section 38.1
Fat-Soluble Vitamins
909
The retina consists of a thin layer of photoreceptor cells
connected to the optic nerve via axons, synapses, and the
bodies of several intermediate cells. Other structures of the
eye provide the optical system for focusing light onto the
retina. Visual pigments are present in the approximately
100 million rod and 5 million cone cells (Figure 38-5).
The cones are less sensitive than the rods but are required
for color vision. Three types of cone cells differ with re-
spect to the wavelengths of light to which they are maxi-
mally sensitive. Comparison and integration of inputs from
these three cell types by the visual cortex produce color
vision. At low light levels, only the rods are active and
color vision is lost. A higher pigment content and greater
molar absorptivity of the pigment contribute to the higher
sensitivity of rods.
Rhodopsin has a broad absorption spectrum with a max-
imum at 500 nm, making the rods most sensitive to green
light. In the human retina, cone cell absorption maxima are
445 nm (blue), 535 nm (green), and 570 nm (red). Opsins in
the three types of cone cell have different primary struc-
tures and are coded by three separate genes. Those for
red- and green-sensitive opsins are on the X chromosome,
while that for blue-sensitive opsin is on an autosome. Con-
genital color blindness, which affects about 9% of the male
population, results from the absence of one or more cone
cell types or from a decrease in amount of one of the
pigments. Absence of red or green cones occurs in 2.5%
of males. Absence of blue cones occurs in only 0.001%
of males. Decrease in red cone pigment (protanomaly) or
green cone pigment (deuteranomaly) occurs in 1.3% and
5.0%, respectively, of males. The decrease in pigment type
presumably is due to mutation in a structural gene. Ab-
sence of two or three types of cone cell is extremely rare.
Light reception and energy transduction take place in
the rod outer segments, while cellular metabolism oc-
curs in the rod inner segment, which is rich in glyco-
gen, mitochondria, and rough endoplasmic reticulum. The
outer segment contains 500-1500 flattened membrane
sacs (disks), each about 16 nm thick, which are electri-
cally isolated from the surrounding plasma membrane and
which contain rhodopsin as an integral, transmembrane
protein. The disks are continually replaced from the outer
segment nearest the nucleus. Phagocytosis of old disks by
cells of the retinal pigment epithelium occurs at the top of
the outer segment. In the rhesus monkey, the lifetime of a
disk is 9-13 days. Disks are formed by the sealing off of
invaginations of plasma membrane, and the composition
of their contents differs from that of cytoplasm. Blindness
due to hereditary defects in regeneration of disks has been
observed in rats.
Cone cells are similar in many ways to rod cells
(Figure 38-5). Because cone cells lack distinct disks,
their visual pigments are located on deep invaginations
of the plasma membrane that resemble disks and prob-
ably are formed by a similar mechanism. Disk mem-
branes are composed primarily of phospholipid and
rhodopsin at a molecular ratio of about 70:1. The
phospholipids are 40% phosphatidylcholine, 40% phos-
phatidylethanolamine, and 13% phosphatidylserine. The
membrane has high fluidity because of extensive unsatu-
ration of the lipids, which gives rhodopsin considerable
rotational and translational mobility. Average spacing of
rhodopsin molecules is 5.6 nm.
Bovine rhodopsin (M.W. 39,000) has two asparagine-
linked oligosaccharides with the structure (mannose^
(N-acetylglucosamine). Seven hydrophobic a-helical seg-
ments, each 21-28 amino acids long, are embedded in the
disk membrane and are connected by hydrophilic sequen-
ces that are exposed to either side of the membrane. The
retinaldehyde moiety is within the membrane, halfway
down the first helical segment, attached by an aldimine
linkage to the e-ami no group of lysine 53 residues from
the carboxyl terminus by aldimine linkage (Figure 38-6).
FIGURE 38-6
Diagrammatic representation of bovine rhodopsin embedded in the disk
membrane. Only three of the seven a-helical segments are shown. The
only point of attachment between retinaldehyde and opsin is the indicated
aldimine bond. [Modified and reproduced, with permission, from
D. F. O’Brien, The chemistry of vision.
S c ie n ce
218,
961 (1982), © 1982
by the American Association for the Advancement of Science.]
