Figure 27. . Retinoid cycles: Overview of the two cycles, and flow in the RPE cycle.

Figure 27.

Retinoid cycles: Overview of the two cycles, and flow in the RPE cycle. A, Overview of the flow of retinoid in the two cycles. The areas separated by solid lines represent cellular compartments of a retinal pigment epithelial cell (top), a rod and a cone photoreceptor cell (middle), and a Müller cell (bottom). The ovals surrounding 11-Ral represent rhodopsin (gray) and cone opsins (tricolor). Photoisomerization reactions are shown in red. All other chemical reactions are catalyzed by enzymes. Retinoids are chaperoned by retinoid-binding proteins (not shown) during intercellular and intracellular movement. Abbreviations: at-RE, all-trans retinyl esters; at-Ral, all-trans retinal; at-Rol, all-trans retinol (= vitamin A); 11-Ral, 11-cis retinal; 11-Rol, 11-cis retinol. From Saari (196). B, Flow of retinoid in the RPE cycle. Delivery of 11-cis retinoid is shown by solid arrows, while removal of all-trans retinoid is shown by dashed arrows. Abbreviations: OS, outer segment; IPM, inter-photoreceptor matrix; RPE, retinal pigment epithelium; SER, smooth endoplasmic reticulum. RAL, retinaldehyde; ROL, retinol. For the chemical icons: AL, OL and E denote the aldehyde, alcohol, and ester groups attached to the retinoid hydrocarbon chain. Enzymes (in red): RDH, all-trans retinol dehydrogenase; LRAT; lecithin:retinol acyltransferase; RDH5, 11-cis retinol dehydrogenase. Chaperone proteins (in blue): IRBP, inter-photoreceptor retinol binding protein; CRBP, cellular retinol binding protein; RPE65, retinal pigment epithelium 65 kDa protein, now known to be the isomerohydrolase; CRALBP, cellular retinal binding protein. The ABCA4 transporter is not shown; this rescues the very small proportion of all-trans retinoid that inadvertently reaches the luminal leaflet of the disc membrane, and it has only a minor role in total retinoid cycling. From Lamb & Pugh (139). Gene names are given in boxes; see also Item D.

From: Evolution of Phototransduction, Vertebrate Photoreceptors and Retina

Cover of Webvision
Webvision: The Organization of the Retina and Visual System [Internet].
Kolb H, Fernandez E, Nelson R, editors.
Salt Lake City (UT): University of Utah Health Sciences Center; 1995-.

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