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G R Mohn

Publications and source records attributed to G R Mohn.

At least 55 records · Page 3Linked to original sources
PubMed1984

The direct mutagenic activity of alpha, omega-dihalogenoalkanes in Salmonella typhimurium. Strong correlation between chemical properties and mutagenic activity.

A series of 18 alpha, omega-dihalogenoalkanes (kappa(CH2)n kappa with n = 1-6 and kappa = Cl, Br, I) was tested for direct mutagenic activity in Salmonella strains TA1530, TA1535 and TA100 using spot-test procedures. The results indicate that the mutagenic behaviour of these compounds is strongly dependent upon the carbon chain length as well as the type of halogen involved. This behaviour correlates with the leaving group ability and the degree of neighbouring group participation in nucleophilic displacement reactions of the different halogen atoms.

Hydrocarbons, Halogenated↗
PubMed1984

Factors influencing the mutagenic activity of the colon carcinogen 1,2-dimethylhydrazine in Salmonella typhimurium strain TA 1535 in vitro.

The colon carcinogen 1,2-dimethylhydrazine (SMDH), a non-mutagen in the standard Ames assay, has been shown in previous experiments to become weakly mutagenic in Salmonella TA 1535 in vitro, when specific test conditions were used. The present studies were performed to determine more precisely the nature of metabolic factors and experimental conditions for optimal mutagenesis of SDMH in the same strain of Salmonella. First, it was confirmed that both the presence of rat liver S9 fractions (25 microliters/ml incubation mixture) and prolonged pre-incubation periods in liquid medium of at least 120 min were necessary to elicit SDMH mutagenesis. In contrast to results obtained with dimethylnitrosamine, which served as a model compound for the activation through oxidative, cytochrome P-450- and NADPH-dependent enzymatic processes, the activation of SDMH to mutagenic factors was not dependent on the presence of NADPH: in fact, NADPH strongly reduced the SDMH-induced mutation yields. It was also observed that growth of the indicator bacteria is an important prerequisite for mutation induction by SDMH. Aminoacetonitrile and disulfiram, two inhibitors of SDMH metabolism and carcinogenicity in mammals, also strongly inhibited SDMH mutagenesis in the present in vitro assay. It can, therefore, be concluded that (i) the right test protocol is of crucial importance for the detection of SDMH as a bacterial mutagen, and (ii) that activation pathways in vitro are (partially) different from presumed in vivo metabolism and activation.

1,2-Dimethylhydrazine↗
PubMed1983

Induction of repairable DNA damage in Escherichia coli and interaction with DNA in vitro by the radical cation of chlorpromazine.

Studies were performed to determine the DNA interactions of and the induction of cytotoxic effects by the radical cation (CPZ+.) formed enzymatically from chlorpromazine (CPZ): in the presence of native DNA the lifetime of CPZ+. is markedly increased. The decreased reactivity of CPZ+. in the presence of native DNA and the concomitant increased viscosity of CPZ+.-DNA complexes strongly support the assumption that CPZ+. does form intercalation complexes with DNA. The relative strong bacteriotoxicity of CPZ+. hindered the accurate determination of mutagenesis in various Salmonella indicator strains, but a test for repairable DNA damage in Escherichia coli using various repair-deficient strains indicated that the cytotoxic action of CPZ+. is in part due to DNA alterations which can be excised in wild-type DNA repair-proficient strains. After activation of CPZ with long wavelength UV light, genetic effects are observed in S. typhimurium strain TA98, as well as in the E. coli tester strains. The possible role of CPZ+. in the photosensitization of CPZ is discussed.

Biotransformation↗
PubMed1983

A differential DNA-repair test using mixtures of E. coli K12 strains in liquid suspension and animal-mediated assays.

The feasibility of performing tests for repairable DNA damage in animal assay procedures was investigated by using repair-proficient and repair-deficient derivatives of E. coli K12 strain 343/113, including mutations in the uvrB, recA, polA and dam genes. To avoid variations in the relative recovery of viable cells from different samples, the strains were further marked with auxotrophic growth requirements, so that mixtures could be treated and the survival of each strain determined individually on media containing the corresponding growth factors. Spot tests were performed with the various strains to re-assess the necessity of using a combination of repair deficiencies, when genotoxic agents of differing mode of action are to be detected. Liquid suspension tests on mixtures of the different strains, furthermore, confirmed that the survival of the individual strains can be determined separately on selective media after treatment with methyl methanesulfonate (MMS) and methyl nitrosourea (MNU). These tests were also used to demonstrate that dimethyl nitrosamine (DMNA) is activated by Aroclor-1254-induced rat-liver S9 fractions to genotoxic products, as measured by the low survival of a recA derivative compared with the repair-proficient wild-type strain. Intrasanguineous host-mediated assays using the present derivatives of E. coli K12/343/113 showed that the various strains, injected simultaneously into mice, could be recovered in amounts sufficient for the individual determination of the relative survival in liver, spleen, lungs, kidneys, pancreas and the blood stream of the host animals. Using a mixture of the repair-proficient parent and the recA derivative inoculated into mice that were subsequently treated with MMS, NMU or DMNA, we found that these chemicals induce a larger decrease in survival in the recA strain as compared with the wild-type in cells recovered from the liver and the spleen. The order of genotoxic potency so determined was DMNA greater than MNU greater than MMS; this is similar to the ranking of the carcinogenicity of these compounds in rodents and probably also reflects the various degrees of DNA alkylation in cells of the livers of the treated animals. The general usefulness of the host-mediated differential DNA repair assay for detecting genotoxic factors in various organs of animals remains to be assessed by using chemical mutagens of different modes of action.

Animals↗
PubMed1983

Molecular dosimetry of the chemical mutagen ethyl methanesulfonate. Quantitative comparison of the mutagenic potency in Neurospora crassa and Saccharomyces cerevisiae.

Extending previous work with E. coli and mammalian cells in culture, forward-mutation frequencies induced by ethyl methanesulfonate (EMS) were quantitatively compared in Neurospora crassa and Saccharomyces cerevisiae under standardized conditions. Concomitantly, the actual dose to DNA was measured by determining the amount of radioactivity bound to DNA after treatment with tritium-labeled EMS. After exposure to EMS (2.5-50 mM), alkylation levels in N. crassa and S. cerevisiae were similar to those previously determined in E. coli and cultured mammalian cells. Consistently, there was a slightly less than proportional increase of the DNA alkylation level with the exposure concentration of the mutagen. Forward mutagenesis induced in yeast and N. crassa showed exponential kinetics with exponents of 1.5 and 2.6, respectively. These results are similar to those previously reported with E. coli, which differed from the results with cultured mammalian cells, where a linear dose-effect relationship between exposure and genetic effect was observed. These differences may reflect differences in the fate of EMS-induced adducts by cellular DNA repair systems, but are not due to initial differences in DNA alkylation levels. The fate and persistence of specific DNA adducts potentially responsible for pre-mutagenic changes are under investigation.

Animals↗
PubMed1983

Induction of chromosomal alterations as an assay for cytostatic drug activity in plasma.

Information about the extent and persistence of cytostatic activity in blood plasma after administration of a cytostatic drug into the body is needed for a better evaluation of the inter-individual variations in drug metabolism and disposition. As an assay for cytotoxic activity, a test system was chosen in which Chinese hamster ovary cells (CHO) were incubated with plasma containing active metabolites of cyclophosphamide (from human patients or rats), after which the frequencies of induced sister-chromatid exchanges per cell were determined. The treatment with plasma increased the frequencies of SCEs very effectively at concentrations of metabolites that were negative in the Salmonella typhimurium back-mutation test with strain TA100. The results obtained indicate that the SCE test system offers the possibility to follow the cytotoxic activity of plasma at various time intervals after administration of cyclophosphamide.

Animals↗
PubMed1981

On the involvement of singlet oxygen in mutation induction by 8-methoxypsoralen and UVA irradiation in Escherichia coli K-12.

The possible mutagenic effects induced by single oxygen, which is formed during UVA irradiation of bacterial cells pretreated with 8-methoxypsoralen (8-MOP), were investigated. As genetic endpoint, black mutation from arg-56 to arg+ was assayed in strain Escherichia coli K-12/343/113/uvrB; this system, in preliminary experiments, was rather sensitive to 8-MOP-induced photodynamic effects. To assess the involvement of singlet oxygen (1O2) in the mutation induction process, 2 tests were applied, namely, comparative mutation induction in D2O and H2O media (pH 7.0) and quenching of 1O2 with 1,4-diazabicyclo[2.2.2]octane (DABCO). When photodynamy was performed with the indicator cells suspended in D2O buffer, the mutagenic effect was substantially higher than that obtained with cells suspended in H2O buffer; this increase was even more pronounced when the incubation mixtures were thoroughly oxygenated before irradiation. D2O itself was not mutagenic under th present experimental conditions. Addition of DABCO in concentrations of 0.1--10 mM to the irradiation mixtures effectively reduced the number of 8-MOP-induced mutant yields by about 40%. DABCO itself had no effect on cell viability or on spontaneous mutation frequency under our experimental conditions. From these 2 sets of results, and from the preliminary findings that the photomutagenic effect of 8-MOP is higher in the uvrB derivative than in the corresponding excision-repair-proficient parent strain, which is in concordance with previous observations in other E coli strains, it can be concluded that 1O2 generated upon UVA irradiation of 8-MOP solutions is probably responsible for part of the observed genetic effects.

DNA Repair↗
PubMed1981

Bacterial systems for carcinogenicity testing.

During the past 30 years, bacterial test systems have been extensively refined in their ability to detect not only mutagenic agents but, in many cases, carcinogenic ones as well. Since many carcinogens are known to be activated within the mammalian body, major improvements in bacterial test systems were made when representative parts of mammalian metabolism were included as part of the test protocol. Presently, systems of great simplicity and convenience are available for the efficient detection of gene mutations, lysogenic induction of prophages, and differential DNA repair. These qualities render bacterial systems potentially useful in distinguishing between carcinogens and non-carcinogens, in characterizing induced mutation spectra, and possibly in quantifying mutagenic potency that may be used to predict tumor-initiating potency. Sensitive strains of Salmonella typhimurium. Escherichia coli and Bacillus subtilis with altered DNA-repair capacities have been constructed which accurately identify many carcinogens. Comparative studies have shown that techniques using these strains can be standardized to some extent and that the majority of carcinogens are active in all adequately sensitive genetic systems. Because of this redundancy, it may be sufficient to employ only one standardized set of tester strains and methodology. However, serveral classes of known carcinogens are undetected or underestimated when assayed in standard testing procedures. Some of these chemicals can be efficiently recognized as mutagens upon varying the methodology, the genetic endpoint, or the mammalian activation system. Thus, to modify and adjust the experimental protocol to the particular type of chemical under study and to calibrate the system with appropriate carcinogenic and non-carcinogenic reference compounds is advisable. It is noteworthy that chemical carcinogens which probably act by non-genotoxic mechanisms thus far remain undetected in bacterial tests. Newly developed systems which measure specific types of genetic events, such as transpositions of DNA segments and derepression of genes, presently are being tested for their ability to detect such carcinogens. A final matter of growing concern is the increasing number of environmental chemicals that are found to be mutagenic in bacteria but for which information about carcinogenic activity in vivo is insufficient. The possible use of bacteria for quantifying mutagenic potency and extrapolating this information to tumor-initiating potency can be envisaged in three ways: (i) direct extrapolation from standard in vitro tests, (ii) indirect extrapolation making use of an in vitro/in vivo comparison of induced effects (the parallelogram method) as devised by Sobels [138] on the basis of identical dose (to DNA), and (iii) host-mediated assays to assess mutagenic potency of carcinogens in selected organs of mammals...

Bacillus subtilis↗
PubMed1980

Preliminary studies on the ability of Drosophila microsomal preparations to activate mutagens and carcinogens.

Subcellular fractions from Drosophila melanogaster, known to have several xenobiotic-metabolizing enzymatic activities, were investigated with respect to their ability to biotransform compounds that require metabolic activation before exerting mutagenic effects. Nitrofurazone, dimethylnitrosamine, cyclophosphamide and 2-acetylaminofluorene were activated to mutagens upon incubation with Drosophila microsomes or 20000 x g supernatant: mutagenicity was observed in Chinese hamster ovary cells, Escherichia coli strains 343/113/R-9 and 343/113/uvrB, and Salmonella typhimurium TA1538. Under the conditions used, microsomal preparations of Drosophila were not able to activate benzo[a]pyrene to a mutagen for Salmonella typhimurium TA98. The spectrum of mutagenic effects observed shows some correlation with the known mutagenicity of these compounds in vivo in Drosophila melanogaster. Drosophila microsomes appeared to be at least as active as rat-liver microsomes when compared in this type of mutagenicity testing.

Animals↗
PubMed1980

Molecular dosimetry of the chemical mutagen ethyl methanesulfonate: quantitative comparison of mutation induction in Escherichia coli, V79 Chinese hamster cells and L5178Y mouse lymphoma cells, and some cytological results in vitro and in vivo.

Molecular dosimetry studies were carried out to measure the extent of binding of radio-labeled ethyl groups to the DNA of Escherichia coli, V79 Chinese hamster cells and L5178Y mouse lymphoma cells treated with ethyl methanesulfonate (EMS). The results show that (1) the amount of ethylation of the DNA is similar in these cells when treatment conditions are identical, (2) the relationship between dose to DNA (ethylations per nucleotide) versus exposure (mM applied concentration) is non-linear in the sense that less alkylation of the DNA is observed at the higher exposures than would be predicted on the basis of proportionality between dose to DNA and exposure, and (3) the non-linearity of the genetic response in the bacterial cells is not reflected in a non-linearity of the alkylation of the DNA in those cells. Quantitative comparison of the frequencies of gene mutations in the various systems shows that the mutation frequency per unit of DNA alkylation is heterogeneous among the mammalian cell systems and that the frequencies observed in the bacterial cells fall within the range observed with mammalian cells. Alkylation of the DNA in the bone marrow, testis and liver of Swiss random-bred mice was also measured. The results support the conclusion that the distribution of the compound to the various tissues is rapid and probably uniform. Quantitative assessment of the cytological data (micronuclei, sister-chromatid exchanges, etc.) on the basis of dose was not as useful because of the low efficiency of EMS for inducing cytologically observable damage.

Animals↗
PubMed1980

Mutagenic activation of dibromomethane and diiodomethane by mammalian microsomes and glutathione S-transferases.

The influence of mammalian metabolizing enzymes on the mutagenic activity of dibromomethane and diiodomethane was investigated by using Salmonella typhimurium strain TA100 as indicator. The 2 compounds are known to be metabolized via an oxidative pathway catalysed by microsomal enzymes as well as through direct enzymatic conjugation with glutathione; both pathways possibly give rise to reactive electrophilic intermediates. In mutagenicity plate assays with pre-incubation, dibromo- and diiodo-methane were directly mutagenic towards strain TA100; their mutagenic activity was enhanced upon incubation either with rat-liver microsomes or with the cytosol fraction of the same organ, containing the glutathione S-transferases. These data can be taken as an indication that both microsomal oxidation and conjugation to glutathione are indeed responsible for the mammalian mutagenic activation of dihalomethanes.

Biotransformation↗