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L-Kynurenine (KYN), a tryptophan metabolite, is metabolized to kynurenic acid (KYNA), which is an antagonist of N-methyl-D-aspartate and alpha7 nicotinic acetylcholine receptors, by kynurenine aminotransferase (KAT) I and KAT II. In this study, optically pure KYN, namely L-KYN or D-KYN, was administered intraperitoneally to male Sprague-Dawley rats (16.3 micromol kg(-1)), and the change in plasma KYNA was investigated by using column-switching high-performance liquid chromatography (HPLC) with fluorescence detection. Unexpectedly, no remarkable alteration in the plasma KYNA was observed when a natural isomer, L-KYN, was administered, whereas plasma KYNA concentration was unequivocally increased when an unnatural isomer, D-KYN, was administered. Serum protein bindings of L-KYN and D-KYN were also studied, and the protein binding of L-KYN (approximately 65%) in rat serum was larger than that of D-KYN (approximately 12%), suggesting that D-KYN may be easily incorporated and metabolized in tissues during blood circulation to generate KYNA in mammals. In addition, the increase in plasma KYNA by the administration of D-KYN was suppressed in rats pretreated with a selective inhibitor of D-amino acid oxidase (DAAO), 5-methylpyrazole-3-carboxylic acid (80 mg/kg). These results suggest that DAAO might be responsible for the production of KYNA from D-KYN in vivo.

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