Cross-linking of proteins by maillard processes—characterization and detection of a lysine-arginine cross-link derived from d-glucose

Covalently cross-linked proteins are among the major modifications caused by the advanced Maillard reaction. So far, the chemical nature of these aggregates is largely unknown. Investigations are reported on how the cross-linking unit 2-ammonio-6-{2-[(4-ammonio-5-oxido-5-oxopentyl)amino]-6,7-dihydroxy-4,5,6,7,8,8a-hexahydroimidazo[4,5- b]azepin-4-yl} hexanoate ( 7), designated as glucosepan, can be identified and quantified from d-glucose-bovine serum albumin (BSA) incubations. Independent synthesis and unequivocal structural characterization are given for glucosepan 7. A protocol was established for its determination by LC–MS with electrospray ionization (ESI). BSA and d-glucose were incubated at 37 °C, pH 7.4 for eight weeks and the time-dependent formation of 7 was observed. Since glucosepan 7 is unstable under acid proteolytic conditions, BSA was cleaved enzymatically. The maximum value obtained from a solution containing 50 g/L BSA and 100 mM d-glucose after eight weeks incubation time corresponds to an arginine derivatization rate of 1.38±0.07 mmol 7/mol Arg (equivalent to 31.7±1.6 mmol 7/mol BSA). From these results, it seems justified to expect 7 to play an important role in the cross-linking of proteins in vivo as well as in foodstuffs. The structural similarity of glucosepan 7 and pentosidine 1 made it obvious to also look for an eventual parallelism in the respective formation pathways.