Kynurenine Aminotransferase Activity Research Articles

Effects of Fenofibrate and Gemfibrozil on Kynurenic Acid Production in Rat Kidneys In Vitro: Old Drugs, New Properties.

Kidney dysfunction significantly increases the cardiovascular risk, even in cases of minor functional declines. Hypertriglyceridemia is the most common lipid abnormality reported in patients with kidney disorders. PPAR-α (peroxisome proliferator-activated receptor-α) agonists called fibrates are the main agents used to lower triglyceride levels. Kynurenic acid (KYNA) is a tryptophan (Trp) derivative directly formed from L-kynurenine (L-KYN) by kynurenine aminotransferases (KATs). KYNA is classified as a uremic toxin, the level of which is correlated with kidney function impairments and lipid abnormalities. The aim of this study was to analyze the effect of the most commonly used triglyceride-lowering drugs, fenofibrate and gemfibrozil, on KYNA production and KAT activity in rat kidneys in vitro. The influence of fenofibrate and gemfibrozil on KYNA formation and KAT activity was tested in rat kidney homogenates in vitro. Fenofibrate and gemfibrozil at 100 µM-1 mM significantly inhibited KYNA synthesis in rat kidney homogenates. Both fibrates directly affected the KAT I and KAT II isoenzyme activities in a dose-dependent manner at similar concentrations. The presented results reveal the novel mechanism of action of fibrates in the kidneys and suggest their potential role in kidney function protection beyond the well-known anti-hyperlipidemic effect.

Kynurenic Acid Levels and Kynurenine Aminotransferase I, II and III Activities in Ganglia, Heart and Liver of Snail Helix Pomatia.

Kynurenic acid (KYNA), a tryptophan metabolite along the kynurenine pathway, is an endogenous antagonist of glutamate ionotropic excitatory amino acid (EAA) receptors and the α7 nicotinic acetylcholine receptor (nAChR). The involvement of KYNA in various pathological conditions and during the aging process is significant. KYNA synthesis from L-kynurenine (L-KYN), through the action of several kynurenine aminotransferases (KATs), is present in the central nervous system (CNS) and periphery of mammals. We were interested in investigating the ability of the brain and peripheral organs of Helix pomatia snails to synthesize KYNA, in an in vitro study. In comparative studies between rat and snail, we looked for the synthesis of KYNA in the liver. We then looked for an effect of age on KYNA synthesis. Ten shell parameters of the Helix pomatia snail were used to establish an Age Rating Scale (ARS), i.e. body weight, shell weight, shell length, width and height, shell opening length and width, lip width, number of shell turns and external shell growth rings. An age of the snails was determined according to the ARS and the snails were divided into three groups, i.e. young, middle and old age. Homogenates of dissected regions, i.e. cerebral ganglia (CG), subpharyngeal ganglia (SG) consisting of pedal, visceral and pleural ganglia, heart and liver, were examined. KYNA was measured by high performance liquid chromatography (HPLC) and KAT activities were measured by an enzymatic method. With respect to ARS, an evaluation of the age of the snails between young (1-2 years), middle (5-7 years) and old (9-13 years) showed significant differences (p<0.001). Analysis of KYNA levels in different snail tissues, i.e. CG, SG, heart and liver, showed an occurrence in the low femtomolar range. Marked and significant increases of KYNA were found in the liver of middle and old age groups. In the SG, KYNA decreased significantly with age. There were no differences in KYNA levels between groups in CG and heart. The lowest KAT activity was found in CG and SG (5 pmol/mg/h), while in heart and liver the values were visibly higher (between 8 and 80 pmol/mg/h). Only in the liver, and exceptionally only for KAT I, the activity increased significantly with age, i.e. up to 14 years. No age-related changes in KAT I, II and III activities were found in CG and SG. Snail liver shows a different pattern of KAT activities compared to the rat liver. Regions of the CNS and periphery of the snail Helix pomatia are able to synthesize KYNA due to KAT activities. In the snail liver, KAT I activity increased with age. Notably, there was no age-related increase in KAT activities in the heart and especially in the CNS of Helix pomatia, indicating significant differences from mammals. A moderate KYNA metabolism in the Helix pomatia snail in the periods studied, up to 14 years, could be a physiological phenomenon that protects organs from possible functional insufficiency due to high KYNA levels, as has been suggested. It is reasonable to search for the factor(s) that could regulate the concentration of KYNA in the body of the snail.

#3508 FUROSEMIDE DECREASES KYNURENIC ACID SYNTHESIS IN RAT KIDNEY IN VITRO

Abstract Background and Aims Loop diuretics are highly potent agents, particularly beneficial in patients with impaired kidney function or heart failure exacerbation. Furosemide, due to fast onset and short time of action, remains the most commonly used drug from this class of diuretics. Inhibition of sodium-potassium-chloride cotransporter (NKCC2) in the thick ascending limb of Henle's loop is the main mechanism of furosemide's action. Other effects of furosemide, like modulation of potassium channels activity or activation of prostaglandin synthesis, were previously described. It was postulated that furosemide may exacerbate kidney injury in a dose dependent manner. Kynurenic acid (KYNA) is a tryptophan derivative synthesized from L-kynurenine (L-KYN) by kynurenine aminotransferases (KATs). KAT I and KAT II are the main KAT isoenzymes. Non-selective antagonism towards ionotropic glutamatergic receptors, as well as activation of aryl hydrocarbon receptors and the α7 nicotinic acetylcholine receptors are known mechanisms of KYNA’s action. KYNA was shown to regulate water and electrolyte balance through natriuretic effect. By some researchers KYNA, together with other tryptophan metabolites, is classified as uremic toxin. We aimed to analyze the influence of furosemide, widely used loop diuretic, on KYNA’s synthesis and the activity of both KATs in rat kidney in vitro. Method The influence of furosemide on KYNA’s production with KAT I and KAT II activity was tested in rat kidney homogenates in vitro after 2 hours incubation in the presence of KYNA precursor L-KYN and furosemide. Diuretic was examined at increasing concentrations: 1 μM, 10 μM, 50 μM, 100 μM, 500 μM and 1 mM. Enzymatic KYNA’s production was measured by high performance liquid chromatography (HPLC) with fluorometric detector. All applicable international, national and institutional guidelines for the care and use of animals were followed. Results Furosemide at 500 μM and 1 mM lowered KYNA formation in kidney homogenates in vitro to 80% (p &amp;lt; 0.05) and 70% (p &amp;lt; 0.05) of control value, respectively. At 1 mM concentration furosemide inhibited kidney KAT I activity in vitro to 38% (p &amp;lt; 0.01) of control value. Similarly, furosemide at 500 μM and 1 mM concentration decreased kidney KAT II activity in vitro to 53% (p &amp;lt; 0.01) and 18% (p &amp;lt; 0.001) of control value, respectively. Conclusion Our study presents a novel mechanism of action of furosemide. Inhibition of KYNA synthesis in the kidney by furosemide may have influence on kidney function.

The influence of cyclooxygenase inhibitors on kynurenic acid production in rat kidney: a novel path for kidney protection?

BackgroundKidney diseases have become a global health problem, affecting about 15% of adults and being often under-recognized. Immunological system activation was shown to accelerate kidney damage even in inherited disorders. The kynurenine pathway is the main route of tryptophan degradation. A metabolite of kynurenine (KYN), kynurenic acid (KYNA), produced by kynurenine aminotransferases (KATs), was reported to affect fluid and electrolyte balance as a result of natriuresis induction. The accumulation of KYNA was shown in patients with impaired kidney function and its level was related to the degree of kidney damage. Cyclooxygenase (COX) inhibitors are well-known analgesics and most of them demonstrate an anti-inflammatory effect. Their main mechanism of action is prostaglandin synthesis blockade, which is also responsible for their nephrotoxic potential. Since the KYN pathway is known to remain under immunological system control, the purpose of this study was to analyze the effect of 9 COX inhibitors on KYNA production together with KATs’ activity in rat kidneys in vitro.MethodsExperiments were carried out on kidney homogenates in the presence of L-KYN and the selected compound in 6 various concentrations.ResultsAmong the examined COX inhibitors only acetaminophen did not change KYNA production in rat kidneys in vitro. Additionally, acetaminophen did not affect the activity of KAT I and KAT II, whereas acetylsalicylic acid and ibuprofen inhibited only KAT II. The remaining COX inhibitors decreased the activity of both KATs in rat kidneys in vitro.ConclusionOur study provides novel mechanisms of COX inhibitors action in the kidney, with possible implications for the treatment of kidney diseases.Graphical abstract

Effect of Ibuprofen on Kynurenic Acid Production and Kynurenine Aminotransferases Activity in Rat Kidneys

Effect of Ibuprofen on Kynurenic Acid Production and Kynurenine Aminotransferases Activity in Rat Kidneys

Kynurenic acid and kynurenine aminotransferase are potential biomarkers of early neurological improvement after thrombolytic therapy: A pilot study.

Biomarkers for predicting treatment response to thrombolysis in acute ischemic stroke are currently lacking. Both, animal models and clinical studies have provided evidence that the kynurenine (KYN) pathway is activated in ischemic stroke. In our pilot study, we aimed to investigate whether KYN pathway enzymes and metabolites could serve as potential biomarkers for treatment response in the hyperacute phase of ischemic stroke. We included 48 acute ischemic stroke patients who received thrombolysis. Blood samples were taken both before and 12 h after treatment. Concentrations of 11 KYN metabolites were determined using ultra-high-performance liquid chromatography-mass spectrometry. To assess the treatment response, we used early neurological improvement (ENI), calculated as the difference between the admission and discharge National Institutes of Health Stroke Scale (NIHSS) scores. We performed receiver operating characteristic (ROC) analysis for KYN pathway metabolites and enzymes that showed a correlation with ENI. In the samples taken before thrombolysis, significantly lower concentrations of kynurenic acid (KYNA) and kynurenine aminotransferase (KAT) activity were found in patients who had ENI (p = 0.01 and p = 0.002, respectively). According to the ROC analysis, the optimal cut-off value to predict ENI for KYNA was 37.80 nM (sensitivity (SN) 69.2%, specificity (SP) 68.4%) and 0.0127 for KAT activity (SN 92.3%, SP 73.7%). Our research is the first clinical pilot study to analyze changes in the KYN pathway in ischemic stroke patients who received thrombolytic treatment. Based on our results, baseline KYNA concentration and KAT activity could serve as potential biomarkers to predict early treatment response to thrombolysis.

Multiple roles of haem in cystathionine β-synthase activity: implications for hemin and other therapies of acute hepatic porphyria.

The role of haem in the activity of cystathionine β-synthase (CBS) is reviewed and a hypothesis postulating multiple effects of haem on enzyme activity under conditions of haem excess or deficiency is proposed, with implications for some therapies of acute hepatic porphyrias. CBS utilises both haem and pyridoxal 5′-phosphate (PLP) as cofactors. Although haem does not participate directly in the catalytic process, it is vital for PLP binding to the enzyme and potentially also for CBS stability. Haem deficiency can therefore undermine CBS activity by impairing PLP binding and facilitating CBS degradation. Excess haem can also impair CBS activity by inhibiting it via CO resulting from haem induction of haem oxygenase 1 (HO 1), and by induction of a functional vitamin B6 deficiency following activation of hepatic tryptophan 2,3-dioxygenase (TDO) and subsequent utilisation of PLP by enhanced kynurenine aminotransferase (KAT) and kynureninase (Kynase) activities. CBS inhibition results in accumulation of the cardiovascular risk factor homocysteine (Hcy) and evidence is emerging for plasma Hcy elevation in patients with acute hepatic porphyrias. Decreased CBS activity may also induce a proinflammatory state, inhibit expression of haem oxygenase and activate the extrahepatic kynurenine pathway (KP) thereby further contributing to the Hcy elevation. The hypothesis predicts likely changes in CBS activity and plasma Hcy levels in untreated hepatic porphyria patients and in those receiving hemin or certain gene-based therapies. In the present review, these aspects are discussed, means of testing the hypothesis in preclinical experimental settings and porphyric patients are suggested and potential nutritional and other therapies are proposed.

Species-specific neuronal localization of kynurenine aminotransferase-2 in the mouse cerebellum

The immunohistochemical pattern of kynurenine aminotransferase-2 (KAT-2) - the key role enzyme in the production of neuroactive and neuroprotective kynurenic acid (KYNA) - was studied in the cerebellum of mice. It is known from literature that KAT-2 is localized mainly in astrocytes in different parts of the cerebrum. Kynurenine aminotransferase (KAT) activity in the cerebellum is relatively low and alternative production routes for KYNA have been described there. Therefore we examined the immunohistochemical pattern of KAT-2 in this part of the brain. Surprisingly, the cellular localization of KAT-2 in mice was proven to be unique; it localized characteristically in Purkinje cells and in some other types of neurons (not identified) but was not found in astrocytes nor microglia. The exclusive neuronal, but not glial localization of KAT-2 in the cerebellum is novel and may be related to its low activity and to the alternative pathways for KYNA production that have been described.

Effect of Gemfibrozil on Kynurenine Aminotransferases Activity and Kynurenic Acid Production in Rat Kidney

Effect of Gemfibrozil on Kynurenine Aminotransferases Activity and Kynurenic Acid Production in Rat Kidney

Kynurenine, Tetrahydrobiopterin, and Cytokine Inflammatory Biomarkers in Individuals Affected by Diabetic Neuropathic Pain.

Neuropathic pain is a common complication of diabetes with high morbidity and poor treatment outcomes. Accumulating evidence suggests the immune system is involved in the development of diabetic neuropathy, whilst neuro-immune interactions involving the kynurenine (KYN) and tetrahydrobiopterin (BH4) pathways have been linked to neuropathic pain pre-clinically and in several chronic pain conditions. Here, using a multiplex assay, we quantified serum levels of 14 cytokines in 21 participants with type 1 diabetes mellitus, 13 of which were classified as having neuropathic pain. In addition, using high performance liquid chromatography and gas chromatography-mass spectrometry, all major KYN and BH4 pathway metabolites were quantified in serum from the same cohort. Our results show increases in GM-CSF and IL-8, suggesting immune cell involvement. We demonstrated increases in two inflammatory biomarkers: neopterin and the KYN/TRP ratio, a marker of indoleamine 2,3-dioxygenase activity. Moreover, the KYN/TRP ratio positively correlated with pain intensity. Total kynurenine aminotransferase activity was also higher in the diabetic neuropathic pain group, indicating there may be increased production of the KYN metabolite, xanthurenic acid. Overall, this study supports the idea that inflammatory activation of the KYN and BH4 pathways occurs due to elevated inflammatory cytokines, which might be involved in the pathogenesis of neuropathic pain in type 1 diabetes mellitus. Further studies should be carried out to investigate the role of KYN and BH4 pathways, which could strengthen the case for therapeutically targeting them in neuropathic pain conditions.

Experimental hypothyroidism raises brain kynurenic acid – Novel aspect of thyroid dysfunction

Hypothyroidism frequently manifests with altered mood and disturbed cognition. Kynurenic acid may influence cognition through antagonism of N-methyl-d-aspartate receptors (NMDA) and α7 nicotinic receptors. In here, thyroid hormones effects on kynurenic acid synthesis in rat cortical slices and on kynurenine aminotransferases (KATs) activity in semi-purified cortical homogenates were studied. Furthermore, brain kynurenic acid levels and KATs activities were evaluated in experimental model of hypothyroidism, induced by chronic administration of 0.05% propylthiouracil in drinking water. In vitro, L-thyroxine (T4) and 3,3,5-triiodothyronine (T3), reduced kynurenic acid synthesis and KATs activities (IC50 ~ 50–150 μM). In vivo, propylthiouracil increased cortical, hippocampal and striatal, but not cerebellar kynurenic acid content (192%, 142% and 124% of control, respectively), despite uniformly decreased KAT II activity and lower cortical and striatal KAT I activity. T4 application to hypothyroid animals restored kynurenic acid levels to control values and reversed enzymatic changes. T4 alone did not change brain kynurenic acid levels, despite increased activities of brain KATs. Hence, thyroid hormones modulate kynurenic acid levels by two opposing mechanisms, stimulation of KATs activity, most probably transcriptional, and direct, post-translational inhibition of KATs. Lack of correlation between KATs activity and kynurenic acid level may reflect the influence of T4 on organic anion transporter and result from impaired removal of kynurenic acid from the brain during hypothyroidism. Our data reveal novel mechanism linked with thyroid hormones deficiency and imply the potential involvement of increased brain kynurenic acid in the hypothyroidism-related cognitive disturbance.

Differences in Kynurenine Metabolism During Depressive, Manic, and Euthymic Phases of Bipolar Affective Disorder.

The kynurenine pathway is involved in inflammatory diseases. Alterations of this pathway were shown in psychiatric entities as well. The aim of this study was to determine whether specific changes in kynurenine metabolism are associated with current mood symptoms in bipolar disorder. Sum scores of the Hamilton Depression Scale, Beck Depression Inventory, and Young Mania Rating Scale were collected from 156 bipolar individuals to build groups of depressive, manic and euthymic subjects according to predefined cut-off scores. Severity of current mood symptoms was correlated with activities of the enzymes kynurenine 3-monooxygenase (ratio of 3-hydroxykynurenine/ kynurenine), kynurenine aminotransferase (ratio of kynurenic acid/ kynurenine) and kynureninase (ratio of 3-hydroxyanthranilic acid/ 3-hydroxykynurenine), proxied by ratios of serum concentrations. Individuals with manic symptoms showed a shift towards higher kynurenine 3-monooxygenase activity (χ2 = 7.14, Df = 2, p = .028), compared to euthymic as well as depressed individuals. There were no differences between groups regarding activity of kynurenine aminotransferase and kynureninase. Within the group of depressed patients, Hamilton Depression Scale and kynurenine aminotransferase showed a significant negative correlation (r = -0.41, p = .036), displaying lower metabolism in the direction of kynurenic acid. Depression severity in bipolar disorder seems to be associated with a decreased synthesis of putative neuroprotective kynurenic acid. Furthermore, higher kynurenine 3-monooxygenase activity in currently manic individuals indicates an increased inflammatory state within bipolar disorder with more severe inflammation during manic episodes. The underlying pathophysiological mechanisms of the different affective episodes could represent parallel mechanisms rather than opposed processes.

Tryptophan Metabolism, Inflammation, and Oxidative Stress in Patients with Neurovascular Disease.

Atherosclerosis is a leading cause of major vascular events, myocardial infarction, and ischemic stroke. Tryptophan (TRP) catabolism was recognized as an important player in inflammation and immune response having together with oxidative stress (OS) significant effects on each phase of atherosclerosis. The aim of the study is to analyze the relationship of plasma levels of TRP metabolites, inflammation, and OS in patients with neurovascular diseases (acute ischemic stroke (AIS), significant carotid artery stenosis (SCAS)) and in healthy controls. Blood samples were collected from 43 patients (25 with SCAS, 18 with AIS) and from 25 healthy controls. The concentrations of twelve TRP metabolites, riboflavin, neopterin (NEO, marker of inflammation), and malondialdehyde (MDA, marker of OS) were measured by liquid chromatography–tandem mass spectrometry (LC-MS/MS). Concentrations of seven TRP metabolites (TRP, kynurenine (KYN), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HAA), anthranilic acid (AA), melatonin (MEL), tryptamine (TA)), NEO, and MDA were significantly different in the studied groups. Significantly lower concentrations of TRP, KYN, 3-HAA, MEL, TA, and higher MDA concentrations were found in AIS compared to SCAS patients. MDA concentration was higher in both AIS and SCAS group (p < 0.001, p = 0.004, respectively) compared to controls, NEO concentration was enhanced (p < 0.003) in AIS. MDA did not directly correlate with TRP metabolites in the study groups, except for 1) a negative correlation with kynurenine acid and 2) the activity of kynurenine aminotransferase in AIS patients (r = −0.552, p = 0.018; r = −0.504, p = 0.033, respectively). In summary, TRP metabolism is clearly more deregulated in AIS compared to SCAS patients; the effect of TRP metabolites on OS should be further elucidated.

Kynurenine Pathway as a New Target of Cognitive Impairment Induced by Lead Toxicity During the Lactation

The immature brain is especially vulnerable to lead (Pb2+) toxicity, which is considered an environmental neurotoxin. Pb2+ exposure during development compromises the cognitive and behavioral attributes which persist even later in adulthood, but the mechanisms involved in this effect are still unknown. On the other hand, the kynurenine pathway metabolites are modulators of different receptors and neurotransmitters related to cognition; specifically, high kynurenic acid levels has been involved with cognitive impairment, including deficits in spatial working memory and attention process. The aim of this study was to evaluate the relationship between the neurocognitive impairment induced by Pb2+ toxicity and the kynurenine pathway. The dams were divided in control group and Pb2+ group, which were given tap water or 500 ppm of lead acetate in drinking water ad libitum, respectively, from 0 to 23 postnatal day (PND). The poison was withdrawn, and tap water was given until 60 PND of the progeny. The locomotor activity in open field, redox environment, cellular function, kynurenic acid (KYNA) and 3-hydroxykynurenine (3-HK) levels as well as kynurenine aminotransferase (KAT) and kynurenine monooxygenase (KMO) activities were evaluated at both 23 and 60 PND. Additionally, learning and memory through buried food location test and expression of KAT and KMO, and cellular damage were evaluated at 60 PND. Pb2+ group showed redox environment alterations, cellular dysfunction and KYNA and 3-HK levels increased. No changes were observed in KAT activity. KMO activity increased at 23 PND and decreased at 60 PND. No changes in KAT and KMO expression in control and Pb2+ group were observed, however the number of positive cells expressing KMO and KAT increased in relation to control, which correlated with the loss of neuronal population. Cognitive impairment was observed in Pb2+ group which was correlated with KYNA levels. These results suggest that the increase in KYNA levels could be a mechanism by which Pb2+ induces cognitive impairment in adult mice, hence the modulation of kynurenine pathway represents a potential target to improve behavioural alterations produced by this environmental toxin.

Prolonged therapy with antidepressants increases hippocampal level of kynurenic acid and expression of Kat1 and Kat2 genes.

Accumulating data suggest an important role of disturbed kynurenine pathway and altered glutamatergic transmission in the pathogenesis of depression. In here, we focused on detailed analyses of kynurenic acid (KYNA) status in vivo following single and 14-day administration of selected tricyclic antidepressant drugs (TCAs) and serotonin selective reuptake inhibitors (SSRIs) in rats. The effect of antidepressants on serum and brain KYNA levels, as well as on the activity of kynurenine aminotransferases (KATs I and II) and expression of Kat1 and Kat2 genes mRNA was studied in three brain regions. Chronic, but not acute, application of antidepressants invariably stimulated KYNA production in hippocampus (amitriptyline, imipramine, fluoxetine and citalopram) and sporadically in cortex (amitriptyline, fluoxetine), whereas no change in KYNA level was observed in striatum. Cortical and hippocampal expression of Kat1 and Kat2 genes was increased after chronic, but not single administration of all studied antidepressants. The activity of semi-purified enzymatic proteins, KAT I and II, was not paralleling changes of Kat1 and Kat2 genes. Our data indicate that prolonged administration of antidepressants targets expression of KYNA biosynthetic enzymes. Furthermore, post-translational modulation of KATs seems to play an important role in tuning of KYNA synthesis within brain structures. We suggest that consistent increase of hippocampal KYNA levels may represent hallmark of antidepressant activity. Mechanisms governing region- and drug-selective action of antidepressants require further investigations.

P.5.d.008 - Ten-day cognitive training program combined with transcranial direct current stimulation (tDCS) in stroke rehabilitation – a sham controlled study

The renin-angiotensin system (RAS) is commonly known to regulate blood pressure, water and electrolyte homeostasis, however it also exerts paracrine and autocrine actions on the kidney. Angiotensin-converting enzyme inhibitors (ACE-Is), alongside their hypotensive properties, have been shown to decrease kidney function decline in animal models of nephropathy.Glutamate (GLU) is the main stimulatory neurotransmitter in the central nervous system, however its importance in the periphery should also be considered. Activation of renal GLU receptors has been linked to normal kidney function and also renal injury. The wide spectrum GLU receptor antagonist kynurenic acid (KYNA) possesses neuroprotective and central hypotensive effects, however its actions outside the brain are less well recognized. KYNA is a tryptophan metabolite synthesized from kynurenine by kynurenine aminotransferases (KATs). The purpose of this study was to examine the influence of three ACE-Is: lisinopril, perindopril and ramipril on KYNA production and KATs activity in rat kidney in vitro.The effect of ACE-Is on KYNA production and KATs activity was examined in rat kidney homogenates. KYNA was detected by high-performance liquid chromatography (HPLC) and quantified fluorometrically.All examined ACE-Is: lisinopril, perindopril and ramipril decreased KYNA production in rat kidney in vitro. KAT I activity was decreased by lisinopril and ramipril whereas the activity of KAT II was lowered by ramipril.Our study shows that ACE-Is can decrease KYNA production in rat kidney in vitro. Further studies are required to determine the clinical importance of the inhibitory action of ACE-Is on KYNA synthesis in the kidney.

Kynurenine aminotransferase activity of Aro8/Aro9 engage tryptophan degradation by producing kynurenic acid in Saccharomyces cerevisiae

Kynurenic acid (KA) is a tryptophan (Trp) metabolite that is synthesised in a branch of kynurenine (KYN) pathway. KYN aminotransferase (KAT) catalyses deamination of KYN, yielding KA. Although KA synthesis is evolutionarily conserved from bacteria to humans, the cellular benefits of synthesising KA are unclear. In this study, we constructed a KAT-null yeast mutant defective in KA synthesis to clarify the cellular function of KA. Amino acid sequence analysis and LC/MS quantification of KA revealed that Aro8 and Aro9 are the major KATs. KA was significantly decreased in the aro8Δ aro9Δ double mutant. We found that aro8Δ aro9Δ cells did not exhibit obvious defects in growth or oxidative stress response when proper amounts of amino acids are supplied in the media. We further found that aro8Δ aro9Δ cells were sensitive to excess Trp. The Trp sensitivity was not rescued by addition of KA, suggesting that Trp sensitivity is not due to the loss of KA. In conclusion, we propose that KAT activity is required for detoxification of Trp by converting it to the less toxic KA.

Comprehensive analysis of the tryptophan metabolome in urine of patients with acute intermittent porphyria.

Acute intermittent porphyria (AIP) is a rare metabolic disorder due to a deficiency of porphobilinogen deaminase, the third enzyme of the heme biosynthetic pathway. This low enzymatic activity may predispose to the appearance of acute neurological attacks. Seminal studies suggested that AIP was associated with changes in tryptophan homeostasis with inconclusive results. Therefore, the aim of this study was to analyze the urinary metabolome of AIP patients focusing on tryptophan metabolism using state-of-the-art technology. This was a case-control study including a group of 25 AIP patients with active biochemical disease and increased excretion of heme-precursors and 25 healthy controls. Tryptophan and related compounds and metabolites including: large neutral amino acids (LNAAs), serotonin, kynurenine, kynurenic acid and anthranilic acid were quantified in urine by liquid chromatography tandem-mass spectrometry (LC-MS/MS). Twenty-nine biological markers (including metabolic ratios and absolute concentrations) were compared between patients and controls. Significant differences were found in the tryptophan-kynurenine metabolic pathway. Compared to controls, AIP patients showed: (a) increased urinary excretion of kynurenine and anthranilic acid (P<0.005); (b): elevation of the kynurenine/tryptophan ratio (P<0.001) and (c): decrease of the kynurenic acid/kynurenine ratio (P=0.001). In contrast, no differences were found in the serotonin metabolic pathway independently of the markers and ratios used. The results of the study demonstrate that there is an imbalance in the kynurenine metabolic pathway in AIP patients, with an increase of the kynurenine/tryptophan ratio in urine and a reduction of the kynurenic acid/kynurenine ratio. The modified ratios suggest induction of indoleamine 2,3-deoxygenase and decreased activity of kynurenine aminotransferase in the liver. The results confirm that LC-MS/MS is useful for the characterization of the urinary metabolome of hepatic porphyrias.

The effect of three angiotensin-converting enzyme inhibitors on kynurenic acid production in rat kidney in vitro

The renin-angiotensin system (RAS) is commonly known to regulate blood pressure, water and electrolyte homeostasis, however it also exerts paracrine and autocrine actions on the kidney. Angiotensin-converting enzyme inhibitors (ACE-Is), alongside their hypotensive properties, have been shown to decrease kidney function decline in animal models of nephropathy. Glutamate (GLU) is the main stimulatory neurotransmitter in the central nervous system, however its importance in the periphery should also be considered. Activation of renal GLU receptors has been linked to normal kidney function and also renal injury. The wide spectrum GLU receptor antagonist kynurenic acid (KYNA) possesses neuroprotective and central hypotensive effects, however its actions outside the brain are less well recognized. KYNA is a tryptophan metabolite synthesized from kynurenine by kynurenine aminotransferases (KATs). The purpose of this study was to examine the influence of three ACE-Is: lisinopril, perindopril and ramipril on KYNA production and KATs activity in rat kidney in vitro. The effect of ACE-Is on KYNA production and KATs activity was examined in rat kidney homogenates. KYNA was detected by high-performance liquid chromatography (HPLC) and quantified fluorometrically. All examined ACE-Is: lisinopril, perindopril and ramipril decreased KYNA production in rat kidney in vitro. KAT I activity was decreased by lisinopril and ramipril whereas the activity of KAT II was lowered by ramipril. Our study shows that ACE-Is can decrease KYNA production in rat kidney in vitro. Further studies are required to determine the clinical importance of the inhibitory action of ACE-Is on KYNA synthesis in the kidney.

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

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