A feeding trial was conducted to investigate the impacts of deficient and excess dietary
threonine levels on
weight gain, plasma
enzymes activities, immune responses and expressions of immune-related genes in the intestine of juvenile blunt snout bream. Triplicate groups of fish (initial weight 3.01 ± 0.01 g, 30 fish per tank) were fed with deficient (0.58%), optimum (1.58%) and excess (2.58%)
threonine level diets to near satiation four times a day for 9 weeks. A mixture of l-
amino acids was supplemented to simulate the whole body
amino acid pattern of blunt snout bream, except for
threonine. The results showed that both deficiency and excess
threonine level diets significantly (P < 0.05) reduced the
weight gain of blunt snout bream. Excess dietary
threonine level triggered plasma
aspartate aminotransferase (AST) and
alanine aminotransferase (ALT) activities (P < 0.05); whereas
superoxide dismutase (SOD) activity was not significantly influenced by imbalanced-dietary
threonine level (P > 0.05). Plasma
complement component 3 (C3) and component 4 (C4) concentrations were significantly depressed by the deficiency of dietary
threonine (P < 0.05). Dietary
threonine regulated the target of
rapamycin (TOR), eukaryotic translation
initiation factor 4E-binding
protein 2 (4E-BP2), tumour
necrosis factor alpha (TNF-α) and
copper-
zinc superoxide dismutase (Cu/Zn-SOD) gene expressions in the intestine of blunt snout bream, which may go further to explain the adverse effects of a deficient and/or an excess dietary
threonine level on growth, immunity and health of fish. Furthermore, the present study also suggests that an optimum dietary
threonine could play an important role in improving growth, enhancing immune function and maintaining health of fish.