Primary Structure of the N-Glycosidically Linked Sialoglycans of Secretory Immunoglobulins A from Human Milk

The alkali-stable sialoglycopeptides of secretory immunoglobulins A from human milk have been separated from the alkali-labile glycopeptides by gel filtration and from the asialoglycopeptides by ion-exchange chromatography. The structures of five of them have been determined on the basis of the results obtained by methylation analysis, mass spectrometry and 360 MHz 1H-NMR spectroscopy. For glycopeptide B, the following structure has been found: (formula; see text) The other glycopeptides can be considered as extensions of this structure. The following extensions to Gal-6' are proposed: NeuAc(alpha 2-6) (glycopeptide A), Gal(beta 1-3) (glycopeptide D) and Fuc(alpha 1-6) (glycopeptide E). Furthermore, in glycopeptide C a fucose residue in (alpha 1-3) linkage to GlcNAc-5' could be traced.