Antioxidant capacity of the neurohormone melatonin.

The aim of this study was to elucidate the antioxidant behaviour of melatonin (M) and determine its activity-structure relationship. M or 5-metoxy-N acetyltriptamine is a neurohormone secreted by the pineal gland, which plays a proven role in maintaining sleep-wake rhythms. The antioxidant capacity of M was analysed using the oxygen radical absorbance capacity (ORAC) assay. Furthermore, spectral measurements for aerobic photolytic reaction of neutral red (NR) and degree of inhibition of photolysis with M, glutathione (GSH), ascorbic acid (AA) and vitamin E analogue Trolox were studied at room temperature 25 degrees C, using visible (VIS) and ultra-violet (UV) radiations. In the ORAC assay 2,2-azobis (2-amidino-propane)dihydrochloride (AAPH) a peroxyl radical generator, ROO degrees ; H2O2-Cu2+, mainly a hydroxyl radical generator, degrees OH; and Cu2+ a transition metal were used. Although some studies indicated that M is a powerful antioxidant, no one has compared its antioxidant capacities with GSH, E-vitamin and AA, using three free radical (FR) generators in an assay which utilizes an area-under curve technique and thus combines both inhibition time and inhibition degree of FR action by an antioxidant into a single quantity. In the current study, we used ORAC assay with three FR generators. The assay is based on propensity of the fluorescence emitted by the protein beta-phycoerythrin (beta-PE) from porphyridium cruentum to be quenched when exposed to FR action. M in our experiments acted as a universal antioxidant against ROO degrees and degrees OH radicals. Also, M served as an antioxidant in the presence of Cu2+. M, which is a lipid-soluble compound, was a twice more powerful antioxidant than vitamin E, and four times than AA or GSH. Furthermore, M inhibited aerobic photolysis of NR photoinduced with VIS and UV rays faster and more effectively, than AA, GSH or vitamin E. AA with NR, under aerobic conditions during irradiation with VIS and UV acted as a pro-oxidant. M may be the premier molecule to protect the cells from oxidative stress.