: OBJECTIVES: The ability of melatonin to protect protein and lipid against oxidative damage induced by an ascorbate-Fe(3+)-EDTA (AFE) system which generates the hydroxyl radical was investigated using bovine serum albumin (BSA) and phosphatidylcholine (PC) liposomes, respectively, and compared with the protective effects of reduced glutathione and alpha-tocopherol. The comparison study was also performed using PC liposomes containing BSA. METHODS: BSA, PC liposomes or their mixtures were exposed to the HO.-generating system of AFE composed of 0.1 mM EDTA-Fe(3+) and 0.5 mM ascorbate in 0.1 M phosphate buffer, pH 7.4, at 37 degrees C for 1 h. Oxidative damage of BSA was determined by measuring the carbonyl content and the fragmentation of protein by the reaction with dinitrophenylhydrazine (DNPH) and electrophoresis, respectively. Lipid peroxidation of PC liposomes was indicated by the quantity of malondialdehyde and 4hydroxyalkenals. RESULTS: Melatonin inhibited protein damage as indicated by the reduced formation of carbonyl groups and fragmentation of BSA by AFE as effectively as did glutathione while alpha-tocopherol was ineffective. Melatonin also prevented lipid peroxidation to the same extent as did alpha-tocopherol in PC liposomes. CONCLUSION: Both BSA and PC lipid exposed to AFE are effectively protected by melatonin while hydrophilic glutathione and hydrophobic alpha-tocopherol are as effective as melatonin only in one target, i.e., BSA or PC lipid, respectively.