Effects of dietary soy protein concentrate meal on growth, immunity, enzyme activity and protein metabolism in relation to gene expression in large yellow croaker Larimichthys crocea

Abstract

A 56-day feeding trial was conducted to investigate the effects of dietary soy protein concentrate meal (SPC) levels on growth performance, body composition, antioxidant and immune responses, enzyme activities and protein metabolism in relation to gene expression in Larimichthys crocea (initial weight 10.50±0.04g). Five isonitrogenous and isolipidic diets (45% protein and 10% lipid) were formulated by replacing 0 (the control), 25%, 50%, 75% and 100% of fish meal protein with SPC protein (designed SPC0, SPC25T, SPC50T, SPC75T and SPC100T, respectively). Lysine and methionine were added to the feed except the control diet, and the levels in the treatment diets were the same. Each diet was randomly fed twice daily (05:00 and 17:00) to apparent satiation to triplicate groups (60 fish per cage). The survival rate (SR) ranged from 90.56 to 93.89%, with no significant differences observed among dietary treatments (P>0.05). In addition, no significant differences were found for weight gain ratios (WGR), specific growth rates (SGR), feed conversion ratios (FCR) and feed intakes (FI) (P>0.05). Fish muscle protein and moisture contents did not show any statistical differences among the dietary treatments (P>0.05). Lipid contents first increased and then decreased, and the SPC25T diet was the highest. Ash contents first decreased and then increased, and the SPC100T diet was the highest. Fish whole-body composition analysis showed that with increasing dietary SPC levels, protein contents did not show any statistical differences among the dietary treatments (P>0.05). Lipid contents were significantly lower of SPC100T than SPC0 and SPC25T diets (P<0.05), and the highest was in the SPC0 diet. The contents of moisture in SPC100T diet were observed the highest, and significantly higher than the control diet (P<0.05). Ash contents of SPC50T diet were significant higher than SPC0 and SPC25T diets (P<0.05), but no significant differences were found among the diets of SPC50T, SPC75T and SPC100T (P>0.05). Significantly lower liver total superoxide dismutase (t-SOD) activity was found for the SPC100T diet than the SPC25T and SPC50T diets (P<0.05). There were no significant differences among the dietary diets with regard to malondialdehyde (MDA), catalase (CAT), and glutathione peroxidase （GPx） (P>0.05). Serum lysozyme (LYZ) activity was significantly lower in the SPC100T diet than in the SPC25T and SPC75T diets (P<0.05). There were no significant differences among the diets for alkaline phosphatase (AKP), complement C3 and C4 (P>0.05). Regarding digestive enzymes, no significant differences were found for hintgut alpha-amylase and lipase activity between the dietary treatments and the control diet (P>0.05). Among all diets, trypsin activity was the highest in fish fed the SPC75 diet, and significantly higher than SPC50T (P<0.05), but there were no significant differences among the SPC0, SPC25T, SPC75T and SPC100T diets (P>0.05). For liver protein metabolism enzymes, no significant differences were found in alanine aminotransferase (ALT) and aspartate amino-transferase (AST) among all dietary diets (P>0. 05). With increasing dietary SPC levels, hepatic insulin-like growth factor-I (IGF-I) expression decreased, and significant differences were observed between SPC50T, SPC75T, SPC100T and the control diets (P<0.05). Target of rapamycin (TOR) expression in the liver decreased, and there were significant differences between the treatment and the control diets (P<0.05).In conclusion, the results of this study suggest that SPC can completely replace fish meal protein as feed for large yellow croaker with no significant negative effects on growth performance.