Model for the structure of the gastric mucous gel

Abstract

THE surface of the stomach is covered by a layer of mucous gel which protects the underlying mucosa from the harmful, acidic stomach contents. The principal component of the gel has been isolated from pig gastric mucus, purified, and shown to be a glycoprotein of molecular weight 2×10⁶ (Table 1)^1,2. This glycoprotein consists of four equal sized subunits (molecular weight 5 × 10⁵) joined by disulphide bridges^3,4. Each glycoprotein subunit consists of a protein core, 14% by weight of the glycoprotein, with carbohydrate side chains attached. The protein core consists of two regions, one rich in serine, threonine and proline and bearing all the carbohydrate, the other having an amino acid composition characteristic of a globular protein. The latter contains cystine residues which bridge the four subunits. Mild reducing agents and proteolytic enzymes each split the glycoprotein into four subunits^4,5. The carbohydrate chains, approximately 15 residues in a branched structure, account for 82% by weight of the glygoprotein and carry ester sulphate residues. The problem therefore is to explain how such a glycoprotein molecule can associate to form a gel which in vivo is essentially impermeable to proteolytic enzymes and which can act as a barrier to protons probably by supporting a pH gradient.