Current Topics in Medicinal Chemistry - Volume 1, Issue 6, 2001

Oxidative stress occurs in the brain due to stroke, Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, trauma, aging and other conditions. Analysis of the effects of oxidative stress can involve quantitation of brain GSH, GSSG, NADPH and NADP. Reliable and rapid assays have been developed for these compounds and will be presented in detail. The assays have been used to analyze the effects of brain oxidative stress. Thermodynamic calculations can be performed to find the observed electrochemical potentials of the GSSG / GSH and the NADP / NADPH couples during oxidative stress. The biochemical consequences of these thermodynamic changes in the cell will be discussed as well as the defense mechanisms available to the cell to recover from oxidative stress.

In experimental models of brain injury of the ischemia-reperfusion type, there is a period of time in which the formation of oxidative damage exceeds its repair. Simultaneously, the expression of immediate early genes is induced to activate the expression of late effector genes. Drugs that reduce the need to repair during this transient period of time also attenuate neuronal death after brain injury. An example discussed in this review is the activator protein-1 (AP-1), the product of the c-fos gene and other imme-diate early genes. What is the effect of a delayed expression of these genes in relationship to the process of cell death? This short period presents a window of opportunity to study the effects of oxidative damage on gene expression in the brain and specific deficiencies in gene repair that have been associated with particular neurological disorders.

Extensive nerve cell death occurs during the development of the central nervous system as well as in episodes of trauma and in neurodegenerative disease. The mechanistic details of how these cells die are poorly understood. Here we describe a unique oxidative stress-induced programmed cell death pathway called oxytosis, and outline pharmacological approaches which interfere with its execution. Oxidative glutamate toxicity, in which exogenous glutamate inhibits cystine uptake through the cystine / glutamate antiporter leading to a depletion of glutathione, is used as an example of oxytosis. It is shown that there is a sequential requirement for de novo macromolecular synthesis, lipoxygenase activation, reactive oxygen species production, and the opening of cGMP-gated channels which allow the influx of extracellular calcium. The translation initiation factor eIF2α plays a central role in this pathway by regulating the levels of glutathione. Finally, examples are given in which the reduction in glutathione, the production of reactive oxygen species, and calcium influx can be experimentally manipulated to prevent cell death. Data are reviewed which suggest that oxytosis may be involved in nerve cell death associated with nervous system trauma and disease.

The deposition of abnormal protein fibrils is a prominent pathological feature of many different ‘protein conformational’ diseases, including some important neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), motor neurone disease and the ‘prion’ dementias. Some of the fibril-forming proteins or peptides associated with these diseases have been shown to be toxic to cells in culture. A clear understanding of the molecular mechanisms responsible for this toxicity should shed light on the probable link between protein deposition and cell loss in these diseases. In the case of the β-amyloid (Aβ), which accumulates in the brain in AD, there is good evidence that the toxic mechanism involves the production of reactive oxygen species (ROS). By means of an electron spin resonance (ESR) spin-trapping method, we have shown recently that solutions of Aβ liberate readily detectable amounts of hydroxyl radicals upon incubation in vitro followed by the addition of small amounts of Fe(II). We have also obtained similar results with a-synuclein, which accumulates in Lewy bodies in PD. Our data suggest that hydrogen peroxide accumulates during Aβ or α-synuclein incubation and that this is subsequently converted to hydroxyl radicals, on addition of Fe (II), by Fenton's reaction. Consequently, we now support the idea that one of the fundamental molecular mechanisms underlying the pathogenesis of cell death in AD, PD, and possibly some other protein conformational diseases, could be the direct production of ROS during formation of the abnormal protein aggregates. This hypothesis suggests a novel approach to the therapy of this group of diseases.

Dopamine oxidation is proposed to be a significant contributor to mesostriatal dopaminergic neurodegeneration, although the mechanisms are not fully resolved. Recent results from in vitro and in vivo models have suggested that some products from mercapturic acid pathway (MAP) metabolism of oxidized dopamine (DA) may contribute to dopaminergic neurodegeneration, and that at least one product of this pathway, 5-S-cysteinyldopamine (Cys-DA), is elevated in patients with advanced Parkinson's disease (PD). Here we review recent findings on MAP enzymes and their products in rodent brain and in diseased regions of brain from patients with mesostriatal dopaminergic neurodegeneration. We also review the current data and our recent findings on the neurobiological activity of MAP metabolites of oxidized DA. We conclude that human striatum has limited enzymatic capacity for mercapturate formation, that levels of MAP products of oxidized DA are significantly elevated in PD patients with advanced dopaminergic neurodegeneration but not in patients with less severe degeneration, and that Cys-DA interferes with trafficking of DA in vitro and in vivo. These results indicate that Cys-DA may interfere with DA trafficking in patients with advanced dopaminergic neurodegeneration.

Toxic metals (lead, cadmium, mercury and arsenic) are widely found in our environment. Humans are exposed to these metals from numerous sources, including contaminated air, water, soil and food. Recent studies indicate that transition metals act as catalysts in the oxidative reactions of biological macromolecules therefore the toxicities associated with these metals might be due to oxidative tissue damage. Redox-active metals, such as iron, copper and chromium, undergo redox cycling whereas redox-inactive metals, such as lead, cadmium, mercury and others deplete cells' major antioxidants, particularly thiol-containing antioxidants and enzymes. Either redox-active or redox-inactive metals may cause an increase in production of reactive oxygen species (ROS) such as hydroxyl radical (HO.), superoxide radical (O2.-) or hydrogen peroxide (H2O2). Enhanced generation of ROS can overwhelm cells' intrinsic antioxidant defenses, and result in a condition known as “oxidative stress“. Cells under oxidative stress display various dysfunctions due to lesions caused by ROS to lipids, proteins and DNA. Consequently, it is suggested that metal-induced oxidative stress in cells can be partially responsible for the toxic effects of heavy metals. Several studies are underway to determine the effect of antioxidant supplementation following heavy metal exposure. Data suggest that antioxidants may play an important role in abating some hazards of heavy metals. In order to prove the importance of using antioxidants in heavy metal poisoning, pertinent biochemical mechanisms for metal-induced oxidative stress should be reviewed.

Cells rely on several transition metals to regulate a wide range of metabolic and signaling functions. The diversity and efficiency of their physiological functions are derived from atomic properties that are specific to transition metals, most notably an incomplete inner valence subshell. These properties impart upon these elements the ability to fluctuate among a variety of positively charged ionic forms, and a chemical flexibility that allows them to impose conformational changes upon the proteins to which they bind. By this means, transition metals can serve as the catalytic centers of enzymes for redox reactions including molecular oxygen and endogenous peroxides. This review addresses the consequences of the aberrant translocation of the redox-capable essential transition elements, iron, copper, and manganese, upon the brain with an emphasis on uncontrolled and deleterious oxidative events. The potential of metal-protein interactions in facilitating such events, and their association with the physiologically redox-inert metals zinc and aluminum, are related to their postulated contribution to the pathology of neurodegeneration.

Neural tissue is especially sensitive to oxidative stress, which is considered a prominent factor in both acute and chronic neurodegenerative diseases and traumatic brain insults. On this basis, therapeutical strategies centered on antioxidants and on drugs able to scavenge excess free radicals and to re-establish the redox equilibrium, have been proposed for treatment of several brain pathologies. The present paper shortly summarizes the main sources of free radical production in the brain and reviews some of the recent data on mechanisms of cellular transduction through which free radicals are believed to damage cells and, eventually, to bring them to death. Some of the most promising therapeutical perspectives for treatment of oxidative stress in neurodegeneration, are then considered. Their choice is the result of a selection, that is unavoidably due to the enormous amount of the literature data, based on personal evaluation as well as on the personal experimental experience of the author. Four main categories of possible therapeuticals are considered: inhibitors of antioxidant enzymes, endogenous antioxidants and their precursors, vitamins and related compounds, other natural antioxidants from fruits and vegetables. Some theoretical and practical issues relevant to the adoption of antioxidant therapies for neurodegeneration are highlited, with particular reference to the fact that a basal production of free radicals must be maintained in the brain due to the host of essential cellular functions subserved by them. In this connection, it seems advisable that future antioxidant strategies for neurodegeneration are based on mixtures of agents able to modulate multiple mechanisms of free radical production and scavenging, without dangerously hampering essential physiological defense based on free radical cellular signaling.

Flavonoids are a group of naturally occuring compounds which are widely distributed in nature. Epidemiological evidence suggests an inverse relationship between dietary intake of flavonoids and cardiovascular risk. The biological activities of flavonoids are related to their antioxidative effects. But a number of studies have found both anti and prooxidant effects for many of these compounds. This review article presents the synthetic pathways of flavonoids and discusses the structure-activity relationships between, xanthine oxidase inhibitive activities and their chemical structures, between the antioxidant and prooxidant activities and the chemical structure. Then we will show the antioxidant properties of new flavonoids in a few models. In these compounds one or two di-tert-butylhydroxyphenyl (DBHP) groups replace the catechol moiety at the position 2 of the benzopyrane heterocycle. New structures are compared with quercetin and BHT in an LDL-oxidation system, in protecting cultured bovine aortic endothelial cells against mO-LDL cytotoxicity and on myocardial functional recovery during reperfusion after 30 min global ischemia in isolated rat hearts.