Pyrimidine Studies I. Effect of DON (6-Diazo-5-oxo-l-norleucine) on Incorporation of Precursors into Nucleic Acid Pyrimidines

Summary The incorporation of ureidosuccinic and orotic acid into the cytosine moiety of the nucleic acids of some mammalian tissues was significantly depressed by DON (6-diazo-5-oxo-l-norleucine) relative to the effect on nucleic acid uracil and thymine. This “cytosine effect” was found in the case of liver, intestine, and tumor of rats bearing human tumor transplants H.S. #1 and H.Ep. #3 following single-dose intraperitoneal administration at a level of 3 mg/kg. This effect was also found in in vitro incorporation studies with rat liver as well as with tumor slices. These results suggest that DON (or a compound derived biosynthetically from the latter) is interfering along a metabolic pathway leading from a uracil-containing metabolite to one containing the cytosine moiety. When uniformly labeled cytidylic acid-C 14 was used as a precursor in vitro with tumor slices, DON had no appreciable effect on the incorporation of radiocarbon into the nucleic acid pyrimidines. When uniformly labeled uridylic acid-C 14 was used, the incorporation of radiocarbon into RNA cytosine was appreciably reduced, relative to uracil. These results are in accord with the mechanism suggested above. The addition of relatively large concentrations of glutamine in an in vitro system partially reversed this effect of DON.