Regulation of carbamoyl-phosphate synthase (ammonia) in liver in relation to urea cycle activity

Maternal Liver Metabolic Response to Chronic Vitamin D Deficiency Is Determined by Mouse Strain Genetic Background

BACKGROUND

Liver metabolite concentrations have the potential to be key biomarkers of systemic metabolic dysfunction and overall health. However, for most conditions we do not know the extent to which genetic differences regulate susceptibility to metabolic responses. This limits our ability to detect and diagnose effects in heterogeneous populations.

OBJECTIVES

Here, we investigated the extent to which naturally occurring genetic differences regulate maternal liver metabolic response to vitamin D deficiency (VDD), particularly during perinatal periods when such changes can adversely affect maternal and fetal health.

METHODS

We used a panel of 8 inbred Collaborative Cross (CC) mouse strains, each with a different genetic background (72 dams, 3-6/treatment group, per strain). We identified robust maternal liver metabolic responses to vitamin D depletion before and during gestation and lactation using a vitamin-D-deficient (VDD; 0 IU vitamin D3/kg) or -sufficient diet (1000 IU vitamin D3/kg). We then identified VDD-induced metabolite changes influenced by strain genetic background.

RESULTS

We detected a significant VDD effect by orthogonal partial least squares discriminant analysis (Q2 = 0.266, pQ2 = 0.002): primarily, altered concentrations of 78 metabolites involved in lipid, amino acid, and nucleotide metabolism (variable importance to projection score ≥1.5). Metabolites in unsaturated fatty acid and glycerophospholipid metabolism pathways were significantly enriched [False Discovery Rate (FDR) <0.05]. VDD also significantly altered concentrations of putative markers of uremic toxemia, acylglycerols, and dipeptides. The extent of the metabolic response to VDD was strongly dependent on genetic strain, ranging from robustly responsive to nonresponsive. Two strains (CC017/Unc and CC032/GeniUnc) were particularly sensitive to VDD; however, each strain altered different pathways.

CONCLUSIONS

These novel findings demonstrate that maternal VDD induces different liver metabolic effects in different genetic backgrounds. Strains with differing susceptibility and metabolic response to VDD represent unique tools to identify causal susceptibility factors and further elucidate the role of VDD-induced metabolic changes in maternal and/or fetal health for ultimately translating findings to human populations.

Modulation of rat liver urea cycle and related ammonium metabolism by sex and cafeteria diet

Liver amino acid metabolism decreased with cafeteria diet through lower ammonium production (even lower in females) and urea cycle activity.

Required allosteric effector site for N-acetylglutamate on carbamoyl-phosphate synthetase I

Carbamoyl-phosphate synthetase I (CPSase I) catalyzes the entry and rate-limiting step in the urea cycle, the pathway by which mammals detoxify ammonia. One facet of CPSase I regulation is a requirement for N-acetylglutamate (AGA), which induces an active enzyme conformation and does not participate directly in the chemical reaction. We have utilized labeling with carbodiimide-activated [14C]AGA to identify peptides 120-127, 234-237, 625-630, and 1351-1356 as potentially being near the binding site for AGA. Identification of peptide 1351-1356 confirms the previous demonstration (Rodriquez-Aparicio, L. B., Guadalajara, A. M., and Rubio, V.(1989) Biochemistry 28, 3070-3074) that the C-terminal region is involved in binding AGA. Identification of peptides 120-127 and 234-237 constitutes the first evidence that the N-terminal region of the synthetase is involved in ligand binding. Since peptides 631-638 and 1327-1348 have been identified near the ATP site of CPSase I (Potter, M. D., and Powers-Lee, S. G.(1992) J. Biol. Chem. 267, 2023-2031), the present finding of involvement of peptides 625-630 and 1351-1356 at an "allosteric" activator site was unexpected. The idea that portions of the AGA effector site might be derived from an ancestral glutamine substrate site via a gene duplication and diversification event was considered.

The regulation of N-acetylglutamate synthetase in rat liver by protein intake

Urea cycle enzymes are subjected to regulation by dietary proteins. We have shown that this is also the case for N-acetylglutamate synthetase (EC 2.3.1.1.) (NAGS). Four different groups (n = 7) of male Wistar rats received either a low protein (8.7%) or a high (32% and 51%) protein diet and a control diet of 17% protein. The NAGS-activity in the liver, assayed after 15 days of feeding the different diets, increased from 25 +/- 7 (controls, 17% protein) to 31 +/- 5 (32% protein) and to 52 +/- 17 (51% protein) nmoles.min-1.g-1 wet weight. It decreased in the group with low protein diet (8.7%) to 5 +/- 3. The ratio of the arginine stimulated to the unstimulated enzyme activity remained constant over the range of protein intake. Similar changes were observed for carbamylphosphate synthetase, ornithine carbamyltransferase and arginase. As it is known for these enzymes adaptive mechanisms in relation to variations in dietary protein consumption also could be demonstrated for the enzyme NAGS.

Effect of growth hormone on rat liver N-acetyl-L-glutamate

Earlier studies have revealed, upon hypophysectomy, a specific increase in mitochondrial urea cycle enzymes, namely carbamyl phosphate synthetase and ornithine transcarbamylase. Administration of growth hormone to hypophysectomized rats brought these enzyme activities back to normal. Since growth hormone plays a role in the formation of citrulline and ultimately urea, in the present study its effect on the levels of N-acetyl-L-glutamate, an allosteric activator of carbamyl phosphate synthetase has been investigated. A significant increase in N-acetyl-L-glutamate concentration in rat liver on hypophysectomy and its reversal back to normal levels on growth hormone administration was reported. These results suggest that the lack of growth hormone tends to amplify urea production by the liver.

Binding of N-acetyl-L-glutamate to rat liver carbamoyl phosphate synthetase (ammonia)

The binding of N-acetyl-L-glutamate, the physiological allosteric activator, to rat liver carbamoyl-phosphate synthetase (ammonia) was studied by techniques of rate of dialysis and of ultracentrifugation in the Airfuge. There is one binding site for acetylglutamate per enzyme monomer (Mr 165 000). K+, Mg2+ (free) and ATP were required to demonstrate binding. The concentrations of ATP required indicate that binding of ATPA (the ATP molecule that yields Pi) is needed. HCO-3 was not essential, but it enhanced binding of acetylglutamate. Glycerol also favored binding. Plots of Kd values versus the reciprocal of free Mg2+ and ATP concentrations are linear and indicate that ATPA, K+ and Mg2+ bind before acetylglutamate. In the presence of these ligands and HCO-3, ammonia increased drastically the Kd value for acetylglutamate, whereas in absence of HCO-3 ammonia had little effect. This suggests that acetylglutamate dissociates with the products and explains the higher Km for acetylglutamate in the synthetase (overall) reaction than in the ATPase (partial) reaction. In the absence of ATP acetylglutamate was bound with high affinity if ADP and carbamoyl phosphate were present. ADP or carbamoyl phosphate alone did not promote substantial binding. Binding of acetylglutamate at low concentration was slow; it was accelerated at higher concentrations of the activator. Exchange of bound acetylglutamate with acetylglutamate in solution was fast. A scheme proposed earlier for allosteric activation of the enzyme [Rubio, V., Britton, H. G. and Grisolia, S. (1983) Eur. J. Biochem. (in preparation)] is refined to incorporate the new information. Binding of ATPA, K+ and Mg2+ and formation of 'active CO2' (the central complex) are greatly favored by acetylglutamate.

Arginine activation of N-acetylglutamate synthetase in mouse liver. Enhancement of the sensitivity in vivo by parenteral treatment with inhibitors of nucleic acid and protein synthesis

N-Acetyl-L-glutamate synthetase catalyzes the synthesis of N-acetyl-L-glutamate, an allosteric and essential activator of carbamoyl-phosphate synthetase I in the liver of ureotelic animals. The enzyme is activated specifically by arginine. We report here that the sensitivity of the synthetase to activation by arginine increases markedly after intraperitoneal administration to mice of inhibitors of nucleic acid and protein synthesis, including actinomycin D, aurintricarboxylic acid, cycloheximide, emetine and puromycin. The effects of cycloheximide were investigated in detail and an amino acid analysis was made of the homogenate of freeze-clamped livers of control or cycloheximide-treated mice.

Transport of N-acetylglutamate in rat-liver mitochondria

The permeability properties of the rat-liver mitochondrial membrane for N-acetylglutamate, the activator of carbamoyl-phosphate synthetase (ammonia), were studied. 1. Transport of N-acetylglutamate into the mitochondria was only observed in partially or fully de-energized mitochondria and when the extramitochondrial concentration was unphysiologically high (in the mM range). However, even under these conditions the intramitochondrial concentration of N-acetylglutamate was much lower than that outside. 2. Mitochondrial N-acetylglutamate efflux only occurs when the mitochondria are in an energized state. At 25 degrees C, at an intramitochondrial N-acetylglutamate concentration of 0.7-1.0 nmol/mg protein, efflux proceeds at a rate of about 0.05 nmol X min-1 X mg protein-1. This is 10-fold lower than the maximal rate of N-acetylglutamate synthesis in the mitochondria. 3. Homologous exchange between intramitochondrial N-[14C]acetylglutamate and extramitochondrial unlabelled N-acetylglutamate could not be demonstrated. 4. It is concluded that transport of N-acetylglutamate in vivo is effectively unidirectional, out of the mitochondria. This behaviour is in accordance with the physiological requirement for efflux of N-acetylglutamate from the mitochondria in order to be degraded in the cytosol.

Carbamoyl phosphate synthetase I of human liver. Purification, some properties and immunological cross-reactivity with the rat liver enzyme

The purification of mitochondrial carbamoyl phosphate synthetase I (carbon-dioxide: ammonia ligase (ADP-forming, carbamate-phosphorylating), EC 6.3.4.16) from small samples of human liver is described. The enzyme is composed of a single polypeptide of Mr 160 000 +/- 500 as shown by SDS-polyacrylamide gel electrophoresis in the presence of reducing agents. The synthetase migrates in polyacrylamide gradient gels in the absence of detergents at a rate corresponding to a Mr of 165 000. Estimates of the molecular weight of the native enzyme by gel filtration and density gradient centrifugation yield a value of 178 000. The results indicate that the enzyme exists predominantly as monomeres. Amino acids composition, isoelectric point, stability, Km values and the ability to catalyze partial reactions have been measured and compared with known properties of carbamoyl phosphate synthetases from other sources. From the available data a high degree of evolutionary conservation of the ammonia-dependent synthetase is suggested. This is also supported by the demonstration of extensive immunological cross-reactivity between the human and rat enzymes.