Reaction Mechanism of Covalent Modification of Phosphatidylethanolamine Lipids by Reactive Aldehydes 4-Hydroxy-2-nonenal and 4-Oxo-2-nonenal

Membrane Lipid Reshaping Underlies Oxidative Stress Sensing by the Mitochondrial Proteins UCP1 and ANT1

Oxidative stress and ROS are important players in the pathogenesis of numerous diseases. In addition to directly altering proteins, ROS also affects lipids with negative intrinsic curvature such as phosphatidylethanolamine (PE), producing PE adducts and lysolipids. The formation of PE adducts potentiates the protonophoric activity of mitochondrial uncoupling proteins, but the molecular mechanism remains unclear. Here, we linked the ROS-mediated change in lipid shape to the mechanical properties of the membrane and the function of uncoupling protein 1 (UCP1) and adenine nucleotide translocase 1 (ANT1). We show that the increase in the protonophoric activity of both proteins occurs due to the decrease in bending modulus in lipid bilayers in the presence of lysophosphatidylcholines (OPC and MPC) and PE adducts. Moreover, MD simulations showed that modified PEs and lysolipids change the lateral pressure profile of the membrane in the same direction and by the similar amplitude, indicating that modified PEs act as lipids with positive intrinsic curvature. Both results indicate that oxidative stress decreases stored curvature elastic stress (SCES) in the lipid bilayer membrane. We demonstrated that UCP1 and ANT1 sense SCES and proposed a novel regulatory mechanism for the function of these proteins. The new findings should draw the attention of the scientific community to this important and unexplored area of redox biochemistry.

Computational Elucidation of the Solvent-Dependent Addition of 4-Hydroxy-2-nonenal (HNE) to Cysteine and Cysteinate Residues

The lipid peroxidation end product, 4-hydroxy-2-nonenal (HNE), is a secondary mediator of oxidative stress due to its strong ability to form adducts to the side chains of lysine, histidine, and cysteine residues (Cys) at increasing reactivities. This reaction can take place in various cellular environments and may be dependent on solvent. Moreover, approximately 10% of cysteine residues within the cells exist as the negatively charged cysteinate, which may also have a distinct reactivity toward HNE. In this study, quantum chemical calculations are used to investigate the reactivity of HNE toward Cys and cysteinate in three distinct solvent environments to mimic the aqueous, polar, and hydrophobic regions within the cell. Water enhances the reactivity of HNE to cysteine compared to that of the polar and hydrophobic solvents, and the reactivity of HNE is further augmented when Cys is first ionized to cysteinate. This is also confirmed by the transition state rate constant calculations. This study reveals the role of solvent polarity in these reactions and how cysteinate can account for the seemingly high reactivity of HNE toward Cys compared to other amino acid residues and demonstrates how a strong nucleophile can enhance the reactivity of an antioxidant analogue of the Cys residue.

Chemistry and Biochemistry Aspects of the 4-Hydroxy-2,3-trans-nonenal

4-hydroxy-2,3-trans-nonenal (C₉H₁₆O₂), also known as 4-hydroxy-2E-nonenal (C₉H₁₆O₂; HNE) is an α,β-unsaturated hydroxyalkenal. HNE is a major aldehyde, formed in the peroxidation process of ω-6 polyunsaturated fatty acids (ω-6 PUFAs), such as linoleic and arachidonic acid. HNE is not only harmful but also beneficial. In the 1980s, the HNE was regarded as a “toxic product of lipid peroxidation” and the “second toxic messenger of free radicals”. However, already at the beginning of the 21st century, HNE was perceived as a reliable marker of oxidative stress, growth modulating factor and signaling molecule. Many literature data also indicate that an elevated level of HNE in blood plasma and cells of the animal and human body is observed in the course of many diseases, including cancer. On the other hand, it is currently proven that cancer cells divert to apoptosis if they are exposed to supraphysiological levels of HNE in the cancer microenvironment. In this review, we briefly summarize the current knowledge about the biological properties of HNE.

Lipid oxidation and aldehyde formation during in vitro gastrointestinal digestion of roasted scallop (Patinopecten yessoensis) - the role of added antioxidant of bamboo leaves

This study investigated lipid oxidation and aldehyde formation in roasted scallop during in vitro gastrointestinal digestion, and the effects of co-digestion of antioxidant of bamboo leaves (AOB) on this process. The results showed that the contents of lipid hydroperoxides (LOOH), conjugated dienes (CD), and Schiff bases (SB) were increased during gastrointestinal digestion. Besides, malondialdehyde (MDA) levels and total aldehyde formation decreased initially at the gastric stage but increased at the intestinal stage. The results of HPLC-ESI-MS/MS analysis showed that the contents of hexanal (HEX), trans, trans-2,4-octadienal (ODE), trans, trans-2,4-decadienal (DDE), 4-hydroxyhexenal (HHE) and 4-hydroxynonenal (HNE) in the digestive juices were all initially decreased and then increased during gastrointestinal digestion. Meanwhile, the content of acrolein, propanal, and trans-2-pentenal at the end of intestinal digestion was lower than that in the initial stage of gastric digestion. Additionally, the digestion of roasted scallop caused significant oxidation of polyunsaturated fatty acids (PUFAs) and release of free fatty acids (FFA) in the intestinal phase, which were positively related to aldehyde production. However, co-digestion of AOB significantly reduced lipid oxidation and formation of lipid oxidation products (LOOH, CD, SB, and aldehyde) during gastrointestinal digestion, indicating that the addition of AOB was effective in reducing gastrointestinal lipid oxidation.

Formation and disappearance of aldehydes during simulated gastrointestinal digestion of fried clams

The formation and disappearance of aldehydes during simulated gastrointestinal digestion (SD) of fried clams was investigated in order to shed light on the underlying mechanism. Results from the thiobarbituric acid reactive substance (TBARS) and fluorometric assays using a specific aldehyde probe indicated that the SD (with lipase) of fried clams initially reduced (at the gastric stage), but subsequently increased (mainly at the intestinal stage) the contents of total aldehydes. Meanwhile, eight specific aldehydes including propanal, acrolein, trans-2-pentenal, hexanal, trans,trans-2,4-octadienal, trans,trans-2,4-decadienal, 4-hydroxy-hexenal and 4-hydroxy-nonenal in the digested meal were determined by using a high-performance liquid chromatography-tandem electrospray ionization mass spectrometry (HPLC-ESI-MS/MS) method. Results indicated that the changes in the trend of the contents of the eight aforementioned aldehydes were similar to those of total aldehydes during SD (with lipase) of fried clams. However, a similar SD process without lipase time-dependently reduced the contents of total and individual aldehydes. Moreover, lipid classes and free fatty acids (FFAs) in the digested meal were determined to reveal the degree of hydrolysis of lipids during the SD process. Results indicated that the SD (with lipase) of fried clams significantly hydrolyzed triacylglycerols (TAG) and polar lipids (PL) and produced FFAs, but the SD process without lipase resulted in negligible lipid hydrolysis. Thus, our results demonstrated a positive correlation between lipid hydrolysis and aldehyde generation during the SD of fried clams. Alternatively, unsaturated FFAs instead of TAG and PL could have served as the main precursors for aldehyde generation due to their high oxidative susceptibility.

Lipid peroxidation derived reactive carbonyl species in free and conjugated forms as an index of lipid peroxidation: limits and perspectives

Reactive carbonyl species (RCS) formed by lipidperoxidation as free forms or as enzymatic and non-enzymatic conjugates are widely used as an index of oxidative stress. Besides general measurements based on derivatizing reactions, more selective and sensitive MS based analyses have been proposed in the last decade. Untargeted and targeted methods for the measurement of free RCS and adducts have been described and their applications to in vitro and ex vivo samples have permitted the identification of many biological targets, reaction mechanisms and adducted moieties with a particular relevance to RCS protein adducts. The growing interest in protein carbonylation can be explained by considering that protein adducts are now recognized as being involved in the damaging action of oxidative stress so that their measurement is performed not only to obtain an index of lipid peroxidation but also to gain a deeper insight into the molecular mechanisms of oxidative stress. The aim of the review is to discuss the most novel analytical approaches and their application for profiling reactive carbonyl species and their enzymatic and non-enzymatic metabolites as an index of lipid-oxidation and oxidative stress. Limits and perspectives will be discussed.

Sample preparation approaches for qualitative and quantitative analysis of lipid-derived electrophile modified proteomes by mass spectrometry

Lipid-derived electrophile (LDE) modifications, which are covalent modifications of proteins by endogenous LDEs, are essential types of protein posttranslational modifications. LDE modifications alter the protein structure and regulate their biological processes in cells. LDE modifications of proteins are also closely associated with several diseases and function as potential biomarkers for clinical diagnosis. The crucial step in studying the LDE modifications is to enrich the LDE modified proteins/peptides from complex biological samples with high efficiency and high selectivity and quantify modified proteins/peptides with high accuracy. In this review, we summarize the recent progress in MS-based proteomic technologies to globally identify and quantify LDE modified proteomes, mainly focusing on discussing the qualitative and quantitative technologies.

The Role of Phosphatidylethanolamine Adducts in Modification of the Activity of Membrane Proteins under Oxidative Stress

Reactive oxygen species (ROS) and their derivatives, reactive aldehydes (RAs), have been implicated in the pathogenesis of many diseases, including metabolic, cardiovascular, and inflammatory disease. Understanding how RAs can modify the function of membrane proteins is critical for the design of therapeutic approaches in the above-mentioned pathologies. Over the last few decades, direct interactions of RA with proteins have been extensively studied. Yet, few studies have been performed on the modifications of membrane lipids arising from the interaction of RAs with the lipid amino group that leads to the formation of adducts. It is even less well understood how various multiple adducts affect the properties of the lipid membrane and those of embedded membrane proteins. In this short review, we discuss a crucial role of phosphatidylethanolamine (PE) and PE-derived adducts as mediators of RA effects on membrane proteins. We propose potential PE-mediated mechanisms that explain the modulation of membrane properties and the functions of membrane transporters, channels, receptors, and enzymes. We aim to highlight this new area of research and to encourage a more nuanced investigation of the complex nature of the new lipid-mediated mechanism in the modification of membrane protein function under oxidative stress.

Covalent modification of phosphatidylethanolamine by 4-hydroxy-2-nonenal increases sodium permeability across phospholipid bilayer membranes

Reactive aldehydes (RAs), such as 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE), produced by cells under conditions of oxidative stress, were shown to react with phosphatidylethanolamine (PE) in biological and artificial membranes. They form RA-PE adducts, which affect the function of membrane proteins by modifying various biophysical properties of the membrane. The ratio of protein to lipid in biological membranes is different, but can reach 0.25 in the membranes of oligodendrocytes. However, the impact of RA-PE adducts on permeability (P) of the neat lipid phase and molecular mechanism of their action are poorly understood. In this study, we showed that HNE increased the membrane P for ions, and in particular for sodium. This effect depended on the presence of DOPE, and was not recorded for the more toxic compound, ONE. Molecular dynamics simulations suggested that HNE-PE and ONE-PE adducts anchored different positions in the lipid bilayer, and thus changed the membrane lipid area and bilayer thickness in different ways. Sodium permeability, calculated in the presence of double HNE-PE adducts, was increased by three to four orders of magnitude when compared to P_Na in adduct - free membranes. A novel mechanism by which HNE alters permeability of the lipid membrane may explain the multiple toxic or regulative effects of HNE on the function of excitable cells, such as neurons, cardiomyocytes and neurosensory cells under conditions of oxidative stress.

Vibrational spectroscopy combined with molecular dynamics simulations as a tool for studying behavior of reactive aldehydes inserted in phospholipid bilayers

Vibrational Fourier-transform infrared (FTIR) spectroscopy aided with molecular dynamics (MD) simulations is used for studying the interaction of several reactive aldehydes (RAs), nonanal (NA), 2-nonenal (NE), 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE), with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. The results obtained by the combination of these two techniques, supported also by electron paramagnetic resonance (EPR) spectroscopy, show that NA has the strongest stabilization in the bilayer, followed by less stabilized NE, HNE and ONE. We also revealed that HNE readily makes hydrogen bonds to carbonyl groups of POPC (but not to phosphate groups), in contrast to other RAs which are hydrogen bond acceptors and do not make hydrogen bonds with lipids. A combination of FTIR spectroscopy and MD simulations is sensitive to small chemical changes in the structures of RAs, thus making it a valuable tool for studying the weak interactions between compounds inserted to phospholipid bilayers.

Mass spectrometry strategies to unveil modified aminophospholipids of biological interest

The biological functions of modified aminophospholipids (APL) have become a topic of interest during the last two decades, and distinct roles have been found for these biomolecules in both physiological and pathological contexts. Modifications of APL include oxidation, glycation, and adduction to electrophilic aldehydes, altogether contributing to a high structural variability of modified APL. An outstanding technique used in this challenging field is mass spectrometry (MS). MS has been widely used to unveil modified APL of biological interest, mainly when associated with soft ionization methods (electrospray and matrix-assisted laser desorption ionization) and coupled with separation techniques as liquid chromatography. This review summarizes the biological roles and the chemical mechanisms underlying APL modifications, and comprehensively reviews the current MS-based knowledge that has been gathered until now for their analysis. The interpretation of the MS data obtained by in vitro-identification studies is explained in detail. The perspective of an analytical detection of modified APL in clinical samples is explored, highlighting the fundamental role of MS in unveiling APL modifications and their relevance in pathophysiology.

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

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