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

Urine sampling of the free amino acid pool serves to reflect plasma enrichm ent and is used as a noninvasive means to determine isotope enrichment in s tudies of amino acid metabolism. We determined the effect of D-[C-13]phenyl alanine and D-[C-13]lysine content of tracers on urinary amino acid enrichm ent following oral infusion of L-[C-13]phenylalanine in 18 preterm infants and L-[1-C-13]lysine in seven healthy adult females. Urinary [C-13]phenylal anine enrichment was higher (P <.0001) for L-[C-13]phenylalanine containing 0.4% D-[C-13]phenylalanine (28.6 +/- 7.1) versus L-[1-C-13]phenylalanine t hat contained undetectable D-[C-13]phenylalanine (10.2 +/- 1.5), D-[C-13]ph enylalanine. 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-[C-13]phenylalanine, and was abse nt from the urine of infants receiving tracer with undetectable [C-13]pheny lalanine. Urinary L-[C-13]phenylalanine enrichment did not differ between t racer groups (9.8 +/- 1.5 and 9.8 +/- 2.5). In adult females, the use of L- [1-C-13]lysine (1.6% D-lysine) resulted in a higher (P <.02) urine total L, D-[C-13]lysine enrichment compared with plasma enrichment (40.8 +/- 4.1 v 1 1.1 +/- 0.7). This study demonstrates the significant presence of D-[C-13]a mino acids in urine that originate as contaminants from commercially manufa ctured tracers, as a result of renal tubular discrimination of D-amino acid s. A tracer containing detectable amounts of D-[C-13]isomer cannot be recom mended for any study in which urine will be used to reflect enrichment in t he arterial plasma pool. Copyright (C) 1999 by W.B. Saunders Company.