title: Isotopic enrichment of amino acids in urine following oral infusions of L-[1-(13)C]phenylalanine and L-[1-(13)C]lysine in humans: confounding effect of D-[13C]amino acids.
abstract: 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-(13)C]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-(13)C]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-(13)C]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.
authors.affiliation: Research Institute, The Hospital for Sick Children, and Department of Nutritional Sciences, University of Toronto, Ontario, Canada.
authors.forename: P B
authors.initials: PB
authors.lastname: Darling
authors.forename: R
authors.initials: R
authors.lastname: Bross
authors.forename: L J
authors.initials: LJ
authors.lastname: Wykes
authors.forename: R O
authors.initials: RO
authors.lastname: Ball
authors.forename: P B
authors.initials: PB
authors.lastname: Pencharz
mesh_terms: D000284:Administration, Oral; D000328:Adult; D000596:Amino Acids / Q000652:urine; D002247:Carbon Isotopes; D015986:Confounding Factors, Epidemiologic; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007234:Infant, Premature; D008239:Lysine / Q000008:administration & dosage / Q000737:chemistry* / Q000652:urine*; D010649:Phenylalanine / Q000008:administration & dosage / Q000737:chemistry* / Q000652:urine*; D013237:Stereoisomerism
chemical_list: D000596:Amino Acids; D002247:Carbon Isotopes; D010649:Phenylalanine; D008239:Lysine