Vitamin a transport between retina and pigment epithelium—an interstitial protein carrying endogenous retinol (interstitial retinol-binding protein)

Abstract

We have demonstrated and partially characterized an interstitial retinol-binding protein (IRBP) confined to bovine interphotoreceptor matrix (IPM). The native protein is a concanavalin A-binding glycoprotein with a mol. wt of 260 k as measured by gel-filtration and size-exclusion high-performance liquid chromatography. On SDS-gels, its mol. wt is 140–145 k. Since the protein is glycosylated, this value is probably too high. Hence the native protein may be a dimer consisting of two identical subunits. The endogenous ligand has been analyzed by high-performance liquid chromatography—it consists mainly of all- trans retinol. Occasionally, retinal and 11- cis retinol are also associated with it. The amount of retinol bound to IRBP increases when the eyes are illuminated. The total binding capacity was estimated to represent 4–5% of the retinol released from a total rhodopsin bleach. We have established that, like serum retinol-binding protein, IRBP can also bind retinoic acid, although it has not been established that retinoic acid is an endogenous ligand. The fluorescence emission λ max for IRBP with its native ligand is at 470 nm and the excitation λ max for this fluorescence is at 333 nm. Other retinoid carriers in the interphotoreceptor matrix have molecular weights of about 15 and 33 k. These probably correspond to cellular retinol- and retinal-binding proteins, respectively. Since both proteins have been identified in the pigment epithelium and retina cytosols, their presence in the IPM could be a result of cell damage. We conclude that interstitial retinol-binding protein is the best candidate for a transport protein carrying retinol between the rod outer segments and the pigment epithelium.