A comparison of cell survival, mutation and persistence of putative promutagenic lesions in Chinese hamster cells exposed to BNU or MNU

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Abstract
The ability of N-n-butyl-N-nitrosourea (BNU) and N-methyl-N-nitrosourea (MNU) to induce cytotoxicity and mutation has been compared in the Chinese hamster cell lines V79A-2 and V79/79. The kinetics of cytotoxicity is resolvable into two phases, a rapid phase occurring within 1 h at pH 7.4 and 37.degree. C that is probably due to alkylation and 37.degree. C that is probably due to alkylation and a phase of progressive cytotoxicity involving long-lived species. The latter component is larger with BNU than with MNU. Using shorterm exposure in which alkylation toxicity predominates, mutations were observed at two loci. Thioguanine-resistant mutants were induced at similar frequencies in V79A-2 and V79/79 but more ouabain-resistant mutants were induced in V79A-2 than in V79/79. Fewer mutants were induced at each locus per surviving cell by BNU compared with MNU. The major potentially miscoding adduct, O6-alkylguanine, was measured by radioimmunoassay and its persistence determined. The methyl adduct persists in V79A-2 but is removed with a half time of .apprx. 7 h in V79/79. In contrast, the butyl adduct was removed from both V79-2 and V79/79 with half times of 28 and 19 h, respectively. No O6-alkylguanine DNA alkyltransferase (AT) activity could be detected in extracts of either cell line. Thus Chinese hamster cells appear to repair O6-alkylguanine by a mechanism(s) other than by AT.