Processing of Vitamin A in The Visual Cycle.

<p>Enzymatic processing within the visual cycle begins with delivery of vitamin A (all-<i>trans</i>-retinol) from the blood circulation. Upon entry into the RPE, all-<i>trans</i>-retinol is converted to a retinyl ester through the activity of lecithin retinol acyl transferase (1). The resulting all-<i>trans</i>-retinyl ester pool represents a storage form of vitamin A upon which RPE65 acts to generate 11-<i>cis</i>-retinol (2); 11-<i>cis</i>-retinol is then oxidized by an 11-<i>cis</i>-specific retinol dehydrogenase to form the visual chromophore, 11-<i>cis</i>-retinal (3). The visual chromophore is delivered to rod and cone outer segments (4) where it combines with opsins to form visual pigments (e.g., rhodopsin). Light activation of rhodopsin initiates visual transduction processes and liberates all-<i>trans</i>-retinal as a photoproduct. Reduction of all-<i>trans</i>-retinal, via all-<i>trans</i>-retinal dehydrogenase, produces all-<i>trans</i>-retinol (5), which is transferred back to the RPE for recycling. The continued activity of RPE65 in the light state ensures sustained levels of rhodopsin, closure of ion channels through transducin activation, and reduced oxygen demand.