DETECTION AND IDENTIFICATION OF A PROTEIN-BOUND IMIDAZOLONE RESULTINGFROM THE REACTION OF ARGININE RESIDUES AND METHYLGLYOXAL

A ninhydrin-positive compound was detected in acid hydrolysates of var ious alkali-treated bakery products (pretzels, snack bars), eluting im mediately after pyridosine in amino acid chromatograms. Following prep arative isolation from a food sample and independent synthesis, the co mpound was unequivocally identified by fast-atom bombardement-mass spe ctrometry, H-1- and C-13-nuclear magnetic resonance as a protein-bound imidazolone, existing in two tautomeric forms, namely -(5-methyl-4-ox o-5-hydroimidazol-2-yl)-L-ornithine and (4-methyl-5-oxo-4-hydroimidazo l-2-yl)-L-ornithine. The acid-labile amino acid derivative is formed b y direct condensation of the guanido group of arginine and methylglyox al, a sugar degradation product, and represents a previously unknown p ost-translational protein modification. For a number of commercially a vailable alkali-treated bakery products, the amounts of the imidazolon e after complete enzymic digestion ranged between 900 and 1300 mg per 100 g protein, indicating that between 20 and 30% of the arginyl resid ues might react with methylglyoxal during the bakery process.