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18
Contribution of G. Wald:
photochemical sensitivity
regulation mechanisms
of rods and cones
This hypothesis of Kühne and Hecht must be considered an important
insight with regard to biochemical processes underlying dark adapta-
tion. Yet, it only represented a first step towards an understanding
of the highly complex processes involved in the photochemical
sensitivity regulation mechanisms of rods and cones. Obviously, a
deeper understanding would require more information on both the
molecular structure of the rod and cone photopigments and the
bleaching and regeneration processes generated by light.
18.1 M of l e c u l a r b a s i s of f b l e ac h i n g
a n d r e g e n e r at i of n of f p h of t of p i g m e n t s
in rods and cones
Inspired by Hecht, Wald, in the early 1930s, set out to throw light
on these largely unexplored research topics (see Wald, 1949a , 1958 ,
1968 ). His profound discoveries and insights earned him the Nobel
Prize which he shared with Granit and Hartline in 1967.
Firstly, he discovered vitamin A in the retina (Wald, 1933 ,
1934 / 1935 ). Shortly thereafter, he concluded that the photopigment
rhodopsin was a conjugated carotenoid-protein engaged in a bleaching-
regeneration cycle when acted upon by light (Wald, 1934 , 1935 /1936).
Thus, in line with the hypothesis of Kühne and Hecht that light
decomposes rhodopsin into its two precursors, Wald presumed that
the carotenoids, all-trans retinal (vitamin A aldehyde) and vitamin A
represented both photoproducts and precursors of rhodopsin.
A few years later, in 1937, Wald made two new important
discoveries:
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