UPTAKE OF 3-HYDROXYKYNURENINE MEASURED IN RAT-BRAIN SLICES AND IN A NEURONAL CELL-LINE

The uptake of 3-hydroxykynurenine (3HK), a tryptophan metabolite with reported convulsant and cytotoxic properties, has been investigated in a neuronally derived hybrid cell line and in tissue slices prepared from rat brain. In both systems, the observed uptake was temperature-dependent and inhibited in the presence of large neutral amino acids. The apparent K(m) and V(max) determined for 3HK uptake into N18 RE-105 cells were 1.65 mM and 25.5 nmol/(min x mg protein), respectively. The uptake of 3HK into rat brain slices could be resolved into two components on the basis of their requirements for sodium. Kinetic analyses performed using hippocampal slices revealed a K(m) of 1.1 mM and V(max) of 18.8 nmol/(h x mg protein) for the sodium-independent process and a K(m) of 4.8 mM and V(max) of 54.5 nmol/(h x mg protein) for the sodium-dependent process. While sodium-dependent uptake was abolished following treatment with metabolic inhibitors, sodium-independent uptake was only slightly impaired. Sodium-independent uptake was inhibited in the presence of the non-metabolizable amino acids, aminoisobutyric acid (AIB) and aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH), but not by N-methylated amino acid substrates. Sodium-dependent uptake was insensitive to AIB and was completely abolished by BCH. These results indicate that an uptake process for 3HK is present in the mammalian brain, and suggest that the sodium-dependent component of 3HK transport may be mediated by a system which has not previously been described in CNS tissue.