Cross-linking of proteins by Maillard processes: Characterization and detection of lysine-arginine cross-links derived from glyoxal and methylglyoxal

alpha-Dicarbonyl compounds, such as glyoxal and methylglyoxal, are crucial intermediates in the browning and cross-linking of proteins by reducing sug ars in the course of the Maillard reaction. The cross linking units 2-ammon io-6-({2-[(4-ammonio-5-oxido-5-oxopentyl)amino]-4,5-dihydro-1H-imidazol-5-y lidene}amino)hexanoate (9) and 2-ammonio-6-({2-[(4-ammonio-5-oxido-5-oxopen tyl)amino]-4-methyl-4,5-dihydro-1H-imidazol-5-ylidene}amino)hexanoate (10), designated as GODIC and MODIC, are identified and quantified from glyoxal/ methylglyoxal-bovine serum albumin (BSA) incubations. Independent syntheses and unequivocal structural characterization are given for 9 and 10. A prot ocol was established for their determination by liquid chromatography-mass spectrometry (LC-MS) with electrospray ionization (ESI). BSA and the respec tive alpha-dicarbonyl compound were incubated at 37 degrees C, pH 7.4 for 1 week, and the time-dependent formation of 9 and 10 was observed. The maxim um value obtained from a solution containing 50 g/L BSA and 2mM glyoxal or methylglyoxal after a 7-day incubation period corresponds to an arginine de rivatization quota of 13.0 +/- 0.32 mmol 9;mol Arg or 3.0 +/- 0.12 mmol 10/ mol Arg. The cross-links 9 and 10 were also detected in a D-glucose-BSA inc ubation. From these results, it seems justified to assign an important role to 9 and 10 in the cross-linking of proteins in vivo as well as in foodstu ffs. In an additional model study, formation of 9 and 10 was compared to th at of the imidazolium cross-links GOLD 3 and MOLD 3. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.