Free D-amino acids in human plasma in relation to senescence and renal diseases.

Minute but appreciable amounts of D-amino acids were detected in normal human plasma. The content was significantly higher in an elderly population (age 76 +/- 6 years, mean +/- SD, n = 41) than in a younger group (age 42 +/- 4 years, n = 26), i.e. 6.9 +/- 4.8 nmol/ml (mean +/- SD, range 0-18.8 nmol/ml) and 2.5 +/- 1.8 nmol/ml (range 0-6.3 nmol/ml) for the elderly and the younger groups, respectively. Elevation of plasma D-amino acid level was observed in a group of patients with renal disease (3.6-52.6 nmol/ml), in proportion to the serum level of creatinine (n = 50, r = 0.726, P less than 0.001), beta 2-microglobulin (n = 34, r = 0.551, P less than 0.005), and to glomerular filtration rate (n = 39, r = 0.556, P less than 0.001).