The EPR signal recorded in reaction medium containing L-lysine and methylglyoxal is supposed to come from the anion radical (semidione) of methylglyoxal and cation radical of methylglyoxal dialkylimine. These free-radical intermediates might be formed as a result of electron transfer from dialkylimine to methylglyoxal. The EPR signal was observed in a nitrogen atmosphere, whereas only trace amounts of free radicals were registered under aerobic conditions. It has been established that the decay of methylglyoxal anion radical on aeration of the medium is inhibited by superoxide dismutase. Using the methods of EPR spectroscopy and lucigenin-dependent chemiluminescence, it has been shown that nonenzymatic generation of free radicals including superoxide anion radical takes place during the interaction of L-lysine with methylglyoxal--an intermediate of carbonyl stress--at different (including physiological) pH values. In the course of analogous reaction of L-lysine with malondialdehyde (the secondary product of the free radical derived oxidation of lipids), the formation of organic free radicals or superoxide radical was not observed.

The Maillard reaction is the key process in protein modification during pathologies connected with carbonyl stress. It was shown in system modeling that Maillard reaction interaction of L-lysine (L-lys) with methylglyoxal (MG) led to the formation of compounds reducing methemoglobin (metHb). Under the above conditions and in the presence of S-nitrosoglutathione (GSNO), metHb nitrosylation took place. Processes of metHb reduction and nitrosylation had the lag phase that was dependent on the presence of oxygen (O2) in the reaction mixture. Oxygen interacting with organic free radicals of the Maillard reaction inhibited hemoglobin (Hb) reduction and hence Hb nitrosylation during the first minutes of the reaction. It was also shown that the yield of organic free-radical intermediates of the L-lys with MG was increased in the presence of GSNO and metHb. All effects described could be a result of the formation of active red-ox GSNO derivates in the Maillard reaction. These derivates are probably mediators of one-electron oxidation of dialkylimine by MG. Anion radicals of S-nitrosothiols can function as such mediators.

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