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

Targeting prominin2 transcription to overcome ferroptosis resistance in cancer

Understanding how cancer cells resist ferroptosis is a significant problem that impacts ongoing efforts to stimulate ferroptosis as a therapeutic strategy. We reported that prominin2 is induced by ferroptotic stimuli and functions to resist ferroptotic death. Although this finding has significant implications for therapy, specific prominin2 inhibitors are not available. We rationalized that the mechanism by which prominin2 expression is induced by ferroptotic stress could be targeted, expanding the range of options to overcome ferroptosis resistance. Here, we show that that 4-hydroxynonenal (4HNE), a specific lipid metabolite formed from the products of lipid peroxidation stimulates PROM2 transcription by a mechanism that involves p38 MAP kinase-mediated activation of HSF1 and HSF1-dependent transcription of PROM2. HSF1 inhibitors sensitize a wide variety of resistant cancer cells to drugs that induce ferroptosis. Importantly, the combination of a ferroptosis-inducing drug and an HSF1 inhibitor causes the cytostasis of established tumors in mice, although neither treatment alone is effective. These data reveal a novel approach for the therapeutic induction of ferroptosis in cancer.

4-Hydroxy-2-nonenal (4-HNE) and Its Lipation Product 2-Pentylpyrrole Lysine (2-PPL) in Peanuts

After synthesis of a deuterated 4-hydroxy-2-nonenal (4-HNE) standard, the formation of 4-HNE during heating of peanut oil and whole peanuts, respectively, was measured by GC-MS. Whereas a significant increase in 4-HNE levels was observed for peanut oil, the amount of 4-HNE decreased when whole peanuts were roasted due to lipation reactions with amino acid side chains of the proteins. The ε-amino group of lysine was identified as the favored reaction partner of 4-HNE. After heating N(α)-acetyl-l-lysine and 4-HNE, a Schiff base, a novel pyridinium derivative, a 2-pentylpyrrol derivative and, following reduction and hydrolysis, a reduced, cyclized Michael adduct were identified. 2-Amino-6-(2-pentyl-1H-pyrrol-1-yl)hexanoic acid (2-PPL) was synthesized and quantitated in peanut proteins, which had been incubated with various amounts of 4-HNE by HPLC-ESI-MS/MS after enzymatic hydrolysis. At low 4-HNE concentrations the modification of lysine could be entirely explained by the formation of 2-PPL. Additionally, 2-PPL was quantified for the first time in peanut samples, and an increase depending on the roasting time was observed. 2-PPL represents a suitable marker to evaluate the extent of food protein lipation by 4-HNE.

S-adenosyl-l-methionine protection of acetaminophen mediated oxidative stress and identification of hepatic 4-hydroxynonenal protein adducts by mass spectrometry

Acetaminophen (APAP) hepatotoxicity is protected by S-adenosyl-l-methionine (SAMe) treatment 1hour (h) after APAP in C57/Bl6 mice. This study examined protein carbonylation as well as mitochondrial and cytosolic protein adduction by 4-hydroxynonenal (4-HNE) using mass spectrometry (MS) analysis. Additional studies investigated the leakage of mitochondrial proteins and 4-HNE adduction of these proteins. Male C57/Bl6 mice (n=5/group) were divided into the following groups and treated as indicated: Veh (15ml/kg water, ip), SAMe (1.25mmol/kg, ip), APAP (250mg/kg), and SAMe given 1h after APAP (S+A). APAP toxicity was confirmed by an increase (p<0.05) in plasma ALT (U/l) and liver weight/10g body weight relative to the Veh, SAMe and S+A groups 4h following APAP treatment. SAMe administered 1h post-APAP partially corrected APAP hepatotoxicity as ALT and liver weight/10g body weights were lower in the S+A group compared the APAP group. APAP induced leakage of the mitochondrial protein, carbamoyl phosphate synthase-1 (CPS-1) into the cytosol and which was reduced in the S+A group. SAMe further reduced the extent of APAP mediated 4-HNE adduction of CPS-1. MS analysis of hepatic and mitochondrial subcellular fractions identified proteins from APAP treated mice. Site specific 4-HNE adducts were identified on mitochondrial proteins sarcosine dehydrogenase and carbamoyl phosphate synthase-1 (CPS-1). In summary, APAP is associated with 4-HNE adduction of proteins as identified by MS analysis and that CPS-1 leakage was greater in APAP treated mice. SAMe reduced the extent of 4-HNE adduction of proteins as well as leakage of CPS-1.

Lipoxidation adducts with peptides and proteins: deleterious modifications or signaling mechanisms?

Protein lipoxidation refers to the modification by electrophilic lipid oxidation products to form covalent adducts, which for many years has been considered as a deleterious consequence of oxidative stress. Oxidized lipids or phospholipids containing carbonyl moieties react readily with lysine to form Schiff bases; alternatively, oxidation products containing α,β-unsaturated moieties are susceptible to nucleophilic attack by cysteine, histidine or lysine residues to yield Michael adducts, overall corresponding to a large number of possible protein adducts. The most common detection methods for lipoxidized proteins take advantage of the presence of reactive carbonyl groups to add labels, or use antibodies. These methods have limitations in terms of specificity and identification of the modification site. The latter question is satisfactorily addressed by mass spectrometry, which enables the characterization of the adduct structure. This has allowed the identification of lipoxidized proteins in physiological and pathological situations. While in many cases lipoxidation interferes with protein function, causing inhibition of enzymatic activity and increased immunogenicity, there are a small number of cases where lipoxidation results in gain of function or activity. For certain proteins lipoxidation may represent a form of redox signaling, although more work is required to confirm the physiological relevance and mechanisms of such processes. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Liquid chromatography/electrospray ionisation mass spectrometric tracking of 4-hydroxy-2(E)-nonenal biotransformations by mouse colon epithelial cells using [1,2-13C2]-4-hydroxy-2(E)-nonenal as stable isotope tracer

4-Hydroxy-2(E)-nonenal (HNE), a product of lipid peroxidation, has been extensively studied in several areas, including metabolism with radio-isotopes and quantification in various matrices with deuterium-labelled HNE as standard. The aim of this work was to evaluate the relevance of (13)C-labelled HNE in biotransformation studies to discriminate metabolites from endogens by liquid chromatography/electrospray ionisation mass spectrometry (LC/ESI-MS). (13)C-Labelled HNE was synthesised in improved overall yield (20%), with the incorporation of two labels in the molecule. Immortalised mouse colon epithelial cells were incubated with 2:3 molar amounts of HNE/(13)C-HNE in order to gain information on the detection of metabolites in complex media. Our results demonstrated that the stable isotope m/z values determined by mass spectrometry were relevant in distinguishing metabolites from endogens, and that metabolite structures could be deduced. Six conjugate metabolites and 4-hydroxy-2(E)-nonenoic acid were identified, together with an incompletely identified metabolite. Stable-isotope-labelled HNE has already been used for quantification purposes. However, this is the first report on the use of (13)C-labelled HNE as a tracer for in vitro metabolism. (13)C-Labelled HNE could also be of benefit for in vivo studies.

Molecular mechanisms of 4-hydroxy-2-nonenal and acrolein toxicity: nucleophilic targets and adduct formation

Acrolein and 4-hydroxy-2-nonenal (HNE) are byproducts of lipid peroxidation and are thought to play central roles in various traumatic injuries and disease states that involve cellular oxidative stress, for example, spinal cord trauma, diabetes, and Alzheimer's disease. In this review, we will discuss the chemical attributes of acrolein and HNE that determine their toxicities. Specifically, these aldehydes are classified as type 2 alkenes and are characterized by an alpha,beta-unsaturated carbonyl structure. This structure is a conjugated system that contains mobile pi-electrons. The carbonyl oxygen atom is electronegative and can promote the withdrawal of mobile electron density from the beta-carbon atom causing regional electron deficiency. On the basis of this type of electron polarizability, both acrolein and HNE are considered to be soft electrophiles that preferentially form 1,4-Michael type adducts with soft nucleophiles. Proteomic, quantum mechanical, and kinetic data will be presented, indicating that cysteine sulfhydryl groups are the primary soft nucleophilic targets of acrolein and HNE. This is in contrast to nitrogen groups on harder biological nucleophiles such as lysine or histidine residues. The toxicological outcome of adduct formation is not only dependent upon residue selectivity but also the importance of the targeted amino acid in protein function or structure. In attempting to discern the toxicological significance of a given adduct, we will consider the normal roles of cysteine, lysine, and histidine residues in proteins and the relative merits of corresponding adducts in the manifestations of diseases or toxic states. Understanding the molecular actions of acrolein and HNE could provide insight into many pathogenic conditions that involve initial cellular oxidative stress and could, thereby, offer new efficacious avenues of pharmacological defense.

Conformational interconversion of the trans-4-hydroxynonenal-derived (6S,8R,11S) 1,N(2)-deoxyguanosine adduct when mismatched with deoxyadenosine in DNA

The (6S,8R,11S) 1,N(2)-HNE-dGuo adduct of trans-4-hydroxynonenal (HNE) was incorporated into the duplex 5'-d(GCTAGCXAGTCC)-3'.5'-d(GGACTAGCTAGC)-3' [X = (6S,8R,11S) HNE-dG], in which the lesion was mismatched opposite dAdo. The (6S,8R,11S) adduct maintained the ring-closed 1,N(2)-HNE-dG structure. This was in contrast to when this adduct was correctly paired with dCyd, conditions under which it underwent ring opening and rearrangement to diastereomeric minor groove cyclic hemiacetals [ Huang , H. , Wang , H. , Qi , N. , Lloyd , R. S. , Harris , T. M. , Rizzo , C. J. , and Stone , M. P. ( 2008 ) J. Am. Chem. Soc. 130 , 10898 - 10906 ]. The (6S,8R,11S) adduct exhibited a syn/anti conformational equilibrium about the glycosyl bond. The syn conformation was predominant in acidic solution. Structural analysis of the syn conformation revealed that X(7) formed a distorted base pair with the complementary protonated A(18). The HNE moiety was located in the major groove. Structural perturbations were observed at the neighbor C(6).G(19) and A(8).T(17) base pairs. At basic pH, the anti conformation of X(7) was the major species. The 1,N(2)-HNE-dG intercalated and displaced the complementary A(18) in the 5'-direction, resulting in a bulge at the X(7).A(18) base pair. The HNE aliphatic chain was oriented toward the minor groove. The Watson-Crick hydrogen bonding of the neighboring A(8).T(17) base pair was also disrupted.

Low-density lipoprotein has an enormous capacity to bind (E)-4-hydroxynon-2-enal (HNE): detection and characterization of lysyl and histidyl adducts containing multiple molecules of HNE

(E)-4-Hydroxynon-2-enal (HNE), an electrophilic bifunctional cytotoxic lipid peroxidation product, forms covalent adducts with nucleophilic side chains of amino acid residues. HNE-derived adducts have been implicated in many pathophysiological processes including atherosclerosis, diabetes, and Alzheimer’s disease. Tritium- and deuterium-labeled HNE (d₄-HNE) were used orthogonally to study adduction with proteins and individual nucleophilic groups of histidyl, lysyl, and cysteine residues. Using tritium-labeled HNE, we detected the binding of 486 molecules of HNE per low-density lipoprotein (LDL) particle, significantly more than the total number of all reactive nucleophiles in the LDL particle. This suggests the formation of adducts that incorporate multiple molecules of HNE with some nucleophilic amino acid side chains. We also found that the reaction of a 1:1 mixture of d₄-HNE and d₀-HNE with N-acetylhistidine, N-acetyl-Gly-Lys-OMe, or N-acetyl cysteine generates 1:1, 2:1, and 3:1 adducts, which exhibit unique mass spectral signatures that aid in structural characterization. A domino-like reaction of initial 1:1 HNE Michael adducts of histidyl or lysyl nucleophiles with multiple additional HNE molecules forms 2:1 and 3:1 adducts that were structurally characterized by tandem mass spectrometry.

Covalent modification of cytochrome c exposed to trans,trans-2,4-decadienal

Modification of biomolecules by reactive aldehydes is believed to play a role in biological processes, including aging, atherosclerosis, and Alzheimer's disease. Here, the modification of cytochrome c promoted by trans, trans-2,4-decadienal (DDE) was investigated. Matrix-assisted laser desorption/ionization time-of-flight experiments indicated increases in the molecular weight of cytochrome c, consistent with the formation of DDE adducts. Our data show that the protein modification was time-, pH-, and DDE concentration-dependent, leading to the formation of at least six adducts after 2 h of incubation at pH 7.4. Electrospray ionization quantitative TOF mass spectrometry analysis of tryptic digests indicated that His-33, Lys-39, Lys-72, and Lys-100 were modified by DDE. These adducts could have significant effects considering that His-33, Lys-72, and Lys-100 are present in clusters of basic amino acid residues, which are believed to participate in the interaction of cytochrome c with cardiolipin in the inner mitochondrial membrane and cytochrome c oxidase. A blue shift in the cytochrome c Soret band from 409 to 406 nm was also observed after DDE reaction, indicating heme crevice opening and displacement of heme sixth ligand (Met-80) coordination in modified protein. The covalent modifications in cytochrome c could play a role in mitochondrial dysfunction associated with oxidative stress.

Interaction with and effects on the profile of proteins of Botrytis cinerea by C6 aldehydes

The natural volatile compounds cis-3-hexenal (c-3-H) and trans-2-hexenal (t-2-H) have significant antifungal activity with potential for use as postharvest fumigants of fruits and vegetables. However, the nature of their interaction with fungi and impact on fungal growth at the molecular level are largely unknown. The sites of interaction of these six carbon (C6) aldehydes with Botrytis cinerea, a common pathogen of many plant species, was characterized using 3H-labeled c-3-H and t-2-H. Radiolabeled C6 aldehydes were produced with lipoxygenase and hydroperoxide lyase extracts using [9,10,12,13,15,16-3H6]linolenic acid as a substrate. Following exposure of B. cinerea cultures to radiolabeled C6 aldehydes, radiolabel was recovered in protein-enriched but not lipid-enriched fractions. Radiolabel was incorporated at higher levels (6-fold per milligram of fresh weight and 4-fold per microgram of protein) into conidia than mycelia. About 95% of the radiolabeled aldehyde recovered in the protein fraction was from the surface of the fungal tissue, while 5% was from protein in internal tissue (cell wall, membrane, and cytosol). Exposure to t-2-H at both 5.4 and 85.6 micromol affected the protein profile of B. cinerea, changing the intensity of over one-third of all proteins. Both up-regulation and down-regulation of specific proteins were observed by two-dimensional gel electrophoresis, indicating a clear effect of t-2-H on changes in the protein profile of B. cinerea. This is the first evidence that fungal proteins are targets of the volatile C6 aldehydes and that sublethal levels of the aldehydes cause changes in the protein profile of a fungus.

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.

Enantioselective metabolism of trans-4-hydroxy-2-nonenal by brain mitochondria

Trans-4-hydroxy-2-nonenal (HNE) is a product of lipid peroxidation with many cellular effects. HNE possesses a stereogenic center at the C4 carbon that influences the metabolism and alkylation targets of HNE. We tested the hypothesis that rat brain mitochondria metabolize HNE in an enantioselective manner after exposure to racemic HNE. The study of HNE chirality, however, is hindered by the lack of facile methods to chromatographically resolve (R)-HNE and (S)-HNE. We used a chiral hydrazine, (S)-carbidopa, as a derivatization reagent to form diastereomers with (R)-HNE and (S)-HNE that were separated by reverse-phase HPLC. After exposure to racemic HNE, rat brain mitochondria metabolized HNE enantioselectively with a higher rate of (R)-HNE metabolism. By using the purified enantiomers of HNE, we found that this enantioselective metabolism of HNE was the result of higher rates of enzymatic oxidation of (R)-HNE by aldehyde dehydrogenases compared to (S)-HNE. Conjugation of HNE to glutathione was a minor metabolic pathway and was not enantioselective. These studies demonstrate that the chirality of HNE affects its mitochondrial metabolism and potentially other processes in the central nervous system.

Prostaglandin H2-derived adducts of proteins correlate with Alzheimer's disease severity

The formation of cyclooxygenase-derived lipid adducts of protein in brains of patients who had Alzheimer's disease has been investigated. The enzymatic product of the cyclooxygenases, prostaglandin H2, rearranges in part to highly reactive gamma-ketoaldehydes, levuglandin (LG) E(2) and LGD(2). These gamma-ketoaldehydes react with free amines on proteins to yield a covalent adduct. Utilizing analysis of the levuglandinyl-lysine adducts by liquid chromatography-tandem mass spectrometry, we now find that this post-translational modification is increased significantly in the hippocampus in Alzheimer's disease. The magnitude of the increase correlates with the pathological evidence of severity.

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.

Localization of isoketal adducts in vivo using a single-chain antibody

Isoketals are highly reactive gamma-ketoaldehydes formed by the oxidation of arachidonic acid that rapidly adduct to proteins. To investigate the formation of isoketal adducts in vivo, we isolated and characterized a single-chain antibody from a phage displayed recombinant ScFv library that bound a model peptide adducted with synthetic 15-E2-isoketal. Recognition of isoketal adduct by this anti-isoketal adduct single-chain antibody was essentially independent of the amino acid sequence of adducted peptides or proteins. The antibody did not cross-react with 4-hydroxynonenal or 4-oxononanal adducts or with 15-F2t-isoprostane (8-iso-prostaglandin F2alpha). We investigated the formation of isoketal adducts in a well-established model of oxidative injury, hyperoxia. Exposure to >98% oxygen for 7 h dramatically increased both the number of immunoreactive airway epithelial cells and the intensity of immunoreactivity compared with animals exposed to normal room air (21% oxygen). We conclude that isoketal adducts form in epithelial cells as a result of high oxygen exposure and that this single-chain antibody provides a valuable tool to localize the formation of isoketal adducts in tissues in vivo.

Mitochondrial oxidation of 4-hydroxy-2-nonenal in rat cerebral cortex

4-hydroxy-trans-2-nonenal (HNE) is a neurotoxic product of lipid peroxidation whose levels are elevated in multiple neurodegenerative diseases and CNS trauma. The detoxification of HNE may take the route of glutathione conjugation to the C3 carbon and the oxidation or reduction of the C1 aldehyde. In this work, we examined whether the oxidation of HNE to its corresponding carboxylic acid, 4-hydroxy-trans-2-nonenoate (HNEAcid) was detoxifying event, if it occurred in rat cerebral cortex, and in which subcellular compartments. Our results show that HNEAcid did not form protein adducts and was non-toxic to Neuro 2A cells. HNEAcid formation occurred in rat cerebral cortex slices following exposure to HNE in a time-dependent and dose-dependent fashion. Homogenate studies indicated that HNEAcid formation was NAD+ dependent. Subcellular fractionation demonstrated that mitochondria had the highest specific activity for HNEAcid formation with a KM of 21 micro m HNE. These data indicate that oxidation of HNE to its corresponding acid is a major detoxification pathway of HNE in the CNS and that mitochondria play a role in this process.

Oxidation of 4-hydroxynonenal in rat brain slices

4-Hydroxy-2-nonenal (HNE) is implicated as a neurotoxic 'second messenger' of oxidative damage in Alzheimer's disease (AD). The mechanism of HNE toxicity is due to alkylation of cellular nucleophilic groups. The C1 aldehyde is key to the alkylation ability of HNE. Oxidation of the C1 aldehyde to 4-hydroxy-2-nonenoic acid is catalyzed by aldehyde dehydrogenases. In this work, we tested the hypothesis that HNE oxidation to HNEAcid occurs in rat cerebral cortex utilizing rat cerebral cortical slices exposed extracellularly to HNE. HNEAcid formation occurs in a dose dependent manner with approximately 18-25% of the HNE consumed accounted for by HNEAcid formation. HNEAcid was found exclusively in the incubation media, suggesting that HNEAcid is exported from the cells of the slice. These data demonstrate that HNE detoxification through the oxidation pathway occur in the cerebral cortex.

Inhibition of succinic semialdehyde dehydrogenase activity by alkenal products of lipid peroxidation

Lipid peroxidation causes the generation of the neurotoxic aldehydes acrolein and 4-hydroxy-trans-2-nonenal (HNE). These products are elevated in neurodegenerative diseases and acute CNS trauma. Previous studies demonstrate that mitochondrial class 2 aldehyde dehydrogenase (ALDH2) is susceptible to inactivation by these alkenals. In the liver and brain another mitochondrial aldehyde dehydrogenase, succinic semialdehyde dehydrogenase (SSADH/ALDH5A1), is present. In this study, we tested the hypothesis that aldehyde products of lipid peroxidation inhibit SSADH activity using the endogenous substrate, succinic semialdehyde (SSA, 50 microM). Acrolein potently inhibited SSADH activity (IC(50)=15 microM) in rat brain mitochondrial preparations. This inhibition was of an irreversible and noncompetitive nature. HNE inhibited activity with an IC(50) of 110 microM. Trans-2-hexenal (HEX) and crotonaldehyde (100 microM each) did not inhibit activity. These data suggest that acrolein and HNE disrupt SSA metabolism and may have subsequent effects on CNS neurochemistry.

In vivo assessment of lipid peroxidation products associated with age-related neurodegenerative diseases

Numerous in vitro and cell culture experiments indicate that oxidative damage decreases astrocyte glutamate transport activity, and it has been proposed that products of lipid peroxidation, particularly 4-hydroxy-2-nonenal, may contribute to neurodegenerative diseases via inhibition of glutamate or glucose transporter activity. We have directly tested the hypothesis that lipid peroxidation products impair glutamate and glucose transport in vivo. Lipid peroxidation products that irreversibly modify protein lysyl residues caused a two- to sixfold elevation in extracellular glutamate in striatum and cerebral cortex of both freely moving and anesthetized rats undergoing microdialysis. No concomitant change in extracellular glucose concentrations was observed. Furthermore, lipid peroxidation product-evoked extracellular glutamate appeared to be derived from nonneuronal sources. Our results demonstrate a biochemical mechanism whereby oxidative damage products can increase extracellular glutamate levels in vivo, providing support for the proposal that oxidative damage leads to inhibition of glutamate transport and thereby may contribute to the progression of neurodegenerative diseases.

Expression and activities of aldo-keto oxidoreductases in Alzheimer disease

A reactive intermediate generated by lipid peroxidation, 4-hydroxy-2-nonenal (HNE), has received considerable attention as a potential effector of oxidative damage and Abeta peptide-mediated neurotoxicity in Alzheimer disease (AD). However, little is known about aldo-keto oxidoreductases, a group of enzymes that constitute a major detoxifying pathway for HNE and related reactive aldehydes in human brain. We have determined the regional, cellular, and class distribution in human brain of the 4 major aldo-keto oxidoreductases that detoxify HNE: aldehyde dehydrogenase (ALDH): aldose reductase; aldehyde reductase: and alcohol dehydrogenase (ADH). Of these 4 enzymes, only ALDH and aldose reductase were expressed in cerebral cortex. hippocampus, basal ganglia, and midbrain: all 4 enzymes were present in cerebellum. In cerebrum and hippocampus, aldose reductase was localized to pyramidal neurons and mitochondrial class 2 ALDH was localized to glia and senile plaques. ALDH, but not aldose reductase, activity was significantly increased in temporal cortex from patients with AD compared to age-matched controls. These results suggest that in brain regions involved in AD, neurons and glia utilize different mechanisms to detoxify HNE, and that increased ALDH activity is a protective response of cerebral cortex to AD.

Acrolein inhibits respiration in isolated brain mitochondria

Lipid peroxidation is elevated in diseased regions of brain in several neurodegenerative diseases. Acrolein (2-propenal) is a major cytotoxic product of lipid peroxidation and its adduction to neuronal proteins has been demonstrated in diseased brain regions from patients with Alzheimer's disease. Mitochondrial abnormalities are implicated in several neurodegenerative disorders, and mitochondria are targets of alkenal adduction in vivo. We examined the effects of acrolein upon multiple endpoints associated with the mitochondrial involvement in neurodegenerative disease. Acrolein inhibited state 3 respiration with an IC(50) of approx. 0.4 micromol/mg protein; however, there was no reduction in activity of complexes I-V. This inhibition was prevented by glutathione and N-acetylcysteine. Acrolein did not alter mitochondrial calcium transporter activity or induce cytochrome c release. These studies indicate that acrolein is a potent inhibitor of brain mitochondrial respiration.

Congeners of N(alpha)-acetyl-L-cysteine but not aminoguanidine act as neuroprotectants from the lipid peroxidation product 4-hydroxy-2-nonenal

Increased generation of neurotoxic lipid peroxidation products is proposed to contribute to the pathogenesis of Alzheimer's disease (AD). Current antioxidant therapies are directed at limiting propagation of brain lipid peroxidation. Another approach would be to scavenge the reactive aldehyde products of lipid peroxidation. N(alpha)-acetyl-L-cysteine (NAC) and aminoguanidine (AG) react rapidly and irreversibly with 4-hydroxy-2-nonenal (HNE) in vitro, and both have been proposed as potential scavengers of HNE in biological systems. We have compared NAC, AG, and a series of congeners as scavengers of HNE and as neuroprotectants from HNE. Our results showed that while both NAC and AG had comparable chemical reactivity with HNE, only NAC and its congeners were able to block HNE-protein adduct formation in vitro and in neuronal cultures. Moreover, NAC and its congeners, but not AG, effectively protected brain mitochondrial respiration and neuronal microtubule structure from the toxic effects of HNE. We conclude that NAC and its congeners, but not AG, may act as neuroprotectants from HNE.

Release of deuterated nonenal during beer aging from labeled precursors synthesized in the boiling kettle

The use of labeled nonenal enabled the demonstration that the appearance of the cardboard flavor in finished beer comes from lipid auto-oxidation during wort boiling and not from lipoxygenasic activity during mashing. Free trans-2-nonenal produced by linoleic acid auto-oxidation in the kettle disappears, owing to retention by wort amino acids and proteins. This binding linkage protects trans-2-nonenal from yeast reduction but is reversible, allowing release of the compound at lower pH during aging. Labeled trans-2-nonenal is detected after aging when deuterated precursors form in the boiling kettle. The amount of alkenal released correlates with the concentration of reversible associations in the pitching wort. This work brings new illumination to the formation of trans-2-nonenal and overturns many previous hypotheses. It also explains why a reduction in the beer pH intensifies the cardboard flavor.

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.

4-Hydroxy-2(E)-nonenal inhibits CNS mitochondrial respiration at multiple sites

A destructive cycle of oxidative stress and mitochondrial dysfunction is proposed in neurodegenerative disease. Lipid peroxidation, one outcome of oxidative challenge, can lead to the formation of 4-hydroxy-2(E)-nonenal (HNE), a lipophilic alkenal that forms stable adducts on mitochondrial proteins. In this study, we characterized the effects of HNE on brain mitochondrial respiration. We used whole rat brain mitochondria and concentrations of HNE comparable to those measured in patients with Alzheimer's disease. Our results showed that HNE inhibited respiration at multiple sites. Complex I-linked and complex II-linked state 3 respirations were inhibited by HNE with IC50 values of approximately 200 microM HNE. Respiration was apparently diminished owing to the inhibition of complex III activity. In addition, complex II activity was reduced slightly. The lipophilicity and adduction characteristics of HNE were responsible for the effects of HNE on respiration. The inhibition of respiration was not prevented by N-acetylcysteine or aminoguanidine. Studies using mitochondria isolated from porcine cerebral cortex also demonstrated an inhibition of complex I- and complex II-linked respiration. Thus, in neurodegenerative disease, oxidative stress may impair mitochondrial respiration through the production of HNE.