Formation of lipid-protein adducts in low-density lipoprotein by fluxes of peroxynitrite and its inhibition by nitric oxide

Decreased proteasomal cleavage at nitrotyrosine sites in proteins and peptides

Removal of moderately oxidized proteins is mainly carried out by the proteasome, while highly modified proteins are no longer degradable. However, in the case of proteins modified by nitration of tyrosine residues to 3-nitrotyrosine (NO₂Y), the role of the proteasome remains to be established. For this purpose, degradation assays and mass spectrometry analyses were performed using isolated proteasome and purified fractions of native cytochrome c (Cyt c) and tyrosine nitrated proteoforms (NO₂Y74-Cyt c and NO₂Y97-Cyt c). While Cyt c treated under mild conditions with hydrogen peroxide was preferentially degraded by the proteasome, NO₂Y74- and NO₂Y97-Cyt c species did not show an increased degradation rate with respect to native Cyt c. Peptide mapping analysis confirmed a decreased chymotrypsin-like cleavage at C-terminal of NO₂Y sites within the protein, with respect to unmodified Y residues. Additionally, studies with the proteasome substrate suc-LLVY-AMC (Y-AMC) and its NO₂Y-containing analog, suc-LLVNO₂Y-AMC (NO₂Y-AMC) were performed, both using isolated 20S-proteasome and astrocytoma cell lysates as the proteasomal source. Comparisons of both substrates showed a significantly decreased proteasome activity towards NO₂Y-AMC. Moreover, NO₂Y-AMC, but not Y-AMC degradation rates, were largely diminished by increasing the reaction pH, suggesting an inhibitory influence of the additional negative charge contained in NO₂Y-AMC secondary to nitration. The mechanism of slowing of proteasome activity in NO₂Y-contaning peptides was further substantiated in studies using the phenylalanine and nitro-phenylalanine peptide analog substrates. Finally, degradation rates of Y-AMC and NO₂Y-AMC with proteinase K were the same, demonstrating the selective inability of the proteasome to readily cleave at nitrotyrosine sites. Altogether, data indicate that the proteasome has a decreased capability to cleave at C-terminal of NO₂Y residues in proteins with respect to the unmodified residues, making this a possible factor that decreases the turnover of oxidized proteins, if they are not unfolded, and facilitating the accumulation of nitrated proteins.

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.

Arachidonic Acid and Nitroarachidonic: Effects on NADPH Oxidase Activity

Arachidonic acid (AA) is a polyunsaturated fatty acid that participates in the inflammatory response mainly through bioactive-lipids formation in macrophages and also in the phagocytic NADPH oxidase 2 (NOX2) activation. NOX2 is the enzyme responsible for a huge superoxide formation in macrophages, essential to eliminate pathogens inside the phagosome. The oxidase is an enzymatic complex comprised of a membrane-bound flavocytochrome b ₅₅₈ (gp91^phox/p22^phox), three cytosolic subunits (p47^phox, p40^phox and p67^phox) and a Rac-GTPase. The enzyme becomes active when macrophages are exposed to appropriate stimuli that trigger the phosphorylation of cytosolic subunits and its migration to plasmatic membrane to form the active complex. It is proposed that AA stimulates NOX2 activity through AA interaction with different components of the NADPH oxidase complex. In inflammatory conditions, there is an increase in reactive oxygen and nitrogen species that results in the production of nitrated derivatives of AA, such as nitroarachidonic acid (NO₂-AA). NO₂-AA is capable to inhibit NOX2 activity by interfering with p47^phox migration to the membrane without affecting phosphorylation of cytosolic proteins. Also, NO₂-AA is capable to interact with protein disulfide isomerase (PDI), which is involved on NOX2 active complex formation. It has been demonstrated that NO₂-AA forms a covalent adduct with PDI that could prevent the interaction with NOX2 and it would explain the inhibitory effects of the fatty acid upon NOX2. Together, current data indicate that AA is an important activator of NOX2 formed in the early events of the inflammatory response, leading to a massive production of oxidants that may, in turn, promote NO₂-AA formation and shutting down the oxidative burst. Hence, AA and its derivatives could have antagonistic roles on NOX2 activity regulation.

Chronic Cadmium Exposure Accelerates the Development of Atherosclerosis and Induces Vascular Dysfunction in the Aorta of ApoE-/- Mice

Cadmium exposure is related to cardiovascular diseases, including hypertension, atherosclerosis, increased oxidative stress, endothelial dysfunction, and specific biochemical changes induced by this metal. Thus, we aimed to investigate whether cadmium exposure induces endothelial dysfunction, accelerates atherosclerotic plaque formation in the aorta, and enhances oxidative stress in apolipoprotein E knockout (ApoE^-/-) mice. Experiments were performed in 14-week-old male wild-type and ApoE^-/- mice. ApoE^-/- mice received cadmium (CdCl₂ 100 mg/L in drinking water for 28 days) or vehicle (distilled water). After treatment, vascular reactivity to phenylephrine, acetylcholine, and sodium nitroprusside was analyzed using isolated aorta. Bone marrow cells were isolated to assess the production of nitric oxide and reactive oxygen and nitrogen species. ApoE^-/- cadmium-treated mice had higher cholesterol levels than non-exposed mice. Cadmium exposure decreased the vasodilatation response to acetylcholine in aortic ring of ApoE^-/- mice, though no changes in phenylephrine or sodium nitroprusside responses were observed. L-NAME reduced vasodilator responses to acetylcholine; this effect was lower in ApoE^-/- cadmium-treated mice, suggesting reduction in nitric oxide (NO) bioavailability. Moreover, in bone marrow cells, cadmium decreased cytoplasmic levels of NO and increased superoxide anions, hydrogen peroxide, and peroxynitrite in ApoE^-/- mice. Morphological analysis showed that cadmium exposure increased plaque deposition in the aorta by approximately 3-fold. Our results suggest that cadmium exposure induces endothelial dysfunction in ApoE^-/- mice. Moreover, cadmium increased total cholesterol levels, which may promote the early development of atherosclerosis in the aorta of ApoE^-/- mice. Our findings support the hypothesis that cadmium exposure might increase the risk of atherosclerosis.

Short-Term Exposure to High Sucrose Levels near Weaning Has a Similar Long-Lasting Effect on Hypertension as a Long-Term Exposure in Rats

Adverse conditions during early developmental stages permanently modify the metabolic function of organisms through epigenetic changes. Exposure to high sugar diets during gestation and/or lactation affects susceptibility to metabolic syndrome or hypertension in adulthood. The effect of a high sugar diet for shorter time lapses remains unclear. Here we studied the effect of short-term sucrose ingestion near weaning (postnatal days 12 and 28) (STS) and its effect after long-term ingestion, for a period of seven months (LTS) in rats. Rats receiving sucrose for seven months develop metabolic syndrome (MS). The mechanisms underlying hypertension in this model and those that underlie the effects of short-term exposure have not been studied. We explore NO and endothelin-1 concentration, endothelial nitric oxide synthase (eNOS) expression, fatty acid participation and the involvement of oxidative stress (OS) after LTS and STS. Blood pressure increased to similar levels in adult rats that received sucrose during short- and long-term glucose exposure. The endothelin-1 concentration increased only in LTS rats. eNOS and SOD2 expression determined by Western blot and total antioxidant capacity were diminished in both groups. Saturated fatty acids and arachidonic acid were only decreased in LTS rats. In conclusion, a high-sugar diet during STS increases the hypertension predisposition in adulthood to as high a level as LTS, and the mechanisms involved have similarities (participation of OS and eNOS and SOD expression) and differences (fatty acids and arachidonic acid only participate in LTS and an elevated level of endothelin-1 was only found in LTS) in both conditions. Changes in the diet during short exposure times in early developmental stages have long-lasting effects in determining hypertension susceptibility.

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.

Nitro-fatty acids: formation, redox signaling, and therapeutic potential

SIGNIFICANCE

Nitrated derivatives of unsaturated fatty acids (nitro-fatty acids) are being formed and detected in human plasma, cell membranes, and tissue, triggering signaling cascades via covalent and reversible post-translational modifications of susceptible nucleophilic amino acids in transcriptional regulatory proteins and enzymes.

RECENT ADVANCES

Nitro-fatty acids modulate metabolic as well as inflammatory signaling pathways, including the p65 subunit of nuclear factor κB and the transcription factor peroxisome proliferator-activated receptor-γ. Moreover, nitro-fatty acids can activate heat shock as well as phase II antioxidant responses. As electrophiles, they also activate the Nuclear factor erythroid 2-related factor 2 pathway.

CRITICAL ISSUES

We first discuss the mechanisms of nitro-fatty acid formation as well as their key chemical and biochemical properties, including their capacity to release nitric oxide and exert antioxidant actions. The electrophilic properties of nitro-fatty acids to activate anti-inflammatory signaling pathways are discussed in detail. A critical issue is the influence of nitroarachidonic acid on prostaglandin endoperoxide H synthases, modulating inflammatory processes through redirection of arachidonic acid metabolism and signaling.

FUTURE DIRECTIONS

Based on this information, we analyze in vivo data supporting nitro-fatty acids as promising pharmacological tools to prevent inflammatory diseases associated with oxidative and nitrative stress conditions. A key future issue is to evaluate whether nitro-fatty acid supplementation would be useful for human diseases linked to inflammation as well as their potential toxicity when administered by long periods of time.

Antioxidant activity of uruguayan propolis. In vitro and cellular assays

The antioxidant capacity of propolis from the southern region of Uruguay was evaluated using in vitro as well as cellular assays. Free radical scavenging capacity was assessed by ORAC, obtaining values significantly higher than those of other natural products (8000 μmol Trolox equiv/g propolis). ORAC values correlated well with total polyphenol content (determined by Folin-Ciocalteu method) and UV absorption. Total polyphenol content (150 mg gallic acid equiv/g propolis) and flavonoids (45 mg quercetin equiv/g propolis) were similar to values reported for southern Brazilian (group 3) and Argentinean propolis. Flavonoid composition determined by RP-HPLC indicates a strong poplar-tree origin. Samples high in polyphenols efficiently inhibit low-density lipoprotein lipoperoxidation and tyrosine nitration. In addition, Uruguayan propolis was found to induce the expression of endothelial nitric oxide synthase and inhibit endothelial NADPH oxidase, suggesting a potential cardiovascular benefit by increasing nitric oxide bioavailability in the endothelium.

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.

Biology of platelet-activating factor acetylhydrolase (PAF-AH, lipoprotein associated phospholipase A2)

INTRODUCTION

This article is focused on platelet-activating factor acetylhydrolase (PAF-AH), a lipoprotein bound, calcium-independent phospholipase A(2) activity also referred to as lipoprotein-associated phospholipase A(2) or PLA(2)G7. PAF-AH catalyzes the removal of the acyl group at the sn-2 position of PAF and truncated phospholipids generated in settings of inflammation and oxidant stress.

DISCUSSION

Here, I discuss current knowledge related to the structural features of this enzyme, including the molecular basis for association with lipoproteins and susceptibility to oxidative inactivation. The circulating form of PAF-AH is constitutively active and its expression is upregulated by mediators of inflammation at the transcriptional level. This mechanism is likely responsible for the observed up-regulation of PAF-AH during atherosclerosis and suggests that increased expression of this enzyme is a physiological response to inflammatory stimuli. Administration of recombinant forms of PAF-AH attenuate inflammation in a variety of experimental models. Conversely, genetic deficiency of PAF-AH in defined human populations increases the severity of atherosclerosis and other syndromes. Recent advances pointing to an interplay among oxidized phospholipid substrates, Lp(a), and PAF-AH could hold the key to a number of unanswered questions.

Long-chainn-3 polyunsaturated fatty acid from fish oil modulates aortic nitric oxide and tocopherol status in the rat

In spite of their high oxidisability, long-chainn-3 PUFA protect against CVD. Dietary fatty acids modulate the fatty acid composition of lipoproteins involved in atherosclerosis. We thought that if long-chainn-3 PUFA were able to increase NO production by the aorta, then by its antioxidant activity the NO will prevent lipid peroxidation. However, the beneficial effect of NOin vivoon VLDL+LDL oxidation would only be possible if NO could diffuse to their lipidic core. Rats were fed maize oil- or fish oil as menhaden oil- (MO) rich diets for 8 weeks, to study the effects of MO on aortic NO production, NO diffusion into VLDL+LDL, the extent of oxidation in native VLDL+LDL and their oxidisabilityex vivo. Aortic NO production and its α-tocopherol content were increased andn-3 PUFA were incorporated into the VLDL+LDL. In spite of the higher peroxidisability and the low α-tocopherol in native VLDL+LDL from rats fed MO, native VLDL+LDL from the two groups shared similar electrophoretic patterns, conjugated dienes, thiobarbituric acid-reactive substances, total antioxidant capacity, and NO diffusibility on VLDL+LDL, indicative of anin vivoprotection against oxidation. However, these results do not correlate with theex vivooxidisability of VLDL+LDL, as NO is lacking. Thus, thein vivobeneficial effects can be explained by increased α-tocopherol in aorta and by a compensatory effect of NO on VLDL+LDL against the low α-tocopherol levels, which may contribute to the anti-atherogenic properties of fish oil.

Tyrosine nitration, dimerization, and hydroxylation by peroxynitrite in membranes as studied by the hydrophobic probe N-t-BOC-l-tyrosine tert-butyl ester

Protein tyrosine oxidation mechanisms in hydrophobic biocompartments (i.e., biomembranes, lipoproteins) leading to nitrated, dimerized, and hydroxylated products are just starting to be appreciated. This chapter reports on the use of the hydrophobic tyrosine analog N-t-BOC-l-tyrosine tert-butyl ester (BTBE) incorporated to phosphatidyl choline liposomes to study peroxynitrite-dependent tyrosine oxidation processes in model biomembranes. The probe proved to be valuable in defining the role of biologically relevant variables in the oxidation process, including the action of hydrophilic and hydrophobic peroxynitrite and peroxynitrite-derived free radical scavengers, transition metal catalysts, carbon dioxide, molecular oxygen, pH, and fatty acid unsaturation degree. Moreover, detection of the BTBE phenoxyl radical and relative product distribution yields of 3-nitro-, 3,3'-di-, and 3-hydroxy-BTBE in the membrane fully accommodate with a free radical mechanism of tyrosine oxidation, with physical chemical and biochemical determinants that in several respects differ of those participating in aqueous environments. The methods presented herein can be extended to explore the reaction mechanisms of tyrosine oxidation by other biologically relevant oxidants and in other hydrophobic biocompartments.

The chemistry of peroxynitrite: implications for biological activity

In biological systems, nitric oxide (NO) combines rapidly with superoxide (O2-) to form peroxynitrite ion (ONOO-), a substance that has been implicated as a culprit in many diseases. Peroxynitrite ion is essentially stable, but its protonated form (ONOOH, pKa = 6.5 to 6.8) decomposes rapidly via homolysis of the O-O bond to form about 28% free NO2 and OH radicals. At physiological pH and in the presence of large amounts of bicarbonate, ONOO- reacts with CO2 to produce about 33% NO2 and carbonate ion radicals (CO3-) in the bulk of the solution. The quantitative role of OH/CO3(-) and NO2 radicals during the decomposition of peroxynitrite (ONOOH/ONOO-) under physiological conditions is described in detail. Specifically, the effect of the peroxynitrite dosage rate on the yield and distribution of the final products is demonstrated. By way of an example, the detailed mechanism of nitration of tyrosine, a vital aromatic amino acid, is delineated, showing the difference in the nitration yield between the addition of authentic peroxynitrite and its continuous generation by NO and O2- radicals.

NO* chemistry: a diversity of targets in the cell

NO(*) alone is a poorly reactive species; however, it is able to undergo secondary reactions to form highly oxidizing and nitrating species, NO(2)(*), N(2)O(3), and ONOO(-). These secondary reactive nitrogen species (RNS) are capable of modifying a diversity of biomolecular structures in the cell. The chemical properties of individual RNS will be discussed, along with their ability to react with amino acids, metal cofactors, lipids, cholesterol, and DNA bases and sugars. Many of the identified RNS-induced modifications have been observed both in vitro and in vivo. Several of these chemical modifications have been attributed with a functional role in the cell, such as the modulation of enzyme activity. Other areas in the field will be discussed, including the ability of RNS to react with metabolites, RNA, and substrates in the mitochondrion, and the cellular removal/repair of RNS-modified structures.

Lipid nitration and formation of lipid-protein adducts: biological insights

Lipid-protein adducts are formed during oxidative and nitrative stress conditions associated with increasing lipid and protein oxidation and nitration. The focus of this review is the analysis of interactions between oxidative-modified lipids and proteins and how lipid nitration can modulate lipid-protein adducts formation. For this, two biologically-relevant models will be analysed: a) human low density lipoprotein, whose oxidation is involved in the early steps of atherogenesis, and b) alpha-synuclein/lipid membranes system, where lipid-protein adducts are being associated with the develop of Parkinson disease and other synucleinopathies.

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.

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.

Inhibition of endogenous nitric oxide in the heart enhances matrix metalloproteinase-2 release

Matrix metalloproteinase (MMP) activity is upregulated in pathologies such as atherosclerosis during which endogenous nitric oxide (NO) biosynthesis is reduced. Diminished levels of NO, an antioxidant species, may result in higher oxidative stress. Oxidants are capable of activating MMPs from their zymogen forms. We examined whether basal biosynthesis of NO in the coronary circulation regulates MMP-2 activity. In isolated rat hearts perfused with Krebs-Henseleit buffer at a constant flow of 10 ml min(-1), we measured the release of MMP-2 into the coronary effluent by gelatin zymography. The main gelatinolytic activity of 72-kDa corresponds to MMP-2. Infusion of the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) concentration dependently increased coronary perfusion pressure (CPP) (by 48+/-11 mmHg with 100 microM) and enhanced the release of the 72-kDa MMP-2 in the effluent. Coinfusion of the NO donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 1 microM) with L-NAME abolished both the increase in CPP and the enhanced MMP-2 release. The thromboxane A2 mimetic U46619 increased CPP to the same extent as L-NAME without increasing 72-kDa activity in the effluent, suggesting that MMP-2 release is not caused simply by enhanced perfusion pressure. Infusion of either L-NAME or U46619 did not significantly enhance LDH release. L-NAME infusion concentration dependently increased the level of lipid hydroperoxides in homogenates prepared from the perfused hearts. Coinfusion of SNAP prevented this increase. These data reveal another cytoprotective mechanism of endogenous NO biosynthesis in the heart, the inhibition of MMP-2 release.

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.

Septic diaphragmatic dysfunction is prevented by Mn(III)porphyrin therapy and inducible nitric oxide synthase inhibition

OBJECTIVE

Decreased diaphragmatic contractility and organ failure observed during sepsis is mediated by an overproduction of nitric oxide ((.)NO)-derived species, mitochondria being a major target of oxidative and nitrative stress. We tested the potential protective effects of (a) a novel synthetic antioxidant, the manganese(III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl) porphyrin (MnTE-2-PyP(5+)) and (b) the inducible (.)NO synthase inhibitor aminoguanidine (AG) on a rat model of sepsis.

SETTING

University research laboratories.

SUBJECTS AND INTERVENTIONS

Sepsis was induced by cecal ligation and perforation in rats.

MEASUREMENTS AND RESULTS

Systemic hemodynamics, pulmonary gas exchange, in vitro diaphragmatic function and mitochondrial respiration were evaluated. Moreover, plasma and mitochondrial oxidative and nitrative stress parameters were investigated. Sepsis determined diaphragmatic dysfunction and a significant decrease in mitochondrial coupling and respiration. Oxidative stress was evidenced by decreased plasma antioxidants and increased lipid oxidation. Tyrosine nitration was increased in the plasma and mitochondria of the septic animals. These alterations were ameliorated or prevented by either MnTE-2-PyP(5+) or AG.

CONCLUSIONS

Our results demonstrate that overproduction of (.)NO and (.)NO-derived reactive species play a critical role in mitochondrial impairment and diaphragmatic function during sepsis. More importantly, AG but mainly the novel metalloporphyrin MnTE-2-PyP(5+) were able to ameliorate diaphragmatic and mitochondrial dysfunction and could contribute to preventing organ failure during severe sepsis.

Enalapril increases mitochondrial nitric oxide synthase activity in heart and liver

Heart and liver mitochondria isolated from rats treated with enalapril, 3-30 mg/kg/day in the drinking water for 7-120 days, showed a time- and dose-dependent increased nitric oxide (NO) production in the range of 14-250%. Heart and liver mitochondria from control rats produced 0.69 and 0.50 nmol of NO/min/mg of protein, respectively, as determined by dual wavelength spectrophotometry (577-591 nm) following hemoglobin oxidation to methemoglobin. The response to enalapril treatment, attributed to a gene-mediated up-regulation of mitochondrial nitric oxide synthase (mtNOS) activity, was half-maximal at 5-6 days and was maintained up to 120 days. Enalapril-treated animals showed an increased mtNOS functional activity in heart mitochondria that inhibited state 3 O(2) uptake (from 22% in control rats to 43%) and increased state 4 hydrogen peroxide (H(2)O(2)) production (from 30% in control rats to 52%). Calculated heart intramitochondrial NO and H(2)O(2) steady-state concentrations were increased 66% and 20%, respectively, by enalapril treatment. Signaling pathways dependent on mitochondrial NO and H(2)O(2) may account for the beneficial effects of enalapril in aging mammals.

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

We have studied the role of three Mn(III)porphyrins differing in charge, alkyl substituent length and reactivity, on LDL exposed to low fluxes of peroxynitrite (PN) in the presence of uric acid. Mn(III)porphyrins (5 microM, MnTE-2-PyP(5+), MnTnOct-2-PyP(5+), and MnTCPP(3-)) plus uric acid (300 microM) inhibited cholesteryl ester hydroperoxide formation, changes in REM as well as spared alpha- and gamma-tocopherol. MnTnOct-2-PyP(5+), the more lipophilic compound, was the most effective in protecting LDL lipids, while MnTCPP(3-) exerted the lesser protection. Mn(III)porphyrins react fast with PN ( approximately 10(5)-10(7) M(-1) s(-1)) to yield a O=Mn(IV) complex. The stoichiometry of uric acid consumption was approximately 1.7 moles per mol of PN, in agreement with reactions with both the O=Mn(IV) complex and nitrogen dioxide. A shift from an anti- to a pro-oxidant action of the Mn(III)porphyrin was observed after uric acid was significantly consumed, supporting competition reactions between LDL targets and uric acid for the O=Mn(IV) complex. Overall, the data is consistent with the catalytic reduction of PN in a cycle that involves a one electron oxidation of Mn(III) to Mn(IV) by PN followed by the reduction back to Mn(III) by uric acid. These antioxidant effects should predominate under in vivo conditions having plasma uric acid concentration range between 150 and 500 microM.

On-line EPR study of free radicals induced by peroxidase/H(2)O(2) in human low-density lipoprotein

The aim of this study was to use direct electron paramagnetic resonance (EPR) spectroscopy at 37 degrees C and spin trapping techniques to study radical species formed during horseradish peroxidase/H(2)O(2)-initiated low-density lipoprotein (LDL) oxidation. Using direct EPR, we obtained evidence for the formation not only of the alpha-tocopheroxyl radical but also of a protein radical(s), assigned to a tyrosyl radical(s) of apolipoprotein B-100 (apo B-100). Spin trapping with 2-methyl-2-nitrosopropane revealed (i) the formation of a mobile adduct with beta-hydrogen coupling assigned to a lipid radical and (ii) a partially immobilised adduct detected in LDL as well as in apo B-100, assigned after proteolytic digestion to the trapping of a radical centred on a tertiary carbon atom of an aromatic residue, probably tyrosine. Our results support the hypothesis that radicals are initiators of the oxidative process, and show that their formation is an early event in peroxidase-mediated oxidation. We also tested the effects of resveratrol (RSV), a polyphenolic antioxidant present in red wine. Our data indicate that 1-10 microM RSV is able to accelerate alpha-tocopherol consumption, conjugated dienes formation and the decay kinetics of LDL-centred radicals. Since phenols are substrates for peroxidases, this result may be ascribed to a RSV-mediated catalysis of peroxidase activity.

Interactions of nitric oxide and peroxynitrite with low-density lipoprotein

Nitric oxide (*NO) is a free radical species that diffuses and concentrates in the hydrophobic core of low-density lipoprotein (LDL) to serve as a potent inhibitor of lipid oxidation processes. Peroxynitrite (PN), the product of the diffusion-limited reaction between *NO and superoxide (O2*-) represents a relevant mediator of oxidative modifications in LDL. The focus of this review is the analysis of interactions between *NO and PN and its secondary reactions with oxygen radicals on LDL oxidation, which are relevant in the development of the early steps as well as progression of atherosclerosis. We propose that the balance between rates of PN and *NO production, which greatly depends on oxidative stress processes within the vascular wall, will critically determine the final extent of oxidative LDL modifications leading or not to scavenger receptor-mediated LDL uptake and foam cell formation.

Diffusion of nitric oxide into low density lipoprotein

A key early event in the development of atherosclerosis is the oxidation of low density lipoprotein (LDL) via different mechanisms including free radical reactions with both protein and lipid components. Nitric oxide (( small middle dot)NO) is capable of inhibiting LDL oxidation by scavenging radical species involved in oxidative chain propagation reactions. Herein, the diffusion of ( small middle dot)NO into LDL is studied by fluorescence quenching of pyrene derivatives. Selected probes 1-(pyrenyl)methyltrimethylammonium (PMTMA) and 1-(pyrenyl)-methyl-3-(9-octadecenoyloxy)-22,23-bisnor-5-cholenate (PMChO) were chosen so that they could be incorporated at different depths of the LDL particle. Indeed, PMTMA and PMChO were located in the surface and core of LDL, respectively, as indicated by changes in fluorescence spectra, fluorescence quenching studies with water-soluble quenchers and the lifetime values (tau(o)) of the excited probes. The apparent second order rate quenching constants of ( small middle dot)NO (k(NO)) for both probes were 2.6-3.8 x 10(10) m(-1) s(-1) and 1.2 x 10(10) m(-1) s(-1) in solution and native LDL, respectively, indicating that there is no significant barrier to the diffusion of ( small middle dot)NO to the surface and core of LDL. Nitric oxide was also capable of diffusing through oxidized LDL. Considering the preferential partitioning of ( small middle dot)NO in apolar milieu (6-8 for n-octanol:water) and therefore a larger ( small middle dot)NO concentration in LDL with respect to the aqueous phase, a corrected k(NO) value of approximately 0.2 x 10(10) m(-1) s(-1) can be determined, which still is sufficiently large and consistent with a facile diffusion of ( small middle dot)NO through LDL. Applying the Einstein-Smoluchowsky treatment, the apparent diffusion coefficient (D(')NO) of ( small middle dot)NO in native LDL is on average 2 x 10(-5) cm(2) s(-1), six times larger than that previously reported for erythrocyte plasma membrane. Thus, our observations support that ( small middle dot)NO readily traverses the LDL surface accessing the hydrophobic lipid core of the particle and affirm a role for ( small middle dot)NO as a major lipophilic antioxidant in LDL.