Identification of MUC5B, MUC5AC and small amounts of MUC2 mucins in cysticfibrosis airway secretions

To investigate the genetic identities of the mucins secreted in cystic fibr osis (CF) airways, sputum was collected from five individuals. Samples were separated into gel and sol phases by high-speed centrifugation and the gel phase was extracted in 6 M guanidinium chloride. The 'insoluble' residue r emaining after extraction of the gel phase was brought into solution by red uction/alkylation. Density-gradient centrifugation in CsCl revealed polydis perse distributions of sialic acid-containing mucins in the gel phase, inso luble residue and sol phase fractions and the degree of variation between t he different individuals was low. Antibodies recognizing MUC5AC and MUC5B i dentified these mucins in each of the fractions. MUC2, however, was present only as a component of the insoluble residue from the gel which accounted for less than 4 % by mass of the total mucins. MUC5B and MUC5AC from the ge l phase were large oligomeric species composed of disulphide-bond linked su bunits and MUC5B was present as two populations with different charge densi ties which are likely to correspond to MUC5B 'glycoforms'. The sol phase co ntained, in addition to MUC5AC and MUC5B, mainly smaller mucins which did n ot react with the antisera and which were probably degraded. MUC5AC appeare d to be enriched in the sol, suggesting that this mucin may be more suscept ible to proteolytic degradation than MUC5B. The mucins present in sputum re mained broadly similar during acute exacerbation and following antibiotic t reatment, although the relative amount of an acidic MUC5B glycoform was dec reased during infection.