Evaluation of laboratory and environmental exposure systems for protein modification upon gas pollutants and environmental factors

Chemical modifications of proteins induced by ambient ozone (O3) and nitrogen oxides (NOx) are of public health concerns due to their potential to trigger respiratory diseases. The laboratory and environmental exposure systems have been widely used to investigate their relevant mechanism in the atmosphere. Using bovine serum albumin (BSA) as a model protein, we evaluated the two systems and aimed to reduce the uncertainties of both the reactants and products in the corresponding kinetic study. In the laboratory simulation system, the generated gaseous pollutants showed negligible losses. Ten layers of BSA were coated on the flow tube with protein extraction recovery of 87.4%. For environmental exposure experiment, quartz fiber filter was selected as the upper filter with low gaseous O3 (8.0%) and NO2 (1.7%) losses, and cellulose acetate filter was appropriate for the lower filter with protein extraction efficiency of 95.2%. The protein degradation process was observed without the exposure to atmospheric oxidants and contributed to the loss of protein monomer mass fractions, while environmental factors (e.g., molecular oxygen and ultraviolet) may cause greater protein monomer losses. Based on the evaluation, the study exemplarily applied the two systems to protein modification and both showed that O3 promotes the protein oligomerization and nitration, while increased temperature can accelerate the oligomerization and increased relative humidity can inhibit the nitration in the environmental exposure samples. The developed laboratory and environmental systems are suitable for studying protein modifications formed under different atmospheric conditions. A combination of the two will further reveal the actual mechanism of protein modifications.

Multiphase reactions of proteins in the air: Oligomerization, nitration and degradation of bovine serum albumin upon ambient exposure

Proteins in atmospheric aerosol can react with atmospheric pollutants such as ozone (O3) and nitrogen dioxide (NO2) in the atmosphere via the reactions of oxidation, nitration, and cross-linking etc. Currently, the reactions have been more thoroughly studied in the laboratory but rarely investigated in the ambient environment. In this study, we used bovine serum albumin (BSA) as the model protein to conduct the exposure experiment in the ambient environment in southern China, an area with increasing oxidative capacity, to investigate the reactions of proteins in the atmosphere. We observed the occurrence of oligomerization, nitration and degradation of BSA upon exposure. The mass fraction of BSA monomer decreased by 5.86 ± 1.61% after exposure and those of dimers, trimers and higher oligomers increased by 1.04 ± 0.49%, 1.37 ± 0.74% and 3.40 ± 1.06%, respectively. Simultaneously, the nitration degrees of monomers, dimers, trimers and higher oligomers increased by 0.42 ± 0.15%, 0.53 ± 0.15%, 0.55 ± 0.28% and 2.15 ± 1.01%, respectively. The results show that oligomerization was significantly affected by O3 and temperature and nitration was jointly affected by O3, temperature and relative humidity, indicating the important role of atmospheric oxidants in the atmospheric reactions of protein. Atmospheric degradation of BSA was observed with the release of free amino acids (FAAs) such as glycine, alanine, serine and methionine. Glycine was the dominant FAA with a molar yield ranging from ∼8% to 33% for BSA. The estimated stoichiometric coefficient (α) of glycine is 10-7-10-6 for the degradation of BSA upon O3. Our observation suggests the occurrence of protein reactions in the oxidative ambient environment, leading to the production of nitrated products, oligomers and low molecular weight products such as peptides and FAAs. This study may deepen the current understanding of the atmospheric reaction mechanisms and reveal the influence of environmental factors in the atmosphere.

Tyrosine nitration enhances the allergenic potential of house dust mite allergen Der p 2

Nitration of allergenic proteins caused by atmospheric pollutants O3 and NO2 may enhance their allergenic potential. In the study, the influence of nitration was investigated on the allergenicity of Der p 2, which is a main allergen from house dust mites and plays an important role in allergenic rhinitis and asthma. The results reveal that nitrated Der p 2 enhanced the IgE-binding capacity, upregulated the mRNA expression and release of IL-6 and IL-8 from bronchial epithelial cells, and induced higher levels of specific-IgE, TH2 cytokines and white blood cells in mice. Besides, nitrated Der p 2 caused more severe oxidative stress and allergenic symptoms in mice. It is concluded that nitration enhanced the allergenicity of Der p 2 through not only directly inducing higher amount of specific-IgE and stronger responses of TH2 cytokines, but also indirectly aggravating allergic symptoms by oxidative stress and adjuvant-like activation airway epithelial cells. The study suggests that the contribution of nitration to the promotion in allergenicity should not be ignored when precisely assessing the risk of house dust mite allergens in real environment.

Digestive interaction between dietary nitrite and dairy products generates novel nitrated linolenic acid products

Nitrated fatty acids are important anti-inflammatory and protective lipids formed in the gastric compartment, with conjugated linoleic acid (rumenic acid, RA, 9Z,11E-18:2) being the primary substrate for lipid nitration. The recently reported identification of nitrated rumelenic acid (NO2-RLA) in human urine has led to hypothesize that rumelenic acid (RLA, 9Z,11E,15Z-18:3) from dairy fat is responsible for the formation of NO2-RLA. To evaluate the source and mechanism of NO2-RLA formation, 15N labeled standards of NO2-RLA were synthesized and characterized. Afterward, milk fat with different RA and RLA levels was administered to mice in the presence of nitrite, and the appearance of nitrated fatty acids in plasma and urine followed. We confirmed the formation of NO2-RLA and defined the main metabolites in plasma, urine, and tissues. In conclusion, RLA obtained from dairy products is the main substrate for forming this novel electrophilic lipid reported to be present in human urine.

Characterization of leucine aminopeptidase (LAP) activity in sweet pepper fruits during ripening and its inhibition by nitration and reducing events

KEY MESSAGE

Pepper fruits contain two leucine aminopeptidase (LAP) genes which are differentially modulated during ripening and by nitric oxide. The LAP activity increases during ripening but is negatively modulated by nitration. Leucine aminopeptidase (LAP) is an essential metalloenzyme that cleaves N-terminal leucine residues from proteins but also metabolizes dipeptides and tripeptides. LAPs play a fundamental role in cell protein turnover and participate in physiological processes such as defense mechanisms against biotic and abiotic stresses, but little is known about their involvement in fruit physiology. This study aims to identify and characterize genes encoding LAP and evaluate their role during the ripening of pepper (Capsicum annuum L.) fruits and under a nitric oxide (NO)-enriched environment. Using a data-mining approach of the pepper plant genome and fruit transcriptome (RNA-seq), two LAP genes, designated CaLAP1 and CaLAP2, were identified. The time course expression analysis of these genes during different fruit ripening stages showed that whereas CaLAP1 decreased, CaLAP2 was upregulated. However, under an exogenous NO treatment of fruits, both genes were downregulated. On the contrary, it was shown that during fruit ripening LAP activity increased by 81%. An in vitro assay of the LAP activity in the presence of different modulating compounds including peroxynitrite (ONOO-), NO donors (S-nitrosoglutathione and nitrosocyteine), reducing agents such as reduced glutathione (GSH), L-cysteine (L-Cys), and cyanide triggered a differential response. Thus, peroxynitrite and reducing compounds provoked around 50% inhibition of the LAP activity in green immature fruits, whereas cyanide upregulated it 1.5 folds. To our knowledge, this is the first characterization of LAP in pepper fruits as well as of its regulation by diverse modulating compounds. Based on the capacity of LAP to metabolize dipeptides and tripeptides, it could be hypothesized that the LAP might be involved in the GSH recycling during the ripening process.

Nitric oxide synthase-guided genome mining identifies a cytochrome P450 enzyme for olefin nitration in bacterial specialized metabolism

The biological signaling molecule nitric oxide (NO) has recently emerged as a metabolic precursor for the creation of microbial natural products with diversified structures and biological activities. Within the biosynthetic gene clusters (BGCs) of these compounds, genes associated with NO production pathways have been pinpointed. In this study, we employ a nitric oxide synthase (NOS)-guided genome mining strategy for the targeted discovery of NO-derived bacterial natural products and NO-utilizing biocatalysts. We show that a conserved NOS-containing BGC, distributed across several actinobacterial genomes, is responsible for the biosynthesis of lajollamycin, a unique nitro-tetraene-containing antibiotic whose biosynthetic mechanism remains elusive. Through a combination of in vivo and in vitro studies, we unveil the first cytochrome P450 enzyme capable of catalyzing olefin nitration in natural product biosynthesis. These results not only expand the current knowledge about biosynthetic nitration processes but also offer an efficient way for targeted identification of NO-utilizing metabolic pathways and novel nitrating biocatalysts.

Simplifying Nitration Chemistry with Bench-stable Organic Nitrating Reagents

Nitro compounds play a crucial role in academia and industries, serving as building blocks for the synthesis of drugs, agrochemicals, and materials. Nitration, a fundamental process in organic synthesis, has undergone significant evolution since the 19th century. While electrophilic nitration dominates historically, recent decades have seen a focus on new reagents and their reactivity modes for achieving mild and robust synthesis of nitro compounds. Our group has a longstanding interest in developing cost-effective, readily available, recyclable nitrating reagents derived from organic scaffolds. These reagents serve as a controllable source of nitryl radical and nitronium ion species, enabling mild and practical nitration of hydrocarbons with exceptional functional group tolerance. This account details the development of nitrating reagents and their diverse applications in catalytic nitration across various classes of organic molecules.

Infrared spectroscopic and thermographic evidence for the interaction of ethylene and 1,3-butadiene with N2O4 at cryogenic temperatures

The possibility of the reaction of ethylene at cryogenic temperatures with dinitrogen tetroxide (N2O4), at least at -40 °C, has been demonstrated. An infrared (IR) spectroscopic analysis of the condensed reaction products was carried out, which showed the presence of substances containing organic nitro compounds and organic nitrates. The latter particularly means the possibility of the formation of explosive substances, as in the known explosive cryogenic reaction of N2O4 with butadiene. The fundamental possibility of such a reaction is of importance for the organization of cryogenic production of gases from raw materials containing ethylene and nitrogen oxide impurities. The sensitivity of the experimental setup was demonstrated by using the example of the known reaction of N2O4 with butadiene. The temperature and time characteristics of the latter reaction have been found. To determine the temperature range of possible reactions of ethylene with N2O4, thermograms of the process of heating the reactive mixture at a constant rate were analyzed. To ensure the sensitivity of the qualitative analysis of thermograms of the reaction of ethylene with N2O4 in the linear heating mode, a quasi-differential method was proposed that uses the temperature of the cryo-thermostat coolant, which varies linearly with time, as the reference temperature. Some recently published theoretical results concerning the mechanisms of N2O4 isomerization are discussed which may be useful for further studies in establishing the mechanisms and predicting the ratios of the products and their yields for the reaction of N2O4 with olefins under particular experimental conditions.

A metabolic perspective on nitric oxide function in melanoma

Nitric oxide (NO) generated from nitric oxide synthase (NOS) exerts a dichotomous effect in melanoma, suppressing or promoting tumor progression. This dichotomy is thought to depend on the intracellular NO concentration and the cell type in which it is generated. Due to its central role in the metabolism of multiple critical constituents involved in signaling and stress, it is crucial to explore NO's contribution to the metabolic dysfunction of melanoma. This review will discuss many known metabolites linked to NO production in melanoma. We discuss the synthesis of these metabolites, their role in biochemical pathways, and how they alter the biological processes observed in the melanoma tumor microenvironment. The metabolic pathways altered by NO and the corresponding metabolites reinforce its dual role in melanoma and support investigating this effect for potential avenues of therapeutic intervention.

Review on nitric oxide at the forefront of rapid systemic signaling in mitigation of salinity stress in plants: Crosstalk with calcium and hydrogen peroxide

Soil salinity is a global issue that limits plant growth in agricultural fields and contributes to food crisis. Salt stressors impede plant's ionic, osmotic, and oxidative balance, as well as a variety of physiological functions. Exposure to salinity stress manifest considerable ROS clustering, entailing modification in performance of various organelles. To deal with salinity, plants use a variety of coping strategies, such as osmoregulation, ion-homeostasis, increased antioxidant synthesis, and so on. Nitric oxide (NO) is a pivotal signalling molecule that helps facilitate salt stress-induced physiological plant responses. A variety of evidences point to NO being produced under similar stress conditions and with similar kinetics as hydrogen peroxide (H2O2). The interplay between H2O2 and NO has important functional implications for modulating plant transduction processes. Besides, NO and calcium (Ca2+)-dependent pathways also have some connection in salt stress response mechanisms. Extensive crosstalk between NO and Ca2+ signalling pathways is investigated, and it suggests that almost every type of Ca2+ channel is under the tight control of NO, and NO acts as a Ca2+ mobilising compound and aids in signal reliance. The review provides insights into understanding recent advances regarding NO's, Ca2+ and H2O2 role in salt stress reduction with entwine signaling mechanisms.

Catalytic ipso-Nitration of Organosilanes Enabled by Electrophilic N-Nitrosaccharin Reagent

Nitroaromatic compounds represent one of the essential classes of molecules that are widely used as feedstock for the synthesis of intermediates, the preparation of nitro-derived pharmaceuticals, agrochemicals, and materials on both laboratory and industrial scales. We herein disclose the efficient, mild, and catalytic ipso-nitration of organotrimethylsilanes, enabled by an electrophilic N-nitrosaccharin reagent and allows chemoselective nitration under mild reaction conditions, while exhibiting remarkable substrate generality and functional group compatibility. Additionally, the reaction conditions proved to be orthogonal to other common functionalities, allowing programming of molecular complexity via successive transformations or late-stage nitration. Detailed mechanistic investigation by experimental and computational approaches strongly supported a classical electrophilic aromatic substitution (SE Ar) mechanism, which was found to proceed through a highly ordered transition state.

Exposure to peroxynitrite impacts the ability of anastellin to modulate the structure of extracellular matrix

The extracellular matrix (ECM) of tissues consists of multiple proteins, proteoglycans and glycosaminoglycans that form a 3-dimensional meshwork structure. This ECM is exposed to oxidants including peroxynitrite (ONOO-/ONOOH) generated by activated leukocytes at sites of inflammation. Fibronectin, a major ECM protein targeted by peroxynitrite, self-assembles into fibrils in a cell-dependent process. Fibrillation of fibronectin can also be initiated in a cell-independent process in vitro by anastellin, a recombinant fragment of the first type-III module in fibronectin. Previous studies demonstrated that modification of anastellin by peroxynitrite impairs its fibronectin polymerization activity. We hypothesized that exposure of anastellin to peroxynitrite would also impact on the structure of ECM from cells co-incubated with anastellin, and influence interactions with cell surface receptors. Fibronectin fibrils in the ECM of primary human coronary artery smooth muscle cells exposed to native anastellin are diminished, an effect which is reversed to a significant extent by pre-incubation of anastellin with high (200-fold molar excess) concentrations of peroxynitrite. Treatment with low or moderate levels of peroxynitrite (2-20 fold molar excess) influences interactions between anastellin and heparin polysaccharides, as a model of cell-surface proteoglycan receptors, and modulates anastellin-mediated alterations in fibronectin cell adhesiveness. Based on these observations it is concluded that peroxynitrite has a dose-dependent influence on the ability of anastellin to modulate ECM structure via interactions with fibronectin and other cellular components. These observations may have pathological implications since alterations in fibronectin processing and deposition have been associated with several pathologies, including atherosclerosis.

Enantiocomplementary synthesis of β-adrenergic blocker precursors via biocatalytic nitration of phenyl glycidyl ethers

Enantiopure β-nitroalcohols, as an important class of nitro-containing compounds, are essential building blocks in pharmaceutical and organic chemistry, particularly for the synthesis of β-adrenergic blockers. In this study, we present the successful protein engineering of halohydrin dehalogenase HHDHamb for the enantioselective bio-nitration of various phenyl glycidyl ethers to the corresponding chiral β-nitroalcohols, using the inexpensive, commercially available, and safer nitrite as a nitrating agent. The chiral (R)- and (S)-1-nitro-3-phenoxypropan-2-ols were synthesized by the several enantiocomplementary HHDHamb variants through the whole-cell biotransformation, which showed good catalytic efficiency (up to 43% isolated yields) and high optical purity (up to >99% ee). In addition, we also demonstrated that the bio-nitration method was able to tolerate the substrate at a high concentration of 1000 mM (150 g/L). Furthermore, representative synthesis of two optically active enantiomers of the β-adrenergic blocker metoprolol was successfully achieved by utilizing the corresponding chiral β-nitroalcohols as precursors.

The inflammatory oxidant peroxynitrous acid modulates the structure and function of the recombinant human V3 isoform of the extracellular matrix proteoglycan versican

Continued oxidant production during chronic inflammation generates host tissue damage, with this being associated with pathologies including atherosclerosis. Atherosclerotic plaques contain modified proteins that may contribute to disease development, including plaque rupture, the major cause of heart attacks and strokes. Versican, a large extracellular matrix (ECM) chondroitin-sulfate proteoglycan, accumulates during atherogenesis, where it interacts with other ECM proteins, receptors and hyaluronan, and promotes inflammation. As activated leukocytes produce oxidants including peroxynitrite/peroxynitrous acid (ONOO^-/ONOOH) at sites of inflammation, we hypothesized that versican is an oxidant target, with this resulting in structural and functional changes that may exacerbate plaque development. The recombinant human V3 isoform of versican becomes aggregated on exposure to ONOO^-/ONOOH. Both reagent ONOO^-/ONOOH and SIN-1 (a thermal source of ONOO^-/ONOOH) modified Tyr, Trp and Met residues. ONOO^-/ONOOH mainly favors nitration of Tyr, whereas SIN-1 mostly induced hydroxylation of Tyr, and oxidation of Trp and Met. Peptide mass mapping indicated 26 sites with modifications (15 Tyr, 5 Trp, 6 Met), with the extent of modification quantified at 16. Multiple modifications, including the most extensively nitrated residue (Tyr¹⁶¹), are within the hyaluronan-binding region, and associated with decreased hyaluronan binding. ONOO^-/ONOOH modification also resulted in decreased cell adhesion and increased proliferation of human coronary artery smooth muscle cells. Evidence is also presented for colocalization of versican and 3-nitrotyrosine epitopes in advanced (type II-III) human atherosclerotic plaques. In conclusion, versican is readily modified by ONOO^-/ONOOH, resulting in chemical and structural modifications that affect protein function, including hyaluronan binding and cell interactions.

Identification and quantification of protein nitration sites in human coronary artery smooth muscle cells in the absence and presence of peroxynitrous acid/peroxynitrite

Peroxynitrous acid/peroxynitrite (ONOOH/ONOO^-) is a powerful oxidizing/nitrating system formed at sites of inflammation, which can modify biological targets, and particularly proteins. Here, we show that multiple proteins from primary human coronary artery smooth muscle cells are nitrated, with LC-MS peptide mass mapping providing data on the sites and extents of changes on cellular and extracellular matrix (ECM) proteins. Evidence is presented for selective and specific nitrations at Tyr and Trp on 11 cellular proteins (out of 3668, including 205 ECM species) in the absence of added reagent ONOOH/ONOO^-, with this being consistent with low-level endogenous nitration. A number of these have key roles in cell signaling/sensing and protein turnover. With added ONOOH/ONOO^-, more proteins were modified (84 total; with 129 nitrated Tyr and 23 nitrated Trp, with multiple modifications on some proteins), with this occurring at the same and additional sites to endogenous modification. With low concentrations of ONOOH/ONOO^- (50 μM) nitration occurs on specific proteins at particular sites, and is not driven by protein or Tyr/Trp abundance, with modifications detected on some low abundance proteins. However, with higher ONOOH/ONOO^- concentrations (500 μM), modification is primarily driven by protein abundance. ECM species are major targets and over-represented in the pool of modified proteins, with fibronectin and thrombospondin-1 being particularly heavily modified (12 sites in each case). Both endogenous and exogenous nitration of cell- and ECM-derived species may have significant effects on cell and protein function, and potentially be involved in the development and exacerbation of diseases such as atherosclerosis.

Cavin-2 loss exacerbates hypoxia-induced pulmonary hypertension with excessive eNOS phosphorylation and protein nitration

Pulmonary hypertension (PH) is associated with a poor prognosis even in recent years. Caveolin-1 (CAV1), a caveolae-associated protein, is a causal gene in PH. Cavin-2, one of the other caveolae-associated proteins, forms protein complexes with CAV1 and influences each other's functions. However, the role of Cavin-2 in PH has not been thoroughly investigated. To clarify the role of Cavin-2 in PH, we exposed Cavin-2-deficient (Cavin-2 KO) mice to hypoxia. A part of the analyses was confirmed in human pulmonary endothelial cells (HPAECs). After 4-week 10% O₂ hypoxic exposure, we performed physiological, histological, and immunoblotting analyses. Right ventricular (RV) systolic pressure elevation and RV hypertrophy were exacerbated in Cavin-2 KO mice with hypoxia-induced PH (Cavin-2 KO PH mice). The vascular wall thickness of pulmonary arterioles was aggravated in Cavin-2 KO PH mice. Cavin-2 loss reduced CAV1 and induced sustained endothelial nitric oxide synthase (eNOS) hyperphosphorylation in the Cavin-2 KO PH lungs and HPAECs. NOx production associated with eNOS phosphorylation was also increased in the Cavin-2 KO PH lung and HPAECs. Furthermore, the nitration of proteins, including protein kinase G (PKG), was raised in the Cavin-2 KO PH lungs. In conclusion, we revealed that Cavin-2 loss exacerbated hypoxia-induced PH. Our results suggest that Cavin-2 loss leads to sustained eNOS hyperphosphorylation in pulmonary artery endothelial cells via CAV1 reduction, resulting in Nox overproduction-mediated nitration of proteins, including PKG, in smooth muscle cells.

Site-specific tyrosine nitration of group 1 allergens of house dust mite Dermatophagoides farinae (der f 1) and Dermatophagoides pteronyssinus (der p 1) in indoor dusts

Nitration can enhance the allergenicity of proteins. The nitration status of house dust mite (HDM) allergens in indoor dusts, however, remains to be elucidated. In the study, site-specific tyrosine nitration degrees of the two important HDM allergens Der f 1 and Der p 1 in indoor dust samples were investigated by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). The measured concentrations of native and nitrated allergens in the dusts were in the range of 0.86-29 μg g^-1 for Der f 1 and from below the detection limit to 29 μg g^-1 for Der p 1. Site-specific analysis revealed that all ten tyrosine residues in Der f 1 and Der p 1 were nitrated to different degrees in the investigated samples. The preferred nitration sites were Y₅₆ in Der f 1 and Y₃₇ in Der p 1 with the nitration degrees of 7.6-84% and 17-96% among the detected tyrosine residues, respectively. The measurements reveal high site-specific nitration degrees for tyrosine in Der f 1 and Der p 1 detected in the indoor dust samples. Further investigations are required to find out if the nitration really aggravates the health effects of HDM allergens and if the effects are tyrosine site-dependent.

Engineering Cytochrome P450BM3 Enzymes for Direct Nitration of Unsaturated Hydrocarbons

Applications of the peroxidase activity of cytochrome P450 enzymes in synthetic chemistry remain largely unexplored. We present herein a protein engineering strategy to increase cytochrome P450BM3 peroxidase activity for the direct nitration of aromatic compounds and terminal aryl-substituted olefins in the presence of a dual-functional small molecule (DFSM). Site-directed mutations of key active-site residues allowed the efficient regulation of steric effects to limit substrate access and, thus, a significant decrease in monooxygenation activity and increase in peroxidase activity. Nitration of several phenol and aniline compounds also yielded ortho- and para-nitration products with moderate-to-high total turnover numbers. Besides direct aromatic nitration by P450 variants using nitrite as a nitrating agent, we also demonstrated the use of the DFSM-facilitated P450 peroxidase system for the nitration of the vinyl group of styrene and its derivatives.

nNOS-induced tyrosine nitration of TRKB impairs BDNF signaling and restrains neuronal plasticity

Nitric oxide (NO) has been long recognized as an important modulator of neural plasticity, but characterization of the molecular mechanisms involved - specially the guanylyl cyclase-independent ones - has been challenging. There is evidence that NO could modify BDNF-TRKB signaling, a key mediator of neuronal plasticity. However, the mechanism underlying the interplay of NO and TRKB remains unclear. Here we show that NO induces nitration of the tyrosine 816 in the TRKB receptor in vivo and in vitro, and that post-translational modification inhibits TRKB phosphorylation and binding of phospholipase Cγ1 (PLCγ1) to this same tyrosine residue. Additionally, nitration triggers clathrin-dependent endocytosis of TRKB through the adaptor protein AP-2 and ubiquitination, thereby increasing translocation of TRKB away from the neuronal surface and directing it towards lysosomal degradation. Accordingly, inhibition of nitric oxide increases TRKB phosphorylation and TRKB-dependent neurite branching in neuronal cultures. In vivo, chronic inhibition of neuronal nitric oxide synthase (nNOS) dramatically reduced TRKB nitration and facilitated TRKB signaling in the visual cortex, and promoted a shift in ocular dominance upon monocular deprivation - an indicator of increased plasticity. Altogether, our data describe and characterize a new molecular brake on plasticity, namely nitration of TRKB receptors.

House dust mite allergens and nitrated products: Identification and risk assessment in indoor dust

Air pollutants can potentially lead to nitration of allergic proteins, thus promoting sensitization of these allergens. However, little is currently known about the nitration status of house dust mite (HDM) allergens. We identified the occurrence of nitrated products of two major HDM allergens Der f 1 and Der p 1 in dust samples collected from college dormitories in eastern China and assessed their associated health risk. The results showed that both non-nitrated and nitrated forms of the two allergens were detected in the dust in the range of non-detected (ND)-10.6, 1.44-15.4, ND-22.4, ND-7.28 µg/g for non-nitrated Der f 1, nitrated Der f 1, non-nitrated Der p 1 and nitrated Der p 1, respectively. The median rates of nitration were determined as 74.0% for Der f 1 and 20.4% for Der p 1 at consideration of one nitration site. Further analysis reveals that the levels of HDM allergens and their nitrated products were found to be generally higher during winter, in dormitories of lower altitude and with female occupants. Furthermore, the calculated risk indexes were at considerably high levels. Our findings suggest that nitrated HDM allergens have already accumulated in the environment at such significant levels and their associated health risk calls for our immediate attention.

Nitration of chemokine CXCL8 acts as a natural mechanism to limit acute inflammation

Chemokine CXCL8 is a key facilitator of the human host immune response, mediating neutrophil migration, and activation at the site of infection and injury. The oxidative burst is an important effector mechanism which leads to the generation of reactive nitrogen species (RNS), including peroxynitrite. The current study was performed to determine the potential for nitration to alter the biological properties of CXCL8 and its detection in human disease. Here, we show peroxynitrite nitrates CXCL8 and thereby regulates neutrophil migration and activation. The nitrated chemokine was unable to induce transendothelial neutrophil migration in vitro and failed to promote leukocyte recruitment in vivo. This reduced activity is due to impairment in both G protein-coupled receptor signaling and glycosaminoglycan binding. Using a novel antibody, nitrated CXCL8 was detected in bronchoalveolar lavage samples from patients with pneumonia. These findings were validated by mass spectrometry. Our results provide the first direct evidence of chemokine nitration in human pathophysiology and suggest a natural mechanism that limits acute inflammation.

Fast and biphasic 8-nitroguanine production from guanine and peroxynitrite

Guanine (Gua), among purines, is a preferred oxidation/nitration target because of its low one-electron redox potential. The reactive oxygen/nitrogen species peroxynitrite (ONOO^-), produced in vivo by the reaction between nitric oxide (^•NO) and superoxide radical (O₂^•‒), is responsible for several oxidative modifications in biomolecules, including nitration, nitrosation, oxidation, and peroxidation. In particular, the nitration of Gua, although detected, as well as its reaction kinetics have been seldom investigated. Thus, we studied the concentration- and temperature-dependent formation of 8-nitroguanine (8-NitroGua) in phosphate buffer (pH 7.40) using stopped-flow spectrophotometry. Traces showed a biexponential behavior, with best-fit rate constants: k_fast = 4.4 s^-1 and k_slow = 0.41 s^-1 (30 °C, 400 μM both Gua and ONOO^-). k_fast increased linearly with the concentration of both reactants whereas k_slow was concentration-independent. Linear regression analysis of k_fast as a function of Gua and ONOO^- concentration yielded values of 2.5-6.3 × 10³ M^-1s^-1 and 1.5-3.5 s^-1 for the second-order (slope) and first-order (ordinate) rate constants, respectively (30 °C). Since ONOO^- is a short-lived species, its decay kinetics was also taken into account for this analysis. The 8-NitroGua product was stable for at least 4 h, so no spontaneous denitration was observed. Stopped-flow assays using antioxidants and free-radical scavengers suggested a mixed direct/indirect reaction mechanism for 8-NitroGua formation. Gua nitration by ONOO^- was also observed in the presence of physiologically relevant CO₂ concentrations. The reaction product identity, its yield (∼4.2%, with 400 μM ONOO^- and 200 μM Gua), and the reaction mechanism were unequivocally determined by HPLC-MS/MS experiments. In conclusion, 8-NitroGua production at physiologic pH reached significant levels in a few hundred milliseconds, suggesting that the process might be kinetically relevant in vivo and can likely cause permanent nitrative damage to DNA bases.

Nitrative inactivation of thioredoxin-1 loses its protective effect in bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is common in the development of inflammatory lung diseases with no effective clinical drug treatment currently. As an essential redox enzyme, thioredoxin (Trx) has been reported to be involved in pulmonary fibrosis, but the mechanism is to be revealed. Therefore, in bleomycin-indued pulmonary fibrosis model in C57 mice, Trx activity and nitrated Trx were examined.,p38-MAPK apoptosis pathway was determined in lung tissues. Additionally, before BLM administration, C57/BL6 mice were treated with aminoguanidine (AG, a peroxynitrite scavenger), recombinant human Trx-1 (rhTrx-1), or SIN-1 (a peroxynitrite donor) nitrated Trx-1 (N-Trx-1). In bleomycin (BLM)-induced pulmonary fibrosis model in C57/BL6 mice, we observed that nitrated Trx increased, while its activity decreased, with the increase of alveolar epithelial cells (AECs)apoptosis by p38-MAPK pathway. We demonstrated that AG or rhTrx-1, but not N-Trx-1 significantly reduced pulmonary fibrosis. Taken together, the results above revealed that blockade of Trx-1 nitration, or supplementation of exogenous rhTrx-1, might represent novel therapies to attenuate pulmonary fibrosis in idiopathic pulmonary fibrosis patients.

GLP-1 receptor nitration contributes to loss of brain pericyte function in a mouse model of diabetes

AIMS/HYPOTHESIS

We have previously shown that diabetes causes pericyte dysfunction, leading to loss of vascular integrity and vascular cognitive impairment and dementia (VCID). Glucagon-like peptide-1 (GLP-1) receptor agonists (GLP-1 RAs), used in managing type 2 diabetes mellitus, improve the cognitive function of diabetic individuals beyond glycaemic control, yet the mechanism is not fully understood. In the present study, we hypothesise that GLP-1 RAs improve VCID by preventing diabetes-induced pericyte dysfunction.

METHODS

Mice with streptozotocin-induced diabetes and non-diabetic control mice received either saline (NaCl 154 mmol/l) or exendin-4, a GLP-1 RA, through an osmotic pump over 28 days. Vascular integrity was assessed by measuring cerebrovascular neovascularisation indices (vascular density, tortuosity and branching density). Cognitive function was evaluated with Barnes maze and Morris water maze. Human brain microvascular pericytes (HBMPCs), were grown in high glucose (25 mmol/l) and sodium palmitate (200 μmol/l) to mimic diabetic conditions. HBMPCs were treated with/without exendin-4 and assessed for nitrative and oxidative stress, and angiogenic and blood-brain barrier functions.

RESULTS

Diabetic mice treated with exendin-4 showed a significant reduction in all cerebral pathological neovascularisation indices and an improved blood-brain barrier (p<0.05). The vascular protective effects were accompanied by significant improvement in the learning and memory functions of diabetic mice compared with control mice (p<0.05). Our results showed that HBMPCs expressed the GLP-1 receptor. Diabetes increased GLP-1 receptor expression and receptor nitration in HBMPCs. Stimulation of HBMPCs with exendin-4 under diabetic conditions decreased diabetes-induced vascular inflammation and oxidative stress, and restored pericyte function (p<0.05).

CONCLUSIONS/INTERPRETATION

This study provides novel evidence that brain pericytes express the GLP-1 receptor, which is nitrated under diabetic conditions. GLP-1 receptor activation improves brain pericyte function resulting in restoration of vascular integrity and BBB functions in diabetes. Furthermore, the GLP-1 RA exendin-4 alleviates diabetes-induced cognitive impairment in mice. Restoration of pericyte function in diabetes represents a novel therapeutic target for diabetes-induced cerebrovascular microangiopathy and VCID.

Influence of plasma halide, pseudohalide and nitrite ions on myeloperoxidase-mediated protein and extracellular matrix damage

Myeloperoxidase (MPO) mediates pathogen destruction by generating the bactericidal oxidant hypochlorous acid (HOCl). Formation of this oxidant is however associated with host tissue damage and disease. MPO also utilizes H₂O₂ to oxidize other substrates, and we hypothesized that mixtures of other plasma anions, including bromide (Br^-), iodide (I^-), thiocyanate (SCN^-) and nitrite (NO₂^-), at normal or supplemented concentrations, might modulate MPO-mediated HOCl damage. For the (pseudo)halide anions, only SCN^- significantly modulated HOCl formation (IC₅₀ ∼33 μM), which is within the normal physiological range, as judged by damage to human plasma fibronectin or extracellular matrix preparations detected by ELISA and LC-MS. NO₂^- modulated HOCl-mediated damage, in a dose-dependent manner, at physiologically-attainable anion concentrations. However, this was accompanied by increased tyrosine and tryptophan nitration (detected by ELISA and LC-MS), and the overall extent of damage remained approximately constant. Increasing NO₂^- concentrations (0.5-20 μM) diminished HOCl-mediated modification of tyrosine and methionine, whereas tryptophan loss was enhanced. At higher NO₂^- concentrations, enhanced tyrosine and methionine loss was detected. These analytical data were confirmed in studies of cell adhesion and metabolic activity. Together, these data indicate that endogenous plasma levels of SCN^- (but not Br^- or I^-) can modulate protein modification induced by MPO, including the extent of chlorination. In contrast, NO₂^- alters the type of modification, but does not markedly decrease its extent, with chlorination replaced by nitration. These data also indicate that MPO could be a major source of nitration in vivo, and particularly at inflammatory sites where NO₂^- levels are often elevated.

Photochemical synthesis of pink silver and its use for monitoring peptide nitration via surface enhanced Raman spectroscopy (SERS)

Oxidative stress may cause extended tyrosine posttranslational modifications of peptides and proteins. The 3-nitro-L-tyrosine (Nit), which is typically formed, affects protein behavior during neurodegenerative processes, such as Alzheimer's and Parkinson's diseases. Such metabolic products may be conveniently detected at very low concentrations by surface enhanced Raman spectroscopy (SERS). Previously, we have explored the SERS detection of the Nit NO₂ bending vibrational bands in a presence of hydrogen chloride (Niederhafner et al., Amino Acids 53:517-532, 2021, ibid). In this article, we describe performance of a new SERS substrate, "pink silver", synthesized photochemically. It provides SERS even without the HCl induction, and the acid further decreases the detection limit about 9 times. Strong SERS bands were observed in the asymmetric (1550-1475 cm^-1) and symmetric (1360-1290 cm^-1) NO stretching in the NO₂ group. The bending vibration was relatively weak, but appeared stronger when HCl was added. The band assignments were supported by density functional theory modeling.

Syntheses and high selective cytotoxicity of dehydroabietylamine C-ring nitration derivatives

To find more effective anticancer agents, a series of novel dehydroabietylamine (DA) derivatives were synthesized, focusing on C-ring nitro modifications and C-18 imide introduction. Their cytotoxic activities against human tumor cell line HeLa (cervix), MCF-7 (breast), A549 (lung), HepG2 (liver), and nonmalignant cell line HUVEC (umbilical vein) in vitro were screened. The C-18 imide heterocyclic compounds 1, 2, and C-ring 14-nitro substituted 14 exhibited moderate to good cytotoxic activities and significant selectivity towards malignant cell lines. More importantly, they were significantly less cytotoxic to nonmalignant cells (HUVEC) than the parent compound and positive control doxorubicin hydrochloride (DOX). Meantime the mechanism of cytotoxicity of DA derivatives was studied. Annexin V-FITC/PI double-staining analysis suggested that cytotoxicity of compounds 2 and 14 was associated with early apoptosis induction. The interaction between compounds and DNA (herring sperm DNA) was studied using absorption spectral analysis and ethidium bromide (EB) fluorescence displacement experiments, the results exhibited that the binding of the compound to DNA was in the intercalative mode. The structure-activity relationship discussion implied that introduction of the nitro-group, especially the 14-nitro group, can significantly improve the cytotoxicity of dehydroabietylimide compounds. The relatively high cytotoxicity and significant high selectivity of compounds 2 and 14 indicated that they were particularly noteworthy. NO released amounts indicated that the amounts of NO released by the compounds bearing nitro-group were quite well associated positive correlation with their cytotoxic activity, which provide a new strategy for structure design of DA anticancer agents in the future.

Photodegradation of benzophenones sensitized by nitrite

Benzophenone UV filters (BPs) are a group of contaminants of emerging concern due to their widespread occurrence and adverse effects on aquatic ecosystems. In this study, the transformation of BPs by nitrite sensitized photodegradation was comprehensively investigated. OH and NO₂ generated by nitrite photolysis reacted with BPs, forming hydroxylated and nitrated products, respectively. Kinetic modeling revealed that the steady-state concentrations of NO₂ were approximately six orders of magnitude higher than those of OH in the UV/nitrite process, although the second-order rate constants of NO₂ reactions with BPs were six orders of magnitude lower. With the increase in nitrite concentration, BPs degradation was accelerated, and the contribution of NO₂ increased as well. At initial nitrite concentration of 10 μM, the contributions of OH and NO₂ to the degradation of 2,4-dihydroxybenzophenone (BP1) were 66.1% and 21.5%, respectively. However, NO₂ only contributed a tiny fraction to the degradation of 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid (BP4), due to the presence of an electron-withdrawing sulfonate group in the molecule. Natural organic matter (NOM) inhibited the nitrite sensitized degradation of BPs, due to light screening and radical scavenging effects. This study suggests that BPs can be effectively transformed in sunlit waters in the presence of nitrite, leading to nitrated products.

Site-selective and metal-free C-H nitration of biologically relevant N-heterocycles

The site-selective and metal-free C-H nitration reaction of quinoxalinones and pyrazinones as biologically important N-heterocycles with t-butyl nitrite is described. A wide range of quinoxalinones were efficiently applied in this transformation, providing C7-nitrated quinoxalinones without undergoing C3-nitration. From the view of mechanistic point, the radical addition reaction exclusively occurred at the electron-rich aromatic region beyond electron-deficient N-heterocycle ring. This is a first report on the C7-H functionalization of quinoxalinones under metal-free conditions. In contrast, the nitration reaction readily takes place at the C3-position of pyrazinones. This transformation is characterized by the scale-up compatibility, mild reaction conditions, and excellent functional group tolerance. The applicability of the developed method is showcased by the selective reduction of NO₂ functionality on the C7-nitrated quinoxalinone product, providing aniline derivatives. Combined mechanistic investigations aided the elucidation of a plausible reaction mechanism.

Expanding the Substrate Scope of Nitrating Cytochrome P450 TxtE by Active Site Engineering of a Reductase Fusion

Aromatic nitration reactions are a cornerstone of organic chemistry, but are challenging to scale due to corrosive reagents and elevated temperatures. The cytochrome P450 TxtE nitrates the indole 4-position of l-tryptophan at room temperature using NO, O2 and NADPH, and has potential to be developed into a useful aromatic nitration biocatalyst. However, its narrow substrate scope (requiring both the α-amino acid and indole functionalities) have hindered this. Screening of an R59 mutant library of a TxtE-reductase fusion protein identified a variant (R59C) that nitrates tryptamine, which is not accepted by native TxtE. This variant exhibits a broader substrate scope than the wild type enzyme and is able to nitrate a range of tryptamine analogues, with significant alterations to the aromatic and aminoethyl moieties.

Clinical relevance of nitrated beta 2-glycoprotein I in antiphospholipid syndrome: Implications for thrombosis risk

Β₂-Glycoprotein I (β₂GPI) is an important anti-thrombotic protein and is the major auto-antigen in the antiphospholipid syndrome (APS). The clinical relevance of nitrosative stress in post translational modification of β₂GPI was examined.The effects of nitrated (n)β₂GPI on its anti-thrombotic properties and its plasma levels in primary and secondary APS were determined with appropriate clinical control groups. β2-glycoprotein I was nitrated at tyrosines 218, 275 and 309. β₂-glycoprotein I binds to lipid peroxidation modified products through Domains IV and V. Nitrated β₂GPI loses this binding (p < 0.05) and had diminished activity in inhibiting platelet adhesion to vWF under high shear flow (p < 0.01). Levels of nβ₂GPI were increased in patients with primary APS compared to patients with either secondary APS (p < 0.05), autoimmune disease without APS (p < 0.05) or non-autoimmune patients with arterial thrombosis (p < 0.01) and healthy individuals (p < 0.05).In conclusion tyrosine nitration of plasma β₂GPI is demonstrated and has important implications with regards to the pathophysiology of platelet mediated thrombosis in APS. Elevated plasma levels of nβ₂GPI in primary APS may be a risk factor for thrombosis warranting further investigation.

Abnormal nitration and S-sulfhydration modification of Sp1-CSE-H2S pathway trap the progress of hyperhomocysteinemia into a vicious cycle

Sp1-CSE-H2S pathway plays an important role in homocysteine-metabolism, whose disorder can result in hyperhomocysteinemia. H2S deficiency in hyperhomocysteinemia has been reported, while the underlying mechanism and whether it in turn affects the progress of hyperhomocysteinemia are unclear. This study focused on the post-translational modification of Sp1/CSE and revealed four major findings: (1) Homocysteine-accumulation augmented CSE's nitration, inhibited its bio-activity, thus caused H2S deficiency. (2) H2S deficiency inhibited the S-sulfhydration of Sp1, down-regulated CSE and decreased H2S further, which in turn weakened CSE's own S-sulfhydration. (3) CSE was S-sulfhydrated at Cys84, Cys109, Cys172, Cys229, Cys252, Cys307 and Cys310, among which the S-sulfhydration of Cys172 and Cys310 didn't affect its enzymatic activity, while the S-sulfhydration of Cys84, Cys109, Cys229, Cys252 and Cys307 was necessary for its bio-activity. (4) H2S deficiency trapped homocysteine-metabolism into a vicious cycle, which could be broken by either blocking nitration or restoring S-sulfhydration. This study detected a new mechanism that caused severe hyperhomocysteinemia, thereby provided new therapeutic strategies for hyperhomocysteinemia.

Mass spectrometry-based study defines the human urine nitrolipidome

Nitrated fatty acids (NO2-FA) are an endogenous class of signaling mediators formed mainly during digestion and inflammation. The signaling actions of NO2-FA have been extensively studied, but their detection and characterization lagged. Several different nitrated fatty acid species have been reported in animals and humans, but their formation remains controversial, and a systemic approach to define the endogenous pool of NO2-FA is needed. Herein, we screened for endogenous NO2-FA in urine from healthy human volunteers as this is the main excretion route for NO2-FA and its metabolites, and it provides an excellent matrix for evaluation. Only isomers of two fatty acids, conjugated linoleic and linolenic acid were found to be nitrated. Several, previously unknown, nitrated species were identified and confirmed using high-resolution mass spectrometry, fragmentation analysis, and compared to synthetic nitrated standards, the main group corresponding to nitrated conjugated linolenic acid (NO2-CLnA). In contrast, we were unable to confirm the presence of previously reported nitrated omega-3's, oleic acid, arachidonic acid and α- and γ-linolenic acid, suggesting that their biological formation and presence in humans should be re-evaluated. Metabolite analysis of NO2-CLnA in human urine identified cysteine adducts and β-oxidation products, which were compared to the metabolic products of nitrated standards obtained using primary mouse hepatocytes. Importantly, NO2-CLnA isomers belong to two defined groups, are electrophilic, participate in Michael addition reactions and account for 39% of total urinary NO2-FA, highlighting their relative abundance and possible role in cell signaling.

Monitoring peptide tyrosine nitration by spectroscopic methods

Oxidative stress can lead to various derivatives of the tyrosine residue in peptides and proteins. A typical product is 3-nitro-L-tyrosine residue (Nit), which can affect protein behavior during neurodegenerative processes, such as those associated with Alzheimer's and Parkinson's diseases. Surface enhanced Raman spectroscopy (SERS) is a technique with potential for detecting peptides and their metabolic products at very low concentrations. To explore the applicability to Nit, we use SERS to monitor tyrosine nitration in Met-Enkephalin, rev-Prion protein, and α-synuclein models. Useful nitration indicators were the intensity ratio of two tyrosine marker bands at 825 and 870 cm^-1 and a bending vibration of the nitro group. During the SERS measurement, a conversion of nitrotyrosine to azobenzene containing peptides was observed. The interpretation of the spectra has been based on density functional theory (DFT) simulations. The CAM-B3LYP and ωB97XD functionals were found to be most suitable for modeling the measured data. The secondary structure of the α-synuclein models was monitored by electronic and vibrational circular dichroism (ECD and VCD) spectroscopies and modeled by molecular dynamics (MD) simulations. The results suggest that the nitration in these peptides has a limited effect on the secondary structure, but may trigger their aggregation.

RAC1 nitration at Y32 IS involved in the endothelial barrier disruption associated with lipopolysaccharide-mediated acute lung injury

Acute lung injury (ALI), a devastating illness induced by systemic inflammation e.g., sepsis or local lung inflammation e.g., COVID-19 mediated severe pneumonia, has an unacceptably high mortality and has no effective therapy. ALI is associated with increased pulmonary microvascular hyperpermeability and alveolar flooding. The small Rho GTPases, RhoA and Rac1 are central regulators of vascular permeability through cytoskeleton rearrangements. RhoA and Rac1 have opposing functional outcome: RhoA induces an endothelial contractile phenotype and barrier disruption, while Rac1 stabilizes endothelial junctions and increases barrier integrity. In ALI, RhoA activity is increased while Rac1 activity is reduced. We have shown that the activation of RhoA in lipopolysaccharide (LPS)-mediated ALI, is dependent, at least in part, on a single nitration event at tyrosine (Y)³⁴. Thus, the purpose of this study was to determine if the inhibition of Rac1 is also dependent on its nitration. Our data show that Rac1 inhibition by LPS is associated with its nitration that mass spectrometry identified as Y³², within the switch I region adjacent to the nucleotide-binding site. Using a molecular modeling approach, we designed a nitration shielding peptide for Rac1, designated NipR2 (nitration inhibitor peptide for the Rho GTPases 2), which attenuated the LPS-induced nitration of Rac1 at Y³², preserves Rac1 activity and attenuates the LPS-mediated disruption of the endothelial barrier in human lung microvascular endothelial cells (HLMVEC). Using a murine model of ALI induced by intratracheal installation of LPS we found that NipR2 successfully prevented Rac1 nitration and Rac1 inhibition, and more importantly attenuated pulmonary inflammation, reduced lung injury and prevented the loss of lung function. Together, our data identify a new post-translational mechanism of Rac1 inhibition through its nitration at Y³². As NipR2 also reduces sepsis induced ALI in the mouse lung, we conclude that Rac1 nitration is a therapeutic target in ALI.

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Endotoxin exposure induces site specific nitration of Rac1 at Y³² via peroxynitrite stress.

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Rac1 nitration at Y³² leads to persistent Rac GTPase inhibition and endothelial barrier disruption.

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Novel Rac1 nitration shielding peptide, NipR2 blocks Rac1 nitration and rescues endotoxin induced lung inflammation.

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NipR2 is potentially an effective therapy for sepsis induced lung injury by targeting Rac1 Y³² nitration.

Plant catalases as NO and H2S targets

Catalase is a powerful antioxidant metalloenzyme located in peroxisomes which also plays a central role in signaling processes under physiological and adverse situations. Whereas animals contain a single catalase gene, in plants this enzyme is encoded by a multigene family providing multiple isoenzymes whose number varies depending on the species, and their expression is regulated according to their tissue/organ distribution and the environmental conditions. This enzyme can be modulated by reactive oxygen and nitrogen species (ROS/RNS) as well as by hydrogen sulfide (H2S). Catalase is the major protein undergoing Tyr-nitration [post-translational modification (PTM) promoted by RNS] during fruit ripening, but the enzyme from diverse sources is also susceptible to undergo other activity-modifying PTMs. Data on S-nitrosation and persulfidation of catalase from different plant origins are given and compared here with results from obese children where S-nitrosation of catalase occurs. The cysteine residues prone to be S-nitrosated in catalase from plants and from bovine liver have been identified. These evidences assign to peroxisomes a crucial statement in the signaling crossroads among relevant molecules (NO and H2S), since catalase is allocated in these organelles. This review depicts a scenario where the regulation of catalase through PTMs, especially S-nitrosation and persulfidation, is highlighted.

Absolute quantitative analysis of intact and oxidized amino acids by LC-MS without prior derivatization

The precise characterization and quantification of oxidative protein damage is a significant challenge due to the low abundance, large variety, and heterogeneity of modifications. Mass spectrometry (MS)-based techniques at the peptide level (proteomics) provide a detailed but limited picture due to incomplete sequence coverage and imperfect enzymatic digestion. This is particularly problematic with oxidatively modified and cross-linked/aggregated proteins. There is a pressing need for methods that can quantify large numbers of modified amino acids, which are often present in low abundance compared to the high background of non-damaged amino acids, in a rapid and reliable fashion. We have developed a protocol using zwitterionic ion-exchange chromatography coupled with LC-MS to simultaneously quantify both parent amino acids and their respective oxidation products. Proteins are hydrolyzed with methanesulfonic acid in the presence of tryptamine and purified by strong cation exchange solid phase extraction. The method was validated for the common amino acids (excluding Gln, Asn, Cys) and the oxidation products 3-chlorotyrosine (3-ClTyr), 3-nitrotyrosine (3-NO₂Tyr), di-tyrosine, N^ε-(1-carboxymethyl)-l-lysine, o,o’-di-tyrosine, 3,4,-dihydroxyphenylalanine, hydroxy-tryptophan and kynurenine. Linear standard curves were observed over ~3 orders of magnitude dynamic range (2–1000 pmol for parent amino acids, 80 fmol–20 pmol for oxidation products) with limit-of-quantification values as low as 200 fmol (o,o’-di-tyrosine). The validated method was used to quantify Tyr and Trp loss, and formation of 3-NO₂Tyr on the isolated protein anastellin treated with peroxynitrous acid, and for 3-ClTyr formation (over a 2 orders of magnitude range) in cell lysates and complex protein mixtures treated with hypochlorous acid.

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Identification and quantification of oxidative protein damage is a major challenge.

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A versatile LC-MS assay is reported that involves hydrolysis to free amino acids.

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Quantification is possible for both parent amino acids and products in single runs.

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A dynamic range of 2-3 orders of magnitude is available for most analytes.

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Example of use with pure proteins, extracellular matrix and cell lysates are given.

Enzyme Immobilization over Polystyrene Surface Using Cysteine Functionalized Copper Nanoparticle as a Linker Molecule

The work focus on the development of a simple and efficient method of enzyme immobilization over a polystyrene surface using cysteine functionalized copper nanoparticle as linker molecule. The polystyrene surface is activated by generating -NO₂ groups by the process of nitration reaction. The nitrated polystyrene plate then is silanized with (3-mercaptopropyl) trimethoxysilane (MPTS) followed with the coupling of cysteine-capped copper nanoparticles on the silanized surface through thiol moiety. A nanoparticle layer is thus created over the polystyrene surface which is efficiently used for covalent immobilization of urease via an amino group of cysteine through glutaraldehyde treatment. The technique resulted in an enhancement in the enzymatic activity by 72.37% over the soluble counterpart. The immobilized enzyme also exhibited appreciable reusability of about 10 times with activity retention of 82% of its initial activity. Immobilization also offered an increased thermal and pH stability to the immobilized enzyme over the soluble enzyme.

Liquid chromatography/tandem mass spectrometry characterization of nitroso, nitrated and nitroxidized cardiolipin products

Cardiolipins (CL) are anionic dimeric phospholipids bearing four fatty acids, found in inner mitochondrial membrane as structural components and are involved in several processes as oxidative phosphorylation or apoptotic signalling. As other phospholipids, CL can be modified by reactive oxygen species (ROS) and reactive nitrogen species (RNS), which can modulate various cellular functions. Modifications of CL by RNS remain largely unstudied although other nitrated lipids are emerging as bioactive molecules. In this work, we developed a C30-LC-HRMS/MS methodology to identify the nitrated and nitroxidized tetralinoleoyl-cardiolipin (TLCL), using a biomimetic model of nitration, and to disclose specific fragmentation pathways under HCD MS/MS. Using this lipidomics approach, we were able to separate and identify nitro, nitroso, nitronitroso, and nitroxidized TLCL derivatives, comprising 11 different nitrated compounds. These products were identified using accurate mass measurements and the fragmentation pattern acquired in higher-energy collision dissociation (HCD)-tandem MS/MS experiments. These spectra showed classifying fragmentation pathways, yielding phosphatidic acid (PA^-), lysophosphatidic acid (LPA^-), and carboxylate fragment ions with the modifying moiety. Remarkably, the typical neutral losses associated with the added moieties were not observed. In conclusion, this work has developed a new method for the identification of nitroso, nitrated and nitroxidized cardiolipin products by using a C30LC-MS platform method, potentially allowing their detection in biological samples.

Nitration of Drp1 provokes mitophagy activation mediating neuronal injury in experimental autoimmune encephalomyelitis

Active autophagy/mitophagy could mediate neurodegeneration and motor disabilities in multiple sclerosis (MS). Mitochondrial recruitment of dynamin-related protein 1 (Drp1) is a crucial step to initiate mitophagy. Peroxynitrite (ONOO^-) could be a player in MS pathology but the mechanisms remain unknown. We used animal model of experimental autoimmune encephalomyelitis (EAE) and tested whether ONOO^- mediates Drp1 assembly in mitochondria for mitophagy and aggravates MS pathology. We found that autophagy/mitophagy activation was coincidently increased with axonal damage, apoptosis and disease progression in active EAE mice, which were remarkably attenuated by mitochondrial division/mitophagy inhibitor Mdivi-1. Importantly, increased ONOO^- production was companied with Drp1 mitochondrial recruitment, PINK1/Parkin-mediated mitophagy, axonal degeneration and neuronal cell death, which were reversed by peroxynitrite decomposition catalyst (PDC). Furthermore, ONOO^- production induced Drp1 nitration, promoted Drp1 assembly and mitochondrial recruitment for mitophagy activation, contributing to the EAE pathology. Together, we conclude that ONOO^- serves as a key mediator in Drp1 nitration modification and assembly for facilitating mitophagy activation. Targeting ONOO^--mediated Drp1 assembly and mitochondrial recruitment could be an important therapeutic strategy for multiple sclerosis treatment.

In search of the mRNA modification landscape in plants

BACKGROUND

Precise regulation of gene expression is indispensable for the proper functioning of organisms in both optimal and challenging conditions. The most commonly known regulative mechanisms include the modulation of transcription, translation and adjustment of the transcript, and protein half-life. New players have recently emerged in the arena of gene expression regulators - chemical modifications of mRNAs.

MAIN TEXT

The latest studies show that modified ribonucleotides affect transcript splicing, localization, secondary structures, interaction with other molecules and translation efficiency. Thus far, attention has been focused mostly on the most widespread mRNA modification - adenosine methylation at the N⁶ position (m⁶A). However, initial reports on the formation and possible functions of other modified ribonucleotides, such as cytosine methylated at the 5' position (m⁵C), 8-hydroxyguanosine (8-OHG) and 8-nitroguanosine (8-NO₂G), have started to appear in the literature. Additionally, some reports indicate that pseudouridine (Ψ) is present in mRNAs and might perform important regulatory functions in eukaryotic cells. The present review summarizes current knowledge regarding the above-mentioned modified ribonucleotides (m⁶A, m⁵C, 8-OHG, 8-NO₂G) in transcripts across various plant species, including Arabidopsis, rice, sunflower, wheat, soybean and potato.

CONCLUSIONS

Chemical modifications of ribonucleotides affect mRNA stability and translation efficiency. They thus constitute a newly discovered layer of gene expression regulation and have a profound effect on the development and functioning of various organisms, including plants.

Human myeloperoxidase (hMPO) is expressed in neurons in the substantia nigra in Parkinson's disease and in the hMPO-α-synuclein-A53T mouse model, correlating with increased nitration and aggregation of α-synuclein and exacerbation of motor impairment

α-Synuclein (αSyn) is central to the neuropathology of Parkinson's disease (PD) due to its propensity for misfolding and aggregation into neurotoxic oligomers. Nitration/oxidation of αSyn leads to dityrosine crosslinking and aggregation. Myeloperoxidase (MPO) is an oxidant-generating enzyme implicated in neurodegenerative diseases. In the present work we have examined the impact of MPO in PD through analysis of postmortem PD brain and in a novel animal model in which we crossed a transgenic mouse expressing the human MPO (hMPO) gene to a mouse expressing human αSyn-A53T mutant (A53T) (hMPO-A53T). Surprisingly, our results show that in PD substantia nigra, the hMPO gene is expressed in neurons containing aggregates of nitrated αSyn as well as MPO-generated HOCl-modified epitopes. In our hMPO-A53T mouse model, we also saw hMPO expression in neurons but not mouse MPO. In the mouse model, hMPO was expressed in neurons colocalizing with nitrated αSyn, carbamylated lysine, nitrotyrosine, as well as HOCl-modified epitopes/proteins. RNAscope in situ hybridization confirmed hMPO mRNA expression in neurons. Interestingly, the hMPO protein expressed in hMPO-A53T brain is primarily the precursor proMPO, which enters the secretory pathway potentially resulting in interneuronal transmission of MPO and oxidative species. Importantly, the hMPO-A53T mouse model, when compared to the A53T model, exhibited significant exacerbation of motor impairment on rotating rods, balance beams, and wire hang tests. Further, hMPO expression in the A53T model resulted in earlier onset of end stage paralysis. Interestingly, there was a high concentration of αSyn aggregates in the stratum lacunosum moleculare of hippocampal CA2 region, which has been associated in humans with accumulation of αSyn pathology and neural atrophy in dementia with Lewy bodies. This accumulation of αSyn aggregates in CA2 was associated with markers of endoplasmic reticulum (ER) stress and the unfolded protein response with expression of activating transcription factor 4 (ATF4), C/EBP homologous protein (CHOP), MPO, and cleaved caspase-3. Together these findings suggest that MPO plays an important role in nitrative and oxidative damage that contributes to αSyn pathology in synucleinopathies.

Reductive modification of genetically encoded 3-nitrotyrosine sites in alpha synuclein expressed in E.coli

Tyrosine nitration is a post-translational protein modification relevant to various pathophysiological processes. Chemical nitration procedures have been used to generate and study nitrated proteins, but these methods regularly lead to modifications at other amino acid residues. A novel strategy employs a genetic code modification that allows incorporation of 3-nitrotyrosine (3-NT) during ribosomal protein synthesis to generate a recombinant protein with defined 3-NT-sites, in the absence of other post-translational modifications. This approach was applied to study the generation and stability of the 3-NT moiety in recombinant proteins produced in E.coli. Nitrated alpha-synuclein (ASYN) was selected as exemplary protein, relevant in Parkinson's disease (PD). A procedure was established to obtain pure tyrosine-modified ASYN in mg amounts. However, a rapid (t_1/2 = 0.4 h) reduction of 3-NT to 3-aminotyrosine (3-AT) was observed. When screening for potential mechanisms, we found that 3-NT can be reduced enzymatically to 3-AT, whilst biologically relevant low molecular weight reductants, such as NADPH or GSH, did not affect 3-NT. A genetic screen for E.coli proteins, involved in the observed 3-NT reduction, revealed the contribution of several, possibly redundant pathways. Green fluorescent protein was studied as an alternative model protein. These data confirm 3-NT reduction as a broadly-relevant pathway in E.coli. In conclusion, incorporation of 3-NT as a genetically-encoded non-natural amino acid allows for generation of recombinant proteins with specific nitration sites. The potential reduction of the 3-NT moiety by E.coli, however, requires attention to the design of the purification strategy for obtaining pure nitrated protein.

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3-nitrotyrosine in proteins is reduced to 3-aminotyrosine in E.coli.

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3-aminotyrosine is stable in E.coli.

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3-nitrotyrosine reduction is independent of the harboring protein but affected by protein structure.

Analysis of Bisulfite Via a Nitro Derivative of Cyanine-3 (NCy3) in the Microfluidic Channel

NCy3, a derivative of Cyanine 3 with a nitro substituent, showed a high reactivity to bisulfite in aqueous media, instantly leading to ratiometric change of absorption spectra and significant fluorescence quenching. Applied in the microfluidic channel, NCy3 functionalize as a sensitive approach for quantitative detection of bisulfite, particularly for samples with a small volume.

Nitrated alpha-synuclein in minor salivary gland biopsies in Parkinson's disease

Alpha-synuclein (α-SYN) is found in peripheral autonomic neuronal network apart from brain in Parkinson's disease (PD). Nitrated α-SYN is an undesirable modification associated with oxidative and nitrative damage and has been found extensively in brain, gastrointestinal(GI) tract and blood cells in PD. We aim to investigate the presence of nitrated α-SYN in minor salivary gland biopsy in PD. Patients with PD and age-matched controls underwent minor salivary gland biopsy. Motor impairment was assessed by Hoehn-Yahr (H-Y) stage and Unified Parkinson's Disease Rating Scale (UPDRS) Part III in off-state. 11C-methyl-N-2b-carbomethoxy-3b-(4-fluorophenyl) tropane (11C-CFT) DAT-PET scan was performed in all subjects. Immunohistochemical staining for nitrated α-SYN was performed in the minor salivary gland tissues. The minor salivary gland tissues of 8 PD cases and 7 controls with early stage (H-Y stage 1-2) were detected. All PD patients showed asymmetrical and reduction of ¹¹C-CFT uptake in the caudate, anterior and posterior putamen, while all control subjects showed normal DAT-PET scan. Positive nitrated α-SYN immunostaining was observed in all PD patients (8/8,100%) but not in control subjects (0/7). The results were consistent well with that of DAT-PET. These nitrated alpha-synuclein positive structures were mainly located in the periacinar stroma in PD patients. Our result suggests that nitrated α-SYN exists in the early stage and is probably a promising biomarker for PD. Minor salivary gland is an ideal site for α-SYN nitration detection. Despite of the small number of subjects, attention should be given to α-SYN nitration in PD and more investigations on nitrated α-SYN in different sites and large sample using should be explored in future.

Sevoflurane preconditioning induces tolerance to brain ischemia partially via inhibiting thioredoxin-1 nitration

Background

Sevoflurane preconditioning induces brain ischemic tolerance, but the mechanism remains poorly elucidated. Nitration is an important form of post-translational modification in pathological signaling. This study was to investigate the role of thioredoxin-1 (Trx-1) nitration in neuroprotection effect induced by sevoflurane preconditioning in a transient stroke model in rats.

Methods

Adult male Sprague–Dawley rats were preconditioned with 2% sevoflurane or vehicle oxygen exposure, 1 h per day, for 5 consecutive days. At 24 h after the last exposure, rats were subjected to focal brain ischemia induced by middle cerebral artery occlusion (MCAO) for 90 min, followed by 72-h reperfusion. Trx-1 expression and activity, as well as the content of nitrotyrosine at penumbra were detected at 24 h after preconditioning and 2, 8, 24, 72 h after MCAO. Nitrated Trx-1 was examined by immunoprecipitation at 8 h after MCAO. The role of Trx-1 nitration in ischemic tolerance was assessed by administration of nitrated human-Trx-1 prior to MCAO. Neurological scores, brain infarct volumes and TUNEL staining were evaluated at 24 h after reperfusion.

Results

Ischemic stroke decreased Trx-1 activity but not the expression in penumbra tissue. The content of nitrotyrosine was elevated after MCAO. Preconditioning with sevoflurane increased Trx-1 activity and reduced its nitration at 8 h after MCAO in comparison with vehicle preconditioning. The decrement of Trx-1 activity was correlated with its nitration level. Exogenous administration of nitrated human-Trx-1 reversed the brain ischemic tolerance of sevoflurane preconditioning, exacerbating brain infarct volume, neurobehavioral defects and apoptosis, while administration of human-Trx-1 had no effect on the sevoflurane preconditioning-induced neuroprotection.

Conclusion

Ischemic stroke reduces Trx-1 activity via post-translational nitrative modulation in rats. Sevoflurane preconditioning induces brain ischemic tolerance and anti-apoptosis by partially preserving Trx-1 activity via inhibiting nitration.

Endogenous hydrogen sulfide (H2S) is up-regulated during sweet pepper (Capsicum annuum L.) fruit ripening. In vitro analysis shows that NADP-dependent isocitrate dehydrogenase (ICDH) activity is inhibited by H2S and NO

Like nitric oxide (NO), hydrogen sulfide (H₂S) has been recognized as a new gasotransmitter which plays an important role as a signaling molecule in many physiological processes in higher plants. Although fruit ripening is a complex process associated with the metabolism of reactive oxygen species (ROS) and nitrogen oxygen species (RNS), little is known about the potential involvement of endogenous H₂S. Using sweet pepper (Capsicum annuum L.) as a model non-climacteric fruit during the green and red ripening stages, we studied endogenous H₂S content and cytosolic l-cysteine desulfhydrase (L-DES) activity which increased by 14% and 28%, respectively, in red pepper fruits. NADPH is a redox compound and key cofactor required for cell growth, proliferation and detoxification. We studied the NADPH-regenerating enzyme, NADP-isocitrate dehydrogenase (NADP-ICDH), whose activity decreased by 34% during ripening. To gain a better understanding of its potential regulation by H₂S, we obtained a 50-75% ammonium sulfate-enriched protein fraction containing the NADP-ICDH protein; with the aid of in vitro assays in the presence of H₂S, we observed that 2 and 10 mM NaHS used as H₂S donors resulted in a decrease of up to 36% and 45%, respectively, in NADP-ICDH activity, which was unaffected by reduced glutathione (GSH). On the other hand, peroxynitrite (ONOO^-), S-nitrosocyteine (CysNO) and DETA-NONOate, with the last two acting as NO donors, also inhibited NADP-ICDH activity. In silico analysis of the tertiary structure of sweet pepper NADP-ICDH activity (UniProtKB ID A0A2G2Y555) suggests that residues Cys133 and Tyr450 are the most likely potential targets for S-nitrosation and nitration, respectively. Taken together, the data reveal that the increase in the H₂S production capacity of red fruits is due to higher L-DES activity during non-climacteric pepper fruit ripening. In vitro assays appear to show that H₂S inhibits NADP-ICDH activity, thus suggesting that this enzyme may be regulated by persulfidation, as well as by S-nitrosation and nitration. NO and H₂S may therefore regulate NADPH production and consequently cellular redox status during pepper fruit ripening.

The fibrils untwisting limits the rate of cellulose nitration process

The rate of cellulose nitration is lower compared to the low molecular weight substances. Theoretical estimates assess that the rate of the nitrating agents' diffusion cannot provide for the characteristic time of cellulose nitration, as the densely packed regions are too small. However, the electrostatic barrier between the nitrating mixture and the microcrystallites makes the latter inaccessible for the nitronium ion. The cellulose nitration rate decreases and the transformation of elementary fibril structure occurs at the same degree of substitution corresponding to a complete nitration of the fibrils' surface. The supercoiled macromolecules in the elementary fibrils cannot dissociate without untwisting. The fibrils' untwisting as well as their swelling are very slow. Thus, we propose that the nanofibrils' untwisting limits the rate of the nitronium ion transport into the cellulose nanofibrils and, thus, the rate of the nitration reaction as a whole.

Nitro-fatty acid formation and metabolism

Nitro-fatty acids (NO₂-FA) are pleiotropic modulators of redox signaling pathways. Their effects on inflammatory signaling have been studied in great detail in cell, animal and clinical models primarily using exogenously administered nitro-oleic acid. While we know a considerable amount regarding NO₂-FA signaling, endogenous formation and metabolism is relatively unexplored. This review will cover what is currently known regarding the proposed mechanisms of NO₂-FA formation, dietary modulation of endogenous NO₂-FA levels, pathways of NO₂-FA metabolism and the detection of NO₂-FA and corresponding metabolites.

Exposure of tropoelastin to peroxynitrous acid gives high yields of nitrated tyrosine residues, di-tyrosine cross-links and altered protein structure and function

Elastin is an abundant extracellular matrix protein in elastic tissues, including the lungs, skin and arteries, and comprises 30-57% of the aorta by dry mass. The monomeric precursor, tropoelastin (TE), undergoes complex processing during elastogenesis to form mature elastic fibres. Peroxynitrous acid (ONOOH), a potent oxidising and nitrating agent, is formed in vivo from superoxide and nitric oxide radicals. Considerable evidence supports ONOOH formation in the inflamed artery wall, and a role for this species in the development of human atherosclerotic lesions, with ONOOH-damaged extracellular matrix implicated in lesion rupture. We demonstrate that TE is highly sensitive to ONOOH, with this resulting in extensive dimerization, fragmentation and nitration of Tyr residues to give 3-nitrotyrosine (3-nitroTyr). This occurs with equimolar or greater levels of oxidant and increases in a dose-dependent manner. Quantification of Tyr loss and 3-nitroTyr formation indicates extensive Tyr modification with up to two modified Tyr per protein molecule, and up to 8% conversion of initial ONOOH to 3-nitroTyr. These effects were modulated by bicarbonate, an alternative target for ONOOH. Inter- and intra-protein di-tyrosine cross-links have been characterized by mass spectrometry. Examination of human atherosclerotic lesions shows colocalization of 3-nitroTyr with elastin epitopes, consistent with TE or elastin modification in vivo, and also an association of 3-nitroTyr containing proteins and elastin with lipid deposits. These data suggest that exposure of TE to ONOOH gives marked chemical and structural changes to TE and altered matrix assembly, and that such damage accumulates in human arterial tissue during the development of atherosclerosis.