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Characterization of rat brain kynurenine aminotransferases I and II
Author(s) -
Guidetti Paolo,
Okuno Etsuo,
Schwarcz Robert
Publication year - 1997
Publication title -
journal of neuroscience research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.72
H-Index - 160
eISSN - 1097-4547
pISSN - 0360-4012
DOI - 10.1002/(sici)1097-4547(19971101)50:3<457::aid-jnr12>3.0.co;2-3
Subject(s) - kynurenine , kynurenic acid , transamination , glutamine , biochemistry , enzyme , transaminase , endogeny , chemistry , amino acid , biology , tryptophan
The endogenous neuroprotectant kynurenic acid (KYNA) is produced by irreversible transamination of L‐kynurenine (KYN). In the brain, two distinct kynurenine aminotransferases (KAT I and KAT II) are responsible for the formation of KYNA. The present experiments were designed to examine the respective roles of the two KATs in the normal rat brain. To this end, the two enzymes were partially purified, and their characteristics were examined. KAT I (identical with glutamine transaminase K) had an optimal pH of 9.5, preferred pyruvate as a cosubstrate and was potently inhibited by glutamine. KAT II (identical with L‐α‐aminoadipate transaminase) had a neutral optimal pH, showed no preference for pyruvate, and was essentially insensitive to inhibition by glutamine. KAT II was selectively inhibited by quisqualic acid (IC 50 : 520 μM). The endogenous substrate 3‐hydroxykynurenine had an approximately 10‐fold preference for KAT II. The distinct properties of the two enzymes made it possible to measure brain KAT I and KAT II in parallel by using dialyzed tissue homogenate (to remove interfering endogenous amino acids). Under these conditions, both enzymes presented essentially the same apparent K m values as the partially purified enzymes. In lesioned, neuron‐depleted brain tissue and in brain regions other than the cerebellum, KYNA derived primarily from KAT II at physiologic pH. In summary, the present study describes a simple methodology for the simultaneous determination of the two KYNA‐producing enzymes in small rat brain tissue samples and provides baseline values for future work in experimentally challenged animals. J. Neurosci. Res. 50:457–465, 1997. © 1997 Wiley‐Liss, Inc.