BINDING AND MODIFICATION OF PROTEINS BY METHYLGLYOXAL UNDER PHYSIOLOGICAL CONDITIONS - A KINETIC AND MECHANISTIC STUDY WITH N-ALPHA-ACETYLARGININE, N-ALPHA-ACETYLCYSTEINE, AND N-ALPHA-ACETYLLYSINE, AND BOVINE SERUM-ALBUMIN

The physiological alpha-oxoaldehyde methylglyoxal binds and modifies a rginine, lysine, and cysteine residues in proteins. The kinetics and m echanism of these reactions were investigated with N alpha-acetylamino acids and bovine serum albumin at pH 7.4 and 37 degrees C. The reacti on of methylglyoxal with N alpha-acetylarginine involved the initial r eversible formation of glycosylamine and 4,5-dihydroxy-5-methylimidazo lidine derivatives, with further slow irreversible conversion to an im idazolone, N alpha-acetyl-N delta-(5-methyl-4-imidazolon-2-yl) ornithi ne. The imidazolone was fluorescent with an excitation lambda(max), va lue of 320 nn and an emission lambda(max) value of 398 nm. Methylglyox al reacted reversibly with N alpha-acetyllysine to form glycosylamine and bisglycosylamine derivatives. Further reaction of these glycosylam ines occurred to form brown, fluorescent oligomers that were not chara cterized, Methylglyoxal reacted rapidly and reversibly with N alpha-ac etylcysteine to farm the hemithioacetal adduct. The reaction of methyl glyoxal with bovine serum albumin (BSA) at pH 7.4 and 37 degrees C inv olved the reversible and irreversible formation of methylglyoxal-BSA a dducts. Irreversible modification of BSA occurred mainly on arginine r esidues to form imidazolone, The formation of methylglyoxal-modified p roteins involves glycoxidation leading to advanced glycation end produ ct-like fluorescence. It is expected to be increased in diabetes melli tus and may be linked to the development of diabetic complications.