Antioxidant therapeutics: Pandora′s box

Evolution has favored the utilization of dioxygen (O₂) in the development of complex multicellular organisms. O₂ is actually a toxic mutagenic gas that is highly oxidizing and combustible. It is thought that plants are largely to blame for polluting the earth′s atmosphere with O₂ owing to the development of photosynthesis by blue-green algae over 2 billion years ago. The rise of the plants and atmospheric O₂ levels placed evolutionary stress on organisms to adapt or become extinct. This implies that all the surviving creatures on our planet are mutants that have adapted to the “abnormal biology” of O₂. Much of the adaptation to the presence of O₂ in biological systems comes from well-coordinated antioxidant and repair systems that focus on converting O₂ to its most reduced form, water (H₂O), and the repair and replacement of damaged cellular macromolecules. Biological systems have also harnessed O₂′s reactive properties for energy production, xenobiotic metabolism, and host defense and as a signaling messenger and redox modulator of a number of cell signaling pathways. Many of these systems involve electron transport systems and offer many different mechanisms by which antioxidant therapeutics can alternatively produce an antioxidant effect without directly scavenging oxygen-derived reactive species. It is likely that each agent will have a different set of mechanisms that may change depending on the model of oxidative stress, organ system, or disease state. An important point is that all biological processes of aerobes have coevolved with O₂ and this creates a Pandora′s box for trying to understand the mechanism(s) of action of antioxidants being developed as therapeutic agents.     

Metabolic stability of superoxide adducts derived from newly developed cyclic nitrone spin traps

Reactive oxygen species are by-products of aerobic metabolism involved in the onset and evolution of various pathological conditions. Among them, the superoxide radical is of special interest as the origin of several damaging species such as H₂O₂, hydroxyl radical, or peroxynitrite (ONOO⁻). Spin trapping coupled with ESR is a method of choice to characterize these species in chemical and biological systems and the metabolic stability of the spin adducts derived from reaction of superoxide and hydroxyl radicals with nitrones is the main limit to the in vivo application of the method. Recently, new cyclic nitrones bearing a triphenylphosphonium or permethylated β-cyclodextrin moiety have been synthesized and their spin adducts demonstrated increased stability in buffer. In this article, we studied the stability of the superoxide adducts of four new cyclic nitrones in the presence of liver subcellular fractions and biologically relevant reductants using an original setup combining a stopped-flow device and an ESR spectrometer. The kinetics of disappearance of the spin adducts were analyzed using an appropriate simulation program. Our results highlight the interest of the new spin trapping agents CD-DEPMPO and CD-DIPPMPO for specific detection of superoxide with high stability of the superoxide adducts in the presence of liver microsomes.     

Synthesis, characterization and application of Ru(BINAP) (Acac) (MNAA) (MeOH), a highly effective catalyst for the asymmetric hydrogenation of 2-(6′-methoxynaphth-2′-yl)acrylic acid

Ru(S-BINAP) (Acac) (MNAA) (MeOH) (1) (where MNAA (2) = 2-(6′-methoxynaphth-2′-yl)acrylate anion), a highly effective catalyst for the asymmetric hydrogenation of 2-(6′-methoxynaphth-2′-yl) acrylic acid (3), was isolated from a dichloromethane/methanol (vol./vol. = 1/4) solution of Ru(S-BINAP) (Acac)₂ and excess of 2-(6′-methoxynaphth-2′-yl)acrylic acid after the solution was exposed to visible light for 2 weeks. On side by side comparison studies, the rate of the hydrogenation of 3 catalyzed by 1 was found to be substantially faster than the same reaction catalyzed by Ru(S-BINAP) (OAc)₂. The molecular structure of 1 was unambiguously characterized by single crystal X-ray diffraction.     

DJ-1 Is a Copper Chaperone Acting on SOD1 Activation

Lack of oxidative stress control is a common and often prime feature observed in many neurodegenerative diseases. Both DJ-1 and SOD1, proteins involved in familial Parkinson disease and amyotrophic lateral sclerosis, respectively, play a protective role against oxidative stress. Impaired activity and modified expression of both proteins have been observed in different neurodegenerative diseases. A potential cooperative action of DJ-1 and SOD1 in the same oxidative stress response pathway may be suggested based on a copper-mediated interaction between the two proteins reported here. To investigate the mechanisms underlying the antioxidative function of DJ-1 in relation to SOD1 activity, we investigated the ability of DJ-1 to bind copper ions. We structurally characterized a novel copper binding site involving Cys-106, and we investigated, using different techniques, the kinetics of DJ-1 binding to copper ions. The copper transfer between the two proteins was also examined using both fluorescence spectroscopy and specific biochemical assays for SOD1 activity. The structural and functional analysis of the novel DJ-1 copper binding site led us to identify a putative role for DJ-1 as a copper chaperone. Alteration of the coordination geometry of the copper ion in DJ-1 may be correlated to the physiological role of the protein, to a potential failure in metal transfer to SOD1, and to successive implications in neurodegenerative etiopathogenesis.     

Reactions of Low Valent Transition-Metal Complexes with Hydrogen Peroxide. Are they “Fenton-Like” or Not? 3. The Case of Fe(II){N(CH2CO2)3}(H2O)2

Usually the toxicity of superoxide is attributed lo its ability to reduce metal ions and subsequently reoxidation of the metal by hydrogen peroxide yields deleterious oxidizing species. As many other nontoxic biological reductants reduce metal compounds, we suggest that part of the mechanism of superoxide toxicity results from its ability to oxidize metal ions bound to biological targets, which subsequently degrade the target via an intramolecular electron Transfer reaction.     

A Manganese-rich Environment Supports Superoxide Dismutase Activity in a Lyme Disease Pathogen,Borrelia burgdorferi

The Lyme disease pathogen Borrelia burgdorferi represents a novel organism in which to study metalloprotein biology in that this spirochete has uniquely evolved with no requirement for iron. Not only is iron low, but we show here that B. burgdorferi has the capacity to accumulate remarkably high levels of manganese. This high manganese is necessary to activate the SodA superoxide dismutase (SOD) essential for virulence. Using a metalloproteomic approach, we demonstrate that a bulk of B. burgdorferi SodA directly associates with manganese, and a smaller pool of inactive enzyme accumulates as apoprotein. Other metalloproteins may have similarly adapted to using manganese as co-factor, including the BB0366 aminopeptidase. Whereas B. burgdorferi SodA has evolved in a manganese-rich, iron-poor environment, the opposite is true for Mn-SODs of organisms such as Escherichia coli and bakers'' yeast. These Mn-SODs still capture manganese in an iron-rich cell, and we tested whether the same is true for Borrelia SodA. When expressed in the iron-rich mitochondria of Saccharomyces cerevisiae, B. burgdorferi SodA was inactive. Activity was only possible when cells accumulated extremely high levels of manganese that exceeded cellular iron. Moreover, there was no evidence for iron inactivation of the SOD. B. burgdorferi SodA shows strong overall homology with other members of the Mn-SOD family, but computer-assisted modeling revealed some unusual features of the hydrogen bonding network near the enzyme''s active site. The unique properties of B. burgdorferi SodA may represent adaptation to expression in the manganese-rich and iron-poor environment of the spirochete.     

Comparative Studies on a Superoxide Dismutase Exhibiting Enzymatic Activity with Iron and Manganese as Active Cofactor

The SOD of Propionibacterium freudenreichii, ssp. shermanii belongs to a new group of SOD'S capable of retaining activity with either Fe or Mn as active metal cofactor.

Both enzymes exhibit identical secondary structure and immunological determinants. Hydrogen peroxide irreversibly inhibits both enzymes. The protein moiety of the Fe-and Mn-SOD could be digested with trypsin to a single active fragment.     

Antioxidant Systems in Tumour Cells: The Levels of Antioxidant Enzymes,Ferritin, and Total Iron in a Human Hepatoma Cell Line

The human hepatoma cell line Hep 3B, which has the hepatitis B virus genome, shows over 80% decrease of copper/zinc superoxide dismutase activity, over 90% decrease of manganese superoxide dismutase activity, over 70% decrease of catalase activity, absence of glutathione peroxidase and glutathione S-transferase activities, over 270-fold increase of ferritin content and 25-fold increase of total iron compared to normal autopsy liver. These conditions of low antioxidant enzyme activities and iron overload are those which support the accumulation of oxygen free-radicals and DNA damage commonly considered to be carcinogenic mechanisms.     

Specific detection of intramitochondrial superoxide produced by either cell activation or apoptosis by employing a newly developed cell-permeative lucigenin derivative, 10,10′-dimethyl-9,9′-biacridinium bis(monomethyl terephthalate)

Here we developed a new cell-permeative lucigenin derivative, 10,10′-dimethyl-9,9′-biacridinium bis(monomethyl terephthalate) (MMT), to detect intracellular superoxide production. Both MMT and lucigenin were specific to superoxide among reactive oxygen species tested. Although lucigenin barely penetrated into cells, MMT accumulated in mitochondria in a variety of cells such as neutrophils. By employing MMT, we found that, upon activation of neutrophils with phorbol myristate acetate, superoxide was generated extracellularly as well as intramitochondrially and that such intramitochondrial superoxide production was dependent on oxidative phosphorylation. We also found that, during apoptosis, superoxide was gradually produced in mitochondria in association with phosphatidylserine exposure and that the kinetics of superoxide production was very heterogeneous at the single-cell level. Thus this study demonstrates that MMT could serve as a specific probe for intramitochondrial superoxide in either activated or apoptotic cells.