Turnover of thymidine in the DNA of marrow and intestinal mucosa in mice, and its response to administration of 5-fluorouracil.

A method has been investigated for increasing the degree of in vivo labeling of deoxyribonucleic acid (DNA) achieved with tritiated thymidine, by prior administra tion of 5-fluorouracil. 5-Fluorouracil, in therapeutic doses, produced a doubling of the incorporation of thymidine into DNA in bone marrow, but had a negligible effect on incorporation of DNA in the intestinal mucosa, except under conditions of starvation and very high dosage. Oral administration of thymidine did not result in proportionately heavier labeling of DNA in the intestinal mucosa. Even when the material was introduced directly into an isolated loop of intestine, minimal direct utilization was observed. Isolation of the liver, with the establishment of a porto-femoral shunt, produced no change in DNA labeling, thus indicating that conjugation of thymidine is not a preliminary step to its utilization.