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In spite of the predominance of cones in the chicken retina,
more rhodopsin than iodopsin was found. In fact, Wald estimated the
concentration of iodopsin in single cones to be hundreds of times less
than that of rhodopsin in single rods. Thereby, he could explain the
relatively low sensitivity of cones and why cones had to be the organ
of day vision. Also, he held that the Purkinje shift obtained with the
chicken retina could be completely accounted for by a transfer from
dependence upon the absorption spectrum of iodopsin in bright light
to that of rhodopsin in dim light (see Wald, 1949a ).
Wald soon concluded that the cone photopigment iodopsin,
like rhodopsin and porphyropsin, was a conjugated carotenoid-
protein. However, the important question of whether the same
chromophores represented both the rod and cone systems or whether
the chromophore of the cone photopigments represented a third
variant of retinal remained unanswered.
At last, though, in the 1950s, Wald came up with a sweeping
generalization (see Wald, 1958 , 1968 ):
With regard to the molecular structure, all visual photopigments
found in the outer segment of vertebrate rods and cones consisted of
one of only two types of retinal (the rhodopsin variant retinal 1 and the
porphyropsin variant retinal 2 ) bound as a chromophore to different
proteins called opsins. Wald also recognized two families of opsin, those
of the rods and those of the cones, so that, in all, four major photopig-
ments could be synthesized in vertebrate rods and cones. Yet, in order
to synthesize the visual pigments the retinal component had to be in
a special shape, the 11-cis configuration. This configuration showed
both a bend and a twist in the side chain of the retinal molecule which
thereby fitted closely to the surface of the opsins.
Along with this parallelism of structure, the different visual
photopigments were found to exhibit an extraordinary parallelism
of the bleaching-regeneration cycle. We may summarize Wald's
description of these two processes as follows:
The only action of light was to isomerize the chromophore
of the photopigment from the 11-cis to the all-trans configuration
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