Clues to the non-carcinogenicity of certain N-Nitroso compounds: Role of alkylated DNA bases.

N-Nitroso compounds (NOC) are known for the carcinogenicity of most members. However, 13% of 332 NOC reviewed in 1984 were found to be non-carcinogenic. The non-carcinogenicity of all N-nitrosamines with even one tertiary alkyl group is notable. Clues to the lack of carcinogenicity include (a) inability to generate the reactive ultimate carcinogen which alkylates DNA bases, and (b) inability of the alkylated DNA base to mispair during DNA replication. This DFT study probes a three-stage process for the induction of mutations, including (a) N-deprotonation of O-alkylated DNA bases formed by attack of the carcinogen, (b) adoption of a conformer by the O-alkylated base conducive to mutagenic base mispairing, and (c) creation of the base mismatch involving the O-alkylated base. These three criteria are applied to the products of methylation, ethylation, isopropylation and tert-butylation at the N7-G, O6-G and O4-T sites. The N-deprotonation criterion differentiates the non-mutagenic N7-alkylguanines from the promutagenic O6-alkylguanines and O4-alkylthymines. All the O-alkylated bases except O4-tert-butylthymine are predicted as capable of adopting a conformer conducive to successful mispairing. O4-tert-butylthymine is predicted as incapable of creating a base mismatch by H-bonding with guanine, pointing to the non-mutagenic effects of tert-butylation of the O4-T site. By extrapolating to all tertiary alkyl groups, this explains why tert-alkylating N-nitrosamines are carcinogenically inactive. These results also highlight the carcinogenic role of alkylation at the O4-T site rather than at the O6-G site.