Stimulatory Effects of Inosine and Deoxyinosine on the Incorporation of Uracil-2-¹⁴C, 5-Fluorouracil-2-¹⁴C, and 5-Bromouracil-2-¹⁴C into Nucleic Acids by Ehrlich Ascites Tumor Cells in Vitro

The incorporation of uracil-2-¹⁴C into the nucleic acids of Ehrlich ascites tumor cells is increased 10- to 50-fold by the addition of inosine. Maximum stimulation is obtained at an inosine concentration of 1 mm. Actinomycin D (50 μg/ml) abolishes incorporation. The effect of inosine seems unlikely to be mediated by an enhancement of synthesis of ATP, 5-phosphoribosyl-1-pyrophosphate, or nucleic acids by de novo pathways. Unlike uracil incorporation, the labeling of nucleic acid by uridine-2-14C is extensive in the absence of inosine. The incorporation of 5-fluorouracil-2-¹⁴C into RNA also is greatly increased by inosine. Inosine has little effect on the incorporation of 5-bromouracil-2-¹⁴C into nucleic acids, whereas deoxyinosine is a potent stimulator. Deoxyinosine, on the other hand, is a relatively weak enhancer of uracil incorporation. These results suggest that ribose-1-phosphate and deoxyribose-l-phosphate may be rate limiting for the incorporation of uracil (or 5-fluorouracil) and 5-bromouracil into RNA and DNA respectively. Inosine and deoxyinosine serve as precursors of ribose-1-phosphate and deoxyribose-1-phosphate These interconversions involve purine and pyrimidine nucleoside phosphorylases. The possible application of these findings in chemotherapy is discussed.