DNA TURNOVER AND THYMIDINE RE-UTILIZATION IN MOUSE TISSUES

Mice were injected intravenously with tritiated thymidine (TdR) and ¹²⁵I-labeled iododeoxyuridine (IUdR) in low doses to provide a simultaneous labeling of tissue DNA with non-toxic amounts of these two precursors. The total activity per organ and the ratio of the two isotopes was measured in the DNA at various times between 1 and 15 days after the injection. Since TdR from dying cells is re-utilized more efficiently than IUdR from the same cells, more labeled TdR than IUdR was retained in the tissue DNA in these experiments. From the slopes of the regression lines, the true rate of turnover of replicating tissue DNA and the per cent re-utilization of TdR were calculated. Re-utilization of TdR varied from 37 to 60 % in the six tissues examined.