Kynurenic acid (KYNA) is a tryptophan metabolite and represents the only known endogenous compound acting as an antagonist to excitatory amino acid receptors in the mammalian CNS. Blocking of these receptors in CNS by KYNA affects cardiac function. As it is not known whether human heart is able to synthesize this neuromodulatory amino acid, we investigated the biosynthesizing enzyme of kynurenine aminotransferase (KAT) in the human heart and compared the activity with that of the human brain. The activities of heart and brain KATs were assayed by the conversion of L-kynurenine (L-KYN) to KYNA and quantitated by HPLC with fluorescence detection. Using either pyruvate or 2-oxoglutarate as cosubstrates, heart KAT was found to have a shallow pH optimum between 8 and 9. Highest heart KAT activity was seen in the presence of 2-oxoglutarate, followed by pyruvate, 2-oxoadipate, and 2-oxoisocaproate. Kinetic analyses, performed at pH 8.5, and using various concentrations of L-KYN (from 0.125 to 22.8 mM) in the presence of 2-oxoglutarate (1 and 5 mM) or pyruvate (5 mM) revealed apparentKmvalues in the millimolar range, for L-KYN 1.5, 27, and 20 mM, respectively. Heart KAT activities were compared with those in human brain KAT I and KAT II showing different pH optima 7.4 and 9.6, respectively. In contrast to brain KAT I, heart KAT activity was not inhibited by an excess of 2 mM L-tryptophan, L-glutamine, or L-phenylalanine at pH 9.6, as well as at pH 8 or 7.4. Our study demonstrates that human heart is capable of synthesizing KYNA from low concentrations of L-KYN selectively. A shallow pH optimum of KAT activity,i.e.between 8.0 and 9.0, pronounced 2-oxoacid specificity, and a lack of sensitivity to inhibition by L-glutamine, L-phenylalanine, and L-tryptophan indicate that the heart KAT system displays enzymatic characteristics different from those of human brain KAT I or KAT II. Fluctuation of L-KYN and 2-oxoacid levels may markedly influence the KYNA synthesis and subsequent KYNA effect on cardiac activity. KYNA synthesis in the human heart suggests a neurophysiologic role. Our studies form the basis for purification and further characterization of KAT protein in human heart as well as for physiologic studies.Abbreviations: KYNA,kynurenic acid;KAT,kynurenine aminotransferase [f]L-KYN, L-kynurenine;NMDA,N-methyl-D-aspartate;AMPA,α-amino-3-hydroxy-5-methyl-4-isoxazole propionate;AMP,2 amino-2-methyl-1-propanol;EAA,excitatory amino acid