Evaluation of D-amino acid oxidase activity in rat kidney using a D-kynurenine derivative, 6-methylthio-D-kynurenine: An in vivo microdialysis study.

D-Amino acid oxidase (DAO), a D-amino acid metabolizing enzyme, is reportedly associated with the psychiatric disease schizophrenia, suggesting a role for DAO inhibitors in its treatment. We have previously reported that DAO catalyzes the conversion of nonfluorescent 6-methylthio-D-kynurenine (MeS-D-KYN) to fluorescent 5-methylthiokynurenic acid (MeS-KYNA) in vitro. The present study aimed to determine the potential of MeS-D-KYN in evaluating DAO activity in vivo using renal microdialysis technique in rats. Male Sprague-Dawley rats were subjected to linear microdialysis probe implantation in the left kidney. Continuous perfusion of MeS-D-KYN was maintained, and DAO activity in the kidney cortex was evaluated by measuring the MeS-KYNA content in the microdialysate. The microdialysate was collected every 30 min and analyzed by high-performance liquid chromatography with fluorescence detection, monitored at 450 nm with an excitation wavelength of 364 nm. A significant production of MeS-KYNA was observed during, but not before, infusion of MeS-D-KYN, indicating that this compound is not endogenous. MeS-KYNA production was suppressed by the co-infusion of DAO inhibitor, 5-chlorobenzo[d]isoxazol-3-ol (CBIO), suggesting that MeS-D-KYN was converted to MeS-KYNA by renal DAO. Moreover, oral administration of CBIO effectively suppressed DAO activity in a dose-dependent manner. DAO converted MeS-D-KYN to MeS-KYNA in vivo, suggesting the potential of this compound in evaluating DAO activity. The use of the renal microdialysis technique developed in this study facilitates the monitoring of DAO activity in live experimental animals.

Diversity-Oriented Biosynthesis Yields l-Kynurenine Derivative-Based Neurological Drug Candidate Collection.

Natural product libraries with a remarkable range of biological activities play pivotal roles in drug discoveries due to their extraordinary structural complexity and immense diversity. l-Kynurenine (l-Kyn)-based derivatives are privileged pharmacophores that exhibit diverse therapeutic implications in neurological disorders. However, the difficulty in obtaining l-Kyn analogues with different skeletal structures has recently led to a decline in its medicinal research. Herein, we report a two-step, one-pot protocol for diversity-oriented biosynthesis of a collection of previously intractable l-Kyn-like compounds. The success of these challenging transformations mainly depends on unlocking the new catalytic scope of tryptophan 2,3-dioxygenases, followed by rational site-directed mutagenesis to modify the substrate domains further. As a result, 18 kynurenine analogues with diverse molecular scaffolds can be rapidly assembled in a predictable manner with 20-83% isolated yields, which not only fill the voids of the catalytic profile of tryptophan 2,3-dioxygenases with an array of substituent groups (e.g., F, Cl, Br, I, CH3, OCH3, and NO2) but also update the current understanding of its substrate spectrum. Our work highlights the great potential of existing enzymes in addressing long-standing synthetic challenges for facilitating the development or discovery of new drug candidates. Furthermore, our approach enables translating the reaction parameters from Eppendorf tubes to 1 L scale, affording l-4-Cl-Kyn and l-5-Cl-Kyn both on a gram scale with more than 80% isolated yields, and provides a promising alternative to further industrial applications.

Tryptophan catabolites and depression in the general population: results from the Gutenberg Health Study.

Previous studies reported significantly altered tryptophan catabolite concentrations in major depression. Thus, tryptophan catabolites were considered as potential biomarkers of depression and their modulators as potential targets for psychopharmacotherapy. However, the results were based mainly on studies with small sample sizes limiting their generalizability. Against this background, we investigated the relationship of peripheral tryptophan catabolites with depression in a population-based sample with n = 3,389 participants (with fasting status ≥ 8 h and C-reactive protein < 10 mg/L). N = 248 had clinically significant depression according to a PHQ-9 score of ≥ 10, n = 1,101 subjects had mild depressive symptoms with PHQ-9 scores between 5 and 9, and n = 2,040 had no depression. After multivariable adjustment, clinically significant depression was associated with lower kynurenine and kynurenic acid. Spearman correlation coefficients of the tryptophan catabolites with the severity of depression were very small (rho ≤ 0.080, p ≤ 0.015). None of the tryptophan catabolites could diagnostically separate depressed from not depressed persons. Concerning linear associations, kynurenine and kynurenic acid were associated only with the severity and the cognitive dimension of depression but not its somatic dimension. Tryptophan catabolites were not associated with persistence or recurrence of depression at the 5 year follow-up. The results replicated the association between kynurenine and kynurenic acid with depression. However, the associations were small raising doubts about their clinical utility. Findings underline the complexity of the relationships between depression and tryptophan catabolites. The search for subgroups of depression with a potentially higher impact of depression might be warranted.

Simultaneous quantification of tryptophan metabolites by liquid chromatography tandem mass spectrometry during early human pregnancy.

OBJECTIVES: In this study we describe the development and validation of a liquid chromatography mass spectrometry method (LC-MS/MS) to quantify five tryptophan (TRP) metabolites within the kynurenine- and serotonin pathway and apply the method to serum samples of women in the first trimester of pregnancy. A secondary aim was to investigate the correlation between body mass index (BMI) and the five analytes. METHODS: A LC-MS/MS was developed for the analysis of TRP, kynurenine (KYN), 5-hydroxytryptophan (5-HTP), hydroxytryptamine (5-HT), and 5-hydroxyindole acetic acid (5-HIAA). Serum samples (n=374) were analyzed of pregnant women (median gestational age: 8 ± 2 weeks) participating in a subcohort of the Rotterdam Periconceptional Cohort (Predict study). RESULTS: The LC-MS/MS method provided satisfactory separation of the five analytes (7 min run). For all analytes R2 was >0.995. Within- and between-run accuracies were 72-97% and 79-104%, and the precisions were all <15% except for the between-run precisions of the low QC-samples of 5-HTP and 5-HT (both 16%). Analyte concentrations were determined in serum samples of pregnant women (median (IQR)); TRP (µmol/L): 57.5 (13.4), KYN (µmol/L): 1.4 (0.4), 5-HTP (nmol/L): 4.1 (1.2), 5-HT (nmol/L): 615 (323.1), and 5-HIAA (nmol/L): 39.9 (17.0). BMI was negatively correlated with TRP, 5-HTP, and 5-HIAA (TRP: r=-0.18, p<0.001; 5-HTP: r=-0.13, p=0.02; natural log of 5-HIAA: r=-0.11, p=0.04), and positively with KYN (r=0.11, p=0.04). CONCLUSIONS: The LC-MS/MS method is able to accurately quantify kynurenine- and serotonin pathway metabolites in pregnant women, providing an opportunity to investigate the role of the TRP metabolism in the (patho)physiology of pregnancy.

The Kynurenine Pathway and Kynurenine 3-Monooxygenase Inhibitors.

Under normal physiological conditions, the kynurenine pathway (KP) plays a critical role in generating cellular energy and catabolizing tryptophan. Under inflammatory conditions, however, there is an upregulation of the KP enzymes, particularly kynurenine 3-monooxygenase (KMO). KMO has garnered much attention due to its production of toxic metabolites that have been implicated in many diseases and disorders. With many of these illnesses having an inadequate or modest treatment, there exists a need to develop KMO inhibitors that reduce the production of these toxic metabolites. Though prior efforts to find an appropriate KMO inhibitor were unpromising, the development of a KMO crystal structure has provided the opportunity for a rational structure-based design in the development of inhibitors. Therefore, the purpose of this review is to describe the kynurenine pathway, the kynurenine 3-monooxygenase enzyme, and KMO inhibitors and their potential candidacy for clinical use.

The interplay between aryl hydrocarbon receptor, H. pylori, tryptophan, and arginine in the pathogenesis of gastric cancer.

Several risk factors are known to be involved in the initiation and development of gastric cancer. Among them, H. pylori is one of the most prominent with multiple virulence factors contributing to its pathogenicity. In this study, we have discussed an interesting immunological cycle exploring the interplay between H. pylori, aryl hydrocarbon receptor (AHR), tryptophan, arginine, and the metabolites of these two amino acids in the development of gastric cancer. AHR is a ligand-activated transcription factor which acts as a regulator for a diverse set of genes and has various types of exogenous and endogenous ligands. The tryptophan metabolite, kynurenine, is one of these ligands that can interact with AHR, leading to immune suppression and subsequently, susceptibility to gastric cancer. On the other hand, H. pylori downregulates the expression of AHR and AHR repressor (AHRR), leading to increased inflammatory cytokine production. A metabolite of the kynurenine pathway, xanthurenic acid, is a potent inhibitor of a terminal enzyme in the synthetic pathway of tetrahydrobiopterin (BH4). BH4, itself, is a cofactor in the process of nitric oxide (NO) production from arginine that has been shown to have immune-enhancing properties. Arginine has also been evidenced to have anti-tumoral function through inducing apoptosis in gastric cell lines; however, controversy exists regarding the anti-tumor role of arginine and BH4, since they are also associated with increased NO production, subsequently promoting tumor angiogenesis. Hence, although several synergistic connections result in immunity improvement, these correlations can also act as a double-edged sword, promoting tumor development. This emphasizes on the need for further investigations to better understand this complex interplay.

Differential Distribution of Tryptophan-Metabolites in Fetal and Maternal Circulations During Normotensive and Preeclamptic Pregnancies.

Preeclampsia (PE) is a hypertensive pregnancy, which is a leading cause of maternal and fetal morbidity and mortality during pregnancy. L-Tryptophan (Trp) is an essential amino acid, which can be metabolized into various biologically active metabolites. However, the levels of many circulating Trp-metabolites in human normotensive pregnancies (NT) and PE are undetermined. This study quantified the levels of Trp-metabolites in maternal and umbilical vein sera from women with NT and PE. Paired maternal and umbilical blood samples were collected from singleton pregnant patients. Twenty-five Trp-metabolites were measured in serum samples using liquid chromatography with tandem mass spectrometry. The effects of L-kynurenine (Kyn) and indole-3-lactic acid (ILA), on function of human umbilical vein endothelial cells (HUVECs), were also determined. Twenty Trp-metabolites were detected. The levels of 9 Trp-metabolites including Kyn and ILA were higher (P < 0.05) in umbilical vein than maternal serum, whereas 2 (5-hydroxy-L-tryptophan and serotonin) were lower (P < 0.05) in umbilical vein compared to maternal serum. PE significantly (P < 0.05) elevated ILA levels in maternal and umbilical vein sera. Kyn dose-dependently decreased (P < 0.05) cell viability. Kyn and ILA dose- and time-dependently (P < 0.05) increased monolayer integrity in HUVECs. These data suggest that these Trp-metabolites are important in regulating endothelial function during pregnancy, and the elevated ILA in PE may antagonize increased endothelial permeability occurring in PE.

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.

Targeting Indoleamine 2,3-Dioxygenase 1: Fighting Cancers via Dormancy Regulation.

The connection between indoleamine 2,3-dioxygenase 1 (IDO1) and tumour dormancy - a quiescent state of tumour cells which has been consistently linked to metastasis and cancer recurrence - is rarely discussed despite the pivotal role of IDO1 in cancer development and progression. Whilst the underlying mechanisms of IDO1-mediated dormancy are elusive, we summarize the IDO1 pathways which potentially contribute to dormancy in this review. Critically, distinct IDO1 activities are involved in dormancy initiation and maintenance; factors outside the well-studied IDO1/kynurenine/aryl hydrocarbon receptor axis, including the mammalian target of rapamycin and general control nonderepressible 2, appear to be implicated in dormancy. We also discuss various strategies for cancer treatment via regulating IDO1-dependent dormancy and suggest the application of nanotechnology to deliver effective treatment.

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.

Structural Basis of Selective Human Indoleamine-2,3-dioxygenase 1 (hIDO1) Inhibition.

hIDO1 is a heme-dioxygenase overexpressed in the tumor microenvironment and is implicated in the survival of cancer cells. Metabolism of tryptophan to N-formyl-kynurenine by hIDO1 leads to immune suppression to result in cancer cell immune escape. In this article, we discuss the discovery of selective hIDO1 inhibitors for therapeutic intervention that have been promoted to clinical trials and for which crystallographic structural information is available for the respective inhibitor-enzyme complex. The structural insights are based on the complex crystal structures and the relative biological data profiles. The structural basis of selective hIDO1 inhibition, as discussed herein, opens new avenues to the discovery of novel inhibitors with improved activity profiles, selectivity, and distinct structure frameworks.

Lanosterol reduces the aggregation propensity of ultraviolet-damaged human γD-crystallins: a molecular dynamics study.

Ultraviolet (UV) radiation-induced oxidation of tryptophan (Trp) to kynurenine (KN) (TRP > KN) in human γD-crystallins (HγD-Crys) promotes the conversion of proteins into partially unfolded species that act as important precursors for sequential large-scale aggregation. Herein, we report that lanosterol shows protective activity to the structure of the TRP > KN mutant HγD-Crys, particularly its N-terminal domain (N-td), by using all-atom molecular dynamics simulations. The Trp68 > KN mutation significantly destabilizes the originally highly stable "Tyr55-Trp68-Tyr62" cluster, thereby causing loop2, where the mutation occurs, to become very flexible. The large fluctuation of loop2 induces cracks, which appear on the protein surface, resulting in the intrusion of water molecules into the hydrophobic core of the N-td. This event eventually triggers the unfolding of the N-td. However, lanosterol can suppress the large fluctuation of loop2 to protect the structural stability of the mutant N-td, thus reducing the aggregation propensity of the TRP > KN mutant HγD-Crys. This structure protective activity of lanosterol arises from its capability to preferentially bind to the hydrophobic regions near loop2. Thus, lanosterol acts as a "water blocker" to prevent the invasion of solvent molecules into the hydrophobic core. These findings provide some valuable insights into the development of potential lanosterol-based drugs for cataract prevention and treatment.

Copper binding by a unique family of metalloproteins is dependent on kynurenine formation.

Some methane-oxidizing bacteria use the ribosomally synthesized, posttranslationally modified natural product methanobactin (Mbn) to acquire copper for their primary metabolic enzyme, particulate methane monooxygenase. The operons encoding the machinery to biosynthesize and transport Mbns typically include genes for two proteins, MbnH and MbnP, which are also found as a pair in other genomic contexts related to copper homeostasis. While the MbnH protein, a member of the bacterial diheme cytochrome c peroxidase (bCcP)/MauG superfamily, has been characterized, the structure and function of MbnP, the relationship between the two proteins, and their role in copper homeostasis remain unclear. Biochemical characterization of MbnP from the methanotroph Methylosinus trichosporium OB3b now reveals that MbnP binds a single copper ion, present in the +1 oxidation state, with high affinity. Copper binding to MbnP in vivo is dependent on oxidation of the first tryptophan in a conserved WxW motif to a kynurenine, a transformation that occurs through an interaction of MbnH with MbnP. The 2.04-Å-resolution crystal structure of MbnP reveals a unique fold and an unusual copper-binding site involving a histidine, a methionine, a solvent ligand, and the kynurenine. Although the kynurenine residue may not serve as a CuI primary-sphere ligand, being positioned ∼2.9 Å away from the CuI ion, its presence is required for copper binding. Genomic neighborhood analysis indicates that MbnP proteins, and by extension kynurenine-containing copper sites, are widespread and may play diverse roles in microbial copper homeostasis.

Methylene blue and ascorbate interfere with the accurate determination of the kinetic properties of IDO2.

Indoleamine 2,3-dioxygenases (IDOs) catalyze the oxidative cleavage of L-tryptophan (Trp) to N-formylkynurenine. Two IDOs, IDO1 and IDO2, are present in vertebrates. IDO1 is a high-affinity Trp-degrading enzyme involved in several physiological processes. By comparison, IDO2 generally has been reported to have low affinity (high Km -value) for Trp, and the enzyme's in vivo function remains unclear. Using IDOs from different species, we show that compared with ferrous-oxy (Fe2+ -O2 ) IDO1, Fe2+ -O2 IDO2 is substantially more stable and engages in multiple turnovers of the reaction in the absence of a reductant. Without reductant, Fe2+ -O2 IDO2 showed Km -values in the range of 80-356 µM, that is, values substantially lower than reported previously and close to the physiological concentrations of Trp. Methylene blue and ascorbate (Asc), used commonly as the reducing system for IDO activity determination, significantly affected the enzymatic activity of IDO2: In combination, the two reductants increased the apparent Km - and kcat -values 8- to 117-fold and 2-fold, respectively. Asc alone both activated and inhibited IDO2 by acting as a source of electrons and as a weak competitive inhibitor, respectively. In addition, ferric (Fe3+ ) IDO1 and IDO2 exhibited weak dioxygenase activity, similar to tryptophan 2,3-dioxygenase. Our results shed new light in the enzymatic activity of IDO2, and they support the view that this isoform of IDO also participates in the metabolism of Trp in vivo.

A high-yielding solid-phase total synthesis of daptomycin using a Fmoc SPPS stable kynurenine synthon.

A high-yielding total synthesis of daptomycin, an important clinical antibiotic, is described. Key to the development of this synthesis was the elucidation of a Camps cyclization reaction that occurs in the solid-phase when conventionally used kynurenine (Kyn) synthons, such as Fmoc-l-Kyn(Boc,CHO)-OH and Fmoc-l-Kyn(CHO,CHO)-OH, are exposed to 20% 2-methylpiperidine (2MP)/DMF. During the synthesis of daptomycin, this side reaction was accompanied by intractable peptide decomposition, which resulted in a low yield of Dap and a 4-quinolone containing peptide. The Camps cyclization was found to occur in solution when Boc-l-Kyn(Boc,CHO)-Ot-Bu and Boc-l-Kyn(CHO,CHO)-OMe were exposed to 20% 2MP/DMF giving the corresponding 4-quinolone amino acid. In contrast, Boc-l-Kyn(CHO)-OMe was stable under these conditions, demonstrating that removing one of the electron withdrawing groups from the aforementioned building blocks prevents enolization in 2MP/DMF. Hence, a new synthesis of daptomycin was developed using Fmoc-l-Kyn(Boc)-OH, which is prepared in two steps from Fmoc-l-Trp(Boc)-OH, that proceeded with an unprecedented 22% overall yield. The simplicity and efficiency of this synthesis will facilitate the preparation of analogs of daptomycin. In addition, the elucidation of this side reaction will simplify preparation of other Kyn-containing natural products via Fmoc SPPS.

HPLC Gradient Retention of Tryptophan and its Metabolites on Three Stationary Phases in Context of Lipophilicity Assessment.

This paper is a continuation of lipophilicity research on 14 compounds (tryptophan, kynurenine pathway products, auxin pathway products, serotonin pathway products, tryptamine, as well as two synthetic auxin analogs): indole-2-acetic acid sodium salt (IAA), serotonin, 5-hydroxy-L-tryptophan, tryptamine, L-tryptophan, L-kynurenine (KYN), kynurenic acid (KYA), 3-hydroxy-DL-kynurenine, naphtyl-1-acetamide, indole-3-propionic acid (IPA), naphthalene-1-acetic acid (NAA), indole-3-butyric acid (IBA), indole-3-pyruvic acid (IPV), as well as melatonin. They were chromatographed in high performance liquid chromatography gradient conditions on tree stationary phases (C18, CN, DIOL) using three modifiers on each phase (methanol, acetonitrile and acetone). The resulting retention data was correlated with computational lipophilicity indices. Six compounds were proven to be ionized in neutral pH physiological conditions (IAA, KYA, IPA, NAA, IBA and IPV) and they were rechromatographed with acidic mobile phase to enhance the resulting dataset. It can be concluded that the retention times are highly correlated with lipophilicity regardless of used modifier and column and the main differentiating trend can be only connected to presence of naphthalene or indole ring. The principal component analysis, additive linear modeling, as well as multiplicative trilinear parallel factor analysis (PARAFAC) modeling helped to understand the internal structure of the obtained results.

Inhibitory mechanism of sinensetin on α-glucosidase and non-enzymatic glycation: Insights from spectroscopy and molecular docking analyses.

Inhibition of α-glucosidase and non-enzymatic glycation is regarded as an effective method to prevent and treat type 2 diabetes and its complications. In this study, the inhibition of sinensetin on α-glucosidase and non-enzymatic glycation was studied with multi-spectroscopic techniques and molecular docking analysis. The results of fluorescence spectroscopy analysis indicated that sinensetin quenched the endogenous fluorescence of α-glucosidase in static manner. The binding of sinensetin with α-glucosidase was a spontaneous process primarily driven by hydrophobic interaction. At 298 K, the binding constant was (5.70 ± 0.12) × 104 L·mol-1 and the binding site number was 1. The conformation of α-glucosidase was altered by sinensetin, which was revealed by circular dichroism (CD), FTIR spectra, synchronous fluorescence and three-dimensional (3D) fluorescence spectroscopy methods. Molecular docking analysis demonstrated that sinensetin interacted with the amino acid residues of α-glucosidase, which might prevent the entrance of substrate, leading to the decrease of catalytic efficiency of α-glucosidase. Furthermore, glycation assays showed that sinensetin stabilized the structure of bovine serum albumins (BSA), interacted with BSA, strongly inhibited the formation of dityrosine, N'-formylkynurenine and advanced glycation end products (AGEs). This study provided useful information concerning sinensetin preventing and treating type 2 diabetes and its related complications.

The Uniqueness of Tryptophan in Biology: Properties, Metabolism, Interactions and Localization in Proteins.

Tryptophan (Trp) holds a unique place in biology for a multitude of reasons. It is the largest of all twenty amino acids in the translational toolbox. Its side chain is indole, which is aromatic with a binuclear ring structure, whereas those of Phe, Tyr, and His are single-ring aromatics. In part due to these elaborate structural features, the biosynthetic pathway of Trp is the most complex and the most energy-consuming among all amino acids. Essential in the animal diet, Trp is also the least abundant amino acid in the cell, and one of the rarest in the proteome. In most eukaryotes, Trp is the only amino acid besides Met, which is coded for by a single codon, namely UGG. Due to the large and hydrophobic π-electron surface area, its aromatic side chain interacts with multiple other side chains in the protein, befitting its strategic locations in the protein structure. Finally, several Trp derivatives, namely tryptophylquinone, oxitriptan, serotonin, melatonin, and tryptophol, have specialized functions. Overall, Trp is a scarce and precious amino acid in the cell, such that nature uses it parsimoniously, for multiple but selective functions. Here, the various aspects of the uniqueness of Trp are presented in molecular terms.

Unconventional [2 + 3] Cyclization Involving [1,4]-Sulfonyl Transfer to Construct Polysubstituted Fluorazones as Inhibitors of Indoleamine 2,3-Dioxygenase 1.

An unconventional [2 + 3] cyclization of pyridinium ylides with 2-ylideneoxindoles has been developed for the facile construction of pharmacologically interesting polysubstituted 9H-pyrrolo[1,2-a]indol-9-ones (fluorazones). Mechanistic studies revealed that the reaction, which has a broad substrate scope, proceeds via intermolecular [1,4]-sulfonyl transfer. Moreover, biological evaluation showed that polysubstituted fluorazone 3ak potently inhibits indoleamine 2,3-dioxygenase 1, kynurenine production, and immunotolerance in tumors.

Phakefustatins A-C: Kynurenine-Bearing Cycloheptapeptides as RXRα Modulators from the Marine Sponge Phakellia fusca.

A neutral-loss scanning mass method was used to explore new kynurenine-containing cycloheptapeptides, phakefustatins A-C (1-3), from the marine sponge Phakellia fusca. Their structures were elucidated by spectroscopic analysis and the advanced Marfey's method. 1 was total synthesized via a final-stage ozonolysis strategy by the combination of solid/solution-phase synthesis. Phakefustatin A (1) was identified as a RXRα modulator to inhibit cancer cell growth, and its pharmacophores could be Kyn and guanidine groups.