Protective effect of melatonin in a non-septic shock model induced by zymosan in the rat

Melatonin and inflammatory bowel disease: From basic mechanisms to clinical application

Inflammatory bowel disease is a chronic inflammatory disease and has periods of recurrence and remission. Improper immune responses to gut flora bacteria, along with genetic susceptibility, appear to be involved in causing this complex disease. It seems dysbiosis and oxidative stress may also be involved in IBD pathogenesis. A significant number of clinical studies have shown an interesting association between sleep disturbances and IBD. Studies in animal models have also shown that sleep deprivation has a significant effect on the pathogenesis of IBD and can aggravate inflammation. These interesting findings have drawn attention to melatonin, a sleep-related hormone. Melatonin is mainly produced by the pineal gland, but many tissues in the body, including the intestines, can produce it. Melatonin can have an interesting effect on the pathogenesis of IBD. Melatonin can enhance the intestinal mucosal barrier, alter the composition of intestinal bacteria in favor of bacteria with anti-inflammatory properties, regulate the immune response, alleviate inflammation and attenuate oxidative stress. It seems that, melatonin supplementation is effective in relieving inflammation and healing intestinal ulcers in IBD animal models. Some clinical studies have also shown that melatonin supplementation as an adjuvant therapy may be helpful in reducing disease activity in IBD patients. In this review article, in addition to reviewing the effects of sleep disturbances and melatonin on key mechanisms involved in the pathogenesis of IBD, we will review the findings of clinical studies regarding the effects of melatonin supplementation on IBD treatment.

Mechanisms and clinical evidence to support melatonin's use in severe COVID-19 patients to lower mortality

The fear of SARS-CoV-2 infection is due to its high mortality related to seasonal flu. To date, few medicines have been developed to significantly reduce the mortality of the severe COVID-19 patients, especially those requiring tracheal intubation. The severity and mortality of SARS-CoV-2 infection not only depend on the viral virulence, but are primarily determined by the cytokine storm and the destructive inflammation driven by the host immune reaction. Thus, to target the host immune response might be a better strategy to combat this pandemic. Melatonin is a molecule with multiple activities on a virus infection. These include that it downregulates the overreaction of innate immune response to suppress inflammation, promotes the adaptive immune reaction to enhance antibody formation, inhibits the entrance of the virus into the cell as well as limits its replication. These render it a potentially excellent candidate for treatment of the severe COVID-19 cases. Several clinical trials have confirmed that melatonin when added to the conventional therapy significantly reduces the mortality of the severe COVID-19 patients. The cost of melatonin is a small fraction of those medications approved by FDA for emergency use to treat COVID-19. Because of its self-administered, low cost and high safety margin, melatonin could be made available to every country in the world at an affordable cost. We recommend melatonin be used to treat severe COVID-19 patients with the intent of reducing mortality. If successful, it would make the SARS-CoV-2 pandemic less fearful and help to return life back to normalcy.

Role of nitric oxide on zymosan-induced inhibition of crop emptying in chicks

Zymosan, a component of yeast cell walls, reduces feed passage through the digestive tract in chicks (Gallus gallus), although the mechanism mediating this effect is poorly understood. Nitric oxide (NO) is associated with a variety of biological actions including effects on the immune system. In addition, it has been suggested that NO is involved in relaxation of the digestive tract and affects feed passage in mammals. It is therefore possible that NO might be related to zymosan-induced reduction of feed passage in chicks. However, the role of NO on the effect of zymosan feed passage has not been clarified yet. Thus, the purpose of the present study was to investigate whether NO is associated with zymosan-induced alteration of feed passage in chicks. Intraperitoneal (IP) injection of zymosan significantly increased plasma nitrate and nitrite (NOx) concentrations at 6 h after injection. Zymosan-induced elevation of plasma NOx concentration was abolished by co-injection of S-methylisothiourea (SMT), a selective inhibitor for inducible NO synthase (iNOS), indicating that zymosan facilitated the induction of iNOS. Furthermore, because zymosan increased iNOS mRNA expression in the digestive tract, NO is likely associated with the effect of zymosan on the digestive tract. IP injection of NO donors significantly decreased crop emptying rate, suggesting that NO functions as an inhibitor of crop emptying. This result implied that zymosan stimulates NO production by the induction of iNOS in the digestive tract and thereby inhibits crop emptying rate. However, the co-injection of SMT did not attenuate the inhibitory effect of zymosan on crop emptying. The present study provides evidence that some changes in the digestive tract caused by zymosan are mediated by iNOS-induced NO in chicks, but NO does not mediate the effect of zymosan on feed passage through the crop.

Effect of zymosan on feed passage in the digestive tract in chicks

1. The purpose of the present study was to examine whether zymosan, which is a component of fungi, affects feed passage through the digestive tract in chicks (Gallus gallus).2. Intraperitoneal (IP) injection of 2.5 mg zymosan significantly reduced the crop-emptying rate and this effect was similar to that of 100 µg lipopolysaccharide (LPS). Zymosan affected phenol red transit from the proventriculus.3. Zymosan significantly affected the gene expression of interleukin-1β (IL-1β), IL-6, IL-8 and histidine decarboxylase in various regions of the digestive tract.4. The present study suggested that zymosan retarded feed passage through the digestive tract in chick and interleukins and histamine may be participating in this process.

FeTPPS Reduces Secondary Damage and Improves Neurobehavioral Functions after Traumatic Brain Injury

Traumatic brain injury (TBI) determinate a cascade of events that rapidly lead to neuron's damage and death. We already reported that administration of FeTPPS, a 5,10,15,20-tetrakis (4-sulfonatophenyl) porphyrin iron III chloride peroxynitrite decomposition catalyst, possessed evident neuroprotective effects in a experimental model of spinal cord damage. The present study evaluated the neuroprotective property of FeTPPS in TBI, using a clinically validated model of TBI, the controlled cortical impact injury (CCI). We observe that treatment with FeTPPS (30 mg/kg, i.p.) reduced: the state of brain inflammation and the tissue hurt (histological score), myeloperoxidase activity, nitric oxide production, glial fibrillary acidic protein (GFAP) and pro-inflammatory cytokines expression and apoptosis process. Moreover, treatment with FeTPPS re-established motor-cognitive function after CCI and it resulted in a reduction of lesion volumes. Our results established that FeTPPS treatment decreases the growth of inflammatory process and the tissue injury associated with TBI. Thus our study confirmed the neuroprotective role of FeTPPS treatment on TBI.

A role for melatonin in maintaining the pro- and anti-inflammatory balance by influencing leukocyte migration and apoptosis in carp

Melatonin is responsible for the synchronization of many physiological processes, including the immune response. Here we focus on the expression of melatonin MT1 receptors in/on leukocytes, and on the effects of melatonin administration on the inflammatory processes of carp. For the first time, we showed that fish leukocytes express MT1 receptors, implicating direct responsiveness to melatonin stimulation. Moreover, both in vitro and in vivo, melatonin modulated the immune response. The most potent effects of melatonin concerned the regulation of leukocyte migration. Melatonin reduced chemotaxis of leukocytes towards CXC chemokines in vitro. In vivo, during zymosan induced peritonitis, i.p. administration of melatonin reduced the number of neutrophils. This correlated with a melatonin-induced decrease of gene expression of the CXCa chemokine. Moreover, melatonin induced a decrease of the respiratory burst in inflammatory leukocytes. Although these data do suggest a potent anti-inflammatory function for this hormone, melatonin-induced inhibition of leukocyte apoptosis clearly indicates towards a dual function. These results show that also in carp, melatonin performs a pleiotropic and extra-pineal function that is important in maintaining the delicate pro- and anti-inflammatory balance during infection. They furthermore demonstrate that neuroendocrine-immune interaction via melatonin is evolutionary conserved.

Melatonin inhibits granulocyte adhesion to ICAM via MT3/QR2 and MT2 receptors

Neutrophils are cells of the innate immune system that first respond and arrive to the site of infection. Melatonin modulates acute inflammatory responses by interfering with leukocyte recruitment. It is known that melatonin modulates granulocyte migration though the endothelial layer thereby acting on the endothelial cell. Here we investigated whether melatonin could modulate granulocyte infiltration by acting directly on granulocytes. Granulocyte infiltration into the peritoneal cavity was investigated in mice kept at normal light/dark conditions and mice kept under constant lighting. To induce migration of neutrophils from the blood into the injury site via the endothelial layer, a bacterial product N-formyl-l-methionyl- l-leucyl- l-phenylalanine (fMLP) was injected into the peritoneal cavity. We found that the number of infiltrated granulocytes during the dark time was lower than that during the light time. It did not depend on circadian time. Moreover, the expression of an adhesion molecule, CD18, on granulocytes, was also lower during the dark time as compared with the light time. We have found that melatonin inhibited fMLP-induced CD18 up-regulation. Importantly, melatonin also inhibited the integrin-mediated granulocyte adhesion to intercellular adhesion molecule-coated plates. This study additionally showed that melatonin receptors MT2 and MT3/quinone reductase 2 (QR2) are expressed on granulocytes. Interestingly, melatonin increases the expression of its MT3/QR2 receptor. The fMLP-mediated CD18 up-regulation was inhibited by melatonin via MT2 receptor and the integrin-mediated granulocyte adhesion was inhibited by melatonin via MT3/QR2 and MT2 receptors. In conclusion, we show that melatonin suppresses granulocyte migration via endothelium by acting directly on granulocytes.

Fundamental issues related to the origin of melatonin and melatonin isomers during evolution: relation to their biological functions

Melatonin and melatonin isomers exist and/or coexist in living organisms including yeasts, bacteria and plants. The levels of melatonin isomers are significantly higher than that of melatonin in some plants and in several fermented products such as in wine and bread. Currently, there are no reports documenting the presence of melatonin isomers in vertebrates. From an evolutionary point of view, it is unlikely that melatonin isomers do not exist in vertebrates. On the other hand, large quantities of the microbial flora exist in the gut of the vertebrates. These microorganisms frequently exchange materials with the host. Melatonin isomers, which are produced by these organisms inevitably enter the host's system. The origins of melatonin and its isomers can be traced back to photosynthetic bacteria and other primitive unicellular organisms. Since some of these bacteria are believed to be the precursors of mitochondria and chloroplasts these cellular organelles may be the primary sites of melatonin production in animals or in plants, respectively. Phylogenic analysis based on its rate-limiting synthetic enzyme, serotonin N-acetyltransferase (SNAT), indicates its multiple origins during evolution. Therefore, it is likely that melatonin and its isomer are also present in the domain of archaea, which perhaps require these molecules to protect them against hostile environments including extremely high or low temperature. Evidence indicates that the initial and primary function of melatonin and its isomers was to serve as the first-line of defence against oxidative stress and all other functions were acquired during evolution either by the process of adoption or by the extension of its antioxidative capacity.

Melatonin: a well-documented antioxidant with conditional pro-oxidant actions

Melatonin (N-acetyl-5-methoxytryptamine), an indoleamine produced in many organs including the pineal gland, was initially characterized as a hormone primarily involved in circadian regulation of physiological and neuroendocrine function. Subsequent studies found that melatonin and its metabolic derivatives possess strong free radical scavenging properties. These metabolites are potent antioxidants against both ROS (reactive oxygen species) and RNS (reactive nitrogen species). The mechanisms by which melatonin and its metabolites protect against free radicals and oxidative stress include direct scavenging of radicals and radical products, induction of the expression of antioxidant enzymes, reduction of the activation of pro-oxidant enzymes, and maintenance of mitochondrial homeostasis. In both in vitro and in vivo studies, melatonin has been shown to reduce oxidative damage to lipids, proteins and DNA under a very wide set of conditions where toxic derivatives of oxygen are known to be produced. Although the vast majority of studies proved the antioxidant capacity of melatonin and its derivatives, a few studies using cultured cells found that melatonin promoted the generation of ROS at pharmacological concentrations (μm to mm range) in several tumor and nontumor cells; thus, melatonin functioned as a conditional pro-oxidant. Mechanistically, melatonin may stimulate ROS production through its interaction with calmodulin. Also, melatonin may interact with mitochondrial complex III or mitochondrial transition pore to promote ROS production. Whether melatonin functions as a pro-oxidant under in vivo conditions is not well documented; thus, whether the reported in vitro pro-oxidant actions come into play in live organisms remains to be established.

The role of melatonin in the cells of the innate immunity: a review

Melatonin is the major secretory product synthesized and secreted by the pineal gland and shows both a wide distribution within phylogenetically distant organisms from bacteria to humans and a great functional versatility. In recent years, a considerable amount of experimental evidence has accumulated showing a relationship between the nervous, endocrine, and immune systems. The molecular basis of the communication between these systems is the use of a common chemical language. In this framework, currently melatonin is considered one of the members of the neuroendocrine-immunological network. A number of in vivo and in vitro studies have documented that melatonin plays a fundamental role in neuroimmunomodulation. Based on the information published, it is clear that the majority of the present data in the literature relate to lymphocytes; thus, they have been rather thoroughly investigated, and several reviews have been published related to the mechanisms of action and the effects of melatonin on lymphocytes. However, few studies concerning the effects of melatonin on cells belonging to the innate immunity have been reported. Innate immunity provides the early line of defense against microbes and consists of both cellular and biochemical mechanisms. In this review, we have focused on the role of melatonin in the innate immunity. More specifically, we summarize the effects and action mechanisms of melatonin in the different cells that belong to or participate in the innate immunity, such as monocytes-macrophages, dendritic cells, neutrophils, eosinophils, basophils, mast cells, and natural killer cells.

Hepatoprotective actions of melatonin: Possible mediation by melatonin receptors

Melatonin, the hormone of darkness and messenger of the photoperiod, is also well known to exhibit strong direct and indirect antioxidant properties. Melatonin has previously been demonstrated to be a powerful organ protective substance in numerous models of injury; these beneficial effects have been attributed to the hormone’s intense radical scavenging capacity. The present report reviews the hepatoprotective potential of the pineal hormone in various models of oxidative stress in vivo, and summarizes the extensive literature showing that melatonin may be a suitable experimental substance to reduce liver damage after sepsis, hemorrhagic shock, ischemia/reperfusion, and in numerous models of toxic liver injury. Melatonin’s influence on hepatic antioxidant enzymes and other potentially relevant pathways, such as nitric oxide signaling, hepatic cytokine and heat shock protein expression, are evaluated. Based on recent literature demonstrating the functional relevance of melatonin receptor activation for hepatic organ protection, this article finally suggests that melatonin receptors could mediate the hepatoprotective actions of melatonin therapy.

The changing biological roles of melatonin during evolution: from an antioxidant to signals of darkness, sexual selection and fitness

Melatonin is a molecule present in a multitude of taxa and may be ubiquitous in organisms. It has been found in bacteria, unicellular eukaryotes, macroalgae, fungi, plants and animals. A primary biological function of melatonin in primitive unicellular organisms is in antioxidant defence to protect against toxic free radical damage. During evolution, melatonin has been adopted by multicellular organisms to perform many other biological functions. These functions likely include the chemical expression of darkness in vertebrates, environmental tolerance in fungi and plants, sexual signaling in birds and fish, seasonal reproductive regulation in photoperiodic mammals, and immunomodulation and anti-inflammatory activity in all vertebrates tested. Moreover, its waning production during aging may indicate senescence in terms of a bio-clock in many organisms. Conversely, high melatonin levels can serve as a signal of vitality and health. The multiple biological functions of melatonin can partially be attributed to its unconventional metabolism which is comprised of multi-enzymatic, pseudo-enzymatic and non-enzymatic pathways. As a result, several bioactive metabolites of melatonin are formed during its metabolism and some of the presumed biological functions of melatonin reported to date may, in fact, be mediated by these metabolites. The changing biological roles of melatonin seem to have evolved from its primary function as an antioxidant.

Chemotactic effect of melatonin on leukocytes

Melatonin seems to be an important stimulatory factor of the immune system. This indolamine is capable of inducing activation of leukocytes. Tissue leukocyte infiltration is a key feature of inflammatory and immune responses; however, there is no information about the effect of melatonin on leukocyte chemotaxis. Therefore, the aim of this study was to examine the in vitro and in vivo effects of melatonin on leukocyte chemotaxis, on modulation of leukocyte chemotaxis to other chemoattractants and on the in vivo induction of leukocyte chemokines. Neutrophils and mononuclear leukocytes (PBMC) were isolated by a discontinuous gradient on Hystopaque. Chemotaxis was performed in blind well Boyden's chambers. In vivo chemotaxis was determined after intraperitoneal injection of melatonin into rats. Leukocyte chemotactic response and leukocyte chemokine expression were determined in human volunteers treated with 20 mg daily of melatonin. Increased neutrophils and PBMC chemotaxis in response to 1.2 nm melatonin was observed in vitro. Peritoneal leukocytes were found increased after melatonin injection. Humans treated with melatonin showed an increased neutrophil chemotactic response to a physiological chemoattractant and increased expression of intracellular chemokines; however, decreased chemotactic response and no chemokine expression were observed in PBMC. These data suggest that melatonin could have a relevant role during the tissue leukocyte infiltration in inflammatory and immune responses.

Protective effect of Hypericum perforatum in zymosan-induced multiple organ dysfunction syndrome: Relationship to its inhibitory effect on nitric oxide production and its peroxynitrite scavenging activity

Hypericum perforatum is a medicinal plant species containing many polyphenolic compounds, namely flavonoids and phenolic acids. Since polyphenolic compounds have high antioxidant potential, we have investigated the effects of H. perforatum extract on the development of multiple organ dysfunction syndrome caused by zymosan (500 mg/kg, administered i.p. as a suspension in saline) in mice. Organ failure and systemic inflammation in rats was assessed 18 h after administration of zymosan and/or H. perforatum extract and monitored for 12 days (for loss of body weight and mortality). Treatment of mice with H. perforatum extract (30 mg/kg i.p., 1 and 6h after zymosan) attenuated the peritoneal exudation and the migration of polymorphonuclear cells caused by zymosan, pulmonary, intestinal and pancreatic injury, and renal dysfunction as well as the increase in myeloperoxidase in the lung and intestine. Immunohistochemical analysis for inducible nitric oxide synthase (iNOS), nitrotyrosine, and poly(ADP-ribose) (PAR) revealed positive staining in lung and intestine tissues obtained from zymosan-injected mice. The degree of staining for nitrotyrosine, iNOS, and PAR was markedly reduced in tissue sections obtained from zymosan-treated mice, which received H. perforatum extract. In conclusion, this study provides evidence, for the first time, that H. perforatum extract attenuates the degree of zymosan-induced multiple organ dysfunction syndrome in mice.

The immune-pineal axis: a shuttle between endocrine and paracrine melatonin sources

The time course of the innate immunological response involves a pro-inflammatory phase followed by an anti-inflammatory phase. Pro-inflammatory responses serve as a defense against several stressor conditions, and sequential processes that shut down these responses are necessary to avoid exacerbation or the development of chronic diseases. In the present review, we put together recent data that show that the pineal gland is a player in bidirectional control of the inflammatory response. Healthy organisms stay in standby mode, ready to react. The nocturnal melatonin surge impairs the rolling and adherence of leukocytes to endothelial layers, limiting cell migration, and stimulates nocturnal production of IL-2 by T helper lymphocytes, exerting an immunostimulatory effect. Otherwise, the release of TNF-alpha from activated macrophages suppresses the nocturnal melatonin surge, allowing a full cell migration and inhibiting IL-2 production. In sequence, activated mononuclear and polymorphonuclear cells produce melatonin in a paracrine manner at the site of injury, which scavenges free radicals and collaborates to resolve the inflammatory response. The sequential diminution of TNF-alpha production is followed by the recovery of the nocturnal melatonin surge and IL-2 production. In summary, the immune-pineal axis, implicated in the sequential involvement of the melatonin produced by the pineal gland and immune-competent cells, is an integral participant of the innate immune response.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Control of cyclooxygenase-2 transcriptional activation by pro-inflammatory mediators

Cyclooxygenase-2 (COX-2) plays a key role in diverse inflammatory conditions. Its cellular levels depend on transcriptional activation by pro-inflammatory mediators. The mechanism by which phorbol esters and cytokines activate COX-2 gene expression has been extensively characterized. Several endogenous molecules and natural products have been reported to inhibit COX-2 expression by targeting at the transcriptional activation induced by pro-inflammatory mediators. This review highlights the importance of C/EBP beta and NF-kappa B in COX-2 transcriptional activation by proinflammatory mediators and as targets of inhibition by endogenous molecules such as melatonin and natural products including salicylate and polyphenols.

Protective effect of melatonin on contractile activity and oxidative injury induced by ischemia and reperfusion of rat ileum

Free radicals derived from molecular oxygen have been reported to be responsible for changes in motility and mucosal damage observed in intestinal ischemia-reperfusion injury. Melatonin has been considered as an antioxidant that prevents injuries resulted from I/R in various tissues. The present study was designed to determine the effect of melatonin on the contractile responses of acetylcholine (Ach) and KCl, on malondialdehyde (MDA), a product of lipid peroxidation, and reduced glutathione (GSH) levels and to assess histopathological changes in the smooth muscle of terminal ileum subjected to ischemia-reperfusion. The intestinal ischemia-reperfusion was induced by occlusion of superior mesenteric artery of rat for 30 min, followed by a period of reperfusion for 3 h. Melatonin at doses of 10 or 50 mg/kg was administered via the tail vein in 5 min prior to reperfusion. Following reperfusion, segments of terminal ileum were rapidly taken and transferred into isolated organ bath and responses to Ach and KCl were recorded. Samples of terminal ileum were also taken for measuring the MDA and GSH levels. EC50 values of these contracting substances were seriously reduced in the ischemia-reperfusion group compared to that of the sham-operated control group. The decreased contraction response to Ach and KCl was significantly ameliorated by a dosage of 50 mg/kg of melatonin, while not by a dosage of 10 mg/kg. Similar pattern of the effect was observed in the tissue levels of MDA and GSH as well as in histological improvement. Melatonin appeared to be restoring the amounts of tissue MDA and GSH back to about control levels. These results suggest that the high dose of melatonin not only physiologically but also biochemically and morphologically could be useful to normalize contractility injured by oxidative stress in intestinal ischemia-reperfusion.

Neutrophils as a specific target for melatonin and kynuramines: effects on cytokine release

A growing body of evidence suggests that the pineal hormone, melatonin, has immunomodulatory properties, although very little is known about its effect on leukocytes. Therefore, we aimed to investigate the effect of melatonin and its oxidation product N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) on cytokine production by neutrophils and peripheral blood mononuclear cells (PBMCs). AFMK (0.001-1 mM) inhibits the lipopolysaccharide (LPS)-mediated production of tumor necrosis factor-alpha (TNF-alpha) and interleukin-8 (IL-8) more efficiently in neutrophils than PBMCs. Moreover, the inhibitory activity of AFMK is stronger than that of melatonin. Interestingly, monocytes efficiently oxidize melatonin to AFMK. We conclude that neutrophils are one of the main targets for melatonin and that at least part of the effects described for melatonin on immune cells may be due to its oxidation product, AFMK. We also consider that the oxidation of melatonin may be an important event in the cross-talking between neutrophils and monocytes.

Melatonin increases survival and inhibits oxidative and amyloid pathology in a transgenic model of Alzheimer's disease

Increased levels of a 40-42 amino-acid peptide called the amyloid beta protein (A beta) and evidence of oxidative damage are early neuropathological markers of Alzheimer's disease (AD). Previous investigations have demonstrated that melatonin is decreased during the aging process and that patients with AD have more profound reductions of this hormone. It has also been recently shown that melatonin protects neuronal cells from A beta-mediated oxidative damage and inhibits the formation of amyloid fibrils in vitro. However, a direct relationship between melatonin and the biochemical pathology of AD had not been demonstrated. We used a transgenic mouse model of Alzheimer's amyloidosis and monitored over time the effects of administering melatonin on brain levels of A beta, abnormal protein nitration, and survival of the mice. We report here that administration of melatonin partially inhibited the expected time-dependent elevation of beta-amyloid, reduced abnormal nitration of proteins, and increased survival in the treated transgenic mice. These findings may bear relevance to the pathogenesis and therapy of AD.

Melatonin reduces memory changes and neural oxidative damage in mice treated with D-galactose

To investigate the role of melatonin in D-galactose-induced amnesic mice, the avoidance/escape and water maze tests were performed to evaluate their learning and memory function. Spectrophotometry was employed to determine the content of thiobarbituric acid-reactive substances (TBARS) and the activities of antioxidative enzymes in the brain. The present results demonstrate that D-galactose-induced amnesic mice had significantly decreased learning and memory function. The reduced activities of superoxide dismutase and glutathione peroxidase and increased levels of TBARS were found in brain tissue of the amnesic mice. Melatonin, administered (ig) at doses of 0.1, 1, or 10 mg/kg to the D-galactose-treated mice for 3 months, was sufficient to block these changes. These data suggest that D-galactose is involved in accelerating the brain aging process by elevating free radical generation and reducing antioxidative enzyme activities in vivo. Furthermore, the antioxidative activity of melatonin on the D-galactose-treated mice may account for, at least partially, the improvement of learning and memory function in the aging and amnesic model.

Melatonin and N-acetylserotonin inhibit leukocyte rolling and adhesion to rat microcirculation

The hormone melatonin produced by the pineal gland during the daily dark phase regulates a variety of biological processes in mammals. The aim of this study was to determine the effect of melatonin and its precursor N-acetylserotonin on the microcirculation during acute inflammation. Arteriolar diameter, blood flow rate, leukocyte rolling and adhesion were measured in the rat microcirculation in situ by intravital microscopy. Melatonin alone or together with noradrenaline did not affect the arteriolar diameter or blood flow rate. Melatonin inhibited both leukocyte rolling and leukotriene B(4) induced adhesion while its precursor N-acetylserotonin inhibits only leukocyte adhesion. The rank order of potency of agonists and antagonist receptor selective ligands suggested that the activation of MT(2) and MT(3) melatonin binding sites receptors modulate leukocyte rolling and adhesion, respectively. The effect of melatonin and N-acetylserotonin herein described were observed with concentrations in the range of the nocturnal surge, providing the first evidence for a possible physiological role of these hormones in acute inflammation.

Pharmacological action of melatonin in shock, inflammation and ischemia/reperfusion injury

A vast amount of circumstantial evidence implicates oxygen-derived free radicals (especially, superoxide and hydroxyl radical) and high-energy oxidants (such as peroxynitrite) as mediators of inflammation, shock and ischemia/reperfusion injury. The aim of this review is to describe recent developments in the field of oxidative stress research. The first part of the review focuses on the roles of reactive oxygen species in shock, inflammation and ischemia/reperfusion injury. The second part of the review described the pharmacological action of melatonin in shock, ischemia/reperfusion, and inflammation. The role of reactive oxygen species: Immunohistochemical and biochemical evidence demonstrate the production of reactive oxygen species in shock, inflammation and ischemia/reperfusion injury. Reactive oxygen species can initiate a wide range of toxic oxidative reactions. These include the initiation of lipid peroxidation, direct inhibition of mitochondrial respiratory chain enzymes, inactivation of glyceraldehyde-3 phosphate dehydrogenase, inhibition of membrane sodium/potassium ATP-ase activity, inactivation of membrane sodium channels, and other oxidative modifications of proteins. All these toxicities are likely to play a role in the pathophysiology of shock, inflammation and ischemia and reperfusion. Treatment with melatonin has been shown to prevent in vivo the delayed vascular decompensation and the cellular energetic failure associated with shock, inflammation and ischemia/reperfusion injury. Reactive oxygen species (e.g., superoxide, peroxynitrite, hydroxyl radical and hydrogen peroxide) are all potential reactants capable of initiating DNA single-strand breakage, with subsequent activation of the nuclear enzyme poly (ADP-ribose) synthetase (PARS), leading to eventual severe energy depletion of the cells, and necrotic-type cell death. Recently, it has been demonstrated that melatonin inhibits the activation of poly (ADP-ribose) synthetase, and prevents the organ injury associated with shock, inflammation and ischemia and reperfusion.

Tyrosine nitration: localisation, quantification, consequences for protein function and signal transduction

The nitration of free tyrosine or protein tyrosine residues generates 3-nitrotyrosine the detection of which has been utilised as a footprint for the in vivo formation of peroxynitrite and other reactive nitrogen species. The detection of 3-nitrotyrosine by analytical and immunological techniques has established that tyrosine nitration occurs under physiological conditions and levels increase in most disease states. This review provides an updated, comprehensive and detailed summary of the tissue, cellular and specific protein localisation of 3-nitrotyrosine and its quantification. The potential consequences of nitration to protein function and the pathogenesis of disease are also examined together with the possible effects of protein nitration on signal transduction pathways and on the metabolism of proteins.

Melatonin and its relation to the immune system and inflammation

Melatonin (N-acetyl-5-methoxytryptamine) was initially thought to be produced exclusively in the pineal gland. Subsequently its synthesis was demonstrated in other organs, for example, the retinas, and very high concentrations of melatonin are found at other sites, for example, bone marrow cells and bile. The origin of the high level of melatonin in these locations has not been definitively established, but it is likely not exclusively of pineal origin. Melatonin has been shown to possess anti-inflammatory effects, among a number of actions. Melatonin reduces tissue destruction during inflammatory reactions by a number of means. Thus melatonin, by virtue of its ability to directly scavenge toxic free radicals, reduces macromolecular damage in all organs. The free radicals and reactive oxygen and nitrogen species known to be scavenged by melatonin include the highly toxic hydroxyl radical (.OH), peroxynitrite anion (ONOO-), and hypochlorous acid (HOCl), among others. These agents all contribute to the inflammatory response and associated tissue destruction. Additionally, melatonin has other means to lower the damage resulting from inflammation. Thus, it prevents the translocation of nuclear factor-kappa B (NF-kappa B) to the nucleus and its binding to DNA, thereby reducing the upregulation of a variety of proinflammatory cytokines, for example, interleukins and tumor neurosis factor-alpha. Finally, there is indirect evidence that melatonin inhibits the production of adhesion molecules that promote the sticking of leukocytes to endothelial cells. By this means melatonin attenuates transendothelial cell migration and edema, which contribute to tissue damage.

Mechanism of reaction of melatonin with human myeloperoxidase

Recently, it was suggested that melatonin (N-acetyl-5-methoxytryptamine) is oxidized by activated neutrophils in a reaction most probably involving myeloperoxidase (Biochem. Biophys. Res. Commun. (2000) 279, 657-662). Myeloperoxidase (MPO) is the most abundant protein of neutrophils and is involved in killing invading pathogens. To clarify if melatonin is a substrate of MPO, we investigated the oxidation of melatonin by its redox intermediates compounds I and II using transient-state spectral and kinetic measurements at 25 degrees C. Spectral and kinetic analysis revealed that both compound I and compound II oxidize melatonin via one-electron processes. The second-order rate constant measured for compound I reduction at pH 7 and pH 5 are (6.1 +/- 0.2) x 10(6) M(-1) s(-1) and (1.0 +/- 0.08) x 10(7) M(-1) s(-1), respectively. The rates for the one-electron reduction of compound II back to the ferric enzyme are (9.6 +/- 0.3) x 10(2) M(-1) s(-1) (pH 7) and (2.2 +/- 0.1) x 10(3) M(-1) s(-1) (pH 5). Thus, melatonin is a much better electron donor for compound I than for compound II. Steady-state experiments showed that the rate of oxidation of melatonin is dependent on the H(2)O(2) concentration, is not affected by superoxide dismutase, and is quickly terminated by sodium cyanide. Melatonin can markedly inhibit the chlorinating activity of MPO at both pH 7 and pH 5. The implication of these findings in the activated neutrophil is discussed.

Actions of melatonin in the reduction of oxidative stress. A review

Melatonin was discovered to be a direct free radical scavenger less than 10 years ago. Besides its ability to directly neutralize a number of free radicals and reactive oxygen and nitrogen species, it stimulates several antioxidative enzymes which increase its efficiency as an antioxidant. In terms of direct free radical scavenging, melatonin interacts with the highly toxic hydroxyl radical with a rate constant equivalent to that of other highly efficient hydroxyl radical scavengers. Additionally, melatonin reportedly neutralizes hydrogen peroxide, singlet oxygen, peroxynitrite anion, nitric oxide and hypochlorous acid. The following antioxidative enzymes are also stimulated by melatonin: superoxide dismutase, glutathione peroxidase and glutathione reductase. Melatonin has been widely used as a protective agent against a wide variety of processes and agents that damage tissues via free radical mechanisms.

Beneficial effects of n-acetylcysteine on ischaemic brain injury

1. Nitric oxide (NO), peroxynitrite, formed from NO and superoxide anion, poly (ADP-ribole) synthetase have been implicated as mediators of neuronal damage following focal ischaemia. Here we have investigated the effects of n-acetylcysteine (NAC) treatment in Mongolian gerbils subjected to cerebral ischaemia. 2. Treatment of gerbils with NAC (20 mg kg(-1) 30 min before reperfusion and 1, 2 and 6 h after reperfusion) reduced the formation of post-ischaemic brain oedema, evaluated by water content. 3. NAC also attenuated the increase in the brain levels of malondialdehyde (MDA) and the increase in the hippocampus of myeloperoxidase (MPO) caused by cerebral ischaemia. 4. Positive staining for nitrotyrosine was found in the hippocampus in Mongolian gerbils subjected to cerebral ischaemia. Hippocampus tissue sections from Mongolian gerbils subjected to cerebral ischaemia also showed positive staining for poly (ADP-ribose) synthetase (PARS). The degree of staining for nitrotyrosine and for PARS were markedly reduced in tissue sections obtained from animals that received NAC. 5. NAC treatment increased survival and reduced hyperactivity linked to neurodegeneration induced by cerebral ischaemia and reperfusion. 6. Histological observations of the pyramidal layer of CA1 showed a reduction of neuronal loss in animals that received NAC. 7. These results show that NAC improves brain injury induced by transient cerebral ischaemia.

Melatonin as a pharmacological agent against neuronal loss in experimental models of Huntington's disease, Alzheimer's disease and parkinsonism

This review summarizes the experimental findings related to the neuroprotective role of melatonin. In particular, it focuses on research directed at models of Huntington's disease, Alzheimer's disease and Parkinsonism. Melatonin has been shown to be highly effective in reducing oxidative damage in the central nervous system; this efficacy derives from its ability to directly scavenge a number of free radicals and to function as an indirect antioxidant. In particular, melatonin detoxifies the highly toxic hydroxyl radical as well as the peroxyl radical, peroxynitrite anion, nitric oxide, and singlet oxygen, all of which can damage macromolecules in brain cells. Additionally, melatonin stimulates a variety of antioxidative enzymes including superoxide dismutase, glutathione peroxidase and glutathione reductase. One additional advantage melatonin has in reducing oxidative damage in the central nervous system is the ease with which to crosses the blood-brain barrier. This combination of actions makes melatonin a highly effective pharmacological agent against free radical damage. The role of physiological levels of melatonin in forestalling oxidative damage in the brain is currently being tested.

The effect of melatonin on liver superoxide dismutase activity, serum nitrate and thyroid hormone levels

Melatonin is a main neurohormone of the pineal gland. The effects of melatonin on the level of serum thyroid-stimulating hormone (TSH), thyroxine (T(4)), triiodothyronine (T(3)), nitrate, melatonin and liver superoxide dismutase (SOD) activity were examined in rats. Melatonin was injected at the dose of 10 mg/kg for 7 days, 2 h before turning the lights off. Rats were decapitated at 10:00 a.m. and 02:00 a.m., which are the times of the lowest and highest serum melatonin levels, respectively. Blood and tissue samples were collected. Decreased TSH, T(3), T(4) and nitrate levels were determined in the melatonin-injected and nighttime groups. Melatonin levels showed a diurnal rhythm. SOD activity increased in the melatonin-treated group. The results demonstrate that increased SOD activity, and reduced serum TSH, T(3), T(4) and nitrate levels correlated with the serum melatonin levels.

Beneficial effects of melatonin in a rat model of splanchnic artery occlusion and reperfusion

The aim of the present study was to investigate the protective effect of the pineal secretary product melatonin in a model of splanchnic artery occlusion shock (SAO). SAO shock was induced in rats by clamping both the superior mesenteric artery and the celiac trunk for 45 min, followed thereafter by release of the clamp (reperfusion). At 60 min after reperfusion, animals were sacrificed for tissue histological examination and biochemical studies. There was a marked increase in the oxidation of dihydrorhodamine 123 to rhodamine (a marker of peroxynitrite-induced oxidative processes) in the plasma of the SAO-shocked rats after reperfusion, but not during ischemia alone. Immunohistochemical examination demonstrated a marked increase in the immunoreactivity to nitrotyrosine, an index of nitrogen species such as peroxynitrite, in the necrotic ileum in shocked rats. SAO-shocked rats developed a significant increase of tissue myeloperoxidase and malondialdehyde activity, and marked histological injury to the distal ileum. SAO shock was also associated with a significant mortality (0% survival at 2 hr after reperfusion). Reperfused ileum tissue sections from SAO-shocked rats showed positive staining for P-selectin, which was mainly localized in the vascular endothelial cells. Ileum tissue sections obtained from SAO-shocked rats with anti-intercellular adhesion molecule (ICAM-1) antibody showed a diffuse staining. Melatonin (applied at 3 mg/kg, 5 min prior to reperfusion, followed by an infusion of 3 mg/kg per hr), significantly reduced ischemia reperfusion injury in the bowel as evaluated by histological examination. This prevented the infiltration of neutrophils into the reperfused intestine, is evidenced by reduced myeloperoxidase activity and reduced lipid peroxidation. This was evaluated by malondialdehyde activity which reduced the production of peroxynitrite during reperfusion, markedly reduced the intensity and degree of P-selectin and ICAM-1 in tissue section from SAO-shocked rats and improved their survival. Taken together, our results clearly demonstrate that melatonin treatment exerts a protective effect and part of this effect may be due to inhibition of the expression of adhesion molecule and peroxynitrite-related pathways and subsequent reduction of neutrophil-mediated cellular injury.

The protective role of endogenous melatonin in carrageenan-induced pleurisy in the rat

Peroxynitrite, a potent cytotoxic oxidant formed by the reaction of nitric oxide (NO) with the superoxide anion, was recently proposed to play a major pathogenic role in the inflammatory process. Here we have investigated the effects of endogenous melatonin, a known scavenger of peroxynitrite, in rats subjected to carrageenan-induced pleurisy. Endogenous melatonin was depleted in rats maintained on 24 h light cycle for 1 wk. In vivo depletion of endogenous melatonin enhanced the carrageenan-induced degree of pleural exudation and polymorphonuclear leukocyte migration in rats subjected to carrageenan-induced pleurisy. Lung myeloperoxidase activity and lipid peroxidation were significantly increased in melatonin-deprived rats. However, the inducible NO synthase in lung samples was unaffected by melatonin depletion. Immunohistochemical analysis for nitrotyrosine revealed a positive staining in lungs from carrageenan-treated rats that was markedly enhanced in melatonin-deprived rats. Furthermore, melatonin depletion significantly increased peroxynitrite formation as measured by the oxidation of the fluorescent dye dihydrorhodamine 123, enhanced DNA damage and the decrease in mitochondrial respiration and reduced the cellular levels of NAD+ in macrophages harvested from the pleural cavity of rats subjected to carrageenan-induced pleurisy. In vivo treatment with exogenous melatonin (15 mg/kg intraperitoneal) significantly reversed the effects of melatonin depletion. Thus, endogenous melatonin plays an important protective role against carrageenan-induced local inflammation.

Augmentation of indices of oxidative damage in life-long melatonin-deficient rats

The chief pineal secretory product, melatonin, is an efficient free radical scavenger and antioxidant. The current study tested whether the life-long reduction of endogenous melatonin levels due to pinealectomy would influence the accumulation of oxidatively damaged products as the animals aged. Rats were either pinealectomized or sham operated when they were 2-months-old. At 25 months of age these animals were killed along with 2-month-old controls. Aging in the pineal-intact animals was associated with increased levels of lipid peroxidation products (malondialdehyde and 4-hydroxyalkenals in the lung, kidney and skin), rises in an oxidatively damaged DNA product (8-hydroxy-deoxyguanosine in liver, kidney and pancreas), and in the levels of protein carbonyls (in the liver). Likewise, advanced age was associated with a significant decrease in membrane fluidity (increased membrane rigidity) of hepatic microsomes in pineal-intact rats. For all of these parameters and in a number of organs, pinealectomy caused further increases in the indices of oxidative damage. Consistent with previous suggestions, the implications of these findings is that aging is associated with the augmented accumulation of oxidatively damaged macromolecules and that these increases are exaggerated when a relative melatonin deficiency is induced by pinealectomy. The findings are consistent with the idea that the accelerated accumulation of oxidatively damaged products after pinealectomy was due to reduction in melatonin since it functions as a free radical scavenger and antioxidant. On the other hand, other pineal secretory products that were reduced as a consequence of pineal removal may have also been responsible for some of the observed changes.

Pineal indoleamines and vitamin E reduce nitric oxide-induced lipid peroxidation in rat retinal homogenates

Oxidative damage to retinal cell membranes can lead to sight-threatening ocular diseases. Pineal indoleamines are naturally located and synthesized in the retina, and they possibly protect the retina from oxidative cell damage. In this study, we compared the efficacy of three different pineal indoleamines (melatonin, N-acetylserotonin, and pinoline) with vitamin E, a well-known antioxidant, against nitric oxide (NO)-induced lipid peroxidation (LPO) in rat retinal homogenates. The possible synergistic effect of these agents was also studied. Retinal homogenates were incubated with sodium nitroprusside, which releases NO*. The LPO product, malondialdehyde (MDA), provided an index of cell damage. The results show that vitamin E and indoleamines significantly reduced MDA levels in a dose-dependent manner. When vitamin E was combined with the indoleamines, the protection was synergistically enhanced. In summary, under conditions where cellular homogenates are used (a) vitamin E and the three pineal indoleamines protected the retinal cells from NO-induced LPO damage; (b) the efficacies of each of these compounds had the following relationships: vitamin E > N-acetylserotonin > pinoline > melatonin; (c) vitamin E acted synergistically with indoleamines in combating oxidative retinal damage. Whether these same associations would exist in vivo after treatment with these compounds is unknown. The pharmacological potential of indoleamines, possibly in combination with vitamin E, in preventing retinal pathogenesis deserves further investigation.

Physiological levels of melatonin contribute to the antioxidant capacity of human serum

This work evaluates whether physiological concentrations of the pineal secretory product melatonin contribute to the total antioxidant status (TAS) of human serum. Day and nighttime serum samples were collected from healthy volunteers ranging from 2 to 89 years of age and used to measure melatonin and TAS. Results showed that both melatonin and TAS in human serum exhibited 24 hr variations with nocturnal peak values at 01:00 hr. Moreover, exposure of volunteers to light at night resulted in clear decreases of both TAS and melatonin. Furthermore, when melatonin was removed from sera collected at night, the TAS value of the sample was reduced to basal daytime values. In aging studies, it was found that nocturnal serum values of TAS and melatonin exhibited maximal values during the first four decades; thereafter, these values decreased as age advanced. In 60-year-old individuals, day/night differences in serum melatonin and TAS levels were clearly diminished, by more than 80%, with these differences being completely abolished in older individuals. Our results suggest that melatonin contributes to the total antioxidative capability of human serum. This antioxidant contribution of melatonin is reduced as age advances correlating with the age-related reduction of melatonin.

Regulation of prostaglandin production in carrageenan-induced pleurisy by melatonin

The aim of the present study was to investigate the effect of melatonin on the production of the inflammatory mediator prostaglandins in a model of acute inflammation, carrageenan-induced pleurisy, where prostaglandins are known to play a crucial role. The results show that melatonin (12.5, 25, and 50 mg/kg, intraperitoneally, 15 min before the carrageenan) inhibits the inflammatory response (pleural exudate formation, polymorphonuclear cell infiltration, and prostaglandin production) in a dose-dependent manner. Immunohistochemical examination demonstrated a marked increase in the immunoreactivity to cyclooxygenase-2 (COX-2) in the lungs of carrageenan-treated rats. Pretreatment of carrageenan-treated rats with melatonin (50 mg/kg, intraperitoneally, 15 min before the carrageenan) prevented COX-2 expression. The present results demonstrate that melatonin exerts potent anti-inflammatory effects. Part of these anti-inflammatory effects may be related to a reduction of prostaglandin production during the inflammatory process.

Pharmacological aspects of N-acetyl-5-methoxytryptamine (melatonin) and 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (pinoline) as antioxidants: reduction of oxidative damage in brain region homogenates

Oxygen consumption is a necessity for all aerobic organisms, but oxygen is also a toxic molecule that leads to the generation of free radicals. The brain consumes a high percentage of the oxygen inhaled (18.5%), and it contains large amounts of unsaturated fatty acids, which makes it highly susceptible to lipid peroxidation. Melatonin (N-acetyl-5-methoxytryptamine), the main secretory product of the pineal gland, is a free radical scavenger that was found to protect against lipid peroxidation in many experimental models. Another compound found in the pineal gland is pinoline (6-methoxy-1,2,3,4-tetrahydro-beta-carboline). Pinoline is structurally related to melatonin. Evidence suggests that pinoline may have an antioxidant capacity similar to that of melatonin. In this study, the ability of pinoline to protect against H2O2-induced lipid peroxidation of different rat brain homogenates (frontal cortex, striatum, cerebellum, hippocampus, and hypothalamus) was investigated. The degree of lipid peroxidation was assessed by estimating the levels of thiobarbituric acid reactive substances, malondialdehyde (MDA) and 4-hydroxyalkenals (4-HDA). Pinoline's antioxidant capacity was compared with that of melatonin. Both melatonin and pinoline reduced the level of MDA and 4-HDA in a dose-dependent manner in all brain regions tested. To compare the antioxidant capacities, percent-inhibition curves were created, and the IC50 values were calculated. The IC50 values for melatonin were higher in all brain regions than were those for pinoline. The IC50 values for melatonin in the five different brain regions ranged from 0.16 mM-0.66 mM, and for pinoline, they ranged from 0.04 mM-0.13 mM. The possibility of synergistic interactions between melatonin and pinoline were also determined using the method of Berenbaum. Little evidence for either synergistic, additive, or antagonistic interactions between melatonin and pinoline was found.

Protective effects of melatonin in zymosan-activated plasma-induced paw inflammation

The aim of the present study was to investigate the protective effect of the pineal hormone melatonin in a model of acute local inflammation (zymosan-activated plasma-induced paw oedema), in which oxyradicals, nitric oxide (NO) and peroxynitrite are known to play a crucial role in the inflammatory process. The intraplantar injection of zymosan-activated plasma elicited an inflammatory response that was characterized by a time-dependent increase in paw oedema, neutrophil infiltration and increased levels of nitrite/nitrate in the paw exudate. The maximal increase in paw volume was observed at 3 h after administration (maximal in paw volume: 1.34 +/- 0.09 ml). At this time point, myeloperoxidase activity and lipid peroxidation were markedly increased in the zymosan-activated plasma-treated paw (226 +/- 10.2 mU/100 mg wet tissue, 31 +/- 2.1 mM/mg wet tissue, respectively). However, zymosan-activated plasma-induced paw oedema was significantly reduced in a dose-dependent manner by treatment with melatonin (given at 62.5 and 125 microg/paw) at 1, 2, 3, 4 h after injection of zymosan-activated plasma. Melatonin treatment also caused a significant reduction of the myeloperoxidase activity and lipid peroxidation and inhibited nitrite/nitrate levels in the paw exudate. The paw tissues were also examined immunohistochemically for the presence of nitrotyrosine (a marker of peroxynitrite formation). At 3 h following injection of zymosan-activated plasma, staining for nitrotyrosine was also found to be localised in the inflamed paw tissue. Treatment with melatonin (125 microg/paw) reduced the appearance of nitrotyrosine in the tissues. Our findings support the view that melatonin exerts anti-inflammatory effects.

Pathophysiology of nitric oxide and related species: free radical reactions and modification of biomolecules

Since its initial discovery as an endogenously produced bioactive mediator, nitric oxide (.NO) has been found to play a critical role in the cellular function of nearly all organ systems. Furthermore, aberrant production of .NO or reactive nitrogen species (RNS) derived from .NO, has been implicated in a number of pathological conditions, such as acute lung disease, atherosclerosis and septic shock. While .NO itself is fairly non-toxic, secondary RNS are oxidants and nitrating agents that can modify both the structure and function of numerous biomolecules both in vitro, and in vivo. The mechanisms by which RNS mediate toxicity are largely dictated by its unique reactivity. The study of how reactive nitrogen species (RNS) derived from .NO interact with biomolecules such as proteins, carbohydrates and lipids, to modify both their structure and function is an area of active research, which is lending major new insights into the mechanisms underlying their pathophysiological role in human disease. In the context of .NO-dependent pathophysiology, these biochemical reactions will play a major role since they: (i) lead to removal of .NO and decreased efficiency of .NO as an endothelial-derived relaxation factor (e.g. in hypertension, atherosclerosis) and (ii) lead to production of other intermediate species and covalently modified biomolecules that cause injury and cellular dysfunction during inflammation. Although the physical and chemical properties of .NO and .NO-derived RNS are well characterised, extrapolating this fundamental knowledge to a complicated biological environment is a current challenge for researchers in the field of .NO and free radical research. In this review, we describe the impact of .NO and .NO-derived RNS on biological processes primarily from a biochemical standpoint. In this way, it is our intention to outline the most pertinent and relevant reactions of RNS, as they apply to a diverse array of pathophysiological states. Since reactions of RNS in vivo are likely to be vast and complex, our aim in this review is threefold: (i) address the major sources and reactions of .NO-derived RNS in biological systems, (ii) describe current knowledge regarding the functional consequences underlying .NO-dependent covalent modification of specific biomolecules, and (iii) to summarise and critically evaluate the available evidence implicating these reactions in human pathology. To this end, three areas of special interest have been chosen for detailed description, namely, formation and role of S-nitrosothiols, modulation of lipid oxidation/nitration by RNS, and tyrosine nitration mechanisms and consequences.