Structure-activity relationships in a series of melatonin analogues with the low-density lipoprotein oxidation model

Potentiating the Benefits of Melatonin through Chemical Functionalization: Possible Impact on Multifactorial Neurodegenerative Disorders

Although melatonin is an astonishing molecule, it is possible that chemistry will help in the discovery of new compounds derived from it that may exceed our expectations regarding antioxidant protection and perhaps even neuroprotection. This review briefly summarizes the significant amount of data gathered to date regarding the multiple health benefits of melatonin and related compounds. This review also highlights some of the most recent directions in the discovery of multifunctional pharmaceuticals intended to act as one-molecule multiple-target drugs with potential use in multifactorial diseases, including neurodegenerative disorders. Herein, we discuss the beneficial activities of melatonin derivatives reported to date, in addition to computational strategies to rationally design new derivatives by functionalization of the melatonin molecular framework. It is hoped that this review will promote more investigations on the subject from both experimental and theoretical perspectives.

Effect of N-Amide Substitution on Antioxidative Activities of Melatonin Derivatives

Five N-amide substituted melatonin (MLT) derivatives were synthesized and evaluated for antioxidative activities, and compounds 9–12 showed higher electron spin resonance (ESR) response than MLT. 4-Bromobenzoyl and naphthoyl derivatives (10 and 11) presented stronger hydroxyl radical inhibitory effect than MLT in Fenton reaction. The substitution at the N1-position on the MLT core structure with acetyl (8), benzoyl (9), 4-bromobenzoyl (10), and naphthoyl (11) and N2-substitution with 4-bromobenzoyl (12) decreased the reducing power of the derivatives in ferric reducing antioxidant power (FRAP) assay. Compounds 8–11 also presented lower antioxidant capacity than their parent compound in 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) disodium salt (ABTS) assay; whereas, compound 12 presented radical scavenging activity similarly to MLT. All aryl derivatives (9–12) showed higher ability to quench peroxyl radicals than MLT about three times, especially the benzoylated derivatives (9 and 10) that presented the highest ability in oxygen radical absorbance capacity (ORAC) assay.

Melatonin and its metabolites vs oxidative stress: From individual actions to collective protection

Oxidative stress (OS) represents a threat to the chemical integrity of biomolecules including lipids, proteins, and DNA. The associated molecular damage frequently results in serious health issues, which justifies our concern about this phenomenon. In addition to enzymatic defense mechanisms, there are compounds (usually referred to as antioxidants) that offer chemical protection against oxidative events. Among them, melatonin and its metabolites constitute a particularly efficient chemical family. They offer protection against OS as individual chemical entities through a wide variety of mechanisms including electron transfer, hydrogen transfer, radical adduct formation, and metal chelation, and by repairing biological targets. In fact, many of them including melatonin can be classified as multipurpose antioxidants. However, what seems to be unique to the melatonin's family is their collective effects. Because the members of this family are metabolically related, most of them are expected to be present in living organisms wherever melatonin is produced. Therefore, the protection exerted by melatonin against OS may be viewed as a result of the combined antioxidant effects of the parent molecule and its metabolites. Melatonin's family is rather exceptional in this regard, offering versatile and collective antioxidant protection against OS. It certainly seems that melatonin is one of the best nature's defenses against oxidative damage.

Melatonin: A Versatile Protector against Oxidative DNA Damage

Oxidative damage to DNA has important implications for human health and has been identified as a key factor in the onset and development of numerous diseases. Thus, it is evident that preventing DNA from oxidative damage is crucial for humans and for any living organism. Melatonin is an astonishingly versatile molecule in this context. It can offer both direct and indirect protection against a wide variety of damaging agents and through multiple pathways, which may (or may not) take place simultaneously. They include direct antioxidative protection, which is mediated by melatonin's free radical scavenging activity, and also indirect ways of action. The latter include, at least: (i) inhibition of metal-induced DNA damage; (ii) protection against non-radical triggers of oxidative DNA damage; (iii) continuous protection after being metabolized; (iv) activation of antioxidative enzymes; (v) inhibition of pro-oxidative enzymes; and (vi) boosting of the DNA repair machinery. The rather unique capability of melatonin to exhibit multiple neutralizing actions against diverse threatening factors, together with its low toxicity and its ability to cross biological barriers, are all significant to its efficiency for preventing oxidative damage to DNA.

Protective effects of 2,3-diaryl-substituted indole-based cyclooxygenase-2 inhibitors on oxidative modification of human low density lipoproteins in vitro

It has been suggested that 2,3-diaryl-substituted indole-based cyclooxygenase-2 (COX-2) inhibitors (2,3-diaryl-indole coxibs) do not only appear as potent anti-inflammatory agents but also show the ability to scavenge reactive oxygen species (ROS). This led to the hypothesis that 2,3-diaryl-indole coxibs also may act as potent inhibitors of oxidative modification of low-density lipoprotein (LDL), which is considered a key factor in atherogenesis. The aim of this study was to explore i) the reactivity of a series of new synthesized 2,3-diaryl-indoles with several well characterized LDL oxidation systems and ii) subsequent effects on an inflammatory/atherogenic microenvironment. The results demonstrate that under the present experimental conditions 2,3-diaryl-indoles showed potent ROS scavenging activity and were able to markedly inhibit LDL oxidation. Subsequently, this led to a substantial decrease of modified LDL uptake by scavenger receptors in THP-1 macrophages in vitro and in rats in vivo. Moreover, modified LDL-mediated monocyte/neutrophil adhesion to endothelial cells, macrophage NFκB activation, as well as macrophage and endothelial cell cytokine release was diminished in vitro. The reduction of modified LDL-induced atherogenic effects by antioxidant 2,3-diaryl-indole coxibs may widen the therapeutic window of COX-2 targeted treatment.

Phenolic Melatonin-Related Compounds: Their Role as Chemical Protectors against Oxidative Stress

There is currently no doubt about the serious threat that oxidative stress (OS) poses to human health. Therefore, a crucial strategy to maintain a good health status is to identify molecules capable of offering protection against OS through chemical routes. Based on the known efficiency of the phenolic and melatonin (MLT) families of compounds as antioxidants, it is logical to assume that phenolic MLT-related compounds should be (at least) equally efficient. Unfortunately, they have been less investigated than phenols, MLT and its non-phenolic metabolites in this context. The evidence reviewed here strongly suggests that MLT phenolic derivatives can act as both primary and secondary antioxidants, exerting their protection through diverse chemical routes. They all seem to be better free radical scavengers than MLT and Trolox, while some of them also surpass ascorbic acid and resveratrol. However, there are still many aspects that deserve further investigations for this kind of compounds.

Distinct roles of N-acetyl and 5-methoxy groups in the antiproliferative and neuroprotective effects of melatonin

Melatonin (N-acetyl-5-methoxytryptamine) is a highly pleiotropic hormone with antioxidant, antiproliferative, oncolytic and neuroprotective properties. Here, we present evidence that the N-acetyl side chain plays a key role in melatonin's antiproliferative effect in HT22 and sw-1353 cells, but it does so at the expense of antioxidant and neuroprotective properties. Removal of the N-acetyl group enhances the antioxidant and neuroprotective properties of the indole, but it can lead to toxic methamphetamine-like effects in several cell lines. Inhibition of NFkB mimicked melatonin's antiproliferative and antioxidant effects, but not neuroprotection. Our results strongly suggest that neuroprotective and antiproliferative effects of melatonin rely on different parts of the molecule and are likely mediated by different mechanisms. We also predict that melatonin metabolism by target cells could determine whether melatonin inhibits cell proliferation, prevents toxicity or induces cell death (e.g. apoptosis or autophagy). These observations could have important implications for the rational use of melatonin in personalized medicine.

Development of Antioxidant COX-2 Inhibitors as Radioprotective Agents for Radiation Therapy-A Hypothesis-Driven Review

Radiation therapy (RT) evolved to be a primary treatment modality for cancer patients. Unfortunately, the cure or relief of symptoms is still accompanied by radiation-induced side effects with severe acute and late pathophysiological consequences. Inhibitors of cyclooxygenase-2 (COX-2) are potentially useful in this regard because radioprotection of normal tissue and/or radiosensitizing effects on tumor tissue have been described for several compounds of this structurally diverse class. This review aims to substantiate the hypothesis that antioxidant COX-2 inhibitors are promising radioprotectants because of intercepting radiation-induced oxidative stress and inflammation in normal tissue, especially the vascular system. For this, literature reporting on COX inhibitors exerting radioprotective and/or radiosensitizing action as well as on antioxidant COX inhibitors will be reviewed comprehensively with the aim to find cross-points of both and, by that, stimulate further research in the field of radioprotective agents.

Regulatory roles of serotonin and melatonin in abiotic stress tolerance in plants

Understanding the physiological and biochemical basis of abiotic stress tolerance in plants has always been one of the major aspects of research aiming to enhance plant productivity in arid and semi-arid cultivated lands all over the world. Growth of stress-tolerant transgenic crops and associated agricultural benefits through increased productivity, and related ethical issues, are also the major concerns of current research in various laboratories. Interesting data on the regulation of abiotic stress tolerance in plants by serotonin and melatonin has accumulated in the recent past. These two indoleamines possess antioxidative and growth-inducing properties, thus proving beneficial for stress acclimatization. Present review shall focus on the modes of serotonin and melatonin-induced regulation of abiotic stress tolerance in plants. Complex molecular interactions of serotonin and auxin-responsive genes have suggested their antagonistic nature. Data from genomic and metabolomic analyses of melatonin-induced abiotic stress signaling have lead to an understanding of the regulation of stress tolerance through the modulation of transcription factors, enzymes and various signaling molecules. Melatonin, nitric oxide (NO) and calmodulin interactions have provided new avenues for research on the molecular aspects of stress physiology in plants. Investigations on the characterization of receptors associated with serotonin and melatonin responses, are yet to be undertaken in plants. Patenting of biotechnological inventions pertaining to serotonin and melatonin formulations (through soil application or foliar spray) are expected to be some of the possible ways to regulate abiotic stress tolerance in plants. The present review, thus, summarizes the regulatory roles of serotonin and melatonin in modulating the signaling events accompanying abiotic stress in plants.

The uremic toxin indoxyl sulfate acts as a pro- or antioxidant on LDL oxidation

Uremic toxins have been shown to play a role in chronic kidney disease (CKD) associated oxidative stress. Oxidative stress and inflammation have been associated with increased risk of cardiovascular disease in uraemia. The oxidative modification of LDL may play a role in early atherogenesis. Enhanced LDL oxidation has been found in uremic patients which may account for accelerated atherosclerosis observed in CKD. The uremic toxin indoxyl sulfate (IS) has been reported to exert oxidative and antioxidative activity. Thus, in the present study we have investigated the influence of IS on the atherogenic modifications of LDL exposed in vitro to different oxidising systems. The transition metal ion (Cu(2+)) and hemin/H2O2 induced lipid oxidation reactions monitored by conjugated diene formation, were inhibited by the presence of IS, which points to possible antioxidant effects by this uremic toxin. A protective effect of IS on LDL apoprotein modification by the exposure to the product of the myeloperoxidase/H2O2/Cl(-) system HOCl, was also observed as estimated by protein carbonyl formation. In contrast, a marked increase in conjugated dienes and lipid hydroperoxides was observed when lipid oxidation was initiated by the free radical generator AAPH in presence of IS. The GC-MS analysis revealed the formation of indole-2,3-dione and 6,12-dihydro-6,12-dioxo-indolo[2,1-b]quinazoline (tryptanthrin) in IS/AAPH reaction. A scheme for the generation of tryptanthrin from IS via indoxyl radicals is proposed, which may facilitate LDL lipid oxidation. Our observations add further insight in the Janus-faced properties of this important uremic toxin.

2,3-Diaryl-substituted indole based COX-2 inhibitors as leads for imaging tracer development

A series of 2,3-diaryl-substituted indoles containing a fluorine or methoxy group was synthesized via Fischer indole synthesis, McMurry cyclization, or Bischler–Möhlau reaction to identify potential leads for PET radiotracer development.

THEORETICAL STUDY OF THE SUBSTITUENT EFFECTS ON THE REACTION ENTHALPIES OF THE ANTIOXIDANT MECHANISMS OF STOBADINE DERIVATIVES IN THE GAS-PHASE AND WATER

In this paper the reaction enthalpies of three antioxidant action mechanisms, HAT, SET–PT, and SPLET, for mono-substituted Stobadines were calculated in gas-phase and water. Results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP), and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In comparison to gas-phase, water attenuates the substituent effect on all reaction enthalpies. Results show that IP and BDE values can be successfully correlated with the indolic N–H bond length after electron abstraction, R(N–H+•), and the partial charge on the indolyl radical nitrogen atom, q( N ). Furthermore, calculated IP and PA values for mono-substituted Stobadines show linear dependence on the energy of the highest occupied molecular orbital (E HOMO ) of studied molecules in the two environments. SPLET represents the thermodynamically preferred mechanism in water.

Cigarette smoke-induced differential regulation of glutathione metabolism in bronchial epithelial cells is balanced by an antioxidant tetrapeptide UPF1

Airway epithelium is a principal target for inhaled oxidants like cigarette smoke, which induce epithelial injury and thereby provoke pathogenesis of chronic airway diseases. Alterations in airway epithelial glutathione (GSH) metabolism are central in causing a loss of reducing environment, however, data are scarce on epithelial cells from larger bronchi. We showed a transient depletion of intracellular GSH in human bronchial epithelial cells after exposure to cigarette smoke condensate (CSC), which later followed by a prolonged elevation. Of the GSH-regulating enzymes, CSC increased mRNA expression of both catalytic (GCLC) and modifier (GCLM) subunits of glutamate-cysteine ligase. UPF1, a tetrapeptide GSH analogue, 4-methoxy-L-tyrosinyl-γ-L-glutamyl-L-cysteinyl-glycine, known to possess a 50-fold higher hydroxyl radical scavenging efficiency than does GSH, normalized the intracellular GSH level in the human bronchial epithelial cells under oxidative stress caused by CSC. UPF1 restored the GCLM and GSH reductase mRNA levels, which were significantly augmented by CSC treatment, back to the level of untreated control cells, referring to a successful establishment of control by UPF1 upon the over-accumulation of GSH. Moreover, UPF1 showed a significantly more potent antioxidant capacity than did N-acetyl-L-cysteine (NAC) and, compared to NAC, demonstrated a better potential to assure the whole GSH homeostasis in human bronchial epithelial cells. The current study suggests that UPF1 is capable of maintaining intracellular GSH level under CSC-induced oxidative stress in bronchial epithelial cells via balanced control over GSH-regulating enzymes, reflecting an improved perception of cellular redox conditions and thereby warranting improved adjustment of GSH accumulation.

Diverse Effects of Glutathione and UPF Peptides on Antioxidant Defense System in Human Erythroleukemia Cells K562

The main goal of the present paper was to examine the influence of the replacement of γ-Glu moiety to α-Glu in glutathione and in its antioxidative tetrapeptidic analogue UPF1 (Tyr(Me)-γ-Glu-Cys-Gly), resulting in α-GSH and UPF17 (Tyr(Me)-Glu-Cys-Gly), on the antioxidative defense system in K562 cells. UPF1 and GSH increased while UPF17 and α-GSH decreased the activity of CuZnSOD in K562 cells, at peptide concentration of 10 μM by 42% and 38% or 35% and 24%, respectively. After three-hour incubation, UPF1 increased and UPF17 decreased the intracellular level of total GSH. Additionally, it was shown that UPF1 is not degraded by γ-glutamyltranspeptidase, which performs glutathione breakdown. These results indicate that effective antioxidative character of peptides does not depend only on the reactivity of the thiol group, but also of the other functional groups, and on the spatial structure of peptides.

Melatonin and structurally-related compounds protect synaptosomal membranes from free radical damage

Since biological membranes are composed of lipids and proteins we tested the in vitro antioxidant properties of several indoleamines from the tryptophan metabolic pathway in the pineal gland against oxidative damage to lipids and proteins of synaptosomes isolated from the rat brain. Free radicals were generated by incubation with 0.1 mM FeCl₃, and 0.1 mM ascorbic acid. Levels of malondialdehyde (MDA) plus 4-hydroxyalkenal (4-HDA), and carbonyl content in the proteins were measured as indices of oxidative damage to lipids and proteins, respectively. Pinoline was the most powerful antioxidant evaluated, with melatonin, N-acetylserotonin, 5-hydroxytryptophan, 5-methoxytryptamine, 5-methoxytryptophol, and tryptoline also acting as antioxidants.

Design, synthesis and properties of novel powerful antioxidants, glutathione analogues

Glutathione (GSH) is the major low-molecular weight antioxidant in mammalian cells. Thus, its analogues carrying similar and/or additional positive properties might have clinical perspectives. Here, we report the design and synthesis of a library of tetrapeptidic GSH analogues called UPF peptides. Compared to cellular GSH our designed peptidic analogues showed remarkably higher hydroxyl radical scavenging ability (EC(50) of GSH: 1,231.0 +/- 311.8 microM; EC(50) of UPF peptides: from 0.03 to 35 microM) and improved antiradical efficiency towards a stable alpha,alpha-diphenyl-beta-picrylhydrazyl (DPPH) radical. The best of UPF peptides was 370-fold effective hydroxyl radical scavengers than melatonin (EC(50): 11.4 +/- 1.0 microM). We also found that UPF peptides do not influence the viability and membrane integrity of K562 human erythroleukemia cells even at 200 microM concentration. Dimerization of GSH and UPF peptides was compared in water and in 0.9% saline solutions. The results, together with an earlier finding that UPF1 showed protective effects in global cerebral ischemia model in rats, suggest that UPF peptides might serve both as potent antioxidants as well as leads for design of powerful non-peptidic antioxidants that correct oxidative stress-driven events.

N-[(Dihydroxyphenyl)acyl]serotonins as potent inhibitors of tyrosinase from mouse and human melanoma cells

A series of N-acyl derivatives of tyramine, tryptamine, and serotonin were synthesized and tested on anti-melanogenic activity. The serotonin derivatives such as N-caffeoylserotonin (3) and N-protocatechuoylserotonin (9) were inhibitory to tyrosinase from mouse B16 and human HMV-II melanoma cells, while the corresponding derivatives of tryptamine and 5-methoxytryptamine were almost inactive or less active than the serotonin derivatives. The inhibitory activity of the serotonin derivatives increased with increasing number of phenolic hydroxyl groups in the acyl moiety. Melanin formation in the culture of B16 cells was suppressed by 3 and 9 with no cytotoxicity in the concentration range tested (IC(50)=15, 3 and 111muM for 3, 9, and kojic acid, respectively). Thus the N-acylserotonin derivatives having a dihydroxyphenyl group are potential anti-melanogenic agents. Their inhibition of tyrosinase is primarily performed through the 5-hydroxyindole moiety and further strengthened by the phenolic hydroxyl groups in the acyl moiety.

Quantification of the water/lipid affinity of melatonin and a pinoline derivative in lipid models

This study assessed the location of melatonin (N-acetyl-5-methoxytryptamine) and of a pinoline derivative (GWC22) [6-ethyl-1-(3-methoxyphenyl)-2-propyl-1,2,3,4-tetrahydro-beta-carboline], when present in lipid assemblies such as linoleate micelles, phosphatidylcholine liposomes or low density lipoproteins (LDL). The efficiency of radical scavenging by these compounds is highly dependent on their partitioning between the lipidic and aqueous phases. We determined the proportion of melatonin or GWC22 in the aqueous and lipid phases of each system (concentrations of the antioxidants ranging between 3 x 10(-5) and 10(-4) m) by assaying melatonin or GWC22 by HPLC/UV detection, or by fluorescence for melatonin in micelles. Our results show that melatonin and GWC22 were preferentially located in the aqueous phase of micelles (68.4% and 59.0%, respectively), whereas only 30.5% of melatonin and 39.0% of GWC22 were found in the lipid phase. By contrast, in phosphatidylcholine liposomes, both compounds were essentially present in the lipid phase (73.5% for melatonin and 79.1% for GWC22, versus 25.9% and 19.5% in the aqueous phase, respectively). In the case of LDL, 99.9% of the melatonin added was found in the methanol/water extracting phase containing phospholipids, unesterified cholesterol and apolipoprotein B100. The partitioning of melatonin and GWC22 in linoleate micelles gave new insights on the marked protective effect of GWC22 towards radiation-induced lipid peroxidation and allowed us to determine more accurately the lower limit values of the reaction rate constants of the two molecules studied with lipid peroxyl radicals, i.e. k(LOO.+melatonin)) >or= 9.0 x 10(4)m(-1)s(-1) and k(LOO.+GWC22) >or= 3.5 x 10(5)m(-1)s(-1).

Comparison of the stability of glutathione and related synthetic tetrapeptides by HPLC and capillary electrophoresis

Glutathione and related peptides are interesting targets as protectors of biological systems against an oxidative injury. Two novel glutathione analogues, UPF1 and UPF15, have been designed and synthesised. As a result of different reactions taking place, the thiol-containing compounds oxidise to disulfides. In this study, the stability of UPF1, UPF15 and glutathione in various solutions was investigated by using HPLC and CE. The results showed that UPF1 and UPF15 are powerful hydroxyl radical scavengers and their dimerisation process velocity is higher than that of glutathione.

Increased Susceptibility of Low-Density Lipoprotein to Ex Vivo Oxidation in Mice Transgenic for Human Apolipoprotein B Treated with 1 Melatonin-Related Compound Is Not Associated with Atherosclerosis Progression

Considerable evidence supports the hypothesis that LDL oxidation has an important role in atherosclerosis. It has been demonstrated that the feeding of hypercholesterolemic mice on an atherogenic diet supplemented with melatonin highly increases the surface of atherosclerotic lesions in aorta and the sensitivity of atherogenic lipoprotein to ex vivo oxidation even though high melatonin doses inhibit lipoprotein oxidation in vitro. A melatonin-related compound (DTBHB: N-[2-(5-methoxy-1H-indol-3-yl)ethyl]-3,5-di-tert-butyl-4-hydroxybenzamide) has been reported to strongly inhibit lipid peroxidation in vitro. In the present study, DTBHB treatment considerably increased the sensitivity of atherogenic lipoproteins to ex vivo oxidation but did not modify atherosclerotic lesion development in mice. Moreover, DTBHB treatment did not induce detectable lipidic alteration. These data confirm that the capacity of molecules to inhibit atherogenic lipoprotein oxidation in vitro offers no prediction of their capacity to inhibit in vivo atherosclerosis development.

Antiradical and antioxidant activities of alkaloids isolated from Mahonia aquifolium. Structural aspects

The antioxidant activities of three alkaloids isolated from Mahonia aquifolium--berberine, jatrorrhizine, and magnoflorine--were studied with respect to their structural aspects, particularly the presence and the position of -OH groups, steric conditions of unpaired electron delocalization and parameters of lipophilicity and hydration energy. The antiradical activities of the compounds tested were evaluated as the reactivities toward free stable alpha,alpha'-diphenyl-beta-picrylhydrazyl radical (DPPH). The antioxidant features of the alkaloids tested were investigated in heterogeneous membrane system of DOPC liposomes stressed by peroxidative damage induced by AAPH azoinitiator. Both alkaloids bearing free phenolic groups--jatrorrhizine and magnoflorine--showed better activities in both systems used than berberine not bearing any readily abstractable hydrogen on its skeleton. The former two showed antiperoxidative efficiency in DOPC liposomal membrane comparable to that of an effective scavenger of peroxyl radicals--stobadine-and higher than that of Trolox. We conclude that the favorable antioxidant features of the hydroxylated alkaloids are most probably ensured by the combination of reasonably high antiradical reactivity with high lipophilicity, however, the solvation process was found to markedly interfere with these beneficial effects.

Antioxidant activity of melatonin and a pinoline derivative on linoleate model system

This study aimed at investigating the in vitro protective effects of GWC22, a novel pinoline derivative [6-ethyl-1-(3-methoxyphenyl)-2-propyl-1,2,3,4-tetrahydro-beta-carboline] chlorhydrate, against radiation-induced oxidation of linoleate initiated by hydroxyl radicals ((*)OH). Using linoleate micelles (10(-2) m) as lipid model, two indexes of peroxidation have been measured, i.e. conjugated dienes and hydroperoxides. Similar determinations were performed with melatonin in order to compare the protective effects of the two compounds. It was observed that, the higher the concentration of GWC22 (or melatonin) (3 x 10(-5) to 10(-4) m), the stronger the antioxidant ability. In these in vitro assays, GWC22 showed a better antioxidant effect than melatonin for a given antioxidant concentration. A reaction scheme has been proposed to explain the inhibitory effect of an antioxidant via the propagating steps of the lipid peroxidation. Indeed, we have suggested that melatonin and GWC22 may compete with the fatty acid to scavenge lipid peroxyl radicals (LOO(*)). We have estimated a lower limit for the LOO(*) rate constant for GWC22 (>/=1.4 x 10(5)/m/s) and for melatonin (>/=2.8 x 10(4)/m/s) assuming that the k-value of the propagating step in linoleate (LOO(*) + linoleate) was 1.4 x 10(3)/m/s. The difference of reactivity between melatonin and GWC22 in this model system is assumed to be related to their relative lipophilicity.

Antioxidant activity and neuroprotective effects of zolpidem and several synthesis intermediates

Structural relationship between the antioxidant melatonin and the non-benzodiazepine hypnotic zolpidem (ZPD) suggests possible direct antioxidant and neuroprotective properties of this compound. In the present work, these effects were analyzed for zolpidem and four of its synthesis intermediates. In vitro assays include lipid peroxidation and protein oxidation studies in liver and brain homogenates. Intracellular antioxidant effects were analyzed by evaluation of free radical formation prevention in HT-22 hippocampal cells treated with glutamate 10mM and measured by flow cytometer DCF fluorescence. The neuroprotective effect of these compounds was evaluated as neuronal death prevention of HT-22 cells treated with the same concentration of glutamate. Zolpidem was found to prevent induced lipid peroxidation in rat liver and brain homogenates showing figures similar to melatonin, although it failed to prevent protein oxidation. ZPD-I was the most effective out of the several zolpidem intermediates studied as it prevented lipid peroxidation with an efficiency higher than melatonin or zolpidem and with an effectiveness similar to estradiol and trolox. ZPD-I prevents protein oxidation, which trolox is known to be unable to prevent. When cellular experiments were undertaken, ZPD-I prevented totally the increase of intracellular free radicals induced by glutamate 10mM in culture medium for 12h, while zolpidem and ZPD-III partially prevented this increase. Also the three compounds protected hippocampal neurons from glutamate-induced death in the same conditions, being their comparative efficacy, ZPD-III > ZPD-I = ZPD.

Protective effect of N-acetyl-serotonin on the nonenzymatic lipid peroxidation in rat testicular microsomes and mitochondria

N-acetyl-serotonin, the immediate precursor of melatonin in the tryptophan metabolic pathway in the pineal gland, has been reported to be an antioxidant. The aim of this study was to test the in vitro protective effect of N-acetyl-serotonin on the ascorbate-Fe(++) induced lipid peroxidation of polyunsaturated fatty acids (PUFAs) located in testis microsomes and mitochondria. We assayed increasing concentrations (0-10 mM) of N-acetyl-serotonin in testis microsomes and (0-1 mM) of N-acetyl-serotonin in testis mitochondria. Control experiments were performed by incubating microsomal and mitochondrial membranes with N-acetyl-serotonin in the absence of lipid peroxidation-inducing drugs. Special attention was paid to the changes produced on the highly PUFAs C20:4 n6 and C22:5 n6. The light emission (chemiluminescence) used as a marker of lipid peroxidation was similar in both organelles when the control and peroxidized groups were compared. N-acetyl-serotonin reduced lipid peroxidation in testicular microsomes or mitochondria for both C20:4 n6 and C22:5 n6. Both long chain PUFAs were protected when N-acetyl-serotonin was incorporated either into microsomes or mitochondria. The N-acetyl-serotonin concentration required to inhibit by approximately 70% lipid peroxidation process was 10 mM in microsomes and between 0.50 and 1 mM in mitochondria. IC 50 values calculated from the inhibition curve of N-acetyl-serotonin on the chemiluminescence rates were higher in microsomes (4.50 mM) than in mitochondria (0.25 mM). In these experimental conditions, N-acetyl-serotonin was about 18 times more potent in testicular mitochondria in inhibiting the oxidative processes than it was in testicular microsomes. These results suggest that the protective role of N-acetyl-serotonin in preserving the long PUFAs may be related to its ability to reduce lipid peroxidation.

Structural aspects of antioxidant activity of substituted pyridoindoles

Stobadine and its two structural analogues, dehydrostobadine and N-acetylated stobadine were used to examine how structural alteration in the close proximity of the indolic nitrogen would influence the antioxidant activity of the substituted pyridoindoles. The compounds were tested for their efficiency to scavenge stable free radicals of alpha,alpha'-diphenyl-beta-picrylhydrazyl as well as for their ability to prevent 2,2'-azobis-(2-amidinopropane)hydrochloride induced peroxidation of dioleoyl phosphatidylcholine liposomes. The results proved that the substituted pyridoindoles can act as potent scavengers of peroxyl radicals both in aqueous and lipid phases, the antioxidant activity being comparable with that of Trolox. Structural changes in the proximity of the indolic nitrogen were found crucial for the radical scavenging efficiency: aromatisation of the pyridoindole skeleton in dehydrostobadine lowered the antioxidant activity, while acetylation of the indolic nitrogen completely abolished the ability to scavenge peroxyl radicals. The results are in agreement with the notion that the antioxidant activity of stobadine and of the related pyridoindoles may be mediated via the indolic nitrogen centre. When stobadine and Trolox were present simultaneously in liposomal incubations, Trolox spared stobadine in a dose-dependent manner; a direct interaction of Trolox with stobadinyl radical appears to be a plausible explanation with possible consequences for the antioxidant capacity of stobadine under in vivo conditions, where re-cycling of stobadine by vitamin E might occur.

Melatonin related compounds inhibit lipid peroxidation during copper or free radical-induced LDL oxidation

This study was designed to evaluate the protective effect of two melatonin related compounds towards low density lipoproteins (LDL) oxidation initiated in vitro either by defined free radicals [i.e. superoxide anion (O2*-) and ethanol-derived peroxyl radicals (RO(2)(*))] produced by gamma radiolysis or by copper ions. The compounds studied were N-[2-(5-methoxy-1H-indol-3-yl)ethyl]-3,5-di-tert-butyl-4-hydroxybenzamide (DTBHB) and (R,S)-1-(3-methoxyphenyl)-2-propyl-1,2,3,4-tetrahydro-beta-carboline (GWC20) which is a pinoline derivative. Their effects were compared with those of melatonin at the same concentration (100 micromol/L). None of the three tested compounds protected endogenous LDL alpha-tocopherol from oxidation by RO(2)(*)/O(2)(*)- free radicals. By contrast, they all protected beta-carotene from the attack of these free radicals with GWC20 being the strongest protector. Moreover, melatonin and DTBHB partially inhibited the formation of products derived from lipid peroxidation (conjugated dienes and thiobarbituric acid-reactive substances or TBARS) while GWC20 totally abolished this production. As previously shown, melatonin (at the concentration used) inhibited copper-induced LDL oxidation by increasing 1.60-fold the lag phase duration of conjugated diene formation over the 8 hr of the experimental procedure, however, DTBHB and GWC20 were much more effective, because they totally prevented the initiation of the propagation phase of LDL oxidation. It would be interesting to test in vivo if DTBHB and GWC20 which exhibit a strong capacity to inhibit in vitro LDL oxidation would reduce or not atherosclerosis in animals susceptible to this pathology.

Daily melatonin supplementation in mice increases atherosclerosis in proximal aorta

Considerable evidence supports the hypothesis that LDL oxidation plays an important role in atherosclerosis. Even though high melatonin doses inhibit LDL oxidation in vitro, the effect of melatonin on atherosclerosis has never been studied. We have demonstrated that the feeding of hypercholesterolemic mice with an atherogenic diet supplemented with melatonin highly increases the surface of atherosclerotic lesions in the proximal aorta. These observations occur without detectable lipidic or glucidic phenotype alteration. Melatonin treatment increased highly the sensitivity of atherogenic lipoprotein to Cu(2+) and gamma-radiolysis generated oxyradical ex vivo oxidation during the fasting period. Moreover, these altered lipoproteins were less recognized by the LDL receptor metabolic pathway of murine fibroblasts while they transferred many more cholesteryl esters to murine macrophages. This study suggests that caution should be taken as regards high melatonin dosage in hypercholesterolemic patients.

Synthesis and pharmacological analysis of high affinity melatonin receptor ligands

We report the synthesis and radioligand binding analysis of a series of naphthalenic melatonin receptor ligands, N-[2-(7-alkoxy-2-methoxy-1-naphthyl)ethyl]propionamide. This series of ligands exhibits subpicomolar binding affinity to both MT1 and MT2 melatonin receptors expressed in chinese hamster ovary (CHO) cells.

Design, synthesis and biological evaluation of novel beta-substituted indol-3-yl ethylamido melatoninergic analogues

A series of new melatonin analogues have been synthesized. Interestingly, two of the new compounds, 11c and 11e, which did not show any appreciable affinity for the melatonin receptor, were found to be potent inhibitors of lipid peroxidation in rat liver microsomes. Analogue 11c, in particular, is a better antioxidant than melatonin.

N-acetylserotonin suppresses hepatic microsomal membrane rigidity associated with lipid peroxidation

N-acetylserotonin, the immediate precursor of melatonin in the tryptophan metabolic pathway in the pineal gland, has been reported to be an antioxidant. The aim of this work was to test the effect of N-acetylserotonin in stabilizing biological membranes against oxidative stress. Hepatic microsomal membranes from male adult rats were incubated with N-acetylserotonin (0.001-3 mM) before inducing lipid peroxidation using FeCl(3), ADP and NADPH. Control experiments were done by incubating microsomal membranes with N-acetylserotonin in the absence of lipid peroxidation-inducing drugs. Membrane fluidity was assessed by fluorescence spectroscopy and malonaldehyde plus 4-hydroxyalkenals concentrations were measured to estimate the degree of lipid peroxidation. Free radicals induced by the combination of FeCl(3)+ADP+NADPH produced a significant decrease in the microsomal membrane fluidity, which was associated with an increase in the malonaldehyde plus 4-hydroxyalkenals levels. These changes were suppressed in a concentration-dependent manner when N-acetylserotonin was added in the incubation buffer. In the absence of lipid peroxidation, N-acetylserotonin (0.001-3 mM) did not change membrane fluidity nor malonaldehyde plus 4-hydroxyalkenals levels. These results suggest that the protective role of N-acetylserotonin in preserving optimal levels of fluidity of the biological membranes may be related to its ability to reduce lipid peroxidation.