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 d e 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 def ined diet that contained either uniformly [(UC)-C-13]-labeled nucleoti des or [(UC)-C-13]-algal amino acids isolated from algal biomass. Ribo nucleic acid and protein were isolated from mucosa, liver and fetus. N ucleosides and amino acids were isolated and converted to their trimet hylsilyl and n-propyl ester, heptaflurobutyramide derivatives, respect ively. The isotopic enrichments of all isotopomers were determined by gas chromatography-mass spectrometry. In the mice that ingested [(UC)- C-13]-nucleotides, the isotopic enrichment of [(UC)-C-13]-purines (0.0 3-0.2 mol/100 mol) was significantly (P < 0.001) less than that of [(U C)-C-13]-uridine (1.5-4.2 mol/100 mol). [C-13(5)]-Purines (0.1-0.8 mol /100 mol) and [C-13(4)]-uridine (0.2-0.5 mol/100 mol) were detected, s howing that some dietary bases and ribose were incorporated via the sa lvage pathway. In mice that ingested (UC)-C-13-amino acids, the isotop ic enrichment (2-4.6 mol/100 mol) of the [C-13(5)]-purines, which deri ve from [(UC)-C-13]-glycine, was between 73 (liver) and 113% (fetus) o f protein-bound C-13(2)-glycine. The isotopic enrichment (0.8-1.6 mol/ 100 mol) of [C-13(3)]-uridine, an isotopomer that derives from [(UC)-C -13]-aspartate, was 50 (liver) to 126% (mucosa) of [C-13(4)]-protein-b ound aspartate. The results suggest that a large majority of the bases incorporated into maternal and fetal ribonucleic acids derive from sy nthesis de novo.