The effects of nitric oxide on the oxidations of l-dopa and dopamine mediated by tyrosinase and peroxidase

Nitrosative stress in Parkinson's disease

Parkinson’s Disease (PD) is a neurodegenerative disorder characterized, in part, by the loss of dopaminergic neurons within the nigral-striatal pathway. Multiple lines of evidence support a role for reactive nitrogen species (RNS) in degeneration of this pathway, specifically nitric oxide (NO). This review will focus on how RNS leads to loss of dopaminergic neurons in PD and whether RNS accumulation represents a central signal in the degenerative cascade. Herein, we provide an overview of how RNS accumulates in PD by considering the various cellular sources of RNS including nNOS, iNOS, nitrate, and nitrite reduction and describe evidence that these sources are upregulating RNS in PD. We document that over 1/3 of the proteins that deposit in Lewy Bodies, are post-translationally modified (S-nitrosylated) by RNS and provide a broad description of how this elicits deleterious effects in neurons. In doing so, we identify specific proteins that are modified by RNS in neurons which are implicated in PD pathogenesis, with an emphasis on exacerbation of synucleinopathy. How nitration of alpha-synuclein (aSyn) leads to aSyn misfolding and toxicity in PD models is outlined. Furthermore, we delineate how RNS modulates known PD-related phenotypes including axo-dendritic-, mitochondrial-, and dopamine-dysfunctions. Finally, we discuss successful outcomes of therapeutics that target S-nitrosylation of proteins in Parkinson’s Disease related clinical trials. In conclusion, we argue that targeting RNS may be of therapeutic benefit for people in early clinical stages of PD.

Validation of a Reversed Phase UPLC-MS/MS Method to Determine Dopamine Metabolites and Oxidation Intermediates in Neuronal Differentiated SH-SY5Y Cells and Brain Tissue

Dopamine is a key neurotransmitter in the pathophysiology of various neurological disorders such as addiction or Parkinson's disease. Disturbances in its metabolism could lead to dopamine accumulation in the cytoplasm and an increased production of o-quinones and their derivatives, which have neurotoxic potential and act as precursors in neuromelanin synthesis. Thus, quantification of the dopaminergic metabolism is essential for monitoring changes that may contribute to disease development. Here, we developed and validated an UPLC-MS/MS method to detect and quantify a panel of eight dopaminergic metabolites, including the oxidation product aminochrome. Our method was validated in differentiated SH-SY5Y cells and mouse brain tissue and was then employed in brain samples from humans and rats to ensure method reliability in different matrices. Finally, to prove the biological relevance of our method, we determined metabolic changes in an in vitro cellular model of dopamine oxidation/neuromelanin production and in human postmortem samples from Parkinson's disease patients. The current study provides a validated method to simultaneously monitor possible alterations in dopamine degradation and o-quinone production pathways that can be applied to in vitro and in vivo experimental models of neurological disorders and human brain samples.

Nucleus accumbens invulnerability to methamphetamine neurotoxicity

Methamphetamine (Meth) is a neurotoxic drug of abuse that damages neurons and nerve endings throughout the central nervous system. Emerging studies of human Meth addicts using both postmortem analyses of brain tissue and noninvasive imaging studies of intact brains have confirmed that Meth causes persistent structural abnormalities. Animal and human studies have also defined a number of significant functional problems and comorbid psychiatric disorders associated with long-term Meth abuse. This review summarizes the salient features of Meth-induced neurotoxicity with a focus on the dopamine (DA) neuronal system. DA nerve endings in the caudate-putamen (CPu) are damaged by Meth in a highly delimited manner. Even within the CPu, damage is remarkably heterogeneous, with ventral and lateral aspects showing the greatest deficits. The nucleus accumbens (NAc) is largely spared the damage that accompanies binge Meth intoxication, but relatively subtle changes in the disposition of DA in its nerve endings can lead to dramatic increases in Meth-induced toxicity in the CPu and overcome the normal resistance of the NAc to damage. In contrast to the CPu, where DA neuronal deficiencies are persistent, alterations in the NAc show a partial recovery. Animal models have been indispensable in studies of the causes and consequences of Meth neurotoxicity and in the development of new therapies. This research has shown that increases in cytoplasmic DA dramatically broaden the neurotoxic profile of Meth to include brain structures not normally targeted for damage. The resistance of the NAc to Meth-induced neurotoxicity and its ability to recover reveal a fundamentally different neuroplasticity by comparison to the CPu. Recruitment of the NAc as a target of Meth neurotoxicity by alterations in DA homeostasis is significant in light of the numerous important roles played by this brain structure.

Removal of bisphenol A and its derivatives from aqueous medium through laccase-catalyzed treatment enhanced by addition of polyethylene glycol

In this study, enzymatic removal of bisphenol A (BPA) from the aqueous medium was investigated through the generation of water-insoluble oligomers, and this procedure was applied to removal of bisphenol derivatives. The experimental parameters, such as the temperature, pH value, enzyme concentration, and concentration and molecular weight of polyethylene glycol (PEG), were determined for the laccase-catalyzed treatment of BPA. The optimum conditions were determined to be pH 7.0 and 40°C in the absence of PEG. Water-insoluble oligomers generated under these conditions were readily removed by filtration or centrifugation. The optimum pH value was decreased to 5.0 in the presence of PEG and the laccase dose was reduced to one-fiftieth of that in the absence of PEG. This indicates that the addition of PEG protects the enzymatic activity and prevents capture of laccase molecules in the oligomers. The oligomers generated in the presence of PEG were removed from the aqueous medium by filtration with a membrane filter or by centrifugation. The oligomers were completely filtrated out with a filter paper by decreasing the pH value to 3.0. In addition, several bisphenol derivatives were also treated and subsequently removed by adjusting the laccase dose in the presence of PEG using the above procedure.

Dual effect of nitric oxide on phenoloxidase-mediated melanization

The study has demonstrated a dual effect of nitric oxide on phenoloxidase (PO)-mediated DOPA oxidation and melanization process. NO generated at low rates proportionally increased in PO-mediated DOPA oxidation. Competitive PO inhibitor, phenylthiourea, resulted in significant inhibition of NO-mediated DOPA oxidation. Further analysis using fluorescent and EPR methods demonstrated that the effect of NO on DOPA oxidation is explained by oxidation of NO to NO2 at the active site of PO followed by oxidation of DOPA by NO2. On the contrary, the bolus addition of NO gas solution resulted in a significant decrease in observed PO activity. Similar dose-dependent effect of NO was observed for the insect's haemocytes quantified as percentage of melanized cells after treatment with nitric oxide. In conclusion, the results of the study suggest that NO may have a significant regulatory role on melanization process in invertebrates as well as in human and result in protective or damaging effects.

Ov-16 [4-(3,4-dihydroxybenzoyloxymethyl)phenyl-O-β-D-glucopyranoside] inhibits melanin synthesis by regulating expressions of melanogenesis-regulated gene and protein

Ov-16 (4-(3,4-dihydroxybenzoyloxymethyl)phenyl-O-β-D-glucopyranoside), a polyphenolic glycoside that is isolated from oregano (Origanum vulgare L.), can scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radicals. This investigation is the first to study in detail the hypopigmentary properties of Ov-16. It demonstrates that 0-1000 μg/ml Ov-16 inhibits the activity of mushroom tyrosinase (Tyr) in a concentration-dependent manner. The inhibitionary Tyr kinetics of Ov-16 towards the oxidation of L-DOPA was found to be uncompetitive. Following the treatment of human skin premalignant kerationcyte HaCaT cells, human skin fibroblast Hs68 cells and mice melanoma B16 cells with Ov-16 (0-100 μg/ml), cell viability was >98%, suggesting that Ov-16 is non-toxic. Ov-16 can reduce cellular Tyr activity, DOPA oxidase activity and melanin synthesis in B16 cells that are stimulated by the α-melanocyte-stimulating hormone (α-MSH). Moreover, Ov-16 inhibited the production of melanin in Streptomyces bikiniensis without affecting the growth of the microorganism. The treatment of B16 cells with Ov-16 considerably reduced the gene expressions of melanocortin-1 receptor (Mc1r), microphthalmia-associated transcription factor (Mitf), Tyr, tyrosinase-related proteins-2 (Trp-2) and Trp-1, as determined by RT-PCR. The expressions of Mc1r, Mift, Tyr, Trp-2 and TrpP-1 protein in Ov-16-treated B16 cells were also significantly reduced, as determined by western blotting and fluorescent staining analysis. These results suggest that Ov-16 exhibits hypopigmentary performance.

The role of melanization and cytotoxic by-products in the cellular immune responses of Drosophila against parasitic wasps

The cellular innate immune response of several species of Drosophila terminates with the encasement of large foreign objects within melanotic capsules comprised of several layers of adhering blood cells or hemocytes. This reaction is manifested by various Drosophila hosts in response to infection by endoparasitic wasps (i.e., parasitoids). Creditable assessments of the factor(s) causing, or contributing to, parasite mortality have long been considered as cytotoxic elements certain molecules associated with enzyme-mediated melanogenesis. However, observations that warrant additional or alternative considerations are those documenting parasitoid survival despite melanotic encapsulation, and those where parasitoids are destroyed with no evidence of this host response. Recent studies of the production of some reactive intermediates of oxygen and nitrogen during infection provide a basis for proposing that these molecules constitute important components of the immune arsenal of Drosophila. Studies of the virulence factors injected by female wasps during oviposition that suppress the host response will likely facilitate identification of the cytotoxic molecules as well as the cell-signaling pathways that regulate their synthesis.

Increases in cytoplasmic dopamine compromise the normal resistance of the nucleus accumbens to methamphetamine neurotoxicity

Methamphetamine (METH) is a neurotoxic drug of abuse that damages the dopamine (DA) neuronal system in a highly delimited manner. The brain structure most affected by METH is the caudate-putamen (CPu) where long-term DA depletion and microglial activation are most evident. Even damage within the CPu is remarkably heterogenous with lateral and ventral aspects showing the greatest deficits. The nucleus accumbens (NAc) is largely spared of the damage that accompanies binge METH intoxication. Increases in cytoplasmic DA produced by reserpine, L-DOPA or clorgyline prior to METH uncover damage in the NAc as evidenced by microglial activation and depletion of DA, tyrosine hydroxylase (TH), and the DA transporter. These effects do not occur in the NAc after treatment with METH alone. In contrast to the CPu where DA, TH, and DA transporter levels remain depleted chronically, DA nerve ending alterations in the NAc show a partial recovery over time. None of the treatments that enhance METH toxicity in the NAc and CPu lead to losses of TH protein or DA cell bodies in the substantia nigra or the ventral tegmentum. These data show that increases in cytoplasmic DA dramatically broaden the neurotoxic profile of METH to include brain structures not normally targeted for damage by METH alone. The resistance of the NAc to METH-induced neurotoxicity and its ability to recover reveal a fundamentally different neuroplasticity by comparison to the CPu. Recruitment of the NAc as a target of METH neurotoxicity by alterations in DA homeostasis is significant in light of the important roles played by this brain structure.

Class III peroxidases in plant defence reactions

When plants are attacked by pathogens, they defend themselves with an arsenal of defence mechanisms, both passive and active. The active defence responses, which require de novo protein synthesis, are regulated through a complex and interconnected network of signalling pathways that mainly involve three molecules, salicylic acid (SA), jasmonic acid (JA), and ethylene (ET), and which results in the synthesis of pathogenesis-related (PR) proteins. Microbe or elicitor-induced signal transduction pathways lead to (i) the reinforcement of cell walls and lignification, (ii) the production of antimicrobial metabolites (phytoalexins), and (iii) the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Among the proteins induced during the host plant defence, class III plant peroxidases (EC 1.11.1.7; hydrogen donor: H(2)O(2) oxidoreductase, Prxs) are well known. They belong to a large multigene family, and participate in a broad range of physiological processes, such as lignin and suberin formation, cross-linking of cell wall components, and synthesis of phytoalexins, or participate in the metabolism of ROS and RNS, both switching on the hypersensitive response (HR), a form of programmed host cell death at the infection site associated with limited pathogen development. The present review focuses on these plant defence reactions in which Prxs are directly or indirectly involved, and ends with the signalling pathways, which regulate Prx gene expression during plant defence. How they are integrated within the complex network of defence responses of any host plant cell will be the cornerstone of future research.

A novel prophenoloxidase 2 exists in shrimp hemocytes

The prophenoloxidase (proPO)-activating system in crustaceans and other arthropods is regarded as a constituent of the immune system and plays an important role in defense against pathogens. Hitherto in crustaceans, only one proPO gene per species has been identified. Here we report the identification of a novel proPO-2 (LvproPO-2) from the hemocytes of Litopenaeus vannamei, which shows 72% identity to proPO-1 (LvproPO-1) cloned previously. Northern blotting analysis and quantitative real-time PCR reveal that LvproPO-2 is mainly expressed in the hemocytes, and its expression is down-regulated in shrimp challenged with white spot syndrome virus (WSSV). Western blotting analysis shows that most LvproPO-2/LvPO-2 (L. vannamei phenoloxidase-2) exists in the hemocytes, but not in plasma of L. vannamei. LvproPO-2/LvPO-2 could be detected on the hemocyte surface and the nucleus of hemocytes by indirect immunofluorescence assay (IFA). These findings provide insight into the molecular biological basis for further studying on the defense mechanism of shrimp innate immunity, especially on the proPO-activating system and melanization cascade of shrimp.

Dopamine disposition in the presynaptic process regulates the severity of methamphetamine-induced neurotoxicity

Methamphetamine (METH) is well known for its ability to cause damage to dopamine (DA) nerve endings of the striatum. The mechanisms by which METH causes neurotoxicity are not fully understood, but likely candidates are increased oxidative and nitrosative stress and mitochondrial dysfunction. Microglial activation is also emerging as an important element of the METH neurotoxic cascade, and it appears that extensive cross-talk between these cells and DA nerve endings is an early event in this process. It may seem paradoxical, but DA itself is also thought to be an essential factor in the neuronal damaging effects of METH, but issues relating to its precise role in this regard remain unanswered. We present in this overview a summary of studies that tested how alterations in the disposition of presynaptic DA (injections of reserpine, L-DOPA, or clorgyline) modulate METH neurotoxicity. In all cases, these drugs significantly increased the magnitude of microglial activation as well as the severity of damage to striatal DA nerve endings caused by METH. The enhancement of METH effects in striatum by reserpine, L-DOPA, and clorgyline persisted for 14 days and showed no evidence of recovery. These data establish that subtle shifts in the newly synthesized pool of DA can cause substantial changes in the severity of METH-induced neurotoxicity. DA released into the synapse by METH is very likely the source of downstream reactants that provoke microglial activation and the ensuing damage to DA nerve endings.

Myeloperoxidase: a target for new drug development?

Myeloperoxidase (MPO), a member of the haem peroxidase-cyclooxygenase superfamily, is abundantly expressed in neutrophils and to a lesser extent in monocytes and certain type of macrophages. MPO participates in innate immune defence mechanism through formation of microbicidal reactive oxidants and diffusible radical species. A unique activity of MPO is its ability to use chloride as a cosubstrate with hydrogen peroxide to generate chlorinating oxidants such as hypochlorous acid, a potent antimicrobial agent. However, evidence has emerged that MPO-derived oxidants contribute to tissue damage and the initiation and propagation of acute and chronic vascular inflammatory disease. The fact that circulating levels of MPO have been shown to predict risks for major adverse cardiac events and that levels of MPO-derived chlorinated compounds are specific biomarkers for disease progression, has attracted considerable interest in the development of therapeutically useful MPO inhibitors. Today, detailed information on the structure of ferric MPO and its complexes with low- and high-spin ligands is available. This, together with a thorough understanding of reaction mechanisms including redox properties of intermediates, enables a rationale attempt in developing specific MPO inhibitors that still maintain MPO activity during host defence and bacterial killing but interfere with pathophysiologically persistent activation of MPO. The various approaches to inhibit enzyme activity of MPO and to ameliorate adverse effects of MPO-derived oxidants will be discussed. Emphasis will be put on mechanism-based inhibitors and high-throughput screening of compounds as well as the discussion of physiologically useful HOCl scavengers.

Chromatographic analysis of dopamine metabolism in a Parkinsonian model

The present study examined the metabolism of released dopamine from rat striatum upon chronic rotenone exposure. The sample separation was carried out by two-dimensional, reversed-phase and ion pair reversed-phase chromatography using on-line solid phase extraction enrichment. Reduced dopamine content and decreased extracellular level of [(3)H] and endogenous dopamine evoked by electrical stimulation indicated the injury of dopaminergic pathway. Sensitivity of dopaminergic neurons were increased to oxidative stress with enhanced release of dopamine and formation of oxidized metabolite dopamine quinone (DAQ). Utilizing multidimensional detection, EC at -100 mV reduction potential, the method has been applied for identification of DAQ and aminochrome (DAC).

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Production and utilization of hydrogen peroxide associated with melanogenesis and tyrosinase-mediated oxidations of DOPA and dopamine

The synthesis and involvement of H(2)O(2) during the early stages of melanogenesis involving the oxidations of DOPA and dopamine (diphenolase activity) were established by two sensitive and specific electrochemical detection systems. Catalase-treated reaction mixtures showed diminished rates of H(2)O(2) production during the autoxidation and tyrosinase-mediated oxidation of both diphenols. Inhibition studies with the radical scavenger resveratrol revealed the involvement in these reactions of additional reactive intermediate of oxygen (ROI), one of which appears to be superoxide anion. There was no evidence to suggest that H(2)O(2) or any other ROI was produced during the tyrosinase-mediated conversion of tyrosine to DOPA (monophenolase activity). Establishing by electrochemical methods the endogenous production H(2)O(2) in real time confirms recent reports, based in large part on the use of exogenous H(2)O(2), that tyrosinase can manifest both catalase and peroxidase activities. The detection of ROI in tyrosinase-mediated in vitro reactions provides evidence for sequential univalent reductions of O(2), most likely occurring at the enzyme active site copper. Collectively, these observations focus attention on the possible involvement of peroxidase-H(2)O(2) systems and related ROI-mediated reactions in promoting melanocytotoxic and melanoprotective processes.

Induction of the Protective Antioxidant Response Element Pathway by 6-Hydroxydopamine In Vivo and In Vitro

Parkinson's disease, a progressive neurodegenerative disorder, is characterized by loss of midbrain dopaminergic neurons. The etiology of sporadic Parkinson's disease is unknown; however, oxidative stress is thought to play a major role in disease pathogenesis. Little is known regarding the transcriptional changes that occur in Parkinson's disease. The antioxidant response element is a cis-acting enhancer sequence that is upstream of many phase II detoxification and antioxidant genes. Here we show that 6-hydroxydopamine, a mitochondrial inhibitor used to model Parkinson's disease, activates the antioxidant response element both in cultured neurons and in the striatum and brainstem of 6-OHDA-lesioned mice. Pretreatment with antioxidants or NMDA receptor antagonists reduced but did not abolish activation. Further induction of this pathway with tert-butylhydroquinone was able to significantly reduce cell death due to 6-OHDA in vitro. These observations indicate that 6-OHDA activates the antioxidant response element through components of oxidative stress, excitotoxicity, and potential structural factors. Further induction of this endogenous defense mechanism may suggest a novel therapeutic venue in Parkinson's disease.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

The separation and quantification of aminochromes using high-pressure liquid chromatography with electrochemical detection

There is increasing evidence that oxidative stress plays an important role in the pathogenesis of many neurodegenerative diseases including Parkinson's disease (PD). In particular there is support for the participation of oxidized catecholamines in PD. Catecholamines are highly reactive and are readily oxidized to aminochromes. While aminochromes have been shown to be toxic, their formation in oxidative stress and subsequent participation in disease has yet to be confirmed. We propose that the characterization of aminochromes, specifically dopaminochrome, is important in clarifying the role that oxidized catecholamines play in PD. We have developed a novel method for the separation and quantification of aminochromes using high-pressure liquid chromatography with electrochemical detection (HPLC-ED). Our method utilizes the separation principles employed in measuring catecholamines by HPLC except that the electrochemical detection of aminochromes is achieved by reversing the detector's electrode. We have used this method to separate and quantify aminochrome standards, prepared by oxidizing catecholamines with sodium periodate (NaIO(4)) and we have also shown that aminochromes can be measured in plasma and cell lysates. Furthermore, we have characterized aminochromes to facilitate forthcoming studies on aminochromes and the role oxidized catecholamines may play in neurodegenerative disease.

Iron accelerates the conversion of dopamine-oxidized intermediates into melanin and provides protection in SH-SY5Y cells

Parkinson's disease (PD) is characterized by the selective loss of dopaminergic neurons in the substantia nigra (SN), and it has been suggested that dopamine is one of the main endogenous toxins in the genesis of PD. We demonstrated that thiol antioxidants (the reduced form of glutathione, N-acetyl-L-cysteine, and L-cysteine), which conjugate with one dopamine oxidation intermediate, o-quinone, provided almost complete protection from dopamine-mediated toxicity in SH-SY5Y, a human neuroblastoma cell line. In contrast, catalase partially provided protection against cell death caused by dopamine. These data suggest that the generation of dopamine oxidation intermediates, rather than hydrogen peroxide, plays a pivotal role in dopamine-induced toxicity. Iron accumulated in the SN of patients with PD can cause dopaminergic neuronal degeneration by enhancing oxidative stress. However, we found that iron reduced the total amounts of dopamine oxidation intermediates and enhanced the formation of melanin, a final product of dopamine oxidation. Also, addition of iron inhibited dopamine-induced cytotoxicity. These results suggest that iron can provide protection when it accelerates the conversion of dopamine oxidation intermediates.

Tetrahydrobiisoquinoline Derivatives by Reaction of Dopamine with Glyoxal: A Novel Potential Degenerative Pathway of Catecholamines under Oxidative Stress Conditions

In 0.1 M phosphate buffer, pH 7.4, dopamine reacts with glyoxal, a cytotoxic and genotoxic alpha-oxoaldehyde produced by oxidative degradation of carbohydrates, to give three main products, two of which could be isolated and identified as the isomeric tetrahydrobiisoquinolines 1 and 2 by extensive two-dimensional NMR and mass spectrometric analysis. Time course studies indicated that 1 is the first intermediate in the process and changes slowly to 2 via an unstable species that escaped all efforts at isolation and structural identification. Products 1 and 2 were detected also among the species formed by the interaction of dopamine with oxidized carbohydrates, such as glucose, ribose, and fructose. Mechanistic evidence suggests that the formation of 1 proceeds by an unusual reaction pathway involving intramolecular cyclization of a double Schiff base intermediate followed by glyoxal-induced oxidation of the resulting octahydrobiisoquinoline intermediate (4). Subsequent conversion of 1 to 2 would involve a complex redox mechanism depending on an initial oxidation step. Product 2 was only poorly toxic to PC12 cells, whereas its methylated derivative 3 was as toxic as salsolinol, an established neurotoxin. Overall, these results throw light on a novel pathway of dopamine modification of potential relevance to the mechanisms underlying neurodegenerative changes in Parkinson's disease and other disorders characterized by a prooxidant state.

Aluminum Facilitation of the Iron-Mediated Oxidation of DOPA to Melanin

Aluminum, a trivalent cation unable to undergo redox reactions, is shown to faciliate iron-initiated DOPA oxidation in the melanin pathway under acidic condition of pH 5.5, which is a favored medium for aluminum facilitation of iron-induced lipid peroxidation. In the process of oxidation of DOPA to melanin in the presence of the metal ions, Fe3+ and H2O2 oxidize DOPA to dopachrome (DC), then Al3+ catalyzes the conversion of DC to 5,6-dihydroxyindole (DHI) and finally Fe3+ oxidizes DHI to indole-5,6-quinone (IQ), which polymerizes immediately to melanochrome and melanin. The reactions involve the intermediate complexes of metal ions and DOPA or its derivative. The present results indicate that aluminum can enhance the oxidative stress on iron-mediated DOPA oxidation in melanin pathway under acidic condition through the cooperation of iron and aluminum ions.

Separation and detection methods for covalent drug–protein adducts

Covalent binding of reactive metabolites of drugs to proteins has been a predominant hypothesis for the mechanism of toxicity caused by numerous drugs. The development of efficient and sensitive analytical methods for the separation, identification, quantification of drug-protein adducts have important clinical and toxicological implications. In the last few decades, continuous progress in analytical methodology has been achieved with substantial increase in the number of new, more specific and more sensitive methods for drug-protein adducts. The methods used for drug-protein adduct studies include those for separation and for subsequent detection and identification. Various chromatographic (e.g., affinity chromatography, ion-exchange chromatography, and high-performance liquid chromatography) and electrophoretic techniques [e.g., sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and capillary electrophoresis], used alone or in combination, offer an opportunity to purify proteins adducted by reactive drug metabolites. Conventionally, mass spectrometric (MS), nuclear magnetic resonance, and immunological and radioisotope methods are used to detect and identify protein targets for reactive drug metabolites. However, these methods are labor-intensive, and have provided very limited sequence information on the target proteins adducted, and thus the identities of the protein targets are usually unknown. Moreover, the antibody-based methods are limited by the availability, quality, and specificity of antibodies to protein adducts, which greatly hindered the identification of specific protein targets of drugs and their clinical applications. Recently, the use of powerful MS technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight) together with analytical proteomics have enabled one to separate, identify unknown protein adducts, and establish the sequence context of specific adducts by offering the opportunity to search for adducts in proteomes containing a large number of proteins with protein adducts and unmodified proteins. The present review highlights the separation and detection technologies for drug-protein adducts, with an emphasis on methodology, advantages and limitations to these techniques. Furthermore, a brief discussion of the application of these techniques to individual drugs and their target proteins will be outlined.

Mitochondrial damage by nitric oxide is potentiated by dopamine in PC12 cells

Mitochondrial damage in PC12 cells, a model for dopaminergic cells, was examined in terms of the contribution of oxidative stress, nitric oxide (*NO), and dopamine to impairment of mitochondrial respiratory control (RC). A kinetic analysis suggested that the oxidative deamination of dopamine catalyzed by monoamine oxidase (MAO) was not a significant source of hydrogen peroxide, because of constrains imposed by the low cytosolic level of dopamine. *NO induced irreversible damage of mitochondrial complex I in PC12 cells: this damage followed a sigmoid response on *NO concentration with a well-defined threshold level. Dopamine did not elicit damage of mitochondria in PC12 cells; however, the amine potentiated the effects of *NO at or near the threshold level, thus leading to irreversible impairment of mitochondrial respiration. This synergism between *NO and dopamine was not observed at *NO concentrations below the threshold level. Depletion of dopamine from the storage vesicles by reserpine protected mitochondria from *NO damage. Dopamine oxidation by *NO increased with pH, and occurred at modest levels at pH 5.5. In spite of this, calculations showed that the oxidation of dopamine in the storage vesicles (pH 5.5) was higher than that in the cytosol (pH 7.4), due to the higher dopamine concentration in the storage vesicles (millimolar range) compared to that in the cytosol (micromolar range). It is suggested that storage vesicles may be the cellular sites where the potential for dopamine oxidation by *NO is higher. These data provide further support to the hypothesis that dopamine renders dopaminergic cells more susceptible to the mitochondrial damaging effects of *NO. In the early stages of Parkinson's disease, *NO production increases until reaching a point near the threshold level that induces neuronal damage. Dopamine stored in dopaminergic cells may cause these cells to be more susceptible to the deleterious effects of *NO, which involve irreversible impairment of mitochondrial respiration.

Oxidative conversion of 6-nitrocatecholamines to nitrosating products: a possible contributory factor in nitric oxide and catecholamine neurotoxicity associated with oxidative stress and acidosis

Oxidation of 6-nitrodopamine (1) and 6-nitronorepinephrine (2), as well as of the model compounds 4-nitrocatechol and 4-methyl-5-nitrocatechol, with horseradish peroxidase (HRP)/H(2)O(2), lactoperoxidase (LPO)/H(2)O(2), Fe(2+)/H(2)O(2), Fe(2+)-EDTA/H(2)O(2) (Fenton reagent), HRP or Fe(2+)/EDTA in combination with D-glucose-glucose oxidase, or Fe(2+)/O(2), resulted in the smooth formation of yellowish-brown pigments positive to the Griess assay. In the case of 1, formation of the Griess positive pigment (GPP-1) promoted by HRP/H(2)O(2) proceeded through the intermediacy of two main dimeric species that could be isolated and identified as 3 and the isomer 4, featuring the 4-nitro-6,7-dihydroxyindole system linked to a unit of 1 through ether bonds. Spectroscopic (FAB-MS, (1)H NMR) and chemical analysis of GPP-1 indicated a mixture of oligomeric species related to 3 and 4 in which oxidative modification of the nitrocatechol moiety of 1 led to the generation of reactive nitro groups supposedly linked to sp(3) hybridized carbons. In the pH range 3-6, GPP-1 induced concentration- and pH-dependent nitrosation of 2,3-diaminonaphthalene, but very poor (up to 2%) nitration of 600 microM tyrosine. At pH 7.4, 1 exerted significant toxicity to PC12 cells, while GPP-1 proved virtually innocuous. By contrast, when assayed on Lactobacillus bulgaricus cells at pH 3.5, 1 was inactive whereas GGP-1 caused about 70% inhibition of cell growth. Overall, these results hint at novel pH-dependent mechanisms of nitrocatecholamine-induced cytotoxicity of possible relevance to ischemia- or inflammation-induced catecholaminergic neuron damage.