The sea cucumber Stichopus japonicus protein was hydrolyzed by pepsin, trypsin, papain, acid protease and neutral protease, respectively, to get five kinds of peptide fractions: pepsin peptides (PP), trypsin peptides (TP), acid protease peptides (AP), neutral protease peptides (NP) and papain peptide (PAP). Antioxidative activities of all peptide fractions were evaluated by hydroxyl radical-(ÁOH) and Superoxide anion (O 2 Á 2 )-scavenging activity.Trypsin peptide (TP) exhibited the highest antioxidative activity compared to other peptide fractions. In considering scavenging effects on hydroxyl radicals (ÁOH) and Superoxide anions (O 2 Á 2 ), TP was employed for isolation, purification and identification of antioxidant peptide. To purify and characterize antioxidative peptide, two steps gel filtration, one-step ion-exchange column chromatography and reversed-phase high-performance liquid chromatography (RP-HPLC) were used. The purified antioxidative peptide TP2b-1 was a novel peptide and was sequenced as GPEPTGPTGAPQWLR, in which the low molecular weight and some amino acid constituents played important role in the radical-scavenging effects according reports. The IC 50 values of TP2b-1 were 138.9 lM on ÁOH and 353.9 lM on O 2 Á 2 .

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Background: The intestine is the first line of defense against bacteria, toxins and antigens presents in the lumen of the instine. Aeromonas veronii is an opportunistic pathogen that has been related to mortality in fish. However, the protective mechanisms of intestinal damage caused by Aeromonas veronii are poorly defined. Thus, the objetive of this study was to evaluate the influence of dietary valine (Val) on growth, antioxidant capacity, and endoplasmic reticulum stress-mediated apoptosis in intestines of the juvenile largemouth bass Micropterus punctulatus after infection with Aeromonas veronii (A. veronii). Methods: A total of 720 fish (37.99 ± 0.01 g) were randomly allocated to six groups with three replicates each group, with different levels of Val (11.0, 14.0, 17.0, 20.1, 23.1, and 26.1 g/kg diet, respectively) for 77 d, and then challenged with A. veronii. Results: In the present study, we demonstrated that dietary Val: (1) increased the percent weight gain (PWG), specific growth rate (SGR), feed intake (FI), feed efficiency (FE), protein efficiency ratio (PER), protein production value (PPV), lipid production value (LPV), and ash production value (APV) (P< 0.05); (2) improved survival rate (SR) and alleviated intestinal injury; (3) reduced the levels of reactive oxygen species (ROS), malondialdehyde (MDA), and protein carbonyl (PC) (P< 0.05), and improved the intestinal antioxidant capacity by enhancing the mRNA levels of Cu/Zn superoxide dismutase (SOD1), catalase (CAT), glutathione-S-transferase (GST), glutathione peroxidase (GPx), glutathione reductase (GR), and γ-glutamyl cysteine ligase catalytic subunit (GCLC) (P< 0.05); (4) increased nuclear NFE2-related factor 2 (Nrf2) mRNA and protein levels (P< 0.05); (5) reduced intestinal DNA fragments, and down-regulated the mRNA levels of 78 KD glucose regulated protein (GRP78), activating transcription factor 6 (ATF6), inositol requiring enzyme-1 (IRE1), protein kinase RNA-dependent-like endoplasmic reticulum kinase (PERK), eukaryotic initiation factor 2α (eIF2α), activating transcription factor 4 (ATF4), C/EBP homologous protein (CHOP), cysteine-aspartic protease 9 (caspase 9), and cysteine-aspartic protease 3 (caspase 3), while up-regulating that of B-cell lymphoma-2 (Bcl2) (P< 0.05); (6) decreased phosphorylation of PERK (p-PERK) and phosphorylation of eIF2α (p-eIF2α), caspase 9, and caspase 3 protein levels. Conclusion: Dietary optimal Val level enhanced the growth performance and the intestinal antioxidant capacity via the Nrf2 signaling pathway after infection with A. veronii. Furthermore, this study is the first to demonstrate the role of Val in reducing ERS-mediated apoptosis through modulation of the PERK/eIF2α signaling pathway. Finally, Quadratic regression analysis of the SGR indicated that the dietary Val requirement of largemouth bass (37.99-162.92 g) was 21.50 g/kg diet, corresponding to 46.64 g/kg protein.