INHIBITORS OF KYNURENINE HYDROXYLASE AND KYNURENINASE INCREASE CEREBRAL FORMATION OF KYNURENATE AND HAVE SEDATIVE AND ANTICONVULSANT ACTIVITIES

Kynurenate is an endogenous antagonist of the ionotropic glutamate rec eptors. It is synthesized from kynurenine, a tryptophan metabolite, an d a significant increase in its brain concentration could be useful in pathological situations. We attempted to increase its neosynthesis by modifying kynurenine catabolism. Several kynurenine analogues were sy nthesized and tested as inhibitors of kynurenine hydroxylase (E.C.1.14 .13.9) and of kynureninase (E.C.3.7.1.3), the two enzymes which cataly se the conversion of kynurenine to excitotoxin quinolinate. Among thes e analogues we observed that nicotinylalanine, a compound whose pharma cological properties have previously been reported, had an IC50 of 900 +/- 180 mu M as inhibitor of kynurenine hydroxylase and of 800 +/- 12 0 mu M as inhibitor of kynureninase. In the search for more potent mol ecules we noticed that meta-nitrobenzoylalanine had an IC50 of 0.9 +/- 0.1 mu M as inhibitor of kynurenine hydroxylase and of 100 +/- 12 mu M as inhibitor of kynureninase. When administered to rats meta-nitrobe nzoylalanine (400 mg/kg) significantly increased the concentration of kynurenine (up to 10 times) and kynurenate (up to five times) in the b rain. Similar results were obtained in the blood and in the liver. Fur thermore meta-nitrobenzoylalanine increased in a dose dependent, long lasting (up to 13 times and up to 4 h) manner the concentration of kyn urenate in the hippocampal extracellular fluid, as evaluated with a mi crodialysis technique. This increase was associated with a decrease in the locomotor activity and with protection from maximal electroshock- induced seizures in rats or from audiogenic seizures in DBA/2 mice. Th e conclusions drawn from the present study are: (i) meta-nitrobenzoyla lanine is a potent inhibitor of kynurenine hydroxylase also affecting kynureninase; (ii) the inhibition of these enzymes causes a significan t increase in the brain extracellular concentration of kynurenate; (ii i) this increase is associated with sedative and anticonvulsant action s, suggesting a functional antagonism of the excitatory amino acid rec eptors.