Formation of mucous gel from the high molecular weight mucoprotein of gastric mucus

Abstract

The highly viscous and gelatinous nature of gastric mucus is shown to depend upon the properties of a mucoprotein of protein content 13.6 % and molecular weight 2 × 10⁶. At 25°C, the viscosity of this mucoprotein is strongly concentration dependent and at concentrations about 4 g l.^–1 rises asymptotically, indicating the onset of a reversible sol-gel transition. Reducing agents (mercaptoethanol) and trypsin split the mucoprotein into four and two sub-units of molecular weight 5.2 × 10⁵ and 9.8 × 10⁵ respectively. These reductions in molecular size are accompanied by loss of the viscous and gelatinous properties. The viscosity of the mucoprotein undergoes two sharp changes in KCl, one between 0 and 0.05 M KCl which is attributed to a polyelectrolyte charge shielding phenomenon. The second transition near 1.0 M KCl is interpreted as a marked increase in asymmetry accompanied by contraction and reduced solvation. Increasing concentrations of CsCl reduce the viscosity of the mucoprotein, bringing about a transition which is essentially complete by 0.1 M CsCl and is as great in extent as both transitions in KCl. In CsCl the highly expanded mucoprotein is contracted to a molecule having the same symmetry but an even smaller volume and lower solvation than in high KCl concentrations. Both salt-induced transitions are non-reversible. A mechanism is discussed in terms of the effect of the Cs⁺, K⁺ and Cl^– ions on water structure and water-mucoprotein interactions. Guanidinium chloride causes partial expansion of the CsCl treated material with a limited aggregation of four mucoprotein molecules on its removal. The viscosity at finite concentration increases markedly and reversibly with temperature and at 45°C the threshold concentration for reversible gel formation is reduced to 2 g l.^–1. There is no evidence for intramolcular changes. The temperature dependence is characteristic of intermolecular entropy driven interactions and the results are interpreted to show that the mechanism of gel formation in gastric mucus involves micelle junctions.