Colon kallikrein was localized in the goblet cells of cat and man by a variety of immunocytochemical techniques. No evidence of this enzyme was found in other sites in this organ. The possible physiological significance of kallikrein in the gastrointestinal tract and of the many related serine proteases is discussed.
The sensitive immunoperoxidase method was used to localize kallikrein in the submandibular gland of the cat, guinea pig, dog, and man. In every instance, kallikrein was localized in the apical region of duct cells, a location suggesting its secretion into the duct system. There was no evidence of the enzyme in other cells or interstitial tissue in the gland. The major source of submandibular gland kallikrein, therefore, must be of ductal origin. The classification of kallikreins with the widely distributed group of serine proteases is discussed, as is their possible significance in regulating physiological processes by specific and limited proteolysis.
Recent studies on the localization of the serine protease salivary kallikrein have led to the conclusion that it has a ductal localization and to the possibility that it is located there in small "secretory" granules. Until now, the latter inference has been based entirely on circumstantial evidence. In the present study, however, direct evidence from immunolocalization studies at the ultrastructural level establishes the localization of this enzyme in the apical duct granules in the cat submandibular gland. These granules stained only with immune sera to cat submandibular gland kallikrein and were the only subcellular structures that did so. They showed the "studded" appearance characteristic of the electron-dense aggregates of peroxidase-antiperoxidase seen in this type of immune reaction.
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