Fluorescence determination of d- and l-tryptophan concentrations in rat plasma following administration of tryptophan enantiomers using HPLC with pre-column derivatization

Similar to l-tryptophan (l-Trp), d-Trp can be converted to unique metabolites in the mammalian body. In the present study, the difference in the plasma half-life (t1/2) between Trp enantiomers was investigated by following the alterations in the plasma concentration of d- or l-Trp after intraperitoneal (i.p.) administration of each enantiomer to male Sprague–Dawley rats (100 mg/kg). The investigation was performed using reversed-phase high-performance liquid chromatography (HPLC) and pre-column fluorescence derivatization with a chiral fluorescent labeling reagent, R(−)-4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazole (R(−)-DBD-PyNCS). The t1/2 value of d-Trp was significantly smaller than that of l-Trp, suggesting that d-Trp was eliminated from the plasma more rapidly than l-Trp. In addition, a significant increase in the plasma concentration of l-Trp was observed following administration of d-Trp, whereas no d-Trp was detected after l-Trp administration. Furthermore, the increase in the plasma concentration of l-Trp was significantly suppressed by pretreatment with an inhibitor of d-amino acid oxidase (DAAO), 3-methylpyrazole-5-carboxylic acid, which suggests that DAAO was involved in the conversion of d-Trp to l-Trp in vivo.