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

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