Regeneration of Visual Pigments the Retinal Cycle

Fig. 4.18 The rhodopsin cycle. The bleaching of rhodopsin yields all-trans retinene (now more currently indicated as retinal), which must be isomerized to neoretinene b before it can regenerate the visual pigment. Alternatively, having been reduced to all trans vitamin A, the latter in turn must be isomerized to neovitamin Ab before it takes part in rhodopsin synthesis. Rod vision thus depends on the continuous stereoisomerization of all-trans retinene or vitamin A, or on the continuous replacement of these substances by new supplies of vitamin Ab from external sources (adapted from [196, 197])...

The retinal pigment epithelium cells (RPE) of the retina form an epithelial cell layer that takes up retinol from choroid capillaries and stores it as retinyl esters, to be used as substrate for the generation of 11-ds-retinal. In the layer of rod and cone photoreceptor cells adjacent to the RPE, 11-ds-retinal combines covalently with the protein opsin to generate the visual pigment rhodopsin in rods and, similarly, iodopsin in cones. Each rod outer segment is densely packed with some 10 molecules of rhodopsin per cell. The small quantity of vitamin A stored in the retina would be inadequate to maintain vision were it not for the visual cycle, a process in which 11-ds-retinal is regenerated after... 

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