Sulfur Amino Acid Nutrition of the Growing Chick: Effect of Age on the Capacity of Cystine to Spare Dietary Methionine

GRABER et al. (1971) reported a constant methionine requirement for maximal gain of the growing chick from 2 to 8 weeks of age. Since the crystalline amino acid diet (14.5% protein equivalent) used in this study was devoid of cystine, the methionine requirement value also represented the total sulfur amino acid (T.S.A.A.) need. However, since protein synthesis requires cysteine as well as methionine, part of the methionine had to be converted to cysteine. The need for cysteine can be met by supplying cysteine or cystine in the diet. During the process of protein synthesis, disulfide bonds are formed between cysteine molecules resulting in cystine formation. Certain tissues, e.g., keratoid tissue, have a high concentration of cystine (Block and Weiss, 1956). Hartsook and Mitchell (1956) stated that with increasing age keratin synthesis assumes an increasingly important role in protein synthesis. Hence, the cystine need may be affected differently by age than…