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Reaction mechanism of melatonin oxidation by reactive oxygen species in vitro
Author(s) -
BonnefontRousselot Dominique,
Collin Fabrice,
Jore Daniel,
GardèsAlbert Monique
Publication year - 2011
Publication title -
journal of pineal research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.881
H-Index - 131
eISSN - 1600-079X
pISSN - 0742-3098
DOI - 10.1111/j.1600-079x.2010.00847.x
Subject(s) - melatonin , reactive oxygen species , mechanism (biology) , in vitro , oxygen , chemistry , biology , biochemistry , neuroscience , organic chemistry , philosophy , epistemology
Melatonin ( N ‐acetyl‐5‐hydroxytryptamine) is a pineal hormone widely known for its antioxidant properties, both in vivo and by direct capture of free radicals in vitro. Although some metabolites and oxidation products of melatonin have been identified, the molecular mechanism by which melatonin exerts its antioxidant properties has not been totally unravelled. This study investigated the reaction mechanism of oxidation of melatonin by radio‐induced reactive oxygen species, generated by gamma radiolysis of water for aqueous solutions of melatonin (from 20 to 200 μ m ), in the presence or absence of molecular oxygen. The hydroxyl radical was found to be the unique species able to initiate the oxidation process, leading to three main products, e.g. N 1 ‐acetyl‐ N 2 ‐formyl‐5‐methoxykynurenin (AFMK), N 1 ‐acetyl‐5‐methoxykynurenin (AMK) and hydroxymelatonin (HO‐MLT). The generation of AFMK and HO‐MLT strongly depended on the presence of molecular oxygen in solution: AFMK was the major product in aerated solutions (84%), whereas HO‐MLT was favoured in the absence of oxygen (86%). Concentrations of AMK remained quite low, and AMK was proposed to result from a chemical hydrolysis of AFMK in solution. A K ‐value of 1.1 × 10 −4 was calculated for this equilibrium. Both hydrogen peroxide and superoxide dismutase had no effect on the radio‐induced oxidation of melatonin, in good accordance for the second case with the poor reactivity of the superoxide anion towards melatonin. Finally, a reaction mechanism was proposed for the oxidation of melatonin in vitro.