Dysregulation of adipogenesis and disrupted lipid metabolism by the antidepressants citalopram and sertraline

Selective Serotonin Reuptake Inhibitors (SSRIs) are widely used medications for the treatment of major depressive disorder. However, long-term SSRI use has been associated with weight gain and altered lipid profiles. These findings suggest that SSRIs may have negative effects on metabolism. Exposure to certain chemicals called 'obesogens' are known to promote lipid accumulation and obesity by modulating adipogenesis. Here, we investigated whether citalopram (CIT) and sertraline (SER) interfere with the process of adipogenesis, using human mesenchymal stem cells (MSCs) in a 2D and a 3D model. Assessment of intracellular lipid accumulation by fluorescence staining was used as a measure for enhanced adipogenesis. To explore possible mechanisms behind SSRIs' effects, receptor mediated activity was studied using responsive cell lines for various nuclear receptors. Furthermore, RNA sequencing was performed in the 3D model, followed by differential gene expression and pathway analysis. A dose dependent increase in lipid accumulation was observed in both models with CIT and SER. For the 3D model, the effect was seen in a range close to reported steady-state plasma concentrations (0,065-0,65 μM for SER and 0,12-0,92 μM for CIT). Pathway analysis revealed unexpected results of downregulation in adipogenesis-related pathways and upregulation in phospholipids and lysosomal pathways. This was confirmed by an observed increase in lysosomes in the 2D model. Our findings suggest lysosomal dysfunction and disrupted lipid metabolism in mature adipocytes, leading to excessive phospholipid synthesis. Moreover, important adipogenic processes are inhibited, potentially leading to dysfunctional adipocytes, which might have implications in maintenance of a healthy metabolic balance.

Evaluation of Endocrine Related Adverse Effects of Non-Endocrine Targeted Pharmaceuticals in Cellular Systems

BACKGROUND

Prenatal period is a critical developmental phase that is sensitive to hormonal disruption by natural and/or exogenous hormones. Some pharmaceuticals frequently prescribed and used safely during pregnancy are shown to interact with the developmental programming of fetus, resulting in endocrine-related adverse effects.

OBJECTIVE

In this research, we aimed to determine the endocrine disrupting potential of paracetamol, indomethacin, alpha-methyldopa and pantoprazole which are frequently prescribed pharmaceuticals during pregnancy.

METHODS

In vitro aromatase inhibitory, estrogen receptor (ER) agonist/antagonist (E-Screen assay) and hormone biosynthesis modulatory effects (H295R steroidogenesis assay) of the selected pharmaceuticals were evaluated. Furthermore, their effects on viability of MCF-7/BUS and H295R cells were also evaluated by MTT assay.

RESULTS

None of the pharmaceuticals affected H295R cell viability. Only indomethacin reduced MCF-7/BUS cell viability at 100µM and 300µM. Among the tested pharmaceuticals, only paracetamol and indomethacin showed aromatase inhibitory activity with IC 50 values of 14.7 x 10 -5 M and 57.6 x 10 -5 M, respectively. Moreover, indomethacin displayed a biphasic ER agonist effect. ER antagonist effects of indomethacin and pantoprazole were confirmed by performing two stepped E-Screen assay. After the partial validation of the H295R steroidogenesis assay with forskolin and prochloraz, the effects of pharmaceuticals on synthesis of testosterone (T) and estradiol (E2) levels were tested. Alpha-methyldopa increased E2 at all tested concentrations and T at 1.48 and 4.4µM. Contrarily other tested pharmaceuticals did not affect steroidogenesis.

CONCLUSION

Present data suggest that all tested pharmaceuticals may have potential endocrine disrupting effect, which should be considered when used in pregnancy.

The Role of Biotransformation in the Activity of Endocrine Disruptors

An "endocrine disruptor" has been broadly defined as an exogenous chemical that interferes with the production, release, transport, metabolism, binding, action, or elimination of endogenous hormones which are responsible for homeostasis, reproduction, development or behaviour. Diverse groups of chemicals such as pharmaceuticals, phytoestrogens, natural hormones, and synthetic chemicals such as pesticides, plasticizers, phthalates, parabens, polychlorinated/polybrominated biphenyls, bisphenols are shown to interfere with the endocrine system and defined as EDs in the last three decades. As for all chemicals, the biotransformation of EDs has a decisive role in their potential toxic effects. Humans are exposed to vast amounts of diverse chemicals throughout their life. Fortunately, most of the chemicals are converted, via biotransformation reactions catalysed by enzymes, into more hydrophilic metabolites, which are readily excreted in urine or bile. Biotransformation reactions resulting in less toxic metabolites are known as detoxification. However, some biotransformation reactions are called bioactivation in which more toxic metabolites are formed. In the case of EDs, metabolites formed via bioactivation usually have a higher affinity for a hormone receptor or induce/inhibit an enzyme involved in the synthesis or catabolism of an endogenous hormone more dramatically compared to their parent compound. In the present review, the role of bioactivation in endocrine modulating effects of chemicals from all groups of Eds is highlighted, namely endogenous estrogens, phytoestrogens, synthetic/industrial chemicals, and pharmaceuticals.

In vitro evaluation of estrogenic, antiestrogenic and antitumor effects of amentoflavone

Apigenin, a flavonoid, is reported to act as an estrogen receptor (ER) agonist and inhibit aromatase enzyme. However, amentoflavone, a biflavonoid bearing two apigenin molecules, has not been evaluated for its endocrine modulatory effects. Besides, it is highly consumed by young people to build muscles, enhance mood and lose weight. In the present study, apigenin was used as a reference molecule and ER mediated as well as ER-independent estrogenic/antiestrogenic activity of amentoflavone was investigated. Antitumor activity of amentoflavone was also investigated in both ER positive (MCF-7 BUS) and triple-negative (MDA-MB-231) breast cancer cells and its cytotoxicity was evaluated in human breast epithelial cells (MCF-10A). Our data confirmed ER agonist, aromatase inhibitory and cytotoxic effects of apigenin in breast cancer cells, where no ER mediated estrogenic effect and physiologically irrelevant, slight, aromatase inhibition was found for amentoflavone. Although selective cytotoxicity of amentoflavone was found in MCF-7 BUS cells, it does not seem to be an alternative to the present cytotoxic drugs. Therefore, neither an adverse effect, mediated by an estrogenic/antiestrogenic effect of amentoflavone nor a therapeutical benefit would be expected from amentoflavone. Further studies could be performed to investigate its in vivo effects.

New indole-7-aldehyde derivatives as melatonin analogues; synthesis and screening their antioxidant and anticancer potential

Over the last decade, there has been substantial interest in the use of melatonin (MLT) and MLT-like compounds in the treatment of several diseases. MLT can scavenge different reactive oxygen species and can also stimulate the synthesis of antioxidant enzymes. Our ongoing study relies on changing the groups in the different modifiable sites of the indole ring to increase the antioxidant activity. In this study a new approach for substitution of indole ring as indole based MLT analogue was proposed. We report the synthesis and characterization of a series of new indole-7-aldehyde hydrazide/hydrazone derivatives as indole-based MLT analogues. Anticancer potential of the compounds were evaluated both by their antioxidant and CYP1 inhibitory activities. In vitro antioxidant capacity of the compounds was investigated both in a cell-based (DCFH assay) and a cell-free (DPPH assay) assay. Potential inhibitory effects of the compounds on CYP1 catalytic activity were investigated via EROD assay. Cytotoxic activity of the compounds was further evaluated by the MTT assay in CHO-K1 cells. MLT analogues having an o-halogenated aromatic moiety exhibited effective antioxidant properties without having any cytotoxic effect. In conclusion, MLT derivatives represent promising scaffolds for discovery of effective antioxidant agents.

Behaviour of 9-Ethyl-9H-carbazole Hydrazone Derivatives Against Oxidant Systems

Antioxidants are helpful in prevention of several diseases related with oxidative stress including neurodegenerative disorders. In recent studies, carbazoles were given proof of promising antioxidant activities. In this article, 9-ethyl-9H-carbazole hydrazone derivatives were synthesized, characterized and their in vitro antioxidant activity and possible cytotoxic effects were investigated. Furthermore, protective effect of the synthesized derivatives against amyloid β-induced damage in PC12 neuronal cells was examined by using MTT assay. The newly synthesized carbazoles were found to have radical scavenging activity with a varying potency both in cell-free and cell-based in vitro assays. Several compounds, especially such as 3d and 3e, 3m and 3n bearing two halogen groups on the phenyl ring, were found to have cytotoxic activity. However, their cytotoxic activities were not higher than that of melatonin. Several compounds also significantly protected neuronal PC12 cells against amyloid β-induced damage, which can be defined as neuroprotective agents. (4-(2-((9-Ethyl-9H-carbazol-3-yl)methylene)hydrazinyl)benzonitrile) 3r was found as the most active compound with both radical scavenging activity and neuroprotective effects against amyloid β-induced damage. These findings might provide an alternative strategy for developing novel carbazole derivatives for management of neurodegenerative diseases, such as Alzheimer's disease.

The Role of Oxidative Stress Modulators in Breast Cancer

BACKGROUND

Latest data from International Agency for Cancer Research shows that breast cancer is the leading cancer site in women and is the leading cause of death among female cancers. Induction of reactive oxygen species (ROS) and oxidative stress as a consequence of impaired balance between prooxidants and antioxidants are suggested to be involved in induction and progression of breast cancer. Cancer cells are found to exhibit higher levels of ROS compared to normal cells. However increased antioxidant defence which balances the oxidative status within the cancer cells suggests that high ROS levels may prevent tumorigenesis via various mechanisms. These contradictory roles of ROS and oxidative stress in breast cancer let scientists investigate potential oxidative stress modulators as anticancer strategies.

CONCLUSIONS

In the present review we address the mechanisms of ROS production in breast cancer cells, the role of impaired oxidative status as well as the benefits of introducing oxidative stress modulators in therapeutic strategies in breast cancer. This review is focusing more on melatonin which we have been working on during the last decade. Our data, in accordance with the literature, suggest an important role for melatonin in breast cancer prevention and adjuvant therapy.

In vitro antioxidant/prooxidant effects of combined use of flavonoids

The present study was undertaken to investigate the individual and combined antioxidant or prooxidant effects of genistein, daidzein and quercetin in human erythrocytes and rat microsomes in vitro. Their reducing potential against oxidation of a redox sensitive fluorescent probe, their protective effect against H₂O₂-induced membrane lipid peroxidation and their inhibitory effect on AAPH-induced hemolysis were evaluated. Genistein and daidzein were prooxidant in erythrocytes but antioxidant in microsomes where their metabolites might have been formed which suggests the importance of metabolic capacity in in vitro models to predict the physiological situation. Quercetin showed antioxidant effects in all models and conditions. Prooxidant effect of 'genistein-daidzein mixture', at their concentrations reflecting the real life, was suppressed by addition of quercetin to the mixture. Our study shows that flavonoids can exert prooxidant effects depending on the conditions, but the mixture effect should be considered while assessing their effects and safety in humans.

Development, characterization, and in vivo assessment of mucoadhesive nanoparticles containing fluconazole for the local treatment of oral candidiasis

This study aimed to develop a suitable buccal mucoadhesive nanoparticle (NP) formulation containing fluconazole for the local treatment of oral candidiasis. The suitability of the prepared formulations was assessed by means of particle size (PS), polydispersity index, and zeta potential measurements, morphology analysis, mucoadhesion studies, drug entrapment efficiency (EE), in vitro drug release, and stability studies. Based on the optimum NP formulation, ex vivo drug diffusion and in vitro cytotoxicity studies were performed. Besides, evaluation of the antifungal effect of the optimum formulation was evaluated using agar diffusion method, fungicidal activity-related in vitro release study, and time-dependent fungicidal activity. The effect of the optimum NP formulation on the healing of oral candidiasis was investigated in an animal model, which was employed for the first time in this study. The zeta potential, mucoadhesion, and in vitro drug release studies of various NP formulations revealed that chitosan-coated NP formulation containing EUDRAGIT(®) RS 2.5% had superior properties than other formulations. Concerning the stability study of the selected formulation, the formulation was found to be stable for 6 months. During the ex vivo drug diffusion study, no drug was found in receptor phase, and this is an indication of local effect. The in vitro antifungal activity studies showed the in vitro efficacy of the NP against Candida albicans for an extended period. Also, the formulation had no cytotoxic effect at the tested concentration. For the in vivo experiments, infected rabbits were successfully treated with local administration of the optimum NP formulation once a day. This study has shown that the mucoadhesive NP formulation containing fluconazole is a promising candidate with once-a-day application for the local treatment of oral candidiasis.

Novel indole-based melatonin analogues substituted with triazole, thiadiazole and carbothioamides: studies on their antioxidant, chemopreventive and cytotoxic activities

Melatonin (MLT) is a well-known free-radical scavenger, involving in the prevention of cellular damage that can lead to cancer, ageing and a variety of neurodegenerative diseases. Research on MLT-related compounds has been required to optimise the maximum pharmaceutical activity with the lowest side effects. In our ongoing research, we have synthesized new indole-based MLT analogues as potential antioxidant agents by modifying the MLT molecule. In this study, we build on previous findings, through the synthesis, characterization and in vitro antioxidant profiling of a series of new indole-based MLT analogues which possess triazole, thiadiazole and carbothioamides on the third position on the indole ring. In vitro antioxidant activity was investigated by evaluating their reducing effect against oxidation of a redox sensitive fluorescent probe and their radical scavenging activity was assessed via the DPPH assay. In addition, in vitro cytotoxic effects of newly synthesized compounds were investigated in CHO-K1 cells using the MTT assay.

Melatonin, its metabolites and its synthetic analogs as multi-faceted compounds: antioxidant, prooxidant and inhibitor of bioactivation reactions

It is known that melatonin (MLT) and some of its metabolites act as antioxidants by scavenging free radicals as well as increasing the activity of antioxidant enzymes in the body. MLT is suggested to exert beneficial effects via various mechanisms in the treatment of many diseases, such as cancer, neurodegenerative diseases, epilepsy, diabetes mellitus and obesity. People working in nightshift exhibit decreased MLT levels that are suggested to be related with increased risk of hormone-related diseases. Similarly blind people were found to have increased MLT levels protecting against many diseases. This review briefly summarizes the published reports supporting these beneficial effects of MLT. Furthermore the present review involves recent developments related to the antioxidant effect of remarkable and multi-faceted molecule MLT as well as its metabolites and its synthesized analogues. The role of MLT as an inhibitor of bioactivation reactions is also discussed.

Antioxidant activity of indole-based melatonin analogues in erythrocytes and their voltammetric characterization

Melatonin (MLT) is a strong free-radical scavenger, which protects the body from the effects of oxidants. In recent years, MLT have been described resulting in much attention in the development of synthetic compounds possessing. As a part of our ongoing study a series of indole-based MLT analogue hydrazide/hydrazone derivatives were synthesized, characterized and in vitro antioxidant activity was investigated by evaluating their reducing effect against oxidation of a redox sensitive fluorescent probe. Membrane stabilizing effect of all compounds was also investigated by lactate dehydrogenase leakage assay. Furthermore voltammetric methods have been applied to the synthesized compounds to characterize oxidation potentials to get insight into their metabolism owing to the oxidation mechanisms taking place at the electrode and in the body share similar principles.

Novel indole-based analogs of melatonin: synthesis and in vitro antioxidant activity studies

The aim of this study was to synthesize and examine possible in vitro antioxidant effects of indole-based melatonin analogue compounds. As a part of our ongoing study nineteen indole hydrazide/hydrazone derivatives were synthesized, characterized and their in vitro antioxidant activity was investigated by three different assays: by evaluating their reducing effect against oxidation of a redox sensitive fluorescent probe, by examining their protective effect against H₂O₂-induced membrane lipid peroxidation and by determining their inhibitory effect on AAPH–induced hemolysis of human erythrocytes. The results indicated significant strong antioxidant activity for most of the compounds, when compared to melatonin.

Synthesis and evaluation ofin vitroantioxidant capacities of some benzimidazole derivatives

New, except 1d, melatonin analogue benzimidazole derivatives were synthesized and characterized in the present study. The potential role of melatonin as an antioxidant by scavenging and detoxifying ROS raised the possibility that compounds that are analogous to melatonin can also be used for their antioxidant properties. Therefore the antioxidant effects of the newly synthesized compounds were investigated in vitro by means of their inhibitory effect on hydrogen peroxide-induced erythrocyte membrane lipid peroxidation (EMLP) and on various erythrocyte antioxidant enzymes viz. superoxide dismutase (SOD), catalase (CAT) and glucose-6-phosphate dehydrogenase (G6PD). The synthesized benzimidazole derivatives showed remarkable antioxidant activity in vitro in the H2O2-induced EMLP system. Furthermore their effects on various antioxidant enzymes are discussed and evaluated from the perspective of structure- activity relationships.

Application of lipid peroxidation and protein oxidation biomarkers for oxidative damage in mammalian cells. A comparison with two fluorescent probes

We recently developed two biomarker sets for oxidative damage: one for determination of lipid peroxidation (LPO) degradation products; acetaldehyde, propanal, butanal, pentanal, hexanal, heptanal, octanal, nonanal, malondialdehyde and acetone, by a gas chromatography-electron capture detection method, and the other for protein oxidation products such as o,o'-dityrosine, by an isotope dilution high performance liquid chromatography-tandem mass spectrometry method. In the present study, we explored the possibility to utilize these biomarkers for determining the oxidative damage in liver mammalian cells in vitro. Two different treatments were chosen for inducing oxidative stress in Chinese Hamster ovary cells: menadione and copper plus hydrogen peroxide (Cu2+/H2O2). Cells were incubated with the model compounds in the presence or absence of vitamin E and C, and cytotoxicity was evaluated by a nuclear-dye method. Results were compared to two fluorescent probes, H2DCF-DA and C11 -BODIPY581/591, which have been used for determining the formation of free radicals in the cells. From ten LPO degradation products, eight were increased significantly following incubation with menadione in cell lysate or incubation media. Menadione-induced oxidative stress was also confirmed by oxidation of fluorescent probes. However, no increased formation of protein oxidation products was observed. Vitamin E and C did not diminish the formation of LPO degradation products that were increased by menadione. Although Cu2+/H2O2 did not induce oxidation of fluorescent probes, it induced formation of six out of ten LPO degradation products. Vitamin E and C did not diminish the formation of LPO degradation products; vitamin C even substantially increased the formation of acetaldehyde and propanal, which is in line with its reported prooxidant action under certain conditions. Vitamin C also caused two-fold increase in Cu2+/H2O2-induced o,o'-dityrosine formation when applied simultaneously. In conclusion, our present results show that the LPO biomarker set can be used for evaluation of oxidant capacity and the toxic potential of various chemicals in an in vitro cell model. These biomarkers might even be more sensitive than measuring protein oxidation products or oxidation of fluorescent probes.

Screening the Oxidative Potential of Several Mono- and Di-Halogenated Biphenyls and Biphenyl Ethers in Rat Hepatocytes

The present study was designed to investigate the potential of reactive oxygen species (ROS) generating and subsequent ROS-mediated lipid peroxidation (LPO) inducing effect of several mono- and di-halogenated biphenyls and biphenyl ethers in rat hepatocytes in vitro. For this aim, 4-chloro- and 4-bromo biphenyl (4-CB and 4-BB), 4-OH, 4'-BB, 4-bromo diphenylether (4-BDE), 4,4'-dichlorobiphenyl (4,4'-DCB), 4,4'-dibromobiphenyl (4,4'-DBB), and 3,4-dichlorobiphenyl (3,4-DCB) were incubated with freshly isolated rat hepatocytes. Their oxidative potential was evaluated by detecting the intracellular ROS formation by oxidant-sensing fluorescent probes (2',7'-dichlorofluorescein diacetate and C(11)-BODIPY(581/591)) using a multiplate reader and determining the levels of eight LPO products (formaldehyde, malondialdehyde, propanal, butanal, pentanal, hexanal, octanal, and nonanal) by a gas chromatography-electron capture detection. 4-BDE was found to be active both in cytoplasm and in the cell membrane in terms of inducing the formation of ROS. Another important finding was the increase in ROS-inducing potential of 4-BB when the same concentration of the hydroxylated derivative, 4-OH,4'-BB, was incubated with hepatocytes. 4-BDE was also found to be the most effective among all tested compounds in inducing LPO where 4-OH, 4'-BB was again more potent than its unmetabolized form, 4-BB. Lactate dehydrogenase leakage analyses indicated that all tested compounds are cytotoxic; 4-BDE caused the highest LDH leakage compared to other mono-halogenated biphenyls tested. Our results suggest that ROS formation by chlorinated biphenyls and mono-hydroxylated bromobiphenyls, and concomitant induction of LPO might be involved in the cytotoxic effects of these industrial pollutants. Similar effects of mono-BDE are also reported, which is a novel observation.

Misincorporation of free m-tyrosine into cellular proteins: a potential cytotoxic mechanism for oxidized amino acids

In vitro studies demonstrate that the hydroxyl radical converts L-phenylalanine into m-tyrosine, an unnatural isomer of L-tyrosine. Quantification of m-tyrosine has been widely used as an index of oxidative damage in tissue proteins. However, the possibility that m-tyrosine might be generated oxidatively from free L-phenylalanine that could subsequently be incorporated into proteins as an L-tyrosine analogue has received little attention. In the present study, we demonstrate that free m-tyrosine is toxic to cultured CHO (Chinese-hamster ovary) cells. We readily detected radiolabelled material in proteins isolated from CHO cells that had been incubated with m-[14C]tyrosine, suggesting that the oxygenated amino acid was taken up and incorporated into cellular proteins. m-Tyrosine was detected by co-elution with authentic material on HPLC and by tandem mass spectrometric analysis in acid hydrolysates of proteins isolated from CHO cells exposed to m-tyrosine, indicating that free m-tyrosine was incorporated intact rather than being metabolized to other products that were subsequently incorporated into proteins. Incorporation of m-tyrosine into cellular proteins was sensitive to inhibition by cycloheximide, suggesting that protein synthesis was involved. Protein synthesis using a cell-free transcription/translation system showed that m-tyrosine was incorporated into proteins in vitro by a mechanism that may involve L-phenylalanine-tRNA synthetase. Collectively, these observations indicate that m-tyrosine is toxic to cells by a pathway that may involve incorporation of the oxidized amino acid into proteins. Thus misincorporation of free oxidized amino acids during protein synthesis may represent an alternative mechanism for oxidative stress and tissue injury during aging and disease.

Correlation between clinical indicators of lead poisoning and oxidative stress parameters in controls and lead-exposed workers

The present study was undertaken to investigate the involvement of oxidative damage in lead-induced toxicity in humans and to enlighten whether oxidative stress indicators are correlated with the known indices of lead toxicity. For these purposes, selected oxidative stress parameters along with some clinical indices of lead poisoning were determined in blood of battery plant workers and control subjects. Workers had significantly increased erythrocyte malondialdehyde (MDA) levels, catalase and glucose-6-phosphate dehydrogenase (G6PD) activities, and decreased blood glutathione:glutathione disulfide ratio compared to the controls. Increased blood lead concentrations and zinc protoporphyrin (ZPP) levels, and decreased delta-aminolevulinic acid dehydratase (ALAD) activity were used as clinical indices of lead toxicity. Statistically significant correlation between oxidative stress parameters and clinical indices implies that disrupted prooxidant/antioxidant balance might contribute to lead-induced toxicity in erythrocytes. A significant correlation was found between ALAD activity and blood lead levels in human subjects. Similarly significant correlation between ALAD activity and erythrocyte MDA concentrations was shown. Present data indicates that ALAD can serve as a valuable biomarker of oxidative stress in lead-exposed hematological system as well as being a biochemical indicator of lead exposure.

Oxidative stress in a phenylketonuria animal model

Oxidative stress is seen in various metabolic disorders for unknown reasons. Oxidative stress is defined as an imbalance between pro-oxidant and antioxidant status in favor of the former. This study investigated whether oxidative stress exists in phenylketonuria (PKU) using the BTBR-Pah(enu2) animal model for PKU. Animals (14-24 weeks old) were sacrificed and brain and red blood cells (RBCs) were obtained aseptically. The lipid peroxidation by-product, evaluated as malondialdehyde (MDA), was significantly higher in the brains and RBCs of PKU animals (n = 6) than in controls (n = 6). Glutathione/glutathione disulfide, a good indicator for tissue thiol status, was significantly decreased both in the brains and RBCs. Some antioxidant enzymes were also analyzed in RBCs, including glucose-6-phosphate dehydrogenase (G6PD), which provides the RBC's main reducing power, reduced nicotinamide adenine dinucleotide phosphate (NADPH), and catalase detoxifies H2O2 by catalyzing its reduction to O2 and H2O. Both catalase and G6PD were significantly increased in the RBCs of PKU animals.

Toxic metals and oxidative stress part I: mechanisms involved in metal-induced oxidative damage

Toxic metals (lead, cadmium, mercury and arsenic) are widely found in our environment. Humans are exposed to these metals from numerous sources, including contaminated air, water, soil and food. Recent studies indicate that transition metals act as catalysts in the oxidative reactions of biological macromolecules therefore the toxicities associated with these metals might be due to oxidative tissue damage. Redox-active metals, such as iron, copper and chromium, undergo redox cycling whereas redox-inactive metals, such as lead, cadmium, mercury and others deplete cells' major antioxidants, particularly thiol-containing antioxidants and enzymes. Either redox-active or redox-inactive metals may cause an increase in production of reactive oxygen species (ROS) such as hydroxyl radical (HO.), superoxide radical (O2.-) or hydrogen peroxide (H2O2). Enhanced generation of ROS can overwhelm cells' intrinsic antioxidant defenses, and result in a condition known as "oxidative stress". Cells under oxidative stress display various dysfunctions due to lesions caused by ROS to lipids, proteins and DNA. Consequently, it is suggested that metal-induced oxidative stress in cells can be partially responsible for the toxic effects of heavy metals. Several studies are underway to determine the effect of antioxidant supplementation following heavy metal exposure. Data suggest that antioxidants may play an important role in abating some hazards of heavy metals. In order to prove the importance of using antioxidants in heavy metal poisoning, pertinent biochemical mechanisms for metal-induced oxidative stress should be reviewed.