Studies of the reaction between proteins and reducing sugars in the “dry” state IV. Decomposition of the amino-sugar complex and the reaction of acetylated casein with glucose

Abstract

The decomposition of a casein-glucose complex in which the glucose was attached almost exclusively to the protein amino groups (70% of the total) was studied in conjunction with the reaction between acetylated casein (containing virtually no free amino groups) and glucose at 37° C and 69% or 85% relative humidity. The complex browned rapidly, at a rate which indicated that decomposition of carbohydrate attached to the protein amino groups could account for most of the darkening of a casein-glucose mixture at 37° C. The reducing power characteristic of the amino-glucose complex also decreased and insolubility developed, the change in each case being more rapid at 85% than at 69% R.H. Only a small proportion of the bound carbohydrate became dialysable, and practically no water was produced. Arginine disappeared extremely slowly, indicating that indirect reaction by cross-linking with carbohydrate already attached to amino groups was probably not an important factor in the casein-glucose system. Acetylated casein stored with glucose browned only very slowly at 37° C, confirming the importance] of free amino groups in this process. Arginine (and apparently other non-lysine side chains) reacted with glucose, although more slowly than in casein, and ferricyanide reducing power increased. At 60° C all three changes were accelerated about 20 times; very pronounced darkening occurred, much glucose combined with the protein and all the arginine groups were rapidly destroyed, notwithstanding the absence of free amino groups from the system. In the early stages of the reaction of casein with glucose at 37° C and 69% R.H. approximately one molecule of water was liberated for each molecule of glucose reacting, the ratio subsequently rising to 1.28 after 30 days and 1.5 after 80 days.