Vitamins C and E and LDL oxidation

Actions of "antioxidants" in the protection against atherosclerosis

This review addresses the role of oxidative processes in atherosclerosis and its resulting cardiovascular disease by focusing on the outcome of antioxidant interventions. Although there is unambiguous evidence for the presence of heightened oxidative stress and resulting damage in atherosclerosis, it remains to be established whether this represents a cause or a consequence of the disease. This critical question is complicated further by the increasing realization that oxidative processes, including those related to signaling, are part of normal cell function. Overall, the results from animal interventions suggest that antioxidants provide benefit neither generally nor consistently. Where benefit is observed, it appears to be achieved at least in part via modulation of biological processes such as increase in nitric oxide bioavailability and induction of protective enzymes such as heme oxygenase-1, rather than via inhibition of oxidative processes and lipid oxidation in the arterial wall. Exceptions to this may be situations of multiple/excessive stress, the relevance of which for humans is not clear. This interpretation is consistent with the overall disappointing outcome of antioxidant interventions in humans and can be rationalized by the spatial compartmentalization of cellular oxidative signaling and/or damage, complex roles of oxidant-producing enzymes, and the multifactorial nature of atherosclerosis.

Both paraoxonase-1 genotype and activity do not predict the risk of future coronary artery disease; the EPIC-Norfolk Prospective Population Study

BACKGROUND

Paraoxonase-1 (PON1) is an antioxidant enzyme, that resides on high-density lipoprotein (HDL). PON1-activity, is heavily influenced by the PON1-Q192R polymorphism. PON1 is considered to protect against atherosclerosis, but it is unclear whether this relation is independent of its carrier, HDL. In order to evaluate the atheroprotective potential of PON1, we assessed the relationships among PON1-genotype, PON1-activity and risk of future coronary artery disease (CAD), in a large prospective case-control study.

METHODOLOGY/PRINCIPAL FINDINGS

Cases (n = 1138) were apparently healthy men and women aged 45-79 years who developed fatal or nonfatal CAD during a mean follow-up of 6 years. Controls (n = 2237) were matched by age, sex and enrollment time. PON1-activity was similar in cases and controls (60.7+/-45.3 versus 62.6+/-45.8 U/L, p = 0.3) and correlated with HDL-cholesterol levels (r = 0.16, p<0.0001). The PON1-Q192R polymorphism had a profound impact on PON1-activity, but did not predict CAD risk (Odds Ratio [OR] per R allele 0.98[0.84-1.15], p = 0.8). Using conditional logistic regression, quartiles of PON1-activity showed a modest inverse relation with CAD risk (OR for the highest versus the lowest quartile 0.77[0.63-0.95], p = 0.01; p-trend = 0.06). PON1-activity adjusted for Q192R polymorphism correlated better with HDL-cholesterol (r = 0.26, p<0.0001) and more linearly predicted CAD risk (0.79[0.64-0.98], p = 0.03; p-trend = 0.008). However, these relationships were abolished after adjustment for HDL (particles-cholesterol-size) and apolipoproteinA-I (0.94[0.74-1.18], p-trend = 0.3).

CONCLUSIONS/SIGNIFICANCE

This study, shows that PON1-activity inversely relates to CAD risk, but not independent of HDL, due to its close association with the HDL-particle. These data strongly suggest that a low PON1-activity is not a causal factor in atherogenesis.

Regulatory role of vitamins E and C on extracellular matrix components of the vascular system

The protective effect of vitamins E (alpha-tocopherol) and C (L-ascorbic acid) in the prevention of cardiovascular disease (CVD) has been shown in a number of situations but a secure correlation is not universally accepted. Under certain conditions, both, L-ascorbic acid and alpha-tocopherol can exhibit antioxidant properties and thus may reduce the formation of oxidized small molecules, proteins and lipids, which are a possible cause of cellular de-regulation. However, non-antioxidant effects have also been suggested to play a role in the prevention of atherosclerosis. Vitamin E and C can modulate signal transduction and gene expression and thus affect many cellular reactions such as the proliferation of smooth muscle cells, the expression of cell adhesion and extracellular matrix molecules, the production of O(2)(-) by NADPH-oxidase, the aggregation of platelets and the inflammatory response. Vitamins E and C may modulate the extracellular matrix environment by affecting VSMC differentiation and the expression of connective tissue proteins involved in vascular remodeling as well as the maintenance of vascular wall integrity. This review summarizes individually the molecular activities of vitamins E and C on the cells within the connective tissue of the vasculature, which are centrally involved in the maintenance of an intact vascular wall as well as in the repair of atherosclerotic lesions during disease development.

Interactive effects of polyphenols, tocopherol and ascorbic acid on the Cu2+–mediated oxidative modification of human low density lipoproteins

BACKGROUND

Only limited knowledge is available about any interactions between phenolic compounds and other antioxidants in inhibiting LDL oxidation. Many foods and beverages contain high levels of phenolic compounds; therefore, these compounds should not be considered in isolation from each other.

AIM OF THE STUDY

The aim of this study was to examine the structure-antioxidant activity relationship of quercetin, caffeic acid, epicatechin, hesperetin and phloretin as well as alpha-tocopherol and ascorbic acid through their ability to interact with copper ions.

METHODS

Isolated human LDL were incubated with single antioxidants or a combination of two and the kinetics of lipid peroxidation were assessed by measurement of conjugated diene formation (lag phase) via monitoring the absorbance at 234 nm after addition of copper ions. In addition, the degree of oxidation of the LDL protein moiety was followed by tryptophan fluorescence and carbonyl content measurements.

RESULTS

Alpha-tocopherol and ascorbic acid showed a lower antioxidant activity in all test systems as compared to polyphenols at equimolar concentrations. Quercetin was the most effective compound in all three systems (p < 0.001 for lag phase and carbonyl content determination). A significant (p < 0.001) prolongation of the lag phase was found when combinations of ascorbic acid/quercetin, ascorbic acid/epicatechin, epicatechin/caffeic acid, and quercetin/epicatechin were tested as compared to the sum of the individual effects. Concerning the effects on LDL protein oxidation, the results from carbonyl content and the tryptophan fluorescence measurements showed that the combination of quercetin and caffeic acid revealed the strongest inhibitory effect (p < 0.001 carbonyl content; p < or = 0.002 tryptohan fluorescence) on protein oxidation which was higher than the effect of the single compounds.

CONCLUSIONS

The results of the present study indicate that a combination of different antioxidants can be superior to the action of single antioxidants in protecting LDL lipid and protein moiety against oxidation. However, the substances may act by different antioxidative mechanisms, which are not necessarily complementary.

The total peroxyl radical trapping potential in serum - an assay to define the stage of atherosclerosis

Lipid peroxides were identified among the factors that contribute to the atherosclerotic plaque formation in the arterial wall. We hypothesised that a correlation may exist between the content of antioxidant constituents in the serum and the gravity of atherosclerosis. To this purpose, we have determined the serum total peroxyl radical- trapping potential (TRAP), which is the combined capacity of all antioxidants to neutralize free radicals in serum and followed its variation in hyperlipemic animals in correlation with the stage of atherosclerosis. In addition, we compared TRAP values in the sera of coronary heart disease (CHD) patients, with or without type II diabetes mellitus. Results showed that after 18 weeks of hyperlipemic diet, the mean TRAP values measured in sera isolated from hyperlipemic hamsters exhibited an about 44% decrease, in good agreement with the increase of serum cholesterol and triglycerides. In the 3 groups of CHD patients, TRAP values decreased with about 10% in sera of stable angina patients, 20% in unstable patients, as compared with normal subjects. The lowest TRAP values were detected in the sera of patients with acute myocardial infarction. The results obtained for different experimental animals and for CHD patients sera indicate that the TRAP method, as adapted in our laboratory, is a reliable and reproducible assay, fit to be used in clinical studies as an ex vivo measurable parameter that correlates with the stage of the atherosclerosis.

Effect of anti-oxidant treatment and cholesterol lowering on resting arterial tone, metabolic vasodilation and endothelial function in the human forearm: a randomized, placebo-controlled study

1. The aim of the present study was to determine whether anti-oxidant therapy with vitamin E and/or cholesterol-lowering therapy with simvastatin would augment resting forearm blood flow (FBF) and metabolic vasodilation in response to exercise and improve endothelial function in young patients with hypercholesterolaemia. 2. Endothelium-dependent and -independent, nitric oxide (NO)-mediated vasodilation have been shown to be impaired in young, otherwise healthy subjects with hypercholesterolaemia. Recent experimental and clinical studies suggest that vascular function may be improved with anti-oxidant or cholesterol- lowering therapy, although these treatments may be synergistic. 3. We compared FBF at rest, in response to isotonic exercise, the endothelium-dependent vasodilator acetylcholine (ACh), the endothelium-independent vasodilator sodium nitroprusside (SNP) and the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) in 26 young, otherwise healthy volunteers (mean (+/-SD) age 29+/-7 years; 14 female, 12 male) with hypercholesterolaemia, before and after 6 months treatment with vitamin E, simvastatin and/or placebo. Treatment was randomized, double-blinded in a 2 x 2 factorial design. Forearm blood flow was measured using venous occlusion plethysmography. 4. Vitamin E therapy increased plasma alpha-tocopherol from 39.5+/-9.6 to 75.7+/-33.8 micromol/L (P < 0.001). Simvastatin reduced total cholesterol from 6.9+/-1.7 to 4.9+/-0.8 mmol/L and low- density lipoprotein (LDL) from 4.8+/-1.7 to 3.0+/-0.7 mmol/L (both P < 0.001), although total and LDL-cholesterol also decreased slightly in the placebo group. Vitamin E increased resting FBF from 2.1+/-0.3 to 2.4+/-0.3 mL/100 mL per min (P = 0.04) and decreased resting forearm vascular resistance from 42.1+/-4.2 to 36.1+/-3.4 units (P = 0.01), but the reduction in resting FBF with L-NMMA was not affected. Vasodilation in response to isotonic exercise, ACh and SNP was similar before and after treatment in the placebo, vitamin E, simvastatin and in the combined vitamin E-simvastatin groups. NG-Monomethyl-L-arginine infusion reduced resting FBF and functional hyperaemia in response to exercise and these responses were not altered by treatment. 5. These data suggest that while vitamin E therapy augments resting FBF and reduces forearm vascular resistance in young hypercholesterolaemic subjects, these effects may not be via NO-dependent pathways. Metabolic vasodilation and responses to the NO-mediated vasodilators ACh and SNP were not favourably affected by anti-oxidant or cholesterol-lowering therapy, either alone or in combination.

A healthy body, a healthy mind: long-term impact of diet on mood and cognitive function

Certain dietary risk factors for physical ill health are also risk factors for depression and cognitive impairment. Although cholesterol lowering has been suggested to increase vulnerability to depression, there is better support for an alternative hypothesis that intake of n-3 long-chain polyunsaturated fatty acids can affect mood (and aggression). Possible mechanisms for such effects include modification of neuronal cell membrane fluidity and consequent impact on neurotransmitter function. Stronger evidence exists concerning a role for diet in influencing cognitive impairment and cognitive decline in older age, in particular through its impact on vascular disease. For example, cognitive impairment is associated with atherosclerosis, type 2 diabetes and hypertension, and findings from a broad range of studies show significant relationships between cognitive function and intakes of various nutrients, including long-chain polyunsaturated fatty acids, antioxidant vitamins, and folate and vitamin B12. Further support is provided by data on nutrient status and cognitive function. Almost all this evidence, however, comes from epidemiological and correlational studies. Given the problem of separating cause and effect from such evidence, and the fact that cognitive impairment and cognitive decline (and depression) are very likely to be significant factors contributing to the consumption of a poor diet, greater emphasis should now be placed on conducting intervention studies. An efficient approach to this problem could be to include assessments of mood and cognitive function as outcome measures in studies designed primarily to investigate the impact of dietary interventions on markers of physical health.

Vitamin C and the risk of acute myocardial infarction

BACKGROUND

Low-fat soluble-antioxidant status is associated with an increased risk of heart disease.

OBJECTIVE

The aim of this study was to examine whether low plasma concentrations of vitamin C confer an independent risk of acute myocardial infarction (AMI).

DESIGN

Male patients (n = 180) aged <65 y with a first AMI and without an existing diagnosis of angina (>6 mo) who were admitted within 12 h after onset of symptoms were compared with apparently healthy volunteers (n = 177). Plasma concentrations and dietary intakes of vitamin C were determined during hospitalization and 3 mo later.

RESULTS

Compared with the control subjects, the patients had higher total cholesterol and lower HDL-cholesterol concentrations and more of them smoked. The relative risk of AMI for the lowest compared with the highest quintile of plasma vitamin C during hospitalization (14.5 and >60.5 micromol/L, respectively) was 8.37 (95% CI: 3.28, 21. 4) after adjustment for classic risk factors. At 3 mo, mean (+/-SEM) plasma vitamin C concentrations in patients had increased significantly, from 19.6 +/- 1.2 to 35.1 +/- 1.9 micromol/L (P < 0. 001) and no longer conferred a risk of AMI [relative risk: 1.02 (95% CI: 0.51, 2.03)]. Habitual dietary vitamin C intake of patients (before AMI) did not differ significantly from that of control subjects. The increase in plasma vitamin C after recovery from the infarction could not be explained by a similarly large increase in dietary vitamin C.

CONCLUSIONS

A low plasma concentration of vitamin C was not associated with an increased risk of AMI, irrespective of smoking status. The apparent risk of AMI due to a low plasma vitamin C concentration was distorted by the acute phase response.

Diet controversies in lipid therapy

Increasing awareness of the relationship between diet and disease has prompted a notable increase in nutrition research. The focus of many of these studies continues to be on amount and type of fat in the diet. At the same time, a great deal of attention is being directed at other dietary components and their mode of action. The results are promising. More definitive answers must await future clinical trial data. However, the total dietary approach, including compliance measures, remains the foundation on which other nutrition recommendations should be based.

Mechanistic aspects of the relationship between low-level chemiluminescence and lipid peroxides in oxidation of low-density lipoprotein

In this study oxidation of low-density lipoprotein (LDL) induced by different Cu2+ concentrations was investigated. Lipid peroxidation was assessed by monitoring low-level chemiluminescence (LL-CL), conjugated diene hydroperoxide (CD) and alpha-tocopherol (TocOH), the major lipophilic antioxidant in LDL. At high Cu2+ concentration, LDL oxidation was characterised by CD formation, LL-CL emission and TocOH consumption. At low Cu2+ concentration, CD formation was independent of LL-CL and occurred in the presence of TocOH. Thus, two different mechanisms lead to lipid peroxide formation in LDL. The combination of CD assay and LL-CL monitoring makes it possible to distinguish the autocatalytic mechanism of CD formation and that associated with TocOH, found at a high and a low rate of initiation, respectively.

Critical evaluation of plasma and LDL oxidant-trapping potential in hemodialysis patients

BACKGROUND

We investigated whether the total peroxyl radical-trapping antioxidant potential (TRAP) assay, which has recently been proposed as a gauge of oxidative stress, could serve to evaluate plasma and low density lipoprotein (LDL) antioxidant state in hemodialysis (HD) patients.

METHODS

TRAP was determined by the lag time of the chemiluminescence reaction induced by azo-initiator-catalyzed linoleic acid peroxidation in the plasma and corresponding LDL preparations of 23 HD patients and 22 healthy subjects. Antioxidant systems, including glutathione peroxidase (GSH-Px), ascorbate, vitamin E, and uric acid, oxidative stress markers including malondialdehyde (MDA), carbonyls, and advanced oxidation protein products (AOPP), and lipids, including cholesterol and triglycerides, were also determined in the plasma.

RESULTS

Both plasma and LDL-TRAP were significantly increased in HD patients despite decreased GSH-Px and ascorbate and increased MDA, carbonyl, and AOPP plasma levels. Plasma TRAP values were closely related to both uric acid and AOPP levels, and LDL-TRAP values were related to triglycerides and AOPP levels. In vitro studies showed that: (a) plasma TRAP of control plasma increased regularly with supplementation of uric acid, although not reaching that of HD plasma with similar uric acid levels; (b) the addition of human serum albumin-AOPP also regularly increased control plasma TRAP, but was close to that of HD plasma with similar AOPP levels; and (c) LDL-TRAP was increased following LDL enrichment with triglycerides.

CONCLUSION

Our study demonstrates that TRAP is not a relevant parameter for evaluating plasma or LDL antioxidant capacity in HD patients, due to the high plasma levels of uric acid, triglycerides and AOPP, which by themselves do not exert efficient antioxidant activity in vivo, but in vitro are able to scavenge the peroxyl radicals involved in the TRAP assay.

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.