PURIFICATION, PROPERTIES, AND ANALYSIS OF HUMAN ASTHMATIC BRONCHIAL MUCIN

A high molecular weight, mucin-type glycoprotein has been isolated from a sample of human bronchial secretion obtained from an asthmatic patient. The glycoprotein elutes in the void volume of a Sepharose 4B column and its mobility is unchanged in the presence of dithiothreitol. Examination of the material in the analytical ultracentrifuge under equilibrium conditions gave an estimated minimal molecular weight of 1.8 × 106 with aggregation to 10 × 106 or greater. Analysis showed the predominant amino acids to be serine, threonine and proline with a low content of methionine and cysteine; glucosamine and galactosamine were present in approximately equimolar amounts and comprised 28% by weight of the glycoprotein. Composition analysis after alkaline borohydride treatment showed that the saccharide chains were O-glycosidically linked through N-acetylgalactosamine to both serine and threonine residues in the peptide backbone. Carbohydrate analysis by gas-liquid chromatography identified galactose, fucose, glucosamine, galactosamine and sialic acid in an approximate molar ratio of 3:3:2:2:1. The sialic acid is present as N-acetylneuraminic acid. A portion (7%) of the saccharides are present as galactosyl→N-acetyl-galactosaminyl residues linked to the protein core. A glycopeptide fraction was isolated following pronase digestion and had a molecular weight of 1.5 × 105. This value was not significantly changed by either removal of sialic acid or exposure to guanidinium chloride. These data support the presence of large clusters of oligosaccharides which are covalently linked to the serine and threonine residues of the peptide. © 1979, American Chemical Society. All rights reserved.