Vitamin A is delivered to retinal pigment epithelium cells (RPE) as the alcohol form. Regeneration of visual pigments requires retinoid isomerization, oxidation of the alcohol to the aldehyde form, and delivery of specific retinoid forms to the retina. A """"""""visual cycle"""""""" involving suttling of retinoids from phororeceptor outer segments (ROS) to RPE and back again during cycles of light and dark exposure is known to exist in the eye. This movement requires transfer of retinoids, which are poorly soluble in water, through an aqueous phase, the interphotoreceptor matrix (IPM). It had been suggested, therefore, that the interphotoreceptor retinoid binding protein (IRBP), a soluble IPM protein that binds retinoids, serves as a delivery vehicle in this cycle. The overall goal of this project is to study the mechanism by which retinoids are transported inside the eye an the role of IRBP in this process. To investigate these questions, the physical-chemical characteristics of the interactions of all-trans- and 11-cis-retinol and retinaldehyde with the various environments which they encounter in the eye: cellular membranes, binding protein and water will be studied. The rate constants of the dissociation and association of retinoids interacting with synthetic lipid bilayers, with the outer membranes of RPE and ROS, and with IRBP will be measured. Equilibrium binding affinities and the thermodynamic properties of binding of retinoids to these phases will also be studied. Rates of direct transfer of retinoids between IRBP and the membranes will be measured in order to clarify whether such transfer occurs spontaneously, or whether direct interactions between IRBP and the membranes of target cells at either end of the IPM play a role in the in situ movement of retinoids. The process of diffusion of retinoids in aqueous phases will be studied in detail. Diffusion constants of retinoids will be measured in the presence of various concentrations of proteins and dextrans (macromolecules that do not bind retinoids) to clarify the effects of the properties of an aqueous phase (e.g. viscosity, presence of obstructing macromolecules) on retinoid diffusion. The effects of IRBP and of serum albumin (a """"""""non-specific"""""""" retinoid binding protein) on diffusion under various conditions will be studied. These studies will establish the factors that determine rates and extents of retinoid distribution in the eye and will help to clarify parameters that govern the diffusion of small hydrophobic ligands in aqueous phases in vivo.
