Responses to graded levels of leucine and branched-chain amino acid imbalance in tiger puffer Takifugu rubripes

In fish, it has been debatable whether branched-chain amino acid (BCAA) imbalance will show antagonistic effects on the growth and the concentrations of plasma or tissue BCAAs. The objective of this study were to investigate the effects of incremental levels of dietary leucine on the growth (trial 1) and potential antagonistic effects between BCAAs (trial 2) in tiger puffer. This study was composed of two separate trials. In trial 1, six experimental diets were prepared by the basal diet containing incremental levels of leucine (19.3, 24.9, 28.6, 32.6, 36.3 and 40.7 g kg(-1) of dry diet). In trial 2, five experimental diets were formulated to contain a proper amount of leucine (29.0 g kg(-1) of dry diet), a deficiency of leucine (12.6 g kg(-1) of dry diet), an excess of leucine (55.9 g kg(-1) of dry diet), an excess of isoleucine (43.2 g kg(-1) of dry diet) and an excess of valine (49.6 g kg(-1) of dry diet). All diets in two trials were fed to fish in triplicate net cages and polyethylene tanks, respectively. In trial 1, growth increase relative to initial body weight was 206.2-227.5%, while growth performance and body condition indices were not significantly affected by different levels of dietary leucine and did not show significantly linear or quadratic relationships with incremental levels of dietary leucine. In trial 2, growth performance also did not exhibit significant differences in fish fed imbalanced BCAAs diets. Compared with a proper amount of leucine group, the activities of branched-chain alpha-keto acid dehydrogenase enzyme complex (BCKDH) and BCKDH kinase increased in the leucine or valine excessive diet. The expressions of amino acid transporters (BOAT1 and y(+)LAT2) and peptide transporter 1 (PepT1) were down-regulated by excessive dietary leucine, isoleucine or valine. Free valine concentration in plasma and muscle increased in the leucine deficient diet compared to a proper amount and an excess of leucine in diets. In conclusion, the growth of tiger puffer was not affected by the diets containing 12.6-55.9 g kg(-1) leucine. An excess of dietary leucine, isoleucine or valine possibly resulted in antagonism between BCAAs based on the activities of enzymes related to BCAA catabolism and the expressions of amino acid transporters, although the growth was not significantly depressed by one excessive BCAA when the other two BCAAs met the requirements for tiger puffer. In addition, plasma and muscle valine levels indicated that the antagonism of valine to leucine were observed in the leucine deficient diet, but not in the leucine excessive diet.