Inactivation of N-methyl-N'-nitro-N-nitrosoguanidine in the Ames Salmonella/microsome test

Mechanisms of inhibition of N-nitroso compounds-induced mutagenicity

In this review we describe the mechanisms of the inhibitory effects of various chemical agents towards the mutagenicity of N-nitroso compounds, including direct-acting mutagens such as N-nitroso derivatives of alkylureas, alkylnitroguanidines and alkylurethanes, and promutagenic nitrosamines. Possible mechanisms by which the inhibitors may exert their effects outside and inside the target cells include chemical and enzymatic deactivation of the mutagen, inhibition of metabolic activation of nitrosamines, scavenging mutagenic products, inhibition of cellular uptake, induction of detoxifying mechanisms, protecting nucleophilic centers in DNA and modulating DNA repair.

Inhibitors of N-nitroso compounds-induced mutagenicity

N-Nitroso compounds are environmental mutagens that are present in the air, water, soil etc. or can be formed by nitrosation of various nitrosatable compounds. The present paper gives a survey of inhibitors of N-nitroso compounds-induced mutagenicity. Inhibitors covered include: thiols, metals, vitamins, phenolic acids, complex mixtures of plant, animal and human origin, organic solvents, inhibitors of mixed-function oxidases etc. Data on inhibitors that prevent the formation of N-nitroso compounds are not covered in this review.

In vitro enhancement of the mutagenicity of 4-nitro-o-phenylenediamine by plant S-9

The direct-acting mutagen 4-nitro-o-phenylenediamine (NOP) was activated in vitro by pea or tobacco S-9 into a more potent mutagen in Salmonella typhimurium strain TA98. NOP mutagenicity was not altered by Aroclor 1254-induced rat liver S-9. The plant S-9 activation of NOP was heat-sensitive but was not NADPH-dependent, did not involve superoxide radicals, and was not inhibited by CO. A direct relationship between plant peroxidase and NOP activation was established. Several purified peroxidases including horseradish peroxidase, chloroperoxidase, and lactoperoxidase also activated NOP. The perodoxidative process was not H2O2-dependent but was partially inhibited by a peroxidase inhibitor.

Complex effects of retinol on the metabolic activation of 2-aminofluorene

It has been reported that vitamin A alcohol (retinol) inhibits the mutagenicity of certain carcinogenic agents requiring metabolic activation by mammalian microsomal enzymes in the Ames Salmonella assay. We have found that the effect of retinol on the mutagenicity of 2-aminofluorene (2AF), a carcinogen requiring metabolic activation, is complex and dependent both on the ratio of retinol (microgram per plate) to S9 (microgram of protein per plate), as well as the absolute concentration of retinol. Retinol had no effect on the mutagenicity of 4-nitro-o-phenylenediamine (4NPDA), a direct-acting mutagen. However, the mutagenicity of 4NPDA was inhibited by S9 and this inhibition was not affected by retinol.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Alteration of the mutagenicity of human fecal extracts by hepatic microsomal enzymes

Human fecal extracts obtain substances mutagenic to Salmonella typhimurium TA 100. The mutagenicity of these extracts can be reduced enzymatically by inducible mammalian microsomal enzymes. Liver homogenates from rats administered the polychlorinated biphenyl mixture of Aroclor 1254 and corn oil were both effective, relative to the quantity of microsomal protein available in the enzyme preparation.

Methods for analysis of the mutagenicity of indirect mutagens/carcinogens in eukaryotic cells

The review discusses the variety of methods for activation of indirect mutagens/carcinogens and testing them in cell cultures, especially in mammalian cell cultures. After the necessity for including metabolizing components in mutagenicity tests has been pointed out, the enzymes that transform foreign compounds metabolically, and the factors influencing them, are described. In the main section the various methods of activating indirect mutagens/carcinogens are presented. The methods of including in vivo metabolism in mutagenicity tests are: Analysis of cells from organisms contaminated with a chemical (III.l.a); body fluid-mediated mutagenesis (III.l.b); host-mediated assay (III.l.c). The following activation systems are suitable for including in vitro metabolism of test compounds in mutagenicity tests: Liver and lung perfusion (III.2.a.alpha); organ slices and homogenates (III.2.a.beta); subcellular fractions (III.2.a.gamma); cultivated cells (cell-mediated mutagenesis) (III.2.b); nonenzymatic activation systems (III.2.c). Finally the main factors that influence the metabolism of test substances are summarized. Two factors illustrate the mutagenicity tests with regard to the metabolism of mammalian livers and the methods of performing mutagenicity tests in man.