Retinal pigment epithelium retinol

The first two of these roles appear to focus on the conformal chemistry associated with the external profile of the molecules. The latter role focuses on the internal, quantum-mechanical structure of the molecules. While retinol, in its metabolic role as a vitamin participates in the manufacture of components of the disks of the Outer Segment of the Photoreceptor cells, it primary role is the last one. It acts as the critical chromogen, independent of any vitamin or hormonal role, leading to the production of chromophores in the retinal pigment epithelium (RPE) cells of the eye. Morton noted this role specifically In the retina, retinol is indubitably a precursor. ... 

Under conditions of low light intensity, the all-trans-retinaldehyde released from rhodopsin is reduced to aU-fra s-retinol, which is then transported to the retinal pigment epithelium bound to the interphotoreceptor RBP. This protein also binds fatty acids, including palmitate and docosa-hexaenoic acid (C22 6 co3), which is known to be essential for vision and which comprises some 50% of the phospholipid of photoreceptor cells. 

In the retinal pigment epithelium, palmitate is bound to the fatty acid binding site of the interphotoreceptor RBP, and the retinoid binding site has a high affinity for 11 -ds-retinaldehyde, which is to be transported to the photoreceptor cells. In the photoreceptor cells, the palmitate is displaced by docosahex-aenoic acid, which causes a conformational change in the protein, so that it no longer binds 11 -ds-retinaldehyde, which is delivered to the photoreceptor cells and binds all-fraws-retinol for transport back to the pigment epithelium. Here, the docosahexaenoic acid is displaced by palmitate, and the affinity of the protein for 11-ds-retinaldehyde is restored (Palczewski and Saari, 1997 Tschanz and Noy, 1997). 

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). 

Retinal pigment epithelium (RPE), 3925, 3927-3929, 3937, 3939 3941, 3943-3947 Retinoids, 2675, 2695 Retinol, 3369 Retinopathy, 1450 Retroaldol-aldol mechanism, 2700 Retrochalcones, 1869, 1874, 1875 Retrograde amnesia, 292 Retronecine, 1058, 1064 Retro-Prince reaction, 3075, 3076 Retrorsine, 1061, 1063, 1385 Reversed phase... 

Livrea MA, Tesoriere L, Bongiomo A (1991) AW-trans to ll-cis retinol isomerization in nuclear membrane fraction from bovine retinal pigment epithelium. Exp Eye Res 52 451-459... 

Saari J, Bredberg L (1989) Lecith/n retinol acetyltransferase in retinal pigment epithelium. J Biol Chem 264 ... 

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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