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chapter 38
Vitamin Metabolism
In the dark, rhodopsin has a red color, which changes to
pale yellow upon exposure to light. This bleaching takes
place within a few milliseconds after absorption of a pho-
ton and occurs because
a\\-trans
retinaldehyde bound to
opsin is unstable. At low temperatures, bleaching proceeds
more slowly, and seven intermediates have been identi-
fied (Figure 38-7). The structures and order of appearance
of these intermediates have yet to be clarified. The final
R hodopsin
h v
H ypsorhodopsin
B athorhodopsin
i
Lum irhodopsin
I
M etarhodopsin I
I
M etarhcdopsin II
Pararhodopsin
I
N -R etinylideneopsin
H20
FIGURE 38-7
Bleaching of rhodopsin in bovine retina. Absorption of light by
11
-cfs-retinaldehyde initiates the configurational change to all
-trans
retinaldehyde, culminating in hydrolysis of the bond between
retinaldehyde and Lys 53' of opsin. The order of formation of the
intermediates is tentative. The symbol
h v
represents a photon of visible
light. [Modified and reproduced, with permission, from C. D. B. Bridges,
Retinoids in photosensitive systems. In:
T he R etin o id s,
Vol. 2,
M. B. Sporn, A. B. Roberts, and D. S. Goodman, eds. Academic Press,
San Diego, 1984).]
FIGURE 38-8
Regeneration of 11 -c/.s-retinaldchyde (11-cTsRal). Following light
exposure, rhodopsin (opsin + 11-mRaI) breaks down to opsin and
a \\-trans
retinaldehyde (atRal) initiating a neural signal. The restoration of
11-ci.sRaI begins with the atRal combining with phosphatidylethanolamine
(PE) to form the protonatal Schiff base N-retinylidine-PE complex. This
complex is transported across the disk membrane by an ATP-binding
cassette protein, exclusively found in the retinal outer segment (ABCR).
Defects in ABCR cause early-onset maculopathy
(S ta rg a rd t’s disease).
After reduction of atRal to all tran.v-retinol (atRol) by atRol dehydrogenase
in the retinal outer segment, atRol is converted to 11-c/sRal in a series of
reactions occuring in the retinal pigment epithelial cell (RPE). Also shown
in the bottom is the phagocytosis of disk by RPE.
compound (perhaps N-retinylideneopsin) is unstable and
hydrolyzes to al
Vtrans
retinaldehyde plus opsin.
The regeneration of rhodopsin following exposure to
bright light requires the following steps. The all
-trans
retinaldehyde combines with phosphotidylethanolamine
(PE) to form a protonated Schiff base N-retinylidene-PE.
This complex is then exported out of the disk by an
ATP-binding
cassette
(ABC)
transporter
exclusively
located in the retinol disk membrane (ABCR). Outside
the disk the all
-trans
retinaldehyde is reduced to
a\\-trans
retinol by a dehydrogenase and transported to retinal
