Reaction of melatonin with lipoperoxyl radicals in phospholipid bilayers.

Melatonin, at 5 to 500 microM was incorporated in unilamellar soybean phosphatidylcholine (PC) liposomes, the peroxidation of which was induced by 2,2'-azobis (2-amidinopropane-hydrochloride) (AAPH), and measured as production of conjugated diene lipid hydroperoxides. Concentration as low as 5 and 10 microM were poorly effective in reducing lipid peroxidation. Melatonin at 30 to 500 microM caused short inhibition periods, increasing with, but not linearly related to concentration, with a concurrent net decrease of the propagation rate. The time course of melatonin oxidation, measured as loss of fluorescence, was studied during the AAPH-stimulated peroxidation of soybean PC liposomes, or when melatonin was incorporated in nonperoxidable unilamellar dimirystoyl phosphatidylcholine (DMC) liposomes. Consumption kinetics of 30 microM melatonin were linear with time in DMC liposomes and disappearance of melatonin occurred at a rate of 0.058 M(-8) s(-1). On the other hand, the consumption of melatonin during the oxidation of soybean PC liposomes, was not linear with time. The rate of disappearance was calculated as 0.19 M(-8) s(-1) at the beginning of the propagation phase, then it slowed down to reach the same rate observed in DMC liposomes. This evidence suggests a reaction with lipid-derived peroxyl radicals, possibly in addition to reaction with peroxyl radicals derived from AAPH. Scavenging of lipoperoxyl radicals by melatonin was also evident in experiments where melatonin was incorporated in multilamellar soybean PC liposomes and peroxidation was initiated by 2,2 '-azobis (2,4-dimethyl-valeronitrile). The antioxidant activity of melatonin in soybean PC liposomes is much lower than that of alpha-tocopherol, under comparable assay conditions. However, a combination of melatonin and alpha-tocopherol, at 5 microM, resulted in a synergistic antioxidant effect. Time course of alpha-tocopherol consumption, monitored in the absence and in the presence of melatonin, showed a significant decrease of the consumption rate when compounds were combined, indicating some protection by melatonin. Regeneration mechanisms were not evident and depletion of alpha-tocopherol was coincident with the inhibition time.