Free d-Amino Acids in Salivary Gland in Rat

Simple Summary Parotid, submandibular, and sublingual glands in rat were found to contain high concentrations of d-aspartic acid and low ones of d-serine and d-alanine. In addition to d-amino acid oxidase and d-aspartate oxidase, serine racemase was also detected in all three of these major salivary glands, as were N-methyl-d-aspartic acid receptor subunits NR1 and NR2D, but not NR2A, NR2B, or NR2C. Free d-amino acids, which are enantiomers of l-amino acids, are found in mammals, including humans, and play an important role in a range of physiological functions in the central nervous system and peripheral tissues. Several d-amino acids have been observed in saliva, but their origin and the enzymes involved in their metabolism and catabolism remain to be clarified. In the present study, large amounts of d-aspartic acid and small amounts of d-serine and d-alanine were detected in all three major salivary glands in rat. No other d-enantiomers were detected. Protein expression of d-amino acid oxidase and d-aspartate oxidase, the enzymes responsible for the oxidative deamination of neutral and dicarboxylic d-amino acids, respectively, were detected in all three types of salivary gland. Furthermore, protein expression of the d-serine metabolic enzyme, serine racemase, in parotid glands amounted to approximately 40% of that observed in the cerebral cortex. The N-methyl-d-aspartic acid subunit proteins NR1 and NR2D were detected in all three major salivary glands. The results of the present study suggest that d-amino acids play a physiological role in a range of endocrine and exocrine function in salivary glands.