BIOCHEMICAL STUDIES ON HUMAN TRACHEOBRONCHIAL MUCIN GLYCOPROTEINS

Two mucin glycoproteins with distinct amino acid and carbohydrate compositions were isolated from normal tracheobronchial secretions after gel filtration and CsBr density gradient centrifugation. The buoyant density difference between the high (1.46) and low (1.34) density, carbohydrate-rich mucin glycoproteins could be accounted for by differences in the amount of non-covalently associated lipid. The low density mucin glycoprotein contained three times more non-covalently associated lipid than the high density mucin glycoprotein. Analysis of mucin-associated lipid indicated the presence of neutral lipids, glycolipids and phospholipids.;Both high and low density mucin glycoproteins were large, with molecular weights ranging from 1-7 x 10('6), and exhibited considerable polydispersity and/or aggregation. The molecular size and polydispersity/aggregation were decreased by reduction/alkylation or performic acid oxidation, suggesting a role for disulfide cross-linking in the maintenance of mucin structure.;A glycopeptide of molecular weight (TURN)200,000 was isolated from high density mucin glycoprotein after digestion with Pronase. This glycopeptide was highly enriched in serine and threonine, but contained only trace amounts of cysteine; the glycopeptide exhibited negligible polydispersity. These studies suggested a mucin structure which consists of clustered carbohydrate-rich and carbohydrate-poor (or free) domains. The carbohydrate-poor domains are involved in intramolecular disulfide cross-linking.;Deglycosylation of high density mucin glycoprotein with trifluoromethanesulfonic acid resulted in 60% loss of total carbohydrate from the glycoprotein. The molecular weight of the deglycosylated mucin was (TURN)600,000; polydispersity and/or aggregation were negligible. These results implied a role for carbohydrate self-association in the maintenance of the mucin structure.;The oligosaccharides of high density mucin glycoprotein were heterogeneous in size and charge. By a combination of anion exchange chromatography, gel filtration and thin layer chromatography, at least 50 oligosaccharides were detected which ranged in size from di- to saccharides containing 10 or more sugars. Both neutral oligosaccharides and acidic oligosaccharides (containing sulfate and/or sialic acid) were found.;Studies using primary explants of tracheal epithelia cultured in vitro indicated the presence of components similar in molecular size and buoyant density to those isolated from tracheobronchial secretions.