Sucrase-alpha-dextrinase in diabetic BioBreed rats: reversible alteration of subunit structure

BioBreed (BB) Wistar rats develop diabetes mellitus, which closely resembles the human disease, in 50% of progeny. Intestinal sucrase-alpha-dextrinase, a glycoprotein hydrolase of the enterocyte's brush border consisting of 140-kDa alpha-dextrinase and 125-kDa sucrase subunits, is essential for surface digestion of carbohydrate nutrients. Although its catalytic characteristics were found to be maintained in the diabetic state, the structure of the subunits, as compared with normal Wistar rats, was altered in the BB rat within 2 days of the onset of diabetes. Its capacity to react in a solid-phase immunoassay was reduced by 50%; when examined by 6% acrylamide electrophoresis, the sucrase subunit was increased in mass by 5 kDa and, in some BB rats, the dextrinase subunit was reduced by 5 kDa. Intact rats labeled intraintestinally with [35S]methionine displayed the alteration within 6 h of synthesis, indicating that nonenzymatic glycosylation could not account for the structural change. This mass change was not seen in streptozotocin-induced diabetes and was independent of the plasma glucose concentration or the degree of acidosis. Deglycosylation with peptide N-glycosidase indicated that the N-linked chains of the normal dextrinase subunit (11 kDa) have twice the mass of those in the BB rat (6 kDa) and that the sucrase subunit may have an increased mass of O-linked chains. Overall, these experiments point to changes in glycosylation as a mechanism of structural alteration in congenital diabetes. Despite persistence of the insulin-dependent diabetes, the subunit pattern eventually became indistinguishable from normal, but at differential rates (21 days and 35 days, respectively, for sucrase and dextrinase subunits).