Structural studies of the N-linked sugar chains of human rhodopsin

Human rhodopsin is a glycoprotein containing two N-linked sugarchains. After the isolation and purification of rhodop-sinsfrom human retinas, structural studies of their N-linked sugarchains were performed. The sugar moieties, quantitatively releasedas ollgosaccharides from the polypeptide backbone by hydrazmolysis,were converted to radioactive oligosaccharides by reductionwith NaB³H₄ after N-acetylation. As indicated by high-voltagepaper electrophoresis, 96% of the sugar chains were free ofsialic add and the remaining were sialylated derivatives. Structuralstudies of each oligosaccharide by lectin affinity column chromatography,and sequential exoglycosidase digestion in combination withmethylation analysis, revealed that almost all of the oligosaccharideswere hybrid-type sugar chains. While the major oligosaccharidespecies of bovine and human rhodopsin are identical, in contrastto the sugar chains of bovine rhodopsin, human rhodopsin alsocontains sialylated isomers and a high concentration of a galactosylatedisomer. These results suggest that species-specific processingof the sugar chains of rhodopsin occurs. galactosylation human rhodopsin hybrid-type sugar chain N-linked oligosaecharide