EVIDENCE FOR INCORPORATION OF INTACT DIETARY PYRIMIDINE (BUT NOT PURINE) NUCLEOSIDES INTO HEPATIC RNA

The absorption and metabolism of dietary nucleic acids have received l ess attention than those of other organic nutrients, largely because o f methodological difficulties. We supplemented the rations of poultry and mice with the edible alga Spirulina platensis, which had been unif ormly labeled with C-13 by hydroponic culture in (CO2)-C-13. The ratio ns were ingested by a hen for 4 wk and by four mice for 6 days; two mi ce were fed a normal diet and two were fed a nucleic acid-deficient di et. The animals were killed and nucleosides were isolated from hepatic RNA. The isotopic enrichment of all mass isotopomers of the nucleosid es was analyzed by selected ion monitoring of the negative chemical io nization mass spectrum and the labeling pattern was deconvoluted by re ference to the enrichment pattern of the tracer material. We found a d istinct difference in the C-13 enrichment pattern between pyrimidine a nd purine nucleosides; the isotopic enrichment of uniformly labeled [M + 9] isotopomers of pyrimidines exceeded that of purines [M + 10] by >2 orders of magnitude in the avian nucleic acids and by 7- and 14-fol d in the murine nucleic acids. The purines were more enriched in lower mass isotopomers, those less than [M + 3], than the pyrimidines. Our results suggest that large quantities of dietary pyrimidine nucleoside s and almost no dietary purine nucleosides are incorporated into hepat ic nucleic acids without hydrolytic removal of the ribose moiety. In a ddition, our results support a potential nutritional role for nucleosi des and suggest that pyrimidines are conditionally essential organic n utrients.