Neonatal nutrition,adult antioxidant defences and sexual attractiveness in the zebra finch

Early nutrition has recently been shown to have pervasive, downstream effects on adult life-history parameters including lifespan, but the underlying mechanisms remain poorly understood. Damage to biomolecules caused by oxidants, such as free radicals generated during metabolic processes, is widely recognized as a key contributor to somatic degeneration and the rate of ageing. Lipophilic antioxidants (carotenoids, vitamins A and E) are an important component of vertebrate defences against such damage. By using an avian model, we show here that independent of later nutrition, individuals experiencing a short period of low-quality nutrition during the nestling period had a twofold reduction in plasma levels of these antioxidants at adulthood. We found no effects on adult external morphology or sexual attractiveness: in mate-choice trials females did not discriminate between adult males that had received standard- or lower-quality diet as neonates. Our results suggest low-quality neonatal nutrition resulted in a long-term impairment in the capacity to assimilate dietary antioxidants, thereby setting up a need to trade off the requirement for antioxidant activity against the need to maintain morphological development and sexual attractiveness. Such state-dependent trade-offs could underpin the link between early nutrition and senescence.     

Comparison of two methods for noninvasive determination of carotenoids in human and animal skin: Raman spectroscopy versus reflection spectroscopy

Based on compelling in vivo and in vitro studies on human skin, carotenoids are thought to be of great interest as powerful antioxidants acting to prevent free‐radical‐induced damages, including premature skin ageing and the development of skin diseases such as cancer. Among the available techniques that are suitable for noninvasive determination of carotenoids in human skin, are resonance Raman spectroscopy (RRS) and reflection spectroscopy (RS). For RS, a LED‐based miniaturized spectroscopic system (MSS) was developed for noninvasive measurement of carotenoids in human skin. The optimization and subsequent calibration of the MSS was performed with the use of RRS. A strong correlation between the carotenoid concentration determined by the RS and for the RRS system was achieved for human skin in vivo (R = 0.88) and for bovine udder skin in vitro (R = 0.81). (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A novel method for evaluating free radical scavenging abilities of antioxidants using ultraviolet induction of bacteriophage lambda

A novel biological method used to evaluate free radical scavenging abilities of antioxidants using ultraviolet (UV) induction of bacteriophage lambda in lysogenic Escherichia coli kappa12 (lambda+) has been developed. This method is based on the induction of bacteriophage lambda from lysogenic cycle to lytic cycle by ultraviolet irradiation, and formation of free radicals during the course of induction. In the experiments, 10(8)cells/ml and 30s (39J/m2) were determined as the cell density of the lysogenic bacterium and UV irradiation time, respectively. The reliability of this method was demonstrated by electron paramagnetic resonance (EPR) spectroscopy and spin trapping with DMPO. This method had good reproducibility with intra-day variations (RSD, %) of less than 4% and inter-day variations (RSD, %) of less than 5%, respectively. By this method, the free radical scavenging abilities (ID50) of well-known antioxidants such as glutathione, superoxide dismutase (SOD), catalase (CAT) and carotenoids were determined quantitatively. The results were consistent with the ones obtained by conventional methods for evaluating free radical scavenging abilities. This developed method is reliable and uses common instruments and inexpensive, stable reagents, thus it could be utilized as a routine laboratory quantitative assay to screen a large number of substances that have potential to scavenge the free radical.     

The interaction of dietary carotenoids with radical species

Dietary carotenoids react with a wide range of radicals such as CCl3O2*, RSO2*, NO2*, and various arylperoxyl radicals via electron transfer producing the radical cation of the carotenoid. Less strongly oxidizing radicals, such as alkylperoxyl radicals, can lead to hydrogen atom transfer generating the neutral carotene radical. Other processes can also arise such as adduct formation with sulphur-centered radicals. The oxidation potentials have been established, showing that, in Triton X-100 micelles, lycopene is the easiest carotenoid to oxidize to its radical cation and astaxanthin is the most difficult. The interaction of carotenoids and carotenoid radicals with other antioxidants is of importance with respect to anti- and possibly pro-oxidative reactions of carotenoids. In polar environments the vitamin E (alpha-tocopherol) radical cation is deprotonated (TOH*+ --> TO* + H+) and TO* does not react with carotenoids, whereas in nonpolar environments such as hexane, TOH*+ is converted to TOH by hydrocarbon carotenoids. However, the nature of the reaction between the tocopherol and various carotenoids shows a marked variation depending on the specific tocopherol homologue. The radical cations of the carotenoids all react with vitamin C so as to "repair" the carotenoid.     

Ageing free radicals and cellular stress

A number of theories have attempted to account for ageing processes in various species. Following the < rate of living > theory of Pearl, Harman suggested fifty years ago that the accumulation of oxidants could explain the alteration of physical and cognitive functions with ageing. Oxygen metabolism leads to reactive species, including free radicals, which tend to oxidize surrounding molecules such as DNA, proteins and lipids. As a consequence various functions of cells and tissues can be altered, leading to DNA instability, protein denaturation and accumulation of lipid byproducts. Oxidative stress is an adaptive process which is triggered upon oxidant accumulation and which comprises the induction of protective and survival functions. Experimental evidence suggests that the ageing organism is in a state of oxidative stress, which supports the free radical theory. A number of other theories have been proposed ; some of these are actually compatible with the free radical theory. Caloric restriction is among the best models to increase life span in many species. While the relationship between caloric restriction and corrected metabolic rate is controversial, the decrease in ROS production by mitochondria appears to be experimentally supported. The ROS and mitochondrial theories of ageing appear to be compatible. Genetic models of increased life span, particularly those affecting the Foxo pathway, are usually accompanied by an increased resistance to oxidative insult. The free radical theory is not consistent with programmed senescence theories involving the cell division dependent decrease in telomere length ; however, oxidants are known to alter telomere structure. An appealing view of the role of oxidative stress in ageing is the trade-off principle which states that a phenotypic trait can be evolutionarily conserved because of its positive effects on development, growth or fertility, and despite its negative effect on somatic functions and ageing. It is likely that most cellular stresses which comprise adaptive and toxic functions follow such a rule.     

Products of DNA, protein and lipid oxidative damage in relation to vitamin C plasma concentration

Oxidative stress plays an important role in the pathogenesis of numerous chronic age-related free radical-induced diseases. Improved antioxidant status minimizes oxidative damage to DNA, proteins, lipids and other biomolecules. Diet-derived antioxidants such as vitamin C, vitamin E, carotenoids and related plant pigments are important in antioxidative defense and maintaining health. The results of long-term epidemiological and clinical studies suggest that protective vitamin C plasma concentration for minimum risk of free radical disease is higher than 50 micromol/l. Products of oxidative damage to DNA (DNA strand breaks with oxidized purines and pyrimidines), proteins (carbonyls) and lipids (conjugated dienes of fatty acids, malondialdehyde) were estimated in a group of apparently healthy adult non-smoking population in dependence on different vitamin C plasma concentrations. Under conditions of protective plasma vitamin C concentrations (>50 micromol/l) significantly lower values of DNA, protein and lipid oxidative damage were found in comparison with the vitamin C-deficient group (<50 micromol/l). The inhibitory effect of higher fruit and vegetable consumption (leading to higher vitamin C intake and higher vitamin C plasma concentrations) on oxidation of DNA, proteins and lipids is also expressed by an inverse significant correlation between plasma vitamin C and products of oxidative damage. The results suggest an important role of higher and frequent consumption of protective food (fruit, vegetables, vegetable oils, nuts, seeds and cereal grains) in prevention of free radical disease.     

EPR spin trapping detection of carbon-centered carotenoid and beta-ionone radicals

Free radical intermediates were detected by the electron paramagnetic resonance spin trapping technique upon protonation/deprotonation reactions of carotenoid and beta-ionone radical ions. The hyperfine coupling constants of their spin adducts obtained by spectral simulation indicate that carbon-centered radicals were trapped. The formation of these species was shown to be a result of chemical oxidation of neutral compounds by Fe(3+) or I(2) followed by deprotonation of the corresponding radical cations or addition of nucleophilic agents to them. Bulk electrolysis reduction of beta-ionone and carotenoids also leads to the formation of free radicals via protonation of the radical anions. Two different spin adducts were detected in the reaction of carotenoid polyenes with piperidine in the presence of 2-methyl-2-nitroso-propane (MNP). One is attributable to piperidine radicals (C(5)H(10)N*) trapped by MNP and the other was identified as trapped neutral carotenoid (beta-ionone) radical produced via protonation of the radical anion. Formation of these radical anions was confirmed by ultraviolet-visible spectroscopy. It was found that the ability of carotenoid radical anions/cations to produce neutral radicals via protonation/deprotonation is more pronounced for unsymmetrical carotenoids with terminal electron-withdrawing groups. This effect was confirmed by the radical cation deprotonation energy (H(D)) estimated by semiempirical calculations. The results indicate that the ability of carotenoid radical cations to deprotonate decreases in the sequence: beta-ionone > unsymmetrical carotenoids > symmetrical carotenoids. The minimum H(D) values were obtained for proton abstraction from the C(4) atom and the C(5)-methyl group of the cyclohexene ring. It was assumed that deprotonation reaction occurs preferentially at these positions.     

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.     

Endogenous antioxidant changes in the myocardium in response to acute and chronic stress conditions

Oxygen is a diradical and because of its unique electronic configuration, it has the potential to form strong oxidants (e.g. superoxide radical, hydrogen peroxide and hydroxyl radical) called oxygen free radicals or partially reduced forms of oxygen (PRFO). These highly reactive oxygen species can cause cellular injury by oxidizing lipids and proteins as well as by causing strand breaks in nucleic acids. PRFO are produced in the cell during normal redox reactions including respiration and there are various antioxidants in the cell which scavenge these radicals. Thus in order to maintain a normal cell structure and function, a proper balance between free radical production and antioxidant levels is absolutely essential. Production of PRFO in the myocardium is increased during variousin vivo as well asin vitro pathological conditions and these toxic radicals are responsible for causing functional, biochemical and ultrastructural changes in cardiac myocytes. Indirect evidence of free radical involvement in myocardial injury is provided by studies in which protection against these alterations is seen in the presence of exogenous administration of antioxidants. Endogenous myocardial antioxidants have also been reported to change under various physiological as well as pathophysiological conditions. It appears that endogenous antioxidants respond and adjust to different stress conditions and failure of these compensatory changes may also contribute in cardiac dysfunction. Thus endogenous and/or exogenous increase in antioxidants might have a therapeutic potential in various pathological conditions which result from increased free radical production.     

Substrate contribution on free radical scavenging capacity of carotenoid extracts produced from <Emphasis Type="Italic">Blakeslea trispora</Emphasis> cultures

Blakeslea trispora produces carotenoids mixtures consisting mainly of lycopene, γ-carotene and β-carotene, together with trace amounts of other carotenoid precursors. The yield of these carotenoids and their composition are greatly affected by culture substrate. The scavenging capacity of carotenoids extract from cultures of B. trispora growing in various substrates was estimated using the 2,2-diphenyl-1-picrylhydrazyl method. Fractions enriched in β-carotene, γ-carotene and lycopene, obtained after column chromatography in alumina basic II, were also examined. Substrates containing starch and oils mixture, Ni²⁺, and that with pantothenic acid presented higher antioxidant activity. An increase in the antioxidant activity of the crude carotenoid extract compared to that of the isolated fractions enriched in β-carotene, γ-carotene and lycopene respectively, observed in most samples, indicated a possible synergistic effect. The results are of interest and by expanding this study to more substrates and other microorganisms- producing antioxidants, a formulation of extract with high free radical scavenging potential could be produced.     

Antioxidant and prooxidant properties of carotenoids

The ability of dietary carotenoids such as beta-carotene and lycopene to act as antioxidants in biological systems is dependent upon a number of factors. While the structure of carotenoids, especially the conjugated double bond system, gives rise to many of the fundamental properties of these molecules, it also affects how these molecules are incorporated into biological membranes. This, in turn, alters the way these molecules interact with reactive oxygen species, so that the in vivo behavior may be quite different from that seen in solution. The effectiveness of carotenoids as antioxidants is also dependent upon their interaction with other coantioxidants, especially vitamins E and C. Carotenoids may, however, lose their effectiveness as antioxidants at high concentrations or at high partial pressures of oxygen. It is unlikely that carotenoids actually act as prooxidants in biological systems; rather they exhibit a tendency to lose their effectiveness as antioxidants.     

Oxidants and antioxidants in proliferative senescence.

In terms of the amount of experimental research it has generated the free radical theory of ageing is one of the most popular hypotheses to explain this ubiquitous phenomenon. From the theory two postulates were derived: either cellular defence mechanisms against free radical-dependent oxidants deteriorate during ageing of cells, or essential, unrepairable damages are imparted to the cell by oxidants regardless of the activity of antioxidant defence systems. The many reports dealing with a putative breakdown in antioxidant defence systems failed to positively support this postulate. However, a minor depletion in cellular glutathione by exposure to a model lipophilic peroxide led to a significant decrement in DNA and protein synthesis. In other words, the glutathione redox cycle is intrinsically fallible with respect to defending the cellular DNA replication system against this model lipophilic peroxide. Interestingly, after ageing in culture cells a partial uncoupling of the NADPH-producing and -consuming systems tends to take place. Experiments involving the addition of antioxidants to the culture medium have failed to significantly extend the lifespan of cultured diploid somatic cells. The level of antioxidants appears to be a modulator rather than a primary determinant of cellular ageing in culture. Several lines of evidence suggest that DNA damages accumulate during ageing of the organism, but no oxidant-related DNA damage has been pinpointed in the cultured cell system. Human mutants with defects in antioxidant enzymes have not shown conclusive signs of accelerated ageing. Cells from patients with Werner's syndrome (progeria of the adult), on the other hand, do not suffer from a defect in their antioxidant defence system, nor do they accumulate more than normal amounts of autofluorescent products resulting from lipid peroxidation. The recent finding that Werner's syndrome constitutes a mutator phenotype may prompt the comparison of oxidant- and ageing-related mutation spectra in order to investigate a mutational theory of ageing as a new derivative from the free radical hypothesis.     

Conflict of evidence: carotenoids and other micronutrients in the prevention and treatment of cognitive impairment

Cognitive impairment is a common age-related disorder which affects in the stadium and type Alzheimer's Disease (AD) a steadily growing number of patients. AD is not curable and is not being easily diagnosed in its preclinical phase. This work aims at highlighting the complex though promising rationale for the use of selected micronutrients against age-related cognitive impairment and its progression. The advances made in the last decades in both defining the etiopathogenesis of cognitive impairment and in revealing mechanisms of action underlying possible preventive effects of several vitamins and micronutrients--likely related to antioxidant activity and modulation of cellular signaling--is being accompanied by conflicting results of most clinical trials. Therefore, available data do not currently support the use of substances such as carotenoids, polyphenols, vitamin D, curcumin, vitamin E, vitamin C, or lipoic acid in AD prevention and/or treatment. This might be partly due to the fact that cognitive impairment and especially AD are extremely complex disorders. The main obstacle to the inclusion of micronutrients among anticognitive impairment drug strategies, however, is that studies conducted so far are poorly comparable and probably underestimate of the role of vascular damage in age-related cognitive impairment. A possible clinical benefit of these substances in AD is not disproved to date, thus further better designed studies are needed.     

Natural substances and Alzheimer's disease: from preclinical studies to evidence based medicine

Over the last 10 years, the potential therapeutic effects of nutraceuticals to prevent or delay Alzheimer's disease were proposed. Among dietary antioxidants curcumin, Ginkgo biloba and carnitines were extensively studied for their neuroprotective effects. The rationale for this alternative therapeutic approach was based on several preclinical studies which suggested the neuroprotective effects for curcumin, Ginkgo biloba and acetyl-l-carnitine due to either a free radical scavenging activity or the inhibition of pro-inflammatory pathways or the potentiation of the cell stress response. However, although these are interesting premises, clinical studies were not able to demonstrate significant beneficial effects of curcumin, Ginkgo biloba and acetyl-l-carnitine in improving cognitive functions in Alzheimer's disease patients. The aim of this review is to summarize the main pharmacologic features of curcumin, Ginkgo biloba and carnitines as well as to underlie the main outcomes reached by clinical studies designed to demonstrate the efficacy of these natural substances in Alzheimer's disease patients. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.     

Testing the shared-pathway hypothesis in the carotenoid-based coloration of red crossbills

The mechanisms involved in the production of red carotenoid-based ornaments of vertebrates are still poorly understood. These colorations often depend on enzymatic transformations (ketolation) of dietary yellow carotenoids, which could occur in the inner mitochondrial membrane (IMM). Thus, carotenoid ketolation and cell respiration could share biochemical pathways, favoring the evolution of ketocarotenoid-based ornaments as reliable indices of individual quality under sexual selection. Captive male red crossbills (Loxia curvirostra Linnaeus) were exposed to redox-active compounds designed to penetrate and act in the IMM: an ubiquinone (mitoQ) or a superoxide dismutase mimetic (mitoTEMPO). MitoQ can act as an antioxidant but also distort the IMM structure, increasing mitochondrial free radical production. MitoQ decreased yellow carotenoids and tocopherol levels in blood, perhaps by being consumed as antioxidants. Contrarily, mitoTEMPO-treated birds rose circulating levels of the second most abundant ketocarotenoid in crossbills (i.e., canthaxanthin). It also increased feather total red ketocarotenoid concentration and redness, but only among those birds exhibiting a redder plumage at the start of the study, that is, supposedly high-quality individuals. The fact that mitoTEMPO effects depended on original plumage color suggests that the red-ketocarotenoid-based ornaments indicate individual quality as mitochondrial function efficiency. The findings would thus support the shared pathway hypothesis.     

Independent and interactive association of blood antioxidants and oxidative damage in elderly people

Oxidative stress is recognized as one of the major contributors to the increased risk of several diseases. Many recent population studies have established a close link between antioxidant defense and lowered risk of morbidity and mortality from cancer and heart disease, but little is known about the cooperative interactions of antioxidants. We examined the cross-sectional independent and interactive association of serum lipid-soluble antioxidant levels and free radical scavenging enzymes to serum malondialdehyde (MDA) levels, as a marker of oxidative damage. The participants were 160 nonsmoker institutionalized elderly. Upper tertile values of erythrocyte-superoxide-dismutase (E-SOD) constituted the strongest-associated single compound with a 74% decreased risk of high MDA. Upper tertiles of carotenoids and alpha-tocopherol independently showed a similar lowering of risk of about 57%. The highest tertiles of lycopene and either beta-carotene or alpha-tocopherol simultaneously reveal a higher decreased risk for oxidative damage (74 and 71%, respectively), very similar to those in the upper tertiles of all these three vitamins (75%). This study represents one of the few attempts to date to understand the interactive effect between antioxidants and suggests that lipid-soluble antioxidants act not individually, but rather cooperatively with each other. The efficacy of this interaction is more effective when lycopene is present.     

Cytoprotective properties of traditional Chinese medicinal herbal extracts in hydrogen peroxide challenged human U373 astroglia cells

Age is the leading risk factor for many of the most prevalent and devastating diseases including neurodegenerative diseases. A number of herbal medicines have been used for centuries to ameliorate the deleterious effects of ageing-related diseases and increase longevity. Oxidative stress is believed to play a role in normal ageing as well as in neurodegenerative processes. Since many of the constituents of herbal extracts are known antioxidants, it is believed that restoring oxidative balance may be one of the underlying mechanisms by which medicinal herbs can protect against ageing and cognitive decline. Based on the premise that astrocytes are key modulators in the progression of oxidative stress associated neurodegenerative diseases, 13 herbal extracts purported to possess anti-ageing properties were tested for their ability to protect U373 human astrocytes from hydrogen peroxide induced cell death. To determine the contribution of antioxidant activity to the cytoprotective ability of extracts, total phenol content and radical scavenging capacities of extracts were examined. Polygonum multiflorum, amongst others, was identified as possessing potent antioxidant and cytoprotective properties. Not surprisingly, total phenol content of extracts was strongly correlated with antioxidant capacity. Interestingly, when total phenol content and radical scavenging capacities of extracts were compared to the cytoprotective properties of extracts, only moderately strong correlations were observed. This finding suggests the involvement of multiple protective mechanisms in the beneficial effects of these medicinal herbs.     

Antioxidant potentials of vitamin A and carotenoids and their relevance to heart disease

Despite being one of the first vitamins to be discovered, the full range of biological activities for vitamin A remains to be defined. Structurally similar to vitamin A, carotenoids are a group of nearly 600 compounds. Only about 50 of these have provitamin A activity. Recent evidence has shown vitamin A, carotenoids and provitamin A carotenoids can be effective antioxidants for inhibiting the development of heart disease. Vitamin A must be obtained from the diet: green and yellow vegetables, dairy products, fruits and organ meats are some of the richest sources. Within the body, vitamin A can be found as retinol, retinal and retinoic acid. Because all of these forms are toxic at high concentrations, they are bound to proteins in the extracellular fluids and inside cells. Vitamin A is stored primarily as long chain fatty esters and as provitamin carotenoids in the liver, kidney and adipose tissue. The antioxidant activity of vitamin A and carotenoids is conferred by the hydrophobic chain of polyene units that can quench singlet oxygen , neutralize thiyl radicals and combine with and stabilize peroxyl radicals. In general, the longer the polyene chain, the greater the peroxyl radical stabilizing ability. Because of their structures, vitamin A and carotenoids can autoxidize when O2 tension increases, and thus are most effective antioxidants at low oxygen tensions that are typical of physiological levels found in tissues. Overall, the epidemiological evidence suggests that vitamin A and carotenoids are important dietary factors for reducing the incidence of heart disease. Although there is considerable discrepancy in the results from studies in humans regarding this relationship, carefully controlled experimental studies continue to indicate that these compounds are effective for mitigating and defending against many forms of cardiovascular disease. More work, especially concerning the relevance of how tissue concentrations, rather than plasma levels, relate to the progression of tissue damage in heart disease is required. This review assembles information regarding the basic structure and metabolism of vitamin A and carotenoids as related to their antioxidant activities. Epidemiological, intervention trials and experimental evidence about the effectiveness of vitamin A and carotenoids for reducing cardiovascular disease is also reviewed.     

Changes in radical scavenging activity of normal,endoreduplicated and depolyploid root tip cells of Allium cepa

The plant cell responds to abiotic stress conditions by adjusting its cellular metabolism and various defensive mechanisms. Cellular metabolism involves changes in the cell cycle, in which the cell undergoes repeated rounds of endocycles leading to polyploidization. Defense mechanisms such as role of antioxidants are a key to understand plant adaptation. The present work describes endoreduplication and radical scavenging activity as two different defense mechanisms adapted by plants for their survival under stress condition. The work describes linkage of these two processes with each other under abiotic stress. Endoreduplicated root tip cells of Allium cepa were depolyploidized by exogenous phytohormones. Further, free radical scavenging activity from normal, endoreduplicated and depolyploidized root tips cells was observed to understand the role of phytohormones. Elevated free radical scavenging potential was observed in endoreduplicated cells compared to normal and depolyploidized cells. Based on these results, it was concluded that endoreduplication and antioxidant pathways are linked with each other through phytohormonal activities. The concentration of auxin and cytokinin regulates the activity of ascorbate oxidase enzyme, which in turn maintains the concentration of AsA within the cell. AsA level directs the prolyl-hydroxylation process of cell division proteins in quiescent center cells either toward endoreduplication process or cell division process.     

Carotenoids are degraded by free radicals but do not affect lipid peroxidation in unilamellar liposomes under different oxygen tensions

It has been questioned whether carotenoids can act as antioxidants in biological membranes. Biological membranes can be modeled for studies of lipid peroxidation using unilamellar liposomes. Both carotenoid depletion and lipid peroxidation were increased with increasing oxygen tension in unilamellar liposomes. Carotenoids in such liposomes were found to be very sensitive to degradation by free radicals generated from iron and 2,2'-azobis(2-amidinopropane) dihydrochloride, but they were not protective against lipid peroxidation. Lycopene and beta-carotene were more sensitive to free radical attack than lutein, zeaxanthin, and beta-cryptoxanthin.