Changes in Salivary Amino Acid Composition During Aging

Background: It has been suggested that the features of saliva reflect the physiological and psychological state of primates as well as subprimates, however, studies revealing the relationship between aging and the concentrations of salivary amino acids are limited. In order to better understand their physiological role, age-related changes of salivary amino acids were investigated. Materials and Methods: Forty-five participants including 5 children (6.60±1.67 (5-9) years old), 20 adults (46.55±14.68 (23-64) years old), and 20 senior citizens (71.60±4.27 (66-82) years old) took part in this study. Whole saliva (one sample per each person) was collected in the daytime (10:00-11:00 or 14:00-15:00). Salivary amino acids were recovered after deproteinization with 5% trichloroacetic acid and determined by an amino acid analyzer. Results: Glycine was the most abundant amino acid in the saliva. Glycine and lysine levels increased significantly (p<0.05) with aging, regardless of gender difference. When the glycine and lysine levels were plotted, much higher correlation (p<0.001) was observed. On the other hand, there was no significant correlation between the salivary concentration of glutamic acid or histidine and age. Conclusion: Salivary amino acid levels may be regarded as markers of aging. Saliva contains various physiologically active substances and cells that maintain homeostasis. Several amino acids in saliva may affect biological responses. Caries-free adults show elevated levels of lysine and arginine in the saliva, as compared with caries-susceptible adults (1). A significant relationship has been reported between the concentration of ammonium and caries prevalence (2). Glycine stimulated the production of prostaglandin E 2 and cyclooxygenase-2 protein in interleukin-1β-stimulated human gingival fibroblast (3). Supplementation of pigs with tryptophan in the diet reduced the basal plasma cortisol and noradrenaline concentrations (4). The major volatile substance of tobacco smoke, acetaldehyde, easily dissolves into saliva during smoking. This acetaldehyde can be totally removed by a cysteine-containing tablet which is sucked during smoking (5). Endogenous glutamate may alter hedonic response to suprathreshold umami substances (6). There are specific receptors for inhibitory amino acids (glycine and γ-aminobutyric acid (GABA)) and stimulatory amino acids (glutamic acid) in taste buds, and these amino acids exert their effects via their respective receptors (7-9). Changes in salivary composition correlate with disease susceptibility, disease state, or both. However, the use of saliva for diagnostic purposes is complicated by the gland-specific effects of circadian rhythm or diurnal variation, and therefore, studies of the circadian rhythm of saliva amino acid concentrations have been limited (10, 11). We recently investigated the diurnal changes of the salivary amino acids in three undergraduate students (12). The results showed that there was no apparent changes in most of salivary amino acid levels except for during eating and sport, and the ratio of glycine/glutamic acid declining under stressful condition during examinations (12). This study suggests that salivary amino acid levels may be useful to evaluate stressful conditions. In order to better understand their physiological role, we investigated here the possible changes in the salivary amino acid composition during aging.