Cystine is the first limiting amino acid for utilization of endogenous amino acids in chicks fed a protein-free diet

Three bioassays were conducted to determine the limiting order of amino aci ds for endogenous amino acid utilization in chicks fed a protein-free diet. The studies were conducted during the period 10 to 21 d posthatching. Expe riment 1 was a deletion assay in which a protein-free basal diet was supple mented with an amino acid mixture containing methionine, cystine, threonine , arginine, phenylalanine and glutamine. Each amino acid, or methionine + c ystine together, was then deleted singly from the amino acid mixture. Suppl ementing the protein-free basal diet with the amino acid mixture reduced we ight loss. Deletion of methionine and cystine from the amino acid mixture i ncreased (P < 0.05) weight loss. Deleting threonine from the amino acid mix ture also resulted in weight loss that was intermediate between the amino a cid-supplemented diet and the protein-free basal diet, indicating it was se cond limiting after sulfur amino acids. Experiments 2 and 3 were amino acid addition assays. Additions of methionine or cystine to the protein-free ba sal diet, either singly or in combination, resulted in lower rates of weigh t loss and protein depletion. Addition of threonine to the diet supplemente d with methionine and cystine further reduced weight loss. These studies in dicate that sulfur amino acids are the first-limiting amino acids for utili zation of endogenous amino acids. However, our results clearly demonstrate that the primary need is for cystine, and not for methionine per se. (C) 19 99 Elsevier Science Inc.