Melatonin and nitric oxide.

Melatonin is a product of the amino acid tryptophan in the pineal gland. Once synthesized, the specific mechanisms governing the release of melatonin from the pineal gland and its functions are largely unknown. Besides its regulatory role in circadian rhythms in mammals, because of its widespread subcellular distribution, melatonin contributes to the reduction of oxidative damage in both the lipid and the aqueous environments of the cell. This postulate is widely supported by the experimental observations showing that melatonin protects lipids in membranes, proteins in the cytosol, and DNA in the nucleus and mitochondria from free radical damage. Melatonin thus reduces the severity of disease conditions where free radicals are implicated. The direct free radical scavenging effects of melatonin are receptor independent. It has recently been shown that it has an ability to scavenge free radicals, including hydroxyl radicals, hydrogen peroxide, peroxyl radicals, singlet oxygen and nitric oxide (NO) and peroxynitrite anion. An excessive amount of NO, a free radical which is generated by the inducible form of NO synthase, is known to cause cytotoxic changes in cells. Hence, NO synthase is considered a pro-oxidative enzyme, and any factor that reduces its activity would be considered an antioxidant. Recent studies have shown that melatonin inhibits the activity of NO synthase, beside its NO and peroxynitrite scavenging activity. Thus, inhibition of NO production may be another means whereby melatonin reduces oxidative damage under conditions, such as ischemia-reperfusion, sepsis, etc, where NO seems to be important in terms of the resulting damage.