readily oxidized by air, especially in the presence
of iron and other transition metal ions. The
resulting tocopherylquinone has no biological
activity. To prevent this loss of biological potency
in nutritional supplements, vitamin E is pre-
sented in the esterified form. In the gastrointesti-
nal tract, the ester is enzymatically hydrolysed
and free tocopherol is absorbed (Traber & Sies
1996).
Carotenoids
Carotenoid designates a long-chain molecule
300 nutrition and exercise
with 40 carbon atoms and an extensive con-
jugated system of double bonds. Plant and
microorganism-derived carotenoids are efficient
scavengers of several forms of ROS (Handelman
1996). The major forms of carotenoids that have
been studied for their antioxidant properties
include a-, b- and g-carotene, lycopene, b-
cryptoxanthin, lutein, zeaxanthin, astaxanthin,
canthaxanthin, violaxanthin, and b-carotene-5,6-
epoxide. Photosynthetic plant leaves are rich in
carotenoids typical of the choloroplast, con-
taining predominantly b-carotene, lutein, and
epoxycarotenoids, e.g. violaxanthin. StorageROO + Chr-OHDecay to non-radical productsROOH + Chr-OChr-O + RO Products
Chr-O + ROO Products
Chr-O + Chr-O ProductsRegeneration–recycling by vitamin C
Chr-O + ascorbic Chr-OH +
semiascorbyl
radicalacid10 Gauss10 GaussESR
signalFormationFig. 22.2Interaction of vitamin E and C during the course of the lipid peroxidation chain reaction termination.
During termination of the lipid peroxidation reaction, vitamin E may be oxidized to its corresponding radical
configuration (Chr-O•) that has no antioxidant potency and may be even toxic. Vitamin C may regenerate Chr-O•
to Chr-OH and itself be oxidized to the semiascorbyl radical. Spontaneous radical–radical recombination may lead
to the decay of some radicals to non-reactive products. Chr-OH, chromanol head with the phenolic OH in
tocopherol; ESR, electron spin resonance spectroscopy; ROO•, peroxyl radical; semiascorbyl radical, one electron
oxidation product of vitamin C or ascorbic acid.