Some aspects of tyrosine secondary metabolism

Neurobiological and Pharmacological Perspectives of D3 Receptors in Parkinson’s Disease

The discovery of the D3 receptor (D3R) subtypes of dopamine (DA) has generated an understandable increase in interest in the field of neurological diseases, especially Parkinson’s disease (PD). Indeed, although DA replacement therapy with l-DOPA has provided an effective treatment for patients with PD, it is responsible for invalidating abnormal involuntary movements, known as L-DOPA-induced dyskinesia, which constitutes a serious limitation of the use of this therapy. Of particular interest is the finding that chronic l-DOPA treatment can trigger the expression of D1R–D3R heteromeric interactions in the dorsal striatum. The D3R is expressed in various tissues of the central nervous system, including the striatum. Compelling research has focused on striatal D3Rs in the context of PD and motor side effects, including dyskinesia, occurring with DA replacement therapy. Therefore, this review will briefly describe the basal ganglia (BG) and the DA transmission within these brain regions, before going into more detail with regard to the role of D3Rs in PD and their participation in the current treatments. Numerous studies have also highlighted specific interactions between D1Rs and D3Rs that could promote dyskinesia. Finally, this review will also address the possibility that D3Rs located outside of the BG may mediate some of the effects of DA replacement therapy.

Comparative genomics and proteogenomics highlight key molecular players involved in Frankia sporulation

Sporulation is a microbial adaptive strategy to resist inhospitable conditions for vegetative growth and to disperse to colonise more favourable environments. This microbial trait is widespread in Actinobacteria. Among them, Frankia strains are able to differentiate sporangia in pure culture, while others can sporulate even when in symbiosis with sporulation occurring within host cells. The molecular determinants controlling Frankia sporulation have not been yet described. In order to highlight, for the first time, the molecular players potentially involved in Frankia sporulation, we conducted (i) a comparison of protein contents between Frankia spores and hyphae and (ii) a comparative genomic analysis of Frankia proteomes with sporulating and non-sporulating Actinobacteria. Among the main results, glycogen-metabolism related proteins, as well as oxidative stress response and protease-like proteins were overdetected in hyphae, recalling lytic processes that allow Streptomyces cells to erect sporogenic hyphae. Several genes encoding transcriptional regulators, including GntR-like, appeared up-regulated in spores, as well as tyrosinase, suggesting their potential role in mature spore metabolism. Finally, our results highlighted new proteins potentially involved in Frankia sporulation, including a pyrophosphate-energized proton pump and YaaT, described as involved in the phosphorelay allowing sporulation in Bacillus subtilis, leading us to discuss the role of a phosphorelay in Frankia sporulation.

Protein oxidation: identification and utilisation of molecular markers to differentiate singlet oxygen and hydroxyl radical-mediated oxidative pathways

The effect of reactive oxidation species (ROS) on tryptophan or tyrosine was investigated by qualitatively determining the major detectable oxidation products generated by hydroxyl radicals, produced by the Fenton process, or singlet oxygen, generated by exposure to green light in the presence of Rose Bengal, on these photosensitive amino acids in synthetic pentapeptides. Based on mass spectrometric analysis it would appear that the hydroxyl radical favours a pathway leading to the formation of tryptophandione-based products from tryptophan. In contrast singlet oxygen attack appears to favour the formation of kynurenine-type products from tryptophan. Specific oxidative products observed proteomically are therefore potentially able to discriminate between predominant ROS-mediated pathways. To validate these findings, a keratin-enriched extract was exposed to UVB light under aqueous conditions. The observation of the conversion of tryptophan to hydroxytryptophan in marker peptides, and the absence of singlet-oxygen specific modifications, suggested that under these conditions oxidative degradation occurred primarily via hydroxyl radical attack. These observations provide the first direct proteomic evidence of the dominant photodegradation pathways in wet wool.

Molecular Pathogenesis and Pathophysiology of Parkinson’s Disease: New Targets for New Therapies

Parkinson’s disease (PD) is a complex chronic neurodegenerative disease of unknown etiology. A conceptual framework for all chronic diseases involves a series of channels or pathways (aging, genetic, environment, oxidative stress, mitochondrial damage, protein aggregation, etc.) and their interactions. Those channels with specificities may explain the ‘developmental’ program that through transcriptional reprogramming results in stressed dopamine neurons that eventually become dysfunctional or die, giving rise to the clinical manifestations of PD. In Chapter 2 we review the molecular mechanisms of those channels that may be implicated in the pathogenesis of PD and the pathophysiology of the disease based on the anatomo‐physiological complexity of the basal ganglia. This illustrates that understanding the molecular mechanisms of a disease may not be enough, or we have to reach an adequate system level to understand the disease process. Finally, we suggest that common therapies used for the treatment of other chronic diseases may be useful for the treatment (or help to advance the understanding) of PD, as well as new targets for new therapies that may be useful in the prevention of, or to stop the progression of, PD and other synucleinopathies.

The search for an endogenous schizogen: the strange case of taraxein

In 1956, Dr. Robert Galbraith Heath, Chair of Psychiatry and Neurology at Tulane University School of Medicine in New Orleans, announced that he and colleagues had discovered a protein they called taraxein in the blood of schizophrenic patients that caused symptoms of schizophrenia when injected into healthy volunteers. Heath's claim received wide public and professional attention. Researchers quickly tried to confirm the discovery. These efforts, which were rigorous and in some cases conducted in consultation with the Tulane researchers, failed. Nevertheless, for the next four decades Heath continued to defend his claim. This article recounts the scientific developments that led up to Heath's putative discovery and it explores the scientific findings for and against the taraxein theory of schizophrenia.

Profiling of residue-level photo-oxidative damage in peptides

Protein and peptide oxidation is a key feature in the progression of a variety of disease states and in the poor performance of protein-based products. The present work demonstrates a mass spectrometry-based approach to profiling degradation at the amino acid residue level. Synthetic peptides containing the photosensitive residues, tryptophan and tyrosine, were used as models for protein-bound residue photodegradation. Electrospray ionisation tandem mass spectrometry (ESI-MS/MS) was utilised to characterise and provide relative quantitative information on the formation of photoproducts localised to specific residues, including the characterisation of low abundance photomodifications not previously reported, including W + 4O modification, hydroxy-bis-tryptophandione and topaquinone. Other photoproducts observed were consistent with the formation of tyrosine-derived dihydroxyphenylalanine (dopa), trihydroxyphenylalanine, dopa-quinone and nitrotyrosine, and tryptophan-derived hydroxytryptophan, dihydroxytryptophan/N-formylkynurenine, kynurenine, hydroxyformylkynurenine, tryptophandiones, tetrahydro-beta-carboline and nitrotryptophan. This approach combined product identification and abundance tracking to generate a photodegradation profile of the model system. The profile of products formed yields information on formative mechanisms. Profiling of product formation offers new routes to identify damage markers for use in tracking and controlling oxidative damage to polypeptides.

Mediterranean shrubs as potential antioxidant sources

Several Mediterranean shrubs, both autochthonous and naturalized, have been traditionally used as food, flavouring and/or spicing agents, and as phytopharmaceuticals. The interest around 'natural' and 'biological' products is steadily increasing in developed countries. Therefore, it seems reasonable to screen some shrubs with regard to the actual information about their content of phytochemicals, in relation to both real and putative beneficial properties, and with particular concern to their 'antioxidant' power. Moreover, striking molecules from the examined shrubs are compared according to their occurrence in the various genera. Also, their structures and structure/activity relationship are discussed in the light of possible practical application.

Methenyltetrahydrofolate synthetase is a high-affinity catecholamine-binding protein

Recombinant mouse 5,10-methenyltetrahydrofolate synthetase (MTHFS) was expressed in Escherichia coli and shown to co-purify with a chromophore that had a lambda(max) at 320nm. The chromophore remained bound to MTHFS during extensive dialysis, but dissociated from MTHFS when its substrate, 5-formyltetrahydrofolate, was bound. The chromophore was identified as an oxidized catecholamine by mass spectrometry and absorption spectroscopy. Purified recombinant mouse MTHFS and rabbit liver MTHFS proteins were shown to bind oxidized N-acetyldopamine (NADA) tightly. The addition of NADA to cell culture medium accelerated markedly folate turnover and decreased both folate accumulation and total cellular folate concentrations in MCF-7 cells. Expression of the MTHFS cDNA in MCF-7 cells increased the concentration of NADA required to deplete cellular folate. The results of this study are the first to identify a link between catecholamines and one-carbon metabolism and demonstrate that NADA accelerates folate turnover and impairs cellular folate accumulation in MCF-7 cells.

Crystal structure of prephenate dehydrogenase from Aquifex aeolicus. Insights into the catalytic mechanism

The enzyme prephenate dehydrogenase catalyzes the oxidative decarboxylation of prephenate to 4-hydroxyphenylpyruvate for the biosynthesis of tyrosine. Prephenate dehydrogenases exist as either monofunctional or bifunctional enzymes. The bifunctional enzymes are diverse, since the prephenate dehydrogenase domain is associated with other enzymes, such as chorismate mutase and 3-phosphoskimate 1-carboxyvinyltransferase. We report the first crystal structure of a monofunctional prephenate dehydrogenase enzyme from the hyper-thermophile Aquifex aeolicus in complex with NAD+. This protein consists of two structural domains, a modified nucleotide-binding domain and a novel helical prephenate binding domain. The active site of prephenate dehydrogenase is formed at the domain interface and is shared between the subunits of the dimer. We infer from the structure that access to the active site is regulated via a gated mechanism, which is modulated by an ionic network involving a conserved arginine, Arg250. In addition, the crystal structure reveals for the first time the positions of a number of key catalytic residues and the identity of other active site residues that may participate in the reaction mechanism; these residues include Ser126 and Lys246 and the catalytic histidine, His147. Analysis of the structure further reveals that two secondary structure elements, beta3 and beta7, are missing in the prephenate dehydrogenase domain of the bifunctional chorismate mutase-prephenate dehydrogenase enzymes. This observation suggests that the two functional domains of chorismate mutase-prephenate dehydrogenase are interdependent and explains why these domains cannot be separated.

Expression of NAD(P)H:quinone oxidoreductase in the normal and Parkinsonian substantia nigra

Dopamine (DA) autooxidation, and consequent formation of neurotoxic DA-derived quinones and reactive oxygen species, has been implicated in dopaminergic cell death and, hence, in the pathogenesis of Parkinson's disease (PD). Stimulation of pathways involved in the detoxication of DA-quinones in the brain is hypothesized to be an effective means to limit oxidative stress and to confer neuroprotection in PD. In this respect, the inducible flavoprotein NAD(P)H:quinone oxidoreductase (NQO1) is of particular interest as it is directly implicated in the detoxication of DA-quinones and, in addition, has broad spectrum anti-oxidant properties. To study the potential pathophysiological role of NQO1 in PD, the cellular expression of NQO1 was examined in the mesencephalon of PD patients and age-matched controls. In the substantia nigra pars compacta (SNpc), NQO1 was found to be expressed in astroglial and endothelial cells and, albeit less frequently, also in dopaminergic neurons. Moreover, while overt NQO1 immunoreactivity was absent in the surrounding nervous tissue, in the Parkinsonian SNpc a marked increase in the astroglial and neuronal expression of NQO1 was consistently observed.

Neuroprotection for Parkinson's disease: a new approach for a new millennium

Parkinson's disease (PD) is the only neurodegenerative disorder in which pharmacological intervention has resulted in a marked decrease in morbidity and a significant delay in mortality. However, the medium to long-term efficacy of this pharmacotherapy, mainly consisting of dopaminomimetics like L -dopa and dopamine receptor agonists, suffers greatly from the unrelenting progression of the disease process underlying PD, i.e., the degeneration of neuromelanin-containing, dopaminergic neurones in the substantia nigra. Efforts concentrated on understanding the mechanisms of dopaminergic cell death in Parkinson's disease have led to identification of a large variety of pathogenetic factors, including excessive release of oxygen free radicals during enzymatic dopamine breakdown, impairment of mitochondrial function, production of inflammatory mediators, loss of trophic support, and apoptosis. Therapeutic approaches aimed at correcting these abnormalities are currently being evaluated on their efficacy as neuroprotectants for PD. Here, we focus on the process of dopamine auto-oxidation, the chain of reactions leading to the formation of neuromelanin, as an often overlooked, yet obvious pathogenetic factor. In particular, we discuss the option of drug-mediated stimulation of endogenous mechanisms responsible for the detoxification of dopamine auto-oxidation products as a novel means of neuroprotection in Parkinson's disease.

The physiopathological significance of ceruloplasmin. A possible therapeutic approach

This article reviews and comments on the physiological roles of ceruloplasmin (Cp). We show that, in addition to its ascertained involvement in iron homeostasis, the protein, by virtue of its unique structure among multicopper oxidases, is likely involved in other processes of both an enzymatic and a nonenzymatic nature. In particular, based on the analysis of the kinetic parameters, on the one hand, and of the side-products of the oxidation, on the other, we propose that the long-recognized ability of Cp to interact with and oxidize non-iron substrates may be of physiological relevance. The striking example of 6-hydroxydopamine oxidation is presented, where we show that the catalytic action is carried out readily under physiological conditions, without release of potentially toxic oxygen intermediates.

Drug treatment of Parkinson's disease. Time for phase II

Parkinson's disease (PD) is a neurodegenerative syndrome for which at present no cure is available; therapy consists mainly of amelioration of the symptoms with L-Dopa and/or dopamine (DA) agonists. Development of an effective causal therapy should be focussed on preventing or at least retarding the neurodegenerative process underlying the disease. At the cellular level, PD is characterized by degeneration of neuromelanin-containing dopaminergic neurons in the substantia nigra. Neuromelanin formation is the outcome of a process generally known as DA autooxidation, a chain of oxidation reactions in which highly neurotoxic DA-quinones are produced. The level of these DA-quinones, as estimated by the occurrence of their cysteinyl conjugates, is reported to be increased in the Parkinsonian substantia nigra. Hence, stimulation of pathways implicated in the detoxication of DA-quinones in the brain may provide neuroprotection in PD. Besides their inactivation through non-enzymatic antioxidants such as ascorbic acid and glutathione, DA-quinones are efficiently inactivated enzymatically by NAD(P)H:quinone oxidoreductase (NQO) and glutathione transferase(s), both of which are expressed in the human substantia nigra. The activity of these enzymes, which belong to the group of phase II biotransformation enzymes, can be up-regulated by a large variety of compounds. These compounds, including dithiolethiones, phenolic anti-oxidants, and isothiocyanates, have been shown to be active both in vitro and in vivo. Thus, considering the role of phase II biotransformation enzymes, in particular NQO and glutathione transferase(s), in the detoxication of DA-quinones, we propose that phase II enzyme inducers warrant evaluation on their neuroprotective potential in PD.