Comparison of mutagenicity of N-methyl-N-nitrosourea and N-ethyl-N-nitrosourea in human diploid fibroblasts.

We have studied the mutagenic efficiencies of N-methyl-N-nitrosourea (MNU) and N-ethyl-N-nitrosourea (ENU) in four strains of human diploid fibroblasts by assaying induction of resistance to thioguanine (TG). MNU was observed to be a very weak mutagen whereas ENU was confirmed to be extremely potent in all strains. Resistance to the mutagenic effects of MNU was attributed to the efficient manner by which human fibroblasts can remove the hypothesized promutagenic base, O6-methylguanine, from their DNA following methylation.