pH-dependent Gelation of Gastric Mucin
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0897-J02-04.1
pH- dependent Gelation of Gastric Mucin R. Bansil1, J. Celli, B. Chasan, S. Erramilli and Z. Hong2 Dept. of Physics, Boston University, Boston MA 02215 N. H. Afdhal, K. R. Bhaskar and B. S. Turner3 Div. of Gastroenterology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston MA 02115 ABSTRACT We discuss the mechanism by which gastric mucin forms a gel at low pH, which serves to protect the stomach from being damaged by the acidic gastric juice that it secretes. Frequency dependence of viscoelastic moduli of pig gastric mucin gels obtained by microscopic dynamic light scattering is presented. Atomic Force Microscopy provides direct visual evidence to indicate that mucin broken into its subunits does not gel at low pH. INTRODUCTION For past several years we have been engaged in a collaborative research program to understand a fundamental puzzle, namely, “Why does the stomach not digest itself?” The answer is that the mucus gel provides this protection1, 2. Our focus is on understanding the biophysical mechanism which causes the mucus to gel and to obtain a detailed molecular picture3, 4, 5, 6, 7, 8. Mucins comprise a family of large polymeric glycoproteins (typically containing 70-80% carbohydrate grafted on a protein backbone with the sugar chains arrayed like the bristles of a bottle-brush) found in the mucus secretions that cover epithelial surfaces of the gastrointestinal, respiratory and genitourinary tracts2, 9, 10. Their cytoprotective function is due in large part to their ability to form gels. Mucins also have pharmacological applications, such as providing specific target sites in the development of cancer vaccines, and using mucoadhesive properties for optimizing drug delivery by different routes. As mentioned above, mucin has a complex architecture9,10. The basic apoprotein consists of heavily glycosylated subunits with poorly glycosylated regions at the ends of the apoprotein. These cysteine rich regions are linked via S-S bonds forming a high molecular weight (about 2 -20 million Dalton) somewhat akin to a linear multi-block copolymer. The sugar side chains are negatively charged, making it a polyelectrolyte. In 1
Corresponding Author (email: [email protected]). Current Address: Boston University School of Medicine, Boston MA 02118 3 Molecular and Cellular Biology and Biochemistry Ph. D Program, Boston University, Boston MA 02215. 2
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such a complex molecule gelation/aggregation could arise from interactions of sugar side chains, or from the protein backbone in the non-glycosylated parts. To address these issues we have examined the gelation behavior of purified porcine gastric mucin (PGM) in the native form, as well as from bio-chemically modified preparations to, (i) remove the sugar (deglycosylated PGM), and (ii) reduce into subunits (reduced PGM) either by protease digestion or by chemically breaking the S-S bonds. Viscosity measurements3 revealed a 100-fold increase in the viscosity of PGM solutions (10 mg/ml) as pH was lowered from 7 to 2. The viscosity chang
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