Peroxidation Of Phosphatidylcholine Research Articles

Development of curcumin-loaded composite phospholipid ethosomes for enhanced skin permeability and vesicle stability.

Ethosomes are widely applied as the carriers for the transdermal delivery of hydrophobic and hydrophilic drugs. Herein, curcumin-loaded ethosomes (CE) with different phospholipid composition were formulated and thoroughly compared. A significant interaction between the unsaturated phosphatidylcholine (PC) and saturated hydrogenated phosphatidylcholine (HPC) was found by molecular simulation and differential scanning calorimetry (DSC), which led to the reduction of PC peroxidation with the presence of HPC. Subsequently, the composite phospholipid ethosomes containing curcumin were prepared for the first time to evaluate their properties in comparison with the conventional ethosomes composed of PC (CE-P) or HPC (CE-H). CE with PC/HPC ratio of 1:1 (CE-P1H1) with the best vesicle stability and flexibility significantly decreased the uptake by HaCaT cells compared to CE-H and free curcumin, indicating reduced skin cell toxicity. Compared with free curcumin, CE-P1H1 had the highest transdermal efficiency (p<0.001), followed by CE-P (p<0.05), partly due to the fact that CE-P1H1 could disturb lipid domain of stratum corneum (SC). Moreover, CE-P1H1 was found to promote curcumin for deep penetration of the skin via the hair follicles route. Our study has shown that using composite phospholipid ethosomes as lipid vesicular carriers could enhance transdermal penetration of drugs and increase in the vesicle stability.

Docosahexaenoic and arachidonic acid peroxidation: It's a within molecule cascade

Peroxidation is a well-known natural phenomenon associated with both health and disease. We compared the peroxidation kinetics of phosphatidylcholine (PC) molecules with different fatty acid compositions (i.e. 18:0, 18:1n-9, 18:2n-6, 20:4n-6 and 22:6n-3 at the sn-2 and 16:0 at sn-1 position) either as molecules free in solution or formed into liposomes. Fatty acid levels, oxygen consumption plus lipid hydroperoxide and malondialdehyde production were measured from the same incubations, at the same time during maximal elicitable peroxidation. PCs with highly peroxidizable fatty acids (i.e. 20:4n-6 and 22:6n-3) in the same incubation were found to be either fully peroxidized or intact. Rates of peroxidation of PCs with multiple bisallylic groups (i.e. 20:4n-6 and 22:6n-3) peroxidized at 2-3 times the rate per bisallylic bond than the same phospholipid with 18:2n-6. The results suggest that propagation of peroxidation (H-atom transfer) is firstly an intramolecular process that is several-fold faster than intermolecular peroxidation. PCs in solution peroxidized twice as fast as those in liposomes suggesting that only half of the phospholipids in liposomes were available to peroxidize i.e. the outer leaflet. Experiments on liposomes suggest that even after heavy peroxidation of the outer leaflet the inner leaflet is unaffected, indicating how cells may protect themselves from external peroxidation and maintain control over internal peroxidation. Intramolecular peroxidation may produce highly concentrated, localized sites of peroxidation product that together with internal control of peroxidation of the inner leaflet of membranes provide new insights into how cells control peroxidation at the membrane level.

Evaluation of physical integrity of lipid bilayer under oxidative stress: application of fluorescence microscopy and digital image processing.

Membrane damage as a result of oxidative stress is quantified using digital image heterogeneity analysis of single giant unilamellar vesicles (GUVs) composed of soy phosphatidylcholine (PC), which were found to undergo budding when containing chlorophyll a (Chla) as photosensitizer in the lipid bilayer. Based on digital image heterogeneity analysis, a dimensionless scalar parameter "entropy" for the budding process was found to change linearly during an initial budding stage. Photo-induced peroxidation of PC to form linoleoyl hydroperoxides, further leading to domains of higher polarities in GUVs, was suggested to initiate the budding process. The effect on budding process of GUVs was suggested for use in assays for evaluation of potential protectors of lipid bilayer integrity under oxidative stress, and "entropy" seemed to be a valid descriptor of such membranal integrity. The one-step procedure for quantification of prooxidative effects and antioxidative protection provided by drug candidates and potential food ingredients in membranes could be easily automated for direct measurement of oxidative and antioxidative effects on cellular integrity.

Effect of fermentation conditions on the antioxidant compounds and antioxidant capacity of Lupinus angustifolius cv. zapaton

To study the effect of fermentation on the antioxidant compounds and antioxidant capacity of Lupinus angustifolious cv. zapaton, two different types of fermentation processes were performed. Solid-state fermentations in cracked seeds carried out by Aspergillus oryzae, Rhizopus oryzae and Bacillus subtilis and liquid state fermentations in flour and cracked seeds carried out by the microbial population present in the seed (natural fermentation) or by L. plantarum inocula. Antioxidant compounds that were quantified included vitamin C by micellar electrokinetic capillary electrophoresis, vitamin E isomers by high performance liquid chromatography, total phenolic compounds (TPC) by spectrophotometry and reduced glutathione (GSH) by spectrofluorimetry. The antioxidant capacity was analysed by determining the superoxide dismutase-like activity (SOD-like activity), Peroxyl Radical-Trapping Capacity (PRTC) and Trolox Equivalent Antioxidant Capacity (TEAC) and by in vitro methods using unilamellar liposomes of egg yolk phosphatidylcholine (PC). In general, fermentation process produced a reduction in vitamin C, vitamin E activity, GSH and SOD-like activity, however TPC, PRTC, TEAC and inhibition of PC peroxidation increased under most of the fermentation conditions. Optimal results to obtain functional lupin products were achieved in cracked seeds fermented with B. subtilis where increases in TPC content, PRTC, inhibition of PC peroxidation and TEAC content of 490, 669, 492 and 224%, respectively, were found. Also, fermentation carried out with L. plantarum in lupin flours and naturally in cracked seeds caused smaller, although significant (P ≤ 0.05) increases in TPC, PRTC, inhibition of PC and TEAC (80–148, 50–90, 23 and 45–65%, respectively).

Fermentation as a Bio-Process To Obtain Functional Soybean Flours

The effect of fermentation on the antioxidant compounds [vitamins C and E, total phenolic compounds (TPC), and reduced glutathione (GSH)], and antioxidant capacity [superoxide anion scavenging activity (SOD-like activity), peroxyl radical-trapping capacity (PRTC), inhibition of phosphatidylcholine (PC) peroxidation, and Trolox equivalent antioxidant capacity (TEAC)] of soybean (Glycine max cv. Merit) was studied. Fermentation was carried out in solid state in cracked seeds inoculated with Aspergillus oryzae, Rhizopus oryzae, Bacillus subtilis, and Lactobacillus plantarum and in liquid state either in cracked seeds or milled soybean flours fermented naturally by only the microorganisms present in the seeds or by inoculation with L. plantarum. Vitamin C was not detected in the studied samples. Fermentation caused a decrease in vitamin E activity, except when cracked seed was fermented with A. oryzae, R. oryzae, or B. subtilis that increased 31, 30, and 89%, respectively. Fermentation produced an increase in TPC content and did not affect or reduce the GSH content. Fermentation decreased SOD-like activity drastically, while PRTC increased except when it was carried out naturally in cracked seed. TEAC values rose sharply when soybeans were fermented with B. subtilis. Processed soybean extracts inhibited PC peroxidation in comparison with the control assay. On the basis of the results obtained, the relative contributions of vitamin E, TPC, and GSH to antioxidant capacity were calculated and results showed a very high TPC contribution and a low contribution of GSH and vitamin E activity. Optimum results for functional soybean flours were achieved when fermentation was carried out with B. subtilis inoculum.

UV-Filter combinations under UV-A exposure: Concomitant quantification of over-all spectral stability and molecular integrity

Efficient UV-absorbing molecules are designed to protect against UV-light over-exposure. However, upon UV exposure they may change spectral performance or act as photooxidants via generation of free radicals and reactive oxygen species alone or in combination with others. Therefore, information about their photointegrity which comprises (i) stable absorbance and (ii) absence of UV-induced molecular breakdown, is fundamental. In this study, seven commonly used UV-A, UV-B and broad spectrum UV-AB filters and their combinations, were incorporated into phosphatidylcholine (PC)-based liposomes and exposed to UV-A (275 kJ/m 2). Spectral integrity, evaluated by recording UV-absorbance spectra of the extracted filter molecules and molecular integrity, assessed indirectly via quantification of UV-A induced PC peroxidation, revealed that spectral stability of filter molecules alone or in combination (e.g. trianilino p-carboxyethylhexyl triazine, EHT plus ethylhexyl p-methoxycinnamate, OMC) does not necessarily imply absence of radical generation and that spectral lability does not necessarily have to lead to radical generation and molecular decay (e.g. OMC). This simple system capable of discriminating between essentially photostable and photounstable UV-absorbing molecules alone and in mixtures, might be useful for determining the influence of UV-protection as well as of photostability of UV-absorbers with regard to UV-induced genotoxic/phototoxic and photoageing-related, radical-based processes.

Evaluation of the antioxidant properties of fruits

Twenty-eight different fruits were analysed for antioxidant activities using two different methods, one that determines the inhibition of ascorbate/iron-induced peroxidation of phosphatidylcholine, by means of thiobarbituric acid-reactive substances (TBARS) measurement, and another that evaluates the scavenging of the radical cation of 2,2′-azinobis (3-ethyl-benzothiazoline-6-sulphonate) (ABTS) relative to Trolox C, a water-soluble vitamin E analogue. The values of the antioxidant capacities of the analysed fruits were in a wide range from 50 g to more than 5 mg for the IW 50 (dry weight causing 50% inhibition of lipidic peroxidation) and from it 406 μmol/g for the TEAC (μmol Trolox equivalents g −1). There was no significant correlation between antioxidant activity (IW 50 and TEAC) and the contents of flavanol of the samples. The antioxidant activities of the analysed fruits cannot only be attributed to their flavanol contents, but to the result of the action of different antioxidant compounds present in the fruits and to possible synergic and antagonist effects still unknown.

Positional Isomers of Monohydroperoxides Produced by Peroxidation of Phosphatidylcholine Containing an Arachidonoyl Moiety in a Liposomal Suspension.

Monohydroperoxides derived from the peroxidation of 1-palmitoyl-2-arachidonoyl-phosphatidylcholine (PC-AA) in a liposomal suspension had a characteristic isomeric composition. Of the two possible isomers derived from a pentadienyl-radical intermediate, the isomer with a hydroperoxy group on the carboxyl-terminal side was at a substantially lower level than the other.

Preparation and characterization of 8a-(phosphatidylcholine-dioxy)-alpha-tocopherones and their formation during the peroxidation of phosphatidylcholine in liposomes.

alpha-Tocopherol was reacted with the phosphatidylcholines (PCs), 1-palmitoyl-2-linoleoyl-3-sn-PC (PLPC), 1-palmitoyl-2-linolenoyl-3-sn-PC, 1-palmitoyl-2-arachidonoyl-3-sn-PC (PAPC) and 1-stearoyl-2-arachidonoyl-3-sn-PC, in the presence of the free radical initiator, 2,2'-azobis (2,4-dimethylvaleronitrile), at 37 degrees C. The addition products of alpha-tocopherol with the PC peroxyl radicals were isolated and identified as 8a-(PC-dioxy)-alpha-tocopherones, in which the peroxyl radicals derived from each PC molecule attacked the 8a-position of the alpha-tocopheroxyl radical. The antioxidative efficiency of alpha-tocopherol against the peroxidation of PLPC and PAPC in liposomes was assessed by the formation of the reaction products of alpha-tocopherol. When alpha-tocopherol was oxidized in the presence of the water-soluble free radical initiator, 2,2'-azobis (2-amidinopropane) dihydrochloride, epoxy-alpha-tocopherylquinones were mainly produced together with 8a-(PC-dioxy)-alpha-tocopherones and alpha-tocopherylquinone. The yield of alpha-tocopherylquinone was increased by treating each sample with dilute acid which indicates the presence of tocopherone precursors other than the 8a-(PC-dioxy)-alpha-tocopherones. The same products were also detected from iron-dependent peroxidation, although the yields were very low.

Nitric Oxide Prevents Oxidative Damage Produced bytert-Butyl Hydroperoxide in Erythroleukemia Cells via Nitrosylation of Heme and Non-heme Iron: ELECTRON PARAMAGNETIC RESONANCE EVIDENCE

We studied protective effects of NO against tert-butylhydroperoxide (t-BuOOH)-induced oxidations in a subline of human erythroleukemia K562 cells with different intracellular hemoglobin (Hb) concentrations. t-BuOOH-induced formation of oxoferryl-Hb-derived free radical species in cells was demonstrated by low temperature EPR spectroscopy. Intensity of the signals was proportional to Hb concentrations and was correlated with cell viability. Peroxidation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin metabolically labeled with oxidation-sensitive cis-parinaric acid was induced by t-BuOOH. An NO donor, (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]-diazen-1-iu m-1, 2-diolate], produced non-heme iron dinitrosyl complexes and hexa- and pentacoordinated Hb-nitrosyl complexes in the cells. Nitrosylation of non-heme iron centers and Hb-heme protected against t-BuOOH-induced: (a) formation of oxoferryl-Hb-derived free radical species, (b) peroxidation of cis-parinaric acid-labeled phospholipids, and (c) cytotoxicity. Since NO did not inhibit peroxidation induced by an azo-initiator of peroxyl radicals, 2, 2'-azobis(2,4-dimethylvaleronitrile), protective effects of NO were due to formation of iron-nitrosyl complexes whose redox interactions with t-BuOOH prevented generation of oxoferryl-Hb-derived free radical species.

Kinetic studies on the formation of phosphatidylcholine hydroperoxides in large unilamellar vesicles by azo compounds

Peroxidation of phosphatidylcholine (PC) in large unilamellar vesicles (LUV) was studied by measuring PC-hydroperoxides (PC-OOH) formed by azo compound-initiated radical chain reaction. When the peroxidation was induced by a water-soluble azo compound, the rate of PC-OOH formation in LUV comprising of egg yolk PC was larger than that in multilamellar vesicles (MLV) of the same origin. On the other hand the opposite trend was observed when a lipid-soluble azo compound was used. The kinetic parameters of free radical oxidation of egg yolk PC were compared with those of soybean PC in LUV with a water-soluble azo compound or a lipid-soluble azo compound and in tert-butyl alcohol solution with a lipid-soluble azo compound. In LUV, the oxidizability per molecule and the kinetic chain length of egg yolk PC were much lower than those of soybean PC independent of the azo-compounds. In solution, the difference in these parameters between the two PC species was small. The oxidizability per bis-allylic hydrogen of egg yolk PC was also lower than that of soybean PC in spite of the same values from both PCs in solution. These data strongly suggest that molecular species substantially affect the oxidation kinetics of PC in unilamellar vesicles, as compared with those in solution.

Iron (III) Stimulation of Lipid Hydroperoxide-Dependent Lipid Peroxidation

In an experimental system where both Fe2+ autoxidation and generation of reactive oxygen species is negligible, the effect of FeCl2 and FeCl3 on the peroxidation of phosphatidylcholine (PC) liposomes containing different amounts of lipid hydroperoxides (LOOH) was studied; Fe2+ oxidation, oxygen consumption and oxidation index of the liposomes were measured. No peroxidation was observed at variable FeCl2/FeCl3 ratio when PC liposomes deprived of LOOH by triphenyl-phosphine treatment were utilized. By contrast, LOOH containing liposomes were peroxidized by FeCl2. The FeCl2 concentration at which Fe2+ oxidation was maximal, defined as critical Fe2+ concentration [Fe2+]*, depended on the LOOH concentration and not on the amount of PC liposomes in the assay. The LOOH-dependent lipid peroxidation was stimulated by FeCl3, addition; the oxidized form of the metal increased the average length of radical chains, shifted to higher values the [Fe2+]* and shortened the latent period. The iron chelator KSCN exerted effects opposite to those exerted by FeCl3 addition. The experimental data obtained indicate that the kinetics of LOOH-dependent lipid peroxidation depends on the Fe2+/Fe3+ ratio at each moment during the time course of lipid peroxidation. The results confirm that exogenously added FeCl3 does not affect the LOOH-independent but the LOOH-deendent lipid peroxidation; and suggest that the Feg, endogenously generated exerts a major role in the control of the LOOH-dependent lipid peroxidation.

Oxidized Phosphatidylcholines That Modify Proteins: ANALYSIS BY MONOCLONAL ANTIBODY AGAINST OXIDIZED LOW DENSITY LIPOPROTEIN

Oxidatively modified low density lipoprotein (OxLDL) is known to be involved in atherogenesis. We have previously developed a murine monoclonal antibody, FOH1a/DLH3, which recognized oxidatively modified lipoproteins as well as foam cells in human atherosclerotic lesions (Itabe, H., Takeshima, E., Iwasaki, H., Kimura, J., Yoshida, Y., Imanaka, T., and Takano, T. (1994) J. Biol. Chem. 269, 15274-15279). The antigen of this monoclonal antibody was formed by peroxidation of phosphatidylcholine (PC), and the antigenic oxidized PC (OxPC) derivatives are thought to form complexes with polypeptides including apolipoproteins. OxLDL was measured by a sensitive sandwich enzyme-linked immunosorbent assay using the monoclonal antibody and anti-human apolipoprotein B antibody, in which antigenic OxPC competed with OxLDL. When antigenic activities of PC analogs were tested by the competition assay, 1-palmitoyl-2-(9-oxononanoyl) PC (9-CHO PC) and the hydroperoxide of egg PC potently inhibited the detection of OxLDL. 1-Palmitoyl-2-linoleoyl PC was oxidized with ferrous ion and ascorbic acid, and the antigenic products were purified from the OxPC extracts on high pressure liquid chromatography columns and subsequently analyzed by laser desorption mass spectrometry. Molecular weight determination and retention times of high pressure liquid chromatography suggest that one of these products was 9-CHO PC. Other products are thought to be 8-carbon aldehyde, dihydroxy, and ketohydroxy derivatives of PC. When a C-terminal 16-mer synthetic peptide of the 70-kDa peroxisomal membrane protein was simply incubated with 9-CHO PC, it was found to be reactive in a sandwich enzyme-linked immunosorbent assay using FOH1a/DLH3 and an anti-peptide antiserum. These results suggest that the anti-OxLDL monoclonal antibody FOH1a/DLH3 reacts with several oxidized products of PC including aldehyde derivatives of PC, which covalently modify polypeptides.

A monoclonal antibody against oxidized lipoprotein recognizes foam cells in atherosclerotic lesions. Complex formation of oxidized phosphatidylcholines and polypeptides.

In this study we have used homogenates of human atheromatous plaque as immunogen to establish a murine monoclonal antibody which recognizes oxidized low density lipoprotein (LDL). This monoclonal antibody, FOH1a/DLH3, reacted with oxidized LDL, but enzyme-linked immunosorbent assay showed that it had no reaction with native, acetylated, or malonaldehyde-treated LDL. The antibody cross-reacted with oxidized high density lipoprotein, suggesting that specific sequences of the apolipoprotein B are not essential for antigen recognition by the antibody. Immunohistochemical analysis of thin paraffin sections from human coronary arteries showed that foam cells derived from macrophages in atherosclerotic lesions were heavily stained by this antibody. Several other structures in the lesions, including swollen collagen fibers, cellular debris in necrotic cores, and endothelial cells, were moderately stained. The epitope of this antibody was characterized by a model antigen-generating system using ferrous ion-induced peroxidation of lipids. When the total lipid fraction extracted from LDL was treated with the ferrous ion-induced peroxidation system, the reaction mixture was recognized by the antibody. Antigenic product(s) was produced only when phosphatidylcholine (PC) was treated with the ferrous ion-induced peroxidation, other lipids failed to react to the antibody. To investigate the possible formation of a complex of antigenic product with a polypeptide, a synthetic peptide and a rabbit antiserum against the peptide were used. Reaction mixture of ferrous ion-induced peroxidation of PC in the presence of the peptide was added to a microtiter well precoated with the monoclonal antibody FOH1a/DLH3. After washing, the peptide remaining in the well was detected with rabbit antiserum against the peptide, whereas no reactivity was observed when peptide alone was added to the well. Binding of the antigenic complex to the precoated monoclonal antibody was competed by oxidized PC produced in the absence of any polypeptide. We conclude that oxidized phospholipid product(s) is the epitope of the monoclonal antibody and that the oxidized phospholipid forms complexes with polypeptides. The antigenic materials are detected in foam cells in atherosclerotic lesions.

Phosphatidylcholine peroxidation and liver cancer in mice fed a choline-deficient diet with ethionine

A high incidence of hepatocellular carcinoma (HCC) was observed in mice fed a choline-deficient diet containing 0.1% ethionine (CDE) for 19 months. HCC was present in 85% of CDE mice and in 22% of choline-deficient (CD) mice not receiving ethionine. This strong hepatocarcinogenicity of the CDE diet was concomitant with a severe decrease in plasma and liver α-tocopherol (Toc) to 60 and 35%, respectively, of those contained in choline-supplemented (CS) control mice. We previously found that this dietary-induced HCC was preceded at 4-week feeding by a depletion of Toc and a remarkable increase of phosphatidylcholine hydroperoxide (PCOOH) in the livers of CDE mice. When HCC was prominent in CDE mice, PCOOH was still elevated. Mouse glutathione S-transferase (GST) M II isozyme, which is related to rat GST-P form, a positive marker for rat hepatic preneoplastic and neoplastic lesions, revealed an inverse histochemical pattern as that seen in rats (i.e., the HCC lesions tended to decreased staining). The aforementioned results taken together indicate that decreases in Toc and enhanced PC peroxidation are important events in CDE-induced mice liver tumors.

Peroxidation and phospholipase A 2 hydrolytic susceptibility of liposomes consisting of mixed species of phosphatidylcholine and phosphatidylethanolamine

The relationship between lipid peroxidation and phospholipase A 2 (PLA 2) hydrolytic activity was studied using unilamellar vesicles (liposomes) as model membranes. Hydrolytic specificity was examined using vesicles prepared with pure bovine heart phosphatidylcholine (PC), bovine heart phosphatidylethanolamine (PE), or mixturesof these phospholipids, using two preparative procedures, i.e., sonication or extrusion. Lipid peroxidation was induced vesicles with cumene hydroperoxide and hematin at 37°C. Determinationsof the extent of peroxidation by means of diene conjugate conjugate derived from second derivative spectra or by polarographic measurement of oxygen consumption rates provided a basis for comparing the extent of peroxidation of each phospholipid species to their subsequent hydrolysis by PLA 2 (from Crotalus adamanteus). The extent of hydrolysis was determined through the release of arachidonic acid from either PC or PE. The PE distribution among the outer vs. inner leaflet of the membrane bilayer was nearly equal in sonicated vesicles, whereas most of the phospholipid was incorporated into the inner leaflet in extruded vesicles. The proportion of PE found in the inner leaflet progressively increased as the ratio of PE to PC increased in both sonicated and extruded vesicle preparations. Lipid peroxidation had no effect on PE distribution under the conditions examined. There was a clear preference for PC peroxidation for all vesicle compositions tested and PC was preferentially hydrolyzed by PLA 2. This effect is proposed to result from a perturbation of membrane structure following peroxidation with assimilation of PC into PLA 2-susceptible domains whereas PE peroxidation and hydrolysis is less affected in mixed PC/PE vesicles. Lipid peroxidation imposes an additional hydrolytic susceptibility over the effects exerted through the mixing of these phospholipids which is based on structural changes rather than formation of specific substrates for PLA 2 PLA 2.

Liver phosphatidylcholine hydroperoxidation provoked by ethionine‐containing choline‐deficient diet in mice

It is shown that peroxidation of phosphatidylcholine (PC) is enhanced in liver of mice fed a hepatocarcinogenic choline-deficient diet containing 0.1% w/w ethionine. Mice were divided into 4 groups and fed for 4 weeks one of the following diets: choline-supplemented; choline-supplemented containing ethionine; choline-deficient; and choline-deficient containing ethionine. Phosphatidylcholine hydroperoxide (PCOOH) of liver lipids was measured by high performance liquid chromatography using a chemiluminescence detector. Mice fed a choline-deficient diet containing ethionine showed 6-fold higher PCOOH levels than the choline-supplemented control mice: the PCOOH/PC molar ratios of liver lipids were 32.3 X 10(-5) and 5.6 X 10(-5), respectively. In addition to this remarkable degree of lipid peroxidation in liver of mice fed the choline-deficient diet containing ethionine, we also observed a significant liver fatty infiltration, a decrease in plasma and liver alpha-tocopherol, and an increase in liver injury-indicative enzyme activities. Also, marker enzymes for hepatocarcinogenesis, glucose-6-phosphatase and gamma-glutamyl transpeptidase were affected. These data suggest that enhanced hydroperoxidation of phosphatidylcholine may participate in hepatocarcinogenesis provoked by choline deficiency in the presence of ethionine.

Quinone formation from carcinogenic benzo[ a]pyrene mediated by lipid peroxidation in phosphatidylcholine liposomes

The behavior of benzo[ a]pyrene (B[ a]P) during peroxidation of phosphatidylcholine (PC) liposomes initiated by an azo compound was investigated to examine the mechanism of quinone formation from carcinogenic B[ a]P mediated by nonenzymatic lipid peroxidation occurring in vivo. B[ a]P had a retarding effect on the peroxidation of polyunsaturated fatty acid moiety of PC. The major oxidation products which accumulated in the peroxidized liposomes were B[ a]P 1,6-, 3,6-, and 6,12-quinone. Antioxidants acting as scavengers of chain-propagating lipid peroxy radicals effectively prevented not only lipid peroxidation but also B[ a]P oxidation in the liposomal suspension. PC hydroperoxides, the primary products of PC oxidation, did not react with B[ a]P in the absence of the azo compound, indicating that lipid peroxy radicals, not lipid hydroperoxides, are responsible for the formation of these quinones. The experiments using 18O 2 gas and 18O-labeled methyl linoleate hydroperoxides demonstrated that B[ a]P quinones are formed by incorporating molecular oxygen and their origin is partly due to the lipid peroxy radical. The mechanism proposed for the formation of B[ a]P quinones mediated by peroxidation of membrane lipids involves a direct attack of the lipid peroxy radical on B[ a]P and subsequent autocatalytic oxidation. Weak carcinogenic and noncarcinogenic pentacyclic aromatic hydrocarbons showed little reactivity to the lipid peroxy radical in the liposomes. Thus, the facility of the peroxidative attack on B[ a]P may be related to the powerful carcinogenic activity of this substance.