Characterization, Localization, and Biosynthesis of an Interstitial Retinol‐Binding Glycoprotein in the Human Eye

: Human eyes contain an Mr 135K retinol‐binding protein that is analogous to interstitial retinol‐binding protein (IRBP) in the subretinal space of bovine eyes. It is a glycoprotein, because it binds 125I‐concanavalin A, 125I‐wheat germ agglutinin and 125I‐Lens culinaris hemagglutinin. It does not bind Ricinus communis agglutinin I. After desialation, it binds Ricinus communis agglutinin I, but loses its capacity to bind wheat germ agglutinin. These observations, coupled with the known specificities of these lectins, suggest that at least one of the oligosaccharide chains is a sialated, biantennary complex type containing fucose. Both by direct analysis of dissected ocular tissues and by immunocytochemistry it was shown that human interstitial retinol binding protein is an extracellular protein that is confined predominantly to the subretinal space. Monkey retinas incubated in vitro in medium containing [3H]leucine were shown to synthesize and secrete this protein into the medium, a conclusion that was confirmed by immunoprecipitation with an immunoglobulin fraction prepared from rabbit antibovine IRBP serum. Virtually no other labeled proteins were detectable in the medium. It is concluded that interstitial retinol‐binding protein meets many of the requirements for a putative transport protein implicated in the transfer of retinol between the pigment epithelium and retina during the visual cycle, and that the neural retina may play an important role in regulating its amount in the subretinal space.