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.