Protein Pages
Recovery and Supply of the Chromophore
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Recent update from: 02.07.2006

This page introduces you to the proteins and pathways involved in the retinoid metabolism of the visual cascade.
If you click on a protein on the right, you will receive a description of the chosen step and further pages providing information on that protein can be accessed.
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Supply of retinoids to photoreceptors is one of the crucial points in retinal nutrition. Retinoids are supplied as vitamin A (retinol, Rol) which is taken up from food in the small intestine by Cellular Retinol-binding Protein 2 (CRBP2) and delivered by a Serum Retinol-binding Protein (SRBP, RBP4)/transthyretin via the choroidal vasculature to the target RPE cells. (2), SRBP-Rol binds to a cell surface receptor (p63) ( (2), (1), (3)) which supports unloading of Rol to SRBP Cellular Retinol-binding Protein 1 (CRBP1) (9).

CRBP1 is an intracellular transporter of atRol which carries the retinoid inside the cell. After esterification of atRol by LRAT Lecithin/Retinol Acyltransferase (LRAT) to form retinyl esters (Ry). The all-trans retinyl ester (atRy) is converted by Isomerohydrolase to 11cRy.

REH Ry is the major storage form of retinol in the RPE (99% of which 75% are stored in the 11cRy pool in RPE). For further steps in the biochemical pathways it is converted to 11cRol and an acyl rest by Retinol Ester Hydrolase (REH). ((14), (18)). This proteins is localised in RPE membranes.

11cRol, its aldehyde 11cRAl, and their all-trans isomers are different forms of retinoids cycling in the process of vision. 11cRol is oxidized by RDH5RDH11 11-cis Retinol Dehydrogenase (RDH) to 11cRAl in the RPE. 11-cis-RDH belongs to the family of short chain alcohol dehydrogenases. It is a integral membrane-bound stereospecific enzyme of the microsomal subfraction which uses NADH as electron source (19). 11-cis-RDH has been immunolocalised to the RPE (20).

CRAlBP 11cRol and 11cRAl are carried by Cellular Retinaldehyde-binding Protein (CRAlBP) to and from RDH (5). CRAlBP is a watersoluble intracellular protein that interacts closely with11-cis-RDH (7). It shows positive immunoreactivity in RPE and Müller cells (9). Additionally CRAlBP acts as a substrate transporter for several other proteins in the retinoid metabolism like Retinyl Ester Synthetase (6) , or LRAT.

In photoreceptor outer segments retinoids are bound to photo pigment apoproteins where they are used as light perceiving ligands. Photons lead to isomerization of 11cRAl to atRAl which changes the inactive form of RHO to the active meta II form. (see also: Visual Cascade)

For recovery of the chromophor atRAl is released from meta II to all-trans-RDH which catalyses the reduction of atRAl to atRol at very slow reaction rates in ROS using NADPH as a cofactor (11)RDH12. This is an indication that inside the retinol cycle more than one RDH are in use since 11-cis-RDH utilizes NADH as proton donor (19).

(11)ABCR/ABCA4In photoreceptors ABC-Cassette Transporter Retina (ABCR) is discussed to be involved in atRAl metabolism. According to Sun et al. ABCR might extract atRAl from ROS disks which is linked to PE to suppress biochemical noise in the visual cascade . Thus explaining A2-E (a derivate of atRAl and PE and a major component of lipofuscin) accumulation in RPE of Stargardt and AMD patients deficient of ABCR activity. A2-E accumulated in ROS disk would therefore be accumulated in RPE after digestion of ROS

atRol diffuses through the plasma membrane into the interphotoreceptor space where it is bound to Interphotoreceptor Retinoid-binding Protein (IRBP ) (16). IRBP , which is synthesised by the neural retina and secreted by photoreceptor cells, is the predominant protein of the IPM (8) (10). It predominantly binds atRAl and atRol (4), although IRBP can also bind other lipid ligands (13) . Due to its retinoid binding properties IRBP is the protein that shuttles the chromophore between the RPE and the photoreceptor outer segment, from and to regeneration (13).

Edwards et al. considered IRBP to contribute to an atRAl sink in IPM that constitutes a constant supply of chromophore to the photoreceptor and protection of retinoids from degradation.

RPE65 A meanwhile solved role in retinoid metabolism plays , a RPE-specific protein of microsomal membranes (12) . Recent data indicate RPE65 to be the central protein in atRol isomeration when it was identified as the long sought isomerohydrolase(15). is assisted by RDH5 and CRALBP since free 11-cis retinol is water insoluble and instable when not bound to a transport protein.

  1. Bavik,C.O., Busch,C., and Eriksson,U. Characterization of a plasma retinol-binding protein membrane receptor expressed in the retinal pigment epithelium Characterization of a plasma retinol-binding protein membrane receptor expressed in the retinal pigment epithelium. 1992; J.Biol.Chem. 267: 23035-23042.
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  2. Bavik,C.O., Eriksson,U., Allen,R.A., and Peterson,P.A. Identification and partial characterization of a retinal pigment epithelial membrane receptor for plasma retinol-binding protein. 1991; J.Biol.Chem. 266: 14978-14985.
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  3. Bavik,C.O., Levy,F., Hellman,U., Wernstedt,C., and Eriksson,U. The retinal pigment epithelial membrane receptor for plasma retinol-binding protein. Isolation and cDNA cloning of the 63-kDa protein. 1993; J.Biol.Chem. 268: 20540-20546.
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  4. Chen,Y. and Noy,N. Equilibrium and kinetic studies in the interactions of Interphotoreceptor Retinoid-binding Protein (IRBP). 1994; Invest.Ophthalmol.Vis.Sci. 35: S2061 Goto Top
  5. Crabb,J.W., Gaur,V.P., Garwin,G.G., Marx,S.V., Chapline,C., Johnson,C.M., and Saari,J.C. Topological and epitope mapping of the cellular retinaldehyde- binding protein from retina. 1991; J.Biol.Chem. 266: 16674-16683.
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  6. Crabb,J.W., Goldflam,S., Harris,S.E., and Saari,J.C. Cloning of the cDNAs encoding the cellular retinaldehyde-binding protein from bovine and human retina and comparison of the protein structures. 1988; J.Biol.Chem. 263: 18688-18692.
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  7. Crabb,J.W., Johnson,C.M., Carr,S.A., Armes,L.G., and Saari,J.C. The complete primary structure of the cellular retinaldehyde- binding protein from bovine retina. 1988; J.Biol.Chem. 263: 18678-18687.
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  8. Edwards,J.F., Storts,R.W., Joyce,J.R., Shelton,J.M., and Menzies,C.S. Juvenile-onset neuronal ceroid-lipofuscinosis in Rambouillet sheep. 1994; Vet.Pathol. 31: 48-54.
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  9. Eisenfeld,A.J., Bunt Milam,A.H., and Saari,J.C. Localization of retinoid-binding proteins in developing rat retina. 1985; Exp.Eye Res. 41: 299-304.
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  10. Gonzalez Fernandez,F., Kittredge,K.L., Rayborn,M.E., Hollyfield,J.G., Landers,R.A., Saha,M., and Grainger,R.M. Interphotoreceptor retinoid-binding protein (IRBP), a major 124 kDa glycoprotein in the interphotoreceptor matrix of Xenopus laevis. Characterization, molecular cloning and biosynthesis. 1993; Journal.of.Cell Science. 105: 7-21. Link Goto Top
  11. Haeseleer,F., Huang,J., Lebioda,L., Saari,J.C., and Palczewski,K. Molecular characterization of a novel short-chain dehydrogenase/reductase that reduces all-trans-retinal. 1998; J.Biol.Chem. 273: 21790-21799.
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  12. Hamel,C.P., Tsilou,E., Pfeffer,B.A., Hooks,J.J., Detrick,B., and Redmond,T.M. Molecular cloning and expression of RPE65, a novel retinal pigment epithelium-specific microsomal protein that is post- transcriptionally regulated in vitro. 1993; J.Biol.Chem. 268: 15751-15757.
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  13. Liou,G.I., Ma,D.P., Yang,Y.W., Geng,L., Zhu,C., and Baehr,W. Human interstitial retinoid-binding protein. Gene structure and primary structure. 1989; J.Biol.Chem. 264: 8200-8206.
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  14. Mata,J.R., Mata,N.L., and Tsin,A.T. Substrate specificity of retinyl ester hydrolase activity in retinal pigment epithelium. 1998; J.Lipid Res. 39: 604-612. Link Goto Top
  15. Moiseyev,G., Chen,Y., Takahashi,Y., Wu,B.X., and Ma,J.X. RPE65 is the isomerohydrolase in the retinoid visual cycle. 2005; Proc.Natl.Acad.Sci.U.S.A. 102: 12413-12418. Link Goto Top
  16. Palczewski,K., Jager,S., Buczylko,J., Crouch,R.K., Bredberg,D.L., Hofmann,K.P., Asson-Batres,M.A., and Saari,J.C. Rod outer segment retinol dehydrogenase: Substrate specificity and role in phototransduction. 1994; Biochemistry. 33: 13741-13750.
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  17. Pfeffer,B.A., Clark,V.M., Flannery,J.G., and Bok,D. Membrane receptors for retinol-binding protein in cultured human retinal pigment epithelium. 1986; Invest Ophthalmol.Vis.Sci. 27: 1031-1040. Link Goto Top
  18. Sierra,D.A., Tsin,A.T.C., Mata,N.L., and Villazana,E.T. Subcellular Distribution of 11-cis LRAT, 11-cis RD, and 11-cis REH in Bovine Pigment Epithelium. 1998; Invest.Ophthalmol.Vis.Sci. 39: S39 Goto Top
  19. Simon,A., Hellman,U., Wernstedt,C., and Eriksson,U. The retinal pigment epithelial-specific 11-cis retinol dehydrogenase belongs to the family of short chain alcohol dehydrogenases. 1995; J.Biol.Chem. 270: 1107-1112.
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  20. Simon,A., Lagercrantz,J., Bajalica Lagercrantz,S., and Eriksson,U. Primary structure of human 11-cis retinol dehydrogenase and organization and chromosomal localization of the corresponding gene. 1996; Genomics. 36: 424-430. Link Goto Top
  21. Sun,H., Molday,R.S., and Nathans,J. Retinal Stimulates ATP Hydrolysis by Purified and Reconstituted ABCR, the Photoreceptor-specific ATP-binding Cassette Transporter Responsible for Stargardt Disease. 1999; J.Biol.Chem. 274: 8269-8281.
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This site is maintained and edited by
Dr. rer. medic. Markus Preising, Dipl.Biol.
Molecular Genetics Laboratory
Department of Paediatric Ophthalmology, Strabismology and Ophthalmogenetics
University of Regensburg
Head: Prof. Dr. med. Birgit Lorenz