Kinetic Radical-scavenging Activity of Melatonin

Abstract

Carbon-centred free radicals can be involved in damage to biological systems under hypoxic/anoxic conditions as well as in ischaemia/reperfusion injury. The antioxidant activities of melatonin against carbon-centred radicals are poorly understood. The aim of this study was to investigate the antioxidant properties of melatonin against carbon-centred radicals in a biomimetic model system consisting of growing methyl methacrylate (MMA) radicals (poly-MMA radicals, PMMA^•). The kinetics of the polymerization of MMA initiated by thermal decomposition of 2,2′-azobis(isobutyronitrile) (AIBN; R^• radical) or benzoyl peroxide (BPO; PhCOO^• radical) in the presence of melatonin were investigated by the induction period method under nearly anaerobic conditions. As melatonin concentrations increased, the length of the induction period (IP) increased, but for the BPO system the IP reached a plateau at a molar ratio of BPO to melatonin of 5:1, indicating that the oxidation of melatonin by PhCOO^• was limited. At low concentrations of melatonin, the stoichiometric factor (n, the number of free radicals trapped by the antioxidant moiety) for melatonin was approximately 2, but as the melatonin concentration increased the n value decreased markedly to 0.1. These observations suggest that melatonin may possess catalytic activity contributing to radical avoidance. The initial rate of polymerization (Rp) in the BPO system was markedly suppressed by high concentrations of melatonin, suggesting a strong interaction between oxidative end-products formed from melatonin and PMMA^•. Under conditions where n was about 2, the k_inh values for melatonin in the BPO system and the AIBN system were 6.58×10⁴ M^-1s^-1 and 2.49×10³ M^-1s^-1, respectively. In the BPO system, the k_inh of melatonin was of a similar magnitude to that of α-tocopherol, whereas in the AIBN system the k_inh of melatonin was 100-fold greater than that of tocopherol. The present findings suggest that melatonin may be able to scavenge harmful carbon-centred radicals in vivo.