Kynurenine and Related Metabolites in St.John's Wort (Hypericum perforatum).

The incorporation of tritium-label into quinolinic acid (QUIN), kynurenic acid (KYNA), and other kynurenine (KYN) pathway metabolites was studied in normal and QUIN-lesioned rat striata after a focal injection of [5-3H]KYN in vivo. The time course of metabolite accumulation was examined 15 min to 4 h after injection of [5-3H]KYN, and the concentration dependence of KYN metabolism was studied in rats killed 2 h after injection of 1.5-1,500 microM [5-3H]KYN. Labeled QUIN, KYNA, 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid, and xanthurenic acid (XA) were recovered from the striatum in every experiment. Following injection of 15 microM [5-3H]KYN, a lesion-induced increase in KYN metabolism was noted. Thus, the proportional recoveries of [3H]KYNA (5.0 vs. 1.8%), [3H]3-HK (20.9 vs. 4.5%), [3H]XA (1.5 vs. 0.4%), and [3H]QUIN (3.6 vs. 0.6%) were markedly elevated in the lesioned striatum. Increases in KYN metabolism in lesioned tissue were evident at all time points and KYN concentrations used. Lesion-induced increases of the activities of kynurenine-3-hydroxylase (3.6-fold), kynureninase (7.6-fold), kynurenine aminotransferase (1.8-fold), and 3-hydroxyanthranilic acid oxygenase (4.2-fold) likely contributed to the enhanced flux through the pathway in the lesioned striatum. These data provide evidence for the existence of a functional KYN pathway in the normal rat brain and for a substantial increase in flux after neuronal ablation. This method should be of value for in vivo studies of cerebral KYN pathway function and dysfunction.

L-Kynurenine (L-KYN) is a central metabolite of tryptophan degradation: known as kynurenine pathway, it is a cascade of enzymatic steps generating biologically active compounds, through which more than 95% of the tryptophan is catabolized. The early phase of the catabolic steps takes place mainly in the liver and the kidneys. However, the metabolization of L-KYN can effectively proceeds in the brain. The blood brain barrier strongly limits the penetrability of the kynurenine metabolites from the periphery to the central nervous system, since most of them can only be transferred by passive diffusion with a very low efficacy. One clear exception is the L-KYN, which can enter the brain with the aid of a large neutral amino acid transporter. Thus, the cerebral kynurenine metabolism is very responsive to the peripheral level of the L-KYN. Preclinical studies have shown that growth in the level of systemic L-KYN is particularly associated with a dose-dependent increase of its direct downstream metabolite kynurenic acid (KYNA) in the central nervous system. Evidence suggests that in the physiologically intact brain the most prominent and rapid change after peripheral L-KYN administration is the peak elevation of KYNA. KYNA is a complex neuromodulator, antioxidant and neuroprotective endogenous molecule. Elevation of brain KYNA content is correlated with attenuation in the concentration of extracellular glutamate, dopamine and acetylcholine in distinct cortical and subcortical brain regions. KYNA influences neurotransmission through multiple actions at the pre- and postsynaptic site. KYNA directly attenuates neurotransmitter release, partly by inhibiting α7 nicotinic acetylcholine (α7nACh) receptor located on presynaptic terminals, and partly by stimulating G-protein-coupled receptor 35 (GPR35) localized on neurons and astrocytes. Thus, even the modest fluctuations in endogenous KYNA can bi-directionally control the extracellular levels of glutamate. KYNA hinders postsynaptic N-methyl-D-aspartate (NMDA) receptor currents by competitive antagonism at allosteric glycine binding site of NMDA receptor. Moreover, in the periphery and in the brain during neuroinflammation, KYNA promotes anti-inflammatory responses due to activation of aryl hydrocarbon receptor and GPR35 receptor expressed by immune-cells, as well as it presumably also modulates neuronal survival through extrasynaptic NMDA receptor blockade. Besides its receptor-mediated actions, KYNA itself is a potent antioxidant. Therefore, elevation of brain KYNA level, either by administration of L-KYN or pharmacological manipulation of the availability of the kynurenine pathway enzymes, has become an attractive strategy to attenuate neuroinflammatory responses and to protect against glutamate induced excitotoxicity associated with ischemic brain injury. Accordingly, we and our collaborators achieved neuroprotection by the administration of L-KYN sulfate (L-KYNs) in experimental models of neurodegenerative diseases and ischemic stroke. Decades after the discovery of the neurotoxic and convulsant properties of glutamate, it has become clear that glutamate hypofunction is also pathogenic and therefore undesirable. Accordingly, in preclinical studies acute or chronic elevation of brain KYNA content, achieved partly by the peripheral administration of L-KYN, has been suggested to trigger alteration in the behavior of rodents: animals expressed hypoactivity or spatial working memory deficit. Moreover, pre- and postnatal chronic L-KYN exposure provoked long-lasting neurochemical and behavioral abnormalities manifested in adulthood. However, the results assessing the behavioral effects of the kynurenerg manipulations emerged from studies that focused mainly on rats, after various-dose of L-KYNs treatment. Implementing similar experiments in mice is of particular importance, because such data is almost absent from the literature. Additionally, the available information concerning the effects of kynurenerg manipulation beyond neuroprotection is quite incomplete, since study on dose-dependent responses to various L-KYNs treatment is not available. On a top of these, L-KYN and KYNA were attributed a direct role in the regulation of the systemic circulation. Namely, L-KYN was identified as an endothelium-derived vasodilator, contributing to peripheral arterial relaxation and regulation of blood pressure during systemic inflammation in rats. Furthermore, intravenous administration of low-dose L-KYN (1 mg/kg) has been shown to increase cerebral blood flow (CBF) in conscious rabbits. Therefore, we hypothesized that acute elevation of systemic L-KYN concentration may exert potential effects on mean arterial blood pressure (MABP) and on resting CBF in the adult mouse brain...

Objective To investigate the clinical significance of serum kynurenine (KYN) and kynurenic acid (KYNA) determined in patients with schizophrenia(SZ)by observing the differences of serum KYN and KYNA between schizophrenia and healthy controls.Methods The serum KYN and KYNA level in schizophrenia and healthy controls were measured by high performance liquid chromatography-fluorescence (HPLC-FLD) and the content ratio of KYN to KYNA were calculated.Results The serum KYN levels had not significantly difference between schizophrenia[(2.148±0.605)μmol/L] and controls[(2.108±0.592)μmol/L]( t =0.675,P >0.05).In schizophrenia the serum KYNA [(24.231±6.831)nmol/L] was significantly decreased compare with controls [(29.420±7.595)nmol/L]( t =1.657,P <0.05), while the content ratio of KYN to KYNA (111.496±23.797) was higher than the controls(80.330±17.639) ( t =1.654,P <0.05).In paranoid schizophrenia the serum KYN was lower than it in hebephrenic schizophrenia. In paranoid schizophrenia the serum KYNA was higher than it in simple and hebephrenic schizophrenia.In hebephrenic schizophrenia the content ratio of KYN to KYNA was higher than it in paranoid and undifferentiated schizophrenia.Conclusion The serum KYNA levels and KYN/KYNA ratio in schizophrenia can be used for clinic diagnosis and therapy of schizophrenia. Key words: Schizophrenia; Kynurenine; Kynurenic acid; High performance liquid chromatography

During inflammation, the kynurenine pathway (KP) metabolises the essential amino acid tryptophan (TRP) potentially contributing to excitotoxicity via the release of quinolinic acid (QUIN) and 3-hydroxykynurenine (3HK). Despite the importance of excitotoxicity in the development of secondary brain damage, investigations on the KP in TBI are scarce. In this study, we comprehensively characterised changes in KP activation by measuring numerous metabolites in cerebrospinal fluid (CSF) from TBI patients and assessing the expression of key KP enzymes in brain tissue from TBI victims. Acute QUIN levels were further correlated with outcome scores to explore its prognostic value in TBI recovery.MethodsTwenty-eight patients with severe TBI (GCS ≤ 8, three patients had initial GCS = 9–10, but rapidly deteriorated to ≤8) were recruited. CSF was collected from admission to day 5 post-injury. TRP, kynurenine (KYN), kynurenic acid (KYNA), QUIN, anthranilic acid (AA) and 3-hydroxyanthranilic acid (3HAA) were measured in CSF. The Glasgow Outcome Scale Extended (GOSE) score was assessed at 6 months post-TBI. Post-mortem brains were obtained from the Australian Neurotrauma Tissue and Fluid Bank and used in qPCR for quantitating expression of KP enzymes (indoleamine 2,3-dioxygenase-1 (IDO1), kynurenase (KYNase), kynurenine amino transferase-II (KAT-II), kynurenine 3-monooxygenase (KMO), 3-hydroxyanthranilic acid oxygenase (3HAO) and quinolinic acid phosphoribosyl transferase (QPRTase) and IDO1 immunohistochemistry.ResultsIn CSF, KYN, KYNA and QUIN were elevated whereas TRP, AA and 3HAA remained unchanged. The ratios of QUIN:KYN, QUIN:KYNA, KYNA:KYN and 3HAA:AA revealed that QUIN levels were significantly higher than KYN and KYNA, supporting increased neurotoxicity. Amplified IDO1 and KYNase mRNA expression was demonstrated on post-mortem brains, and enhanced IDO1 protein coincided with overt tissue damage. QUIN levels in CSF were significantly higher in patients with unfavourable outcome and inversely correlated with GOSE scores.ConclusionTBI induced a striking activation of the KP pathway with sustained increase of QUIN. The exceeding production of QUIN together with increased IDO1 activation and mRNA expression in brain-injured areas suggests that TBI selectively induces a robust stimulation of the neurotoxic branch of the KP pathway. QUIN’s detrimental roles are supported by its association to adverse outcome potentially becoming an early prognostic factor post-TBI.

To provide a more comprehensive clinic marker of tryptophan (TRP) catabolism in patients with systemic lupus erythematosus (SLE), we developed a simple and efficient method that simultaneously measured serum TRP, kynurenine (KYN), and kynurenic acid (KYNA) using high performance liquid chromatography with fluorescence detection (HPLC-FD). A simple and specific high performance liquid chromatography (HPLC) method was developed for simultaneously quantitative determination of TRP, KYN and KYNA with fluorescence detection (FD) using programmed wavelength and on-column fluorescence derivatization. Thirty patients with SLE and 80 healthy control subjects were analyzed for serum TRP metabolites using the assay we developed. The tryptophan breakdown index (TBI) and neuroprotective ratio (NPR) were calculated. The retention time of KYN, KYNA and TRP were 8.5 min, 13.7 min and 17.6 min, respectively. The linear range for TRP was 0.245-196 micromol/L, the limit of detection (LOD) was 0.001 micromol/L and average recovery was 103.71%. The linear range for KYN was 0.049-98 v/L, the LOD was 0.0245 micromol/L, and average recovery was 97.45%. The linear range for KYNA was 1.05-2093 nmol/L, the LOD was 0.05 nmol/L, and average recovery was 100.60%. Inter-day and intra-day relative standard deviations (SDs) were <5%. Phenylalanine, tyrosine, 5-hydroxytryptamine and creatinine did not interfere with the method. The results showed great differences in TRP, KYN and KYNA contents and TBI between patients with SLE and healthy controls, but little difference in NPR. The method is simple, fast, accurate, and meets the requirements for simultaneous determination of TRP, KYN and KYNA in serum.

To evaluate the potential contribution of circulating kynurenines to brain kynurenine pools, the rates of cerebral uptake and mechanisms of blood-brain barrier transport were determined for several kynurenine metabolites of tryptophan, including L-kynurenine (L-KYN), 3-hydroxykynurenine (3-HKYN), 3-hydroxyanthranilic acid (3-HANA), anthranilic acid (ANA), kynurenic acid (KYNA), and quinolinic acid (QUIN), in pentobarbital-anesthetized rats using an in situ brain perfusion technique. L-KYN was found to be taken up into brain at a significant rate [permeability-surface area product (PA) = 2-3 x 10(-3) ml/s/g] by the large neutral amino acid carrier (L-system) of the blood-brain barrier. Best-fit estimates of the Vmax and Km of saturable L-KYN transfer equalled 4.5 x 10(-4) mumol/s/g and 0.16 mumol/ml, respectively. The same carrier may also mediate the brain uptake of 3-HKYN as D,L-3-HKYN competitively inhibited the brain transfer of the large neutral amino acid L-leucine. For the other metabolites, uptake appeared mediated by passive diffusion. This occurred at a significant rate for ANA (PA, 0.7-1.6 x 10(-3) ml/s/g), and at far lower rates (PA, 2-7 x 10(-5) ml/s/g) for 3-HANA, KYNA, and QUIN. Transfer for KYNA, 3-HANA, and ANA also appeared to be limited by plasma protein binding. The results demonstrate the saturable transfer of L-KYN across the blood-brain barrier and suggest that circulating L-KYN, 3-HKYN, and ANA may each contribute significantly to respective cerebral pools. In contrast, QUIN, KYNA, and 3-HANA cross the blood-brain barrier poorly, and therefore are not expected to contribute significantly to brain pools under normal conditions.

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

Background and Aims Patients with end stage kidney disease (ESKD) are at high risk of premature death, mainly due to cardiovascular diseases. Hemodialysis (HD) and hemodiafiltration (HDF) procedures, although performed to minimize ESKD related organ damage, have limited efficacy regarding removal of selected uremic toxins and improving patients’ outcomes. Products of tryptophan (Trp) degradation, synthesized through the kynurenine (KYN) pathway, represent one of the least studied substances in patients with kidney diseases. In animal models, a direct effect of KYN on hypertension and kidney function decline has been proposed. Interestingly, kynurenic acid (KYNA), which is formed from KYN, was reported to induce natriuresis and lower heart rate in hypertensive rats. However, in multiple studies in humans, serum level of KYN, KYNA, and another Trp metabolite 3-hydroxykynurenine (3-OHKYN) was shown to correlate with kidney function impairment and its complications. The aim of this study was to investigate the removal of Trp and its metabolites: KYN, KYNA and 3-OHKYN in patients treated by HD or HDF, as well as the influence of patients’ medical conditions on KYN pathway product clearance. Method The study was conducted in 50 patients with ESKD (median age 65 years, 44% of participants were male, the median dialysis vintage was 29.5 months), receiving HD (70% patients) or HDF (30% of patients) procedures. Dialysis parameters, related with patient (type of vascular access, body mass gain, blood flow) and procedure (dialysate flow, ultrafiltration volume, substitution volume, type and surface of dialyzer), together with patients’ clinical data and medical history have been investigated. Serum level of Trp, KYN, KYNA and 3-OHKYN were measured before and after a dialysis session through high performance liquid chromatography. Results The serum concentration of Trp (predialysis 6.2 μM vs postdialysis 7.1 μM, p = 0.133) and 3-OHKYN (53.9 nM vs 30.9 nM, p = 0.0981) remained unchanged, whereas the concentration of KYN (1.6 μM vs 0.7 μM, p = 0.0002), KYNA (418.4 nM vs 245.9 nM, P &amp;lt; 0.0001) and KYN/Trp ratio (0.4 vs 0.1, p = 0.0002) significantly decreased after dialysis procedures. Hypertension was associated with improved KYN clearance (2.4 μM vs 0.8 μM, p = 0.0005) and KYN/Trp ratio (0.4 vs 0.1, p = 0.0007), but not KYNA, although in patients without hypertension higher predialysis KYNA concentration was observed (584.6 nM vs 367.4 nM, p = 0.0044). On the other hand, in patients with heart failure or atherosclerosis lower predialysis serum KYN concentration and higher KYNA levels were found, however both comorbidities did not correlate with the clearance of these substances. Serum KYN removal was higher in HD than in HDF treated patients (2.4 μM vs 0.7 μM, p = 0.0005), but the clearance of other Trp metabolites did not differ between both dialysis modalities. Conclusion Our study found incomplete removal of Trp metabolites in ESKD patients treated by HD and HDF procedures. Targeted strategies to lower selected KYN pathway metabolites concentration may be beneficial in patients with ESKD.

IntroductionThe kynurenine pathway of tryptophan catabolism has come into the spotlight of schizophrenia research since its catabolites exert neuroactive effects. A strong body of evidence suggests that kynurenic acid, a catabolite of kynurenine pathway, acts as the only endogenous NMDA receptor antagonist leading to the weakening of circuits in layer III of dorsolateral prefrontal cortex of schizophrenia patients. Studies exploring the levels of kynurenic acid and other metabolites of tryptophan in peripheral blood did not yield any definite conclusions.ObjectivesPrimary objective of this study was to assess differences in concentrations of key constituents of kynurenic pathway in blood plasma – tryptophan (TRP), kynurenine (KYN) and kynurenic acid (KYNA) between schizophrenia patients (SCZ) and healthy controls (HC). Secondary objective was to explore correlations between these concentrations and clinical characteristics.MethodsIn our two-centre prospective case-control study we measured plasma concentrations of TRP, KYN and KYNA in 36 healthy controls (HC) and 38 schizophrenia (SCZ) patients during acute exacerbation and remission and explored the correlations with clinical parameters using PANSS scale. The patients were matched with HC by age, sex and body mass index and exclusion criteria included obesity class 2 or higher, any concomitant organic mental or neurological disorder, acute or chronic inflammatory disease, and use of immunomodulatory drugs or psychoactive substances.ResultsTRP concentrations were significantly higher in HC than in SCZ patients in acute phase (p&lt;0,001) and remission (p&lt;0,001), while SCZ patients in acute phase had significantly higher TRP levels than in remission (p&lt;0,01). Levels of KYNA and KYN were significantly lower in SCZ patients than in HC both in acute phase and remission, all with high statistical significance (p&lt;0,001). There was no statistically significant difference between acute phase and remission neither for KYN (p&gt;0,05), nor for KYNA (p&gt;0,05). There was no correlation of plasma levels of TRP, KYN and KYNA with total PANSS score, PANSS positive scale score, PANSS negative scale score and PANSS general psychopathology scores, both in acute phase and remission (p&gt;0,05). Also, there was no correlation between plasma levels of TRP, KYN and KYNA in SCZ patients in remission with improvements measured with PANSS scale (p&gt;0,05).ConclusionsAlthough there are concerns about the value of measurement of metabolites of kynurenine pathway in the peripheral blood, our data suggest that significantly decreased levels of KYN and KYNA could suggest that disrupted TRP degradation in SCZ patients may be reflected in the peripheral blood as well. Further studies of peripheral levels of kynurenine pathway metabolites on larger samples should also explore effects of antipsychotic therapy, but also their correlation with other clinical parameters such as neurocognition.Disclosure of InterestNone Declared

The incorporation of L-kynurenine (L-KYN) into kynurenic acid (KYNA) was examined in rat brain slices. KYNA was measured in the slices and in the incubation medium after purification by ion-exchange and HPLC chromatography. In pilot experiments, the formation of KYNA was confirmed by gas chromatography. KYNA was produced stereoselectively from L-KYN, and approximately 90% of the newly synthesized KYNA was recovered from the incubation medium. Intracellular KYNA was not actively retained by the tissue and was lost from the cells upon repeated washes. Thus, regulation of the levels of extracellular KYNA appears to occur at the level of L-KYN uptake and/or kynurenine transaminase, the biosynthetic enzyme of KYNA. KYNA production from L-KYN was linear up to 4 h and reached a plateau at a L-KYN concentration of 250 microM. The process was effectively inhibited by the transaminase inhibitor aminooxyacetic acid (IC50, approximately 25 microM), and showed pronounced regional distribution (hippocampus greater than cortical areas greater than thalamus much greater than cerebellum). The conversion of L-KYN to KYNA was dependent on oxygenation and on the presence of glucose in the incubation medium. Neither deletion of Ca2+ or Mg2+ nor addition of 20 mM Mg2+ had any effect. However, KYNA production was significantly attenuated in the absence of Cl- or in the presence of 50 mM K+ in the incubation medium. In Na+-free medium, the production of KYNA from L-KYN was increased by 30%.(ABSTRACT TRUNCATED AT 250 WORDS)

ObjectivesAge-related macular degeneration (AMD), in which oxidative stress, inflammation and metabolic imbalances play a role in its pathogenesis, is one of the leading causes of irreversible vision loss. The kynurenine (KYN) pathway, one of the principal routes of tryptophan (TRP) metabolism, constitutes an important mechanism in retinal neurodegeneration. Based on this information, our study aimed to compare the serum TRP, KYN, kynurenic acid (KYNA), 3-hydroxykynurenine (3HK), 3-hydroxyanthranilic acid (3HAA) and, quinolinic acid (QA) levels of AMD patients and to investigate the diagnostic values ​​of these biomarkers.MethodsSerum samples were collected from AMD patients and control groups. TRP, KYN, KYNA, 3HK, 3HAA, and QA levels were measured using a commercial ELISA method. KYN pathway activity, KYN/TRP and, KYNA/3HK ratios were also assessed. Mann-Whitney U test, ROC analysis, Spearman correlation were applied for statistical comparisons.ResultsAccording to our results, 3HK was significantly higher in the AMD group, while TRP, KYN, QA, and KYNA/3HK ratio were higher in the control. ROC analysis revealed 3HK to be the strongest discriminatory marker. The KYNA/3HK ratio also provided significant diagnostic value. Correlation analysis revealed strong negative correlations between 3HK and KYN, QA, and especially KYNA/3HK. Conversely, strong positive correlations were found between KYN and KYNA/3HK, and between TRP, KYN, QA, and KYNA.ConclusionKYN pathway metabolites exhibit significant alterations in patients with AMD. 3HK levels and the reduction of the KYNA/3HK ratio suggest a disruption of the neurotoxic–neuroprotective balance and imply that KYN pathway dysfunction may play a role in the pathogenesis of AMD. Among the biomarkers examined, 3HK displayed the highest diagnostic performance, while the KYNA/3HK ratio emerged as an additional biological indicator. These findings indicate that 3HK and the KYNA/3HK ratio may serve as potential biomarker candidates for the early diagnosis and monitoring of AMD.

Aim: Chronic inflammation is closely associated with tryptophan (TRP)-kynurenine (KYN) metabolic pathway. However, TRP-KYN pathway has not been fully elucidated in psoriasis, a systemic inflammatory disease with skin lesions and extracutaneous manifestations. Herein, we studied comprehensively serum profiles of TRP-KYN pathway metabolites in psoriatic patients (PSOs) to clarify the involvement of this pathway in the pathophysiology of psoriasis and to evaluate serum biomarkers reflecting systemic inflammation in PSOs. Methods: The concentrations of main TRP metabolites, TRP, KYN, 3-hydroxykynurenine (3HK), kynurenic acid (KYNA), 3-hydroxyanthranilic acid (3HAA), and anthranilic acid (AA), were determined by high-performance liquid chromatography in the sera from 65 PSOs and 35 healthy controls (HCs). The levels of these metabolites and the ratios of metabolites were compared between these subjects. The correlations between these values and the psoriasis area severity index (PASI) scores were analyzed. Skin samples from PSOs and HCs were subjected to immunohistochemical staining for kynureninase. Cytokine concentrations were comprehensively measured in the same samples and the correlations between the cytokine levels and TRP-KYN pathway metabolite levels were examined. Results: Serum TRP, KYN, and KYNA concentrations were lower and the 3HAA concentrations were higher in PSOs than in HCs. The ratios of 3HK/KYN, 3HAA/3HK, and 3HK/AA were higher in PSOs than in HCs. The AA levels and the ratio of AA/KYN were weakly positively correlated, and TRP, KYNA, and 3HK levels and the ratios of KYNA/KYN and 3HAA/AA were weakly negatively correlated with the PASI scores. The AA, KYN, and KYNA levels were positively correlated with the interferon gamma-induced protein 10 (IP-10) concentrations. Kynureninase expression was enhanced in the epidermis, both involved and uninvolved skin. Conclusions: Serum profiles of TRP-KYN pathway metabolites differed between PSOs and HCs. TRP-KYN pathway-associated processes, including kynureninase activation, may be involved in the pathogenesis of psoriasis, and thus serve as targets for psoriasis therapy.

Background: Chronic infection with Toxoplasma gondii (TOXO) results in microcysts in the brain that are controlled by inflammatory activation and subsequent changes in the kynurenine pathway. TOXO seropositivity is associated with a heightened risk of schizophrenia (SCZ) and with cognitive impairments. Latency of the acoustic startle response, a putative index of neural processing speed, is slower in SCZ. SCZ subjects who are TOXO seropositive have slower latency than SCZ subjects who are TOXO seronegative. We assessed the relationship between kynurenine pathway metabolites and startle latency as a potential route by which chronic TOXO infection can lead to cognitive slowing in SCZ.Methods: Fourty-seven SCZ subjects and 30 controls (CON) were tested on a standard acoustic startle paradigm. Kynurenine pathway metabolites were measured using liquid chromatography-tandem mass spectrometry were kynurenine (KYN), tryptophan (TRYP), 3-hydroxyanthranilic acid (3-OHAA), anthranilic acid (AA), and kynurenic acid (KYNA). TOXO status was determined by IgG ELISA.Results: In univariate ANCOVAs on onset and peak latency with age and log transformed startle magnitude as covariates, both onset latency [F(1,61) = 5.76; p = 0.019] and peak latency [F(1,61) = 4.34; p = 0.041] were slower in SCZ than CON subjects. In stepwise backward linear regressions after stratification by Diagnosis, slower onset latency in SCZ subjects was predicted by higher TRYP (B = 0.42; p = 0.008) and 3-OHAA:AA (B = 3.68; p = 0.007), and lower KYN:TRYP (B = −185.42; p = 0.034). In regressions with peak latency as the dependent variable, slower peak latency was predicted by higher TRYP (B = 0.47; p = 0.013) and 3-OHAA:AA ratio (B = 4.35; p = 0.010), and by lower KYNA (B = −6.67; p = 0.036). In CON subjects neither onset nor peak latency was predicted by any KYN metabolites. In regressions stratified by TOXO status, in TOXO positive subjects, slower peak latency was predicted by lower concentrations of KYN (B = −8.08; p = 0.008), KYNA (B = −10.64; p = 0.003), and lower KYN:TRYP ratios (B = −347.01; p = 0.03). In TOXO negative subjects neither onset nor peak latency was predicted by any KYN metabolites.Conclusions: KYN pathway markers predict slowing of startle latency in SCZ subjects and in those with chronic TOXO infection, but this is not seen in CON subjects nor TOXO seronegative subjects. These findings coupled with prior work indicating a relationship of slower latency with SCZ and TOXO infection suggest that alterations in KYN pathway markers may be a mechanism by which neural processing speed, as indexed by startle latency, is affected in these subjects.

Introduction The kynurenine (KYN) pathway has been implicated in depression and neurotoxicity. Derived from tryptophan, KYN can be further degraded along one of two distinct branches. The KYN-KYNA branch is regulated by the enzyme kynurenine aminotransferase (KAT) and is considered neuroprotective, as it degrades KYN into the non-blood brain barrier (BBB) transportable metabolite kynurenic acid (KYNA). The KYN-NAD branch is regulated by the enzyme kynurenine monooxygenase (KMO) and is considered neurotoxic as it degrades KYN into the BBB transportable metabolite 3-hydroxykynurenine (3-HK) and, further in the cascade, quinolinic acid (QUIN). Recent studies have shown the importance of muscle health on directing kynurenine metabolism towards the neuroprotective branch, highlighting a novel muscle-to-brain axis. Specifically, exercise induced increases in the transcription factor PCG-1⍺ amplifies the content of KAT enzymes that convert KYNA from KYN. Duchenne muscular dystrophy (DMD) is an X-linked severe muscle disorder caused by a loss of dystrophin leading to muscle fragility, wasting, and weakness. In light of recent evidence revealing the cognitive and depressive behaviours in DMD patients and in the preclinical mdx mouse, we sought to determine whether KYN metabolism as well as PGC-1α and KAT content would be altered in the mdx model. Methods 8-10 week old male mdx and wild-type (DBA/2J) mice were purchased from Jackson laboratories. Behavioural changes (ie., grooming activity, food and water intake) were measured using a Promethion metabolic cage system along with fear and anxiety-like behaviour during a novel object recognition test (NORT). Mice were euthanized and serum KYN and KYNA were measured using commercially available ELISA kits. Extensor digitorum longus muscle was collected, homogenized, and Western blotting was performed for PGC-1⍺, KAT1, and KAT3. Results Metabolic cage results showed that fine activity (-5%), water intake (-25%), and food intake (-60%) were lower across light and dark stages in mdx mice compared to WT mice (main effect of genotype, p<0.0001 for all measures). The mdx mice also spent more time in the corners of the NORT arenas compared to their WT counterparts (+270s, p<0.0001). Though the change in serum KYN was insignificant across mdx and WT mice, the concentration of serum KYNA was lower in the mdx mice (-57%, p = 0.01), therefore causing a lower KYN:KYNA ratio in mdx mice compared with WT (-56%, p = 0.01). Western blotting demonstrated a reduction in PGC-1⍺ (-65%, p = 0.002) and KAT1 (-35%, p = 0.02) content in mdx mice compared to WT mice, whereas the KAT3 content was elevated in mdx mice (1.5-fold, p = 0.05). Conclusion Our results of lowered serum KYN:KYNA concentrations from mdx mice (compared to WT) correspond well with changes in affective and anxiety-related behaviours. The observed reduction in muscle PGC-1⍺ and KAT1 content likely contributes to these changes in KYN:KYNA ratio. Though KAT3 was upregulated in mdx muscle compared to WT, this could represent a failed compensatory response.

The causes of reduced aerobic exercise capacity (ExCap) in chronic kidney disease (CKD) are multifactorial, possibly involving the accumulation of tryptophan (TRP) metabolites such as kynurenine (KYN) and kynurenic acid (KYNA), known as kynurenines. Their relationship to ExCap has yet to be studied in CKD. We hypothesised that aerobic ExCap would be negatively associated with plasma levels of TRP, KYN and KYNA in CKD. We included 102 patients with non-dialysis CKD stages 2-5 (CKD 2-3, n = 54; CKD 4-5, n = 48) and 54 healthy controls, age- and sex-matched with the CKD 2-3 group. ExCap was assessed as peak workload during a maximal cycle ergometer test. Plasma KYN, KYNA and TRP were determined by high-performance liquid chromatography. Kidney function was evaluated by glomerular filtration rate (GFR) and estimated GFR. The CKD 2-3 group and healthy controls repeated tests after five years. The association between TRP, KYN, KYNA and ExCap in CKD was assessed using a generalised linear model. At baseline, there were significant differences between all groups in aerobic ExCap, KYN, KYNA, TRP and KYN/TRP. KYNA increased in CKD 2-3 during the follow-up period. In CKD 2-5, KYNA, KYN/TRP and KYNA/KYN were all significantly negatively associated with ExCap at baseline, whereas KYN and TRP were not. Kynurenines were significantly correlated with GFR (p < 0.001 for all). Including GFR in the statistical model, no kynurenines were independently associated with ExCap at baseline. At follow-up, the increase in KYN and KYN/TRP was related to a decrease in ExCap in CKD 2-3. After adjusting for GFR, increase in KYN/TRP remained an independent significant predictor of a decline in ExCap in CKD 2-3. Aerobic ExCap was inversely associated with plasma levels of kynurenines in CKD at baseline and follow-up.