Fibronectin: source of mannose in a highly purified respiratory mucin

Human bronchial mucin, solubilized in an aqueous solution of sodium azide and protease inhibitors, was purified by molecular sieve chromatography. The mucin was purified by Sepharose 4B and 2B column chromatography. Chemical analyses of this preparation showed a typical mucin profile of amino acids and carbohydrates, except for the presence of an appreciable amount of mannose. Sodium dodecyl sulfate-polyacrylamide gel (5%) electrophoresis of this material showed a high Mr glycoprotein at the top of the gel and two additional bands with mobilities of fibronectin subunits (230 and 210 kD). The fibronectin was separated from the mucin by geletin-Sepharose column chromatography, and the fibronectin eluted from the column was immunologically similar to fibronectin purified from human serum. Ion-exchange chromatography of purified mucin resulted in neutral and acidic fractions. The neutral mucin was the major component. Chemical composition of these two fractions indicated that the amount of threonine, serine, and sialic acid was higher in the acidic fraction, whereas the neutral fraction contained more proline, aspartic acid, leucine, glycine, fucose, and galactose than the acidic fraction.