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. Prev iously, we reported that the mutagenicity of N-nitrosodimethylamine (NDMA) was 10 times higher than that of N-nitrosodiethylamine (NDEA) in the Drosop hila wing spot test. To find out how to explain this difference, we have me asured the levels of O-alkylated bases in the DNA of exposed Drosophila lar vae. Third instar larvae were fed for 3 or 6 h with NDMA or NDEA. Part of t he treated larvae were grown to adult flies to score their wings for the pr esence of mutant spots. From the remaining larvae, DNA was isolated and dig ested to deoxyribonucleosides, and the digest fractionated by high-performa nce liquid chromatography (HPLC). The amounts of specific alkyldeoxyribonuc leosides present in the fractions were quantified by a radioimmunoassay (RI A) using monoclonal antibodies. Dose-dependent O-6-methylguanine, O-6-ethyl guanine and O-4-ethylthymine formations were found to be correlated with th e induction frequencies of mutant wing spots. At the same exposure dose, th e values of O-6-alkylde oxyguanosine/10(6) deoxyguanosine were similar for NDMA and NDEA: on feeding 20 mu mol/1.5 mi feeding solution, the values for NDMA were 4.0 with 3 h and 18.5 with 6 h of exposure; with 20 mu mol 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 tota l 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 mutagenici ties and the alkylation rates observed for O-4-alkylthymidines. These resul ts suggest that the difference between the mutagenic potencies of NDMA and NDEA cannot be explained by the amounts of O-alkyl adducts formed. Differen t mechanisms are considered by which NDMA and NDEA may produce the genetic effects observed. (C) 1999 Elsevier Science B.V. All rights reserved.