GLYCOLIPID AND GLYCOPROTEIN METABOLISM IN EYE TISSUE

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Little is known of the metabolic fate of rhodopsin following the phagocytosis by the retinal pigment epithelium (RPE) of rod outer segment disc membranes. Our previous studies have shown the presence in bovine RPE of an enzyme system which cleaves purine rhodopsin to a single glycopeptide. These studies were performed with rhodopsin labeled in its corbohydrate groups. We wish now to introduce a more general label into this molecule by reductive methylation and follow the fate of the rest of this glycoprotein. Speical attention will be paid to the capacity of the RPE of the dystrophic rat to carry out the catabolism of rhodopsin. This may provide information concerning the metabolic capacity of dystrophic ocular tissue not previously available. The carbohydrate chains of rhodopsin are unique as compared to other asparagine-linked glycoproteins. Their abridged size suggests the presence in the retina of special control mechanisms regulating their biosynthesis. We wish to investigate this process. We shall investigate the nature of the oligosaccharide-lipid precursors of the sugar chains of rhodopsin, as well as attempt to isolate "pre-rhodopsin" intermediates formed during the processing phase of rhodopsin glycosylation. We shall also investigate the presence of the activity of glycosyltransferases concerned with the final assembly of the biosynthesis of the saccharide chains of rhodopsin and the means used by the retina to control the nature of these groups. As a result of our previous studies dealing with the glycosylation of rhodopsin, evidence was obtained indicating the presence of unglycosylated rhodopsin in the rod outer segment disc membranes. We wish to explore this possibility further. The production of unglycosylated rhodopsin, as brought about by the use of the antibiotic, tunicamycin, will be examined in vitro in bovine retina and in the retina of the young rat, as well as in vivo in the rat. In addition to these studies, the effect of glycosylation on the stability of rhodopsin will be examined. Many of the aspects of the dolichol pathway of glycoprotein biosynthesis in the retinal have been described by our laboratory. We wish to continue our investigations dealing with mechanisms regulating the biosynthesis of the "early" GlcNAc-lipids.