N-carbamylglutamate markedly enhances ureagenesis in N-acetylglutamate deficiency and propionic acidemia as measured by isotopic incorporation and blood biomarkers

Abstract

N-acetylglutamate (NAG) is an endogenous essential cofactor for conversion of ammonia to urea in the liver. Deficiency of NAG causes hyperammonemia and occurs because of inherited deficiency of its producing enzyme, NAG synthase (NAGS), or interference with its function by short fatty acid derivatives. N-carbamylglutamate (NCG) can ameliorate hyperammonemia from NAGS deficiency and propionic and methylmalonic acidemia. We developed a stable isotope (13)C tracer method to measure ureagenesis and to evaluate the effect of NCG in humans. Seventeen healthy adults were investigated for the incorporation of (13)C label into urea. [(13)C]urea appeared in the blood within minutes, reaching maximum by 100 min, whereas breath (13)CO(2) reached a maximum by 60 min. A patient with NAGS deficiency showed very little urea labeling before treatment with NCG and normal labeling thereafter. Correspondingly, plasma levels of ammonia and glutamine decreased markedly and urea tripled after NCG treatment. Similarly, in a patient with propionic acidemia, NCG treatment resulted in a marked increase in urea labeling and decrease in glutamine, alanine, and glycine. These results provide a reliable method for measuring the effect of NCG on nitrogen metabolism and strongly suggest that NCG could be an effective treatment for inherited and secondary NAGS deficiency.

Figures

Figure 1

Increase over time of isotopic enrichment in breath 13CO2 (A) plasma [13C]urea (B) and the concentration of plasma [13C]urea (C) in healthy adult volunteers who were administered orally 0.33 mmol/kg (27 mg/kg) of [1-13C]sodium-acetate. Results are mean ± SEM of 33 individual studies in 17 subjects.

Figure 2

Increase over time of isotopic enrichment in breath 13CO2 (A) plasma [13C]urea (B) and the concentration of plasma [13C]urea (C) in a patient with NAGS deficiency before (○) and after (●) 3-d treatment with N-carbamylglutamate.

Figure 3

Plasma levels of ammonia (A), blood urea nitrogen (B) and glutamine (C) in a patient with NAGS deficiency before (A-blue, B-light green, C-magenta) and after (A-red, B-dark green, C-orange) 3-d treatment with N-carbamylglutamate.

Figure 4

Increase over time of isotopic enrichment in breath 13CO2 (A) plasma [13C]urea (B) and the concentration of plasma [13C]urea (C) in a patient with propionic acidemia before (○) and after (●) 3-d treatment with N-carbamylglutamate.