Re: What prevents pepsin from digesting the walls of the stomach?

Rick,

  Sorry for the delay in response as I was on vacation.

  The short answer to your question is that mucin is secreted throughout 
the gastrointestinal tract from mucous neck cells and in an alkaline medium 
in the stomach mucosa.  Further the viscosity of the medium that contains 
mucin is rather insoluble and forces the pepsin and hydrochloric acid into 
an aqueous phase further protecting the stomach lining.
  mucin: polypeptide glycoprotein with 4 subunits connected by disulfide 
bonds that is resistant to degradation by pepsin and other proteolytic 
enzymes due to the saccharide moieties attached to OH amino acid side 
chains (ie. serine, threonine, & tyrosine).  Note that tyrosine is one of 
the side chain aromatics required by pepsin (see below).


FYI regarding pepsin:
Cleaves peptide bonds adjacent to aromatic side chains (phenylalanine, 
tyrosine & tryptophan).  Pepsinogen I (acid secreting region) and 
Pepsinogen II (pyloric region) are precursors.  Pepsinogen interestingly 
can also be released by the mucous neck cells in small quantites.  
Pepsinogen, however, is normally released in larger quantities during 
digestion of proteins by the peptic cells.

Sources:  Ganong Review of Physiology
          Guyton Medical Physiology
          Stryer Biochemistry