The efficacy of vitamin E and melatonin as antioxidants against lipid peroxidation in rat retinal homogenates

Free radical-induced oxidation can cause severe cell damage in biological systems. Melatonin, a pineal secretory product, is a recently identified antioxidant that protects cells from the damaging effects of free radicals. We compared the effect of melatonin and vitamin E, another antioxidant, against lipid peroxidation (LPO) in rat retinal homogenates. The aim was to characterize the antioxidative efficacy of melatonin in retina, a tissue highly susceptible to oxidative damage. The LPO product, malondialdehyde (MDA), was determined to provide an index of cell damage in vitro. After the incubation with iron(II) ions, the free radical scavenging effectiveness of four different concentrations (i.e., 0.5, 1.0, 2.0, and 4.0 mM) of vitamin E and melatonin were determined by comparing the final levels of MDA. Lipid peroxidation product levels were significantly reduced in a dose-response manner by all concentrations of vitamin E. Melatonin, in concentrations of either 2.0 or 4.0 mM, also significantly reduced LPO. Statistical analysis of the data showed that vitamin E treatment always yielded a lower level of LPO products than did the same concentration of melatonin. The concentrations of each agent required to inhibit 50% of the lipid damage (IC50) were 0.69 mM and 4.98 mM for vitamin E and melatonin, respectively. Both vitamin E and melatonin protect the retina against LPO in a dose-dependent manner. Although the IC50 value for melatonin is about 7.2 times higher than that of vitamin E, melatonin's pharmacological and physiological role in the treatment and/or prevention of certain retinal diseases in vivo should be further investigated.