Abstract
The present study explored the impact of dietary isoleucine (Ile) on fish growth and flesh quality and revealed a possible role of muscle antioxidant defense in flesh quality in relation to dietary Ile. Grass carp (weighing 256.8±3.5 g) were fed diets containing six graded levels of Ile (3.8, 6.6, 9.3, 12.5, 15.2 and 18.5 g/kg) for eight weeks. The results indicated that compared with Ile deficiency (3.8 g/kg diets) and excess (18.5 g/kg diets) groups, 9.3–15.2 g Ile/kg diet supplementations promoted fish growth and muscle fat deposition, whereas 6.6–15.2 g Ile/kg diets supplementation enhanced muscle nutrients (protein and total EAAs) deposition. Furthermore, muscle shear force, pH value, and hydroxyproline concentration were improved by 9.3–12.5, 9.3 and 9.3 g Ile/kg diet supplementations, respectively. However, muscle cooking loss, lactate content, and activities of cathepsin B and L were decreased by 6.6–15.2, 9.3–12.5, 9.3–12.5 and 9.3–15.2 g Ile/kg diet supplementations, respectively. Additionally, 6.6–15.2 and 6.6–12.5 g Ile/kg diet supplementations attenuated malondialdehyde and protein carbonyl contents, respectively. The activities of copper/zinc superoxide dismutase (Cu/Zn-SOD) and glutathione peroxidase (GPx), and glutathione content were enhanced by 6.6–9.3, 6.6–12.5 and 6.6–15.2 g Ile/kg diet supplementations, respectively. Moreover, the relative mRNA expressions of antioxidant enzymes, including Cu/Zn-SOD (6.6–12.5 g/kg diets) and GPx (12.5 g/kg diets), as well as antioxidant-related signaling molecules, including NF-E2-related factor 2 (Nrf2) (6.6–12.5 g/kg diets), target of rapamycin (6.6–12.5 g/kg diets), ribosomal S6 protein kinase 1 (9.3–12.5 g/kg diets) and casein kinase 2 (6.6–12.5 g/kg diets), were up-regulated when Ile diet supplementations were administered at these levels, respectively, whereas the relative mRNA expression of Kelch-like ECH-associated protein 1 was down-regulated with 9.3 g Ile/kg diet supplementations. Collectively, the present study indicated that optimum isoleucine improved flesh quality, partly due to the activation of antioxidant defense through the Nrf2 signaling pathway.
Highlights
Between 1980 and 2010, the global per capita consumption of fish increased by more than 58% with the rapid development of aquaculture [1]
The Nrf2 mRNA levels were the highest for fish that were fed 12.5 g Ile/ kg diet, followed by 6.6 and 9.3 g Ile/kg diet (P,0.05) (Fig. 4), and no significant differences were found among the other groups (P.0.05) (Fig. 4)
The results suggest that Ile-improved Cu/Zn-SOD and GPx activities may be attributed to the up-regulation of their mRNA expressions in fish muscle
Summary
Between 1980 and 2010, the global per capita consumption of fish increased by more than 58% with the rapid development of aquaculture [1]. Isoleucine (Ile), one type of branched-chain amino acids, is an essential nutrient of all fish species studied and primarily deposited in body protein, notably in skeletal muscle protein [6]. Different from non-branched-chain amino acids, Ile largely escapes firstpass hepatic catabolism and is catabolized primarily in skeletal muscle leading to the formation of glutamate [6]. Acerete et al [10] reported that high plasma cortisol levels might be responsible for flesh deterioration in European sea bass (Dicentrarchus labrax). These findings indicated a likely correlation between Ile and fish flesh quality, which warrants investigation
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