The mechanism of nucleophilic substitution of alkylpyrroles in the presence of oxygen

Red-to-NIR emissive radical cations derived from simple pyrroles

Red-to-near-infrared (NIR) fluorophores are highly desirable in bio-imaging studies with advantages of high tissue penetration ability and less interference from auto-fluorescence. However, their preparation usually requires tedious synthetic procedures, which seriously restrict their applications. Thus, the direct preparation of red-to-NIR fluorophores from easily available substrates is highly desirable. Compared with the conventional closed-shell fluorophores, radical cations feature a large red-shift absorption, but only very few of them are fluorescent and they suffer from high instability. Herein, we proposed a convenient strategy for the preparation of red-to-NIR fluorophores through air oxidation of electron-rich 2,5-dimethylpyrroles to in situ generate red-to-NIR emissive radical cations, which can be stabilized by adsorption on silica gel-coated thin layer chromatography (TLC) plates or encapsulated in cucurbit[7]uril (CB[7]). The radical cations derived from pyrroles were verified using electron paramagnetic resonance (EPR) spectroscopy, theoretical calculations and one-electron oxidation experiments. Moreover, the pyrrole-derived radical cations encapsulated in CB[7] can be used for mitochondrial imaging in living cells with high specificity and in vivo imaging with long-term stability. The easily available pyrrole-derived radical cations with red-to-NIR emission are thus promising for biomedical applications.

Revisited Mechanism of Reaction between a Model Lysine Amino Acid Side Chain and 4-Hydroxynonenal in Different Solvent Environments

We revisit the mechanism of reaction between a model lysine side chain and reactive aldehyde 4-hydroxynonenal in different solvents with an increasing water content. We show by model organic reactions and qualitative spectrometric analysis that a nonpolar pyrrole adduct is dominantly formed in non-aqueous solvents dichloromethane and acetonitrile. On the other hand, in aqueous acetonitrile and neat water, other polar products are also isolated, including Michael adducts, hemiacetal adducts, and pyridinium salt adducts, at the same time as the ratio of nonpolar products to polar products is decreasing. The experiments are supported by detailed quantum chemical calculations of the reaction mechanism with different computational setups showing that the pyrrole adduct is the most thermodynamically stable product compared to Michael adducts and hemiacetal adducts and also indicating that water molecules released along the reaction pathway are catalyzing reaction steps involving proton transfer. Finally, we also identify the mechanism of the pyridinium salt adduct that is formed only in aqueous solutions.

Molecular Characterization of Brown Carbon in Biomass Burning Aerosol Particles

Emissions from biomass burning are a significant source of brown carbon (BrC) in the atmosphere. In this study, we investigate the molecular composition of freshly emitted biomass burning organic aerosol (BBOA) samples collected during test burns of sawgrass, peat, ponderosa pine, and black spruce. We demonstrate that both the BrC absorption and the chemical composition of light-absorbing compounds depend significantly on the type of biomass fuels. Common BrC chromophores in the selected BBOA samples include nitro-aromatics, polycyclic aromatic hydrocarbon derivatives, and polyphenols spanning a wide range of molecular weights, structures, and light absorption properties. A number of biofuel-specific BrC chromophores are observed, indicating that some of them may be used as source-specific markers of BrC. On average, ∼50% of the light absorption in the solvent-extractable fraction of BBOA can be attributed to a limited number of strong BrC chromophores. The absorption coefficients of BBOA are affected by solar photolysis. Specifically, under typical atmospheric conditions, the 300 nm absorbance decays with a half-life of ∼16 h. A "molecular corridor" analysis of the BBOA volatility distribution suggests that many BrC compounds in the fresh BBOA have low saturation mass concentration (<1 μg m^-3) and will be retained in the particle phase under atmospherically relevant conditions.

Revealing Brown Carbon Chromophores Produced in Reactions of Methylglyoxal with Ammonium Sulfate

Atmospheric brown carbon (BrC) is an important contributor to light absorption and climate forcing by aerosols. Reactions between small water-soluble carbonyls and ammonia or amines have been identified as one of the potential pathways of BrC formation. However, detailed chemical characterization of BrC chromophores has been challenging and their formation mechanisms are still poorly understood. Understanding BrC formation is impeded by the lack of suitable methods which can unravel the variability and complexity of BrC mixtures. This study applies high performance liquid chromatography (HPLC) coupled to photodiode array (PDA) detector and high resolution mass spectrometry (HRMS) to investigate optical properties and chemical composition of individual BrC components produced through reactions of methylglyoxal (MG) and ammonium sulfate (AS), both of which are abundant in the atmospheric environment. A direct relationship between optical properties and chemical composition of 30 major BrC chromophores is established. Nearly all of these chromophores are nitrogen-containing compounds that account for >70% of the overall light absorption by the MG+AS system in the 300-500 nm range. These results suggest that reduced-nitrogen organic compounds formed in reactions between atmospheric carbonyls and ammonia/amines are important BrC chromophores. It is also demonstrated that improved separation of BrC chromophores by HPLC will significantly advance understanding of BrC chemistry.

The 8-Pyrrole-Benzothiazinones Are Noncovalent Inhibitors of DprE1 from Mycobacterium tuberculosis

8-Nitro-benzothiazinones (BTZs), such as BTZ043 and PBTZ169, inhibit decaprenylphosphoryl-β-d-ribose 2'-oxidase (DprE1) and display nanomolar bactericidal activity against Mycobacterium tuberculosis in vitro. Structure-activity relationship (SAR) studies revealed the 8-nitro group of the BTZ scaffold to be crucial for the mechanism of action, which involves formation of a semimercaptal bond with Cys387 in the active site of DprE1. To date, substitution of the 8-nitro group has led to extensive loss of antimycobacterial activity. Here, we report the synthesis and characterization of the pyrrole-benzothiazinones PyrBTZ01 and PyrBTZ02, non-nitro-benzothiazinones that retain significant antimycobacterial activity, with MICs of 0.16 μg/ml against M. tuberculosis. These compounds inhibit DprE1 with 50% inhibitory concentration (IC50) values of <8 μM and present favorable in vitro absorption-distribution-metabolism-excretion/toxicity (ADME/T) and in vivo pharmacokinetic profiles. The most promising compound, PyrBTZ01, did not show efficacy in a mouse model of acute tuberculosis, suggesting that BTZ-mediated killing through DprE1 inhibition requires a combination of both covalent bond formation and compound potency.

Role of N-acetylcysteine in protecting against 2,5-hexanedione neurotoxicity in a rat model: changes in urinary pyrroles levels and motor activity performance

The interference of N-acetylcysteine (NAC) on 2,5-hexanedione (2,5-HD) neurotoxicity was evaluated through behavioral assays and the analysis of urinary 2,5-HD, dimethylpyrrole norleucine (DMPN), and cysteine-pyrrole conjugate (DMPN NAC), by ESI-LC-MS/MS, in rats exposed to 2,5-HD and co-exposed to 2,5-HD and NAC. Wistar rats were treated with 4 doses of: 400mg 2,5-HD/kg bw (group I), 400mg 2,5-HD/kg bw+200mg NAC/kg bw (group II), 200mg NAC/kg bw (group III) and with saline (group IV). The results show a significant decrease (p<0.01) in urinary DMPN and free 2,5-HD, a significant increase (p<0.01) in DMPN NAC excretion, and a significant recovery (p<0.01) on motor activity in rats co-exposed to 2,5-HD+NAC, as compared with rats exposed to 2,5-HD alone. Taken together, our findings suggest that at the studied conditions NAC protects against 2,5-HD neurotoxicity and DMPN may be proposed as a new sensitive and specific biomarker of 2,5-HD neurotoxicity in animals treated with a toxic amount of 2,5-hexanedione.

Generation and detection of levuglandins and isolevuglandins in vitro and in vivo

Levuglandins (LGs) and isolevuglandins (isoLGs), formed by rearrangement of endoperoxide intermediates generated through the cyclooxygenase and free radical induced oxidation of polyunsaturated fatty acids (PUFAs), are extraordinarily reactive, forming covalent adducts incorporating protein lysyl ε-amino groups. Because they accumulate, these adducts provide a dosimeter of oxidative injury. This review provides an updated and comprehensive overview of the generation of LG/isoLG in vitro and in vivo and the detection methods for the adducts of LG/isoLG and biological molecules in vivo.

Isolevuglandins covalently modify phosphatidylethanolamines in vivo: detection and quantitative analysis of hydroxylactam adducts

Levuglandins (LGs) and isolevuglandins (isoLGs, also called "isoketals" or "isoKs") are extraordinarily reactive products of cyclooxygenase- and free radical-induced oxidation of arachidonates. We now report the detection in vivo and quantitative analysis of LG/isoLG adducts that incorporate the amino group of phosphatidylethanolamines (PEs) into LG/isoLG-hydroxylactams. Notably, LC-MS/MS detection of these hydroxylactams is achieved with samples that are an order of magnitude smaller and sample processing is much simpler and less time consuming than required for measuring protein-derived LG/isoLG-lysyl lactams. A key feature of our protocol is treatment of biological phospholipid extracts with phospholipase A(2) to generate mainly 1-palmitoyl-2-lysoPE-hydroxylactams from heterogeneous mixtures of phospholipids with a variety of acyl groups on the 2 position. Over 160% higher mean levels of LG/isoLG-PE-hydroxylactam (P<0.001) were detected in liver from chronic ethanol-fed mice (32.4+/-6.3 ng/g, n=6) compared to controls (12.1+/-1.5 ng/g, n=4), and mean levels in plasma from patients with age-related macular degeneration (5.2+/-0.4 ng/ml, n=15) were elevated approximately 53% (P<0.0001) compared to those of healthy volunteers (3.4+/-0.1 ng/ml, n=15). Just as LG/isoLG-protein adducts provide a dosimeter of oxidative injury, this study suggests that LG/isoLG-PE-hydroxylactams are potential biomarkers for assessing risk for oxidative stress-stimulated diseases.

Comparative covalent protein binding of 2,5-hexanedione and 3-acetyl-2,5-hexanedione in the rat

2,5-Hexanedione (HD) is the metabolite implicated in n-hexane neurotoxicity. This gamma-diketone reacts with protein lysine amines to form 2,5-dimethylpyrrole adducts. Pyrrole adduction of neurofilaments (NF) and/or other axonal proteins was proposed as a critical step in the neuropathy. While pyrrole adduction is widely accepted as necessary, subsequent pyrrole oxidation, which may result in protein cross-linking, was alternatively postulated as the critical mechanistic step. Previous studies have indicated that 3-acetyl-2,5-HD (AcHD), an analogue that forms pyrroles that do not oxidize, was not neurotoxic in rats. However, relative levels of pyrrole adduction of NF or other axonal proteins were not reported. In the present study, groups of 6 male Wistar rats were given saline, [1,6-(14)C]-HD (3 mmol/kg/d), or [5-(14)C]-AcHD (0.1 mmol/kg/d), i.p. for 21 d. HD- and AcHD-treated rats lost 10% and gained 14% body weight, respectively, compared to a 22% gain for control rats. At termination, HD- and AcHD-treated rats exhibited mean scores of 3.5 and 1.4, respectively, for hindlimb weakness (0-5 scale). Incorporation of radiolabel from HD was 27.8 +/- 3.9, 13.9 +/- 2.6, and 7.8 +/- 0.6 nmol/mg in plasma protein, purified globin, and axonal cytoskeletal proteins, respectively, compared to 0.6 +/- 0.1, 1.6 +/- 0.5, and 1.0 +/- 0.1 for AcHD. Binding of HD to the NF-L, -M, and -H subunit proteins from treated animals was 4-, 24-, and 13-fold higher, respectively, that that of AcHD, indicating differing stoichiometry and patterns of NF adduction for the two diketones. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis of globin and NF proteins did not demonstrate protein cross-linking for either diketone at the dose levels and time period examined. These results indicate that that the lack of neurotoxicity previously reported for AcHD may reflect differences in adduct levels at critical axonal target sites rather than an inability to form cross-linking adducts. Based on these data, further studies are required to fully assess the neurotoxic potency of AcHD and other non-cross-linking analogues as compared to HD.

Modulation of protein function by isoketals and levuglandins

Oxidative stress, defined as an increase in reactive oxygen species, leads to peroxidation of polyunsaturated fatty acids and generates a vast number of biologically active molecules, many of which might contribute in some way to health and disease. This chapter will focus on one specific class of peroxidation products, the levuglandins and isoketals (also called isolevuglandins). These gamma-ketoaldehydes are some of the most reactive products derived from the peroxidation of lipids and exert their biological effects by rapidly adducting to primary amines such as the lysyl residues of proteins. The mechanism of their formation and remarkable reactivity will be described, along with evidence for their increased formation in disease conditions linked with oxidative stress and inflammation. Finally, the currently known effects of these gamma-ketoaldehydes on cellular function will then be discussed and when appropriate compared to the effects of alpha,beta-unsaturated fatty aldehydes, in order to illustrate the significant differences between these two classes of peroxidation products that modify proteins.

PGH2-derived levuglandin adducts increase the neurotoxicity of amyloid beta1-42

The body of evidence indicating that oligomers of amyloid beta(1-42) (Abeta(1-42)) produce toxicity to neurons, together with our demonstration that prostaglandin H(2) (PGH(2)) oligomerizes amyloid beta(1-42), led to the examination of the neurotoxicity of amyloid beta(1-42) treated with PGH(2). The neurotoxic effects of Abeta(1-42) incubated with PGH(2) was examined in primary cultures of cerebral neurons of mice, monitoring the reduction of 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) as an indicator of cell toxicity. Whereas Abeta(1-42) itself, incubated for 24 h, has little or no effect on MTT reduction, Abeta(1-42) 24 h after exposure to PGH(2) produced a marked inhibition of MTT reduction, comparable with the inhibition resulting from Abeta(1-42) that has been oligomerized by incubation for 6 days. Similar results were obtained when Abeta(1-42) was incubated with levuglandin E(2) (LGE(2)), a reactive aldehyde formed by spontaneous rearrangement of PGH(2). The oligomers formed from reaction of Abeta(1-42) with LGE(2) exhibit immunochemical similarity with amyloid-derived diffusible ligands (ADDLs), as determined by analysis of the products of reaction of Abeta(1-42) with LGE(2) using western blotting with an antibody that is selective for ADDLs.

Cyclooxygenase-dependent lipid-modification of brain proteins

Substantial evidence indicates that both beta-amyloid and cyclooxygenase activity contribute to the pathogenesis of Alzheimer disease. The immediate product of the cyclooxygenases, prostaglandin H2, rapidly rearranges in aqueous solution, with approximately 20% being converted to levuglandins E2 and D2. These gamma-ketoaldehydes are highly reactive and rapidly adduct to accessible amine groups on macromolecules, particularly the epsilon-amine of lysine residues on proteins. The immediate LG-lysine adducts are themselves reactive, and can covalently crosslink proteins. PGH2, acting via LGs, accelerates the formation of the type of oligomers of amyloid beta that has been associated with neurotoxicity. In this review, we discuss the cyclooxygenase-dependent lipid-modification of proteins by levuglandins in vitro, in cells in culture and in vivo in transgenic mice over-expressing COX in the brain.

Distinguishing levuglandins produced through the cyclooxygenase and isoprostane pathways

The cyclooxygenase (COX) pathway generates enantiomerically pure levuglandin (LG) E(2) by a rearrangement of the prostaglandin (PG) endoperoxide PGH(2). The isoprostane pathway generates racemic LGE(2) together with stereoisomers, designated collectively as isoLGE(2), through free radical-induced lipid oxidation. Within seconds, both LGs and isoLGs are rapidly sequestered by protein adduction. In theory, the diastereomeric purity of LGE(2)-protein adduct-derived lysyl lactams can reveal the relative contributions of the COX and isoprostane pathways to LGE(2) stereoisomer production in vivo. Notably, however, the detection of LGE(2)-protein adducts does not provide a basis for inferring their formation through the isoprostane pathway in vivo unless the COX pathway can be rigorously excluded. In contrast, LGE(2)structural isomers, designated collectively as iso[n]LGE(2)s, are produced exclusively through the isoprostane pathway. Immunoassays that selectively recognize iso[n]LGE(2)-protein adducts are the only tools available to unambiguously detect and quantify the production of isolevuglandins in vivo through free radical-induced oxidation of arachidonates.

Isoketals: highly reactive γ-ketoaldehydes formed from the H2-isoprostane pathway

Oxidation of arachidonic acid leads to the formation of highly reactive gamma-ketoaldehydes now termed isoketals. Isoketals react with proteins at a rate that far exceeds other well studied products of lipid peroxidation such as 4-hydroxynonenal and demonstrate a remarkable proclivity to crosslink these proteins. For these reasons, isoketals have the potential to significantly alter protein function and contribute to disease processes. This article reviews the chemistry of isoketal formation, of their adduction to proteins, and of their proclivity to crosslink proteins, as well as their effects on protein function, and their potential role in diseases associated with oxidative injury.

2,5-hexanedione-induced testicular injury

Now in its third decade of mechanistic investigation, testicular injury caused by 2,5-hexanedione (2,5-HD) exposure is a well-studied model with a rich database. The development of this model reflects the larger changes that have moved biology from a branch of chemistry into the molecular age. Critically examined in this review is the proposed mechanism for 2,5-HD-induced testicular injury in which germ cell maturation is disrupted owing to alterations in Sertoli cell microtubule-mediated functions. The goal is to evaluate the technical and conceptual approaches used to assess 2,5-HD-induced testicular injury, to highlight unanswered questions, and to identify fruitful avenues of future research.

Evidence of zinc protection against 2,5-hexanedione neurotoxicity: correlation of neurobehavioral testing with biomarkers of excretion

Risk prevention of human exposure against n-hexane neurotoxicity is relevant towards the protective measures to be proposed in occupational toxicology. Metabolic studies have identified 2,5-hexanedione (2,5-HD) as the main neurotoxic metabolite of n-hexane, which reacts with amino groups of lysine in axonal neurofilaments forming 2,5-dimethylpyrrole adducts, which are responsible for n-hexane neurotoxicity. In the present study, we have investigated the interaction of zinc with 2,5-HD, by correlating the decrease of pyrrole derivatives excretion with changes of neurobehavioral effects. Two subchronic experiments (11 and 8 weeks of exposure) were performed in Wistar rats exposed to different doses of 2,5-HD (200, 400 mg/kg per day) and to the mixture of 2,5-HD + zinc acetate (200 + 300 mg/kg per day) and (400 + 500 mg/kg per day). The results obtained show a significant increase in the excretion of pyrroles in the groups exposed to 2,5-HD alone as compared to controls, and a significant decrease in the excretion of pyrrole derivatives in the groups of rats co-exposed to 2,5-HD + zinc acetate when compared to the rats exposed to 2,5-HD alone. These biochemical changes were immediately evident after the first day of exposure. Simultaneously, neurobehavioral testing (rearing and ambulation in open field) was performed weekly in the same groups of rats. The results demonstrated a significant decrease in neurobehavioral dysfunction in rats co-exposed to 2,5-HD and zinc acetate. At the end of the exposure period, pyrroles levels returned to control values progressively, and the recovery of the neurotoxic effects was gradually established depending on the dose of exposure. The results suggest that zinc is a potential chemo-protector against 2,5-HD neurotoxicity which was identified by neurobehavioral testing. Moreover, pyrrole derivatives are good predictive biochemical biomarkers of 2,5-HD exposure and could be used as a complementary tool to characterize its neurotoxic effects.

Cross-linking of proteins by 3-(trifluoromethyl)-2,5-hexanedione. Model studies implicate an unexpected amine-dependent defluorinative substitution pathway competing with pyrrole formation

Protein modification by the neurotoxic gamma-diketone 3-methyl-2,5-hexanedione (3-MHD) and its analogue 3-(trifluoromethyl)-2,5-hexanedione (3-TFMHD) was examined. Unlike 3-MHD, which forms lysine-based pyrroles that lead to autoxidation-dependent protein cross-linking, 3-TFMHD forms an autoxidatively inert pyrrole. The surprising finding that 3-TFMHD was nonetheless as effective as 3-MHD in cross-linking ribonuclease A suggested that protein lysine condensation with 3-TFMHD could take an alternate course competing with pyrrole formation. Model studies using neopentylamine led to the isolation of the expected 1-(2,2-dimethylpropyl)-2,5-dimethyl-3-(trifluoromethyl)pyrrole as well as the neopentylamine-3-TFMHD 2:1 adducts N,N'-bis(2,2-dimethylpropyl)-2-amino-3-acetyl-5-methylpyrrole (major) and N,N'-bis(2,2-dimethylpropyl)-3-(1-aminoethylidene)-5-methyl-4-pyrrolin-2-one (minor). The formation of these 2:1 adducts, the lysine analogues of which are believed to be mainly responsible for the observed protein cross-linking, is proposed to proceed via Schiff base formation, enamine fluoride elimination, second amine condensation, and hydrolysis.

Characterization of the lysyl adducts of prostaglandin H-synthases that are derived from oxygenation of arachidonic acid

These investigations characterize the covalent binding of reactive products of prostaglandin H-synthases (PGHSs) to the enzyme and to other molecules. The intermediate product of oxygenation of arachidonic acid by the PGHSs, prostaglandin (PG) H2, undergoes rearrangement to the highly reactive gamma-keto aldehydes, levuglandin (LG) E2 and D2. We previously have demonstrated that LGE2 reacts with the epsilon-amine of lysine to form both the lysyl-levuglandin Shiff base and the pyrrole-derived lysyl-levuglandin lactam adducts. We now demonstrate that these lysyl-levuglandin adducts are formed on the PGHSs following the oxygenation of arachidonic acid; after reduction of the putative Schiff base, proteolytic digestion of the enzyme, and isolation of the adducted amino acid residues, these adducts were identified by liquid chromatography-tandem mass spectrometry. The reactivity of the LGs is reflected by the finding that virtually all of the LG predicted to be formed from PGH2 can be accounted for as adducts of the PGH-synthase and that oxygenation of arachidonic acid by PGH-synthases also leads to the formation of adducts of other proteins present in the reaction solution. The reactivity of the PGH-synthase adducts themselves is demonstrated by the formation of intermolecular cross-links.

Novel 2,5-hexanedione analogues. Substituent-induced control of the protein cross-linking potential and oxidation susceptibility of the resulting primary amine-derived pyrroles

The neurotoxic gamma-diketone, 2,5-hexanedione (2,5-HD), induces neurofilamentous swellings at prenodal sites in proximal axons as a consequence of pyrrolation of lysine epsilon-amino groups on neurofilament proteins. However, there is disagreement as to whether pyrrole formation and the associated alteration of noncovalent interactions is sufficient to cause neurofilament accumulation, or whether pyrrole autoxidation and subsequent protein-protein cross-linking is an obligatory event. To investigate gamma-diketones that might form pyrroles inert to autoxidative-induced cross-linking, we synthesized 1,1,1-trifluoro-2,5-hexanedione, 3-(trifluoromethyl)-2,5-hexanedione (3-TFMHD), and two 3-(dialkylaminocarbonyl)-2,5-diketones and assessed their rates of pyrrole formation with amines, the oxidation susceptibility of the resulting pyrroles, and the protein cross-linking potential in vitro, relative to those of 3-methyl-2,5-hexanedione. 1,1,1-Trifluoro-2,5-hexanedione does not form pyrroles, but the three 2,5-HD analogues with an electron-withdrawing 3-substituent all rapidly formed pyrroles that were inert to autoxidation. Although 3-TMFHD nonetheless still induced cross-linking of ribonuclease A, by a nonoxidative mechanism independent of the pyrrole, the two 3-(dialkylaminocarbonyl)-2,5-diketones did not exhibit any protein cross-linking. As these two gamma-diketones possess aqueous-organic partitioning properties similar to those of 2,5-HD, they should serve as useful mechanistic probes in further studies.

Rapid cross-linking of proteins by 4-ketoaldehydes and 4-hydroxy-2-alkenals does not arise from the lysine-derived monoalkylpyrroles

Exposure of proteins to 4-hydroxy-2-nonenal (HNE) results in conversion of lysines in part to 2-pentylpyrroles that can be formed in higher yield by exposure to the isomeric 4-oxononanal. Since both HNE and 4-oxononanal cause protein cross-linking, and since pyrrolation of proteins by gamma-diketones is also known to result in protein cross-linking, it has been considered that the initially formed 2-pentylpyrroles are responsible for the protein cross-linking seen for HNE and 4-oxononanal. Here we show that protein-bound 2-alkylpyrrole products associated with modification by 4-hydroxy-2-alkenals and 4-oxoalkanals, possessing only monoalkyl substitution, induce undetectable levels of autoxidation-mediated protein cross-linking over time periods where the parent aldehydes effect extensive protein cross-linking, which then must be occurring through alternative mechanisms. Finally, using both RNase and BSA, our finding that reductive methylation of lysines blocks protein cross-linking induced by either HNE or 4-oxononanal (and development of fluorescence in the case of HNE) implicates the obligatory role of lysines in the cross-linking reactions.

Structural elucidation of a 2:2 4-ketoaldehyde-amine adduct as a model for lysine-directed cross-linking of proteins by 4-ketoaldehydes

4-Ketoaldehydes react with lysine-based amines to form the same pyrroles that form in low yield by treatment of proteins with 4-hydroxy-2-alkenals generated endogenously during lipid peroxidation. Since pyrroles are susceptible to oxidative coupling, it has been presumed that the cross-linking of proteins observed for 4-ketoaldehydes involves the intermediacy of the corresponding lysine-derived pyrroles. However, the monoalkylpyrroles generated from simple 4-ketoaldehydes do not exhibit oxidative coupling within the time frame that 4-ketoaldehydes induce substantial protein cross-linking. Model studies have revealed that a side reaction competes with the formation of pyrroles from 4-ketoaldehydes and amines. In this paper, we show that this side reaction involves a non-redox aldol-like condensation process resulting in 2:2 adducts, which have been structurally characterized as isomeric pyrrolo[3.2. 1]azabicyclooctanes. Formation of these 2:2 adducts in competition with pyrrole formation is proposed to represent the major pathway for rapid lysine-dependent cross-linking of proteins by simple 4-ketoaldehydes.

Characterization of the lysyl adducts formed from prostaglandin H2 via the levuglandin pathway

Prostaglandin H(2) has been demonstrated to rearrange to gamma-ketoaldehyde prostanoids termed levuglandins E(2) and D(2). As gamma-dicarbonyl molecules, the levuglandins react readily with amines. We sought to characterize the adducts formed by synthetic levuglandin E(2) and prostaglandin H(2)-derived levuglandins with lysine. Using liquid chromatography/electrospray mass spectrometry, we found that the reaction predominantly produces lysyl-levuglandin Schiff base adducts that readily dehydrate to form lysyl-anhydrolevuglandin Schiff base adducts. These adducts were characterized by examination of their mass spectra, by analysis of the products of their reaction with sodium cyanide, sodium borohydride, and methoxylamine and by the mass spectra derived from collision-induced dissociation in tandem mass spectrometry. The Schiff base adducts also are formed on peptide-bound lysyl residues. In addition, synthetic levuglandin E(2) and prostaglandin H(2)-derived levuglandins produced pyrrole-derived lactam and hydroxylactam adducts upon reaction with lysine as determined by tandem mass spectrometry. A marked time dependence in the formation of these adducts was observed: Schiff base adducts formed very rapidly and robustly, whereas the lactam and hydroxylactam adducts formed more slowly but accumulated throughout the time of the experiment. These findings provide a basis for investigating protein modification induced by oxygenation of arachidonic acid by the cyclooxygenases.

The lipid peroxidation product 4-hydroxynonenal inhibits neurite outgrowth, disrupts neuronal microtubules, and modifies cellular tubulin

Oxidative stress is believed to be an important factor in the development of age-related neurodegenerative diseases such as Alzheimer's disease (AD). The CNS is enriched in polyunsaturated fatty acids and is therefore particularly vulnerable to lipid peroxidation. Indeed, accumulation of lipid peroxidation products has been demonstrated in affected regions in brains of AD patients. Another feature of AD is a change in neuronal microtubule organization. A possible causal relationship between lipid peroxidation products and changes in neuronal cell motility and cytoskeleton has not been investigated. We show here that 4-hydroxy-2(E)-nonenal (HNE), a major product of lipid peroxidation, inhibits neurite outgrowth and disrupts microtubules in Neuro 2A cells. The effect of HNE on microtubules was rapid, being observed after incubation times as short as 15 min. HNE can react with target proteins by forming either Michael adducts or pyrrole adducts. 4-Oxononanal, an HNE analogue that can form only pyrrole adducts but not Michael adducts, had no effect on the microtubules. This suggests that the HNE-induced disruption of microtubules occurs via Michael addition. We also show that cellular tubulin is one of the major proteins modified by HNE and that the HNE adduction to tubulin occurs via Michael addition. Inhibition of neurite outgrowth, disruption of microtubules, and tubulin modification were observed at pathologically relevant HNE concentrations and were not accompanied by cytotoxicity. Our results show that these are proximal effects of HNE that may contribute to cytoskeletal alterations that occur in AD.

Reactions of 4-hydroxy-2(E)-nonenal and related aldehydes with proteins studied by carbon-13 nuclear magnetic resonance spectroscopy

In order to understand the modifications of proteins produced by aldehydes of lipid peroxidation, [1-13C]-2(E)-hexenal, [1-13C]-4-oxopentanal, and a mixture of [1-13C]- and [2-13C]-4-hydroxynon-2(E)-enal were synthesized and the reaction of each of the labeled aldehydes with bovine serum albumin was analyzed by 13C NMR spectroscopy. Protein nucleophiles add to the 3-position of hexenal, and the resulting propanal moieties appear to undergo aldol condensation, form imine cross-links with lysyl residues, or lead to pyridinium rings. During the reaction of 4-oxopentanal with the lysyl residues of bovine serum albumin, only 1-alkyl-2-methylpyrrole and a possible intermediate leading to the pyrrole were observed. Hydroxypyrrolidine cross-links such as 25 could not be detected, leaving the pyrrole as the mediator of protein cross-linking. The Michael adducts are the major products in the reaction between 4-hydroxynon-2-enal and proteins. They exist almost exclusively in the cyclic hemiacetal form and do not appear to cross-link through imine formation with lysyl residues. A minor pathway involves the reaction of 4-hydroxynon-2-enal with the lysyl amino groups of protein resulting in 2-pentylpyrrole adducts that may mediate protein cross-linking. The Michael adducts appear not to be the direct source of the pyrrole, but the imine 32 and the enamine 35 are likely intermediates toward the five-membered ring.

(Carboxyalkyl)pyrroles in human plasma and oxidized low-density lipoproteins

Free-radical oxidation of human plasma low-density lipoprotein (LDL) produces (carboxyalkyl)pyrrole (CAP) epitopes that were detected with enzyme-linked immunosorbent assays using antibodies raised against keyhole limpet hemocyanin (KLH)-bound 2-(omega-carboxyheptyl)-pyrrole (CHP) and 2-(omega-carboxypropyl)pyrrole (CPP). These antibodies exhibit high structural selectivity (< 0.5% cross-reactivity) in competitive binding inhibition assays with the corresponding human serum albumin (HSA)-bound pyrroles. No cross-reactivity was detected for HSA-bound 2-pentylpyrrole, an epitope that is generated by a reaction of 4-hydroxy-2-nonenal (HNE) with protein lysyl residues. Oxidation of either arachidonic or linoleic acid in the presence of HSA produced an HNE-derived 2-pentylpyrrole epitope. However, only oxidation of linoleic acid formed HSA-bound CHP, while only oxidation of arachidonic acid generated HSA-bound CPP. Since ester hydrolysis with KOH markedly elevated levels of immunoreactive epitopes detected in oxidized LDL, the CAPs are presumably generated by reactions of oxidized cholesteryl esters, triglycerides, and phospholipids with LDL protein, and only some of these oxidized esters are hydrolyzed, e.g., by phospholipase activity associated with LDL. Protein-bound CHP immunoreactivity was detected in human plasma, and levels are significantly elevated in renal failure and atherosclerosis patients compared with healthy volunteers. This provides the first evidence for the biological occurrence of protein-bound CAPs in vivo and further suggests that free-radical oxidation of polyunsaturated lipids produces hydroxyalkenal carboxylate esters whose gamma-hydroxy-alpha,beta-unsaturated aldehyde functionality and reactivity resemble that of HNE.

Characterization of glutathione conjugates of pyrrolylated amino acids and peptides by liquid chromatography-mass spectrometry and tandem mass spectrometry with electrospray ionization

High-performance liquid chromatography (HPLC) coupled with electrospray mass spectrometry (ES-MS) and tandem mass spectrometry (MS-MS) was used to identify the products formed upon reaction of lysine-containing peptides with the neurotoxicant 2,5-hexanedione (2,5-HD). In addition, secondary autoxidative reaction products of the resultant alkylpyrroles with the biological thiol, glutathione, were characterized. ES mass spectra of the HPLC-separated conjugates showed intense [M+H]+ ions as well as several ions formed by amide and C-S bond cleavage. The glutathione conjugates of pyrrolylated amino acids and peptides were analyzed by ES ionization and MS-MS, and product-ion spectra showed fragmentation pathways typical of glutathione conjugates. ES-MS-MS analysis of a synthetic nonapeptide modeling a sequence found in neurofilament proteins showed pyrrole formation after incubation with 2,5-HD, and sequence ions were used to assign the position of the pyrrole adduct. Subsequent reaction of the pyrrolylated peptide with reduced glutathione was evidenced by a shift in m/z of the sequence ions of the reaction products with or without prior methylation. The results demonstrate the utility of ES-MS and ES-MS-MS in the characterization of xenobiotic-modified peptides and confirm that stable pyrrole-thiol conjugates are formed by the reaction of biological thiols with pyrrolylated peptides.

Solid-state 13C-NMR spectroscopy of adduction products of 2,5-hexanedione with ribonuclease, albumin, and rat neurofilament protein

The Paal-Knorr condensation reaction between the gamma-diketone 2,5-hexanedione (2,5-HD) and epsilon-amine moieties of proteins of various molecular weight, including ribonuclease (RNase), bovine serum albumin (BSA) and rat neurofilament (NF), has been investigated by solid-state 13C-NMR spectroscopy. These proteins all reacted with 2,5-HD with the formation of 2,5-dimethylpyrrole (2,5-DMP) derivatives. The size and complexity of the protein affected the rate of formation of 2,5-DMP derivatives. Using the selective reducing reagent NaCNBH3, the Paal-Knorr reaction intermediates were trapped by conversion into amines, which were identified by solid-state NMR spectroscopy. The secondary autoxidation reaction following the formation of 2,5-DMP derivatives was also studied by solid-state NMR spectroscopy.

Pathogenetic studies of hexane and carbon disulfide neurotoxicity

Two commonly employed solvents, n-hexane and carbon disulfide (CS2), although chemically dissimilar, result in identical neurofilament-filled swellings of the distal axon in both the central and peripheral nervous systems. Whereas CS2 is itself a neurotoxicant, hexane requires metabolism to the gamma-diketone, 2,5-hexanedione (HD). Both HD and CS2 react with protein amino functions to yield initial adducts (pyrrolyl or dithiocarbamate derivatives, respectively), which then undergo oxidation or decomposition to an electrophile (oxidized pyrrole ring or isothiocyanate), that then reacts with protein nucleophiles to result in protein cross-linking. It is postulated that progressive cross-linking of the stable neurofilament during its anterograde transport in the longest axons ultimately results in the accumulation of neurofilaments within axonal swellings. Reaction with additional targets appears to be responsible for the degeneration of the axon distal to the swellings.