Origin of D-serine present in urine of mutant mice lacking D-amino-acid oxidase activity

Urine of ddY/DAO(-) mice lacking D-amino-acid oxidase contained 5.7 times more serine than that of normal ddY/DAO(+) mice. Most of the serine was D-isomer. The origin of this D-serine was examined. Oral administration of 0.02% amoxicillin and 0.004% minocycline to the ddY/DAO(-) mice for 7 days did not reduce the urinary D-serine, indicating that the D-serine was not of intestinal bacterial origin. When the mouse diet was changed to one with different compositions, the urinary D-serine was considerably reduced. Furthermore, starvation of the ddY/DAO(-) mice for 24 hours reduced the urinary D-serine to 33% of the original level. These results indicate that most of the urinary D-serine comes from the diet. However, the urine of the starved ddY/DAO(-) mice still contained 4.6 times more D-serine than that of the ddY/DAO(+) mice, suggesting a part of the D-serine have an endogenous origin.