Characterization of the Maillard reaction products of β-lactoglobulin glucosylated in mild conditions

beta -Lactoglobulin (BLG) was heated in the presence of glucose in mild conditions leading to nonenzymatic glycation through the Maillard reaction. The characterization of the chromophores formed during heating at 60C showed that the modification of the UV-VIS spectrum of modified proteins was mainly due to a condensation reaction of glucose. Amino acid analysis showed that at least four amino acids (lysine, arginine, histidine and tyrosine) were modified during the reaction. Modification of SDS-PAGE pattern indicated the prevalence of a covalent polymerization of the BLG monomers independent from disulfide bridge formation. Solubility of the glucose -beta -lactoglobulin conjugates was higher than that of heated protein at pH 5. A shift of the minimum of solubility toward acidic pH, relative to the shift of the pi of modified proteins, was observed as a function of time of glycation. Tryptic hydrolysis of the modified proteins revealed a slow glycation of Lys 69 as compared to Lys 70, which is consistent with its three-dimensional orientation and its potential biological role.