Effects Of Different Dietary Methionine/cystine Ratios On Growth Performance And MTOR Signaling Pathway In Grouper Epinephelus Coioides

A study was conducted to investigate the effects of different dietary methionine/cystine ratios on growth performance,body composition,immunological index,antioxidant index and relative expression of mTOR pathway gene in grouper(Epinephelus coioides).Six experimental diets with a fixed level of methionine and cystine(1.78%),but varied methionine/cystine ratios of 0.76,1.06,1.43,1.93,2.77,4.50 were formulated,fish meal and soy protein isolate as the protein source.The groupers with an initially weighting of about 11.66 g were randomly assigned to four tanks with a stocking density of 30 fish per tank and fed one of diets to apparent satiation by hand twice daily for 8 weeks.The results are summarized as follows:The weight gain rate,specific growth rate,feed efficiency and protein efficiency ratio of grouper were first increased and then decreased with the dietary decrease of methionine/cystine ratio(P<0.05),and they reached a peak at the methionine/cystine ratio of 2.77,there was no significant difference in weight gain rate and specific growth rate between groups fed with the diets at the ratio of methionine to cystine of 1.43-4.50.Dietary methionine/cystine ratio did not affect feed intake,hepatopancrea somatic index,condition factor,survival rate,moisture content,crude lipid and ash contents(P> 0.05).The crude protein content was lowest at the ratio of methionine to cystine of 0.76 among groups,and the value was significantly lower than that of other groups(P<0.05).Dietary methionine/cystine ratio did not affect the activity of methionine synthase,betaine homocysteine methyltransferase,S-adenosylhomocysteine hydrolase in the liver(P>0.05).The activity of cystothione beta synthase and glutamic pyruvic transaminase increased gradually with the increase of methionine/cystine ratio(P < 0.05),however,the activity of glutamic oxalacetic transaminase increased first and then decreased(P<0.05),and reached a peak at the methionine/cystine ratio of 1.93.The immunoglobulin M and the complement C3 content,the alkaline phosphatase and the lysozyme activities increased first and then decreased with the increase of methionine/cystine ratio,the activity of acid phosphatase was the highest when methionine/cystine ratio was 1.43,the difference in serum immunological indexes was not significant(P>0.05)between groups with the diets at the methionine/cystine ratio of 1.43-4.50.The glutathione peroxidase activity,catalase activity,total antioxidant capacity and malondialdehyde content increased first and then decreased with the increase of dietary methionine/cystine ratio(P<0.05),superoxide dismutase activity was the highest at themethionine/cystine ratio of 1.93.When fed the diets at the methionine/cystine ratio of 1.93-2.77,the grouper had better growth performance and antioxidant capacity.There was no significant difference between dietary methionine/cystine ratio groups in the relative expression levels of RagA/B,RagC/D,Raptor and m LST8 m RNAs(P > 0.05).The relative expression level of V-ATPase mRNAs was irregular with different methionine/cystine ratio.When methionine/cystine ratio was 4.50,the Ragulator mRNAs were down-regulated significantly in comparison with other groups(P < 0.05),however,the mTOR,S6 K,eIF4E,eIF4 B and S6 m RNAs were down-regulated significantly when dietary methionine/cystine ratio was 0.76,the higher methionine/cystine ratio(4.50)also led to a significantly down-regulation in S6 mRNA.The mRNA levels of 4E-BP were significantly down-regulated and then significantly up-regulated with the increase of dietary methionine/cystine ratio.The relative expression levels of V-ATPase,RagA/B,mLST8,mTOR,S6 K and eIF4 E mRNAs in the muscle of the grouper were up-regulated first and then down-regulated with the increase of methionine/cystine ratio(P< 0.05),the relative expression level of 4E-BP mRNAs were the opposite.The relative expression levels of RagC/D,eIF4 B and S6 mRNAs in muscle were significantly lower than that in other groups when the ratio of methionine/cystine was 0.76,the relative expression level of Ragulator mRNAs in muscle was the same as in liver.The relative expression level of Raptor mRNA in muscle was increased with increasing the methionine/cystine ratio(P < 0.05).The results showed that the different dietary methionine/cystine ratios significantly affected the mRNA expression related to mTOR signaling pathway,the suitable ratio of methionine/cystine(1.93-2.77)activated mTORC1 and up-regulated the expression of the genes controlling protein synthesis in the mTOR signaling pathway,and thus promoted protein synthesis in the liver and muscle.