Characterization of advanced glycation end products: mass changes in correlation to side chain modifications.

Advanced glycation end products (AGEs) that arise from the reaction of sugars with protein side chains are supposed to be involved in the pathogenesis of several diseases; therefore, the effects of AGEs on cells are the objective of numerous investigations. Because AGE modifications are an extremely heterogeneous group of side chain modifications, the exact characterization of an AGE-modified protein is impossible. To gain a deeper understanding about AGE formation kinetics and structures, AGEs can be characterized with respect to the degree of modification, specific side chain modifications, absorbance and fluorescence characteristics, and changes in the protein structure and molecular weight. For this study, human serum albumin (HSA)-AGEs derived from different concentrations of glucose, methyl glyoxal, and glyoxylic acid were used. The molecular mass of the obtained AGEs was determined using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The mass data were compared with earlier results concerning the degree of lysine and arginine side chain modifications and AGE-specific fluorescence and absorbance data. The molecular masses were found to gradually increase with increasing concentrations of the individual modifier without reaching a plateau. The mass increase correlates very well with the AGE-specific absorbance at 360 nm and with the degree of side chain modifications. The mass spectrometric data prove, for the first time, that an increasing absorbance at 360 nm is directly correlated to a mass increase during the AGE formation process.