7. THE DISTRIBUTION AND EVOLUTION OF VISUAL SYSTEMS
319
The specific differences in the opsins confer distinctive properties upon
them and upon the visual pigments derived from them. These properties
are only beginning to be explored; but already we can recognize small
and large specific differences in absorption spectrum, in the rate of
combination of opsin with neo-b retinene (18), in the temperature range
and energy of activation for denaturation (19), and in a few instances
differences in isoelectric point (frog, 4.53 (20); cattle, about 5.4) (21).
Half-times for the synthesis of rhodopsin in solution are already known
to vary with species from 10-20 seconds (alligator, scup) to 4-6 minutes
(frog, cattle, chicken) (25°) (18). The chicken cone pigment, iodopsin,
is synthesized in scdution more than 500 times as fast as chicken
rhodopsin (22); but the rhodopsins of the butterfish and cusk are synthesized as rapidly as chicken iodopsin (23). We find not only a wide
range of rates of synthesis of rhodopsin, but at the upper extreme they
overlap with the generally more rapid rates of synthesis of iodopsin.
A comparable situation involving absorption spectra has lately caused
an untoward amount of concern in some quarters. In part the reason
for this is that while their chemistry was still obscure the visual pigments
came to be known and classified on the basis of their color. Their names
are color names: rhodopsin means rose-colored eye pigment; porphyropsin, purple; iodopsin, violet; cyanopsin, blue eye pigment. From the
beginning it was recognized that the absorption spectra vary somewhat,
yet for a long time all the known rhodopsins could be characterized as
having A max between the limits 500 ± 2 τημ, and porphyropsins
522 ± 2 τημ. Lately these limits have expanded, as inevitably they must
as the size of sample is increased. Human rhodopsin lies at 493 τημ (24),
gecko rhodopsin at 524 πΐμ (25), quite apart from the discovery of new
natural groupings of rhodopsins, as in deep-sea fishes (A max about 480
πΐμ) (26) and invertebrates (octopus, 475 πΐμ (27); euphausiids, 465
ναμ) (28). Certain freshwater fishes may have porphyropsins with A max
533 ναμ (32, 33), and the iodopsins of certain marine fishes, rather than
lying at 562 m^, seem to be at 520-530 πΐμ (29).
Most of the known rhodopsins of amphibia, reptiles, mammals, and
surface forms of marine fish, however, center close to 500 τημ, and it is
probable that the great bulk of such rhodopsins have A max lying between
the limits 498 and 502 τημ. This is now an old story, and grown tedious.
Only rhodopsins with highly divergent X max now seem worth writing
papers about, so that from now on the literature will abound in aberrant
spectra. One should not be misled by this into a distorted view of the
situation as a whole. Nor is it necessary to be confused by the observation that gecko rhodopsin lies at as long a wavelength as the usual porphyropsin, or that the rhodopsins of deep-sea fishes look orange instead
of rose-colored. We can retain the old color names which are traditional
319
The specific differences in the opsins confer distinctive properties upon
them and upon the visual pigments derived from them. These properties
are only beginning to be explored; but already we can recognize small
and large specific differences in absorption spectrum, in the rate of
combination of opsin with neo-b retinene (18), in the temperature range
and energy of activation for denaturation (19), and in a few instances
differences in isoelectric point (frog, 4.53 (20); cattle, about 5.4) (21).
Half-times for the synthesis of rhodopsin in solution are already known
to vary with species from 10-20 seconds (alligator, scup) to 4-6 minutes
(frog, cattle, chicken) (25°) (18). The chicken cone pigment, iodopsin,
is synthesized in scdution more than 500 times as fast as chicken
rhodopsin (22); but the rhodopsins of the butterfish and cusk are synthesized as rapidly as chicken iodopsin (23). We find not only a wide
range of rates of synthesis of rhodopsin, but at the upper extreme they
overlap with the generally more rapid rates of synthesis of iodopsin.
A comparable situation involving absorption spectra has lately caused
an untoward amount of concern in some quarters. In part the reason
for this is that while their chemistry was still obscure the visual pigments
came to be known and classified on the basis of their color. Their names
are color names: rhodopsin means rose-colored eye pigment; porphyropsin, purple; iodopsin, violet; cyanopsin, blue eye pigment. From the
beginning it was recognized that the absorption spectra vary somewhat,
yet for a long time all the known rhodopsins could be characterized as
having A max between the limits 500 ± 2 τημ, and porphyropsins
522 ± 2 τημ. Lately these limits have expanded, as inevitably they must
as the size of sample is increased. Human rhodopsin lies at 493 τημ (24),
gecko rhodopsin at 524 πΐμ (25), quite apart from the discovery of new
natural groupings of rhodopsins, as in deep-sea fishes (A max about 480
πΐμ) (26) and invertebrates (octopus, 475 πΐμ (27); euphausiids, 465
ναμ) (28). Certain freshwater fishes may have porphyropsins with A max
533 ναμ (32, 33), and the iodopsins of certain marine fishes, rather than
lying at 562 m^, seem to be at 520-530 πΐμ (29).
Most of the known rhodopsins of amphibia, reptiles, mammals, and
surface forms of marine fish, however, center close to 500 τημ, and it is
probable that the great bulk of such rhodopsins have A max lying between
the limits 498 and 502 τημ. This is now an old story, and grown tedious.
Only rhodopsins with highly divergent X max now seem worth writing
papers about, so that from now on the literature will abound in aberrant
spectra. One should not be misled by this into a distorted view of the
situation as a whole. Nor is it necessary to be confused by the observation that gecko rhodopsin lies at as long a wavelength as the usual porphyropsin, or that the rhodopsins of deep-sea fishes look orange instead
of rose-colored. We can retain the old color names which are traditional