Mucin: Aggregation and colloidal interactions of relevance to some biomedical problems

Mucus is the viscoelastic secretion that lines, most epithelial surfaces forming a protective, lubricating barrier. The viscoelastic properties of mucus arise from mucin, a glycoprotein of molecular weight ranging from 2–10 million. In this paper we address two problems where the aggregation and interactions of mucin with colloidal particles is of physiological relevance. The first deals with gastric mucin and the question of the mechanisms that prevent the stomach from digesting itself. Using dynamic light scattering techniques we show that solutions of gastric mucin aggregate below pH 4. Very large aggregates with 100-fold slower diffusion constants than the mucin monomer are observed at pH 2. Viscosity measurements indicate that mucin will eventually gel at low pH, thus acting as a diffusional, barrier and protecting the stomach. The second problem is concerned with the role of mucin in the nucleation of cholesterol crystals which lead to gallstone formation. Using dynamic light scattering we have shown that mucin at relatively low concentrations (4 mg/ml) promotes the fusion of phospholipid + cholesterol vesicles. The time evolution of the fusion process was measured. No changes in the aggregation state of the gallbladder mucin were observed during the fusion process, suggesting that this phenomenon is related to physico-chemical interactions between the polymer (mucin) and the colloidal particle (vesicle).