The nitrogen-sparing effect of methionine in chicks receiving a protein-free diet supplemented with arginine: effect of various methionine substituents.

1. The nitrogen-sparing effect of methionine in chicks fed on a protein-free diet containing arginine was examined in three 10-d trials. Chicks received either a protein-free diet, this diet supplemented with arginine and methionine or diets containing arginine in which the methionine was replaced by various methionine-related compounds. 2. Body-weight changes, nitrogen retentions and uric acid-nitrogen excretion indicated that the methyl moiety was unlikely to contribute to nitrogen sparing activity and that cysteine, or possibly glutathione, played an important role. The possible mechanism of the nitrogen-sparing effect is discussed.