N(G)-nitro-L-arginine and its methyl ester inhibit brain synthesis of kynurenic acid possibly via nitric oxide-independent mechanism.

The effect of nitric oxide synthase (NOS) inhibitors on the brain production of endogenous glutamate receptor antagonist, kynurenic acid, was estimated in vitro. Under standard incubation conditions N(G)-nitro-L-arginine, but not N(G)-nitro-L-arginine methyl ester, up to 5 mM, or 7-nitroindazole, up to 100 microM, inhibited de novo synthesis of kynurenic acid in cortical slices. However, during prolonged incubation, N(G)-nitro-L-arginine methyl ester also reduced the production of kynurenic acid. The substrate for NOS, L-arginine (up to 5 mM), did not influence kynurenic acid synthesis and did not reverse the N(G)-nitro-L-arginine-evoked changes, suggesting that the observed effects are not related to disturbed generation of NO. Enzymatic studies revealed that N(G)-nitro-L-arginine and its methyl ester blocked the activity of brain kynurenine aminotransferase (KAT) I. The activity of KAT II was diminished only by N(G)-nitro-L-arginine. Kinetic analyses have shown that N(G)-nitro-L-arginine and its methyl ester reduce Vmax and increase Km of KAT I, whereas N(G)-nitro-L-arginine diminishes Vmax of KAT II. In conclusion, we report that N(G)-nitro-L-arginine and its methyl ester impair brain synthesis of kynurenic acid, probably via NO-independent mechanism, what could contribute, at least partially, to the enhancement of neurotoxicity or seizures observed in some experimental designs based on their use.