Ribonucleic Acid Nucleotides in Maternal and Fetal Tissues Derive almost Exclusively From Synthesis de novo in Pregnant Mice

The contributions of dietary nucleotides and nucleotides synthesized de novo to ribonucleic acid synthesis in vivo were estimated by feeding, from d 13 to 18 of gestation, two groups of five pregnant mice a defined diet that contained either uniformly [U13C]-labeled nucleotides or [U13C]-algal amino acids isolated from algal biomass. Ribonucleic acid and protein were isolated from mucosa, liver and fetus. Nucleosides and amino acids were isolated and converted to their trimethylsilyl and n-propyl ester, heptaflurobutyramide derivatives, respectively. The isotopic enrichments of all isotopomers were determined by gas chromatography-mass spectrometry. In the mice that ingested [U13C]-nucleotides, the isotopic enrichment of [Ul3C]-purines (0.03-0.2 mol/100 mol) was significantly (P < 0.001) less than that of [U13C]-uridine (1.5-4.2 mol/100 mol). [13C5]-Purines (0.1-0.8 mol/100 mol) and [13C4]-uridine (0.2-0.5 mol/100 mol) were detected, showing that some dietary bases and ribose were incorporated via the salvage pathway. In mice that Ingested U13C-amino acids, the isotopic enrichment (2-4.6 mol/100 mol) of the [13C2]-purines, which derive from [Ul3C]-glycine, was between 73 (liver) and 113% (fetus) of protein-bound 13C2-glycine. The isotopic enrichment (0.8-1.6 mol/100 mol) of [13C3]-uridine, an isotopomer that derives from [U13C]-aspartate, was 50 (liver) to 126% (mucosa) of [13C4]-protein-bound aspartate. The results suggest that a large majority of the bases incorporated into maternal and fetal ribonucleic acids derive from synthesis de novo.