The incorporation of O4-methylthymidine into V79A cell DNA when present in the cell culture medium.

When Chinese hamster V79A cells are cultured in the presence of O4-methyl-[6-3H]-thymidine the incorporation of this modified nucleoside into newly synthesised DNA is observed. The radioactivity incorporated has been identified as O4-methylthymidine by digesting the DNA to 3'-monophosphates with spleen phosphodiesterase followed by treatment with alkaline phosphatase to give O4-methylthymidine as the major radioactive product. The radioactivity associated with the latter co-chromatographs with authentic O4-methylthymidine in several chromatographic systems. Once incorporated the modified nucleoside appears to be rapidly removed from the DNA with half-life of 2-3 h. There is no evidence of demethylation of the O4-methylthymidine to give thymidine, in either the culture medium or within the cells once incorporated into DNA. Although the levels of incorporation observed are low, (being only 125 O4-methylthymidine residues per 10(8) thymidine residues at a modified nucleoside concentration of 10(-5) M), they may still be relevant as similar levels are apparently produced in the DNA of the cultured cells on treatment with biologically significant doses of carcinogenic alkylating agents.