Sources of free radicals

Macrophage-produced peroxynitrite induces antibiotic tolerance and supersedes intrinsic mechanisms of persister formation

Staphylococcus aureus is a leading human pathogen that frequently causes chronic and relapsing infections. Antibiotic tolerant persister cells contribute to frequent antibiotic failure in patients. Macrophages represent an important niche during S. aureus bacteremia and recent work has identified a role for oxidative burst in the formation of antibiotic tolerant S. aureus. We find that host-derived peroxynitrite, the reaction product of superoxide and nitric oxide, is the main mediator of antibiotic tolerance in macrophages. Using a collection of S. aureus clinical isolates, we find that, despite significant variation in persister formation in pure culture, all strains were similarly enriched for antibiotic tolerance following internalization by activated macrophages. Our findings suggest that host interaction strongly induces antibiotic tolerance and may negate bacterial mechanisms of persister formation, established in pure culture. These findings emphasize the importance of studying antibiotic tolerance in the context of bacterial interaction with the host suggest that modulation of the host response may represent a viable therapeutic strategy to sensitize S. aureus to antibiotics.

Melatonin: an ancient molecule that makes oxygen metabolically tolerable

Melatonin is remarkably functionally diverse with actions as a free radical scavenger and antioxidant, circadian rhythm regulator, anti-inflammatory and immunoregulating molecule, and as an oncostatic agent. We hypothesize that the initial and primary function of melatonin in photosynthetic cyanobacteria, which appeared on Earth 3.5-3.2 billion years ago, was as an antioxidant. The evolution of melatonin as an antioxidant by this organism was necessary as photosynthesis is associated with the generation of toxic-free radicals. The other secondary functions of melatonin came about much later in evolution. We also surmise that mitochondria and chloroplasts may be primary sites of melatonin synthesis in all eukaryotic cells that possess these organelles. This prediction is made on the basis that mitochondria and chloroplasts of eukaryotes developed from purple nonsulfur bacteria (which also produce melatonin) and cyanobacteria when they were engulfed by early eukaryotes. Thus, we speculate that the melatonin-synthesizing actions of the engulfed bacteria were retained when these organelles became mitochondria and chloroplasts, respectively. That mitochondria are likely sites of melatonin formation is supported by the observation that this organelle contains high levels of melatonin that are not impacted by blood melatonin concentrations. Melatonin has a remarkable array of means by which it thwarts oxidative damage. It, as well as its metabolites, is differentially effective in scavenging a variety of reactive oxygen and reactive nitrogen species. Moreover, melatonin and its metabolites modulate a large number of antioxidative and pro-oxidative enzymes, leading to a reduction in oxidative damage. The actions of melatonin on radical metabolizing/producing enzymes may be mediated by the Keap1-Nrf2-ARE pathway. Beyond its direct free radical scavenging and indirect antioxidant effects, melatonin has a variety of physiological and metabolic advantages that may enhance its ability to limit oxidative stress.

Hypobromous acid, a powerful endogenous electrophile: Experimental and theoretical studies

Hypobromous acid (HOBr) is an inorganic acid produced by the oxidation of the bromide anion (Br(-)). The blood plasma level of Br(-) is more than 1,000-fold lower than that of chloride anion (Cl(-)). Consequently, the endogenous production of HOBr is also lower compared to hypochlorous acid (HOCl). Nevertheless, there is much evidence of the deleterious effects of HOBr. From these data, we hypothesized that the reactivity of HOBr could be better associated with its electrophilic strength. Our hypothesis was confirmed, since HOBr was significantly more reactive than HOCl when the oxidability of the studied compounds was not relevant. For instance: anisole (HOBr, k2=2.3×10(2)M(-1)s(-1), HOCl non-reactive); dansylglycine (HOBr, k2=7.3×10(6)M(-1)s(-1), HOCl, 5.2×10(2)M(-1)s(-1)); salicylic acid (HOBr, k2=4.0×10(4)M(-1)s(-1), non-reactive); 3-hydroxybenzoic acid (HOBr, k2=5.9×10(4)M(-1)s(-1), HOCl, k2=1.1×10(1)M(-1)s(-1)); uridine (HOBr, k2=1.3×10(3)M(-1)s(-1), HOCl non-reactive). The compounds 4-bromoanisole and 5-bromouridine were identified as the products of the reactions between HOBr and anisole or uridine, respectively, i.e. typical products of electrophilic substitutions. Together, these results show that, rather than an oxidant, HOBr is a powerful electrophilic reactant. This chemical property was theoretically confirmed by measuring the positive Mulliken and ChelpG charges upon bromine and chlorine. In conclusion, the high electrophilicity of HOBr could be behind its well-established deleterious effects. We propose that HOBr is the most powerful endogenous electrophile.

Pulse radiolysis studies on the reaction of the reduced vitamin B₁₂ complex Cob(II)alamin with superoxide

O₂.- scavenger: The rate constant for the rapid reaction of the ROS superoxide with the reduced vitamin B₁₂ radical complex cob(II)alamin was directly determined to be 3.8×10(8) M⁻¹ s⁻¹. This rate was independent of pH over the range 5.5-8.7. These results have implications for studying the use of B₁₂ supplements to combat diseases associated with oxidative stress.

Vitamin B12 protects against superoxide-induced cell injury in human aortic endothelial cells

Superoxide (O(2)(•-)) is implicated in inflammatory states including arteriosclerosis and ischemia-reperfusion injury. Cobalamin (Cbl) supplementation is beneficial for treating many inflammatory diseases and also provides protection in oxidative-stress-associated pathologies. Reduced Cbl reacts with O(2)(•-) at rates approaching that of superoxide dismutase (SOD), suggesting a plausible mechanism for its anti-inflammatory properties. Elevated homocysteine (Hcy) is an independent risk factor for cardiovascular disease and endothelial dysfunction. Hcy increases O(2)(•-) levels in human aortic endothelial cells (HAEC). Here, we explore the protective effects of Cbl in HAEC exposed to various O(2)(•-) sources, including increased Hcy levels. Hcy increased O(2)(•-) levels (1.6-fold) in HAEC, concomitant with a 20% reduction in cell viability and a 1.5-fold increase in apoptotic death. Pretreatment of HAEC with physiologically relevant concentrations of cyanocobalamin (CNCbl) (10-50nM) prevented Hcy-induced increases in O(2)(•-) and cell death. CNCbl inhibited both Hcy and rotenone-induced mitochondrial O(2)(•-) production. Similarly, HAEC challenged with paraquat showed a 1.5-fold increase in O(2)(•-) levels and a 30% decrease in cell viability, both of which were prevented with CNCbl pretreatment. CNCbl also attenuated elevated O(2)(•-) levels after exposure of cells to a Cu/Zn-SOD inhibitor. Our data suggest that Cbl acts as an efficient intracellular O(2)(•-) scavenger.

Vitamin B(12) and redox homeostasis: cob(II)alamin reacts with superoxide at rates approaching superoxide dismutase (SOD)

We report a kinetic study of the reaction between superoxide and an important intracellular form of vitamin B(12), cob(II)alamin. Superoxide is implicated in the pathophysiology of many inflammatory diseases, whereas vitamin B(12) derivatives are often beneficial in their treatment. We found that cob(II)alamin reacts with superoxide at rates approaching those of superoxide dismutase itself, suggesting a probable mechanism by which vitamin B(12) protects against chronic inflammation and modulates redox homeostasis.

Superoxide, peroxynitrite and oxidative/nitrative stress in inflammation

A considerable body of evidence suggests that formation of potent reactive oxygen species and resulting oxidative/nitrative stress play a major role in acute and chronic inflammation and pain. Much of the knowledge in this field has been gathered by the use of pharmacological and genetic approaches. In this mini review, we will evaluate recent advances made towards understanding the roles of reactive oxygen species in inflammation, focusing in particular on superoxide and peroxynitrite. Given the limited space to cover this broad topic, here we will refer the reader to comprehensive review articles whenever possible.

Alterations in plasma antioxidants during reperfusion of the ischemic small intestine in rats

The aim of this study was to evaluate the contribution of three plasma antioxidants (albumin, uric acid, SH groups) to the plasma total peroxyl radical-trapping antioxidant capacity (TRAP) in 2 and 4 h of intestinal reperfusion in rats. TRAP increased significantly both after 2 and 4 h of reperfusion. Neither albumin nor SH groups contributed significantly to this increase. TRAP was strongly influenced by the increase in uric acid concentration and also probably by the cell destruction caused by oxidative stress. Since the TRAP increase was accompanied by an increase in the level of thiobarbituric acid reactive substances (TBARS, a marker of lipid peroxidation), we can conclude that even such a large increase in TRAP is not sufficient to prevent the progression of lipid peroxidation and oxidative stress.

Role of oxidative stress in neurodegeneration: recent developments in assay methods for oxidative stress and nutraceutical antioxidants

Reactive oxygen species (ROS) are produced in the course of normal metabolism and they serve important physiological functions. However, because of their high reactivity, accumulation of ROS beyond the immediate needs of the cell may affect cellular structure and functional integrity, by bringing about oxidative degradation of critical molecules, such as the DNA, proteins, and lipids. Although cells possess an intricate network of defense mechanisms to neutralize excess ROS and reduce oxidative stress, some tissues, especially the brain, are much more vulnerable to oxidative stress because of their elevated consumption of oxygen and the consequent generation of large amounts of ROS. For the same reason, the mitochondrial DNA (mtDNA) of brain cells is highly susceptible to structural alterations resulting in mitochondrial dysfunction. Several lines of evidence strongly suggest that these effects of ROS may be etiologically related to a number of neurodegenerative disorders. Nutraceutical antioxidants are dietary supplements that can exert positive pharmacological effects on specific human diseases by neutralizing the negative effects of ROS. The present communication concentrates on a review of recent concepts and methodological developments, some of them based on the results of work from our own laboratory, on the following aspects: (1) the complex interactions and complementary interrelationships between oxidative stress, mitochondrial dysfunction, and various forms of neural degeneration; (2) fractionation and isolation of substances with antioxidant properties from plant materials, which are extensively used in the human diet and, therefore, can be expected to be less toxic in any pharmacological intervention; (3) recent developments in methodologies that can be used for the assay of oxidative stress and determination of biological activities of exogenous and endogenous antioxidants; and (4) presentation of simple procedures based on polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) of the resulting amplicon for investigations of structural alterations in mtDNA.

On the selectivity of superoxide dismutase mimetics and its importance in pharmacological studies

The list of pathophysiological conditions associated with the overproduction of superoxide expands every day. Much of the knowledge compiled on the role of this radical in disease has been gathered using the native superoxide dismutase enzyme and, more recently, by the use of superoxide dismutase knockout models or transgenic models that overexpress the various isoforms of the enzyme. Although the native enzyme has shown promising anti-inflammatory properties in both preclinical and clinical studies, there were drawbacks and issues associated with its use as a therapeutic agent and pharmacological tool. Based on the concept that removal of superoxide modulates the course of inflammation, synthetic, low-molecular-weight mimetics of the superoxide dismutase enzymes that could overcome some of the limitations associated with the use of the native enzyme have been designed. In this review, we will discuss the advances made using various superoxide dismutase mimetics that led to the proposal that superoxide (and/or the product of its interaction with nitric oxide, peroxynitrite) is an important mediator of inflammation, and to the conclusion that superoxide dismutase mimetics can be utilized as therapeutic agents in diseases of various etiologies. The importance of the selectivity of such compounds in pharmacological studies will be discussed.

Oxidative DNA damage: endogenous and chemically induced

A variety of types of DNA oxidation occur endogenously and mediated by xenobiotics. Certain forms are mutagenic and carcinogenic and may lead to other pathologies.

Glutathione depletion and the production of reactive oxygen species in isolated hepatocyte suspensions

Diethyl maleate (DEM) (5 mM) and ethyl methanesulfonate (EMS) (35 mM) treatments rapidly depleted cellular reduced glutathione (GSH) below detectable levels (1 nmol/10(6) cells), and induced lipid peroxidation and necrotic cell death in freshly isolated rat hepatocytes. In hepatocytes incubated with 2.5 mM DEM and 10 mM EMS, however, the complete depletion of cellular GSH observed was not sufficient to induce lipid peroxidation or cell death. Instead, DEM- and EMS-induced lipid peroxidation and cell death were dependent on increased reactive oxygen species (ROS) production as measured by increases in dichlorofluorescein fluorescence. The addition of antioxidants (vitamin E succinate and deferoxamine) prevented lipid peroxidation and cell death, suggesting that lipid peroxidation is involved in the sequence of events leading to necrotic cell death induced by DEM and EMS. To investigate the subcellular site of ROS generation, the cytochrome P450 inhibitor, SKF525A, was found to reduce EMS-induced lipid peroxidation but did not protect against the loss of cell viability, suggesting a mitochondrial origin for the toxic lipid peroxidation event. In agreement with this conclusion, mitochondrial electron transport inhibitors (rotenone, thenoyltrifluoroacetone and antimycin A) increased EMS-induced lipid peroxidation and cell death, while the mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone, blocked EMS- and DEM-mediated ROS production and lipid peroxidation. Furthermore, EMS treatment resulted in the significant loss of mitochondrial alpha-tocopherol shortly after its addition, and this loss preceded losses in cellular alpha-tocopherol levels. Treatment of hepatocytes with cyclosporin A, a mitochondrial permeability transition inhibitor, oxypurinol, a xanthine oxidase inhibitor, or BAPTA-AM, a calcium chelator, provided no protection against EMS-induced cell death or lipid peroxidation. Our results indicate that DEM and EMS induce cell death by a similar mechanism, which is dependent on the induction of ROS production and lipid peroxidation, and mitochondria are the major source for this toxic ROS generation. Cellular GSH depletion in itself does not appear to be responsible for the large increases in ROS production and lipid peroxidation observed.

Aging and oxygen toxicity: Relation to changes in melatonin

Melatonin (N-acetyl-5-methoxytryptamine) is a chemical mediator produced in the pineal gland and other sites in the body. The melatonin found in the blood is derived almost exclusively from the pineal gland. Since the pineal synthesizes melatonin primarily at night, blood levels of the indole are also higher at night (5-15 fold) than during the day. Some individuals on a nightly basis produce twice as much melatonin as others of the same age. Throughout life, the melatonin rhythm gradually wanes such that, in advanced age, melatonin production is usually at a minimum. Melatonin was recently found to be a free radical scavenger and antioxidant. It has been shown, in the experimental setting, to protect against both free radical induced DNA damage and oxidative stress-mediated lipid peroxidation. Pharmacologically, melatonin has been shown to reduce oxidative damage caused by such toxins as the chemical carcinogen safrole, carbon tetrachloride, paraquat, bacterial lipopolysaccharide, kainic acid, δ-aminolevulinic and amyloid β peptide of Alzheimer's disease as well as a model of Parkinson's disease involving the drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Additionally, the oxidative damage caused by agents such as ionizing radiation and excessive exercise is reduced by melatonin. Since free radical-induced molecular injury may play a significant role in aging, melatonin's ability to protect against it suggests a potential function of melatonin in deferring aging and age-related, free radical-based diseases. Besides its ability to abate oxidative damage, other beneficial features of melatonin may be important in combating the signs of aging; these include melatonin's immune-stimulating function, its sleep-promoting ability, its function as an anti-viral agent, and general protective actions at the cellular level. Definitive tests of the specific functions of physiological levels of melatonin in processes of aging are currently being conducted.

Accumulation of purine catabolites in solid tumors exposed to therapeutic hyperthermia

Intensified adenosine triphosphate (ATP) degradation following therapeutic hyperthermia is often observed in solid tumors. As a result, accumulation of purine catabolites can be expected together with formation of protons at several stages during degradation to the final product, uric acid. Proton formation in turn can contribute to the development of heat-induced acidosis. Furthermore, oxidation of hypoxanthine and xanthine may result in generation of reactive oxygen species, which may lead to DNA damage, lipid peroxidation and protein denaturation, thus also contributing to heat-induced cytotoxicity. In hyperthermia experiments a tumor-size-dependent, significant increase in the levels of the following catabolites has been demonstrated: [symbol: see text] [IMP + GMP] (sum of guanosine and inosine monophosphate levels), inosine, hypoxanthine, xanthine and uric acid, along with a drop in ATP and guanosine triphosphate (GTP) levels. These data suggest that formation of reactive oxygen species and protons during purine degradation may indeed play a significant role in the antitumor effect of hyperthermia.

Iron-induced tissue damage and cancer: the role of reactive oxygen species-free radicals

Oxygen is poisonous, but we cannot live without it. The high oxidizing potential of oxygen molecules (dioxygen) is a valuable source of energy for the organism and its reactivity is low; that is, spin forbidden. However, the dioxygen itself is a 'free radical' and, especially in the presence of transition metals, it is a major promoter of radical reactions in the cell. Humans survive only by virtue of their elaborate defense mechanisms against oxygen toxicity. Iron is the most abundant transition metal in the human body. Because iron shows wide variation in redox potential with different co-ordination ligands, it may be used as a redox intermediate in many biological mechanism. However, it is precisely this redox activeness that makes iron a key participant in free radical production. The current research on the relationship between iron and cancer is briefly reviewed. Research results are reported here which indicate that iron, when bound to certain ligands, can cause free-radical mediated tissue damage and become carcinogenic. The present study also suggests that iron may also have a significant role in spontaneous human cancer.