title: Rod outer segment retinol formation is independent of Abca4, arrestin, rhodopsin kinase, and rhodopsin palmitylation.
abstract: OBJECTIVE
  The reactive aldehyde all-trans retinal is released in rod photoreceptor outer segments by photoactivated rhodopsin and is eliminated through reduction to all-trans retinol. This study was undertaken to determine whether all-trans retinol formation depends on Abca4, arrestin, rhodopsin kinase, and the palmitylation of rhodopsin, all of which are factors that affect the release and sequestration of all-trans retinal.
  
  
  METHODS
  Experiments were performed in isolated retinas and single living rods derived from 129/sv wild-type mice and Abca4-, arrestin-, and rhodopsin kinase-deficient mice and in genetically modified mice containing unpalmitylated rhodopsin. Formation of all-trans retinol was measured by imaging its fluorescence and by HPLC of retina extracts. The release of all-trans retinal from photoactivated rhodopsin was measured in purified rod outer segment membranes according to the increase in tryptophan fluorescence. All experiments were performed at 37°C.
  
  
  RESULTS
  The kinetics of all-trans retinol formation in the different types of genetically modified mice are in reasonable agreement with those in wild-type animals. The kinetics of all-trans retinol formation in 129/sv mice are similar to those in C57BL/6, although the latter are known to regenerate rhodopsin much more slowly. The release of all-trans retinal from rhodopsin in purified membranes is significantly faster than the formation of all-trans retinol in intact cells and is independent of the presence of the palmitate groups.
  
  
  CONCLUSIONS
  The regeneration of rhodopsin and the recycling of its chromophore are not strongly coupled. Neither the activities of Abca4, rhodopsin kinase, and arrestin, nor the palmitylation of rhodopsin affects the formation of all-trans retinol.
authors.affiliation: Department of Ophthalmology, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
authors.forename: Lorie R
authors.initials: LR
authors.lastname: Blakeley
authors.forename: Chunhe
authors.initials: C
authors.lastname: Chen
authors.forename: Ching-Kang
authors.initials: CK
authors.lastname: Chen
authors.forename: Jeannie
authors.initials: J
authors.lastname: Chen
authors.forename: Rosalie K
authors.initials: RK
authors.lastname: Crouch
authors.forename: Gabriel H
authors.initials: GH
authors.lastname: Travis
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Koutalos
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000818:Animals; D019393:Arrestin / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D051606:G-Protein-Coupled Receptor Kinase 1 / Q000378:metabolism*; D054878:Lipoylation; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D012243:Rhodopsin / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D014801:Vitamin A / Q000096:biosynthesis*
chemical_list: D018528:ATP-Binding Cassette Transporters; C512841:Abca4 protein, mouse; D019393:Arrestin; D014801:Vitamin A; D012243:Rhodopsin; D051606:G-Protein-Coupled Receptor Kinase 1