Comparative study on the effects of L-methionine or 2-hydroxy-4-(methylthio) butanoic acid as dietary methionine source on growth performance and anti-oxidative responses of turbot (Psetta maxima)

A growth trial was conducted to evaluate the effects of L-methionine (L-Met) or 2-hydroxy-4-(methylthio) butanoic acid (HMTBa) as dietary methionine source on growth, whole-body composition, and ascorbic acid concentrations in serum and liver of turbot (Psetta maxima). Five levels (0.3, 0.6, 0.9, 1.2 and 1.5% dry matter) of L-Met or HMTBa were added to a practical basal diet, which was limiting in methionine (0.59%) and cystine (0.42%). The basal control diet and 10 experimental diets were fed to groups (n = 5) of juvenile turbot (initial weight: 5.6 g), which were reared in a flow-through seawater system. Fish were fed twice daily to satiation for 75 days. Fish fed the basal diet displayed significantly (P < 0.05) lower specific growth rate (SGR), feed efficiency (FE), feed intake (FI), protein efficiency ratio (PER), productive protein value (PPV), whole-body protein and lipid contents, but higher whole-body moisture and ash contents compared to those fed with L-Met or HMTBa supplemented diets. Ascorbic acid concentrations in serum increased significantly with dietary L-Met or HMTBa levels (P < 0.05), but not in liver (P > 0.05). On the basis of SGR or FI, the dietary total methionine requirement of juvenile turbot was estimated to be 1.58 and 1.59% (3.31 and 3.27% of dietary protein) based on L-Met or 1.56% and 1.49% (3.25% and 3.19% of dietary protein) based on HMTBa, respectively, using second-order polynomial regression analysis. Furthermore, fish fed HMTBa, in addition, showed higher PER and ascorbic acid concentrations in serum than those fed L-Met. In conclusion, turbot can use HMTBa as effectively as or better than L-Met to achieve a higher maximum performance.