title: Reduction of all-trans retinal to all-trans retinol in the outer segments of frog and mouse rod photoreceptors.
abstract: The first step in the Visual Cycle, the series of reactions that regenerate the vertebrate visual pigment rhodopsin, is the reduction of all-trans retinal to all-trans retinol, a reaction that requires NADPH. We have used the fluorescence of all-trans retinol to study this reduction in living rod photoreceptors. After the bleaching of rhodopsin, fluorescence (excitation, 360 nm; emission, 457 or 540 nm) appears in frog and wild-type mouse rod outer segments reaching a maximum in 30-60 min at room temperature. With this excitation and emission, the mitochondrial-rich ellipsoid region of the cells shows strong fluorescence as well. Fluorescence measurements at different emission wavelengths establish that the outer segment and ellipsoid signals originate from all-trans retinol and reduced pyridine nucleotides, respectively. Using outer segment fluorescence as a measure of all-trans retinol formation, we find that in frog rod photoreceptors the NADPH necessary for the reduction of all-trans retinal can be supplied by both cytoplasmic and mitochondrial metabolic pathways. Inhibition of the reduction reaction, either by retinoic acid or through suppression of metabolic activity, reduced the formation of retinol. Finally, there are no significant fluorescence changes after bleaching in the rod outer segments of Rpe65(-/-) mice, which lack 11-cis retinal.
authors.affiliation: Department of Physiology and Biophysics, University of Colorado School of Medicine, Denver, Colorado, USA.
authors.forename: Chunhe
authors.initials: C
authors.lastname: Chen
authors.forename: Efthymia
authors.initials: E
authors.lastname: Tsina
authors.forename: M Carter
authors.initials: MC
authors.lastname: Cornwall
authors.forename: Rosalie K
authors.initials: RK
authors.lastname: Crouch
authors.forename: Sukumar
authors.initials: S
authors.lastname: Vijayaraghavan
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Koutalos
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D005260:Female; D036681:Fluorescence Recovery After Photobleaching / Q000379:methods; D007536:Isomerism; D007700:Kinetics; D008027:Light; D008297:Male; D008657:Metabolic Clearance Rate; D051379:Mice; D008810:Mice, Inbred C57BL; D008856:Microscopy, Fluorescence / Q000379:methods; D009249:NADP / Q000378:metabolism*; D010084:Oxidation-Reduction / Q000528:radiation effects; D010786:Photoreceptor Cells; D011894:Rana pipiens; D012172:Retinaldehyde / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism* / Q000528:radiation effects*; D013045:Species Specificity; D014801:Vitamin A / Q000378:metabolism*
chemical_list: D014801:Vitamin A; D009249:NADP; D012172:Retinaldehyde