N-carbamylglutamate enhancement of ureagenesis leads to discovery of a novel deleterious mutation in a newly defined enhancer of the NAGS gene and to effective therapy.
ABSTRACT N-acetylglutamate synthase (NAGS) catalyzes the conversion of glutamate and acetyl-CoA to NAG, the essential allosteric activator of carbamyl phosphate synthetase I, the first urea cycle enzyme in mammals. A 17-year-old female with recurrent hyperammonemia attacks, the cause of which remained undiagnosed for 8 years in spite of multiple molecular and biochemical investigations, showed markedly enhanced ureagenesis (measured by isotope incorporation) in response to N-carbamylglutamate (NCG). This led to sequencing of the regulatory regions of the NAGS gene and identification of a deleterious single-base substitution in the upstream enhancer. The homozygous mutation (c.-3064C>A), affecting a highly conserved nucleotide within the hepatic nuclear factor 1 (HNF-1) binding site, was not found in single nucleotide polymorphism databases and in a screen of 1,086 alleles from a diverse population. Functional assays demonstrated that this mutation decreases transcription and binding of HNF-1 to the NAGS gene, while a consensus HNF-1 binding sequence enhances binding to HNF-1 and increases transcription. Oral daily NCG therapy restored ureagenesis in this patient, normalizing her biochemical markers, and allowing discontinuation of alternate pathway therapy and normalization of her diet with no recurrence of hyperammonemia. Inc.
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ABSTRACT: The defect in nitrogen excretion in patients with inborn errors of urea synthesis can be controlled by exploiting the biosynthetic pathways of readily excretable non-urea metabolites which contain nitrogen derived from ammonium, alanine, glutamate, and glutamine. Two classes of such metabolites are the urea-cycle intermediates--including citrulline, argininosuccinic acid, and arginine--and the aminoacid acylation products--hippuric acid (the glycine conjugate of benzoic acid) and phenylactylglutamine (the glutamine conjugate of phenylactic acid). Thus the urea cycle may serve as a model for the development of excretion pathways of toxic precursors which accumulate in inborn errors of metabolism.The Lancet 10/1979; 2(8140):452-4. · 39.06 Impact Factor
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ABSTRACT: Carbamyl glutamate injected into normal rats produced no change in blood urea levels. Rats fed a high-protein diet or starved for 48 h had increased blood urea. Carbamyl glutamate injection induced a further increase in the levels of urea in their blood. Also, carbamyl glutamate administered in the drinking water of normal mice produced an increase in blood urea, which was accompanied by a decrease in blood ammonia. The application of these findings to the treatment of urea cycle enzymopathies is discussed.European Journal of Pediatrics 02/1985; 143(3):196-7. · 1.91 Impact Factor
Article: Hyperammonemia.[show abstract] [hide abstract]
ABSTRACT: A symptomatic elevation in plasma ammonium concentration, termed hyperammonemia, is associated with numerous congenital and acquired conditions (Table 11). In some cases, such as urea cycle disorders, ammonia is the principal toxin. In other instances, such as portal systemic encephalopathy, it is but one of a number of metabolic disturbances, However, in either case hyperammonemic episodes should be treated aggressively to prevent coma, subsequent brain damage, or death. This involves restricting protein intake, providing adequate calories, and giving agents that remove accumulated nitrogen. Long-term therapy relies on diagnosing the specific disease rate. This rarely requires invasive procedures such as liver biopsy. In most cases measurement of plasma amino acids and urinary organic acids will identify the defect. Treatment involving restriction of nitrogen intake, vitamin supplementation, or stimulation of alternative pathways of waste nitrogen excretion can then be instituted. Early therapy, especially in patients with neonatal-onset hyperammonemia, is imperative to avoid severe brain damage. On this basis, the plasma ammonium level should be determined in virtually every newborn with lethargy, hypotonia, poor feeding, seizures, and/or respiratory distress of unclear origin (Table 12).Current Problems in Pediatric and Adolescent Health Care 12/1984; 14(11):1-69.