Retinal pigment epithelium retinyl esters

All-trans retinol then diffuses out of the outer segments (rod or cone) and into the retinal pigmented epithelium (RPE). The diffusion of all-trans-retinol out of the outer segments and into RPE cells is facilitated by interstitial retinoid binding protein (IRBP), which is localized in the extracellular matrix of the retina (22). IRBP binds all-trans-retinol as it diffuses out of the outer segment cells and is believed to facilitate transport of aM-trans-retinol to the RPE cell. Additionally, RPE cells directly acquire retinol from serum as described earlier. In both cases, retinol in the RPE cell is rapidly esterified by LRAT in a lecithin-dependent mechanism (22). Retinyl esters provide both a mechanism of storage for the RPE cell and/or provide a substrate for the isomerization reaction (22). 

The retinoids in dark-adapted eyes occur predominantly in the photoreceptor cell outer segments, which contain visual pigment, and in the adjacent retinal pigment epithelium (RPE) cells, which contain variable amounts of retinyl esters. These esters consist mainly of palmitate (A4) and stearate and are sometimes sequestered in lipid droplets (Bridges, 1975 Young and Bok, 1979 Alvarez etal., 1981 Bridges etal., 1982). The only major isomers present aieall-trans and 11-cis. The 11-cis isomer is restricted to ocular tissues. 

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... 

---