Protein degradation and synthesis measured with multiple amino acid tracers in post-absorptive and protein-anabolic conditions. Tessari, Paolo, Rocco Barazzoni, Michela Zanetti, Monica Vettore, Sylvie Normand, Daniela Bruttomesso, Bernard Beaufrere. Dept. of Metabolism, University of Padova, Italy; INSERM Unit[grave]e 197, Lyon, France; and Laboratoire de Nutrition Humaine, Centre de Recherche en Nutrition Humaine, Clermont-Ferrand, France
APStracts 3:0137E, 1996.
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 following an anabolic stimulus in healthy volunteers. Protein anabolism was stimulated by insulin plus a BCAA-enriched, AAA deficient amino acid solution, which increased Leu (from 145+/-9 to 266+/-10 [mu]mol/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 [mu]mol/kg x min), 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 [mu]mol/kg x min) and the phenylalanine tracer (0.57+/-0.03), agreed well with the known abundance of these amino acids in body protein(s). Following insulin and amino acids, PD was suppressed (p<0.001) using all tracers. However, while percent suppression of endogenous Leu Ra ( 1.49+/-0.10 [mu]mol/kg x min, -26+/-5%) and Phe Ra ( 0.53+/-0.02 [mu]mol/kg x min, -20+/-2%) were comparable, endo-genous Tyr Ra was suppressed to a larger extent ( 0.23+/-0.02 [mu]mol/ /kg x min, -46+/-3%, p=0.038). PS was stimulated using the Leu (+24+/-7%, p<0.02) but not the Phe (+6+/-4%, NS) data. We conclude that isotopes of different essential amino acids provide comparable estimates of PD and PS in the post-absorptive state. However, their responses to an anabolic stimulus may differ, possibly depending on exogenous amino acid availability and/or the resulting plasma levels. 

Received 26 June 1995; accepted in final form 7 June 1996.
APS Manuscript Number E293-5.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 July 1996