In vivo rates of protein synthesis in Atlantic salmon (Salmo salar L.) smolts determined using a stable isotope flooding dose technique

In vivo rates of protein synthesis in fish have predominantly been measured using a single flooding dose injection of a solution containing H-3-Phenylalanine as a tracer. However, use of a radiolabelled tracer restricts the application of this technique to controlled laboratory conditions. In this study, the flooding dose technique was used to successfully measure in vivo rates of protein synthesis in Atlantic salmon (Salmo salar L.) smelts utilising a stable isotope labelled amino acid, L-[N-15]-Phenylalanine, as an alternative tracer Methodologies an presented allowing the N-15 enrichment in the free amino acid pool and in body protein to be measured using mass spectrometry. In this study, the fractional rates of protein consumption (k(r)), synthesis (k(s)) and growth (k(g)) Of Atlantic salmon smelts (37 +/- 1.4 g) were measured as 4.7, 3.3 and 1.6% day(-1) respectively. Protein synthesis retention efficiency (k(g)k(s)) and protein growth efficiency (k(g)k(r))were calculated as 48.5 and 34.0% respectively. The k(s), k(g)k(s) and k(g)k(r). values obtained in this study were found to be within the range of values available in the literature for juvenile salmonid fish of a comparable size. The use of L-[N-15]-Phenylalanine provides an alternative tracer to radiolabelled amino acids for measuring in vivo rates of protein synthesis in fish using the flooding dose' technique and also allows studies of fish protein metabolism to be carried out in environmentally sensitive (field and laboratory) conditions where the use of radiolabels is prohibited.