Galactosylation of rhodopsin by the human retina

The major oligosaccharide chains of bovine, frog and human rhodopsins are abridged, hybrid, asparagine-linked structures that contain only mannose and N-acetylglucosamine as the constituent sugars. Isomers that also contain galactose have been detected in varying amounts in rhodopsins from different species. As part of studies to examine the mechanisms used by the human retina to control the glycosylation of rhodopsin, the kinetics of the retinal galactosyltransferase was examined using Golgi-enriched fractions of human retina as the enzyme source and rhodopsin, opsin, and the oligosaccharide isolated from rhodopsin as acceptors. These reactions were compared to those using bovine retina and rat liver Golgi. A comparison of the Vmax/Km ratios revealed that the efficiency of the human retinal galactosyltransferase was some 20-fold lower than that of the rat liver. In addition, consistent with the higher state of galactosylation of human as compared to bovine rhodopsin, greater efficiency was observed with the human preparation. While the conformation of the visual pigment exerted an effect, its low galactosylation was not due to a major directing influence on glycosylation by the polypeptide matrix as indicated by the even lower activity toward the isolated oligosaccharide. The galactosylated oligosaccharides obtained by these procedures were identified by chromatographic methods. The relatively low activity of the retinal galactosyltransferases observed in this study may help explain the limited glycosylation that is typical of rhodopsin.