A Role for Xanthurenic Acid in the Control of Brain Dopaminergic Activity

Xanthurenic acid (XA) is a metabolite of the kynurenine pathway (KP) synthetized in the brain from dietary or microbial tryptophan that crosses the blood-brain barrier through carrier-mediated transport. XA and kynurenic acid (KYNA) are two structurally related compounds of KP occurring at micromolar concentrations in the CNS and suspected to modulate some pathophysiological mechanisms of neuropsychiatric and/or neurodegenerative diseases. Particularly, various data including XA cerebral distribution (from 1 µM in olfactory bulbs and cerebellum to 0.1–0.4 µM in A₉ and A₁₀), its release, and interactions with G protein-dependent XA-receptor, glutamate transporter and metabotropic receptors, strongly support a signaling and/or neuromodulatory role for XA. However, while the parent molecule KYNA is considered as potentially involved in neuropsychiatric disorders because of its inhibitory action on dopamine release in the striatum, the effect of XA on brain dopaminergic activity remains unknown. Here, we demonstrate that acute local/microdialysis-infusions of XA dose-dependently stimulate dopamine release in the rat prefrontal cortex (four-fold increase in the presence of 20 µM XA). This stimulatory effect is blocked by XA-receptor antagonist NCS-486. Interestingly, our results show that the peripheral/intraperitoneal administration of XA, which has been proven to enhance intra-cerebral XA concentrations (about 200% increase after 50 mg/kg XA i.p), also induces a dose-dependent increase of dopamine release in the cortex and striatum. Furthermore, our in vivo electrophysiological studies reveal that the repeated/daily administrations of XA reduce by 43% the number of spontaneously firing dopaminergic neurons in the ventral tegmental area. In the substantia nigra, XA treatment does not change the number of firing neurons. Altogether, our results suggest that XA may contribute together with KYNA to generate a KYNA/XA ratio that may crucially determine the brain normal dopaminergic activity. Imbalance of this ratio may result in dopaminergic dysfunctions related to several brain disorders, including psychotic diseases and drug dependence.

Xanthurenic Acid Is the Main Pigment of Trichonephila clavata Gold Dragline Silk

Spider silk is a natural fiber with remarkable strength, toughness, and elasticity that is attracting attention as a biomaterial of the future. Golden orb-weaving spiders (Trichonephila clavata) construct large, strong webs using golden threads. To characterize the pigment of golden T. clavata dragline silk, we used liquid chromatography and mass spectrometric analysis. We found that the major pigment in the golden dragline silk of T. clavata was xanthurenic acid. To investigate the possible function of the pigment, we tested the effect of xanthurenic acid on bacterial growth using gram-negative Escherichia coli and gram-positive Bacillus subtilis. We found that xanthurenic acid had a slight antibacterial effect. Furthermore, to investigate the UV tolerance of the T. clavata threads bleached of their golden color, we conducted tensile deformation tests and scanning electron microscope observations. However, in these experiments, no significant effect was observed. We therefore speculate that golden orb-weaving spiders use the pigment for other purposes, such as to attract their prey in the sunlight.

Functional analysis of novel sulfotransferases in the silkworm Bombyx mori

Sulfoconjugation plays a vital role in the detoxification of xenobiotics and in the metabolism of endogenous compounds. In this study, we aimed to identify new members of the sulfotransferase (SULT) superfamily in the silkworm Bombyx mori. Based on amino acid sequence and phylogenetic analyses, two new enzymes, swSULT ST1 and swSULT ST2, were identified that appear to belong to a distinct group of SULTs including several other insect SULTs. We expressed, purified, and characterized recombinant SULTs. While swSULT ST1 sulfated xanthurenic acid and pentachlorophenol, swSULT ST2 exclusively utilized xanthurenic acid as a substrate. Based on these results, and those concerning the tissue distribution and substrate specificity toward pentachlorophenol analyses, we hypothesize that swSULT ST1 plays a role in the detoxification of xenobiotics, including insecticides, in the silkworm midgut and in the induction of gametogenesis in silkworm ovary and testis. Collectively, the data obtained herein contribute to a better understanding of SULT enzymatic functions in insects.

An intracellular membrane protein GEP1 regulates xanthurenic acid induced gametogenesis of malaria parasites

Gametocytes differentiation to gametes (gametogenesis) within mosquitos is essential for malaria parasite transmission. Both reduction in temperature and mosquito-derived XA or elevated pH are required for triggering cGMP/PKG dependent gametogenesis. However, the parasite molecule for sensing or transducing these environmental signals to initiate gametogenesis remains unknown. Here we perform a CRISPR/Cas9-based functional screening of 59 membrane proteins expressed in the gametocytes of Plasmodium yoelii and identify that GEP1 is required for XA-stimulated gametogenesis. GEP1 disruption abolishes XA-stimulated cGMP synthesis and the subsequent signaling and cellular events, such as Ca2+ mobilization, gamete formation, and gametes egress out of erythrocytes. GEP1 interacts with GCα, a cGMP synthesizing enzyme in gametocytes. Both GEP1 and GCα are expressed in cytoplasmic puncta of both male and female gametocytes. Depletion of GCα impairs XA-stimulated gametogenesis, mimicking the defect of GEP1 disruption. The identification of GEP1 being essential for gametogenesis provides a potential new target for intervention of parasite transmission.

Mangiferin improves hepatic damage-associated molecular patterns, lipid metabolic disorder and mitochondrial dysfunction in alcohol hepatitis rats

This study was conducted to investigate the beneficial effects and possible mechanism of action of mangiferin (MF) in alcohol hepatitis (AH) rats. Building on our previous study, the damage-associated molecular patterns (DAMPs), lipid metabolic disorder and mitochondrial dysfunction were investigated. MF effectively regulated the abnormal liver function, the levels of alcohol, FFAs and metal elements in serum. More importantly, MF improved the expression levels of mRNA and protein of PPAR-γ, OPA-1, Cav-1, EB1, NF-κB p65, NLRP3, Cas-1 and IL-1β, and decreased the positive protein expression rates of HSP90, HMGB1, SYK, CCL20, C-CAS-3, C-PARP and STARD1. Additionally, MF decreased the levels of fumarate, cAMP, xanthurenic acid and d-glucurone-6,3-lactone, and increased the levels of hippuric acid and phenylacetylglycine, and then adjusted the changes of phenylalanine metabolism, TCA cycle and ascorbate and aldarate metabolic pathways. The above results suggested that MF can effectively prevent AH by modulating specific AH-associated genes, potential biomarkers and metabolic pathways in AH rats, etc.

3-Hydroxykynurenine Transamination Leads to the Formation of the Fluorescent Substances in Human Lenses

The levels of alanine, aspartate and glutamine transaminase increase considerably in some diseases. We measured the activity of these enzymes and of the transaminase of 3-hydroxykynurenine, an aminoacid, which acts as a UV lens filter. Alanine and glutamine transaminases (carboxypeptidase) were not detected in normal and cataractous human lenses, and aspartate transaminase was found only in the cortex of normal lenses. 3-hydroxykynurenine transaminase was not found in lenses from persons below thirty years of age, but was found in lenses at about fifty years of age, and in cataractous lenses. Transamination of 3-hydroxykynurenine leads to the formation of xanthurenic acid and its derivatives. These substances appear to be responsible for the increase of lens fluorescence during cataract development.

Xanthurenic Acid Formation from 3-Hydroxykynurenine in the Mammalian Brain: Neurochemical Characterization and Physiological Effects

Xanthurenic acid (XA), formed from 3-hydroxykynurenine (3-HK) in the kynurenine pathway of tryptophan degradation, may modulate glutamatergic neurotransmission by inhibiting the vesicular glutamate transporter and/or activating Group II metabotropic glutamate receptors. Here we examined the molecular and cellular mechanisms by which 3-HK controls the neosynthesis of XA in rat, mouse and human brain, and compared the physiological actions of 3-HK and XA in the rat brain. In tissue homogenates, XA formation from 3-HK was observed in all three species and traced to a major role of kynurenine aminotransferase II (KAT II). Transamination of 3-HK to XA was also demonstrated using human recombinant KAT II. Neosynthesis of XA was significantly increased in the quinolinate-lesioned rat striatum, indicating a non-neuronal localization of the process. Studies using rat cortical slices revealed that newly produced XA is rapidly released into the extracellular compartment, and that XA biosynthesis can be manipulated experimentally in the same way as the production of kynurenic acid from kynurenine (omission of Na+ or glucose, depolarizing conditions, or addition of 2-oxoacids). The synthesis of XA from 3-HK was confirmed in vivo by striatal microdialysis. In slices from the rat hippocampus, both 3-HK and XA reduced the slopes of dentate gyrus field EPSPs. The effect of 3-HK was reduced in the presence of the KAT inhibitor aminooxyacetic acid. Finally, both 3-HK and XA reduced the power of gamma-oscillatory activity recorded from the hippocampal CA3 region. Endogenous XA, newly formed from 3-HK, may therefore play a physiological role in attentional and cognitive processes.

Evaluation of the antioxidant properties of tryptophan and its metabolites in in vitro assay

BACKGROUND

Human milk contains a number of nutrients and bioactive ingredients which play an important role in the growth and development of infants. One important nutrient and bioactive ingredient of human milk is L-tryptophan. L-Tryptophan is an essential aromatic α-amino acid and is required in the diet of children and adult humans. As an essential amino acid, it is needed for protein synthesis and as a precursor of key biomolecules such as serotonin, melatonin, tryptamine, niacin, quinolinic acid and kynurenic acid, nicotinamide adenine dinucleotide. The aim of the study was to evaluate the antioxidant, anti-inflammatory and antiproliferative properties of tryptophan isolated from enzymatic hydrolysates from human milk and its metabolites on human glioma U251 cells and to evaluate the effects of human recombinant (hrIFNγ) on molecular ions of tryptophan and its metabolites in human glial U251 cells.

METHODS

The cytotoxicity was determined by MTT (3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide) assay. The antioxidant property was assessed by the oxygen radical scavenging capacity (ORAC) method. The anti-inflammatory effect was determined by the enzyme-linked immunosorbent assay (ELISA) against cytokines IL-6 and TNF-α. The effects of recombinant human (rhIFNγ) on molecular ions of tryptophan and its catabolites were evaluated by mass spectrometry. The tryptophan was isolated from milk peptides following enzymatic digestion, followed by separation by chromatographic and mass spectrometric methods.

RESULTS

Tryptophan from human milk exhibited profoundly higher oxygen radical absorption capacity (7,986±468 µm Trolox equivalent (TE)/g) than that of whole human milk (80.4±13.3 µm TE/g). Tryptophan showed a moderate degree of anti-inflammatory activity against TNF-α and IL-6. rhIFNγ inhibited tryptophan metabolism. A low concentration of L-tryptophan (10-25 μg/mL) inhibited nearly 25% of cell growth. When U251 cells were treated with 25 μg/mL L-tryptophan and subsequently challenged with 30 ng/mL of human recombinant IFNγ, a significant inhibitory effect on cell growth was observed. Low concentrations of Xanthurenic acid, L-kynurenine, and 3-OH DL kynurenine were found to inhibit cell growth except melatonin and 3-OH anthranilic acid. Melatonin was a strong inducer of TNF-α in RAW cells, whereas 3-OH kynurenine at 25, 50 and 100 µg/mL inhibited IL-6 in RAW cells. No significant change was observed in the IL-8 profile in tryptophan-treated U251 cells except that L-kynurenine at 10 µg/mL produced significantly high level of an inflammatory cytokine IL-8. Melatonin, 3-OH, DL kynurenine at high concentrations (100 µg/mL) induced proliferation of U251 cells. Melatonin seemed to show synergistic effects with recombinant human IFNγ (rhINFγ) in promoting growth of human glioma cells. While treatment of U251 cells with tryptophan alone and subsequent treatment with rhIFNγ inhibited the growth of human cancer glioma cells, and conversely melatonin combined with rhIFNγ promoted growth of the U251 cells.

CONCLUSIONS

The findings from this study suggest that human milk-derived tryptophan and its metabolites possess strong antioxidant properties. Such effects might play a significant role in regulating the cell proliferation and growth of human cancer cells in a concentration-dependent manner.

Tetrahydrobiopterin Biosynthesis as a Potential Target of the Kynurenine Pathway Metabolite Xanthurenic Acid

Tryptophan metabolites in the kynurenine pathway are up-regulated by pro-inflammatory cytokines or glucocorticoids, and are linked to anti-inflammatory and immunosuppressive activities. In addition, they are up-regulated in pathologies such as cancer, autoimmune diseases, and psychiatric disorders. The molecular mechanisms of how kynurenine pathway metabolites cause these effects are incompletely understood. On the other hand, pro-inflammatory cytokines also up-regulate the amounts of tetrahydrobiopterin (BH4), an enzyme cofactor essential for the synthesis of several neurotransmitter and nitric oxide species. Here we show that xanthurenic acid is a potent inhibitor of sepiapterin reductase (SPR), the final enzyme in de novo BH4 synthesis. The crystal structure of xanthurenic acid bound to the active site of SPR reveals why among all kynurenine pathway metabolites xanthurenic acid is the most potent SPR inhibitor. Our findings suggest that increased xanthurenic acid levels resulting from up-regulation of the kynurenine pathway could attenuate BH4 biosynthesis and BH4-dependent enzymatic reactions, linking two major metabolic pathways known to be highly up-regulated in inflammation.

New insights into tryptophan and its metabolites in the regulation of bone metabolism

Osteoporosis, a debilitating disease caused by an imbalance between the action of osteoblasts and osteoclasts, is becoming an increasing problem in today's aging population. Although many advances in this field have addressed certain aspects of disease progression and pain management, new approaches to treatment are required. This review focuses on the influence of tryptophan, its metabolites and their influence on bone remodeling. Tryptophan is a precursor to serotonin, melatonin, kynurenines and niacin. Changes of tryptophan levels were noticed in bone metabolic diseases. Moreover, some works indicate that tryptophan plays a role in osteoblastic differentiation. Serotonin can exert different effects on bones, which depend on site of serotonin synthesis. Gut-derived serotonin inhibits bone formation, whereas brain-derived serotonin enhances bone formation and decreases bone resorption. Melatonin, increased differentiation of human mesenchymal stem cells into the osteoblastic cell lineage. Results of melatonin action on bone are anabolic and antiresorptive. Activation of the second tryptophan metabolic pathway, the kynurenine pathway, is associated with osteoblastogenesis and can be implicated in the occurrence of bone diseases. Oxidation products like kynurenine stopped proliferation of bone marrow mesenchymal stem cells. This may result in inhibition of osteoblastic proliferation and differentiation. Kynurenic acid acts as an antagonist at glutamate receptors, which are expressed on osteoclasts. Quinolinic acid activates N-methyl-D-aspartate receptors. 3-hydroxyanthranilic acid exhibits pro-oxidant and antioxidant activity. Decreased concentration of 3-hydroxyanthranilic acid can be one of the causes of osteoporosis. 3-hydroxykynurenine reduced the viability of osteoblast-like cells. Picolinic acid exerted osteogenic effect in vitro. Kynurenine derivatives exert various effects on bones. Discovery of the exact mechanism of action of tryptophan metabolites on bones may take us a step closer to understanding the complicated mechanism of bone metabolism, which in turn may result in finding a new, effective therapy for treating bone diseases.

[Kynurenines in pathogenesis of endogenous psychiatric disorders]

The essential amino acid tryptophan is metabolized on the methoxyindole pathway to serotonin, melatonin and 5-hydroxyindoleacetic acid and on the kynurenine pathway to kynurenine and related neuroactive metabolites, including 3-hydroxykynurenine, kynurenic acid, quinolinic acid and xanthurenic acid. Kynurenine and related metabolites play a significant role in the pathogenesis of major depressive disorder and schizophrenia. This paper is the review of literature data on the most modern state of this problem.

The antioxidant role of xanthurenic acid in the Aedes aegypti midgut during digestion of a blood meal

In the midgut of the mosquito Aedes aegypti, a vector of dengue and yellow fever, an intense release of heme and iron takes place during the digestion of a blood meal. Here, we demonstrated via chromatography, light absorption and mass spectrometry that xanthurenic acid (XA), a product of the oxidative metabolism of tryptophan, is produced in the digestive apparatus after the ingestion of a blood meal and reaches milimolar levels after 24 h, the period of maximal digestive activity. XA formation does not occur in the White Eye (WE) strain, which lacks kynurenine hydroxylase and accumulates kynurenic acid. The formation of XA can be diminished by feeding the insect with 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl] benzenesulfonamide (Ro-61-8048), an inhibitor of XA biosynthesis. Moreover, XA inhibits the phospholipid oxidation induced by heme or iron. A major fraction of this antioxidant activity is due to the capacity of XA to bind both heme and iron, which occurs at a slightly alkaline pH (7.5-8.0), a condition found in the insect midgut. The midgut epithelial cells of the WE mosquito has a marked increase in occurrence of cell death, which is reversed to levels similar to the wild type mosquitoes by feeding the insects with blood supplemented with XA, confirming the protective role of this molecule. Collectively, these results suggest a new role for XA as a heme and iron chelator that provides protection as an antioxidant and may help these animals adapt to a blood feeding habit.

Xanthurenic acid is an endogenous substrate for the silkworm cytosolic sulfotransferase, bmST1

Sulfotransferase enzymes are known to regulate physiologically active substances such as steroids and catecholamines in mammals. Although invertebrates also express sulfotransferases, their biological function is mostly unclear. In a previous study, we reported that 4-nitrocatechol and the gallete ester are substrates for the silkworm sulfotransferase bmST1. The K(m) of bmST1 for these substrates is high. However, endogenous substrates of bmST1 have not yet been determined. We therefore investigated endogenous bmST1 substrates and carried out a detailed expression profile analysis of bmST1. We found that xanthurenic acid, a tryptophan metabolite, is a possible endogenous substrate of bmST1. The K(m) of bmST1 for xanthurenic acid is low, in the μM range, which is lower than that for previously reported substrates. Additionally, xanthurenic acid is a tryptophan metabolite that characteristically shows toxicity in vivo. High dose administration of xanthurenic acid resulted in inhibition of cuticular biosynthesis. The expression of the bmST1 gene reached a maximal level in the Malpighian tubule at the 4th molting stage, when amino acid metabolism might be activated. Our results suggest that bmST1 plays a role in detoxification of xanthurenic acid in the silkworm.

The immune effects of TRYCATs (tryptophan catabolites along the IDO pathway): relevance for depression - and other conditions characterized by tryptophan depletion induced by inflammation

Immune activation is accompanied by induction of indoleamine (2,3)-dioxygenase (IDO), an enzyme which degrades tryptophan, a phenomenon which plays a role in the pathophysiology of major depression and post-natal depression and anxiety states. TRYCATs - tryptophan catabolites along the IDO pathway - such as kynurenine, kynurenic acid, xanthurenic acid, and quinolinic acid, have multiple effects, e.g. apoptotic, anti- versus pro-oxidant, neurotoxic versus neuroprotective, and anxiolytic versus anxiogenic effects. The aim of the present study was to study the immune effects of the above TRYCATS. Toward this end we examined the effects of the above TRYCATs on the LPS + PHA-induced production of interferon-gamma (IFNgamma), interleukin-10 (IL-10), and tumor necrosis factor-alpha (TNFalpha) in 18 normal volunteers. We found that the production of IFNgamma was significantly decreased by all 4 catabolites. Xanthurenic acid and quinolinic acid decreased the production of IL-10. Kynurenine, kynurenic acid, and xanthurenic acid, decreased the IFNgamma/IL-10 production ratio, whereas quinolinic acid increased this ratio. Kynurenic acid significantly reduced the stimulated production of TNFalpha. It is concluded that kynurenine, kynurenic acid, and xanthurenic acid have anti-inflammatory effects trough a reduction of IFNgamma, whereas quinolinic acid has pro-inflammatory effects in particular via significant decreases in IL-10. Following inflammation-induced IDO activation, some TRYCATs, i.e. kynurenine, kynurenic acid, and xanthurenic acid, exert a negative feedback control over IFNgamma production thus downregulating the initial inflammation, whereas an excess of quinolinic acid further aggravates the initial inflammation.

The tryptophan oxidation pathway in mosquitoes with emphasis on xanthurenic acid biosynthesis

Oxidation of tryptophan to kynurenine and 3-hydroxykynurenine (3-HK) is the major catabolic pathway in mosquitoes. However, 3-HK is oxidized easily under physiological conditions, resulting in the production of reactive radical species. To overcome this problem, mosquitoes have developed an efficient mechanism to prevent 3-HK from accumulating by converting this chemically reactive compound to the chemically stable xanthurenic acid. Interestingly, 3-HK is a precursor for the production of compound eye pigments during the pupal and early adult stages; consequently, mosquitoes need to preserve and transport 3-HK for compound eye pigmentation in pupae and adults. This review summarizes the tryptophan oxidation pathway, compares and contrasts the mosquito tryptophan oxidation pathway with other model species, and discusses possible driving forces leading to the functional adaptation and evolution of enzymes involved in the mosquito tryptophan oxidation pathway.

Plasmodium development in white-eye (kh(w)) and transformed strains (kh43) of Aedes aegypti (Diptera: Culicidae)

Xanthurenic acid (XA) has been implicated as an inducer in vivo of exflagellation in Plasmodium spp. Consequently, the development of Plasmodium gallinaceum was assessed in a white-eye mosquito strain, kh(w), of Aedes aegypti (L.), which is deficient in XA because of a mutation of the gene encoding the enzyme kynurenine hydroxylase, and in a transformed line of kh(w) mosquitoes that carry the wild-type cn+ gene of Drosophila melanogaster Meigen and express a functional enzyme necessary for XA production. Although XA was not detectable in kh(w) mosquitoes by using high-pressure liquid chromatography with electrochemical detection, parasites were able to develop. Transformed kh(w) mosquitoes failed to consistently support parasite development at higher prevalences and mean intensities than did the nontransformed kh(w) lines, even though XA was detectable. These data suggest that factors other than XA may play a role in initiating Plasmodium development in vivo.

The effects of blood feeding and exogenous supply of tryptophan on the quantities of xanthurenic acid in the salivary glands of Anopheles stephensi (Diptera: Culicidae)

Xanthurenic acid (XA), produced as a byproduct during the biosynthesis of insect eye pigment (ommochromes), is a strong inducer of Plasmodium gametogenesis at very low concentrations. In previous studies, it was shown that XA is present in Anopheles stephensi (Diptera: Culicidae) mosquito salivary glands and that during blood feeding the mosquitoes ingested their own saliva into the midgut. Considering these two facts together, it is therefore likely that XA is discharged with saliva during blood feeding and is swallowed into the midgut where it exerts its effect on Plasmodium gametocytes. However, the quantities of XA in the salivary glands and midgut are unknown. In this study, we used high performance liquid chromatography with electrochemical detection to detect and quantify XA in the salivary glands and midgut. Based on the results of this study, we found 0.28+/-0.05 ng of XA in the salivary glands of the mosquitoes, accounting for 10% of the total XA content in the mosquito whole body. The amounts of XA in the salivary glands reduced to 0.13+/-0.06 ng after mosquitoes ingested a blood meal. Approximately 0.05+/-0.01 ng of XA was detected in the midgut of nonblood fed An. stephensi mosquitoes. By adding synthetic tryptophan as a source of XA into larval rearing water (2 mM) or in sugar meals (10 mM), we evaluated whether XA levels in the mosquito (salivary glands, midgut, and whole body) were boosted and the subsequent effect on infectivity of Plasmodium berghei in the treated mosquito groups. A female specific increase in XA content was observed in the whole body and in the midgut of mosquito groups where tryptophan was added either in the larval water or sugar meals. However, XA in the salivary glands was not affected by tryptophan addition to larval water, and surprisingly it reduced when tryptophan was added to sugar meals. The P. berghei oocyst loads in the mosquito midguts were lower in mosquitoes fed tryptophan treated sugar meals than in mosquitoes reared on tryptophan treated larval water. Our results suggest that mosquito nutrition may have a significant impact on whole body and midgut XA levels in mosquitoes. We discuss the observed parasite infectivity results in relation to XA's relationship with malaria parasite development in mosquitoes.

Identification and biochemical characterization of the Anopheles gambiae 3-hydroxykynurenine transaminase

Spontaneous oxidation of 3-hydroxykynureine (3-HK), a metabolic intermediate of the tryptophan degradation pathway, elicits a remarkable oxidative stress response in animal tissues. In the yellow fever mosquito Aedes aegypti the excess of this toxic metabolic intermediate is efficiently removed by a specific 3-HK transaminase, which converts 3-HK into the more stable compound xanthurenic acid. In anopheline mosquitoes transmitting malaria, xanthurenic acid plays an important role in Plasmodium gametocyte maturation and fertility. Using the sequence information provided by the Anopheles gambiae genome and available ESTs, we adopted a PCR-based approach to isolate a 3-HK transaminase coding sequence from the main human malaria vector A. gambiae. Tissue and developmental expression analysis revealed an almost ubiquitary profile, which is in agreement with the physiological role of the enzyme in mosquito development and 3-HK detoxification. A high yield procedure for the expression and purification of a fully active recombinant version of the protein has been developed. Recombinant A. gambiae 3-HK transaminase is a dimeric pyridoxal 5'-phosphate dependent enzyme, showing an optimum pH of 7.8 and a comparable catalytic efficiency for both 3-HK and its immediate catabolic precursor kynurenine. This study may be useful for the identification of 3-HK transaminase inhibitors of potential interest as malaria transmission-blocking drugs or effective insecticides.

Neurokynurenines--seizures or/and anxiety in children with epilepsy?

Neurokynurenines (NK) possessing various neuroactivities, are involved in many abnormalities in stress, anxiety, depression, alcoholism, epilepsy, neurologic diseases. Level of the excitatory NK, e.g. kynurenine (KYN) and quinolinic acid (QUIN), is elevated in many of those diseases, and a correlation between a rise of NK and severity of symptoms has been registered. In some of those diseases a rise of the level of the inhibitory NK, e.g. kynurenic (KYNA) and xanthurenic (XAN) acids, has been observed as well. However, that rise was smaller than that of the excitatory NK. Those changes are resulted in a shift of the balance between the excitatory and the inhibitory NK towards prevalence of the formers. Just a ratio between them determines a final integrative functional (or clinical) effect. Actually, a rise of the level of the excitatory and inhibitory NK is not specific for any disorder ( the same is true for catecholamines, serotonin, neuropeptides etc). To differentiate a relative role of NK in a neurological disorder it is important to check symptoms of stress and anxiety and their severity together with symptoms of a disease and their dynamics. This seems to be promising for understanding whether any shifts in concentrations of NK, e.g. KYN, QUIN, XAN or KYNA, are related to symptoms of a disorder or severity of accompanying stress and anxiety. As a pilot trial, 30 children (4-14 years old) with epileptic seizures were studied. Excretion of XAN (as a putative anticonvulsant) was measured in 24-h urine samples and concentration of XAN in blood serum on admission to the hospital after a seizure-episode and before discharge after treatment. Symptoms of anxiety were rated. We found that while the levels of XAN are normalized at achieving a clinical remission (seizures), symptoms of anxiety and stress are increased. Our preliminary results suggest that that XAN is involved in the formation of late psychic pathology in children with epilepsy.

The Pseudomonas siderophore quinolobactin is synthesized from xanthurenic acid, an intermediate of the kynurenine pathway

To cope with iron deficiency fluorescent pseudomonads produce pyoverdines which are complex peptidic siderophores that very efficiently scavenge iron. In addition to pyoverdine some species also produce other siderophores. Recently, it was shown that Pseudomonas fluorescens ATCC 17400 produces the siderophore quinolobactin, an 8-hydroxy-4-methoxy-2-quinoline carboxylic acid (Mossialos, D., Meyer, J.M., Budzikiewicz, H., Wolff, U., Koedam, N., Baysse, C., Anjaiah, V., and Cornelis, P. (2000) Appl Environ Microbiol 66: 487-492). The entire quinolobactin biosynthetic, transport and uptake gene cluster, consisting out of two operons comprising 12 open reading frames, was cloned and sequenced. Based on the genes present and physiological complementation assays a biosynthetic pathway for quinolobactin is proposed. Surprisingly, this pathway turned out to combine genes derived from the eukaryotic tryptophan-xanthurenic acid branch of the kynurenine pathway and from the pathway for the biosynthesis of pyridine-2,6-bis(thiocarboxylic acid) from P. stutzeri, PDTC. These results clearly show the involvement of the tryptophan-kynurenine-xanthurenic acid pathway in the synthesis of an authentic quinoline siderophore.

Gamete interruptus; a novel calcium-dependent kinase is essential for malaria sexual reproduction

Malaria parasites undergo sexual fertilization minutes after the bloodmeal enters the mosquito midgut. In this issue of Cell, Billker et al. (2004) describe a new Plasmodium calcium-dependent protein kinase essential for gamete formation, and show that it is required for parasite transmission.

Calcium and a Calcium-Dependent Protein Kinase Regulate Gamete Formation and Mosquito Transmission in a Malaria Parasite

Transmission of malaria parasites to mosquitoes is initiated by the obligatory sexual reproduction of the parasite within the mosquito bloodmeal. Differentiation of specialized transmission stages, the gametocytes, into male and female gametes is induced by a small mosquito molecule, xanthurenic acid (XA). Using a Plasmodium berghei strain expressing a bioluminescent calcium sensor, we show that XA triggers a rapid rise in cytosolic calcium specifically in gametocytes that is essential for their differentiation into gametes. A member of a family of plant-like calcium dependent protein kinases, CDPK4, is identified as the molecular switch that translates the XA-induced calcium signal into a cellular response by regulating cell cycle progression in the male gametocyte. CDPK4 is shown to be essential for the sexual reproduction and mosquito transmission of P. berghei. This study reveals an unexpected function for a plant-like signaling pathway in cell cycle regulation and life cycle progression of a malaria parasite.

The photosensitiser xanthurenic acid is not present in normal human lenses

UV light has often been investigated as a risk factor for the most common cause of blindness, human age-related cataract. One mechanism whereby UV light could induce cataract is via the action of photosensitisers. In this regard, xanthurenic acid has recently been highlighted since it has been reported to be present in the human lens and, in model studies, it markedly enhances the photo-oxidation of proteins by wavelengths of light that penetrate the cornea. In this study we used HPLC and mass spectrometry to examine whether xanthurenic acid is indeed present in human lenses and, if so, the effect of age on its lenticular concentration. Xanthurenic acid could be formed artefactually by incubation of 3-hydroxykynurenine (3OHKyn) yellow, a known autoxidation product of the lenticular UV filter, 3OHKyn, in the presence of air and light, however, it could not be detected in any human lenses studied. Therefore, it appears unlikely that xanthurenic acid plays a role in lens aging or human cataract.

Accumulation of toxic products degradation of kynurenine in hemodialyzed patients

In patients that developed a chronic renal failure the augmentation in tryptophan degradation is reflected in the increase in plasma metabolites of kynurenine pathway. Hemodialysis is one of therapeutic approaches that significantly reduce all plasma kynurenine metabolites in uremic patients. In spite of haemodialysis, plasma concentration of kynurenine, kynurenic acid, 3-hydroxykynurenine, anthranilic acid, xanthurenic acid and quinolinic acid were still elevated in uremic patients in comparison with healthy volunteers. These data shows significant disturbances in kynurenine metabolism in uremic patients. Accumulation of these substances in uremic blood is capable to account for certain uremic symptoms.

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

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

Characterization and identification of exflagellation-inducing factor in the salivary gland of Anopheles stephensi (Diptera: Culicidae)

Gamete activation factor (GAF) induces exflagellation of Plasmodium microgametes. We found GAF in the salivary glands of female mosquitoes, Anopheles stephensi. The exflagellation was induced in a concentration-dependent manner in the supernatant of salivary gland's crude homogenate. The exflagellation-inducing activity in the salivary gland was higher than that in the midgut and the head. GAF in the salivary glands was found to be heat stable and low molecular weight (<3000 molecular weight). Analysis of the supernatant by capillary electrophoresis and UV absorbance profile showed that the salivary glands contained xanthurenic acid, which was previously identified as GAF in the head of A. stephensi. The exflagellation-inducing activity in the salivary gland declined immediately after a blood meal, implying that GAF was in the saliva, and was delivered into the midgut together with the blood and induced exflagellation in the midgut.

Xanthurenic acid inhibits metal ion-induced lipid peroxidation and protects NADP-isocitrate dehydrogenase from oxidative inactivation

Vitamin B6 deficiency increases the lipid peroxidation and the synthesis of xanthurenic acid from tryptophan. Antioxidant properties of xanthurenic acid were examined in relation to the coordination of transition metals. Xanthurenic acid inhibited the formation of thiobarbituric acid-reactive substances as a marker of iron-mediated lipid peroxidation and copper-dependent oxidation of low density lipoprotein. NADP-isocitrate dehydrogenase (EC 1.1.1.42), a principal NADPH-generating enzyme for the antioxidant defense system, was inactivated by reduced iron and copper, and xanthurenic acid protected the enzyme from the Fe2+-mediated inactivation. Xanthurenic acid may participate in the enhanced regeneration of reduced glutathione by stimulating the NADPH supply. Xanthurenic acid further enhanced the autooxidation of Fe2+ ion. Other tryptophan metabolites such as kynurenic acid and various quinoline compounds did not inhibit the lipid peroxidation and the inactivation of NADP-isocitrate dehydrogenase, and they showed little or no effect on the Fe2+ autooxidation. The antioxidant properties of xanthurenic acid are related to the metal-chelating activity and probably to the enhanced oxidation of reduced transition metals as a prooxidant, and this action may be due to the electron deficient nature of this compound.

Pathological apoptosis by xanthurenic acid, a tryptophan metabolite: activation of cell caspases but not cytoskeleton breakdown

BACKGROUND

A family of aspartate-specific cysteinyl proteases, named caspases, mediates programmed cell death, apoptosis. In this function, caspases are important for physiological processes such as development and maintenance of organ homeostasis. Caspases are, however, also engaged in aging and disease development. The factors inducing age-related caspase activation are not known. Xanthurenic acid, a product of tryptophan degradation, is present in blood and urine, and accumulates in organs with aging.

RESULTS

Here, we report triggering of apoptotic key events by xanthurenic acid in vascular smooth muscle and retinal pigment epithelium cells. Upon exposure of these cells to xanthurenic acid a degradation of ICAD/DFF45, poly(ADP-ribose) polymerase, and gelsolin was observed, giving a pattern of protein cleavage characteristic for caspase-3 activity. Active caspase-3, -8 and caspase-9 were detected by Western blot analysis and immunofluorescence. In the presence of xanthurenic acid the amino-terminal fragment of gelsolin bound to the cytoskeleton, but did not lead to the usually observed cytoskeleton breakdown. Xanthurenic acid also caused mitochondrial migration, cytochrome C release, and destruction of mitochondria and nuclei.

CONCLUSIONS

These results indicate that xanthurenic acid is a previously not recognized endogenous cell death factor. Its accumulation in cells may lead to accelerated caspase activation related to aging and disease development.

Photochemical studies on xanthurenic acid

The tryptophan metabolite xanthurenic acid (Xan) has been isolated from aged human cataractous lenses. The photophysical properties of Xan were examined to determine if it is a potential chromophore for age-related cataractogenesis. We found that Xan produces singlet oxygen (psi delta = 0.17 in CD3OD) with the same efficiency as the lenticular chromophore N-formyl kynurenine and quenches singlet oxygen at a rate similar (2.1 x 10(7); CD3OD) to other tryptophan metabolites found in the eye. As the mechanisms of induction of cataracts may also involve redox reactions, the interactions of hydrated electrons (e(aq)-), the azide radical (N3*) and hydroxyl radical (OH*) with Xan were studied using the technique of pulse radiolysis. The reaction rate constants of e(aq)-, N3* and OH* with Xan were found to be of the same order of magnitude as other tryptophan metabolites. The rate constant for reaction of Xan with e(aq)- solvated electrons was found to be diffusion controlled (k = 1.43 x 10(10) M(-1) s(-1); the reaction with N3* was very fast (k = 4.0 x 10(9) M(-1) s(-1)); and with OH* was also near diffusion controlled (k = 1.0 x 10(10) M(-1) s(-1)). Superoxide O2*- production by irradiated Xan in methanol was detected by electron paramagnetic resonance and substantiated by determining that the enhanced rate of oxygen consumption of Xan irradiated in the presence of furfuryl alcohol was lowered by superoxide dismutase.

Xanthurenic acid provokes formation of unfolded proteins in endoplasmic reticulum of the lens epithelial cells

The role of xanthurenic acid in a cell is unknown, but it is suspected to provoke several diseases. This study shows that accumulation of xanthurenic acid in the lens epithelial cells leads to an overexpression of endoplasmic reticulum (ER) resident stress chaperones proteins, glucose-regulated protein (Grp94), and calreticulin. Both chaperones proteins are overexpressed in the presence of unfolded proteins. A formation of the unfolded protein in the presence of xanthurenic acid may take place due to covalent binding of xanthurenic acid to protein. Grp94 is responsible for scavenging of the unfolded proteins. The results suggest that Grp94 scavenged xanthurenic acid-modified proteins, and for this reason become preferentially yellow-stained in the presence of yellow xanthurenic acid. Such a modified Grp94 is weakly recognized by anti-Grp94 antibody. An end point of the xanthurenic acid accumulation in the cell is the cell death. In conclusion xanthurenic acid can lead to cell pathology.

Identification of xanthurenic acid as the putative inducer of malaria development in the mosquito

Malaria is transmitted from vertebrate host to mosquito vector by mature sexual blood-living stages called gametocytes. Within seconds of ingestion into the mosquito bloodmeal, gametocytes undergo gametogenesis. Induction requires the simultaneous exposure to at least two stimuli in vitro: a drop in bloodmeal temperature to 5 degrees C below that of the vertebrate host, and a rise in pH from 7.4 to 8.0-8.2. In vivo the mosquito bloodmeal has a pH of between 7.5 and 7.6. It is thought that in vivo the second inducer is an unknown mosquito-derived gametocyte-activating factor. Here we show that this factor is xanthurenic acid. We also show that low concentrations of xanthurenic acid can act together with pH to induce gametogenesis in vitro. Structurally related compounds are at least ninefold less effective at inducing gametogenesis in vitro. In Drosophila mutants with lesions in the kynurenine pathway of tryptophan metabolism (of which xanthurenic acid is a side product), no alternative active compound was detected in crude insect homogenates. These data could form the basis of the rational development of new methods of interrupting the transmission of malaria using drugs or new refractory mosquito genotypes to block parasite gametogenesis.

[The effect of wheat bran and green meal supplements on vitamin B6 metabolism of sows fed suboptimal vitamin B6 supply]

The aim of these investigations was to examine the influence of "natural fibrous feedstuffs" as wheat bran and alfalfa meal on criteria of vitamin B6 metabolism of adult sows subjected to a low vitamin B6 supply. Two experiments were conducted in two periods with 12 sows (180 kg BW) and 3 groups each. The supplements were in the first experiment 0 g, 225 g and 675 g wheat bran, and in the second experiment 0 g, 575 g and 1150 g alfalfa meal to a compound feed, low in vitamin B6 content. The criteria were fecal and urinary vitamin B6 concentration and excretion, vitamin B6 concentration in blood, hematological criteria, activity of aspartate aminotransferase in erythrocytes (EAST) and xanthurenic acid excretion in the tryptophan load test. Vitamin B6 concentration in feces amounted 10-12 micrograms/g DM and was neither influenced by quality or amount of the fibrous products. Vitamin B6 excretion was increased by each supplement and 60-70% of vitamin B6 was excreted via feces. Fecal vitamin B6 excretion was enlarged linearly by increasing fibrous supplementation. Bacterially fermentable substrates from wheat bran induced a higher bacterial vitamin B6 synthesis compared to cellulose.

Metabolism of [5-(3)H]kynurenine in the developing rat brain in vivo: effect of intrastriatal ibotenate injections

Two metabolites of the kynurenine pathway of tryptophan degradation, the neurotoxin quinolinic acid (QUIN) and the neuroprotectant kynurenic acid (KYNA), may play a role in the initiation or propagation of brain diseases. In order to study their disposition during the acute and chronic stages of neurodegeneration, effects of an excitotoxic insult on the de novo synthesis of several kynurenine pathway metabolites were examined in vivo. Neuronal injury and lesions were produced in 7-day (PND 7), 14-day (PND 14) and young adult rats by an intrastriatal injection of the excitotoxin ibotenic acid. At 2 h, 2, 7 and 28 days later, the formation of tritiated KYNA, 3-hydroxykynurenine (3HK), xanthurenic acid and QUIN was assessed after an acute intrastriatal injection of their common bioprecursor, [5-(3)H]kynurenine. In all three age groups, the acute insult resulted in a shift towards enhanced KYNA formation, as indicated by 2-4 fold decreases in the 3HK/KYNA and QUIN/KYNA ratios in ibotenate-treated striata. At later post-lesion intervals, age-specific several-fold changes were observed in the flux through both the KYNA and QUIN branches of the kynurenine pathway. With aging, kynurenine conversion to QUIN and especially to 3HK, became increasingly more prominent, though KYNA synthesis was substantially activated as well. The acute toxin-induced changes in kynurenine metabolism, the propensity of the lesioned immature striatum to increase KYNA production preferentially, and the pronounced lesion-induced long-term increases in cerebral KYNA, 3HK and QUIN formation may participate in the modulation of NMDA receptor function following injury. In particular, changes in the production of these kynurenine pathway metabolites may play a role in mechanisms involved in endogenous neuroprotection, delayed neurodegeneration and regenerative processes.

Zinc, copper and iron levels in tissues of the vitamin B6 deficient rat

Tissue zinc (Zn), copper (Cu) and iron (Fe) were determined in three groups of young male Wistar rats that received a daily pyridoxine hydrochloride (PN.HCl) intake of 45, 23 and 0 micrograms respectively in their diets over 8 weeks. No significant differences were found in the Zn and Cu levels in the liver, kidney, skeletal and cardiac tissue of all 3 groups. The Fe levels were significantly higher in the heart and liver and significantly lower in the skeletal muscle of the group receiving no PN.HCl in the diet (P < 0.05). This study indicates that the increased fecal excretion of Zn and Cu observed during a previous balance study on the above vitamin B6 deficient group of animals may be due to a decreased absorption of these elements from the diet rather than their excretion from tissue stores. The changes in Fe levels in the heart, liver and skeletal muscle points towards some alteration in tissue stores of this element during a vitamin B6 deficiency.

Xanthurenic acid derivative formation in the lens is responsible for senile cataract in humans

BACKGROUND

The tryptophan degradation pathway leads to NAD production via 3-hydroxykynurenine. Kynurenine aminotransferase (KAT) transforms 3-hydroxykynurenine into xanthurenic acid. In this study, we measured the activity of KAT in human lenses and studied the consequences of xanthurenic acid formation

METHODS

KAT activity was determined by the method of Tobes. Fluorescence spectroscopy and SDS-PAGE were used for the protein studies. Thin-layer chromatography and infrared and fast atom bombardment spectrometry were used for substance characterization.

RESULTS

The KAT activity was detected in senile cataratous lenses, but was absent in the young lenses. Xanthurenic acid at physiological pH exists in equilibrium with its tautomeric form reported by us as oxo-xanthurenic acid (OXA), which is oxidized to di-oxoxanthurenic acid (DOXA), a naphthoquinone-like substance. The incubation of DOXA with crystallins in a solution of physiological pH led to crystallin crosslinking and formation of conjugates with glutathione.

CONCLUSIONS

Xanthurenic acid is formed in human lenses. Its tautomerization and oxidation leads to a naphthoquinone-like substance, DOXA. DOXA provoked formation of conjugates with glutathione and crosslinking of crystallins. Thus, KAT activity seems to be the initial event in senile cataract formation in humans.

Deamination of 3-hydroxykynurenine in bovine lenses: a possible mechanism of cataract formation in general

BACKGROUND

3-Hydroxykynurenine, a metabolite of tryptophan, acts as UV filter in the human lens. In this study, we looked for this substance and its metabolites in young and old bovine lenses, because of their possible role in the formation of cataract.

METHODS

The substances were detected by HPLC analysis. The fluorescent substance formed from 3-hydroxykynurenine was characterized by thin-layer chromatography followed by reaction with ninhydrin, UV and fluorescence spectrum analysis, and atom bombardment for molecular mass determination. The kynurenine aminotransferase activity was determined by the method of Tobes.

RESULTS

3-Hydroxykynurenine was detected at concentrations of 0.07, 0.19, and 1.14 micrograms/g of tissue in the bovine iris/ciliary body, retina, and transparent bovine lenses respectively. 3-Hydroxykynurenine was deaminated in old bovine eyes but not in calf eyes. In old eyes, kynurenine aminotransferase activity was 2.7, 3.5, and 9.6 mumol/g of tissue per h in retina, iris/ciliary body, and lens respectively.

CONCLUSION

The deamination of 3-hydroxykynurenine resulted in the formation of a fluorescent substance which was identified as oxidized xanthurenic acid. This substance, accumulating in the bovine lens and interacting with lens proteins, could induce cataract formation.

Physiological role of 3-hydroxykynurenine and xanthurenic acid upon crustacean molting

Studies with crabs (Charybdis japonica) and crayfish (Procambarus clarkii) revealed that the tryptophan metabolites, 3-hydroxy-L-kynurenine (3-OH-K) and xanthurenic acid (XA), common secretory products of the X-organ-sinus gland complex of eyestalks from several decapods, regulated the molting of crustaceans in species-nonspecific fashion. Injection of 3-OH-K to the eyestalk-ablated crayfish delayed the onset of the first molt and lengthened the interval between the first and second molts. These lines of evidence were in accord with previous accounts of the so-called "molt inhibiting hormone" (MIH) effect. Removal of eyestalks caused a change in the conversion capacity of exogenous 3-OH-K to XA in the hemolymph. The peak in transformation capacity was followed by a peak in the titer of 20-hydroxyecdysone or molting hormone. Moreover, the seasonal profiles of the XA and ecdysone titers in Charybdis japonica exhibited a staggered relationship in the tissues tested. The ratio of XA to 3-OH-K, which is expected to indicate the apparent 3-OH-Kase activity, fluctuated seasonally and locally. When the Y-organ with the adhering tissues (Y-organ complex or YOC) was incubated during the period of high XA titer, the YOC produced 100 times more ecdysone than before incubation. It is suggested that ecdysteroidogenesis in situ was suppressed during this period by XA, but incubation of the YOC lead to a dramatic acceleration in ecdysone synthesis by overriding this inhibitory effect. XA profoundly repressed ecdysteroidogenesis in the YOC culture. Thus, XA is the ecdysone biosynthesis inhibitor (EBI) and 3-OH-K the precursor in crustaceans. An interfering effect of XA to a biocatalyst cytochrome P-450 system was postulated for the inhibition mechanism of ecdysteroidogenesis.

On kynureninase activity

1) In Mg-deficient rats, kynureninase activity is decreased. 2) p-Hydroxyphenylpyruvate inhibits kynureninase activity. 3) -SH groups in the apoenzyme of kynureninase play a very important role in the enzymatic reaction. 4) 3-Hydroxykynurenine may be a very important regulative metabolite in the 3-hydroxykynurenine----xanthurenic acid pathway.

Biosynthesis of xanthurenic acid 8-O-beta-D-glucoside in Drosophila. Characterization of the xanthurenic acid:UDP-glucosyltransferase activity

Xanthurenic acid 8-glucoside is a side metabolite of the tryptophan-xanthommatin pathway in Drosophila. From 3-hydroxykynurenine, two biosynthetic pathways can be envisaged, one via xanthurenic acid, and another via 3-O-glucoside of 3-hydroxykynurenine. In this report evidence is presented to show that the synthesis takes place via xanthurenic acid. (a) We have demonstrated that the Drosophila melanogaster vermilion purple mutant (unable to synthesize 3-hydroxykynurenine) synthesizes xanthurenic acid 8-glucoside when fed with xanthurenic acid; and (b) the activities required for its synthesis via xanthurenic acid have been found (3-hydroxykynurenine transaminase and xanthurenic acid:UDP-glucosyltransferase). This is the first time that a UDP-glucosyltransferase activity that utilizes xanthurenic acid has been demonstrated. The enzyme in crude extracts from Drosophila sordidula shows the following characteristics. (a) It has optimal activity at 35 degrees C at pH 7.1 (in buffer Tris-HCl), and in the presence of a divalent cation (Mg2+ or Mn2+); (b) the activity is inhibited by xanthurenic acid (above 1.5 mM), UDP, D-gluconic acid 1,5-lactone, and Triton X-100; (c) it is localized in both the microsomal and the soluble fractions; (d) the specific activity is two times higher in heads than in bodies; and (e) the activity is enhanced in flies fed with phenobarbital.

Effects of a dietary excess of leucine and of the addition of leucine and 2-oxo-isocaproate on the metabolism of tryptophan and niacin in isolated rat liver cells

1. Feeding rats on a low-tryptophan, niacin-free, high-leucine diet resulted in impaired synthesis from tryptophan of the nicotinamide nucleotide coenzymes, NAD and NADP, and N1-methyl nicotinamide in isolated hepatocytes, compared with cells from animals fed on a low-tryptophan, niacin-free control diet providing an appropriate amount of leucine. This was accompanied by reduced accumulation of the tryptophan metabolites kynurenine, 3-hydroxykynurenine and xanthurenic acid. 2. With hepatocytes from animals fed on the low-tryptophan, niacin-free control diet, the addition of leucine to the incubation medium resulted in reduced synthesis of niacin from tryptophan, and a small increase in the accumulation of 3-hydroxykynurenine. 3. With hepatocytes from animals fed on the low-tryptophan, niacin-free control diet, the addition of 2-oxoisocaproate to the incubation medium resulted in increased synthesis of NAD(P) and niacin, and increased accumulation of 3-hydroxykynurenine. 4. The results suggest that a dietary excess of leucine alters the activity of the enzymes of tryptophan----niacin metabolism, or the uptake of tryptophan into the liver, in a different manner from the simple inhibition and activation seen in experiments in vitro. 5. Differences between studies in isolated hepatocytes and intact animals suggest that extra-hepatic metabolism of tryptophan, catalysed by indoleamine dioxygenase, is more important than has been believed hitherto.

Kynurenine metabolism and xanthurenic acid formation in vitamin B6-deficient rat after tryptophan injection

Normal and vitamin B6-deficient rats received an intraperitoneal injection of 30 mg/100g of body wt, and the contents of metabolites in kidney or plasma and the related enzyme activities in kidney were determined. The contents of kynurenine and 3-hydroxykynurenine in B6-deficient rat plasma and kidney were much higher than those in normal rat. The changes of those contents in plasma were parallel to those in kidney, but not in liver. The contents of kynurenic acid and xanthurenic acid in B6-deficient liver, plasma, and kidney were also much higher than those in normal rats. However, the changes of those contents in plasma were parallel to those in liver, but not in kidney. Xanthurenic acid and kynurenic acid accumulated to a much greater extent in kidney than in plasma and liver. Kidney kynureninase activity was very low, but kynurenine aminotransferase activities were very high. These observations indicated that the production of xanthurenic acid after tryptophan injection was favorable in B6-deficient kidney with respect to enzyme activities and substrate concentrations, and suggested that kidney took up kynurenine or 3-hydroxykynurenine from blood and after conversion of them it excreted xanthurenic or kynurenic acid into urine.

Endogenous inhibitor of ecdysone synthesis in crabs

Attempts to isolate the molt-inhibiting hormone (MIH) of crustaceans from crab eyestalks (ES) resulted in the characterization of xanthurenic acid as an inhibitor of ecdysone biosynthesis in the cultured Y-organ-complex (YOC) homogenate. It was also found that 3-hydroxy-L-kynurenine present in the ES is transformed into xanthurenic acid in the YOC and body fluid. Its mode of inhibitory action in ecdysone biosynthesis is probably inactivation of cytochrome P-450.

[Tryptophan metabolism and oligophrenia (author's transl)]

After dealing with the biochemistry of tryptophan metabolism the most important results obtained in humans are presented. Special emphasis is given to the hereditary defects of tryptophan metabolism associated with mental retardation and convulsions due to lack of pyridoxine. The author's findings demonstrate the existence of a hereditary disturbance of the tryptophan metabolism via kynurenine in a certain part of oligophrenic patients. This metabolic defect can be controlled by high doses of vitamin B6. Furthermore investigations conducted with a view to interpreting these results are discussed, especially the determination of kynureninase activity, serotonin blood levels and pyridine nucleotide synthesis.