Evaluation of the antioxidant effect of melatonin by flow injection analysis-luminol chemiluminescence

The antioxidant effect of melatonin was evaluated by using a flow injection analysis chemiluminescence (FIA-CL) method to measure alone its direct antioxidant behaviour in aqueous solutions. Inhibition by melatonin, DL-tryptophan or serotonin (5-hydroxytryptamine, 5-HT) of the steady CL emissions of various oxidants with luminol was measured. LogIC(50) values of melatonin were calculated: -2.6+/-0.1M for superoxide (O(2) radical(-)), -3.0+/-0.02 M, for hydrogen peroxide (H(2)O(2)), -4.7+/-0.04 M, for hypochlorous anion ((-)OCl), -2.8+/-0.1M, for hydoxyl (radical OH) and -2.0+/-0.03 M, for peroxynitrite (ONOO(-)). Thus melatonin exerted a limited direct antioxidant activity towards reactive oxygen species (ROS) in aqueous solutions, broadly comparable to that of tryptophan, but less than that of serotonin and of major antioxidants such as ascorbic acid. This suggests that where the benefits of melatonin are correctly attributed to antioxidant behaviour, this is mediated largely through antioxidant enzymes. The pharmacological implications are discussed.