Effects of dietary isoleucine levels on activities of digestive enzymes and immune index of Trachinotus ovatus

We investigated the effects of dietary isoleucine on the digestive enzymes and immune index of juvenile golden pompano, Trachinotus ovatus. Six diets with different concentrations of L-isoleucine (13.2 g · kg^-1, 15.7 g · kg^-1, 18.2 g · kg^-1, 20.7 g ·kg^-1, 23.2 g · kg^-1 and 25.7 g · kg^-1 dry diet) were formulated to contain 430 g · kg^-1 crude protein with fish meal, soybean meal, peanut meal and pre-coated crystalline amino acids. Each diet was randomly assigned to triplicate treatments of 20 fish [initial body weight was (6.36±0.03)g] for eight weeks. The intestinal amylase and pepsase activities, serum T-AOC and LZM activities, IgM concentrations increased with increasing isoleucine concentration up to 18.2 g · kg^-1, whereas decreased in the groups exceeding 20.7 g · kg^-1isoleucine significantly. Those parameters of the fish fed with diets of 15.7 g · kg^-1and 23.2 g · kg^-1isoleucine were significantly higher than those fed with diets of 13.2 g · kg^-1and 25.7 g · kg^-1isoleucine, while significantly lower than groups containing 18.2 g · kg^-1and 20.7 g · kg^-1isoleucine. There is an opposite change trend in serum urea and MDA concentration, i.e., significantly lower in 18.2 g · kg^-1and 20.7 g · kg^-1isoleucine groups than those in the other groups. The SOD activity rate increased with increasing isoleucine concentration up to 18.2 g · kg^-1, and there was no significant difference when the dietary isoleucine exceeded 18.2 g · kg^-1. So diet with 18.2~20.7 g · kg^-1isoleucine can improve the digestive performance and immune function of T.ovatus.