The Metabolism And The Neurochemical Effect Of D-kynurenine In Central Nervous System

Objective:Kynurenine (KYN) is a metabolite of the essential amino acid tryptophan (TRP). There are two isomers:D-kynurenine (D-KYN) and L-kynurenine (L-KYN). A substantial literature has elucidated that L-KYN and its metabolites are relative important in the process of neurology and neuropathology. Recent study indicated that the level of D-kynurenine (D-KYN), the enatiomer of L-KYN, could be elevated under infectious condition and there is relatively limited information about the metabolism of D-KYN. In view of the potential exposure to D-KYN, it is felt that more information on the metabolism of the D-isomer would be desirable. In present study, we aimed to investigate the possible metabolite, the mechanism and the neurochemical effect of D-KYN metabolism.Method:1. In vitro experiments:rat, mouse and human brain and liver tissue homogenates were incubated with D-KYN, newly produced KYN A and3-HK production were measured by HPLC respectively.(2) To elucidate the ability of kynurenine aminotransferase (KAT) and D-aminoacid oxidase (D-AAO) to convert D-KYN to KYNA, partially purified KATs and pure D-AAO were incubated with D-KYN, newly produced KYNA production were measured by HPLC.(3) Using non-specific KAT inhibitor aminooxyacetic acid (AOAA), D-AAO inhibitor kojic acid (KA) and kynurenine3-monooxygenase inhibitor Ro61-8048to examine the mechanisms involved in the production of KYNA from D-KYN.2. In microdialysis experiments, D-KYN was infused to rat prefrontal cortex, striatum and cerebellum, the effect on extracellular KYNA concentrations and dopamine was measured by HPLC.3. To investigate the effect of systemic administered D-KYN on the levels of KYNA and3-HK, D-KYN was i.p. injected to mice, the concentration of KYNA and3-HK were measured in the plasma, liver, forebrain and cerebellum. The time and dose dependent production and the optical property of3-HK were tested.Results:Rat, mouse and human brain and liver had the capability to convert D-KYN to KYNA in vitro, and cerebellum produced much more KYNA from D-KYN than forebrain. Both partially purified KATs and pure D-AAO could convert D-KYN to KYNA in vitro. In forebrain, the conversion from D-KYN to KYNA was mainly catalyzed by KAT, while in cerebellum, most of the production of KYNA was from D-AAO. Rat and mouse brain and liver can also convert D-KYN to3-HK in vitro, which was probably catalyzed by kynurenine3-monooxygenase (KMO).Local perfusion of D-KYN in to rat striatum significantly elevated extracellular KYNA level to4times of baseline level and decreased extracellular dopamine level by30%. Co-perfusion of AOAA or KA attenuated aobut40%the effect of D-KYN respectively. Similar to striatum, reverse dialysis of D-KYN in to rat prefrontal cortex and cerebellum sisnificantly elevated extracellular KYNA concentration, especially in cerebellum where KYNA concentration inceased about200times. Co-perfusion of kojic acid attenuated about60%and95%the effect of D-KYN respectively.Systemically administered D-KYN (30mg/kg) was converted to KYNA and3-HK in mice in all four tissues tested. Cerebellum produced much more KYNA than forebrain, but L-KYN (30mg/kg) was still the more effective precursor for KYNA. Surprisingly, D-KYN was more potent as a precursor of3-HK in the liver than L-KYN. The production of KYNA and3-HK from i.p. injection of D-KYN was dose and time dependent after a higher dose (300mg/kg) of D-KYN treatment in six time points (5,15,30,120and240min).Conclusions:1. In central nervous system, D-KYN could serve as a precursor of KYNA, which was more effective in cerebellum.2. The conversion of D-KYN to KYNA is catalyzed by both KAT and D-AAO.3. Local perfusion of D-KYN into rat brain not only increased the extracellar level of KYNA, but also decreased the the extracellar level of dopamine.4. D-KYN can also be converted to D-3-HK which was probalbaly catalyzed by KMO. Therefore, D-KYN, which might be elevated in centraunder infections, could eventually increase the level of KYNA and D-3-HK in brain, thus plays important physiological and pathological roles.