Antagonistic effects of branched-chain amino acids induced by excess protein-bound leucine in diets for rainbow trout ( Oncorhynchus mykiss)

Abstract

Three feeding experiments were conducted to evaluate the antagonistic effects induced by excess leucine (Leu), derived from intact protein (zein or corn gluten meal (CGM)), on growth and protein utilization of fingerling rainbow trout ( Oncorhynchus mykiss). In Experiment 1, three semipurified diets containing wheat gluten+limiting essential amino acids (EAA), zein+EAA, or zein+EAA+isoleucine (Ile)+valine (Val), were fed for 6 weeks by self-feeders. In Experiment 2, the three diets used in Experiment 1 and a zein diet with EAA and levels of Ile and Val enrichment at one half the amounts used in Experiment 1 were fed by hand for 6 weeks. In Experiment 3, a fish meal diet and three non-fish meal diets containing 23.5% CGM supplemented either with non-essential amino acids, EAA, or EAA+Ile+Val, were fed by hand for 60 days. In Experiment 1, growth and N retention of fish fed the zein diets, regardless of the Ile and Val enrichment, were inferior ( P<0.05) to fish fed the wheat gluten diet due largely to reduced feed intake. In Experiment 2, fish fed zein diets with both levels of Ile and Val enrichment attained growth and N retention not significantly different ( P>0.05) from those of fish fed the wheat gluten diet. In Experiment 3, fish fed the two non-fish meal diets with EAA supplementation, regardless of the Ile and Val enrichment, showed identical growth and protein utilization to those of fish fed the fish meal diet. Free Ile and Val concentrations in the plasma, liver and muscle of fish fed the zein and CGM diet without Ile and Val enrichment were lower than those of fish fed the wheat gluten diet and fish meal diet, respectively, whereas the Ile and Val enrichment restored the concentrations. These results suggest that antagonisms induced by excess Leu from zein occur in rainbow trout. A practical level of CGM inclusion had no adverse effect on growth performance in a 60-day feeding, however, certain antagonisms appear in tissue branched-chain amino acid (BCAA) concentrations.