AJP - Gastrointestinal and Liver Physiology, Vol 259, Issue 4 681-G686, Copyright © 1990 by American Physiological Society
Lipid binding to gastric mucin: protective effect against oxygen radicals
D. H. Gong, B. Turner, K. R. Bhaskar and J. T. Lamont
Evans Memorial Department of Clinical Research, Boston University School of Medicine, Massachusetts 02118.
Gastric mucus forms a viscous gel overlying the gastric mucosa and is
thought to protect the underlying mucosa from noxious agents such as acid,
proteases, and bile salts. A common property of mucin, the principal
glycoprotein in mucous secretions, is its ability to bind lipids. The
purpose of this study was to determine if lipids bound to gastric mucin
protect the mucin from oxygen radical attack. Pig gastric mucin, partially
purified by Sepharose 4B gel chromatography, was found to contain large
amounts of free fatty acids and cholesterol as well as lesser amounts of
sphingomyelin and phospholipids. Purified mucin obtained by
density-gradient ultracentrifugation in a CsCl gradient contained only
trace amounts of fatty acids but no other lipids. Exposure to the oxygen
radical-generating system iron/ascorbate caused a marked reduction in
viscosity of purified mucin but did not affect partially purified mucin,
suggesting that bound lipids shielded the mucin from attack by oxygen
radicals. Using discontinuous sucrose-gradient ultracentrifugation in the
presence of liposomes containing [3H]palmitic acid, we demonstrated that
mucin is capable of binding fatty acids. We also observed a striking
increase in solution viscosity of gastric mucin at low pH, a feature that
might contribute to the ability of mucin to form a protective diffusion
barrier for the underlying epithelium.
