Kynurenine Aminotransferase Activity Research Articles

Angiotensin-converting enzyme inhibitors modulate kynurenic acid production in rat brain cortex in vitro

It is well established that the renin-angiotensin system (RAS) is present in the brain and that glutamate activates the brain centers responsible for blood pressure control. An antagonist of glutamate, kynurenic acid (KYNA) was shown to decrease blood pressure after intracerebral administration. KYNA is an endogenous metabolite of tryptophan produced from the breakdown of kynurenine by kynurenine aminotransferases (KAT), mainly within astrocytes.The purpose of this study was to evaluate the influence of three angiotensin-converting enzyme inhibitors (lisinopril, perindopril and ramipril) on KYNA production and KAT activity in the rat brain cortex in vitro.The effect of the angiotensin-converting enzyme inhibitors on KYNA production was examined on rat brain cortical slices incubated for 2h in the presence of l-kynurenine and the angiotensin-converting enzyme inhibitors. To analyze KAT I and KAT II activity, brain cortical homogenates were incubated for 2h with L-kynurenine and the tested drugs. KYNA was separated by HPLC and quantified fluorometrically.Among the examined angiotensin-converting enzyme inhibitors, lisinopril increased KYNA production, perindopril was ineffective, and ramipril decreased KYNA synthesis in rat brain cortical slices. Lisinopril increased KAT I activity and perindopril did not affect it. However, ramipril lowered KAT I activity in rat brain cortex in vitro. Neither lisinopril nor perindopril affected KAT II activity, but ramipril decreased KAT II activity in the rat brain cortex in vitro.Our study reveals that angiotensin-converting enzyme inhibitors show various influences on KYNA production in rat brain cortical slices and activity of KATs.

Tryptophan Metabolism in Rat Liver after Administration of Tryptophan, Kynurenine Metabolites, and Kynureninase Inhibitors

Rat liver tryptophan (Trp), kynurenine pathway metabolites, and enzymes deduced from product/substrate ratios were assessed following acute and/or chronic administration of kynurenic acid (KA), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HAA), Trp, and the kynureni-nase inhibitors benserazide (BSZ) and carbidopa (CBD). KA activated Trp 2,3-dioxygenase (TDO), possibly by increasing liver 3-HAA, but inhibited kynurenine aminotransferase (KAT) and kynureninase activities with 3-HK as substrate. 3-HK inhibited kynureninase activity from 3-HK. 3-HAA stimulated TDO, but inhibited kynureninase activity from K and 3-HK. Trp (50 mg/kg) increased kynurenine metabolite concentrations and KAT from K, and exerted a temporary stimulation of TDO. The kynureninase inhibitors BSZ and CBD also inhibited KAT, but stimulated TDO. BSZ abolished or strongly inhibited the Trp-induced increases in liver Trp and kynurenine metabolites. The potential effects of these changes in conditions of immune activation, schizophrenia, and other disease states are discussed.

Effect of irbesartan on kynurenic acid production and activity of kynurenine aminotransferases in rat brain cortex

Effect of irbesartan on kynurenic acid production and activity of kynurenine aminotransferases in rat brain cortex

Alternative kynurenic acid synthesis routes studied in the rat cerebellum.

Kynurenic acid (KYNA), an astrocyte-derived, endogenous antagonist of α7 nicotinic acetylcholine and excitatory amino acid receptors, regulates glutamatergic, GABAergic, cholinergic and dopaminergic neurotransmission in several regions of the rodent brain. Synthesis of KYNA in the brain and elsewhere is generally attributed to the enzymatic conversion of L-kynurenine (L-KYN) by kynurenine aminotransferases (KATs). However, alternative routes, including KYNA formation from D-kynurenine (D-KYN) by D-amino acid oxidase (DAAO) and the direct transformation of kynurenine to KYNA by reactive oxygen species (ROS), have been demonstrated in the rat brain. Using the rat cerebellum, a region of low KAT activity and high DAAO activity, the present experiments were designed to examine KYNA production from L-KYN or D-KYN by KAT and DAAO, respectively, and to investigate the effect of ROS on KYNA synthesis. In chemical combinatorial systems, both L-KYN and D-KYN interacted directly with peroxynitrite (ONOO−) and hydroxyl radicals (OH•), resulting in the formation of KYNA. In tissue homogenates, the non-specific KAT inhibitor aminooxyacetic acid (AOAA; 1 mM) reduced KYNA production from L-KYN and D-KYN by 85.1 ± 1.7% and 27.1 ± 4.5%, respectively. Addition of DAAO inhibitors (benzoic acid, kojic acid or 3-methylpyrazole-5-carboxylic acid; 5 μM each) attenuated KYNA formation from L-KYN and D-KYN by ~35% and ~66%, respectively. ONOO− (25 μM) potentiated KYNA production from both L-KYN and D-KYN, and these effects were reduced by DAAO inhibition. AOAA attenuated KYNA production from L-KYN + ONOO− but not from D-KYN + ONOO−. In vivo, extracellular KYNA levels increased rapidly after perfusion of ONOO− and, more prominently, after subsequent perfusion with L-KYN or D-KYN (100 μM). Taken together, these results suggest that different mechanisms are involved in KYNA production in the rat cerebellum, and that, specifically, DAAO and ROS can function as alternative routes for KYNA production.

Experimental diabetes mellitus type 1 increases hippocampal content of kynurenic acid in rats

BackgroundDiabetes mellitus (DM) is frequently associated with peripheral and central complications and has recently emerged as a risk factor for cognitive impairment and dementia. Kynurenic acid (KYNA), a unique tryptophan derivative, displays pleiotropic effects including blockade of ionotropic glutamate and α7 nicotinic receptors. Here, the influence of experimental diabetes on KYNA synthesis was studied in rat brain. MethodsDM was induced by i.p. administration of streptozotocin (STZ). Five weeks later, KYNA content and the activity of semi-purified kynurenine aminotransferases (KATs) were measured in frontal cortex, hippocampus and striatum of diabetic and insulin-treated rats, using HPLC-based methods. ResultsHippocampal but not cortical or striatal KYNA concentration was considerably increased during DM, either untreated or treated with insulin (220% and 170% of CTR, respectively). The activity of kynurenine aminotransferase I (KAT I) was not affected by DM in all of the studied structures. KAT II activity was moderately increased in cortex (145% of CTR) and hippocampus (126% of CTR), but not in striatum of diabetic animals. Insulin treatment normalized cortical but not hippocampal KAT II activity. ConclusionsA novel factor potentially implicated in diabetic hippocampal dysfunction has been identified. Observed increase of KYNA level may stem from the activation of endogenous neuroprotection, however, it may also have negative impact on cognition.

Serum indoleamine 2,3-dioxygenase and kynurenine aminotransferase enzyme activity in patients with ischemic stroke

We investigated the activation and pathophysiological roles of indoleamine 2,3-dioxygenase (IDO) and kynurenine aminotransferase (KAT) in patients with ischemic stroke. Patients were recruited from the acute stroke unit of a general hospital within 24hours post-stroke. The immune transmission turbidity method was used to determine the concentration of serum high-sensitivity C-reactive protein (hsCRP), apolipoprotein A-1 and apolipoprotein B. The concentrations of triglyceride, cholesterol, high density lipoprotein (HDL), low density lipoprotein and non-esterified fatty acids were determined using an enzymatic method. Tryptophan (TRP), kynurenine (KYN) and kynurenine acid (KYNA) concentrations were determined by high performance liquid chromatography. The National Institutes of Health Stroke Scale (NIHSS) was used to assess the neurological deficits at admission and 3weeks post-stroke. The IDO and KAT activity ratio were calculated by KYN/TRP and KYNA/KYN, respectively. The correlation between hsCRP and IDO, KAT and NIHSS score was also analyzed. A total of 81 patients with ischemic stroke and 35 normal controls were recruited. Lower TRP, KYNA, HDL and KAT activity ratio were found in the stroke group compared to the control group (p<0.05). The levels of hsCRP and IDO activity ratio were much higher in the stroke group than the control group (p<0.01). The IDO activity in patients with ischemic stroke showed a positive correlation with hsCRP (r=0.425, p=0.027). In addition, hsCRP and IDO levels were positively associated with the NIHSS score both at admission and 3weeks post-stroke. These data suggest an inflammatory response characterized by up-regulated IDO activation in ischemic stroke, which might be closely relevant to its pathophysiology.

P.2.d.006 How does amitriptyline treatment affect coenzyme Q levels and oxidative stress in patients diagnosed with depressive disorder?

We investigated the activation and pathophysiological roles of indoleamine 2,3-dioxygenase (IDO) and kynurenine aminotransferase (KAT) in patients with ischemic stroke. Patients were recruited from the acute stroke unit of a general hospital within 24 hours post-stroke. The immune transmission turbidity method was used to determine the concentration of serum high-sensitivity C-reactive protein (hsCRP), apolipoprotein A-1 and apolipoprotein B. The concentrations of triglyceride, cholesterol, high density lipoprotein (HDL), low density lipoprotein and non-esterified fatty acids were determined using an enzymatic method. Tryptophan (TRP), kynurenine (KYN) and kynurenine acid (KYNA) concentrations were determined by high performance liquid chromatography. The National Institutes of Health Stroke Scale (NIHSS) was used to assess the neurological deficits at admission and 3 weeks post-stroke. The IDO and KAT activity ratio were calculated by KYN/TRP and KYNA/KYN, respectively. The correlation between hsCRP and IDO, KAT and NIHSS score was also analyzed. A total of 81 patients with ischemic stroke and 35 normal controls were recruited. Lower TRP, KYNA, HDL and KAT activity ratio were found in the stroke group compared to the control group (p < 0.05). The levels of hsCRP and IDO activity ratio were much higher in the stroke group than the control group (p < 0.01). The IDO activity in patients with ischemic stroke showed a positive correlation with hsCRP (r = 0.425, p = 0.027). In addition, hsCRP and IDO levels were positively associated with the NIHSS score both at admission and 3 weeks post-stroke. These data suggest an inflammatory response characterized by up-regulated IDO activation in ischemic stroke, which might be closely relevant to its pathophysiology.

Biochemical and structural characterization of mouse mitochondrial aspartate aminotransferase, a newly identified kynurenine aminotransferase-IV

Mammalian mAspAT (mitochondrial aspartate aminotransferase) is recently reported to have KAT (kynurenine aminotransferase) activity and plays a role in the biosynthesis of KYNA (kynurenic acid) in rat, mouse and human brains. This study concerns the biochemical and structural characterization of mouse mAspAT. In this study, mouse mAspAT cDNA was amplified from mouse brain first stand cDNA and its recombinant protein was expressed in an Escherichia coli expression system. Sixteen oxo acids were tested for the co-substrate specificity of mouse mAspAT and 14 of them were shown to be capable of serving as co-substrates for the enzyme. Structural analysis of mAspAT by macromolecular crystallography revealed that the cofactor-binding residues of mAspAT are similar to those of other KATs. The substrate-binding residues of mAspAT are slightly different from those of other KATs. Our results provide a biochemical and structural basis towards understanding the overall physiological role of mAspAT in vivo and insight into controlling the levels of endogenous KYNA through modulation of the enzyme in the mouse brain.

Development of a microplate fluorescence assay for kynurenine aminotransferase

Inhibition of kynurenine aminotransferases (KATs) is a strategy to therapeutically reduce levels of kynurenic acid (KYNA), an endogenous antagonist of glutamatergic N-methyl- d-aspartate (NMDA) and cholinergic α 7 nicotinic receptors. Several methods of measuring KAT activity in vitro have been developed, but none is well-suited to high throughput and automation. In this article, we describe a modification of existing high-performance liquid chromatography (HPLC)-based methods that enables the development of a 96-well microplate assay in both enzyme- and cell-based formats using human KAT I as an example. KYNA enzymatically produced from l-kynurenine is measured directly in a reaction mixture fluorimetrically.

Kynurenine metabolism in multiple sclerosis

Objective--Excitatory amino acid receptors are involved in the normal physiology of the brain, and may play a role in the pathogenesis of neurological disorders such as Huntington's disease, Parkinson's disease, amyotrophic lateral sclerosis, etc. It has been demonstrated that the blockade of one of these receptors ameliorates the symptoms of experimental allergic encephalomyelitis, an animal model of multiple sclerosis (MS). In a recent study, a decreased level of kynurenic acid was found in the cerebrospinal fluid of patients with MS. The only known endogenous excitotoxin receptor antagonist is the tryptophan metabolite kynurenic acid. Another metabolite is quinolinic acid, which exerts different action: it is an excitotoxin receptor agonist. The ratio of these two metabolites is determined by the activities of kynurenine aminotransferase I and II (KAT I and KAT II). In this study, we measured the activities of these enzymes and the concentration of kynurenic acid in the red blood cells (RBC) and in the plasma of patients with MS. KAT activities were detected both in the RBC and in the plasma. As compared with the control subjects, the KAT I and KAT II activities were significantly higher in the RBC of the patients. The concentration of kynurenic acid is elevated in the plasma of MS patients, and there is a tendency to an elevation in the RBC. These changes may indicate a compensatory protective mechanism against excitatory neurotoxic effects. Our data demonstrate the involvement of the kynurenine system in the pathogenesis of MS, which may predict a novel therapeutic intervention.

Tryptophan metabolism and oxidative stress in patients with Huntington's disease

Abnormalities in the kynurenine pathway may play a role in Huntington's disease (HD). In this study, tryptophan depletion and loading were used to investigate changes in blood kynurenine pathway metabolites, as well as markers of inflammation and oxidative stress in HD patients and healthy controls. Results showed that the kynurenine : tryptophan ratio was greater in HD than controls in the baseline state and after tryptophan depletion, indicating increased indoleamine dioxygenase activity in HD. Evidence for persistent inflammation in HD was provided by elevated baseline levels of C-reactive protein, neopterin and lipid peroxidation products compared with controls. The kynurenate : kynurenine ratio suggested lower kynurenine aminotransferase activity in patients and the higher levels of kynurenine in patients at baseline, after depletion and loading, do not result in any differences in kynurenic acid levels, providing no supportive evidence for a compensatory neuroprotective role for kynurenic acid. Quinolinic acid showed wide variations in blood levels. The lipid peroxidation data indicate a high level of oxidative stress in HD patients many years after disease onset. Levels of the free radical generators 3-hydroxykynurenine and 3-hydroxyanthranilic acid were decreased in HD patients, and hence did not appear to contribute to the oxidative stress. It is concluded that patients with HD exhibit abnormal handling of tryptophan metabolism and increased oxidative stress, and that these factors could contribute to ongoing brain dysfunction.

Dual effect of DL‐homocysteine and S‐adenosylhomocysteine on brain synthesis of the glutamate receptor antagonist, kynurenic acid

Increased serum level of homocysteine, a sulfur-containing amino acid, is considered a risk factor in vascular disorders and in dementias. The effect of homocysteine and metabolically related compounds on brain production of kynurenic acid (KYNA), an endogenous antagonist of glutamate ionotropic receptors, was studied. In rat cortical slices, DL-homocysteine enhanced (0.1-0.5 mM) or inhibited (concentration inducing 50% inhibition [IC50]=6.4 [5.5-7.5] mM) KYNA production. In vivo peripheral application of DL-homocysteine (1.3 mmol/kg intraperitoneally) increased KYNA content (pmol/g tissue) from 8.47 +/- 1.57 to 13.04 +/- 2.86 (P <0.01; 15 min) and 11.4 +/- 1.72 (P <0.01; 60 min) in cortex, and from 4.11 +/- 1.54 to 10.02 +/- 3.08 (P <0.01; 15 min) in rat hippocampus. High concentrations of DL-homocysteine (20 mM) applied via microdialysis probe decreased KYNA levels in rabbit hippocampus; this effect was antagonized partially by an antagonist of group I metabotropic glutamate receptors, LY367385. In vitro, S-adenosylhomocysteine acted similar to but more potently than DL-homocysteine, augmenting KYNA production at 0.03-0.08 mM and reducing it at > or =0.5 mM. The stimulatory effect of S-adenosylhomocysteine was abolished in the presence of the L-kynurenine uptake inhibitors L-leucine and L-phenyloalanine. Neither the N-methyl-D-aspartate (NMDA) antagonist CGS 19755 nor L-glycine influenced DL-homocysteine- and S-adenosylhomocysteine-induced changes of KYNA synthesis in vitro. DL-Homocysteine inhibited the activity of both KYNA biosynthetic enzymes, kynurenine aminotransferases (KATs) I and II, whereas S-adenosylhomocysteine reduced only the activity of KAT II. L-Methionine and L-cysteine, thiol-containing compounds metabolically related to homocysteine, acted only as weak inhibitors, reducing KYNA production in vitro and inhibiting the activity of KAT II (L-cysteine) or KAT I (L-methionine). The present data suggest that DL-homocysteine biphasically modulates KYNA synthesis. This seems to result from conversion of compound to S-adenosylhomocysteine, also acting dually on KYNA formation, and in part from the direct interaction of homocysteine with metabotropic glutamate receptors and KYNA biosynthetic enzymes. It seems probable that hyperhomocystemia-associated brain dysfunction is mediated partially by changes in brain KYNA level.

Cysteine and keto acids modulate mosquito kynurenine aminotransferase catalyzed kynurenic acid production

Kynurenine aminotransferase (KAT) catalyzes the formation of kynurenic acid (KYNA), the natural antagonist of ionotropic glutamate receptors. This study tests potential substrates and assesses the effects of amino acids and keto acids on the activity of mosquito KAT. Various keto acids, when simultaneously present in the same reaction mixture, display a combined effect on KAT catalyzed KYNA production. Moreover, methionine and glutamine show inhibitory effects on KAT activity, while cysteine functions as either an antagonist or an inhibitor depending on the concentration. Therefore, the overall level of keto acids and cysteine might modulate the KYNA synthesis. Results from this study will be useful in the study of KAT regulation in other animals.

Carbamazepine enhances brain production of kynurenic acid in vitro

Disturbed formation of kynurenic acid, an endogenous antagonist of glutamate ionotropic receptors, might contribute to the pathogenesis of seizures. Here, the effect of anticonvulsant drug, carbamazepine on the production of kynurenic acid was studied. Carbamazepine (0.5–3 mM) enhanced kynurenic acid synthesis in rat cortical slices and also increased the activity of kynurenine aminotransferase (KAT) I at 0.1–3.0 mM concentration. Thus, anticonvulsant drugs, such as carbamazepine, might act partially via stimulation of kynurenic acid production.

Content of Kynurenic Acid and Activity of Kynurenine Aminotransferases in Mammalian Eyes

The present study investigated the kynurenic acid (KYNA) contents and kynurenine aminotransferase (KAT I and II) activity in structures of the human, monkey, rabbit and bovine eye. KYNA levels were investigated with HPLC and detected fluorimetrically. The activity of KAT I and II was assayed as quantitative analysis of newly synthesized KYNA in vitro. Mean KYNA levels (±SD) in the human retina and vitreous body were 36.8 ± 7.6 and 33.1 ± 6.2 pmol/g wet tissue weight, respectively. In human eyes, KAT I activity in the vitreous body was 0.57 ± 0.28, that of KAT II was 2.56 ± 0.69. KAT I activity in the retina was 3.42 ± 1.17 and that of KAT II 10.75 ± 9.2. (KAT activity is expressed as KYNA synthesis in picomoles per gram wet tissue weight per hour.) The values of KYNA and KAT observed in other mammalian species tested were in the same range. In conclusion, KYNA and KAT enzymatic activity are present in the structures of human and other mammalian eyes.

FK506 attenuates 1-methyl-4-phenylpyridinium- and 3-nitropropionic acid-evoked inhibition of kynurenic acid synthesis in rat cortical slices

Kynurenic acid (KYNA), the only known endogenous glutamate antagonist, is produced in the brain by kynurenine aminotransferases (KATs) I and II. Mitochondrial toxins, 1-methyl-4-phenylpyridinium (MPP+) and 3-nitropropionic acid (3-NPA), were previously shown to reduce KYNA synthesis via interference with KAT I and II. Data presented here demonstrate that immunophilin ligand, FK506 (10-130 µM), but not CsA (1-50 µM), or ryanodine receptor blocker, dantrolene (1-100 µM), enhances the formation of KYNA in cortical slices. FK506, but not CsA or dantrolene, abolished the inhibition of KYNA synthesis evoked by MPP+ and 3-NPA. None of studied compounds influenced the activity of KAT I and KAT II. FK506 is the first among currently used drugs that might stimulate KYNA synthesis. This effect does not seem to arise from the interference with KATs or calcineurin activity.

L-Cysteine sulphinate, endogenous sulphur-containing amino acid, inhibits rat brain kynurenic acid production via selective interference with kynurenine aminotransferase II

In the present study the effect of endogenous sulphur-containing amino acids, l-cysteine sulphinate, l-cysteate, l-homocysteine sulphinate and l-homocysteate, on the production of glutamate receptor antagonist, kynurenic acid (KYNA), was evaluated. The experiments comprised the measurements of (a) KYNA synthesis in rat cortical slices and (b) the activity of KYNA biosynthetic enzymes, kynurenine aminotransferases (KATs). All studied compounds reduced KYNA production and inhibited the activity of KAT I and/or KAT II, thus acting most probably intracellularly. l-Cysteine sulphinate in very low, micromolar concentrations selectively affected the activity of KAT II, the enzyme catalyzing approximately 75% of KYNA synthesis in the brain. l-Cysteine sulphinate potency was higher than other studied sulphur-containing amino acids, than l-aspartate, l-glutamate, or any other known KAT II inhibitor. Thus, l-cysteine sulphinate might act as a modulator of KYNA formation in the brain.

Formation of kynurenic and xanthurenic acids from kynurenine and 3-hydroxykynurenine in the dinoflagellate Lingulodinium polyedrum: role of a novel, oxidative pathway

The dinoflagellate Lingulodinium polyedrum (syn. Gonyaulax polyedra) was used as a model organism for studying the effects of high and low physiological oxidative stress on the formation of kynurenic and xanthurenic acids from kynurenine and 3-hydroxykynurenine. Cell were incubated with the precursors and exposed to light (high physiological stress due to photosynthetically formed oxidants) or kept in darkness (low stress). In cultures of less than 0.5 ml cell volume/l of medium, cells took up approximately one half of 0.1 mM extracellular kynurenine within 18 h. The amino acid was partially converted to kynurenic acid, most of which was released to the medium; however, intracellular concentrations of the product were by approximately 10-fold higher than extracellular levels. Rates of kynurenic acid release exceeded by far those explained by kynurenine and tryptophan aminotransferase activities, the latter representing an additional source of kynurenic acid formation via indole-3-pyruvic acid. Light enhanced the release of kynurenic acid by approximately 4-fold; these rates were further increased by exposure to continuous light. Diurnal rhythmicity of kynurenic acid release was clearly exogenous and did not match with the circadian pattern of kynurenine or tryptophan aminotransferase activities; no rhythm was detected in constant darkness. Similar findings were obtained on turnover of 3-hydroxykynurenine to xanthurenic acid and release of the product to the medium. However, light/dark differences were relatively smaller, and additional products were formed, according to HPLC data obtained with electrochemical detection. Results are most easily explained on the basis of a recently discovered pathway of kynurenic acid formation from kynurenine, involving either non-enzymatic oxidation by H 2O 2 or, at higher rates, enzymatic catalysis by hemoperoxidase. A corresponding mechanism may exist for the hydroxylated analogue.

1-Methyl-4-phenylpyridinium and 3-nitropropionic acid diminish cortical synthesis of kynurenic acid via interference with kynurenine aminotransferases in rats

The aim of the present study was to evaluate the effect of mitochondrial inhibitors, 1-methyl-4-phenylpyridinium (MPP +) and 3-nitropropionic acid (3-NPA), on the brain production of endogenous glutamate antagonist, kynurenic acid (KYNA). MPP + and 3-NPA dose-dependently impaired the synthesis of KYNA in rat cortical slices. Enzymatic studies revealed that MPP + inhibits in a concentration-dependent manner the activity of kynurenine aminotransferase II (KAT II), but not the activity of kynurenine aminotransferase I (KAT I). 3-NPA impaired the activity of both enzymes, KAT I and KAT II. Thus, MPP +- and 3-NPA-evoked neurotoxicity may be at least partially associated with the depletion of KYNA.

Isolation, characterization, and functional expression of kynurenine aminotransferase cDNA from the yellow fever mosquito, Aedes aegypti

This study describes the molecular and biochemical characterization of kynurenine aminotransferase (KAT) from Aedes aegypti mosquitoes. Through screening an A. aegypti larval cDNA library, a 1695-bp full-length cDNA clone with a 1434-bp open reading frame (ORF) was isolated. Its deduced amino acid sequence consists of 477 amino acid residues with a predicted molecular mass of 53,490 and the amino acid sequence shares 47 and 42% identity with KATs from Homo sapiens and Rattus norvegicus, respectively. This putative A. aegypti KAT (AeKAT) has four potential N-glycosylation sites (Asn-Xxx-Trp/Ser) and a typical mitochondrial leader sequence consisting of 49 amino acids at its NH 2-terminus with a putative cleavage site between Met-49 and Ser-50. A consensus pyridoxal 5′-phosphate (PLP) binding domain (Gly-Ser-Ala-Gly-Lys-Thr-Phe-Ser) is present in the central region of the ORF. Using a baculovirus/insect cell expression system, a full-length AeKAT and a truncated AeKAT, lacking the mitochondrial leader sequence, were expressed. The full-length AeKAT expressed in Sf9 insect cells is insoluble and has no detectable KAT activity, but the truncated AeKAT is soluble and capable of catalyzing the transamination of kynurenine to kynurenic acid in the presence of pyruvate as an amino group acceptor. However, the expressed truncated protein is not active to 3-hydroxykynurenine. Northern analysis indicates that transcription of the AeKAT occurs at all stages during mosquito development, but higher levels of mRNA are observed during the pupal and adult stages. Results indicate that a specific KAT is present in mosquitoes and the enzyme may play an important physiological role in A. aegypti.