rTMS reduces spatial learning and memory deficits induced by sleep deprivation possibly via suppressing the expression of kynurenine 3-monooxygenase in rats

INTRODUCTION

Impairment of learning and memory caused by sleep deprivation is a common symptom that significantly affects quality of life. Repetitive transcranial magnetic stimulation (rTMS) is a promising approach to exert a positive effect on cognitive impairment. However, there is less known about the mechanism of rTMS for learning and memory induced by chronic REM sleep deprivation (CRSD). This study was to detect the effects of rTMS on spatial learning and memory deficits by CRSD and explore possible mechanism.

METHODS

Sixty male Sprague-Dawley rats were randomly divided into four groups: wide platform (Control), sleep deprivation (SD), sleep deprivation + rTMS (TMS), and sleep deprivation + sham rTMS (Sham-TMS). Morris water maze (MWM) and open field test (OFT) assessed spatial learning and memory and anxiety of rats with pre/post-intervention. Golgi staining and transmission electron microscope (TEM) were used to observe structural variations of synapses in the hippocampus. The alteration in gene expression of different groups was analyzed by RNA-sequencing (RNA-Seq), and the key gene was screened and identified by quantitative polymerase chain reaction (qPCR) and subsequently verified with western blotting and immunofluorescence.

RESULTS

The behavioral test showed spatial learning and memory decreased and anxiety increased in the SD group compared to the Control and TMS groups. Moreover, rTMS improved spine density, ultrastructural damage, and quantities of synapses. In accordance with RNA-Seq, 56 differentially expressed genes (DEGs) were identified by comparing alternations in four groups and concentrated on kynurenine 3-monooxygenase (KMO). The expression of KMO increased significantly in rats of the SD group compared to the Control and TMS groups identified by qPCR, western blotting, and immunofluorescence.

CONCLUSION

1 Hz rTMS alleviated spatial learning and memory deficits induced by CRSD probably via down-regulating the expression of KMO and improving the structure and quantity of synapses in the hippocampus of rats.

Effect of kynurenine 3-hydroxylase inhibition on the dyskinetic and antiparkinsonian responses to levodopa in Parkinsonian monkeys

Homeostatic interactions between dopamine and glutamate are central to the normal physiology of the basal ganglia. This relationship is altered in Parkinsonism and in levodopa-induced dyskinesias (LID), resulting in an upregulation of corticostriatal glutamatergic function. Kynurenic acid (KYNA), a tryptophan metabolite with antagonist activity at ionotropic glutamate receptors and the capability to inhibit glutamate release presynaptically, might therefore be of therapeutic value in LID. To evaluate this hypothesis, we used a pharmacological tool, the kynurenine 3-hydroxylase inhibitor Ro 61-8048, which raises KYNA levels acutely. Ro 61-8048 was tested in MPTP cynomolgus monkeys with a stable parkinsonian syndrome and reproducible dyskinesias after each dose of levodopa. Serum and CSF concentrations of KYNA and its precursor kynurenine increased dose-dependently after Ro 61-8048 administration, alone or in combination with levodopa. Coadministration of Ro 61-8048 with levodopa produced a moderate but significant reduction in the severity of dyskinesias while maintaining the motor benefit. These results suggest that elevation of KYNA levels through inhibition of kynurenine 3-hydroxylase constitutes a promising novel approach for managing LID in Parkinson's disease.

Localisation of indoleamine 2,3-dioxygenase and kynurenine hydroxylase in the human placenta and decidua: implications for role of the kynurenine pathway in pregnancy

Indoleamine 2,3-dioxygenase (IDO) has been implicated in contributing to immunotolerance in early pregnancy, but the presence in the term placenta of mRNAs for enzymes that produce other biologically active kynurenine end-products suggests other functions for kynurenine pathway metabolites. The aim of this study was to investigate the localisation of two key enzymes - IDO and kynurenine hydroxylase (KYN-OHase) - in first trimester decidua and in the human placenta across pregnancy. Using immunocytochemistry, it was shown that there was strong expression of IDO and KYN-OHase in stromal and glandular epithelial cells of first trimester decidua. In first and second trimester placenta, IDO and KYN-OHase were localised to the syncytiotrophoblast, stroma and macrophages. IDO and KYN-OHase mRNAs were also identified, and the enzymes appear to be functional because kynurenine and 3-hydroxy-anthranilic acid (respective products of the activity of these enzyme) were released into the medium when first trimester placental explants were maintained in culture for 48h. In term placenta, both IDO and KYN-OHase immunoreactivities were confined mainly to vascular endothelial cells of villous blood vessels, and to macrophages within the fetal villus, whereas syncytial staining was very weak or absent. The shift of expression of these enzymes away from the syncytiotrophoblast to fetal endothelial cells in terminal villi suggests that the function of the enzymes may change from a role in immunosuppression at the maternal-fetal interface in early pregnancy, to one associated with regulation of fetoplacental blood flow or placental metabolism in late gestation.

Prolonged survival of a murine model of cerebral malaria by kynurenine pathway inhibition

C57BL/6J mice infected with Plasmodium berghei ANKA develop neurological dysfunction and die within 7 days of infection. We show that treatment of infected mice with a kynurenine-3-hydroxylase inhibitor prevents them from developing neurological symptoms and extends their life span threefold until severe anemia develops.

The Drosophila melanogaster cinnabar gene is a cell autonomous genetic marker in Aedes aegypti (Diptera: Culicidae)

The cinnabar gene of Drosophila melanogaster (Meigen) encodes for kynurenine hydroxylase, an enzyme involved in ommochrome biosynthesis. This gene is commonly included as a visible genetic marker in gene vectors used to create transgenic Aedes aegypti (L.) that are homozygous for the khw allele, the mosquito homolog of cinnabar. Unexpectedly, the phenotype of cells expressing kynurenine hydroxylase in transgenic Ae. aegypti is cell autonomous as demonstrated by the recovery of insects heterozygous for the kynurenine hydroxylase transgene with mosaic eye color patterns. In addition, a transgenic gynandromorph was recovered in which one-half of the insect was expressing the kynurenine hydroxylase transgene, including one eye with red pigmentation, whereas the other half of the insect was homozygous khw and included a white eye. The cell autonomous behavior of cinnabar in transgenic Ae. aegypti is unexpected and increases the utility of this genetic marker.

Kynurenine 3-mono-oxygenase inhibitors reduce glutamate concentration in the extracellular spaces of the basal ganglia but not in those of the cortex or hippocampus

Kynurenine 3-mono-oxygenase (KMO, kynurenine hydroxylase) inhibitors increase brain kynurenic acid (KYNA) synthesis and cause pharmacological actions possibly mediated by a reduced activity of excitatory synapses. We used in vivo microdialysis and passive avoidance to study the effects of local KYNA or systemic KMO inhibitor administration on glutamate (GLU) neurotransmission. Local application of KYNA (30-100 nM) through reverse microdialysis reduced GLU content in caudate and cortical dialysates by 75 and 55%, respectively. No changes were found in the hippocampus. Systemic administration of Ro 61-8048 (4-40 mg/kg) increased KYNA levels in dialysates obtained from the cortex (from 10.3 +/- 1.9 to 45.5 +/- 15 nM), caudate (from 2.4 +/- 0.8 to 9.5 +/- 0.9 nM) and hippocampus (from 7.7 +/- 1.7 to 19.2 +/- 3.5 nM). It also caused a parallel robust decrease in GLU levels in the dialysates collected from the caudate (from 2.2 +/- 0.5 to 0.63 +/- 0.05 microM) but not in those collected from the parietal cortex or the hippocampus. In a passive avoidance paradigm, the administration of the NMDA receptor antagonist MK-801 (0.1 mg/kg) reduced, while Ro 61-8048 (4-80 mg/kg) did not change the latency time of entering into the dark compartment on the recall trial. Our data show that KMO inhibitors increase brain KYNA synthesis and selectively reduce GLU extracellular concentration in the basal ganglia.

A genomic screen in yeast implicates kynurenine 3-monooxygenase as a therapeutic target for Huntington disease

Huntington disease is a fatal neurodegenerative disorder caused by expansion of a polyglutamine tract in the protein huntingtin (Htt), which leads to its aggregation in nuclear and cytoplasmic inclusion bodies. We recently identified 52 loss-of-function mutations in yeast genes that enhance the toxicity of a mutant Htt fragment. Here we report the results from a genome-wide loss-of-function suppressor screen in which we identified 28 gene deletions that suppress toxicity of a mutant Htt fragment. The suppressors are known or predicted to have roles in vesicle transport, vacuolar degradation, transcription and prion-like aggregation. Among the most potent suppressors was Bna4 (kynurenine 3-monooxygenase), an enzyme in the kynurenine pathway of tryptophan degradation that has been linked directly to the pathophysiology of Huntington disease in humans by a mechanism that may involve reactive oxygen species. This finding is suggestive of a conserved mechanism of polyglutamine toxicity from yeast to humans and identifies new candidate therapeutic targets for the treatment of Huntington disease.

Tryptophan metabolism along the kynurenine pathway in diet-induced and genetic hypercholesterolemic rabbits

BACKGROUND

The activity of nicotinic acid in hypercholesterolemia has been poorly understood. In man, nicotinic acid derives for the most part from tryptophan along the tryptophan-nicotinic acid pathway, also called the kynurenine pathway, kynurenine being the key metabolite in this process. In the present paper, we investigated if, in animals with hypercolesterolemia, degradation of tryptophan to nicotinic acid along the kynurenine pathway was perturbated.

METHODS

Liver, kidney and intestine enzyme activities of the tryptophan-nicotinic acid pathway in normolipidemic, diet-induced hyperlipidemic New Zealand and heritable hypercholesterolemic Watanabe (WHHL) rabbits were determined.

RESULTS

Liver tryptophan 2,3-dioxygenase (TDO) activity was present only as a holoenzyme and was higher in the controls than in the hyperlipidemic and Watanabe rabbits, but no difference was present between the group fed an atherogenic hyperlipidic diet and the WHHL rabbits. Small intestine indole 2,3-dioxygenase (IDO) did not vary significantly among the three groups but was higher in comparison with liver TDO activity. In liver, kynurenine 3-monooxygenase and kynurenine-oxoglutarate transaminase activities did not show any significant difference among the three groups of rabbits. Kynureninase and 3-hydroxyanthranilate 3,4-dioxygenase activities per g of fresh tissue decreased significantly in the group of hyperlipidemic and in WHHL rabbits. In the kidneys, kynurenine 3-monooxygenase and kynureninase activity did not change significantly in the three groups of rabbits; kynurenine-oxoglutarate transaminase activity per g of fresh tissue decreased in both hyperlipidemic groups, but no significant difference was observed between hyperlipidemic and Watanabe rabbits. 3-Hydroxyanthranilate 3,4-dioxygenase activity in kidney was decreased markedly in hyperlipidemic and Watanabe rabbits, but there was no difference between the two hypercholesterolemic groups. Aminocarboxymuconate-semialdehyde decarboxylase activity did not change. Thus 3-hydroxyanthranilate 3,4-dioxygenase may be an important regulatory mechanism in the control of the flow of tryptophan along the kynurenine pathway to NAD in hypercholesterolemic rabbits.

CONCLUSIONS

This study first demonstrates that in rabbits, hypercholesterolemia, both diet- or genetically induced, can influence the enzyme activities of the tryptophan-nicotinic acid pathway leading to a decreased formation of nicotinic acid, and thus NAD.

The kynurenine 3-hydroxylase inhibitor Ro 61-8048 improves dystonia in a genetic model of paroxysmal dyskinesia

The effects of the novel kynurenine 3-hydroxylase inhibitor 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl]benzenesulfonamide (Ro 61-8048) on severity of dystonia were examined in dt(sz) mutant hamsters, an animal model of paroxysmal dystonia, in which stress precipitates dystonic episodes. Ro 61-8048 (50, 100 and 150 mg/kg i.p.) significantly reduced the severity of dystonia in dt(sz) hamsters without leading to marked central side effects. Determinations of kynurenic acid concentrations in brain homogenates demonstrated that Ro 61-8048 (100 mg/kg i.p.) provoked a two- to threefold increase of the endogeneous broad spectrum glutamate receptor antagonist kynurenic acid in the striatum, cerebellum and brainstem of mutant hamsters. The antidystonic efficacy of Ro 61-8048 at well-tolerated doses suggests that kynurenine 3-hydroxylase inhibitors should be considered as new therapeutic candidates for the treatment of dyskinesias.

Action Site of Bisphenol A as Metabolic Disruptor Lies in the Tryptophan-Nicotinamide Conversion Pathway

We have reported that the administration of bisphenol A to rats reduces the conversion ratio of tryptophan to nicotinamide. In the present paper, we show that bisphenol A, a monomer of polycarbonate plastics, inhibits the enzyme activity of kynurenine 3-hydroxylase. Namely, the conversion ratio of tryptophan to nicotinamide is reduced through the inhibition of kynurenine 3-hydroxylase activity by bisphenol A.

High efficiency, site-specific excision of a marker gene by the phage P1 cre-loxP system in the yellow fever mosquito, Aedes aegypti

The excision of specific DNA sequences from integrated transgenes in insects permits the dissection in situ of structural elements that may be important in controlling gene expression. Furthermore, manipulation of potential control elements in the context of a single integration site mitigates against insertion site influences of the surrounding genome. The cre-loxP site-specific recombination system has been used successfully to remove a marker gene from transgenic yellow fever mosquitoes, Aedes aegypti. A total of 33.3% of all fertile families resulting from excision protocols showed evidence of cre-loxP-mediated site-specific excision. Excision frequencies were as high as 99.4% within individual families. The cre recombinase was shown to precisely recognize loxP sites in the mosquito genome and catalyze excision. Similar experiments with the FLP/FRT site-specific recombination system failed to demonstrate excision of the marker gene from the mosquito chromosomes.

Analysis of the wild-type and mutant genes encoding the enzyme kynurenine monooxygenase of the yellow fever mosquito, Aedes aegypti

Kynurenine 3-monooxygenase (KMO) catalyses the hydroxylation of kynurenine to 3-hydroxykynurenine. KMO has a key role in tryptophan catabolism and synthesis of ommochrome pigments in mosquitoes. The gene encoding this enzyme in the yellow fever mosquito, Aedes aegypti, is called kynurenine hydroxylase (kh) and a mutant allele that produces white eyes has been designated khw. A number of cDNA clones representative of wild-type and mutant genes were isolated. Sequence analyses of the wild-type and mutant cDNAs revealed a deletion of 162 nucleotides in the mutant gene near the 3'-end of the deduced coding region. RT-PCR analyses confirm the transcription of a truncated mRNA in the mutant strain. The in-frame deletion results in a loss of 54 amino acids, which disrupts a major alpha-helix and which probably accounts for the loss of activity of the enzyme. Recombinant Ae. aegypti KMO showed high substrate specificity for kynurenine with optimum activity at 40 degrees C and pH = 7.5. Kinetic parameters and inhibition of KMO activity by Cl- and pyridoxal-5-phosphate were determined.

Peripheral distribution of kynurenine metabolites and activity of kynurenine pathway enzymes in renal failure

We investigated L-kynurenine distribution and metabolism in rats with experimental chronic renal failure of various severity, induced by unilateral nephrectomy and partial removal of contralateral kidney cortex. In animals with renal insufficiency the plasma concentration and the content of L-tryptophan in homogenates of kidney, liver, lung, intestine and spleen were significantly decreased. These changes were accompanied by increase activity of liver tryptophan 2,3-dioxygenase, the rate-limiting enzyme of kynurenine pathway in rats, while indoleamine 2,3-dioxygenase activity was unchanged. Conversely, the plasma concentration and tissue content of L-kynurenine, 3-hydroxykynurenine, and anthranilic, kynurenic, xanthurenic and quinolinic acids in the kidney, liver, lung, intestine, spleen and muscles were increased. The accumulation of L-kynurenine and the products of its degradation was proportional to the severity of renal failure and correlated with the concentration of renal insufficiency marker, creatinine. Kynurenine aminotransferase, kynureninase and 3-hydroxyanthranilate-3,4-dioxygenase activity was diminished or unchanged, while the activity of kynurenine 3-hydroxylase was significantly increased. We conclude that chronic renal failure is associated with the accumulation of L-kynurenine metabolites, which may be involved in the pathogenesis of certain uremic syndromes.

Different structural organization of the encephalopsin gene in man and mouse

Encephalopsin, also called Panopsin, is a recently discovered extraretinal photoreceptor, which may play a role in non-visual photic processes such as the entrainment of circadian rhythm or the regulation of pineal melatonin production. Based on RT-PCR data and comparative genomic sequence analysis, we show that the human OPN3 gene consists of six exons and expresses various splice variants, while the murine homologue contains four exons and produces just one splice form. Furthermore, the human OPN3 gene overlaps with the neighboring KMO gene on a genomic as well as on an RNA level, whereas the corresponding genes in mouse lie close together but do not overlap. This finding is of particular interest, since differences in gene organization between man and mouse, that have been reported so far, occur within gene clusters, i.e. the number of genes within a certain cluster may differ between man and mouse. OPN3 provides an exception to this rule, since it is positionally uncoupled from other genes of the opsin family.

Modulation of the Kynurine Pathway of Tryptophan Metabolism in Search for Neuroprotective Agents. Focus on Kynurenine-3-Hydroxylase

A novel potent and selective kynurenine-3-hydroxylase inhibitor is descibed along a preliminary evaluation in a in vivo gerbil model of its ability to increase the kynurenine and kynurenic acid concentration in both plasma and brain. These data support the notion that kynurenine-3-hydroxylase inhibitors may have a sustained therapeutic potential in those diseases characterized by unbalance in the QUIN/KYNA branches of the kynurenine pathway.

Enzyme activities Along the kynurenine pathway in mice

Tryptophan metabolism was studied in adult male Swiss mice by determining enzyme activities along the kynurenine pathway. The following enzymes were assayed: liver tryptophan 2,3-dioxygenase, small intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase. Liver tryptophan 2,3-dioxygenase was present only as a holoenzyme: similar results were obtained in the absence or in the presence of the cofactor haematin. The specific activity of small intestine indole 2,3-dioxygenase was higher than that of tryptophan 2,3-dioxygenase. As superoxide dismutase was very active in mouse intestine, this enzyme may be one of the rate controlling factors of the indole 2,3 dioxygenase activity. Kynurenine 3-monooxygenase appeared to be very active. Kidneys showed higher activity than liver. Instead, kynureninase was more active in liver, but activity was lower than that demonstrated by the other enzymes of the kynurenine pathway. Conversely, kynurenine-oxoglutarate transaminase was much more active in kidney than in liver. However, the most active enzyme along the kynurenine pathway was 3-hydroxyanthranilate 3,4-dioxygenase, with liver showing the highest activity; aminocarboxymuconate-semialdehyde decarboxylase, which showed similar values in both liver and kidney, showed activity markedly lower than 3-hydroxyanthranilate 3,4-dioxygenase. Serum tryptophan appeared to be 87% bound to proteins. Results demonstrate that, in mouse, tryptophan is mainly metabolised along the kynurenine pathway. Therefore, mouse is a suitable animal model for studying tryptophan metabolism in the pathological field.

Kynurenine Pathway Enzymes in Guinea pigs

In some animals, the administration of repeated doses of tryptophan can cause death. It has been reported that guinea pig does not survive repeated doses of tryptophan, due to the absence of the hormonal induction mechanism of liver tryptophan 2,3-dioxygenase (TDO). Therefore, it was of interest to investigate if guinea pig is an animal model suitable for studying tryptophan metabolism. The activities of the enzymes of the kynurenine pathway were determined. Liver TDO was present only as a holoenzyme; kynurenine 3-monooxygenase showed similar, but very high, activity in both liver and kidney. Liver and kidney kynureninase values were also similar, whereas kynurenine-oxoglutarate transaminase activity was higher in kidney than in liver. 3-Hydroxyanthranilate 3,4-dioxygenase gave similar, but very high, values in both liver and kidney, whereas aminocarboxymuconate-semialdehyde decarboxylase activity was double in kidney with respect to liver, but much lower than that of 3-hydroxyanthranilate 3,4-dioxygenase. Total and free tryptophan concentrations in serum were also determined. The free fraction was about 10% of total tryptophan.

Studies on the neuroprotective action of kynurenine mono-oxygenase inhibitors in post-ischemic brain damage

Kynurenine 3-mono-oxygenase (KMO) inhibitors facilitate kynurenic acid (KYNA) neosynthesis and reduce the formation of 3OH-kynurenine (3-HK) and quinolinic acid (QUIN). They also attenuate post-ischemic brain damage and decrease glutamate (Glu) content in brain extracellular spaces. To investigate KMO mechanism(s) of neuroprotection, we performed experiments in gerbils subjected to bilateral carotid occlusion and in organotypic rat hippocampal slice cultures exposed to oxygen and glucose deprivation (OGD). In gerbils, direct application of KYNA (100 nM, through reverse microdialysis in the hippocampus) completely prevented the increase in Glu output induced by transient (5 min) occlusion of the carotids. In rat hippocampal slices exposed for 30 min to OGD, KMO inhibitors (m-nitrobenzoyl)-alanine (mNBA, 30-100 microM) or 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfonamide (Ro 61-8048, 1-10 microM) reduced post-ischemic neuronal death and increased KYNA concentrations in the incubation medium. KYNA may antagonize glycineb or alpha7 nicotinic acetylcholine receptors but the concentrations in the incubation medium never reached values that could efficiently antagonize receptor function. On the contrary, 3-HK (1-10 microM) added to slices exposed to OGD in the presence of KMO inhibitors completely prevented the neuroprotective effects of the inhibitors. Our findings suggest that KMO inhibitors reduce OGD-induced pyramidal cell death by decreasing 3-HK (and possibly QUIN) synthesis.

Kidney and Liver Kynurenine Pathway Enzymes in Chronic Renal Failure

It has been suggested that kynurenine pathway may be an important pathological factor during the chronic renal failure (CRF) development. Therefore in the present study, in rats with end-stage of chronic renal failure, we measured the plasma and tissues (kidney and liver) concentrations of tryptophan, kynurenine, 3-hydroxykynurenine. We also evaluated the activity of tryptophan 2,3-dioxygenase in the liver (TDO), indoleamine 2,3-dioxygenase in kidney (IDO) and kynurenine 3-hydroxylase (HK) in the liver and in the kidney. The plasma and tissues tryptophan concentrations were decreased, whereas the concentrations of its metabolites increased when compared to control group. The increase in the TDO and 3-HK activity was observed, while IDO activity remains unchanged. In conclusion, the increase in the activity of TDO and HK along with disturbances of renal excreting function may be responsible for the elevation in the kynurenine and 3-hydroxykynurenine concentrations in experimental chronic renal failure.

Tryptophan Metabolism in Rabbits

Enzyme activities involved in tryptophan metabolism along the kynurenine pathway were studied in male New Zealand white rabbits. Activities are expressed both as specific activity and per g of fresh tissue. Liver tryptophan 2,3-dioxygenase activity (TDO), when assayed in either the absence (holoenzyme) or presence of added haematin (apoenzyme), did not change. Therefore, in rabbit, TDO was present only in holoenzyme form. Small intestine indole 2,3-dioxygenase was significantly higher than liver TDO. Mitochondrial kynurenine 3-monooxygenase was higher in liver than in kidney. Kynureninase activity was similar in both tissues, whereas kynurenine-oxoglutarate transaminase was markedly higher in kidney than in liver. 3-Hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase activities were higher in kidney than in liver. However, the former enzyme showed much higher activity than the latter. These findings suggest that, in rabbit, tryptophan is mainly metabolised along the kynurenine pathway although the apo-TDO enzyme is lacking, as high indole 2,3-dioxygenase activity can obviate this lack.

cDNA cloning, functional expression and characterization of kynurenine 3-hydroxylase of Anopheles stephensi (Diptera: Culicidae)

Kynurenine 3-hydroxylase (K3H) is a NADPH-dependent flavin monooxygenase involved in the tryptophan pathway. Xanthurenic acid (XA) is a metabolite of this pathway and has recently been identified as a gamete activating factor (GAF) of the malarial parasite. We cloned K3H cDNA from Anopheles stephensi (AsK3H), because anopheline mosquitoes are a vector of the human malaria parasite, Plasmodium falciparum and the catalytic function of AsK3H in XA production. Recombinant AsK3H protein was expressed in Sf-9 cells using the baculovirus system and its enzymatic properties were characterized. The specific activities of crude cell lysate and affinity purified protein were 94.9 +/- 6.2 and 865.6 +/- 10.5 nmol/min/mg protein, respectively. The optimum pH of AsK3H was 7.0. Analysis of AsK3H gene expression using RT-PCR revealed that AsK3H was constitutively expressed in egg, larva, pupa and adult.

Kynurenine 3-mono-oxygenase inhibitors attenuate post-ischemic neuronal death in organotypic hippocampal slice cultures

Kynurenine 3-mono-oxygenase (KMO) inhibitors reduce 3-hydroxykynurenine (3-HK) and quinolinic acid (QUIN) neosynthesis and facilitate kynurenine metabolism towards kynurenic acid (KYNA) formation. They also reduce tissue damage in models of focal or transient global cerebral ischemia in vivo. We used organotypic hippocampal slice cultures exposed to oxygen and glucose deprivation (OGD) to investigate KMO mechanism(s) of neuroprotective activity. Exposure of the slices to 30 min of OGD caused CA1 pyramidal cell death and significantly decreased the amount of KYNA released in the incubation medium. The KMO inhibitors (m-nitrobenzoyl)-alanine (30-100 micro m) or 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfonamide (1-10 micro m) reduced post-ischemic neuronal death and increased KYNA concentrations in slice incubation media. The maximal concentration of KYNA detected in the incubation media of slices treated with KMO inhibitors was approximately 50 nm and was too low to efficiently interact with alpha7 nicotinic acetylcholine receptors or with the glycineb site of N-methyl-d-aspartate (NMDA) receptors. On the other hand, the addition of either 3-HK or QUIN (1-10 micro m) to OGD-exposed hippocampal slices prevented the neuroprotective activity of KMO inhibitors. Our results suggest that KMO inhibitors reduce the neuronal death found in the CA1 region of organotypic hippocampal slices exposed to 30 min of OGD by decreasing the local synthesis of 3-HK and QUIN.

FcepsilonRI induces the tryptophan degradation pathway involved in regulating T cell responses

FcepsilonRI is suspected to play a pivotal role in the pathophysiology of atopic disorders such as atopic dermatitis. In search for genes differentially regulated by FcepsilonRI on APCs, a differential cDNA bank of receptor-stimulated and unstimulated monocytes was established. By means of suppression subtractive hybridization, we identified kynurenine 3-monooxygenase and subsequently indoleamine 2,3-dioxygenase (IDO) to be overexpressed in FcepsilonRI-activated monocytes. IDO is the rate-limiting enzyme in the catabolism of the essential amino acid tryptophan. We show that cross-linking of FcepsilonRI on monocytes results in low tryptophan concentrations associated with impaired T cell stimulatory capacity. Importantly, T cell suppression could be prevented by the addition of tryptophan or inhibition of IDO. Moreover, stimulation of T cells by FcepsilonRI-activated monocytes was increased compared with T cell stimulation by nonactivated monocytes if exogenous supply of tryptophan was available. We speculate that the expression of IDO by FcepsilonRI(+) APCs in vivo allows these cells to regulate T cell responses in atopic disorders by inhibiting or stimulating T cell proliferation, depending on the metabolic environment.

Enzyme activities along the tryptophan-nicotinic acid pathway in alloxan diabetic rabbits

Recent data from our laboratory have indicated that the rabbit is a suitable animal model for the study of enzyme activities of the tryptophan-nicotinic acid pathway. We report here the pattern of tryptophan metabolism in rabbits made diabetic with alloxan treatment, and hypercholesterolemic with a high-cholesterol diet. A group of rabbits with only hypercholesterolemia was also considered. The enzymes assayed were: liver tryptophan 2,3-dioxygenase (TDO), intestine indoleamine 2,3-dioxygenase (IDO), liver and kidney kynurenine 3-monooxygenase, kynurenine-oxoglutarate transaminase, kynureninase, 3-hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase.TDO showed a reduction of specific activity in liver of diabetic-hyperlipidemic and hyperlipidemic rabbits compared to controls. Intestine IDO activities and liver and kidney kynurenine monooxygenase were unchanged with respect to controls.Kynurenine-oxoglutarate transaminase and kynureninase activities were reduced in the kidneys, but not in the liver, of diabetic-hyperlipidemic rabbits. The main finding was the reduction of 3-hydroxyanthranilate 3,4-dioxygenase activity (expressed as activity per g of fresh tissue) in the liver and kidneys of diabetic-hypercholesterolemic and hyperlipidemic rabbits compared to controls. Conversely, aminocarboxymuconate-semialdehyde decarboxylase activity was significantly higher in diabetic hypercholesterolemic rabbits in comparison with control and hypercholesterolemic rabbits. These data demonstrate that also in diabetic rabbits there is an alteration of tryptophan metabolism at the level of 3-hydroxyanthranilic acid-->nicotinic acid step. Also dyslipidemia seems to be involved in enzyme activity variations of the tryptophan metabolism along the kynurenine pathway.

Characterization of the kynurenine 3-monooxygenase gene corresponding to the white egg 1 mutant in the silkworm Bombyx mori

Kynurenine 3-monooxygenase (KMO, EC 1.14.13.9), which catalyzes the oxidation of kynurenine to 3-hydroxykynurenine, is involved in the synthesis of ommochrome pigments in insects. A silkworm mutant, white egg 1 ( w-1), has been shown to be deficient in this enzyme activity. The mutant is characterized morphologically by its white eyes and the fact that the females lay white eggs. To analyze the relationship between the KMO gene and the mutation, we first determined the entire sequence of a full-length 2.0-kb cDNA and examined its expression pattern in the wild type. The cDNA sequence contains one ORF encoding a polypeptide of 456 amino acids, and transcripts were detected in the larval Malpighian tubules and the pupal ovaries, but not in other tissues. Southern analysis and nucleotide sequencing showed that the KMO gene is present in a single copy and consists of ten exons distributed over a 16-kb region. Comparison of the transcripts between the wild type and mutant silkworms showed that the wild type expressed a single transcript, whereas the mutant exhibited markedly reduced amounts of two transcripts with sizes of 2.0 kb and 1.8 kb. Nucleotide sequence analysis of these mutant transcripts indicated that sequences corresponding to the ninth and tenth exons were missing. Inverse PCR and Southern analysis of the mutant gene demonstrated that the corresponding genomic region was deleted in the w-1 mutant.

[Enzymes of the kynurenine pathway]

In this review, the role of kynurenine pathway enzymes (tryptophan 2,3-dioxygenase, indoleamine 2,3-dioxygenase, formamidase, kynurenine aminotransferase, kynurenine 3-hydroxylase, kynureninase, 3-hydroxyanthranilic acid oxygenase, picolinic carboxylase, quinolinic acid phosphoribosyltransferase) in the synthesis of tryptophan products degradation was described. Taking into account the importance of disorders, in which kynurenine metabolites may play pathogenic role. It was been postulated that kynurenine metabolites may be involved in pathogenesis of many diseases of clinical importance. Therefore evaluation of correlations between kynurenine metabolites and other variables, both clinical and biochemical, would be of interest. Attention was paid to the potential use of those biochemical parameter as biomarkers of many disease activity.

Pharmacological elevation of endogenous kynurenic acid levels activates nigral dopamine neurons

Inhibitors of kynurenine 3-hydroxylase have previously been used to increase endogenous levels of kynurenic acid, an excitatory amino acid receptor antagonist. In the present electrophysiological study PNU 156561A was utilized to elevate endogenous concentrations of kynurenic acid and subsequent effects on the firing pattern of dopamine (DA) neurons of rat substantia nigra (SN) were analyzed. Pretreatment with PNU 156561A (40 mg/kg, i.v., 5-7 h) caused a five-fold increase in endogenous kynurenic acid levels in whole brain five to seven hours after administration and also evoked a significant increase in firing rate and bursting activity of nigral DA neurons. The results of the present study show that a moderate increase in endogenous kynurenic acid levels produces significant actions on the tonic glutamatergic control of the firing pattern of nigral DA neurons, and implicate kynurenine 3-hydroxylase inhibitors as novel antiparkinsonian agents.

Cloning and characterization of the Tribolium castaneum eye-color genes encoding tryptophan oxygenase and kynurenine 3-monooxygenase

The use of eye-color mutants and their corresponding genes as scorable marker systems has facilitated the development of transformation technology in Drosophila and other insects. In the red flour beetle, Tribolium castaneum, the only currently available system for germline transformation employs the exogenous marker gene, EGFP, driven by an eye-specific promoter. To exploit the advantages offered by eye-pigmentation markers, we decided to develop a transformant selection system for Tribolium on the basis of mutant rescue. The Tribolium orthologs of the Drosophila eye-color genes vermilion (tryptophan oxygenase) and cinnabar (kynurenine 3-monooxygenase) were cloned and characterized. Conceptual translations of Tc vermilion (Tcv) and Tc cinnabar (Tccn) are 71 and 51% identical to their respective Drosophila orthologs. We used RNA interference (RNAi) to show that T. castaneum larvae lacking functional Tcv or Tccn gene products also lack the pigmented eyespots observed in wild-type larvae. Five available eye-color mutations were tested for linkage to Tcv or Tccn via recombinational mapping. No linkage was found between candidate mutations and Tccn. However, tight linkage was found between Tcv and the white-eye mutation white, here renamed vermilion(white) (v(w)). Molecular analysis indicates that 80% of the Tcv coding region is deleted in v(w) beetles. These observations suggest that the Tribolium eye is pigmented only by ommochromes, not pteridines, and indicate that Tcv is potentially useful as a germline transformation marker.

Synthesis and release of neurotoxic kynurenine metabolites by human monocyte-derived macrophages

We studied the regulation of the kynurenine pathway of tryptophan metabolism in human monocyte-derived macrophages (MDM) with the aim of evaluating macrophage involvement in inflammatory neurological disorders. Cultured MDM metabolized tryptophan and released kynurenine metabolites, including the excitotoxin quinolinic acid (QUIN). Lipopolysaccharides (LPS) or the pro-inflammatory cytokines INFgamma and TNFalpha increased, while IL 4 or IL 10 inhibited the rate of tryptophan metabolism and the release of QUIN. The incubation media of INFgamma-exposed MDM caused neuronal death in primary cultures of mixed cortical cells. Glutamate receptor antagonists or poly(ADP-ribose) polymerase inhibitors significantly reduced this death, thus suggesting new possibilities for the treatment of neuronal damage in neuroinflammatory disorders.

Enzyme activities involved in tryptophan metabolism along the kynurenine pathway in rabbits

The following enzyme activities of the tryptophan-nicotinic acid pathway were studied in male New Zealand rabbits: liver tryptophan 2,3-dioxygenase, intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase. Intestine superoxide dismutase and serum tryptophan were also determined. Liver tryptophan 2,3-dioxygenase exists only as holoenzyme, but intestine indole 2,3-dioxygenase is very active and can be considered the key enzyme which determines how much tryptophan enters the kynurenine pathway also under physiological conditions. The elevated activity of indole 2,3-dioxygenase in the rabbit intestine could be related to the low activity of superoxide dismutase found in intestine. Kynurenine 3-monooxygenase appeared more active than kynurenine-oxoglutarate transaminase and kynureninase, suggesting that perhaps a major portion of kynurenine available from tryptophan may be metabolized to give 3-hydroxyanthranilic acid, the precursor of nicotinic acid. In fact, 3-hydroxyanthranilate 3,4-dioxygenase is much more active than the other previous enzymes of the kynurenine pathway. In the rabbit liver 3-hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase show similar activities, but in the kidney 3-hydroxyanthranilate 3,4-dioxygenase activity is almost double. These data suggest that in rabbit tryptophan is mainly metabolized along the kynurenine pathway. Therefore, the rabbit can also be a suitable model for studying tryptophan metabolism in pathological conditions.

Kynurenine 3-mono-oxygenase activity and neurotoxic kynurenine metabolites increase in the spinal cord of rats with experimental allergic encephalomyelitis

Kynurenine 3-mono-oxygenase, one of the key enzymes of the "kynurenine pathway", catalyses the formation of 3-hydroxykynurenine and may direct the neo-synthesis of quinolinic and kynurenic acids. While 3-hydroxykynurenine and quinolinic acid have neurotoxic properties, kynurenic acid antagonizes excitotoxic neuronal death. Here we report that the expression and activity of kynurenine 3-mono-oxygenase significantly increased in the spinal cord of rats with experimental allergic encephalopathy, an experimental model of multiple sclerosis. As a consequence of this increase, the spinal cord content of 3-hydroxykynurenine and quinolinic acid reached neurotoxic levels. We also report that systemic administration of Ro 61-8048, a selective kynurenine 3-mono-oxygenase inhibitor, reduced the increase of both 3-hydroxykynurenine and quinolinic acid, and caused accumulation of kynurenic acid. In the brain and spinal cord of the controls, kynurenine 3-mono-oxygenase immunoreactivity was located in granules (probably mitochondria) present in the cytoplasm of both neurons and astroglial cells. In the spinal cord of rats with experimental allergic encephalopathy, however, cells with a very intense kynurenine 3-mono-oxygenase immunoreactivity, also able to express class II major histocompatibility complex and inducible nitric oxide synthase, were found in perivascular, subependymal and subpial locations. These cells (most probably macrophages) were responsible for the large increase in 3-hydroxykynurenine and quinolinic acid found in the spinal cords of affected animals. The results show that cells of the immune system are responsible for the increased formation of 3-hydroxykynurenine and quinolinic acid, two neurotoxic metabolites that accumulate in the central nervous system of rats with experimental allergic encephalomyelitis. They also demonstrate that selective kynurenine 3-mono-oxygenase inhibitors reduce the neo-synthesis of these toxins.

Effects of oxygen on kynurenine-3-monooxygenase activity

Kynurenine-3-monooxygenase (KM), the third enzyme in the kynurenine (KYN) pathway from tryptophan to quinolinic acid (QA), is a monooxygenase requiring oxygen, NADPH and FAD for the catalytic oxidation of L-kynurenine to 3-hydroxykynurenine and water. KM is innately low in the brain and similar in activity to indoleamine oxidase, the rate-limiting pathway enzyme. Accumulation in the CNS of QA, a known excitotoxin, is proposed to cause convulsions in several pathologies. Thus, we theorized that hyperbaric oxygen (HBO) induced convulsions arise from increased QA via oxygen K, effects on this pathway [Brown OR, Draczynska-Lusiak. Oxygen activation and inactivation of quinolinate-producing and iron-requiring 3-hydroxyanthranilic acid oxidase: a role in hyperbaric oxygen-induced convulsions? Redox Report 1995; 1: 383-385]. To complement prior studies on the effects of oxygen on pathway enzymes, in this paper we report the effects of oxygen on KM. Brain and liver KM enzyme are not known to be identical, and some systemically-produced KYN pathway intermediates can permeate the brain and might stimulate the brain pathway. Thus, KM from both brain and liver was assayed at various oxygen substrate concentrations to evaluate, in vitro, the potential effects of increases in oxygen, as would occur in mammals breathing therapeutic and convulsive HBO. In crude tissue extracts, KM was not activated during incubation in HBO up to 6 atm. The effects of oxygen as substrate on brain and liver KM activity was nearly identical: activity was nil at zero oxygen with an apparent oxygen Km of 20-22 microM. Maximum KM activity occurred at about 1000 microM oxygen and decreased slightly to plateau from 2000 to 8000 microM oxygen. This compares to approximately 30-40 microM oxygen typically reported for brain tissue of humans or rats breathing air, and an unknown but surely much lower value (perhaps below 1 microM) intracellularly at the site of KM. Thus HBO, as used therapeutically and at convulsive pressures, likely stimulates flux through the KM-catalyzed step of the KYN pathway in liver and in brain and could increase brain QA, by Km effects on brain KM, or via increased KM pathway intermediates produced systemically (in liver) and transported into the brain.

Screening marine natural products for selective inhibitors of key kynurenine pathway enzymes

Kynurenine, a metabolite of tryptophan along the 'kynurenine pathway', is at a branch point of the pathway which can lead to the synthesis of both quinolinic acid (QUIN) and kynurenic acid (KYNA). KYNA is an antagonist of glutamate receptors; however, QUIN is a selective agonist of NMDA receptors, and has been shown to act as an excitotoxic agent. A high QUIN/KYNA ratio has been implicated in a variety of neurological diseases in which excitotoxic neuronal cell death is found, e.g. AIDS-related dementia, stroke, etc. Inhibiting the key enzymes of this pathway (i.e. kynureninase and kynurenine 3-hydroxylase) would lower the QUIN/KYNA ratio, which may potentially have neuroprotective effects. We have developed high through-put assays for kynurenine pathway enzymes which allow us to screen extracts from marine organisms for selective enzyme inhibitors. Active metabolites are purified, isolated and identified by HPLC, high-field NMR and mass spectral techniques. Extracts from a sponge of the Aka species were found to contain a selective inhibitor of kynureninase. We have recently purified and identified the active principal as being serotonin sulfate. Related indoleamines, serotonin and 5-hydroxyindoleacetic acids are inactive. This finding may be suggestive of a novel interaction between the serotoninergic and excitatory amino acid pathways.

Pharmacologically elevated levels of endogenous kynurenic acid prevent nicotine-induced activation of nigral dopamine neurons

Previous studies have shown that systemically administered nicotine is associated with an activation of rat midbrain dopamine neurons. The aim of the present electrophysiological study was to investigate if manipulation of brain kynurenic acid, an endogenous excitatory amino acid receptor antagonist, can affect the response of nigral dopamine neurons to nicotine. A potent inhibitor of kynurenine 3-hydroxylase, PNU 156561A (40 mg/kg, i.v., 4-7 h), was utilized to increase the levels of kynurenic acid in rat brain. This treatment, which caused a fourfold increase in brain kynurenic acid levels, abolished the increase in firing rate and burst activity of nigral dopamine neurons as induced by nicotine (25-400 microg/kg, i.v.). It is proposed that the excitation of dopamine neurons in the substantia nigra following nicotine administration is an indirect effect, mediated by glutamate release. In addition, our data highlight the role of brain kynurenic acid as a potentially important modulator of basic glutamatergic responses in brain.

Kynurenine 3-hydroxylase inhibition in rats: effects on extracellular kynurenic acid concentration and N-methyl-D-aspartate-induced depolarisation in the striatum

Inhibition of kynurenine 3-hydroxylase suppresses quinolinic acid synthesis and, therefore, shunts all kynurenine metabolism toward kynurenic acid (KYNA) formation. This may be a pertinent antiexcitotoxic strategy because quinolinic acid is an agonist of NMDA receptors, whereas kynurenic acid antagonises all ionotropic glutamate receptors with preferential affinity for the NMDA receptor glycine site. We have examined whether the kynurenine 3-hydroxylase inhibitor Ro 61-8048 increases extracellular (KYNA) sufficiently to control excessive NMDA receptor function. Microdialysis probes incorporating an electrode were implanted into the striatum of anaesthetised rats, repeated NMDA stimuli were applied through the probe, and the resulting depolarisation was recorded. Changes in extracellular KYNA were assessed by HPLC analysis of consecutive dialysate samples. Ro 61-8048 (42 or 100 mg/kg) markedly increased the dialysate levels of KYNA. The maximum increase (from 3.0 +/- 1.0 to 31.0 +/- 6.0 nM; means +/- SEM, n = 6) was observed 4 h after administration of 100 mg/kg Ro 61-8048, but the magnitude of the NMDA-induced depolarisations was not reduced. A separate study suggested that extracellular KYNA would need to be increased further by two orders of magnitude to become effective in this preparation. These results challenge the notion that kynurenine 3-hydroxylase inhibition may be neuroprotective, primarily through accumulation of KYNA and subsequent attenuation of NMDA receptor function.

Evidence that kynurenine pathway metabolites mediate hyperbaric oxygen-induced convulsions

Metabolism of tryptophan (TRP) through the kynurenine (KYN) pathway in brain, liver, and kidney produces intermediates including the neuroactive agonist quinolinic acid (QA) and the antagonists kynurenic acid (KA) and anthranilic acid (AA) for N-methyl D-aspartate (NMDA) receptors in the central nervous system. We hypothesized that elevated concentrations of QA, KA, or AA can moderate the convulsions that are observed during exposure of rats to hyperbaric oxygen (HBO). We found that i.p. administration of TRP or KYN (both of which cross the blood-brain barrier) had no effect on HBO-induced seizures. However, AA (administered i.p.) or gavage administration of the KYN pathway blocking drug Ro 61-8048, both of which enter the brain from the circulatory system, affect the time to first convulsion and/or coma during HBO in a manner consistent with a modulatory role for seizure activity.

Mechanism of increases in L-kynurenine and quinolinic acid in renal insufficiency

Marked increases in metabolites of the L-tryptophan-kynurenine pathway, L-kynurenine and quinolinic acid (Quin), were observed in serum and cerebrospinal fluid (CSF) of both the rat and human with renal insufficiency. The mechanisms responsible for their accumulation after renal insufficiency were investigated. In patients with chronic renal insufficiency, elevated levels of serum L-kynurenine and Quin were reduced by hemodialysis. In renal-insufficient rats, Quin and L-kynurenine levels in serum, brain, and CSF were also increased parallel to the severity of renal insufficiency. Urinary excretion of Quin (3.5-fold) and L-kynurenine (2.8-fold) was also increased. Liver L-tryptophan 2,3-dioxygenase activity (TDO), a rate-limiting enzyme of the kynurenine pathway, was increased in proportion to blood urea nitrogen and creatinine levels. Kynurenine 3-hydroxylase and quinolinic acid phosphoribosyltransferase were unchanged, but the activities of kynureninase, 3-hydroxyanthranilate dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase (ACMSDase) were significantly decreased. Systemic administrations of pyrazinamide (ACMSDase inhibitor) increased serum Quin concentrations in control rats, demonstrating that changes in body ACMSDase activities in response to renal insufficiency are important factors for the determination of serum Quin concentrations. We hypothesize the following ideas: that increased serum L-kynurenine concentrations are mainly due to the increased TDO and decreased kynureninase activities in the liver and increased serum Quin concentrations are due to the decreased ACMSDase activities in the body after renal insufficiency. The accumulation of CSF L-kynurenine is caused by the entry of increased serum L-kynurenine, and the accumulation of CSF Quin is secondary to Quin from plasma and/or Quin precursor into the brain.

Kynurenergic manipulations influence excitatory synaptic function and excitotoxic vulnerability in the rat hippocampus in vivo

Competing enzymatic mechanisms degrade the tryptophan metabolite L-kynurenine to kynurenate, an inhibitory and neuroprotective compound, and to the neurotoxins 3-hydroxykynurenine and quinolinate. Kynurenine 3-hydroxylase inhibitors such as PNU 156561 shift metabolism towards enhanced kynurenate production, and this effect may underlie the recently discovered anticonvulsant and neuroprotective efficacy of these drugs. Using electrophysiological and neurotoxicological endpoints, we now used PNU 156561 as a tool to examine the functional interplay of kynurenate, 3-hydroxykynurenine and quinolinate in the rat hippocampus in vivo. First, population spike amplitude in area CA1 and the extent of quinolinate-induced excitotoxic neurodegeneration were studied in animals receiving acute or prolonged intravenous infusions of L-kynurenine, PNU 156561, (L-kynurenine+PNU 156561) or kynurenate. Only the latter two treatments, but not L-kynurenine or PNU 156561 alone, caused substantial inhibition of evoked responses in area CA1, and only prolonged (3h) infusion of (L-kynurenine+PNU 156561) or kynurenate was neuroprotective. Biochemical analyses in separate animals revealed that the levels of kynurenate attained in both blood and brain (hippocampus) were essentially identical in rats receiving extended infusions of L-kynurenine alone or (L-kynurenine+PNU 156561) (4 and 7microM, respectively, after an infusion of 90 or 180min). However, addition of the kynurenine 3-hydroxylase inhibitor resulted in a significant decrement in the formation of 3-hydroxykynurenine and quinolinate in both blood and brain. These data suggest that the ratio between kynurenate and 3-hydroxykynurenine and/or quinolinate in the brain is a critical determinant of neuronal excitability and viability. The anticonvulsant and neuroprotective potency of kynurenine 3-hydroxylase inhibitors may therefore be due to the drugs' dual action on both branches of the kynurenine pathway of tryptophan degradation.

Neuroprotective effects of kynurenine-3-hydroxylase inhibitors in models of brain ischemia

The neuroprotective effects of two kynurenine hydroxylase inhibitors, (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfona mide (Ro 61-8048), were studied in vitro and in vivo. In organotypic hippocampal slice cultures deprived of oxygen and glucose, these inhibitors significantly reduced neuronal damage. In gerbils subjected to bilateral carotid occlusion for 5 min, the administration of mNBA (400 mg/kg i.p., 3 times) or Ro 61-8048 (40 mg/kg i.p., 3 times) dramatically decreased the percentage of damaged pyramidal neurones in the hippocampal CA1 region. Finally, in rats with permanent occlusion of the middle cerebral artery, mNBA (200-400 mg/kg i.p.) and Ro 61-8048 (40 mg/kg i.p.) administration reduced the infarct volume. Our results demonstrate that ischemic neuronal damage may be significantly decreased by inhibiting kynurenine hydroxylase.

Purification of L-kynurenine 3-monooxygenase from mitochondrial outer membrane of pig liver

The aim of the present work is to obtain the homogeneous L-kynurenine 3-monooxygenase (Fpk) enzyme preparation by a simple and rapid immunoaffinity purification method. Fpk was purified by monoclonal antibody (mAb) immunoabsorbent column. The column was prepared using hydrazide-activated agarose beads (Affi-Gel Hz) to which IgG molecules were coupled via carbohydrate moieties located on the Fc region and peroxidized with periodate. Partially purified Fpk was charged on the column and after washing the column with buffer containing 0.5 M NaCl and 0.5% Triton-X-100 and then with buffer alone, the enzyme was eluted with acidic elution buffer. Despite the loss of the catalytic activity due to the acidic elution, the immunoaffinity preparation may be useful for the analysis of the chemical structure of Fpk and for the production of the polyclonal antibody toward Fpk.

Kynurenine pathway metabolism in human astrocytes

The involvement of astrocytes in Kynurenine pathway (KP) metabolism is still poorly understood. In the present study, we investigated the ability of human fetal astrocytes in vitro to produce quinolinic and picolinic acids using mass spectrometry. In parallel, we estimated the level of expression of five major KP enzymes using RT-PCR. The results demonstrated that astrocytes express most KP enzymes, except for kynurenine-hydroxylase. This in vitro study provides novel informations regarding the ability of human fetal astrocytes to degrade L-tryptophan along the KP.

Quantification of anthranilic acid and its related enzyme activity in several different species

Anthranilic acid (AA) has been attracted considerable attention as one of the L-tryptophan-kynurenine pathway metabolites in the central nervous system. In this study, the concentration of L-kynurenine (L-KYN) and AA in serum and CSF, and its related enzyme activities were determined in several species. In rabbits, CSF AA concentrations were lower and serum AA concentrations were slightly higher than those in other species. However, the concentrations of L-KYN were substantially higher in rabbits in both serum and CSF compared with other species. Tissue enzyme activities varied among species. In rabbits, lung IDO activities were higher, but liver kynurenine 3-hydroxylase activities were lower than those of the other species tested. Furthermore, brain kynurenine 3-hydroxylase activities were higher in gerbils than those in other species. These results clearly demonstrated that kynurenine pathway enzyme activities and metabolite concentrations vary with species.

Ommochrome genesis in an albino strain of a terrestrial isopod

The contents of tryptophan (Trp) metabolites and the activities of the enzymes involved in ommochrome biosynthesis were measured in an albino strain of a terrestrial isopod Armadillidium vulgare. There was little difference between the Trp content in the albino mutant and that in the wild type, although the contents of 3-hydroxykynurenine (3-OH-Kyn), 3-hydroxyanthranilic acid (3-OH-AA) and xanthommatin in the albino were significantly lower than those in the wild type. Tryptophan 2,3-dioxygenase (TDO) activity in the albino was extremely low, while the activities of Kyn-3-hydroxylase and kynureninase did not differ significantly between the two phenotypes. The extremely low activity of TDO is probably one of main reasons why almost no ommochrome pigment is produced in the albino mutant.

Modulation and function of kynurenic acid in the immature rat brain

Using in vivo and in vitro paradigms, the regulation and function of the brain metabolite kynurenic acid (KYNA) was examined in rats on postnatal days (PND) 7 and 14. As shown previously in adult rats, glucose removal and d-amphetamine (d-Amph) administration caused decreases in KYNA formation, while exposure to pyruvate up-regulated KYNA synthesis. The effect of glucose deprivation was substantially blunted in immature animals. In PND 14 rats, d-Amph pre-treatment exacerbated the excitotoxic effects of an intrastriatal N-methyl-D-aspartate (NMDA) injection. This potentiation was prevented by m-nitrobenzoylalanine, a kynurenine 3-hydroxylase inhibitor that also antagonized the KYNA reduction caused by d-Amph. These and additional experiments with the competitive NMDA receptor antagonist CGP 40116 indicate the existence of a functionally significant, novel high-affinity receptor for KYNA in the brain.

Functional characterization and mechanism of action of recombinant human kynurenine 3-hydroxylase

The mitochondrial outer membrane enzyme kynurenine 3-hydroxylase (K3H) is an NADPH-dependent flavin mono-oxygenase involved in the tryptophan pathway, where it catalyzes the hydroxylation of kynurenine. K3H was transiently expressed in COS-1 cells as a glutathione S-transferase (GST) fusion protein, and the pure recombinant protein (rec-K3H) was obtained with a specific activity of about 2000 nmol.min-1.mg-1. Rec-K3H was shown to have an optimum pH at 7.5, to use NADPH more efficiently than NADH, and to contain one molecule of non-covalently bound FAD per molecule of enzyme. The mechanism of the rec-K3H-catalyzed reaction was investigated by overall initial-rate measurements, and a random mechanism in which combination of the enzyme with one substrate does not influence its affinity for the other is proposed. Further kinetic studies revealed that K3H activity was inhibited by both pyridoxal phosphate and Cl-, and that NADPH-catalyzed oxidation occurred even in the absence of kynurenine if 3-hydroxykynurenine was present, suggesting an uncoupling effect of 3-hydroxykynurenine with peroxide formation. This observation could be of clinical interest, as peroxide formation could explain the neurotoxicity of 3-hydroxykynurenine in vivo.

Synthesis and SAR of 4-aryl-2-hydroxy-4-oxobut-2-enoic acids and esters and 2-amino-4-aryl-4-oxobut-2-enoic acids and esters: potent inhibitors of kynurenine-3-hydroxylase as potential neuroprotective agents

The synthesis and structure-activity relationship of a series of 4-aryl-2-hydroxy-4-oxobut-2-enoic acids and esters and 2-amino-4-aryl-4-oxobut-2-enoic acids and esters as potent inhibitors of kynurenine-3-hydroxylase are described. These compounds are the most potent inhibitors of the kynurenine-3-hydroxylase enzyme so far disclosed. Additionally methyl 4-(3-chlorophenyl)-2-hydroxy-4-oxobut-2-enoate (2d), 4-(3-chlorophenyl)-2-hydroxy-4-oxobut-2-enoic acid (3d), methyl 4-(3-fluorophenyl)-2-hydroxy-4-oxobut-2-enoate (2f), and 4-(3-fluorophenyl)-2-hydroxy-4-oxobut-2-enoic acid (3f) prevent the increase in the interferon-gamma-induced synthesis of quinolinic acid in primary cultures of cultured human peripheral blood monocyte-derived macrophages.

Ommochrome deficiency in an albino strain of a terrestrial isopod, Armadillidium vulgare

In order to clarify the cause of ommochrome deficiency in an albino strain of the terrestrial isopod, Armadillidium vulgare, levels of xanthommatin, 3-hydroxykynurenine, 3-hydroxyanthranilic acid and tryptophan in whole body extracts of the albino and the wild type individuals were determined together with enzyme activities of kynurenine-3-hydroxylase, kynureninase and tryptophan-2,3-dioxygenase. Xanthommatin could not be detected in the albinos. The levels of 3-hydroxykynurenine and 3-hydroxyanthranilic acid were determined by high-performance liquid chromatography (HPLC) with electrochemical detection and were markedly low in the albinos compared with the wild type individuals. In contrast to those, the tryptophan levels determined by HPLC with fluorescence detection did not differ significantly between the two phenotypes. In the albino A. vulgare, kynurenine-3-hydroxylase activity was lower and kynureninase activity was higher than in the wild type, although the differences were not statistically significant. Tryptophan-2,3-dioxygenase activity in the albinos was less than 10% that in the wild type. Thus, ommochrome deficiency in the albino A. vulgare is considered to be caused by the extremely low activity of tryptophan-2,3-dioxygenase.

Quinolinic acid formation in immune-activated mice: studies with (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(-3-nitrophenyl)thiazol-2yl]-benzenesul fonamide (Ro 61-8048), two potent and selective inhibitors of kynurenine hydroxylase

The role of kynurenine hydroxylase activity in the neo-formation of the excitotoxin quinolinic acid (QUIN) has been studied in mice by using (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(-3-nitrophenyl)thiazol-2yl]-benzenesulf onamide (Ro 61-8048), two potent and selective inhibitors of this enzyme. Immune-stimulation with pokeweed mitogen (PWM, 200 microg i.v., 12 h) induced a robust increase in kynurenine (KYN) and its metabolites kynurenic acid (KYNA) and QUIN in blood and brain. When incubated in a medium containing KYN but not tryptophan, spleen, lung and liver (but not brain) slices accumulated a measurable amount of QUIN in the supernatant. Slices obtained from PWM treated animals had a ten-fold increase in QUIN accumulation in spleen, no changes in lung and a 40% decrease in liver, suggesting that the spleen contributes to the increased QUIN levels found in the blood and brain of immune-stimulated mice. Large doses of kynurenine hydroxylase inhibitors increased KYN and KYNA, but unexpectedly did not decrease QUIN content in control blood and brain. When tested in organ slices obtained from either controls or immune-stimulated animals, mNBA (1-1000 microM) and Ro 61-8048 (0.1-100 microM) strongly reduced QUIN neo-formation, suggesting that, in vitro, kynurenine hydroxylase activity is required for QUIN neosynthesis. Indeed, after repeated doses of mNBA or Ro 61-8048, QUIN content in blood and brain of immune-stimulated animals significantly decreased. Our results suggest that, under basal conditions, sufficient QUIN synthesis may occur through kynurenine hydroxylase-independent pathways. In immune-stimulated animals, however, kynurenine hydroxylase inhibitors significantly reduce blood and brain accumulation of QUIN.

Kynurenine hydroxylase inhibitors reduce ischemic brain damage: studies with (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfonamide (Ro 61-8048) in models of focal or global brain ischemia

Two kynurenine hydroxylase inhibitors, (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2yl]-benzenesulfona mide (Ro 61-8048), have been tested as neuroprotective agents on brain lesions induced by bilateral carotid occlusion in gerbils or by middle cerebral artery occlusion in rats. The percentage of lesioned pyramidal neurones found in the hippocampal CA1 region of gerbils subjected to bilateral carotid occlusion for 5 minutes decreased from 92+/-10% in vehicle-treated animals to 7+/-6% after mNBA (400 mg/kg intraperitoneally, three times at 1, 30, and 180 minutes after occlusion) or to 10+/-11% after Ro 61-8048 (40 mg/kg intraperitoneally, three times). A significant reduction in infarct volumes also was found when the kynurenine hydroxylase inhibitors were given to rats after permanent middle cerebral artery occlusion (from 207+/-111 mm3 in vehicle-treated rats to 82+/-18 and to 62+/-57 mm3 in rats treated with mNBA, 400 mg/kg intraperitoneally, or with Ro 61-8048, 40 mg/kg intraperitoneally, respectively). The administration of mNBA (400 mg/kg intraperitoneally) or Ro 61-8048 (40 mg/kg intraperitoneally) to gerbils with a dialysis probe in their dorsal hippocampus or to rats with a dialysis probe in their parietal cortex significantly increased kynurenic acid concentration in the dialysates. The data suggest that inhibition of kynurenine hydroxylase could be a new avenue to reduce neuronal loss in brain ischemia.

The kynurenine metabolic pathway in the eye: studies on 3-hydroxykynurenine, a putative cataractogenic compound

The rabbit lens has an elevated content of 3-hydroxykynurenine (30HKYN) in spite of a very low activity of the enzymes leading to its synthesis. The iris/ciliary body, on the contrary, has very high activity of 30HKYN synthesizing enzymes but a content of 30HKYN lower than that of the lens. These observations suggest that 30HKYN is formed in the iris/ ciliary body, released into the aqueous humor and then taken up into the lens where it may be used for the synthesis of UV filtering products. An excessive accumulation of 30HKYN in the lens has been associated with cataract formation. We found that available selective inhibitors of kynurenine hydroxylase reduced 30HKYN synthesis in both the lens and the iris/ciliary body.