Mutagenicities of N-nitrosodimethylamine and N-nitrosodiethylamine in Drosophila and their relationship to the levels of O-alkyl adducts in DNA

N-Nitrosodialkylamines are potent carcinogens in experimental animals. Previously, we reported that the mutagenicity of N-nitrosodimethylamine (NDMA) was 10 times higher than that of N-nitrosodiethylamine (NDEA) in the Drosophila wing spot test. To find out how to explain this difference, we have measured the levels of O-alkylated bases in the DNA of exposed Drosophila larvae. Third instar larvae were fed for 3 or 6 h with NDMA or NDEA. Part of the treated larvae were grown to adult flies to score their wings for the presence of mutant spots. From the remaining larvae, DNA was isolated and digested to deoxyribonucleosides, and the digest fractionated by high-performance liquid chromatography (HPLC). The amounts of specific alkyldeoxyribonucleosides present in the fractions were quantified by a radioimmunoassay (RIA) using monoclonal antibodies. Dose-dependent O6-methylguanine, O6-ethylguanine and O4-ethylthymine formations were found to be correlated with the induction frequencies of mutant wing spots. At the same exposure dose, the values of O6-alkylde- oxyguanosine/106 deoxyguanosine were similar for NDMA and NDEA: on feeding 20 micromol/1.5 ml feeding solution, the values for NDMA were 4.0 with 3 h and 18.5 with 6 h of exposure; with 20 micromol NDEA, the corresponding values were 5.4 with 3 h and 14.6 with 6 h of exposure. The wing spot frequencies were very different; however, with NDMA, the total numbers of spots/wing were 3.5 (3 h) and 15 (6 h), and with NDEA 0.8 (3 h) and 0.9 (6 h). Similar discrepancies exist as well between the mutagenicities and the alkylation rates observed for O4-alkylthymidines. These results suggest that the difference between the mutagenic potencies of NDMA and NDEA cannot be explained by the amounts of O-alkyl adducts formed. Different mechanisms are considered by which NDMA and NDEA may produce the genetic effects observed.