João Batista Teixeira da Rocha

Toxicology and pharmacology of selenium: emphasis on synthetic organoselenium compounds

The advance in the area of synthesis and reactivity of organoselenium, as well as the discovery that selenium was the cause of severe intoxication episodes of livestock in the 1930s and the subsequent determination that selenium was an essential trace element in the diet for mammals, has motivated intense studies of the biological properties of both organic and inorganic selenium compounds. In this review, we shall cover a wide range of toxicological and pharmacological effects, in which organoselenium compounds are involved but the effects of inorganic compounds were not discussed in detail here. The molecular toxicity of inorganic selenium was described in relation to its interaction with endogenous –SH groups to allow a comparison with that of synthetic organoselenium compounds. Furthermore, in view of the recent points of epidemiological evidence that overexposure to selenium can facilitate the appearance of chronic degenerative diseases, we also briefly revised the history of selenium toxicity and physiology and how environmental selenium can reach inside the mammalian cells. The biological narrative of the element selenium, in the last century, has been marked by a contrast between its toxic and its beneficial effects. Thus, the potential therapeutic use of simple organoselenium compounds has not yet been sufficiently explored and, consequently, we cannot discard this class of compounds as promising pharmaceutical agents. In effect, the future of the organochalcogens as pharmacological agents will depend on more detailed toxicological studies in the oncoming years.

Mechanisms of methylmercury-induced neurotoxicity: evidence from experimental studies

Neurological disorders are common, costly, and can cause enduring disability. Although mostly unknown, a few environmental toxicants are recognized causes of neurological disorders and subclinical brain dysfunction. One of the best known neurotoxins is methylmercury (MeHg), a ubiquitous environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. In the aquatic environment, MeHg is accumulated in fish, which represent a major source of human exposure. Although several episodes of MeHg poisoning have contributed to the understanding of the clinical symptoms and histological changes elicited by this neurotoxicant in humans, experimental studies have been pivotal in elucidating the molecular mechanisms that mediate MeHg-induced neurotoxicity. The objective of this mini-review is to summarize data from experimental studies on molecular mechanisms of MeHg-induced neurotoxicity. While the full picture has yet to be unmasked, in vitro approaches based on cultured cells, isolated mitochondria and tissue slices, as well as in vivo studies based mainly on the use of rodents, point to impairment in intracellular calcium homeostasis, alteration of glutamate homeostasis and oxidative stress as important events in MeHg-induced neurotoxicity. The potential relationship among these events is discussed, with particular emphasis on the neurotoxic cycle triggered by MeHg-induced excitotoxicity and oxidative stress. The particular sensitivity of the developing brain to MeHg toxicity, the critical role of selenoproteins and the potential protective role of selenocompounds are also discussed. These concepts provide the biochemical bases to the understanding of MeHg neurotoxicity, contributing to the discovery of endogenous and exogenous molecules that counteract such toxicity and provide efficacious means for ablating this vicious cycle.

Metals, oxidative stress and neurodegeneration: a focus on iron, manganese and mercury.

Essential metals are crucial for the maintenance of cell homeostasis. Among the 23 elements that have known physiological functions in humans, 12 are metals, including iron (Fe) and manganese (Mn). Nevertheless, excessive exposure to these metals may lead to pathological conditions, including neurodegeneration. Similarly, exposure to metals that do not have known biological functions, such as mercury (Hg), also present great health concerns. This review focuses on the neurodegenerative mechanisms and effects of Fe, Mn and Hg. Oxidative stress (OS), particularly in mitochondria, is a common feature of Fe, Mn and Hg toxicity. However, the primary molecular targets triggering OS are distinct. Free cationic iron is a potent pro-oxidant and can initiate a set of reactions that form extremely reactive products, such as OH. Mn can oxidize dopamine (DA), generating reactive species and also affect mitochondrial function, leading to accumulation of metabolites and culminating with OS. Cationic Hg forms have strong affinity for nucleophiles, such as -SH and -SeH. Therefore, they target critical thiol- and selenol-molecules with antioxidant properties. Finally, we address the main sources of exposure to these metals, their transport mechanisms into the brain, and therapeutic modalities to mitigate their neurotoxic effects.

Involvement of glutamate and reactive oxygen species in methylmercury neurotoxicity

This review addresses the mechanisms of methylmercury (MeHg)-induced neurotoxicity, specifically examining the role of oxidative stress in mediating neuronal damage. A number of critical findings point to a central role for astrocytes in mediating MeHg-induced neurotoxicity as evidenced by the following observations: a) MeHg preferentially accumulates in astrocytes; b) MeHg specifically inhibits glutamate uptake in astrocytes; c) neuronal dysfunction is secondary to disturbances in astrocytes. The generation of reactive oxygen species (ROS) by MeHg has been observed in various experimental paradigms. For example, MeHg enhances ROS formation both in vivo (rodent cerebellum) and in vitro (isolated rat brain synaptosomes), as well as in neuronal and mixed reaggregating cell cultures. Antioxidants, including selenocompounds, can rescue astrocytes from MeHg-induced cytotoxicity by reducing ROS formation. We emphasize that oxidative stress plays a significant role in mediating MeHg-induced neurotoxic damage with active involvement of the mitochondria in this process. Furthermore, we provide a mechanistic overview on oxidative stress induced by MeHg that is triggered by a series of molecular events such as activation of various kinases, stress proteins and other immediate early genes culminating in cell damage.

Polyamines reduces lipid peroxidation induced by different pro-oxidant agents.

Polyamines, among other functions, are considered to act as a free radical scavenger and antioxidant. The quinolinic acid (QA), sodium nitroprusside (SNP) and iron (Fe+2) stimulate production of free radicals and lipid peroxidation. In the present study, we investigated the free radical and/or aldehyde scavenger effects of polyamines spermine and spermidine on thiobarbituric acid reactive species (TBARS) production induced by QA, SNP, Fe+2/EDTA system and free Fe2+ in rat brain. Spermine and spermidine inhibited QA-induced TBARS production; however spermine was a better antioxidant than spermidine. Spermine also inhibited SNP-, Fe+2/EDTA- and free Fe2+-induced TBARS production, but had a modest effect. Spermidine, in turn, also discretely inhibited SNP-, Fe+2/EDTA- and free Fe2+-induced TBARS production. In the presence of MK-801, QA-induced TBARS production was considerably more inhibited by polyamines. In addition, arcaine does not affect the reducer effect of polyamines. The present findings suggest that the observed effects of polyamines are not related to the activation of NMDA receptor but with their antioxidant and free radical scavenger properties.

Anti-inflammatory and antinociceptive activity of diphenyl diselenide.

Abstract:Objective and design: Ebselen, an organoselenium compound is able to modulate the inflammatory response in rodents. In the present study, the anti-inflammatory and antinociceptive activity of diaryl diselenides and ebselen was studied.¶Materials: Adult male Wistar rats and albino mice were treated with diaryl diselenides and ebselen in different doses.¶Methods: Carrageenin-induced paw edema, tail-flick, formalin, acetic acid-induced abdominal writhing and capsaicin models of pain were carried out. Data were analyzed by ANOVA followed by Duncans multiple range when appropriate.¶Results: In all models, the most promising profile was displayed by diphenyl diselenide, which produced anti-inflammatory and antinociceptive activity significantly higher than ebselen. Diphenyl diselenide also produced dose-dependent antinociception when assessed in acetic acid-induced abdominal constriction, tail-flick test or formalin and capsaicin-induced nociception.¶Conclusion: The data presented here provide evidence that administration of diphenyl diselenide produced anti-inflammatory and antinociceptive activity.

Protective role of aryl and alkyl diselenides on lipid peroxidation

The concept that selenium-containing molecules may be better nucleophiles (and therefore antioxidants) than classical antioxidants has led to the design of synthetic organoselenium compounds. In the present study we appraised the antioxidant potential, thiol peroxidase activity, and rate of dithiotreitol and reduced glutathione oxidation of simple organodiselenide compounds in rats and mice. The present results demonstrate that alkyl and aryl diselenides are antioxidant compounds. We verified that the substitution on the aromatic moiety of diphenyl diselenide or the replacement of on aryl group by an alkyl substitute on diselenides changes their antioxidant and thiol peroxidase-like properties. The diaryl diselenides (PhSe)(2) and (p-ClPhSe)(2) presented higher thiol peroxidase activity and demonstrated better antioxidant potential than the other diselenides tested. In fact, the results revealed that alkyl diselenides, at low concentrations, were prooxidants and that aryl diselenides did not present this effect. Alkyl diselenides [(C(2)H(5)Se)(2) and (C(3)H(7)Se)(2)] demonstrated a higher potential for -SH group oxidation than aryl diselenides. In addition, this study demonstrated that diselenide protection against lipid peroxidation was different in mice and rats. The compounds tested acted more as antioxidants in the brains of mice than in the brains of rats.

Oxidative stress in MeHg-induced neurotoxicity

Methylmercury (MeHg) is an environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. Although the molecular mechanisms mediating MeHg-induced neurotoxicity are not completely understood, several lines of evidence indicate that oxidative stress represents a critical event related to the neurotoxic effects elicited by this toxicant. The objective of this review is to summarize and discuss data from experimental and epidemiological studies that have been important in clarifying the molecular events which mediate MeHg-induced oxidative damage and, consequently, toxicity. Although unanswered questions remain, the electrophilic properties of MeHg and its ability to oxidize thiols have been reported to play decisive roles to the oxidative consequences observed after MeHg exposure. However, a close examination of the relationship between low levels of MeHg necessary to induce oxidative stress and the high amounts of sulfhydryl-containing antioxidants in mammalian cells (e.g., glutathione) have led to the hypothesis that nucleophilic groups with extremely high affinities for MeHg (e.g., selenols) might represent primary targets in MeHg-induced oxidative stress. Indeed, the inhibition of antioxidant selenoproteins during MeHg poisoning in experimental animals has corroborated this hypothesis. The levels of different reactive species (superoxide anion, hydrogen peroxide and nitric oxide) have been reported to be increased in MeHg-exposed systems, and the mechanisms concerning these increments seem to involve a complex sequence of cascading molecular events, such as mitochondrial dysfunction, excitotoxicity, intracellular calcium dyshomeostasis and decreased antioxidant capacity. This review also discusses potential therapeutic strategies to counteract MeHg-induced toxicity and oxidative stress, emphasizing the use of organic selenocompounds, which generally present higher affinity for MeHg when compared to the classically studied agents.

Antioxidant Properties of New Chalcogenides Against Lipid Peroxidation in Rat Brain

Ebselen (2-phenyl- 1,2-benzisoselenazole-3 (2H)-one) is a seleno-organic compound with antioxidant properties, and anti-inflammatory actions. Recently, ebselen improved the outcome of acute ischemic stroke in humans. In the present study, the potential antioxidant capacity of organochalcogenide compounds diphenyl diselenide (PhSe)2, diphenyl ditelluride (PhTe)2, diphenyl disulfide (PhS)2, p-Cl-diphenyl diselenide (pCl-PhSe)2, bis-[S-4-isopropyl 2-phenyl oxazoline] diselenide (AA-Se)2, bis-[S-4-isopropyl 2-phenyl oxazoline] ditelluride (AA-Te)2 and bis-[S-4-isopropyl 2-phenyl oxazoline] disulfide (AA-S)2 was compared with that of ebselen (a classical antioxidant). Spontaneous and quinolinic acid (QA)- (2 mM) and sodium nitroprusside (SNP)- (5 μM)-induced thiobarbituric reactive species (TBARS) production by rat brain homogenates was determined colorimetrically. TBARS formation was reduced by ebselen, (PhSe)2, (PhTe)2, (AA-Se)2, (AA-S)2 and (pCl- PhSe)2 to basal rates. The concentrations of these compounds needed to inhibit TBARS formation by 50% (lC50) are 1.71 μM, 3.73 μM, 1.63 μM, 9.85 μM, > 33.3 μM, 23.2 μM and 4.83 μM, respectively for QA. For TBARS production induced by SNP the lC50 was 2.02 μM, 12.5 μM, 2.80 μM, > 33.3 μM, 24.5 μM and 7.55 μM, respectively. The compounds (AA-Te)2 and (PhS)2 have no antioxidant activity and pro-oxidant activity, respectively. These results suggest that (AA-Se)2 and (AA-S)2 can be considered as potential pharmaceutical antioxidant agents.

Diphenyl diselenide and diphenyl ditelluride differentially affect δ-aminolevulinate dehydratase from liver, kidney, and brain of mice

In the present study, the inhibitory effect of diphenyl diselenide and diphenyl ditelluride after in vitro, acute (a single dose), or chronic exposure (14 doses) was examined in mice 24 hours after the last administration. In vitro, diphenyl diselenide, and diphenyl ditelluride inhibited δ‐aminolevulinate dehydratase (δ‐ALA‐D) from brain, liver, and kidney with a similar potency (IC50 5–10 μM), and at 120 μM, they increased the rate of dithiothreitol (DTT) and reduced glutathione (GSH) oxidation. After a single dose (sc), diphenyl diselenide (1 mmol/kg) inhibited the liver (22%, p < 0.01) and brain (27%, p < 0.01) δ‐ALA‐D, but it did not inhibit the kidney enzyme. After a single dose (sc), diphenyl ditelluride (0.5 mmol/kg) inhibited liver (46%, p < 0.01), kidney (21%, p < 0.05), and brain (39%, p < 0.01) δ‐ALA‐D. Chronic exposure to diphenyl diselenide (0.125 and 0.250 mmol/kg) caused significant (p < 0.05) increase in liver and liver‐to‐body weight ratio and inhibited liver (40 and 60%, respectively) and brain (21 and 40%, respectively) δ‐ALA‐D. Kidney δ‐ALA‐D was not inhibited significantly after exposure to diphenyl diselenide. Total nonprotein −SH concentration was decreased only in liver of animals exposed for 14 days to selenide. Chronic exposure to diphenyl ditelluride (0.010 and 0.025 mmol/kg) caused significant (p < 0.05) inhibition of liver (28 and 42%, respectively) and brain (23 and 54%, respectively) δ‐ALA‐D. Kidney δ‐ALA‐D was not inhibited significantly by diphenyl ditelluride. Total nonprotein −SH concentration was decreased to a different extent after acute or chronic treatment with diphenyl ditelluride depending on analyzed tissue. Hemoglobin content was decreased significantly by 17 and 22% after chronic treatment with 0.125 and 0.25 mmol/kg diphenyl diselenide, respectively. Chronic exposure to 0.010 mmol/kg diphenyl ditelluride caused a reduction of 17% in hemoglobin content that tended to be significant (p < 0.10). These results suggest that δ‐ALA‐D inhibition after exposure to organochalcogens may perturb heme‐dependent metabolic pathway and contribute to the toxicological properties of these compounds.