Protein crosslinking by the Maillard reaction: Dicarbonyl-derived imidazolium crosslinks in aging and diabetes

alpha-Dicarbonyl compounds that arise from various metabolic pathways react with proteins to form a variety of adducts in a reaction known as the Mail lard reaction. These adducts are collectively known as advanced glycation e nd products or AGEs, Methylglyoxal (MG) and glyoxal (GXL) are two such dica rbonyls, They react with proteins to produce lysine-lysine imidazolium cros slinking AGEs. The imidazolium crosslinks derived from MG (MOLD-methylglyox al-lysine dimer) and GXL (GOLD-glyoxal-lysine dimer) are present in human t issue proteins. In this study, we report an HPLC method for the simultaneou s quantification of GOLD and MOLD in biological specimens. The method consi sts of reverse-phase HPLC of acid-hydrolyzed proteins, collection of eluate -containing imidazoliums, phenylisothiocyanate derivatization, followed by a second reverse-phase HPLC. This method was linear for both the imidazoliu m compounds in the range of 0.5-300 pmol. The levels of GOLD and MOLD in ag ing lenses (20 to 80 years) were trace-8.4 pmol and 15-93 pmol per milligra m of protein, respectively. Cataractous lenses showed significantly higher levels of both GOLD and MOLD (mean a SD, 14.5 +/- 1.8 and 141 +/- 18.4 pmol per milligram of protein, P < 0.05). Brunescent lenses had the highest lev els of imidazolium crosslinks (GOLD, 18.36 +/- 2.5; and MOLD, 179.2 +/- 32. 3 pmol per milligram of protein, P < 0.05). The GOLD and MOLD levels were h igher in diabetic plasma proteins when compared to that of normal (GOLD, 17 .5 +/- 6.34 pmol per milligram of protein vs 43.5 +/- 15.96 pmol per millig ram of protein; and MOLD, 172.5 +/- 32.53 pmol per milligram of protein vs 273 +/- 62.67 pmol per milligram of protein, P < 0.05), GOLD and MOLD are s ignificant in terms of tissue damage in aging and diabetes because they rep resent protein crosslinking by compounds that are major precursors of AGEs. Our method can be used for quantification of imidazolium crosslinks in tis sue proteins to assess alpha-dicarbonyl-mediated protein damage in vivo. (C ) 1999 Academic Press.