Effects of Replacing Dietary Fishmeal with Enzyme-Treated Soybean Protein on Growth Performance, Serum Biochemical Indices, Antioxidant, Digestive Enzyme Activities, Intestinal Barrier Integrity and Relative mRNA Expression Levels of Juvenile Largemo

The enzyme-treated soybean protein (ETSP) could improve their application as an ingredient and substitute for fishmeal (FM) in aquafeeds. In the present research, an eight-week feeding trial was carried out to investigate the effects of using ETSP as a particle substitution in FM on growth performance, serum biochemical indices, antioxidant, intestinal barrier integrity and relative mRNA expression levels in largemouth bass (Micropterus salmoides). A cohort of 450 fish (15.52 ± 0.04 g/fish) were randomly divided into five groups with five replacement levels of ETSP: 0, 12.5, 25.0, 37.5 and 50.0% (control, H2-12.5, H2-25.0, H2-37.5 and H2-50.0). Results showed that final body weight (FBW), weight gain (WG), and specific growth rate (SGR) reach the highest value, and the feed conversion ratio (FCR) reaches lowest in H2-25.0 group. The activities of total superoxide dismutase (T-SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) were remarkably increased by replacing FM with ETSP at certain level, while malondialdehyde (MDA) concentration significantly decreased (p < 0.05). Higher levels of total protein (TP) and albumin (ALB) as well as lower activity of aspartate aminotransferase (AST) were detected with ETSP replacement. Histological examination revealed that villus height in mid-intestine was the highest, while this factor in pyloric caeca was not affected. The activity of diamine oxidase (DAO) and the contents of D-lactate (D-Lac) and lipopolysaccharide (LPS) in serum were gradually decreased when replacing FM with ETSP in the diets. In addition, replacement of FM with ETSP at 25% improved intestinal barrier function by upregulating gene expression related to the tight junctions (occludin, claudins, ZO-1 and ZO-2) and anti-inflammatory cytokines (IL-10) while downregulating pro-inflammatory factors (IL-8 and TNFα). Moreover, proper amount of ETSP instead of FM can upregulate mRNA expression of LAT1, PEPT1 and PEPT2 in the intestine. In conclusion, at the fishmeal level used in the diet tested in the present study, 25% replacement is the most beneficial for juvenile largemouth bass.