Immunoelectron microscopy of carbonic anhydrase isozyme VI in rat submandibular gland: comparison with isozymes I and II.

Carbonic anhydrase (CA) was purified from the saliva of pilocarpine-treated rats by inhibitor-affinity chromatography, and its localization in the rat submandibular gland was studied by the indirect immunoperoxidase technique using a monoclonal antibody (MAb) raised against the enzyme. SDS-polyacrylamide gel electrophoresis of the CA VI gave three bands of 33, 39, and 42 KD. Enzyme digestion experiment showed that the 42 KD molecule was degraded into the 39 KD molecule and the 39 KD molecule into the 33 KD molecule. The cleavage of the 42 KD molecule was independent and that of the 39 KD molecule was dependent on endo-beta-N-acetylglucosaminidase F. The 42 KD molecule was detected in the CA purified from the pilocarpine-treated but not the untreated salivary gland. The MAb recognized all the three components of the enzyme. Immunostaining for CA VI was seen in the cytosol and secretory granules of serous acinar cells and in the duct luminal contents. Staining specific for erythrocyte CA (CA I and CA II) was observed in the cytosol of the epithelial cells of granular, striated, and excretory ducts. Among these duct cells, the agranular varieties in the granular and excretory ducts were essentially devoid of the immunoreactivity.