Amino Acid Metabolism in Gilthead Seabream Is Affected by the Dietary Protein to Energy Ratios

Abstract

The dietary protein to energy ratio (P/E) has proven to influence protein utilization and/or growth in several fish species. This study intended to unravel the bioavailability and metabolic fate of lysine and methionine in gilthead seabream (Sparus aurata) juveniles fed plant diets with different P/E ratios. Seabream juveniles were fed two isonitrogenous diets (45% crude protein) differing in crude lipids (20 and 14%): LowP/E ( P / E ratio = 20.0 mg protein k J − 1 ) and HighP/E ( P / E ratio = 21.4 mg protein k J − 1 ). After three weeks, fish ( 11.6 ± 4.3 g ) were tube-fed the respective diet labelled with 14C-protein (L-amino acid mixture), 14C-lysine, or 14C-methionine. Protein, lysine, and methionine utilization were determined based on the proportion of 14C-amino acid evacuated, retained in the free or protein-bound fraction of liver and muscle, or catabolized. This study revealed that a decrease in P/E ratio resulted in lower amino acid evacuation (p <0.05), contributing to a more efficient amino acid uptake. Results indicate that amino acids are retained as protein in the liver and not only temporarily available in the free pool. The amount of free amino acids retained in the muscle of LowP/E fed fish was significantly higher than in HighP/E fish ( p < 0.05 ) due to a simultaneous higher retention of lysine and methionine, without affecting the overall protein retention. Methionine catabolism was significantly lower than lysine or protein independently of the P/E ratio ( p < 0.05 ), reinforcing that this amino acid is preferentially spared for metabolic functions and not used as energy source. In contrast, increasing the dietary P/E ratio decreased lysine catabolism and increased its availability for growth. The bioavailability and metabolism of individual amino acids should be considered when optimizing P/E ratios in diets for gilthead seabream juveniles. Formulating diets with optimum P/E ratios will improve diet utilization and fish performance.

Metabolic Fate Is Defined by Amino Acid Nature in Gilthead Seabream Fed Different Diet Formulations

“…The fact that methionine was preferentially retained in the muscle of fish that were fed all experimental diets reinforces the importance of methionine availability in this tissue for growth and other metabolic purposes. A recent study in seabream juveniles that were fed diets differing in the protein to energy ratios revealed that the higher retention of methionine in the muscle, when compared with protein or lysine, was due to its presence in the amino acid free pool and not in the protein-bound fraction, reinforcing the crucial role of methionine in various metabolic functions in addition to protein synthesis [18].…”

“…The metabolic fate of the indispensable amino acids as a function of their nature (ketogenic and/or glucogenic) and of the dietary treatment was determined after three weeks of feeding the experimental diets. The metabolic flux assays were performed according to the methodology previously published in detail by Teodósio et al [18]. This method was adapted from Costas [19], which was modified for juvenile fish from the procedure described by Rønnestad et al [20], a modification from the original method published by Rust et al [21].…”

Effects of Dietary Protein and Lipid Levels in Practical Formulation on Growth, Feed Utilization, Body Composition, and Serum Biochemical Parameters of Growing Rockfish Sebastes schlegeli

“…Excess dietary protein lead to catabolism of amino acids into energy [ 59 ]. Fish are able to selectively retain or catabolize specific amino acids according to the dietary protein to energy ratio [ 60 ]. In the present study, growing rockfish selectively retained histidine but catabolized phenylalanine, lysine, and tyrosine when they received high-protein diets, which was in accord with the findings recorded in giant trevally Caranx ignobilis [ 61 ], N. diacanthus [ 47 ], and chu's croaker Nibea coibor [ 62 ].…”

“…Excess dietary protein lead to catabolism of amino acids into energy [59]. Fish are able to selectively retain or catabolize specific amino acids according to the dietary protein to energy ratio [60].…”