This survey summarizes the findings, accumulated within the last 2 years, concerning melatonin's role in defending against toxic free radicals. Free radicals are chemical constituents that have an unpaired electron in their outer orbital and, because of this feature, are highly reactive. Inspired oxygen, which sustains life, also is harmful because up to 5% of the oxygen (O2) taken in is converted to oxygen-free radicals. The addition of a single electron to O2 produces the superoxide anion radical (O2-.); O2-. is catalytic-reduced by superoxide dismutase, to hydrogen peroxide (H2O2). Although H2O2 is not itself a free radical, it can be toxic at high concentrations and, more importantly, it can be reduced to the hydroxyl radical (.OH). The .OH is the most toxic of the oxygen-based radicals and it wreaks havoc within cells, particularly with macromolecules. In recent in vitro studies, melatonin was shown to be a very efficient neutralizer of the .OH; indeed, in the system used to test its free radical scavenging ability it was found to be significantly more effective than the well known antioxidant, glutathione (GSH), in doing so. Likewise, melatonin has been shown to stimulate glutathione peroxidase (GSH-Px) activity in neural tissue; GSH-PX metabolizes reduced glutathione to its oxidized form and in doing so it converts H2O2 to H2O, thereby reducing generation of the .OH by eliminating its precursor. More recent studies have shown that melatonin is also a more efficient scavenger of the peroxyl radical than is vitamin E. The peroxyl radical is generated during lipid peroxidation and propagates the chain reaction that leads to massive lipid destruction in cell membranes. In vivo studies have demonstrated that melatonin is remarkably potent in protecting against free radical damage induced by a variety of means. Thus, DNA damage resulting from either the exposure of animals to the chemical carcinogen safrole or to ionizing radiation is markedly reduced when melatonin is co-administered. Likewise, the induction of cataracts, generally accepted as being a consequence of free radical attack on lenticular macromolecules, in newborn rats injected with a GSH-depleting drug are prevented when the animals are given daily melatonin injections. Also, paraquat-induced lipid peroxidation in the lungs of rats is overcome when they also receive melatonin during the exposure period. Paraquat is a highly toxic herbicide that inflicts at least part of its damage by generating free radicals.(ABSTRACT TRUNCATED AT 400 WORDS)

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Carbon tetrachloride (CCl4) exerts its toxic effects by the generation of free radicals. In this study we investigated whether melatonin, a potent free radical scavenger, could prevent the deleterious effects of CCl4. Liver homogenates and liver microsomes were incubated with CCl4 in the presence of melatonin and lipid peroxidation and glucose-6 phosphatase (G6Pase) activity were determined. All doses of CCl4 (1, 0.5, 0.1 mM) produced significantly high levels of lipid peroxidation, as reflected by increased levels of malonaldehyde and 4-hydroxyalkenals, in both liver homogenates and liver microsomes. These doses of CCl4 concommitantly reduced the activity of microsomal G6Pase. Co-incubation with melatonin dose-dependently (2, 1, 0.5 mM) inhibited the production of lipid peroxidation, but it was unable to restore the activity of G6Pase. In in vivo studies, rats were also treated with melatonin (10 mg/kg, i.p.), given 30 min before and 60 min after the administration of CCl4 (5 ml/kg, i.p.). Significantly elevated levels of lipid peroxidation were measured in the liver and kidney. Melatonin prevented the CCl4-induced lipid peroxidation in the kidney, but not in the liver. These data suggest that melatonin may provide protection against some of the damaging effects of CCl4, possibly due to its ability to scavenge toxic free radicals.