Amino acid absorption and protein synthesis responses of turbot Scophthalmus maximus to lysine and leucine in free, dipeptide and tripeptide forms

The study aimed to investigate the effects of crystalline Lys and Leu, Lys-Leu dipeptide and Lys-Leu-Lys/Leu-Lys-Leu tripeptide, on growth, postprandial Lys and Leu concentrations, gene expression of peptide and amino acid (AA) transporters and related gene expression of protein synthesis pathway in turbot. Three diets (CAA, Di and Tri) contained Lys and Leu as free (Lys and Leu), dipeptide (Lys-Leu) and tripeptide (Lys-Leu-Lys/Leu-Lys-Leu) forms, respectively. Improved growth was observed in the Di group compared with the CAA group. For peptide and AAs transporters, PepT1, B(0)AT1 and y(+)LAT2 mRNA levels were affected in proximal intestine by dietary treatments. Free Lys and Leu concentrations in the CAA group were significantly higher than that of the Di and Tri groups at 6 hr postfeeding in serum and at 2, 6 and 24 hr postfeeding in muscle. For protein synthesis pathway in muscle, Akt2, TOR and S6k1 gene expression were the highest in the Di group and the lowest in the CAA group, whereas MuRF1 relative expression was the highest in the Tri group. In conclusion, dietary Lys-Leu dipeptide was utilized more efficiently than free Lys and Leu or Lys-Leu-Lys/Leu-Lys-Leu tripeptide for turbot by regulating AAs transport, postprandial AAs concentration and the synthesis of muscle proteins.