RELATIONSHIP BETWEEN DNA ALKYLATION AND SPECIFIC-LOCUS MUTATION-INDUCTION BY N-METHYL-N-NITROSOUREA AND N-ETHYL-N-NITROSOUREA IN CULTURED CHINESE-HAMSTER OVARY CELLS (CHO-HGPRT SYSTEM)

Chinese hamster ovary (CHO) cells in culture were utilized to determine the cytotoxicity, specific-locus mutation induction, and DNA alkylation which result from treatment of the cells with a range of concentrations of N-methyl-N-nitrosourea (MNU) or N-ethyl-N-nitrosourea (ENU). With [3H]MNU over the concentration range 0.43-13.7 mM, methylation of DNA was found to increase linearly, with a mean value of 56.7 pmol residue per μmol nucleoside per mM. With [1-3H]ENU over the concentration range 1.7-26.8 mM, ethylation was linear, with a mean value of 3.8 pmol residue per μmol nucleotide per mM. Mutation induction at the hypoxanthineguanine phosphoribosyl transferase (HGPRT) locus was quantified by determination of the frequency of resistance to 6-thioguanine under stringentlydefined selection conditions. The mutation frequency increased linearly with MNU or ENU concentration (0.01-2.0 mM); mean values were 2800 and 840 mutants per 106 clonable cells per mM, respectively. At equal levels of DNA alkylation, ENU was found to be approx. 4.5 times as mutagenic as MNU. © 1979.