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d-Amino acids are now recognized to be widely present in mammals. Renal d-amino-acid oxidase (DAO) is associated with conversion of d-amino acids to the corresponding α-keto acids, but its contribution in vivo is poorly understood because the α-keto acids and/or l-amino acids formed are indistinguishable from endogenous compounds. First, we examined whether DAO is indispensable for conversion of d-amino acids to their α-keto acids by using the stable isotope tracer technique. After a bolus intravenous administration of d-[2H7]leucine to mutant mice lacking DAO activity (ddY/DAO−) and normal mice (ddY/DAO+), elimination of d-[2H7]leucine and formation of α-[2H7]ketoisocaproic acid ([2H7]KIC) and l-[2H7]leucine in plasma were determined. The ddY/DAO− mice, in contrast to ddY/DAO+ mice, failed to convert d-[2H7]leucine to [2H7]KIC and l-[2H7]leucine. This result clearly revealed that DAO was indispensable for the process of chiral inversion of d-leucine. We further investigated the effect of renal mass reduction by partial nephrectomy on elimination of d-[2H7]leucine and formation of [2H7]KIC and l-[2H7]leucine. Renal mass reduction slowed down the elimination of d-[2H7]leucine. The fraction of conversion of d-[2H7]leucine to [2H7]KIC in sham-operated rats was 0.77, whereas that in five-sixths-nephrectomized rats was 0.25. The elimination behavior of d-[2H7]leucine observed in rats suggested that kidney was the principal organ responsible for converting d-leucine to KIC. [ABSTRACT FROM AUTHOR]