Summary.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.
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