Effects of crystalline methionine and calcium 2-hydroxy-4-(methylthio) butyrate on growth, antioxidant ability and intestinal protease activities of Japanese seabass (Lateolabrax japonicus)

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.