Effects of different dietary levels of soybean protein hydrolysates on the growth performance, antioxidant capacity and relative mRNA expression levels of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ x Epinephelus lanceolatus ♂)

An 8-week feeding trial was conducted to investigate the effect of different dietary levels of soybean protein hydrolysates (SPH) on the growth performance, antioxidant capacity, non-specific immunity and mRNA expression levels of insulin-like growth factor I (IGF-I), target of rapamycin (TOR) and oligopeptide transporter 1 (PepT1) for juvenile hybrid grouper (average weight: 12.5 +/- 0.40 g). Five iso-protein and iso-lipid diets were formulated by adding 0, 10, 20, 40 and 60 g/kg of SPH, respectively. The SPH2-6 significantly reduced feed conversion ratio and enhanced weight gain rate, specific growth rate and protein efficiency ratio (p < .05). The treatment groups significantly decreased the total protein (p < .05). The cholesterol, low-density lipoprotein and high-density lipoprotein of fish fed with SPH6 were significantly lower (p < .05) than control. Fish fed with SPH4 and SPH6 had significantly higher catalase, superoxide dismutase and alkaline phosphatase activities, as well as significantly lower malondialdehyde than the control (p < .05). The levels of aspartate aminotransferase, alanine transaminase, lysozyme, lipase, trypsin, pepsin, complement 3 and complement 4 showed a similar trend of initially increasing and then decreasing. The SPH2-6 significantly increased IGF-I and TOR mRNA expressions in liver as well as PepT1 in intestine (p < .05). The optimal SPH inclusion level was estimated to be 42.99 g/kg diet for juvenile hybrid grouper.