| Abstract: | Many chemicals are not mutagenic per se, but when metabolized by mammalian tissues yield mutagenic products. Dimethylnitrosamine (DMN) is such a promutagen. It has no effect on cell growth or mutant frequency when incubated alone with L5178Y mouse lymphoma cells, but exerts both mutagenic and toxic effects when incubated in a microsome reaction mixture. Microsomes were prepared from C3H/f We 16-wk-old male mice by the calcium preciptation technique. L5178Y continuously cultured mouse lymphoma cells heterozygous for thymidine kinase (TK+/-) were incubated for 15 min with calcium-precipitated microsomes and various concentrations of DMN in appropriate reaction mixtures. After a 48-hr expression time, treated cells were cloned in soft agar with and without bromodeoxyuridine (BUdR) (50 mug/ml); 10 days later colonies grown to greater than about 200 mum diameter were counted. The frequency of BUdR-resistant (mutant) colonies increased linearly with the DMN concentration. A reconstruction experiment showed that the assay conditions did not significantly alter the relationship between parent and BUdR-resistant cells in growth and cloning efficiency. The smallest dose of DMN used in these experiments was 100mumol/liter, the one-sided (100 mumol greater than control frequency) -p value is 0.036. The locus is extremely sensitive to mutagenesis by DMN compared with other known mutagens at similar levels of cell survival. |
