How different constituents of low density lipoprotein determine its oxidizability by copper: a correlational approach

Dual signaling evoked by oxidized LDLs in vascular cells

The oxidative theory of atherosclerosis relies on the modification of low density lipoproteins (LDLs) in the vascular wall by reactive oxygen species. Modified LDLs, such as oxidized LDLs, are thought to participate in the formation of early atherosclerotic lesions (accumulation of foam cells and fatty streaks), whereas their role in advanced lesions and atherothrombotic events is more debated, because antioxidant supplementation failed to prevent coronary disease events and mortality in intervention randomized trials. As oxidized LDLs and oxidized lipids are present in atherosclerotic lesions and are able to trigger cell signaling on cultured vascular cells and macrophages, it has been proposed that they could play a role in atherogenesis and atherosclerotic vascular remodeling. Oxidized LDLs exhibit dual biological effects, which are dependent on extent of lipid peroxidation, nature of oxidized lipids (oxidized phospholipids, oxysterols, malondialdehyde, α,β-unsaturated hydroxyalkenals), concentration of oxidized LDLs and uptake by scavenger receptors (e.g. CD36, LOX-1, SRA) that signal through different transduction pathways. Moderate concentrations of mildly oxidized LDLs are proinflammatory and trigger cell migration and proliferation, whereas higher concentrations induce cell growth arrest and apoptosis. The balance between survival and apoptotic responses evoked by oxidized LDLs depends on cellular systems that regulate the cell fate, such as ceramide/sphingosine-1-phosphate rheostat, endoplasmic reticulum stress, autophagy and expression of pro/antiapoptotic proteins. In vivo, the intimal concentration of oxidized LDLs depends on the influx (hypercholesterolemia, endothelial permeability), residence time and lipid composition of LDLs, oxidative stress intensity, induction of defense mechanisms (antioxidant systems, heat shock proteins). As a consequence, the local cellular responses to oxidized LDLs may stimulate inflammatory or anti-inflammatory pathways, angiogenic or antiangiogenic responses, survival or apoptosis, thereby contributing to plaque growth, instability, complication (intraplaque hemorrhage, proteolysis, calcification, apoptosis) and rupture. Finally, these dual properties suggest that oxLDLs could be implicated at each step of atherosclerosis development, from early fatty streaks to advanced lesions, depending on the nature and concentration of their oxidized lipid content.

Effects of cyclodextrins on the structure of LDL and its susceptibility to copper-induced oxidation

Cyclodextrins (CDs) have long been widely used as drug/food carriers and were recently developed as drugs for the treatment of diseases (e.g. Niemann-Pick C1 and cancers). It is unknown whether cyclodextrins may influence the structure of low-density lipoprotein (LDL), its susceptibility to oxidation, and atherogenesis. In this study, four widely used cyclodextrins including α-CD, γ-CD, and two derivatives of β-CD (HPβCD and MβCD) were recruited. Interestingly, agarose gel electrophoresis (staining lipid and protein components of LDL with Sudan Black B and Coomassie brilliant blue, respectively but simultaneously) shows that cyclodextrins at relatively high concentrations caused disappearance of the LDL band and/or appearance of an additional protein-free lipid band, implying that cyclodextrins at relatively high concentrations can induce significant electrophoresis-detectable lipid depletion of LDL. Atomic force microscopy (AFM) detected that MβCD (as a representative of cyclodextrins) induced size decrease of LDL particles in a dose-dependent manner, further confirming the lipid depletion effects of cyclodextrins. Moreover, the data from agarose gel electrophoresis, conjugated diene formation, MDA production, and amino group blockage of copper-oxidized LDL show that cyclodextrins can impair LDL susceptibility to oxidation. It implies that cyclodextrins probably help to inhibit atherogenesis by lowering LDL oxidation.

Algae consumption and risk of type 2 diabetes: Korean National Health and Nutrition Examination Survey in 2005

The purpose of this study is to examine the relationship between dietary algae (seaweed) consumption and the risk of Type 2 diabetes mellitus in the Korean population. We analyzed data from the Korean National Health and Nutrition Examination Survey in 2005, a nationally representative survey. The study participants included 3,405 males and females aged 20-65 y. Participants were classified into four groups according to the quartiles of total algae consumption frequencies. Proportional odds models were used to assess the relationship between algae consumption and the risk of having diabetes or prediabetes, after adjustment for age, family history of diabetes, education, smoking, alcohol consumption, physical activity, body mass index, waist circumference, triglycerides, total energy intake and food group intakes. The frequency of algae consumption was positively correlated to the consumption of legumes, fruits, fish, and dairy products in both genders (p<0.001). The odds ratios (95% confidence interval) for diabetes vs. prediabetes vs. normoglycemia was 0.66 (0.43-0.99) for males and 0.80 (0.51-1.24) for females in the highest quartile of algae consumption compared to the lowest quartile. Our results suggest that dietary algae consumption may decrease the risk of diabetes mellitus in Korean men. A well-designed prospective study is needed to confirm this association.

HDL3-Mediated Inactivation of LDL-Associated Phospholipid Hydroperoxides Is Determined by the Redox Status of Apolipoprotein A-I and HDL Particle Surface Lipid Rigidity

Objectives— Small dense HDL3 particles of defined lipidome and proteome potently protect atherogenic LDL against free radical-induced oxidation; the molecular determinants of such antioxidative activity in these atheroprotective, antiinflammatory particles remain indeterminate.

Methods and Results— Formation of redox-active phosphatidylcholine hydroperoxides (PCOOH) and redox-inactive phosphatidylcholine hydroxides (PCOH) was initiated in LDL by free radical-induced oxidation. Human HDL3 inactivated LDL-derived PCOOH (−62%, P <0.01) and enhanced accumulation of PCOH (2.1-fold, P <0.05); in parallel, HDL3 accumulated minor amounts of PCOOH. Enzyme-deficient reconstituted dense HDL potently inactivated PCOOH (−43%, P <0.01). HDL3-mediated reduction of PCOOH to PCOH occurred concomitantly with oxidation of methionine residues in HDL3-apolipoprotein AI (apoAI). Preoxidation of methionine residues by chloramine T markedly attenuated PCOOH inactivation (−35%); by contrast, inhibition of HDL3-associated enzymes was without effect. PCOOH transfer rates from oxidized LDL to phospholipid liposomes progressively decreased with increment in the rigidity of the phospholipid monolayer.

Conclusions— The redox status of apoAI and surface lipid rigidity represent major determinants of the potent HDL3-mediated protection of LDL against free radical-induced oxidation. Initial transfer of PCOOH to HDL3 is modulated by the surface rigidity of HDL3 particles with subsequent reduction of PCOOH to PCOH by methionine residues of apoAI.

LDL resistance to oxidation: Effects of lipid phenotype, autologous HDL and alanine

BACKGROUND

Although LDL resistance to copper-induced oxidation is a time-honoured method, how it is modulated by the physiologic variability of lipid phenotype and what influences the protective action of homologous HDL and exogenous alanine is still unclear.

METHODS

In 159 subjects without severe dyslipidemias, LDL resistance to copper-induced oxidation (lag phase) was measured under standardised conditions, with alanine and with autologous HDL.

RESULTS

Lag phase was normally distributed and averaged 68+/-10 min (range: 40-105 min). Both VLDL-triglycerides (37+/-5, 52+/-7, 59+/-7, 53+/-5 mg/dl, p<0.05) and LDL-triglycerides (27+/-2, 27+/-1, 30+/-2, 35+/-3 mg/dl, p<0.01) increased across quartiles of lag phase. The relative LDL enrichment in triglycerides (triglycerides percent or triglycerides/cholesterol ratio) was strongly related to lag phase (r=0.29 and r=0.31, p<0.0005 for both) independently of age, gender, BMI, and presence of diabetes or hypertension. The protective effect of HDL was variable (+42+/-18 min) and largely dependent on the capacity of HDL to resist oxidation (r=0.69, p<0.0001). Alanine induced a rather constant lag phase prolongation (+32+/-7 min) that was weakly related only to baseline lag phase (r=0.17, p<0.05).

CONCLUSIONS

Relative triglyceride abundance protects LDL from ex-vivo oxidation, HDL particles protect LDL mainly through substrate dilution and alanine probably through a direct anti-oxidant effect.

Nutritional, Dietary and Postprandial Oxidative Stress

Nutritional, or dietary oxidative stress denotes a disturbance of the redox state resulting from excess oxidative load or from inadequate nutrient supply favoring prooxidant reactions. Low intake or impaired availability of dietary antioxidants including vitamins E and C, carotenoids, polyphenols, and other micronutrients (e.g., selenium) weakens the antioxidant network. Postprandial oxidative stress, as a subform of nutritional oxidative stress, ensues from sustained postprandial hyperlipidemia and/or hyperglycemia and is associated with a higher risk for atherosclerosis, diabetes, and obesity. In Western societies, a significant part of the day is spent in the postprandial state. Unsaturated fatty acids incorporated into LDL and oxidized LDL are an atherogenic factor. Lipid hydroperoxides present in the diet are absorbed, contributing to the prooxidant load. In hyperlipidemic and hyperglycemic subjects, endothelium-dependent vasodilation is impaired in the postprandial state, making postprandial oxidative stress an important factor modulating cardiovascular risk. Postprandial oxidative stress is attenuated when dietary antioxidants are supplied together with a meal rich in oxidized or oxidizable lipids. Ingestion of dietary polyphenols, e.g., from wine, cocoa, or tea, improves endothelial dysfunction and lowers the susceptibility of LDL lipids to oxidation. Polyphenols affect endothelial function not solely as antioxidants but also as modulatory signaling molecules.

Pro-oxidative effect of α-tocopherol in the oxidation of LDL isolated from co-antioxidant-depleted non-diabetic hemodialysis patients

The association between the antioxidants in LDL and the oxidizability of LDL assessed by the oxidation lag time during copper ion-catalyzed oxidation was investigated in 69 non-diabetic hemodialysis patients and 23 healthy volunteers. The concentrations of co-antioxidants, including ubiquinol-10, lycopene and beta-carotene, in LDL were significantly lower in the hemodialysis patients than in the healthy volunteers, while there was no difference in the alpha-tocopherol concentration between the groups. The lag time showed a significantly positive correlation with the alpha-tocopherol level (r = 0.62, P < 0.01) in the healthy subjects, but a significantly negative correlation (r = -0.38, P < 0.05) in the hemodialysis patients. Furthermore, in vitro incubation of LDL with alpha-tocopherol prolonged the lag time in the healthy subjects, but shortened it in the hemodialysis patients. These results suggested that the alpha-tocopherol might exert the pro-oxidative effect in co-antioxidant-depleted LDL that was isolated from the hemodialysis patients. Despite such co-antioxidant depletion and the pro-oxidative effect of alpha-tocopherol, the lag time in the hemodialysis patients was not statistically different from that in the healthy volunteers. This might have been because the polyunsaturated fatty acids concentration, another determinant of the lag time, in LDL was less in the hemodialysis patients than in the healthy controls.

Plasma and CSF markers of oxidative stress are increased in Parkinson's disease and influenced by antiparkinsonian medication

We determined systemic oxidative stress in Parkinson's disease (PD) patients, patients with other neurological diseases (OND) and healthy controls by measurement of in vitro lipoprotein oxidation and levels of hydro- and lipophilic antioxidants in plasma and cerebrospinal fluid (CSF). Additionally, we investigated the influence of levodopa (LD) and dopamine agonist therapy (DA) on the oxidative status in PD patients. We found increased oxidative stress, seen as higher levels of lipoprotein oxidation in plasma and CSF, decrease of plasma levels of protein sulfhydryl (SH) groups and lower CSF levels of alpha-tocopherol in PD patients compared to OND patients and controls. Levodopa treatment did not significantly change the plasma lipoprotein oxidation but LD monotherapy tended to result in an increase of autooxidation and in a decrease of plasma antioxidants with significance for ubiquinol-10. DA monotherapy was significantly associated with higher alpha-tocopherol levels. Patients with DA monotherapy or co-medication with DA showed a trend to lower lipoprotein oxidation. These data support the concept of oxidative stress as a factor in the pathogenesis of PD and might be an indicator of a potential prooxidative role of LD and a possible antioxidative effect of DA in PD treatment.

Effects of Atorvastatin and Simvastatin on Low-Density Lipoprotein Subtraction Profile, Low-Density Lipoprotein Oxidizability, and Antibodies to Oxidized Low-Density Lipoprotein in Relation to Carotid Intima Media Thickness in Familial Hypercholesterolemia

Background

Little is known about the effects of statins on the quality of circulating low-density lipoprotein (LDL) in relation to atherosclerosis progression.

Methods

In a double-blind, randomized trial of 325 patients with familial hypercholesterolemia (FH), we assessed the effects of high-dose atorvastatin (80 mg) and conventional-dose simvastatin (40 mg) on LDL subtraction profile ( n = 289), LDL oxidizability ( n = 121), and circulating autoantibodies to oxidized LDL ( n = 220). Progression of atherosclerosis was measured by carotid intima media thickness (IMT) ( n = 325).

Results

At baseline, the patients showed an intermediate LDL subtraction profile composed of three LDL subtractions (LDL1, LDL2, LDL3), with LDL2 as the predominant subtraction. A strong negative correlation was found between plasma triglycerides and the LDL subtraction profile ( r = - .64, p = .000). Both plasma levels of triglycerides and small dense LDL3 correlated weakly with baseline IMT ( r = .11, p = .04 and r = .15, p = .01, respectively; n = 289). No association was found between baseline IMT and oxidation parameters or circulating antibodies to oxidized LDL. Atorvastatin reduced triglycerides, LDL cholesterol, and all LDL subtractions to a greater extent than did simvastatin and led to regression of carotid IMT. However, LDL subtraction pattern and plasma levels of autoantibodies to oxidized LDL remained unchanged in both treatment groups, and LDL oxidizability increased minimally to a similar extent in both groups. Significant treatment differences were found for the rate of in vitro oxidation of LDL and the amount of dienes formed during in vitro oxidation of LDL, which both decreased more following atorvastatin than after simvastatin.

Conclusion

Change of IMT after statin treatment was associated with baseline IMT ( r = .41), LDL cholesterol ( r = -.20), and the amount of dienes formed during in vitro oxidation of LDL ( r = .28) but not with plasma levels of antibodies to oxidized LDL, in vitro LDL oxidizability, and LDL subtraction profile.

Extent of copper LDL oxidation depends on oxidation time and copper/LDL ratio: chemical characterization

The aim of our study was to determine, as a function of [Cu(2+)]/[LDL] ratios (0.5 and 0.05) and of oxidation phases, the extent of LDL oxidation by assessing the lipid and apo B oxidation products. The main results showed that: (i) kinetics of conjugated diene formation presented four phases for Cu(2+)/LDL ratio of 0.5 and two phases for [Cu(2+)]/[LDL] ratio of 0.05; (ii) oxidation product formation (cholesteryl ester and phosphatidylcholine hydroperoxides, apo B carbonyl groups) occurred early in the presence of endogenous antioxidants, under both copper oxidation conditions; (iii) apo B carbonylated fragments appeared when antioxidants were totally consumed at [Cu(2+)]/[LDL] ratio of 0.5; and (iv) antioxidant concentrations were stable, oxysterol formation was negligible, and no carbonylated fragment was detected at [Cu(2+)]/[LDL] ratio of 0.05. Depending on the copper/LDL ratio, oxidized LDL differ greatly in the nature of lipid peroxidation product and the degree of apo B fragmentation.

Lipoprotein oxidation, plasma total antioxidant capacity and homocysteine level in patients with multiple sclerosis

Free radical-mediated peroxidation of biological molecules, especially of lipids, is implicated in the pathogenesis of a number of diseases like multiple sclerosis. Low concentration of antioxidant vitamins: beta carotene, retinol, alpha tocopherol and ascorbic acid have been observed in serum or cerebrospinal fluid of multiple sclerosis patients. On the basis of these observations, we studied the potential lipoprotein oxidation and total antioxidant capacity in the pathogenesis of multiple sclerosis. Lipoprotein oxidizability for plasma in vitro, serum levels of autoantibodies against oxidized low-density lipoproteins, plasma total homocysteine levels with vitamin B12 and folate, and plasma total antioxidant capacity were measured in twenty four patients with multiple sclerosis and twenty four healthy sex- and age-matched person as control. In multiple sclerosis patients during an attack, a significant increase in both in vitro lipid oxidizability for plasma and in the levels of autoantibodies against oxidized low-density lipoproteins, and a strong decrease in plasma total antioxidant capacity were detected. Plasma total homocysteine levels were significantly higher in multiple sclerosis patients whose plasma vitamin B12 and folate levels were lower but not statistically significant, than controls. The present study indicates that lipoprotein oxidation may be important factor in the course of multiple sclerosis and in vitro measurements of plasma oxidation kinetics as an indication for lipoprotein oxidation might be useful as an additional tool for the clinical diagnosis of multiple sclerosis.

Neonatal blood plasma is less susceptible to oxidation than adult plasma owing to its higher content of bilirubin and lower content of oxidizable Fatty acids

Newborn infants are susceptible to a range of problems attributed to excessive production of free radicals. Because of a higher content of antioxidants, above all bilirubin, and a lower content of oxidizable lipids, newborn plasma should be better protected against oxidation than adult plasma. To test this hypothesis, we measured the susceptibility of plasma to in vitro oxidation in microsamples (7 microL) from 57 healthy newborns and 18 adults. Heparin plasma was diluted 150-fold and oxidized by 50 microM Cu2+. Oxidation was monitored as an increase in sample absorbance at 234 nm. Plasma oxidizability was found to be significantly lower in newborns than in adults. Accordingly, the level of bilirubin, an important antioxidant, was significantly higher, and the level of polyunsaturated fatty acids, a major substrate of lipid peroxidation, was significantly lower in newborn plasma. In addition, plasma oxidizability correlated positively with the level of polyunsaturated fatty acids and negatively with that of bilirubin. These data indicate that plasma is better protected against oxidative stress in newborns than in adults, owing to its higher content of antioxidants like bilirubin and its lower content of oxidizable lipids.

Effect of fish oil on LDL oxidation and plasma homocysteine concentrations in health

Oxidation of low-density lipoprotein (LDL) and hyperhomocysteinemia are believed to play a role in therogenesis. Whether n-3 polyunsaturated fatty acids increase LDL susceptibility to oxidation or influence homocysteine (Hcy) metabolism has long been a subject of controversy. In this study, we evaluated the effect of 8 weeks of dietary supplementation with 6 g/day of fish oil (FO; 3 g of n-3 fatty acids) on plasma lipoproteins, in vitro LDL peroxidation, antioxidant status, and plasma Hcy concentrations in 16 normolipidemic subjects. FO rapidly and significantly (P < .01) decreased plasma total and very low density lipoprotein triglyceride concentrations and had no effect on LDL or high-density-lipoprotein cholesterol. The mean lag time before onset of Cu(2+)-induced LDL oxidation, as well as plasma and LDL alpha-tocopherol and beta-carotene concentrations, was unchanged. However, changes in plasma aminothiol concentrations occurred during the study. Specifically, a progressive and significant increase in total Hcy plasma concentrations was observed (13.4% and 20% after 4 and 8 weeks, respectively; P < .01). Total glutathione concentrations were significantly higher after 8 weeks (P < .05). The tHcy increase was not associated with changes in plasma folate or vitamin B(12) concentrations. However, concentrations of plasma nitric oxide metabolites (NO(x) = NO(2) + NO(3)) were significantly higher than at baseline after 8 weeks of FO intake (74%; P < .01). Further, the changes in total Hcy and NO(x) plasma concentrations observed after 8 weeks of FO were found to be significantly correlated (r = .78, P < .001). With this study, we report for the first time the apparent interaction of n-3 fatty acids and nitric oxide on Hcy metabolism.

Comparison of the effects of alpha-tocopherol, ubiquinone-10 and probucol at therapeutic doses on atherosclerosis in WHHL rabbits

Oxidative modification of lipoproteins may trigger and maintain atherogenesis. We compared the effects of different antioxidants (alpha-tocopherol, probucol, ubiquinone-10) at doses similar to those used in humans in Watanabe Heritable Hyperlipidemic (WHHL) rabbits for 12 months. Aortic lesions were analyzed for their extent and cellular composition of lesions, mean thickness of fibrous caps and density of smooth muscle cells therein, content of antioxidants, non-oxidized and oxidized lipids. Compared to controls, probucol significantly lowered the extent and macrophage content of lesions and increased the existence and smooth muscle cell density of fibrous caps. alpha-Tocopherol supplementation increased the aortic content of vitamin E, but had no decreasing effect on either the accumulation of macrophage-specific antigen in the aorta or lesion size. Nevertheless, both probucol and alpha-tocopherol significantly decreased in vitro LDL oxidizability, measured under typically strong oxidative conditions. Ubiquinone-10 supplement increased lesion size and the fraction of lesions containing fibrous caps; however, LDL oxidizability remained unaffected by ubiquinone-10 treatment. None of the antioxidants tested lowered oxidized lipids within aortic tissue; however, long-term treatment with probucol provided the most effective anti-atherosclerotic effect, while alpha-tocopherol may be pro-atherogenic and ubiquinone-10 exerts ambivalent effects. Our data suggest that (i) widely used oxidation measures, such as ex-vivo LDL oxidizability, do not reflect the degree of atherosclerosis; and (ii) long-term beneficial effects of relatively low doses of antioxidants may be outweighed by high levels of plasma cholesterol in WHHL rabbits.

Gemfibrozil lowers plasma lipids and increases polyunsaturated fatty acid content and oxidative susceptibility of lipoproteins in hypertriglyceridemia

BACKGROUND

Gemfibrozil is an effective drug in the treatment of hypertriglyceridemia and its effects on morbidity and mortality seem out of proportion to its lipid lowering actions. There is considerable interest in its potential effects on lipoprotein fatty acid composition and consequent effect on oxidative susceptibility. Experimental results are not conclusive regarding whether gemfibrozil alters lipid composition or oxidative susceptibility of lipoproteins in humans. Here we investigate this question using different methodology than employed in previous investigations.

METHODS

Eleven hypertriglyceridemic individuals completed a 12-week course of gemfibrozil therapy (600 mg twice daily) intended to primarily evaluate a new way of assessing lipoprotein susceptibility to oxidation in relation to changes in the fatty acid profile. We measured susceptibility of lipoproteins in the plasma macromolecule fraction to copper-mediated oxidation. In addition, plasma lipids were separated into phospholipid (PL), cholesterol ester (CE) and triglyceride (TG) fractions and the fatty acid composition of these classes determined by gas-liquid chromatography. The relation between changes in lipid concentration, fatty acid composition and oxidative parameters (principally lag time) was examined by correlational analysis.

RESULTS

Triglyceride concentrations and total cholesterol concentrations responded appropriately to gemfibrozil (lowered by 55% and 15%, respectively). Polyunsaturated fatty acid (PUFA) proportion increased significantly in cholesterol ester and phospholipid fractions of plasma lipids at the expense of saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA). Oxidative parameters also changed significantly. Lag time (LT) and maximal extent of oxidation showed the most significant changes. Lag time, the principle measure of lipoprotein susceptibility to oxidation, was decreased by gemfibrozil. The increase in polyunsaturated fatty acid content in phospholipid and cholesterol ester significantly correlated with decreased lag time.

CONCLUSION

These data support the notion that gemfibrozil increases the proportion of polyunsaturated fatty acids in plasma lipids and that this increase is associated with an increase in lipoprotein oxidative susceptibility as measured by lag time in hypertriglyceridemia.

Increased lipid peroxidation in cerebrospinal fluid and plasma from patients with Creutzfeldt-Jakob disease

Oxidative pathomechanisms play an important role in neurodegenerative diseases like Alzheimer's disease (AD). It has been shown that lipid peroxidation in cerebrospinal fluid (CSF) and plasma is increased in AD. To assess the role of oxidative stress in Creutzfeldt-Jakob disease (CJD), we investigated the oxidizability of lipids, the lipid composition and the levels of the antioxidants ascorbate and alpha-tocopherol in CSF and plasma of 15 CJD patients and 12 neurologically healthy controls. CSF and plasma lipid peroxidation was increased in CJD patients and polyunsaturated fatty acids were reduced in CSF of these patients. Ascorbate levels were lower in CSF and plasma of CJD patients, while alpha-tocopherol was found to be decreased in CSF but not in plasma. These results support the hypothesis that oxidative mechanisms are involved in the pathogenesis of CJD and provide a rationale for the use of antioxidants in the therapy of this disease.

Simple and rapid assessment of lipoprotein susceptibility to oxidation in the macromolecule fraction of plasma

BACKGROUND

Susceptibility of lipoprotein to oxidation is usually studied using purified lipoproteins. However, for large clinical studies or routine clinical assessment, a rapid, less time-consuming method that minimizes hydroperoxide accumulation in lipoprotein during preparation is desirable. Methods using whole plasma are complicated by the presence of an anticoagulant that may affect oxidation and the presence of antioxidants in the plasma aqueous phase. Lipoproteins in serum are relatively unprotected from initiation of oxidation.

METHODS

We studied copper-mediated oxidation of a macromolecule fraction of plasma prepared by simple molecular sieve chromatography.

RESULTS

LDL was more susceptible to oxidation than plasma macromolecules. The lag times approached zero as the copper concentration increased. The propagation rate was linear for PM vs. the copper concentration while LDL became saturated at 10-20 micromol/l. The interassay CVs for the lag phase rate, lag time, PR and maximal DeltaA(max) were 23%, 7.7%, 4.9%, and 3.3%, respectively.

CONCLUSIONS

This procedure should be applicable to large numbers of individuals in investigations regarding the effects of drugs and diets on lipid composition and oxidative susceptibility.

Resistance of human cerebrospinal fluid to in vitro oxidation is directly related to its amyloid-beta content

Amyloid-beta (A beta) peptide, a major constituent of senile plaques and a hallmark of Alzheimer's disease (AD), is normally secreted by neurons and can be found in low concentrations in cerebrospinal fluid (CSF) and plasma where it is associated with lipoproteins. However, the physiological role of A beta secretion remains unknown. We measured the resistance to in vitro oxidation of CSF obtained from 20 control subjects and 30 patients with AD, and correlated it with CSF levels of antioxidants, lipids and A beta. We found that the oxidative resistance, expressed as a duration of the oxidation lag-phase, was directly related to CSF levels of A beta 1-40, A beta 1-42 and ascorbate and inversely to levels of fatty acids. These data suggest that, besides ascorbate, A beta is another major physiological antioxidant for CSF lipoproteins.

17beta-estradiol affects in vivo the low density lipoprotein composition, particle size, and oxidizability

The aim of this study was to explore the possible modifications induced by 17beta-estradiol (E(2)) in vivo on low-density lipoprotein (LDL) lipid composition, particle size, and oxidizability. For this purpose, women were recruited from an in vitro fertilization program, ranging their plasma E(2) levels from less than 12 pg/ml to more than 2000 pg/ml at the end of the treatment. The LDL lipid constituents were analyzed by thin layer chromatography and image analysis, and the LDL diameter was calculated from the lipid data. The results showed that high plasma E(2) levels were associated with smaller LDL particles, with lower amounts of free and esterified cholesterol and an increased relative content of alpha-tocopherol. The hormonal treatment produced a remodelation of the LDL acyl composition, rendering a lipoprotein enriched in saturated fatty acids, with a poorer polyunsaturated fatty acid content. These alterations in the physicochemical properties of LDL paralleled changes in the susceptibility of LDL to in vitro oxidation induced by both Cu(2+) and the peroxyl radical generator, 2,2'-azobis (2-amidinopropane), these changes being mainly reflected in a reduced maximum oxidation rate. The in vivo changes in the physicochemical properties of LDL induced by E(2) could explain some of the antiatherogenic actions of estrogens.

Time-course of oxidation of lipids in human cerebrospinal fluid in vitro

Oxidative mechanisms play an important role in the pathogenesis of Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. To assess whether the oxidation of brain lipoproteins plays a role in the development of these pathologies, we investigated whether the lipoproteins of human cerebrospinal fluid (CSF) are susceptible to oxidative modification in vitro. We studied oxidation time-course for up to 100 h of human CSF in the absence (autooxidation) or presence of exogenous oxidants. Autooxidation of diluted CSF was found to result in a slow accumulation of lipid peroxidation products. The time-course of lipid hydroperoxide accumulation revealed three consecutive phases, lag-phase, propagation phase and plateau phase. Qualitatively similar time-course has been typically found in human plasma and plasma lipoproteins. Autooxidation of CSF was accelerated by adding exogenous oxidants, delayed by adding antioxidants and completely inhibited by adding a chelator of transition metal ions. Autooxidation of CSF also resulted in the consumption of endogenous ascorbate, alpha-tocopherol, urate and linoleic and arachidonic acids. Taking into account that (i) lipid peroxidation products measured in our study are known to be derived from fatty acids, and (ii) lipophilic antioxidants and fatty acids present in CSF are likely to be located in CSF lipoproteins, we conclude that lipoproteins of human CSF are modified in vitro during its autooxidation. This autooxidation appears to be catalyzed by transition metal ions, such as Cu(II) and Fe(III), which are present in native CSF. These data suggest that the oxidation of CSF lipoproteins might occur in vivo and play a role in the pathogenesis of neurodegenerative diseases.

Increased lipoprotein oxidation in Alzheimer's disease

Oxidation has been proposed to be an important factor in the pathogenesis of Alzheimer's disease (AD) and amyloid beta is considered to induce oxidation. In biological fluids, including cerebrospinal fluid (CSF), amyloid beta is found complexed to lipoproteins. On the basis of these observations, we investigated the potential role of lipoprotein oxidation in the pathology of AD. Lipoprotein oxidizability was measured in vitro in CSF and plasma from 29 AD patients and found to be significantly increased in comparison to 29 nondemented controls. The levels of the hydrophilic antioxidant ascorbate were significantly lower in CSF and plasma from AD patients. In plasma, alpha-carotene was significantly lower in AD patients compared to controls while alpha-tocopherol levels were indistinguishable between patients and controls. In CSF, a nonsignificant trend to lower alpha-tocopherol levels among AD patients was found. Polyunsaturated fatty acids, the lipid substrate for oxidation, were significantly lower in the CSF of AD patients. Our findings suggest that (i) lipoprotein oxidation may be important in the development of AD and (ii) the in vitro measurement of lipid peroxidation in CSF might become a useful additional marker for diagnosis of AD.

Higher cholesterol in human LDL is associated with the increase of oxidation susceptibility and the decrease of antioxidant defence: experimental and simulation data

Increased low-density lipoprotein (LDL) cholesterol is a recognized risk factor for atherosclerosis. There is also strong evidence that oxidatively modified LDL initiates the development of this pathological process and the administration of antioxidants might have a protective effect. However, the appropriate trials did not provide completely consistent results. We found in this study that the oxidation kinetics and also the antioxidant effectiveness are different depending on the cholesterol content in LDL. Higher cholesterol in LDL causes an acceleration of its oxidation as well as an increase of resistance to the antioxidative effect of ascorbic acid. In searching for a theoretical background of this dual impact of cholesterol in LDL, computer simulation of LDL oxidation was used. It was found that the pre-existing level of lipid hydroperoxides together with the total amount of oxidizable lipid substrate associated with the cholesterol level in LDL were satisfactory prerequisites for a best fit to the experimental data. In conclusion, this study provides at least a partial explanation for some failures to arrest, by administration of antioxidants, the progression of atherosclerosis in animal and human hypercholesterolemia.

The lag phase

Peroxidation of lipids in membranes and lipoproteins proceeds through the classical free radical sequence encompassing initiation, propagation, and termination phases which are expressed by a lag phase in which little oxidation occurs, followed by a rapid increase in autocatalysis by chain-propagating intermediates and, finally, a decrease in the rate of oxidation. The lag phase is lengthened by preventive or chain-breaking antioxidants, which scavenge the initiation reaction or intercept the chain-carrying species. Hence, the lag phase in lipid peroxidation processes reflects the antioxidant status of membranes and lipoproteins and, as a corollary, their resistance to oxidation. A large number of lipid peroxidation studies with different membranes attest to the complex free radical network underlying this process. The type of initiator and the steady-state level of oxygen are important factors that affect differently the rates of the individual steps of peroxidation. Equally complex are the factors that influence the lag phase preceding the oxidation of LDL. Lipid peroxyl radicals play a key role in the dynamics of lipid peroxidation: on the one hand, the lag phase is best defined for chain-breaking compounds able to reduce peroxyl radicals; on the other hand, the overall time course of lipid peroxidation is largely influenced by the rate constants for propagation reactions and termination involving peroxyl radical recombination.

Oxidation resistance of LDL is correlated with vitamin E status in beta-thalassemia intermedia

The alteration of the oxidant/antioxidant balance may affect the susceptibility of low density lipoproteins (LDL) to oxidation in haemolytic disorders such as thalassemia. Thirty patients affected by beta-thalassemia intermedia were examined, and compared with age-matched healthy controls. The mean amount of vitamin E in the thalassemic LDL was lower than control (p < 0.0001), either when it was calculated on the base of LDL protein (61% decrease) or cholesterol (25% decrease). The LDL resistance to Cu2+-induced oxidation, evaluated as the length of the lag phase before the onset of conjugated diene (CD) lipid hydroperoxide production, was 20% lower than control. Other parameters of LDL susceptibility to oxidation, such as the rate of lipid peroxidation, Rp, and the total amount of conjugated dienes produced, CDmax, were only slightly lower than control, which can be explained by a lower content of peroxidable lipids in the thalassemic LDL. Total LDL cholesterol was 1.08 x 10(3) and 2.07 x 10(3) mol/mol LDL in thalassemic and in control LDL, respectively. The length of the lag phase in thalassemic LDL shows a strongly positive correlation with its vitamin E content (r = 0.732; p < 0.0001). The r2-value of 0.53 provides evidence that more than 50% of the lag phase is determined by vitamin E. Oxidizability of LDL lipids may explain 22-24% of the lag phase, as calculated by the inverse correlation between the length of the lag phase and CDmax (r = -0.474; p = 0.008; r2 = 0.22) and Rp (r = -0.499; p = 0.005; r2 = 0.24). In multiple regression analysis, the lag phase was predictable to 66% by vitamin E plus CDmax, and to 60% by vitamin E plus Rp. Plasma vitamin E was 53% lower in thalassemia patients compared to control and positively correlated with vitamin E in the LDL (r = 0.677; p < 0.0001). None of the correlations above were observed in control subjects. In conclusion, beta-thalassemia is associated with very low levels of vitamin E in plasma and in LDL, a condition that renders these particles more susceptible to in vitro oxidative modification and may account for atherogenesis-related vascular diseases described in thalassemia. The present data on a statistically significant correlation between abnormally low vitamin E and oxidizability of LDL contribute substantially to the hypothesis that vitamin E is a pathophysiologically important determinant of antioxidative protection of LDL.

Copper can promote oxidation of LDL by markedly different mechanisms

Oxidation of LDL (0.1 microM) in PBS with copper concentrations ranging from 0.03 to 10 microM, equal to 0.3-100 Cu2+/LDL, was investigated by monitoring the formation of conjugated dienes at 234 nm. With all 8 LDL samples examined, the kinetics changed strongly at submicromolar Cu2+ concentrations. Based on time-course of the formation of conjugated dienes, cholesteryl linoleate hydroxides and hydroperoxides as well as the antioxidant consumption, two oxidation types were distinguished. Type A oxidations, observed at relatively high Cu2+ concentrations of 10-100 Cu2+/ LDL, represented the conventional kinetics of LDL oxidation with an inhibition period (= lag-time) followed by a propagation phase. In contrast, type C oxidations proceeded after a negligibly short lag time followed by a distinct propagation phase. The rate of this propagation increased rapidly to 0.5 mol diene/mol LDL and then slowed down in the presence of alpha-,gamma-tocopherols and carotenoids, which were consumed faster than tocopherols. The increase in diene absorption was due to the formation of both hydroxides and hydroperoxides suggesting a high initial decomposition of hydroperoxides. At submicromolar concentrations of about 0.1 to 0.5 microM, type C and type A oxidation can be combined resulting in 4 consecutive oxidation phases, i.e. 1st inhibition and 1st propagation (belonging to type C), followed by 2nd inhibition and 2nd propagation (belonging to type A). Increasing copper concentrations lowered the 1st propagation and shortened the 2nd inhibition periods until they melted into one apparent kinetic phase. Decreasing [Cu2+] increased the 1st propagation and 2nd inhibition but lowered the 2nd propagation phase until it completely disappeared. A threshold copper concentration, denoted as Cu(lim), can be calculated as a kinetic constant based on the Cu2+-dependence for the rate of 2nd propagation. Below Cu(lim), LDL oxidation proceeds only via type C kinetics. The Cu2+-dependence of the oxidation kinetics suggests that LDL contains two different Cu2+ biding sites. Cu2+ at the low-affinity binding sites, with half-saturation at 5-50 Cu2+/LDL, initiates and accelerates the 2nd propagation by decomposing lipid hydroperoxides. Cu2+ bound to the high-affinity binding sites, with half-saturation at 0.3-2.0 Cu2+/LDL, is responsible for the 1st propagation. Arguments in favor and against this propagation being due to tocopherol mediated peroxidation (TMP) are discussed. If the lag-time concept is extended to the conjugated diene curves seen for combined oxidation profiles, then a true inhibition phase does not apply to this time interval, but instead represents the time elapsed before the onset of the 2nd propagation phase.

How different constituents of human plasma and low density lipoprotein determine plasma oxidizability by copper

Lipoprotein oxidation induced in vitro in whole plasma is expected to represent a more relevant model of the lipoprotein oxidation in the arterial wall than the in vitro oxidation of single isolated lipoproteins, e.g. low density lipoprotein (LDL). However, it remains unclear, how lipoprotein oxidation occurring in plasma is related to chemical composition and properties of the latter as well as to those of individual plasma lipoproteins. The present study was undertaken to characterize, how different constituents of human plasma contribute to the oxidizability of plasma lipoproteins oxidized directly in plasma samples. Oxidizability of plasma lipoproteins was assessed as oxidizability of whole heparin plasma and was measured spectrophotometrically as an increase in absorbance at 234 nm. To relate plasma oxidizability to its chemical composition and properties, plasma hydrophilic and lipophilic antioxidants, fatty acids, total lipids and TRAP were measured. To relate plasma oxidizability to the properties of individual lipoproteins, chemical composition and oxidizability were evaluated for LDL. We found that the oxidation kinetics of heparin plasma (diluted 150-fold and oxidized by 50 microM Cu2+) was characterized by three consecutive phases similar to the lag-, propagation and decomposition phases of LDL oxidation. Plasma oxidizability measured as different characteristics of these phases correlated negatively with plasma initial SH-groups, albumin, ascorbate, bilirubin, alpha-tocopherol, ubiquinol-10, free cholesterol, monounsaturated and saturated fatty acid content and positively with plasma initial total cholesterol, cholesterol ester and polyunsaturated fatty acid content. Plasma oxidizability measured as a rate of conjugated diene accumulation after different periods of oxidation correlated negatively with plasma initial albumin, urate, alpha-carotene and beta-carotene content. A positive correlation between oxidizabilities of whole plasma and LDL (isolated from the same plasma samples and oxidized by 14 mol Cu2+/mol LDL) was found. These data show that the oxidizability of plasma samples is critically determined by their chemical composition. They also suggest that the plasma oxidizability measured as an increase in absorbance at 234 nm may be used as a practical measure of the oxidizability of plasma lipoproteins.

Plasma ubiquinol-10 is decreased in patients with hyperlipidaemia

Ubiquinol-10, the reduced form of ubiquinone-10 (coenzyme Q10), is a potent lipophilic antioxidant present in nearly all human tissues. The exceptional oxidative lability of ubiquinol-10 implies that it may represent a sensitive index of oxidative stress. The present study was undertaken to assess the hypothesis that the level of ubiquinol-10 in human plasma can discriminate between healthy subjects and patients who are expected to be subjected to an increased oxidative stress in vivo. Using a newly developed method, we measured plasma ubiquinol-10 in 38 hyperlipidaemic patients with and without further complications, such as coronary heart disease, hypertension, or liver disease, and in 30 healthy subjects. The oxidizability of plasma samples obtained from hyperlipidaemic patients was found to be increased in comparison with control subjects, suggesting that the patients were subjected to a higher oxidative stress in vivo than the controls. Plasma ubiquinol-10, expressed as a percentage of total ubiquinol-10 + ubiquinone-10 or normalized to plasma lipids, was lower in the patients than in controls (P = 0.001 and 0.008, respectively). The proportion of ubiquinol-10 decreased in the order young controls > aged controls > hyperlipidaemic patients without complications > hyperlipidaemic patients with complications (P = 0.003). A negative correlation was found between the proportion of ubiquinol-10 and plasma triglycerides. The hyperlipidaemic patients with hypertension had a lower proportion of ubiquinol-10 than subjects without. When the study population was divided into smokers and non-smokers, plasma ubiquinol-10 was found to be reduced amongst smokers, independently of whether it was expressed as a percentage of total ubiquinol-10 + ubiquinone-10 (P = 0.006) or normalized to plasma lipids (P = 0.009). These data suggest that the level of ubiquinol-10 in human plasma may represent a sensitive index of oxidative stress in vivo especially indicative of early oxidative damage. Measuring plasma ubiquinol-10 can be proposed as a practical approach to assess oxidative stress in humans.

Whole plasma oxidation assay as a measure of lipoprotein oxidizability

Lipoprotein oxidation induced in vitro in whole plasma is expected to be a more relevant model of the lipoprotein oxidation in the arterial wall than the in vitro oxidation of single isolated lipoproteins, e.g., low density lipoprotein (LDL). However, it is unclear, whether the oxidizability of whole plasma may serve as an adequate measure of the oxidizability of plasma lipoproteins. We measured the oxidizability of whole plasma diluted 150-fold as an absorbance increase at 234 nm known to reflect the level of conjugated dienes in the samples. Plasma oxidation was induced by Cu(II), 2,2'-azobis-(2-amidinopropane) hydrochloride (AAPH), lipoxygenase or myeloperoxidase+H2O2. Oxidizability of human plasma measured in the presence of Cu(II) was found to correlate with the oxidizability of LDL measured in the common Cu(II)-based LDL oxidation assay. The plasma oxidizability also correlated positively with plasma oxidizable fatty acid and negatively with plasma antioxidant content. Supplementation of human plasma with different antioxidants (albumin, urate, ascorbate, bilirubin, alpha-tocopherol and ubiquinol-10) in vitro decreased its oxidizability. Supplementation of Watanabe heritable hyperlipidaemic rabbits with different antioxidants (vitamin E, ubiquinone-10, probucol, carvedilol) in vivo lowered the oxidizability of rabbit plasma in comparison with rabbits fed standard diet. When plasma from hyperlipidaemic patients with or without coronary heart disease and from age-matched healthy controls was studied, the plasma oxidizability was found to be highest in the patients with coronary heart disease and lowest in the controls. Taken together, these data indicate that the plasma oxidation assay (i) provides information similar to that obtained using the common LDL oxidation assay, (ii) upgrades the latter, taking into account the effect of hydrophilic antioxidants on lipoprotein oxidation and characterizing the oxidizability of all plasma lipoproteins, and (iii) offers important practical advantages, such as fast and simple sample processing, low amount of plasma required and avoidance of artefactual oxidation during lipoprotein isolation. We propose the measurement of plasma oxidizability at 234 nm as an adequate practical index of the oxidizability of plasma lipoproteins.

Alpha-tocopherol as a reductant for Cu(II) in human lipoproteins. Triggering role in the initiation of lipoprotein oxidation

Initiation of lipid peroxidation by Cu(II) requires reduction of Cu(II) to Cu(I) as a first step. It is unclear, however, whether this reaction occurs in the course of lipoprotein oxidation. It is also unknown which reductant, if any, can drive the reduction of Cu(II) in this case. We found that Cu(II) was rapidly reduced to Cu(I) by all major human lipoproteins (high, low, and very low density lipoproteins (HDL, LDL, and VLDL), and chylomicrons). Cu(II)-reducing activity was associated with a lipid moiety of the lipoproteins. The rates of Cu(II) reduction by different lipoproteins were similar when the lipoproteins were adjusted to similar alpha-tocopherol concentrations. Enriching lipoproteins with alpha-tocopherol considerably increased the rate of CU(II) reduction. CU(II) reduction by alpha-tocopherol-deficient LDL isolated from a patient with familial inherited vitamin E deficiency was found to occur much slower in comparison with LDL isolated from a donor with a normal plasma level of alpha-tocopherol. Initial rate of CU(II) reduction by alpha-tocopherol-deficient LDL was found to be zero. Enriching LDL with ubiquinol-10 to concentrations close to those of alpha-tocopherol did not influence the reaction rate. When LDL was treated with ebselen to eliminate preformed lipid hydroperoxides, the reaction rate was also not changed significantly. CU(II) reduction was accompanied by a consumption of lipoprotein alpha-tocopherol and accumulation of conjugated dienes in the samples. Increasing alpha-tocopherol content in lipoproteins slightly decreased the rate of conjugated diene accumulation in LDL and HDL and considerably increased it in VLDL. The results suggest that alpha-tocopherol plays a triggering role in the lipoprotein oxidation by CU(II), providing its initial step as follows: alpha TocH + CU(II) --> alpha Toc. + Cu(I) + H+. This reaction appears to diminish or totally eliminate the antioxidative activity of alpha-tocopherol in the course of lipoprotein oxidation.