BLUE AND ULTRAVIOLET LIGHT-ABSORBING OPSIN FROM THE RETINAL-PIGMENT EPITHELIUM

The retinal pigment epithelium (RPE) contains an abundant opsin that i s distinct from rhodopsin and cone visual pigments and is able to bind the retinaldehyde chromophore. The putative retinal G protein-coupled receptor (RGR) was isolated in digitonin solution from bovine RPE mic rosomes and copurified consistently with a minor 34-kDa protein. The a bsorption spectrum of RGR revealed endogenous pH-sensitive absorbance in the blue and near-ultraviolet regions of light, Membrane-bound RGR was incubated with exogenously added all-trans-retinal and formed two long-lived pH-dependent photopigments with absorption maxima of 469 +/ - 2.4 and 370 +/- 7.3 nm. The effects of hydrogen ion concentration su ggest that the blue and near-UV photopigments are tautomeric forms of RGR, in which an all-transretinal Schiff base is protonated or unproto nated, respectively. The RPE pigment was also demonstrable by its reac tivity to hydroxylamine in the dark. The retinaldehyde-RGR conjugate a t neutral pH favors the near-UV pigment and is a novel light-absorbing opsin in the vertebrate eye.