New functions of enzymatic cottonseed protein: Improvement of intestinal digestion and absorption, structure and microbial composition of juvenile yellow catfish (Pelteobagrus fulvidraco).

Enzymatic cottonseed protein (ECP) is a functional peptide product reported to have antioxidant and immunomodulatory properties. This study focused on how the intestinal apical junction complex (AJC) structure, microbial composition, growth performance, and digestive and absorptive capacity were affected by adding ECP to diets with a 3% crude protein (CP) reduction. A total of 1260 healthy juvenile yellow catfish weighing 5.90 ± 0.05 g were randomly assigned to seven experimental groups, each consisting of three replicates. A normal protein (NP) diet (42% CP) or one of six low protein (LP) diets (39% CP) with differing amounts of ECP (0%, 1%, 2%, 3%, 4%, and 5%, respectively) given for ten weeks. Dietary CP levels that were lowered by 3% (from 42% to 39%) disrupted the intestinal AJC structure and reduced growth performance, including specific growth rate (SGR; P = 0.042) and feed intake (FI; P = 0.001) and digestive and absorptive capacity, including the activities of alkaline phosphatase (AKP; P < 0.001), γ-glutamyl transpeptidase (γ-GT; P < 0.001), Na(+)/K(+)-ATPase (P < 0.001), pepsin (P < 0.001), lipase (P = 0.009), amylase (P < 0.001), chymotrypsin (P < 0.001), and carboxypeptidase (CPs; P = 0.005). Adding 2% ECP to the LP diets (39% CP) improved SGR (P = 0.001), FI (P < 0.001), pepsin activity (P < 0.001), the activities of γ-GT (P < 0.001), AKP (P < 0.001), lipase (P < 0.001), amylase (P < 0.001), Na(+)/K(+)-ATPase (P < 0.001), chymotrypsin (P < 0.001), aminopeptidases (Aps; P < 0.001), and CPs (P < 0.001). The factors that control the expression of peptide transporter 1 (PepT1), such as nuclear factor erythroid 2-related factor 2 (nrf2), Janus kinase 2 (jak2), caudal type homeobox transcription factor 2 (CDX2), and specificity protein 1 (SP1), as well as those that affect its function, such as protein sodium/hydrogen exchange isoform 3 (NHE3) and PDZ domain containing 1 (PDZK1), may be responsible for the increase in the expression of PepT1 (P = 0.008). Furthermore, the Ras homolog family member A (RhoA)/Rho-associated kinase (ROCK) signaling pathway may be involved in improving intestinal AJC structure. ECP also increased the relative abundance of beneficial bacteria, including Fusobacteria, Firmicutes, Cetobacterium, and Acinetobacter, while decreasing that of pathogenic bacteria, such as Spirochaetes, Actinomycetes, Brevinema, and Plesiomonas. A 3% decrease in dietary CP levels and supplementation with 2% ECP restored growth performance, intestinal structural indices, and digestive and absorptive capacity to levels equivalent to or better than those of the NP. Furthermore, chymotrypsin and percentage weight gain broken-line regression analysis revealed that the optimal ECP additions for juvenile catfish were 2.08% and 2.2%, respectively.