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

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

Neuron (2005-06-15)
Vladimir J Kefalov, Maureen E Estevez, Massahiro Kono, Patrice W Goletz, Rosalie K Crouch, M Carter Cornwall, King-Wai Yau
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.

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9-cis-Retinal, vitamin A analog
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