Retinal pigment epithelium retinoid

The retina comprises two principal components, the non-neural retinal pigment epithelium and the neural retina. The retinal pigment epithelium is an essential component of the visual system both structurally and functionally. It is important for the turnover and phagocytosis of photoreceptor outer segments, the metabolism of retinoids, the exchange of nutrients between the photoreceptors, and the choroidal blood vessels and the maintenance of an efficient outer blood-retinal barrier. 

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). 

Carlson, A., and Bok, D. (19%). I rornotion of the release of ll-ci s-retinal from cultured retinal pigment epithelium by interphotoreceptOr retinoid-binding protein. Biachemisiry 31, 9056-9062,... 

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... 

Saari, J. C., Bredburg, L, and Garwin, G. G. (1982) Identification of the endogenous retinoids associated with three cellular retinoid-binding proteins from bovine retina and retinal pigment epithelium J Biol. Chem 257, 13,329-13,333. 

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. 

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