摘要：
To investigate the bioavailability and requirement of methionine, this experiment was conducted to study the effects of DL-methionine (DLM) or calcium 2-hydroxy-4-(methylthio) butyrate (MHA) on growth, antioxidant and intestinal protease activities of Japanese seabass (Lateolabrax japonicus). Fish [initial body weight: (5.67±0.05) g] were fed one of nine experimental diets containing 0%, 0.2%, 0.4%, 0.6% or 0.8%exogenous methionine (control, MHA 0.2, MHA 0.4, MHA 0.6, MHA 0.8, DLM 0.2, DLM 0.4, DLM 0.6 and DLM 0.8) for 8 weeks. After the feeding experiments, growth performance, body composition,antioxidant, intestinal protease and blood biochemistry were determined. The results showed that weight gain ratio (WGR) and specific growth rate (SGR) were significantly affected by dietary methionine level and source. Weight gain ratio increased with increasing level of DLM and MHA from 0% to 0.6% and then decreased. Moreover, weight gain of fish fed with MHA diet was higher than that of fish fed with diet contained the same DLM level. Dietary methionine level could significantly affect feed conversion ratio (FCR) and whole-body composition of fish. FCR and crude lipid content decreased and then increased with methionine levels increasing. The opposite is true for crude protein. In addition, superoxide dismutase activity was decreased in the methionine group than that in the control group. The activities of catalase and glutathione reductase in liver also increased and then decreased as methionine levels increased, but the contents of glutathione and malondialdehyde were just opposite. Serum alanine transaminase levels decreased and then increased with the increase of methionine levels. Cholesterol and triacylglycerol, aspartate aminotransferase activities decreased in the methionine group than that in the control group. Lysozyme activity was significantly higher in the methionine group than that in the control group. The activity of intestinal protease increased as DLM and MHA increased from 0% to 0.6% and then decreased with further increases of methionine levels. In conclusion, the optimal level of dietary MHA or DLM supplement for seabass was estimated at 0.6% on the basis of WGR and SGR. Based on the second-order regression analysis of SGR, the optimal methionine for seabass was 1.57%. The bioavailability of MHA was higher than that of DLM (134.15%). The supplement of DLM or MHA could improve the hepatic antioxidant ability of fish, which would benefit the health of liver.%本实验研究了饲料羟基蛋氨酸钙(MHA)与DL-蛋氨酸(DLM)对花鲈生长、抗氧化能力及肠道蛋白酶活性的影响,旨在比较两种外源蛋氨酸的生物效价及在饲料中适宜的蛋氨酸添加水平.饲料中添加外源蛋氨酸有效含量为0%、0.2%、0.4%、0.6%、0.8%的MHA或DLM,配制9种实验饲料(对照组、MHA 0.2、MHA 0.4、MHA 0.6、MHA 0.8、DLM 0.2、DLM 0.4、DLM 0.6和DLM 0.8).用该9种饲料分别投喂初始体质量为(5.67±0.05)g的花鲈8周后,采集样品进行分析.结果显示,蛋氨酸形式与水平均显著影响鱼体增重率、特定生长率,随饲料蛋氨酸水平的增加,鱼体增重率呈先升高后降低的趋势,并在蛋氨酸添加量为0.6%时达到最大值;此外,羟基蛋氨酸钙组鱼体增重率均高于同水平DL-蛋氨酸组.蛋氨酸水平显著影响饲料系数与鱼体组成,饲料系数与鱼体粗脂肪含量随蛋氨酸水平升高呈现先降低后升高的趋势,而粗蛋白与此相反,水分与灰分则差异不显著.随蛋氨酸水平的升高,肝脏超氧化物歧化酶(SOD)与对照组相比呈升高的趋势,过氧化氢酶(CAT)、谷胱甘肽还原酶(GR)活性呈先升高后降低的趋势,而还原型谷胱甘肽(GSH)、丙二醛(MDA)与之相反.随蛋氨酸水平的升高,血清谷丙转氨酶(ALT)的含量先降低后升高,胆固醇(TC)、甘油三酯(TG)、谷草转氨酶(AST)的活性与对照组相比呈降低的趋势.蛋氨酸添加组溶菌酶(LZM)的活性显著高于对照组.随饲料蛋氨酸水平的增加,肠道蛋白酶活性均呈先升高后降低的趋势,并在0.6%添加量时达到最高.研究表明,饲料添加外源蛋氨酸会显著促进花鲈的生长,其中添加0.6%水平的蛋氨酸,花鲈的增重率和特定生长率最高;对特定生长率与饲料蛋氨酸水平进行回归分析,得出花鲈饲料适宜的蛋氨酸水平为1.57%;以特定生长率为评价指标,羟基蛋氨酸钙的生物学效价为DL-蛋氨酸的134.15%;添加外源蛋氨酸可以提高花鲈肝脏抗氧化能力,有利于鱼体的肝脏健康.
