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Fig. 6.15.
Urucum fruits (
Bixa orellana
L.).
Urucum extract has large amounts of
carotenoids with a large diversity of chemi-
cal structures (other structures are bixin,
zeaxanthin, canthaxanthin, astaxanthin and
lutein; Fig. 6.14). It has been used by South
American native civilizations as a colouring
dye for the human body and food, guaran-
teeing increased photo-protection from the
caustic effects of photosensitized reactions
in the tropical and equatorial areas.
Nowadays, carotenoids are used in food
supplements and have been shown to pro-
tect against sunlight-induced erythema in
human skin (Gollnick
et al
., 1996; Biesalski
and Obermueller-Jevic, 2001) and photo-
induced immunosuppression (Fuller
et al
.,
1992; Herraiz
et al
., 1998).
Because photosensitized oxidations
may be mediated both by singlet oxygen
and by free radicals, multiple antioxidant
actions of carotenoids may contribute to its
protective effect (Krinsky, 1989). Both sing-
let oxygen and free radicals produced dur-
ing photosensitized oxidations can cause
cellular damage by reacting with DNA and
proteins, or by inducing lipid peroxidation.
Carotene seems to protect against photosen-
sitized tissue injury both by scavenging free
radicals and by quenching singlet oxygen
(Krinsky, 1979). In fact, b-carotene is clini-
cally used to prevent photosensitized tissue
damage
in
humans
with
porphyria
(Mathews-Roth, 1986).
The analysis of specific products has
been used to monitor singlet oxygen quench-
ing in experimental models of phototox-
icity, and has demonstrated that carotenoids
can quench singlet oxygen by physical and
chemical processes. The photo-oxidation of
0.1 M of 2-methyl-2-pentene is inhibited by
95% by 10
−4
mol.l
−1
of b-carotene (Foote
et al
., 1970). The rate constant for quench-
ing of
1
O
2
by b-carotene is k
q
= 1 × 10
10
l.
mol
−1
s
−1
that accounts for most of its sup-
pression ability (Schmidt, 2004). The reac-
tion products of singlet oxygen oxidation
of b-carotene shows a series of compounds
including ionone and apocarotenal, endoper-
oxide, dihydroperoxides, chain-cleavage and
carbonyl-containing products (Fig. 6.16).
The formation of the endoperoxide (b-caro-
tene-5,8-endoperoxide) is typical of 1,4-cyclo-
addition reactions of singlet oxygen with
cis
-dienes. This confirms in an unequivocal
way that the oxidation mechanism is through
1
O
2
(Stratton
et al
., 1993).
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