Chemistry Reference
In-Depth Information
The antioxidant properties of carotenoids are facilitated by the presence of multiple, closely
spaced energy levels between the excited states and the ground state of the molecules. Thus, triplet-
triplet energy transfer from photosensitizer to carotenoid occurs, thereby preventing energy transfer
from photosensitizer to oxygen and the formation of singlet oxygen (Martin et al., 1999). Carotenoids
are also the excellent physical quenchers of singlet oxygen, the energy of singlet molecular oxygen
being transferred to the carotenoid molecule to yield ground state oxygen, and a triplet-excited
carotenoid. Energy transfer in this way is possible because the triplet energy level of the carotenoid
is lower than the energy level of singlet oxygen.
16.3 PHOTOREACTIVE BISRETINOID COMPOUNDS IN
PHOTORECEPTOR OUTER SEGMENTS
Given the presence of lutein and zeaxanthin in photoreceptor outer segments and because of the
known antioxidant properties of carotenoids (Khachik et al., 1997; Landrum and Bone, 2001),
questions arise as to whether there are specii c molecules in photoreceptor outer segments toward
which carotenoid protection might be directed. Photoreactive molecules that could be important in
this regard are the bisretinoid molecules that form in photoreceptor outer segments as the precur-
sors of retinal pigment epithelial (RPE) lipofuscin (Figure 16.1). One of these molecules is A2PE
(Figure 16.1), a phosphatidyl-pyridinium bisretinoid pigment that forms through a biogenic cas-
cade (Liu et al., 2000; Parish et al., 1998) that involves reactions between the membrane phos-
pholipid phosphatidylethanolamine (PE) and all-
trans
-retinal, the latter being generated upon
the photoisomerization of 11-
cis
-retinal (Figure 16.2). Intermediates in biogenic pathway include
11-
cis
-retinal
Light
11-
cis
-retinol
OPL
all-
trans
-retinal
ONL
Visual
cycle
all-
trans
-retinyl
ester
PE
Lipofuscin
precursors
all-
trans
-retinol
RPE
Phagocytosis
Lipofuscin
accumulation
in RPE
FIGURE 16.2 (See color insert following page 336.)
Intersection of the visual (retinoid) cycle and
pathway for RPE lipofuscin formation. The photoisomerization of 11-
cis
-retinal leads to the release of all-
trans
-retinal from rhodopsin. All-
trans
-retinal for the most part is reduced to all-
trans
-retinol and remains
in the visual cycle for reconversion to 11-
cis
-retinal. All-
trans
-retinal can also react inaptly, leave the visual
cycle, and forming lipofuscin precursors. At least some of the reactions leading to the lipofuscin pathway are
between all-
trans
-retinal and PE in a 2:1 ratio. After the phagocytosis of shed outer segment membrane by
RPE, lipofuscin accumulates in the latter cells. RPE lipofuscin detected as autol uorescence in monkey retina
imaged by l uorescence microscopy (left). Nuclei are stained with DAPI. The autol uorescence adjacent to
the RPE is at the level of outer segments and is likely attributable to lipofuscin precursors that form in outer
segments. Outer nuclear layer (ONL); outer plexiform layer (OPL).
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