320
GEORGE WALD
and familiar, but need now to adopt definitions based on the known
chemistry. From this point of view we shall define a rhodopsin as a
visual pigment that contains neo-b retinenei bound to rod opsin, and
porphyropsin as a pigment containing neo-b retinene 2 bound to the
same class of protein. Since this composition involves a protein, specific
differences are expected; they are an important and essential complication of the subject, and should not be the source of either elation or
confusion.
What the specific differences in opsins imply chemically is an interesting question, the details of which still need to be explored. In such a
molecule the prosthetic group is bound at a particular site, involving
directly a small constellation of amino acids, ordinarily a very small
fragment of the whole protein. This fragment probably maintains a
measure of constancy throughout one entire family of proteins. Thus
it is probable that a specific constellation of amino acids characterizes
the rod opsins, and another constellation the cone opsins. Apart from
the binding site, all the remainder of the protein can vary endlessly, as
one goes from one species to another. Differences in composition in the
neighborhood of the binding site probably are the source of specific differences in the properties and behavior of the chromophore; differences
in composition that lie more remote can still affect such general properties as isoelectric point, solubility, and molecular weight. The final
searching out of specific differences may call for such delicate procedures as the immunological production of specific antibodies, themselves
proteins; though present methods of amino acid analysis and the determination of amino acid sequences have begun to make it possible to
approach this problem directly.
III. Habitat Relations: Freshwater Fishes versus Marine Fishes
and Land Vertebrates
The rods of freshwater teleosts contain porphyropsin, which bleaches
to form successively retinene 2 and vitamin A 2 (30, 31) (Fig. 4). Only
about a dozen forms have yet been reported in the literature; but they
are widely distributed, and as yet include no exception to this condition.
In almost all these animals the A max of porphyropsin lies at 520-526 πΐμ,
(i.e., 523 ± 3 τημ) (31). In several instances Dartnall has reported
longer A max : 533 πΐμ in the tench, pike (32), and bleak (33). There is
nothing intrinsically unlikely in finding porphyropsins at these wavelengths; yet these values are derived by the manipulation of quite impure
preparations; and must be coupled with conflicting reports that tench
porphyropsin lies at about 520 τημ (34) and pike porphyropsin at 525
m/i (35).*
* Note added in proof: Crescitelli (35a) has confirmed 533 ηΐμ as the X max of
tench porphyropsin.
GEORGE WALD
and familiar, but need now to adopt definitions based on the known
chemistry. From this point of view we shall define a rhodopsin as a
visual pigment that contains neo-b retinenei bound to rod opsin, and
porphyropsin as a pigment containing neo-b retinene 2 bound to the
same class of protein. Since this composition involves a protein, specific
differences are expected; they are an important and essential complication of the subject, and should not be the source of either elation or
confusion.
What the specific differences in opsins imply chemically is an interesting question, the details of which still need to be explored. In such a
molecule the prosthetic group is bound at a particular site, involving
directly a small constellation of amino acids, ordinarily a very small
fragment of the whole protein. This fragment probably maintains a
measure of constancy throughout one entire family of proteins. Thus
it is probable that a specific constellation of amino acids characterizes
the rod opsins, and another constellation the cone opsins. Apart from
the binding site, all the remainder of the protein can vary endlessly, as
one goes from one species to another. Differences in composition in the
neighborhood of the binding site probably are the source of specific differences in the properties and behavior of the chromophore; differences
in composition that lie more remote can still affect such general properties as isoelectric point, solubility, and molecular weight. The final
searching out of specific differences may call for such delicate procedures as the immunological production of specific antibodies, themselves
proteins; though present methods of amino acid analysis and the determination of amino acid sequences have begun to make it possible to
approach this problem directly.
III. Habitat Relations: Freshwater Fishes versus Marine Fishes
and Land Vertebrates
The rods of freshwater teleosts contain porphyropsin, which bleaches
to form successively retinene 2 and vitamin A 2 (30, 31) (Fig. 4). Only
about a dozen forms have yet been reported in the literature; but they
are widely distributed, and as yet include no exception to this condition.
In almost all these animals the A max of porphyropsin lies at 520-526 πΐμ,
(i.e., 523 ± 3 τημ) (31). In several instances Dartnall has reported
longer A max : 533 πΐμ in the tench, pike (32), and bleak (33). There is
nothing intrinsically unlikely in finding porphyropsins at these wavelengths; yet these values are derived by the manipulation of quite impure
preparations; and must be coupled with conflicting reports that tench
porphyropsin lies at about 520 τημ (34) and pike porphyropsin at 525
m/i (35).*
* Note added in proof: Crescitelli (35a) has confirmed 533 ηΐμ as the X max of
tench porphyropsin.