Protein degradation and synthesis measured with multiple amino acid tracers in vivo

Whether tracers of different essential amino acids yield the same estimates of body protein turnover is still uncertain. Therefore, we have simultaneously determined leucine (Leu; using [14C]Leu), phenylalanine (Phe; using [13C]Phe), and tyrosine (Tyr; using [2H2]Tyr) rates of appearance (Ra) from proteolysis (PD), as well as Leu and Phe disposal, into protein synthesis (PS) both before and after an anabolic stimulus in healthy volunteers. Protein anabolism was stimulated by insulin plus a branched-chain amino acid-enriched aromatic amino acid-deficient amino acid solution, which increased Leu (from 145 +/- 9 to 266 +/- 10 mumol/l) but decreased Phe (from 57 +/- 2 to 46 +/- 3) and Tyr (from 58.7 +/- 5.5 to 21.0 +/- 2.2) concentrations. Postabsorptive endogenous Leu Ra (2.04 +/- 0.12 mumol.kg-1.min-1), Phe Ra (0.66 +/- 0.03), and Tyr Ra (0.45 +/- 0.06), as well as rates of PS determined with the leucine (1.65 +/- 0.10 mumol.kg-1.min-1) and the phenylalanine tracer (0.57 +/- 0.03), agreed well with the known abundance of these amino acids in body protein(s). After insulin and amino acids, PD was suppressed (P < 0.001) using all tracers. However, although percent suppression of endogenous Leu Ra (-->1.49 +/- 0.10 mumol.kg-1.min-1, 26 +/- 5%) and Phe Ra (-->0.53 +/- 0.02 mumol.kg-1.min-1, -20 +/- 2%) were comparable, endogenous Tyr Ra was suppressed to a larger extent (-->0.23 +/- 0.02 mumol.kg-1.min-1, -46 +/- 3% P = 0.038). PS was stimulated using the Leu (+24 +/- 7%, P < 0.02) but not the Phe (+6 +/- 4%, not significant) data. We conclude that isotopes of different essential amino acid: provide comparable estimates of PD and PS in the postabsorptive state. However, their responses to an anabolic stimulus may differ, possibly depending on exogenous amino acid availability and/or the resulting plasma levels.