No abstract

The incorporation of thymidine (thymine deoxyriboside) into deoxyribonucleic acid (DNA) of proliferating cells has been studied in the past with various isotopic labels and biochemical techniques. Reichard and Estborn showed that thymidine-N15 was a specific precursor of DNA thymine in rats (1). Friedkin, Tilson, Roberts and Wood (2, 3) used thymidine-2-CI4 to show that this compound was incorporated into DNA thymidine of proliferating embryo and animal tissues with negligible partition of radioactivity into other components of DNA or into ribonucleic acid.The recent synthesis of tritium-labeled thymidine by Taylor, Woods and Hughes (4) and in vitro by serial autoradiographic evaluation of labeled cell populations (6-9). In the course of these in vivo investigations it became important to assess the availability time of tritiated thymidine for incorporation into DNA following rapid intravenous injection. It is essential for serial study of proliferating cell populations that the H3-thymidine label be available for incorporation into new DNA for only a small part of the DNA synthesis time, and that afterward there be no further label availability. This short availability time of H3-thymidine then permits study of the various components of the generation cycles (preand postmitotic rests, mitosis and DNA synthesis) by serial study of the appearance of labeled mitotic figures, the appearance of labeled nondividing cells and the decrease of labeling upon subsequent successive mitoses. From these data a kinetic analysis should determine the flow rate between compartments and the time spent in each state (8,9).The metabolic studies reported here indicate that the effective availability time for intravenously administered H3-thymidine is a matter of minutes. The metabolic fate of this tritium-labeled compound was examined by analyses of serial specimens of plasma, stool and urine as well as by evaluation of autoradiograms of tissues. These studies revealed that catabolism of some H3-thymidine to tritiated water (THO) as well as to nonvolatile tritiated compounds occurs. From the data to be presented here it appears that at least 50 per cent of injected H3-thymidine was retained in vivo and was presumably incorporated into new DNA of proliferating cells.

No abstract

No abstract

No abstract