Studies on the Human Glomerular Basement Membrane: Composition, Nature of the Carbohydrate Units and Chemical Changes in Diabetes Mellitus

Abstract

The basement membrane has been isolated in purified form from pooled normal human glomeruli and shown by detailed analyses to be composed of glycoprotein material. The peptide portion was characterized by the presence of large amounts of glycine, as well as by the occurrence of a substantial number of hydroxyproline, hydroxylysine, and cystine residues. Approximately 7 per cent of the weight of the membrane was found to be made up of sugar residues. The membrane was extensively solubilized by digestion with bacterial collagenase, and a study of glycopeptides obtained after additional Pronase treatment indicated the occurrence of glucosylgalactose disaccharide units linked to hydroxylysine as well as heteropolysaccharides consisting of N-acetylneuraminic acid, fucose, galactose, mannose and hexosamines similar to those previously found in the bovine glomerular basement membrane. Chemical analyses of glomeruli obtained from diabetic and nondiabetic kidneys indicated that the former contained more basement membrane-like material. Furthermore basement membranes isolated from the diabetic glomeruli had a composition different from those obtained from control subjects. The diabetic membranes were found to have a significant increase (P < 0.01) in their hydroxylysine content and in the number of glucosylgalactose disaccharides linked to this amino acid. The increase in hydroxylysine was accompanied by a decrease in the lysine level so that the sum of these amino acids remained unchanged from the normal. Changes of lesser significance were observed in hydroxyproline, glycine, valine and tyrosine with the former two amino acids showing an increase and the latter two a decrease from the nondiabetic level. No change was found in the number of the heteropolysaccharide units. These results suggest an overproduction in the diabetic state of those subunits of the basement membrane that are rich in hydroxylysmejand its glycosidically linked disaccharide unit. Such alterations in the subunit composition may lead to an abnormal packing of the peptide chains and thereby form the basis for the detective filtration function observed in diabetes.