Effects And Mechanisms Of Different Levels Of Methionine On Growth Performance And Flesh Quality As Well As Mechanism In On-growing Grass Carp (Ctenopharyngodon Idella)

In this experiment,540 grass carp(initial weight was 178.47 ± 0.36 g)were randomly assigned 6 treatments(total 18 cages,each treatment with triplicate cages and 30 fish per cages),which were fed diets with graded methionine level of 2.54,4.85,7.43,10.12,12.4015.11 g/kg diet,respectively.The aim of current experiment was to explore the impact of graded methionine levels on growth performance and flesh quality as well as related mechanisms,and determinine the methionine requirement according to flesh quality related indices for on-growing grass carp.Results showed that compared with dietary methionine deficiency or excess,appropriate level of methionine in the diet obviously elevated feed intake,feed efficiency,percentage weight gain(PWG),specific growth rate,glutamate-oxaloacetate transaminase(GOT)and glutamate-pyruvate transaminase activities in the hepatopancreas and muscle(P < 0.05),and decreased the plasma ammonia content of on-growing grass carp(P < 0.05),indicating that methionine could improved amino acid utilization and growth performance of grass carp.In addition,compared with dietary methionine deficiency or excess,appropriate dietary methionine level improved flesh quality of on-growing grass carp:(1)Significantly increased muscle crude protein,crude lipid,unsaturated fatty acid(C18: 1n9 c and C20: 5n-3,etc.),flavor amino acid(aspartic acid,glutamic acid,glycine and serine,etc.)and inosine monophosphate contents(P < 0.05),and significantly improved the physicochemical properties of muscle(water-holding capacity,hardness and p H)(P <0.05).(2)Increased muscle protein content might be related to the up-regulation of TOR,S6K1 m RNA levels and t-TOR,p-TOR(Ser2448)protein levels as well as the down-regulation of 4E-BP1 m RNA level by methionine.(3)Increased muscle water-holding capacity was potentially associated with methionine down-regulated P38 MAPK,JNK,Fas L,Apaf-1,Bax and caspase-(2,3,7,8,and 9),and up-regulated IAP,Blc-2 and Mcl-1 m RNA levels,further reduced apoptosis.(4)Increased muscle firmness might be related to methionine reduced cathepsin B and L activities,and activated TGF-β1/Smads(down-regulated TNF-α and up-regulated TGF-β1,Smad4,Smad2 m RNA levels,and up-regulated Smad4,t-Smad2,p-Smad2(Ser467)protein levels)and CK2/TOR(up-regulated CK2α,CK2β,TOR m RNA levels and t-TOR,p-TOR(Ser2448)protein levels)signaling pathways,futher up-regulated type I collagen(up-regulated Col1α1 and Col1α2 m RNA levels)gene expression,subsequently elevated collagen content.(5)Increased muscle p H might be ascribed to methionine decreased lactic acid content.(6)Decreased the contents of ROS,MDA,PC in the muscle,which might be related to methionine up-regulated the m RNA and protein levels of Nrf2,down-regulated the m RNA levels of Keap1a(not Keap1b)to up-regulate the expression of antioxidant enzyme genes(except Cu Zn SOD and GSTP1),increase the activities of antioxidant enzymes(Except Cu Zn SOD),up-regulated the m RNA levels of γGLC-m and γGLC-c to increase GSH content,further improved the antioxidant capacity of muscle.In summary,compared with dietary methionine deficiency or excess,appropriate level of methionine in the diet elevated growth performance and improved flesh quality of on-growing grass carp.The improvement of flesh quality by methionine was closely related to increased muscle nutrient and flavor substance contents,and improved muscle physicochemical properties by decreasing apoptosis,enhancing the ability of collagen synthesis and antioxidant.In addition,based on PWG,muscle GOT activity,collagen and MDA contents,the methionine requirements of on-growing grass carp(178-626 g)were9.56,9.76,9.28 and 9.37 g/kg diets(33.26,33.96,32.29 and 32.60 g/kg proteins).