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(a configuration without the bend and twist, but containing the
same chemical constituents). Since the all-trans configuration did
not fit into the surface of the opsin like that of the 11-cis retinal,
this transformation resulted in significant changes in the molecular
structure of the photopigment leading eventually to a complete
separation between the all-trans retinal and the opsin. A cascade of
change took place. Pre-lumi-rhodopsin, lumi-rhodopsin, meta-rho-
dopsin I and meta-rhodopsin II all represented successive, stepwise,
and rapid changes in the molecular structure before the compound
was hydrolyzed, liberating all-trans retinal. Visual excitation had
occurred by the time meta II had been formed (at about 1 ms), since
subsequent changes were too slow to be involved.
Finally, the all-trans retinal was converted to vitamin A by
an enzymatic process (retinal reductase as apoenzyme and DPN-H
2
as coenzyme) in which the retinal molecule received two hydrogen
atoms from the coenzyme, reducing its carbonyl group to the primary
alcohol group of vitamin A. Retinal
1
was reduced to vitamin A
1
and
retinal
2
to vitamin A
2
(see Wald,
1949b
). Later, it was found that other
coenzymes may also be used in the enzymatic process (see Wald,
1968
).
Up to meta II the all-trans chromophore remained attached to
opsin at the same site. As long as this was the case, absorption of a new
photon could isomerize the all-trans chromophore to 11-cis retinal
and thereby immediately regenerate the photopigment. In fact, the
absorption of a second photon by any of the all-trans intermediates of
bleaching could re-isomerize the chromophore to 11-cis retinal and
thereby regenerate the photopigment. Somewhat surprisingly, then,
absorption of light may bleach as well as regenerate photopigments.
Yet, in accord with the hypothesis of Kühne, the photopig-
ments were also found to regenerate after complete bleaching from
the vitamin A stage. Thus, retinal was continuously formed in the
dark by an enzymatic process in which vitamin A was oxidized by
alcohol dehydrogenase (vitamin A
1
and A
2
were, respectively, oxidized
to retinal
1
and retinal
2
). Wald, like Kühne, knew that the pigment
