Immunohistochemical demonstration of acidic mammalian chitinase in the mouse salivary gland and gastric mucosa

Acidic mammalian chitinase (AMCase) is the sole chitinolytic enzyme that has been identified thus far in the gastrointestinal tract of mammals. AMCase mRNA expression has been demonstrated in the salivary gland and stomach of mice and in the stomach of humans, while a bovine homologue of AMCase is produced in the liver and secreted into the blood. The present study using antibody raised against bovine AMCase demonstrates the cellular distribution of AMCase in salivary and gastric secretions at the protein level. Immunostaining using mouse tissues detected intense immunoreactivity for AMCase in serous-type secretory cells of the parotid gland and von Ebner's gland. Gastric chief cells, localized at the bottom of gastric glands, were also immunoreactive for AMCase. Electron-microscopically, the immunoreactivity was localized in granules in the apical cytoplasm of these secretory cells, and not in other structures. Western blot analysis confirmed the existence of AMCase in the parotid gland and stomach, and in their secretions in mice. However, no immunoreactive band was clearly detectable in immunoblots of the human parotid saliva and gastric juice. At least in the mouse, AMCase is secreted into the saliva and gastric juice, and may function as a digestive enzyme or play a defensive role against chitinous pathogens.