Interphotoreceptor retinol-binding proteins: possible transport vehicles between compartments of the retina

In the eye, vitamin A (retinol) is mainly stored in the retinal pigment epithelium(RPE) although its primary function is in the visual process in the photoreceptor organelles of the neural retina (NR). It is well established that during light adaptation, the amount of retinol drops in the NR but rises in the RPE. During dark adaptation, the converse occurs. This indicates a migration of retinoid between the two tissues, the direction of which is dictated by the state of light or dark adaptation of the photoreceptors. The mechanism by which this migration is effected is unknown. We now present evidence that at least one protein exists in the subretinal space or on the cell surfaces which demonstrates many of the unique characteristics one would expect of an interphotoreceptor retinol-binding protein and could function as a vitamin A transport vehicle between NR and RPE.