The efficacy of vitamin E and melatonin as antioxidants against lipid peroxidation in rat retinal homogenates

Abstract: Free radical-induced oxidation can cause severe cell damage in biological systems. Melatonin, a pineal secretory product, is a recently identified antioxidant that protects cells from the damaging effects of free radicals. We compared the effect of melatonin and vitamin E, another antioxidant, against lipid peroxidation (LPO) in rat retinal homogenates. The aim was to characterize the antioxidative efficacy of melatonin in retina, a tissue highly susceptible to oxidative damage. The LPO product, malondialdehyde (MDA), was determined to provide an index of cell damage in vitro. After the incubation with iron(II) ions, the free radical scavenging effectiveness of four different concentrations (i.e., 0.5, 1.0, 2.0, and 4.0 mM) of vitamin E and melatonin were determined by comparing the final levels of MDA. Lipid peroxidation product levels were significantly reduced in a dose-response manner by all concentrations of vitamin E. Melatonin, in concentrations of either 2.0 or 4.0 mM, also significantly reduced LPO. Statistical analysis of the data showed that vitamin E treatment always yielded a lower level of LPO products than did the same concentration of melatonin. The concentrations of each agent required to inhibit 50% of the lipid damage (IC50) were 0.69 mM and 4.98 mM for vitamin E and melatonin, respectively. Both vitamin E and melatonin protect the retina against LPO in a dose-dependent manner. Although the IC50 value for melatonin is about 7.2 times higher than that of vitamin E, melatonin's pharmacological and physiological role in the treatment and/or prevention of certain retinal diseases in vivo should be further investigated.     

Teratogenic effects of diabetes mellitus in the rat. Prevention by vitamin E

Summary We wanted to determine whether administration of vitamin E could reduce the production of free radicals which could play a role in the teratogenic effects of diabetes mellitus. Diabetes was induced in Wistar rats by the intravenous administration of streptozotocin. The animals were divided into six groups: one with no supplement (D) and two, supplemented during pregnancy either with oral vitamin E (150 mg/day) (D + E) or with a placebo (safflower oil) (D + O). Three other groups were kept under the same conditions, but were treated with insulin: D + I, D + I + E and D + I + O. There were three groups of matched controls: C, C + E and C + O. All animals were killed on day 11.5 of pregnancy. In C animals the percentages of reabsorptions and malformations were 1.3 and 2%, respectively, comapred with 23.6, 24.3, 6.2 and 13.2%, respectively in D and D + I groups. The crown-rump length, number of somites, and protein and DNA content were higher in C animals than in the diabetic rats, independent of insulin treatment. When vitamin E was administered no changes in these parameters were observed in C and D + I animals; however, in the D mothers it reduced the rate of embryo malformations to 4.6% and increased the crown-rump length and the number of somites. However, vitamin E did not modify the protein and DNA content and the percentage of reabsorptions. In conclusion, administration of vitamin E to diabetic animals decreases the rate of embryo malformations and increases their size and maturation, supporting a role for free radicals in the teratogenic effects of diabetes.     

Low vitamin E status is a potential risk factor for insulin-dependent diabetes mellitus

Objectives. To study the association of vitamin E status with occurrence of insulin-dependent diabetes mellitus (IDDM). Design. A case–control study nested within a 21-year follow-up study. Subjects. Nineteen incident IDDM patients with an average age of 28 years and three individually matched controls per patient. Main outcome measure. Serum concentrations of alpha-tocopherol. Results. Serum alpha-tocopherol concentration at the baseline examination was inversely associated with IDDM occurring 4–14 years later. The cholesterol-adjusted relative risk of IDDM between the highest and lowest thirds of the vitamin concentration was 0.12 (95% confidence interval = 0.02– 0.85). Conclusions. The finding corroborates the hypothesis of a protective effect of vitamin E against development of IDDM. Because of the relatively old age of the patients in the present population, further epidemiological studies on the topic are warranted.     

Endothelial dysfunction in Type 1 diabetes mellitus: relationship with LDL oxidation and the effects of vitamin E.

AIMS: To examine the hypothesis that increased susceptibility of low density lipoproteins (LDL) to oxidation predisposes to endothelial dysfunction in patients with Type 1 diabetes mellitus. METHODS: A cross-sectional study of 46 non-nephropathic diabetic and 39 control subjects and in the diabetic patients, a 3-month duration, randomized, placebo-controlled double-blind trial of vitamin E 500 U/day. Flow-mediated vasodilatation (FMD) was measured in the forearm by high resolution ultrasound. LDL oxidation by Cu2+ was measured in vitro. RESULTS: Diabetic patients had greater basal and reactive forearm blood flow (geometric mean (SD%) flow (ml/min) 110.15 (19.19%) vs. 74.99 (23.17%); P=0.045, and 344.35 (20.84%) vs. 205.17 (21.48%); P=0.007), compared with controls, but there was no difference in FMD (median (interquartile range) 0.00 (-0.01-0.02) vs. 0.02 (-0.01-0.02) cm2; P=0.78). Diabetic LDL oxidation lag time correlated with postdilatation brachial artery area (r= 0.32; P=0.05) but not with FMD. Lag-times and total LDL oxidation by Cu2+, lipoprotein and vitamin E concentrations were similar in diabetic and control groups. Antibody titres to oxidized LDL (oxLDL) were higher in non-diabetic than diabetic subjects, and were unrelated to FMD. In diabetic patients, vitamin E increased mean (SD) plasma vitamin E levels (24.0 (6.5) to 47.5 (7.5) gmol/l; P=0.0006) and resulted in increased FMD (delta 0.00 (-0.02-0.01) vs. 0.01 (0.01-0.02)) cm2; P=0.0036), but no changes in LDL Cu2+ oxidation profiles were observed. CONCLUSIONS: FMD is no different in Type 1 diabetic and non-diabetic subjects and nor are indices of lipid peroxidation and in vitro LDL oxidation although levels of antibody to oxLDL are lower in diabetes. Vitamin E supplementation increases plasma vitamin E levels and may enhance FMD in diabetes but, in the absence of changes in LDL oxidation, this may not be mediated by reduced oxidation of LDL.     

Preservation of the antioxidant status in chemically-induced diabetes mellitus by melatonin

Oxidant stress is believed to be enhanced in patients with diabetes mellitus, which may lead to endothelial dysfunction and the development of atherosclerosis. In diabetes, hyperglycemia drives non-enzymatic glycation and oxidation of proteins and lipids which enhances the formation of advanced glycation end products (AGEs), which may be involved in the pathogenesis of diabetic vascular disease. The macrovascular complications of diabetes seem to be due to enhanced cellular oxidant stress by the interaction of AGEs with their receptor. It would be worthwhile to devise methods to reduce this oxidant stress. In alloxan-induced diabetic rats lipid peroxidation products were increased, while levels of nitric oxide glutathione peroxidase and superoxide dismutase were reduced. Melatonin restored these biochemical abnormalities to normalcy independent of hyperglycemia. This model can be used to study the role of oxidant stress in the development of macrovascular complications in diabetes mellitus.     

Inhibition of adriamycin-induced human platelet lipid peroxidation by vitamin E

Adriamycin® is an important cancer chemotherapeutic agent whose clinical use is compromised by potentially lethal cardiotoxicity. In the mouse, cardiotoxicity is related to the peroxidation of cardiac lipids. Both phenomena, however, can be reduced by pretreatment of the animal with vitamin E. Using the platelet as our model we have demonstrated that Adriamycin induces lipid peroxidation of human platelets in a dose-dependent manner. Platelet malonyldialdehyde (MDA) production was used as an indicator of lipid peroxidation. Adriamycin at 5, 10, 50, and 100 μg/ml produced 1.40 ± 0.2 (1 SD), 2.23 ± 0.41, 4.23 ± 0.4, and 6.13 ± 0.43 nmoies MDA per 10⁹ platelets, respectively. Vitamin E both in vitro and in vivo inhibited this lipid peroxidation. In vitro, vitamin E at concentrations of 0.01, 0.05, and 0.1 mg/ml inhibited platelet MDA formation by 14 ± 2, 29 ± 5, and 38 ± 6%, respectively. In six controls who ingested 1,600 units vitamin E daily for two weeks, platelet MDA formation induced by N-ethyl maleimide, thrombin, and Adriamycin was decreased by 11-20%. In similar studies, the ingestion of 10 grains acelyl salicylic acid (ASA) also inhibited platelet lipid peroxidation induced by these same agents. ASA and vitamin E were not additive in their inhibition of MDA formation induced by N-ethyl maleimide or thrombin. They were additive, however, in their inhibition of Adriamycin-induced lipid peroxidation, which suggests that the effect of vitamin E on Adriamycin-induced platelet lipid peroxidation is not due to inhibition of platelet cyclooxygenase. In the light of these observations on the inhibitory effect of vitamin E on lipid peroxidation in human platelets, further studies appear warranted on the clinical effects of E in inhibiting cardiac lipid peroxidation and concomitant cardiotoxicity in humans on Adriamycin therapy.     

Oxidative stress and antioxidant status in type 1 diabetes mellitus

OBJECTIVES: To test the hypothesis that type 1 diabetes is associated with increased oxidative stress and/or antioxidant status by investigating concentrations of 8-iso-prostaglandin F2alpha (8-iso-PGF2alpha) in urine and plasma and malondialdehyde (MDA) in plasma as indicators of lipid peroxidation in vivo, and antioxidant status in diabetic subjects compared with healthy control subjects. DESIGN AND SUBJECTS: Thirty-eight subjects with type 1 diabetes mellitus and 41 healthy age- and sex-matched control subjects were included in the study. Blood and urine samples were obtained and analysed for 8-iso-PGF2alpha with a newly developed radioimmunoassay, as well as for MDA, total antioxidant capacity (TAOC) and serum tocopherol levels. RESULTS: None of the variables of lipid peroxidation showed any significant difference between the two groups. Similarly, there were no significant correlations between the levels of 8-iso-PGF2alpha or MDA, and degree of glycemic control (HbA1c). Total antioxidant capacity in plasma was 16% lower amongst the subjects with type 1 diabetes than in the control group (P < 0.0005). Lipid corrected levels of alpha-tocopherol in serum were significantly increased in type 1 diabetic subjects (P < 0.05), as were gamma-tocopherol levels (P < 0.005). CONCLUSIONS: In spite of lower total antioxidant defence, our results do not support the oxidative stress hypothesis for type 1 diabetes mellitus. The higher tocopherol levels suggest that no vitamin E supplementation is necessary for subjects with type 1 diabetes mellitus.     

Protective effects of melatonin,vitamin E and N-acetylcysteine against acetaminophen toxicity in mice: a comparative study

Acetaminophen (AA) is a commonly used analgesic and antipyretic drug; however, when used in high doses, it causes fulminant hepatic necrosis and nephrotoxic effects in both humans and experimental animals. It has been reported that the toxic effects of AA are the result of oxidative reactions that take place during its metabolism. In this study we investigated if melatonin, vitamin E or N-acetylcysteine (NAC) are protective against AA toxicity in mice. The doses of the antioxidants used were as follows: melatonin (10 mg/kg), vitamin E (30 mg/kg) and NAC (150 mg/kg). Blood urea nitrogen (BUN), serum creatinine, alanine aminotransferase (ALT), aspartate aminotransferase (AST) levels in blood, and glutathione (GSH), malondialdehyde (MDA), oxidized protein levels and myeloperoxidase (MPO) activity in liver and kidney tissues were measured. BUN and serum creatinine, ALT and AST levels which were increased significantly following AA treatment decreased significantly after pretreatment with either vitamin E, melatonin or NAC; however, they were not reduced to control levels. ALT and AST levels were significantly higher at 4 hr compared with the 24 hr levels after AA administration. However, BUN and creatinine levels were significantly elevated only at 24 hr. GSH levels were reduced while MDA, MPO and oxidized protein levels were increased significantly following AA administration. These changes were reversed by pretreatment with either melatonin, vitamin E or NAC. Liver toxicity was higher at 4 hr, whereas nephrotoxicity appeared to be more severe 24 hr after treatment with AA. Vitamin E was the least efficient agent in reversing AA toxicity while melatonin, considering it was given as at lower dose than either vitamin E or NAC, was the most effective. This may be the result of the higher efficacy of melatonin in scavenging various free radicals and also because of its ability in stimulating the antioxidant enzymes.     

The effects of vitamin E on platelet activity in beta-thalassaemia patients

A double-blind, crossover, placebo-controlled study of the effect of vitamin E on platelet functions was performed on nine splenectomized and 16 non-splenectomized beta-thalassaemia/haemoglobin E (beta-thalassaemia/HbE) patients. The patients were supplemented with a daily dose of vitamin E (525 IU) for 3 months. The functions of platelets were assessed by adenosine diphosphate (ADP)-induced platelet aggregation and adenosine triphosphate release. Plasma alpha-tocopherol, plasma thiobarbituric reactive substances (TBARs) and serum ferritin levels represented patients' antioxidant status, lipid peroxidation status and iron status respectively. Before experimentation, all patients had low plasma alpha-tocopherol levels. The splenectomized patients showed severe iron overload iron, had higher plasma TBAR levels and their platelets were more reactive to ADP than those of non-splenectomized patients. Three months of daily vitamin E supplementation resulted in a significant increase in plasma alpha-tocopherol levels and reduction in plasma TBAR levels in all patients. Serum ferritin levels of the patients were not altered; however, vitamin E reduced the platelet reactivity of the splenectomized patients towards normal levels. The influence of vitamin E on platelet reactivity may result in delaying hypoxaemia and pulmonary occlusion that commonly occurs in splenectomized beta-thalassaemia/HbE patients.     

Effects of high-dose vitamin E supplementation on oxidative stress and microalbuminuria in young adult patients with childhood onset type 1 diabetes mellitus

INTRODUCTION: The aim of this study was to evaluate the effects of high-dose vitamin E supplementation (1200 mg/day) on reducing both microalbuminuria (MA) and oxidative stress in patients with type 1 diabetes mellitus (T1DM) and persistent MA. METHODS: We performed a 12-month, randomized, placebo-controlled, double-blind cross-over trial in ten Caucasian young adults (7m/3f; mean age 18.87 +/- 2.91 years) with T1DM and persistent MA.At baseline and at end of the treatment period, determination of albumin excretion rate (AER) and HbA(1c) and evaluation of the oxidant/antioxidant status were performed. RESULTS: At the beginning of the study, AER and HbA(1c) were not significantly different between the vitamin E and placebo group. No differences in terms of oxidant and antioxidant status were found between the two groups. This was associated with no significantly different urinary VEGF and TGF-beta levels. After 6 months, no significant differences in AER were observed between the two groups (p = 0.59). However, plasma and LDL-vitamin E content were significantly higher in the vitamin E group compared to the placebo group (p = 0.0001 and p = 0.004, respectively). This was associated with a significantly longer lag phase (p = 0.002) and lower MDA (p = 0.049). However, no statistically significant differences were detected in terms of VEGF and TGF-beta urinary levels. CONCLUSION: These data demonstrate that high-dose vitamin E supplementation reduces markers of oxidative stress and improves antioxidant defence in young patients with T1DM. However, although it positively affects the oxidant/antioxidant status, vitamin E supplementation does not reduce AER in patients with T1DM and persistent MA.     

Role of vitamin D in the pathogenesis of type 2 diabetes mellitus

Vitamin D deficiency has been shown to alter insulin synthesis and secretion in both humans and animal models. It has been reported that vitamin D deficiency may predispose to glucose intolerance, altered insulin secretion and type 2 diabetes mellitus. Vitamin D replenishment improves glycaemia and insulin secretion in patients with type 2 diabetes with established hypovitaminosis D, thereby suggesting a role for vitamin D in the pathogenesis of type 2 diabetes mellitus. The presence of vitamin D receptors (VDR) and vitamin D–binding proteins (DBP) in pancreatic tissue and the relationship between certain allelic variations in the VDR and DBP genes with glucose tolerance and insulin secretion have further supported this hypothesis. The mechanism of action of vitamin D in type 2 diabetes is thought to be mediated not only through regulation of plasma calcium levels, which regulate insulin synthesis and secretion, but also through a direct action on pancreatic β-cell function. Therefore, owing to its increasing relevance, this review focuses on the role of vitamin D in the pathogenesis of type 2 diabetes mellitus.     

The role of vitamin D in protecting type 1 diabetes mellitus

The relationship between autoimmune diabetes or type 1 diabetes mellitus and vitamin D has been reported in the literature. Many factors, environmental and genetic, have been known, as risk factors, to cause both type 1 diabetes and vitamin D deficiency. Vitamin D treatment has improved or prevented type 1 diabetes mellitus in animals and humans. Vitamin D also has been known to protect from autoimmune diseases in animal models. Therefore, it would be interesting to review the role of vitamin D in type 1 diabetes mellitus.     

Melatonin versus vitamin E as protective treatment against oxidative stress after extra-hepatic bile duct ligation in rats

The aims of the present study were first to compare the effects of melatonin and vitamin E on the cholestasis syndrome and their protective effect on liver injury, and second, to evaluate the activity of antioxidant enzymes after treatment with these antioxidant drugs. Cholestasis was achieved in adult male Wistar rats by double ligature and section of the extra-hepatic biliary duct. Hepatic and plasma oxidative stress markers were evaluated by changes in the amount of lipid peroxides, measured as malondialdehyde (MDA) and reduced glutathione (GSH) in plasma and homogenates of hepatic tissue. Serum bilirubin, alkaline phosphatase (AP), and gamma-glutamyl-transpeptidase (GGT) were used to evaluate the severity of cholestasis, and serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were used to evaluate the hepatic injury. Both vitamin E and melatonin were administrated 1 day before and 7 days after bile duct ligation. Acute ligation of the bile duct was accompanied by a significant increased in MDA and a decrease in GSH levels in both plasma and liver, as well as a significant reduction in antioxidant enzymes activities. The overall analysis of both treatments showed that melatonin (500 microg/kg daily) offered significantly better protection against cholestasis and a superior protective effect on hepatic injury than did vitamin E (15 mg/kg daily). Although vitamin E treatment resulted in a reduction of parameters of oxidative stress, the results were significantly better after a much lower daily dose of melatonin. Moreover, melatonin treatment was associated with a significant recovery of antioxidative enzymes. In conclusion, the present paper demonstrates a correlation between the intensity of biliary tract obstruction and increased free radical generation, as well as a direct correlation between the level of oxidative stress and the biochemical markers of liver injury. Melatonin (at a much lower dose than vitamin E) was much more efficient than vitamin E in reducing the negative parameters of cholestasis, the degree of oxidative stress and provided a significantly greater hepatoprotective effect against the liver injury secondary to the acute ligation of the biliary duct.     

Characterization of the antioxidant effects of melatonin and related indoleamines in vitro

Abstract: The antioxidant and possible pro-oxidant effects of melatonin and related indoleamines (tryptophan, serotonin, N-acetylserotonin, and 5-methoxytryptamine) were studied in vitro. In two model membrane systems, i.e., phospholipid liposomes and rat erythrocytes, lipid peroxidation induced by Fe²⁺ and H₂O₂, respectively, were reduced by the tested indoleamines except for tryptophan. The 5-hydroxy-indoleamines, serotonin, and N-acetylserotonin exhibited pro-oxidant actions in the bleomycin assay by reducing Fe³⁺ to Fe²⁺, which leads to DNA damage. The 5-methoxy-indoleamines, melatonin and 5-methoxytryptamine, were devoid of any pro-oxidant actions in this assay. Serotonin, but not N-acetylserotonin, scavenged the superoxide anion. None of the indoleamines tested had any reactivity towards H₂O₂. All the indoleamines, including tryptophan, were, however, shown to react with hypochlorous acid. These findings support an antiperoxidative and antioxidant action of melatonin which is devoid of pro-oxidant effect on non-lipid substrates.     

The protective effects of melatonin from oxidative damage induced by amyloid beta-peptide 25-35 in middle-aged rats

This work investigated the ability of melatonin to prevent oxidative damage in brain tissue induced by injection of beta-amyloid peptide 25-35 (Abeta25-35) in middle-aged rats. The Morris water maze was used to evaluate the cognitive function of the rats. Thiobarbituric acid-reactive substances and antioxidative enzymes (superoxide dismutase and glutathione peroxidase) activities were measured. It was found that injection of (Abeta25-35) (20 microg) into the rat hippocampus caused an increase in the latency (the time to find the platform), the total swimming distance to the platform, and the starting angles in (Abeta25-35)-treated rats. Furthermore, a significant rise in lipid peroxidation and decrease in antioxidative enzyme activities in brain tissue were found. Melatonin (0.1, 1, and 10 mg/kg, i.g. x 10 days) improved the spatial resolution of amnesic rats in the Morris water maze test. Meanwhile, melatonin antagonized the lipid peroxidation in both the mitochondria (P < 0.01) at the doses of 0.1, 1.0, and 10 mg/kg and in the cytoplasm at the doses of 0.1 and 1.0 mg/kg. Also in the amnesic rats, melatonin (0.1, 1.0, and 10 mg/kg. i.g. x 10 days) stimulated the antioxidative enzyme activities. The results show that melatonin effectively reduced lipid peroxidation and enhanced the antioxidative enzyme activities in Abeta(25-35)-treated rats, which may contribute to the improvement of rats' learning and memory impaired by Abeta(25-35).     

人参皂苷Re对糖尿病大鼠的心肌保护作用及其机制探讨

目的探讨人参皂苷Re对糖尿病大鼠的心肌保护作用及机制。方法取40只Wistar大鼠高脂高糖饲料喂养,腹腔注射链脲佐菌素（STZ）35mg／kg制作糖尿病模型,将34只造模成功大鼠随机分为模型组12只,人参皂苷11只,吡格列酮组11只,分别予以相应量纯净水、人参皂苷Re25mg／（kg·d）、吡格列酮10mg／（kg·d）灌胃,连续给药4周;另取10只正常大鼠予标准饲料喂养4周作为对照组。干预4周后处死各组大鼠,分别检测血糖、血脂,血清和心肌组织中超氧化物歧化酶（SOD）活性和丙二醛（MDA）水平,并行心肌组织病理学观察。结果人参皂苷组和吡格列酮组空腹血糖和血清总胆固醇水平高于对照组,（P〈0．01）,低于模型组（P〈0．01）,两组间无统计学差异;人参皂苷组和吡格列酮组SOD活性低于对照组,MDA水平高于对照组（P均〈0．05）;血清和心肌组织中SOD活性均高于模型组（P〈0．01）、MDA水平均低于模型组（P〈0．05）,但两组间无统计学差异。人参皂苷组和吡格列酮组心肌组织结构损伤明显轻于模型组,心肌细胞排列较模型组整齐。结论人参皂苷Re对糖尿病大鼠具有心肌保护作用;抗脂质过氧化作用是其作用机制之一。     

维生素E、维生素C对糖尿病患者心血管保护作用的研究

本文测定了57例2型糖尿病患者(2-DM组)和57例健康人(对照组)血浆内维生素E、维生素C的水平与低密度脂蛋白和高密度脂蛋白的变化情况，结果发现两组维生素E的水平无显著差异；维生素C水平2-DM组低于对照组。认为补充维生素E及维生素C可增强机体的抗氧化能力，调节脂质代谢，从而降低了高血压、冠心病、心肌梗死的发病率。     

维生素E抗衰老作用的实验研究

目的 探讨维生素E对大鼠衰老过程的影响。方法 肝、肾和脑组织中过氧化脂质（LPO）测定分别采用硫代巴比妥酸法和磷钨酸法。结果 肝、肾和脑组织中各剂量组LPO含量低于相应的对照组，而脑组织中只有200ppm、600ppm组与对照组有差异。GSH-Px活性只有400ppm组与相应对照组间有差异。结论 维生素上有抗衰老作用。     

Protective effects of melatonin in reducing oxidative stress and in preserving the fluidity of biological membranes: a review

Free radicals generated within subcellular compartments damage macromolecules which lead to severe structural changes and functional alterations of cellular organelles. A manifestation of free radical injury to biological membranes is the process of lipid peroxidation, an autooxidative chain reaction in which polyunsaturated fatty acids in the membrane are the substrate. There is considerable evidence that damage to polyunsaturated fatty acids tends to reduce membrane fluidity. However, adequate levels of fluidity are essential for the proper functioning of biological membranes. Thus, there is considerable interest in antioxidant molecules which are able to stabilize membranes because of their protective effects against lipid peroxidation. Melatonin is an indoleamine that modulates a wide variety of endocrine, neural and immune functions. Over the last two decades, intensive research has proven this molecule, as well as its metabolites, to possess substantial antioxidant activity. In addition to their ability to scavenge several reactive oxygen and nitrogen species, melatonin increases the activity of the glutathione redox enzymes, that is, glutathione peroxidase and reductase, as well as other antioxidant enzymes. These beneficial effects of melatonin are more significant because of its small molecular size and its amphipathic behaviour, which facilitates ease of melatonin penetration into every subcellular compartment. In the present work, we review the current information related to the beneficial effects of melatonin in maintaining the fluidity of biological membranes against free radical attack, and further, we discuss its implications for ageing and disease.     

Glutamate induces oxidative stress not mediated by glutamate receptors or cystine transporters: protective effect of melatonin and other antioxidants

Glutamate is responsible for most of the excitatory synaptic activity and oxidative stress induction in the mammalian brain. This amino acid is increased in the substantia nigra in parkinsonism due to the lack of dopamine restraint to the subthalamic nucleus. Parkinson's disease also shows an increase of iron levels in the substantia nigra and a decrease of glutathione, the antioxidant responsible for the ascorbate radical recycling. Considered together, these facts could make the antioxidant ascorbate behave as a pro-oxidant in parkinsonism. Since both glutamate and ascorbate are present in the synaptosomes and neurons of substantia nigra, we tested 1) if glutamate is able to induce oxidative stress independently of its excitatory activity, and 2) if ascorbate may have synergistic effects with glutamate when these two molecules co-exist. Brains were homogenized in order to disrupt membranes and render membrane receptors and intracellular signaling pathways non-functional. In these homogenates glutamate induced lipid peroxidation, indicating that this amino acid also may cause oxidative stress not mediated by its binding to glutamate receptors or cystine transporters. Ascorbate also induced lipid peroxidation thus behaving as a pro-oxidant. Both substances together produced an additive effect but they did not synergize. Given that melatonin is a potent physiological antioxidant with protective effects in models of neurotoxicity, we tested the role of this secretory product on the pro-oxidant effect of both compounds given separately or in combination. We also checked the protective ability of several other antioxidants. Pharmacological doses of melatonin (millimolar), estrogens, pinoline and trolox (micromolar) prevented the oxidant effect of glutamate, ascorbate, and the combination of both substances. Potential therapeutic application of these results is discussed.