THE EFFECT OF AMINO-ACID INFUSION ON LEG PROTEIN-TURNOVER ASSESSED BY L-[N-15]PHENYLALANINE AND L-[1-C-13]LEUCINE EXCHANGE

Abstract. A stable isotope technique depending on the use of [15N]phenylalanine and [1‐13C]leucine to assess exchange was utilized to measure the components of protein turnover of the human leg and the effects of amino acid infusion. Eight healthy subjects (28.5 ±2.5 years) were studied when post‐absorptive in the basal state and again during infusion of a mixed amino acid solution (55 g 1−1, 1.52 ml kg−1 h−1). During the basal period leucine oxidation by the leg was 4.4 ± 2.0 nmol 100 g−1 min−1 and this increased threefold during amino acid infusion (13.6 ± 3.1 nmol 100 g−1 min−1, mean ± SEM, P=0.003). Amino acid infusion abolished the net negative balance between incorporation of leucine into, and release from, protein (basal, — 31.8 ± 5.8; during infusion, +3.1 ±7.1 nmol 100 g−1 P = 0.001). Phenylalanine exchange showed a similar pattern (basal, —13.7 ±1.8; during infusion, ‐0.8±3.0 nmol 100 g−1 min−1, P=0.003). Basal entry of leucine into leg protein (i.e. protein synthesis) was 70.0± 10.8 nmol 100 g−1 min−1 and this increased during amino acid infusion to 87.3 + 14.1 nmol 100 g−1 min−1 (P = 0.11). Phenylalanine entry to protein also increased with amino acid infusion (29.1±4.5 vs. 38.3± 5.8 nmol 100 g−1 min−1, P=0.09). Release from protein of leucine (101.8 ± 9.1 vs. 84.2±9.l nmol 100 g−1 min−1, P=0.21) and of phenylalanine (42.8 ± 4.2 vs. 39.1±4.2 nmol 100 g−1 min−1, P=0.50) was unchanged by amino acid infusion. The results suggest that, in the post‐absorptive state in man, infusion of mixed amino acids, without additional energy substrates; reverses negative amino acid balance by a mechanism which includes stimulation of muscle protein synthesis but which does not alter protein breakdown. Interpretation of the results obtained concurrently on whole‐body protein turnover suggests that the increase in muscle protein synthesis contributes substantially to the whole‐body increase, but the fall in whole‐body breakdown with exogenous amino acids is independent of changes in muscle. © 1980 European Society for Clinical Investigation