M.P. Terrón

Melatonin is a highly conserved molecule. Its presence can be traced back to ancient photosynthetic prokaryotes. A primitive and primary function of melatonin is that it acts as a receptor-independent free radical scavenger and a broad-spectrum antioxidant. The receptor-dependent functions of melatonin were subsequently acquired during evolution. In the current review, we focus on melatonin metabolism which includes the synthetic rate-limiting enzymes, synthetic sites, potential regulatory mechanisms, bioavailability in humans, mechanisms of breakdown and functions of its metabolites. Recent evidence indicates that the original melatonin metabolite may be N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) rather than its commonly measured urinary excretory product 6-hydroxymelatonin sulfate. Numerous pathways for AFMK formation have been identified both in vitro and in vivo. These include enzymatic and pseudo-enzymatic pathways, interactions with reactive oxygen species (ROS)/reactive nitrogen species (RNS) and with ultraviolet irradiation. AFMK is present in mammals including humans, and is the only detectable melatonin metabolite in unicellular organisms and metazoans. 6-hydroxymelatonin sulfate has not been observed in these low evolutionary-ranked organisms. This implies that AFMK evolved earlier in evolution than 6-hydroxymelatonin sulfate as a melatonin metabolite. Via the AFMK pathway, a single melatonin molecule is reported to scavenge up to 10 ROS/RNS. That the free radical scavenging capacity of melatonin extends to its secondary, tertiary and quaternary metabolites is now documented. It appears that melatonin's interaction with ROS/RNS is a prolonged process that involves many of its derivatives. The process by which melatonin and its metabolites successively scavenge ROS/RNS is referred as the free radical scavenging cascade. This cascade reaction is a novel property of melatonin and explains how it differs from other conventional antioxidants. This cascade reaction makes melatonin highly effective, even at low concentrations, in protecting organisms from oxidative stress. In accordance with its protective function, substantial amounts of melatonin are found in tissues and organs which are frequently exposed to the hostile environmental insults such as the gut and skin or organs which have high oxygen consumption such as the brain. In addition, melatonin production may be upregulated by low intensity stressors such as dietary restriction in rats and exercise in humans. Intensive oxidative stress results in a rapid drop of circulating melatonin levels. This melatonin decline is not related to its reduced synthesis but to its rapid consumption, i.e. circulating melatonin is rapidly metabolized by interaction with ROS/RNS induced by stress. Rapid melatonin consumption during elevated stress may serve as a protective mechanism of organisms in which melatonin is used as a first-line defensive molecule against oxidative damage. The oxidative status of organisms modifies melatonin metabolism. It has been reported that the higher the oxidative state, the more AFMK is produced. The ratio of AFMK and another melatonin metabolite, cyclic 3-hydroxymelatonin, may serve as an indicator of the level of oxidative stress in organisms.

This review summarizes some of the recent findings concerning the long-held tenet that the enzyme, N-acetyltransferase, which is involved in the production of N-acetylserotonin, the immediate precursor of melatonin, may in fact not always control the quantity of melatonin generated. New evidence from several different laboratories indicates that hydroxyindole-O-methyltransferase, which O-methylates N-acetylserotonin to melatonin may be rate-limiting in some cases. Also, the review makes the point that melatonin's actions are uncommonly widespread in organs due to the fact that it works via membrane receptors, nuclear receptors/binding sites and receptor-independent mechanisms, i.e., the direct scavenging of free radicals. Finally, the review briefly summarizes the actions of melatonin and its metabolites in the detoxification of oxygen and nitrogen-based free radicals and related non-radical products. Via these multiple processes, melatonin is capable of influencing the metabolism of every cell in the organism.

This brief review considers some of the cardiac diseases and conditions where free radicals and related reactants are believed to be causative. The report also describes the beneficial actions of melatonin against oxidative cardiovascular disorders. Based on the data available, melatonin seems to have cardioprotective properties via its direct free radical scavenger and its indirect antioxidant activity. Melatonin efficiently interacts with various reactive oxygen and reactive nitrogen species (receptor independent actions) and it also upregulates antioxidant enzymes and downregulates pro-oxidant enzymes (receptor-dependent actions). Moreover, melatonin enters all cells and subcellular compartments and crosses morphophysiologic barriers. These findings have implications for the protective effects of melatonin against cardiac diseases induced by oxidative stress. Melatonin attenuates molecular and cellular damages resulting from cardiac ischemia/reperfusion in which destructive free radicals are involved. Anti-inflammatory and antioxidative properties of melatonin are also involved in the protection against a chronic vascular disease, atherosclerosis. The administration of melatonin, as a result of its antioxidant features, has been reported to reduce hypertension and cardiotoxicity induced by clinically used drugs. The results described herein help to clarify the beneficial effects of melatonin against these conditions and define the potential clinical applicability of melatonin in cardiovascular diseases.

The functional versatility and diversity of melatonin has exceeded everyone's expectations. The evidence is substantial that melatonin has multiple receptor-mediated and receptor-independent actions. Considering the unexpectedly widespread distribution of cellular membrane receptors as well as the existence of nuclear binding sites/receptors and the fact that some of melatonin's actions are receptor-independent means that melatonin likely functions in every cell with which it comes in contact. This is highlighted by the fact that there are no morpho-physiological barriers to melatonin, e.g., the blood-brain barrier. In addition to its widespread actions, melatonin synthesis occurs in widely diverse tissues with its production not being relegated to the pineal gland. This should not be unexpected given that it is present throughout the animal kingdom including species that lack a pineal gland, e.g., insects, and in single cell organisms. In this review, only a few of melatonin's effects that involve the interaction of the indoleamine with receptors are described. These functions include the control of seasonal reproduction, modulation of sleep processes and influences on bone growth and osteoporosis. Among the actions of melatonin that are likely receptor independent and that are reviewed herein include its ability to neutralize free radicals which leads to a reduction in cataract formation, reducing oxidative stress due to exposure to hyperbaric hyperoxia, ameliorating hyperthyroidism and abating the toxicity of sepsis and septic shock. These actions alone speak to the diversity of beneficial effects of melatonin; however, the review is no way near exhaustive in terms of what melatonin is capable of doing. Because of its ubiquitous benefits, the pharmaceutical industry is developing melatonin analogues which interact with melatonin receptors. Clearly, the intent of the drugs is to take advantage of some of melatonin's numerous beneficial effects.

Craniocerebral trauma (CCT) is the most frequent cause of morbidity-mortality as a result of an accident. The probable origins and etiologies are multifactorial and include free radical formation and oxidative stress, the suppression of nonspecific resistance, lymphocytopenia (disorder in the adhesion and activation of cells), opportunistic infections, regional macro and microcirculatory alterations, disruptive sleep-wake cycles and toxicity caused by therapeutic agents. These pathogenic factors contribute to the unfavorable development of clinical symptoms as the disease progresses. Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine endogenously produced in the pineal gland and in other organs and it is protective agent against damage following CCT. Some of the actions of melatonin that support its pharmacological use after CCT include its role as a scavenger of both oxygen and nitrogen-based reactants, stimulation of the activities of a variety of antioxidative enzymes (e.g. superoxide dismutase, glutathione peroxidase, glutathione reductase and catalase), inhibition of pro-inflammatory cytokines and activation-adhesion molecules which consequently reduces lymphocytopenia and infections by opportunistic organisms. The chronobiotic capacity of melatonin may also reset the natural circadian rhythm of sleep and wakefulness. Melatonin reduces the toxicity of the drugs used in the treatment of CCT and increases their efficacy. Finally, melatonin crosses the blood-brain barrier and reduces contusion volume and stabilizes cellular membranes preventing vasospasm and apoptosis of endothelial cells that occurs as a result of CCT.