N-Acetylglutamate Content in Liver and Gut of Normal and Fasted Mice, Normal Human Livers, and Livers of Individuals with Carbamyl Phosphate Synthetase or Ornithine Transcarbamylase Deficiency

Abstract

ABSTRACT. N-acetylglutamate (NAG) content was measured in homogenates of liver and small intestine obtained from normal and 24-h starved syngeneic mice. Subsequently, NAG was determined in normal, and in carbamyl phosphate synthetase I and ornithine transcarbamylase enzyme-deficient human liver tissue homogenates. The method used in this study, which is direct and highly specific, used anion exchange extraction, gas chromatographic separation, and mass spectrometric detection and quantitation. Hepatic NAG content in the fed animals was In three patients with apparently complete carbamyl phosphate synthetase I or ornithine transcarbamylase deficiency, hepatic NAG levels were lower than controls (2.2-12.8 nmoljg tissue 42.3-140.7 nmol/g protein), two patients with ornithine transcarbamylase deficiency had levels similar to the controls and one patient with carbamyl phosphate synthetase I deficiency had elevated levels (98.4 nmoljg tissue, 1185.5 nmoljg protein). The livers of two patients with cirrhosis and hyperammonemia contained amounts of NAG within the range of normal livers. The marked variability in tissue NAG concentrations in various nutritional and metabolic conditions favors the hypothesis that NAG plays a role in the regulation of urea synthesis. Hepatic NAG levels are markedly reduced in some but not all patients with defects in urea cycle enzymes. (Pediatr Res 27: 408-412,1990) Abbreviations NAG, N-acetylglutamate NAGS, N-acetylglutamate synthase

Regulation of [15N]Urea Synthesis from [5-15N]Glutamine

“…1), as well as a simultaneous measurement of isotopic enrichment in each of these metabolites. Table III illustrates that the level of NAG at physiologic pH is in good agreement with the level obtained in previous studies using GC-MS or indirect chemical determination (3,29). However, Lund and Wiggins (3) did not detect a correlation between NAG level and rates of urea synthesis from glutamine, whereas our current data suggest a correlation between changes in [ 15 N] urea synthesis from [5-15 N]glutamine (Table I) and changes in the level of NAG with alteration of the incubation pH or in the presence of glucagon (Table III).…”

“…Level of N-Acetylglutamate in Isolated Hepatocytes-A major difficulty in our understanding of the role of NAG in the regulation of urea synthesis is the lack of a reliable and precise method for determination of the NAG level in tissues (3,29). In the current study, we have used the t-BDMS derivative of NAG and GC-MS to directly measure this metabolite with a stable isotope dilution method (41,42).…”

“…Metabolism of pyruvate via the pyruvate dehydrogenase pathway may stimulate urea synthesis secondary to formation of acetyl-CoA, a precursor of N-acetylglutamate (NAG), the primary activator of carbamyl phosphate synthetase (6,4,(27)(28)(29)(30)(31)(32). Previous studies have demonstrated a short term regulation of urea synthesis through rapid changes in NAG levels because carbamyl phosphate synthetase is inactive when NAG is absent (27,28,32).…”

The Study of Carbamoyl Phosphate Synthetase 1 Deficiency Sheds Light on the Mechanism for Switching On/Off the Urea Cycle

“…The P1439L mutation also increased K a NAG by about 15 folds. These important decreases in the affinity of the enzyme for NAG should account for the clinical deficiency, given the NAG levels prevailing in the liver (Tuchman and Holzknecht, 1990). The same should be the case for the T1443A and Y1491H clinical mutations, found earlier Díez-Fernández et al 2013) to increase K a NAG by about 160 folds and about 50 folds, respectively (Table 1).…”

N-acetyl-glutamic acid: Evaluation of acute and 28-day repeated dose oral toxicity and genotoxicity

“…In mammals, NAG is produced enzymatically by NAG synthase using glutamate and acetyl-CoA where it functions as an essential allosteric cofactor of CPSI and regulates the incorporation of ammonia into the urea cycle (Tuchman and Holzknecht, 1990). The importance of NAG in nitrogen metabolism is highlighted by individuals that suffer from NAGS deficiency and become hyperammonemic because CPSI is inactive in the absence of NAG .…”