Melatonin was recently reported to be an effective free radical scavenger and antioxidant.
Melatonin is believed to scavenge the highly toxic hydroxyl radical, the peroxynitrite anion, and possibly the peroxyl radical.
Also, secondarily, it reportedly scavenges the superoxide anion radical and it quenches singlet oxygen.
Additionally, it stimulates mRNA levels for superoxide dismutase and the activities of glutathione peroxidase, glutathione reductase and glucose-6-phosphate dehydrogenase (all of which are antioxidative enzymes), thereby increasing its antioxidative capacity.
Also, melatonin, at least at some sites, inhibits nitric oxide synthase, a pro-oxidative enzyme.
In both in vivo and in vitro experiments melatonin has been shown to reduce lipid peroxidation and oxidative damage to nuclear DNA.
While these effects have been observed primarily using pharmacological doses of melatonin, in a small number of experiments melatonin has been found to be physiologically relevant as an antioxidant as well.
The efficacy of melatonin in inhibiting oxidative damage has been tested in a variety of neurological disease models where free radicals have been implicated as being in part causative of the condition.
Thus, melatonin has been shown prophylactically to reduce amyloid beta protein toxicity of Alzheimer's disease, to reduce oxidative damage in several models of Parkinson's disease (dopamine auto-oxidation, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 6-hydroxydopamine), to protect against glutamate excitotoxicity, to reduce ischemia-reperfusion injury, to lower neural damage due to gamma-aminolevulinic acid (phorphyria), hyperbaric hyperoxia and a variety of neural toxins.
Since endogenous melatonin levels fall markedly in advanced age, the implication of these findings is that the loss of this antioxidant may contribute to the incidence or severity of some age-associated neurodegenerative diseases.
Melatonin also prevents electron leakage from the mitochondrial electron transport chain thereby diminishing free radical generation; this process is referred to as radical avoidance.
The fact that melatonin and its metabolites are all efficient radical scavengers indicates that melatonin is a precursor molecule for a variety of intracellular reducing agents.
In specific reference to the brain, melatonin also has an advantage over some other antioxidants given that it readily passes through the blood-brain-barrier.
This, coupled with the fact that it and its by-products are particularly efficient detoxifiers of reactive species, make these molecules of major importance in protecting the brain from oxidative/nitrosative abuse.