Protection against oxidative protein damage induced by metal-catalyzed reaction or alkylperoxyl radicals: Comparative effects of melatonin and other antioxidants

Melatonin is a well-known hydroxyl radical (^·OH) scavenger that protects DNA and lipids from free radical attack. In this paper, we studied the ability of melatonin to prevent oxidative damage to bovine serum albumin (BSA) induced by two different paradigms: the metal-catalyzed oxidation (MCO) induced by Cu²⁺/H₂O₂ and the alkoxyl and alkylperoxyl radicals formed by the azo initiator 2,2′-azobis(2-amidinopropane) hydrochloride (AAPH, 40 mM). The protective effects of melatonin were compared with 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox), glutathione (GSH), ascorbate, 3,4′,5-trihydroxy-trans-stilbene (resveratrol, 0.1 μM-4 mM) and mannitol (50 μM-100 mM). Melatonin efficiently prevented protein modification induced by both models, as assayed by polyacrylamide gel electrophoresis and carbonyl content. Both trolox and ascorbate had an obvious pro-oxidant effect in the Cu²⁺/H₂O₂ model, whereas both prevented BSA damage induced by AAPH. In the MCO model, the efficacy of GSH in terms of protein protection was higher than melatonin at relatively high concentrations (250 μM-4 mM); however, at lower concentrations (50-250 μM), the efficacy of melatonin was superior to GSH. D-Mannitol (50 μM-100 mM) and resveratrol did not protect BSA from the site-specific damage induced by Cu²⁺/H₂O₂. On the other hand, the relative protective efficiency in the AAPH model was melatonin≃trolox>GSH>ascorbate.