The presence of high concentrations of free D-amino acids in human saliva

Free neutral (D)-ammo acids have previously been detected in human plasma, usually accounting for less than 2% of the total free amino acid concentration ((D)-amino acid ratio) [Nagata, Y, Masui, R., Akino, T., 1992a. The presence of free (D)-serine, (D)-alanine and (D)-proline in human plasma. Experientia 48, 986-988. Nagata, Y, Yamamoto, K., Shimojo, T., 1992b. Determination of (D)- and L-amino acids in mouse kidney by high-performance liquid chromatography. Journal of Chromatography 575, 147-152. Nagata, Y, Yamamoto, K., Shimajo, T., Konno, R., Yasumura, Y, Akino, T., 1992c. The presence of free (D)-alanine, (D)-proline and (D)-serine in mice. Biochimca et Biiophysica Acta 1115, 208-211]. In the present study to search for the source of free (D)-amino acids, (D)- and (L)-enantiomers of the major non-essential amino acids, i.e., the free form of serine, alanine, proline, aspartate and glutamate were analyzed by HPLC in human saliva, submandibular glands and oral epithelial cells. The (D)-enantiomer ratios to total of free alanine or proline were 3 5% and 20%, respectively, in saliva. The ratios of the other (D)-amino acids were less than 11%. The effect of ingested food and oral bacteria on the saliva amino acid levels was suggested to be insignificant. (D)-Alanine and (D)-aspartate were also detected in the submandibular gland in ratios up to 5%, and (D)-alanine and (D)-proline were found in oral epithelial cells in ratios of 18% and 5%, respectively. The submandibular gland and oral epithelial cells are suggested to be possible sources of the saliva (D)-alanine and (D)-aspartate. (c) 2005 Elsevier Inc. All rights reserved.