A brain-permeable inhibitor of the neurodegenerative disease target kynurenine 3-monooxygenase prevents accumulation of neurotoxic metabolites

Breast cancer metastasis progression is associated with elevated activity of kynurenine monooxygenase and kynureninase

INTRODUCTION

Metastasis remains the major cause of death in breast cancer (BrCa) and lacks specific treatment strategies. The kynurenine pathway (KP) has been suggested as a key mechanism facilitating progression of BrCa. While KP activity has been explored in primary BrCa, its role in metastasis remains unclear. To better understand this, we examined changes in the KP of BrCa with no metastasis compared to BCa that produced local or distant metastases. Given that the cancer cell secretome plays a role in metastasis, we also investigated the relationship between changes in KP activity and serum proteins of patients with local or distant metastases.

METHODS

To investigate changes in the KP in BrCa, with and without metastasis, we quantified KP metabolites in blood sera collected from patients with stage 1 BrCa (n = 34), BrCa with local metastases (n = 46), BrCa with distant metastases (n = 20) and healthy controls (n = 39). The serum protein profile of the BrCa patients with local or distant metastasis was determined before correlation analyses were carried out to examine the relationship between changes in the KP and cancer serum proteins using SPSS.

RESULTS

We found that the KP was elevated in BrCa patients with local and distant metastasis compared to healthy controls and stage 1 BrCa patients. The activity of kynurenine monooxygenase (KMO) and kynureninase (KYNU) A was positively associated with disease stage and was higher compared to healthy controls. Proteome analysis in patients with local or distant metastasis revealed the dysregulation of 14 proteins, 9 of which were up-regulated and 5 down-regulated at the distant metastasis stage. Importantly, three of these proteins have not been previously linked to BrCa metastasis. In the correlation studies between the KP profile, cancer serum proteins and metastasis status, KYNU A had the greatest number of significant associations with cancer serum protein, followed by KMO.

CONCLUSION

Our findings reveal that the KP was regulated differently at various stages of BrCa and was more dysregulated in patients with local or distant metastasis. These KP activity changes showed a significant association with cancer serum proteins in BrCa patients with local or distant metastasis, highlighting the potential role of KP in BrCa metastasis.

Revolutionizing our understanding of Parkinson's disease: Dr. Heinz Reichmann's pioneering research and future research direction

Millions of individuals around the world are afflicted with Parkinson's disease (PD), a prevalent and incapacitating neurodegenerative disorder. Dr. Reichmann, a distinguished professor and neurologist, has made substantial advancements in the domain of PD research, encompassing both fundamental scientific investigations and practical applications. His research has illuminated the etiology and treatment of PD, as well as the function of energy metabolism and premotor symptoms. As a precursor to a number of neurotransmitters and neuromodulators that are implicated in the pathophysiology of PD, he has also investigated the application of tryptophan (Trp) derivatives in the disease. His principal findings and insights are summarized and synthesized in this narrative review article, which also emphasizes the challenges and implications for future PD research. This narrative review aims to identify and analyze the key contributions of Reichmann to the field of PD research, with the ultimate goal of informing future research directions in the domain. By examining Reichmann's work, the study seeks to provide a comprehensive understanding of his major contributions and how they can be applied to advance the diagnosis and treatment of PD. This paper also explores the potential intersection of Reichmann's findings with emerging avenues, such as the investigation of Trp and its metabolites, particularly kynurenines, which could lead to new insights and potential therapeutic strategies for managing neurodegenerative disorders like PD.

Gut-brain axis and neurodegeneration: mechanisms and therapeutic potentials

This paper reviews the effects of gut microbiota in regulating neurodegenerative diseases through controlling gut-brain axis. Specific microbial populations and their metabolites (short-chain fatty acids and tryptophan derivatives) regulate neuroinflammation, neurogenesis and neural barrier integrity. We then discuss ways by which these insights lead to possible interventions - probiotics, prebiotics, dietary modification, and fecal microbiota transplantation (FMT). We also describe what epidemiological and clinical studies have related certain microbiota profiles with the courses of neurodegenerative diseases and how these impact the establishment of microbiome-based diagnostics and individualized treatment options. We aim to guide microbial ecology research on this key link to neurodegenerative disorders and also to highlight collaborative approaches to manage neurological health by targeting microbiome-related factors.

Structural and Functional Characterization of a Novel Class A Flavin Monooxygenase from Bacillus niacini

A gene cluster responsible for the degradation of nicotinic acid (NA) in Bacillus niacini has recently been identified, and the structures and functions of the resulting enzymes are currently being evaluated to establish pathway intermediates. One of the genes within this cluster encodes a flavin monooxygenase (BnFMO) that is hypothesized to catalyze a hydroxylation reaction. Kinetic analyses of the recombinantly purified BnFMO suggest that this enzyme catalyzes the hydroxylation of 2,6-dihydroxynicotinic acid (2,6-DHNA) or 2,6-dihydroxypyridine (2,6-DHP), which is formed spontaneously by the decarboxylation of 2,6-DHNA. To understand the details of this hydroxylation reaction, we determined the structure of BnFMO using a multimodel approach combining protein X-ray crystallography and cryo-electron microscopy (cryo-EM). A liganded BnFMO cryo-EM structure was obtained in the presence of 2,6-DHP, allowing us to make predictions about potential catalytic residues. The structural data demonstrate that BnFMO is trimeric, which is unusual for Class A flavin monooxygenases. In both the electron density and coulomb potential maps, a region at the trimeric interface was observed that was consistent with and modeled as lipid molecules. High-resolution mass spectral analysis suggests that there is a mixture of phosphatidylethanolamine and phosphatidylglycerol lipids present. Together, these data provide insights into the molecular details of the central hydroxylation reaction unique to the aerobic degradation of NA in Bacillus niacini.

The Biology and Biochemistry of Kynurenic Acid, a Potential Nutraceutical with Multiple Biological Effects

Kynurenic acid (KYNA) is an antioxidant degradation product of tryptophan that has been shown to have a variety of cytoprotective, neuroprotective and neuronal signalling properties. However, mammalian transporters and receptors display micromolar binding constants; these are consistent with its typically micromolar tissue concentrations but far above its serum/plasma concentration (normally tens of nanomolar), suggesting large gaps in our knowledge of its transport and mechanisms of action, in that the main influx transporters characterized to date are equilibrative, not concentrative. In addition, it is a substrate of a known anion efflux pump (ABCC4), whose in vivo activity is largely unknown. Exogeneous addition of L-tryptophan or L-kynurenine leads to the production of KYNA but also to that of many other co-metabolites (including some such as 3-hydroxy-L-kynurenine and quinolinic acid that may be toxic). With the exception of chestnut honey, KYNA exists at relatively low levels in natural foodstuffs. However, its bioavailability is reasonable, and as the terminal element of an irreversible reaction of most tryptophan degradation pathways, it might be added exogenously without disturbing upstream metabolism significantly. Many examples, which we review, show that it has valuable bioactivity. Given the above, we review its potential utility as a nutraceutical, finding it significantly worthy of further study and development.

The Complex World of Kynurenic Acid: Reflections on Biological Issues and Therapeutic Strategy

It has been unequivocally established that kynurenic acid has a number of actions in a variety of cells and tissues, raising, in principle, the possibility of targeting its generation, metabolism or sites of action to manipulate those effects to a beneficial therapeutic end. However, many basic aspects of the biology of kynurenic acid remain unclear, potentially leading to some confusion and misinterpretations of data. They include questions of the source, generation, targets, enzyme expression, endogenous concentrations and sites of action. This essay is intended to raise and discuss many of these aspects as a source of reference for more balanced discussion. Those issues are followed by examples of situations in which modulating and correcting kynurenic acid production or activity could bring significant therapeutic benefit, including neurological and psychiatric conditions, inflammatory diseases and cell protection. More information is required to obtain a clear overall view of the pharmacological environment relevant to kynurenic acid, especially with respect to the active concentrations of kynurenine metabolites in vivo and changed levels in disease. The data and ideas presented here should permit a greater confidence in appreciating the sites of action and interaction of kynurenic acid under different local conditions and pathologies, enhancing our understanding of kynurenic acid itself and the many clinical conditions in which manipulating its pharmacology could be of clinical value.

Modelling the elusive conformational activation in kynurenine 3-monooxygenase

Kynurenine 3-monooxygenase (KMO) regulates the levels of important physiological intermediates in the kynurenine pathway [Guillemin, et al., Journal of Neuroscience, 2007, 27, 12884], which is the major route for L-tryptophan catabolism. Its catalytic activity (hydroxylation) is dependent on the formation of a short-lived intermediate that forms after the reduction of the coenzyme FAD. The reduction takes place fast when the substrate binds to KMO. Crystal structures of the apo form and in complex with an effector inhibitor, which prevents the hydroxylation of the substrate but also stimulates KMO like the substrate, and a competitive inhibitor, which suppresses the substrate hydroxylation, are available for the resting in conformation only. The active out conformational state that enables the reduction of FAD at an exposed location of KMO after its stimulation by an effector, however, was implicated but not resolved experimentally and has remained elusive so far. Molecular dynamics simulations of apo KMO and the inhibitor-KMO complexes are carried out using extensive multi-dimensional umbrella sampling to explore the free-energy surface of the coenzyme FAD's conformational conversion from the in state (buried within the active site) to the out state. This allows a discussion and comparison with the experimental results, which showed a significant increase in the rate of reduction of FAD in the presence of an effector inhibitor and absence of enzymatic function in the presence of a competitive inhibitor [Kim, et al., Cell Chemical Biology, 2018, 25, 426]. The free-energy barriers associated with those conformational changes and structural models for the active out conformation are obtained. The interactions during the conformational changes are determined to identify the influence of the effector.

Identification of potent inhibitors of kynurenine-3-monooxygenase from natural products: In silico and in vitro approaches

Existing inhibitors of kynurenine-3-monooxygenase (KMO) have side effects and poorly cross the blood-brain barrier. Therefore, the discovery of new molecules targeting KMO isnecessary.This study aims to develop a novel therapeutic drug targeting KMO using computational methods and experimental validation of natural compounds.The results of our study show that the top four compounds, namely, 3'-Hydroxy-alpha-naphthoflavone exhibited the best docking scores with KMO (-10.0 kcal/mol), followed by 3'-Hydroxy-ss-naphthoflavone (-9.9 kcal/mol), genkwanin (-9.2 kcal/mol) and apigenin(-9.1 kcal/mol) respectively. Molecular dynamics was used to assess the stability of the primary target, KMO, and inhibitor complexes. We found stable interactions of 3'-Hydroxy-ss-naphthoflavone and apigenin with KMO up to 100 ns. Further, kinetic measurements showed that 3'-Hydroxy-alpha-naphthoflavone and 3'-Hydroxy-ss-naphthoflavone induce competitive inhibition with a good IC50 activity (15.85 ± 0.98 μM and 18.71 ± 0.78, respectively), while Genkwanin and Apigenin exhibit non-competitive inhibition mechanism (21.61 ± 0.97 μM and 24.14 ± 1.00 μM, respectively).Drug-likeness features and ADME analysis features also showed that the top four compounds could be used as potential candidates to replace the synthetic KMO inhibitor drugs with known side effects and poor brain-blood barrier penetration.

DFT modeling of water-assisted hydrogen peroxide formation from a C(4a)-(hydro)peroxyflavin

The cofactor of a class A monooxygenase is reduced at an external location of the enzyme and is subsequently pulled back into the active site after the reduction. This observation brings the question; is there any defense mechanism of the active site of a monooxygenase against the formation of the harmful hydrogen peroxide from the reactive C(4a)-(hydro)peroxide intermediate? In this study, the barrier energies of one to three water molecule-mediated uncoupling reaction mechanisms in water exposed reaction conditions were determined. These were found to be facile barriers. Secondly, uncoupling was modeled in the active site of kynurenine 3-monooxygenase complex which was represented with 258 atoms utilizing cluster approach. Comparison of the barrier energy of the cluster model to the models that represent the water exposed conditions revealed that the enzyme does not have an inhibitory reaction site architecture as the compared barrier energies are roughly the same. The main defense mechanism of KMO against the formation of the hydrogen peroxide is deduced to be the insulation, and without this insulation, the monooxygenation would not take place as the barrier height of the hydrogen peroxide formation within the active site is almost half of that of the monooxygenation.

Nutrient Metabolites Associated With Low D3Cr Muscle Mass, Strength, and Physical Performance in Older Men

BACKGROUND

The relationship between amino acids, B vitamins, and their metabolites with D3-creatine (D3Cr) dilution muscle mass, a more direct measure of skeletal muscle mass, has not been investigated. We aimed to assess associations of plasma metabolites with D3Cr muscle mass, as well as muscle strength and physical performance in older men from the Osteoporotic Fractures in Men cohort study.

METHODS

Out of 1 425 men (84.2 ± 4.1 years), men with the lowest D3Cr muscle mass (n = 100), slowest walking speed (n = 100), lowest grip strength (n = 100), and a random sample (n = 200) serving as a comparison group to the low groups were included. Metabolites were analyzed using liquid chromatography-tandem mass spectrometry. Metabolite differences between the low groups and random sample and their relationships with the muscle outcomes adjusted for confounders and multiple comparisons were assessed using t-test/Mann-Whitney-Wilcoxon and partial correlations, respectively.

RESULTS

For D3Cr muscle mass, significant biomarkers (p < .001) with ≥10% fold difference and largest partial correlations were tryptophan (Trp; r = 0.31), kynurenine (Kyn)/Trp; r = -0.27), nicotinamide (Nam)/quinolinic acid (Quin; r = 0.21), and alpha-hydroxy-5-methyl-tetrahydrofolate (hm-THF; r = -0.25). For walking speed, hm-THF, Nam/Quin, and Quin had the largest significance and fold difference, whereas valine (r = 0.17), Trp (r = 0.17), HKyn/Xant (r = -0.20), neopterin (r = -0.17), 5-methyl-THF (r = -0.20), methylated folate (r = -0.21), and thiamine (r = -0.18) had the strongest correlations. Only hm-THF was correlated with grip strength (r = -0.21) and differed between the low group and the random sample.

CONCLUSIONS

Future interventions focusing on how the Trp metabolic pathway or hm-THF influences D3Cr muscle mass and physical performance declines in older adults are warranted.

The function of previously unappreciated exerkines secreted by muscle in regulation of neurodegenerative diseases

The initiation and progression of neurodegenerative diseases (NDs), distinguished by compromised nervous system integrity, profoundly disrupt the quality of life of patients, concurrently exerting a considerable strain on both the economy and the social healthcare infrastructure. Exercise has demonstrated its potential as both an effective preventive intervention and a rehabilitation approach among the emerging therapeutics targeting NDs. As the largest secretory organ, skeletal muscle possesses the capacity to secrete myokines, and these myokines can partially improve the prognosis of NDs by mediating the muscle-brain axis. Besides the well-studied exerkines, which are secreted by skeletal muscle during exercise that pivotally exert their beneficial function, the physiological function of novel exerkines, e.g., apelin, kynurenic acid (KYNA), and lactate have been underappreciated previously. Herein, this review discusses the roles of these novel exerkines and their mechanisms in regulating the progression and improvement of NDs, especially the significance of their functions in improving NDs' prognoses through exercise. Furthermore, several myokines with potential implications in ameliorating ND progression are proposed as the future direction for investigation. Elucidation of the function of exerkines secreted by skeletal muscle in the regulation of NDs advances the understanding of its pathogenesis and facilitates the development of therapeutics that intervene in these processes to cure NDs.

Therapeutic potential of targeting kynurenine pathway in neurodegenerative diseases

Kynurenine pathway (KP), the primary pathway of L-tryptophan (Trp) metabolism in mammals, contains several neuroactive metabolites such as kynurenic acid (KA) and quinolinic acid (QA). Its imbalance involved in aging and neurodegenerative diseases (NDs) has attracted much interest in therapeutically targeting KP enzymes and KP metabolite-associated receptors, especially kynurenine monooxygenase (KMO). Currently, many agents have been discovered with significant improvement in animal models but only one aryl hydrocarbon receptor (AHR) agonist 30 (laquinimod) has entered clinical trials for treating Huntington's disease (HD). In this review, we describe neuroactive KP metabolites, discuss the dysregulation of KP in aging and NDs and summarize the development of KP regulators in preclinical and clinical studies, offering an outlook of targeting KP for NDs treatment in future.

Quantum and Classical Evaluations of Carboxylic Acid Bioisosteres: From Capped Moieties to a Drug Molecule

Using the Quantum Theory of Atoms in Molecules, the average electron density (AED) tool was developed and employed to quantitatively evaluate the similarities between bioisosteric moieties in drug design. Bioisosteric replacements are valuable in drug molecules to fine-tune their pharmacokinetic and pharmacodynamic properties while maintaining their biological activity. This study was performed on non-classical bioisosteres of carboxylic acid. It was found that the AED of a given bioisostere is generally transferable, within less than 5% difference, irrespective of its environment. It was shown that the AED tool succeeds at depicting not only the similarities of bioisosteric groups but also at highlighting, as counter examples, the differences in non-bioisosteric groups. For the first time, the AED was used to evaluate bioisosterism in an FDA-approved drug molecule, furosemide, and in five analogues of this medicine. In one of the analogues, non-bioisosteric moieties were exchanged, and in four of the analogues, carboxylic acid was replaced with either furan or sulfonamide, and vice versa. It was also found that irrespective of the pH, the AED tool consistently reproduced experimental predictions. The distinct power of the AED tool in quantitatively and precisely measuring the similarity among bioisosteric groups is contrasted with the relatively ambiguous bioisosteric evaluations through the classical qualitative electrostatic potential (ESP) maps. The ESP maps were demonstrated to fail, even qualitatively, in depicting the similarities, in some cases.

Gram-Scale Preparation of Cannflavin A from Hemp (Cannabis sativa L.) and Its Inhibitory Effect on Tryptophan Catabolism Enzyme Kynurenine-3-Monooxygenase

Inhibitors targeting kynurenine-3-monooxygenase (KMO), an enzyme in the neurotoxic kynurenine pathway (KP), are potential therapeutics for KP metabolites-mediated neuroinflammatory and neurodegenerative disorders. Although phytochemicals from Cannabis (C. sativa L.) have been reported to show modulating effects on enzymes involved in the KP metabolism, the inhibitory effects of C. sativa compounds, including phytocannabinoids and non-phytocannabinoids (i.e., cannflavin A; CFA), on KMO remain unknown. Herein, CFA (purified from hemp aerial material at a gram-scale) and a series of phytocannabinoids were evaluated for their anti-KMO activity. CFA showed the most active inhibitory effect on KMO, which was comparable to the positive control Ro 61-8048 (IC50 = 29.4 vs. 5.1 μM, respectively). Furthermore, a molecular docking study depicted the molecular interactions between CFA and the KMO protein and a biophysical binding assay with surface plasmon resonance (SPR) technique revealed that CFA bound to the protein with a binding affinity of 4.1×10−5 M. A competitive SPR binding analysis suggested that CFA and Ro 61-8048 bind to the KMO protein in a competitive manner. Our findings show that C. sativa derived phytochemicals, including CFA, are potential KMO inhibitors, which provides insight into the development of therapeutics targeting the KP and its related pathological conditions.

The tryptophan metabolism, kynurenine pathway and oxidative stress - Implications for glioma pathobiology

The kynurenine pathway receives increasing attention due to its involvement in central nervous system pathologies, i.a. neurodegenerative and psychiatric disorders, but also due to the contribution to the pathomechanism of neoplasms, including brain tumors.The present review focuses on kynurenine pathway activity in gliomas, brain tumors of glial origin. The upregulation of kynurenine pathway enzyme, indoleamine 2,3-dioxygenase (IDO), resulting in a decreased level of tryptophan and augmented kynurenine synthesis (increased (KYN/Trp ratio) are the most recognised hallmark of malignant transformation, characterised with immunomodulatory adaptations, providing an escape from defence mechanisms of the host, growth-beneficial milieu and resistance to some therapeutics. The review addresses, however, the oxidative/nitrosative stress-associated mechanisms of tryptophan catabolism, mainly the kynurenine pathway activity, linking them with glioma pathobiology.

Kynurenine Metabolism and Alzheimer's Disease: The Potential Targets and Approaches

L-tryptophan, an essential amino acid, regulates protein homeostasis and plays a role in neurotransmitter-mediated physiological events. It also influences age-associated neurological alterations and neurodegenerative changes. The metabolism of tryptophan is carried majorly through the kynurenine route, leading to the production of several pharmacologically active enzymes, substrates, and metabolites. These metabolites and enzymes influence a variety of physiological and pathological outcomes of the majority of systems, including endocrine, haemopoietic, gastrointestinal, immunomodulatory, inflammatory, bioenergetic metabolism, and neuronal functions. An extensive literature review of PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was carried out to understand the nature of the extensive work done on the kynurenine metabolites that influence cellular redox potential, immunoregulatory mechanisms, inflammatory pathways, cell survival channels, and cellular communication in close association with several neurodegenerative changes. The imbalanced state of kynurenine pathways has found a close association to several pathological disorders, including HIV infections, cancer, autoimmune disorders, neurodegenerative and neurological disorders including Parkinson's disease, epilepsy and has found special attention in Alzheimer's disease (AD). Kynurenine pathway (KP) is intricately linked to AD pathogenesis owing to the influence of kynurenine metabolites on excitotoxic neurotransmission, oxidative stress, uptake of neurotransmitters, and modulation of neuroinflammation, amyloid aggregation, microtubule disruption, and their ability to induce a state of dysbiosis. Pharmacological modulation of KP pathways has shown encouraging results, indicating that it may be a viable and explorable target for the therapy of AD.

Computational Survey of Recent Experimental Developments in the Hydroxylation Mechanism of Kynurenine 3-Monooxygenase

Recently, two new mechanistic proposals for the kynurenine 3-monooxygenase (KMO) catalyzed hydroxylation reaction of l-Kynurenine (l-Kyn) have been proposed. According to the first proposal, instead of the distal oxygen, the proximal oxygen of the hydroperoxide intermediate of flavin adenine dinucleotide (FAD) is transferred to the substrate ring. The second study proposes that l-Kyn participates in its base form in the reaction. To address these proposals, the reaction was reconsidered with a 386 atom quantum cluster model that is based on a recent X-ray structure (PDB id: 6FOX). The computations were carried out at the UB3LYP/6-311+G(2d,2p)//UB3LYP/6-31G(d,p) level with solvation (polarizable continuum model) and dispersion (DFT-D3(BJ)) corrections. To supplement the results of the density functional theory (DFT) calculations, molecular dynamics (MD) simulations of the protein-substrate complex were employed. The comparison of a proximal oxygen transfer mechanism to the distal oxygen transfer mechanism revealed that the former requires too high of a barrier energy while the latter validated our previous results. According to the MD simulations, the hydroperoxy moiety does not favor an alignment that might promote the proximal oxygen transfer mechanism. In the second part of the study, hydroxylation reaction with the base form of l-Kyn was sought. Although DFT calculations confirmed a much more facile reaction with the base form of l-Kyn, a mechanism which would allow the deprotonation of the l-Kyn before the oxygen transfer could not be determined with the X-ray-based positions. A concerted mechanism with both the oxygen transfer and the deprotonation required a high barrier energy. A stepwise mechanism involving the deprotonation of l-Kyn was found, starting from an MD frame. The overall barrier of the oxygen transfer step of this model was found to be in the range of that of with neutral l-Kyn. MD simulations supported the idea of ineffectiveness of the nearby shell surrounding the utilized active site core on the deprotonation of l-Kyn.

The Kynurenine Pathway as a Potential Target for Neuropathic Pain Therapy Design: From Basic Research to Clinical Perspectives

Accumulating evidence suggests the key role of the kynurenine pathway (KP) of the tryptophan metabolism in the pathogenesis of several diseases. Despite extensive research aimed at clarifying the mechanisms underlying the development and maintenance of neuropathic pain, the roles of KP metabolites in this process are still not fully known. Although the function of the peripheral KP has been known for several years, it has only recently been acknowledged that its metabolites within the central nervous system have remarkable consequences related to physiology and behavior. Both the products and metabolites of the KP are involved in the pathogenesis of pain conditions. Apart from the neuroactive properties of kynurenines, the KP regulates several neurotransmitter systems in direct or indirect ways. Some neuroactive metabolites are known to have neuroprotective properties (kynurenic acid, nicotinamide adenine dinucleotide cofactor), while others are toxic (3-hydroxykynurenine, quinolinic acid). Numerous animal models show that modulation of the KP may turn out to be a viable target for the treatment of diseases. Importantly, some compounds that affect KP enzymes are currently described to possess analgesic properties. Additionally, kynurenine metabolites may be useful for assessing response to therapy or as biomarkers in therapeutic monitoring. The following review describes the molecular site of action and changes in the levels of metabolites of the kynurenine pathway in the pathogenesis of various conditions, with a particular emphasis on their involvement in neuropathy. Moreover, the potential clinical implications of KP modulation in chronic pain therapy as well as the directions of new research initiatives are discussed.

Tryptophan Catabolism and Inflammation: A Novel Therapeutic Target For Aortic Diseases

Aortic diseases are the primary public health concern. As asymptomatic diseases, abdominal aortic aneurysm (AAA) and atherosclerosis are associated with high morbidity and mortality. The inflammatory process constitutes an essential part of a pathogenic cascade of aortic diseases, including atherosclerosis and aortic aneurysms. Inflammation on various vascular beds, including endothelium, smooth muscle cell proliferation and migration, and inflammatory cell infiltration (monocytes, macrophages, neutrophils, etc.), play critical roles in the initiation and progression of aortic diseases. The tryptophan (Trp) metabolism or kynurenine pathway (KP) is the primary way of degrading Trp in most mammalian cells, disturbed by cytokines under various stress. KP generates several bioactive catabolites, such as kynurenine (Kyn), kynurenic acid (KA), 3-hydroxykynurenine (3-HK), etc. Depends on the cell types, these metabolites can elicit both hyper- and anti-inflammatory effects. Accumulating evidence obtained from various animal disease models indicates that KP contributes to the inflammatory process during the development of vascular disease, notably atherosclerosis and aneurysm development. This review outlines current insights into how perturbed Trp metabolism instigates aortic inflammation and aortic disease phenotypes. We also briefly highlight how targeting Trp metabolic pathways should be considered for treating aortic diseases.

The kynurenine pathway in chronic diseases: a compensatory mechanism or a driving force?

The kynurenine (KYN) pathway (KP) of tryptophan (TRP) metabolism is dysregulated in inflammation-driven pathologies including oncological and brain diseases [e.g., multiple sclerosis (MS), depression] and thus is a promising therapeutic target. Both pathological and compensatory mechanisms underlie disease-associated KP activation. There is growing evidence for bioenergetic roles of certain KP metabolites such as kynurenic acid (KA), or quinolinic acid (QA) as an NAD⁺ precursor, which may explain its frequently observed 'pathological' overactivation. Disease- and tissue-specific aspects, negative feedback on inflammatory signals, and the balance of downstream metabolites are likely to be decisive factors in the interpretation of an imbalanced KP. Therapeutic strategies should consider the compensatory actions and bioenergetic roles of KP metabolites to successfully design future theragnostic approaches aimed at attenuating disease progression.

Galantamine-Memantine Combination and Kynurenine Pathway Enzyme Inhibitors in the Treatment of Neuropsychiatric Disorders

The kynurenine pathway (KP) is a major route for L-tryptophan (L-TRP) metabolism, yielding a variety of bioactive compounds including kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN), and picolinic acid (PIC). These tryptophan catabolites are involved in the pathogenesis of many neuropsychiatric disorders, particularly when the KP becomes dysregulated. Accordingly, the enzymes that regulate the KP such as indoleamine 2,3-dioxygenase (IDO)/tryptophan 2,3-dioxygenase, kynurenine aminotransferases (KATs), and kynurenine 3-monooxygenase (KMO) represent potential drug targets as enzymatic inhibition can favorably rebalance KP metabolite concentrations. In addition, the galantamine-memantine combination, through its modulatory effects at the alpha7 nicotinic acetylcholine receptors and N-methyl-D-aspartate receptors, may counteract the effects of KYNA. The aim of this review is to highlight the effectiveness of IDO-1, KAT II, and KMO inhibitors, as well as the galantamine-memantine combination in the modulation of different KP metabolites. KAT II inhibitors are capable of decreasing the KYNA levels in the rat brain by a maximum of 80%. KMO inhibitors effectively reduce the central nervous system (CNS) levels of 3-HK, while markedly boosting the brain concentration of KYNA. Emerging data suggest that the galantamine-memantine combination also lowers L-TRP, kynurenine, KYNA, and PIC levels in humans. Presently, there are only 2 pathophysiological mechanisms (cholinergic and glutamatergic) that are FDA approved for the treatment of cognitive dysfunction for which purpose the galantamine-memantine combination has been designed for clinical use against Alzheimer's disease. The alpha7 nicotinic-NMDA hypothesis targeted by the galantamine-memantine combination has been implicated in the pathophysiology of various CNS diseases. Similarly, KYNA is well capable of modulating the neuropathophysiology of these disorders. This is known as the KYNA-centric hypothesis, which may be implicated in the management of certain neuropsychiatric conditions. In line with this hypothesis, KYNA may be considered as the "conductor of the orchestra" for the major pathophysiological mechanisms underlying CNS disorders. Therefore, there is great opportunity to further explore and compare the biological effects of these therapeutic modalities in animal models with a special focus on their effects on KP metabolites in the CNS and with the ultimate goal of progressing to clinical trials for many neuropsychiatric diseases.

Kynurenine pathway metabolites in cerebrospinal fluid and blood as potential biomarkers in Huntington's disease

Converging lines of evidence from several models, and post-mortem human brain tissue studies, support the involvement of the kynurenine pathway (KP) in Huntington's disease (HD) pathogenesis. Quantifying KP metabolites in HD biofluids is desirable, both to study pathobiology and as a potential source of biomarkers to quantify pathway dysfunction and evaluate the biochemical impact of therapeutic interventions targeting its components. In a prospective single-site controlled cohort study with standardised collection of cerebrospinal fluid (CSF), blood, phenotypic and imaging data, we used high-performance liquid-chromatography to measure the levels of KP metabolites-tryptophan, kynurenine, kynurenic acid, 3-hydroxykynurenine, anthranilic acid and quinolinic acid-in CSF and plasma of 80 participants (20 healthy controls, 20 premanifest HD and 40 manifest HD). We investigated short-term stability, intergroup differences, associations with clinical and imaging measures and derived sample-size calculation for future studies. Overall, KP metabolites in CSF and plasma were stable over 6 weeks, displayed no significant group differences and were not associated with clinical or imaging measures. We conclude that the studied metabolites are readily and reliably quantifiable in both biofluids in controls and HD gene expansion carriers. However, we found little evidence to support a substantial derangement of the KP in HD, at least to the extent that it is reflected by the levels of the metabolites in patient-derived biofluids.

Neuroinflammation and the Kynurenine Pathway in CNS Disease: Molecular Mechanisms and Therapeutic Implications

Diseases of the central nervous system (CNS) remain a significant health, social and economic problem around the globe. The development of therapeutic strategies for CNS conditions has suffered due to a poor understanding of the underlying pathologies that manifest them. Understanding common etiological origins at the cellular and molecular level is essential to enhance the development of efficacious and targeted treatment options. Over the years, neuroinflammation has been posited as a common link between multiple neurological, neurodegenerative and neuropsychiatric disorders. Processes that precipitate neuroinflammatory conditions including genetics, infections, physical injury and psychosocial factors, like stress and trauma, closely link dysregulation in kynurenine pathway (KP) of tryptophan metabolism as a possible pathophysiological factor that 'fuel the fire' in CNS diseases. In this study, we aim to review emerging evidence that provide mechanistic insights between different CNS disorders, neuroinflammation and the KP. We provide a thorough overview of the different branches of the KP pertinent to CNS disease pathology that have therapeutic implications for the development of selected and efficacious treatment strategies.

Multiomics Investigation of Hypertension and White Matter Hyperintensity as a Source of Vascular Dementia or a Comorbidity to Alzheimer's Disease

BACKGROUND

Age-related comorbidity is common and significantly increases the burden for the healthcare of the elderly. Alzheimer's disease (AD) and hypertension are the two most prevalent age-related conditions and are highly comorbid. While hypertension is a risk factor for vascular dementia (VD), hypertension with AD (ADHyp+) is often characterized as probable vascular dementia. In the absence of imaging and other diagnostic tests, differentiating the two pathological states is difficult.

OBJECTIVE

Our goals are to (1) identify differences in CSF-based vascular dementia profiles, if any, between individuals who have AD only (ADHyp-), and individuals with ADHyp+ using CSF levels of amyloid β, tau and p-tau, and (2) compare genome-wide DNA profiles of ADHyp- and ADHyp+ with an unaffected control population.

METHOD

Genotype and clinical data were used to compare healthy controls to AD Hyp- vs. AD Hyp+. We compared the CSF biomarkers followed by evaluating genome wide profiles in three groups, and mapped SNPs to genes based on position and lowest p-value. The significant genes were examined for co-expression and known disease networks.

RESULTS

We found no differences between Aβ, tau and p-tau levels between ADHyp- and ADHyp+. We found TOMM40 to be associated with ADHyp- as expected but not with ADHyp+. Interestingly, SLC9A3R2 polymorphism was associated with ADHyp+, and significant gene expression changes were observed for neighboring genes.

CONCLUSION

Through this exploratory study using a novel cohort stratification design, we highlight the genetic differences in clinically similar phenotypes, indicating the utility of genetic profiling in aiding differential diagnosis of ADHyp+ and VD.

Inflammation control and improvement of cognitive function in COVID-19 infections: is there a role for kynurenine 3-monooxygenase inhibition?

The novel respiratory virus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), emerged during late 2019 and spread rapidly across the world. It is now recognised that the nervous system can be affected in COVID-19, with several studies reporting long-term cognitive problems in patients. The metabolic pathway of tryptophan degradation, known as the kynurenine pathway (KP), is significantly activated in patients with COVID-19. KP metabolites have roles in regulating both inflammatory/immune responses and neurological functions. In this review, we speculate on the effects of KP activation in patients with COVID-19, and how modulation of this pathway might impact inflammation and reduce neurological symptoms.

Pharmacophore-Based Virtual Screening of Novel Competitive Inhibitors of the Neurodegenerative Disease Target Kynurenine-3-Monooxygenase

The pathogenesis of several neurodegenerative diseases such as Alzheimer’s or Huntington’s disease has been associated with metabolic dysfunctions caused by imbalances in the brain and cerebral spinal fluid levels of neuroactive metabolites. Kynurenine monooxygenase (KMO) is considered an ideal therapeutic target for the regulation of neuroactive tryptophan metabolites. Despite significant efforts, the known KMO inhibitors lack blood–brain barrier (BBB) permeability and upon the mimicking of the substrate binding mode, are subject to produce reactive oxygen species as a side reaction. The computational drug design is further complicated by the absence of complete crystal structure information for human KMO (hKMO). In the current work, we performed virtual screening of readily available compounds using several protein–ligand complex pharmacophores. Each of the pharmacophores accounts for one of three distinct reported KMO protein-inhibitor binding conformations. As a result, six novel KMO inhibitors were discovered based on an in vitro fluorescence assay. Compounds VS1 and VS6 were predicted to be BBB permeable and avoid the hydrogen peroxide production dilemma, making them valuable, novel hit compounds for further drug property optimization and advancement in the drug design pipeline.

Ablation of kynurenine 3-monooxygenase rescues plasma inflammatory cytokine levels in the R6/2 mouse model of Huntington's disease

Kynurenine 3-monooxygenase (KMO) regulates the levels of neuroactive metabolites in the kynurenine pathway (KP), dysregulation of which is associated with Huntington's disease (HD) pathogenesis. KMO inhibition leads to increased levels of neuroprotective relative to neurotoxic metabolites, and has been found to ameliorate disease-relevant phenotypes in several HD models. Here, we crossed KMO knockout mice to R6/2 HD mice to examine the effect of KMO depletion in the brain and periphery. KP genes were dysregulated in peripheral tissues from R6/2 mice and KMO ablation normalised levels of a subset of these. KP metabolites were also assessed, and KMO depletion led to increased levels of neuroprotective kynurenic acid in brain and periphery, and dramatically reduced neurotoxic 3-hydroxykunurenine levels in striatum and cortex. Notably, the increased levels of pro-inflammatory cytokines TNFa, IL1β, IL4 and IL6 found in R6/2 plasma were normalised upon KMO deletion. Despite these improvements in KP dysregulation and peripheral inflammation, KMO ablation had no effect upon several behavioural phenotypes. Therefore, although genetic inhibition of KMO in R6/2 mice modulates several metabolic and inflammatory parameters, these do not translate to improvements in primary disease indicators-observations which will likely be relevant for other interventions targeted at peripheral inflammation in HD.

Full-length in meso structure and mechanism of rat kynurenine 3-monooxygenase inhibition

The structural mechanisms of single-pass transmembrane enzymes remain elusive. Kynurenine 3-monooxygenase (KMO) is a mitochondrial protein involved in the eukaryotic tryptophan catabolic pathway and is linked to various diseases. Here, we report the mammalian full-length structure of KMO in its membrane-embedded form, complexed with compound 3 (identified internally) and compound 4 (identified via DNA-encoded chemical library screening) at 3.0 Å resolution. Despite predictions suggesting that KMO has two transmembrane domains, we show that KMO is actually a single-pass transmembrane protein, with the other transmembrane domain lying laterally along the membrane, where it forms part of the ligand-binding pocket. Further exploration of compound 3 led to identification of the brain-penetrant compound, 5. We show that KMO is dimeric, and that mutations at the dimeric interface abolish its activity. These results will provide insight for the drug discovery of additional blood-brain-barrier molecules, and help illuminate the complex biology behind single-pass transmembrane enzymes.

N-(6-phenylpyridazin-3-yl)benzenesulfonamides as highly potent, brain-permeable, and orally active kynurenine monooxygenase inhibitors

Huntington's disease (HD) is one of the serious neurodegenerative diseases and no disease modifiers are available to date. The correction of unbalanced kynurenine pathway metabolites may be useful to treat disease progression and kynurenine monooxygenase (KMO) is considered an ideal drug target. A couple of KMO inhibitors have been reported, but their brain permeability was very poor. We found pyridazinylsulfonamide as a novel lead compound, and it was optimized to the brain-permeable and highly potent KMO inhibitor 12, which was equipotent with CHDI-340246 and superior to CHDI-340246 in terms of brain penetration. Compound 12 was effective in R6/2 mice (HD model mice), i.e. neuroprotective kynurenic acid was increased, whereas neurotoxic 3-hydroxykynurenine was suppressed. In addition, impaired cognitive function was improved. Therefore, the brain-permeable KMO inhibitor was considered to be a disease modifier for HD treatment.

A palette of fluorophores that are differentially accumulated by wild-type and mutant strains of Escherichia coli: surrogate ligands for profiling bacterial membrane transporters

Our previous work demonstrated that two commonly used fluorescent dyes that were accumulated by wild-type Escherichia coli MG1655 were differentially transported in single-gene knockout strains, and also that they might be used as surrogates in flow cytometric transporter assays. We summarize the desirable properties of such stains, and here survey 143 candidate dyes. We eventually triage them (on the basis of signal, accumulation levels and cost) to a palette of 39 commercially available and affordable fluorophores that are accumulated significantly by wild-type cells of the 'Keio' strain BW25113, as measured flow cytometrically. Cheminformatic analyses indicate both their similarities and their (much more considerable) structural differences. We describe the effects of pH and of the efflux pump inhibitor chlorpromazine on the accumulation of the dyes. Even the 'wild-type' MG1655 and BW25113 strains can differ significantly in their ability to take up such dyes. We illustrate the highly differential uptake of our dyes into strains with particular lesions in, or overexpressed levels of, three particular transporters or transporter components (yhjV, yihN and tolC). The relatively small collection of dyes described offers a rapid, inexpensive, convenient and informative approach to the assessment of microbial physiology and phenotyping of membrane transporter function.

New frontiers in flavin-dependent monooxygenases

Flavin-dependent monooxygenases catalyze a wide variety of redox reactions in important biological processes and are responsible for the synthesis of highly complex natural products. Although much has been learned about FMO chemistry in the last ~80 years of research, several aspects of the reactions catalyzed by these enzymes remain unknown. In this review, we summarize recent advancements in the flavin-dependent monooxygenase field including aspects of flavin dynamics, formation and stabilization of reactive species, and the hydroxylation mechanism. Novel catalysis of flavin-dependent N-oxidases involving consecutive oxidations of amines to generate oximes or nitrones is presented and the biological relevance of the products is discussed. In addition, the activity of some FMOs have been shown to be essential for the virulence of several human pathogens. We also discuss the biomedical relevance of FMOs in antibiotic resistance and the efforts to identify inhibitors against some members of this important and growing family enzymes.

Microbiota metabolites modulate the T helper 17 to regulatory T cell (Th17/Treg) imbalance promoting resilience to stress-induced anxiety- and depressive-like behaviors

Chronic stress disrupts immune homeostasis while gut microbiota-derived metabolites attenuate inflammation, thus promoting resilience to stress-induced immune and behavioral abnormalities. There are both peripheral and brain region-specific maladaptations of the immune response to chronic stress that produce interrelated mechanistic considerations required for the design of novel therapeutic strategies for prevention of stress-induced psychological impairment. This study shows that a combination of probiotics and polyphenol-rich prebiotics, a synbiotic, attenuates the chronic-stress induced inflammatory responses in the ileum and the prefrontal cortex promoting resilience to the consequent depressive- and anxiety-like behaviors in male mice. Pharmacokinetic studies revealed that this effect may be attributed to specific synbiotic-produced metabolites including 4-hydroxyphenylpropionic, 4-hydroxyphenylacetic acid and caffeic acid. Using a model of chronic unpredictable stress, behavioral abnormalities were associated to strong immune cell activation and recruitment in the ileum while inflammasome pathways were implicated in the prefrontal cortex and hippocampus. Chronic stress also upregulated the ratio of activated proinflammatory T helper 17 (Th17) to regulatory T cells (Treg) in the liver and ileum and it was predicted with ingenuity pathway analysis that the aryl hydrocarbon receptor (AHR) could be driving the synbiotic's effect on the ileum's inflammatory response to stress. Synbiotic treatment indiscriminately attenuated the stress-induced immune and behavioral aberrations in both the ileum and the brain while in a gut-immune co-culture model, the synbiotic-specific metabolites promoted anti-inflammatory activity through the AHR. Overall, this study characterizes a novel synbiotic treatment for chronic-stress induced behavioral impairments while defining a putative mechanism of gut-microbiota host interaction for modulating the peripheral and brain immune systems.

Advantages of brain penetrating inhibitors of kynurenine-3-monooxygenase for treatment of neurodegenerative diseases

Kynurenine-3-monooxygenase (KMO) is an important therapeutic target for several brain disorders that has been extensively studied in recent years. Potent inhibitors towards KMO have been developed and tested within different disease models, showing great therapeutic potential, especially in models of neurodegenerative disease. The inhibition of KMO reduces the production of downstream toxic kynurenine pathway metabolites and shifts the flux to the formation of the neuroprotectant kynurenic acid. However, the efficacy of KMO inhibitors in neurodegenerative disease has been limited by their poor brain permeability. Combined with virtual screening and prodrug strategies, a novel brain penetrating KMO inhibitor has been developed which dramatically decreases neurotoxic metabolites. This review highlights the importance of KMO as a drug target in neurological disease and the benefits of brain permeable inhibitors in modulating kynurenine pathway metabolites in the central nervous system.

Tryptophan Metabolism as a Pharmacological Target

L-Tryptophan is an essential amino acid required for protein synthesis. It undergoes an extensive and complex metabolism along several pathways, resulting in many bioactive molecules acting in various organs through different action mechanisms. Enzymes involved in its metabolism, metabolites themselves, or their receptors, represent potential therapeutic targets, which are the subject of dynamic research. Disruptions in L-tryptophan metabolism are reported in several neurological, metabolic, psychiatric, and intestinal disorders, paving the way to develop drugs to target it. This review will briefly describe L-tryptophan metabolism and present and discuss the most recent pharmacological developments targeting it.

Structural Similarities between Some Common Fluorophores Used in Biology, Marketed Drugs, Endogenous Metabolites, and Natural Products

It is known that at least some fluorophores can act as 'surrogate' substrates for solute carriers (SLCs) involved in pharmaceutical drug uptake, and this promiscuity is taken to reflect at least a certain structural similarity. As part of a comprehensive study seeking the 'natural' substrates of 'orphan' transporters that also serve to take up pharmaceutical drugs into cells, we have noted that many drugs bear structural similarities to natural products. A cursory inspection of common fluorophores indicates that they too are surprisingly 'drug-like', and they also enter at least some cells. Some are also known to be substrates of efflux transporters. Consequently, we sought to assess the structural similarity of common fluorophores to marketed drugs, endogenous mammalian metabolites, and natural products. We used a set of some 150 fluorophores along with standard fingerprinting methods and the Tanimoto similarity metric. Results: The great majority of fluorophores tested exhibited significant similarity (Tanimoto similarity > 0.75) to at least one drug, as judged via descriptor properties (especially their aromaticity, for identifiable reasons that we explain), by molecular fingerprints, by visual inspection, and via the "quantitative estimate of drug likeness" technique. It is concluded that this set of fluorophores does overlap with a significant part of both the drug space and natural products space. Consequently, fluorophores do indeed offer a much wider opportunity than had possibly been realised to be used as surrogate uptake molecules in the competitive or trans-stimulation assay of membrane transporter activities.

A novel role for kynurenine 3-monooxygenase in mitochondrial dynamics

The enzyme kynurenine 3-monooxygenase (KMO) operates at a critical branch-point in the kynurenine pathway (KP), the major route of tryptophan metabolism. As the KP has been implicated in the pathogenesis of several human diseases, KMO and other enzymes that control metabolic flux through the pathway are potential therapeutic targets for these disorders. While KMO is localized to the outer mitochondrial membrane in eukaryotic organisms, no mitochondrial role for KMO has been described. In this study, KMO deficient Drosophila melanogaster were investigated for mitochondrial phenotypes in vitro and in vivo. We find that a loss of function allele or RNAi knockdown of the Drosophila KMO ortholog (cinnabar) causes a range of morphological and functional alterations to mitochondria, which are independent of changes to levels of KP metabolites. Notably, cinnabar genetically interacts with the Parkinson's disease associated genes Pink1 and parkin, as well as the mitochondrial fission gene Drp1, implicating KMO in mitochondrial dynamics and mitophagy, mechanisms which govern the maintenance of a healthy mitochondrial network. Overexpression of human KMO in mammalian cells finds that KMO plays a role in the post-translational regulation of DRP1. These findings reveal a novel mitochondrial role for KMO, independent from its enzymatic role in the kynurenine pathway.

Kynurenine Pathway in Chronic Kidney Disease: What’s Old, What’s New, and What’s Next?

Impaired kidney function and increased inflammatory process occurring in the course of Chronic Kidney Disease (CKD) contribute to the development of complex amino-acid alterations. The essential amino-acid tryptophan (TRP) undergoes extensive metabolism along several pathways, resulting in the production of many biologically active compounds. The results of many studies have shown that its metabolism via the kynurenine pathway is potently increased in the course of CKD. Metabolites of this pathway exhibit differential, sometimes opposite, roles in several biological processes. Their accumulation in the course of CKD may induce oxidative cell damage which stimulates inflammatory processes. They can also modulate the activity of numerous cellular signaling pathways through activation of the aryl hydrocarbon receptor, leading to the disruption of homeostasis of various organs. As a result, they can contribute to the development of the systemic disorders accompanying the course of chronic renal failure. This review gathers and systematizes reports concerning the knowledge connecting the kynurenine pathway metabolites to systemic disorders accompanying the development of CKD.

In silico methods predict new blood-brain barrier permeable structure for the inhibition of kynurenine 3-monooxygenase

Kynurenine 3-monooxygenase (KMO) regulates the levels of bioactive substances in the kynurenine pathway of tryptophan catabolism and its activity is tied to so many diseases that finding an appropriate inhibitor for KMO has become an urgent task. This especially proved to be difficult for the central nervous system related diseases due to the requirement that the supposed inhibitor should be both blood brain barrier permeable and should not cause hydrogen peroxide as a harmful side product. In this in silico study, we present our step-wise approach, whose starting point is based on the important experimental observations. To tackle the problem, a library of 7561938 structures was obtained from Zinc15 database utilizing the tranche browser. From this library, a subset of 501777 structures was determined with the considerations of their functional groups that constrain their applicability. Then, the binding affinity ranking of this set of structures was determined via virtual screening. Starting from the structures whose affinities are the highest among this subset, the ADMET properties were checked through in silico methods and the binding properties of the selected inhibitor candidates were further investigated via molecular dynamics simulations and MM/GBSA calculations. According to the computational results of this study, ZINC_71915355 has passed all the evaluations and is a potentially BBB permeable structure that can inhibit KMO. Additionally, ZINC_19827377 was identified as a new potential KMO inhibitor which may be more suitable for peripheral administration. From the in silico study presented herein, ZINC_71915355 and ZINC_19827377 structures, which showed high binding affinity without harmful H₂O₂ production, along with the tailored properties can now serve as powerful candidates for KMO inhibition and these hits are worth of further experimental validation.

Hydrogen movements in the oxidative half-reaction of kynurenine 3-monooxygenase from Pseudomonas fluorescens reveal the mechanism of hydroxylation

Kynurenine 3-monoxygenase (KMO) catalyzes the conversion of l-kynurenine (L-Kyn) to 3-hydroxykynurenine (3-OHKyn) in the pathway for tryptophan catabolism. We have investigated the effects of pH and deuterium substitution on the oxidative half-reaction of KMO from P. fluorescens (PfKMO). The three phases observed during the oxidative half reaction are formation of the hydroperoxyflavin, hydroxylation and product release. The measured rate constants for these phases proved largely unchanging with pH, suggesting that the KMO active site is insulated from exchange with solvent during catalysis. A solvent inventory study indicated that a solvent isotope effect of 2-3 is observed for the hydroxylation phase and that two or more protons are in flight during this step. An inverse isotope effect of 0.84 ± 0.01 on the rate constant for the hydroxylation step with ring perdeutero-L-Kyn as a substrate indicates a shift from sp² to sp³ hybridization in the transition state leading to the formation of a non-aromatic intermediate. The pH dependence of transient state data collected for the substrate analog meta-nitrobenzoylalanine indicate that groups proximal to the hydroperoxyflavin are titrated in the range pH 5-8.5 and can be described by a pKa of 8.8. That higher pH values do not slow the rate of hydroxylation precludes that the pKa measured pertains to the proton of the hydroperoxflavin. Together, these observations indicate that the C4a-hydroperoxyflavin has a pKa ≫ 8.5, that a non-aromatic species is the immediate product of hydroxylation and that at least two solvent derived protons are in-flight during oxygen insertion to the substrate aromatic ring. A unifying mechanistic proposal for these observations is proposed.