Free radical scavenging effects of melatonin and serotonin: possible mechanism.

Melatonin has been reported to be a potent free radical scavenger, but the mechanism by which it protects membranes from lipid peroxidation is poorly understood. The present study addresses this problem by comparing the free radical scavenging properties of melatonin and serotonin, two indoles with similar structure, but differing solubilities. Both serotonin and melatonin significantly prevented lipid peroxidation of platelet membranes. Additionally, melatonin significantly decreased the microviscosity (increased the fluidity) of platelet membranes, while serotonin had the opposite effect. These data led us to postulate that serotonin exerts its free radical scavenging action in the aqueous phase, or at the water-membrane interface, while melatonin positions itself within the lipid bilayer where it protects membrane phospholipids against free radical attack.