[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.