Immunohistochemical localization of carbonic anhydrase isoenzymes VI, II, and I in human parotid and submandibular glands.

Human salivary carbonic anhydrase (HCA VI) was purified by inhibitor affinity chromatography and its location in the human parotid and submandibular glands identified, using a polyclonal antiserum raised against the purified enzyme in rabbits in conjunction with the peroxidase-antiperoxidase complex method. The antibodies raised against the purified enzyme in rabbits did not crossreact with the HCA II or I. However, they slightly recognized human IgA; the antiserum was therefore absorbed with human IgA before immunohistochemical use. HCA VI-specific staining was detected in the cytoplasm and particularly in the secretory granules of the serous acinar cells of both parotid and submandibular glands, the staining of the secretory granules being most distinct in paraformaldehyde-fixed tissues. Some epithelial cells and the luminal content of the striated ducts also gave a specific HCA VI staining. Staining specific for HCA II was also found in the granules of the serous acinar cells, particularly in the submandibular gland when Carnoy fluid fixation was used. Slight HCA II-specific staining was also detected in the striated ductal cells in the Carnoy fluid-fixed specimens. No staining specific for HCA I was detected. The results indicate that the serous acinar cells in human parotid and submandibular glands contain abundant HCA II and HCA VI. Interestingly, only HCA VI is secreted into the saliva, although both enzymes appear to be located in structures resembling the secretory granules in the acinar cells. The enzymes probably form a mutually complementary system regulating the salivary buffer capacity.