Free amino acids and related compounds in whole body of juvenile European eel
Anguilla anguilla were determined to elucidate the relationships between levels of free amino acids and dietary protein level, protein intake or protein deposition of the fish. The eel were fed with the diets having seven levels of protein (0.3, 11.5, 21.8, 32.7, 43.8, 54.5 and 64.4%) at 25°C for six weeks. The protein source of these diets was casein fortified with L-forms of crystalline arginine, istidine, lysine, methionine, cystine, threonine and tryptophan. The total contents of free essential amino acid gradually rose as the protein intake increased, and a positive and significant correlation was observed between these parameters. On the other hand, the contents of free nonessential amino acid and ninhydrin reactive substance were more strongly correlated with the levels of protein deposition than those of dietary protein or protein intake. Free amino acids affected by the dietary treatment were as follows: threonine, α-amino-n-butylic acid, cystathionine,
leucine, phenylalanine, ornithine, lysine, arginine and proline, of which the contents were positively and significantly correlated with the levels of dietary protein and protein intake. Whereas, the contents of aspartic acid, glutamic acid, alanine, methionine, tyrosine, β-alanine, histidine and asparagine showed stronger positive correlation with the levels of protein deposition than those of protein intake. Unlike these amino acids, negative and significant correlations were observed between the dietary protein levels and the serine contents, and the protein deposition levels and the taurine contents.
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