Incorporation of 5-fluorouracil into murine bone marrow DNA in vivo.

Although 5-fluorouracil (FUra) is readily incorporated into RNA, the possibility of its being incorporated into DNA in substantial amounts has only recently been recognized. Examination of nucleic acids prepared from tumor-bearing BALB/c X DBA/8 F1 mice labeled with [3H]FUra in vivo revealed very little alkali-stable, acid-precipitable radioactivity in tumor and only small amounts in intestine. However, substantial amounts were detected in bone marrow. Pretreatment of mice with low-dose thymidine (500 mg/kg) increased the incorporation of FUra into RNA but did not change the amount incorporated into alkali-stable material. The net result was a reduction in the fraction of the total in an alkali-stable form. Formation of DNA containing FUra residues is substantially reduced if the mice receive very high doses of thymidine along with the labeled FUra, presumably through competition from an expanded deoxythymidine triphosphate pool. Bone marrow nucleic acids labeled with 32P and [3H]FUra were analyzed by cesium sulfate gradients. Two distinct peaks of tritium radioactivity were observed that band with 32P radioactivity at the densities of RNA and DNA. Pretreatment with alkali destroyed the (32P/3H)RNA peak, but not the DNA peak. Cesium sulfate-purified DNA containing FUra residues was digested with pancreatic DNase and venom phosphodiesterase. The resulting nucleotides were analyzed by high-pressure liquid chromatography. The majority of the radioactivity cochromatographed with 5-fluorodeoxyuridine monophosphate marker. No radioactivity was detected in the regions corresponding to fluorouridine monophosphate or deoxyuridine monophosphate, although radioactivity was detected cochromatographing with deoxythymidine monophosphate. After digestion with alkaline phosphatase, the majority of the radioactivity cochromatographed with fluorodeoxyuridine (and some thymidine). These results confirm previous observations of FUra incorporation into DNA of tissue culture cells.