Chemistry Reference
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unevenly distributed in animal tissue following intake as feed or food. The
upconcentration of zeaxanthin and lutein in the yellow spot in the human eye is
crucial for protection of the visual function against oxidative stress, while the
importance of uptake and distribution of other carotenoids by animals is less
evident except for the function of some carotenoids as provitamin A and for
some birds of other carotenoids as plumage colorants.
1.3.3 Iodide
Iodide became important during early evolution as a protector against ROS, and
iodide/iodine cycling is probably one of the most ancient mechanisms of defence
against poisonous ROS. Coastal seawaters contain approximately 60g iodine per
litre, and algae and kelp accumulate iodide, which together with peroxidases
scavenges ROS (Venturi and Venturi, 2007). Iodide has recently been identified in
Laminariales (kelp), the strongest accumulators of iodine among living
organisms, as an important inorganic antioxidant, which upon oxidative stress
submerged in seawater, scavenge oxygen radicals and peroxides, and in addition
upon exposure to the atmosphere at low tide, reacts with ozone (KÈpper et al.,
2008). Notably, iodide reacts with ozone, singlet oxygen and superoxide radicals
at rates up to 500-fold higher than ascorbate and glutathione. Marine lipids as
synthesized by algae and seaweed rank among the molecules most sensitive to
especially ROS due to their high unsaturation linked to their low melting point
necessitated by the often cold marine environment. Iodide seems throughout
evolution to have had crucial roles for the protection of these energy-dense
compounds of outmost importance for marine life. As may be seen from Table 1.2,
the reaction between the ROS mentioned so far and iodide is all thermo-
dynamically favourable, which is in contrast to the reactions with bromide (or
chloride). The second-order rate constants for reaction with iodide indicate fast
reactions even approaching the diffusion limit except for the reaction with
hydrogen peroxide (KÈpper et al., 2008). For the reaction with hydrogen peroxide,
enzymes like the iodoperoxidases found in kelp facilitate removal of hydrogen
peroxide through formation of iodinated organic compounds like iodo-tyrosine:
2I
ÿ
2H
2
O
2
2tyrosine 2H
! 2 iodo ÿ tyrosine + 4H
2
O
1.15
Table 1.2 Stoichiometry, reaction free energy and second-order rate constant for
reaction of iodide with reactive oxygen species in aqueous solution at ambient conditions
a
Gë (kJ mol
ÿ1
)
k
2
(l mol
ÿ1
s
ÿ1
)
Reaction
I
ÿ
·
OH ! I
·
+ OH
ÿ
1.2 10
10
ÿ37.7
I
ÿ
1
O
2
H
+
H
2
O ! HIO + H
2
O
2
8.7 10
5
ÿ36.4
I
ÿ
HO
2
·
H
+
! I
·
H
2
O
2
1.0 10
8 b
ÿ10.6
I
ÿ
O
3
H
+
! HIO + O
2
1.2 10
9
ÿ210.8
2I
ÿ
H
2
O
2
2H
+
! I
2
+ 2H
2
O
ÿ237.1
0.69
a
Based on KÈpper et al. (2008).
b
Rate constant is for reaction between I
3
ÿ
med O
2
·ÿ
.
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