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 retinas, structural studies of their N-linked sugar chains were performed. The sugar moieties, quantitatively released as oligosaccharides from the polypeptide backbone by hydrazinolysis, were converted to radioactive oligosaccharides by reduction with NaB3H4 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 derivatives. Structural studies of each oligosaccharide by lectin affinity column chromatography, and sequential exoglycosidase digestion in combination with methylation analysis, revealed that almost all of the oligosaccharides were hybrid-type sugar chains. While the major oligosaccharide species of bovine and human rhodopsin are identical, in contrast to the sugar chains of bovine rhodopsin, human rhodopsin also contains sialylated isomers and a high concentration of a galactosylated isomer. These results suggest that species-specific processing of the sugar chains of rhodopsin occurs.