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Deuterium isotope effects in mutagenesis by nitroso compounds
Abstract:
Nitrosamines which have deuterium instead of hydrogen in the position alpha to the nitroso group have been reported to have reduced activity in carcinogenicity tests. This result implies that cleavage of a carbon--hydrogen bond is a limiting step in the reaction mechanism leading to tumor formation. Mutagenicity tests were undertaken with nitrosamines, which require metabolic activation, and with nitrosamides, which are directly acting mutagens, to determine the effect of deuterium substitution on the activity of each type of compound. Two nitrosamides (N-methyl-N'-nitro-N-nitrosoguanidine and methylnitrosourea) and three nitrosamines (dimethylnitrosamine, nitrosomorpholine, and dinitrosopiperazine) and their deuterium-containing analogs were tested for reversion of a nonsense mutation in the tyr locus of Escherichia coli WU 3610 (tyr-, leu-). Nitrosamines activated by rat-liver microsomes, but not nitrosamides, were less active as mutagens when the deuterium atom was present. The results suggest that the metabolic activation of nitrosamines to a mutagenic species involves the loss of hydrogen, a reaction which the nitrosamides, in the absence of enzyme, do not undergo.
Insights
Deuterium substitution reduced mutagenicity in nitrosamines, suggesting hydrogen loss is key to their metabolic activation. Nitrosamides, which act directly, were unaffected by deuterium, highlighting differences in mutagenic mechanisms.
Area of Science:
- Biochemistry
- Toxicology
- Molecular Biology
Background:
- Nitrosamines are known carcinogens, with reduced carcinogenicity observed when deuterium replaces hydrogen alpha to the nitroso group.
- This observation suggests that carbon-hydrogen bond cleavage is a rate-limiting step in tumor formation.
- Mutagenicity testing is crucial for understanding the mechanisms of chemical carcinogenesis.
Purpose of the Study:
- To investigate the effect of deuterium substitution on the mutagenicity of nitrosamines and nitrosamides.
- To determine if deuterium substitution impacts mutagenicity differently between compounds requiring metabolic activation (nitrosamines) and direct-acting mutagens (nitrosamides).
- To elucidate the role of hydrogen cleavage in the metabolic activation of nitrosamines.
Main Methods:
- Mutagenicity assays were performed using Escherichia coli WU 3610 (tyr-, leu-) to test for reversion of a nonsense mutation.
- Two nitrosamides (N-methyl-N'-nitro-N-nitrosoguanidine and methylnitrosourea) and three nitrosamines (dimethylnitrosamine, nitrosomorpholine, and dinitrosopiperazine) and their deuterium analogs were used.
- Nitrosamines were activated using rat-liver microsomes, while nitrosamides were tested directly.
Main Results:
- Deuterium-substituted nitrosamines showed reduced mutagenic activity when metabolically activated by rat-liver microsomes.
- Nitrosamides, which are direct-acting mutagens, did not exhibit reduced mutagenicity with deuterium substitution.
- The presence of deuterium did not affect the mutagenicity of nitrosamides in the absence of enzymatic activation.
Conclusions:
- The metabolic activation of nitrosamines to mutagenic species involves the cleavage of a carbon-hydrogen bond.
- Deuterium substitution at the alpha position interferes with this critical C-H bond cleavage step, reducing mutagenicity.
- Nitrosamides do not undergo this hydrogen cleavage mechanism, explaining their lack of response to deuterium substitution.