An alteration in intestinal membrane glycoprotein hydrolases has been identified in the spontaneously diabetic Biobreed (BB) rat. At least two membrane glycoproteins, sucrase--dextrinase and aminooligopeptidase, are assembled intracellulary and transferred to the membrane as abnormally large macromolecules. Unlike non-enzymatic glycosylation that occurs very slowly as a function of tissue glucose levels, the change in N-or O-linked glycosylation in spontaneous diabetes occurs co-or post-translationally during intracellular assembly. No similar glycoprotein abnormality was found in animals with chemical diabtes due to streptozotocin. Studies have been designed to establish the molecular basis for this glycoprotein abnormality. Membrane glycoproteins (aminooligopeptidase; sucrase-Alpha-dextrinase) from rat intestine (Wistar, BB control and BB diabetic) and from liver and kidney (alkaline phosphatase and leucine aminopeptidase) will be isolated by immunoprecipitation from ER-Golgi, brush border and plasma membranes and their carbohydrate chains characterized by gel filtration, serial lectin affinity chromatography, and QAE-Sephadex chromatography. High performance liquid chromatography will be used before and after exposure to glycosidase probes for final characterization. The glycosylation and deglycosylation pathways involved in co-and post-translational addition of N-linked carbohydrate chains will be studied by step-by-step assay of the appropriate enzymes in purified ER, Golgi and surface (plasma and brush border) membranes. After the abnormality has been characterized in the diabetic BB rat, normal and diabetic human tissues recovered from surgery or autopsy will be studied using slight modifications of the same techniques of oligosaccharide structure. These studies hold promise of elucidating one or more defects in the N- or O-linked glycosylation of glycoproteins in diabetes mellitus. Such alteration in oligosaccharide chains of membrane glycoproteins may be important for the pathogenesis of multisystem damage in the diabetic condition.
