Different dietary protein levels affect flesh quality, fatty acids and alter gene expression of Nrf2-mediated antioxidant enzymes in the muscle of grass carp (Ctenopharyngodon idella)

This study investigated the effects of dietary protein levels on the flesh quality, fatty acids, antioxidant capacity and antioxidant-related signalling molecule gene expression in the muscle of grass carp (Ctenopharyngodon idella). A total of 540 grass carp (264.11 +/- 0.76 g) were fed six diets containing graded levels of protein (143.1, 176.7, 217.2, 257.5, 292.2 and 322.8 g digestible protein kg(-1) diet) for 8 weeks. The results indicated that optimal dietary protein levels: (1) improved physical (or chemical) and flavor quality of grass carp fillets by increasing shear force, pH, hydroxyproline, protein, lipid, free amino acids (aspartic acid, threonine, serine, glutamic acid, glycine, alanine, valine, isoleucine, lysine, histidine, arginine and proline) and nucleotide (5'inosinic acid) contents, and decreasing cooking loss, cathepsin B and L activities and lactate content. (2) enhanced the potential health benefits to humans of grass carp fillets by decreasing the saturated fatty acid (C14:0, C15:0, C16:0, C18:0 and C20:0) concentrations, and increasing unsaturated fatty acids [monounsaturated fatty acids (C14:1 and C16:1) and polyunsaturated fatty acids (C18:2c + t, Cl 8:3n-3, C20:4 and C22:6)] concentrations. (3) enhanced antioxidant capacity of grass carp fillets by decreasing reactive oxygen species, malondialdehyde and protein carbonyl contents, and increasing the activities and gene expression of copper/zinc superoxide dismutase, manganese superoxide dismutase, glutathione peroxidase, glutathione-S-transferase and glutathione reductase and the content of glutathione. (4) regulated antioxidant-related signalling molecules gene expression by up-regulating the mRNA levels of NF-E2-related factor 2, casein kinase 2 alpha and 2 beta, and decreasing Kelch-like-ECH-associated protein 1a and 1b in the muscle of grass carp. In conclusion, optimal dietary protein level improved the physical and flavor characteristics, fatty acid profile, antioxidant capacity and regulated the gene expression of antioxidant-related signalling molecules in fish fillets.