ABSTRACT
In order to maintain visual sensitivity in the light, bleached visual pigment molecules are continuously regenerated by providing them with fresh supplies of 11-ris-retinal. This cyclic process of bleaching and regeneration, isomerization to all-trans and re-isomerization to ll-cis, operates through the interplay of a number of enzymes working in conjunction with intracellular and extracellular retinoid transport proteins. Visual pigments are divisible into two large classes. The rhodopsins are based on 11-cis-retinal and the porphyropsins on 11-cis-3-dehydroretinal. The localization of retinol isomerase in the pigment epithelium, however, would explain the essential role of this layer in rhodopsin regeneration. Retinoids are insoluble and often susceptible to oxidative degradation. Some workers refer to interstitial retinol-binding protein (IRBP) as interphotoreceptor retinoid-binding protein. The transport vehicle, IRBP, has been purified, its primary structure determined and its endogenous retinoid ligands characterized during the physiological processes of light- and dark-adaptation.
