title: Breaking the covalent bond--a pigment property that contributes to desensitization in cones.
abstract: Retinal rod and cone pigments consist of an apoprotein, opsin, covalently linked to a chromophore, 11-cis retinal. Here we demonstrate that the formation of the covalent bond between opsin and 11-cis retinal is reversible in darkness in amphibian red cones, but essentially irreversible in red rods. This dissociation, apparently a general property of cone pigments, results in a surprisingly large amount of free opsin--about 10% of total opsin--in dark-adapted red cones. We attribute this significant level of free opsin to the low concentration of intracellular free 11-cis retinal, estimated to be only a tiny fraction (approximately 0.1 %) of the pigment content in red cones. With its constitutive transducin-stimulating activity, the free cone opsin produces an approximately 2-fold desensitization in red cones, equivalent to that produced by a steady light causing 500 photoisomerizations s-1. Cone pigment dissociation therefore contributes to the sensitivity difference between rods and cones.
authors.affiliation: Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA. vkefalov@jhmi.edu
authors.forename: Vladimir J
authors.initials: VJ
authors.lastname: Kefalov
authors.forename: Maureen E
authors.initials: ME
authors.lastname: Estevez
authors.forename: Massahiro
authors.initials: M
authors.lastname: Kono
authors.forename: Patrice W
authors.initials: PW
authors.lastname: Goletz
authors.forename: Rosalie K
authors.initials: RK
authors.lastname: Crouch
authors.forename: M Carter
authors.initials: MC
authors.lastname: Cornwall
authors.forename: King-Wai
authors.initials: KW
authors.lastname: Yau
mesh_terms: D000465:Algorithms; D000818:Animals; D002352:Carrier Proteins / Q000494:pharmacology; D002478:Cells, Cultured; D003116:Color; D003623:Dark Adaptation; D004224:Diterpenes; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D004594:Electrophysiology; D007814:Larva; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D010775:Photic Stimulation / Q000379:methods; D038761:Photobleaching / Q000187:drug effects / Q000528:radiation effects; D012160:Retina / Q000166:cytology*; D017949:Retinal Cone Photoreceptor Cells / Q000187:drug effects / Q000502:physiology*; D012168:Retinal Pigments / Q000502:physiology*; D012172:Retinaldehyde / Q000378:metabolism* / Q000494:pharmacology; D017299:Rod Opsins / Q000378:metabolism*; D013053:Spectrophotometry / Q000379:methods; D013997:Time Factors; D014562:Caudata; D014785:Vision, Ocular / Q000187:drug effects / Q000502:physiology
chemical_list: C020932:11-cis-retinal-binding protein; D002352:Carrier Proteins; D004224:Diterpenes; D012168:Retinal Pigments; D017299:Rod Opsins; C031390:9-cis-retinal; D012172:Retinaldehyde