Hepatic carbamoyl phosphate metabolism

Abstract

1. The interrelationship between the two carbamoyl phosphate pools in intact hepatocytes and intact liver was studied with respect to de novo pyrimidine synthesis by use of selective inhibitors of the mitochondrial and the cytosolic carbamoyl-phosphate synthetase.2. Inhibition of mitochondrial carbamoyl phosphate synthesis by 4-pentenoate was without effect on galactosamine-stimulated pyrimidine synthesis. Conditions favouring mitochondrial carbamoyl phosphate accumulation, like excess ammonium ions or L-norvaline, led to an increase in pyrimidine synthesis bypassing the feedback inhibition of cytosolic carbamoyl-phosphate synthetase by UTP.3. A stimulation of pyrimidine synthesis was not observed when the carbamoyl phosphate accumulation was due to aspartate deficiency in the presence of aminooxyacetate. The full response of pyrimidine synthesis to excess ammonium ions was restored, even in the presence of aminooxyacetate, when aspartate was substituted. This is explained by an inhibition of aspartate carbamoyltransferase flux [in view of the K,,, (aspartate = 0.7 mmol/l) of this enzyme] resulting from a 90% decrease in aspartate tissue levels.4. Acivicin, the inhibitor of cytosolic carbamoyl-phosphate synthetase, completely abolished the galactosamine-induced stimulation of pyrimidine synthesis, but was without effect on the stimulation of pyrimidine synthesis by ammonium ions and L-norvaline.5. It is concluded that experimental changes in mitochondrial carbamoyl phosphate content exert effects on de novo pyrimidine synthesis; however, it is considered unlikely that relevant amounts of mitochondrial carbamoyl phosphate are used for pyrimidine synthesis under physiological conditions. In addition the data point to a potential regulatory role of aspartate in hepatic pyrimidine synthesis.In the liver of mammals carbamoyl phosphate, as common substrate of the synthesis of arginine and urea and the de novo pyrimidine synthesis, is synthesized by two different enzymes. The mitochondrial carbamoyl-phosphate synthetase (CPS I) supplies carbamoyl phosphate for the synthesis of urea [I -31. It requires ammonium ions as substrate and N-acetyl-L-glutamate as allosteric activator [I, 41. At physiological ammonium ion concentrations the CPS I flux is rate-limiting for urea synthesis [5]. The much less active cytoplasmic glutaminedependent carbamoyl-phosphate synthetase (CPS 11) provides carbamoyl phosphate for the de novo synthesis of pyrimidines [l, 6, 71. This enzyme is the site of feedback inhibition by UTP regulating de novo pyrimidine synthesis [I, 81. Furthermore, it is activated by physiological concentraCorrespondence to 3 .

Role of CPS1 in Cell Growth, Metabolism, and Prognosis in LKB1-Inactivated Lung Adenocarcinoma

“…CPS1 plays a crucial role in removing excessive ammonia by converting ammonium into carbamoyl phosphate in the liver (16), which is a de novo resource of arginine (19) and potential substrate of pyrimidine synthesis (17,18). We demonstrated that knockdown of CPS1 induced accumulation of ammonia, a decrease of metabolites in nucleic acid synthesis pathways, and a decrease of arginine in LKB1-inactivated LADC cell lines.…”

“…Carbamoyl phosphate synthase 1 (CPS1) was identified as a markedly increased protein in LKB1-inactivated LADC cell lines. CPS1 is the first rate-limiting enzyme in the urea cycle, converting ammonium into carbamoyl phosphate, and plays an intricate role in arginine metabolism and pyrimidine metabolism (16)(17)(18)(19). We further investigated the effect of CPS1 through knockdown experiments and the consequences of its overexpression on survival in LADC.…”

Lysine 88 Acetylation Negatively Regulates Ornithine Carbamoyltransferase Activity in Response to Nutrient Signals

“…To further investigate the mechanism by which Lys 88 acetylation reduces OTC activity, we expressed (28,29), the acetylated OTC would have very low activity. It is worth noting that the K88Q mutant not only altered OTC substrate binding but also dramatically decreased the maximum velocity.…”

Intrahepatic amino acid and glucose metabolism in a ?-galactosamine–induced rat liver failure model

“…Previous studies in laboratory animals and isolated hepatocytes treated with D-galactosamine showed that pharmacologic inhibition of ornithine carbamoyltransferase and elevation of free ammonium levels both can increase the availability of mitochondrial carbamoyl phosphate to the cytosolic compartment for synthesis of orotic acid, the precursor of pyrimidine synthesis. [57][58][59] Because D-galactosamine depletes uridine pools in liver, 60 Table 2 are shown on this simplified network. Arrows indicate the direction of reaction assumed in the model.…”