OXIDATION-REDUCTION PROPERTIES OF METHYLGLYOXAL-MODIFIED PROTEIN IN RELATION TO FREE-RADICAL GENERATION

Oxidation-reduction properties of methylglyoxal-modified protein in re lation to free radical generation were investigated. Glycation of bovi ne serum albumin by methylglyoxal generated the protein-bound free rad ical, probably the cation radical of the cross-linked Schiff base, as observed in the reaction of methylglyoxal with L-alanine (Yim, H.-S., Kang, S.-O., Bah, Y. C., Chock, P. B., and Yim, M. B. (1995) J. Biol. Chem. 270, 28228-28233) or with N-alpha-acetyl-L-lysine. The glycated bovine serum albumin showed increased electrophoretic mobility suggest ing that the basic residues, such as lysine, were modified by methylgl yoxal. The glycated protein reduced ferricytochrome c to ferrocytochro me c in the absence of oxygen or added metal ions. This reduction of c ytochrome c was accompanied by a large increase in the amplitude of th e electron paramagnetic resonance signal originated from the protein-b ound free radical. In addition, the glycated protein catalyzed the oxi dation of ascorbate in the presence of oxygen, whereas the protein fre e radical signal disappeared. These results indicate that glycation of protein generates active centers for catalyzing one-electron oxidatio n-reduction reactions. This active center, which exhibits enzyme-like characteristic, was suggested to be the cross-linked Schiff base/the c ross-linked Schiff base radical cation of the protein. It mimics the c haracteristics of the metal-catalyzed oxidation system. The glycated b ovine serum albumin cross-linked further to the cytochrome c in the ab sence of methylglyoxal. The cross-linked cytochrome c maintains its ox idation-reduction properties. These results together indicate that gly cated proteins accumulated in vivo provide stable active sites for cat alyzing the formation of free radicals.