Preferential transport of non-enzymatically glucosylated ferritin across the kidney glomerulus

We have examined the significance of carbohydrate structure to the transglomerular passage of proteins. Carbohydrate-free (non-glycosylated) ferritin, prepared by Concanavalin-A-sepharose affinity chromatography, was perfused into rat kidneys, and was observed to be restricted from transglomerular transport and to accumulate within the lamina rara interna of the glomerular basement membrane. Visibility of the laminar structure of the glomerular basement membrane was enhanced following perfusion fixation containing tannic acid, permitting the observation of charge dense regions within the basement membrane. Non-enzymatically glucosylated ferritin was not restricted by the lamina rara interna and was observed to penetrate the lamina densa and lamina rara externa. Glucosylated ferritin was observed to be sequestered also by epithelial pinocytic vesicles and to be accumulated within multivesicular bodies. Quantitative measurements using fluorescently labelled ferritins indicated the preferential clearance of glucosylated ferritin from the plasma and preferential appearance of glucosylated ferritin in the urine. The differential transport of glucosylated ferritin was not due to the formation of a cationic protein, as isoelectric focusing established that glucosylation of ferritin results in a more anionic protein. These studies suggest that glucosylation of anionic proteins results in their increased transglomerular permeability. This increased protein permeability could contribute to the proteinuria observed in diabetic microangiopathy.