Peroxynitrite flux-mediated LDL oxidation is inhibited by manganese porphyrins in the presence of uric acid

Fatty acid nitration in human low-density lipoprotein

Lipid nitration occurs during physiological and pathophysiological conditions, generating a variety of biomolecules capable to modulate inflammatory cell responses. Low-density lipoprotein (LDL) oxidation has been extensively related to atherosclerotic lesion development while oxidative modifications confer the particle pro-atherogenic features. Herein, we reviewed the oxidation versus nitration of human LDL protein and lipid fractions. We propose that unsaturated fatty acids present in LDL can be nitrated under mild nitration conditions, suggesting an anti-atherogenic role for LDL carrying nitro-fatty acids (NFA).

Data of detection and characterization of nitrated conjugated-linoleic acid (NO2-cLA) in LDL

Under physiological and pathophysiological conditions, lipid nitration occurs generating nitro-fatty acids (NFA) with pleiotropic activities as modulation of inflammatory cell responses. Foam cell formation and atherosclerotic lesion development have been extensively related to low-density lipoprotein (LDL) oxidation. Considering our manuscript “Fatty acid nitration in human low-density lipoprotein” (https://doi.org/10.1016/j.abb.2019.108190), herein we report the oxidation versus nitration of human LDL protein and lipid fractions. Data is shown on LDL fatty acid nitration, in particular, formation and quantitation of nitro-conjugated linoleic acid (NO₂-cLA) under mild nitration conditions. In parallel to NO₂-cLA formation, depletion of endogenous antioxidants, protein tyrosine nitration, and carbonyl formation is observed. Overall, our data propose the formation of a potential anti-atherogenic form of LDL carrying NFA.

Free radical-dependent inhibition of prostaglandin endoperoxide H Synthase-2 by nitro-arachidonic acid

Prostaglandin endoperoxide H synthase (PGHS) is a heme-enzyme responsible for the conversion of arachidonic acid (AA) to prostaglandin H₂ (PGH₂). PGHS have both oxygenase (COX) and peroxidase (POX) activities and is present in two isoforms (PGHS-1 and -2) expressed in different tissues and cell conditions. It has been reported that PGHS activity is inhibited by the nitrated form of AA, nitro-arachidonic acid (NO₂AA), which in turn could be synthesized by PGHS under nitro-oxidative conditions. Specifically, NO₂AA inhibits COX in PGHS-1 as well as POX in both PGHS-1 and -2, in a dose and time-dependent manner. NO₂AA inhibition involves lowering the binding stability and displacing the heme group from the active site. However, the complete mechanism remains to be understood. This review describes the interactions of PGHS with NO₂AA, focusing on mechanisms of inhibition and nitration. In addition, using a novel approach combining EPR-spin trapping and mass spectrometry, we described possible intermediates formed during PGHS-2 catalysis and inhibition. This literature revision as well as the results presented here strongly suggest a free radical-dependent inhibitory mechanism of PGHS-2 by NO₂AA. This is of relevance towards understanding the underlying mechanism of inhibition of PGHS by NO₂AA and its anti-inflammatory potential.

Manganese porphyrin redox state in endothelial cells: Resonance Raman studies and implications for antioxidant protection towards peroxynitrite

Cationic manganese(III) ortho N-substituted pyridylporphyrins (MnP) act as efficient antioxidants catalyzing superoxide dismutation and accelerating peroxynitrite reduction. Importantly, MnP can reach mitochondria offering protection against reactive species in different animal models of disease. Although an LC-MS/MS-based method for MnP quantitation and subcellular distribution has been reported, a direct method capable of evaluating both the uptake and the redox state of MnP in living cells has not yet been developed. In the present work we applied resonance Raman (RR) spectroscopy to analyze the intracellular accumulation of two potent MnP-based lipophilic SOD mimics, MnTnBuOE-2-PyP⁵⁺ and MnTnHex-2-PyP⁵⁺ within endothelial cells. RR experiments with isolated mitochondria revealed that the reduction of Mn(III)P was affected by inhibitors of the electron transport chain, supporting the action of MnP as efficient redox active compounds in mitochondria. Indeed, RR spectra confirmed that MnP added in the Mn(III) state can be incorporated into the cells, readily reduced by intracellular components to the Mn(II) state and oxidized by peroxynitrite. To assess the combined impact of reactivity and bioavailability, we studied the kinetics of Mn(III)TnBuOE-2-PyP⁵⁺ with peroxynitrite and evaluated the cytoprotective capacity of MnP by exposing the endothelial cells to nitro-oxidative stress induced by peroxynitrite. We observed a preservation of normal mitochondrial function, attenuation of cell damage and prevention of apoptotic cell death. These data introduce a novel application of RR spectroscopy for the direct detection of MnP and their redox states inside living cells, and helps to rationalize their antioxidant capacity in biological systems.

Nitroarachidonic acid (NO2AA) inhibits protein disulfide isomerase (PDI) through reversible covalent adduct formation with critical cysteines

BACKGROUND

Nitroarachidonic acid (NO₂AA) exhibits pleiotropic anti-inflammatory actions in a variety of cell types. We have recently shown that NO₂AA inhibits phagocytic NADPH oxidase 2 (NOX2) by preventing the formation of the active complex. Recent work indicates the participation of protein disulfide isomerase (PDI) activity in NOX2 activation. Cysteine (Cys) residues at PDI active sites could be targets for NO₂AA- nitroalkylation regulating PDI activity which could explain our previous observation.

METHODS

PDI reductase and chaperone activities were assessed using the insulin and GFP renaturation methods in the presence or absence of NO₂AA. To determine the covalent reaction with PDI as well as the site of reaction, the PEG-switch assay and LC-MS/MS studies were performed.

RESULTS AND CONCLUSIONS

We determined that both activities of PDI were inhibited by NO₂AA in a dose- and time- dependent manner and independent from release of nitric oxide. Since nitroalkenes are potent electrophiles and PDI has critical Cys residues for its activity, then formation of a covalent adduct between NO₂AA and PDI is feasible. To this end we demonstrated the reversible covalent modification of PDI by NO₂AA. Trypsinization of modified PDI confirmed that the Cys residues present in the active site a' of PDI were key targets accounting for nitroalkene modification.

GENERAL SIGNIFICANCE

PDI may contribute to NOX2 activation. As such, inhibition of PDI by NO₂AA might be involved in preventing NOX2 activation. Future work will be directed to determine if the covalent modifications observed play a role in the reported NO₂AA inhibition of NOX2 activity.

Nitro-Arachidonic Acid Prevents Angiotensin II-Induced Mitochondrial Dysfunction in a Cell Line of Kidney Proximal Tubular Cells

Nitro-arachidonic acid (NO₂-AA) is a cell signaling nitroalkene that exerts anti-inflammatory activities during macrophage activation. While angiotensin II (ANG II) produces an increase in reactive oxygen species (ROS) production and mitochondrial dysfunction in renal tubular cells, little is known regarding the potential protective effects of NO₂-AA in ANG II-mediated kidney injury. As such, this study examines the impact of NO₂-AA on ANG II-induced mitochondrial dysfunction in an immortalized renal proximal tubule cell line (HK-2 cells). Treatment of HK-2 cells with ANG II increases the production of superoxide (O₂^●-), nitric oxide (^●NO), inducible nitric oxide synthase (NOS2) expression, peroxynitrite (ONOO^-) and mitochondrial dysfunction. Using high-resolution respirometry, it was observed that the presence of NO₂-AA prevented ANG II-mediated mitochondrial dysfunction. Attempting to address mechanism, we treated isolated rat kidney mitochondria with ONOO^-, a key mediator of ANG II-induced mitochondrial damage, in the presence or absence of NO₂-AA. Whereas the activity of succinate dehydrogenase (SDH) and ATP synthase (ATPase) were diminished upon exposure to ONOO-, they were restored by pre-incubating the mitochondria with NO₂-AA. Moreover, NO₂-AA prevents oxidation and nitration of mitochondrial proteins. Combined, these data demonstrate that ANG II-mediated oxidative damage and mitochondrial dysfunction is abrogated by NO₂-AA, identifying this compound as a promising pharmacological tool to prevent ANG II–induced renal disease.

An educational overview of the chemistry, biochemistry and therapeutic aspects of Mn porphyrins--From superoxide dismutation to H2O2-driven pathways

Most of the SOD mimics thus far developed belong to the classes of Mn-(MnPs) and Fe porphyrins(FePs), Mn(III) salens, Mn(II) cyclic polyamines and metal salts. Due to their remarkable stability we have predominantly explored Mn porphyrins, aiming initially at mimicking kinetics and thermodynamics of the catalysis of O2(-) dismutation by SOD enzymes. Several MnPs are of potency similar to SOD enzymes. The in vivo bioavailability and toxicity of MnPs have been addressed also. Numerous in vitro and in vivo studies indicate their impressive therapeutic efficacy. Increasing insight into complex cellular redox biology has been accompanied by increasing awareness of complex redox chemistry of MnPs. During O2(-) dismutation process, the most powerful Mn porphyrin-based SOD mimics reduce and oxidize O2(-) with close to identical rate constants. MnPs reduce and oxidize other reactive species also (none of them specific to MnPs), acting as reductants (antioxidant) and pro-oxidants. Distinction must be made between the type of reactions of MnPs and the favorable therapeutic effects we observe; the latter may be of either anti- or pro-oxidative nature. H2O2/MnP mediated oxidation of protein thiols and its impact on cellular transcription seems to dominate redox biology of MnPs. It has been thus far demonstrated that the ability of MnPs to catalyze O2(-) dismutation parallels all other reactivities (such as ONOO(-) reduction) and in turn their therapeutic efficacies. Assuming that all diseases have in common the perturbation of cellular redox environment, developing SOD mimics still seems to be the appropriate strategy for the design of potent redox-active therapeutics.

Therapeutic potential of peroxynitrite decomposition catalysts: a patent review

INTRODUCTION

Peroxynitrite is a cytotoxic oxidant species implicated in a host of pathologies, including inflammatory and neurodegenerative diseases, cancer, radiation injury and chronic pain. With the recognition of the role of peroxynitrite in disease, numerous experimental and therapeutic tools have arisen to probe peroxyntirite's pathophysiological contribution and attenuate its oxidative damage. Peroxynitrite decomposition catalysts (PNDCs) are redox-active compounds that detoxify peroxynitrite by catalyzing its isomerization or reduction to nitrate or nitrite.

AREAS COVERED

This review discusses recent research articles and patents published 1995 - 2014 on the development and therapeutic use of PNDCs. Iron and manganese metalloporphyrin PNDCs attenuate the toxic effects of peroxynitrite and are currently being developed for clinical applications. Additionally, some Mn porphyrin-based PNDCs have optimized pharmaceutical properties such that they exhibit greater peroxynitrite selectivity. Other classes of PNDC agents, including bis(hydroxyphenyl)dipyrromethenes and metallocorroles, have demonstrated preclinical efficacy, oral availability and reduced toxicity risk.

EXPERT OPINION

Interest in the drug-like properties of peroxynitrite-neutralizing agents has grown with the realization that PNDCs will be powerful tools in the treatment of disease. The design of compounds with enhanced oral availability and peroxynitrite selectivity is a critical step toward the availability of safe, effective and selective redox modulators for the treatment of peroxynitrite-associated pathologies.

Rational design of superoxide dismutase (SOD) mimics: the evaluation of the therapeutic potential of new cationic Mn porphyrins with linear and cyclic substituents

Our goal herein has been to gain further insight into the parameters which control porphyrin therapeutic potential. Mn porphyrins (MnTnOct-2-PyP(5+), MnTnHexOE-2-PyP(5+), MnTE-2-PyPhP(5+), and MnTPhE-2-PyP(5+)) that bear the same positive charge and same number of carbon atoms at meso positions of porphyrin core were explored. The carbon atoms of their meso substituents are organized to form either linear or cyclic structures of vastly different redox properties, bulkiness, and lipophilicities. These Mn porphyrins were compared to frequently studied compounds, MnTE-2-PyP(5+), MnTE-3-PyP(5+), and MnTBAP(3-). All Mn(III) porphyrins (MnPs) have metal-centered reduction potential, E1/2 for Mn(III)P/Mn(II)P redox couple, ranging from -194 to +340 mV versus NHE, log kcat(O2(•-)) from 3.16 to 7.92, and log kred(ONOO(-)) from 5.02 to 7.53. The lipophilicity, expressed as partition between n-octanol and water, log POW, was in the range -1.67 to -7.67. The therapeutic potential of MnPs was assessed via: (i) in vitro ability to prevent spontaneous lipid peroxidation in rat brain homogenate as assessed by malondialdehyde levels; (ii) in vivo O2(•-) specific assay to measure the efficacy in protecting the aerobic growth of SOD-deficient Saccharomyces cerevisiae; and (iii) aqueous solution chemistry to measure the reactivity toward major in vivo endogenous antioxidant, ascorbate. Under the conditions of lipid peroxidation assay, the transport across the cellular membranes, and in turn shape and size of molecule, played no significant role. Those MnPs of E1/2 ∼ +300 mV were the most efficacious, significantly inhibiting lipid peroxidation in 0.5-10 μM range. At up to 200 μM, MnTBAP(3-) (E1/2 = -194 mV vs NHE) failed to inhibit lipid peroxidation, while MnTE-2-PyPhP(5+) with 129 mV more positive E1/2 (-65 mV vs NHE) was fully efficacious at 50 μM. The E1/2 of Mn(III)P/Mn(II)P redox couple is proportional to the log kcat(O2(•-)), i.e., the SOD-like activity of MnPs. It is further proportional to kred(ONOO(-)) and the ability of MnPs to prevent lipid peroxidation. In turn, the inhibition of lipid peroxidation by MnPs is also proportional to their SOD-like activity. In an in vivo S. cerevisiae assay, however, while E1/2 predominates, lipophilicity significantly affects the efficacy of MnPs. MnPs of similar log POW and E1/2, that have linear alkyl or alkoxyalkyl pyridyl substituents, distribute more easily within a cell and in turn provide higher protection to S. cerevisiae in comparison to MnP with bulky cyclic substituents. The bell-shape curve, with MnTE-2-PyP(5+) exhibiting the highest ability to catalyze ascorbate oxidation, has been established and discussed. Our data support the notion that the SOD-like activity of MnPs parallels their therapeutic potential, though species other than O2(•-), such as peroxynitrite, H2O2, lipid reactive species, and cellular reductants, may be involved in their mode(s) of action(s).

Association of Uric Acid Genetic Risk Score With Blood Pressure

High levels of serum uric acid are associated with hypertension in observational studies. The aim of this study was to investigate the association of uric acid gene variants with blood pressure. We studied 5791 participants aged ≥55 years from the Rotterdam Study. Thirty gene variants identified for serum uric acid level were used to compile genetic risk score (GRS). We used linear regression models to investigate the association of the uric acid GRS with systolic and diastolic blood pressure in the whole study population and separately in participants with and without comorbidities and medication use. In the age- and sex-adjusted model, each SD increase in uric acid GRS was associated with 0.75 mm Hg lower systolic blood pressure (95% confidence interval, −1.31 to −0.19) and 0.42 mm Hg lower diastolic blood pressure (95% confidence interval, −0.72 to −0.13). The association did not attenuate after further adjustment for antihypertensive medication use and conventional cardiovascular risk factors. In subgroup analysis, the association of uric acid GRS with systolic blood pressure was significantly stronger in participants (n=885) on diuretic treatment ( P for interaction, 0.007). In conclusion, we found that higher uric acid GRS is associated with lower systolic and diastolic blood pressure. Diuretics treatment may modify the association of uric acid genetic risk score and systolic blood pressure. Our study suggests that genome wide association study’s findings can be associated with an intermediate factor or have a pleiotropic role and, therefore, should be applied for Mendelian Randomization with caution.

SOD therapeutics: latest insights into their structure-activity relationships and impact on the cellular redox-based signaling pathways

SIGNIFICANCE

Superoxide dismutase (SOD) enzymes are indispensable and ubiquitous antioxidant defenses maintaining the steady-state levels of O2·(-); no wonder, thus, that their mimics are remarkably efficacious in essentially any animal model of oxidative stress injuries thus far explored.

RECENT ADVANCES

Structure-activity relationship (half-wave reduction potential [E1/2] versus log kcat), originally reported for Mn porphyrins (MnPs), is valid for any other class of SOD mimics, as it is dominated by the superoxide reduction and oxidation potential. The biocompatible E1/2 of ∼+300 mV versus normal hydrogen electrode (NHE) allows powerful SOD mimics as mild oxidants and antioxidants (alike O2·(-)) to readily traffic electrons among reactive species and signaling proteins, serving as fine mediators of redox-based signaling pathways. Based on similar thermodynamics, both SOD enzymes and their mimics undergo similar reactions, however, due to vastly different sterics, with different rate constants.

CRITICAL ISSUES

Although log kcat(O2·(-)) is a good measure of therapeutic potential of SOD mimics, discussions of their in vivo mechanisms of actions remain mostly of speculative character. Most recently, the therapeutic and mechanistic relevance of oxidation of ascorbate and glutathionylation and oxidation of protein thiols by MnP-based SOD mimics and subsequent inactivation of nuclear factor κB has been substantiated in rescuing normal and killing cancer cells. Interaction of MnPs with thiols seems to be, at least in part, involved in up-regulation of endogenous antioxidative defenses, leading to the healing of diseased cells.

FUTURE DIRECTIONS

Mechanistic explorations of single and combined therapeutic strategies, along with studies of bioavailability and translational aspects, will comprise future work in optimizing redox-active drugs.

Late administration of Mn porphyrin-based SOD mimic enhances diabetic complications

Mn(III) N-alkylpyridylporphyrins (MnPs) have demonstrated protection in various conditions where increased production of reactive oxygen/reactive nitrogen species (ROS/RNS), is a key pathological factors. MnPs can produce both pro-oxidative and antioxidative effects depending upon the cellular redox environment that they encounter. Previously we reported (Free Radic. Res. 39: 81–8, 2005) that when the treatment started at the onset of diabetes, Mn(III) meso-tetrakis(N-methylpyridinium-2-yl)porphyrin, MnTM-2-PyP⁵⁺ suppressed diabetes-induced oxidative stress. Diabetes, however, is rarely diagnosed at its onset. The aim of this study was to investigate if MnTM-2-PyP⁵⁺ can suppress oxidative damage and prevent diabetic complications when administered more than a week after the onset of diabetes. Diabetes was induced by streptozotocin. The MnP-based treatment started 8 days after the onset of diabetes and continued for 2 months. The effect of the treatment on activities of glutathione peroxidase, superoxide dismutase, catalase, glutathione reductase, glucose-6-phosphate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, and glyoxalases I and II as well as malondialdehyde and GSH/GSSG ratio were determined in kidneys. Kidney function was assessed by measuring lysozyme and total protein in urine and blood urea nitrogen. Vascular damage was evaluated by assessing vascular reactivity. Our data showed that delayed administration of MnTM-2-PyP⁵⁺ did not protect against oxidative damage and did not prevent diabetic complications. Moreover, MnTM-2-PyP⁵⁺ contributed to the kidney damage, which seems to be a consequence of its pro-oxidative action. Such outcome can be explained by advanced oxidative damage which already existed at the moment the therapy with MnP started. The data support the concept that the overall biological effect of a redox-active MnP is determined by (i) the relative concentrations of oxidants and reductants, i.e. the cellular redox environment and (ii) MnP biodistribution.

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Mn porphyrins (MnP) are among the most potent SOD mimics.

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MnP suppressed diabetes-induced oxidative stress if applied at the onset of diabetes.

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Delayed administration of MnP augmented oxidative stress and diabetic complications.

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The overall in vivo effect of MnP depends on its redox environment.

Nitroarachidonic acid prevents NADPH oxidase assembly and superoxide radical production in activated macrophages

Nitration of arachidonic acid (AA) to nitroarachidonic acid (AANO2) leads to anti-inflammatory intracellular activities during macrophage activation. However, less is known about the capacity of AANO2 to regulate the production of reactive oxygen species under proinflammatory conditions. One of the immediate responses upon macrophage activation involves the production of superoxide radical (O2(•-)) due to the NADPH-dependent univalent reduction of oxygen to O2(•-) by the phagocytic NADPH oxidase isoform (NOX2), the activity of NOX2 being the main source of O2(•-) in monocytes/macrophages. Because the NOX2 and AA pathways are connected, we propose that AANO2 can modulate macrophage activation by inhibiting O2(•-) formation by NOX2. When macrophages were activated in the presence of AANO2, a significant inhibition of NOX2 activity was observed as evaluated by cytochrome c reduction, luminol chemiluminescence, Amplex red fluorescence, and flow cytometry; this process also occurs under physiological mimic conditions within the phagosomes. AANO2 decreased O2(•-) production in a dose- (IC50=4.1±1.8 μM AANO2) and time-dependent manner. The observed inhibition was not due to a decreased phosphorylation of the cytosolic subunits (e.g., p40(phox) and p47(phox)), as analyzed by immunoprecipitation and Western blot. However, a reduction in the migration to the membrane of p47(phox) was obtained, suggesting that the protective actions involve the prevention of the correct assembly of the active enzyme in the membrane. Finally, the observed in vitro effects were confirmed in an in vivo inflammatory model, in which subcutaneous injection of AANO2 was able to decrease NOX2 activity in macrophages from thioglycolate-treated mice.

Methoxy-derivatization of alkyl chains increases the in vivo efficacy of cationic Mn porphyrins. Synthesis, characterization, SOD-like activity, and SOD-deficient E. coli study of meta Mn(III) N-methoxyalkylpyridylporphyrins

Cationic Mn(III) N-alkylpyridylporphyrins (MnPs) are potent SOD mimics and peroxynitrite scavengers and diminish oxidative stress in a variety of animal models of central nervous system (CNS) injuries, cancer, radiation, diabetes, etc. Recently, properties other than antioxidant potency, such as lipophilicity, size, shape, and bulkiness, which influence the bioavailability and the toxicity of MnPs, have been addressed as they affect their in vivo efficacy and therapeutic utility. Porphyrin bearing longer alkyl substituents at pyridyl ring, MnTnHex-2-PyP(5+), is more lipophilic, thus more efficacious in vivo, particularly in CNS injuries, than the shorter alkyl-chained analog, MnTE-2-PyP(5+). Its enhanced lipophilicity allows it to accumulate in mitochondria (relative to cytosol) and to cross the blood-brain barrier to a much higher extent than MnTE-2-PyP(5+). Mn(III) N-alkylpyridylporphyrins of longer alkyl chains, however, bear micellar character, and when used at higher levels, become toxic. Recently we showed that meta isomers are ∼10-fold more lipophilic than ortho species, which enhances their cellular accumulation, and thus reportedly compensates for their somewhat inferior SOD-like activity. Herein, we modified the alkyl chains of the lipophilic meta compound, MnTnHex-3-PyP(5+) via introduction of a methoxy group, to diminish its toxicity (and/or enhance its efficacy), while maintaining high SOD-like activity and lipophilicity. We compared the lipophilic Mn(III) meso-tetrakis(N-(6'-methoxyhexyl)pyridinium-3-yl)porphyrin, MnTMOHex-3-PyP(5+), to a hydrophilic Mn(III) meso-tetrakis(N-(2'-methoxyethyl)pyridinium-3-yl)porphyrin, MnTMOE-3-PyP(5+). The compounds were characterized by uv-vis spectroscopy, mass spectrometry, elemental analysis, electrochemistry, and ability to dismute O(2)˙(-). Also, the lipophilicity was characterized by thin-layer chromatographic retention factor, R(f). The SOD-like activities and metal-centered reduction potentials for the Mn(III)P/Mn(II)P redox couple were similar-to-identical to those of N-alkylpyridyl analogs: log k(cat) = 6.78, and E(1/2) = +68 mV vs. NHE (MnTMOHex-3-PyP(5+)), and log k(cat) = 6.72, and E(1/2) = +64 mV vs. NHE (MnTMOE-3-PyP(5+)). The compounds were tested in a superoxide-specific in vivo model: aerobic growth of SOD-deficient E. coli, JI132. Both MnTMOHex-3-PyP(5+) and MnTMOE-3-PyP(5+) were more efficacious than their alkyl analogs. MnTMOE-3-PyP(5+) is further significantly more efficacious than the most explored compound in vivo, MnTE-2-PyP(5+). Such a beneficial effect of MnTMOE-3-PyP(5+) on diminished toxicity, improved efficacy and transport across the cell wall may originate from the favorable interplay of the size, length of pyridyl substituents, rotational flexibility (the ortho isomer, MnTE-2-PyP(5+), is more rigid, while MnTMOE-3-PyP(5+) is a more flexible meta isomer), bulkiness and presence of oxygen.

Superoxide dismutase mimics: chemistry, pharmacology, and therapeutic potential

Oxidative stress has become widely viewed as an underlying condition in a number of diseases, such as ischemia-reperfusion disorders, central nervous system disorders, cardiovascular conditions, cancer, and diabetes. Thus, natural and synthetic antioxidants have been actively sought. Superoxide dismutase is a first line of defense against oxidative stress under physiological and pathological conditions. Therefore, the development of therapeutics aimed at mimicking superoxide dismutase was a natural maneuver. Metalloporphyrins, as well as Mn cyclic polyamines, Mn salen derivatives and nitroxides were all originally developed as SOD mimics. The same thermodynamic and electrostatic properties that make them potent SOD mimics may allow them to reduce other reactive species such as peroxynitrite, peroxynitrite-derived CO(3)(*-), peroxyl radical, and less efficiently H(2)O(2). By doing so SOD mimics can decrease both primary and secondary oxidative events, the latter arising from the inhibition of cellular transcriptional activity. To better judge the therapeutic potential and the advantage of one over the other type of compound, comparative studies of different classes of drugs in the same cellular and/or animal models are needed. We here provide a comprehensive overview of the chemical properties and some in vivo effects observed with various classes of compounds with a special emphasis on porphyrin-based compounds.

Decreased hepatic ischemia-reperfusion injury by manganese–porphyrin complexes

Reactive oxygen and nitrogen species have been implicated in ischemia-reperfusion (I/R) injury. Metalloporphyrins (MP) are stable catalytic antioxidants that can scavenge superoxide, hydrogen peroxide, peroxynitrite and lipid peroxyl radicals. Studies were conducted with three manganese-porphyrin (MnP) complexes with varying superoxide dimutase (SOD) and catalase catalytic activity to determine if the MnP attenuates I/R injury in isolated perfused mouse livers. The release of the hepatocellular enzymes alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) was maximal at 1 min reperfusion, decreased rapidly and increased gradually by 90 min. Manganese tetrakis-(N-ethyl-2 pyridyl) porphyrin (MnTE-2-PyP) decreased ALT, AST, LDH at 1-90 min reperfusion, while manganese tetrakis-(N-methyl-2 pyridyl) porphyrin (MnTM-2-PyP) and manganese tetrakis-(ethoxycarbonyl) porphyrin (MnTECP) decreased ALT and LDH from 5 to 90 min reperfusion. The release of thiobarbituric acid-reacting substances (TBARS) was diminished by MnTE-2-PyP and MnTM-2-PyP at 90 min. The extent of protein nitration (nitrotyrosine, NT) was decreased in all three MnPs treated livers. These results demonstrate that MnP complexes can attenuate hepatic I/R injury and may have therapeutic implications in disease states involving oxidants.

Chemical biology of peroxynitrite: kinetics, diffusion, and radicals

Peroxynitrite is formed by the very fast reaction of nitric oxide and superoxide radicals, a reaction that kinetically competes with other routes that chemically consume or physically sequester the reagents. It can behave either as an endogenous cytotoxin toward host tissues or a cytotoxic effector molecule against invading pathogens, depending on the cellular source and pathophysiological setting. Peroxynitrite is in itself very reactive against a few specific targets that range from efficient detoxification systems, such as peroxiredoxins, to reactions eventually leading to enhanced radical formation (e.g., nitrogen dioxide and carbonate radicals), such as the reaction with carbon dioxide. Thus, the chemical biology of peroxynitrite is dictated by the chemical kinetics of its formation and decay and by the diffusion across membranes of the species involved, including peroxynitrite itself. On the other hand, most durable traces of peroxynitrite passing (such as 3-nitrotyrosine) are derived from radicals formed from peroxynitrite by routes that represent extremely low-yield processes but that have potentially critical biological consequences. Here we have reviewed the chemical kinetics of peroxynitrite as a biochemical transient species in order to estimate its rates of formation and decay and then its steady-state concentration in different intra- or extracellular compartments, trying to provide a quantitative basis for its reactivity; additionally, we have considered diffusion across membranes to locate its possible effects. Finally, we have assessed the most successful attempts to intercept peroxynitrite by pharmacological intervention in their potential to increment the existing biological defenses that routinely deal with this cytotoxin.

Peroxynitrite-mediated lipid oxidation and nitration: mechanisms and consequences

Lipid oxidation and nitration represents a novel area of research of relevance in the understanding of inflammatory processes. Peroxynitrite, the product of the diffusion-limited reaction between nitric oxide and superoxide anion, mediates oxidative modifications in lipid systems including cell membranes and lipoproteins. In this review, we discuss the mechanisms of lipid oxidation and nitration by peroxynitrite as well as the influence of physiological molecules and cell targets to redirect peroxynitrite reactivity. We also provide evidence to support that oxidation/nitration of lipids results in the formation of novel signaling modulators of key lipid-metabolizing enzymes.

SOD-like activity of Mn(II) beta-octabromo-meso-tetrakis(N-methylpyridinium-3-yl)porphyrin equals that of the enzyme itself

Mn porphyrins are among the most efficient SOD mimics with potency approaching that of SOD enzymes. The most potent ones, Mn(III) N-alkylpyridylporphyrins bear positive charges in a close proximity to the metal site, affording thermodynamic and kinetic facilitation for the reaction with negatively charged superoxide. The addition of electron-withdrawing bromines onto beta-pyrrolic positions dramatically improves thermodynamic facilitation for the O2*- dismutation. We have previously characterized the para isomer, Mn(II)Br(8)TM-4-PyP(4+) [Mn(II) beta-octabromo-meso-tetrakis(N-methylpyridinium-4-yl)porphyrin]. Herein we fully characterized its meta analogue, Mn(II)Br(8)TM-3-PyP(4+) with respect to UV/vis spectroscopy, electron spray mass spectrometry, electrochemistry, O2*- dismutation, metal-ligand stability, and the ability to protect SOD-deficient Escherichia coli in comparison with its para analogue. The increased electron-deficiency of the metal center stabilizes Mn in its +2 oxidation state. The metal-centered Mn(III)/Mn(II) reduction potential, E((1/2))=+468 mV vs NHE, is increased by 416 mV with respect to non-brominated analogue, Mn(III)TM-3-PyP(5+) and is only 12 mV less positive than for para isomer. Yet, the complex is significantly more stable towards the loss of metal than its para analogue. As expected, based on the structure-activity relationships, an increase in E((1/2)) results in a higher catalytic rate constant for the O2*- dismutation, log k(cat)> or =8.85; 1.5-fold increase with respect to the para isomer. The IC(50) was calculated to be < or =3.7 nM. Manipulation of the electron-deficiency of a cationic porphyrin resulted, therefore, in the highest k(cat) ever reported for a metalloporphyrin, being essentially identical to the k(cat) of superoxide dismutases (log k(cat)=8.84-9.30). The positive kinetic salt effect points to the unexpected, unique and first time recorded behavior of Mn beta-octabrominated porphyrins when compared to other Mn porphyrins studied thus far. When species of opposing charges react, the increase in ionic strength invariably results in the decreased rate constant; with brominated porphyrins the opposite was found to be true. The effect is 3.5-fold greater with meta than with para isomer, which is discussed with respect to the closer proximity of the quaternary nitrogens of the meta isomer to the metal center than that of the para isomer. The potency of Mn(II)Br(8)TM-3-PyP(4+) was corroborated by in vivo studies, where 500 nM allows SOD-deficient E. coli to grow >60% of the growth of wild type; at concentrations > or =5 microM it exhibits toxicity. Our work shows that exceptionally high k(cat) for the O2*- disproportionation can be achieved not only with an N(5)-type coordination motif, as rationalized previously for aza crown ether (cyclic polyamines) complexes, but also with a N(4)-type motif as in the Mn porphyrin case; both motifs sharing "up-down-up-down" steric arrangement.

Peroxynitrite detoxification and its biologic implications

Peroxynitrite is a cytotoxic oxidant formed in vivo from the diffusional-controlled reaction between nitric oxide and superoxide radicals. Increased peroxynitrite formation has been related to the pathogenesis of multiple diseases, thus underlining the importance of understanding the mechanisms of its detoxification. In nature, different enzymatic routes for peroxynitrite decomposition have evolved. Among them, peroxiredoxins catalytically reduce peroxynitrite in vitro; modulation of their expression affects peroxynitrite-mediated cytotoxicity, and their content changes in pathologic conditions associated with increased peroxynitrite formation in vivo, thus indicating a physiologic role of these enzymes in peroxynitrite reduction. Selenium-containing glutathione peroxidase also catalyzes peroxynitrite reduction, but its role in vivo is still a matter of debate. In selected cellular systems, heme proteins also play a role in peroxynitrite detoxification, such as its isomerization by oxyhemoglobin in red blood cells. Moreover, different pharmacologic approaches have been used to decrease the toxicity related to peroxynitrite formation. Manganese or iron porphyrins catalyze peroxynitrite decomposition, and their protective role in vivo has been confirmed in biologic systems. Glutathione peroxidase mimetics also rapidly reduce peroxynitrite, but their biologic role is less well established. Flavonoids, nitroxides, and tyrosine-containing peptides decreased peroxynitrite-mediated toxicity under different conditions, but their mechanism of action is indirect.

Peroxynitrite: biochemistry, pathophysiology and development of therapeutics

Peroxynitrite--the product of the diffusion-controlled reaction of nitric oxide with superoxide radical--is a short-lived oxidant species that is a potent inducer of cell death. Conditions in which the reaction products of peroxynitrite have been detected and in which pharmacological inhibition of its formation or its decomposition have been shown to be of benefit include vascular diseases, ischaemia-reperfusion injury, circulatory shock, inflammation, pain and neurodegeneration. In this Review, we first discuss the biochemistry and pathophysiology of peroxynitrite and then focus on pharmacological strategies to attenuate the toxic effects of peroxynitrite. These include its catalytic reduction to nitrite and its isomerization to nitrate by metalloporphyrins, which have led to potential candidates for drug development for cardiovascular, inflammatory and neurodegenerative diseases.

Is uric acid itself a player or a bystander in the pathophysiology of chronic heart failure?

Uric acid (UA) is the end product of purine metabolism in humans. Hyperuricemia is often found in patients with chronic heart failure (CHF). The increase of serum UA level is inversely associated with disease severity, cardiac function and prognosis of CHF. Some researchers found that UA had detrimental impact on the cardiovascular system, including mediating immune response upon cell injury, increasing endotoxin-stimulated tumor necrosis factor-alpha production and hence proinflammatory immune activation, increasing blood pressure, and so on. Other researchers found that UA had important antioxidant properties by scavenging various reactive oxygen species. So far, there is no evidence suggest that UA has detrimental effect on the pathophysiology of CHF. Xanthine oxidase (XO) is an enzyme that produces uric acid during purine metabolism. XO activity is up-regulated in failing heart, and serum UA levels reflect the degree of XO activation in CHF. XO plays an important role in the pathophysiology process of CHF, including myocyte apoptosis, endothelial dysfunction and cardiac mechanoenergetic uncoupling. The therapeutic effect of long-term XO inhibition has been confirmed in animal models and partly in human bodies. We hypothesize that UA itself is not a player but a bystander associated with the activation of XO in the pathophysiology of CHF.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Unconventional neuroprotection against Ca2+ -dependent insults by metalloporphyrin catalytic antioxidants

We evaluated whether both inert and catalytically active metalloporphyrin antioxidants, meso-substituted with either phenyl-based or N-alkylpyridinium-based groups, suppress Ca(2+)-dependent neurotoxicity in cell culture models of relevance to cerebral ischemia. Representatives from both metalloporphyrin classes, regardless of antioxidant strength, protected cultured cortical neurons or PC-12 cultures against the Ca(2+) ionophores ionomycin or A23187, by suppressing neurotoxic Ca(2+) influx. Some metalloporphyrins suppressed excitotoxic Ca(2+) influx indirectly induced by the Ca(2+) ionophores in cortical neurons. Metalloporphyrins did not quench intracellular fluorescence, suggesting localization to the plasma membrane interface and/or interference with Ca(2+) ionophores. Metalloporphyrins suppressed ionomycin-induced Mn(2+) influx, but did not protect cortical neurons against pyrithione, a Zn(2+) ionophore. In other Ca(2+)-dependent paradigms, Ca(2+) influx via plasma membrane depolarization, but not through reversal of plasmalemmal Na(+)/Ca(2+) exchangers, was modestly suppressed by Mn(III)meso-tetrakis(4-benzoic acid)porphyrin (Mn(III)TBAP) or by an inert analog, Zn(II)TBAP. Mn(III)TBAP and Zn(II)TBAP potently protected cortical neurons against long-duration oxygen-glucose deprivation (OGD), performed in the presence of antagonists of NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and L-type voltage-gated Ca(2+) channels, raising the possibility of an unconventional mode of blockade of transient receptor protein melastatin 7 channels by a metalloTBAP family of metalloporphyrins. The present study extends the range of Ca(2+)-dependent insults for which metalloporphyrins demonstrate unconventional neuroprotection. MetalloTBAPs appear capable of targeting an OGD temporal continuum.

Interaction with phospholipids modulates alpha-synuclein nitration and lipid-protein adduct formation

Intracellular aggregates of alpha-syn (alpha-synuclein) represent pathoanatomical hallmarks of neurodegenerative disorders (synucleinopathies). The molecular mechanisms underlying alpha-syn aggregation into filamentous inclusions may involve oxidation and nitration of the protein. Whereas the effects of oxidants and nitrating species on soluble alpha-syn have been studied in detail, the effect of these reactive species on alpha-syn associated with lipids is still unknown. In the present paper, we report that alpha-syn bound to small unilamellar liposomes composed of phosphatidylcholine/phosphatidic acid is resistant to oxidation and nitration when compared with soluble alpha-syn. Additionally, increasing concentrations of unsaturated fatty acids diminished the oxidation and nitration of alpha-syn upon exposure to fluxes of peroxynitrite (8-20 microM x min(-1)). To investigate the effect of oxidized lipids on alpha-syn, the protein was incubated with the bifunctional electrophile 4-HNE [4-hydroxy-2(E)-nonenal]. MS analysis showed the formation of three major products corresponding to the native protein and alpha-syn plus one or two 4-HNE molecules. Trypsin digestion of the modified protein followed by peptide 'finger-printing' revealed that 4-HNE modified the peptide E46GVVHGVATVAEK58. Further analysis of the peptides with liquid chromatography-tandem MS identified the modified residue as His50. The data indicate that the association of alpha-syn with biological membranes protects the protein from oxidation and nitration and thus diminishes the formation of protein molecules capable of forming aggregates. However, products of lipid peroxidation can also modify alpha-syn, generating novel protein adducts that could serve as biomarkers for documenting oxidative processes in human as well as animal and cellular models of alpha-syn aggregation and pathology.

New PEG-ylated Mn(iii) porphyrins approaching catalytic activity of SOD enzyme

Two new tri(ethyleneglycol)-derivatized Mn(III) porphyrins were synthesized with the aim of increasing their bioavailability, and blood-circulating half-life. These are Mn(III) tetrakis(N-(1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)pyridinium-2-yl)porphyrin, MnTTEG-2-PyP5+ and Mn(III) tetrakis(N,N'-di(1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)imidazolium-2-yl)porphyrin, MnTDTEG-2-ImP5+. Both porphyrins have ortho pyridyl or di-ortho imidazolyl electron-withdrawing substituents at meso positions of the porphyrin ring that assure highly positive metal centered redox potentials, E1/2 = +250 mV vs. NHE for MnTTEG-2-PyP5+ and E1/2 = + 412 mV vs. NHE for MnTDTEG-2-ImP5+. As expected, from established E1/2 vs. log kcat(O2 *-) structure-activity relationships for metalloporphyrins (Batinic-Haberle et al., Inorg. Chem., 1999, 38, 4011), both compounds exhibit higher SOD-like activity than any meso-substituted Mn(III) porphyrins-based SOD mimic thus far, log kcat = 8.11 (MnTTEG-2-PyP5+) and log kcat = 8.55 (MnTDTEG-2-ImP5+), the former being only a few-fold less potent in disproportionating O2*- than the SOD enzyme itself. The new porphyrins are stable to both acid and EDTA, and non toxic to E. coli. Despite elongated substituents, which could potentially lower their ability to cross the cell wall, MnTTEG-2-PyP5+ and MnTDTEG-2-ImP5+ exhibit similar protection of SOD-deficient E. coli as their much smaller ethyl analogues MnTE-2-PyP5+ and MnTDE-2-ImP5+, respectively. Consequently, with anticipated increased blood-circulating half-life, these new Mn(III) porphyrins may be more effective in ameliorating oxidative stress injuries than ethyl analogues that have been already successfully explored in vivo.

Allopurinol or Oxypurinol in Heart Failure Therapy – a Promising New Development or End of Story?

The plasma level of the uric acid is frequently elevated in heart failure, due to increased production and/or to reduced renal excretion of this antioxidant metabolite. The transformation of hypoxanthine to xanthine and the conversion of the latter into uric acid, which occur in purine catabolism, are catalysed by xanthine oxidoreductase. The constitutive xanthine dehydrogenase form of this enzyme generally uses NAD(+) as an electron acceptor, whereas the post-translational xanthine oxidase form uses molecular oxygen and yields four units of reactive oxygen species per unit of transformed substrate. Allopurinol and oxypurinol inhibit xanthine oxidoreductase and thus diminish the generation of reactive species and decrease plasma uric acid. In a recent study in patients with NHYA class II-III heart failure, add-on treatment with allopurinol 300 mg/day for 3 months lowered plasma uric acid but failed to improve laboratory exercise performance or the distance walked in 6 minutes. In another recent trial, which was carried out in patients with NHYA class III-IV heart failure, add-on treatment with oxypurinol 600 mg/day for 24 weeks decreased plasma uric acid concentration but did not change a composite of patient outcome and state. These results indicate that the reduction in plasma uric acid caused by allopurinol or oxypurinol does not benefit patients with heart failure. Moreover, the hypothesis that the diminution in the renal excretion of the antioxidant uric acid caused by diuretics may be salutary in cardiac failure is strengthened by the study results considered.

Design, synthesis, and biological characterization of potential antiatherogenic nitric oxide releasing tocopherol analogs

Synthesis and biological characterization of a series of alpha-tocopherol analogs with NO-releasing capacity are reported. The selected NO-donor moieties were nitrooxy and furoxan. All products were tested for their in vitro NO-releasing capacities, vasodilating properties, and antiplatelet activity. They were also capable of preventing LDL oxidation.

Interactions of peroxynitrite with uric acid in the presence of ascorbate and thiols: implications for uncoupling endothelial nitric oxide synthase

It has been suggested that uric acid acts as a peroxynitrite scavenger although it may also stimulate lipid peroxidation. To gain insight into how uric acid may act as an antioxidant, we used electron spin resonance to study the reaction of uric acid and plasma antioxidants with ONOO-. Peroxynitrite reacted with typical plasma concentrations of urate 16-fold faster than with ascorbate and 3-fold faster than cysteine. Xanthine but not other purine-analogs also reacted with peroxynitrite. The reaction between ONOO- and urate produced a carbon-centered free radical, which was inhibited by either ascorbate or cysteine. Moreover, scavenging of ONOO- by urate was significantly increased in the presence of ascorbate and cysteine. An important effect of ONOO- is oxidation of tetrahydrobiopterin, leading to uncoupling of nitric oxide synthase. The protection of eNOS function by urate, ascorbate and thiols in ONOO(-)-treated bovine aortic endothelial cells (BAECs) was, therefore, investigated by measuring superoxide and NO using the spin probe 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine (CMH) and the NO-spin trap Fe[DETC]2. Peroxynitrite increased superoxide and decreased NO production by eNOS indicating eNOS uncoupling. Urate partially prevented this effect of ONOO- while treatment of BAECs with the combination of either urate with ascorbate or urate with cysteine completely prevented eNOS uncoupling caused by ONOO-. We conclude that the reducing and acidic properties of urate are important in effective scavenging of peroxynitrite and that cysteine and ascorbate markedly augment urate's antioxidant effect by reducing urate-derived radicals.

An alternative Ca2+‐dependent mechanism of neuroprotection by the metalloporphyrin class of superoxide dismutase mimetics

This study challenges the conventional view that metalloporphyrins protect cultured cortical neurons in models of cerebral ischemia by acting as intracellular catalytic antioxidants [superoxide dismutase (SOD) mimetics]. High SOD-active Mn(III)porphyrins meso-substituted with N,N'-dimethylimidazolium or N-alkylpyridinium groups did not protect neurons against oxygen-glucose deprivation (OGD), although lower SOD-active and -inactive para isomers protected against N-methyl-D-aspartate (NMDA) exposure. Mn(III)meso-tetrakis(4-benzoic acid)porphyrin (Mn(III)TBAP), as well as SOD-inactive metalloTBAPs and other phenyl ring- or beta-substituted metalloporphyrins that contained redox-insensitive metals, protected cultures against OGD and NMDA neurotoxicity. Crucially, neuroprotective metalloporphyrins suppressed OGD- or NMDA-induced rises in intracellular Ca2+ concentration in the same general rank order as observed for neuroprotection. Results from paraquat toxicity, intracellular fluorescence quenching, electrophysiology, mitochondrial Ca2+, and spontaneous synaptic activity experiments suggest a model in which metalloporphyrins, acting at the plasma membrane, protect neurons against OGD by suppressing postsynaptic NMDA receptor-mediated Ca2+ rises, thereby indirectly preventing accumulation of neurotoxic mitochondrial Ca2+ levels. Though neuroprotective in a manner not originally intended, SOD-inactive metalloporphyrins may represent promising therapeutic agents in diseases such as cerebral ischemia, in which Ca2+ toxicity is implicated. Conventional syntheses aimed at improving the catalytic antioxidant capability and/or intracellular access of metalloporphyrins may not yield improved efficacy in some disease models.

A manganese porphyrin suppresses oxidative stress and extends the life span of streptozotocin-diabetic rats

Enhanced oxidative stress due to hyperglycemia has been implicated in diabetic complications and is considered a major cause of cell and tissue damage. The aim of the present study was to investigate whether synthetic manganese porphyrin, Mn(III) 5,10,15,20-tetrakis(N-methylpyridinium-2-yl)porphyrin (MnTM-2-PyP5+) can ameliorate diabetes-induced oxidative stress and affect life span of diabetic rats. Diabetes was induced by a single (60 mg/kg) intraperitoneal injection of streptozotocin in male Wistar rats. Oxidative stress was monitored by measuring malondialdehyde levels (MDA) in blood plasma and erythrocytes using HPLC. The antioxidant status was assessed by measuring the total radical-trapping potential (TRAP) of blood plasma. Life span of the animals was used as an indication of the overall effect of MnTM-2-PyP5+. MnTM-2-PyP5+ was administered subcutaneously at 1 mg/kg for the duration of the experiment, five times/week followed by one week of rest. Diabetes increased plasma and erythrocyte levels of MDA and decreased TRAP. MnTM-2-PyP5+ had no effect on blood glucose and glycosylated hemoglobin, but significantly increased TRAP and lowered MDA. This Mn porphyrin decreased mortality and markedly extended the life span of the diabetic animals. MnTM-2-PyP5+ suppressed diabetes-induced oxidative stress, which presumably accounts for its beneficial effect on the life span of the diabetic rats. The results indicate that Mn(III) N-alkylpyridylporphyrins can be used as potent therapeutic agents in diabetes.

Reactivity of Peroxynitrite and Nitric Oxide with LDL

Low density lipoprotein (LDL) oxidation by peroxynitrite is a complex process, finely modulated by control of peroxynitrite formation, LDL availability and free-radical scavenging by nitric oxide (*NO), ascorbate and alpha-tocopherol (alpha -TOH). In the presence of CO2, lipid targets are spared at the expense of surface constituents. Since surface damage may lead to oxidation-induced LDL aggregation and particle recognition by scavenger receptors, CO2 cannot be considered an inhibitor of peroxynitrite-dependent LDL modifications. Chromanols, urate and ascorbate cannot scavenge peroxynitrite in the vasculature, although intermediates of urate oxidation and high ascorbate concentrations may do soin vitro. Most if not all of the protection against peroxynitrite-induced LDL oxidation afforded by urate, ascorbate, chromanols and also*NO should be considered to depend on their free radical scavenging abilities, including inactivation of lipid peroxyl radicals (LOO),*NO2, and CO3*-; as well as their capacity to reduce high oxidation states of metal centers. Peroxynitrite direct interception by reduced manganese (II) porphyrins is possibly the most powerful although unspecific strategy to inhibit peroxynitrite reactions. In light of the recent demonstration of nitrated bioactive lipids in vivo, renewed interest in the mechanisms of peroxynitrite- and nitric oxide-mediated lipid nitration and nitrosation is guaranteed.

The increase in serum uric acid concentration caused by diuretics might be beneficial in heart failure

Patients with mild-moderate chronic heart failure (CHF) often have raised levels of serum uric acid (UA). This is due, amongst other factors, to reduced UA excretion by the kidneys, which is partly explained by restriction of sodium intake and treatment with diuretics. The decline in renal function that parallels worsening cardiac function also contributes to elevated serum UA in patients with advanced CHF. However, UA production also appears to be augmented in CHF. Because UA scavenges various reactive oxygen species, diuretic-induced elevations in serum UA could be beneficial in patients with CHF. This concept is supported by the superior performance of antihypertensive therapy with diuretics in preventing heart failure. The present hypothesis may be tested by examining the effects of add-on treatment with a thiazide-type diuretic on morbidity and mortality, or surrogate variables, in asymptomatic patients with left ventricular dysfunction but without fluid retention.

Catalytic antioxidants: a radical approach to new therapeutics

In humans, several pathologies involve the overproduction of reactive oxygen species. Metal-containing catalytic antioxidants have emerged as a novel class of potential therapeutic agents that scavenge a wide range of reactive oxygen species. There are three structural classes of manganese-containing catalytic antioxidants that have efficacy in several oxidative stress models of human disease. The classes are divided based on their in vitro selectivity towards the scavenging of superoxide. The selective catalytic antioxidants include the macrocyclics, whereas the non-selective catalytic antioxidants include the salens and porphyrins. Cardiovascular, neurodegenerative and inflammatory lung disorders are all potentially important targets for catalytic antioxidant therapy.

Tetrahydrobiopterin rapidly reduces the SOD mimic Mn(III) ortho-tetrakis(N-ethylpyridinium-2-yl)porphyrin

Mn(III) ortho-tetrakis(N-ethylpyridinium-2-yl)porphyrin (Mn(III)TE-2-PyP(5+)) effectively scavenges reactive oxygen and nitrogen species in vitro, and protects in vivo, in different rodent models of oxidative stress injuries. Further, Mn(III)TE-2-PyP(5+) was shown to be readily reduced by cellular reductants such as ascorbic acid and glutathione. We now show that tetrahydrobiopterin (BH(4)) is also able to reduce the metal center. Under anaerobic conditions, in phosphate-buffered saline (pH 7.4) at 25 +/- 0.1 degrees C, reduction of Mn(III)TE-2-PyP(5+) occurs through two reaction steps with rate constants k(1) = 1.0 x 10(4) M(-1) s(-1) and k(2) = 1.5 x 10(3) M(-1) s(-1). We ascribe these steps to the formation of tetrahydrobiopterin radical (BH(4)(.+)) (k(1)) that then undergoes oxidation to 6,7-dihydro-8H-biopterin (k(2)), which upon rearrangement gives rise to 7,8-dihydrobiopterin (7,8-BH(2)). Under aerobic conditions, Mn(III)TE-2-PyP(5+) catalytically oxidizes BH(4). This is also true for its longer chain alkyl analog, Mn(III) ortho-tetrakis(N-n-octylpyridinium-2-yl)porphyrin. The reduced Mn(II) porphyrin cannot be oxidized by 7,8-BH(2) or by l-sepiapterin. The data are discussed with regard to the possible impact of the interaction of Mn(III)TE-2-PyP(5+) with BH(4) on endothelial cell proliferation and hence on tumor antiangiogenesis via inhibition of nitric oxide synthase.

New class of potent catalysts of O2.-dismutation. Mn(III) ortho-methoxyethylpyridyl- and di-ortho-methoxyethylimidazolylporphyrins

Three new Mn(III) porphyrin catalysts of O2.-dismutation (superoxide dismutase mimics), bearing ether oxygen atoms within their side chains, were synthesized and characterized: Mn(III) 5,10,15,20-tetrakis[N-(2-methoxyethyl)pyridinium-2-yl]porphyrin (MnTMOE-2-PyP(5+)), Mn(III)5,10,15,20-tetrakis[N-methyl-N'-(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTM,MOE-2-ImP(5+)) and Mn(III) 5,10,15,20-tetrakis[N,N'-di(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTDMOE-2-ImP(5+)). Their catalytic rate constants for O2.-dismutation (disproportionation) and the related metal-centered redox potentials vs. NHE are: log k(cat)= 8.04 (E(1/2)=+251 mV) for MnTMOE-2-PyP(5+), log k(cat)= 7.98 (E(1/2)=+356 mV) for MnTM,MOE-2-ImP(5+) and log k(cat)= 7.59 (E(1/2)=+365 mV) for MnTDMOE-2-ImP(5+). The new porphyrins were compared to the previously described SOD mimics Mn(III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)), Mn(III) 5,10,15,20-tetrakis(N-n-butylpyridinium-2-yl)porphyrin (MnTnBu-2-PyP(5+)) and Mn(III) 5,10,15,20-tetrakis(N,N'-diethylimidazolium-2-yl)porphyrin (MnTDE-2-ImP(5+)). MnTMOE-2-PyP(5+) has side chains of the same length and the same E(1/2), as MnTnBu-2-PyP(5+)(k(cat)= 7.25, E(1/2)=+ 254 mV), yet it is 6-fold more potent a catalyst of O2.-dismutation , presumably due to the presence of the ether oxygen. The log k(cat)vs. E(1/2) relationship for all Mn porphyrin-based SOD mimics thus far studied is discussed. None of the new compounds were toxic to Escherichia coli in the concentration range studied (up to 30 microM), and protected SOD-deficient E. coli in a concentration-dependent manner. At 3 microM levels, the MnTDMOE-2-ImP(5+), bearing an oxygen atom within each of the eight side chains, was the most effective and offered much higher protection than MnTE-2-PyP(5+), while MnTDE-2-ImP(5+) was of very low efficacy.