Optimal dietary methionine requirement of sub-adult turbot (Scophthalmus maximus L.): Growth performance, feed utilization and hepatic lipid metabolism

A 10-week feeding trial was conducted to explore the optimal dietary methionine requirement of sub-adult turbot. The growth performance, feed utilization, hepatic free amino acid profile and the lipid metabolism re-sponses in liver were determined. Five isonitrogenous and isolipidic diets with differential methionine contents (0.81%, 1.34%, 1.82%, 2.27% and 2.82%) were fed to 300 healthy turbot (initial weight: 94.70 +/- 0.05 g). Results showed that fish growth increased markedly with the dietary methionine levels from 0.81% to 1.82% and then reached to a plateau when dietary methionine levels were beyond. Compared with the methionine defi-ciency, adequate dietary methionine improved the crude protein and crude lipid levels in whole body and liver. Meanwhile, increasing dietary methionine also induced the content of methionine in hepatic free amino acid pool, as well as cysteine, leucine, isoleucine, lysine, histidine and phenylalanine. For hepatic lipid metabolism, adequate dietary methionine induced the protein abundance of SREBP1 and the mRNA expressions of soat2, ppar gamma, fas, and dgat1. However, the genes involved in lipolysis showed an opposite trend. On the basis of specific growth rate or feed efficiency ratio, the optimum methionine requirement of sub-adult turbot was estimated to be 1.885% and 1.815% of the diet, using the broken-line regression analysis.