Effects of dietary methionine levels on muscle amino acids composition and serum antioxidant ability of GIFT Oreochromis niloticus

A feeding trial was conducted to evaluate the effects of dietary methionine levels on muscle amino acids composition and serum antioxidant ability of GIFT tilapia (Oreochromis niloticus)initial body weight of (66.76±2.29) g. Six isonitrogenous and isoenergetic semi-purified diets were formulated to contain 32.09% crude protein,17.82% kJ/g gross energy and six levels of methionine 0.26%, 0.55%, 0.85%, 1.14%, 1.44% and 1.73% respectively (dry matter basis). Each diet was randomly assigned to triplicate groups of 25 fish and fed to apparent satiation by hand thrice daily (8:00, 13:00 and 16:00) for 60 days in indoor farming system. The results showed that the all amino acids content, gross of essential amino acid (ΣEAA), gross of total amino acid (ΣTAA), gross of total delicious amino acid (ΣDAA) in muscle initially increased with increasing dietary methionine levels but then decreased. All the indexes were highest when the dietary methionine level was 1.14% (P<0.05). The ratio of essential amino acids to total amino acids (ΣEAA/ΣTAA), the ratio of delicious amino acid to total amino acids (ΣDAA/ΣTAA), the ratio of branched chain amino acid to aromatic amino acids (ΣBCAA/ΣAAA) had no significant differences (P>0.05). With the increase of dietary methionine level, the activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px) in serum first decreased, then increased, at the same time, the malondialdehyde (MDA) in serum first increased, then towards stability. Results of above show that the optimum dietary methionine level could effectively improve the muscle amino acid composition and the quality of the muscles, and enhance the serum antioxidant capacity of GIFT tilapia.