Most of the lipid in purulent sputum is bound to mucus glycoprotein.

Mucus glycoprotein (mucin) is the principal biochemical constituent of sputum. Appreciable quantities of lipid, DNA, and nonmucin proteins are also present, particularly in purulent sputum. Previous studies have shown that purified mucin from respiratory tract secretions contains non-covalently bound lipid. However, it is not known whether lipids in purulent sputum are associated only with mucin or with nonmucin proteins and DNA as well. Purulent sputum was obtained from cystic fibrosis patients. Tracheal aspirates were obtained from noncystic patients with purulent secretions due to Pseudomonas species, as well as from noninfected, noncystic patients who had mucoid airway secretions. The lipid content of unfractionated airway secretions (sputum or tracheal aspirates), gel filtration-purified mucin, and nonmucin components of the airway secretions was analyzed. The purified mucin from all three groups had a significantly higher content of solvent-extractable lipid as compared to unfractionated airway secretions. The nonmucin fractions contained only small amounts of lipid. Density gradient centrifugation verified that the lipid recovered in the purified mucin fraction was complexed with the glycoprotein. The results of this study indicate that most of the lipids in purulent sputum are associated with mucin.