Chemistry Reference
In-Depth Information
Blue light is the most efi cient part of the visible spectrum reaching the adult human retina to trigger
light-induced damage (Boulton et al., 2001; Rozanowska and Sarna, 2005). It has been demonstrated
that blue light initiates production of reactive oxygen species by mitochondria, all-
trans
-retinal, lipo-
fuscin and melanosomes (Boulton et al., 2001; Godley et al., 2005; Rozanowska and Sarna, 2005;
Rozanowska et al., 2002)—molecules and organelles particularly abundant in the inner and outer
segments of photoreceptors and the RPE. Thus, reducing blue light irradiance levels of these parts of
the retina can minimize photoactivation of photosensitizers and subsequent photodamage.
Moreover, carotenoids may quench electronically excited states and scavenge free radicals formed
in the retina, and therefore protect biomolecules from oxidative damage. Due to the low energy level
of the i rst excited triplet state (
3
Car), carotenoids (Car) can act as efi cient acceptors of triplet state
energy from photosensitizers (S) (Equation 15.1), such as all-
trans
-retinal, the photosensitizers of
lipofuscin (Rozanowska et al., 1998), or singlet oxygen (
1
O
2
) (Equation 15.2) (Cantrell et al., 2003):
3
3
Car
+→
S
Car
+
S
(15.1)
1
3
Car
+→ +
O
Car
O
(15.2)
2
2
Carotenoids are particularly valuable as singlet oxygen quenchers. They accept the energy from
the excited state of molecular oxygen, singlet oxygen (
1
O
2
) and, as a result, the oxygen molecule
returns to its ground state, while the carotenoid is left in a triplet state that thermally deactivates to
the ground state (Equation 15.2). Thus, it is a safe physical mode of quenching of
1
O
2
and results
in no chemical modii cation to any of the interacting molecules. The bimolecular rate constants
of interactions of carotenoids with singlet oxygen are close to the diffusion controlled limits, with
zeaxanthin being about twice more effective than lutein (Cantrell et al., 2003).
Carotenoids interact with a number of free radicals either via electron (Equation 15.3) or hydro-
gen (Equation 15.4) transfer, or forming an addition complex (Equation 15.5) (El-Agamey et al.,
2004b):
•
•
−
Car
+→
R
Car
+
R
(15.3)
•
•
Car
+→
R
Car
+
RH
(15.4)
Car
•
•
+→−
Car
R
⎡
R
⎤
(15.5)
⎣
⎦
In case of scavenging of lipid-derived peroxyl radicals (LOO
•
), the radical adduct formed [LOO-
Car]
•
is less reactive than the LOO
•
, so carotenoids act as chain-breaking antioxidants in lipid
peroxidation (Equation 15.6):
•
•
Car
+
LOO
→ −
[LOO
Car]
(15.6)
15.3 RPE AS A MEDIATOR OF SPECIFIC UPTAKE OF
CAROTENOIDS INTO THE RETINA
RPE plays numerous functions essential for proper structure and function of retinal photoreceptors.
They include the maintenance of the blood-retina barrier, selective uptake and transport of nutri-
ents from the blood to the retina and removal of waste products to the blood, enzymatic cleavage
of b-carotene into vitamin A, storage of vitamin A and its metabolic transformations, phagocytosis
and molecular renewal of POS, expression and secretion of growth factors and immunomodula-
tory cytokines (Aizman et al., 2007; Aleman et al., 2001; Crane et al., 2000a,b; Elner et al., 2006;
Holtkamp et al., 2001; Leuenberger et al., 2001; Lindqvist and Andersson, 2002; Maminishkis
et al., 2006; Momma et al., 2003; Strauss, 2005).
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