloring foods and for clinical tests, enhanced the mutagenic activity of Trp-P-1, a carcinogenic pyrolysate of tryptophan, depending on the dose of IC. His⁺ revertants of TA 98 induced by Trp-P-1 were two to four times greater in the presence of 10 to 50 μg/plate of IC than those in the absence of IC.

IC also enhanced the mutagenicity of Trp-P-2, another carcinogenic pyrolysate of tryptophan, while the activities of other mutagens such as MNNG, 4-NQO, AF-2, BP, Glu-P-1 were not affected.     

Mutagenicity and cytotoxicity of congeners of two classes of nitroso compounds in Chinese hamster ovary cells

The induction of mutation by certain nitrosamidines and nitrosamides has been quantitated utilizing the hypoxanthine--guanine phosphoribosyl transferase (HGPRT) locus in Chinese hamster ovary cells. Dose--response relationships for cytotoxicity and mutagenicity are presented for N-methyl-N-nitrosourea (MNU), N-ethyl-N-nitrosourea (ENU), N-butyl-N-nitrosourea (BNU), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG). Based on the concentration of each agent required to kill 90% of the cells, the following order of cytotoxicity was observed: MNNG greater than ENNG greater than MNU greater than ENU greater than BNU. This is the same order of potency as observed for mutation induction per unit concentration of mutagen.