Lysozyme, ribonuclease A, ovalbumin and bovine serum albumin were reacted with reducing sugars under physiological conditions of 37°C and pH 7.4, and polymerization of proteins, changes in amino acid composition and carbonyl compounds formed during the reaction were investigated. Incubation of all the protein-sugar systems resulted in noticeable losses of arginine and lysine residues. Polymerization, as well as impairment of arginine residues of lysozyme, with fructose and aldopentose was higher than with aldohexoses. 3-Deoxyglucosone (3DG) was identified as one of the major carbonyl compounds generated in the reaction systems of proteins with glucose and fructose. The formation of 3DG in protein-fructose systems was 1.3-2 times that in protein-glucose systems. Glucose alone did not polymerize succinylated lysozyme, but fructose did and it impaired only arginine residues. These results indicate that 3DG was generated on the reaction between glucose and free amino groups in proteins, whereas it was generated from fructose alone with the modification of free amino groups. It is strongly suggested that 3DG was the cross-linker responsible for the polymerization of proteins and the attacker of arginine residues.

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