Development of a minimally invasive protocol for the determination of phenylalanine and lysine kinetics in humans during the fed state

The primed, continuous intravenous infusion of amino acids labeled with C-13 together with measurement of isotopic enrichment in plasma is commonly used to study amino acid metabolism. However, a less invasive, oral infusion that also produces an isotopic steady state in CO2 and urine would be useful, particularly for pediatric studies. We measured the C-13 enrichments of expired CO2, plasma and urine free phenylalanine and lysine and estimated flux and oxidation rates in adult humans (n = 12) who received a 4-h oral, primed, equal dose infusion of either L-[1-C-13]phenylalanine, L-[1-C-13]lysine (D-lysine - 1.6%) or L-[1-C-13]lysine (D-lysine less than or equal to 0.2%). Steady fed state conditions were established by feeding subjects eight hourly meals beginning 4 h before the start of the oral infusion protocol. Isotopic plateau in CO2, plasma and urine was achieved within 120 min of phenylalanine or lysine infusion, At isotopic plateau, the mean ratio of plasma to urine enrichment was 1.0 +/- 0.04 (SEM), 0.39 +/- 0.03 and 0.97 +/- 0.02 for L-[1-C-13]phenylalanine, L-[1-C-13]lysine (D-lysine = 1.6%) and L-[1-C-13]lysine (D-lysine less than or equal to 0.2%), respectively. There was good agreement between isotopic enrichment in plasma and urine for L-[1-C-13]phenylalanine and L-[1-C-13]lysine (D-lysine less than or equal to 0.2%). However, use of L-[1-C-13]lysine (D-lysine = 1.6%) resulted in significantly higher enrichment in urine, probably due to renal tubular discrimination of D-lysine. Mean flux rates for phenylalanine and lysine were consistent with the literature. Thus, the oral, primed, equal dose infusion produces the isotopic steady-state condition required for amino acid flux and oxidation determination within an 8-h period.