1. The effect of ammonia on purine and pyrimidine nucleotide biosynthesis was studied in rat liver and brain in vitro. The incorporation of NaH14CO3 into acid-soluble uridine nucleotide (UMP) in liver homogenates and minces was increased 2.5–4-fold on incubation with 10mm-NH4Cl plus N-acetyl-l-glutamate, but not with either compound alone. 2. The incorporation of NaH14CO3 into orotic acid was increased 3–4-fold in liver homogenate with NH4Cl plus acetylglutamate. 3. The 5-phosphoribosyl 1-pyrophosphate content of liver homogenate was decreased by 50% after incubation for 10min with 10mm-NH4Cl plus acetylglutamate. 4. Concomitant with this decrease in free phosphoribosyl pyrophosphate was a 40–50% decrease in the rates of purine nucleotide synthesis, both de novo and from the preformed base. 5. Subcellular fractionation of liver indicated that the effects of NH4Cl plus acetylglutamate on pyrimidine and purine biosynthesis required a mitochondrial fraction. This effect of NH4Cl plus acetylglutamate could be duplicated in a mitochondria-free liver fraction with carbamoyl phosphate. 6. A similar series of experiments carried out with rat brain demonstrated a significant, though considerably smaller, effect on UMP synthesis de novo and purine base reutilization. 7. These data indicate that excessive amounts of ammonia may interfere with purine nucleotide biosynthesis by stimulating production of carbamoyl phosphate through the mitochondrial synthetase, with the excess carbamoyl phosphate in turn increasing pyrimidine nucleotide synthesis de novo and diminishing the phosphoribosyl pyrophosphate available for purine biosynthesis.
- © 1978 London: The Biochemical Society