1. VISUAL PIGMENTS
15
ing to vision are triggered at some early stage in this sequence. The general processes of visual photochemistry are familiar and require no restatement (see Dartnall, 1957, 1962; Morton and Pitt, 1969; Wald, 1959,
1960). Abrahamson and Ostroy ( 1967) have recentIy reviewed the complex chain of events following illumination and have evaluated current
theories about the structure of visual pigments and initiation of vision.
These concepts have been derived from studies of the visual pigments
of frogs and cattle and are not within the scope of the present review.
Interest in the visual pigments of fishes derives primarily from their
great diversity, which is discussed below. These differences are based
on alterations of both parts of the molecule: a series of species-specific
opsins has been described (e.g., Bridges, 1965a; Dartnall and Lythgoe,
1965) and two retinenes are known (Wald, 1959, 1960). Retinene,, the
aldehyde of vitamin Al, has one double bond in its ring structure, whereas
retinene, (derived from vitamin A,) has two. In principle, any opsin can
combine with either retinene; in this way a pair of visual pigments can be
formed. A number of these pairs have been described (Bridges, 1965a;
Dartnall and Lythgoe, 1965; Munz and Schwanzara, 1967). The nomenclature of visual pigments has been a source of confusion (Dartnall, 1962,
p. 389). It is convenient to attach names to the series of visual pigments
based on each of the two retinenes. Visual pigments based on retinene,
may be called “rhodopsins” and those based on retinene,, “porphyropsins,”
without regard to their origin in rods or cones. This usage is operational,
for the origin of extracted visual pigments is usually not known with
certainty. Some authors would prefer to restrict these names to the visual
pigments of rods (e.g., Wald, 1959, 1960)) but this limitation is unworkable at present (see Dartnall, 1962, p. 505).
B. Methods of Study
1. PARTIAL BLEACHING
The primary method for investigation and identification of visual pigments is known as partial bleaching. It has been developed chiefly by
Dartnall, who has described the method in detail (Dartnall, 1962).
Retinal extracts usually contain hemoglobin or other light-absorbing substances, in addition to visual pigments. Identification of any one component in the composite absorbance spectrum is made uncertain by the
presence of the others ( Fig. 6A, curve 1 ) . Fortunately, visual pigments
are photosensitive (i.e., bleached by light), but the usual impurities are
not. (In Fig. 6A, note that as the absorbance of the visual pigment decreases in the visible region, a product of bleaching appears in the ultra-
15
ing to vision are triggered at some early stage in this sequence. The general processes of visual photochemistry are familiar and require no restatement (see Dartnall, 1957, 1962; Morton and Pitt, 1969; Wald, 1959,
1960). Abrahamson and Ostroy ( 1967) have recentIy reviewed the complex chain of events following illumination and have evaluated current
theories about the structure of visual pigments and initiation of vision.
These concepts have been derived from studies of the visual pigments
of frogs and cattle and are not within the scope of the present review.
Interest in the visual pigments of fishes derives primarily from their
great diversity, which is discussed below. These differences are based
on alterations of both parts of the molecule: a series of species-specific
opsins has been described (e.g., Bridges, 1965a; Dartnall and Lythgoe,
1965) and two retinenes are known (Wald, 1959, 1960). Retinene,, the
aldehyde of vitamin Al, has one double bond in its ring structure, whereas
retinene, (derived from vitamin A,) has two. In principle, any opsin can
combine with either retinene; in this way a pair of visual pigments can be
formed. A number of these pairs have been described (Bridges, 1965a;
Dartnall and Lythgoe, 1965; Munz and Schwanzara, 1967). The nomenclature of visual pigments has been a source of confusion (Dartnall, 1962,
p. 389). It is convenient to attach names to the series of visual pigments
based on each of the two retinenes. Visual pigments based on retinene,
may be called “rhodopsins” and those based on retinene,, “porphyropsins,”
without regard to their origin in rods or cones. This usage is operational,
for the origin of extracted visual pigments is usually not known with
certainty. Some authors would prefer to restrict these names to the visual
pigments of rods (e.g., Wald, 1959, 1960)) but this limitation is unworkable at present (see Dartnall, 1962, p. 505).
B. Methods of Study
1. PARTIAL BLEACHING
The primary method for investigation and identification of visual pigments is known as partial bleaching. It has been developed chiefly by
Dartnall, who has described the method in detail (Dartnall, 1962).
Retinal extracts usually contain hemoglobin or other light-absorbing substances, in addition to visual pigments. Identification of any one component in the composite absorbance spectrum is made uncertain by the
presence of the others ( Fig. 6A, curve 1 ) . Fortunately, visual pigments
are photosensitive (i.e., bleached by light), but the usual impurities are
not. (In Fig. 6A, note that as the absorbance of the visual pigment decreases in the visible region, a product of bleaching appears in the ultra-