In vitro degradation of gastric mucin. Carbohydrate side chains protect polypeptide core from pancreatic proteases.

The polypeptide core of mucin glycoprotein subunits resists cleavage by proteases. To determine if the carbohydrate side chains of these subunits protect the underlying polypeptide core from proteolytic cleavage, we compared the effect of pancreatic proteases on hog gastric mucin before and after cleavage of its carbohydrate moieties by bacterial glycosidases from an anaerobic human fecal culture supernate. Hog gastric mucin was resistant to pancreatic proteases: less than 10% of the mucin protein became soluble in 80% vol/vol ethanol after 24-h incubation with trypsin, alpha-chymotrypsin, and elastase, and its elution pattern from Sephadex G-200 remained unchanged after elastase treatment, with 90% eluting at the void volume. By contrast, after removal of 50% of its carbohydrates, mucin was susceptible to pancreatic proteases: 50% of mucin protein became soluble in 80% vol/vol ethanol after 24-h incubation with alpha-chymotrypsin and elastase, and 24% of mucin protein became soluble in 80% vol/vol ethanol after 24-h incubation with trypsin; after elastase treatment, its elution from Sephadex G-200 was markedly retarded. We conclude that the carbohydrate side chains of hog gastric mucin glycoprotein protect the underlying polypeptide core from proteolysis and that degradation of the carbohydrate side chains by glycosidases from fecal bacteria renders the polypeptide core susceptible to pancreatic proteases.