Structural characterization of the oligosaccharide chains of human alpha(1)-microglobulin from urine and amniotic fluid

Human alpha(1)-microglobulin (alpha(1)-m; also called protein HC), a glycop rotein belonging to the lipocalin superfamily, was isolated by sequential a nion-exchange chromatography and gel filtration from the urine of hemodiali zed patients and from amniotic fluid collected in the week 16-18 of pregnan cy. The carbohydrate chains of the protein purified from the two sources, w hich are organized in two Asn-linked and one Thr-linked oligosaccharides, w ere structurally characterized using matrix-assisted laser desorption ioniz ation and electrospray mass spectrometry. The glycans attached to Thr5 are differently truncated NeuHexHexNAc sequences, and O-glycosylation in the am niotic fluid protein is only partial. Asn96 has both diantennary and triant ennary structures attached in the case of urinary alpha(1)-m and only diant ennary glycans in the amniotic fluid protein. The main carbohydrate units a ttached to Asn17 are in both proteins monosialylated and disialylated diant ennary glycans. The position of the oligosaccharide chains in a three-dimen sional model of the protein, produced using the automated Swiss-Model servi ce, is also discussed.