Regulatory Influences on the Glycosylation of Rhodopsin By Human and Bovine Retinas
The retina has assembled oligosaccharide chains on rhodopsin that are unlike those found in other asparagine-linked, complex class of glycoproteins in terms of their abridged size and limited number of sugars. We have examined the kinetic properties of two glycosyltransferases of the retina whose action would be required in order to synthesize oligosaccharide chains of rhodopsin which would be similar in structure to those found in other molecules of this type. We have examined the kinetics of N-acetylglucosaminyltrans-ferase II (GlcNAc-transferase II), required for branching, and galactosyltransferase, required for extending the oligosaccharide chain. Golgi-enriched preparations from human retina, bovine retina and rat liver were used as the enzyme sources and rhodopsin, opsin, and the oligosaccharide isolated from rhodopsin used as acceptors. From an evaluation of the Vmax/Km ratios it was observed that bovine and human retinas have very limited abilities compared to the rat liver to carry out the transfer of GlcNAc and galactose to these acceptors. In keeping with the presence of a relatively high concentration of galactosylated isomers found in human rhodopsin, human retinas had up to 13-fold greater activity of galactosyltransferase than did bovine retina. It was also apparent that the glycosyltransferases were not appreciably influenced by the polypeptide portion of the molecule. A further aspect of metabolic regulation was revealed with the observation that galactosylation of rhodopsin blocked the addition of GlcNAc and thus prevented branching of the oligosaccharide chain. It is suggested that these properties contribute to the assembly of the abridged structures that have been observed in the oligosaccharides of rhodopsins of all species thus far examined. The identification of the substrates and products of the reactions was carried out by chromatographic means.
KeywordsRetinitis Pigmentosa Visual Pigment Human Retina Oligosaccharide Chain Bovine Rhodopsin
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