Presence and origin of large amounts of d-proline in the urine of mutant mice lacking d-amino acid oxidase activity

Using a column-switching HPLC system combining a micro-ODS column and a chiral column, the amounts of d-proline (d-Pro) have been determined in 18 tissues, plasma and urine of mice. To avoid the enzymatic degradation of d-amino acids in vivo, a mutant mouse strain lacking d-amino acid oxidase activity (ddY/DAO⁻ mouse) was used. In the brain, relatively large amounts of d-Pro were observed in the anterior pituitary, posterior pituitary and pineal glands. In the peripheral tissues, the amounts of d-Pro were high in the pancreas and kidney. Above all, it is surprising that the ddY/DAO⁻ mice excreted large amounts of d-Pro in their urine (433 nmol/mL, 20 times that of l-Pro). The origin of d-Pro has also been investigated. By comparing germ-free mice and gnotobiotic mice, intestinal bacteria were shown to have no effect on the urinary d-Pro amount. Concerning the dietary origin, a notable amount of d-Pro was still excreted in the urine after starvation for 4 days, suggesting that some of the d-Pro is produced in the mice. Age-dependent changes in the urinary d-Pro amount have also been investigated from the postnatal 1st month up to 12 months, and ddY/DAO⁻ mice were found to excrete large amounts of d-Pro in the urine constantly throughout their lives. Kenji Hamase is Associate Professor in the Department of Bioanalytical Chemistry, Graduate School of Pharmaceutical Sciences at Kyushu University. His current research interests focus on the development of analytical methods for d-amino acids and the study of their physiological functions and diagnostic values. He received the Japanese Society for Analytical Chemistry Award for Young Scientists in 2003, and the PSJ Award for Young Scientists in 2006.