Breaking the covalent bond--a pigment property that contributes to desensitization in cones

Neuron. 2005 Jun 16;46(6):879-90. doi: 10.1016/j.neuron.2005.05.009.


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.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Algorithms
  • Animals
  • Carrier Proteins / pharmacology
  • Cells, Cultured
  • Color
  • Dark Adaptation
  • Diterpenes
  • Dose-Response Relationship, Drug
  • Dose-Response Relationship, Radiation
  • Electrophysiology
  • Larva
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Membrane Potentials / radiation effects
  • Photic Stimulation / methods
  • Photobleaching / drug effects
  • Photobleaching / radiation effects
  • Retina / cytology*
  • Retinal Cone Photoreceptor Cells / drug effects
  • Retinal Cone Photoreceptor Cells / physiology*
  • Retinal Pigments / physiology*
  • Retinaldehyde / metabolism*
  • Retinaldehyde / pharmacology
  • Rod Opsins / metabolism*
  • Spectrophotometry / methods
  • Time Factors
  • Urodela
  • Vision, Ocular / drug effects
  • Vision, Ocular / physiology


  • 11-cis-retinal-binding protein
  • Carrier Proteins
  • Diterpenes
  • Retinal Pigments
  • Rod Opsins
  • 9-cis-retinal
  • Retinaldehyde