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
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
Lum irhodopsin
M etarhodopsin I
M etarhcdopsin II
N -R etinylideneopsin
Bleaching of rhodopsin in bovine retina. Absorption of light by
-cfs-retinaldehyde initiates the configurational change to all
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).]
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
retinaldehyde plus opsin.
The regeneration of rhodopsin following exposure to
bright light requires the following steps. The all
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
located in the retinol disk membrane (ABCR). Outside
the disk the all
retinaldehyde is reduced to
retinol by a dehydrogenase and transported to retinal
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