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.