The dietary arginine requirement of channel catfish (Ictalurus punctatus) is influenced by endogenous synthesis of arginine from glutamic acid
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Previous studies with young mammals have established that arginine synthesis from glutamate-derived citrulline can be a major endogenous source of arginine. Therefore, an experiment was conducted to re-assess the dietary arginine requirement of juvenile channel catfish and to determine the metabolic effects of including glutamate or glycine to maintain isonitrogenous levels among diets. Two sets of diets were formulated to contain 24 g crude protein/100 g dry weight from casein/gelatin and crystalline amino acids with arginine supplementation in 0.5 increments from 0.5 to 2.0 g/100 g diet. Amino acid nitrogen was maintained equal, within sets, by replacing arginine with aspartate and either glutamate or glycine. Each diet was fed to apparent satiation to triplicate groups of 12 fish initially averaging 11.4 g/fish for 8 weeks. Weight gain (WG), feed efficiency (FE), protein efficiency ratio (PER), protein retention (PR) and survival were significantly (P < 0.05) affected by arginine. At the suboptimal level of dietary arginine, glutamate appeared to contribute arginine through internally derived citrulline based on increased plasma citrulline and arginine concentrations. WG and plasma amino acid concentrations of fish fed diets with glycine suggested that it does not serve as a precursor for citrulline. Based on WG and FE, juvenile channel catfish were found to require arginine at 3.3% to 3.8% of dietary protein, when glutamate was included in the diet. The requirement estimate was 33% higher when glycine replaced glutamate in the diet and was similar to the previously determined arginine requirement of channel catfish at 4.3 g/100 g of dietary protein. These results strongly suggest that dietary glutamate is used for endogenous synthesis of arginine in channel catfish, especially when arginine is deficient in the diet. (C) 2000 Elsevier Science B.V.
