The reaction of 4-hydroxy-2-nonenal with N alpha-acetyl-L-histidine

Connecting the "Dots": From Free Radical Lipid Autoxidation to Cell Pathology and Disease

Our understanding of lipid peroxidation in biology and medicine is rapidly evolving, as it is increasingly implicated in various diseases but also recognized as a key part of normal cell function, signaling, and death (ferroptosis). Not surprisingly, the root and consequences of lipid peroxidation have garnered increasing attention from multiple disciplines in recent years. Here we "connect the dots" between the fundamental chemistry underpinning the cascade reactions of lipid peroxidation (enzymatic or free radical), the reactive nature of the products formed (lipid-derived electrophiles), and the biological targets and mechanisms associated with these products that culminate in cellular responses. We additionally bring light to the use of highly sensitive, fluorescence-based methodologies. Stemming from the foundational concepts in chemistry and biology, these methodologies enable visualizing and quantifying each reaction in the cascade in a cellular and ultimately tissue context, toward deciphering the connections between the chemistry and physiology of lipid peroxidation. The review offers a platform in which the chemistry and biomedical research communities can access a comprehensive summary of fundamental concepts regarding lipid peroxidation, experimental tools for the study of such processes, as well as the recent discoveries by leading investigators with an emphasis on significant open questions.

Effect of subchronic hypobaric hypoxia on oxidative stress in rat heart

We examined the effect of subchronic hypobaric hypoxia in rat heart. Adult male Sprague-Dawley rats were exposed at 25,000 ft for different time periods (2 and 5 days). Susceptibility of their hearts to oxidative stress as well as modulation in gene expression was evaluated. The results showed a crosstalk between reactive oxygen species (ROS) and nitric oxide (NO), initial response was accompanied by increase in ROS generation and development of oxidative stress as confirmed by increased lipid peroxidation, protein oxidation and accumulation of 2, 4-dinitrophenyl hydrazine and 4-hydroxy-2-nonenal adducts. At the same time, glutathione activity decreased; however, antioxidant enzymatic activities of superoxide dismutases, glutathione-S-transferase, and glutathione peroxidase rose in response to 5-days hypoxia. Interestingly, NO level increased till 5 days, however ROS decreased after 5 days; this observation suggests that ROS/NO balance plays an important role in cardioprotection. This observation is further supported by upregulation of antioxidant genes hemeoxygenase (HO-1) and metallothionein (MT). In addition, hypoxia also induces gradual upregulation of hypoxia-inducible transcription factor (HIF-1α), which in turn induces the expression of adaptive genes erythropoiesis, vascular endothelial growth factor, glucose transporter-1, nitric oxide synthase. Collectively, our data suggests a reciprocal regulation of ROS and NO and this effect is mediated by the increase in antioxidant proteins HO-1 and MT. Along with this HIF-1-mediated induction of various cardioprotective genes also plays an important role in acclimatization.

Chemical Nature of Stochastic Generation of Protein-based Carbonyls: Metal-catalyzed Oxidation versus Modification by Products of Lipid Oxidation†

An assessment of 2,4-dinitrophenylhydrazine (DNPH)-detectable protein-based carbonyls is one of the most common assays used to quantify oxidative stress in vitro and in vivo. In this study, we compared, for the lipid-binding protein beta-lactoglobulin, the extent to which carbonyl reactivity could be introduced by adventitious metal-catalyzed oxidation (MCO) in the absence and presence of a polyunsaturated lipid or by treatment with various individual bifunctional lipid oxidation products capable of introducing carbonyls into proteins by adduction to nucleophilic side chains. With metal ions and either O2/reductant or H2O2 as the terminal oxidant, the maximal level of DNPH-detectable carbonyl generation obtainable in several hours was 0.1-0.2 mol carbonyl per mol protein monomer, with Cu(II) being more effective than Fe(II). Exposure instead to bifunctional lipoxidation-derived aldehydes (1-2 mM) generated in some cases in excess of 1 mol carbonyl per mol protein. The rank order of carbonyl incorporation reactivity was acrolein > 4-oxo-2-nonenal > 4-hydroxy-2-nonenal > 2,4-decadienal > malondialdehyde. Protein cross-linking ability followed a somewhat different rank order. Parallel studies on reductively methylated beta-lactoglobulin revealed that His and Cys residues are intrinsically more responsible than Lys residues for carbonyl appearance and that the availability of Lys residues accounts for the reduction of carbonyl content at later time (presumably reflecting cross-linking chemistry) that occurs for acrolein and 4-oxo-2-nonenal. Overall, these results suggest that DNPH reactivity observed physiologically on nonmetalloproteins may arise more from the attachment of lipid-derived products of oxidative stress than from adventitious MCO of side chains. Additional studies carried out to clarify the potential use of DNPH derivatization to tag peptide-based carbonyls for mass spectrometric analysis revealed that DNPH derivatization can reverse under the conditions used for proteolysis.

Aging, lipofuscin formation, and free radical-mediated inhibition of cellular proteolytic systems

Alterations in a wide array of physiological functions are a normal consequence of aging. Importantly, aged individuals exhibit an enhanced susceptibility to various degenerative diseases and appear less able than their young and adult counterparts to withstand (patho)physiological stress. Elucidation of mechanisms at play in the aging process would benefit the development of effective strategies for enhancing the quality of life for the elderly. It is likely that decrements in cellular and physiological function that occur during aging are the net result of numerous interacting factors. The current review focuses on the potential contribution(s) of free radical-mediated modifications to protein structure/function and alterations in the activities of two major proteolytic systems within cells, lysosomes and the proteasome, to the age-dependent accumulation of fluorescent intracellular granules, termed lipofuscin. Specifically, aging appears to influence the interplay between the occurrences of free radical-derived modifications to protein and the ability of cells to carry out critical proteolytic functions. We present immunochemical and ultrastructural evidence demonstrating the occurrence of a fluorescent protein cross-link derived from free radical-mediated reaction(s) within lipofuscin granules of rat cerebral cortex neurons. In addition, we provide evidence that a fluorophore-modified protein present in lipofuscin granules is the alpha subunit of F1F0-ATP synthase, a mitochondrial protein. It has previously been shown that protein(s) bearing this particular fluorescent cross-link are resistant to proteolysis and can inhibit the proteasome in a non-competitive fashion (J. Biol. Chem. 269 (1994a) 21639; FEBS Lett. 405 (1997) 21). Therefore, the current findings demonstrate that free radical-mediated modifications to protein(s) that lead to the production of inhibitor(s) of cellular proteolytic systems are present on specific protein components of lipofuscin. In addition, the mitochondrial origin of one of these proteins indicates specific intracellular pathways likely to be influenced by free radical events and participate in the formation of lipofuscin. The results of these studies are related to previous in vitro and in vivo observations in the field, thus shedding light on potential consequences to cellular function. In addition, future research directions suggested by the available evidence are discussed.

Carnosine inhibits (E)-4-hydroxy-2-nonenal-induced protein cross-linking: structural characterization of carnosine-HNE adducts

(E)-4-Hydroxy-2-nonenal (HNE) is a highly cytotoxic aldehyde generated during peroxidation of lipids, which induces modification and aggregation of low-density lipoproteins and has been found to elicit covalent cross-linking of proteins. Carnosine was previously shown to trap HNE. Results presented here provide evidence that by trapping HNE in stable covalent adducts, carnosine can inhibit HNE-induced protein cross-linking. This trapping effect may be augmented by carnosine-chelating trace transition metal ions that promote oxidative HNE-induced cross-linking. Adducts formed in the reaction of HNE with carnosine have been isolated and structurally characterized. The main carnosine-HNE adduct is shown to be a 13-member cyclic adduct formed through initial Schiff base formation followed by conjugate addition of the imidazole group.

Identification of a New Cross-link and Unique Histidine Adduct from Bovine Serum Albumin Incubated with Malondialdehyde

Malondialdehyde, acetaldehyde, acrolein, and 4-hydroxynonenal are all products of fatty acid oxidation found in the fatty streaks of atherosclerotic arteries due to a lack of antioxidants and an increase in glycation products. Previously identified cross-links derived from these molecules have nearly always required more than one molecule of each type, although this is physiologically less likely than a reaction involving a single molecule. Here we provide indirect but strong evidence for a malondialdehyde-derived cross-link requiring just one malondialdehyde molecule to link arginine and lysine, giving 2-ornithinyl-4-methyl(1epsilon-lysyl)1,3-imidazole following a 4-day incubation of albumin with 8 mm malondialdehyde. This cross-link was identified as its partial degradation product Nepsilon-(2-carboxyl,2-aminoethane)-Nepsilon-methanoyl-lysine by NMR and mass spectrometry. Analysis of plasma from treated diabetic patients revealed that one patient levels had as high as 0.46%, 0.67% of their lysine/arginine residues modified by this cross-link, although others had lower levels. Alkaline hydrolysis of serum albumin also revealed two acid-labile malondialdehyde adducts of histidine in significant quantities, the isomers 4- and 2-ethylidene-histidine. These constituted up to 0.93% of the histidines in treated diabetic patients. Although collagen is readily cross-linked by malondialdehyde, none of these particular products could be found in incubations of collagen with malondialdehyde.

Carnosine is a quencher of 4-hydroxy-nonenal: through what mechanism of reaction?

The aim of this study was to understand the mechanism of action through which carnosine (beta-alanyl-L-histidine) acts as a quencher of cytotoxic alpha,beta-unsaturated aldehydes, using 4-hydroxy-trans-2,3-nonenal (HNE) as a model aldehyde. In phosphate buffer solution (pH 7.4), carnosine was 10 times more active as an HNE quencher than L-histidine and N-acetyl-carnosine while beta-alanine was totally inactive; this indicates that the two constitutive amino acids act synergistically when incorporated as a dipeptide and that the beta-alanyl residue catalyzes the addition reaction of the histidine moiety to HNE. Two reaction products of carnosine were identified, in a pH-dependent equilibrium: (a) the Michael adduct, stabilized as a 5-member cyclic hemi-acetal and (b) an imine macrocyclic derivative. The adduction chemistry of carnosine to HNE thus appears to start with the formation of a reversible alpha,beta-unsaturated imine, followed by ring closure through an intra-molecular Michael addition. The biological role of carnosine as a quencher of alpha,beta-unsaturated aldehydes was verified by detecting carnosine-HNE reaction adducts in oxidized rat skeletal muscle homogenate.

Novel protein modification by kynurenine in human lenses

It is known that human lenses increase in color and fluorescence with age, but the molecular basis for this is not well understood. We demonstrate here that proteins isolated from human lenses contain significant levels of the UV filter kynurenine covalently bound to histidine and lysine residues. Identification was confirmed by synthesis of the kynurenine amino acid adducts and comparison of the chromatographic retention times and mass spectra of these authentic standards with those of corresponding adducts isolated from human lenses following acid hydrolysis. Using calf lens proteins as a model, covalent binding of kynurenine to lens proteins has been shown to proceed via side chain deamination in a manner analogous to that observed for the related UV filter, 3-hydroxykynurenine O-beta-D-glucoside. Levels of histidylkynurenine and lysylkynurenine were low in human lenses in subjects younger than 30, but thereafter increased in concentration with the age of the individual. Post-translational modification of lens proteins by tryptophan metabolites therefore appears to be responsible, at least in part, for the age-dependent increase in coloration and fluorescence of the human lens, and this process may also be important in other tissues in which up-regulation of tryptophan catabolism occurs.

Effect of 4-hydroxy-2(E)-nonenal on soybean lipoxygenase-1

The oxidation of linoleic acid by soybean lipoxygenase-1 (LOX-1) was inhibited in a time-dependent manner by 4-hydroxy-2(E)-nonenal (HNE). Kinetic analysis indicated the effect was due to slow-binding inhibition conforming to an affinity labeling mechanism-based inhibition. After 25 min of preincubation of LOX-1 with and without HNE, Lineweaver-Burk reciprocal plots indicated mixed noncompetitive/competitive inhibition. Low concentrations of HNE influenced the electron paramagnetic resonance (EPR) signal of 13(S)-hydroperoxy-9(Z), 11 (E)-octadecadienoic acid (13-HPODE)-generated Fe3+-LOX-1 slightly, but higher concentrations completely eliminated the EPR signal indicating an active site hindered from access by 13-HPODE. HNE may compete for the active site of LOX-1 because its precursor, 4-hydroperoxy-(2E)-nonenal, is a product of LOX-1 oxidation of (3Z)-nonenal. Also, it was an attractive hypothesis to suggest that HNE may disrupt the active site by forming a Michael adduct with one or more of the three histidines that ligate the iron active site of LOX-1.

Modification of histidine residues by 4,5-epoxy-2-alkenals

The reactions of 4,5(E)-epoxy-2(E)-heptenal with 4-methylimidazole and N(alpha)-acetyl-L-histidine methyl ester were studied to characterize the adducts produced in the modification of histidine residues by epoxyalkenals and to develop a methodology for the determination of these adducts in protein hydrolysates. The reaction products, which were isolated and characterized, resulted in the Michael adducts produced in the addition of one of the imidazolic nitrogens to the carbon-carbon double bond of the epoxyalkenal. Only some of the theoretical isomers were produced. Thus, in the reaction with 4-methylimidazole, the main product was 4, 5-epoxy-3-(4-methylimidazol-1-yl)heptanol (88%), although the formation of 4,5-epoxy-3-(5-methylimidazol-1-yl)heptanol (12%) was also observed. On the other hand, the reaction with N(alpha)-acetyl-L-histidine methyl ester produced exclusively N(alpha)-acetyl-1-[1'-(1' ',2' '-epoxybutyl)-3'-hydroxypropyl]-L-histidine methyl ester. This last compound was used to develop a procedure for the determination of 4, 5(E)-epoxy-2(E)-heptenal-histidine adducts in protein hydrolysates. When this procedure was applied to the analysis of bovine serum albumin treated with 0.01-10 mM 4,5(E)-epoxy-2(E)-heptenal, the formation of the adduct was observed and its concentration increased with the concentration of the aldehyde and the incubation time, and was parallel to the histidine losses observed in the protein after acid hydrolysis as well as to the formation of protein carbonyls. In addition, the number of histidine residues lost in the protein was very similar to the number of adduct residues produced, suggesting that the addition reaction is the major mechanism for histidine losses suffered by proteins following their reaction with epoxyalkenals.

Ability of carnosine and other skeletal muscle components to quench unsaturated aldehydic lipid oxidation products

Breakdown of lipid peroxides results in the formation of aldehydic compounds which are toxic to biological systems and deleterious to food quality. To determine the potential of skeletal muscle compounds to protect biomolecules from lipid oxidation products, the ability of carnosine and various other related compounds to quench monounsaturated and polyunsaturated aldehydes was investigated. Carnosine, the most abundant dipeptide in skeletal muscle, is capable of quenching alpha,beta-monounsaturated aldehydes and 4-hydroxy-2-trans-nonenal (HNE) more effectively than its constituent amino acid. Carnosine (5 mM) reduced 44% of headspace trans-2-hexenal (0.5 mM) after 1 h incubation at 40 degrees C and pH 7.4. Other histidine-containing dipeptides and the amine compounds, spermine and spermidine, had similar or slightly lower quenching activity than carnosine. Glutathione and thioctic acid had superior quenching ability than carnosine, but their overall contribution to aldehyde quenching compared to carnosine is limited due to their lower concentration in skeletal muscle. The results suggest that carnosine could be important for decreasing the toxicity of lipid oxidation products in biological systems and for minimizing rancidity in muscle foods.

Glutathione (GSH) and the toxicity of oxidised low-density lipoprotein to human monocyte-macrophages

Macrophage death, believed to be an important event in the pathogenesis of human atherosclerosis, can be induced by oxidised low-density lipoprotein (LDL) in vitro. Supplementation of the culture medium with 5 mM GSH significantly protected human monocyte-macrophages in vitro against the toxicity of copper-oxidised LDL. Oxidation products of LDL include the aldehyde 4-hydroxynonenal (HNE). We present evidence that conjugation of HNE by GSH contributes to this protection. In the absence of cells, HPLC analysis showed there were marked reductions in the levels of both pure HNE and HNE in copper-oxidised LDL in the presence of GSH. However, GSH did not reverse protein modification, as judged by agarose gel electrophoresis, nor did it influence the depletion of polyunsaturated fatty acids, which were assessed using gas chromatography. The possible implications for human atherosclerosis are discussed.

An evaluation of the role of mitochondria in neurodegenerative diseases: mitochondrial mutations and oxidative pathology, protective nuclear responses, and cell death in neurodegeneration

There is mounting evidence for mitochondrial involvement in neurodegenerative diseases including Alzheimer's and Parkinson's disease and amyotrophic lateral sclerosis. Mitochondrial DNA mutations, whether inherited or acquired, lead to impaired electron transport chain (ETC) functioning. Impaired electron transport, in turn, leads to decreased ATP production, formation of damaging free-radicals, and altered calcium handling. These toxic consequences of ETC dysfunction lead to further mitochondrial damage including oxidation of mitochondrial DNA, proteins, and lipids, and opening of the mitochondrial permeability transition pore, an event linked to cell death in numerous model systems. Although protective nuclear responses such as antioxidant enzymes and bcl-2 may be induced to combat these pathological changes, such a vicious cycle of increasing oxidative damage may insidiously damage neurons over a period of years, eventually leading to neuronal cell death. This hypothesis, a synthesis of the mitochondrial mutations and oxidative stress hypotheses of neurodegeneration, is readily tested experimentally, and clearly points out many potential therapeutic targets for preventing or ameliorating these diseases.

Selective inactivation of alpha-ketoglutarate dehydrogenase and pyruvate dehydrogenase: reaction of lipoic acid with 4-hydroxy-2-nonenal

Previous research has established that 4-hydroxy-2-nonenal (HNE), a highly toxic product of lipid peroxidation, is a potent inhibitor of mitochondrial respiration. HNE exerts its effects on respiration by inhibiting alpha-ketoglutarate dehydrogenase (KGDH). Because of the central role of KGDH in metabolism and emerging evidence that free radicals contribute to mitochondrial dysfunction associated with numerous diseases, it is of great interest to further characterize the mechanism of inhibition. In the present study, treatment of rat heart mitochondria with HNE resulted in the selective inhibition of KGDH and pyruvate dehydrogenase (PDH), while other NADH-linked dehydrogenases and electron chain complexes were unaffected. KGDH and PDH are structurally and catalytically similar multienzyme complexes, suggesting a common mode of inhibition. To determine the mechanism of inhibition, the effects of HNE on purified KGDH and PDH were examined. These studies revealed that inactivation by HNE was greatly enhanced in the presence of substrates that reduce the sulfur atoms of lipoic acid covalently bound to the E2 subunits of KGDH and PDH. In addition, loss of enzyme activity induced by HNE correlated closely with a decrease in the availability of lipoic acid sulfhydryl groups. Use of anti-lipoic acid antibodies indicated that HNE modified lipoic acid in both purified enzyme preparations and mitochondria and that this modification was dependent upon the presence of substrates. These results therefore identify a potential mechanism whereby free radical production and subsequent lipid peroxidation lead to specific modification of KGDH and PDH and inhibition of NADH-linked mitochondrial respiration.

Reaction of N-acetylglycyllysine methyl ester with 2-alkenals: an alternative model for covalent modification of proteins

Among the various reactions of lipid peroxidation products with proteins, 2-alkenals have been shown to react extensively with the epsilon-amino group of lysine residues [Zídek et al. (1997) Chem. Res. Toxicol. 10, 702-710]. To obtain additional information about the kinetic and mechanistic aspects of this modification, a model peptide (N-acetylglycyllysine O-methyl ester) was reacted with 2-hexenal. The reaction products were characterized through a combination of NMR and MS techniques. The structural elucidation efforts have shown the formation of pyridinium salts through the reaction of two or more alkenals with one amino group. Kinetic data were obtained using a continuous infusion of the reaction mixture into an electrospray ionization mass spectrometer. A mechanism is proposed that offers an alternative model for the formation of stable protein cross-links. The reaction progresses through a Schiff base intermediate to form a dihydropyridine species which can be alternatively reduced to form various 3,4- or 2,5-substituted pyridinium species or react with another Schiff base to form a trialkyl-substituted pyridinium structure. The stoichiometry of this structure (aldehyde/amine) is 3:2, in contrast to the widely accepted 1:2. Therefore, it represents another possible cross-linking mechanism for bifunctional products of lipid peroxidation.

Cardiac reperfusion injury: aging, lipid peroxidation, and mitochondrial dysfunction

Cardiac reperfusion and aging are associated with increased rates of mitochondrial free radical production. Mitochondria are therefore a likely site of reperfusion-induced oxidative damage, the severity of which may increase with age. 4-Hydroxy-2-nonenal (HNE), a major product of lipid peroxidation, increases in concentration upon reperfusion of ischemic cardiac tissue, can react with and inactivate enzymes, and inhibits mitochondrial respiration in vitro. HNE modification of mitochondrial protein(s) might, therefore, be expected to occur during reperfusion and result in loss in mitochondrial function. In addition, this process may be more prevalent in aged animals. To begin to test this hypothesis, hearts from 8- and 24-month-old rats were perfused in Langendorff fashion and subjected to periods of ischemia and/or reperfusion. The rate of state 3 respiration of mitochondria isolated from hearts exposed to ischemia (25 min) was approximately 25% less than that of controls, independent of age. Reperfusion (40 min) caused a further decline in the rate of state 3 respiration in hearts isolated from 24- but not 8-month-old rats. Furthermore, HNE modification of mitochondrial protein (approximately 30 and 44 kDa) occurred only during reperfusion of hearts from 24-month-old rats. Thus, HNE-modified protein was present in only those mitochondria exhibiting reperfusion-induced declines in function. These studies therefore identify mitochondria as a subcellular target of reperfusion damage and a site of age-related increases in susceptibility to injury.

Inhibition of NADH-linked mitochondrial respiration by 4-hydroxy-2-nonenal

During the progression of certain degenerative conditions, including myocardial ischemia-reperfusion injury, mitochondria are a source of increased free-radical generation and exhibit declines in respiratory function(s). It has therefore been suggested that oxidative damage to mitochondrial components plays a critical role in the pathology of these processes. Polyunsaturated fatty acids of membrane lipids are prime molecular targets of free-radical damage. A major product of lipid peroxidation, 4-hydroxy-2-nonenal (HNE), is highly cytotoxic and can readily react with and damage protein. In this study, the effects of HNE on intact cardiac mitochondria were investigated to gain insight into potential mechanisms by which free radicals mediate mitochondrial dysfunction. Exposure of mitochondria to micromolar concentrations of HNE caused rapid declines in NADH-linked but not succinate-linked state 3 and uncoupled respiration. The activity of complex I was unaffected by HNE under the conditions of our experiments. Loss of respiratory activity reflected the inability of HNE-treated mitochondria to meet NADH demand during maximum rates of O2 consumption. HNE exerted its effects on intact mitochondria by inactivating alpha-ketoglutarate dehydrogenase. These results therefore identify a potentially important mechanism by which free radicals bring about declines in mitochondrial respiration.

Prevention by 2-mercaptoethane sulfonate and N-acetylcysteine of renal oxidative damage in rats treated with ferric nitrilotriacetate

Ferric nitrilotriacetate (Fe-NTA) is a renal toxicant and carcinogen in rats and mice. We found that its administration results in formation of 4-hydroxy-2-nonenal (HNE) in the renal proximal tubule cells of rats, and 8-hydroxydeoxyguanosine (8-OHdG) adducts in their DNA, suggesting a role for oxidative stress. Since 2-mercaptoethane sulfonate (MESNA) and N-acetylcysteine (NAC), administered orally, have been shown to increase the kidney levels of free thiol groups, their influence on the renal toxicity and carcinogenicity induced by Fe-NTA was examined in the present study. Male Wistar rats were intraperitoneally injected with Fe-NTA (12 mg Fe/kg), and MESNA (100 mg/kg) or NAC (200 mg/kg) was given orally 1 h before and 1 h after this treatment. The animals were killed for tissue analyses 3 h after the Fe-NTA exposure. In accord with our previous reports, HNE-modified protein was detected in the proximal tubules of Fe-NTA-treated rats by means of immunohistochemistry. Likewise, levels of 8-OHdG in the renal nuclear DNA, lipid peroxides as thiobarbituric acid-reactive substances in the kidneys, and blood urea nitrogen and creatinine in the serum were significantly increased by the Fe-NTA treatment. All of these changes were completely inhibited by oral administration of MESNA or NAC. These results suggest that both of these compounds can prevent the oxidative stress induced by Fe-NTA.

Monoclonal antibodies for detection of 4-hydroxynonenal modified proteins

A promising approach to study lipid peroxidation pathology is antibodies recognizing aldehydes which react with and became bound to amino acid side chains of proteins. We present in this study the characterization of several monoclonal antibodies which recognize 4-hydroxynonenal (HNE) modified proteins. Six out of 20 antibodies recognizing HNE modified BSA were able to detect HNE-protein adducts in peroxidized liver microsomes. Two of these antibodies were selected and characterized. Both antibodies could also detect HNE-protein adducts in oxidized low density lipoprotein. They exhibit no detectable cross reaction with proteins modified by malonaldehyde, nonanal, nonanal and 4-hydroxyhexenal. Protein bound 4-hydroxyoctenal and 4-hydroxydecenal were recognized to some extent. Further characterization revealed that the two antibodies are highly selective for HNE bound to histidine with only some cross reaction to HNE bound to lysine and cysteine. Preliminary quantitative ELISA-analysis showed that oxidized microsomes and oxidized LDL contain 12 nmol and 3 nmol HNE-histidine per mg protein respectively.