Ischemic acute kidney injury perturbs homeostasis of serine enantiomers in the body fluid in mice: early detection of renal dysfunction using the ratio of serine enantiomers.

The imbalance of blood and urine amino acids in renal failure has been studied mostly without chiral separation. Although a few reports have shown the presence of D-serine, an enantiomer of L-serine, in the serum of patients with severe renal failure, it has remained uncertain how serine enantiomers are deranged in the development of renal failure. In the present study, we have monitored serine enantiomers using a two-dimensional HPLC system in the serum and urine of mice after renal ischemia-reperfusion injury (IRI), known as a mouse model of acute kidney injury. In the serum, the level of D-serine gradually increased after renal IRI in parallel with that of creatinine, whereas the L-serine level decreased sharply in the early phase after IRI. The increase of D-serine was suppressed in part by genetic inactivation of a D-serine-degrading enzyme, D-amino acid oxidase (DAO), but not by disruption of its synthetic enzyme, serine racemase, in mice. Renal DAO activity was detected exclusively in proximal tubules, and IRI reduced the number of DAO-positive tubules. On the other hand, in the urine, D-serine was excreted at a rate nearly triple that of L-serine in mice with sham operations, indicating that little D-serine was reabsorbed while most L-serine was reabsorbed in physiological conditions. IRI significantly reduced the ratio of urinary D-/L-serine from 2.82 ± 0.18 to 1.10 ± 0.26 in the early phase and kept the ratio lower than 0.5 thereafter. The urinary D-/L-serine ratio can detect renal ischemia earlier than kidney injury molecule-1 (KIM-1) or neutrophil gelatinase-associated lipocalin (NGAL) in the urine, and more sensitively than creatinine, cystatin C, or the ratio of D-/L-serine in the serum. Our findings provide a novel understanding of the imbalance of amino acids in renal failure and offer a potential new biomarker for an early detection of acute kidney injury.

D-amino acid oxidase activity in urine obtained from patients with renal disorders.

D-amino acid oxidase (DAO) is an enzyme released from the kidney, and a method for its enzymatic assay was developed by our group for urine specimens. A clinical evaluation of the test was performed for healthy individuals and patients with various types of nephropathy. 1) The normal reference interval was 0-10.0 micrograms/g Cr for randomly-collected urine. The intra-assay CV was 3.5-9.4%, and the inter-assay CV was 4.5-9.8%. 2) Urinary DAO index correlated well with changes in urinary microalbumin and N-acetyl-beta-glucosaminidase. 3) Chronic renal failure generally showed high levels of DAO, except for nephrotic syndrome, in which changes were minimal after the acute stage. In the nephrotic syndrome, urinary DAO was elevated in the acute stage with high proteinuria, but went down to almost normal as the proteinuria improved, although DAO changes did not correlate with the changes in the degree of proteinuria in lupus nephritis. 4) DAO changes were more sensitive than those of N-acetyl-beta-glucosaminidase or alpha 1-microglobulin in the clinical course of renal disorders. From these findings, DAO was judged to be very useful for monitoring the severity of renal dysfunction.