The conversion of formate-C14 or serine-3-C14 to DNA-thymine and free thymine, thymidine, and thymidylate was studied in vitro with cell suspensions of the Ehrlich ascites tumor, with three lymphatic tumors, and with normal mouse spleen or rat thymus cells. In the presence of deoxycytidine, deoxyuridine, or to a lesser degree cytidine, the total radioactivity of free thymidine plus thymine was increased. In the presence of deoxycytidine, cytidine, and in some cases deoxyuridine, the specific activity of DNA-T was also elevated. Deoxycytidine stimulated the conversion of formate-C14 to DNA-methylcytosine. Non-labeled thymidine reduced the conversion of formate-C14 or serine-3-C14 to DNA-thymine but increased the labeling of free thymidine. A decreased specific activity of DNA-T was also observed in experiments in which uracil-2-C14 was the substrate and deoxyuridine was added to the incubation flasks.

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Aided in part by grants from the Leukemia Society, Inc., and the American Cancer Society.

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