Labile glutamine peptides, and their bearing on the origin of the ammonia set free during the enzymic digestion of proteins

Abstract

THAT the ammonia (amide-N) given by proteins on acid hydrolysis comes in part from acid amides present in the molecule has been recently proved by the isola- tion after enzymic digestion of l-asparagine from edestin In the latter research the amount of d-glutamine present at various stages was estimated by the indirect method of Chibnall and Westall [1932]. At the end of the digestion of the gliadin the amount present was equivalent to 43 % of the total amide-N. More than half of this was lost during precipitation of-bases with phosphotungstic acid and the final yield of crystalline d-glutamine was equivalent to only about 5 % of the total amide-N. They pointed out however that the indirect determination of glutamine amide-N might include labile peptides of d-glutamine as well as this amide. Since Chibnall and Westall [1932] had shown that l-leucyl-d-glutamine was stable under the hydrolytic conditions employed it was suggested that these labile peptides might be those resembling d-glutamine itself in having a free amino-group in the y-position with respect to the amide group.