US EPA

3822159 

Journal Article 

Orotic acid biosynthesis in rat liver: studies on the source of carbamoylphosphate 

Tremblay, GS; Crandall, DE; Knott, CE; Alfant, M 

1977 

Yes 

Archives of Biochemistry and Biophysics
ISSN: 0003-9861
EISSN: 1096-0384 

178 

1 

264-277 

English 

189694 

10.1016/0003-9861(77)90191-6 

Measurements of the incorporation of radiolabeled precursors into orotic acid in tissue slices and minces provided evidence of the participation of the intramitochondrial carbamoylphosphate synthetase (CPSase-I) in the de novo biosynthesis of pyrimidines in rat liver. Ammonia, the only nitrogen source utilized by CPSase-I, markedly stimulated the incorporation of NaH14CO3 into orotic acid in liver slices, and ornithine, which enhances the intramitochondrial consumption of carbamoylphosphate (CP) in citrulline synthesis, antagonized the stimulation by ammonia. Sensitivity of the incorporation of NaH14CO3 into orotic acid to stimulation by ammonia was found to increase with age in concert with the emergence of CPSase-I in the liver during late fetal and neonatal development. Tissues lacking in CPSase-I activity did not exhibit the responses to ammonia and ornithine observed with the adult rat liver. While the occurrence of CPSase-I in the liver contributes extensively toward the exceptionally high capacity of that tissue for the de novo biosynthesis of orotic acid, our results also indicate that the physiological rate of orotic acid biosynthesis in rat liver is approximately one-third of capacity; the incorporation of NaH14CO3 into orotic acid averaged 488 nmol/g of tissue in 3 h in the presence of toxic levels of ammonia, but declined to 160 nmol/g of tissue in 3 h when physiological levels of both ammonia and ornithine were provided. However, the rate of orotic acid biosynthesis observed with physiological concentrations of ammonia and ornithine could be reduced further, to about one-quarter of the physiological rate, by providing additional ornithine; thus, physiological levels of ornithine do not prevent the escape of intramitochondrial CP into the cytoplasm. Finally, over 80% of the incorporation of NaH14CO3 into orotic acid at physiological levels of ammonia and ornithine was found to be ammonia dependent, and all but a small fraction of the ammonia-dependent incorporation could be blocked by providing ornithine in amounts in excess of physiological. These results indicate that CPSase-I is the major source of CP in the biosynthesis of hepatic pyrimidines under normal (physiological) conditions as well as in ammonia toxicity. © 1977. 

carbamate kinase; carbamoyl phosphate; orotic acid; radioisotope; in vitro study; isotope tracing; liver; rat; theoretical study; Ammonia; Ammonium Chloride; Animal; Bicarbonates; Carbamates; Carbamoyl-Phosphate Synthase (Ammonia); Carbamyl Phosphate; Carbon Radioisotopes; Comparative Study; Glutamine; Liver; Mitochondria, Liver; Ornithine; Orotic Acid; Phosphotransferases; Putrescine; Rats; Support, U.S. Gov't, P.H.S.