Structural analysis of the carbohydrate moieties of human tamm-horsfall glycoprotein

Abstract

Glycopeptides present in a pronase digest of human Tamm-Horsfall glycoprotein were fractionated by chromatography on DEAE-Sephadex A25 in 0.1 acetic acid. The separated glycopeptides were characterised by 500-MHz 1H-n.m.r. spectroscopy, in conjunction with sugar and amino acid analysis, and they were shown to be of the N-glycosylic, N-acetyllactosamine type. Each fraction consisted mainly of a tetra-antennary entity having various degrees of complexity, with lesser amounts of the triantennary structure, and even smaller amounts of the diantennary type. There was extensive heterogeneity in non-reducing terminal groups in each of the glycopeptides, whereas the peptide portions were similar. The extent to which any one of the galactose residues in the N-acetyllactosamine units was substituted, and the type of substitution, differed. The substituents were α-NeuAc-(2→6), α-NeuAc-(2→3), and α-NeuAc-(2→3)[β-GalNAc-(1→4)]. The carbohydrate moieties of the glycoprotein were heterogeneous also because of an uneven distribution of the fucose residues, which were attached to GlcNAc residues, both that linked to asparagine and one or more of those present in the N-acetyllactosamine units. The α-NeuAc-(2→3)[β-GalNAc-(1→4)]-β-Gal-(1→ sequence forms, at least in part, the Sda immunodeterminant. The pKa of the carboxyl group of the sialic acid residue in this entity is lower than that for molecules lacking GalNAc in this position. Thus, the difference in the number of Sda determinants carried by the glycopeptides enabled the latter to be fractionated on DEAE-Sephadex.