7. THE DISTRIBUTION AND EVOLUTION OF VISUAL SYSTEMS
313
arthropods, and vertebrates, we find that the primary visual functions have passed to a group of carotenoid derivatives—haplocarotenoids—having
20 carbon atoms in the molecule. Furthermore, unlike the plant carotenoids which are synthesized de novo,
these molecules or the plant carotenoids from which they are made
must be supplied in the diet. They therefore have the position in
the animal economy of vitamins, the vitamins A. So far as we know,
all the photosensitive pigments involved in animal vision are formed
by joining aldehydes of the vitamins A—the so-called retinenes—
to proteins. Even here, however, one finds the larger plant carotenoids
and some of their more highly oxidized derivatives—such as astaxanthin,
C40H52O4—in the role of accessory pigments, concerned with filtering
and absorbing the light which strikes the visual receptors. So it is that
the filter pigments of the cones of many day birds and turtles are carotenoids (7) and the pigment of the yellow patch (macula lutea) of the
human retina is also a carotenoid (3).
What properties particularly fit the carotenoids and vitamins A for
photoreception? I think in the first instance the fact that they are pigments and readily form further pigments by association with proteins.
Second, these pigments tend to absorb light maximally in the range of
wavelengths in which the energy of sunlight reaching the surface of the
earth is most concentrated, a point already considered in Fig. 1. In this
the carotenoids and their derivatives are not unique; the phycobilins
would have done just as well. A third property is beginning to emerge
as of the utmost importance. Unlike other common natural pigments—
porphyrins, phycobilins, anthocyanins—the color of which is associated
with conjugated systems bound more or less rigidly in rings, the
carotenoids and vitamins A have long straight-chain conjugated systems, which readily undergo cis-trans or geometrical isomerization (8).
That is, these molecules can undergo a variety of drastic changes in
shape, involving only changes in geometrical configuration about their
double bonds. These changes are readily induced by simple absorption
of light. The capacity of a carotenoid to interact with a protein depends
intimately upon its shape; a change in shape can modify or abolish such
interaction; and both the interaction and its cessation are readily invoked by light. This is the fundamental basis of the formation and
bleaching of the visual pigments (9, 10); and may be the key to the
special suitability of carotenoids in all types of photoreception.
II. Visual Systems of Vertebrates
The biochemistry of vertebrate vision has at present reached the
following simple position (11). Two vitamins A are known—A x and A 2
313
arthropods, and vertebrates, we find that the primary visual functions have passed to a group of carotenoid derivatives—haplocarotenoids—having
20 carbon atoms in the molecule. Furthermore, unlike the plant carotenoids which are synthesized de novo,
these molecules or the plant carotenoids from which they are made
must be supplied in the diet. They therefore have the position in
the animal economy of vitamins, the vitamins A. So far as we know,
all the photosensitive pigments involved in animal vision are formed
by joining aldehydes of the vitamins A—the so-called retinenes—
to proteins. Even here, however, one finds the larger plant carotenoids
and some of their more highly oxidized derivatives—such as astaxanthin,
C40H52O4—in the role of accessory pigments, concerned with filtering
and absorbing the light which strikes the visual receptors. So it is that
the filter pigments of the cones of many day birds and turtles are carotenoids (7) and the pigment of the yellow patch (macula lutea) of the
human retina is also a carotenoid (3).
What properties particularly fit the carotenoids and vitamins A for
photoreception? I think in the first instance the fact that they are pigments and readily form further pigments by association with proteins.
Second, these pigments tend to absorb light maximally in the range of
wavelengths in which the energy of sunlight reaching the surface of the
earth is most concentrated, a point already considered in Fig. 1. In this
the carotenoids and their derivatives are not unique; the phycobilins
would have done just as well. A third property is beginning to emerge
as of the utmost importance. Unlike other common natural pigments—
porphyrins, phycobilins, anthocyanins—the color of which is associated
with conjugated systems bound more or less rigidly in rings, the
carotenoids and vitamins A have long straight-chain conjugated systems, which readily undergo cis-trans or geometrical isomerization (8).
That is, these molecules can undergo a variety of drastic changes in
shape, involving only changes in geometrical configuration about their
double bonds. These changes are readily induced by simple absorption
of light. The capacity of a carotenoid to interact with a protein depends
intimately upon its shape; a change in shape can modify or abolish such
interaction; and both the interaction and its cessation are readily invoked by light. This is the fundamental basis of the formation and
bleaching of the visual pigments (9, 10); and may be the key to the
special suitability of carotenoids in all types of photoreception.
II. Visual Systems of Vertebrates
The biochemistry of vertebrate vision has at present reached the
following simple position (11). Two vitamins A are known—A x and A 2