- Inhibition of the Fe(III)-catalyzed dopamine oxidation by ATP and its relevance to oxidative stress in Parkinson's disease
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Parkinson's disease (PD) is characterized by the progressive degeneration of dopaminergic cells, which implicates a role of dopamine (DA) in the etiology of PD. A possible DA degradation pathway is the Fe(III)-catalyzed oxidation of DA by oxygen, which produces neuronal toxins as side products. We investigated how ATP, an abundant and ubiquitous molecule in cellular milieu, affects the catalytic oxidation reaction of dopamine. For the first time, a unique, highly stable DA-Fe(III)-ATP ternary complex was formed and characterized in vitro. ATP as a ligand shifts the catecholate-Fe(III) ligand metal charge transfer (LMCT) band to a longer wavelength and the redox potentials of both DA and the Fe(III) center in the ternary complex. Remarkably, the additional ligation by ATP was found to significantly reverse the catalytic effect of the Fe(III) center on the DA oxidation. The reversal is attributed to the full occupation of the Fe(III) coordination sites by ATP and DA, which blocks O2 from accessing the Fe(III) center and its further reaction with DA. The biological relevance of this complex is strongly implicated by the identification of the ternary complex in the substantia nigra of rat brain and its attenuation of cytotoxicity of the Fe(III)-DA complex. Since ATP deficiency accompanies PD and neurotoxin 1-methyl-4-phenylpyridinium (MPP+) induced PD, deficiency of ATP and the resultant impairment toward the inhibition of the Fe(III)-catalyzed DA oxidation may contribute to the pathogenesis of PD. Our finding provides new insight into the pathways of DA oxidation and its relationship with synaptic activity.
- Jiang, Dianlu,Shi, Shuyun,Zhang, Lin,Liu, Lin,Ding, Bingrong,Zhao, Bingqing,Yagnik, Gargey,Zhou, Feimeng
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- 6,7-Dihydroxy-1,2,3,4-tetrahydroisoquinoline formation by iron mediated dopamine oxidation: A novel route to endogenous neurotoxins under oxidative stress conditions
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Aerobic oxidation of dopamine mediated by iron ions gives 6,7- dihydroxy-1,2,3,4-tetrahydroisoquinoline (2) and 3,4-dihydroxybenzaldehyde (4) in yields up to 10% and 15%, respectively. Based on 13C labelling experiments, a reaction mechanism is proposed involving oxidative fission of the dopamine side chain to give 4 and formaldehyde, the latter giving 2 by Pictet-Spengler condensation with dopamine. This provides a novel route to endogenous generation of neurotoxic isoquinoline alkaloids under oxidative stress conditions.
- Napolitano, Alessandra,Pezzella, Alessandro,Prota, Giuseppe
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- Generation of the Neurotoxin 6-Hydroxydopamine by Peroxidase/H2O2 Oxidation of Dopamine
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At physiological pH values, oxidation of the neurotransmitter dopamine(DA) by the peroxidase/H2O2 system leads to, besides dopaminochrome and 5,6-dihydroxyindole resulting from oxidative cyclization of dopaminequinone (DQ), significant amounts of the neurotoxin 6-hydroxydopamine (6-OHDA) in the oxidized quinonoid form (topaminequinone, TQ).Formation of TQ was shown to depend critically on the presence of hydrogen peroxide in the reaction medium and was not observed when DA oxidation was carried out using the tyrosinase/O2 system or chemical agents such as periodate or ferricyanide.These and other data suggest that, under the conditions adopted, nucleophilic attack of the hydrogen peroxide anion on DQ leading to TQ significantly competes with the intramolecular cyclization path.In line with this mechanism, the reaction course was not affected by the presence of hydroxyl radical scavengers.Peroxidase/H2O2 oxidation of the model N-acetyldopamine (1) gave, as expected, the 2-hydroxy-1,4-benzoquinone 3 in yields up to 55percent, depending on the catecholamine/H2O2 mole ratio.Likewise, reaction of 4-methyl-1,2-benzoquinone (4) with hydrogen peroxide afforded 2-hydroxy-5-methyl-1,4-benzoquinone (5) in good yields.Collectively, these results would point to the possibility that intraneuronal formation of 6-OHDA is associated with an increased production of hydrogen peroxide under oxidative stress conditions.
- Napolitano, Alessandra,Crescenzi, Orlando,Pezzella, Alessandro,Prota, Giuseppe
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p. 917 - 922
(2007/10/02)
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- One-Electron Redox Potentials of Phenols. Hydroxy- and Aminophenols and Related Compounds of Biological Interest
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The rate constants for reversible electron transfer between a series of substituted phenolate ions and anilines and various substituted phenoxyl or anilino radicals in aqueous solution were measured by observing the formation or depletion of the radicals involved.Nonequilibrium concentrations of the radicals were produced in the presence of the corresponding phenols or anilines by using the pulse radiolysis technique.The relaxation of the system to equilibrium was monitored by optical detection methods.From the equilibrium constants for one-electron transfer, the one-electron redox potentials (E2) for 38 phenolic or anilino type compounds were determined, many of which are natural products.The redox potentials are strongly influenced by electron-donating or -withdrawing substituents at the aromatic system.
- Steenken, S.,Neta, P.
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p. 3661 - 3667
(2007/10/02)
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- Aromatic L-Amino Acid Decarboxylase from Micrococcus percitreus Purification, Crystallization and Properties
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An aromatic L-amino acid decarboxylase was crystallized from the cell free extract of Micrococcus percitreus.The purification procedure included protamine sulfate treatment, ammonium sulfate fractionation, DEAE-Sephadex column chromatography and Sephadex G-200 filtration.Crystals were obtained from a solution of the purified enzyme by addition of ammonium sulfate.The crystalline enzyme preparation was homogeneous as judged by ultracentrifugation and SDS-polyacrylamide gel electrophoresis.The molecular weight was determined to be approximately 101,000.The enzyme was evidently composed of two identical subunits of a molecular weight of 48,000.The enzyme catalyzed the stoichiometric conversion of L-tryptophan to tryptamine and CO2 in the presence of pyridoxal phosphate.The optimum pH was 9.0 for the conversion.The Km value and the maximum velocity of L-tryptophan decarboxylation were 2.4E-3 M and 44 μmol/min/mg of protein, respectively.This enzyme also catalyzed decarboxylation of 5-hydroxy-L-tryptophan, L-phenylalanine, L-tyrosine, 3,4-dihydroxy-L-phenylalanine, L-kynurenine and thier α-methyl amino acid derivatives.
- Nakazawa, Hidetsugu,Kumagai, Hidehiko,Yamada, Hideaki
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p. 2543 - 2552
(2007/10/02)
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