Mechanistic and comparative studies of melatonin and classic antioxidants in terms of their interactions with the ABTS cation radical

Melatonin and classic antioxidants possess the capacity to scavenge ABTS^.+ with IC₅₀s of 4, 11, 15.5, 15.5, 17 and 21 μM for melatonin, glutathione, vitamin C, trolox, NADH and NADPH, respectively. In terms of scavenging ABTS^.+, melatonin exhibits a different profile than that of the classic antioxidants. Classic antioxidants scavenge one or less ABTS^.+, while each melatonin molecule can scavenge more than one ABTS^.+, probably with a maximum of four. Classic antioxidants do not synergize when combined in terms of scavenging ABTS^.+. However, a synergistic action is observed when melatonin is combined with any of the classic antioxidants. Cyclic voltaxnmetry indicates that melatonin donates an electron at the potential of 715 mV. The scavenging mechanism of melatonin on ABTS^.+ may involve multiple-electron donations via intermediates through a stepwise process. Intermediates including the melatoninyl cation radical, the melatoninyl neutral radical and cyclic 3-hydroxymelatonin (cyclic 3-OHM) and N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK) seem to participate in these reactions. More interestingly, the pH of the solution dramatically modifies the ABTS^.+ scavenging capacity of melatonin while pH changes have no measurable influence on the scavenging activity of classic antioxidants. An acidic pH markedly reduces the ABTS^.+ scavenging capacity of melatonin while an increased pH promotes the interaction of melatonin and ABTS^.+. The major melatonin metabolites that develop when melatonin interacts with ABTS^.+ are cyclic 3-OHM and AFMK. Cyclic 3-OHM is the intermediate between melatonin and AFMK, and cyclic 3-OHM also has the ability to scavenge ABTS^.+. Melatonin and the metabolites which are generated via the interaction of melatonin with ABTS^.+, i.e. the melatoninyl cation radical, melatoninyl neutral radical and cyclic 3-OHM, all scavenge ABTS^.+. This unique cascade action of melatonin, in terms of scavenging, increases its efficiency to neutralized ABTS^.+; this contrasts with the effects of the classic antioxidants.