Intracellular retinol-binding proteins from bovine pigment epithelial and photoreceptor cell fractions. Purification of high molecular weight lipoglycoproteins.

Abstract

Cytosol retinol (vitamin A)-binding lipoglycoproteins were isolated from preparations of bovine retinal pigment epithelial cells and from rod photoreceptor outer segment fractions. The lipoglycoproteins had molecular weights of at least 1.5 X 10(6) as determined by gel filtration chromatography. The purified lipoglycoprotein which was obtained from pigment epithelial cells contains at least two kinds of polypeptide subunits (molecular weight about 35,000 and 150,000), while the rod outer segment cytosol lipoglycoprotein contains at least four kinds of polypeptide subunits (molecular weight about 50,000, 75,000, 120,000, and, 200,000). The pigment epithelial cytosol retinol-binding material contains about 30% (by weight) of lipid, while the material obtained from rod photoreceptor outer segment fractions contains about 65% (by weight) of lipid. Both lipoproteins contain predominantly cholesterol and phosphatidylcholine, while cholesterol esters, triglycerides, phosphatidylethanolamine, and phosphatidylserine are absent. The amino acid compositions of the delipidated proteins from pigment epithelial cells and rod outer segment fractions are similar. The neutral and amino sugars are co-valently linked to the polypeptide moiety of the molecule. The absorption and circular dichroism spectra of the lipoproteins obtained from pigment epithelial cells and rod outer segment fractions are different.