Regulatory Properties of Hepatic Tryptophan Oxygenase

Characterization of the structural determinants of the ubiquitin-dependent proteasomal degradation of human hepatic tryptophan 2,3-dioxygenase

Human hepatic tryptophan 2,3-dioxygenase (hTDO) is a homotetrameric hemoprotein. It is one of the most rapidly degraded liver proteins with a half-life (t1/2) of ∼2.3 h, relative to an average t1/2 of ∼2-3 days for total liver protein. The molecular mechanism underlying the poor longevity of hTDO remains elusive. Previously, we showed that hTDO could be recognized and ubiquitinated by two E3 ubiquitin (Ub) ligases, gp78/AMFR and CHIP, and subsequently degraded via Ub-dependent proteasomal degradation pathway. Additionally, we identified 15 ubiquitination K-sites and demonstrated that Trp-binding to an exosite impeded its proteolytic degradation. Here, we further established autophagic-lysosomal degradation as an alternative back-up pathway for cellular hTDO degradation. In addition, with protein kinases A and C, we identified 13 phosphorylated Ser/Thr (pS/pT) sites. Mapping these pS/pT sites on the hTDO surface revealed their propinquity to acidic Asp/Glu (D/E) residues engendering negatively charged DEpSpT clusters vicinal to the ubiquitination K-sites over the entire protein surface. Through site-directed mutagenesis of positively charged patches of gp78, previously documented to interact with the DEpSpT clusters in other target proteins, we uncovered the likely role of the DEpSpT clusters in the molecular recognition of hTDO by gp78 and plausibly other E3 Ub-ligases. Furthermore, cycloheximide-chase analyses revealed the critical structural relevance of the disordered N- and C-termini not only in the Ub-ligase recognition, but also in the proteasome engagement. Together, the surface DEpSpT clusters and the N- and C-termini constitute an intrinsic bipartite degron for hTDO physiological turnover.

Tryptophan 2,3-dioxygenase and indoleamine 2,3-dioxygenase 1 make separate, tissue-specific contributions to basal and inflammation-induced kynurenine pathway metabolism in mice

BACKGROUND

In mammals, the majority of the essential amino acid tryptophan is degraded via the kynurenine pathway (KP). Several KP metabolites play distinct physiological roles, often linked to immune system functions, and may also be causally involved in human diseases including neurodegenerative disorders, schizophrenia and cancer. Pharmacological manipulation of the KP has therefore become an active area of drug development. To target the pathway effectively, it is important to understand how specific KP enzymes control levels of the bioactive metabolites in vivo.

METHODS

Here, we conducted a comprehensive biochemical characterization of mice with a targeted deletion of either tryptophan 2,3-dioxygenase (TDO) or indoleamine 2,3-dioxygenase (IDO), the two initial rate-limiting enzymes of the KP. These enzymes catalyze the same reaction, but differ in biochemical characteristics and expression patterns. We measured KP metabolite levels and enzyme activities and expression in several tissues in basal and immune-stimulated conditions.

RESULTS AND CONCLUSIONS

Although our study revealed several unexpected downstream effects on KP metabolism in both knockout mice, the results were essentially consistent with TDO-mediated control of basal KP metabolism and a role of IDO in phenomena involving stimulation of the immune system.

Prognostic Significance of Tryptophan Catabolism in Adult T-cell Leukemia/Lymphoma

PURPOSE

Indoleamine 2,3-dioxygenase 1 (IDO1: IDO), an enzyme catabolizing tryptophan (Trp) into the kynurenine (Kyn) pathway, is increasingly being recognized as an important microenvironmental factor suppressing antitumor immune responses. The purpose of the present study was to determine the prognostic significance of Trp catabolism in adult T-cell leukemia/lymphoma (ATL).

EXPERIMENTAL DESIGN

We quantified serum Trp and Kyn in 96 ATL patients, 38 human T-cell lymphotropic virus type-1 asymptomatic carriers (HTLV-1 ACs), and 40 healthy adult volunteer controls. The relationships between various clinical parameters including overall survival were analyzed. IDO expression was evaluated in the affected lymph nodes of ATL patients.

RESULTS

Serum Kyn concentrations and Kyn/Trp ratios were significantly higher in HTLV-1 ACs than healthy controls. Both increased significantly with progression from HTLV-1 AC to ATL. However, there were no significant differences in the serum Trp concentrations between ATL patients, HTLV-1 ACs, and controls. IDO was possibly produced by ATL and/or cells of the microenvironment. Multivariate analyses demonstrated that a high serum Kyn/Trp ratio and high Kyn level, but not a high Trp level, were significantly independent detrimental prognostic factors in ATL, as well as in that subset of patients with aggressive variant ATL.

CONCLUSIONS

Quantification of serum Kyn and Trp is useful for predicting prognosis of an individual ATL patient. Furthermore, ATL, especially in patients with a high serum Kyn/Trp ratio, is an appropriate disease for testing novel cancer immunotherapies targeting IDO.

Ligand migration in human indoleamine-2,3 dioxygenase

Human indoleamine 2,3-dioxygenase (hIDO), a monomeric heme enzyme, catalyzes the oxidative degradation of L-tryptophan (L-Trp) and other indoleamine derivatives. Its activity follows typical Michaelis-Menten behavior only for L-Trp concentrations up to 50 μM; a further increase in the concentration of L-Trp causes a decrease in the activity. This substrate inhibition of hIDO is a result of the binding of a second L-Trp molecule in an inhibitory substrate binding site of the enzyme. The molecular details of the reaction and the inhibition are not yet known. In the following, we summarize the present knowledge about this heme enzyme.

Ferryl derivatives of human indoleamine 2,3-dioxygenase

The critical role of the ferryl intermediate in catalyzing the oxygen chemistry of monooxygenases, oxidases, or peroxidases has been known for decades. In contrast, its involvement in heme-based dioxygenases, such as human indoleamine 2,3-dioxygenase (hIDO), was not recognized until recently. In this study, H(2)O(2) was used as a surrogate to generate the ferryl intermediate of hIDO. Spectroscopic data demonstrate that the ferryl species is capable of oxidizing azinobis(3-ethylbenzothiazoline-6-sulfonic acid) but not L-Trp. Kinetic studies reveal that the conversion of the ferric enzyme to the ferryl intermediate facilitates the L-Trp binding rate by >400-fold; conversely, L-Trp binding to the enzyme retards the peroxide reaction rate by ∼9-fold, because of the significant elevation of the entropic barrier. The unfavorable entropic factor for the peroxide reaction highlights the scenario that the structure of hIDO is not optimized for utilizing H(2)O(2) as a co-substrate for oxidizing L-Trp. Titration studies show that the ferryl intermediate possesses two substrate-binding sites with a K(d) of 0.3 and 440 μM and that the electronic properties of the ferryl moiety are sensitive to the occupancy of the two substrate-binding sites. The implications of the data are discussed in the context of the structural and functional relationships of the enzyme.

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

Heme is vital to our aerobic universe. Heme cellular content is finely tuned through an exquisite control of synthesis and degradation. Heme deficiency is deleterious to cells, whereas excess heme is toxic. Most of the cellular heme serves as the prosthetic moiety of functionally diverse hemoproteins, including cytochromes P450 (P450s). In the liver, P450s are its major consumers, with >50% of hepatic heme committed to their synthesis. Prosthetic heme is the sine qua non of P450 catalytic biotransformation of both endo- and xenobiotics. This well-recognized functional role notwithstanding, heme also regulates P450 protein synthesis, assembly, repair, and disposal. These less well-appreciated aspects are reviewed herein.

Spectroscopic studies of ligand and substrate binding to human indoleamine 2,3-dioxygenase

Human indoleamine 2,3-dioxygenase (hIDO) is an intracellular heme-containing enzyme, which catalyzes the initial and rate-determining step of l-tryptophan (l-Trp) metabolism via the kynurenine pathway in nonhepatic tissues. Steady-state kinetic data showed that hIDO exhibits substrate inhibition behavior, implying the existence of a second substrate binding site in the enzyme, although so far there is no direct evidence supporting it. The kinetic data also revealed that the K(m) of l-Trp (15 microM) is approximately 27-fold lower than the K(d) of l-Trp (0.4 mM) for the ligand-free ferrous enzyme, suggesting that O(2) binding proceeds l-Trp binding during the catalytic cycle. With cyanide as a structural probe, we have investigated the thermodynamic and kinetic parameters associated with ligand and substrate binding to hIDO. Equilibrium titration studies show that the cyanide adduct is capable of binding two l-Trp molecules, with K(d) values of 18 microM and 26 mM. The data offer the first direct evidence of the second substrate binding site in hIDO. Kinetic studies demonstrate that prebinding of l-Trp to the enzyme retards cyanide binding by approximately 13-fold, while prebinding of cyanide to the enzyme facilitates l-Trp binding by approximately 22-fold. The data support the view that during the active turnover of the enzyme it is kinetically more favored to bind O(2) prior to l-Trp.

Identification and characterization of novel variants of the tryptophan 2,3-dioxygenase gene: differential regulation in the mouse nervous system during development

Tryptophan 2,3-dioxygenase (TDO), an initial and rate-limiting enzyme for the kynurenine pathway of tryptophan (Trp) metabolism, is thought to play an important role in systemic Trp metabolism as well as in emotional and psychiatric status. In contrast to its predominant expression in the liver, expression of TDO in the brain is poorly understood. Here, we show that tdo mRNA is expressed in various nervous tissues, including the hippocampus, cerebellum, striatum and brainstem. During development, tdo mRNA was differentially regulated in brain tissues. Further, we identified two novel variants of the tdo gene, termed tdo variant1 and variant2. Similar tetramer formation and enzymatic activity were obtained when these forms were expressed in wheat germ and COS-7 cells, respectively. Quantitative real-time RT-PCR revealed that tdo variants were expressed in various nervous tissues, with high expression in the cerebellum and hippocampus, followed by the midbrain. tdo variant2 was the only variant expressed in the cerebellum from postnatal day 4 (P4) to P7, suggesting a unique role for this variant during early postnatal development. Our findings indicate that tdo and its novel variants may play an important role in not only the liver but also in local areas in developing and adult brain.

Cooperative binding of L-trp to human tryptophan 2,3-dioxygenase: resonance Raman spectroscopic analysis

Tryptophan 2,3-dioxygenase (TDO) is a tetrameric enzyme that catalyses the oxidative cleavage of l-tryptophan (l-Trp) to N-formylkynurenine by the addition of O(2) across the 2,3-bond of the indole ring. This reaction is the first and rate-limiting step in the kynurenine pathway in mammals. In the present study, we measured the conformational changes in the haem pocket of recombinant human TDO (rhTDO) in ferric form that are induced by l-Trp binding using both resonance Raman and optical absorption spectroscopies. The deconvolution analysis of the haem Raman bands at various concentrations of l-Trp revealed that the wild-type enzyme exhibits homotropic cooperativity in l-Trp binding, which was confirmed by a change in the optical absorption spectra. Mutation analysis showed that the Y42F mutant abolished the cooperative binding, and that the H76A mutant considerably reduced the catalytic activity. These data and the inter-subunit contacts reported in the bacterial TDO structure suggest that the Y42 of rhTDO is responsible for the cooperative binding of l-Trp by participating in the active site of the adjacent subunit.

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

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

Biochemical mechanisms leading to tryptophan 2,3-dioxygenase activation

Tryptophan 2,3-dioxygenase (TDO) is the first enzyme in the tryptophan oxidation pathway. It is a hemoprotein and its heme prosthetic group is present as a heme-ferric (heme-Fe(3+)) form that is not active. To be able to oxidize tryptophan, the heme-Fe(3+) form of the enzyme must be reduced to a heme-ferrous (heme-Fe(2+)) form and this study describes conditions that promote TDO activation. TDO is progressively activated upon mixing with tryptophan in a neutral buffer, which leads to an impression that tryptophan is responsible for TDO activation. Through extensive analysis of factors resulting in TDO activation during incubation with tryptophan, we conclude that tryptophan indirectly activates TDO through promoting the production of reactive oxygen species. This consideration is supported by the virtual elimination of the initial lag phase when either pre-incubated tryptophan solution was used as the substrate or a low concentration of superoxide or hydrogen peroxide was incorporated into the freshly tryptophan and TDO mixture. However, accumulation of these reactive oxygen species also leads to the inactivation of TDO, so that both TDO activation and inactivation proceed with the specific outcome depending greatly on the concentrations of superoxide and hydrogen peroxide. As a consequence, the rate of TDO catalysis varies depending upon the proportion of the active to inactive forms of the enzyme, which is in a dynamic relationship in the reaction mixture. These data provide some insight towards elucidating the molecular regulation of TDO in vivo.

Fertilization in the sea

Chemical communication between sperm and egg is a key factor mediating sexual reproduction. Dissolved signal molecules that cause sperm to orient and accelerate towards an egg could play pivotal roles in fertilization success,but such compounds are largely undescribed. This investigation considered the behavioral responses of red abalone (Haliotis rufescens) sperm to soluble factors released into sea water by conspecific eggs. Sperm in proximity to individual live eggs swam significantly faster and oriented towards the egg surface. Bioassay-guided fractionation was employed to isolate the chemoattractant, yielding a single pure, fully active compound after reversed-phase and size-exclusion high-performance liquid chromatography. Chemical characterization by nuclear magnetic resonance spectroscopy indicated that the free amino acid L-tryptophan was the natural sperm attractant in H. rufescens.

Eggs released L-tryptophan at concentrations that triggered both activation and chemotaxis in sperm, exhibiting significant activity at levels as low as 10-8 mol l-1. The D-isomer of tryptophan was inactive,showing that the sperm response was stereospecific. Serotonin, a potent neuromodulator and tryptophan metabolite, had no effect on sperm swim speeds or on orientation. In experimental treatments involving an elevated, uniform concentration of tryptophan (10-7 mol l-1) or the addition of tryptophanase, an enzyme that selectively digests tryptophan,sperm failed to navigate towards live eggs. A natural gradient of L-tryptophan was therefore necessary and sufficient to promote recruitment of sperm to the surface of eggs in red abalone.

Structure--function relationships of rat hepatic tryptophan 2,3-dioxygenase: identification of the putative heme-ligating histidine residues

The liver cytosolic enzyme tryptophan 2,3-dioxygenase (TDO) catalyzes the oxidation of L-tryptophan to formylkynurenine and controls the physiological flux of tryptophan into both the serotonergic and kynureninic pathways. This hemoprotein enzyme is composed of four noncovalently bound subunits of equivalent mass and contains two heme moieties per molecule. Electron paramagnetic resonance analyses have indicated that a histidyl nitrogen is involved in heme ligation [Henry et al., (1976) J. Biol. Chem. 251, 1578], but the identity of the His residue(s) is unknown. In an attempt to characterize the active site of the enzyme we have substituted each of the 12 His residues in the rat TDO subunit with Ala, to determine their relative importance in heme binding. Sequence alignment of the rat liver protein with that of known or putative TDO sequences from other organisms reveals that four of the His residues are conserved in eukaryotes, two of which are also conserved in prokaryotes. Our findings indicate that replacement of the evolutionarily conserved His 76 and 328 residues resulted in a dramatic reduction of TDO activity, whereas that of the eukaryotically conserved His70 resulted in a significant reduction relative to that of the wild-type enzyme. On the other hand, replacement of the other eukaryotically conserved His273 residue, while affecting the relative expression of the enzyme, had little effect on its specific activity. Size-exclusion analyses revealed that the His76Ala and His328Ala mutants retained little or no heme, suggesting that these may be key residues in ligating the prosthetic heme moieties. Whether these His residues are both provided by the same TDO subunit or a different TDO subunit remains to be determined.

Tryptophan metabolism and brain function: focus on kynurenine and other indole metabolites

The synthesis of NAD (or NADP) from tryptophan involves a series of enzymes and the formation of a number of intermediates which are collectively called 'kynurenines.' In the late 1970s and early 1980s, it became clear that intraventricular administration of several 'kynurenines' could cause convulsions and that one of the 'kynurenines,' quinolinic acid, was an agonist of a sub-population of NMDA receptors and caused excitotoxic neuronal death. A related metabolite, kynurenic acid, could, on the other hand, reduce excitotoxin-induced neuronal death by antagonising ionotropic glutamate receptors. Since then, modifications in quinolinic and kynurenic acid synthesis have been proposed as a pathogenetic mechanism in Huntington's chorea and epilepsy. It was subsequently shown that a robust activation of the kynurenine pathway and a large accumulation of quinolinic acid in the central nervous system occurred in several inflammatory neurological disorders. More recently, it has been shown that 3OH-kynurenine or 3OH-anthranilic acid, two other kynurenine metabolites, may cause either apoptotic or necrotic neuronal death in cultures and that inhibitors of kynurenine hydroxylase may reduce neuronal death in in vitro and in vivo models of brain ischaemia or excitotoxicity. Finally, it has been reported that indole metabolites, indirectly linked to the kynurenine pathway, are able to modify neuronal function and animal behaviour by interacting with voltage-dependent Na+ channels. Oxindole, one of these metabolites, has sedative and anticonvulsant properties and accumulates in the blood and brain when liver function is impaired. In conclusion, a number of metabolites affecting brain function originate from tryptophan metabolism. Selective inhibitors of their forming enzymes may be useful to understand their role in physiology or as therapeutic agents in pathology.

Enzymology of NAD+ synthesis

Beyond its role as an essential coenzyme in numerous oxidoreductase reactions as well as respiration, there is growing recognition that NAD+ fulfills many other vital regulatory functions both as a substrate and as an allosteric effector. This review describes the enzymes involved in pyridine nucleotide metabolism, starting with a detailed consideration of the anaerobic and aerobic pathways leading to quinolinate, a key precursor of NAD+. Conversion of quinolinate and 5'-phosphoribosyl-1'-pyrophosphate to NAD+ and diphosphate by phosphoribosyltransferase is then explored before proceeding to a discussion the molecular and kinetic properties of NMN adenylytransferase. The salient features of NAD+ synthetase as well as NAD+ kinase are likewise presented. The remainder of the review encompasses the metabolic steps devoted to (a) the salvaging of various niacin derivatives, including the roles played by NAD+ and NADH pyrophosphatases, nicotinamide deamidase, and NMN deamidase, and (b) utilization of niacins by nicotinate phosphoribosyltransferase and nicotinamide phosphoribosyltransferase.

Inhibition of hepatic tryptophan-2,3-dioxygenase: superior potency of melatonin over serotonin

In view of the biogenic amine deficiency hypothesis of depression and the contentious claim that hepatic tryptophan-2,3-dioxygenase (TDO) is the major peripheral determinant of brain tryptophan, brain serotonin (5-HT), and ultimately melatonin, the regulation of TDO by melatonin and 5-HT is investigated and discussed. It is concluded that TDO activity is regulated differentially by melatonin and 5-HT. Melatonin is a competitive inhibitor of TDO and 5-HT is predominantly an allosteric inhibitor. In the presence of 5-HT the effects of melatonin are nullified. However melatonin alone is a more potent inhibitor of TDO. Melatonin is also shown to be a reversible negative effector of TDO.

Mode of inhibition of rat liver tryptophan 2,3-dioxygenase by melatonin

Liver tryptophan 2,3-dioxygenase is a cytosolic enzyme that plays a crucial role in the regulation of circulating levels of tryptophan. Stimulation of the activity of this enzyme by heme enhances the catabolism of tryptophan, making less tryptophan available for uptake into the brain. Melatonin, the major hormone of the pineal gland, is shown to cause competitive inhibition of this enzyme (Ki = 2.70 microM). This structural analog of the substrate L-tryptophan is a negative homotropic cooperative modulator of the enzyme. The enzyme has a Km = 100 microM, and the substrate concentration required for optimum activity was found to be 2.5 mM with substrate inhibition becoming a feature at higher levels of tryptophan.

Characteristics of substrates and inhibitors in binding to rat liver L-tryptophan 2,3-dioxygenase: a Fourier transform infrared and kinetic study

Infrared spectroscopy and steady-state kinetics were applied to rat liver L-tryptophan 2,3-dioxygenase, in order to find relations between the structure and binding characteristics of its substrates and inhibitors. The binding characteristics were reflected by changes in the infrared CO stretch band(s) of an Fe(II)-CO complex of the enzyme upon addition of L-tryptophan and 12 analogs. The CO stretch band around 1961 cm-1 of the complex was not much affected by 1-methyl-D,L-tryptophan, a noncompetitive inhibitor, implying a binding at a site distant from the Fe(II)-CO vicinity. The spectral pattern was significantly changed by any of the other compounds which conserved an indole NH, indicative of its binding to the catalytic site. All substrates, which contained a complete CH(NH2)COOH group in addition to the NH, gave spectra similar to that of an L-tryptophan-bound complex. Spectral changes caused by six inhibitors, which lacked the complete CH(NH2)COOH, were different from one another and from those by the substrates. Hence, for an analog, the indole NH is indispensable to bind to the catalytic site, and the CH(NH2)COOH is important to take a correct configuration appropriate to the catalytic reaction. The reason why L- and D-isomers of 5-hydroxytryptohan are not substrates, in spite of their conservation of the required functional groups and correct binding to the catalytic site, has been ascribed to a possible distortion of the protein structure in the heme pocket due to a strong hydrogen bond from the hydroxyl group to an amino acid side chain.

L-Tryptophan action on hepatic RNA synthesis and enzyme induction

L-Tryptophan increases the activity of hepatic amino acid metabolizing enzymes, affects gluconeogenesis and displays a modulatory effect on several enzymes connected with RNA synthesis. The underlying mechanism differ in individual cases and result in both an increase of enzyme synthesis de novo and a decrease of enzyme degradation. Tryptophan displays a unique effect causing aggregation of hepatic polyribosomes connected with enhanced protein synthesis and preceded by a higher transport of poly (A) messenger RNA from the nucleus to the cytoplasm. The variety of rather specific effects mediated by tryptophan brings to mind hormonal action and the existence of specific tryptophan receptors is predicted.

Tryptophan 2,3-dioxygenase: a review of the roles of the heme and copper cofactors in catalysis

L-Tryptophan, 2,3-dioxygenase (EC 1.13.11.11) has been purified to homogenity from L-tryptophan induced Pseudomonas acidovorans (ATCC 11299b) and from L-tryptophan and cortisone induced rat liver. The enzyme from both sources is composed of four subunits and contains two g-atoms copper and two moles heme per mole tetramer. The proteins from the two sources are not identical. Three oxidation states of tryptophan oxygenase have been isolated: (1) fully oxidized, [Cu(II)]2[Ferriheme]2; (2) half reduced, [Cu(i)]2[ferriheme]2; and (3) fully reduced, [Cu(I)]2[ferroheme]2. Catalytic activity is dependent solely on the presence of Cu(I) in the enzyme, the heme may be either ferro or ferri. The presence of Cu(II) in the enzyme results in a requirement for an exogenous reductant, such as ascorbate, in order to elicit enzymic activity. Ligands, such as cyanide and carbon monoxide, can inhibit catalysis by binding to either or to both the copper and heme moieties. Metal complexing agents, such as bathocuproinesulfonate and bathophenanthrolinesulfonate, can inhibit catalysis by binding to Cu(I) resent only in catalytically active enzyme molecules. During catalysis by the fully reduced form of the enzyme, molecular oxygen binds to the heme moieties, while during catalysis by the half reduced form of the enzyme it does not, presumably binding instead to the Cu(I) moieties. Enzymes that catalyze similar reactions have been purified from other sources. Indoleamine 2,3-dioxygenase appears to be a heme protein, but its copper content is unknown. Pyrrolooxygenases appear to be completely different enzymes, although they have not yet been purified to homegeneity.