[Show abstract][Hide abstract] ABSTRACT: There are several interrelated mechanisms involving iron, dopamine, and neuromelanin in neurons. Neuromelanin accumulates during aging and is the catecholamine-derived pigment of the dopamine neurons of the substantia nigra and norepinephrine neurons of the locus coeruleus, the two neuronal populations most targeted in Parkinson's disease. Many cellular redox reactions rely on iron, however an altered distribution of reactive iron is cytotoxic. In fact, increased levels of iron in the brain of Parkinson's disease patients are present. Dopamine accumulation can induce neuronal death; however, excess dopamine can be removed by converting it into a stable compound like neuromelanin, and this process rescues the cell. Interestingly, the main iron compound in dopamine and norepinephrine neurons is the neuromelanin-iron complex, since neuromelanin is an effective metal chelator. Neuromelanin serves to trap iron and provide neuronal protection from oxidative stress. This equilibrium between iron, dopamine, and neuromelanin is crucial for cell homeostasis and in some cellular circumstances can be disrupted. Indeed, when neuromelanin-containing organelles accumulate high load of toxins and iron during aging a neurodegenerative process can be triggered. In addition, neuromelanin released by degenerating neurons activates microglia and the latter cause neurons death with further release of neuromelanin, then starting a self-propelling mechanism of neuroinflammation and neurodegeneration. Considering the above issues, age-related accumulation of neuromelanin in dopamine neurons shows an interesting link between aging and neurodegeneration.
Full-text · Article · Oct 2015 · Progress in Neurobiology
[Show abstract][Hide abstract] ABSTRACT: Parkinson's disease (PD) etiology is closely linked to the aggregation of α-synuclein (αS). Copper(II) ions can bind to αS and may impact its aggregation propensity. As a consequence, deciphering the exact mode of Cu(II) binding to αS is important in the PD context. Several previous reports have shown some discrepancies in the description of the main Cu(II) site in αS, which are resolved here by a new scenario. Three Cu(II) species can be encountered, depending on the pH and the Cu:αS ratio. At low pH, Cu(II) is bound to the N-terminal part of the protein by the N-terminal amine, the adjacent deprotonated amide group of the Asp2 residue, and the carboxylate group from the side chain of the same Asp2. At pH 7.4, the imidazole group of remote His50 occupies the fourth labile equatorial position of the previous site. At high Cu(II):αS ratio (>1), His50 leaves the coordination sphere of the first Cu site centered at the N-terminus, because a second weak affinity site centered on His50 is now filled with Cu(II). In this new scheme, the remote His plays the role of a molecular switch and it can be anticipated that the binding of the remote His to the Cu(II) ion can induce different folding of the αS protein, having various aggregation propensity.
No preview · Article · Apr 2015 · Inorganic Chemistry
[Show abstract][Hide abstract] ABSTRACT: Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the presence of abnormal α-synuclein (αSyn) deposits in the brain. Alterations in metal homeostasis and metal-induced oxidative stress may play a crucial role in the aggregation of αSyn and, consequently, in the pathogenesis of PD. We have therefore investigated the capability of copper-αSyn6 and copper-αSyn15 peptide complexes, with the 1-6 and 1-15 terminal fragments of the protein, to promote redox reactions that can be harmful to other cellular components. The pseudo-tyrosinase activity of copper-αSyn complexes against catecholic (di-tert-buthylcatechol (DTBCH2) and 4-methylcatechol (4-MC) and phenolic (phenol) substrates is lower compared to that of free copper(II). In particular, the rates (kcat) of DTBCH2 catalytic oxidation are 0.030 s-1 and 0.009 s-1 for the reaction promoted by free copper(II) and [Cu2+-αSyn15], respectively. On the other hand, HPLC/ESI-MS analysis of solutions of αSyn15 incubated with copper(II) and 4-MC showed that αSyn is competitively oxidized with remarkable formation of sulfoxide at Met1 and Met5 residues. Moreover, sulfoxidation of methionine residues, which is related to the aggregation of αSyn, also occurs on peptide not directly bound to copper, indicating that external αSyn can also be oxidized by copper. Therefore, this study strengthens the hypothesis that copper plays an important role in oxidative damage of αSyn which is proposed to be strongly related to the etiology of PD.
[Show abstract][Hide abstract] ABSTRACT: Recent advances in dinuclear copper complexes as mimics of the catalytic centers of tyrosinase and catechol oxidase allowed the reproduction of the structural and mechanistic aspects of the enzymes. However, a challenging objective is the development of chiral complexes for bioinspired enantioselective oxidation reactions. Here, we report the synthesis and characterization of a dinuclear copper(II) complex with a new chiral diamino-m-xylenetetra(benzimidazole) ligand (L55Bu4), which has chiral centers at the four 2-methylbutyl substituents of the benzimidazole rings. The spectral characteristics, ligand binding properties, and reactivity of [CuII2L55Bu4]4+ in the catalytic oxidations of several biogenic catechols (L-/D-dopa, L-/D-DopaOMe, and L-/D-norepinephrine) and thioanisole are reported. The best discriminating properties are displayed towards the DopaOMe derivatives, for which the oxidation rate of the L enantiomer is approximately one order of magnitude larger than that of the opposite D isomer.
No preview · Article · Mar 2015 · Berichte der deutschen chemischen Gesellschaft
[Show abstract][Hide abstract] ABSTRACT: Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the presence of abnormal α-synuclein (αS) deposits in the brain. Alterations in homeostasis and metal-induced oxidative stress may play a crucial role in the progression of αS amyloid assembly and pathogenesis of PD. Contrary to αS, β-synuclein (βS) is not involved in the PD etiology. However, it has been suggested that the βS/αS ratio is altered in PD, indicating that a correct balance of these two proteins is implicated in the inhibition of αS aggregation. αS and βS share similar abilities to coordinate Cu(II). In this study, we investigated and compared the interaction of Cu(I) with the N-terminal portion of βS and αS by means of NMR, circular dichroism, and X-ray absorption spectroscopies. Our data show the importance of M10K mutation, which induces different Cu(I) chemical environments. Coordination modes 3S1O and 2S2O were identified for βS and αS, respectively. These new insights into the bioinorganic chemistry of copper and synuclein proteins are a basis to understand the molecular mechanism by which βS might inhibit αS aggregation.
No preview · Article · Dec 2014 · Inorganic Chemistry
[Show abstract][Hide abstract] ABSTRACT: At cardiovascular level, nitric oxide (NO) controls smooth muscle functions, maintains vascular integrity and exerts anti-hypertensive effect. Metal-nonoates are a recently discovered class of NO donors, with NO release modulated through the complexation of the N-aminoethylpiperazine N-diazeniumdiolate ligand to metal ions and therefore representing a significant innovation with respect to the drugs traditionally used. In this study, we characterized the vascular protective effects of the most effective compound of this class, Ni(PipNONO)Cl, compared to the commercial NONOate derivate DETA/NO. Ni(PipNONO)Cl induced a concentration dependent relaxation of pre-contracted rat aortic rings. The ED50 was 0.67 μM, compared to 4.3 μM obtained with DETA/NO. When tested on cultured microvascular endothelial cells, Ni(PipNONO)Cl exerted a protective effect on the endothelium, promoting cell proliferation and survival in the range of pM. The administration of Ni(PipNONO)Cl to vascular smooth muscle cells reduced cell number, promoting their apoptosis at high concentration (10 μM). Inhibition of smooth muscle cell migration, a hallmark of atherosclerosis, was accompanied by cytoskeletal rearrangement and loss of lamellipodia. When added to isolated platelets, Ni(PipNONO)Cl significantly reduced ADP induced aggregation. Since atherosclerosis is accompanied by an inflammatory environment, cultured endothelial cells were exposed to interleukin-1 beta (IL-1β). In the presence of IL-1β, Ni(PipNONO)Cl inhibited cyclooxygenase.2 (COX-2) and inducible nitric oxide synthase (iNOS) upregulation, reduced endothelial permeability and the platelet and monocyte adhesion markers CD31 and CD40 at the plasma membrane. Overall, these data indicate that Ni(PipNONO)Cl exerts vascular protective effects relevant for vascular dysfunction and prevention of atherosclerosis and thrombosis.
Full-text · Article · Sep 2014 · Journal of Pharmacology and Experimental Therapeutics
[Show abstract][Hide abstract] ABSTRACT: Inspired by catalytic sites of cytochrome c oxidase (CcO) and nitric oxide reductase (NOR), a new series of dinuclear heme–non-heme complexes is described. The complexes are derived from the association between an iron(III)–protoporphyrin IX containing a covalently attached Gly-L-His-OMe residue to one propionic acid substituent (HMGH) and a metal complex with a tridentate amino-bis(benzimidazole) (BBH) ligand, mimicking the tris-histidine coordination of FeB and CuB in NOR and CcO, respectively. Besides the coordination of FeIII and CuII with the BBH ligand, we also explored the role of “non-biomimetic” metals, such as CoII, MnII, or ZnII, in order to establish the priority among the ancillary metal ions in cooperating with the ferric heme and promoting its catalytic activity in oxidation reactions. pH-spectrophotometric titrations show that the presence of the non-heme metal decreases the pKa of water-bound to hemin (pKa = 8.4 ± 0.1), with a larger effect with iron(III), copper(II) and zinc(II) complexes (pKa of 6.4 ± 0.1, 6.0 ± 0.1 and 6.5 ± 0.1, respectively), which suggests that an interaction with the non-heme metal center takes place also at a micromolar range. NMR spectra indicate that the interaction between hemin and the non-heme center is not strong enough to convert the high spin configuration of FeIII–heme to low spin as observed for CcO and NOR enzymes. The dinuclear complex enhances the peroxidase-like activity of heme in kinetic studies performed at pH 5.5, 7.0 (using 3-(4-hydroxyphenyl)-propanoic acid as the substrate) and 9.0 (using o-phenylenediamine). In particular, the stronger effects are observed with FeIII, CuII, and CoII complexes, which increase the turnover rates of hemin throughout the pH range analyzed. At neutral and basic pH the KM value decreases up to one fourth indicating a positive cooperation between HMGH and [M(BBH)]n+ in binding the substrates. Moreover, the presence of the non-heme center facilitates the binding of H2O2 and formation of high valent ˙PFeIVO species. These data show that interaction between the two metal centers occurs with heme in several oxidation states.
Full-text · Article · Jan 2014 · New Journal of Chemistry
[Show abstract][Hide abstract] ABSTRACT: Dopaminergic neurons of the substantia nigra selectively degenerate over the course of Parkinson's disease. These neurons are also the most heavily pigmented cells of the brain, accumulating the dark pigment neuromelanin over a lifetime. The massive presence of neuromelanin in these brain areas has long been suspected as a key factor involved in the selective vulnerability of neurons. The high concentration of neuromelanin in substantia nigra neurons seems to be linked to the presence of considerable amounts of cytosolic dopamine that have not been sequestered into synaptic vesicles. Over the past few years, studies have uncovered a dual nature of neuromelanin. Intraneuronal neuromelanin can be a protective factor, shielding the cells from toxic effects of redox active metals, toxins, and excess of cytosolic catecholamines. In contrast, neuromelanin released by dying neurons can contribute to the activation of neuroglia triggering the neuroinflammation that characterizes Parkinson's disease. This article reviews recent studies on the molecular aspects of neuromelanin of the human substantia nigra.
Full-text · Article · Jan 2014 · Neurotoxicity Research
[Show abstract][Hide abstract] ABSTRACT: The physiological functions of neuroglobin (Ngb), the heme protein of the globin family expressed in the nervous tissue, have not yet been clarified. Besides O2 storage and homoeostasis, Ngb is thought to play a role in neuroprotection as a scavenger of toxic reactive species generated in vivo under conditions of oxidative stress. Herein, the interaction of Ngb with the quinones generated by oxidation of catecholamines (dopamine, norepinephrine) and catechol estrogens (2-hydroxyestradiol, 4-hydroxyestradiol) and implicated in neurodegenerative pathologies like Parkinson's and Alzheimer's diseases, has been investigated. The cytotoxicity of quinones has been ascribed to the derivatization of amino acid residues (mainly cysteine) in proteins through the formation of covalent bonds with the aromatic rings. Combined studies of tandem mass spectrometry and protein unfolding indicate the presence of quinone-promoted modifications in all the Ngb derivatives analyzed (i.e. obtained employing either catecholamines or catechol estrogens as source of the reactive species). Among protein residues, the highest reactivity of cysteines (Cys46, Cys55, and Cys120 in human Ngb) towards quinone species has been confirmed and the dependence of the extent of protein modification on the method employed for catechol oxidation has been observed. When the oxidation reaction proceeds by one-electron steps, the involvement of semiquinone reactivity has been observed. The whole analysis of the data of Ngb modification suggests that the catecholamine oxidation products can extensively modify proteins (likely by catecholamine oligomers). The modification mediated by catechol estrogens is less pronounced but strongly affects the interactions with the solvent and the protein stability.
Full-text · Article · Oct 2013 · Chemical Research in Toxicology
[Show abstract][Hide abstract] ABSTRACT: Stressful situation: Ferric heme binds to Aβ16 giving a mixture of five-coordinate [hemin(Aβ16)] and six-coordinate [hemin(Aβ16)2 ] species, the equilibrium of which depends on the Aβ16/hemin ratio and on temperature. Under oxidative and nitrative stress conditions the heme-Aβ16 complexes promote peroxidase-like reactions causing oxidation and nitration of the Aβ Tyr10 residue. Both dityrosine formation and tyrosine nitration strongly enhance Aβ aggregation.
No preview · Article · Jul 2013 · Angewandte Chemie International Edition
[Show abstract][Hide abstract] ABSTRACT: Hemoglobin (Hb) is the most abundant protein in human blood and we showed that under oxidative/nitrative stress conditions it is susceptible to cysteine oxidation, tyrosine nitration, and formation of a dimer of Hb subunits through tyrosine linkage. In the presence of hydrogen peroxide, Hb and its subunits efficiently convert nitrite into reactive nitrogen species, through reactions that are typical of peroxidases. If an exogenous phenolic substrate is present, Hb promotes its nitration with a fivefold higher efficiency with respect to the peroxidase-like phenol coupling reaction. In the absence of an exogenous substrate, the protein itself undergoes covalent modification. Trypsin treatment of Hb modified under conditions mimicking pathophysiological conditions, followed byHPLC-ESI-MS/MS analysis, allowed detection of nitration of Yα24, Yα42, Yβ130 and Yβ145, and conversion of Cα104, Cβ93 and Cβ112 into cysteine sulfinic acids. As additional biomarkers of nitrative stress, we found a covalent dimer of Hb αβ subunits and covalently linked heme-peptide. The dimer is selectively nitrated at Yα42. In a preliminary analysis of samples of human blood we found that low amounts of the dimer of subunits are always present. Therefore, if a correlation between the extent of Hb subunits coupling and pathological states could be established, this dimer could become an easily detectable biomarker of pathological conditions.
[Show abstract][Hide abstract] ABSTRACT: Mn(II)–Mn(II) cluster Apoenzyme Human prolidase, the enzyme responsible for the hydrolysis of the Xaa-Pro/Hyp peptide bonds, is a key player in the recycling of imino acids during the final stage of protein catabolism and extracellular matrix remodeling. Its metal active site composition corresponding to the maximal catalytic activity is still unknown, although prolidase function is of increasing interest due to the link with carcinogenesis and mutations in prolidase gene cause a severe connective tissue disorder. Here, using EPR and ICP-MS on human recombinant prolidase pro-duced in Escherichia coli (hRecProl), the Mn(II) ion organized in a dinuclear Mn(II)–Mn(II) center was identified as the protein cofactor. Furthermore, thermal denaturation, CD/fluorescence spectroscopy and limited proteoly-sis revealed that the Mn(II) is required for the proper protein folding and that a protein conformational modifi-cation is needed in the transition from apo-to Mn(II)loaded-enzyme. The collected data provided a better knowledge of the human holo-prolidase and, although limited to the recombinant enzyme, the exact identity and organization of the metal cofactor as well as the conformational change required for activity were proven.
[Show abstract][Hide abstract] ABSTRACT: Contrary to earlier claims, the Cu(II) complex with the soluble Aβ16 peptide, and also that with Aβ28 exhibit no phenol monooxygenase (tyrosinase-like) activity; the complexes neither exhibit superoxide dismutase activity.
No preview · Article · Apr 2013 · Chemical Communications
[Show abstract][Hide abstract] ABSTRACT: Prolidase is the only human enzyme responsible for the digestion of iminodipeptides containing proline or hydroxyproline at their C-terminal end, being a key player in extracellular matrix remodeling. Prolidase deficiency (PD) is an intractable loss of function disease, characterized by mutations in the prolidase gene. The exact causes of activity impairment in mutant prolidase are still unknown. We generated three recombinant prolidase forms, hRecProl-231delY, hRecProl-E412K and hRecProl-G448R, reproducing three mutations identified in homozygous PD patients. The enzymes showed very low catalytic efficiency, thermal instability and changes in protein conformation. No variation of Mn(II) cofactor affinity was detected for hRecProl-E412K; a compromised ability to bind the cofactor was found in hRecProl-231delY and Mn(II) was totally absent in hRecProl-G448R. Furthermore, local structure perturbations for all three mutants were predicted by in silico analysis. Our biochemical investigation of the three causative alleles identified in perturbed folding/instability, and in consequent partial prolidase degradation, the main reasons for enzyme inactivity. Based on the above considerations we were able to rescue part of the prolidase activity in patients' fibroblasts through the induction of Heath Shock Proteins expression, hinting at new promising avenues for PD treatment.