Major discrepancies between results obtained with two different methods for evaluating DNA damage: alkaline elution and alkaline unwinding. Possible explanations.

The fluorometric assay of DNA alkaline unwinding, developed by Birnboim and Jevcak (Cancer Res 41: 1889-1892, 1981) was applied to rat liver DNA, after treatment in vivo. N-nitrosodimethylamine, for which DNA damage in rat liver has been extensively investigated, was tested as a standard compound. The results were in complete agreement (both in terms of damage and repair) with data from the literature and with our own results obtained with other methods of detecting DNA alkaline fragmentation. Sensitivity was also of the same order of that of usual methods, with the effect of 0.3 mg/Kg of N-nitrosodimethylamine being detectable. Other DNA damaging carcinogens such as 1,2-dimethylhydrazine, 2-naphthylamine and dacarbazine were also correctly detected. Compounds like nitrofurantoin, benzoin and caprolactam, which appeared clearly positive with the alkaline elution technique, but for which genotoxicity and carcinogenicity are doubtful (nitrofurantoin) or most likely negative (benzoin and caprolactam), gave negative results with this method. This is also in agreement with previous results, observed using a different approach to measuring DNA unwinding. On the basis of these and other observations, we suggest that, under certain conditions, the alkaline elution technique is perhaps not only sensitive to DNA breaks but also to changes in chromatin conformation. Unwinding methods could be more specific in the detection of DNA fragmentations.