The presence of a soluble interphotoreceptor retinol‐binding protein (IRBP) in the retinal interphotoreceptor space

A new, gentle technique has been developed for washing of the retinal interphotoreceptor space (IPS) to obtain soluble components of the extracellular matrix (ECM). Using this method, we have determined that the major soluble constituent of monkey IPS is a 146,000 Mr glycoprotein, which binds 3Hretinol, sediments on sucrose gradients at 7S and has an Rf of 0.42 on native gel electrophoresis. Using size-exclusion high performance liquid chromatography, the apparent molecular weight of the native protein was calculated to be 250,000 daltons. In contrast to previous studies, no 15,000-dalton cellular retinol-binding protein (CRBP) or 33,000-dalton cellular retinaldehyde-binding protein (CRALBP) was observed in the IPS wash, indicating that these proteins are probably not involved in retinol transport between retina and pigment epithelium (PE). In the supernatant fraction of retinal homogenates that contains soluble intracellular proteins as well as extracellular constituents, the 146,000 Mr protein was closely associated with a 93,000 Mr protein that could be separated on SDS-gel electrophoresis; the 93,000 Mr protein was not found in the IPS wash. The 146,000 Mr interphotoreceptor retinol-binding protein (IRBP) may function in extracellular retinol transport in the IPS.