Biochemical and rheological characterization of sputum mucins from a patient with cystic fibrosis.

The studies described herein entail the isolation and characterization of native mucin isolated in the absence of disulfide reducing agents. The rheological characteristics of mucin closely resemble those of whole sputum. The aggregation of mucin molecules is responsible for the high viscosity of concentrated mucin solutions such as sputum. Disulfide reducing agents reduce the viscosity of concentrated mucin solutions by dissociation of soluble mucin aggregates without affecting the insoluble aggregates. Detailed chemical analyses of the mucins have been carried out and indicate that the reduction of interchain disulfides is accompanied, not only by a reduction in mucin viscosity, but by the liberation of two small proteins from the large mucin molecule. Detailed analyses of the cysteine plus cystine content of native mucin indicate that none of the free cysteines are accessible to alkylation in the absence of denaturants. Approximately 80% of the cystine content of the molecule is accessible to disulfide reduction in the absence of denaturants. This finding accounts for the ability of disulfide reducing agents to reduce sputum viscosity under nondenaturing conditions.