STRUCTURAL STUDIES OF THE N-LINKED SUGAR CHAINS OF HUMAN RHODOPSIN

Human rhodopsin is a glycoprotein containing two N-linked sugar chains . After the isolation and purification of rhodopsins from human retina s, structural studies of their N-linked sugar chains were performed. T he sugar moieties, quantitatively released as oligosaccharides from th e polypeptide backbone by hydrazinolysis, were converted to radioactiv e oligosaccharides by reduction with (NaBH4)-H-3 after N-acetylation. As indicated by high-voltage paper electrophoresis, >96% of the sugar chains were free of sialic acid and the remaining were sialylated deri vatives. Structural studies of each oligosaccharide by lectin affinity column chromatography, and sequential exoglycosidase digestion in com bination with methylation analysis, revealed that almost all of the ol igosaccharides were hybrid-type sugar chains. While the major oligosac charide species of bovine and human rhodopsin are identical, in contra st to the sugar chains of bovine rhodopsin, human rhodopsin also conta ins sialylated isomers and a high concentration of a galactosylated is omer. These results suggest that species-specific processing of the su gar chains of rhodopsin occurs.