:  Melatonin, or N‐acetyl‐5‐methoxytryptamine, is a compound derived from tryptophan that is found in all organisms from unicells to vertebrates. This indoleamine may act as a protective agent in disease conditions such as Parkinson's, Alzheimer's, aging, sepsis and other disorders including ischemia/reperfusion. In addition, melatonin has been proposed as a drug for the treatment of cancer. These disorders have in common a dysfunction of the apoptotic program. Thus, while defects which reduce apoptotic processes can exaggerate cancer, neurodegenerative disorders and ischemic conditions are made worse by enhanced apoptosis. The mechanism by which melatonin controls cell death is not entirely known. Recently, mitochondria, which are implicated in the intrinsic pathway of apoptosis, have been identified as a target for melatonin actions. It is known that melatonin scavenges oxygen and nitrogen‐based reactants generated in mitochondria. This limits the loss of the intramitochondrial glutathione and lowers mitochondrial protein damage, improving electron transport chain (ETC) activity and reducing mtDNA damage. Melatonin also increases the activity of the complex I and complex IV of the ETC, thereby improving mitochondrial respiration and increasing ATP synthesis under normal and stressful conditions. These effects reflect the ability of melatonin to reduce the harmful reduction in the mitochondrial membrane potential that may trigger mitochondrial transition pore (MTP) opening and the apoptotic cascade. In addition, a reported direct action of melatonin in the control of currents through the MTP opens a new perspective in the understanding of the regulation of apoptotic cell death by the indoleamine.

Mitochondria do not contain catalase and are therefore largely dependent on reduced glutathione (GSH) and glutathione peroxidases for its antioxidant protection. When GSH levels are greatly decreased, hydrogen peroxide accumulates leading to extensive mitochondrial damage. Melatonin has antioxidant properties and prevents toxic effects of reactive oxygen species by maintaining cellular GSH homeostasis. Thus, we examined the influence of melatonin and other classical antioxidants such as vitamins C and E on GSH content and the activity of the GSH-related enzymes (glutathione peroxidase and reductase) in isolated rat liver and brain mitochondria treated with t-butyl hydroperoxide (t-BHP). In control mitochondria melatonin (100 nM) significantly increases GSH content and glutathione peroxidase and reductase activities. After incubation with 100 µM t-BHP, the mitochondrial hydroperoxides level increased, 90% of mitochondrial GSH was oxidized to GSSH, and the activities of GSH-related enzymes were almost totally inhibited. Melatonin (100 nM) counteracted the changes in GSH, GSH-related enzymes and hydroperoxides induced by t-BHP in cultured mitochondria. In the presence of 100 nM melatonin, the activity of the respiratory chain complexes I and IV, measured in submitochondrial particles prepared from rat liver and brain mitochondria, increased significantly. Vitamin C was virtually without effect, and only 1 mM vitamin E increased GSH and reduced hydroperoxide mitochondrial contents. Our results suggest that melatonin, but not vitamins C and E, prevents the toxic effects of hydroperoxides on mitochondria by regenerating their GSH content.Key words: mitochondrial oxidative damage • antioxidant • free radicals • hydroperoxides • electron transport chain itochondria is the major intracellular source of superoxide anion (O 2•-) and hydrogen peroxide (H 2 O 2 ) because of the fixation of molecular oxygen in the respiratory chain (1, 2). Small inefficiencies in the mitochondrial electron transport M chain produce background levels of radical oxygen species (ROS) that can lead to severe mitochondrial dysfunction and cell death (2-5).Glutathione (GSH) and its related enzymes glutathione peroxidase (GPx) and reductase (GRx) are the main mitochondrial antioxidant system (6-8). Mitochondria do not contain catalase and are therefore largely, if not entirely, dependent on GSH and its recycling enzymes (6). Mitochondria do not synthesize GSH but obtain it by from cytosol through a multicomponent transport system, which explains the remarkable ability of mitochondria to take up and retain GSH (6,9). Studies in liver and kidney preparations have concluded that in chemical-induced oxidative injury involving GSH depletion, it was the depletion of the mitochondrial GSH pool rather that of the cytosolic GSH pool critical for development of irreversible cellular damage (10, 11). The GSH-GSSG status is decisive to maintenance of numerous aspects of mitochondrial function, including membrane structure and integrity, intramitochondrial redox ...

Aerobic cells use oxygen for the production of 90-95% of the total amount of ATP that they use. This amounts to about 40 kg ATP/day in an adult human. The synthesis of ATP via the mitochondrial respiratory chain is the result of electron transport across the electron transport chain coupled to oxidative phosphorylation. Although ideally all the oxygen should be reduced to water by a four-electron reduction reaction driven by the cytochrome oxidase, under normal conditions a small percentage of oxygen may be reduced by one, two, or three electrons only, yielding superoxide anion, hydrogen peroxide, and the hydroxyl radical, respectively. The main radical produced by mitochondria is superoxide anion and the intramitochondrial antioxidant systems should scavenge this radical to avoid oxidative damage, which leads to impaired ATP production.

This brief resume enumerates the multiple actions of melatonin as an antioxidant. This indoleamine is produced in the vertebrate pineal gland, the retina and possibly some other organs. Additionally, however, it is found in invertebrates, bacteria, unicellular organisms as well as in plants, all of which do not have a pineal gland. Melatonin's functions as an antioxidant include: a), direct free radical scavenging, b), stimulation of antioxidative enzymes, c), increasing the efficiency of mitochondrial oxidative phosphorylation and reducing electron leakage (thereby lowering free radical generation), and 3), augmenting the efficiency of other antioxidants. There may be other functions of melatonin, yet undiscovered, which enhance its ability to protect against molecular damage by oxygen and nitrogen-based toxic reactants. Numerous in vitro and in vivo studies have documented the ability of both physiological and pharmacological concentrations to melatonin to protect against free radical destruction. Furthermore, clinical tests utilizing melatonin have proven highly successful; because of the positive outcomes of these studies, melatonin's use in disease states and processes where free radical damage is involved should be increased.

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