Fate of lipid hydroperoxides in blood plasma

Cholesteryl ester hydroperoxide (CE-OOH) and phosphatidylcholine hydroperoxide (PC-OOH) are the major primary oxidation products of lipoproteins. CE-OOH is present in human and rat plasmas while PC-OOH is undetectable. This is likely due to the enzymatic (plasma glutathione peroxidase) and the nonenzymatic (apolipoproteins A and B-100) reducing activities of PC-OOH in plasma, and to the enzymatic conversion of PC-OOH to CE-OOH by lecithin:cholesterol acyltransferase in high density lipoproteins. The regioisomeric distribution of CE-O(O)H in human plasma indicates that free radical-mediated chain oxidation is an ongoing process, even in healthy young individuals.     

Fate of lipid hydroperoxides in blood plasma

Cholesteryl ester hydroperoxide (CE-OOH) and phosphatidylcholine hydroperoxide (PC-OOH) are the major primary oxidation products of lipoproteins. CE-OOH is present in human and rat plasmas while PC-OOH is undetectable. This is likely due to the enzymatic (plasma glutathione peroxidase) and the nonenzymatic (apolipoproteins A and B-100) reducing activities of PC-OOH in plasma, and to the enzymatic conversion of PC-OOH to CE-OOH by lecithin:cholesterol acyltransferase in high density lipoproteins. The regioisomeric distribution of CE-O(O)H in human plasma indicates that free radical-mediated chain oxidation is an ongoing process, even in healthy young individuals.     

Plasma lipid hydroperoxides measurement by an automated xylenol orange method

Lipid hydroperoxides (LH) appear to be good candidates as initial biomarkers of oxidative stress. We describe an automated method to quantify it, based on a known principle: oxidation of Fe II to Fe III by lipid hydroperoxides, under acidic conditions, followed by complexation of Fe III by xylenol orange. This method requires only a 10-microl sample volume of heparinized plasma or serum. It has been carried out automatically, with two reagents, in a two-end-point mode with bichromatic detection at 570 and 700 nm. The within-run precision, measured on a low- and a high-level plasma, was 5.0+/-0.3 and 14.0+/-0.6 microM (n=25 for each series). The between-run precision (one run for 18 days), evaluated on two commercial controls, was 5.6+/-0.5 microM (CV=8.9%) and 7.9+/-0.5 microM (CV=6.3%). The recovery of known amounts of tert-butylhydroperoxide (1 and 2 microM) added to human plasma was 98%. The specificity was demonstrated by the excellent correlation of the values of 42 samples measured either directly, with a simple dilution, or after gel permeation chromatography. The reference interval determined on 21 subjects was 4.9+/-1.7 microM. This was in the upper range of previously published values but our recovery and chromatographic experiments strongly suggest that former methods have underestimated the true content of LH in human plasma.     

Evidence that lipid hydroperoxides inhibit plasma lecithin:cholesterol acyltransferase activity

The oxidation of low density lipoproteins (LDL) has been implicated in the development of atherosclerosis. Recently, we found that polar lipids isolated from minimally oxidized LDL produced a dramatic inhibition of lecithin: cholesterol acyltransferase (LCAT) activity, suggesting that HDL-cholesterol transport may be impaired during early atherogenesis. In this study, we have identified molecular species of oxidized lipids that are potent inhibitors of LCAT activity. Treatment of LDL with soybean lipoxygenase generated small quantities of lipid hydroperoxides (20 +/- 4 nmol/mg LDL protein, n = 3); but when lipoxygenase-treated LDL (1 mg protein/ml) was recombined with the d > 1.063 g/ml fraction of human plasma, LCAT activity was rapidly inhibited (25 +/- 4 and 65 +/- 16% reductions by 1 and 3 h, respectively). As phospholipid hydroperoxides (PL-OOH) are the principal oxidation products associated with lipoxygenase-treated LDL, we directly tested whether PL-OOH inhibited plasma LCAT activity. Detailed dose-response curves revealed that as little as 0.2 and 1.0 mole % enrichment of plasma with PL-OOH produced 20 and 50% reductions in LCAT activity by 2 h, respectively. To gain insight into the mechanism of LCAT impairment, the enzyme's free cysteines (Cys31 and Cys184) and active site residues were "capped" with the reversible sulfhydryl compound, DTNB, during exposure to either minimally oxidized LDL or PL-OOH. Reversal of the DTNB "cap" after such exposures revealed that the enzyme was completely protected from both sources of peroxidized phospholipids. We, therefore, conclude that PL-OOH inhibited plasma LCAT activity by modifying the enzyme's free cysteine and/or catalytic residues. These studies are the first to suggest that PL-OOH may accelerate the atherogenic process by impairing LCAT activity.     

Selective microdetermination of lipid hydroperoxides

A sensitive and selective assay for lipid hydroperoxides was developed based upon the activation by hydroperoxides of the cyclooxygenase activity of prostaglandin H synthase. The assay measures hydroperoxides directly by their stimulatory action on the cyclooxygenase and thus differs from the methods used currently which rely on the measurement of secondary products to estimate the amount of hydroperoxide. The present assay of enzymatic response was approximately linear in the range 10 to 150 pmol of added lipid hydroperoxide. This sensitivity for lipid peroxides is about 50-fold greater than that of the thiobarbiturate assay with fluorescence detection. When applied to samples of human plasma, the enzymatic assay indicated that the concentration of lipid hydroperoxides in normal subjects is 0.5 microM, more than 50-fold lower than estimated by the thiobarbiturate assay (30-50 microM). Nevertheless, the circulating concentration of 0.5 microM lipid hydroperoxide approaches that reported to have deleterious effects upon vascular prostacyclin synthase.     

Preparation of pure lipid hydroperoxides

Increasing evidence of lipid peroxidation in food deterioration and pathophysiology of diseases have revealed the need for a pure lipid hydroperoxide (LOOH) reference as an authentic standard for quantification and as a compound for biological studies in this field. Generally, LOOH is prepared from photo- or enzymatically oxidized lipids; however, separating LOOH from other oxidation products and preparing pure LOOH is difficult. Early studies showed the usability of reaction between hydroperoxide and vinyl ether for preparation of pure LOOH. Because the reactivity of vinyl ether with LOOHs other than fatty acid hydroperoxides has never been reported, here, we employed the reaction for preparation of a wide variety of pure LOOHs. Phospholipid, cholesteryl ester, triacylglycerol, or fatty acid was photo- or enzymatically oxidized; the resultant crude sample containing hydroperoxide was allowed to react with a vinyl ether [2-methoxypropene (MxP)]. Liquid chromatography (LC) and mass spectrometry confirmed that MxP selectively reacts with LOOH, yielding a stable MxP adduct (perketal). The lipophilic perketal was eluted at a position away from that of intact LOOH and identified and isolated by LC. Upon treatment with acid, perketal released the original LOOH, which was finally purified by LC. Using our optimized purification procedures, for instance, we produced 75 mg of pure phosphatidylcholine hydroperoxide (>99%) from 100 mg of phosphatidylcholine. Our developed method expands the concept of the perketal method, which provides pure LOOH references. The LOOHs prepared by the perketal method would be used as "gold standards" in LOOH methodology.     

Chemiluminescent assay of lipid hydroperoxides

The addition of luminol plus a catalyst such as peroxidase or a heme prosthetic group to a solution containing a small quantity of lipid hydroperoxides results in a flash of chemiluminescence, the intensity of which is a function of the hydroperoxide concentrations. Various protocols for lipid hydroperoxide assays have been described and we have studied conditions to increase their sensitivity and specificity. Plasma lipid hydroperoxide determinations require an extraction, since compounds present in plasma interfere with light emission. Moreover, the sensitivity of the assay is by the presence of hydrogen peroxide in the medium, which causes high background values. Catalase does not act on lipid hydroperoxides and can be used to eliminate hydrogen peroxide from the reaction medium. The determination requires a blank tube in which hydroperoxides are destroyed by incubating the sample with haematin plus ascorbate. The increase in the chemiluminescence of the assay tube caused by the presence of lipid hydroperoxides is then compared to the value obtained for an internal standard.     

Automatic determination of hydroperoxides of phosphatidylcholine and phosphatidylethanolamine in human plasma

An automatic method for the determination of hydroperoxides of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) is reported. Sample plasma was deproteinized with a fourfold volume of methanol. After centrifugation, the supernatant was injected directly into an HPLC system without further treatment. The hydroperoxides of PC and PE were concentrated and washed on an ODS column followed by introduction into two analytical columns, a silica gel and an aminopropylsilica gel column, which were connected in series, by column switching. After the separation, they were detected by postcolumn detection with diphenyl-1-pyrenylphosphine. The compounds were determined at picomole levels within 30 min with good reproducibilities. By using only a silica gel column as an analytical column, PC hydroperoxides were determined within 20 min, and samples could be injected into it at 15-min intervals. Those methods made it possible to inject a sample of up to 2 ml at one time and up to 8 ml by repeated injections and to determine phospholipid hydroperoxides in human plasma at picomole levels.     

An inter-laboratory validation of methods of lipid peroxidation measurement in UVA-treated human plasma samples

Abstract

Lipid peroxidation products like malondialdehyde, 4-hydroxynonenal and F₂-isoprostanes are widely used as markers of oxidative stress in vitro and in vivo. This study reports the results of a multi-laboratory validation study by COST Action B35 to assess inter-laboratory and intra-laboratory variation in the measurement of lipid peroxidation. Human plasma samples were exposed to UVA irradiation at different doses (0, 15 J, 20 J), encoded and shipped to 15 laboratories, where analyses of malondialdehyde, 4-hydroxynonenal and isoprostanes were conducted. The results demonstrate a low within-day-variation and a good correlation of results observed on two different days. However, high coefficients of variation were observed between the laboratories. Malondialdehyde determined by HPLC was found to be the most sensitive and reproducible lipid peroxidation product in plasma upon UVA treatment. It is concluded that measurement of malondialdehyde by HPLC has good analytical validity for inter-laboratory studies on lipid peroxidation in human EDTA-plasma samples, although it is acknowledged that this may not translate to biological validity.     

Determination of phospholipid hydroperoxides in human blood plasma by a chemiluminescence-HPLC assay

A chemiluminescence-high performance liquid chromatography (CL-HPLC) system was developed (Miyazawa, T. et al., Anal. Lett., 20, 915-925, 1987) and applied for the hydroperoxide-specific determination of phosphatidylcholine hydroperoxide (PCOOH) in biological tissues such as human blood plasma (Miyazawa, T. et al., Anal Lett 21:1033-1044, 1988; J. Biochem. 103:744-746; 1988). This system involves separation of phosphatidylcholines from plasma total lipids with normal phase silica gel HPLC and post-column detection of hydroperoxide-dependent chemiluminescence of PCOOH. The chemiluminescence is produced by luminol oxidation during a reaction of hydroperoxide and cytochrome c-heme. The high specificity for hydroperoxide base enables a sensitive assay for a large range of PCOOH, with the detection limit of 10 picomole of hydroperoxide-O2. By use of this assay system, the presence of PCOOH in human blood plasma is confirmed quantitatively. The PCOOH concentration of healthy plasma is in the range below 10 nM to 500 nM, and much higher concentrations (500-9000 nM) of PCOOH are observed in the plasma of unhealthy donors.     

Simultaneous determination of phospholipid hydroperoxides and cholesteryl ester hydroperoxides in human plasma by high-performance liquid chromatography with chemiluminescence detection

A method was developed for the simultaneous determination of phosphatidylcholine hydroperoxides (PCOOH) and cholesteryl ester hydroperoxides (CEOOH). Lipid hydroperoxides (LOOH) were quantitatively extracted from human plasma with a mixture of n-hexane and ethyl acetate, and separated by column-switching high-performance liquid chromatography using one aminopropyl column and two octyl columns followed by chemiluminescence detection. LOOHs could be completely separated from each other and detected at picomole levels. The results of method validation tests were satisfactory. This method was then applied to determine LOOH in normal human plasma; the levels of PCOOH and CEOOH found were 36.0±4.0 nM (mean±S.D., n=6) and 12.3±3.1 nM (mean±S.D., n=6), respectively.     

Detection of retinal lipid hydroperoxides in experimental uveitis.

In our on-going studies of experimental uveitis, we previously obtained a preliminary indication of phagocyte-mediated retinal lipid peroxidation by measuring conjugated dienes (CD), thiobarbituric acid reactive substances (TBARS) and fluorescent chromolipids. Using gas chromatography/mass spectrometry (GC/MS), the current study detected hydroperoxide-derived 10-, 11-, 13-, 14-, and 17-hydroxydocosahexaenoic acid (HDHE) in retinal membranes. Docosahexaenoic acid (22:6) is the major polyunsaturated fatty acid (PUFA) in photoreceptor membranes. Hydroperoxides from other retinal PUFA were found also. Arachidonic acid (20:4) yielded 8-, 9-, 11-, 12-hydroxyeicosatetraenoic acid (HETE) as major products. Since 12-HETE could also arise from lipoxygenase catalyzed oxygenation of free 20:4, the source of 12-HETE could be both peroxidative and lipoxygenase pathways. Concomitantly, peroxidative loss of 22:6 and accumulation of 20:4 were also noted. At the peak of inflammation, loss of 22:6 was close to 50% of the original amount in the control retinas. In the same time period, 20:4 increased more than two-fold. The present data suggest that the oxygen radicals derived from phagocytes initiate the retinal lipid peroxidation, and the resultant formation of hydroperoxides, oxidative loss of 22:6 and accumulation of 20:4 appear to serve as amplification factors in subsequent biochemical events, such as chemotaxis of PMNs and activation of cyclooxygenase.     

A modified tetramethylbenzidine method for measuring lipid hydroperoxides

A simple and sensitive spectrophotometric method for measuring lipid peroxides and peroxides in general is described. The method was developed by modifying an existing method based on the peroxidase activity of hemoglobin with tetramethylbenzidine as the electron donor. The modifications resulted in much improved sensitivity and reproducibility. With the modified method lipid peroxides as low as 2 nmol can be measured, a high sensitivity compared with other spectrophotometric methods. The absorbance is linear over a wide range of concentrations. It is suggested that this modified method in combination with the commonly used thiobarbituric acid method will give a better quantitation of lipid peroxidation.     

Detection of in vivo lipid peroxidation using the thiobarbituric acid assay for lipid hydroperoxides

Thiobarbituric acid (TBA) assays which have been modified for detection of lipid hydroperoxides appear to be useful for demonstration of in vivo lipid peroxidation. Since these methods require heating tissue membranes with the buffered TBA, there is a possibility of interference from the detection of autoxidation that occurs during heating. These studies were undertaken to investigate conditions which favor TBA color production from hydroperoxide while limiting autoxidation during the assay. An acetic acid-sodium acetate buffered (pH 3.6) TBA assay was used. Heating linoleic acid hydroperoxide with 50 microM ferric iron or under nitrogen nearly doubled color production compared to heating it with no added iron or under air. The lipid antioxidant butylated hydroxytoluene inhibited color production from fatty acid hydroperoxides. When tissue fractions, including liver and lung microsomes and lung whole membranes, were heated in the assay, color production was greater under air than under nitrogen and was much greater under oxygen. When liver microsomes from carbon tetrachloride-exposed rats were used, color was increased only when oxygen was present in the heating atmosphere. The results with tissue fractions appear to demonstrate autoxidation during color development rather than the presence of preformed hydroperoxides. Finally, it was found that color production from membrane fractions was dependent on the vitamin E content of the membranes. It appears that autoxidation during heating should be limited by heating under nitrogen and not by adding antioxidants, which inhibit color production from hydroperoxides. As the vitamin E effect demonstrates, antioxidant status must be considered, since a change in color production could result from a change in antioxidant content without the accumulation of lipid hydroperoxides.     

Assay of lipid hydroperoxides by activation of cyclooxygenase activity

The bis-dioxygenation of arachidonate to form the hydroperoxide, prostaglandin G2, is catalyzed by the cyclooxygenase activity of prostaglandin H synthase. This activity is stimulated by hydroperoxide, and it can be used to assay picomole amounts of hydroperoxide.     

Dissemination of peroxidative stress via intermembrane transfer of lipid hydroperoxides: model studies with cholesterol hydroperoxides

Lipid hydroperoxides (LOOHs) can be generated in cells when cholesterol (Ch) and other unsaturated lipids in cell membranes are degraded under conditions of oxidative stress. If LOOHs escape reductive detoxification by glutathione-dependent selenoperoxidases, they may undergo iron-catalyzed one-electron reduction to free radical species, thus triggering peroxidative chain reactions which exacerbate oxidative membrane damage. LOOHs are more polar than parent lipids and much longer-lived than free radical precursors or products. Accordingly, intermembrane transfer of LOOHs (analogous to that of unoxidized precursors) might be possible, and this could jeopardize acceptor membranes. We have investigated this possibility, using photoperoxidized [(14)C]Ch-labeled erythrocyte ghosts as cholesterol hydroperoxide (ChOOH) donors and unilamellar liposomes [e.g., dimyristoyl-phosphatidylcholine/Ch, 9:1 mol/mol] as acceptors. ChOOH material consisted mainly of 5alpha-hydroperoxide, a singlet oxygen adduct. Time-dependent transfer of ChOOH versus Ch at 37 degrees C was determined, using high-performance liquid and thin-layer chromatographic methods to analyze liposomal extracts for these species. A typical experiment in which the starting ChOOH/Ch mol ratio in ghosts was approximately 0.05 showed that the initial transfer rate of ChOOH was approximately 16 times greater than that of parent Ch. Using [(14)C]Ch as a reporter in liposome acceptors, we found that transfer-acquired ChOOHs, when exposed to a lipophilic iron chelate and ascorbate, could trigger strong peroxidative chain reactions, as detected by accumulation of [(14)C]Ch oxidation products. These findings support the hypothesis that intermembrane transfer of ChOOHs can contribute to their prooxidant membrane damaging and cytotoxic potential.     

Reduction of lipid hydroperoxides by apolipoprotein B-100.

We have previously isolated two proteins which can reduce phosphatidylcholine hydroperoxide (PC-OOH) from human blood plasma and identified one of the proteins as apolipoprotein A-I (Mashima, R. , et al. (1998) J. Lipid Res. 39, 1133-1140). In the present study we have identified the other protein as apolipoprotein B-100 (apo B-100) by amino acid sequence analysis of its tryptic peptides. The reactivity of lipid hydroperoxides with apo B-100 decreased in the order of PC-OOH > linoleic acid hydroperoxide > cholesteryl ester hydroperoxide under our experimental conditions. Pretreatment of apo B-100 with chloramine T, an oxidant of methionine, diminished the PC-OOH-reducing activity, indicating that some of 78 methionines are responsible for the reduction of PC-OOH. Despite the presence of 6 methionines in albumin, albumin was inactive to reduce PC-OOH. Free methionine was also inactive. These data suggest that the accessibility and binding of lipid hydroperoxides to the protein methionine residues are crucial for reduction of lipid hydroperoxides.     

Limitations of the hemoglobin method for detecting lipid hydroperoxides

The method using peroxidase activity of hemoglobin (Hb) for the determination of lipid peroxides, trilinoleoylglycerol hydroperoxides and phosphatidylcholine hydroperoxides as substrates and tetramethyl benzidine as electron donor for the peroxidase reaction of Hb. The reactivities of these substrates were different. Some electron donors were tested for peroxidase activity of Hb, but none showed a complete reduction of methyl linoleate hydroperoxides. Front these results, the Hb method needs to be carefully applied to biological materials that contain mixtures of different typos of lipid classes.