In vivo monitoring of urea cycle activity with (13)C-acetate as a tracer of ureagenesis

Opladen, T.; Lindner, M.; Das, A. M.; Marquardt, T.; Khan, A.; Emre, S. H.; Burton, B. K.; Barshop, B. A.; Bohm, T.; Meyburg, J.; Zangerl, K.; Mayorandan, S.; Burgard, P.; Durr, U. H.; Rosenkranz, B.; Rennecke, J.; Derbinski, J.; Yudkoff, M.; Hoffmann, G. F.

Mol Genet Metab. 2015 Nov 26; 117(1):19-26

Abstract

BACKGROUND: The hepatic urea cycle is the main metabolic pathway for detoxification of ammonia. Inborn errors of urea cycle function present with severe hyperammonemia and a high case fatality rate. Long-term prognosis depends on the residual activity of the defective enzyme. A reliable method to estimate urea cycle activity in-vivo does not exist yet. The aim of this study was to evaluate a practical method to quantify (13)C-urea production as a marker for urea cycle function in healthy subjects, patients with confirmed urea cycle defect (UCD) and asymptomatic carriers of UCD mutations. METHODS: (13)C-labeled sodium acetate was applied orally in a single dose to 47 subjects (10 healthy subjects, 28 symptomatic patients, 9 asymptomatic carriers). RESULTS: The oral (13)C-ureagenesis assay is a safe method. While healthy subjects and asymptomatic carriers did not differ with regards to kinetic variables for urea cycle flux, symptomatic patients had lower (13)C-plasma urea levels. Although the (13)C-ureagenesis assay revealed no significant differences between individual urea cycle enzyme defects, it reflected the heterogeneity between different clinical subgroups, including male neonatal onset ornithine carbamoyltransferase deficiency. Applying the (13)C-urea area under the curve can differentiate between severe from more mildly affected neonates. Late onset patients differ significantly from neonates, carriers and healthy subjects. CONCLUSION: This study evaluated the oral (13)C-ureagenesis assay as a sensitive in-vivo measure for ureagenesis capacity. The assay has the potential to become a reliable tool to differentiate UCD patient subgroups, follow changes in ureagenesis capacity and could be helpful in monitoring novel therapies of UCD.

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