Structural analysis of hydroperoxides formed by oxidation of phosphatidylcholine with singlet oxygen

Epoxides: an underestimated lipid oxidation product

Immense gains in understanding of mechanisms and effects of lipid oxidation have been achieved in the nearly 90 years over which lipid oxidation has been an active research focus. Even so, the substantial questions still being raised about lipid oxidation in this special issue show clearly that missing pieces remain and must be considered for full accounting of this important reaction in any system. In this context, epoxides are spotlighted as a critical overlooked product of lipid autoxidation - underestimated in analysis, underestimated in presence as a functionally active and competitive intermediate and product of lipid oxidation, and underestimated in potential contributions to impact of lipid oxidation on other molecules and cell functions. Logical reasons for ignoring or not finding epoxides are offered in historical development of lipid oxidation knowledge. Reactions generating lipid epoxides in autoxidation are reviewed, limitations in detecting and tracking epoxides are outlined to explain why epoxides may not be detected when they should be present, and justifications for increased research and analysis of epoxides are argued. The main goal is to provide a context for recognizing epoxides as critical products that must be accounted for in determining the state rather than extent of lipid oxidation and in tracking its consequences in oils, foods, personal care products, and tissues. A secondary goal is to stimulate new research using contemporary analyses to fill in the gaps of knowledge about epoxide formation, structure, and reactions in lipid autoxidation.

Targeted Lipidomics Reveal the Effects of Different Phospholipids on the Phospholipid Profiles of Hepatic Mitochondria and Endoplasmic Reticulum in High-Fat/High-Fructose-Diet-Induced Nonalcoholic Fatty Liver Disease Mice

The lipid alternation in mitochondria and endoplasmic reticulum (ER) might be indicative of their abnormal morphology and function, which contribute to development of nonalcoholic fatty liver disease (NAFLD). However, the influence of dietary phospholipids (PLs) on the PL composition of the organellar membrane is largely unknown. High-fat/high-fructose (HFHF)-diet-induced NAFLD mice were administrated with different PLs (2%, w/w) with specific fatty acids and headgroups, including eicosapentaenoic acid (EPA)-phosphatidylcholine (PC)/phosphatidylethanolamine (PE)/phosphatidylserine (PS), docosahexaenoic acid (DHA)-PC/PE/PS, egg-PC/PE/PS, and soy-PC/PE/PS. After 8 weeks of feeding, PLs dramatically decreased hepatic lipid accumulation, in which EPA/DHA-PS had the best efficiency. Furthermore, lipidomic analysis revealed that the HFHF diet narrowed the difference in PL composition between mitochondria and ER, significantly reduced the PC/PE ratio, and changed the unsaturation of cardiolipin in mitochondria. Dietary PLs reversed these alterations. Heatmap analysis indicated that dietary PL groups containing the same fatty acids clustered together. Moreover, dietary PLs significantly increased the ratio of PC/PE in both hepatic mitochondria and ER, especially EPA-PE. This study showed that fatty acid composition of PLs might represent greater impact on the PL composition of the organellar membrane than headgroups.

Direct exposure of non-equilibrium atmospheric pressure plasma confers simultaneous oxidative and ultraviolet modifications in biomolecules

Thermal plasmas and lasers are used in medicine to cut and ablate tissues and for coagulation. Non-equilibrium atmospheric pressure plasma (NEAPP) is a recently developed, non-thermal technique with possible biomedical applications. Although NEAPP reportedly generates reactive oxygen/nitrogen species, electrons, positive ions, and ultraviolet radiation, little research has been done into the use of this technique for conventional free radical biology. Recently, we developed a NEAPP device with high electron density. Electron spin resonance spin-trapping revealed ^•OH as a major product. To obtain evidence of NEAPP-induced oxidative modifications in biomolecules and standardize them, we evaluated lipid peroxidation and DNA modifications in various in vitro and ex vivo experiments. Conjugated dienes increased after exposure to linoleic and α-linolenic acids. An increase in 2-thiobarbituric acid-reactive substances was also observed after exposure to phosphatidylcholine, liposomes or liver homogenate. Direct exposure to rat liver in saline produced immunohistochemical evidence of 4-hydroxy-2-nonenal- and acrolein-modified proteins. Exposure to plasmid DNA induced dose-dependent single/double strand breaks and increased the amounts of 8-hydroxy-2'-deoxyguanosine and cyclobutane pyrimidine dimers. These results indicate that oxidative biomolecular damage by NEAPP is dose-dependent and thus can be controlled in a site-specific manner. Simultaneous oxidative and UV-specific DNA damage may be useful in cancer treatment.

Phospholipid hydroperoxides are detoxified by phospholipase A2 and GSH peroxidase in rat gastric mucosa

The aim of this study was to determine the metabolic fate of phospholipid hydroperoxides (PLOOH) in rat gastric mucosa. Here we report evidence concerning the mechanism for PLOOH detoxification in gastric mucosa homogenate. Analysis by the TLC blot technique showed that the gastric mucosa has the highest potential to eliminate 1-palmitoyl-2-linoleoyl-phosphatidylcholine hydroperoxides (PL-PtdChoOOH) compared with the intestinal mucosa and liver. Major products detected after incubation with gastric mucosa were the partially reduced linoleic acid hydroperoxides (LAOOH) and lysophosphatidylcholine, indicating the involvement of phospholipase A2 (PLA2) in the elimination pathway. Using unilamellar vesicles, we demonstrated that gastric mucosal PLA2 does not distinguish between PLOOH and intact phospholipids. Although gastric mucosal PLA2 activity efficiently eliminated excess amounts of PLOOH, the complete reduction of LAOOH was dependent on the supply of exogenous GSH. In a separate experiment, administration of egg yolk PtdChoOOH to rats for 6 d significantly elevated GSH peroxidase (GPx) activity in the gastric mucosa. We concluded that excess amounts of PLOOH are efficiently eliminated through the hydrolysis by PLA2, and the subsequent reduction of FA hydroperoxide by GPx is the critical step for complete detoxification of oxidized phospholipids in the stomach.

Development of phosphine reagents for the high-performance liquid chromatographic-fluorometric determination of lipid hydroperoxides

Phosphine reagents were designed and synthesized as a new type of fluorescent reagents for the determination of lipid hydroperoxides in foodstuff and biological materials. All phosphine reagents prepared had no fluorescence but their oxides, which were produced by the reaction of the phosphines with hydroperoxides, had strong fluorescence. Among the phosphine reagents prepared, diphenyl-1-pyrenylphosphine had the most suitable properties as a fluorescent reagent and was successfully applied to the determination of hydroperoxides by batch, flow injection and HPLC post-column methods.

Mechanisms of tubulin modification by phosphatidylcholine hydroperoxides

The interaction of lipid peroxides with cellular proteins has been postulated to contribute to cellular aging. A potential target for such effects is tubulin, the building block of microtubules. We examined the concentration-dependent effects of phosphatidylcholine hydroperoxides on the ability of tubulin to polymerize into microtubules. The results demonstrated that even very low concentrations of peroxides were sufficient to interfere with the tubulin and, therefore, the microtubule function. Decreased tubulin activity (as measured by tubulin GTPase activity) showed correlation with the modification of methionine and cysteine in tubulin and a change in the tubulin conformational state as indicated by fluorescence and ultraviolet spectroscopic measurements. As no effect on electric conductivity was observed, indicating that modulation of ionic binding was not involved, the interaction mechanism may be a hydrophobic one.

Determination of singlet oxygen-specific versus radical-mediated lipid peroxidation in photosensitized oxidation of lipid bilayers: effect of beta-carotene and alpha-tocopherol

Photosensitized oxidation reactions damage tissue by catalyzing the formation of oxyradicals and singlet oxygen. beta-Carotene is hypothesized to exert photoprotective effects by quenching singlet oxygen formed by Type II reactions and by scavenging free radicals formed by Type I reactions. beta-Carotene antioxidant mechanisms were studied in a phospholipid membrane model of photooxidation with a new isotope dilution gas chromatography-mass spectrometry (GC-MS) assay that quantitatively distinguishes singlet oxygen-mediated and radical-mediated lipid peroxidation. This assay measures 9- and 10-hydroxylinoleate methyl esters and was used to generate photooxidation profiles for the photosensitizers methylene blue, Rose Bengal, and tetraphenylporphine. These profiles indicate a shift from Type II to Type I photooxidation mechanisms in later stages of photooxidation. beta-Carotene (0.45 mol %) inhibited singlet oxygen-mediated lipid peroxidation at early stages of methylene blue-sensitized photooxidation. Production of radical-mediated products increased faster than singlet oxygen-mediated products at later stages. beta-Carotene-5,8-endoperoxide, a specific marker for singlet oxygen oxidation of beta-carotene in solution, was unstable under the incubation conditions and was not detected in this system. alpha-Tocopherol (0.45 mol %) was ineffective in inhibiting photosensitized lipid peroxidation, whereas 4.5 mol % alpha-tocopherol inhibited almost all radical-mediated lipid peroxidation as well as early-stage singlet oxygen-mediated lipid peroxidation. Cumene hydroperoxide stimulated radical-mediated lipid peroxidation, indicating that accumulation of hydroperoxides from Type II photooxidation may enhance Type I reactions. These data suggest that singlet oxygen quenching, rather than radical scavenging reactions, accounts for the photoprotective actions of beta-carotene.

Lipid peroxidation inhibits acyl-CoA:-1-acyl-sn-glycero-3-phosphocholine O-acyltransferase but not CTP: phosphocholine cytidylyltransferase activity in rat brain membranes

In brain tissue in vivo peroxidized according to three model systems, we determined two microsomal enzyme activities involved in phospholipid biosynthesis. The first, short-term model, was based on the i.v. administration to normal rats, twice a day, for a period of 1 week, of a sonicated emulsion of a peroxidized mixture of phospholipids and linoleate (4:1, w/w; 500 mg/day; hydroperoxides: 200-250 nmol/mg lipid). The half-life time of the injected toxic lipid species in the blood circulation was about 1 h. At the end of the week's treatment, brain and liver malondialdehyde, conjugated diene and lipid hydroperoxide levels were significantly higher in treated rats than in the controls. The second model consisted of the acute injection of aqueous Fe2+ solution (50 mM) into lateral ventricles, and the collection of brain tissue 2 h later. The third model was based on two consecutive injections of hydroperoxylinoleate (1 mg each) into lateral ventricles over a period of 18 h, and the collection of brain tissue 2 h after the second administration. In brain microsomal membranes prepared from peroxide- or iron-treated rats, lysophosphatidylcholine acyltransferase activity exhibited a significant inhibition. On the contrary, in microsomal preparations derived from the short-term model, CTP:phosphocholine cytidylyltransferase activity was slightly stimulated. Intraventricular injection of linoleate or linoleic acid hydroperoxide left this enzyme activity unchanged. The effect of in vitro membrane peroxidation on both microsomal enzyme activities was investigated. By using an Fe2+ (20 microM)-ascorbate (0.25 mM) peroxidation system, the residual acyltransferase and cytidylyltransferase activities were 80 and 72% of initial activity respectively. Significant dose-dependent inactivation of acyltransferase (maximum loss of 45% of initial activity) was seen when 0.1-10 mumol of photooxidized phospholipids were preincubated with 100 micrograms of microsomal membranes. Unoxidized or photooxidized phospholipids (1 mM) promoted a slight stimulation of cytidylyltransferase activity. Altogether, the results suggest a link between oxygen radical generation and the perturbation of the membrane structure in which the enzymes are located.

Isolation and characterization of addition products of alpha-tocopherol with peroxyl radicals of dilinoleoylphosphatidylcholine in liposomes

alpha-Tocopherol was reacted with peroxyl radicals of phosphatidylcholine at 37 degrees C in liposomes. The phospholipid-peroxyl radicals were generated by the reaction of 1,2-dilinoleoyl-3-sn-phosphatidylcholine with a free radical initiator, 2,2'-azobis(2,4-di-methylvaleronitrile), under air. One peak corresponding to the reaction products of alpha-tocopherol with phosphatidylcholine-peroxyl radicals was isolated by reversed-phase high-performance liquid chromatography. Its structure was identified as a mixture of 8a-(phosphatidylcholine-peroxy)-alpha-tocopherones: 1-[9-(8a-peroxy-alpha-tocopherone)-10,12-octadecadienoyl]-2- linoleoyl-3-sn-phosphatidylcholine,1-[13-(8a-peroxy-alpha-tocopherone )-9,11-octadecadienoyl]-2-linoleoyl-3-sn-phosphatidylcholine, 1-linoleoyl-2-[9-(8a, peroxy-alpha-tocopherone)-10,12-octadecadienoyl]-3-sn-pho sph atidylcholine and 1-linoleoyl-2-[13-(8a-peroxy-alpha-tocopherone)-9,11-octadecadi enoyl]-3-sn- phosphatidylcholine. The results indicate that each alpha-tocopherol can trap two peroxyl radicals during the peroxidation of unsaturated phospholipid in liposomes.

Consumption of carotenoids in photosensitized oxidation of human plasma and plasma low-density lipoprotein

Human plasma and plasma low-density lipoprotein (LDL) were exposed to photoirradiation in the presence of methylene blue (water-soluble photosensitizer) or 12-(1-pyrene)dodecanoic acid (P-12, lipid-soluble photosensitizer). In methylene-blue-sensitized photooxidation of human plasma and LDL, endogenous carotenoids and tocopherols were consumed with the accumulation of cholesteryl ester hydroperoxides (CE-OOH). Xanthophylls (zeaxanthin and lutein) decreased faster than lycopene and carotenes in the case of human plasma. In P-12-sensitized photooxidation of human plasma and LDL, the decrease rate of xanthophylls was slower than that of lycopene and carotenes. A lower level of beta-carotene exerted the effective inhibition of lipid peroxidation and retarded the oxidative loss of alpha-tocopherol, when the phosphatidylcholine liposomes containing these two lipid-soluble antioxidants were subjected to methylene blue- or P-12-sensitized photooxidation. These results suggest that antioxidant activity of carotenoids in photosensitized oxidation (Type II) of human plasma LDL depends on the site of singlet oxygen (1O2) to be generated and that carotenoids can protect tocopherols from the oxidative loss by 1O2 in the plasma.

No evidence for lysophospholipid formation during peroxidation of phospholipids by NADPH-cytochrome P-450 reductase and iron ions

Liposomes comprised of liver microsomal phospholipids and radioactive phosphatidylcholine or phosphatidylethanolamine as tracers were incubated with isolated liver microsomal NADPH-cytochrome P-450 reductase, NADPH and ADP-EDTA-chelated iron ions, a system which stimulates peroxidation of unsaturated fatty acids of phospholipids. Phospholipids and their reaction products were extracted and chromatographed on HPLC. Phosphatidylcholine and phosphatidylethanolamine considerably decreased after 30 min incubation, depending on the enzyme and NADPH as measured by UV absorbance and radioactivity. However, neither a lysophospholipid peak nor a lysophospholipid-like peak were detectable. We suggest that lysophospholipid formation during microsomal lipid peroxidation is exclusively due to phospholipase A2 and not due to peroxidative breakdown of the unsaturated fatty acid in the beta-position of glycerol.

Preparation of hydroperoxy and hydroxy derivatives of rat liver phosphatidylcholine and phosphatidylethanolamine

A convenient method for the preparation of hydroperoxy and hydroxy derivatives of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) is described. PC and PE obtained from rat liver were oxidized with singlet oxygen by using methylene blue as the photosensitizer, and their hydroperoxides were isolated with the aid of reverse phase liquid chromatography. The hydroxy derivatives were obtained by reducing the hydroperoxides with sodium borohydride. The results of gas chromatography mass spectrometry revealed that hydroxy fatty acid components of the hydroxy derivatives were derived from isomeric hydroperoxides of oleic acid, linoleic acid, arachidonic acid and docosahexanoic acid. Normal phase high performance liquid chromatography did not separate the hydroperoxy and hydroxy derivatives from the respective unoxidized phospholipids, although unoxidized PC and PE were separated from each other. However, the hydroperoxy and hydroxy derivatives could be distinguished from unoxidized phospholipid species on reversed phase thin layer chromatography.

High-performance liquid chromatographic determination of phospholipid peroxidation products of rat liver after carbon tetrachloride administration

A method to detect and determine phospholipid peroxidation products in a biological system was developed using reversed-phase high performance liquid chromatography and normal-phase HPLC. Reversed-phase HPLC could separate phosphatidylcholine (PC) hydroperoxides and phosphatidylethanolamine (PE) hydroperoxides of rat liver from the respective phospholipids. A linear relationship was observed between these hydroperoxides and their peak areas on the chromatogram. In the experiment with rats administered CCl4, reversed-phase HPLC gave prominent, large peaks attributable to the peroxidation of phospholipids, and the peroxide level of the liver phospholipids was tentatively determined. Normal-phase HPLC analysis confirmed that both PC and PE in the liver phospholipids were peroxidized after CCl4 treatment. Neither the thiobarbituric acid value of the liver homogenate nor the fatty acid composition of the liver phospholipid fraction showed any significant difference between CCl4-treated and control rats. It is concluded that normal-phase HPLC and reversed-phase HPLC can complement each other to serve as a direct and sensitive method for the determination of lipid peroxide levels in a biological source. However, it was difficult to distinguish phospholipid hydroperoxides from their hydroxy derivatives.

Singlet oxygen in copper-catalyzed lipid peroxidation in erythrocyte membranes

Lipid hydroperoxide was generated in human erythrocyte membranes by irradiation with near ultraviolet (UV) light in the presence of a photosensitizer, hematoporphyrin, but no production of 2-thiobarbituric acid-reactive materials (malonaldehyde and its precursors) was detected. Incubation of the irradiated membranes with CuSO4 led to increased levels of hydroperoxide and formation of malonaldehyde. Hydroperoxides were essential for initiating the Cu(II)-catalyzed peroxidation as no significant activity was observed with nonirradiated membranes and Cu(II) unless an organic peroxide, either t-butyl hydroperoxide or cumene hydroperoxide, was added. Catalytic activity was also found with Fe(II), but not with other metal ions tested. The peroxidation catalyzed with Cu(II) was partially inhibited by several singlet oxygen quenchers but was not affected by superoxide dismutase, catalase or OH radical scavengers. The possible involvement of singlet oxygen in the Cu(II)-catalyzed peroxidation reaction was further supported by a 3-fold enhancement of malonaldehyde production in D2O.

Autoxidation of phosphatidylcholine liposomes

Autoxidation of pure soybean phosphatidylcholine liposomes at 40 C was found to proceed without an observed induction period, but otherwise, the rates of disappearance of the linoleic acid (70% of total) and linolenic acid (6% of total) followed typical autocatalytic kinetics. Incorporation of 0.05 mol % of tocopherol into the liposomes produced an induction period of about 7 hr under the condition used for the incubation. The products formed from the autoxidation of pure soybean phosphatidylcholine liposomes were mostly 9- and 13-hydroperoxyoctadecadienoates (isolated as hydroxy esters). The yield of hydroperoxides with cis,trans configuration was about the same as those with trans,trans configuration throughout incubation period. After extensive autoxidation, a large quantity of trihydroxyoctadecenoate was also produced. When a large quantity of dipalmitoyl phosphatidylcholine was incorporated into soybean phosphatidylcholine liposomes, the rate of autoxidation decreased and was found to conform to apparent first-order kinetics. In this system, the yield of trans,trans hydroperoxides was much greater than that of cis,trans isomers at all stages of autoxidation. Late in the autoxidation of the mixed liposomes, both trihydroxyoctadecenoate and hydroxyepoxyoctadecenoate were produced in substantial quantities.