Enzymatic production of hydroperoxides of unsaturated fatty acids by injury of mammalian cells

The action of peroxyl radicals, powerful deleterious reagents, explains why neither cholesterol nor saturated fatty acids cause atherogenesis and age-related diseases

Cells respond to alterations in their membrane structure by activating hydrolytic enzymes. Thus, polyunsaturated fatty acids (PUFAs) are liberated. Free PUFAs react with molecular oxygen to give lipid hydroperoxide molecules (LOOHs). In case of severe cell injury, this physiological reaction switches to the generation of lipid peroxide radicals (LOO(·)). These radicals can attack nearly all biomolecules such as lipids, carbohydrates, proteins, nucleic acids and enzymes, impairing their biological functions. Identical cell responses are triggered by manipulation of food, for example, heating/grilling and particularly homogenization, representing cell injury. Cholesterol as well as diets rich in saturated fat have been postulated to accelerate the risk of atherosclerosis while food rich in unsaturated fatty acids has been claimed to lower this risk. However, the fact is that LOO(·) radicals generated from PUFAs can oxidize cholesterol to toxic cholesterol oxides, simulating a reduction in cholesterol level. In this review it is shown how active LOO(·) radicals interact with biomolecules at a speed transcending usual molecule-molecule reactions by several orders of magnitude. Here, it is explained how functional groups are fundamentally transformed by an attack of LOO(·) with an obliteration of essential biomolecules leading to pathological conditions. A serious reconsideration of the health and diet guidelines is required.

Is Atherosclerosis a Multifactorial Disease or Is It Induced by a Sequence of Lipid Peroxidation Reactions?

The delivery of not only free cholesterol but also cholesterol esters to cells by low-density lipoprotein (LDL) has hitherto been unstudied. Minor compounds present in mammalian-derived food include cholesterol linoleate and arachidonate. Evidence is presented that these esters are directly incorporated into VLDL and are responsible for the deleterious effects of atherosclerosis. Cholesterol esterified with these polyunsaturated fatty acids (PUFAs) is readily oxidized at the PUFA residue during storage and heating. Apparently, the liver is unable to distinguish between nonoxidized and oxidized cholesterol PUFA esters and also incorporates the latter into VLDL, which is transformed to LDL. When this LDL is transferred to endothelial cells, the toxic products are liberated and induce cell damage. Cell damage is combined with structural changes that influence neighboring cells and cause an influx of Ca2+ ions and activation of phospholipases and lipoxygenases, resulting in production of lipid hydroperoxides (LOOHs). When the level of free PUFAs generated by phospholipases exceeds a certain limit, lipoxygenases commit suicide, causing liberation of iron ions. The latter react with LOOHs and thus induce a switch from enzymatic to nonenzymatic generation of lipid peroxidation (LPO) products. Although the LOO. radicals produced in enzymatic reactions are deactivated within the enzyme complex, LOO. radicals generated in nonenzymatic reactions are able to attack any biological compound, inducing severe damage. Apparently, iron ions and LOOH molecules at the surface of injured cells transfer the nonenzymatic LPO reactions to the phospholipid layer of bypassing lipoproteins, thus explaining why inflammatory diseases, such as diabetes, are combined with atherogenesis.

The relation of lipid peroxidation processes with atherogenesis: A new theory on atherogenesis

The extremely high sensitivity of polyunsaturated fatty acids (PUFAs) to oxygen is apparently used by nature to induce stepwise appropriate cell responses. It is hypothesized that any alteration in the cell membrane structure induces influx of Ca2+ ions. Ca2+ ions are required to activate degrading enzymes, such as phospholipases and lipoxygenases (LOX) that transform PUFAs bound to membrane phospholipids to lipidhydroperoxides (LOOHs). Enzymatic reduction products of LOOHs seem to serve as ligands of proteins, which induce gene activation to initiate a physiological response. Increasing external impact on cells is connected with deactivation of LOX, liberation of the iron ion in its active center followed by cleavage of LOOH molecules to LO * radicals. LO * radicals induce a second set of responses leading to generation of unsaturated aldehydic phospholipids and unsaturated epoxyhydroxy acids that contribute to induction of apoptosis. Finally peroxyl radicals are generated by attack of LO * radicals on phospholipids. The latter attack nearly all types of cell constituents: Amino- and hydroxyl groups are oxidized to carbonyl functions, sugars and proteins are cleaved, molecules containing double bonds such as unsaturated fatty acids or cholesterol suffer epoxidation. LOOH molecules and iron ions at the cell wall of an injured cell are in tight contact with phospholipids of neighboring cells and transfer to these reactive radicals. Thus, the damaging processes proceed and cause finally necrosis except the chain reaction is stopped by scavengers, such as glutathione. Consequently, PUFAs incorporated into phospholipids of the cell wall are apparently equally important for the fate of a single organism as the DNA in the nucleus for conservation of the species. This review intends to demonstrate the connection of cell alteration reactions with induction of lipid peroxidation (LPO) processes and their relation to inflammatory diseases, especially atherosclerosis and a possible involvement of food. Previously it was deduced that food rich in cholesterol and saturated fatty acids is atherogenic, while food rich in n-3 PUFAs was recognized to be protective against vascular diseases. These deductions are in contradiction to the fact that saturated fatty acids withstand oxidation while n-3 PUFAs are subjected to LPO like all other PUFAs. Considering the influence of minor food constituents a new theory about atherogenesis and the influence of n-3 PUFAs is represented that might resolve the contradictory results of feeding experiments and chemical experiences. Cholesterol-PUFA esters are minor constituents of mammalian derived food, but main components of low density lipoprotein (LDL). The PUFA part of these esters occasionally suffers oxidation by heating or storage of mammalian derived food. There are indications that these oxidized cholesterol esters are directly incorporated into lipoproteins and transferred via the LDL into endothelial cells where they induce damage and start the sequence of events outlined above. The deduction that consumption of n-3 PUFAs protects against vascular diseases is based on the observation that people living on a fish diet have a low incidence to be affected by vascular diseases. Fish are rich in n-3 PUFAs; thus, it was deduced that the protective properties of a fish diet are due to n-3 PUFAs. Fish, fish oils, and vegetables contain besides n-3 PUFAs as minor constituents furan fatty acids (F-acids). These are radical scavengers and are incorporated after consumption of these nutrients into human phospholipids, leading to the assumption that not n-3 PUFAs, but F-acids are responsible for the beneficial efficiency of a fish diet.

Do changes in the cell membrane structure induce the generation of lipid peroxidation products which serve as first signalling molecules in cell to cell communication?

Evidence is presented that mammalian and plant cells respond equally to any event which changes their cell membrane structure. Proliferation, wounding or aging induces generation of lipidhydroperoxides from cell wall phospholipids. These are transformed to signalling compounds, some of these induce apoptosis. If the exerted impact exceeds a certain level, the original enzymic reaction switches to a non-enzymic one which produces peroxylradicals. The latter are not liberated enzymically. Peroxylradicals generate a second set of signalling compounds, but cause also severe damage: they epoxidize double bonds, and oxidize proteins, sugars and nucleic acids. Such reactions occur in all inflammatory diseases. Lipidhydoperoxides and their degradation products are incorporated in fat. Apparently, these compounds are transferred partly to LDL. Such LDL is still recognized by the cell LDL receptor. Toxic lipid peroxidation products are therefore introduced into cells and might be able to damage cells from inside long before the typical signs of atherosclerosis and other chronic diseases become visible.

Lipid peroxidation in aging and age-dependent diseases

Aging is related with an increase in oxidation products derived from nucleic acids, sugars, sterols and lipids. Evidence will be presented that these different oxidation products are generated by processes induced by changes in the cell membrane structure (CMS), and not by superoxide, as commonly assumed. CMS activate apparently membrane bound phospholipases A2 in mammals and plants. Such changes occur by proliferation, aging and especially by wounding. After activation of phospholipases, influx of Ca2+ ions and activation of lipoxygenases (LOX) is induced. The LOX transform polyunsaturated fatty acids (PUFAs) into lipid hydroperoxides (LOOHs), which seem to be decomposed by action of enzymes to signalling compounds. Following severe cell injury, LOX commit suicide. Their suicide liberates iron ions that induce nonenzymic lipid peroxidation (LPO) processes by generation of radicals. Radicals attack all compounds with the structural element -CH=CH-CH(2)-CH=CH-. Thus, they act on all PUFAs independently either in free or conjugated form. The most abundant LPO products are derived from linoleic acid. Radicals induce generation of peroxyl radicals, which oxidise a great variety of biological compounds including proteins and nucleic acids. Nonenzymic LPO processes are induced artificially by the treatment of pure PUFAs with bivalent metal ions. The products are separable after appropriate derivatisation by gas chromatography (GC). They are identified by electron impact mass spectrometry (EI/MS). The complete spectrum of LPO products obtained by artificial LPO of linoleic acid is detectable after wounding of tissue, in aged individuals and in patients suffering from age-dependent diseases. Genesis of different LPO products derived from linoleic acid will be discussed in detail. Some of the LPO products are of high chemical reactivity and therefore escape detection in biological surrounding. For instance, epoxides and highly unsaturated aldehydic compounds that apparently induce apoptosis.

Alteration in cellular fatty acid composition as a response to salt, acid, oxidative and thermal stresses in Lactobacillus helveticus

The fundamental question in this study is concerned with whether the increase of unsaturated fatty acids in the cell membrane is a general response of certain thermotolerant strains or species when exposed to superoptimal temperatures, and in combination with other stresses, especially oxidative stress. A strain of Lactobacillus helveticus, a species widely used as a starter in the dairy industry and able to tolerate high temperature and NaCl concentrations as well as acidic conditions, was chosen for this study. Cells of strain CNBL 1156, grown in its natural medium (i.e. milk whey), were exposed for 100 min to sublethal combinations of temperature, NaCl, H(2)O(2) and pH, modulated according to a Central Composite Design. The fatty acid composition of cell lipid extract was identified by GC/MS. Polynomial equations, able to describe the individual interactive and quadratic effects of the independent variables on cell fatty acid composition, were obtained. The results and the mathematical models relative to the individual fatty acids indirectly suggest that desaturase activation or hyperinduction play an important role in the response to heat stress. In fact, the relative proportions of oleic, linoleic and palmitic acids increased with temperature in a range between 38 and 54 degrees C. The fatty acid profiles included vernolic acid (up to 37% of total fatty acids), an epoxide of linoleic acid not previously reported in microbial cells. In particular, this epoxide was present in cells exposed to low pH in combination with high temperatures and oxidative stress. In conclusion, these results provide experimental support to the hypothesis that the increase of an oxygen-consuming desaturase system, with a consequent increase in fatty acid desaturation, is a cellular response to environmental stresses able to protect the cells of this anaerobic micro-organism from toxic oxygen species and high temperatures.

Detection of phospholipid oxidation in oxidatively stressed cells by reversed-phase HPLC coupled with positive-ionization electrospray [correction of electroscopy] MS

Measurement of lipid peroxidation is a commonly used method of detecting oxidative damage to biological tissues, but the most frequently used methods, including MS, measure breakdown products and are therefore indirect. We have coupled reversed-phase HPLC with positive-ionization electrospray MS (LC-MS) to provide a method for separating and detecting intact oxidized phospholipids in oxidatively stressed mammalian cells without extensive sample preparation. The elution profile of phospholipid hydroperoxides and chlorohydrins was first characterized using individual phospholipids or a defined phospholipid mixture as a model system. The facility of detection of the oxidized species in complex mixtures was greatly improved compared with direct-injection MS analysis, as they eluted earlier than the native lipids, owing to the decrease in hydrophobicity. In U937 and HL60 cells treated in vitro with t-butylhydroperoxide plus Fe(2+), lipid oxidation could not be observed by direct injection, but LC-MS allowed the detection of monohydroperoxides of palmitoyl-linoleoyl and stearoyl-linoleoyl phosphatidylcholines. The levels of hydroperoxides observed in U937 cells were found to depend on the duration and severity of the oxidative stress. In cells treated with HOCl, chlorohydrins of palmitoyloleoyl phosphatidylcholine were observed by LC-MS. The method was able to detect very small amounts of oxidized lipids compared with the levels of native lipids present. The membrane-lipid profiles of these cells were found to be quite resistant to damage until high concentrations of oxidants were used. This is the first report of direct detection by LC-MS of intact oxidized phospholipids induced in cultured cells subjected to oxidative stress.

Evidence for the formation of dinor isoprostanes E1 from alpha-linolenic acid in plants

The free radical oxidation of arachidonic acid is known to generate complex metabolites, termed isoprostanes, that share structural features of prostaglandins and exert potent receptor-mediated biological activities. In the present study, we show that alpha-linolenic acid can undergo a similar oxidation process, resulting in a series of isomeric dinor isoprostanes E1. E-ring dinor isoprostane formation from linolenate was found to be catalyzed by soybean lipoxygenase. The main enzymatic products were 13- and 9-hydroperoxylinolenate but in addition, two dinor isoprostane E1 regioisomers were formed with a yield of 0.31%. Identification and quantification of two dinor isoprostane E1 regioisomers in plant cell cultures was achieved by a negative chemical ionization gas chromatography-mass spectrometry method using [18O]dinor isoprostanes E1 as internal standards. Endogenous levels of these compounds were determined in four taxonomically distant plant species and found to be in the range of 4.5 to 60.9 ng/g of dry weight. Thus analogous pathways in animals and plants exist, each leading to a family of prostaglandin-like compounds derived from polyunsaturated fatty acids. It remains to be shown whether the dinor isoprostanes exert biological activities in plants as has been demonstrated for their C20 congeners in mammals.

Linoleic acid peroxidation--the dominant lipid peroxidation process in low density lipoprotein--and its relationship to chronic diseases

Modern separation and identification methods enable detailed insight in lipid peroxidation (LPO) processes. The following deductions can be made: (1) Cell injury activates enzymes: lipoxygenases generate lipid hydroperoxides (LOOHs), proteases liberate Fe ions--these two processes are prerequisites to produce radicals. (2) Radicals attack any activated CH2-group of polyunsaturated fatty acids (PUFAs) with about a similar probability. Since linoleic acid (LA) is the most abundant PUFA in mammals, its LPO products dominate. (3) LOOHs are easily reduced in biological surroundings to corresponding hydroxy acids (LOHs). LOHs derived from LA, hydroxyoctadecadienoic acids (HODEs), surmount other markers of LPO. HODEs are of high physiological relevance. (4) In some diseases characterized by inflammation or cell injury HODEs are present in low density lipoproteins (LDL) at 10-100 higher concentration, compared to LDL from healthy individuals.

Direct observation of lipid hydroperoxides in phospholipid vesicles by electrospray mass spectrometry

Positive ion electrospray ionization mass spectrometry was used to obtain a lipid profile of vesicles prepared from egg yolk lethicin and enriched with arachidonylstearoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine. The vesicles were oxidized by treatment with tert-butylhydroperoxide and iron (II) sulfate, and the formation of hydroperoxides of the polyunsaturated lipid arachidonylstearoyl phosphatidylcholine was observed. The native lipid signal at 832 a.m.u. decreased and new signals appeared at 864, 896, and 928 a.m.u., corresponding to the addition of one (+32), two (+64), and three (+96) molecules of dioxygen. The dihydroperoxide was found to be the most favourable peroxide product, but it appeared that a degradation of the hydroperoxides was occurring concomitant with their formation, and only their net formation was observed. The rate of depletion of the polyunsaturated lipid and the rate of accumulation of the hydroperoxides was found to increase with the Fe2+ concentration between 10 microM and 2 mM, and was also dependent on the tert-butylhydroperoxide concentration. This is the first report of analysis of lipid hydroperoxides by electrospray mass spectrometry, showing that technique offers a sensitive, direct, and informative approach to the study of oxidative damage to biological membranes.

Strong dependence of the lipid peroxidation product spectrum whether Fe2+/O2 or Fe3+/O2 is used as oxidant

Catalytic amounts of Fe2+ or Fe3+ ions are widely applied to induce simulated biological lipid peroxidation reactions. Independently, whether Fe2+ or Fe3+ were used, similar products were obtained. We show in this paper that the product spectrum is indeed very different, whether one ion species, either Fe2+ or Fe3+, is present in excess; thus, decomposition of (13S,9Z,11E) 13-hydroxyperoxy-9, 11-octadecadienoic acid (13S-HPODE) generates in the presence of equimolar amounts of Fe2+ ions mainly the corresponding alcohol (13S, 9Z,11E) 13-hydroxy-9,11-octadecadienoic acid besides 12,13-epoxy-11-hydroxy-9-octadecenoic acid (12,13-epHOD) and 13-oxo-9,11-octa-decadienoic acid (13-KODE), while decomposition of 13S-HPODE with equimolar amounts of Fe3+ produces mainly 12,13-epHOD, hydrolysis products thereof and other oxidized products, e.g., hydroxyoxo acids. In addition, unusually large amounts of aldehydes are formed, e.g., the amount of 4-hydroxy-nonenal was found to exceed that obtained by Fe2+ induced air oxidation for a factor of about 100. Since these further oxidation products are suspected to cause cell damage, liberated Fe3+ ions seem to be responsible for generation of toxic products in inflammatory diseases, e.g., atherosclerosis.

Influence of lipid profile and fatty acid composition on the oxidation behavior of rat and guinea pig low density lipoprotein

Low density lipoprotein (LDL) oxidation is one of the first steps proposed for the development of atherosclerosis. Since lipid profile and fatty acid composition may affect this process, we studied the influence of these factors on the oxidation behavior of rat and guinea pig LDL. Marked compositional differences were observed. Thus, the main lipid carried by rat LDL was triglyceride (TG) (35.8 +/- 5.8%, w/w) whereas total cholesterol (TC) represented 23.8 +/- 3.0%. In contrast, guinea pig LDL contained 13.2 +/- 2% of TG and 44.8 +/- 4.5% of TC. Rat LDL contained higher 20:4(n-6) molar percentages than guinea pig LDL. Thiobarbituric acid reactive substances (TBARS) production (255 +/- 26 and 137 +/- 13 nmol malondialdehyde/mg prot. for rat and guinea pig LDL, respectively) and the maximum rate of conjugated dienes (CD) formation (485 +/- 93 and 77 +/- 11 nmol CD/min/mg protein for rat and guinea pig LDL, respectively) showed that rat LDL are less resistant to oxidation in vitro than guinea pig LDL. The higher oxidation rate of rat LDL seems to be related to its lipid profile, mainly to the high proportion of TG, and to the high content of 20:4(n-6), which is one of the fatty acids most prone to oxidation.

Lipid peroxidation in presence of ebselen

Lipid peroxidation is initiated by cell damage. After homogenisation of porcine heart tissue in aqueous solution we observed the same lipid peroxidation products as detected after heart infarction. We used this observation to study the influence of ebselen (2-phenyl-1,2-benzoisoselenazol-3-(2H)-one) on the generation of oxidatively derived monohydroxy fatty acids and alpha-hydroxyaldehydes, typical lipid peroxidation (LPO) products. Heart tissue was homogenised before and after enzyme destruction and with addition of ebselen. The obtained LPO products were analysed by GC/MS after appropriate derivatisation and quantified by using internal standards. The amount of monohydroxy fatty acids and alpha-hydroxyaldehydes increased considerably in the porcine heart homogenates in which the enzymes were kept active. Addition of ebselen caused an additional significant increase of hydroxy fatty acids, while the increase of aldehydic compounds was less. These results confirm the glutathione peroxidase-like activity of ebselen but demonstrate also that it does not prevent lipid peroxidation.

Increased levels of lipid oxidation products in low density lipoproteins of patients suffering from rheumatoid arthritis

9-Hydroxy-10,12-octadecadienoic acid (9-HODE) and 13-hydroxy-9,11-octadecadienoic acid (13-HODE) are accumulated in the low density lipoproteins of patients suffering from rheumatoid arthritis for a factor of 20-50 compared to healthy individuals of the same age. Both acids, derived by lipid peroxidation of linoleic acid, induce the release of interleukin 1 beta. The latter induces bone degression. The genesis of 9- and 13-HODE seems therefore to be an important factor in the development and progression of rheuma; in addition 9-HODE was reported to be a stimulus of inflammation, comparable to leukotrienes.

Increase in hydroxy fatty acids in human low density lipoproteins with age

The content of hydroxy fatty acids in low density lipoprotein (LDL) of healthy volunteers aged between 22 and 87 years without any signs of atherosclerosis or other age-dependent diseases was investigated. The level of hydroxy fatty acids obtained from LDL increases during life time: clinically healthy persons between 56 and 66 years showed a 3- to 4-fold increase compared to young volunteers. This level increased in samples of probands aged 68 to 74 years compared to samples of young people for a factor of 10-20 and in samples of probands aged 78 to 87 years for a factor of 30-40. These hydroxy acids--generated mainly from hydroperoxy acids on linoleic acid and only partly from arachidonic acid--are obviously parameters of the LDL oxidation stage. About 90% of the total amount of hydroxy fatty acid were free fatty acids. The distribution patterns of the monohydroxy derivatives of linoleic and arachidonic acid indicate that they originate mainly from autocatalytic processes. The individual level of hydroxy acids is probably an indicator of the biological age.

Enzymic lipid peroxidation--a consequence of cell injury?

It is postulated that cell injury activates "dormant" enzymes to produce lipid hydroperoxides. In a first step, membrane lipids are cleaved by esterases. The unsaturated fatty acids thus produced are converted in a second step by lipoxygenases to lipid hydroperoxides (LOOHs). In a third, nonenzymic step, these LOOHs, together with dienoic hydroxy fatty acids produced by enzymic reduction of LOOHs, react with a second oxygen molecule to generate dihydroperoxy-fatty acids and hydroxy-hydroperoxy-fatty acids, which are degraded to alpha-hydroxyladehydic compounds. This last reaction requires production of LO'-radicals by iron ions that also are generated as a result of cell damage. In addition, alpha-hydroxyaldehydes are produced by hydrolysis of plasmalogen epoxides, which are generated by oxidation of plasmalogens with LOO' or by action of epoxidases. We hypothize that alpha-hydroxyaldehydes act as second messengers. The release of lipoxygenase and the consequent lipid hydroperoxidation is postulated to occur in massive cell damage (e.g., myocardial infarction), in chronic diseases such as rheumatism, diabetes and atherosclerosis, in aging, and in control of cell proliferation.