Vertebrate vision starts with photoisomerization of the
11-cis-retinal chromophore to all-trans-retinal.
Biosynthesis of 11-cis-retinal is required to
maintain vision. A key enzyme catalyzing the oxidation
of 11-cis-retinol is 11-cis-retinol dehydrogenase
(11-cis-RDH), which is encoded by the RDH5
gene. 11-cis-RDH is expressed in the RPE and not
in the neural retina. The consequences of a lack of 11-cis-RDH
were studied in a family with fundus albipunctatus. We
identified the causative novel RDH5 mutation,
Arg157Trp, that replaces an amino acid residue conserved
among short-chain alcohol dehydrogenases. Three-dimensional
structure modeling and in vitro experiments suggested
that this mutation destabilizes proper folding and inactivates
the enzyme. Studies using RPE membranes indicated the existence
of an alternative oxidizing system for the production of
11-cis-retinal. In vivo visual consequences
of this null mutation showed complex kinetics of dark adaptation.
Rod and cone resensitization was extremely delayed following
full bleaches; unexpectedly, the rate of cone recovery
was slower than rods. Cones showed a biphasic recovery
with an initial rapid component and an elevated final threshold.
Other unanticipated results included normal rod recovery
following 0.5% bleach and abnormal recovery following bleaches
in the 2–12% range. These intermediate bleaches showed
rapid partial recovery of rods with transitory plateaux.
Pathways in addition to 11-cis-RDH likely provide
11-cis-retinal for rods and cones and can maintain
normal kinetics of visual recovery but only under certain
constraints and less efficiently for cone than rod function.