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 (∼0.1 %) of the pigment content in red cones. With its constitutive transducin-stimulating activity, the free cone opsin produces an ∼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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