Indispensable but Insufficient Role of Renal D ‐Amino Acid Oxidase in Chiral Inversion of N G ‐Nitro‐ D ‐arginine

Unidirectionally chiral inversion of N G ‐nitro‐ D ‐arginine ( D ‐NNA) to its L ‐enantiomer ( L ‐NNA) occurred in rats, and it was blocked markedly ( ca. 80%) by renal vascular ligation, and entirely (100%) by the D ‐amino acid oxidase (DAO) inhibitor sodium benzoate, suggesting that renal DAO is essential for the inversion. However, the doses of sodium benzoate administrated were extremely high ( e.g. , 400 mg/kg) due to its low potency. It is thus possible that sodium benzoate‐mediated blockade of D ‐NNA inversion might be due to its nonspecific (or non‐DAO‐related) effects. In addition, after D ‐NNA was incubated with the pure enzyme of DAO in vitro without tissue homogenates, L ‐NNA was not produced, even though D ‐NNA was disposed. We propose that this occurred because D ‐NNA was first converted to its corresponding α ‐keto acid by DAO and then to L ‐NNA by transaminase(s); however, there was no direct evidence for this process. The goal of this study is to further elucidate the process of D ‐NNA chiral inversion both in vivo and in in vitro tissue homogenates by comparing mutant ddY/DAO −/− mice that lack DAO activity entirely compared to normal ddY/DAO +/+ mice and Swiss mice. Furthermore, the ability to produce L ‐NNA from D ‐NNA‐corresponding α ‐keto acids ( N G ‐nitroguanidino‐2‐oxopentanoic acid) produced by porcine kidney‐derived DAO (pkDAO) was also studied in the DAO inhibitor‐pretreated rats. We found that D ‐NNA chiral inversion occurred in Swiss mice and ddY/DAO +/+ mice both in vivo and in in vitro kidney homogenates, but not in ddY/DAO −/− mice, correlated to their DAO activities. The α ‐keto acid ( N G ‐nitro‐guanidino‐2‐oxopentanoic acid) from D ‐NNA was able to produce L ‐NNA, and subsequent vasoconstriction and pressor responses. These results indicate that the role of renal DAO is indispensible but insufficient for chiral inversion of D ‐NNA and other neutral and polar D ‐amino acids, and unidentified aminotransferase(s) are involved in a subsequent mechanism for the process of chiral inversion.