Improvement of flesh quality, muscle growth and protein deposition in adult grass carp (Ctenopharyngodon idella): The role of tryptophan

Abstract

The muscle is the main edible part of the animal trunk. Previous studies have shown that tryptophan is the limiting amino acid for protein synthesis. This study investigated whether dietary tryptophan could improve growth performance, flesh quality, muscle growth, and protein deposition in adult grass carp (Ctenopharyngodon idella). 450 grass carp (671.31 ± 1.66 g) were fed with six diets (iso‑nitrogenous) with different concentrations of tryptophan (0.25, 1.66, 3.12, 4.69, 5.95, and 7.61 g/kg diet) for 9 weeks. The results revealed that the optimal tryptophan levels (3.12 or 4.69 g/kg diet) increased feed intake (FI), feed efficiency (FE), percent weight gain (PWG), and specific growth rate (SGR) compared with the deficient group (0.25 g/kg diet), indicating that tryptophan improved the growth performance of adult grass carp. The optimal tryptophan levels increased the muscle contents of protein, lipid, protein-bound amino acids, free amino acids (FAA), pH24h, and shear force; improved the fatty acid (FA) profile in muscle; and reduced cooking loss and lactate content, suggesting that tryptophan improved the flesh quality of adult grass carp. The optimal tryptophan levels increased the myofiber diameter, frequency of >50 μm diameter fiber, mRNA levels of MRFs (MyoD, MyoG, Mrf4, and Myf5), and decreased the mRNA levels of MSTN in muscle, indicating that tryptophan promoted muscle growth in adult grass carp. We further found that the promotion of muscle protein deposition by dietary tryptophan may be related to protein synthesis, ubiquitin-proteasome, and autophagic lysosomal pathways. We discovered that dietary tryptophan increased the mRNA abundances of IGF-1, PI3K, AKT, TOR, and S6K1 and decreased the mRNA abundances of 4EBP1, FOXO1a, FOXO3a, MAFBX, MuRF1, UB, ULK1, Beclin1, ATG5, ATG12, p62, LC3–1, and LC3–2, thereby promoting muscle protein deposition in adult grass carp. The above results showed that dietary tryptophan enhanced growth performance, flesh quality, muscle growth, and muscle protein deposition in adult grass carp. Moreover, using broken line regression, the tryptophan requirement of adult grass carp was estimated to be 3.02 g/kg diet (11.64 g/kg protein) based on PWG.