title: Melatonin directly scavenges free radicals generated in red blood cells and a cell-free system: chemiluminescence measurements and theoretical calculations.
abstract: Melatonin, a pineal secretory product, has properties of both direct and indirect powerful antioxidant. The aim of the present study was to compare the radical-scavenging, structural and electronic properties of melatonin and tryptophan, precursor of melatonin. Using the alkoxyl- and peroxyl radical-generating systems [the organic peroxide-treated human erythrocytes and a cell-free system containing the azo-initiator 2,2'-azobis(2-amidinopropane)dihydrochloride], we evaluated the radical-scavenging effects of melatonin and tryptophan. Melatonin rather than tryptophan at concentrations of 100-2000 microM markedly inhibited membrane lipid peroxidation in human erythrocytes treated with organic hydroperoxide as well as radical-induced generation of luminol-dependent chemiluminescence. The apparent Stern-Volmer constants for inhibition of membrane lipid peroxidation by melatonin and tryptophan were estimated to be (0.23+/-0.05) x 10(4) M(-1) and (0.02+/-0.005) x 10(4) M(-1), respectively. The apparent Stern-Volmer constants for inhibition of azo-initiator-derived peroxyl radical generation by melatonin and tryptophan were determined to be (0.42+/-0.05) x 10(4) M(-1) and (0.04+/-0.01) x 10(4) M(-1), respectively. The structural and electronic properties of melatonin and its precursor, tryptophan, were determined theoretically by performing semi-empirical and ab initio calculations. The high radical-scavenging properties of melatonin may be explained by the high surface area value and high dipole moment value. From the thermodynamic standpoint, based on our calculations, N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK), was the most stable end oxidative product of melatonin.
authors.affiliation: Department of Membrane Biochemistry, Institute of Biochemistry, National Academy of Sciences of Belarus, Blvd. Leninskogo Komsomola 50, 230017 Grodno, Belarus. hepato@biochem.unibel
authors.forename: I B
authors.initials: IB
authors.lastname: Zavodnik
authors.forename: A V
authors.initials: AV
authors.lastname: Domanski
authors.forename: E A
authors.initials: EA
authors.lastname: Lapshina
authors.forename: M
authors.initials: M
authors.lastname: Bryszewska
authors.forename: R J
authors.initials: RJ
authors.lastname: Reiter
mesh_terms: D002474:Cell-Free System; D004305:Dose-Response Relationship, Drug; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000378:metabolism; D004912:Erythrocytes / Q000187:drug effects* / Q000378:metabolism; D016166:Free Radical Scavengers / Q000494:pharmacology*; D006801:Humans; D015227:Lipid Peroxidation / Q000187:drug effects; D049449:Luminescence; D008550:Melatonin / Q000494:pharmacology*; D008958:Models, Molecular; D014364:Tryptophan / Q000494:pharmacology; D020122:tert-Butylhydroperoxide / Q000494:pharmacology
chemical_list: D016166:Free Radical Scavengers; D014364:Tryptophan; D020122:tert-Butylhydroperoxide; D008550:Melatonin