ROLE OF THE MAILLARD REACTION IN AGING OF TISSUE PROTEINS - ADVANCED GLYCATION END PRODUCT-DEPENDENT INCREASE IN IMIDAZOLIUM CROSS-LINKS INHUMAN LENS PROTEINS

Dicarbonyl compounds such as glyoxal and methylglyoxal are reactive di carbonyl intermediates in the nonenzymatic browning and cross-linking of proteins during the Maillard reaction. We describe here the quantif ication of glyoxal and methylglyoxal-derived imidazolium cross-links i n tissue proteins. The imidazolium salt cross links, glyoxal-lysine di mer (GOLD) and methylglyoxal-lysine dimer (MOLD), were measured by liq uid chromatography/mass spectrometry and were present in lens protein at concentrations of 0.02-0.2 and 0.1-0.8 mmol/mol of lysine, respecti vely. The lens concentrations of GOLD and MOLD correlated significantl y with one another and also increased with lens age. GOLD and MOLD wer e present at significantly higher concentrations than the fluorescent cross-links pentosidine and dityrosine, identifying them as major Mail lard reaction cross-links in lens proteins. Like the N-carboxy-alkylly sines N-epsilon-(carboxymethyl)lysine and N-epsilon-(carboxyethyl)lysi ne, these cross-links were also detected at lower concentrations in hu man skin collagen and increased with age in collagen. The presence of GOLD and MOLD in tissue proteins implicates methylglyoxal and glyoxal, either free or protein-bound, as important precursors of protein cros s-links formed during Maillard reactions in vivo during aging and in d isease.