What Are the Priorities of Use When an Amino Acid with Multiple Functions Is Deficient in the Diet?
Amino acids are used to synthesize a variety of different body proteins, e.g., myofibrillar, stromal, sarcoplasmic, keratoid, and acute-phase tissue proteins, hormones, enzymes, and specialized proteins such as metallothionein. Also, several amino acids have precursor roles. Concerning the synthesis priority of one type of protein over another when an amino acid is deficient, little is known about this intriguing question. Our work with chickens fed diets deficient in either histidine (43) or cysteine (44) suggested that protein synthesis is prioritized over either carnosine or glutathione synthesis. However, questions of priority remain for many amino acids that have important precursor roles: arginine for urea cycle function and synthesis of protein, creatine, polyamines, and nitric oxide; tyrosine for synthesis of protein, catecholamines, thyroxin, and melanin; tryptophan for synthesis of protein, serotonin, and niacin nucleotides; and glycine for synthesis of protein (contractile v
