VITAMIN A RECEPTORS: RETINOL BINDING IN NEURAL RETINA AND PIGMENT EPITHELIUM

Abstract— With sucrose density gradient analysis, bovine retinal cytosol demonstrates 2S and 7S retinol-binding species; binding in pigment epithelial cytosol is predominantly to a 2S species. Binding of retinol to the 2S component in retina is unaffected by retinoic acid or retinyl palmitate whereas the ester effectively competes for 2S binding in pigment epithelium. Specific retinol binding can also be demonstrated by gel filtration on Sepharose 4B; no high molecular weight (> 100–200,000) retinol binding species are observed by this technique. Both the 2S and 7S binding species in retinal cytosol are protein in nature and differentially susceptible to proteolysis. The 2S and 7S species appear to be separate and distinct since chaotropic agents such as sodium thiocyanate or KCl and CaCl₂ do not seem to convert the 7S species into 2S subunits. Scatchard plot analysis indicates high affinity retinol binding to the 2S receptor. Computer analysis of the binding data yields K _a=3 × 10⁸, n = 1, a molecular size of 16,200 and ΔG⁰=?9.5 kcal/mol.