Auto-oxidation Of Dopamine Research Articles

Sustainable hydrogen peroxide production based on dopamine through Janus-like mechanism transition from chemical to photocatalytic reactions

With increasing interest in hydrogen peroxide (H2O2) as an environmentally friendly oxidant for environmental remediation and a promising liquid fuel, we present herein a novel method for green and sustainable H2O2 production using versatile dopamine-based materials, demonstrating the conversion of H2O2 generation mechanism from a chemical to a photocatalytic process along with the structural transformation of dopamine to polydopamine. During the auto-oxidation of dopamine in the presence of oxygen, which involved the transition of catechol to hydroquinone groups and the ring closure of the amine group, H2O2 was chemically generated at a rate of 1.61 mmol g-1h−1 without an extra electron donor and additional energy sources. In contrast, self-polymerized polydopamine exhibited an effective photocatalytic H2O2 generation at a rate of 0.53 mmol g-1h−1 owing to its visible light absorptivity and semiconducting property. We anticipate that these unique properties of dopamine will provide a new class of organic-based, highly efficient solar-to-H2O2 conversion and sustainable energy systems in the future.

Effects of platinum-coexisting dopamine with X-ray irradiation upon human glioblastoma cell proliferation.

In brain tumors, neurotransmitters and platinum drugs may have some interaction, but their role in radiation therapy remains unclear. We investigated the effects of dopamine in combination with platinum on human glioblastoma U-251MG cells upon X-ray irradiation, comparing with L-DOPA, 2-phenylethylamine and temozolomide. Cell proliferation of U-251MG cells was prominently decreased by dopamine in combination with 10μM platinum upon 4Gy of X-ray irradiation, accompanied with intracellular reactive oxygen species generation and mitotic catastrophe. Platinum alone did not increase intracellular reactive oxygen species. On the other hand, L-DOPA in combination with platinum did not decrease cell proliferation regardless of X-ray irradiation. It was clearly shown that 2-phenylethylamine did not suppress cell proliferation as compared to dopamine. Temozolomide decreased cell proliferation in a dose-dependent manner upon X-ray irradiation. However, the combined administration of temozolomide and platinum did not further decrease cell proliferation. The platinum nanoparticles were gradually taken up by cells after administration as determined by ICP analysis. Our results suggest that platinum-coexisting dopamine led cells to mitotic catastrophe due to increased production of intracellular reactive oxygen species which was boosted by X-ray and platinum-catalyzed auto-oxidation of dopamine, and thereby cell proliferation was suppressed. In addition, normal human fibroblast OUMS-36T-1 cells were subjected to experiments. Regarding the effect of the combined administration of dopamine and platinum on each cell which was exposed to X-ray, cell proliferation was decreased in U-251MG cells by the combined administration of platinum, whereas that was not decreased in OUMS-36T-1 cells. This provides one basic insight into the effects of dopamine in combined with platinum on radiation therapy for glioblastoma.

Converging dopaminergic neurotoxicity mechanisms of antipsychotics, methamphetamine and levodopa.

Drugs affecting dopaminergic neurotransmission may exert toxic and beneficial effects that persist after discontinuation by modulating gene expression in key brain regions. Drug addiction, cravings and the tardive symptoms associated with chronic exposure to antipsychotics are among the most common processes attributed to long-term dopaminergic neurotoxicity. The purpose of this review was to investigate the mechanisms of dopaminergic neurotoxicity induced by neuroleptic drugs, dopamine agonists, levodopa, stimulants and known dopaminergic neurotoxins MATERIALS AND METHODS: A PubMed search for each of the dopaminergic compounds in question was carried out. The heterogenous nature of the relevant preclinical studies precluded a systematic review, so a narrative review was carried out. The dopaminergic neurotoxins 6-oxidopamine and 1-methyl-4-phenyl-tetrahydropyridine (MPTP) promote oxidative stress and inhibit mitochondrial function, while their affinity for the dopamine transporter ensures they are attain toxic intracellular concentrations exclusively in dopaminergic neurons. Stimulants which inhibit the vesicular monoamine transporter such as amphetamine and its derivatives promote oxidative stress by greatly increasing intracellular dopamine concentrations and enabling dopamine autooxidation. Antipsychotics increase dopamine release and turnover by blocking autoinhibitory D2 receptors and lead to upregulation of post-synaptic D2 receptors. Dopamine agonists may slow the progression of Parkinson's disease by reducing dopamine turnover, but downregulation of D2 receptors may underlie their behavioural toxicity. Though the mechanisms have not been completely elucidated yet, it seems drugs which affect dopaminergic neurotransmission may exert long-term effects which reverse slowly upon discontinuation, if at all. Until the nature of these changes is clear it would be best to utilize drugs which affect dopaminergic neurotransmission cautiously especially if prolonged treatment is required.

Anodic Coating of 1.4622 Stainless Steel with Polydopamine by Repetitive Cyclic Voltammetry and Galvanostatic Deposition

Polydopamine exhibits high potential as coating of materials used for medical devices, antifouling, and corrosion prevention. Repetitive cyclic voltammetry on gold electrodes was used to deposit polydopamine by anodic oxidation. Investigation of polydopamine formation in the pH range 5–8 showed that coating at pH 6.5 permits selective polydopamine formation on the electrode, while auto-oxidation of dopamine in the electrolyte could be suppressed. The optimized conditions then were transferred to stainless steel electrodes. At pH 6.5–6.6, an anodic peak current density of 12.9 μA/mm2 was measured using a plane stainless steel anode in 5 mM dopamine solution. During experimental scale-up, anodic coating of 1.4622 steel plates in galvanostatic experiments led to formation of a colored polydopamine surface, which exhibited reduction of the water contact angle by 13.5°. The controlled anodic polydopamine formation is of a particular value for an economic polydopamine coating of the conductive material in larger-scale applications.

L-3,4-dihydroxyphenylalanine (l-DOPA) modulates brain iron, dopaminergic neurodegeneration and motor dysfunction in iron overload and mutant alpha-synuclein mouse models of Parkinson's disease.

Treatment with the dopamine (DA) precursor l-3,4-dihydroxyphenylalanine (l-DOPA) provides symptomatic relief arising from DA denervation in Parkinson's disease. Mounting evidence that DA autooxidation to neurotoxic quinones is involved in Parkinson's disease pathogenesis has raised concern about potentiation of oxidative stress by l-DOPA. The rate of DA quinone formation increases in the presence of excess redox-active iron (Fe), which is a pathological hallmark of Parkinson's disease. Conversely, l-DOPA has pH-dependent Fe-chelating properties, and may act to 'redox silence' Fe and partially allay DA autoxidation. We examined the effects of l-DOPA in three murine models of parkinsonian neurodegeneration: early-life Fe overexposure in wild-type mice, transgenic human (h)A53T mutant α-synuclein (α-syn) over-expression, and a combined 'multi-hit' model of Fe-overload in hA53T mice. We found that l-DOPA was neuroprotective and prevented age-related Fe accumulation in the substantia nigra pars compacta (SNc), similar to the mild-affinity Fe chelator clioquinol. Chronic l-DOPA treatment showed no evidence of increased oxidative stress in wild-type midbrain and normalized motor performance, when excess Fe was present. Similarly, l-DOPA also did not exacerbate protein oxidation levels in hA53T mice, with or without excess nigral Fe, and showed evidence of neuroprotection. The effects of l-DOPA in Fe-fed hA53T mice were somewhat muted, suggesting that Fe-chelation alone is insufficient to attenuate neuron loss in an animal model also recapitulating altered DA metabolism. In summary, we found no evidence in any of our model systems that l-DOPA treatment accentuated neurodegeneration, suggesting DA replacement therapy does not contribute to oxidative stress in the Parkinson's disease brain.

Enzymatically Active Polydopamine @ Alkaline Phosphatase Nanoparticles Produced by NaIO4 Oxidation of Dopamine.

Polydopamine (PDA) deposition, obtained from the oxidation of dopamine and other catecholamines, is a universal way to coat all known materials with a conformal coating which can subsequently be functionalized at will. The structural analogies between polydopamine and eumelanin, the black-brown pigment of the skin, were incited to produce stable polydopamine nanoparticles in solution, instead of amorphous precipitates obtained from the oxidation of dopamine. Herein, we demonstrate that size-controlled and colloidally stable PDA-based nanoparticles can be obtained in acidic conditions, where spontaneous auto-oxidation of dopamine is suppressed, using sodium periodate as the oxidant and a protein, like alkaline phosphatase (ALP), as a templating agent. The size of the PDA@ALP nanoparticles depends on the dopamine/enzyme ratio and the obtained particles display enzymatic activity of alkaline phosphatase, with an activity extending up to two weeks after particle synthesis. The PDA@ALP nanoparticles can be engineered in polyelectrolyte multilayered films to potentially design model biosensors.

The role of free radicals in the aging brain and Parkinson's Disease: convergence and parallelism.

Free radical production and their targeted action on biomolecules have roles in aging and age-related disorders such as Parkinson’s disease (PD). There is an age-associated increase in oxidative damage to the brain, and aging is considered a risk factor for PD. Dopaminergic neurons show linear fallout of 5–10% per decade with aging; however, the rate and intensity of neuronal loss in patients with PD is more marked than that of aging. Here, we enumerate the common link between aging and PD at the cellular level with special reference to oxidative damage caused by free radicals. Oxidative damage includes mitochondrial dysfunction, dopamine auto-oxidation, α-synuclein aggregation, glial cell activation, alterations in calcium signaling, and excess free iron. Moreover, neurons encounter more oxidative stress as a counteracting mechanism with advancing age does not function properly. Alterations in transcriptional activity of various pathways, including nuclear factor erythroid 2-related factor 2, glycogen synthase kinase 3β, mitogen activated protein kinase, nuclear factor kappa B, and reduced activity of superoxide dismutase, catalase and glutathione with aging might be correlated with the increased incidence of PD.

Alterations in striatal oxidative stress level produced by pharmacological manipulation of dopamine as shown by a novel synthetic marker molecule

Oxidative stress (OS) is thought to participate in neurodegenerative diseases such as Parkinson’s disease, but the contribution of dopamine metabolism and auto-oxidation to OS in Parkinson’s and other diseases is not clear. Oxidative stress in rat striatum was measured by microdialysis using a novel synthetic compound composed of tyrosine and linoleic acid (LT), and determination of the oxidation products LT-OOH and LT-epoxy by HPLC-MS. Since LT is non-diffusible through the microdialysis membrane, the oxidized products formed in microdialyzate reflect oxidation state in the extracellular compartment. The extracellular oxidative stress (OS ec) was compared with intracellular oxidative stress (OS ic) as measured by tissue levels of oxidized and reduced glutathione and 7-ketocholesterol. Reserpinization caused an increase in OS ic but a reduction in OS ec. Inhibition of both subtypes of monoamine oxidase (MAO-A and MAO-B) with tranylcypromine caused a reduction in both OS ic and OS ec whereas selective inhibition of MAO-A with clorgyline caused a reduction in Os ic but no change in OS ec. A high dose (10 mg/kg) of amphetamine caused an increase in OS ec whereas a smaller dose (4 mg/kg) caused a reduction in OS ec. Both doses of amphetamine reduced OS ic. The present findings are consistent with a role of monoamine oxidase as well as dopamine auto-oxidation in production of striatal OS.

Dopamine auto‐oxidation aggravates non‐apoptotic cell death induced by over‐expression of human A53T mutant alpha‐synuclein in dopaminergic PC12 cells

In this study, we demonstrated that transient transfection and over-expression of human mutant A53T alpha-synuclein (alpha-syn) could induce expression level- and time-dependent, non-apoptotic cell death in PC12 cells, while wild-type and mutant A30P alpha-syn could not. The non-apoptotic cell death induced by over-expression of A53T alpha-syn in PC12 cells was found to be dopamine (DA) related. It could be alleviated by nerve growth factor but not by chemicals that abrogate endoplasmic reticulum stress. Furthermore, PC12 cell death could be alleviated by N-acetyl-cysteine (NAC) as well as by L-cysteine; but not by cell permeable tyrosinase inhibitors. NAC could prevent DA auto-oxidation and tyrosinase-catalyzed DA oxidation, whereas L-cysteine could potently abrogate DA auto-oxidation but could not prevent tyrosinase-catalyzed DA oxidation. Both NAC and L-cysteine could increase the reduced and total GSH levels, and concurrently decrease the oxidized GSH level in PC12 cells. On the other hand, over-expression of human mutant A53T alpha-syn could decrease the reduced and total GSH levels, and increase the oxidized GSH level in the cells. Taken together, we concluded that auto-oxidation of endogenous DA aggravates non-apoptotic cell death induced by over-expression of human mutant A53T alpha-syn in PC12 cells.

Roles of Glutathione (GSH) in Dopamine (DA) Oxidation Studied by Improved Tandem HPLC Plus ESI-MS

In this study, new procedure with improved tandem HPLC plus ESI-MS was utilized to decipher the protective role of glutathione (GSH) against dopamine (DA) oxidation. We demonstrated that auto-oxidation of DA could produce aminochrome (AM, a cyclized DA quinone), which could be effectively abrogated by reductants, especially by GSH. Furthermore GSH was demonstrated to be able to conjugate with AM to form various conjugates via condensation reactions without enzymatic catalysis. The GSH-AM conjugates tend to aggregate, possibly mediated by conjugated AM structures, but could be inhibited by GSH. We hypothesized that proteins conjugated by AM might facilitate Lewy body formation of Parkinson's disease (PD) in dopaminergic neurons via similar polymerization. We proposed that GSH could protect dopaminergic neurons against DA-induced toxicity via various mechanisms. The imbalance between DA oxidation and GSH protective capacity could be a key factor contributing to PD. Strategies to use GSH analogues, GSH inducers or to control DA oxidation might work to control PD onset and development.

Dopamine as a Potent Inducer of Cellular Glutathione and NAD(P)H:Quinone Oxidoreductase 1 in PC12 Neuronal Cells: A Potential Adaptive Mechanism for Dopaminergic Neuroprotection

Dopamine auto-oxidation and the consequent formation of reactive oxygen species and electrophilic quinone molecules have been implicated in dopaminergic neuronal cell death in Parkinson's disease. We reported here that in PC12 dopaminergic neuronal cells dopamine at noncytotoxic concentrations (50-150 muM) potently induced cellular glutathione (GSH) and the phase 2 enzyme NAD(P)H:quinone oxidoreductase 1 (NQO1), two critical cellular defenses in detoxification of ROS and electrophilic quinone molecules. Incubation of PC12 cells with dopamine also led to a marked increase in the mRNA levels for gamma-glutamylcysteine ligase catalytic subunit (GCLC) and NQO1. In addition, treatment of PC12 cells with dopamine resulted in a significant elevation of GSH content in the mitochondrial compartment. To determine whether treatment with dopamine at noncytotoxic concentrations, which upregulated the cellular defenses could protect the neuronal cells against subsequent lethal oxidative and electrophilic injury, PC12 cells were pretreated with dopamine (150 muM) for 24 h and then exposed to various cytotoxic concentrations of dopamine or 6-hydroxydopamine (6-OHDA). We found that pretreatment of PC12 cells with dopamine at a noncytotoxic concentration led to a remarkable protection against cytotoxicity caused by dopamine or 6-OHDA at lethal concentrations, as detected by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium reduction assay. In view of the critical roles of GSH and NQO1 in protecting against dopaminergic neuron degeneration, the above findings implicate that upregulation of both GSH and NQO1 by dopamine at noncytotoxic concentrations may serve as an important adaptive mechanism for dopaminergic neuroprotection.

Melatonin protects SK‐N‐SH neuroblastoma cells from amphetamine‐induced neurotoxicity

Several hypotheses regarding the mechanism underlying amphetamine-induced neurotoxicity have been proposed. One of them is based on the observation of free radical formation and oxidative stress produced by auto-oxidation of dopamine (DA). The formation of DA-related reactive oxygen species (ROS) such as superoxide and hydroxyl radicals appears to play an important role in amphetamine-induced neurotoxicity. Melatonin, the main secretory product of pineal gland, is well known for its protective effects that are currently attributed mainly to its radical scavenging and antioxidant properties. The present study was conducted to investigate the protective effects of melatonin on d-amphetamine (AMPH)-induced neurotoxicity in cultured human dopaminergic neuroblastoma SK-N-SH cells. Our data indicate that AMPH significantly reduces cell viability, induces oxidative stress (enhances ROS production and malondialdehyde levels), up-regulates alpha-synuclein expression and decreases intracellular ATP levels. However, pretreatment of SK-N-SH cells with melatonin prevents AMPH-induced loss of cell viability and induction of oxidative stress, while reducing alpha-synuclein expression and increasing ATP production. These results suggest that the antioxidant properties of melatonin may provide a protective mechanism against AMPH-induced neuronal degeneration.

Studies of interaction between iron (III) and intermediates of synthetic neuromelanin by means of cyclic voltammetry of and dopamine

Neuromelanin is a complex polymer pigment found primarily in the dopaminergic neurons of the human substantia nigra, whose composition is complex including production of dopamine auto-oxidation, glutathione and a variety of amino acid. Neuromelanin forms stable complex with iron (III). We observed that 5,6-dihydroxyindole and its ramification possessed strong ability of chelating iron (III), and they are the production of dopamine auto-oxidation under physiological pH condition. In the present of l-cysteine, the relative yields of electrochemical oxidation of dopamine also had strong ability of chelating iron (III). The experimental results suggest that 5,6-dihydroxyindole and 5-S-cysteineldopamine play important roles in the process of synthetic neuromelanin chelating iron (III).

Methamphetamine‐induced dopaminergic neurotoxicity is regulated by quinone formation‐related molecules

Recently, the neurotoxicity of dopamine (DA) quinone formation by auto-oxidation of DA has focused on dopaminergic neuron-specific oxidative stress. In the present study, we examined DA quinone formation in methamphetamine (METH)-induced dopaminergic neuronal cell death using METH-treated dopaminergic cultured CATH.a cells and METH-injected mouse brain. In CATH.a cells, METH treatment dose-dependently increased the levels of quinoprotein (protein-bound quinone) and the expression of quinone reductase in parallel with neurotoxicity. A similar increase in quinoprotein levels was seen in the striatum of METH (4 mg/kg X4, i.p., 2 h interval)-injected BALB/c mice, coinciding with reduction of DA transporters. Furthermore, pretreatment of CATH.a cells with quinone reductase inducer, butylated hydroxyanisole, significantly and dose-dependently blocked METH-induced elevation of quinoprotein, and ameliorated METH-induced cell death. We also showed the protective effect of tyrosinase, which rapidly oxidizes DA and DA quinone to form stable melanin, against METH-induced dopaminergic neurotoxicity in vitro and in vivo using tyrosinase null mice. Our results indicate that DA quinone formation plays an important role, as a dopaminergic neuron-specific neurotoxic factor, in METH-induced neurotoxicity, which is regulated by quinone formation-related molecules.

Quinone formation as dopaminergic neuron-specific oxidative stress in the pathogenesis of sporadic Parkinson's disease and neurotoxin-induced parkinsonism.

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by dopaminergic neuron-specific degeneration in the substantia nigra. A number of gene mutations and deletions have been reported to play a role in the pathogenesis of familial PD. Moreover, a number of pathological and pharmacological studies on sporadic PD and dopaminergic neurotoxin-induced parkinsonism have hypothesized that mitochondrial dysfunction, inflammation, oxidative stress, and dysfunction of the ubiquitin-proteasome system all play important roles in the pathogenesis and progress of PD. However, these hypotheses do not yet fully explain the mechanisms of dopaminergic neuron-specific cell loss in PD. Recently, the neurotoxicity of dopamine quinone formation by auto-oxidation of dopamine has been shown to cause specific cell death of dopaminergic neurons in the pathogenesis of sporadic PD and dopaminergic neurotoxin-induced parkinsonism. Furthermore, this quinone formation is closely linked to other representative hypotheses in the pathogenesis of PD. In this article, we mainly review recent studies on the neurotoxicity of quinone formation as a dopaminergic neuron-specific oxidative stress and its role in the etiology of PD, in addition to several neuroprotective approaches against dopamine quinone-induced toxicity.

In vivo Dopamine Pre-Synaptic Receptors and Antioxidant Activities in Patients with Alzheimer’s Disease, Dementia with Lewy Bodies and in Controls

The degeneration of dopaminergic cells in dementia with Lewy bodies (DLB) may provide an important source of additional free radical generation. As a result, the oxidative stress status in DLB could be significantly enhanced. Subsequently, the levels of endogenous antioxidants, which are an indirect measure of free radical activities, may be different in DLB patients when compared with Alzheimer’s disease (AD) patients and controls. In this preliminary study, we measured the activities of superoxide dismutase (SOD), catalase (CAT), glutathione (GLU) and total antioxidant capacity in the blood of DLB, AD and control subjects. The state of nigrostriatal dopaminergic system was also assessed in vivo by using a radioactive ligand with an affinity for the dopamine pre-synaptic receptors and by imaging with single-photon emission tomography. Data obtained showed a decrease in dopamine pre-synaptic receptors in all the brain regions of DLB patients. The levels of SOD did not differ significantly between DLB, AD and control subjects. However, GLU levels were significantly higher in the DLB patients when compared with AD patients (p < 0.05) and controls (p < 0.01). CAT blood levels were also higher in DLB when compared with AD, but this did not reach statistical significance. The results suggest that a different oxidative stress state may exist in DLB. This may occur due to increased free radical production from the degeneration of dopaminergic cells and auto-oxidation of dopamine, the availability of which may be maintained in early-stage DLB disease as a result of the compensatory increase in its turnover from the remaining dopaminergic cells.

Dopamine- or L-DOPA-induced neurotoxicity: the role of dopamine quinone formation and tyrosinase in a model of Parkinson's disease.

Dopamine (DA)- or L-dihydroxyphenylalanine-(L-DOPA-) induced neurotoxicity is thought to be involved not only in adverse reactions induced by long-term L-DOPA therapy but also in the pathogenesis of Parkinson's disease. Numerous in vitro and in vivo studies concerning DA- or L-DOPA-induced neurotoxicity have been reported in recent decades. The reactive oxygen or nitrogen species generated in the enzymatical oxidation or auto-oxidation of an excess amount of DA induce neuronal damage and/or apoptotic or non-apoptotic cell death; the DA-induced damage is prevented by various intrinsic and extrinsic antioxidants. DA and its metabolites containing two hydroxyl residues exert cytotoxicity in dopaminergic neuronal cells mainly due to the generation of highly reactive DA and DOPA quinones which are dopaminergic neuron-specific cytotoxic molecules. DA and DOPA quinones may irreversibly alter protein function through the formation of 5-cysteinyl-catechols on the proteins. For example, the formation of DA quinone-alpha-synuclein consequently increases cytotoxic protofibrils and the covalent modification of tyrosine hydroxylase by DA quinones. The melanin-synthetic enzyme tyrosinase in the brain may rapidly oxidize excess amounts of cytosolic DA and L-DOPA, thereby preventing slowly progressive cell damage by auto-oxidation of DA, thus maintainng DA levels. Since tyrosinase also possesses catecholamine-synthesizing activity in the absence of tyrosine hydroxylase (TH), the double-edged synthesizing and oxidizing functions of tyrosinase in the dopaminergic system suggest its potential for application in the synthesis of DA, instead of TH in the degeneration of dopaminergic neurons, and in the normalization of abnormal DA turnover in the long-term L-DOPA-treated Parkinson's disease patients.

Neuroprotective actions of selegiline.

Selegiline, a selective inhibitor of monoamine oxidase-B (MAO-B), was one of the first adjunct therapies in clinical neurology. A retrospective analysis of data from patients with Parkinson's disease found a significant increase in survival in those treated with selegiline plus L-dopa compared with L-dopa alone. The mechanism of action of selegiline is complex and cannot be explained solely by its MAO-B inhibitory action. Pretreatment with selegiline can protect neurons against a variety of neurotoxins, such as 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP), 6-hydroxydopamine, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), methyl-beta-acetoxyethyl-2-chloroethylamine (AF64A), and 5,6-dihydroxyserotonin, which damage dopaminergic, adrenergic, cholinergic, and sertoninergic neurons, respectively. Selegiline produces an amphetamine-like effect, enhances the release of dopamine, and blocks the reuptake of dopamine. It stimulates gene expression of L-aromatic amino acid decarboxylase, increases striatal phenylethylamine levels, and activates dopamine receptors. Selegiline reduces the production of oxidative radicals, up-regulates superoxide dismutase and catalase, and suppresses nonenzymatic and iron-catalyzed autooxidation of dopamine. Selegiline compensates for loss of target-derived trophic support, delays apoptosis in serum-deprived cells, and blocks apoptosis-related fall in the mitochondrial membrane potential. Most of the aforementioned properties occur independently of selegiline's efficacy to inhibit MAO-B.

Inhibition of tyrosinase reduces cell viability in catecholaminergic neuronal cells.

The biosynthesis of dopamine (DA) in catecholaminergic neurons is regulated by tyrosine hydroxylase, which converts tyrosine into 3, 4-dihydroxyphenylalanine (L-DOPA). In melanocytes, tyrosinase catalyzes both the hydroxylation of tyrosine and the consequent oxidation of L-DOPA to form melanin. Although it has been demonstrated that tyrosinase is also expressed in the brain, the physiological role of tyrosinase in the brain is still obscure. In this study, to investigate the role of tyrosinase in catecholaminergic neuronal cells, we examined the effects of tyrosinase inhibition on the viability of CATH.a and SH-SY5Y cells using tyrosinase inhibitors-specifically, phenylthiourea (PTU) and 5-hydroxyindole (5-HI)-and the transfection of antisense tyrosinase cDNA. Both inhibitors significantly reduced the cell viability of CATH.a cells in a dose-dependent manner. PTU also specifically enhanced DA-induced cell death, but 5-HI did not. This discrepancy in cell death is probably due to the inhibitors' different mechanism of action: 5-HI inhibits the hydroxylation of tyrosine as a competitor for the substrate to induce cell death that may be due to depletion of DA, whereas PTU mainly inhibits the enzymatic oxidation of L-DOPA and DA rather than tyrosine hydroxylation to increase consequently autooxidation of DA. Indeed, the intracellular DA content in CATH.a cells was enhanced by PTU exposure. In contrast, PTU showed no enhancing effects on DA-induced cell death of SH-SY5Y cells, which express little tyrosinase. Furthermore, transfection with antisense tyrosinase cDNA into CATH.a cells dramatically reduced cell viability and significantly enhanced DA-induced cell death. These results suggest that tyrosinase controls the intracellular DA content by biosynthesis or enzymatic oxidation of DA, and the dysfunction of this activity induces cell death by elevation of intracellular DA level and consequent gradual autooxidation of DA to generate reactive oxygen species.

Neuroprotective strategies in Parkinson's disease: protection against progressive nigral damage induced by free radicals.

Brain undergoes neurodegeneration when excess free radicals overwhelm antioxidative defense systems during senescence, head trauma and/or neurotoxic insults. A site-specific accumulation of ferrous citrate-iron complexes in the substantia nigra dopaminergic neurons could lead to exaggerated dopamine turnover, dopamine auto-oxidation, free radical generation, and oxidant stress. Eventually, this iron-catalyzed dopamine auto-oxidation results in the accumulation of neuromelanin, a progressive loss of nigral neurons, and the development of Parkinson's disease when brain dopamine depletion is greater than 80%. Emerging evidence indicates that free radicals such as hydroxyl radicals ((.-)OH) and nitric oxide ((.-)NO) may play opposite role in cell and animal models of parkinsonism. (.-)OH is a cytotoxic oxidant whereas oNO is an atypical neuroprotective antioxidant. (.-)NO and S-nitrosoglutathione (GSNO) protect nigral neurons against oxidative stress caused by 1-methyl-4-phenylpyridinium (MPP(+)), dopamine, ferrous citrate, hemoglobin, sodium nitroprusside and peroxynitrite. MPP(+), the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), increases the nigral uptake of iron complexes and dopamine overflow leading to the generation of (.-)OH, protein oxidation, lipid peroxidation, and associated retrograde degeneration. In addition to GSNO, MPP(+)-induced oxidative neurotoxicity can be prevented by antioxidants including selegiline, 7-nitroindazole, 17beta-estradiol, melatonin, alpha-phenyl-tert-butylnitrone and U78517F. Similar to selegiline, 7-nitroindazole is a MAO-B inhibitor, which blocks the bio-activation of MPTP and oxidative stress. Freshly prepared but not light exposed, (.-)NO-exhausted GSNO is about 100 times more potent than the classic antioxidant glutathione. Via S-nitrosylation, GSNO also inhibits proteolysis and cytotoxicity caused by caspases and HIV-1 protease. Furthermore, in addition to protection against serum deprivation stress, the induction of neuronal NOS1 in human cells increases tolerance to MPP(+)-induced neuro-toxicity since newly synthesized (.-)NO prevents apoptosis possibly through up-regulation of bcl-2 and down regulation of p66(shc). In conclusion, reactive oxygen species are unavoidable by-products of iron-catalyzed dopamine auto-oxidation, which can initiate lipid peroxidation, protein oxidation, DNA damage, and nigral loss, all of which can be prevented by endogenous and exogenous (.-)NO. Natural and man-made antioxidants can be employed as part of preventative or neuroprotective treatments in Parkinson's disease and perhaps dementia complexes as well. For achieving neuroprotection and neuro-rescue in early clinical parkinsonian stages, a cocktail therapy of multiple neuroprotective agents may be more effective than the current treatment with extremely high doses of a single antioxidative agent.