Increased de Novo Pyrimidine Nucleotide Synthesis in Liver Induced by Ammonium Ions in Amounts Surpassing the Urea Cycle Capacity

Abstract

An increased rate of de novo pyrimidine nucleotide synthesis is induced in rat liver by administration of ammonium ions. Specific enzymatic analyses demonstrate a marked elevation of the hepatic contents of orotate, UTP, UDP‐glucose, total uracil and cytosine 5′‐nucleotides. Inhibition of uridylate formation by means of 6‐azauridine suppresses the effect of ammonium ions on uracil nucleotide contents and intensifies the accumulation of orotate. After administration of ammonium ions, de novo uridylate synthesis continues at an elevated rate when the hepatic contents of ammonium ions have returned to the normal range. A mediating role of the elevated glutamine levels in the in‐ crease of pyrimidine synthesis induced by ammonium ions has been excluded by an inhibition of glutamine synthetase by methionine sulfoximine.The following evidence indicates that mitochondrial carbamoyl phosphate synthesis from ammonium ions serves as the source of the excess carbamoyl phosphate channeled into cytoplasmic pyrimidine synthesis and leading to an increased formation of orotate and uridylate: (a) acceleration of mitochondrial carbamoyl phosphate consumption by ornithine in the urea cycle prevents the increase of pyrimidine synthesis induced by ammonium ions; (b) inhibition of ornithine carbamoyltransferase by the ornithine analogs norvaline and 2,4‐diaminobutyrate mimics the effect of excess ammonium ions and leads‐to an increased formation of pyrimidines; (c) ascites hepatoma cells deficient in mitochondrial ammonia‐dependent carbamoyl‐phosphate synthetase are unable to increase their rate of pyrimidine synthesis in the presence of ammonium ions.Ornithine has no inhibitory effect on the elevated rate of pyrimidine synthesis elicited by UTP deficiency. This indicates that an efflux of mitochondrial carbamoyl phosphate plays a negligible role in the absence of excess ammonium ions in intact liver. Surplus ammonium ions, however, lead to a state of increased de novo pyrimidine nucleotide synthesis in which the feedback regulation of this pathway by UTP is bypassed.