Free d-Amino Acids in Salivary Gland in Rat
by
Masanobu Yoshikawa
1,*, 
Takugi Kan
2,
Kosuke Shirose
2,
Mariko Watanabe
2,
Mitsumasa Matsuda
2,
Kenji Ito
2 and
Mitsuru Kawaguchi
3 1
Department of Clinical Pharmacology, School of Medicine, Tokai University, Isehara 259-1193, Japan
2
Department of Anesthesiology, School of Medicine, Tokai University, Isehara 259-1193, Japan
3
Tokyo Dental College, Tokyo 101-0061, Japan
*
Author to whom correspondence should be addressed.
Biology 2022, 11(3), 390; https://doi.org/10.3390/biology11030390
Submission received: 21 January 2022
/
Revised: 21 February 2022
/
Accepted: 28 February 2022
/
Published: 2 March 2022
(This article belongs to the Special Issue Deficiencies of Behavioral or Phenotypes Characteristics Based on Environments, Genetics, Disease and Another Factors)
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.
Abstract
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.
