Metabolism of Nonessential 15N-Labeled Amino Acids and the Measurement of Human Whole-Body Protein Synthesis Rates

Eight 15N-labeled nonessential amino acids plus 15NH4Cl were administered over a 10-h period to four healthy adult males using a primed-constant dosage regimen. The amount of 15N excreted in the urine and the urinary ammonia, hippuric acid and plasma alanine 15N enrichments were measured. There was a high degree of consistency across subjects in the ordering of the nine compounds based on the fraction of 15N excreted (Kendall coefficient of concordance W = 0.83, P less than 0.01). Protein synthesis rates were calculated from the urinary ammonia plateau enrichment and the cumulative excretion of 15N. Glycine was one of the few amino acids that gave similar values by both methods.