A study was designed to investigate the influence of leucine (Leu) levels on growth performance, enzyme activities and relative expression of TOR and 4E-BP genes in rainbow trout, Oncorhynchus mykiss (average initial body weight: 1.60 ± 0.25 g/fish; average initial length 5.20 ± 0.30 cm/fish). Six isonitrogenous (450 g/kg) and energetic (20.90 kJ 100 g⁻¹_, gross energy) experimental diets in gradation of 2.5 g/kg of dry diets were formulated to contain graded Leu levels (10.0, 12.5, 15.0, 17.5, 20.0 and 22.5 g/kg, drymatter basis). 20 fish were stocked in triplicate groups in 75 L trough connected with flow-through system (2–2.5 L/min) and were fed twice daily (09:00 and 17:00 hours) at 5% BW day⁻¹. Quadratic regression analysis of live weight gain (LWG%), feed conversion ratio (FCR), protein efficiency ratio (PER) and body protein deposition (BPD) data indicated Leu requirement at 17.70, 17.42, 17.25 and 17.57 g/kg dry diet, respectively. Highest (p < .05) protein, lowest moisture and intermediate fat contents were noted at 17.5 g/kg Leu fed diet. Significant variation in haematological parameters was also observed. While no significant (p ˃ .05) differences in plasma total protein, cholesterol, triglycerides, albumin and globulin were noted. However, alkaline phosphatase (ALP), alanine transaminase (ALT), aspartate transaminase (AST), glucose, urea and uric acid levels produced significant differences. Except amylase, higher enzymatic activities were observed at 17.5 g/kg Leu diet. Higher relative expression level of target of rapamycin (TOR) and eukaryotic translation initiation factor 4E-binding protein (4E-BP) mRNA was observed at 17.5 and 15.0 g/kg Leu fed diets. Based on the above data, it is recommended that 17.45 g/kg Leu, corresponding to 38.77 g/kg of dietary protein would be useful for optimum growth and also regulates relative gene expression of TOR and 4E-BP signalling pathway.

中文翻译：

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日粮亮氨酸水平对虹鳟、Oncorhynchus mykiss鱼种生长性能、血液生化参数、肝脏特征、肠道酶活性和雷帕霉素信号通路靶点相关基因表达的影响

一项研究旨在调查亮氨酸 (Leu) 水平对虹鳟鱼Oncorhynchus mykiss的生长性能、酶活性以及 TOR 和 4E-BP 基因的相对表达的影响（平均初始体重：1.60 ± 0.25 g/鱼；平均初始长度 5.20 ± 0.30 厘米/鱼）。六种等氮（450 g/kg）和高能（20.90 kJ 100 g ^-1 _，总能量）实验日粮以 2.5 g/kg 的干粮等级配制成含有分级的 Leu 水平（10.0、12.5、15.0、17.5、20.0和 22.5 g/kg，以干物质计）。将 20 条鱼分三组放养在 75 L 槽中，与流通系统相连（2-2.5 L/min），每天喂食两次（09:00 和 17:00），第^{-1天 5% BW}. 活体重增加 (LWG%)、饲料转化率 (FCR)、蛋白质效率比 (PER) 和身体蛋白质沉积 (BPD) 数据的二次回归分析表明，Leu 需要量为 17.70、17.42、17.25 和 17.57 g/kg 干饲料，分别。 在 17.5 g/kg Leu 饲喂日粮中发现最高 ( p < .05) 蛋白质、最低水分和中等脂肪含量。还观察到血液学参数的显着变化。虽然没有显着性（p˃.05) 注意到血浆总蛋白、胆固醇、甘油三酯、白蛋白和球蛋白的差异。然而，碱性磷酸酶（ALP）、丙氨酸转氨酶（ALT）、天冬氨酸转氨酶（AST）、葡萄糖、尿素和尿酸水平产生了显着差异。除淀粉酶外，在 17.5 g/kg Leu 饮食中观察到更高的酶活性。在 17.5 和 15.0 g/kg Leu 饲喂日粮中观察到雷帕霉素 (TOR) 靶点和真核翻译起始因子 4E 结合蛋白 (4E-BP) mRNA 的相对表达水平较高。基于上述数据，建议 17.45 g/kg Leu，对应于 38.77 g/kg 的日粮蛋白质，对于最佳生长是有用的，并且还可以调节 TOR 和 4E-BP 信号通路的相对基因表达。