Linoleic Hydroperoxide Research Articles

Reaction of Phosphatidylcholine Hydroperoxide in Human Plasma: The Role of Peroxidase and Lecithin:Cholesterol Acyltransferase

In order to elucidate the reason why phosphatidylcholine hydroperoxide is unstable in human plasma, 1-palmitoyl-2-linoleoylphosphatidylcholine hydroperoxide (PLPC-OOH) was incubated aerobically in human plasma at 37°C, and its decomposition products were measured. The major product was the corresponding alcohol (PLPC-OH) and this reduction probably occurred by an enzymatic process since no acceleration in ascorbate depletion and no significant decrease in other plasma antioxidants were observed upon addition of PLPC-OOH. Cholesteryl linoleate hydroperoxide and its alcohol (Ch18:2-OH) were also detected as minor products. Similarly, 1-stearoyl-2-arachidonoylphosphatidylcholine hydroperoxide gave its alcohol (SAPC-OH) as a major product and cholesteryl arachidonate hydroperoxide and its hydroxide (Ch20:4-OH) as minor products. These oxidized cholesteryl esters are likely to be produced by the action of lecithin:cholesterol acyltransferase (LCAT) present in high-density lipoprotein (HDL) since (a) incubation of PLPC-OH and SAPC-OH in human plasma gave Ch18:2-OH and Ch20:4-OH, respectively, (b) isolated human HDL converted PLPC-OH to Ch18:2-OH and SAPC-OH to Ch20:4-OH, while isolated human low-density lipoprotein was inactive for this conversion, and (c) formation of oxidized cholesteryl esters in plasma and HDL was inhibited by the LCAT inhibitor 5,5′-dithiobis(2-nitrobenzoic acid). A possible beneficial role of LCAT for converting phosphatidylcholine hydroperoxide to cholesteryl ester hydroperoxide is also discussed.

Phosphatidylcholine synthesis-related enzyme activities of bovine brain microvessels exhibit susceptibility to peroxidation

In microvessels isolated from bovine brain, microsomal enzyme activities involved in phosphatidylcholine biosynthesis and degradation were determined. The microvessels possessed acyl-CoA:1-acyl- sn-glycero-3-phosphocholine (AT) and glycerophosphocholine phosphodiesterase (GroPChoPDE) activity at a higher level compared with bovine and rat brain or rat liver microsomes whereas they expressed CTP: phosphocholine cytidylyltransferase (CT) and choline phosphotransferase (CPT) activity at a lower level. Each enzyme has been characterized in terms of response to inhibitors or activators revealing properties very similar to those in brain and liver microsomes. In the homogenate prepared from t-butylhydroperoxide-treated microvessels (10 min exposure to 10 μM up to 1 mM concentrations), AT and CPT activities exhibited a significant dose-dependent inhibition. In contrast, GroPChoPDE activity was unaffected. CT was inhibited only at 1 mM concentration. Short treatment of microvessels with Fe 2+ (20 μM)-ascorbate (0.25 mM) or 100 μM linoleate hydroperoxide did not have any effect on the activity of the four enzymes. Strong inhibition of all enzymes was noted when the linoleate hydroperoxide system was fortified by Fe 2+ ions (100 μM). AT inactivation was also found when oxidized low density lipoprotein was preincubated with microvessels. On the other hand, oxidized LDL left unchanged CPT and GroPChoPDE activities whereas it promoted a slight stimulation of cytidylyltransferase activity. Overall, the results suggest a link between oxygen radical generation and the perturbation of the microvessel membrane structure in which the four enzymes are incorporated, coupled to a direct sulfhydryl protein modification.

Effect of α‐tocopherol and Trolox on the decomposition of methyl linoleate hydroperoxides

To clarify the mechanisms of antioxidant action, the effect of alpha-tocopherol and its water-soluble carboxylic acid derivative, Trolox, was studied on the decomposition of methyl linoleate hydroperoxides (MeLoOOH). Decomposition rate and the distribution of autoxidation products formed from MeLoOOH were followed by analyzing the volatile and non-volatile products by static headspace gas chromatography and normal-phase high-performance liquid chromatography, respectively. Both alpha-tocopherol and Trolox markedly inhibited the decomposition of MeLoOOH in a concentration-dependent way. In the absence of antioxidants, MeLoOOH was completely decomposed after incubation for 48 h at 60 degrees C, and in the presence of equal molar concentration of antioxidants only 6-7% of initial MeLoOOH was decomposed even after 280 h of incubation. MeLoOOH produced 1.2% methyl linoleate hydroxy compounds (MeLoOOH) in the presence of alpha-tocopherol and 3.8% in the presence of Trolox. Both antioxidants inhibited the formation of volatile decomposition products and the formation of ketodiene compounds. The hydroxy compounds may be formed by the reaction of alkoxy radical and hydrogen donating antioxidants. Conversion of MeLoOOH into stable MeLoOH demonstrated that the antioxidants alpha-tocopherol and Trolox trap alkoxyl radicals by H-donation.

Effects of Superoxide Dismutase and Catalase on Growth of Retinal Pigment Epithelial Cells in vitro following Addition of Linoleic Acid or Linoleic Acid Hydroperoxide

The rod outer segments of the retina that are phagocytized by retinal pigment epithelial (RPE) cells are susceptible to lipid peroxidation because of their high content of polyunsaturated fatty acids. Linoleic hydroperoxides (LHP), synthesized by peroxidation of linoleic acids (LA), produce greater damage to retinal function than does LA. We compared the effects of LHP and LA on the growth of cultured chick embryonic RPE cells and analyzed a model of data sets using multiple linear regression for the number of cells as a function of time. The spectrum of LA had a sharp peak at 205 nm and a broad spectrum at 235 nm, while LHP had only a broad spectrum at 235 nm. Exposure to LA and LHP caused dose-dependent damage of chick embryonic RPE cells: they were significantly more affected by the addition of LHP than LA. The antioxidative enzymes catalase and superoxide dismutase minimized damage to the RPE cells caused by LHP in proportion to the enzyme concentration. However, RPE cells incubated with LA were more affected by the enzymes than when no enzymes were added.

Continuous monitoring of intermediates and final products of oxidation of low density lipoprotein by means of UV-spectroscopy.

The most widely used routine technique for determination of LDL 'oxidizability' is the continuous monitoring of the absorption at 234 nm in the UV spectrum of LDL, following the addition of an oxidation promotor such as copper ions. This absorption is commonly attributed to the conjugated dienes formed upon oxidation as the major intermediate, namely the hydroperoxides of polyunsaturated fatty acids, mostly linoleate hydroperoxides. These, however, are not the only products of oxidation that absorb light at 234 nm. Other products, particularly 7-ketocholesterol, also absorb light at the same wavelength. Furthermore, enals and dienals also absorb in the wavelength range of 210-300 nm. The aim of the present work was to develop a simple spectroscopic method for more detailed investigation of the kinetics of lipoprotein oxidation. The method is based on continuous measurement of the UV spectrum in the wavelength range of 210-300 nm and subsequent decomposition of the spectra into four absorption bands due to hydroperoxides, 7-ketocholesterol, dienals and enals. The sixth derivatives of the spectra, recorded during the first seven hours of copper-induced oxidation of LDL were used to monitor the growth and subsequent decay of the hydroperoxides. The resultant time course, in conjunction with difference spectra obtained after the concentration of these intermediates decay to zero, enabled us to determine the spectra of the other oxidation products and, by that, to evaluate their time dependencies. Based on these results, we present a series of four simple equations that can be used to evaluate the concentrations of the individual products of LDL oxidation from UV absorption measurements of the mixtures ar merely four different wavelengths. The resultant time dependencies of the accumulation of four major products of lipid oxidation are consistent with published data obtained through separation and chemical analysis. This simple method can be used for more meaningful routine kinetic measurements of lipids oxidation.

Coulometric detection in high-performance liquid chromatographic analysis of cholesteryl ester hydroperoxides

A highly sensitive and simple method for the determination of cholesteryl ester hydroperoxides (CHE-OOH) was developed using high-performance liquid chromatography (HPLC) with coulometric electrochemical detection. The lowest detectable level by this technique was 2 pmol for cholesteryl linoleate hydroperoxides at the signal-to-noise ratio of 3. This method was applied to the determination of ChE-OOH presumably present in human plasma. Although ChE-OOH could not be detected, the ChE-OOH level in the fluid was estimated to be less than 27 nM. It was found that the extraction efficiency of an internal standard, cholesteryl nervonate, was decreased by lowering its amount spiked to the plasma. The concentration of ChE-OOH in human plasma and plasma lipoprotein, which were peroxidized with a radical initiator in vitro, could be determined by use of this standard. HPLC-coulometric technique is, therefore, useful to measure the peroxidation of plasma lipids in vitro.

Identification and quantification of regioisomeric cholesteryl linoleate hydroperoxides in oxidized human low density lipoprotein and high density lipoprotein.

Oxidation of human LDL is implicated as an initiator of atherosclerosis. Isolated low density lipoprotein (LDL) and high density lipoprotein (HDL2) were exposed to aqueous radicals generated from the thermolabile azo compound 2,2'-azobis(2-amidinopropane) dihydrochloride. The primary nonpolar lipid products formed from the autoxidation of LDL and HDL were the regioisomeric cholesteryl linoleate hydroperoxides. In LDL oxidations, 9- and 13-hydroperoxides with trans,cis conjugated diene were formed as the major oxidation products if endogenous alpha-tocopheral was present in the LDL. After extended oxidation of LDL, at the time when endogenous alpha-tocopherol was consumed, the two trans,cis conjugated diene hydroperoxides began to disappear and the 9- and 13-hydroperoxides with trans,trans conjugated diene appeared. At very long oxidation times, none of the primary products, the conjugated diene hydroperoxides, were present. In HDL2, which has only very low levels of antioxidants, both the 9- and 13-hydroperoxides with trans,cis conjugated diene and the 9- and 13-hydroperoxides with trans,trans conjugated diene were formed at early stages of oxidation. The corresponding alcohols were also formed in the HDL2 oxidations. A mechanistic hypothesis consistent with these observations is presented.

Study on position and geometric configuration of methyl linoleate hydroperoxide isomers obtained by thermo‐oxidation: Chromatographic analyses of their corresponding hydroxy derivatives

Study on position and geometric configuration of methyl linoleate hydroperoxide isomers obtained by thermo‐oxidation: Chromatographic analyses of their corresponding hydroxy derivatives

Dietary antioxidants and cigarette smoke-induced biomolecular damage: a complex interaction

Epidemiologic evidence suggests that cigarette smoking is a major risk factor for chronic obstructive pulmonary diseases such as chronic bronchitis and emphysema, for carcinogenesis, and for cardiovascular disease. However, the precise mechanisms of these effects are incompletely understood. The gas phase of cigarette smoke contains abundant free radicals including nitric oxide. Hence, cigarette smoke may induce some of its damaging effects by free radical mechanisms. We report that exposure of plasma, a model for respiratory tract lining fluids, to gas-phase cigarette smoke causes depletion of antioxidants, including ascorbate, urate, ubiquinol-10, and alpha-tocopherol, and a variety of carotenoids, including beta-carotene. Gas-phase cigarette smoke induced some lipid peroxidation, as measured by cholesteryl linoleate hydroperoxide (18:2OOH) formation. Ascorbate was effective in preventing 18:2OOH formation. In contrast to the low concentrations of lipid hydroperoxides measured (< 1 mumol/L), protein carbonyl formation, a measure of protein modification, increased by approximately 400 mumol/L after nine puffs of cigarette smoke. Reduced glutathione inhibited protein carbonyl formation, whereas other plasma antioxidants, including ascorbate, were ineffective. alpha, beta-Unsaturated aldehydes (acrolein and crotonaldehyde) in cigarette smoke may react with protein -SH and -NH2 groups by a Michael addition reaction that results in a protein-bound aldehyde functional group. Gas-phase cigarette smoke is capable of converting tyrosine to 3-nitrotyrosine and dityrosine, indicating free radical mechanisms of protein damage by nitrogen oxides. Aldehydes and nitrogen oxides in cigarette smoke may be significant contributors to biomolecular damage, and endogenous antioxidants can attenuate some of these adverse effects.

Exchange of oxidized cholesteryl linoleate between LDL and HDL mediated by cholesteryl ester transfer protein.

This study examines the cholesteryl ester transfer protein (CETP)-mediated exchange of cholesteryl linoleate hydroperoxide (Ch18:2-OOH) and cholesteryl linoleate hydroxide (Ch18:2-OH) between low density lipoprotein (LDL) and high density lipoprotein (HDL). When [3H]Ch18:2-OOH- and [3H]18:2-OH-labeled LDL were incubated at 37 degrees C for 0-24 h with unoxidized HDL and purified CETP, Ch18:2-OOH and Ch18:2-OH accumulated in the HDL. Similarly, when incubations were carried out with [3H]Ch18:2-OOH- and [3H]Ch18:2-OH-labeled HDL, unoxidized LDL, and CETP, Ch18:2-OOH and Ch18:2-OH accumulated in the LDL. Comparable results were obtained for the CETP-mediated transfer of [3H]Ch18:2-OH alone from LDL to HDL. Transfer to HDL of oxidized cholesteryl linoleate from [3H]Ch18:2-OOH- and [3H]Ch18:2-OH-labeled LDL was comparable to that of unoxidized cholesteryl linoleate (Ch18:2). However, the rate of transfer of [3H]Ch18:2-OOH and [3H]Ch18:2-OH from LDL to HDL increased linearly as the molar ratio of acceptor (HDL) to donor (oxidized LDL) particles in the incubation increased from 0.5:1 to 10:1. This increased rate of exchange was accompanied by an increased proportion of the oxidized Ch18:2 being present as the hydroxide rather than hydroperoxide. Further increases in the molar ratio of HDL to oxidized LDL particles neither affected the transfer rate nor the extent of reduction of Ch18:2-OOH to Ch18:2-OH. We therefore conclude that i) CETP mediates bidirectional transfers of Ch18:2-OOH and Ch18:2-OH between HDL and LDL; ii) CETP does not distinguish between Ch18:2-OOH, Ch18:2-OH, and Ch18:2 as it mediates their exchange between HDL and LDL; and iii) association with HDL hastens the reduction of Ch18:2-OOH to Ch18:2-OH.

Microsomal glutathione transferase: Lipid-derived substrates and lipid dependence

Rat liver microsomal glutathione transferase was found to display glutathione peroxidase activity toward a variety of oxidized lipids. 1-Linoleoyl-2-palmitoyl phosphatidylcholine hydroperoxide, 2-linoleoyl-1-palmitoyl phosphatidylcholine hydroperoxide, 2-linoleoyl-1-palmitoyl phosphatidylethanolamine hydroperoxide, and cholesteryl linoleate hydroperoxide all served as substrates (0.02, 0.04, 0.02, and 0.02 μmol/min mg, respectively). The phospholipid hydroperoxide glutathione peroxidase activity of the enzyme was found not require detergent and increased when liposomes containing peroxidized phospholipid were fused with liposomes containing microsomal glutathione transferase. Methyl linoleate ozonide serves as a very efficient substrate for the microsomal glutathione transferase. The unactivated and N-ethylmaleimide-activated enzyme displayed specific activities of 0.74 and 5.9 μmol/min mg, respectively. Upon examination of a series of 4-hydroxyalk-2-enals it was found that the catalytic efficiency of the enzyme increases from the 4-hydroxyhept-2-enal up to the 4-hydroxytetradec-2-enal. The specific activities with the various 4-hydroxyalk-2-enals tested varied between 0.28 and 0.95 μmol/min mg. The phospholipid dependence of the microsomal glutathione transferase was examined in proteoliposomes formed by cholate dialysis. Phosphatidyl choline, phosphatidyl serine, phosphatidyl ethanolamine, and rat liver microsomal phospholipids could all be used successfully to reconstitute the enzyme. In conclusion, microsomal glutathione transferase can detoxify a number of lipid peroxidation products as well as fatty acid ozonide. The results imply a protective role for the enzyme under conditions of oxidative stress.

Free-Radical Scavenging Reactions of .alpha.-Tocopherol during the Autoxidation of Methyl Linoleate in Bulk Phase

Methyl linoleate containing alpha-tocopherol was autoxidized at 37 and 60 degrees C in bulk phase. The reaction products of alpha-tocopherol at 37 degrees C were 4a,5-epoxy-8a-hydroperoxy-alpha-tocopherones (1), a mixture of methyl 9-(8a-dioxy-alpha-tocopherone)-10(E),12(Z)-octadecadienoates and methyl 13-(8a-dioxy-alpha-tocopherone)-9(Z),11(E)-octadecadienoates (2), a mixture of methyl 9-(alpha-tocopheroxy)-10(E),12(Z) -octadecadienoates and methyl 13-(alpha-tocopheroxy)-9(Z),11(E)-octadecadienoates (3), alpha-tocopherol spirodiene dimer (4), and alpha-tocopherol trimer (5). Compound 3 was the major product at 60 degrees C, and the E,E-isomers of 3 were produced in addition to the E,Z-isomers. When methyl linoleate containing a low concentration of alpha-tocopherol was autoxidized at 37 degrees C, alpha-tocopherol suppressed the formation of methyl linoleate hydroperoxides, and the main products of alpha-tocopherol were 1 and 2. In contrast, (alpha-tocopherol at a high concentration induced the accumulation of methyl linoleate hydroperoxides, and the reaction products were 1-5. The autoxidation at 60 degrees C proceeded rapidly, and the main product of alpha-tocopherol was 1. In addition, the accumulation of 3 was observed under air-insufficient conditions. The results indicate that the alpha-tocopheroxyl radical produced can react with both alkyl and alkylperoxyl radicals of unsaturated lipids during the autoxidation. Competing with these trapping reactions, 1 appears to be formed primarily by autoxidation reactions of alpha-tocopherol.

Antioxidant Action of Neuromelanin: The Mechanism of Inhibitory Effect on Lipid Peroxidation

Iron and lipid peroxidation are believed to be involved in the degeneration of pigmented neurons in Parkinson′s disease. Melanin-iron interaction is thought to play a role in iron accumulation and reactivity. The purpose of this study was to examine antioxidant properties of isolated natural and synthetic neuromelanin. Effect of neuromelanin from substantia nigra and its synthetic model, dopamine melanin, on Lipid peroxidation, induced by ferrous ions and free-radical initiators, has been studied in methyl linoleate aqueous dispersions, 2,2′-azobis(amidinopropane)dihydrochloride and 2,2′-azobis(2,4-dimethyl-valeronitrile) were used as water-soluble and lipid-soluble radical initiator, respectively. Rate of oxidation was followed quantitatively by measuring oxygen uptake and accumulation of lipid hydroperoxides. Melanin had a distinct protective effect on lipid peroxidation induced by ferrous ions or water-soluble free-radical initiator but was relatively inefficient when peroxidation was initiated with lipid-soluble compound. It also inhibited iron-catalyzed decomposition of methyl linoleate hydroperoxides in the presence of ascorbate. Extent of the inhibition depended on the ratio of melanin to iron. Taken together, these results provide strong support for the idea that neuromelanin of pigmented neurons can act as a natural antioxidant by sequestering redox-active metal ions.

Ferrocene-modified horseradish peroxidase enzyme electrodes. A kinetic study on reactions with hydrogen peroxide and linoleic hydroperoxide

An enzyme electrode for the determination of hydrogen peroxide and linoleic hydroperoxide, a model compound for lipid hydroperoxides, is described. Horseradish peroxidase bearing covalently attached ferrocene groups as electron transfer mediators is used as the sensing molecule and is adsorbed to the surface of disposable printed carbon electrodes as the base sensor. The electrodes are suitable for the determination of hydrogen peroxide in the range 1–50 µmol l–1 and linoleic hydroperoxide in the range 5–100 µmol l–1. A comparison of the kinetic behaviour of the modified enzyme in solution by spectrophotometry, and on the electrode by amperometry, shows that the catalytic current is limited by mass transport when hydrogen peroxide is the substrate and enzyme kinetics when linoleic hydroperoxide is the substrate.

Lipid Hydroperoxide-Induced Endogenous DNA Adducts in Hamsters: Possible Mechanism of Lipid Hydroperoxide-Mediated Carcinogenesis

DNA of livers or kidneys of rodents contain endogenously modified nucleotides of as yet unknown structure or function. As part of our examination of the origin of endogenous DNA modifications, we examined polar adducts of hamster liver and kidney DNA by 32P-postlabeling analysis (central and upper cuts) and compared them to adducts generated by incubation of DNA with [9Z,11E,(13S)]-13-hydroperoxyoctadecadienoic acid or with malondialdehyde. The central and upper cuts of DNA adduct maps of intact female retired breeder hamsters contained one and three adducts called C1, U1, U2, and U3, respectively, which cochromatographed with similar adducts in DNA incubated with the hydroperoxide or with malondialdehyde. Concentrations of adducts C1, U1, and U2 in organs of 1-month-old hamsters were lower than those in the 5-month-old female retired breeder hamsters. Adduct concentrations increased in tandem with lipid peroxide concentrations in livers and kidneys of hamsters treated with carbon tetrachloride compared to controls. A linear dependence of the concentrations of adducts on those of lipid peroxides was demonstrated independent of the treatment conditions or the organ examined. The polar endogenous adducts U1, U2, and U3 were identified by cochromatography in five different chromatographic systems with adducts induced by linoleic hydroperoxide or malondialdehyde. Adducts U1, U2, or U3 were also formed by incubating malondialdehyde with dAMP or with dGMP, respectively. The dependence of adduct concentrations in vivo on lipid peroxide levels are taken as evidence that decomposition products of lipid peroxides such as malondialdehyde induced endogenous DNA adducts in intact animals. Moreover, these adducts may mediate carcinogenic processes induced by chemicals which raise lipid peroxide levels.

Effect of methyl linoleate hydroperoxides on murine natural killer cell-mediated cytotoxicity.

Methyl linoleate hydroperoxides (MLHPO), a model for lipid hydroperoxides, suppressed murine natural killer (NK) cell-mediated cytotoxicity to YAC-1 cells. When NK cells were cultured with MLHPO, the conjugation of NK cells could not lyse YAC-1 cells. These findings suggest that NK cell-mediated cytotoxicity can be suppressed by lipid hydroperoxides.

Dynamics of Free Radical Formation from the Reaction of Peroxides with Haemproteins as Studied by Stopped-Flow Chemiluminescence

Metal ions such as iron and copper are implicated in the generation of free radicals in vivo, but the dynamics of free radical production in the decomposition of hydrogen peroxide and hydroperoxides by iron complexes have not been shown unequivocally due to their rapid rates. In the present study, we have devised a stopped-flow chemiluminescence system and succeeded in elucidating the time course of the free radical formation in the decomposition of hydrogen peroxide and organic hydroperoxides by haemproteins. The rate of free radical formation was very much dependent on the metal complexes and peroxides. Methemoglobin and cytochrome c decomposed hydrogen peroxide slower than microperoxidase but generated free radicals continuously. Methyl linoleate hydroperoxide was decomposed more rapidly than hydrogen peroxide. The chemiluminescence intensity was directly proportional to the amount of hydroperoxide, and the chemiluminescence progress curve reflected the time course of free radical flux. These results show that this stopped-flow chemiluminescence system is suitable to follow the radical flux from the decomposition of peroxides by metal ions.

Human blood cells support the reduction of low-density-lipoprotein-associated cholesteryl ester hydroperoxides by albumin-bound ebselen.

Ebselen, a glutathione peroxidase mimic capable of reducing simple as well as complex hydroperoxides, including those of phospholipids and cholesteryl esters in intact oxidized low-density lipoprotein (LDLox), requires the presence of low-molecular-mass thiols to be active. In plasma, the drug is thought to be transported as an inactive albumin complex. As formation of LDLox is likely to occur extracellularly, we tested under which conditions ebselen can support reduction of LDLox-associated cholesteryl ester hydroperoxides outside cells. We observed that addition of albumin-bound ebselen to whole blood, but not plasma, resulted in reduction of LDLox-associated cholesteryl linoleate hydroperoxides to the corresponding hydroxides. The observed reduction was rapid and its extent increased with increasing concentrations of ebselen. Physical contact of blood cells with LDLox was not required for this reducing activity. These results demonstrate that, in the presence of blood cells, extracellular ebselen is catalytically active. They suggest that ebselen may be considered as a drug for extracellular targets.

Free radical-mediated oxidation of lipids induced by hemoglobin in aqueous dispersions

In order to elucidate the effects of hemoglobin on the oxidative damage, the oxidations of soybean phosphatidylcholine (PC) liposomes and low density lipoprotein (LDL) in aqueous dispersions induced by ruptured erythrocytes (hemolysate) and methemoglobin were studied. The rate of oxidation of soybean PC liposomes by hemolysate was much faster than that induced by the same concentration of ferric sufate or cupric chloride and increased with the increase in the amount of hemolysate. When soybean PC was treated beforehand with ebselen, which decomposes hydroperoxides but can not scavenge free radicals, the oxidation did not proceed. Furthermore, the oxidation was suppressed by the addition of pentamethylchromanol in liposomal membrane. The reaction of hemoglobin with methyl linoleate hydroperoxide in aqueous dispersions was also studied. It was found that hemolysate and methemoglobin decomposed the hydroperoxides much faster than cupric chloride and ferric sulfate. The electron spin resonance study strongly suggested that both hemolysate and methemoglobin decomposed hydroperoxides rapidly to form alkoxyl and peroxyl radicals. It was also found out by spectrophotometrical study and by using carbon monoxide and potassium cyanide that hemolysate was oxidized by organic hydroperoxide to form methemoglobin and ferryhemoglobin. The rate of oxidation of LDL induced by hemolysate was much slower than that of soybean PC lyposomes, while LDL was oxidized fater than soybean PC liposomes by cupric chloride under the same conditions. However, in the presence of tert-butyl hydroperoxide, the oxidation of LDL induced by hemolysate proceeded rapidly. These results were interpreted by both the chemical reactivity and the accessibility of metal catalyst to the lipid hydroperoxides in LDL and liposomes. It was suggested that hemoglobin released by hemorrhage might play a crucial role in the initiation of oxidative damage in vivo.

Oxidative stability of blends and interesterified blends of soybean oil and palm olein

AbstractImprovement of oxidative stability of soybean oil by blending with a more stable oil was investigated. Autoxidation of blends and interesterified blends (9∶1, 8∶2, 7∶3 and 1∶1, w/w) of soybean oil and palm olein was studied with respect to fatty acid composition, fatty acid location and triacylglycerol composition. Rates of formation of triacylglycerol hydroproxides, peroxide value and volatiles were evaluated. The fatty acid composition of soybean oil was changed by blending. Linolenic and linoleic acids decreased and oleic acid increased. The triacylglycerol composition of blends and interesterified blends was different from that of soybean oil. Relative to soybean oil, LnLL, LLL, LLO, LLP, LOO and LLS triacylglycerols were lowered and POO, POP and PLP were higher in blends and interesterified blends (where Ln, L, O, P and S represent linolenic, linoleic, oleic, palmitic and stearic acids, respectively). Interesterification of the blends leads to a decrease in POO and POP and an increase in LOP. Linoleic acid concentration at triacylglycerol carbon‐2 was decreased by blending and interesterification. Rates of change for peroxide value and oxidation product formation confirmed the improvement of soybean oil stability by blending and interesterification. But, blends were more stable than interesterified blends. Also, the formation of hexanal, the major volatile of linoleate hydroperoxides of soybean oil, was decreased by blending and interesterification.