REACTION OF MELATONIN WITH LIPOPEROXYL RADICALS IN PHOSPHOLIPID-BILAYERS

Melatonin, at 5 to 500 mu M was incorporated in unilamellar soybean ph osphatidylcholine (PC) Liposomes, the peroxidation of which was induce d by 2,2'-azobis (2-amidinopropane-hydrochloride) (AAPH), and measured as production of conjugated diene lipid hydroperoxides. Concentration as low as 5 and 10 mu M were poorly effective in reducing lipid perox idation. Melatonin at 30 to 500 mu mM caused short inhibition periods, increasing with, but not linearly related to concentration, with a co ncurrent net decrease of the propagation rate. The time course of mela tonin oxidation, measured as loss of fluorescence, was studied during the AAPH-stimulated peroxidation of soybean PC liposomes, or when mela tonin was incorporated in nonperoxidable unilamellar dimirystoyl phosp hatidylcholine (DMC) liposomes. Consumption kinetics of 30 mu M melato nin were linear with time in DMC liposomes and disappearance of melato nin occurred at a rate of 0.058 M(-8)s(-1). On, the other hand, the co nsumption of melatonin during the oxidation of soybean PC liposomes, w as 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 slow ed down to reach the same rate observed in s DMC liposomes. This evide nce suggests a reaction with lipid-derived peroxyl radicals, possibly in addition to reaction with peroxyl radicals derived from AAPH. Scave nging of lipoperoxyl radicals by melatonin was also evident in experim ents where melatonin was incorporated in multilamellar soybean PC lipo somes and peroxidation was initiated by 2,2'-azobis (2,4-dimethyl-vale ronitrile). The antioxidant activity of melatonin in soybean PC liposo mes is much lower than that of alpha-tocopherol, under comparable assa y conditions. However, a combination of melatonin and alpha-tocopherol , at 5 mu M, 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 consumptio n rate when compounds were combined, indicating some protection by mel atonin. Regeneration mechanisms were not evident and depletion of alph a-tocopherol was coincident with the inhibition time. (C) 1997 Elsevie r Science Inc.