Inhibitory effect of ethanolamine plasmalogen on iron- and copper-dependent lipid peroxidation

A tale of two lipids: Lipid unsaturation commands ferroptosis sensitivity

Membrane lipids play important roles in the regulation of cell fate, including the execution of ferroptosis. Ferroptosis is a non-apoptotic cell death mechanism defined by iron-dependent membrane lipid peroxidation. Phospholipids containing polyunsaturated fatty acids (PUFAs) are highly vulnerable to peroxidation and are essential for ferroptosis execution. By contrast, the incorporation of less oxidizable monounsaturated fatty acids (MUFAs) in membrane phospholipids protects cells from ferroptosis. The enzymes and pathways that govern PUFA and MUFA metabolism therefore play a critical role in determining cellular sensitivity to ferroptosis. Here, we review three lipid metabolic processes-fatty acid biosynthesis, ether lipid biosynthesis, and phospholipid remodeling-that can govern ferroptosis sensitivity by regulating the balance of PUFAs and MUFAs in membrane phospholipids.

Peroxisome-driven ether-linked phospholipids biosynthesis is essential for ferroptosis

It is well established that ferroptosis is primarily induced by peroxidation of long-chain poly-unsaturated fatty acid (PUFA) through nonenzymatic oxidation by free radicals or enzymatic stimulation of lipoxygenase. Although there is emerging evidence that long-chain saturated fatty acid (SFA) might be implicated in ferroptosis, it remains unclear whether and how SFA participates in the process of ferroptosis. Using endogenous metabolites and genome-wide CRISPR screening, we have identified FAR1 as a critical factor for SFA-mediated ferroptosis. FAR1 catalyzes the reduction of C16 or C18 saturated fatty acid to fatty alcohol, which is required for the synthesis of alkyl-ether lipids and plasmalogens. Inactivation of FAR1 diminishes SFA-dependent ferroptosis. Furthermore, FAR1-mediated ferroptosis is dependent on peroxisome-driven ether phospholipid biosynthesis. Strikingly, TMEM189, a newly identified gene which introduces vinyl-ether double bond into alkyl-ether lipids to generate plasmalogens abrogates FAR1-alkyl-ether lipids axis induced ferroptosis. Our study reveals a new FAR1-ether lipids-TMEM189 axis dependent ferroptosis pathway and suggests TMEM189 as a promising druggable target for anticancer therapy.

Composition of plasmalogens in serum lipoproteins from patients with non-alcoholic steatohepatitis and their susceptibility to oxidation

BACKGROUND

Plasmalogens are ether phospholipids (PL) with an alkenyl group including vinyl ether bound at the sn-1 position and a polyunsaturated fatty acid bound at the sn-2 position, and are susceptible to oxidation. To date, there are no reports on the relationship between plasmalogen in serum lipoproteins and non-alcoholic steatohepatitis (NASH), caused by multiple factors including oxidative stress. Here, we have investigated the distribution of plasmalogens in serum lipoproteins isolated from NASH patients and healthy volunteers.

METHODS

Serum lipoproteins were separated by gel-filtration chromatography, and analyzed for ethanolamine and choline plasmalogens using liquid chromatography-mass spectrometry.

RESULTS

Both plasmalogen levels were higher in HDL than in VLDL or LDL. The plasmalogens/PL ratio was significantly lower in NASH than controls, for all lipoprotein fractions. Ethanolamine plasmalogens containing 20:4 and 22:6 at the sn-2 position and choline plasmalogens containing 16:0 at the sn-1 position were predominant in each group. In oxidation test using LDL from healthy serum, both types of plasmalogens were decreased during the early stages of oxidation.

CONCLUSION

Plasmalogens could be a potential biomarker for evaluating the early stages of oxidation in NASH.

Oxidative environment causes molecular remodeling in embryonic heart-a metabolomic and lipidomic fingerprinting analysis

Environmental factors including pollution affect human health, and the unifying factor in determining toxicity and pathogenesis for a wide array of environmental factors is oxidative stress. Here, we created the oxidative environment with 2,2-azobis (2-amidinopropane) dihydrochloride (AAPH) and consequent cardiac remodeling in chick embryos. The metabolite fingerprint of heart tissue was obtained from Fourier transform infrared (FTIR) spectroscopic analysis. The global lipidomic analysis was done using electrospray ionization coupled with tandem mass spectrometry (ESI-MS/MS) by precursor ion scanning and neutral loss scanning methods. Further, the fatty acid levels were quantified in AAPH-treated H9c2 cardiomyoblasts with gas chromatography-mass spectrometry (GC-MS). Lipidomic fingerprinting study indicated that majority of differentially expressed phospholipids species in heart tissue belonged to ether phosphatidylcholine (ePC) species, and we conclude that excess oxidative environment may alter the phospholipid metabolism at earlier stages of cardiac remodeling.

Myoglobinemia markers with potential applications in forensic sample analysis: lipid markers in myoglobinemia for postmortem blood

The crush syndrome, in which rhabdomyolysis and trauma occur as a result of heat stroke and drug intoxication, can lead to myoglobinemia. This condition can be diagnosed by measuring myoglobin (Mb) levels in blood and urine. However, postmortem Mb levels are unreliable indicators, since blood Mb concentration drastically increases within a very short time after death and urine cannot always be obtained at dissection; this makes it difficult to diagnose myoglobinemia in a corpse. To address this issue, in this study, we used a lipidomics approach to identify markers that can be used to detect myoglobinemia in postmortem blood samples. We found that increases in levels of fatty acid oxides such as stearic, oleic, linoleic, and arachidonic acid and decreases in levels of plasmalogens and phosphatidylethanolamine in the blood were associated with high Mb level. These results demonstrate that postmortem samples are amenable to lipidomics analysis and provide a set of markers other than Mb that can be used for postmortem diagnosis of myoglobinemia.

Interaction of plasmenylcholine with free radicals in selected model systems

Plasmalogens (Plg) - naturally occurring glycerophospholipids with the vinyl-ether group in the sn-1 position are generally viewed as physiological antioxidants. Although there are numerous examples of antioxidant action of plasmalogen in cell cultures and in experimental animals, this hypothesis is far from being satisfactorily proven due to substantial limitations of such studies. Thus, plasmalogen reactivity in cells results in the accumulation of toxic byproducts and the experimental design is usually too complicated to evaluate the protective function of solely one type of lipid molecular species. In this study, experiments were performed in homogenous and heterogeneous model systems consisting of solutions in organic solvents as well as micelles and liposomes containing pure synthetic plasmenylcholines. Under the experimental conditions used, chemical reactivity of plasmalogens could be attributed to specific fatty acid esterification pattern. This is important because the chemical reactivity cannot be separated from physico-chemical properties of the lipids. Time-dependent formation of phospholipid and cholesterol hydroperoxides were determined by iodometric assay and HPLC-EC. EPR oximetry and Clark electrode were employed to detect the accompanying changes in oxygen concentration. Oxidation of the studied lipids was monitored by standard colorimetric TBARS method as well as MALDI-TOF mass spectrometry. Our data indicate that the reactivity of sn-2 monounsaturated vinyl ether lipids in peroxyl radical-induced or iron-catalyzed peroxidation reactions is comparable with that of their diacyl analogs. In samples containing cholesterol and plasmalogens, oxidative processes lead to accumulation of the radical oxidation product of cholesterol. It can be concluded that the antioxidant action of plasmalogens takes place intramolecularly rather than intermolecularly and depends on the degree of unsaturation of esterified fatty acids. Thus, it is questionable if plasmalogens can really be viewed as "endogenous antioxidant", even though they may exhibit, under special conditions, protective effect.

Plasmalogens and fatty alcohols in rhizomelic chondrodysplasia punctata and Sjögren-Larsson syndrome

Plasmalogens are a special class of ether-phospholipids, best recognized by their vinyl-ether bond at the sn-1 position of the glycerobackbone and by the observation that their deficiency causes rhizomelic chondrodysplasia punctata (RCDP). The complex plasmalogen biosynthetic pathway involves multiple enzymatic steps carried-out in peroxisomes and in the endoplasmic reticulum. The rate limiting step in the biosynthesis of plasmalogens resides in the formation of the fatty alcohol responsible for the formation of an intermediate with an alkyl-linked moiety. The regulation in the biosynthesis of plasmalogens also takes place at this step using a feedback mechanism to stimulate or inhibit the biosynthesis. As such, fatty alcohols play a relevant role in the formation of ether-phospholipids. These advances in our understanding of complex lipid biosynthesis brought two seemingly distinct disorders into the spotlight. Sjögren-Larsson syndrome (SLS) is caused by defects in the microsomal fatty aldehyde dehydrogenase (FALDH) leading to the accumulation of fatty alcohols and fatty aldehydes. In RCDP cells, the defect in plasmalogens is thought to generate a feedback signal to increase their biosynthesis, through the activity of fatty acid reductases to produce fatty alcohols. However, the enzymatic defects in either glyceronephosphate O-acyltransferase (GNPAT) or alkylglycerone phosphate synthase (AGPS) disrupt the biosynthesis and result in the accumulation of the fatty alcohols. A detailed characterization on the processes and enzymes that govern these intricate biosynthetic pathways, as well as, the metabolic characterization of defects along the pathway should increase our understanding of the causes and mechanisms behind these disorders.

Erythrocyte phospholipid molecular species and fatty acids of Down syndrome children compared with non-affected siblings

The majority of children with Down syndrome (DS) develop Alzheimer's disease (AD) at an early age. Although long-chainn-3 fatty acids (FA) are protective of neurodegeneration, little is known about the FA status in DS. In the present study, we aimed to investigate whether children with DS presented altered plasma and erythrocyte membrane phospholipids (PL) FA composition, when compared with their non-affected siblings. Venous blood samples were analysed for plasma and erythrocyte membrane FA composition by TLC followed by GC techniques. Lipid molecular species were determined by electrospray ionisation/tandem MS (ESI-MS/MS). FA analysis measured by standard GC showed an increased concentration of MUFA and a decreased concentration of plasmalogens in major PL fractions, but there were no differences in the concentrations of arachidonic acid or DHA. However, as identified by ESI-MS/MS, children with DS had increased levels of the following erythrocyte PL molecular species: 16 : 0–16 : 0, 16 : 0–18 : 1 and 16 : 0–18 : 2n-6, with reduced levels of 16 : 0–20 : 4n-6 species. Children with DS presented significantly higher levels of MUFA in both plasma and erythrocyte membrane, as well as higher levels of saturated and monounsaturated molecular species. Of interest was the almost double proportion of 16 : 0–18 : 2n-6 and nearly half the proportion of 16 : 0–20 : 4n-6 of choline phosphoacylglycerol species in children with DS compared with their non-affected siblings. These significant differences were only revealed by ESI-MS/MS and were not observed in the GC analysis. Further investigations are needed to explore molecular mechanisms and to test the association between the pathophysiology of DS and the risk of AD.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Plasmalogens the neglected regulatory and scavenging lipid species

Plasmalogens are a class of phospholipids carrying a vinyl ether bond in sn-1 and an ester bond in sn-2 position of the glycerol backbone. Although they are widespread in all tissues and represent up to 18% of the total phospholipid mass in humans, their physiological function is still poorly understood. The aim of this review is to give an overview over the current knowledge in plasmalogen biology and pathology with an emphasis on neglected aspects of their involvement in neurological and metabolic diseases. Furthermore a better understanding of plasmalogen biology in health and disease could also lead to the development of better diagnostic and prognostic biomarkers for vascular and metabolic diseases such as obesity and diabetes mellitus, inflammation, neuro-degeneration and cancer.

The role of plasmalogen in the oxidative stability of neutral lipids and phospholipids

The role of ethanolamine plasmalogen extracted from bovine brain (BBEP) in maintaining oxidative stability of bulk soybean oil and liposome made with egg phospholipids (PL) was studied. In a purified soybean oil (PSO), the addition of 200 and 1000 ppm of BBEP promoted lipid oxidation at rates of 0.037 and 0.071 (all rates in ln (PV) h(-1), and PV stands for peroxide value), whereas soy lecithin (SL) added in the same amount showed a trend similar to the PSO blank, which had an oxidation rate of 0.025. The PSO with BBEP was susceptible to cupric ion catalyzed oxidation, in that the oil was oxidized much more quickly than the PSO with SL and cupric ion. In commercial soybean oil (CSO) with the presence of tocopherols, SL at 1000 ppm acted synergistically as an antioxidant with the natural tocopherols, but addition of BBEP accelerated lipid oxidation, as evidenced by the oxidative stability index (OSI) test. In the egg PL liposome, the BBEP caused a fast breakdown of the lipid hydroperoxides and consequently promoted more thiobarbituric acid reactive substance (TBARS) formation. The PL oxidation in the presence of copper in the liposome was not affected by the BBEP, which indicates that the hypothesis of ethanolamine plasmalogen (EthPm) chelating cupric ion as the antioxidation mechanism was not supported. The addition of cumene hydroperoxide to the egg PL liposome promoted lipid oxidation, as indicated by a fast development of PV and TBARS. However, the result with cumene hydroperoxide failed to differentiate the effect of BBEP and SL and their concentration on lipid oxidation. On the basis of the observations from this study, we conclude that EthPm is not an antioxidant but rather a pro-oxidant in a bulk lipid system, and it has no significant antioxidant effect for PL oxidation in the liposome.

Studies on plasmalogen-selective phospholipase A2 in brain

Plasmalogen-selective phospholipase A(2) (PlsEtn-PLA(2)) has been purified from pig brain using multiple column chromatographic procedure. The purified enzyme migrates as a single band on polyacrylamide. It is stimulated by Triton X-100 and inhibited by sodium deoxycholate. Purified PlsEtn-PLA(2) is inhibited by iodoacetate, and this inhibition can be prevented by beta-meracaptoethanol. Treatment of neuronal cell cultures with kainic acid stimulates PlsEtn-PLA(2) activity in a dose-dependent manner, and this stimulation can be blocked by Ly294486, a selective kainic acid receptor antagonist. Activities of PlsEtn-PLA(2) are markedly increased in plasma membrane and synaptosomal plasma membrane fraction prepared from nucleus basalis and hippocampal region of brains from Alzheimer disease patients compared to age-matched controls. It is proposed that accumulation of ceramide and increased expression of cytokines may be responsible for the stimulation of PlsEtn-PLA(2) in Alzheimer disease.

HOCl-mediated glycerophosphocholine and glycerophosphoethanolamine generation from plasmalogens in phospholipid mixtures

Many mammalian tissues and cells contain, in addition to (diacyl) phospholipids, considerable amounts of plasmalogens, which may function as important antioxidants. Apart from the "scavenger" function mediated by the high sensitivity of the vinyl-ether bond, the functional role of plasmalogens is so far widely unknown. Furthermore, there is increasing evidence that plasmalogen degradation products have harmful effects in inflammatory processes. In a previous investigation glycerophosphocholine (GPC) formation was verified as a novel plasmalogen degradation pathway upon oxidation with hypochlorous acid (HOCl), however these investigations were performed in simple model systems. Herein, we examine plasmalogen degradation in a more complex system in order to evaluate if GPC generation is also a major pathway in the presence of other highly unsaturated glycerophospholipids (GPL) representing an additional reaction site of HOCl targets. Using MALDI-TOF mass spectrometry and (31)P NMR spectroscopy, we confirmed that the first step of the HOCl-induced degradation of GPL mixtures containing plasmalogens is the attack of the vinyl-ether bond resulting in the generation of 1-lysophosphatidylcholine (lysoPtdCho) or 1-lysophosphatidylethanolamine. In the second step HOCl reacts with the fatty acyl residue in the sn-2 position of 1-lysoPtdCho. This reaction is about three times faster in comparison to comparable diacyl-GPL. Thus, the generation of GPC and glycerophosphoethanolamine (GPE) from plasmalogens are relevant products formed from HOCl attack on the vinyl-ether bond of plasmalogens under pathological conditions.

Synthesis and antioxidant properties of an unnatural plasmalogen analogue bearing a trans O-vinyl ether linkage

To assess the antioxidant behavior of trans-1, we first synthesized trans-allyl ether 4 by opening an (S)-glycidol derivative with an (E)-alk-2-en-ol, and then produced the unnatural E-enol ether 1 by a stereoselective iridium(I)-catalyzed olefin isomerization. Natural cis-1 was preferentially degraded by HOCl and was more protective than trans-1 against lipid peroxidation induced by a free-radical initiator, demonstrating that the geometry of the 1'-alkenyloxy bond participates in the antioxidant defensive role of 1.

Effect of double bond geometry in sphingosine base on the antioxidant function of sphingomyelin

We previously showed that sphingomyelin (SM) inhibits peroxidation of phosphatidylcholine (PC) and cholesterol. Since SM uniquely has a trans unsaturation in its sphingosine base, we investigated whether this feature is important for its antioxidant function. Substitution of the natural trans Delta(4)-double bond with a cis double bond (cis-SM), however, increased SM's ability to inhibit Cu(2+)-mediated 16:0-18:2 PC oxidation by up to eightfold. Dihydro-SM, which lacks the double bond, was equally effective as trans-SM. In contrast to its effect in the sphingosine base, the presence of a cis double bond in the N-acyl group of trans-SM was not protective. cis-SM also inhibited the oxidation of cholesterol by FeSO_(4)/ascorbate more efficiently than the trans isomer. The enhanced protective effect of cis-SM is selective for metal ion-promoted oxidation, and appears to arise from a decrease in the effective concentration of metal ions. These studies show that the trans double bond of SM is not essential for its antioxidant effects.

Evidence for the reactivity of fatty aldehydes released from oxidized plasmalogens with phosphatidylethanolamine to form Schiff base adducts in rat brain homogenates

The vinyl ether bond of plasmalogens could be among the first target of free radicals attack. Consequently, because of their location in the membranes of cells, plasmalogens represent a first shield against oxidative damages by protecting other macromolecules and are often considered as antioxidant molecules. However, under oxidative conditions their disruption leads to the release of fatty aldehydes. In this paper, we showed using gas chromatography-mass spectrometry (GC-MS) analyses that fatty aldehydes released from plasmalogens after oxidation (UV irradiation and Fe2+/ascorbate) of cerebral cortex homogenates can generate covalent modifications of endogenous macromolecules such as phosphatidylethanolamine (PE), like the very reactive and toxic malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE). These newly formed Schiff base adducts could be responsible for deleterious effects on cells thus making the protective role of plasmalogens potentially questionable.

Therapeutic uses of antioxidant liposomes

This review will focus on the therapeutic uses of antioxidant liposomes. Antioxidant liposomes have a unique ability to deliver both lipid- and water-soluble antioxidants to tissues. This review will detail the varieties of antioxidants which have been incorporated into liposomes, their modes of administration, and the clinical conditions in which antioxidant liposomes could play an important therapeutic role. Antioxidant liposomes should be particularly useful for treating diseases or conditions in which oxidative stress plays a significant pathophysiological role because this technology has been shown to suppress oxidative stress. These diseases and conditions include cancer, trauma, irradiation, retinotherapy or prematurity, respiratory distress syndrome, chemical weapon exposure, and pulmonary infections.

Primary aminophospholipids in the external layer of liposomes protect their component polyunsaturated fatty acids from 2,2'-azobis(2-amidinopropane)- dihydrochloride-mediated lipid peroxidation

We showed in our previous study that docosahexaenoic acid-rich phosphatidylethanolamine in the external layer of small-size liposomes, as a model for biomembranes, protected its docosahexaenoic acid from 2,2'-azobis(2-amidinopropane)dihydrochloride- (AAPH-) mediated lipid peroxidation in vitro. Besides phosphatidylethanolamine, both phosphatidylserine and an alkenyl-acyl analogue of phosphatidylethanolamine, phosphatidylethanolamine plasmalogen, are reported to possess characteristic antioxidant activities. However, there are few reports about the relationship between the protective activity of phosphatidylethanolamine plasmalogen and/or phosphatidylserine against lipid peroxidation and their distribution in a phospholipid bilayer. Furthermore, it is unclear whether phosphatidylethanolamine plasmalogen and/or phosphatidylserine protect their component polyunsaturated fatty acids (PUFAs) from lipid peroxidation. In the present study, we examined the relationship between the transbilayer distribution of aminophospholipids, such as phosphatidylethanolamine rich in arachidonic acid, phosphatidylethanolamine plasmalogen, and phosphatidylserine, and the oxidative stability of their component PUFAs. The transbilayer distribution of these aminophospholipids in liposomes was modulated by coexisting phosphatidylcholine bearing two types of acyl chain: dipalmitoyl or dioleoyl. The amounts of these primary aminophospholipids in the external layer became significantly higher in liposomes containing dioleoylphosphatidylcholine than in those containing dipalmitoylphosphatidylcholine. Phosphatidylethanolamine rich in arachidonic acid, phosphatidylethanolamine plasmalogen or phosphatidylserine in the external layer of liposomes, as well as external docosahexaenoic acid-rich phosphatidylethanolamine, were able to protect their component PUFAs from AAPH-mediated lipid peroxidation.

Structural characterization of plasmenylcholine photooxidation products

Oxidative damage to plasmenyl-type lipids contributes to decreased membrane barrier function, loss of membrane structure and formation of nonlamellar defects in membrane bilayers. Previous results from this laboratory have shown that membrane-soluble sensitizers (e.g. zinc phthalocyanine and bacteriochlorophyll a) mediate the photooxidation of palmitoyl plasmenylcholine (1-O-alk-1'-Z-enyl-2-palmitoyl-sn-glycero-3-phosphocholine; PPlsC) vesicles with the subsequent creation of lamellar defect structures, vesicle contents leakage and membrane-membrane fusion. Because plasmalogen lipids are significant components of sarcoplasma and myelin membranes, we sought to characterize the products of their photooxidation. This study focuses on the photooxidation of PPlsC vesicles in the presence of the water-soluble sensitizer, aluminum phthalocyanine tetrasulfonate (AlPcS4(4-)). Attack of photogenerated singlet oxygen on the 1-O-alkenyl ether linkage of PPlsC lipids was expected to generate dioxetane- and ene-type photoproducts. The products formed during continuous aerobic irradiation (28 mW/cm2, (610 nm) of PPlsC vesicles in the presence of AlPcS4(4-) were separated via reverse-phase high-performance liquid chromatography (HPLC) with electrochemical detection (ECD) or evaporative light-scattering detection (ELSD). Photooxidized dipalmitoyl-phosphatidylcholine-cholesterol vesicles (control) were used to optimize the HPLC-ECD conditions, using 7 alpha-hydroperoxy-cholesterol as standard. HPLC-ECD was found to be most sensitive for PPlsC hydroperoxides, whereas HPLC-ELSD was more sensitive for nonhydroperoxide photoproducts. The three major photoproducts formed during vesicle irradiation were isolated via preparative HPLC and then characterized by 1H-nuclear magnetic resonance and mass spectrometry. 1-Formyl-2-palmitoyl-sn-glycero-3-phosphocholine and 1-hydroxy-2-palmitoyl-sn-glycero-3-phosphocholine were identified as dioxetane cleavage products that coeluted at approximately 3 min. The second fraction (retention time [RT] = 48 min) was identified as a PPlsC allylic hydroperoxide. The third photoproduct, eluting at RT = 64 min, is tentatively identified as an oxidation product arising from allylic hydroperoxide degradation via Hock rearrangement or free radical decomposition.

Plasmalogens protect unsaturated lipids against UV-induced oxidation in monolayer

Oxidative stress results from the attack by free radicals of several cellular targets (proteins, DNA and lipids). The cell equilibrium is a direct consequence of the pro-/antioxidant balance. In order to understand the physiological processes involved in oxidative stress, we followed oxidation of unsaturated lipids using a biomimetic system: Langmuir monolayers. The oxidation mode chosen was UV-irradiation and the lipid model was a polyunsaturated phospholipid: 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC). The monomolecular film technique was used to measure membrane rheology before and after UV-irradiation. We showed that the UV-irradiation of a DLPC monomolecular film led to a molecular area and surface elasticity modulus decrease that attests to the apparition of new molecular species at the air-water interface. The antioxidant effect of a synthetic plasmalogen (1-O-(1'-(Z)-hexadecenyl)-2-O-oleoyl-sn-glycero-3-phosphocholine or P(PLM)OPE) was tested on the oxidation of DLPC. Indeed, for about 25% mol P(PLM)OPE in mixed DLPC/P(PLM)OPE monolayers, a complete inhibition of the molecular area and the surface elasticity modulus decreases was observed in our experimental conditions. Lower P(PLM)OPE quantities delayed but did not prevent the DLPC oxidation in mixed monolayers.

Ingestion of plasmalogen markedly increased plasmalogen levels of blood plasma in rats

Plasmalogens, a subclass of phospholipids, are widely distributed in human and animals, and are taken into the body as food. However, no data exist on the intestinal absorption or fate of ingested plasmalogen. Here, we determined whether dietary plasmalogen is absorbed and whether blood and tissue concentrations increased in normal male Wistar rats by using four separate experiments. Phospholipids containing more than 20 wt% of plasmalogen extracted from the bovine brain were incorporated into test diets (10-15 wt%). In experiment 1, we estimated the absorption rate by measuring the plasmalogen vinyl ether bonds remaining in the alimentary tract of rats after the ingestion of 2 g of test diet containing 91 micromol plasmalogen. The absorption rate of plasmalogen was nearly 80 mol% after 4 h, comparable to the total phospholipid content in the test diet. In experiment 2, we observed no degradation of the plasmalogen vinyl ether bonds under in vitro conditions simulating those of the stomach and small intestinal lumen. In experiment 3 we confirmed a comparable absorption (36 mol%) by using a closed loop of the upper small intestine in anesthetized rats 90 min after injecting a 10 wt% brain phospholipid emulsion. Feeding a test diet containing 10 wt% brain phospholipids for 7 d increased plasmalogen concentration threefold in blood plasma and by 25% in the liver; however, no increases were seen in blood cells, skeletal muscle, brain, lungs, kidneys, or adipose tissue (experiment 4). We concluded that dietary plasmalogen is absorbed from the intestine and contributes to a large increase in plasmalogen levels in blood plasma.

Lymphatic absorption of plasmalogen in rats

Plasmalogen is a subclass of phospholipids that is widely distributed in man and animals. Many physiological roles have been proposed for this lipid; however, there have been no reports on the intestinal absorption of plasmalogen. In the present study, we examined lymphatic absorption of plasmalogen after the duodenal infusion of emulsified brain phospholipids (BPL) containing plasmalogen (22 mol % of total phospholipids) and soyabean lecithin (SPL) (100 g emulsified phospholipid/l). Male Wistar rats with implanted cannulas in the mesenteric lymph duct and the duodenum were kept in a Bollman-type restraining cage, and were infused the emulsion after 1 d recovery with duodenal infusion of a glucose-NaCl solution. Lymphatic plasmalogen output was increased at 2-4 h after the switch to BPL emulsion, and peaked at 4-6 h. However, no increases were observed after SPL infusion. Lymphatic recovery of plasmalogen for 8 h was 198 nmol, which was 0.22 mol % of the total plasmalogen disappeared from the intestine. We did not detect any increases in long-chain fatty aldehydes, which are the degradation product of plasmalogen, either in the blood or the small intestine. We conclude that a small percentage but a significant amount of the plasmalogen was absorbed into the lymph.

Covalent binding of hydroxy-alkenals 4-HDDE, 4-HHE, and 4-HNE to ethanolamine phospholipid subclasses

Lipid oxidation is implicated in a wide range of pathophysiogical disorders, and leads to reactive compounds such as fatty aldehydes, of which the most well known is 4-hydroxy-2E-nonenal (4-HNE) issued from 15-hydroperoxyeicosatetraenoic acid (15-HpETE), an arachidonic acid (AA) product. In addition to 15-HpETE, 12(S)-HpETE is synthesized by 12-lipoxygenation of platelet AA. We first show that 12-HpETE can be degraded in vitro into 4-hydroxydodeca-(2E,6Z)-dienal (4-HDDE), a specific aldehyde homologous to 4-HNE. Moreover, 4-HDDE can be detected in human plasma. Second, we compare the ability of 4-HNE, 4-HDDE, and 4-hydroxy-2E-hexenal (4-HHE) from n-3 fatty acids to covalently modify different ethanolamine phospholipids (PEs) chosen for their biological relevance, namely AA- (20: 4n-6) or docosahexaenoic acid- (22:6n-3) containing diacyl-glycerophosphoethanolamine (diacyl-GPE) and alkenylacyl-glycerophosphoethanolamine (alkenylacyl-GPE) molecular species. The most hydrophobic aldehyde used, 4-HDDE, generates more adducts with the PE subclasses than does 4-HNE, which itself appears more reactive than 4-HHE. Moreover, the aldehydes show higher reactivity toward alkenylacyl-GPE compared with diacyl-GPE, because the docosahexaenoyl-containing species are more reactive than those containing arachidonoyl. We conclude that the different PE species are differently targeted by fatty aldehydes: the higher their hydrophobicity, the higher the amount of adducts made. In addition to their antioxidant potential, alkenylacyl-GPEs may efficiently scavenge fatty aldehydes.

Docosahexaenoic acid abundance in the brain: a biodevice to combat oxidative stress

Docosahexaenoic acid (DHA) (22:6) is a polyunsaturated fatty acid of the n - 3 series which is believed to be a molecular target for lipid peroxides (LPO) formation. Its ubiquitous nature in the nervous tissue renders it particularly vulnerable to oxidative stress, which is high in brain during normal activity because of high oxygen consumption and generation of reactive oxygen species (ROS). Under steady state conditions potentially harmful ROS and LPO are maintained at low levels due to a strong antioxidant defense mechanism, which involves several enzymes and low molecular weight reducing compounds. The present review emphasizes a paradox: a discrepancy between the expected high oxidability of the DHA molecule due to its high degree of unsaturation and certain experimental results which would indicate no change or even decreased lipid peroxidation when brain tissue is supplied or enriched with DHA. The following is a critical review of the experimental data relating DHA levels in the brain to lipid peroxidation and oxidative damage there. A neuroprotective role for DHA, possibly in association with the vinyl ether (VE) linkage of plasmalogens (pPLs) in combating free radicals is proposed.

Platelet phospholipids are differentially protected against oxidative degradation by plasmalogens

The oxidative degradation of phospholipids in the presence and absence of plasmalogens (plasmenyl phosphatidylethanolamine: PPE) was followed by chemical analysis. Human platelet phospholipids, either intact or after removal of PPE by acid treatment, were oxidized with 28 mM 2,2'-azobis(2-amidinopropane di-HCl in Triton X-100 micelles (detergent/phospholipid 5:1, mol/mol). PPE (12% of all phospholipids, mol/mol) disappeared about three times more rapidly than glycerophospholipids, whereas sphingomyelin remained unaltered and the lysophosphatidylethanolamine (lysoPE) generated became progressively more unsaturated. After 60 min oxidation, the FA compositions of PS, PC, and PI were similar in extracts with or without plasmalogens. In contrast, diacyl phosphatidylethanolamine (DPE) became more saturated in the absence of PPE. The rate of phospholipid destruction was always unique to each class, but for all phospholipids slowed down in the presence of PPE. This protective effect increased in the order DPE < PS < PC < PI and did not seem to be simply related to the class unsaturation. Alpha-tocopherol had no influence on the time courses of the quantities and compositions of the phospholipids, even at a molar ratio of alpha-tocopherol to phospholipids four times higher than in platelet membranes. Thus, PPE protected phospholipids efficiently but differentially against peroxidative attack, whereas the contribution of alpha-tocopherol appeared to be negligible even at a concentration four times greater than in platelet membranes.

Therapeutic uses of antioxidant liposomes

This chapter focuses on the use of antioxidant liposomes in the general area of free radical biology and medicine. The term antioxidant liposome is relatively new and refers to liposomes containing lipid-soluble chemical antioxidants, water-soluble chemical antioxidants, enzymatic antioxidants, or combinations of these various antioxidants. The role of antioxidants in health and disease has been extensively discussed, and many excellent reviews and books are available (1–3). Antioxidant liposomes hold great promise in the treatment of many diseases in which oxidative stress plays a prominent role. Oxidative stress is a physiological condition in which the production of damaging free radicals exceeds the in vivo capacity of antioxidant protection mechanisms to prevent pathophysiology. Free radicals are molecules with unpaired electrons, often highly reactive and damaging to biological systems. The biological membranes of subcellular organelles are a major site of free radical damage but proteins and DNA are also significant targets. Moreover, free radicals can alter cellular signal transduction pathways and stimulate the synthesis of inflammatory cytokines. Oxygen radicals and other reactive oxygen species (ROS) arise from the single electron reductions of oxygen.

Plasmalogen degradation by oxidative stress: production and disappearance of specific fatty aldehydes and fatty alpha-hydroxyaldehydes

Plasmalogens are often considered as antioxidant molecules that protect cells from oxidative stress. Their vinyl ether bond could indeed be among the first targets for newly formed radicals. However, the long chain aldehydes released from plasmalogens were seldom studied and possible injurious or harmless effects were poorly examined. Thus, the sensitivity of the vinyl ether bond of plasmalogens was investigated in a cerebral cortex homogenate under UV irradiation- or Fe2+/ascorbate-induced peroxidation. Kinetics of aldehyde production was followed by gas chromatography/mass spectrometry. This confirmed that plasmalogens were highly sensitive to oxidative stress (70% cleavage after 90 min UV irradiation and 30% after 30 min of Fe2+/ascorbate). The aldehydes corresponding to sn-1 position 16:0, 18:0, or 18:1 were poorly detected. Conversely, oxidation of plasmalogens yielded preferentially 15:0, 17:0, and 17:1 aldehydes under UV and the alpha-hydroxyaldehydes 16:0-OH and 18:0-OH following a Fe2+/ascorbate oxidation. Kinetics showed that free aldehydes and above all free alpha-hydroxyaldehydes disappeared from the medium as soon as produced. Consequently, the behavior of these released aldehydes in the tissues has to be investigated in order to ascertain the protective effect of plasmalogens against oxidation.

Plasmalogen levels in serum from patients with impaired carbohydrate or lipid metabolism and in elderly subjects with normal metabolic values

The precise role played by plasmalogen phospholipids (PL) of lipoproteins and cell membranes is not well understood. However, they might act as endogenous antioxidants in defending cell membranes and lipoproteins from reactive oxygen species. A decline of plasmalogen concentrations has been observed in some tissues in normal aging and in some pathologic conditions. For healthy adults, we had reported negative correlations of age with serum plasmalogen PL derived heaxadecanal dimethylacetal (16:0DMA) or octadecanal dimethylacetal (18:0DMA) values. To mark off these age associated changes from disturbances in glucose or lipid metabolism, this study compares the 16:0DMA and 18:0DMA data of serum PL from 118 elderly subjects, aged 57-94 years, and grouped according to the disturbance of glucose or lipid metabolism. Using a new synthetic test mixture of 16:0DMA with oleic acid butylester as a quality control in gas chromatography, the highest 16:0DMA values were found in hypercholesterolemic subjects. However, related to the bulk of serum PL, were the plasmalogens possibly act as antioxidants, the highest values of 16:0DMA/PL were found in controls. A negative correlation of serum triglycerides (TG) with 16:0DMA was detected (n=118). The data suggest a closer association of low DMA values with elevated TG levels as compared to elevated plasma glucose or other serum lipid levels.

Plasmalogens: workhorse lipids of membranes in normal and injured neurons and glia

Plasmalogens are unique glycerophospholipids because they have an enol ether double bond at the sn-1 position of the glycerol backbone. They are found in all mammalian tissues, with ethanolamine plasmalogens 10-fold higher than choline plasmalogens except in muscles. The enol ether double bond at the sn-1 position makes plasmalogens more susceptible to oxidative stress than the corresponding ester-bonded glycerophospholipids. Plasmalogens are not only structural membrane components and a reservoir for second messengers but may also be involved in membrane fusion, ion transport, and cholesterol efflux. Plasmalogens may also act as antioxidants, thus protecting cells from oxidative stress. Receptor-mediated degradation of plasmalogens by plasmalogen-selective phospholipase A2 results in the generation of arachidonic acid, eicosanoids, and platelet activating factor. Low levels of these metabolites have trophic effects, but at high concentration they are cytotoxic and may be involved in allergic response, inflammation, and trauma. Levels of plasmalogens are decreased in several neurological disorders including Alzheimer's disease, ischemia, and spinal cord trauma. This may be due to the stimulation of plasmalogen-selective phospholipase A2. A deficiency of plasmalogens in peroxisomal disorders and Niemann-Pick type C disease indicates that this deficiency may be due to the decreased activity of plasmalogen synthesizing enzymes that occur in peroxisomes.

Inhibition of peroxyl radical-mediated lipid oxidation by plasmalogen phospholipids and alpha-tocopherol

The recently discovered peroxyl radical scavenging properties of plasmalogen phospholipids led us to evaluate their potential interactions with alpha-tocopherol. The oxidative decay of plasmalogen phospholipids and of polyunsaturated fatty acids as induced by peroxyl radicals (generated from 2,2'-azobis-2-amidinopropane hydrochloride; AAPH) was studied in micelles using 1H-NMR and chemical analyses. In comparison with alpha-tocopherol, a 20- to 25-fold higher concentration of plasmalogen phospholipids was needed to induce a similar inhibition of peroxyl radical-mediated oxidation of polyunsaturated fatty acids. Plasmalogen phospholipids and alpha-tocopherol protected each other from oxidative degradation. In low-density lipoproteins (LDL) and micelles supplemented with plasmalogen phospholipids plus alpha-tocopherol, the peroxyl radical-promoted oxidation was additively diminished. The differences in the capacities to inhibit oxidation processes induced by peroxyl radicals between the plasmalogen phospholipids and alpha-tocopherol were less pronounced in the LDL particles than in the micelles. In conclusion, plasmalogen phospholipids and alpha-tocopherol apparently compete for the interaction with the peroxyl radicals. Oxidation processes induced by peroxyl radicals are inhibited in an additive manner in the presence of the two radical scavengers. The contribution of the plasmalogen phospholipids to the protection against peroxyl radical promoted oxidation in vivo is expected to be at least as important as that of alpha-tocopherol.

Role of plasmalogens in the enhanced resistance of LDL to copper-induced oxidation after LDL apheresis

Extracorporeal reduction of plasma low density lipoproteins (LDLs) by LDL apheresis was shown to attenuate the proatherogenic influences of LDL, such as impairment of vasodilation and increased monocyte adhesion to the endothelium. In 16 patients with familial hypercholesterolemia, we analyzed whether LDL apheresis by the heparin precipitation procedure affected the oxidative resistance of LDL. Plasma LDL cholesterol concentrations were reduced by 65% after the apheresis. The lag time of copper-mediated LDL oxidation was increased from 103 to 117 minutes (P<0.0005). The LDL contents of alpha-tocopherol and beta-carotene, as well as the ratio of monounsaturated to polyunsaturated fatty acids in LDL, were not altered. However, the LDL apheresis induced a 15% increase in the LDL contents of plasmalogen phospholipids (P<0.0005), a class of ether phospholipids that were recently shown to prevent lipid oxidation. The phosphatidylcholine (PC) to lysoPC ratio was elevated by 16% after the apheresis (P<0.0005). The percent increase in LDL plasmalogen phospholipids showed a close association with the increased lag time after apheresis (P<0.0005). The LDL plasmalogen contents of the blood samples from patients and from normolipidemic donors were also positively related to the lag time (P<0.005). In vitro loading of LDL with plasmalogen phospholipids resulted in a prolongation of the lag time and an increase in the PC/lysoPC ratio. In conclusion, the rapid rise in LDL contents of plasmalogen phospholipids most probably causes the increase in lag time after LDL apheresis. Plasmalogens appear to play an important role in the oxidation resistance of LDL in vivo.

Synthesis of plasmalogens in eye lens epithelial cells

The present paper describes cloning and sequencing of the mouse cDNA encoding dihydroxyacetonephosphate acyltransferase (DAPAT), the peroxisomal key enzyme of plasmalogen (PM) biosynthesis. Using monospecific antibodies, we localized DAPAT and alkyl dihydroxyacetonephosphate synthase to peroxisomes of mouse lens epithelial cells (LECs) and determined their enzymatic activity. By electrospray ionization mass spectrometry of mouse lens lipid extracts, we identified phosphatidyl ethanolamine including plasmenyl ethanolamine species as major constituents. Our data demonstrate the capacity of LECs to synthesize PMs and the high coincidence between deficiency of PM and early manifestation of cataract in patients with peroxisomal disorders suggests that ether-bonded lipids may play an important role in maintaining lens transparency.

The protective role of plasmalogens in iron-induced lipid peroxidation

The role of plasmalogens in iron-induced lipid peroxidation was investigated in two liposomal systems. The first consisted of total brain phospholipids with and without plasmalogens, and the second of phosphatidylethanolamine/phosphatidylcholine liposomes with either diacyl- or alkenylacyl-phosphatidylethanolamine. By measuring thiobarbituric acid reactive substances, oxygen consumption, fatty acids and aldehydes, we show that plasmalogens effectively protect polyunsaturated fatty acids from oxidative damage, and that the vinyl ether function of plasmalogens is consumed simultaneously. Furthermore, the lack of lag phase, the increased antioxidant efficiency with time, and the experiments with lipid- and water-soluble azo compounds, indicate that plasmalogens probably interfere with the propagation rather than the initiation of lipid peroxidation, and that the antioxidative effect cannot be related to iron chelation.

The biological significance of plasmalogens in defense against oxidative damage

The phospholipid class of plasmalogens is ubiquitously found in considerable amounts as a constituent of mammalian cell membranes and of plasma lipoproteins. Plasmalogens are more susceptible to oxidative reactions compared to their fatty acid ester analogues, due to the reactivity of their enolether function. Studies on plasmalogen-deficient cell lines lead to the proposal that these ether lipids serve as endogenous antioxidants. No clear conclusions regarding the antioxidative effects of plasmalogens could be drawn from studies in patients of different ages with peroxisomal deficiency disorders. A defective peroxisomal plasmalogen synthesis is not necessarily associated with other defects in the metabolism of peroxisomes, as has been established in a cell line recently. In different mammalian tissues a decrease of plasmalogens with age was described. Moreover, an accumulation of plasmalogen oxidation products was measured in brain of old cattle compared to young ones. In pathologic conditions associated with oxidative stress like in spinal cord ischemia and reperfusion, plasmalogen levels varied inversely according to the oxidative burden. Oxidation products of plasmalogens increased with time of ischemia in infarcted porcine heart tissue. Enrichment of lipoproteins with plasmalogens increased their oxidative resistance, which was diminished in the case of LDL particles in patients with coronary arteriosclerosis. In red cell membranes plasmalogens were reduced with donor age and in hyperlipidemia. Under lipid lowering therapy with lovastatin an increase was observed, indicating a possible antioxidative impact of this treatment. Taken together, there is good evidence that plasmalogens are effective as endogenous antioxidants. However, more experimental approaches not confounded by other lipolytic processes are needed to establish this role of plasmalogens.

Isolation and identification of glycated aminophospholipids from red cells and plasma of diabetic blood

Glycosylation is a major pathway for posttranslational modification of tissue protein and begins with nonenzymatic addition of carbohydrate to the primary amino groups. Excessive glycation of tissue protein has been implicated in the pathogenesis of diabetes and ageing. While glycation of aminophospholipids has also been postulated, glycated aminophospholipids have not been isolated. Using normal phase HPLC with on-line electrospray mass spectrometry we found glycated ethanolamine phospholipids to make up 10-16% of the total phosphatidylethanolamine (PE) of the red blood cells and plasma of the diabetic subjects. The corresponding values for glycated PE of control subjects were 1-2%.