Bioavailability of flavan-3-ols and procyanidins: gastrointestinal tract influences and their relevance to bioactive forms in vivo

There is considerable interest in the bioavailability of flavan-3-ols such as tea catechins and cocoa-derived procyanidin components of the diet and their bioactivity in vivo. Their hydrogen-donating abilities and their propensity for nitration make these compounds powerful scavengers of reactive oxygen and nitrogen species. In addition, recent evidence has suggested that these compounds may interact with redox-sensitive cell signaling pathways. However, their bioactivity in vivo will be dependent on the absorption and metabolism of these compounds after ingestion and the reducing properties of resulting metabolites. Many cell, animal, and human studies have shown that flavanol monomers, such as epicatechin, are extensively metabolised to O-methylated forms and/or conjugated to glucuronides and sulphates during absorption into the circulation. The cleavage of higher procyanidin oligomers to mixtures of monomer and dimer in the stomach may act to enhance the potential for their absorption in the small intestine as higher oligomers have very limited absorption. Studies suggest that the major bioactive forms of flavanol monomers and procyanidins in vivo are likely to be metabolites and/or conjugates of epicatechin. One such metabolite, 3'-O-methylepicatechin, has been shown to exert protective effects against oxidative stress-induced cell death. Future studies will continue to concentrate on the exact mechanism of action of the bioactive forms of flavan-3-ols in vivo.

Contrasting influences of glucuronidation and O-methylation of epicatechin on hydrogen peroxide-induced cell death in neurons and fibroblasts

The purpose of this study was to examine the comparative mechanisms by which the dietary form of the flavonoid epicatechin and its predominant in vivo metabolite, epicatechin glucuronide, influence oxidative stress-induced cell death in fibroblasts and neurons. The results demonstrate the contrasting influences of in vivo glucuronidation and methylation on the bioactivity of epicatechin.

Lack of influence of dietary nitrate/nitrite on plasma nitrotyrosine levels measured using a competitive inhibition of binding ELISA assay

The action of peroxynitrite in vivo has been proposed to account for the involvement of nitrotyrosine in the pathogenesis of many diseases. However, it has been demonstrated that nitrite under acidic conditions, similar to those in the human stomach, also has the ability to nitrate tyrosine. Dietary nitrate is also implicated in the progression of gastritis and gastric cancer and elevated levels of nitrate are found in many disease states in which nitrotyrosine may play a role. Thus, we investigated whether the dietary nitrate intake might contribute towards the plasma protein-bound levels of nitrotyrosine. Seven healthy, non-smokers participated in a two-day study consisting of a nitrate-low control day followed by a day during which three nitrate-rich meals were consumed. Maximal urinary excretion was attained 4-6 hours after consumption of a meal and the maximum was proportional to the dose. Plasma nitrate was elevated nine-fold, 1 hour after consumption of a meal containing 128.3 mg nitrate. Plasma nitrated protein levels did not appear to alter significantly from basal 1 hour after supplementation with a nitrate-rich meal. Thus dietary nitrate does not appear to contribute to the levels of plasma nitrated proteins, as determined using a competitive inhibition of binding ELISA assay, but this does not preclude any contribution it may make to the total body burden of nitrotyrosine.

Flavonoids protect neurons from oxidized low-density-lipoprotein-induced apoptosis involving c-Jun N-terminal kinase (JNK), c-Jun and caspase-3

Oxidative stress has been associated with neuronal loss in neurodegenerative diseases and during age-associated cognitive decline. Flavonoids have been proposed to play a useful role in protecting the central nervous system against oxidative and excitotoxic stress, although the mechanism of action is unknown. Using oxidized low-density lipoprotein (oxLDL) as the oxidative insult we investigated the mechanism of neurotoxicity and attempted to identify possible sites of action of two of the most potent protective flavonoids, epicatechin and kaempferol, in cultured primary neurons. Using cultured striatal neurons and selective phosphospecific antibodies we addressed the potential role of extracellular signal-regulated kinases 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK). OxLDL stimulated a Ca(2+)-dependent activation of both ERK1/2 and JNK that was strongly inhibited by pre-treatment with low micromolar concentrations of epicatechin. Neurotoxicity induced by oxLDL, however, was neither reduced nor enhanced by inhibiting ERK1/2 activation with mitogen-activated protein kinase kinase (MEK) inhibitors, suggesting that this cascade is unlikely to be involved in either oxLDL toxicity or the protective effects of flavonoids. oxLDL caused a sustained activation of JNK that resulted in the phosphorylation of the transcription factor c-Jun, which was abolished in neurons pre-treated with flavonoids. Furthermore, oxLDL induced the cleavage of procaspase-3 and increased caspase-3-like protease activity in neurons, an effect which was strongly inhibited by pre-exposure to either epicatechin or kaempferol. In addition, a caspase-3 inhibitor reduced oxLDL-induced neuronal death, implicating an apoptotic mechanism. A major in vivo metabolite of epicatechin, 3'-O-methyl-epicatechin was as effective as epicatechin in protecting neurons. Thus dietary flavonoids might have potential as protective agents against neuronal apoptosis through selective actions within stress-activated cellular responses, including protein kinase signalling cascades.

The mechanisms for nitration and nitrotyrosine formation in vitro and in vivo: impact of diet

The detection of 3-nitro-L-tyrosine residues associated with many disease states, including gastric cancer, has implicated a role for peroxynitrite in vivo, and thus endogenously produced nitric oxide and superoxide. Additionally, dietary nitrate has been suggested to be involved in the pathogenesis of gastric cancer through a mechanism involving reduction to nitrite and subsequent formation of potentially mutagenic nitroso-compounds. Studies have now demonstrated that a multitude of reactive nitrogen species other than peroxynitrite are capable of producing nitrotyrosine. Thus, we have reviewed the evidence that dietary nitrate, amongst other reactive nitrogen species, may contribute to the body burden of nitrotyrosine.

Epicatechin is the primary bioavailable form of the procyanidin dimers B2 and B5 after transfer across the small intestine

Perfusion of isolated small intestine with the procyanidin dimers B2 and B5 extracted from cocoa indicated that both forms of dimer are transferred to the serosal side of enterocytes but only to a very small extent (<1% of the total transferred flavanol-like compounds). However, perfusion of dimer mainly resulted in large amounts of unmetabolised/unconjugated epicatechin monomer being detected on the serosal side (95.8%). The cleavage of dimer during transfer seemed to be energy-dependent, requiring an intact cell system, as incubation with jejunal homogenates failed to yield epicatechin. Low levels methylated dimer were also detected (3.2%), but no conjugates and metabolites of epicatechin indicating that metabolism of monomer and dimer is limited during dimer cleavage/translocation. The methylation of dimer may be by catechol-O-methyltransferase, however, at high concentrations of dimer COMT activity is reduced leading to an inhibition of both monomer and dimer O-methylation.

Flavonoid Antioxidants

In order to ascertain the role of dietary flavonoids as antioxidants in vivo it is necessary to understand the chemical nature of the absorbed forms in the circulation in vivo and how the multiplicity of research findings in vitro reflect the bioactivity of flavonoids in vivo. Only when we gain adequate information on the circulating forms can we begin to understand the targeting to the tissues, whether flavonoids cross the blood-brain barrier, for example, and in what forms. Flavonoids are powerful antioxidants in vitro, but their overall function in vivo has yet to be clarified, whether antioxidant, anti-inflammatory, enzyme inhibitor, enzyme inducer, inhibitor of cell division, or some other role. It should also be emphasised that the reducing properties of flavonoids might contribute to redox regulation in cells, independently of their antioxidant properties, and thus might protect against cell ageing, for example, by working together with the intracellular reductant network. To gain understanding of these issues the factors influencing the absorption of flavonoids in the gastrointestinal tract needs to be established, namely the questions of: de-glycosylation before absorption, conjugation in the small intestine through glucuronidation, sulphation or methylation etc, metabolism and degradation in the colon to smaller phenolic molecules. The forms in which they circulate in vivo will influence their polarity and, thus, their localization and bioactivities in vivo. Finally if antioxidant activities are important, the elucidation of how such properties in vitro relate to the potential for conjugates and metabolites in vivo to act as antioxidants is required. The absorbed flavonoid components might function in the aqueous phase (like vitamin C) or in the lipophilic milieu (as vitamin E) in vivo. This will depend on their polarity properties on uptake, how they are metabolised on absorption, and their resulting structural forms in the circulation.

Epicatechin and its in vivo metabolite, 3'-O-methyl epicatechin, protect human fibroblasts from oxidative-stress-induced cell death involving caspase-3 activation

There is considerable current interest in the cytoprotective effects of natural antioxidants against oxidative stress. In particular, epicatechin, a major member of the flavanol family of polyphenols with powerful antioxidant properties in vitro, has been investigated to determine its ability to attenuate oxidative-stress-induced cell damage and to understand the mechanism of its protective action. We have induced oxidative stress in cultured human fibroblasts using hydrogen peroxide and examined the cellular responses in the form of mitochondrial function, cell-membrane damage, annexin-V binding and caspase-3 activation. Since one of the major metabolites of epicatechin in vivo is 3'-O-methyl epicatechin, we have compared its protective effects with that of epicatechin. The results provide the first evidence that 3'-O-methyl epicatechin inhibits cell death induced by hydrogen peroxide and that the mechanism involves suppression of caspase-3 activity as a marker for apoptosis. Furthermore, the protection elicited by 3'-O-methyl epicatechin is not significantly different from that of epicatechin, suggesting that hydrogen-donating antioxidant activity is not the primary mechanism of protection.

Bioavailability of flavonoids and potential bioactive forms in vivo

Flavonoids are powerful antioxidants in vitro, but their overall functions in vivo have yet to be clarified, whether antioxidant, anti-inflammatory, enzyme inhibitor or inducer, or some other role. The reducing properties of flavonoids might also contribute to redox regulation in cells independently of their antioxidant properties. However, in order to understand their bioactivity in vivo, it is necessary to understand the factors influencing the absorption of flavonoids by the gastrointestinal tract, the nature of the conjugates and metabolites in the circulation and how this influences their antioxidant activities.

Isomerization of lycopene in the gastric milieu

There is considerable interest in the bioavailability of carotenoids from the diet and their bioactivity in vivo. Little is known, however, of the preabsorption events in the gastric lumen on the breakdown or isomerisation of dietary carotenoids. In this study the effects of the acidic environment found in the gastric milieu on lycopene have been investigated. The results show that under these conditions all-trans-lycopene is isomerised to cis-isomers, which may be implicated in enhanced absorption from the small intestine. Furthermore the pH, as well as the food matrix, seems to have an influence on the level of isomerisation of this carotenoid.

Epicatechin and catechin are O-methylated and glucuronidated in the small intestine

There is considerable interest in the bioavailability of polyphenols and their bioactivity in vivo. We have studied the absorption and metabolism of catechin and epicatechin in the small intestine and the comparative transfer across the jejunum and ileum. Perfusion of isolated jejunum with the flavanols resulted in glucuronidation ( approximately 45%), O-methylation: 3'-O-Methyl- and 4'-O-methyl- ( approximately 30%), and O-methyl-glucuronidation ( approximately 20% of total flavanols identified) during transfer across the enterocytes to the serosal side. This demonstrates the activity of catechol-O-methyl transferases in the metabolism of flavanols and suggests that these metabolites and conjugates are likely to enter the portal vein. In contrast, in the case of the ileum, the majority of the flavanols appeared on the serosal side unmetabolised and the total percentage of flavanols transferred was higher than that in the jejunum ( approximately fivefold).

Decomposition of cocoa procyanidins in the gastric milieu

There is considerable interest in the bioavailability of flavonoids and phenolic components of the diet and their bioactivity in vivo. However, little is known of pre-absorption events in the gastric lumen. The effects of the acidic environment, as found in the gastric milieu, on procyanidin oligomers of catechin polyphenols has been investigated. The results show that under these conditions the procyanidin oligomers (trimer to hexamer) are hydrolysed to mixtures of epicatechin monomer and dimer, thus enhancing the potential for their absorption in the small intestine.

Resveratrol is absorbed in the small intestine as resveratrol glucuronide

We have studied the absorption and metabolism of resveratrol in the jejunum in an isolated rat small intestine model. Only small amounts of resveratrol were absorbed across the enterocytes of the jejunum and ileum unmetabolised. The major compound detected on the serosal side was the glucuronide conjugate of resveratrol (96.5% +/- 4.6 of the amount absorbed) indicating the susceptibility of resveratrol to glucuronidation during transfer across the rat jejunum. The presence of the glucuronide was confirmed using HPLC-PDA and nanoES-MS/MS techniques. These findings suggest that resveratrol is most likely to be in the form of a glucuronide conjugate after crossing the small intestine and entering the blood circulation. This will have important implications for the biological functions of resveratrol in vivo.

Ferulic acid excretion as a marker of consumption of a French maritime pine (Pinus maritima) bark extract

French maritime pine (Pinus maritima) bark extract (PBE) is a polyphenol-rich food supplement patented under the name of Pycnogenol and known to have strong antioxidant activity and different beneficial effects on human health. Although its biological properties have begun to be extensively studied both in vitro, in laboratory animals and more recently in humans, little is known about its bioavailability. The present study investigated the urinary excretion of free and conjugated ferulic acid, present in quantitatively detectable amounts in PBE, after oral PBE administration to human subjects. Eleven healthy adult subjects (4 women and 7men) consumed either a single dose (200 mg PBE) or two doses of PBE (100 and 200 mg, respectively) within a 48-h interval. Two days before the oral administration of PBE and during the urine sample collection period volunteers adhered to a diet low in polyphenols. Aliquots of all urine production were collected over 24 h. Free and conjugated ferulic acid was assessed in urine by HPLC using diode array detection. A close association between the dietary intake of PBE and the urinary excretion of ferulic acid was detected. Moreover, the results indicate that a considerable proportion of ferulic acid is excreted as glucuronide or sulfate after PBE consumption, varying over the range 2 to 20% between individuals. The kinetics of excretion associated with the administration of 100 mg PBE was quite similar to that obtained after 200 mg PBE. A a biphasic trend was evident in a number of subjects. All subjects studied here displayed a significant, although variable level of excretion of ferulic acid after supplementation with PBE, Thus, the data provide evidence that at least a part of the phenolic components of PBE are absorbed, metabolized, and eliminated by humans.

Absorption of ferulic acid from low-alcohol beer

Flavonoids and monophenolic compounds have been well-described over recent years for their properties as antioxidants and scavengers of reactive oxygen and nitrogen species. A number of epidemiological studies implicate a role for flavonoids in reducing the risk of coronary heart disease. In particular, the focus has been on flavonol-rich fruit and vegetables and flavonoid-rich beverages, especially tea and red wine. Mechanisms of protection are unclear since the absorption, distribution, metabolism and elimination of dietary phenolics have not yet been extensively investigated. Here we report the bioavailability of ferulic acid, 4-hydroxy-3-methoxy-cinnamic acid, the major hydroxycinnamate in beer. Studies of the pharmacokinetics of urinary excretion of ferulic acid from low alcohol beer consumption in humans have been undertaken. The results show that ferulic acid is absorbed with a peak time for maximal excretion of ca. 8 h and the mean cumulative amount excreted is 5.8 +/- 3.2 mg. These findings are consistent with the uptake of ferulic acid from dietary sources, such as tomatoes, and suggest that ferulic acid is more bioavailable than individual dietary flavonoids and phenolics so far studied.

Moderate supplementation with natural alpha-tocopherol decreases platelet aggregation and low-density lipoprotein oxidation

Previous studies have shown that oral administration of 300 mg alpha-tocopherol/day to healthy volunteers decreases platelet function and enhances their sensitivity to the platelet inhibitor, prostaglandin E(1), when full dose-response curves to a range of agonist concentrations are made. In this study, the effects of oral doses of natural alpha-tocopherol (75, 200 and 400 IU/day) were studied in order to determine whether the same effects might be achieved with lower intakes of vitamin E and whether inhibition is related to the platelet levels of the antioxidant in platelet membranes. Twenty two subjects undertook the supplementation regime, divided into three units of 2 weeks, each cycling through each of the dosages. The results show that uptake of vitamin E by the platelets was optimal at 75 IU/day, correlating with the maximal influence on platelet aggregation and platelet responsiveness to inhibition by PGE1, increased supplemental levels exerting no greater effects.

The small intestine can both absorb and glucuronidate luminal flavonoids

We have studied the perfusion of the jejunum and ileum in an isolated rat intestine model with flavonoids and hydroxycinnamates and the influence of glycosylation on the subsequent metabolism. Flavone and flavonol glucosides and their corresponding aglycones are glucuronidated during transfer across the rat jejunum and ileum and this glucuronidation occurs without the need for gut microflora. Furthermore, this suggests the presence of glycosidases as well as UDP-glucuronyl transferase in the jejunum. In contrast, quercetin-3-glucoside and rutin are mainly absorbed unmetabolised. The results suggest that the more highly reducing phenolics are absorbed predominantly as glucuronides (96.5%+/-4.6) of the amount absorbed, whereas monophenolic hydroxycinnamates and monophenolic B-ring flavonoids are less predisposed to glucuronidation and higher levels of aglycone (88.1%+/-10.1) are detected on absorption through both the jejunum and ileum.

Inhibition of peroxynitrite-mediated oxidation of dopamine by flavonoid and phenolic antioxidants and their structural relationships

The interaction between peroxynitrite and dopamine and the inhibition of this reaction by plant-derived antioxidants have been investigated. Peroxynitrite promoted the oxidation of dopamine to 6-hydroxyindole-5-one as characterised by HPLC and photodiode array spectra, akin to the products of the tyrosinase-dopamine reaction, but no evidence of dopamine nitration was obtained. Although peroxynitrite did not cause nitration of dopamine in vitro, the catecholamine is capable of inhibiting the formation of 3-nitrotyrosine from peroxynitrite-mediated nitration of tyrosine. The plant-derived phenolic compounds, caffeic acid and catechin, inhibited peroxynitrite-mediated oxidation of dopamine. This effect is attributed to the ability of catechol-containing antioxidants to reduce peroxynitrite through electron donation, resulting in their oxidation to the corresponding o-quinones. The antioxidant effect of caffeic acid and catechin was comparable to that of the endogenous antioxidant, glutathione. In contrast, the structurally related monohydroxylated hydroxycinnamates, p-coumaric acid and ferulic acid, which inhibit tyrosine nitration through a mechanism of competitive nitration, did not inhibit peroxynitrite-induced dopamine oxidation. The findings of the present study suggest that certain plant-derived phenolics can inhibit dopamine oxidation.

Do iron and vitamin C co-supplementation influence platelet function or LDL oxidizability in healthy volunteers?

OBJECTIVE

To examine the effect of co-supplementation with iron and vitamin C on antioxidant status, platelet function and low density lipoprotein oxidation in normal healthy volunteers.

DESIGN

The study was carried out with two groups of 20 subjects each acting as their own control, comparing presupplemention with postsupplemention. One group was supplemented with iron and the RDA level of vitamin C and the second group with iron and 260 mg/d vitamin C.

SETTING

The International Antioxidant Research Centre, The Guy's, King's College and St Thomas's School of Biomedical Science, Guy's Campus, London.

SUBJECTS

Forty normal healthy volunteers, recruited from the staff of the Medical School and Hospital in which two volunteers withdrew during the study.

INTERVENTIONS

Subjects in both studies were randomly assigned to one of two groups (5 males and 5 females group) and received supplements containing iron (14 mg/d) and either 60 mg/d (Group A) or 260 mg/d (Group B) vitamin C for 12 wk. Blood samples were taken at 6 wk and 12 wk, and prior to supplementation and analysed for iron and antioxidant status (transferrin bound iron, vitamin C and E, and beta-carotene levels) in both studies. Samples from the first study were analysed for the susceptibility of LDL isolated from plasma to Cu2+-induced oxidation and samples from the second for platelet function.

RESULTS

Transferrin-bound iron was significantly increased (P < 0.05) at 12 wk, in Group A subjects (from 14.9+/-5.3 micromol/1 to 19.5+/-2.3 micromol/l; mean+/-s.d.; n=19), whereas those in Group B showed a significant increase (P < 0.05) after 6 wk (from 15.8+/-4.5 micromol/l to 20.4+/-6.6 micromol/l; n = 19) which decreased at 12 wk (16.3+/-5.0 micromol/l). Plasma total ascorbate significantly increased from an initial level of 59.3+/-21.3 micromol/l to 87.6+/-29.0 micromol/l after 6 wk and 81.7+/-11.4 micromol/l after 12 wk following the Group B supplementation, but only after 12 wk in Group A (from 64.0+/-24.8 micromol/l to 77.2+/-13.2 micromol/l). Plasma alpha-tocopherol concentrations were significantly increased after 6 wk and 12 wk with both levels of supplementation (from 24.2+/-5.71 micromol/l Group A and 23.4+/-5.3 micromol/l Group B to 26.3+/-5.5 micromol/l and 25.71+/-4.7 micromol/1 respectively at 12wk). The mean lag phase to oxidation of low density lipoprotein (LDL) was significantly increased in subjects in Group B after 12 wk ingestion of iron and 260 mg vitamin C (from 80.0+/-14.8 min to 97.2+/-16.9 min; n = 9). Platelet sensitivity to ADP-induced aggregation was significantly decreased (P < 0.05) by 12 wk in Group A (from EC50 2.3 < or = 1.3 microM to 3.7+/-2.2 microM; n = 10), whereas those receiving higher vitamin C showed a significant decrease (P < 0.05; from EC50 1.9+/-0.6 microM to 3.1+/-1.8 microM) after 6wk which subsequently increased towards presupplemental levels (2.6+/-1.6 microM). Platelets from the latter subjects showed a significant reduction in ADP-induced ATP secretion at both 6wk and 12 wk.

CONCLUSION

The results show modest beneficial effects on LDL oxidation and platelet function following supplementation with iron and vitamin C. No evidence for pro-oxidant effects was observed.

Antioxidant activity applying an improved ABTS radical cation decolorization assay

A method for the screening of antioxidant activity is reported as a decolorization assay applicable to both lipophilic and hydrophilic antioxidants, including flavonoids, hydroxycinnamates, carotenoids, and plasma antioxidants. The pre-formed radical monocation of 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS*+) is generated by oxidation of ABTS with potassium persulfate and is reduced in the presence of such hydrogen-donating antioxidants. The influences of both the concentration of antioxidant and duration of reaction on the inhibition of the radical cation absorption are taken into account when determining the antioxidant activity. This assay clearly improves the original TEAC assay (the ferryl myoglobin/ABTS assay) for the determination of antioxidant activity in a number of ways. First, the chemistry involves the direct generation of the ABTS radical monocation with no involvement of an intermediary radical. Second, it is a decolorization assay; thus the radical cation is pre-formed prior to addition of antioxidant test systems, rather than the generation of the radical taking place continually in the presence of the antioxidant. Hence the results obtained with the improved system may not always be directly comparable with those obtained using the original TEAC assay. Third, it is applicable to both aqueous and lipophilic systems.

Bioavailability of ferulic acid

There is a wealth of evidence for the powerful antioxidant properties in vitro of flavonoid components of the diet. However, few studies have been undertaken concerning the hydroxycinnamates, major constituents of fruit, some vegetables, beverages, and grains, particularly the extent to which they are absorbed in vivo from the diet. The study described here has investigated the bioavailability of ferulic acid in humans, from tomato consumption, through the monitoring of the pharmacokinetics of excretion in relation to intake. The results show that the peak time for maximal urinary excretion is approximately 7 h and the recovery of ferulic acid in the urine, on the basis of total free ferulic acid and feruloyl glucuronide excreted, is 11-25% of that ingested.

Inhibition of nitrous acid-dependent tyrosine nitration and DNA base deamination by flavonoids and other phenolic compounds

Exposure of tyrosine or DNA bases to acidic nitrite at low pH results in the nitration of tyrosine and the formation of base deamination products, respectively. At pH 1, hypoxanthine and xanthine are formed from the deamination of adenine and guanine, respectively, whereas under the same conditions, uracil is not detected. The yield of 3-nitrotyrosine derived from interaction of equimolar nitrite and tyrosine at pH 1 is approximately 50% of that obtained from equimolar peroxynitrite-tyrosine interactions at pH 7. 4. The ability of a range of plant phenolic constituents to prevent damage mediated by acidic nitrite was also examined in comparison with the activity of vitamin C. The epicatechin/gallate family of flavonols, constituents of green tea, red wine, etc., demonstrates the most extensive inhibitory properties against both tyrosine nitration and base deamination. The results also show that ascorbic acid is a poor inhibitor of nitration or deamination under acidic conditions such as those of the stomach. The ability of plant phenolics to scavenge reactive nitrogen species derived from acidic nitrite may contribute to the protective effects of tea polyphenols against gastric cancer.

Modulation of the UVA activation of haem oxygenase, collagenase and cyclooxygenase gene expression by epigallocatechin in human skin cells

We have investigated the modifying effects of epigallocatechin, a major polyphenolic constituent of green tea, on ultraviolet-A-activated gene expression in human fibroblasts and keratinocytes using the stress responsive enzymes: haem oxygenase-1, interstitial collagenase and cyclooxygenase-2. Although epigallocatechin strongly reduced ultraviolet-A-induced haem oxygenase-1 activation in skin-derived 'fibroblasts, the same compound activated collagenase and cyclooxygenase expression. In a keratinocyte cell line, ultraviolet-A-mediated haem oxygenase-1 over-expression was low and epigallocatechin failed to modulate it further. In contrast to the results with fibroblasts, ultraviolet-A activation of cyclooxygenase in keratinocytes was reduced by epigallocatechin. The results indicate that the effect of this green tea polyphenol on cellular stress responses is complex and may involve direct effects on signal transduction as well as changes that may be associated with its antioxidant activity.

Peroxynitrite oxidises catechols to o-quinones

Nitration of phenolic compounds is a well-established mechanism on interaction with peroxynitrite. However, while nitration is the predominant reaction for monophenolic hydroxycinnamates, this does not take place with the catechol-containing hydroxycinnamate, caffeic acid. The aim of the present study was to investigate the mechanism of the chemical interaction of caffeic acid with peroxynitrite and to characterise the products formed. A novel compound was detected and characterised as the o-quinone of caffeic acid based on its reaction with nucleophilic thiol compounds, glutathione and L-cysteine. The same novel product was identified following the oxidation of caffeic acid in alkaline solutions confirming the identity of this species as a caffeic acid oxidation product.

Functional food science and defence against reactive oxidative species

This paper assesses critically the science base that underpins the argument that oxidative damage is a significant causative factor in the development of human diseases and that antioxidants are capable of preventing or ameliorating these disease processes. The assessment has been carried out under a number of headings, and some recommendations for future research are made based on the present day knowledge base. The knowledge database (1) Consideration of the basic science that underlies understanding of the role of free radicals in causing cellular pathologies, and the role of antioxidants in preventing this, shows that an imbalance of reactive oxygen species and antioxidant defence systems may lead to chemical modifications of biologically relevant macromolecules. This imbalance provides a logical pathobiochemical mechanism for the initiation and development of several disease states. Experimental data obtained in vivo provide evidence that antioxidants function in systems that scavenge reactive oxygen species and that these are relevant to what occurs in vivo. The relevance in vivo of these observations depends inter alia on knowledge of the uptake and distribution of the antioxidant within the human body, and on what tissue levels of the antioxidant may be expected in relation to dietary levels. (2) There is some way to go until validated precise methods are available for measuring biomarkers of oxidative damage in human subjects in vivo under minimally invasive conditions. With respect to oxidative damage in DNa, HPLC and GC-mass spectrophotometry methods have both merits and limitations. Lipid oxidation products in plasma are best measured as isoprostanes or as lipid hydroperoxides using specific HPLC techniques. Development of isoprostane measurement will advance specificity and precision. The measurement of oxidative damage to proteins has some potential but such methods have not been effectively exploited. (3) Epidemiological studies support the hypothesis that the major antioxidant nutrients vitamin E and vitamin C, and beta-carotene (which may or may not be acting as an antioxidant in vivo), may play a beneficial role in prevention of several chronic disorders. More research is needed on the impact of other non-nutrient compounds, such as other carotenoids and flavonoids, on human health. In general, human intervention studies using hard end-points are the gold standard. Trials are restricted mainly to the major antioxidants and do not allow firm conclusions because of inconsistent findings, an insufficient number of studies and the use of varying doses. There is evidence that large doses of beta-carotene may be deleterious to the health of certain subgroups of the population such as heavy habitual smokers. (4) With respect to the safety of administration of supplementary vitamins, vitamin C is safe at levels of supplementation up to 600 mg/d, and higher levels, up to 2000 mg/d, are without risk. Vitamin E has a very low human toxicity and an intake of 1000 mg/d is without risk; 3200 mg/d has been shown to be without any consistent risk. Large intakes of beta-carotene must be viewed with caution because they have been shown to confer detriment to a population at high risk of lung cancer when administered after many years of high risk (smoking) behaviour. Until further work clarifies the situation in heavy smokers with respect to taking supplements, larger doses should be avoided by such individuals. There is little reliable information about the human toxicology of flavonoids and related non-nutrient antioxidant constituents of the diet. (5) The food industry has long experience in the control of oxidative damage in foods and this experience can be used to advantage for the protection of food antioxidants which are beneficial. Some of these, such as vitamins C and E and beta-carotene, are well known, and strategies for their protection in foods are already exploited by food technologies. (ABSTRACT TRUNCATED)

The effects of iron and vitamin C co-supplementation on oxidative damage to DNA in healthy volunteers

The effects of co-supplementing healthy volunteers with iron (14 mg/day ferrous sulphate) and vitamin C (either 60 mg/day or 260 mg/day as ascorbic acid) on levels of oxidative DNA damage in white blood cells were studied. The subjects were divided into two groups: one group of 20 volunteers with a higher mean initial level of plasma vitamin C (71.9 +/- 14.0 mumol/l) and a second group of 18 volunteers with a lower mean level (50.4 +/- 25.8 mumol/l). In the first group there was a significant rise in several oxidative DNA base damage products and in total oxidative DNA damage in DNA extracted from white blood cells, but not in 8-hydroxyguanine, after 6 weeks of supplementation. However, after 12 weeks levels returned approximately to normal. In the group with the lower initial level of plasma ascorbate, presupplemental levels of oxidative DNA damage were higher and decreased on supplementation with iron and ascorbate. Since oxidative DNA damage has been suggested as a risk factor for the development of cancer, the implications of increased levels in well-nourished subjects after iron/ascorbate supplementation are disturbing in view of the frequent use of dietary supplements containing both iron salts and ascorbate.

Interaction of peroxynitrite with carotenoids and tocopherols within low density lipoprotein

Peroxynitrite is a powerful oxidising and nitrating agent generated in vivo by the combination of nitric oxide and superoxide. Previous studies have shown that on exposure to peroxynitrite, low density lipoprotein (LDL) is modified resulting in both a time- and concentration-dependent change to lipid and protein components. The present investigation highlights the reaction between carotenoids and tocopherols, present within the lipophilic phase of LDL, and peroxynitrite at varying concentrations. It was observed that the carotenoids were consumed by a significantly greater proportion than that of the tocopherols with lycopene (87.2 +/- 11%) being more reactive than beta-carotene (68.2 +/- 5.8%) when exposed to peroxynitrite (50 microM) for 1 min. Among the tocopherols, alpha-tocopherol (54.9 +/- 20.2%) was more extensively depleted than gamma-tocopherol (14.7 +/- 1.09%) at peroxynitrite concentration of 500 microM. It was also observed that peroxynitrite, unlike copper ions, does not lead to significant peroxidation of LDL as determined by the formation of conjugated dienes and thiobarbituric acid-reactive substances.

Comparative mechanisms and rates of free radical scavenging by carotenoid antioxidants

The comparative mechanisms and relative rates of nitrogen dioxide (NO2.), thiyl (RS.) and sulphonyl (RSO2.) radical scavenging by the carotenoid antioxidants lycopene, lutein, zeaxanthin, astaxanthin and canthaxanthin have been determined by pulse radiolysis. All the carotenoids under study react with the NO2. radical via electron transfer to generate the carotenoid radical cation (Car.+). In marked contrast the glutathione and 2-mercaptoethanol thiyl radicals react via a radical addition process to generate carotenoid-thiyl radical adducts [RS-Car].. The RSO2. radical undergoes both radical addition, [RSO2-Car]. and electron abstraction, Car.+. Both carotenoid adduct radicals and radical cations decay bimolecularly. Absolute rate constants for radical scavenging were in the order of approximately 10(7)-10(9) M(-1) s(-1) and follow the sequence HO(CH2)2S. > RSO2. > GS. > NO2.. Although there were some discernible trends in carotenoid reactivity for individual radicals, rate constants varied by no greater than a factor of 2.5. The mechanism and rate of scavenging is strongly dependent on the nature of the oxidising radical species but much less dependent on the carotenoid structure.

Measurement of the antioxidant status of dietary constituents, low density lipoproteins and plasma

There is a considerable interest in antioxidants as bioactive components of food and as nutritional agents with a role in the maintenance of health and in disease prevention. There are situations in which knowledge of the individual levels of specific antioxidant components might be less useful than the total antioxidant potency of the medium concerned, the total antioxidant activity being determined by the combined reducing activities of its constituents. Such situations might be in the understanding of the structure-activity relationships of pure antioxidant compounds, in the determination of the antioxidant contributions of specific dietary components and how this relates to the antioxidant composition and activities of the individual constituents, and in the study of decreases in plasma antioxidant activity in individuals under oxidative stress in specific disease states. This article describes the application of the ABTS radical cation method to the measurement of the total antioxidant activity in order to address these questions.

Non-oxidative modification of low density lipoprotein by ruptured myocytes

In this study, the interaction of ruptured cardiac myocytes with low density lipoprotein (LDL) has been investigated and the consequent extent of uptake by macrophages. The results show that lysate released from ruptured myocytes is capable of inducing LDL oxidation and that the resulting modified form is recognised and degraded by macrophages. Peroxyl radical scavengers inhibit the LDL oxidation but not the macrophage uptake suggesting that LDL can be modified by mechanisms that are independent of oxidative processes by intracellular constituents of cardiac myocytes.

Serum total antioxidant activity after myocardial infarction

Serum total antioxidant activity (TAA), albumin and uric acid were measured on admission, and for the next 2 days in 56 patients suffering myocardial infarction, 20 of whom received streptokinase. The 'antioxidant gap', the difference between the serum TAA and the sum of the serum albumin and uric acid activity, was calculated. No significant changes in serum total antioxidant activity were observed in either group of patients between admission, day 1 and day 2. However, a decline in the 'antioxidant gap' after myocardial infarction was associated with a significantly higher mortality.

Practical approaches to low density lipoprotein oxidation: whys, wherefores and pitfalls

The purpose of this review is to bring together the different approaches for studying the oxidation of low density lipoproteins and try to identify some critical factors which will permit greater comparability between laboratories. These issues are discussed both in terms of the variety of exogenous mediators of oxidation applied (transition metal ions, haem proteins, azo initiators, peroxynitrite, cells etc.) and their raisons d'être, as well as the methodologies (formation of conjugated dienes, hydroperoxides, decomposition products of lipid peroxidation, altered surface charge, macrophage uptake) applicable to the different stages of the oxidation and the factors underlying their accurate execution and interpretation.

The antioxidant properties of theaflavins and their gallate esters--radical scavengers or metal chelators?

The antioxidant properties of theaflavins and their gallate esters were studied by investigating their abilities to scavenge free radicals in the aqueous and lipophilic phases. The total relative antioxidant activities in the aqueous phase were assessed by measuring their direct ABTS.+ radical scavenging abilities, and by their efficacies in inhibiting the degradation of deoxyribose induced by iron. The propensities for enhancing the resistance of LDL to oxidation mediated by Cu2+ were also measured. The results show that the hierarchy of reactivity of these compounds as antioxidants is: theaflavin digallate > 3'-monogallate = 3-monogallate > theaflavin. Spectroscopic studies show that all the compounds chelate iron and copper; enhanced absorbance in the visible region is observed in the case of the iron-digallate complex, but not with copper.

The effects of ascorbic acid and iron co-supplementation on the proliferation of 3T3 fibroblasts

Exposure of 3T3 fibroblasts to FeII reveals a concentration-dependent inhibition of cell proliferation compared to control cells, the apparent threshold for this iron-mediated effect being 5 microM FeII. The inhibition of cell proliferation was accompanied by an enhancement of total malondialdehyde (MDA) levels (as detected directly by hplc) in the cells at higher iron concentrations. The co-supplementation of FeII with varying concentrations of ascorbic acid over the range 5 microM to 240 microM had no significant effect on the threshold for iron toxicity or lipid peroxidation. These results show that there is neither a significant exacerbation of the pro-oxidant effect of FeII nor any protective effect of ascorbate when cultures of 3T3 mouse fibroblasts are exposed to co-supplementation regimes of iron with ascorbic acid.

Differential distribution of ferulic acid to the major plasma constituents in relation to its potential as an antioxidant

The hydroxycinnamates, intermediates in the phenylpropanoid synthetic pathway, are effective in enhancing the resistance of low-density lipoprotein (LDL) to oxidation in the order caffeic acid > ferulic acid > p-coumaric acid. It is unclear whether the mode of action of ferulic acid as an antioxidant is based on its activities in the aqueous or the lipophilic phase. Partitioning of 14C-labelled ferulic acid into plasma and its components, LDL and the albumin-rich fractions, has been studied under conditions of maximum aqueous solubility. The majority of ferulic acid associates with the albumin-rich fraction of the plasma, although a proportion is also found to partition between the LDL and aqueous phases; however, ferulic acid does not associate with the lipid portion of the LDL particle, suggesting that it exerts its antioxidant properties from the aqueous phase. This is of particular interest since the results demonstrate that ferulic acid is a more effective antioxidant against LDL oxidation than the hydrophilic antioxidant ascorbic acid.

Novel monohydroxamate drugs attenuate myocardial reperfusion-induced arrhythmias

The novel monohydroxamates N-methyl hexanoylhydroxamic acid, N-methyl acetohydroxamic acid, and N-methyl butyrohydroxamic acid have antioxidant and iron chelating properties. They attenuated reperfusion-induced contractile dysfunction following long periods of ischaemia (50 min) in the isolated rat heart. Here we compare their effects and that of the trihydroxamate desferrioxamine on reperfusion-induced arrhythmias following short duration ischaemia (10 min). Isolated rat hearts were perfused by the Langendorff method, subjected to regional ischaemia and reperfusion. Arrhythmias induced during the first 5 min of reperfusion were quantified. Drugs (all at 150 microM) were introduced during the last 2 min of ischaemia and remained throughout reperfusion. Although the monohydroxamate- and desferrioxamine-treated hearts showed a reduction in the incidence of ventricular tachycardia and fibrillation, only the reduction in the incidence of sustained fibrillation ( > 3 min duration) in N-methyl acetohydroxamic acid--(27%), N-methyl hexanoylhydroxamic acid--(27%) and desferrioxamine-treated hearts (20%) was statistically significant (p < 0.05 vs control 73%; n = 15). There was a reduction in the severity of the arrhythmias, manifest as a significant increase in the duration of sinus rhythm in all the monohydroxamate-treated hearts, and a significant reduction (vs control 218 +/- 29 s; mean +/- SEM) in the duration of ventricular fibrillation in hearts treated with N-methyl acetohydroxamic acid (101 +/- 31 s) and desferrioxamine (112 +/- 30 s). This improvement was offset by an increase in the duration of ventricular tachycardia, in hearts treated with N-methyl acetohydroxamic acid, N-methyl butyrohydroxamic acid and desferrioxamine. These results suggest that these novel monohydroxamates, particularly N-methyl acetohydroxamic acid, attenuate reperfusion-induced arrhythmias in this model when introduced during the ischaemic period.

Plant polyphenols: free radical scavengers or chain-breaking antioxidants?

There is increasing interest in the biological effects of tea- and wine-derived polyphenols and many studies in vitro and in vivo are demonstrating their antioxidant properties. Tea is a major source of dietary polyphenols and an even richer source of the flavanols, the catechins and catechin/gallate esters. Although there are limited studies on the bioavailability of the polyphenols, the absorption of flavanols in humans has been shown. The studies described in this chapter discuss the relative antioxidant potentials of the polyphenolic flavonoids in vitro against radicals generated in the aqueous phase in comparison with their relative effectiveness as antioxidants against propagating lipid peroxyl radicals, and how their activity influences that of alpha-tocopherol in low-density lipoproteins exposed to oxidative stress.

Polyphenolic flavanols as scavengers of aqueous phase radicals and as chain-breaking antioxidants

The purpose of this investigation was to establish the relative antioxidant activities in vitro of the flavanolic polyphenols, the catechins, and catechin-gallate esters. The relative antioxidant potentials were measured against radicals generated in the aqueous phase and against propagating lipid peroxyl radicals. The results show that in the aqueous phase their order of effectiveness as radical scavengers is epicatechin gallate (ECG) > epigallocatechin gallate (EGCG) > epigallocatechin (EGC) > gallic acid (GA) > epicatechin congruent to catechin; against propagating lipid peroxyl radical species, epicatechin and catechin are as effective as ECG and EGCG, the least efficacious being EGC and GA. This is consistent with their relative abilities to protect against consumption of LDL alpha-tocopherol. The results are discussed in the context of the most relevant antioxidant constituents of green tea extracts.

Antioxidant potential of intermediates in phenylpropanoid metabolism in higher plants

In this study the antioxidant activities of the hydroxycinnamic acids, chlorogenic, caffeic, ferulic and p-coumaric, have been investigated in peroxidising lipid systems mediated by metmyoglobin. The results show that the order of effectiveness in increasing the resistance of LDL to peroxidation, in protecting LDL cholesterol from oxidation and preventing the oxidative modification of the LDL apoprotein B100 is caffeic = chlorogenic > ferulic > p-coumaric acid. Assessment of the rates of reaction of the hydroxycinnamates with ferrylmyoglobin, a product of the reductive decomposition of lipid hydroperoxides, reveals that the compounds are more effective as peroxyl radical scavengers than reductants of ferryl myoglobin in peroxidising LDL systems mediated by haem proteins.

Oxidisability of low density lipoproteins in patients with carotid or femoral artery atherosclerosis

Oxidation of low density lipoprotein (LDL) is implicated in the pathogenesis of atherosclerosis. In this study the susceptibility to oxidation of LDL (from patients with atherosclerosis) is related to the progression of the disease. LDL were isolated from 37 patients with demonstrable atherosclerotic plaques. The susceptibility of LDL to oxidation (induced by an exogenous oxidative stress) was assessed by measuring the breakdown products of lipid peroxidation, the increased formation of conjugated dienes, and changes in surface charge of the apolipoprotein B (apo B). Progression of the atherosclerotic plaque was assessed by measuring the maximum velocity of blood through the narrowest portion of the vessel at inclusion and after one year. Twenty-nine of the 37 samples taken were found to have LDL that were partially oxidised, whereas 8 samples showed LDL whose state of oxidation was within the normal range. Progression of the atherosclerotic plaque occurred in 19 (66%) of the 29 patients whose lipoproteins were partially oxidised compared with only 2 (25%) of the 8 patients with normal lipoproteins (P = 0.055, Fisher's exact test). These data support an association between the progression of atherosclerotic plaques in carotid and femoral vessels and the susceptibility to oxidation of LDL.

The role of lipid hydroperoxides in the myoglobin-dependent oxidation of LDL

It has previously been reported that mb in both the iron-oxo ferryl and the ferric oxidation states can promote lipid peroxidation and lead to oxidative modification of low-density lipoprotein. The mechanism of these oxidation reactions is unclear and could involve either lipid hydroperoxide-dependent or independent reactions. In order to ascertain which of the afore-mentioned mechanisms predominates, the effects of exogenous lipid hydroperoxides on the ability of Mb, in its various oxidation states, to oxidize low-density lipoprotein has been investigated. The results suggest that oxidation proceeds through a one-electron redox cycle between met and ferryl myoglobin and that the reactions of both redox forms are at least partially dependent on lipid hydroperoxides within the LDL particle.

Inhibition of copper-mediated low density lipoprotein peroxidation by quinoline and indolinone nitroxide radicals

Quinoline and indolinone nitroxide radicals are known to be efficient scavengers of oxygen-centred (rate constants (k) between 10(3) and 10(5)/M/sec) and carbon-centred radicals (almost diffusion-controlled rate). In this study, the relative effects of these compounds in protecting low density lipoprotein (LDL) from oxidation induced by copper have been investigated. The extent of lipid peroxidation was assessed by monitoring the increased conjugated diene formation, the altered surface charge of the apolipoprotein B and the generation of aldehydic breakdown products of oxidized LDL. All the nitroxides inhibited LDL peroxidation in a concentration-dependent manner. The corresponding hydroxylamines of the nitroxides were also studied and were shown to inhibit lipid peroxidation to almost the same extent as the parent nitroxide. The data indicates that this class of nitroxide radicals (and their reduced hydroxylamine forms) are effective lipophilic antioxidants with the quinoline nitroxide being more efficient than the indolinone nitroxides.