Identification of novel urinary metabolites of the lipid peroxidation product 4-hydroxy-2-nonenal in rats

Global Profiling of Toxicologically Relevant Metabolites in Urine: Case Study of Reactive Aldehydes

Among the numerous unknown metabolites representative of our exposure, focusing on toxic compounds should provide more relevant data to link exposure and health. For that purpose, we developed and applied a global method using data independent acquisition (DIA) in mass spectrometry to profile specifically electrophilic compounds originating metabolites. These compounds are most of the time toxic, due to their chemical reactivity toward nucleophilic sites present in biomacromolecules. The main line of cellular defense against these electrophilic molecules is conjugation to glutathione, then metabolization into mercapturic acid conjugates (MACs). Interestingly, MACs display a characteristic neutral loss in MS/MS experiments that makes it possible to detect all the metabolites displaying this characteristic loss, thanks to the DIA mode, and therefore to highlight the corresponding reactive metabolites. As a proof of concept, our workflow was applied to the toxicological issue of the oxidation of dietary polyunsaturated fatty acids, leading in particular to the formation of toxic alkenals, which lead to MACs upon glutathione conjugation and metabolization. By this way, dozens of MACs were detected and identified. Interestingly, multivariate statistical analyses carried out only on extracted HRMS signals of MACs yield a better characterization of the studied groups compared to results obtained from a classic untargeted metabolomics approach.

Chemistry and analysis of HNE and other prominent carbonyl-containing lipid oxidation compounds

The process of lipid oxidation generates a diverse array of small aldehydes and carbonyl-containing compounds, which may occur in free form or esterified within phospholipids and cholesterol esters. These aldehydes mostly result from fragmentation of fatty acyl chains following radical oxidation, and the products can be subdivided into alkanals, alkenals (usually α,β-unsaturated), γ-substituted alkenals and bis-aldehydes. Isolevuglandins are non-fragmented di-carbonyl compounds derived from H₂-isoprostanes, and oxidation of the ω-3-fatty acid docosahexenoic acid yield analogous 22 carbon neuroketals. Non-radical oxidation by hypochlorous acid can generate α-chlorofatty aldehydes from plasmenyl phospholipids. Most of these compounds are reactive and have generally been considered as toxic products of a deleterious process. The reactivity is especially high for the α,β-unsaturated alkenals, such as acrolein and crotonaldehyde, and for γ-substituted alkenals, of which 4-hydroxy-2-nonenal and 4-oxo-2-nonenal are best known. Nevertheless, in recent years several previously neglected aldehydes have been investigated and also found to have significant reactivity and biological effects; notable examples are 4-hydroxy-2-hexenal and 4-hydroxy-dodecadienal. This has led to substantial interest in the biological effects of all of these lipid oxidation products and their roles in disease, including proposals that HNE is a second messenger or signalling molecule. However, it is becoming clear that many of the effects elicited by these compounds relate to their propensity for forming adducts with nucleophilic groups on proteins, DNA and specific phospholipids. This emphasizes the need for good analytical methods, not just for free lipid oxidation products but also for the resulting adducts with biomolecules. The most informative methods are those utilizing HPLC separations and mass spectrometry, although analysis of the wide variety of possible adducts is very challenging. Nevertheless, evidence for the occurrence of lipid-derived aldehyde adducts in biological and clinical samples is building, and offers an exciting area of future research.

Enzymatic and non-enzymatic detoxification of 4-hydroxynonenal: Methodological aspects and biological consequences

4-Hydroxynonenal (HNE), an electrophilic end-product deriving from lipid peroxidation, undergoes a heterogeneous set of biotransformations including enzymatic and non-enzymatic reactions. The former mostly involve red-ox reactions on the HNE oxygenated functions (phase I metabolism) and GSH conjugations (phase II) while the latter are due to the HNE capacity to spontaneously condense with nucleophilic sites within endogenous molecules such as proteins, nucleic acids and phospholipids. The overall metabolic fate of HNE has recently attracted great interest not only because it clearly determines the HNE disposal, but especially because the generated metabolites and adducts are not inactive molecules (as initially believed) but show biological activities even more pronounced than those of the parent compound as exemplified by potent pro-inflammatory stimulus induced by GSH conjugates. Similarly, several studies revealed that the non-enzymatic reactions, initially considered as damaging processes randomly involving all endogenous nucleophilic reactants, are in fact quite selective in terms of both reactivity of the nucleophilic sites and stability of the generated adducts. Even though many formed adducts retain the expected toxic consequences, some adducts exhibit well-defined beneficial roles as documented by the protective effects of sublethal concentrations of HNE against toxic concentrations of HNE. Clearly, future investigations are required to gain a more detailed understanding of the metabolic fate of HNE as well as to identify novel targets involved in the biological activity of the HNE metabolites. These studies are and will be permitted by the continuous progress in the analytical methods for the identification and quantitation of novel HNE metabolites as well as for proteomic analyses able to offer a comprehensive picture of the HNE-induced adducted targets. On these grounds, the present review will focus on the major enzymatic and non-enzymatic HNE biotransformations discussing both the molecular mechanisms involved and the biological effects elicited. The review will also describe the most important analytical enhancements that have permitted the here discussed advancements in our understanding of the HNE metabolic fate and which will permit in a near future an even better knowledge of this enigmatic molecule.

Antioxidants and HNE in redox homeostasis

Under physiological conditions, cells are in a stable state known as redox homeostasis, which is maintained by the balance between continuous ROS/RNS generation and several mechanisms involved in antioxidant activity. ROS overproduction results in alterations in the redox homeostasis that promote oxidative damage to major components of the cell, including the biomembrane phospholipids. Lipid peroxidation subsequently generates a diverse set of products, including α,β-unsaturated aldehydes. Of these products, 4-hydroxy-2-nonenal (HNE) is the most studied aldehyde on the basis of its involvement in cellular physiology and pathology. This review summarizes the current knowledge in the field of HNE generation, metabolism, and detoxification, as well as its interactions with various cellular macromolecules (protein, phospholipid, and nucleic acid). The formation of HNE-protein adducts enables HNE to participate in multi-step regulation of cellular metabolic pathways that include signaling and transcription of antioxidant enzymes, pro-inflammatory factors, and anti-apoptotic proteins. The most widely described roles for HNE in the signaling pathways are associated with its activation of kinases, as well as transcription factors that are responsible for redox homeostasis (Ref-1, Nrf2, p53, NFκB, and Hsf1). Depending on its level, HNE exerts harmful or protective effects associated with the induction of antioxidant defense mechanisms. These effects make HNE a key player in maintaining redox homeostasis, as well as producing imbalances in this system that participate in aging and the development of pathological conditions.

4-Hydroxynonenal metabolites and adducts in pre-carcinogenic conditions and cancer

4-hydroxy-2-nonenal (HNE) is an amazing reactive compound, originating from lipid peroxidation within cells but also in food and considered as a "second messenger" of oxidative stress. Due to its chemical features, HNE is able to make covalent links with DNA, proteins and lipids. The aim of this review is to give a comprehensive summary of the chemical properties of HNE and of the consequences of its reactivity in relation to cancer development. The formation of exocyclic etheno-and propano-adducts and genotoxic effects are addressed. The adduction to cellular proteins and the repercussions on the regulation of cell signaling pathways involved in cancer development are reviewed, notably on the Nrf2/Keap1/ARE pathway. The metabolic pathways leading to the inactivation/elimination or, on the contrary, to the bioactivation of HNE are considered. A special focus is given on the link between HNE and colorectal cancer development, due to its occurrence in foodstuffs and in the digestive lumen, during digestion.

Hormetic and regulatory effects of lipid peroxidation mediators in pancreatic beta cells

Nutrient sensing mechanisms of carbohydrates, amino acids and lipids operate distinct pathways that are essential for the adaptation to varying metabolic conditions. The role of nutrient-induced biosynthesis of hormones is paramount for attaining metabolic homeostasis in the organism. Nutrient overload attenuate key metabolic cellular functions and interfere with hormonal-regulated inter- and intra-organ communication, which may ultimately lead to metabolic derangements. Hyperglycemia and high levels of saturated free fatty acids induce excessive production of oxygen free radicals in tissues and cells. This phenomenon, which is accentuated in both type-1 and type-2 diabetic patients, has been associated with the development of impaired glucose tolerance and the etiology of peripheral complications. However, low levels of the same free radicals also induce hormetic responses that protect cells against deleterious effects of the same radicals. Of interest is the role of hydroxyl radicals in initiating peroxidation of polyunsaturated fatty acids (PUFA) and generation of α,β-unsaturated reactive 4-hydroxyalkenals that avidly form covalent adducts with nucleophilic moieties in proteins, phospholipids and nucleic acids. Numerous studies have linked the lipid peroxidation product 4-hydroxy-2E-nonenal (4-HNE) to different pathological and cytotoxic processes. Similarly, two other members of the family, 4-hydroxyl-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), have also been identified as potential cytotoxic agents. It has been suggested that 4-HNE-induced modifications in macromolecules in cells may alter their cellular functions and modify signaling properties. Yet, it has also been acknowledged that these bioactive aldehydes also function as signaling molecules that directly modify cell functions in a hormetic fashion to enable cells adapt to various stressful stimuli. Recent studies have shown that 4-HNE and 4-HDDE, which activate peroxisome proliferator-activated receptor δ (PPARδ) in vascular endothelial cells and insulin secreting beta cells, promote such adaptive responses to ameliorate detrimental effects of high glucose and diabetes-like conditions. In addition, due to the electrophilic nature of these reactive aldehydes they form covalent adducts with electronegative moieties in proteins, phosphatidylethanolamine and nucleotides. Normally these non-enzymatic modifications are maintained below the cytotoxic range due to efficient cellular neutralization processes of 4-hydroxyalkenals. The major neutralizing enzymes include fatty aldehyde dehydrogenase (FALDH), aldose reductase (AR) and alcohol dehydrogenase (ADH), which transform the aldehyde to the corresponding carboxylic acid or alcohols, respectively, or by biding to the thiol group in glutathione (GSH) by the action of glutathione-S-transferase (GST). This review describes the hormetic and cytotoxic roles of oxygen free radicals and 4-hydroxyalkenals in beta cells exposed to nutritional challenges and the cellular mechanisms they employ to maintain their level at functional range below the cytotoxic threshold.

4-Hydroxy-nonenal-A Bioactive Lipid Peroxidation Product

This review on recent research advances of the lipid peroxidation product 4-hydroxy-nonenal (HNE) has four major topics: I. the formation of HNE in various organs and tissues, II. the diverse biochemical reactions with Michael adduct formation as the most prominent one, III. the endogenous targets of HNE, primarily peptides and proteins (here the mechanisms of covalent adduct formation are described and the (patho-) physiological consequences discussed), and IV. the metabolism of HNE leading to a great number of degradation products, some of which are excreted in urine and may serve as non-invasive biomarkers of oxidative stress.

NADP(+)-dependent dehydrogenase activity of carbonyl reductase on glutathionylhydroxynonanal as a new pathway for hydroxynonenal detoxification

An NADP(+)-dependent dehydrogenase activity on 3-glutathionyl-4-hydroxynonanal (GSHNE) was purified to electrophoretic homogeneity from a line of human astrocytoma cells (ADF). Proteomic analysis identified this enzymatic activity as associated with carbonyl reductase 1 (EC 1.1.1.184). The enzyme is highly efficient at catalyzing the oxidation of GSHNE (KM 33 µM, kcat 405 min(-1)), as it is practically inactive toward trans-4-hydroxy-2-nonenal (HNE) and other HNE-adducted thiol-containing amino acid derivatives. Combined mass spectrometry and nuclear magnetic resonance spectroscopy analysis of the reaction products revealed that carbonyl reductase oxidizes the hydroxyl group of GSHNE in its hemiacetal form, with the formation of the corresponding 3-glutathionylnonanoic-δ-lactone. The relevance of this new reaction catalyzed by carbonyl reductase 1 is discussed in terms of HNE detoxification and the recovery of reducing power.

"Twin peaks": searching for 4-hydroxynonenal urinary metabolites after oral administration in rats

4-Hydroxynonenal (HNE) is a cytotoxic and genotoxic lipid oxidation secondary product which is formed endogenously upon peroxidation of cellular n-6 fatty acids. However, it can also be formed in food or during digestion, upon peroxidation of dietary lipids. Several studies have evidenced that we are exposed through food to significant concentrations of HNE that could pose a toxicological concern. It is then of importance to known how HNE is metabolized after oral administration. Although its metabolism has been studied after intravenous administration in order to mimick endogenous formation, its in vivo fate after oral administration had never been studied. In order to identify and quantify urinary HNE metabolites after oral administration in rats, radioactive and stable isotopes of HNE were used and urine was analyzed by radio-chromatography (radio-HPLC) and chromatography coupled with High Resolution Mass Spectrometry (HPLC-HRMS). Radioactivity distribution revealed that 48% of the administered radioactivity was excreted into urine and 15% into feces after 24h, while 3% were measured in intestinal contents and 2% in major organs, mostly in the liver. Urinary radio-HPLC profiles revealed 22 major peaks accounting for 88% of the urinary radioactivity. For identification purpose, HNE and its stable isotope [1,2-(13)C]-HNE were given at equimolar dose to be able to univocally identify HNE metabolites by tracking twin peaks on HPLC-HRMS spectra. The major peak was identified as 9-hydroxy-nonenoic acid (27% of the urinary radioactivity) followed by classical HNE mercapturic acid derivatives (the mercapturic acid conjugate of di-hydroxynonane (DHN-MA), the mercapturic acid conjugate of 4-hydroxynonenoic acid (HNA-MA) in its opened and lactone form) and by metabolites that are oxidized in the terminal position. New urinary metabolites as thiomethyl and glucuronide conjugates were also evidenced. Some analyses were also performed on feces and gastro-intestinal contents, revealing the presence of tritiated water that could originate from beta-oxidation reactions.

Glutathionylated 4-hydroxy-2-(E)-alkenal enantiomers in rat organs and their contributions toward the disposal of 4-hydroxy-2-(E)-nonenal in rat liver

The major route for elimination of 4-hydroxy-2-(E)-nonenal (4-HNE) has long been considered to be through glutathionylation and eventual excretion as a mercapturic acid conjugate. To better quantitate the glutathionylation process, we developed a sensitive LC-MS/MS method for the detection of glutathione (GSH) conjugates of 4-hydroxy-2-(E)-alkenal enantiomers having a carbon skeleton of C5 to C12. The newly developed method enabled us to quantify 4-hydroxy-2-(E)-alkenal-glutathione diastereomers in various organs, i.e., liver, heart, and brain. We identified the addition of iodoacetic acid as a critical step during sample preparation to avoid an overestimation of glutathione-alkenal conjugation. Specifically, we found that in the absence of a quenching step reduced GSH and 4-hydroxy-2-(E)-alkenals react very rapidly during the extraction and concentration steps of sample preparation. Rat liver perfused with d11-4-hydroxy-2-(E)-nonenal (d11-4-HNE) revealed enantioselective conjugation with GSH and transportation out of the liver. In the d11-4-HNE-perfused rat livers, the amount of d11-(S)-4-HNE-GSH released from the rat liver was higher than that of d11-(R)-4-HNE-GSH, and more d11-(R)-4-HNE-GSH than d11-(S)-4-HNE-GSH remained in the perfused liver tissues. Overall, the glutathionylation pathway was found to account for only 8.7% of the disposition of 4-HNE, whereas catabolism to acetyl-CoA, propionyl-CoA, and formate represented the major detoxification pathway.

The lipid peroxidation product 4-hydroxy-2-nonenal: Advances in chemistry and analysis

4-Hydroxy-2-nonenal (HNE) is one of the most studied products of phospholipid peroxidation, owing to its reactivity and cytotoxicity. It can be formed by several radical-dependent oxidative routes involving the formation of hydroperoxides, alkoxyl radicals, epoxides, and fatty acyl cross-linking reactions. Cleavage of the oxidized fatty acyl chain results in formation of HNE from the methyl end, and 9-oxo-nonanoic acid from the carboxylate or esterified end of the chain, although many other products are also possible. HNE can be metabolized in tissues by a variety of pathways, leading to detoxification and excretion. HNE-adducts to proteins have been detected in inflammatory situations such as atherosclerotic lesions using polyclonal and monoclonal antibodies, which have also been applied in ELISAs and western blotting. However, in order to identify the proteins modified and the exact sites and nature of the modifications, mass spectrometry approaches are required. Combinations of enrichment strategies with targetted mass spectrometry routines such as neutral loss scanning are now facilitating detection of HNE-modified proteins in complex biological samples. This is important for characterizing the interactions of HNE with redox sensitive cell signalling proteins and understanding how it may modulate their activities either physiologically or in disease.

Urinary biomarkers of oxidative status

Oxidative damage produced by reactive oxygen species (ROS) has been implicated in the etiology and pathology of many health conditions, including a large number of chronic diseases. Urinary biomarkers of oxidative status present a great opportunity to study redox balance in human populations. With urinary biomarkers, specimen collection is non-invasive and the organic/metal content is low, which minimizes the artifactual formation of oxidative damage to molecules in specimens. Also, urinary levels of the biomarkers present intergraded indices of redox balance over a longer period of time compared to blood levels. This review summarizes the criteria for evaluation of biomarkers applicable to epidemiological studies and evaluation of several classes of biomarkers that are formed non-enzymatically: oxidative damage to lipids, proteins, DNA, and allantoin, an oxidative product of uric acid. The review considers formation, metabolism, and exertion of each biomarker, available data on validation in animal and clinical models of oxidative stress, analytical approaches, and their intra- and inter-individual variation. The recommended biomarkers for monitoring oxidative status over time are F₂-isoprostanes and 8-oxodG. For inter-individual comparisons, F₂-isoprostanes are recommended, whereas urinary 8-oxodG levels may be confounded by differences in the DNA repair capacity. Promising urinary biomarkers include allantoin, acrolein-lysine, and dityrosine.

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.

Chemistry and biochemistry of lipid peroxidation products

Oxidative stress and resulting lipid peroxidation is involved in various and numerous pathological states including inflammation, atherosclerosis, neurodegenerative diseases and cancer. This review is focused on recent advances concerning the formation, metabolism and reactivity towards macromolecules of lipid peroxidation breakdown products, some of which being considered as 'second messengers' of oxidative stress. This review relates also new advances regarding apoptosis induction, survival/proliferation processes and autophagy regulated by 4-hydroxynonenal, a major product of omega-6 fatty acid peroxidation, in relationship with detoxication mechanisms. The use of these lipid peroxidation products as oxidative stress/lipid peroxidation biomarkers is also addressed.

Trans-4-hydroxy-2-hexenal, a product of n-3 fatty acid peroxidation: make some room HNE

Lipid peroxidation yields multiple aldehyde species. Of these, trans-4-hydroxy-2-nonenal (HNE), derived from n-6 poly-unsaturated fatty acids (PUFA) is one of the most studied products of lipid peroxidation. On the other hand, oxidative damage to n-3 PUFA, e.g. docosahexaenoic acid (DHA; 22:6, n-3) and eicosapentaenoic acid, is now recognized as an important effector of oxidative stress and is of particular interest in n-3 rich tissues such as brain and retina. Trans-4-hydroxy-2-hexenal (HHE) is a major alpha,beta-unsaturated aldehyde product of n-3 PUFA oxidation and, like HNE, is an active biochemical mediator resulting from lipid peroxidation. HHE adducts are elevated in disease states, in some cases, at higher levels than the corresponding HNE adduct. HHE has properties in common with HNE, but there are important differences particularly with respect to adduction targets and detoxification pathways. In this review, the biochemistry and cell biology of HHE will be discussed. From this review, it is clear that further study is needed to determine the biochemical and physiological roles of HHE and its related aldehyde, trans-4-oxo-2-hexenal.

Direct and indirect high-performance liquid chromatography enantioseparation of trans-4-hydroxy-2-nonenoic acid

trans-4-Hydroxy-2-nonenoic acid (HNEA) is a marker of lipid peroxidation resulting from the metabolism of trans-4-hydroxy-2-nonenal (HNE). Direct and indirect RP-HPLC methods for the separation of HNEA enantiomers were developed and compared. The indirect method involved pre-column derivatization with a chiral amino agent, (1S,2S)-(+)-2-amino-1-(4-nitrophenyl)-1,3-propanediol, and subsequent separation of diastereomers on a Spherisorb ODS2 column. The direct separation of HNEA enantiomers was performed using the chiral stationary phase, Chiralpak AD-RH. Validation parameters including limit of quantification, linear range, accuracy and precision were determined. The indirect separation method was successfully applied for the determination of enantiomeric ratio of HNEA in rat brain mitochondrial lysate, and showed that HNEA was formed (R)-enantioselectively from HNE.

Rapid detection and identification of N-acetyl-L-cysteine thioethers using constant neutral loss and theoretical multiple reaction monitoring combined with enhanced product-ion scans on a linear ion trap mass spectrometer

A sensitive and specific liquid chromatography-mass spectrometry (LC-MS) method based on the combination of constant neutral loss scans (CNL) with product ion scans was developed on a linear ion trap. The method is applicable for the detection and identification of analytes with identical chemical substructures (such as conjugates of xenobiotics formed in biological systems) which give common CNLs. A specific CNL was observed for thioethers of N-acetyl-L-cysteine (mercapturic acids, MA) by LC-MS/MS. MS and HPLC parameters were optimized with 16 MAs available as reference compounds. All of these provided a CNL of 129 Da in the negative-ion mode. To assess sensitivity, a multiple reaction monitoring (MRM) mode with 251 theoretical transitions using the CNL of 129 Da combined with a product ion scan (IDA thMRM) was compared with CNL combined with a product ion scan (IDA CNL). An information-dependent acquisition (IDA) uses a survey scan such as MRM (multiple reaction monitoring) to generate "informations" and starting a second acquisition experiment such as a product ion scan using these "informations." Th-MRM means calculated transitions and not transitions generated from an available standard in the tuning mode. The product ion spectra provide additional information on the chemical structure of the unknown analytes. All MA standards were spiked in low concentrations to rat urines and were detected with both methods with LODs ranging from 60 pmol/mL to 1.63 nmol/mL with IDA thMRM. The expected product ion spectra were observed in urine. Application of this screening method to biological samples indicated the presence of a number of MAs in urine of unexposed rats, and resulted in the identification of 1,4-dihydroxynonene mercapturic acid as one of these MAs by negative and positive product ion spectra. These results show that the developed methods have a high potential to serve as both a prescreen to detect unknown MAs and to identify these analytes in complex matrix.

Enantioselective metabolism of (R)- and (S)-4-hydroxy-2-nonenal in rat

4-Hydroxy-2-nonenal (HNE) is an endogenous product of lipid peroxidation, which is believed to play a biological role in the pathogenesis of various diseases. HNE is formed as a racemic mixture of (R)- and (S)- enantiomers. These enantiomers differ in their biological properties. The aim of this study was to investigate separately the in vivo metabolism of the two HNE enantiomers in male rats after intravenous administration of the corresponding radiolabeled compounds and to compare the results with those obtained with the racemic mixture. Although the difference in the excretion rates was not statistically significant, the HPLC profiles of urinary metabolites showed qualitative and quantitative differences between the two enantiomers. The level of 3-mercapturic acid-1,4-dihydroxynonane, which is considered as the major urinary metabolite of HNE, was significantly lower in the case of (S)-HNE injected rats. In vitro studies using rat liver cytosolic incubations and HNE-glutathione conjugate as substrate were performed to clarify the intermediate pathways involved in their metabolism. Large differences were obtained in the reduction and retro-Michael conversion steps of the metabolism between the conjugates originating from the two enantiomers.

Fate of 4-hydroxynonenal in vivo: disposition and metabolic pathways

Due to the cytotoxicity of 4-hydroxynonenal (HNE), and to the fact that this major product of lipid peroxidation is a rather long-living compound compared with reactive oxygen species, the capability of organisms to inactivate and eliminate HNE has received increasing attention during the last decade. Several recent in vivo studies have addressed the issue of the diffusion, kinetics, biotransformation and excretion of HNE. Part of these studies are primarily concerned with the toxicological significance of HNE biotransformation and more precisely with the metabolic pathways by which HNE is inactivated and eliminated. The other aim of in vivo metabolic study is the characterisation of end-metabolites, especially in urine, in order to develop specific and non-invasive biomarkers of lipid peroxidation. When HNE is administered intravenously or intraperitoneally, it is mainly excreted into urine and bile as conjugated metabolites, in a proportion that is dependent on the administration route. However, biliary metabolites undergo an enterohepatic cycle that limits the final excretion of faecal metabolites. Only a very low amount of metabolites is found to be bound to macromolecules. The main urinary metabolites are represented by two groups of compounds. One comes from the mercapturic acid formation from (i) 1,4 dihydroxynonene-glutathione (DHN-GSH) which originates from the conjugation of HNE with GSH by glutathione-S-transferases and the subsequent reduction of the aldehyde by a member of aldo-keto reductase superfamily; (ii) the lactone of 4-hydroxynonanoic-GSH (HNA-lactone-GSH) which originates from the conjugation of HNE followed by the oxidation of the aldehyde by aldehyde dehydrogenase; (iii) HNA-GSH which originates from the hydrolysis of the corresponding lactone. The other one is a group of metabolites issuing from the omega-hydroxylation of HNA or HNA-lactone by cytochromes P450 4A, followed eventually, in the case of omega-oxidized-HNA-lactone, by conjugation with GSH and subsequent mercapturic acid formation. Biliary metabolites are GSH or mercapturic acid conjugates of DHN, HNE and HNA. Stereochemical aspects of HNE metabolism are also discussed.

Intracellular metabolism of 4-hydroxynonenal

4-hydroxynonenal (HNE) is a major aldehydic product of lipid peroxidation known to exert a multitude of biological, cytotoxic, and signal effects. Mammalian cells possess highly active pathways of HNE metabolism. The metabolic fate of HNE was investigated in various mammalian cells and organs such as hepatocytes, intestinal enterocytes, renal tubular cells, aortic and brain endothelial cells, synovial fibroblasts, neutrophils, thymocytes, heart, and tumor cells. The experiments were carried out at 37 degrees C at initial HNE concentrations between 1 microM--that means in the range of physiological and pathophysiologically relevant HNE levels--to 100 microM. In all cell types which were investigated, 90-95% of 100 microM HNE were degraded within 3 min of incubation. At 1 microM HNE the physiological blood serum level of about 0.1-0.2 microM was restored already after 10-30 s. As primary products of HNE in hepatocytes and other cell types the glutathione-HNE-1:1-conjugate, the hydroxynonenoic acid and the corresponding alcohol of HNE, the 1,4-dihydroxynonene, were identified. Furthermore, the beta-oxidation of hydroxynonenoic acid including the formation of water was demonstrated. The quantitative share of HNE binding to proteins was low with about 2-8% of total HNE consumption. The glycine-cysteine-HNE, cysteine-HNE adducts and the mercapturic acid from glutathione-HNE adduct were not formed in the most cell types, but in kidney cells and neutrophils. The rapid metabolism underlines the role of HNE degrading pathways in mammalian cells as important part of the secondary antioxidative defense mechanisms in order to protect proteins from modification by aldehydic lipid peroxidation products.

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.

Mercapturate metabolism of 4-hydroxy-2-nonenal in rat and human cerebrum

4-Hydroxy-2-nonenal (HNE), a potent toxin formed in the brain from oxidation of polyunsaturated fatty acids, is increased in Alzheimer disease (AD), where it is a proposed effector of amyloid beta peptide-mediated neurotoxicity. Detoxification of HNE via the mercapturic acid pathway (MAP) is the primary means by which other organs, such as liver, limit its toxic effects. Here we examined the distribution and activity of MAP detoxification for HNE in cerebrum. Our results showed that rat cerebral cortex and especially synaptosomes were less well equipped to detoxify HNE via the MAP than liver. Glutathione transferases (GSTs) catalyze the committed step in the MAP; GST-mu and GST-pi, but not OST-alpha, were detected in neurons and astrocytes in cerebrum from AD patients and controls. MAP activity in frontal cortex of AD patients was modestly but significantly increased compared to controls. These data suggest that lipid peroxidation may present a greater toxic burden to cerebrum than to other organs, and that a component of response to injury in late stage AD is a slight increase in MAP activity.

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.

Reactive intermediates and the dynamics of glutathione transferases

Reactive intermediates are a continuous burden in biology and several defense mechanisms have evolved. Here we focus on the functions of glutathione transferases (GSTs) with the aim to discuss the quantitative aspects of defense against reactive intermediates. Humans excrete approximately 0.1 mmol of thioether conjugates per day. As the amount of GST active sites in liver is approximately 0.5 mmol, it appears that glutathione transferase catalysts are present in tremendous excess. In fact, the known catalytic properties of GSTs reveal that the enzymes can empty the liver glutathione (GSH) pool in a matter of seconds when provided with a suitable substrate. However, based on the urinary output of conjugates (or derivatives thereof), individual GSTs turn over (i.e., catalyze a single reaction) only once every few days. Glutathione transferase overcapacity reflects the fact that there is a linear relation between GST enzyme amount and protection level (provided that GSH is not depleted). Put in a different perspective, a few reactive molecules will always escape conjugation and reach cellular targets. It is therefore not surprising that signaling systems sensing reactive intermediates have evolved resulting in the increase of GSH and GST levels. Precisely for this reason, more moderately reactive electrophiles (Michael acceptors) are receiving growing interest due to their anticarcinogenic properties. Another putative regulatory mechanism involves direct activation of microsomal GST1 by thiol-reactive electrophiles through cysteine 49. The toxicological significance of low levels of reactive intermediates are of interest also in drug development, and here we discuss the use of microsomal GST1 activation as a surrogate detection marker.

gamma-Glutamyltranspeptidase-dependent metabolism of 4-hydroxynonenal-glutathione conjugate

A major pathway for detoxification of the highly reactive lipid peroxidation product, 4-hydroxy-2,3-trans-nonenal (HNE) is through the conjugation with glutathione (GSH). We have studied the metabolism of GS-HNE conjugate by the enzyme gamma-glutamyltranspeptidase (GGT) using its purified form, as well as a GGT-overexpressing fibroblast cell line (V79 GGT). Using mass spectrometry analysis we identified for the first time cysteinylglycine-HNE (CysGly-HNE) as the GGT metabolite of GS-HNE. Furthermore, the GGT-dependent metabolism of GS-HNE in the V79 GGT cell line was associated with a considerable increase of cytotoxicity as compared to a control cell line which does not express GGT (V79 Cl). The cytotoxic effect was dose- and time-dependent (100% cellular death at 200 microM GS-HNE after 24 h incubation) in V79 GGT cells, whereas no decrease of viability was observed in V79 Cl cells. A similar cytotoxic effect was obtained when cells were incubated directly with CysGly-HNE, demonstrating that this GGT-dependent metabolite unlike GS-HNE, exhibits cytotoxic properties.

Spontaneous urinary chemiluminescence and diet. An intervention study in humans

Spontaneous urinary visible chemiluminiscence has been measured in samples from a diet intervention study in humans. For a month, two groups, each of 21 male volunteers, received either a Mediterranean-type diet or an occidental (high-fat) diet. Urinary chemiluminescence levels (in 10(3) cpm) were 8.5 +/- 3.1 and 6.0 +/- 2.0 for the high-fat and Mediterranean-type diets, respectively. The differences obtained were statistically significant. These results reflect differences in the oxidative stress associate to both diets and support the proposal that spontaneous urinary chemiluminiscence constitutes a complementary index of systemic oxidative stress.

Metabolism of 4-hydroxynonenal, a cytotoxic product of lipid peroxidation, in rat precision-cut liver slices

4-Hydroxy-2-nonenal (HNE) is a major aldehydic product of lipid peroxidation known to exert several biological and cytotoxic effects. The in vitro metabolism of [4-(3)H]-HNE by rat precision-cut liver slices was investigated. Liver slices rapidly metabolize HNE - about 85% of 0.1 microM [4-(3)H]-HNE was degraded within 5 min of incubation. The main metabolites of HNE identified were 4-hydroxynonenoic acid (HNA), glutathione-HNE-conjugate (HNE-GSH), glutathione-1,4-dihydroxynonene-conjugate (DHN-GSH) and cysteine-HNE-conjugate (HNE-CYS). Whereas glutathione conjugation demonstrated saturation kinetics (K(m)=412.2+/-152.7 microM and V(max)=12.3+/-2.5 nmol h(-1) per milligram protein), HNA formation was linear up to 500 microM HNE in liver slices. In contrast to previous reports, no trace of the corresponding alcohol of the HNE, 1,4-dihydroxynon-2-ene was detected in the present study. Furthermore, the beta-oxidation of HNA including the formation of tritiated water was demonstrated. The identification of 4-hydroxy-9-carboxy-2-nonenoic acid and 4,9-dihydroxynonanoic acid demonstrated that omega-oxidation significantly contributes to the biotransformation of HNE in liver slices.

Glutathione conjugation of 4-hydroxy-trans-2,3-nonenal in the rat in vivo, the isolated perfused liver and erythrocytes

The formation of glutathione (GSH) conjugates of racemic 4-hydroxy-trans-2,3-nonenal (4-HNE) in the rat in vivo in the perfused rat liver and rat erythrocytes has been studied. An HPLC system was developed for the assay of 4-HNE-glutathione conjugates (HNE-SG). The very sensitive electrochemical detection method (detection limit 5 pmol) can also be used to study endogenously formed HNE-SG. Three diastereomeric HNE-SG conjugates could be separated by this system. Rat liver cytosol catalyzed the formation of 2 of the 3 conjugates. When 17 micromol/kg [(3)H] 4-HNE was injected intravenously in the rat, 21% of the radioactivity was excreted within 90 min in bile and 37% in urine. Most of the 4-HNE in bile was present as 2 of the HNE-SG conjugates (molecular mass 463). In addition, 25% was excreted as a third GSH conjugate (molecular mass of 461), which was identified as the lactone of the 4-hydroxynonenoic acid glutathione conjugate. Erythrocytes in vitro eliminated 4-HNE very rapidly, in part by GSH conjugation, suggesting that they may also play an important role in vivo. To study the role of the liver selectively, we used the recirculating perfused rat liver without erythrocytes in the perfusion medium; the same conjugates were found, but the third conjugate was a minor component. These results present direct evidence for the in vivo formation of 4-HNE glutathione conjugates in which the liver may play an important role.

4-Hydroxynonenal as a biological signal: molecular basis and pathophysiological implications

Reactive oxygen intermediates (ROI) and other pro-oxidant agents are known to elicit, in vivo and in vitro, oxidative decomposition of omega-3 and omega-6 polyunsaturated fatty acids of membrane phospholipids (i.e, lipid peroxidation). This leads to the formation of a complex mixture of aldehydic end-products, including malonyldialdehyde (MDA), 4-hydroxy-2,3-nonenal (HNE), and other 4-hydroxy-2,3-alkenals (HAKs) of different chain length. These aldehydic molecules have been considered originally as ultimate mediators of toxic effects elicited by oxidative stress occurring in biological material. Experimental and clinical evidence coming from different laboratories now suggests that HNE and HAKs can also act as bioactive molecules in either physiological and pathological conditions. These aldehydic compounds can affect and modulate, at very low and nontoxic concentrations, several cell functions, including signal transduction, gene expression, cell proliferation, and, more generally, the response of the target cell(s). In this review article, we would like to offer an up-to-date review on this particular aspect of oxidative stress--dependent modulation of cellular functions-as well as to offer comments on the related pathophysiological implications, with special reference to human conditions of disease.