17beta-estradiol reduces tumor necrosis factor-alpha-mediated LDL accumulation in the artery wall

Estrogens have direct effects on the vascular wall that may prevent the development of atherosclerosis. In particular, estrogens, such as 17beta-estradiol (estradiol), are known to have potent antioxidant activity. Tumor necrosis factor-alpha (TNF) is found in human atheroma and produces oxygen-derived free radicals. These oxygen-derived free radicals may modify low density lipoproteins (LDL) and increase LDL binding in the artery wall. We asked: 1) does TNF increase LDL accumulation in the artery wall and 2) can the TNF-mediated increase in LDL accumulation be prevented by the antioxidant activity of estradiol? Carotid arteries from ovariectomized 3-month-old rats were removed and perfused with fluorescently labeled LDL and arterial LDL flux was measured using quantitative fluorescence microscopy. In six arteries, addition of TNF (10 ng/ml) to the perfusate resulted in a 2.3-fold increase in the rate of LDL accumulation (1.50 +/- 0.37 ng/min per cm2 vs. 3.38 +/- 0.48 ng/min per cm2; P < 0.01). Estradiol (65 pg/ml) and alpha-tocopherol (6 mg/L) both attenuated TNF-mediated LDL accumulation (P < 0.05), indicating that TNF may exert its effects on LDL accumulation through cellular production of oxygen-derived free radicals. These results support an antioxidant role for estradiol in the protection against LDL accumulation in the artery wall and subsequent progression of atherosclerosis.     

Free radical-mediated chain oxidation of low density lipoprotein and its synergistic inhibition by vitamin E and vitamin C

The oxidation of human low density lipoprotein (LDL) initiated by free radical initiator and its inhibition by vitamin E and water-soluble antioxidants have been studied. It was found that the kinetic chain length was considerably larger than 1, suggesting that LDL was oxidized by a free radical chain mechanism. Vitamin E acted as a lipophilic chain-breaking antioxidant. Water-soluble chain-breaking antioxidants such as ascorbic acid and uric acid suppressed the oxidation of LDL initiated by aqueous radicals but they could not scavenge lipophilic radicals within LDL to break the chain propagation. Ascorbic acid acted as a synergistic antioxidant in conjunction with vitamin E.     

Prooxidant and antioxidant properties of Trolox C, analogue of vitamin E, in oxidation of low-density lipoprotein

Trolox C (Trolox), a water-soluble analogue of vitamin E lacking the phytyl chain, was investigated with respect to its effect on the oxidation of low-density lipoprotein (LDL). Trolox was added at different time points of LDL oxidation induced by Cu2+ and aqueous peroxyl radicals. In the case of Cu2+ -induced LDL oxidation, the effect of Trolox changed from antioxidant to prooxidant when added at later time points during oxidation; this transition occurred whenever alpha-tocopherol was just consumed in oxidizing LDL. Thus, in the case of Cu2+ -dependent LDL oxidation, the presence of lipophilic antioxidants in the LDL particle is likely to be a prerequisite for the antioxidant activity of Trolox. When oxidation was induced by peroxyl radicals, as a model of metal-independent oxidation, the effect of Trolox was always antioxidant, suggesting the importance of Cu2+ /Cu+ redox-cycling in the prooxidant mechanism of Trolox. Our data suggest that, in the absence of significant amounts of lipophilic antioxidants, LDL becomes highly susceptible to oxidation induced by transition metals in the presence of aqueous reductants.     

The importance of lipid solubility in antioxidants and free radical generating systems for determining lipoprotein proxidation.

The oxidative modification of low-density lipoprotein (LDL) plays an important role in atherosclerosis. Protecting LDL from oxidation has been shown to reduce the risk of coronary heart disease. In this study, we compared the protective effects of two lipophilic antioxidants (vitamin E and lazaroid) with two hydrophilic antioxidants (trolox and vitamin C) in the presence of several different free radical generating systems. Vitamin E (IC50 = 5.9 microM) and lazaroid (IC50 = 5.0 microM) were more effective in inhibiting lipid peroxidation caused by a Fe-ADP free radical generating system than vitamin C (IC50 = 5.2 x 10(3) microM) and trolox (IC5 = 1.2 x 10(3) microM). Preincubation of lipoproteins with a lipophilic antioxidant increased the protective effect against various free radicals. Preincubation with hydrophilic antioxidants did not have an effect. We also tested the efficacy of the antioxidants when the free radicals were generated within the lipid or the aqueous environment surrounding the LDL. For this purpose, we used the peroxyl generating azo-compounds AMVN (2,2'-azobis(2,4-dimethylvaleronitrile)) and AAPH (2,2'azobis(2-amidinopropane) dihydrochloride). All of the antioxidants tested were more effective against free radicals generated in a water soluble medium than they were against free radicals generated in a lipid environment. In conclusion, our data demonstrate that lipid solubility is an important factor for both the antioxidant and the free radical generating systems in determining the extent of lipid peroxidation in LDL. Our data also demonstrate that antioxidant efficacy in one set of experimental conditions may not necessarily translate into a similar degree of protection in another set of conditions where lipophilicity is a variable.     

Does lycopene offer human LDL any protection against myeloperoxidase activity?

Lycopene is a lipophilic antioxidant that is largely transported in human blood by Low Density Lipoproteins (LDL). One of the early events in the aetiology of atherosclerosis is thought to be the oxidation of LDL. Myeloperoxidase an enzyme secreted by neutrophils and macrophages is thought to oxidise human LDL particles. In this study, isolated human LDL was challenged with myeloperoxidase or copper, and the LDL was screened for lipoperoxidation and oxidation of apolipoprotein B100, depletion of lycopene and oxidation of cholesterol. Myeloperoxidase induced oxidation of LDL through direct interaction with apolipoprotein B100. No lipoperoxidation was observed following myeloperoxidase treatment; however, 7-ketocholesterol was detected indicating the products of myeloperoxidase interact with the surface of the LDL particles. Lycopene does react with the products of myeloperoxidase in solvent, but played no role in protecting against enzyme derived oxidation of human LDL.     

Prooxidant and antioxidant properties of Trolox C,analogue of vitamin E,in oxidation of low-density lipoprotein

Trolox C (Trolox), a water-soluble analogue of vitamin E lacking the phytyl chain, was investigated with respect to its effect on the oxidation of low-density lipoprotein (LDL). Trolox was added at different time points of LDL oxidation induced by Cu²⁺ and aqueous peroxyl radicals. In the case of Cu²⁺ -induced LDL oxidation, the effect of Trolox changed from antioxidant to prooxidant when added at later time points during oxidation; this transition occurred whenever α-tocopherol was just consumed in oxidizing LDL. Thus, in the case of Cu²⁺-dependent LDL oxidation, the presence of lipophilic antioxidants in the LDL particle is likely to be a prerequisite for the antioxidant activity of Trolox.

When oxidation was induced by peroxyl radicals, as a model of metal-independent oxidation, the effect of Trolox was always antioxidant, suggesting the importance of Cu²⁺/Cu⁺ redox-cycling in the prooxidant mechanism of Trolox. Our data suggest that, in the absence of significant amounts of lipophilic antioxidants, LDL becomes highly susceptible to oxidation induced by transition metals in the presence of aqueous reductants.     

Oxidation of low-density lipoprotein by iron at lysosomal pH: implications for atherosclerosis

Low-density lipoprotein (LDL) has recently been shown to be oxidized by iron within the lysosomes of macrophages, and this is a novel potential mechanism for LDL oxidation in atherosclerosis. Our aim was to characterize the chemical and physical changes induced in LDL by iron at lysosomal pH and to investigate the effects of iron chelators and α-tocopherol on this process. LDL was oxidized by iron at pH 4.5 and 37 °C and its oxidation monitored by spectrophotometry and high-performance liquid chromatography. LDL was oxidized effectively by FeSO(4) (5-50 μM) and became highly aggregated at pH 4.5, but not at pH 7.4. The level of cholesteryl esters decreased, and after a pronounced lag, the level of 7-ketocholesterol increased greatly. The total level of hydroperoxides (measured by the triiodide assay) increased up to 24 h and then decreased only slowly. The lipid composition after 12 h at pH 4.5 and 37 °C was similar to that of LDL oxidized by copper at pH 7.4 and 4 °C, i.e., rich in hydroperoxides but low in oxysterols. Previously oxidized LDL aggregated rapidly and spontaneously at pH 4.5, but not at pH 7.4. Ferrous iron was much more effective than ferric iron at oxidizing LDL when added after the oxidation was already underway. The iron chelators diethylenetriaminepentaacetic acid and, to a lesser extent, desferrioxamine inhibited LDL oxidation when added during its initial stages but were unable to prevent aggregation of LDL after it had been partially oxidized. Surprisingly, desferrioxamine increased the rate of LDL modification when added late in the oxidation process. α-Tocopherol enrichment of LDL initially increased the rate of oxidation of LDL but decreased it later. The presence of oxidized and highly aggregated lipid within lysosomes has the potential to perturb the function of these organelles and to promote atherosclerosis.     

The salicylate metabolite gentisic acid, but not the parent drug, inhibits glucose autoxidation-mediated atherogenic modification of low density lipoprotein

Oxidation of low density lipoprotein (LDL) by glucose-derived radicals may play a role in the aetiology of atherosclerosis in diabetes. Salicylate was shown to scavenge certain radicals. In the present study, aspirin, salicylate and its metabolites 2,5- and 2, 3-dihydroxybenzoic acid (DHBA) were tested for their ability to impair LDL oxidation by glucose. Only the DHBA derivatives, when present during LDL modification, inhibited LDL oxidation and the increase in endothelial tissue factor synthesis induced by glucose oxidised LDL. The LDL glycation reaction was not affected by DHBA. The antioxidative action of DHBA may be attributed to free radical scavenging and/or chelation of transition metal ions catalysing glucose autoxidation.     

Coenzyme Q improves LDL resistance to ex vivo oxidation but does not enhance endothelial function in hypercholesterolemic young adults

Oxidative modification of low-density lipoprotein (LDL) may cause arterial endothelial dysfunction in hyperlipidemic subjects. Antioxidants can protect LDL from oxidation and therefore improve endothelial function. Dietary supplementation with coenzyme Q (CoQ(10)) raises its level within LDL, which may subsequently become more resistant to oxidation. Therefore, the aim of this study was to assess whether oral supplementation of CoQ(10) (50 mg three times daily) is effective in reducing ex vivo LDL oxidizability and in improving vascular endothelial function. Twelve nonsmoking healthy adults with hypercholesterolemia (age 34+/-10 years, nine women and three men, total cholesterol 7.4+/-1.1 mmol/l) and endothelial dysfunction (below population mean) at baseline were randomized to receive CoQ(10) or matching placebo in a double-blind crossover study (active/placebo phase 4 weeks, washout 4 weeks). Flow-mediated (FMD, endothelium-dependent) and nitrate-mediated (NMD, smooth muscle-dependent) arterial dilatation were measured by high-resolution ultrasound. CoQ(10) treatment increased plasma CoQ(10) levels from 1.1 +/-0.5 to 5.0+/-2.8 micromol/l (p =.009) but had no significant effect on FMD (4.3+/-2.4 to 5.1+/-3.6 %, p =.99), NMD (21.6+/-6.1 to 20.7+/-7.8 %, p = .38) or serum LDL-cholesterol levels (p = .51). Four subjects were selected randomly for detailed analysis of LDL oxidizability using aqueous peroxyl radicals as the oxidant. In this subgroup, CoQ(10) supplementation significantly increased the time for CoQ(10)H(2) depletion upon oxidant exposure of LDL by 41+/-19 min (p = .04) and decreased the extent of lipid hydroperoxide accumulation after 2 hours by 50+/-37 micromol/l (p =.04). We conclude that dietary supplementation with CoQ(10) decreases ex-vivo LDL oxidizability but has no significant effect on arterial endothelial function in patients with moderate hypercholesterolemia.     

Three different pathways for human LDL oxidation are inhibited in vitro by water extracts of the medicinal herb Achyrocline satureoides

In this study we investigated the antioxidant properties of one herbal preparation widely used in complementary and alternative medicine in large areas of the world: Achyrocline satureoides (AS), popularly known as "marcela". Although rich in flavonoids, the ethnopharmacological uses of this plant do not include atherosclerosis prevention. Furthermore, no study had been conducted so far exploring the antioxidant activity of Achyrocline satureoides vis-à-vis human LDL oxidation, which is the compelling issue in pinpointing potential cardioprotective new uses for a traditional remedy. We explored the effects of AS extracts on human LDL oxidation, employing 3 different systems which are thought to play a role in oxidation of LDL in the arterial wall: copper, peroxynitrite, and lipoxygenase. Oxidation was monitored by conjugate dienes, TBARS formation and aggregation of apoB using SDS-PAGE. In copper-initiated oxidation a dose dependent inhibition of the initiation and propagation of lipid oxidation is shown by an increase in the lag phase for conjugate diene production which was 60 +/- 15 min in the absence and 120 +/- 20 min in the presence of 4 microg/ml AS extracts (p < 0.001). TBARS production was reduced by 95% after 3 h incubation at 5 microg/ml. Aggregation of apoB was abolished at the same concentrations. SIN-1 (3-morpholinosydnonimine) produces peroxynitrite via generation of NO and O2-. When LDL was incubated in its presence, a milder oxidation was observed as compared with Cu2+, and AS produced over 70% inhibition. Finally, we show a striking dose-dependent inhibitory effect of lipoxygenase conjugate diene production, which is over 95% at AS concentrations of 5 microg/ml. When compared with other antioxidants, AS effect is greater but in the same order of magnitude than that of ascorbic acid and similar to the popular herbal tea Ilex paraguariensis. In all three systems employed an effect is already substantiated at a concentration of the AS extract of 4 microg/ml, which corresponds to a 1/100 dilution of the preparations usually drunk.     

The importance of lipid solubility in antioxidants and free radical generating systems for determining lipoprotein peroxidation

The oxidative modification of low-density lipoprotein (LDL) plays an important role in atherosclerosis. Protecting LDL from oxidation has been shown to reduce the risk of coronary heart disease. In this study, we compared the protective effects of two lipophilic antioxidants (vitamin E and lazaroid) with two hydrophilic antioxidants (trolox and vitamin C) in the presence of several different free radical generating systems. Vitamin E (IC₅₀ = 5.9 μM) and lazaroid (IC₅₀ = 5.0 μM) were more effective in inhibiting lipid peroxidation caused by a Fe-ADP free radical generating system than vitamin C (IC₅₀ = 5.2 × 10³ μM) and trolox (IC₅ = 1.2 × 10³ μM). Preincubation of lipoproteins with a lipophilic antioxidant increased the protective effect against various free radicals. Preincubation with hydrophilic antioxidants did not have an effect. We also tested the efficacy of the antioxidants when the free radicals were generated within the lipid or the aqueous environment surrounding the LDL. For this purpose, we used the peroxyl generating azo-compounds AMVN (2,2′-azobis(2,4-dimethylvaleronitrile)) and AAPH (2,2′azobis (2-amidinopropane) dihydrochloride). All of the antioxidants tested were more effective against free radicals generated in a water soluble medium than they were against free radicals generated in a lipid environment. In conclusion, our data demonstrate that lipid solubility is an important factor for both the antioxidant and the free radical generating systems in determining the extent of lipid peroxidation in LDL. Our data also demonstrate that antioxidant efficacy in one set of experimental conditions may not necessarily translate into a similar degree of protection in another set of conditions where lipophilicity is a variable.     

Age-related decrease of dehydroepiandrosterone concentrations in low density lipoproteins and its role in the susceptibility of low density lipoproteins to lipid peroxidation

The incidence of atherosclerosis and related diseases increases with age. The aging process may enhance lipoprotein modification, which leads to an increase in the susceptibility of low density lipoprotein (LDL) and high density lipoprotein (HDL) to oxidation. Dehydroepiandrosterone (DHEA), the most abundant steroid hormone in humans, has been shown to have antiatherogenic effects. This hormone also decreases dramatically with age. In the present study, we were interested in determining the presence of DHEA/DHEAS (dehydroepiandrosterone sulfate) and changes in their concentrations in HDL and LDL lipoproteins with age. Moreover, we studied the susceptibility of LDL to oxidation with age in the presence or absence of vitamin E or DHEA. We demonstrated that vitamin E is unable to restore the decreased resistance to oxidation of LDL from elderly subjects to that of LDL obtained from young subjects. Furthermore, our results provide evidence that DHEA is an integral part of LDL and HDL and disappears to almost nondetectable levels during aging. The DHEA incorporated into the LDL from elderly subjects increased LDL resistance to oxidation in a concentration-dependent manner. The increased resistance provided by DHEA was higher than that with vitamin E. DHEA seems to act either by protecting vitamin E from disappearance from LDL under oxidation or by scavenging directly the free radicals produced during the oxidative process. Our results suggests that DHEA exerts an antioxidative effect on LDL, which could have antiatherogenic consequences. Careful clinical trials of DHEA replacement should determine whether this ex vivo effect could be translated into any measurable antiatherogenic (cardioprotective) action.     

Vitamin C protects low-density lipoprotein from homocysteine-mediated oxidation

Homocysteine, an atherogenic amino acid, promotes iron-dependent oxidation of low-density lipoprotein (LDL). We investigated whether vitamin C, a physiological antioxidant, could protect LDL from homocysteine-mediated oxidation. LDL (0.2 mg of protein/ml) was incubated at 37 degrees C with homocysteine (1000 microM) and ferric iron (10-100 microM) in either the absence (control) or presence of vitamin C (5-250 microM). Under these conditions, vitamin C protected LDL from oxidation as evidenced by an increased lag time preceding lipid diene formation (> or = 5 vs. 2.5 h for control), decreased thiobarbituric acid-reactive substances accumulation (< or = 19 +/- 1 nmol/mg when vitamin C > or = 10 microM vs. 32 +/- 3 nmol/mg for control, p <.01), and decreased lipoprotein anodic electrophoretic mobility. Near-maximal protection was observed at vitamin C concentrations similar to those in human blood (50-100 microM); also, some protection was observed even at low concentrations (5-10 microM). This effect resulted neither from altered iron redox chemistry nor enhanced recycling of vitamin E in LDL. Instead, similar to previous reports for copper-dependent LDL oxidation, we found that vitamin C protected LDL from homocysteine-mediated oxidation through covalent lipoprotein modification involving dehydroascorbic acid. Protection of LDL from homocysteine-mediated oxidation by vitamin C may have implications for the prevention of cardiovascular disease.     

Antioxidant activities of estrogens against aqueous and lipophilic radicals; differences between phenol and catechol estrogens

Natural estrogens have much greater radical-scavenging antioxidant activity than has previously been demonstrated, with activities up to 2.5 times those of vitamin C and vitamin E. The biological significance of this finding remains to be elucidated. In this work the antioxidant activity of a range of estrogens (phenolic, catecholic and stilbene-derived) has been studied. The activity of these substances as hydrogen-donating scavengers of free radicals in an aqueous solution has been determined by monitoring their relative abilities to quench the chromogenic radical cation 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS*+). The results show that the order of reactivity in scavenging this radical in the aqueous phase is dependent on the precise estrogenic structure, with phenolic estrogens being more potent antioxidants than catecholestrogens or diethylstilbestrol. The ability of the same estrogens to scavenge lipid phase radicals has also been assessed, determined by the ex vivo enhancement of the resistance of low-density lipoprotein (LDL) to oxidation; the order of efficacy is different from that in the aqueous phase, with the phenolic estrogens estriol, estrone and 17beta-estradiol being less potent than 2-hydroxyestradiol, 4-hydroxyestradiol, or diethylstilbestrol. In this lipid-based system, phenolic estrogens were found to be unable to regenerate alpha-tocopherol from LDL subjected to oxidative stress, while at the same time 2- and 4-hydroxyestradiol significantly delayed alpha-tocopherol loss. These results indicate that the various estrogens are good scavengers of free radicals generated in both the aqueous and the lipophilic phases. The antioxidant activity of an estrogen depends not only on the hydrophilic or lipophilic nature of the scavenged radical, but also on the phenol and catechol structures of the estrogen compound.     

Reactions of lumiflavin and lumiflavin radicals with .CO2- and alcohol radicals

The kinetics of reaction of oxidized lumiflavin (F0) with the radicals .CO2(-), CH3CHOH, and (CH3)2COH have been investigated at pH 7 and 24 +/- 1 degree C by the pulse radiolysis technique. The radicals have been shown to react with lumiflavin with second-order rate constants of 36 +/- 4, 26 +/- 3, and 20 +/- 3 in units of 10(8) M-1 s-1, respectively. These rate constants are close to the diffusion limit. The main product in each case was the lumiflavin semiquinone radical FH.. By utilization of long pulses (approximately 100 mus), it was shown that the reaction FH. + .AH(alpha) leads to FH- + A(alpha) + H+ [.AH(alpha) = .CO2(-), CH3CHOH, or (CH3)2COH] proceeded for all three types of .AH(alpha) radical with second-order rate constants of 17 (+4,-3), 9 (+5,-3), and 9 (+4,-3), respectively, in the above units. The beta-carbon radical .CH2CH(OH)CH3 added to .FH, forming an alkylated flavin, while the .CH2CH2OH radical appeared to be capable of addition or hydrogen atom donation to .FH.     

Regional material property alterations in porcine femoral arteries with atheroma development

We have developed a novel methodology that permits assessment of regional vascular mechanical property alterations in the presence of atheroma in vivo employing a Yucatan miniswine model with induced lesions. Femoral arteries were imaged with intravascular ultrasound. Image data were segmented and, following three-dimensional reconstruction, underwent finite element and sensitivity analysis with optimization to identify regions with altered vascular mechanical properties. All regions were compared to histological analysis. In 12 animals with 8 weeks of endothelial cell denudation and high cholesterol diet (induced atherosclerosis), the elastic modulus initially decreased with early lesion development and then increased with increasing fibrosis-(elastic modulus-all values x10(4)Pa-mean+/-SEM) histologically normal (non-denuded control segment) elements 9.73+/-0.01, fatty elements 9.53+/-0.01, fibrofatty elements 9.41+/-0.03, and fibrous elements 9.68+/-0.02 (all p<0.001 vs. normal elements). Wall thickness, however, increased with atheroma formation. These data demonstrate decreasing vascular material properties with early lesions, followed by an increase as lesions progress. This methodology permits determination of areas with early atheroma development, follow atheroma progression, and potentially evaluate interventions aimed at decreasing atheroma load and normalizing vascular material properties.     

Paradoxical protective effect of aminoguanidine toward low-density lipoprotein oxidation: inhibition of apolipoprotein B fragmentation without preventing its carbonylation. Mechanism of action of aminoguanidine

Oxidative modification of low-density lipoproteins (LDLs) is an important feature in the initiation and progression of atherosclerosis. Aminoguanidine (AMG), classically described as an inhibitor of advanced glycation end products, turned out to be also efficient in animal models as an antioxidant against lipid peroxidation. The originality of the present study was based on the simultaneous assessment of the oxidation of LDL lipid and protein moieties in order to characterize the molecular sites of AMG protection. Oxidation of the LDL lipid moiety was monitored by measuring conjugated dienes (CD) and hydroperoxide molecular species from cholesteryl esters (CEOOH) and phosphatidylcholines (PCOOH). LDL protein oxidative modifications were assessed by evaluating apoB carbonylation and fragmentation. The LDL oxidation was mediated by water gamma radiolysis, which has the advantage of being quantitative and highly selective with regard to the free radicals produced. Here, we reported that AMG resulted in a protection of LDLs against lipid peroxidation (both in the lag phase and in the propagation phase) and against apoB fragmentation in a concentration-dependent manner, due to the scavenging effect of AMG toward lipid peroxyl radicals. Paradoxically, AMG was poorly efficient against apoB carbonylation that began during the lag phase. We hypothesize that, even in the presence of AMG, a nonnegligible proportion of (*)OH radicals remained able to initiate oxidation of the LDL protein moiety, leading to apoB carbonylation.     

Vitamin C preserves the cardio-protective paraoxonase activity of high-density lipoprotein during oxidant stress

HDL-associated paraoxonase (PON) antioxidant enzyme activity is cardio-protective. We investigated whether vitamin C prevented loss of PON activity from HDL during oxidant stress. HDL was incubated with either hydrophilic or lipophilic peroxyl radical initiators in the absence (control) or presence of vitamin C (50 and 100 micromol/L). Regardless of the type of radical, accumulation of lipid oxidation products in HDL was similar in incubations lacking vitamin C. Loss of PON activity was greater in HDL exposed to hydrophilic, in contrast to lipophilic, radicals, but addition of vitamin C maintained enzyme activity. Vitamin C's capacity to attenuate loss of the HDL ability to prevent atherogenic modification of LDL (assessed as electrophoretic mobility) was, however, modest, and appeared limited only to those incubations in which HDL was exposed to lipophilic radicals. Our results indicate that vitamin C may, under some conditions, prevent loss of cardio-protective function from HDL during oxidant stress.     

Lipolysis-induced iron release from diferric transferrin: Possible role of lipoprotein lipase in LDL oxidation.

Conditions leading to oxidation of LDL in vivo are still unknown. While the occurrence of oxidized lipoproteins and catalytic free iron in advanced atherosclerotic lesions has been demonstrated, the origin of both is unclear. In vivo, iron metabolism is tightly regulated by iron-binding proteins that ensure that virtually no free iron exists. We examined whether physiological events such as lipolysis might reduce pH, facilitate iron release from transferrin (Tf), and promote low density lipoprotein (LDL) oxidation. Lipolysis is brought about by lipoprotein lipase (LpL), a triglyceride hydrolase present on endothelial cell surfaces and in atherosclerotic lesions. LpL hydrolysis of Intralipid lowered pH from 7.40 to 7.00 in 10% human serum and from 7.40 to 6.88 in phosphate-buffered saline. Similar decreases in pH were also observed when very low density lipoproteins were hydrolyzed by LpL. Lipolysis was accompanied by a 2-fold increase in the release of 59Fe from Tf. Tf binding to subendothelial matrix (SEM), a site of key events in atherosclerosis, increased 2-fold as the pH decreased from 7.40 to 6.00. More free iron also bound to SEM as the pH decreased below 7.40. We next tested whether a reduction in pH promotes LDL oxidation. More oxidation products were found in LDL incubated at low pH for 24 h in 10% human serum. Malonaldehyde contents (nmol/mg protein), measured as TBARS, were 7.11 +/- 0.34 at pH 7.40, 7.65 +/- 0.49 at pH 7.00, 9.00 +/- 1.18 at pH 6.50, and 11. 54 +/- 0.63 at pH 6.00.Based on these results, we hypothesize that lipolysis-induced acidic conditions enhance iron release from Tf and increase formation of oxidized LDL.     

The action of defined oxygen-centred free radicals on human low-density lipoprotein.

The effects of defined oxygen-centred free radicals on human low-density lipoprotein (LDL) structure and receptor affinity are discussed in relation to the mechanisms of cell-mediated oxidative modification of LDL. Both hydroxyl (OH.) and hydroperoxyl (HO2.) radicals caused depletion of endogenous alpha-tocopherol and formation of hydroperoxides. Superoxide (O2-.) radicals produced only very limited oxidation, but could potentiate oxidation stimulated by the addition of Cu2+. All these radicals enhanced the net negative charge of intact LDL and induced fragmentation of apolipoprotein B-100 (apo B). OH. also caused cross-linking of apo B. Radical attack decreased the affinity of LDL for the fibroblast apo B/E receptor, but did not enhance its endocytosis by mouse macrophages.