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

Abstract

Three bioassays were conducted to determine the limiting order of amino acids for endogenous amino acid utilization in chicks fed a protein-free diet. The studies were conducted during the period 10 to 21 d posthatching. Experiment 1 was a deletion assay in which a protein-free basal diet was supplemented with an amino acid mixture containing methionine, cystine, threonine, arginine, phenylalanine and glutamine. Each amino acid, or methionine + cystine together, was then deleted singly from the amino acid mixture. Supplementing the protein-free basal diet with the amino acid mixture reduced weight loss. Deletion of methionine and cystine from the amino acid mixture increased (P < 0.05) weight loss. Deleting threonine from the amino acid mixture also resulted in weight loss that was intermediate between the amino acid-supplemented diet and the protein-free basal diet, indicating it was second limiting after sulfur amino acids. Experiments 2 and 3 were amino acid addition assays. Additions of methionine or cystine to the protein-free basal diet, either singly or in combination, resulted in lower rates of weight loss and protein depletion. Addition of threonine to the diet supplemented with methionine and cystine further reduced weight loss. These studies indicate that sulfur amino acids are the first-limiting amino acids for utilization of endogenous amino acids. However, our results clearly demonstrate that the primary need is for cystine, and not for methionine per se.