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Mechanisms and Biomarkers (WG 4) page 45
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Carotenoid Concentration and Interaction with Other Antioxidants - Many dietary
intervention studies that have employed high concentrations of carotenoids, failed to report
any effects. This has also been reported for other antioxidants such as vitamin E. Killion and
colleagues (1996) reported that patients suffering atherosclerosis exhibited altered levels of
vitamin E. Their data also demonstrates a strong association between tissue vitamin E levels
and tissue lipid peroxidation. Similar results were obtained when rats were fed with large
amounts of β-carotene, increased concentrations of peroxides were found in the rat plasma
and liver.
Any effective synergism between the carotenoids, xanthophylls, tocopherols and other cooxidants
would depend upon a balance between all of these components. An increase in
carotenoid concentration could disturb this balance and promote lipid peroxidation. The
induction of lipid peroxidation would generate carotenoid radical adducts. The prevention of
any prooxidant activity would therefore depend upon the ability of tocopherols to reduce the
radicals, and the regeneration of the tocopherols by water soluble antioxidants. Therefore an
increase in the carotenoid concentration or a decrease in associated antioxidants would
enhance the probability of prooxidant activity.
Another untoward effect of an increased concentration of carotenoids is the destabilisation of
plasma membranes. van De Ven (1984) and colleagues showed that when β-carotene was
incorporated into phosphatidylcholine membranes, the orientation of the carotenoid was
dependent upon the composition of the lipid bilayers. In the case of dioleyl lecithin bilayers
the carotenoids had a parallel orientation and a perpendicular orientation was observed in a
soybean lecithin lamellar membranes. The addition of β-carotene to phosphatidylcholine
membranes decreased the rigidity of the membrane by increasing the motional freedom of the
polar lipid headgroups. β-Carotene is distributed homogeneously with a membrane with no
preferred orientation, thus its effect is to decrease the rigidity of the membrane. Whereas,
xanthophylls would behave as ‘gate posts’ spanning the entire membrane bilayer. This
rigidifying effect would decrease the penetration of oxygen in the lipid bilayer. This in turn
would then limit the extent of lipid peroxidation.
Recently lutein and zeaxanthin were incorporated by Sujak (1999) into phosphatidylcholine
liposomes. The liposomes were then challenged with either UV or AAPH (2,2’-azobis(2methylpropionamidine)dihydrochloride).
The conclusions of this study indicated that there
was a decreased penetration barrier for the oxidant AAPH, and the xanthophylls acted by free
radical scavenging mechanisms to protect the liposomes against against oxidative damage.