Characterization of the antioxidant effects of melatonin and related indoleamines in vitro

The antioxidant and possible pro-oxidant effects of melatonin and related indoleamines (tryptophan, serotonin, N-acetylserotonin, and 5-methoxytryptamine) were studied in vitro. In two model membrane systems, i.e., phospholipid liposomes and rat erythrocytes, lipid peroxidation induced by Fe2+ and H2O2, respectively, were reduced by the tested indoleamines except for tryptophan. The 5-hydroxy-indoleamines, serotonin, and N-acetylserotonin exhibited pro-oxidant actions in the bleomycin assay by reducing Fe3+ to Fe2+, which leads to DNA damage. The 5-methoxy-indoleamines, melatonin and 5-methoxytryptamine, were devoid of any pro-oxidant actions in this assay. Serotonin, but not N-acetylserotonin, scavenged the superoxide anion. None of the indoleamines tested had any reactivity towards H2O2. All the indoleamines, including tryptophan, were, however, shown to react with hypochlorous acid. These findings support an antiperoxidative and antioxidant action of melatonin which is devoid of pro-oxidant effect on non-lipid substrates.