Biochemical mechanisms leading to tryptophan 2,3-dioxygenase activation

The association of dietary nitrates/nitrites intake and the gut microbial metabolite trimethylamine N-oxide and kynurenine in adults: a population-based study

Background

Dietary nitrate and nitrite may affect the gut microbiota and its metabolites, such as trimethylamine N-oxide (TMAO) and kynurenine (KYN). However, this association and the exact mechanism are still unclear. Therefore, this study aimed to assess the association between dietary consumption of nitrite and nitrate on TMAO and KYN levels in adults.

Methods

This cross-sectional study was employed on a subsample baseline phase of the Tehran University of Medical Sciences (TUMS) Employee's Cohort Study (TEC). A total of 250 adults aged 18 years or older were included in the current analysis. Data on the dietary intakes were collected using a validated dish-based food frequency questionnaire (FFQ), and dietary intakes of nitrite and nitrate were estimated using the FFQ with 144 items. Serum profiles and TMAO and KYN were measured using a standard protocol.

Results

The findings of this study demonstrate a significant association between the intake of animal sources of nitrate and nitrite and the likelihood of having elevated levels of TMAO and KYN. Specifically, after adjustment, individuals with the highest intake adherence to nitrates from animal sources exhibited increased odds of having the highest level of TMAO (≥51.02 pg/ml) (OR = 1.51, 95% CI = 0.59-3.88, P = 0.03) and KYN (≥417.41 pg/ml) (OR = 1.75, 95% CI = 0.73-4.17, P = 0.02). Additionally, subjects with the highest animal intake from nitrite sources have 1.73 and 1.45 times higher odds of having the highest levels of TMAO and KYN. These results emphasize the potential implications of animal-derived nitrate and nitrite consumption on the levels of TMAO and KYN.

Conclusion

The present evidence indicates that a high level of nitrate and nitrite intake from animal sources can increase the odds of high levels of TMAO and KYN. Further studies suggest that we should better evaluate and understand this association.

Efficient nanoparticle-based CRISPR-Cas13d induced mRNA disruption of an eye pigmentation gene in the white-backed planthopper, Sogatella furcifera

The discovery of the clustered regularly interspaced short palindromic repeat (CRISPR) system has driven gene manipulation technology to a new era with applications reported in organisms that span the tree of life. The utility of CRISPR-mediated editing was further expanded to mRNA following identification of the RNA-targeting Cas13 family of smaller endonuclease proteins. Application of this family to insect research, however, has been more limited. In this study, the smallest Cas13 family member, Cas13d, and guide RNAs (gRNAs) were complexed with a versatile nanomaterial (star polycation, SPc) to generate a proof-of-concept RNA-editing platform capable of disrupting mRNA expression of the eye pigmentation gene tryptophan 2,3-dioxygenase (SfTO) in white-backed planthoppers (WBPHs). The resulting red-eye phenotype was present in 19.76% (with SPc) and 22.99% (without SPc) of the treatment groups and was comparable to the red-eye phenotype generated following conventional RNA interference knockdown (22.22%). Furthermore, the Cas13/gRNA phenotype manifested more quickly than RNA interference. Consistent with the expected Cas13d mechanism, SfTO transcript levels were significantly reduced. Taken together, the results indicate that the SPc-CRISPR-Cas13d/gRNA complex negatively impacted expression of the target gene. These findings confirm the utility of this novel mRNA disruption system in insects and lay the foundation for further development of these tools in the implementation of green agricultural pest management tactics.

Aryl hydrocarbon receptor and Krüppel like factor 10 mediate a transcriptional axis modulating immune homeostasis in mosquitoes

Immune responses require delicate controls to maintain homeostasis while executing effective defense. Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. The Krüppel-like factor 10 (KLF10) is a C2H2 zinc-finger containing transcription factor. The functions of mosquito AhR and KLF10 have not been characterized. Here we show that AhR and KLF10 constitute a transcriptional axis to modulate immune responses in mosquito Anopheles gambiae. The manipulation of AhR activities via agonists or antagonists repressed or enhanced the mosquito antibacterial immunity, respectively. KLF10 was recognized as one of the AhR target genes in the context. Phenotypically, silencing KLF10 reversed the immune suppression caused by the AhR agonist. The transcriptome comparison revealed that silencing AhR and KLF10 plus challenge altered the expression of 2245 genes in the same way. The results suggest that KLF10 is downstream of AhR in a transcriptional network responsible for immunomodulation. This AhR–KLF10 axis regulates a set of genes involved in metabolism and circadian rhythms in the context. The axis was required to suppress the adverse effect caused by the overactivation of the immune pathway IMD via the inhibitor gene Caspar silencing without a bacterial challenge. These results demonstrate that the AhR–KLF10 axis mediates an immunoregulatory transcriptional network as a negative loop to maintain immune homeostasis.

Inhibitory effect of ascorbate on tryptophan 2,3-dioxygenase

Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) catalyze the same reaction, oxidative cleavage of L-tryptophan (L-Trp) to N-formyl-kynurenine. In both enzymes, the ferric (FeIII) form is inactive, and ascorbate (Asc) is frequently used as a reductant in in vitro assays to activate the enzymes by reducing the heme iron. Recently, it has been reported that Asc activates IDO2 by acting as a reductant, however, it is also a competitive inhibitor of the enzyme. Here, the effect of Asc on human TDO (hTDO) is investigated. Similar to its interaction with IDO2, Asc acts as both a reductant and a competitive inhibitor of hTDO in the absence of catalase, and its inhibitory effect was enhanced by the addition of H2O2. Interestingly, however, no inhibitory effect of Asc was observed in the presence of catalase. TDO is known to be activated by H2O2 and a ferryl-oxo (FeIV=O) intermediate (Compound II) is generated during the activation process. The observation that Asc acts as a competitive inhibitor of hTDO only in the absence of catalase can be explained by assuming that the target of Asc is Compound II. Asc seems to compete with L-Trp in an unusual manner.

Binding of l-kynurenine to X. campestris tryptophan 2,3-dioxygenase

The kynurenine pathway is the major route of tryptophan metabolism. The first step of this pathway is catalysed by one of two heme-dependent dioxygenase enzymes - tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) - leading initially to the formation of N-formylkynurenine (NFK). In this paper, we present a crystal structure of a bacterial TDO from X. campestris in complex with l-kynurenine, the hydrolysed product of NFK. l-kynurenine is bound at the active site in a similar location to the substrate (l-Trp). Hydrogen bonding interactions with Arg117 and the heme 7-propionate anchor the l-kynurenine molecule into the pocket. A mechanism for the hydrolysis of NFK in the active site is presented.

The Footprint of Kynurenine Pathway in Neurodegeneration: Janus-Faced Role in Parkinson's Disorder and Therapeutic Implications

Progressive degeneration of neurons and aggravation of dopaminergic neurons in the substantia nigra pars compacta results in the loss of dopamine in the brain of Parkinson's disease (PD) patients. Numerous therapies, exhibiting transient efficacy have been developed; however, they are mostly accompanied by side effects and limited reliability, therefore instigating the need to develop novel optimistic treatment targets. Significant therapeutic targets have been identified, namely: chaperones, protein Abelson, glucocerebrosidase-1, calcium, neuromelanin, ubiquitin-proteasome system, neuroinflammation, mitochondrial dysfunction, and the kynurenine pathway (KP). The role of KP and its metabolites and enzymes in PD, namely quinolinic acid (QUIN), kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranillic acid (3-HAA), kunurenine-3-monooxygenase (KMO), etc. has been reported. The neurotoxic QUIN, N-methyl-D-aspartate (NMDA) receptor agonist, and neuroprotective KYNA-which antagonizes QUIN actions-primarily justify the Janus-faced role of KP in PD. Moreover, KP has been reported to play a biomarker role in PD detection. Therefore, the authors detail the neurotoxic, neuroprotective, and immunomodulatory neuroactive components, alongside the upstream and downstream metabolic pathways of KP, forming a basis for a therapeutic paradigm of the disease while recognizing KP as a potential biomarker in PD, thus facilitating the development of a suitable target in PD management.

The kynurenine pathway in major depression: What we know and where to next

Major depression is a serious psychiatric disorder, occurring in up to 20 % of the population. Despite its devastating burden, the neurobiological changes associated with depression are not fully understood. A growing body of evidence suggests the kynurenine pathway is implicated in the pathophysiology of depression. In this review, we bring together the literature examining elements of the kynurenine pathway in depression and explore the implications for the pathophysiology and treatment of depression, while highlighting the gaps in the current knowledge. Current research indicates an increased potential for neurotoxic activity of the kynurenine pathway in peripheral blood samples but an increased activation of the putative neuroprotective arm in some brain regions in depression. The disconnect between these findings requires further investigation, with a greater research effort on elucidating the central effects of the kynurenine pathway in driving depression symptomology. Research investigating the benefits of targeting the kynurenine pathway centred on human brain findings and the heterogenous subtypes of depression will help guide the identification of effective drug targets in depression.

Tryptophan: A Rheostat of Cancer Immune Escape Mediated by Immunosuppressive Enzymes IDO1 and TDO

Blockade of the immunosuppressive tryptophan catabolism mediated by indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) holds enormous promise for sensitising cancer patients to immune checkpoint blockade. Yet, only IDO1 inhibitors had entered clinical trials so far, and those agents have generated disappointing clinical results. Improved understanding of molecular mechanisms involved in the immune-regulatory function of the tryptophan catabolism is likely to optimise therapeutic strategies to block this pathway. The immunosuppressive role of tryptophan metabolite kynurenine is becoming increasingly clear, but it remains a mystery if tryptophan exerts functions beyond serving as a precursor for kynurenine. Here we hypothesise that tryptophan acts as a rheostat of kynurenine-mediated immunosuppression by competing with kynurenine for entry into immune T-cells through the amino acid transporter called System L. This hypothesis stems from the observations that elevated tryptophan levels in TDO-knockout mice relieve immunosuppression instigated by IDO1, and that the vacancy of System L transporter modulates kynurenine entry into CD4+ T-cells. This hypothesis has two potential therapeutic implications. Firstly, potent TDO inhibitors are expected to indirectly inhibit IDO1 hence development of TDO-selective inhibitors appears advantageous compared to IDO1-selective and dual IDO1/TDO inhibitors. Secondly, oral supplementation with System L substrates such as leucine represents a novel potential therapeutic modality to restrain the immunosuppressive kynurenine and restore anti-tumour immunity.

Oxidative Storm Induced by Tryptophan Metabolites: Missing Link between Atherosclerosis and Chronic Kidney Disease

Chronic kidney disease (CKD) occurrence is rising all over the world. Its presence is associated with an increased risk of premature death from cardiovascular disease (CVD). Several explanations of this link have been put forward. It is known that in renal failure, an array of metabolites cannot be excreted, and they accumulate in the organism. Among them, some are metabolites of tryptophan (TRP), such as indoxyl sulfate and kynurenine. Scientists have become interested in them in the context of inducing vascular damage in the course of chronic kidney impairment. Experimental evidence suggests the involvement of TRP metabolites in the progression of chronic kidney disease and atherosclerosis separately and point to oxidative stress generation as one of the main mechanisms that is responsible for worsening those states. Since it is known that blood levels of those metabolites increase significantly in renal failure and that they generate reactive oxygen species (ROS), which lead to endothelial injury, it is reasonable to suspect that products of TRP metabolism are the missing link in frequently occurring atherosclerosis in CKD patients. This review focuses on reports that shed a light on TRP metabolites as contributing factors to vascular damage in the progression of impaired kidney function.

The introduction of immunosuppressor (TDO inhibitor) significantly improved the efficacy of irinotecan in treating hepatocellular carcinoma

As TDO inhibitors can improve the efficacy of tumor chemotherapeutics, two TDO-targeted conjugates consisting of irinotecan (Ir) and a TDO inhibitor unit were designed and prepared to reverse tumor immune suppression, which could remarkably enhance antitumor activity of Ir by boosting cellular uptakes against TDO overexpressed HepG2 cancer cells. In vitro mechanistic studies demonstrated that compound PVIS-Ir and PVIG-Ir could arrest cell cycle at G2 phase and induce cell apoptosis by mitochondrial apoptotic pathway. Furthermore, compound PVIS-Ir could effectively inhibit TDO protein expression via releasing a TDO inhibitor derivative, which could also completely embed in TDO protein pocket. Further mechanism study indicated that PVIS-Ir could block kynurenine production and deactivate aryl hydrocarbon receptor (AHR), resulting in T-cell activation and proliferation. In vivo studies confirmed that PVIS-Ir could improve tumor immune microenvironment in a murine model. This combinational strategy of chemotherapy and immunotherapy can be a promising way in the treatment of hepatocellular carcinoma. Conjugates obtained by combining an immune checkpoint TDO inhibitor with irinotecan via different linkers could improve tumor immune microenvironment by inhibiting the TDO enzyme expression to block kynurenine production and induce HepG2 cancer cell apoptosis via DNA damage through releasing a TDO inhibitor and irinotecan in cancer cells.

Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

The ‘microbiota-gut-brain axis’ plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

Role of IDO and TDO in Cancers and Related Diseases and the Therapeutic Implications

Kynurenine (Kyn) pathway is a significant metabolic pathway of tryptophan (Trp). The metabolites of the Kyn pathway are closely correlated with numerous diseases. Two main enzymes, indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO or TDO2), regulate the first and rate-limiting step of the Kyn pathway. These enzymes are directly or indirectly involved in various diseases, including inflammatory diseases, cancer, diabetes, and mental disorders. Presently, an increasing number of potential mechanisms have been revealed. In the present review, we depict the structure of IDO and TDO and explicate their functions in various diseases to facilitate a better understanding of them and to indicate new therapeutic plans to target them. Moreover, we summarize the inhibitors of IDO/TDO that are currently under development and their efficacy in the treatment of cancer and other diseases.

Multifunctional platinum(IV) complexes as immunostimulatory agents to promote cancer immunochemotherapy by inhibiting tryptophan-2,3-dioxygenase

Increasing evidences suggested that cisplatin can be involved in a tumor-specific immune response as an immunomodulator to improve antitumor immunity, but the designation and development are limited. Here, we report a series of novel Pt(IV) complexes derived from the conjugation of platinum(II) anticancer agents with an immune checkpoint TDO inhibitor. These complexes not only showed significant cytotoxic effects on the tested cancer cell lines, but also could enhance antitumor immune response. Particularly, complex T2, the mono-conjuagte of oxoplatin and (E)-4-oxo-4-(3-(2-(pyridin-3-yl)vinyl)-1H-indol-1-yl)butanoic acid, displayed 35-fold more potency than cisplatin against TDO-overexpressed HepG-2 cancer cells. Further study indicated that T2 could inhibit TDO via releasing a derivative of a TDO inhibitor and block kynurenine production, resulting in T-cell activation and proliferation in vitro. In vivo tests proved that T2 as a potent immunomodulator could highly promote the antitumor activity of cisplatin and effectively suppress the expression of TDO. This immunochemotherapeutic strategy can be promisingly applied to treat with TDO-overexpressed cancers.

Pt(IV) hybrids containing a TDO inhibitor serve as potential anticancer immunomodulators

Tryptophan 2,3-dioxygenase (TDO), an immunosuppressive enzyme, can involve in immune evasion and tumor tolerance. TDO inhibitors can boost the efficacy of chemotherapeutics by promoting immunity. Herein, a strategy to introduce a TDO inhibitor into Pt(IV) complexes for reversing tumor immune suppression was adopted. A mono-modified Pt(IV) complex, 3, displayed significant antitumor activity against human liver cancer cells. Flow cytometry study revealed that complex 3 could induce cell death via a mitochondrial-dependent apoptosis pathway and arrest the cell cycle at S phase. Furthermore, complex 3 was effective to enhance T-cell immune responses by inhibiting the TDO enzyme expression to block the kynurenine production and inactivating the downstream of aryl hydrocarbon receptor (AHR).

The Kynurenine Pathway As a Novel Link between Allergy and the Gut Microbiome

In the past few decades, the indoleamine 2,3 dioxygenase (IDO) subset of the kynurenine (KYN) pathway of tryptophan (TRP) metabolism has been the subject of much research in the area of immune tolerance. In this review, we aim to incorporate new findings on this pathway in relation to allergy and the gut microbiome, while providing a comprehensive overview of the pathway itself. Stimulated by interferon gamma, IDO acts as a tolerogenic, immunosuppressive enzyme to attenuate allergic responses by the induction of the KYN-IDO pathway, resultant depletion of TRP, and elevation in KYN metabolites. Acting through the aryl hydrocarbon receptor, KYN metabolites cause T-cell anergy and apoptosis, proliferation of Treg and Th17 cells, and deviation of the Th1/Th2 response, although the outcome is highly dependent on the microenvironment. Moreover, new evidence from germ-free mice and human infants shows that gut microbiota and breast milk are key in determining the functioning of the KYN-IDO pathway. As such, we recommend further research on how this pathway may be a critical link between the microbiome and development of allergy.

Elevated Levels of Peripheral Kynurenine Decrease Bone Strength in Rats with Chronic Kidney Disease

The diagnosis and treatment of bone disorders in patients with chronic kidney disease (CKD) represent a clinical challenge. CKD leads to mineral and bone complications starting early in the course of renal failure. Recently, we have observed the positive relationship between intensified central kynurenine turnover and bone strength in rats with subtotal 5/6 nephrectomy (5/6 Nx)-induced CKD. The aim of the present study was to determine the association between peripheral kynurenine pathway metabolites and bone strength in rats with 5/6 Nx-induced CKD. The animals were sacrificed 1 and 3 months after 5/6 Nx or sham operation. Nephrectomized rats presented higher concentrations of serum creatinine, urea nitrogen, and parathyroid hormone both 1 and 3 months after nephrectomy. These animals revealed higher concentrations of kynurenine and 3-hydroxykynurenine in the serum and higher gene expression of aryl hydrocarbon receptor (AhR) as a physiological receptor for kynurenine and AhR-dependent cytochrome in the bone tissue. Furthermore, nephrectomy significantly increased the number of osteoclasts in the bone without affecting their resorptive activity measured in serum. These changes were particularly evident in rats 1 month after 5/6 Nx. The main bone biomechanical parameters of the tibia were unchanged between nephrectomized and sham-operated rats but were significantly increased in older compared to younger animals. A similar trend was observed for geometrical parameters measured with calipers, bone mineral density based on Archimedes' method and image of bone microarchitecture obtained from micro-computed tomography analyses of tibial cortical bone. In nephrectomized animals, peripheral kynurenine levels correlated negatively with the main parameters of bone biomechanics, bone geometry, and bone mineral density values. In conclusion, our data suggest that CKD-induced elevated levels of peripheral kynurenine cause pathological changes in bone structure via AhR pathway. This finding opens new opportunities for the treatment/prevention of osteoporosis in CKD.

Characterization of 2-Oxindole Forming Heme Enzyme MarE, Expanding the Functional Diversity of the Tryptophan Dioxygenase Superfamily

3-Substituted 2-oxindoles are important structural motifs found in many biologically active natural products and pharmaceutical lead compounds. Here, we report an enzymatic formation of the 3-substituted 2-oxindoles catalyzed by MarE in the maremycin biosynthetic pathway in Streptomyces sp. B9173. MarE is a homologue of Fe^II/heme-dependent tryptophan 2,3-dioxygenases (TDOs). Typical TDOs usually catalyze the insertion of two oxygen atoms from O₂ into an indole ring to generate N-formylkynurenine (NFK)-like products. In contrast, MarE catalyzes the insertion of a single oxygen atom from O₂ into an indole ring, to probably generate an epoxyindole intermediate that undergoes an unprecedented 2,3-hydride migration to form 2-oxindole structure. MarE shows substrate robustness to catalyze the conversion of a series of 3-substituted indoles into their corresponding 3-substituted 2-oxindoles. Although containing most key amino acid residues conserved in well-known TDO homologues, MarE falls into a separate new subgroup in the phylogenetic tree. The characterization of MarE and its homologue enriches the functional diversities of TDO superfamily and provides a new strategy for discovering novel natural products containing 3-substituted 2-oxindole pharmacophores by genome mining.

A short history of heme dioxygenases: rise, fall and rise again

It is well established that there are two different classes of enzymes—tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO)—that catalyse the O₂-dependent oxidation of l-tryptophan to N-formylkynurenine. But it was not always so. This perspective presents a short history of the early TDO and IDO literature, the people that were involved in creating it, and the legacy that this left for the future.

(1)H-NMR analysis provides a metabolomic profile of patients with multiple sclerosis

Objective:

To investigate the metabolomic profiles of patients with multiple sclerosis (MS) and to define the metabolic pathways potentially related to MS pathogenesis.

Methods:

Plasma samples from 73 patients with MS (therapy-free for at least 90 days) and 88 healthy controls (HC) were analyzed by ¹H-NMR spectroscopy. Data analysis was conducted with principal components analysis followed by a supervised analysis (orthogonal partial least squares discriminant analysis [OPLS-DA]). The metabolites were identified and quantified using Chenomx software, and the receiver operating characteristic (ROC) curves were calculated.

Results:

The model obtained with the OPLS-DA identified predictive metabolic differences between the patients with MS and HC (R2X = 0.615, R2Y = 0.619, Q2 = 0.476; p < 0.001). The differential metabolites included glucose, 5-OH-tryptophan, and tryptophan, which were lower in the MS group, and 3-OH-butyrate, acetoacetate, acetone, alanine, and choline, which were higher in the MS group. The suitability of the model was evaluated using an external set of samples. The values returned by the model were used to build the corresponding ROC curve (area under the curve of 0.98).

Conclusion:

NMR metabolomic analysis was able to discriminate different metabolic profiles in patients with MS compared with HC. With the exception of choline, the main metabolic changes could be connected to 2 different metabolic pathways: tryptophan metabolism and energy metabolism. Metabolomics appears to represent a promising noninvasive approach for the study of MS.

The kynurenine pathway and neurodegenerative disease

Neuroactive metabolites of the kynurenine pathway (KP) of tryptophan degradation have been closely linked to the pathogenesis of several neurodegenerative diseases. Tryptophan is an essential amino acid required for protein synthesis, and in higher eukaryotes is also converted into the key neurotransmitters serotonin and tryptamine. However, in mammals >95% of tryptophan is metabolized through the KP, ultimately leading to the production of nicotinamide adenosine dinucleotide (NAD(+)). A number of the pathway metabolites are neuroactive; e.g. can modulate activity of several glutamate receptors and generate/scavenge free radicals. Imbalances in absolute and relative levels of KP metabolites have been strongly associated with neurodegenerative disorders including Huntington's, Alzheimer's, and Parkinson's diseases. The KP has also been implicated in the pathogenesis of other brain disorders (e.g. schizophrenia, bipolar disorder), as well as several cancers and autoimmune disorders such as HIV. Pharmacological and genetic manipulation of the KP has been shown to ameliorate neurodegenerative phenotypes in a number of model organisms, suggesting that it could prove to be a viable target for the treatment of such diseases. Here, we provide an overview of the KP, its role in neurodegeneration and the current strategies for therapeutic targeting of the pathway.

Heme-dependent dioxygenases in tryptophan oxidation

L-Tryptophan is an essential amino acid for mammals. It is utilized not only for protein synthesis but also for the biosynthesis of serotonin and melatonin by the serotonin pathway as well as nicotinamide adenine dinucleotide by the kynurenine pathway. Although the kynurenine pathway is responsible for the catabolism of over 90% of l-tryptophan in the mammalian intracellular and extracellular pools, the scientific field was dominated in the last century by studies of the serotonin pathway, due to the physiological significance of the latter's catabolic intermediates and products. However, in the past decade, the focus gradually reversed as the link between the kynurenine pathway and various neurodegenerative disorders and immune diseases is increasingly highlighted. Notably, the first step of this pathway, which is catalyzed by heme-dependent dioxygenases, has been proven to be a potential target for immune regulation and cancer treatment. A thorough understanding of the intriguing chemistry of the heme-dependent dioxygenases may yield insight for the drug discovery of these prevalent illnesses. In this review, we survey enzymatic and mechanistic studies, initially started by Kotake and Masayama over 70 years ago, at the molecular level on the heme-dependent tryptophan dioxygenation reactions.

Enzyme reactivation by hydrogen peroxide in heme-based tryptophan dioxygenase

An intriguing mystery about tryptophan 2,3-dioxygenase is its hydrogen peroxide-triggered enzyme reactivation from the resting ferric oxidation state to the catalytically active ferrous form. In this study, we found that such an odd Fe(III) reduction by an oxidant depends on the presence of L-Trp, which ultimately serves as the reductant for the enzyme. In the peroxide reaction with tryptophan 2,3-dioxygenase, a previously unknown catalase-like activity was detected. A ferryl species (δ = 0.055 mm/s and ΔE(Q) = 1.755 mm/s) and a protein-based free radical (g = 2.0028 and 1.72 millitesla linewidth) were characterized by Mössbauer and EPR spectroscopy, respectively. This is the first compound ES-type of ferryl intermediate from a heme-based dioxygenase characterized by EPR and Mössbauer spectroscopy. Density functional theory calculations revealed the contribution of secondary ligand sphere to the spectroscopic properties of the ferryl species. In the presence of L-Trp, the reactivation was demonstrated by enzyme assays and by various spectroscopic techniques. A Trp-Trp dimer and a monooxygenated L-Trp were both observed as the enzyme reactivation by-products by mass spectrometry. Together, these results lead to the unraveling of an over 60-year old mystery of peroxide reactivation mechanism. These results may shed light on how a metalloenzyme maintains its catalytic activity in an oxidizing environment.

Structure and reaction mechanism in the heme dioxygenases

As members of the family of heme-dependent enzymes, the heme dioxygenases are differentiated by virtue of their ability to catalyze the oxidation of l-tryptophan to N-formylkynurenine, the first and rate-limiting step in tryptophan catabolism. In the past several years, there have been a number of important developments that have meant that established proposals for the reaction mechanism in the heme dioxygenases have required reassessment. This focused review presents a summary of these recent advances, written from a structural and mechanistic perspective. It attempts to present answers to some of the long-standing questions, to highlight as yet unresolved issues, and to explore the similarities and differences of other well-known catalytic heme enzymes such as the cytochromes P450, NO synthase, and peroxidases.

Environmentally induced oxidative stress in aquatic animals

Reactive oxygen species (ROS) are an unenviable part of aerobic life. Their steady-state concentration is a balance between production and elimination providing certain steady-state ROS level. The dynamic equilibrium can be disturbed leading to enhanced ROS level and damage to cellular constituents which is called "oxidative stress". This review describes the general processes responsible for ROS generation in aquatic animals and critically analyses used markers for identification of oxidative stress. Changes in temperature, oxygen levels and salinity can cause the stress in natural and artificial conditions via induction of disbalance between ROS production and elimination. Human borne pollutants can also enhance ROS level in hydrobionts. The role of transition metal ions, such as copper, chromium, mercury and arsenic, and pesticides, namely insecticides, herbicides, and fungicides along with oil products in induction of oxidative stress is highlighted. Last years the research in biology of free radicals was refocused from only descriptive works to molecular mechanisms with particular interest to ones enhancing tolerance. The function of some transcription regulators (Keap1-Nrf2 and HIF-1α) in coordination of organisms' response to oxidative stress is discussed. The future directions in the field are related with more accurate description of oxidative stress, the identification of its general characteristics and mechanisms responsible for adaptation to the stress have been also discussed. The last part marks some perspectives in the study of oxidative stress in hydrobionts, which, in addition to classic use, became more and more popular to address general biological questions such as development, aging and pathologies.

Substrate-protein interaction in human tryptophan dioxygenase: the critical role of H76

The initial and rate-limiting step of the kynurenine pathway in humans involves the oxidation of tryptophan to N-formyl kynurenine catalyzed by two hemeproteins, tryptophan 2,3-dioxygenase (hTDO) and indoleamine 2,3-dioxygenase (hIDO). In hTDO, the conserved H76 residue is believed to act as an active site base to deprotonate the indole NH group of Trp, the initial step of the Trp oxidation reaction. In hIDO, this histidine is replaced by a serine. To investigate the role of the H76, we have studied the H76S and H76A mutants of hTDO. Activity assays show that the mutations cause a decrease in k(cat) and an increase in K(M) for both mutants. The decrease in the k(cat) is accounted for by the replacement of the active site base catalyst, H76, with a weaker base, possibly a water, whereas the increase in K(M) is attributed to the loss of the specific interactions between the H76 and the substrate as well as the protein matrix. Resonance Raman studies with various Trp analogs indicate that the substrate is positioned in the active site by the ammonium, carboxylate, and indole groups, via intricate H-bonding and hydrophobic interactions. This scenario is consistent with the observation that l-Trp binding significantly perturbs the electronic properties of the O(2)-adduct of hTDO. The important structural and functional roles of H76 in hTDO is underscored by the observation that the electronic configuration of the active ternary complex, l-Trp-O(2)-hTDO, is sensitive to the H76 mutations.

Purification and biochemical characterization of a recombinant Anopheles gambiae tryptophan 2,3-dioxygenase expressed in Escherichia coli

In the malaria vector Anopheles gambiae, tryptophan 2,3-dioxygenase (TDO) is the only enzyme able to initiate l-tryptophan degradation through the kynurenine pathway. TDO converts l-tryptophan to N-formylkynurenine by catalyzing the heme-dependent oxidative opening of the substrate indole ring. Despite the central role exerted by kynurenines in the physiology of living organisms, only a few insect TDOs have been subjected to biochemical characterization in vitro. We performed a RT-PCR-based analysis of the tissue distribution of TDO mRNA in A. gambiae that revealed a ubiquitous expression of the gene, thus further underlining the importance of the enzyme in the mosquito biology. We developed an expression/purification procedure yielding pure and active recombinant A. gambiae TDO. Spectral analyses showed that the enzyme was purified in its heme-ferric form that was subsequently used to determining the Michaelis-Menten constants of the TDO catalyzed reaction in the presence of reducing agents. The screening of a number of compounds as potential TDO modulators showed that several kynurenines and other Tryptophan-derived molecules interfere with the enzyme activity in vitro. Our study could contribute to understanding TDO regulation in vivo and to the identification of inhibitors to be used to alter Tryptophan homeostasis in the malaria vector.