The Importance of Chiral Metabolomics

Chronic kidney disease (CKD) is a global health problem. The number of patients with worsening kidney function who eventually need costly kidney replacement therapy or transplantation is increasing. In addition, the risk of life-threatening cardiovascular diseases increases with CKD progression. Preventing patients from progressing to end-stage kidney disease (ESKD) is therefore critical, but unfortunately there are no effective methods to predict progression. Currently, we rely on kidney functions estimated from serum creatinine and some additional information, such as proteinuria, but these are insufficient. Naturally, nephrologists are searching for better biomarkers.

Could amino acids, those vital components of human bodies, help provide the answer? Amino acid levels are influenced by the functions of many organs; kidneys, for instance, regulate the body’s amino acid balances via reabsorption. Scientists have been studying amino acids for over a century – but because people only detected L-forms in nature, D-amino acids were regarded as unnatural and were not studied vigorously. Eventually, sporadic reports arose of the presence of D-amino acids, including in the blood of patients with kidney disease. Some studies also indicated physiological roles for D-amino acids, but once again these reports were sporadic – mainly because of the measurement challenge. Typically, D-amino acids are present in human bodies at trace levels, and the chemically similar nature of amino acid enantiomers makes it difficult to separate them and measure them simultaneously. And because reliable methods to measure D-amino acids are lacking, their functions and presence in tissues have remained a mystery.

It is only recently that methods have been devised to measure D-amino acids precisely via a metabolomic approach. Kenji Hamase and his colleagues went to great lengths to develop a metabolomic platform – based on micro-2D-HPLC – that can precisely detect whole sets of chiral amino acids from human samples (1). In the first dimension, labelled amino acids are separated by reverse-phase separation. Then, the fraction of each amino acid is automatically transferred to the enantioselective (chiral-selective) column for chiral separation. The 2D-HPLC system is powerful enough to quantitatively detect all amino acid enantiomers from clinical samples ranging from around 1 fmol to 100 pmol.

Our research group used chiral amino acid metabolomic profiling to search for prognostic biomarkers of CKD. Our study revealed that D-amino acids, particularly D-serine and D-asparagine, were robustly associated with the progression of CKD to ESKD (2). The risk of progression was elevated from two- to four-fold in those with higher levels – and this trend is seen only in D-forms. The fact that just a trace portion of amino acids have a stronger relationship with disease processes and prognosis strongly supports the importance of chiral separations.

A D-amino acid test could provide a powerful tool for clinicians, helping them identify high-risk CKD patients for intensive care. The development of a device suitable for clinical use – designed to increase throughput – is currently underway. Another important direction for the future will be more detailed research to study the physiology and metabolism of D-amino acids – both of which are poorly understood – so that we can enrich our understanding of kidney diseases. Through chiral metabolomics, I believe that the mysterious world of D-amino acids will turn out to be a fruitful one for clinicians.