title: Fourth module of Xenopus interphotoreceptor retinoid-binding protein: activity in retinoid transfer between the retinal pigment epithelium and rod photoreceptors.
abstract: OBJECTIVE
  Interphotoreceptor retinoid-binding protein (IRBP), an extracellular protein believed to support the exchange of retinoids between the neural retina and retinal pigment epithelium (RPE) in the vertebrate eye, exhibits a modular, i.e., repeat, structure. The present study was undertaken to determine whether an individual module of IRBP has activity in retinoid transfer between the RPE and rod photoreceptors.
  
  
  METHODS
  The retinoid transfer activity of a recombinant protein corresponding to the fourth module of Xenopus laevis IRBP (X4IRBP) was examined in two ways. First, X4IRBP was tested for its ability to support the regeneration of porphyropsin in detached/reattached Xenopus retina/RPE-eyecups. Following illumination and removal of native IRBP, Xenopus eyecups supplemented with 42 microM X4IRBP or (as a control) Ringer's solution were incubated in darkness and then analyzed for regenerated porphyropsin. Second, toad (Bufo marinus) RPE-eyecup preparations were used to evaluate X4IRBP's ability to promote the release of 11-cis retinal from the RPE.
  
  
  RESULTS
  The regeneration of porphyropsin in X4IRBP-supplemented Xenopus retina/RPE-eyecups (0.45 +/- 0.04 nmol; mean +/- SEM, n = 11) exceeded that in controls (0.13 +/- 0.02 nmol, n = 11). For promoting the release of 11-cis retinal from the toad RPE, 42 microM X4IRBP was more effective than equimolar bovine serum albumin although considerably less than that of 26 microM native bovine IRBP.
  
  
  CONCLUSIONS
  The results indicate a low but significant activity of IRBP's fourth module in reactions relevant to retinoid exchange.
authors.affiliation: Department of Ophthalmology, University of Virginia Health Sciences Center, Charlottesville 22908, USA.
authors.forename: F
authors.initials: F
authors.lastname: Gonzalez-Fernandez
authors.forename: C A
authors.initials: CA
authors.lastname: Baer
authors.forename: E
authors.initials: E
authors.lastname: Baker
authors.forename: T I
authors.initials: TI
authors.lastname: Okajima
authors.forename: B
authors.initials: B
authors.lastname: Wiggert
authors.forename: M S
authors.initials: MS
authors.lastname: Braiman
authors.forename: D R
authors.initials: DR
authors.lastname: Pepperberg
mesh_terms: D000818:Animals; D002024:Bufo marinus; D002417:Cattle; D005136:Eye Proteins / Q000494:pharmacology*; D008024:Ligands; D016253:Microscopy, Immunoelectron; D010857:Pigment Epithelium of Eye / Q000187:drug effects* / Q000378:metabolism; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000494:pharmacology; D012168:Retinal Pigments / Q000502:physiology*; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D012172:Retinaldehyde / Q000378:metabolism*; D012177:Retinol-Binding Proteins / Q000378:metabolism / Q000494:pharmacology*; D014785:Vision, Ocular; D014982:Xenopus laevis; D000099214:Retinol-Binding Proteins, Interstitial
chemical_list: D005136:Eye Proteins; D008024:Ligands; D011994:Recombinant Proteins; D012168:Retinal Pigments; D012172:Retinaldehyde; D012177:Retinol-Binding Proteins; D000099214:Retinol-Binding Proteins, Interstitial; C010507:porphyropsin