Inhibition of the Fe(III)-catalyzed dopamine oxidation by ATP and its relevance to oxidative stress in Parkinson's disease

Abstract

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.

Scheme 1. Two Proposed Iron-Induced DA Oxidation Pathways

Figure 1

UV–vis spectra of mixtures of DA, Fe(III), and ATP at different concentration ratios: 2:1:0 (black), 1:1:0 (red), and 1:1:1 (blue) in which the Fe(III) concentration was maintained at 250 μM.

Figure 2

ES–MS spectrum of the DA–Fe(III)–ATP ternary complex. M denotes the ternary complex.

Figure 3

Cyclic voltammograms of DA (top), Fe(III)–DA (middle), and DA–Fe(III)–ATP (bottom) acquired within the potential window between −0.9 and 0.7 V (A) and between −0.8 and 0.1 V (B). In panel (A), the initial potential was scanned from 0.1 V. The concentrations of DA, Fe(III), and ATP were all 250 μM. The scan rates are 50 mV s^–1 in panel (A) and 10 mV s^–1 in panel (B). The arrow indicates the initial scan direction. All measurements were performed in a nitrogen-purged glovebox.

Figure 4

Chromatograms showing the separation of DA from some of its oxidation products in different solutions. The Fe(III)-catalyzed oxidation reactions proceeded for 6 h in the absence and presence of ATP. The initial concentration of DA, Fe(III), and ATP are all 250 μM. Before separation, the sample mixture was diluted 10-fold with 0.1 M HCl.

Figure 5

(A) Variations of DA concentrations under different conditions: DA only (■), in the presence of Fe(III) (●), and in the presence of Fe(III) and ATP (△). (B) Plots of the 6-OHDAQ peak area over that of consumed DA in the presence of 250 μM Fe(III) (○) and 250 μM Fe(III) and 250 μM ATP (●).

Figure 6

Electrospray high-resolution mass spectrum of tissues extracted from the SN region of rat brain showing the presence of the DA–Fe(III)–ATP ternary complex. The inset is an enlarged spectrum of three major isotopic peaks of the complex.

Figure 7

UV–vis spectra of 250 μM DA–Fe(III)–ATP (black curve), a mixture of 250 μM DA, 250 μM ATP, and 0.15 mg/mL ferritin incubated for 0 h (blue), the same ferritin-containing mixture after 21 h of incubation (green), and the same mixture incubated for 21 h followed by removal of ferritin (red).

Figure 8

Viabilities of SH-SY5Y cells in the presence of 250 μM DA, 250 μM DA and 50 μM Fe(III), and a mixture of 250 μM DA, 50 μM Fe(III), and 250 μM ATP after a 12 h incubation. The p value for the cell viabilities of the DA/Fe(III) mixture and the DA/Fe(III)/ATP mixture is 0.002, indicating that the difference between the two mixtures is statistically significant.

Figure 9

Proposed mechanism of the iron-catalyzed DA oxidation by O₂.