Growth and metabolic responses of juvenile grouper (Epinephelus coioides) to dietary methionine/cystine ratio at constant sulfur amino acid levels

Abstract

An eight-week feeding trial was conducted to investigate the effects of dietary methionine/cystine (Met/Cys) ratio on juvenile grouper growth performance, hepatic metabolic enzymes, serum immune-related parameters, liver antioxidant indices, and relative expression of genes in the mTOR pathway. Six isonitrogenous, isocaloric experimental diets with a fixed Met and Cys level (1.78%) and variable Met/Cys ratio (0.76–4.50) were formulated. Each diet was randomly assigned to triplicate groups of 30 juvenile fish with an average initial weight of 11.66 g. The fish were fed twice daily for 2 months. Weight gain and specific growth rates, feed efficiency and intake, and protein efficiency ratio had positive linear and/or quadratic responses to the dietary Met/Cys ratio. In contrast, the variations in dietary Met/Cys ratio had no apparent effect on the hepatosomatic index, condition factor, whole-body moisture, protein, lipid or ash content, or hepatic betaine homocysteine methyltransferase or S-adenosylhomocysteine hydrolase. Hepatic methionine synthase, cystathionine β-synthase, glutathione peroxidase, superoxide dismutase, and catalase activity, serum acid phosphatase, alkaline phosphatase, and lysozyme activity, and serum IgM and C3 content had positive linear and/or quadratic responses to the dietary Met/Cys ratio. In contrast, the malondialdehyde content had negative linear and/or quadratic responses to the dietary Met/Cys ratio. Hepatic RagA/B, RagC/D, Raptor, and eIF4B mRNA levels were unaffected by the dietary Met/Cys ratio. On the contrary, muscular upstream and downstream mTOR signal pathway gene mRNA levels had positive linear and/or quadratic responses to the dietary Met/Cys ratio. However, the 4E-BP mRNA levels had negative linear and quadratic responses to the dietary Met/Cys ratio. Cys could replace up to 41% of the Met in the fish diets. The use of appropriate Met/Cys ratios (range 1.93–2.77) in the feed may help improve grouper growth performance, immune function, and antioxidant ability and positively regulate genes in the mTOR signaling pathway.