Isotopic enrichment of amino acids in urine following oral infusions of l-[1-13C]phenylalanine and l-[1-13C]lysine in humans: Confounding effect of d-[13C]amino acids☆

Add time:09/03/2019    Source:sciencedirect.com

Urine sampling of the free amino acid pool serves to reflect plasma enrichment and is used as a noninvasive means to determine isotope enrichment in studies of amino acid metabolism. We determined the effect of d-[13C]phenylalanine and d-[13C]lysine content of tracers on urinary amino acid enrichment following oral infusion of l-[13C]phenylalanine in 18 preterm infants and l-[1-13C]lysine in seven healthy adult females. Urinary [13C]phenylalanine enrichment was higher (P < .0001) for l-[13C]phenylalanine containing 0.4% d-[13C]phenylalanine (28.6 ± 7.1) versus l-[1-13C]phenylalanine that contained undetectable d-[13C]phenylalanine (10.2 ± 1.5). d-[13C]phenylalanine, measured by chiral column gas chromatography—mass spectrometry (GC-MS), accounted for 10% to 30% (20.5% ± 7%) of total phenylalanine in the urine of infants who received 0.4% d-[13C]phenylalanine, and was absent from the urine of infants receiving tracer with undetectable [13C]phenylalanine. Urinary l-[13C]phenylalanine enrichment did not differ between tracer groups (9.8 ± 1.5 and 9.8 ± 2.5). In adult females, the use of l-[1-13C]lysine (1.6% d-lysine) resulted in a higher (P < .02) urine total l,d-[13C]lysine enrichment compared with plasma enrichment (40.8 ± 4.1 v 11.1 ± 0.7). This study demonstrates the significant presence of d-[13C]amino acids in urine that originate as contaminants from commercially manufactured tracers, as a result of renal tubular discrimination of d-amino acids. A tracer containing detectable amounts of d-[13C]isomer cannot be recommended for any study in which urine will be used to reflect enrichment in the arterial plasma pool.

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