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IRBP?-11cRCHO

interphotoreceptor matrix

atROH-IRBP?

Figure 3 Model of the mammalian visual cycle. ABCR, ATP-binding cassette; atRCHO, all-frans-retinal; atROH, all-frans-retinol; atRE, all-frans-retinyl esters; 11cRCHO, 11-c/s-retinal; 11cROH, 11-c/s-retinol; CRALBP, cellular retinal binding protein; CRBP(I), cellular retinol binding protein, type I; IRBP, interphotoreceptor retinoid binding protein; IMH, isomerohydrolase; LRAT, lecithin:retinol acyltransferase; ROS, rod outer segment; RPE, retinal pigment epithelium; RPE65, RPE protein 65; SRBP, serum retinol binding protein.

Figure 3 Model of the mammalian visual cycle. ABCR, ATP-binding cassette; atRCHO, all-frans-retinal; atROH, all-frans-retinol; atRE, all-frans-retinyl esters; 11cRCHO, 11-c/s-retinal; 11cROH, 11-c/s-retinol; CRALBP, cellular retinal binding protein; CRBP(I), cellular retinol binding protein, type I; IRBP, interphotoreceptor retinoid binding protein; IMH, isomerohydrolase; LRAT, lecithin:retinol acyltransferase; ROS, rod outer segment; RPE, retinal pigment epithelium; RPE65, RPE protein 65; SRBP, serum retinol binding protein.

Retinoids do not require IRBP for transfer between the two membranes, however, because IRBP null mice show no gross abnormalities of the visual cycle, even though they have severe retinal abnormalities. Although retinoids have very limited solubility in the aqueous phase, membranes of the RPE and ROS transfer retinoids between them in the absence of IRBP.

In the ROS, 11cRCHO forms a Schiff's base adduct with a lysine residue in the protein opsin to create rhodopsin. Light isomerizes the 11cRCHO of rhodopsin into atRCHO. The isomerization straightens the retinoid side chain, changing the conformation of rhodopsin. This conformation change initiates a nerve impulse through G proteins and releases atRCHO, regenerating opsin. An ATP transporter ABCR facilitates leaching of atRCHO from rhodopsin, and an ER enzyme, prRDH (photo-receptor retinol dehydrogenase), also an SDR, reduces atRCHO into atROH. atROH cycles back to the RPE and binds with CRBP(I).

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