Optimal methionine supplementation improved the growth, hepatic protein synthesis and lipolysis of grass carp fry (Ctenopharyngodon idella)

Abstract

Six experimental diets with graded methionine levels (0.33%, 0.55%, 0.71%, 0.87%, 1.03%, 1.20%, dry matter) were formulated to quantify the dietary methionine requirement, and assess the influence of dietary methionine levels on the metabolism of protein and lipid in Ctenopharyngodon Idella fry (initial weight 0.36 ± 0.01 g) during an 8-week feeding trial. The results showed that 1.03% dietary methionine significantly improved the growth performance, protein efficiency ratio and feed utilization, compared with 0.33% methionine. Dietary methionine supplementation slightly increased survival rate. 0.55%–0.87% methionine level increased the phosphorylated protein and mRNA levels of phosphatidylinositol 3-kinase (PI3K), target of rapamycin (TOR), eukaryotic initiation factor 4E binding protein-1 and ribosomal protein S6 kinase 1. Compared with 1.20% methionine, 0.55%–0.87% methionine levels significantly increased mRNA levels of hepatic insulin-like growth factor-1 (IGF-1), carnitine palmitoyl transferase-1, peroxisome proliferator-activated receptor α, adipose triglyceride lipase and hormone sensitive lipase. The highest mRNA levels of hepatic acetyl-CoA carboxylase, sterol regulatory element-binding protein 1, stearyl-CoA desaturase and fatty acid synthetase were observed in 0.33% methionine diet. Based on specific growth rate and feed conversion ratio, the dietary methionine requirement of grass carp fry was 1.03% of diet (2.67% of dietary protein), using the broken-line regression analysis. Suitable dietary methionine could improve the growth, feed utilization and lipolysis of fry and promote protein synthesis via the IGF-1/PI3K/TOR pathway. While methionine deficiency tended to enhance hepatic lipid synthesis.