Plasma amino acids reflect the dynamics of amino acids in organs and their levels have clinical significance. Amino acids as clinical indicators have been evaluated as a mixture of d- and l-amino acids because d-enantiomers are believed to be physiologically nonexistent. However, it has become clear that some d-amino acids are synthesized by endogenous enzymes and symbiotic bacteria. Here, using a two-dimensional HPLC system, we measured enantiomers of all proteinogenic amino acids in plasma and urine and analyzed for correlation with other biochemical parameters in humans who underwent health checkups at our institutional hospital. Four d-amino acids (d-asparagine, d-alanine, d-serine, and d-proline) were detected in the plasma, amounting to less than 1% of the quantities of l-amino acids, but in the urine at several tens of percent, showing that d-amino acids have much higher fractional excretion than their L-counterparts. Detected plasma d-amino acids and d-/l-amino acid ratios were well correlated with renal parameters, such as blood urea nitrogen, creatinine, and cystatin C. On the other hand, a set of plasma l-amino acids were associated with body mass index and correlated with metabolic parameters such as liver enzymes, lipids, blood glucose, and uric acid. Thus, chiral resolution of plasma amino acids revealed totally different associations of the enantiomers with organ functions, and warrants further investigation for clinical and laboratory usefulness.
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