Introduction
Kynurenic acid (KYNA) is a metabolite of tryptophan
formed enzymatically along the kynurenine
pathway. The first step in the pathway is catalyzed by
tryptophan 2, 3-dioxygenase (TDO) and indoleamine
2, 3-dioxygenase (IDO), enzymes responsible for
tryptophan degradation to formylkynurenine. TDO is
primarily expressed within the liver and IDO is an
extrahepatic cytokine inducible enzyme. The next
step in the synthesis of kynurenine is catalyzed by
formylkynurenine formamidase. The main end product
of kynurenine catabolism is nicotinamide adenosine
dinucleotide (NAD+
). An another biologically
active product, KYNA, is generated from kynurenine
by kynurenine aminotransferase.1,2
KYNA presence was first demonstrated in urine
by Liebig.3
Nevertheless, the compound was not thoroughly
analyzed until the 1980’s and 1990’s when
researchers indicated that KYNA is an antagonist
of ionotropic glutamate receptors.4–8 Subsequ