The branched-chain amino acid antagonism in chicks.

The effects of dietary supplements of branched-chain amino acids on growth, food consumption and metabolism in chicks were investigated. When an adequate diet contained 1.20, 1.60, 2.25, 3.75, or 5.00% leucine, increasing leucine content caused reduced food consumption and weight gains, coupled with impaired efficiency of food utilization. When the diet deficient in branched-chain amino acids contained 0.98, 1.46, 2.25, 3.75, or 5.00% leucine, increasing leucine resulted in increased food consumption and reduced efficiency of food utilization when levels of leucine up to 3.75% were fed. Excess leucine depressed plasma concentrations of isoleucine and valine. Excesses of isoleucine or valine caused smaller depressions of concentrations of the other two branched-chain amino acids. All these effects were seen during the first 8 days of experiment, after which they diminished or disappeared. Muscle branched-chain amino acid aminotransferase (BCAT) (L-leucine:2-oxoglutarate aminotransferase, EC 2.6.1.6) activity was increased in chicks fed excess leucine but not in those fed excess isoleucine or valine. Hepatic alpha-ketoisocaproic dehydrogenase (KADH) (2-oxoisocaproate:lipoate oxidoreductase, EC 1.2.4.3) activity and muscle polyribosomal aggregation were unaffected by diet. When chicks were fed diets containing either 0.98 or 2.25% leucine, production of 14CO2 from [1-14C]isoleucine and [1-14C]valine was increased in chicks fed the higher level of leucine. The increase was small in both cases, representing approximately 2% of consumed isoleucine and valine. Increased production of 14CO2 was observed within 12 hours of feeding excess leucine; however, BCAT increased only after 2 to 4 days. No differences were seen in excreted 14C or in the relative distribution of 14C along the small intestine. We conclude that the chick is able to adapt in part to excesses of dietary leucine and that the branched-chain amino acid antagonism may involve increased catabolism of the limiting branched-chain amino acids.