Free d-aspartic acid in rat salivary glands

Free d-aspartic acid (d-Asp) has been reported to occur in a wide variety of tissues and cells, exclusively in central nervous system and endocrine tissues. In this manuscript, we demonstrate that large amounts of d-Asp are present in the exocrine tissue, salivary glands. In adult male rats, d-Asp concentrations in parotid and submandibular gland were 212 ± 68 and 233 ± 34 nmol/g wet weight, respectively, and were low (38 ± 20 nmol/g wet weight) in sublingual gland. This result indicates that substantial level of d-Asp exists not only in central nervous system and endocrine tissues but also in exocrine tissues. Furthermore, d-Asp concentration in parotid gland increased transiently at 3 weeks of age and decreased thereafter. In contrast, the d-Asp level in submandibular gland continued to increase gradually from 1 to 7 weeks of age and remained at an adult level after 7 weeks of age. Using anti-d-Asp antibody, immunohistochemical study was done against these glands and it showed that the predominant localization of d-Asp in acinar cells in parotid gland, while d-Asp is specifically located in striated duct cells in submandibular gland. These results suggest that d-Asp may play different roles between the two glands.