A comparison of the utilization of thymine and thymidine for the synthesis of DNA-thymine in novikoff hepatoma cells

A comparison was made between the utilization of thymine and thymidine for the synthesis of DNA in Novikoff hepatoma cells growing in suspension culture. When the cell cultures were switched from exponential growth to a relatively non-growing condition, by resuspending them in culture media minus serum for 18 h, there was an 85% decrease in the rate of thymidine incorporation but only a 15% decrease in the rate of thymine incorporation. Exposure to an alkylating agent (methyl methane sulfonate) resulted in a 79% decrease in thymidine incorporation, while thymine incorporation was decreased only 35%. Thymidine at a concentration equal to its Km for incorporation into DNA (4 × 10⁻⁷ M) had virtually no effect on thymine incorporation. It was not until a thymidine concentration of ten times the K_m was employed that appreciable (40%) decreases in the rate of thymine incorporation were observed. Examination of total cellular DNA or nuclear DNA gave similar results. These studies are interpreted as indicating the presence of multiple precursor pools for the synthesis of DNA-thymine in Novikoff hepatoma cells.