Hydrophobicity of mucin-like glycoproteins secreted by cultured tracheal epithelial cells: association with lipids.

Confluent cultures of primary hamster tracheal surface epithelial cells are highly enriched with secretory cells resembling airway goblet cells. The cultures secrete lipids, most of which appear to be associated with high-molecular-weight mucinlike glycoproteins (HMW MLGP). In the present communication, we examined the nature of the association and analyzed the lipids associated with the secreted HMW MLGP. The HMW MLGP purified in the presence of 4 M guanidinium HCl were highly associated with lipids. When these HMW MLGP were gel filtered using a Sepharose CL-4B column in the presence of detergents, more than 97% of the associated lipids were dissociated from HMW MLGP and eluted in fractions with Kd = 0.70. On the other hand, when [3H]palmitic acid-labeled spent medium was gel filtered, column elution profiles varied among elution buffers, a major change being an appearance of a 3H peak at Kd = 0.70 in the presence of 50 mM sodium acetate, phosphate-buffered saline (PBS), or PBS/sodium dodecyl sulfate, but not with 4 M guanidinium HCl. Lipids associated with MMW MLGP under a "physiological" condition consisted of neutral, phospho-, and glycolipids. We conclude that (1) HMW MLGP secreted from cultured airway epithelial cells are extremely hydrophobic and associated with a variety of lipids, mostly via noncovalent binding, and (2) the degree of the lipid association seems to depend on the ionic environment of the solution.