No abstract

A 12‐week feeding trial was conducted to investigate the potential protective effects of citric acid against soybean meal‐induced intestinal oxidative damage and micro‐ecological imbalance in juvenile turbot (S. maximus L.). Four isonitrogenous and isolipidic experimental diets, that is fish meal‐based diet (FM), FM with 40% fish meal protein replaced with soybean meal protein diet (SBM), supplemented with 1.5% citric acid (1.5% CA) or 3% citric acid (3% CA). Results showed that both citric acid diets significantly enhanced the total antioxidative capacity and the gene expression of superoxide dismutase, glutathione peroxidase and heme oxygenase 1, while also decreasing the malondialdehyde content in the distal intestine. Compared to diet SBM, the genes expression of p53, protein kinase C and Caspase‐3 were remarkably declined by dietary citric acid supplementation, while the genes expression of proliferating cell nuclear antigen and mucins showed an opposite trend. The structural integrity of the distal intestine in fish fed citric acid was showed in the histological results. Sequencing of bacterial 16s rRNA V4 region showed that the profile of intestinal bacteria was altered by dietary citric acid supplementation, which was supported by the diet‐cluster of PCoA and phylogenetic tree. MetaStat analysis indicated that dietary citric acid dramatically reduced the relative abundance of the Vibrio genus. In conclusion, dietary citric acid mitigated soybean meal‐induced intestinal oxidative damage, beneficially alleviated the micro‐ecological imbalance and specifically reduced the relative abundance of the Vibrio genus in the distal intestine of juvenile turbot.

Aldehyde reductase (ALR) plays key roles in the detoxification of toxic aldehyde. In this study, the authors cloned the swamp eel ALR gene using rapid amplification of cDNA ends‐PCR (RACE‐PCR). The recombinant protein (rALR) was expressed in Escherichia coli and purified using a Ni2+‐NTA chelating column. The rALR protein exhibited efficient reductive activity towards several aldehydes, ketones and S‐nitrosoglutathione (GSNO). A spot assay suggested that the recombinant E. coli strain expressing rALR showed better resistance to formaldehyde, sodium nitrite and GSNO stress, suggesting that swamp eel ALR is crucial for redox homeostasis in vivo. Consequently, the authors investigated the effect of rALR on the oxidative parameters of the liver in swamp eels challenged with Aeromonas hydrophila. The hepatic glutathione (GSH) content significantly increased, and the hepatic NO content and levels of reactive oxygen species and reactive nitrogen species significantly decreased when rALR was administered. In addition, the mRNA expression of hepatic Alr, HO1 and Nrf2 was significantly upregulated, whereas the expression levels of NF‐κB, IL‐1β and NOS1 were significantly downregulated in the rALR‐administered group. Collectively, these results suggest that ALR is involved in the response to nitrosative stress by regulating GSH/NO levels in the swamp eel.