Studies on Enzymatic Digestion of Gastric Mucin

Although it is generally assumed that mucin is digested in the intestinal tract, no systematic investigations on the enzymatic hydrolysis of mucin have been reported. We have, therefore, undertaken to study the action of enzymes upon mucin in vitro for the purpose of determining its possible manner of cleavage.

The general procedure consisted in adding a known amount of enzyme to a mucin solution of known concentration at a pH well within the range of activity of the enzyme being tested. Controls consisted of mucin alone and enzyme alone at approximately the same pH. At intervals, usually 2 days, samples were removed and analyzed, until no further breakdown beyond that occurring in the controls could be detected. Proteolysis was followed by the nitrous acid method of Van Slyke. 1 Cleavage of the carbohydrate portion of the molecule was tested for by determining the presence or absence of reducing substances, using the Somogyi-Shaffer-Hartmann method. 2 The extent of cleavage was also determined by the decrease in the amount of mucin precipitable in 70% alcohol, and subsequent analysis of the precipitated material. The activity of all enzymes used was demonstrated by their hydrolysis of suitable substrates. In those cases in which digestion of mucin was observed, a second portion of enzyme was subsequently added to preclude the possibility that cessation of hydrolysis was due to depletion or inactivation of the enzyme.

We have previously reported the isolation of a purified mucin from commercial preparations by a process of peptic digestion followed by 70% alcohol precipitation. 3 Since peptic digestion is used in the preparation of commercial mucin, the additional peptic digestion was employed for the sole purpose of rendering alcohol soluble any extraneous protein material present as a result of improper manufacture, or subsequently added as a diluent.