Melatonin directly scavenges free radicals generated in red blood cells and a cell-free system: Chemiluminescence measurements and theoretical calculations

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 μM 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) × 104 M− 1 and (0.02 ± 0.005) × 104 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) × 104 M− 1 and (0.04 ± 0.01) × 104 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, N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK), was the most stable end oxidative product of melatonin.