Stable isotope dilution analysis of orotic acid and uracil in amniotic fluid

Rapid, sensitive and accurate stable isotope dilution assays were developed for the measurement of orotic acid and uracil in amniotic fluid. The method utilizes [¹⁵N₂]orotic acid and [¹⁵N₂]uracil as internal standards, isolation by liquid partition chromatography and quantitation by chemical ionization selected ion monitoring gas chromatography-mass spectrometry. Orotic acid at a concentration of 0.26 ± 0.05 μmol/1 and uracil at a concentration of 0.55 ± 0.13 μmol/l were detectable in normal amniotic fluid. As affected fetuses with argininosuccinate synthetase or ornithine carbamoyl transferase deficiency showed no significant elevation of orotic acid and/or uracil in their surrounding amniotic fluids, this method unfortunately seemed not to be useful for prenatal diagnosis of these inherited disorders. Nevertheless, it provides significant advantage over available methods for the quantitation of orotic acid and uracil in which the analysis of these compounds must be very accurate, highly specific and sensitive (e.g. detection of heterozygosity for omithine carbamoyl transferase deficiency).