KYNURENIC ACID AND KYNURENINE AMINOTRANSFERASE IN HEART

Kynurenic acid (KYNA) is a tryptophan metabolite and represents the on ly known endogenous compound acting as an antagonist to excitatory ami no acid receptors in the mammalian CNS. Blocking of these receptors in CNS by KYNA affects cardiac function. As it is not known whether huma n heart is able to synthesize this neuromodulatory amino acid, we inve stigated the biosynthesizing enzyme of kynurenine aminotransferase (KA T) 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 con version of L-kynurenine (L-KYN) to KYNA and quantitated by HPLC with f luorescence detection. Using either pyruvate or 2-oxoglutarate as cosu bstrates, heart KAT was found to have a shallow pH optimum between 8 a nd 9. Highest heart KAT activity was seen in the presence of 2-oxoglut arate, followed by pyruvate, 2-oxoadipate, and 2-oxoisocaproate. Kinet ic 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 apparent K-m values in the millimol ar range, for L-KYN 1.5, 27, and 20 mM, respectively. Heart KAT activi ties 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 KA T I, heart KAT activity was not inhibited by an excess of 2 mM L-trypt ophan, L-glutamine, or L-phenylalanine at pH 9.6, as well as at pH 8 o r 7.4. Our study demonstrates that human heart is capable of synthesiz ing KYNA from low concentrations of L-KYN selectively. A shallow pH op timum 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 di splays enzymatic characteristics different from those of human brain K AT I or KAT II. Fluctuation of L-KYN and 2-oxoacid levels may markedly influence the KYNA synthesis and subsequent KYNA effect on cardiac ac tivity. KYNA synthesis in the human heart suggests a neurophysiologic role. Our studies form the basis for purification and further characte rization of KAT protein in human heart as well as for physiologic stud ies.