Radical-scavenging activity of melatonin, either alone or in combination with vitamin E, ascorbate or 2-mercaptoethanol as co-antioxidants, using the induction period method.

Yoshinori Kadoma, Seiichiro Fujisawa

Index: In Vivo 25(1) , 49-53, (2011)

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Abstract

Melatonin shows antioxidant/prooxidant activity but its mechanism of action remains unknown.The radical-scavenging activity of melatonin and various melatonin/co-antioxidant mixtures in a 1:1 molar ratio was evaluated in terms of the length of the induction period (IP) for polymerization of methyl methacrylate (MMA), initiated by thermal decomposition of 2,2'-azobisisobutyronitrile (AIBN) or by benzoyl peroxide (BPO) under nearly anaerobic conditions, which was monitored by differential scanning calorimetry (DSC).The observed IP (A) for a pinoline, L-ascorbyl 2,6-dibutyrate (ASDB), vitamin E (alpha-, beta-, gamma- or delta-T) or 2-mercaptoethanol (2ME) mixture was compared with the calculated total sum of IP (melatonin+each co-antioxidant) (B). For both the AIBN and BPO systems, the A/B for the melatonin/ASDB, beta-T, gamma-T or delta-T mixture was 0.3-0.7, whereas that for the melatonin/2ME mixture was approximately 1. For the AIBN system, the A/B for the melatonin/alpha-T or pinoline mixture was 0.7-0.8. By contrast, for the BPO system, that for the melatonin/alpha-T or pinoline mixture was approximately 1.The prooxidant effect of the melatonin/ascorbate or vitamin E mixtures induced by radical-oxidizing activity may help to explain the anticancer activity of melatonin in biological systems.