An intracellular serine protease, which is believed to initiate the degradation of several intracellular pyridoxal phosphate-dependent enzymes, was localized by immunofluorescence in atypical mast cells of the lamina propria and in intraepithelial cells of the rat small intestine. Some mucus-secreting goblet cells also contained the protease antigen. Atypical mast cells containing the enzyme were present in large numbers beneath the epithelium of bronchioles. All atypical mast cells also contained low levels of the chymotrypsin-like protease of normal mast cells. Both enzymes were consistently present in normal connective tissue mast cells. Amino acid content, molecular weight, and lack of immunologic crossreactivity indicate that the two enzymes are similar but not identical. The cell-specific localization of the intestinal serine protease makes it unlikely that the enzyme has any general role in the degradation of pyridoxal phosphate-dependent enzymes. The function of the enzyme in mast cells, atypical mast cells, and intestinal goblet cells is not known.
