Elucidation of gene structure and physiological function of D-amino-acid oxidase present in the proximal tubules of the kidney

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D-Amino-acid oxidase is present in the proximal tubules of the kidneys of higher animals. It catalyzes oxidative deamination of unnatural D-amino acids. The physiological roles of this enzyme have been unknown because its substrate D-amino acids are considered to be rare in higher animals. The gene structure and physiological functions of this enzyme have been examined in this study.D-Amino-acid oxidase cDNA was able to be amplified by PCR in all the mice of 15 strains examined. There was a single-base pair substitution in the Hindlll recognition sequence in the cDNA in four out of the 15 strains.The Southern hybridization analyzes have shown that the 15 strains of mice carry the D-amino-acid oxidase gene similar in the whole structure but also shown that there are variations in its inside structure among the mouse strains.The expression of the D-amino-acid oxidase gene in the mouse kidney was examined by in situ hybridization. mRNA for this enzyme was only detected in the epithelial cells of the proximal tubules. This result indicates that this enzyme is synthesized, remains, and functions within the cells of the proximal tubules.The mutant mice lacking D-amino-acid oxidase were found to excrete a large amounts of D-alanine and D-serine into their urine. The sources of these D-amino acids were examined. Experiments using the germ-free and gnotobiotic mutant mice have demonstrated that the urinary D-alanine comes from intestinal bacteria. Antibiotic administration did not reduce the urinary D-serine, indicating it was not of bacterial origin. Nutritional experiments have shown that some part of the D-serine comes from the mouse diet but the rest of it has an endogenous origin. Since D-aminoaciduria is not seen in normal mice, the present study using D-amino-acid oxidase-deficient mice indicates one of the physiological roles of this enzyme is the metabolism of D-amino acids.