Bovine transferrin glycopeptide: the relevance of its structure to interaction with the mammalian hepatic lectin that binds asialoglycoproteins

Abstract

After proteolytic digestion of bovine transferrin (phenotype AA), a glycopeptide fraction was isolated by gel filtration and high-voltage electrophoresis. Two glycopeptide bands were recovered, each of which contained 1 residue of aspartic acid, 2 of serine, 2 of galactose, 3 of mannose and 4 of N-acetyl glucosamine together with some fucose. However, the bands differed with respect to sialic acid content (2 and 3 residues respectively). Using the same isolation procedures, a commercial sample of human transferrin yielded 4 glycopeptide bands (2 major, 2 minor) on high-voltage electrophoresis. On analysis, the 2 major bands differed in amino-acid residues but their carbohydrate compositions were very similar. The analyses largely resembled previously published analytical data for 2 human transferrin glycopeptides and the carbohydrate analyses showed many similarities to the bovine transferrin glycopeptide. These data and earlier observations made on bovine and human transferrins indicate that, in contrast with human transferrin, bovine transferrin contains only 1 heterosaccharide chain per molecule. Following tritiation, bovine transferrin asialoglycopeptide was compared with asialoglycopeptides from other serum proteins for their affinity to the rat liver in vivo. Considerable differences were observed and the following order of binding was established for the asialoglycopeptide preparations: bovine transferrin < human transferrin .mchlt. bovine fetuin .mchlt. human .alpha.1-acid glycoprotein.