| Summary: | Free <span style="font-variant: small-caps;">d</span>-amino acids, which are enantiomers of <span style="font-variant: small-caps;">l</span>-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 <span style="font-variant: small-caps;">d</span>-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 <span style="font-variant: small-caps;">d</span>-aspartic acid and small amounts of <span style="font-variant: small-caps;">d</span>-serine and <span style="font-variant: small-caps;">d</span>-alanine were detected in all three major salivary glands in rat. No other <span style="font-variant: small-caps;">d</span>-enantiomers were detected. Protein expression of <span style="font-variant: small-caps;">d</span>-amino acid oxidase and <span style="font-variant: small-caps;">d</span>-aspartate oxidase, the enzymes responsible for the oxidative deamination of neutral and dicarboxylic <span style="font-variant: small-caps;">d</span>-amino acids, respectively, were detected in all three types of salivary gland. Furthermore, protein expression of the <span style="font-variant: small-caps;">d</span>-serine metabolic enzyme, serine racemase, in parotid glands amounted to approximately 40% of that observed in the cerebral cortex. The <i>N</i>-methyl-<span style="font-variant: small-caps;">d</span>-aspartic acid subunit proteins NR1 and NR2D were detected in all three major salivary glands. The results of the present study suggest that <span style="font-variant: small-caps;">d</span>-amino acids play a physiological role in a range of endocrine and exocrine function in salivary glands.
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