Binding and Modification of Proteins by Methylglyoxal under Physiological Conditions

The physiological a-oxoaldehyde methylglyoxal binds and modifies arginine, lysine, and cysteine residues in proteins. The kinetics and mechanism of these reactions were investigated with Na-acetylamino acids and bovine serum albumin at pH 7.4 and 37 “C. The reaction of methylglyoxal with Na-acetylarginine involved the initial reversible formation of glycosylamine and 4,5-dihydroxy5-methylimidazolidine derivatives, with further slow irreversible conversion to an imidazolone, Na-acetyl-NG(5-methyl-4-imidazolon-2-yl)ornithine. The imidazolone was fluorescent with an excitation A, , value of 320 nm and an emission A, value of 398 nm. Methylglyoxal reacted reversibly with Na-acetyllysine to form glycosylamine and bisglycosylamine derivatives. Further reaction of these glycosylamines occurred to form brown, fluorescent oligomers that were not characterized. Methylglyoxal reacted rapidly and reversibly with Na-acetylcysteine to form the hemithioacetal adduct. The reaction of methylglyoxal with bovine serum albumin (BSA) at pH 7.4 and 37 “C involved the reversible and irreversible formation of methylglyoxal-BSA adducts. Irreversible modification of BSA occurred mainly on arginine residues to form imidazolone. The formation of methylglyoxal-modified proteins involves glycoxidation leading to advanced glycation end product-like fluorescence. It is expected to be increased in diabetes mellitus and may be linked to the development of diabetic complications.