Tryptophan Oxidation Research Articles

Structural Characterization of New Microcystins Containing Tryptophan and Oxidized Tryptophan Residues

Microcystins are cyclic peptides produced by cyanobacteria, which can be harmful to humans and animals when ingested. Eight of the (more than) 90 microcystin variants presently characterized, contain the amino acid tryptophan. The well-researched oxidation products of tryptophan; kynurenine, oxindolylalanine, and N-formylkynurenine, have been previously identified in intact polypeptides but microcystin congeners containing oxidized tryptophan moieties have not been reported. Liquid chromatography-tandem mass spectrometric analysis of an extract of Microcystis CAWBG11 led to the tentative identification of two new tryptophan-containing microcystins (MC‑WAba and MC-WL), as well as eight other microcystin analogs containing kynurenine, oxindolylalanine and N‑formylkynurenine (Nfk). Investigation of one of these congeners (MC‑NfkA) by nuclear magnetic resonance spectroscopy was used to verify the presence of Nfk in the microcystin. Liquid chromatography-mass spectrometry analysis of a tryptophan oxidation experiment demonstrated that tryptophan-containing microcystins could be converted into oxidized tryptophan analogs and that low levels of oxidized tryptophan congeners were present intracellularly in CAWBG11. MC-NfkR and MC-LNfk were detected in standards of MC-WR and MC-LW, indicating that care during storage of tryptophan-containing microcystins is required.

Analysis of Redox Activity of Proteins on the Carbon Screen Printed Electrodes

AbstractDirect redox activity of different proteins was investigated on the surface of carbon screen printed electrodes (SPE). The signal attributed to the electrochemical oxidation of amino acid residues (cysteine (Cys), tryptophan (Trp) and tyrosine (Tyr)) was registered at Emax from 0.6 to 0.7 V (vs. Ag/AgCl). Based on the difference in the redox behavior of L‐tyrosine and 3‐nitro‐L‐tyrosine, the selective electrochemical detection of native and nitrated albumins was demonstrated. It was shown that the electrochemical signal correlated with the surface density of electroactive amino acid residues on the protein molecule. A simple electrochemical method for the total protein analysis was proposed.

Inhibiting the photosensitized oxidation of anthracene and tryptophan by means of natural antioxidants

It is shown that model reactions of photosensitized oxidation of anthracene and tryptophan can be used for evaluation and comparison of antioxidant activity of various classes of compounds. Inhibition of the oxidation of substrates in the presence of the familiar antioxidants tocopherol (vitamin E), ascorbic acid (vitamin C), and mixtures of these vitamins with methionine, and in the presence of reputed antioxidants dihydroquercetin and taurine, are considered. It is concluded that all of the above compounds except for taurine have antioxidant properties; i.e., they reduce the rate constants of the photosensitized oxidation of anthracene and tryptophan. It is found that the inhibition of oxidation is associated with the interaction between antioxidants and singlet oxygen. Analysis of the kinetic dependences of the photosensitized oxidation of substrates in the presence of antioxidants reveals that a mixture of vitamins inhibits the process most efficiently, and inhibition occurs at the initial stages due to more active interaction between singlet oxygen and vitamin C

Comprehensive Analysis of Nonenzymatic Post-Translational β-Lactoglobulin Modifications in Processed Milk by Ultrahigh-Performance Liquid Chromatography–Tandem Mass Spectrometry

Nonenzymatic post-translational protein modifications (nePTMs) result in changes of the protein structure that may severely influence physiological and technological protein functions. In the present study, ultrahigh-performance liquid chromatography-electrospray ionization tandem mass spectrometry (UHPLC-ESI-MS/MS) was applied for the systematic identification and site-specific analysis of nePTMs of β-lactoglobulin in processed milk. For this purpose, β-lactoglobulin, which had been heated with lactose under conditions to force nePTM formation (7 d/60 °C), was screened for predicted modifications by using full scans and enhanced resolution scan experiments combined with enhanced product ion scans. Thus, the main glycation, glycoxidation, oxidation, and deamidation products of lysine, arginine, methionine, cysteine, tryptophan, and asparagine, as well as the N-terminus, were identified. Using these MS data, a very sensitive scheduled multiple reaction monitoring method suitable for the analysis of milk products was developed. Consequently, 14 different PTM structures on 25 binding sites of β-lactoglobulin were detected in different milk products.

Tryptophan oxidation photosensitized by pterin

Pterins are normal components of cells and they have been previously identified as good photosensitizers under UV-A irradiation, inducing DNA damage and oxidation of nucleotides. In this work, we have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the oxidation of another class of biomolecules, amino acids, using tryptophan (Trp) as a model compound. Irradiation of Ptr in the UV-A spectral range (350nm) in aerated aqueous solutions containing Trp led to the consumption of the latter, whereas the Ptr concentration remained unchanged. Concomitantly, hydrogen peroxide (H2O2) was produced. Although Ptr is a singlet oxygen (1O2) sensitizer, the degradation of Trp was inhibited in O2-saturated solutions, indicating that a 1O2-mediated process (type II oxidation) was not an important pathway leading to Trp oxidation. By combining different analytical techniques, we could establish that a type I photooxidation was the prevailing mechanism, initiated by an electron transfer from the Trp molecule to the Ptr triplet excited state, yielding the corresponding radical ions (Trp+/Trp(-H) and Ptr−). The Trp reaction products that could be identified by UPLC-mass spectrometry are in agreement with this conclusion.

Investigation of the phototoxicity and cytotoxicity of naproxen, a non-steroidal anti-inflammatory drug, in human fibroblasts

Non-steroidal anti-inflammatory drugs (NSAID) are widely used in the treatment of pain and inflammation associated with several diseases. Naproxen, 2-(6-methoxy-2-naphthyl) propionic acid (NAP), belongs to this pharmacological class and appears to be associated with a high incidence of both photoallergic and phototoxic reactions. In this study, using human fibroblasts, we examined the biological effects of NAP photosensitization induced by UVA, the predominant UV component of sunlight reaching the Earth's surface. We showed that NAP or UVA alone have no cytotoxic effects at the concentrations and doses used in this study. The same result was observed when cells were pre-incubated with NAP but irradiated without NAP. In marked contrast, exposure of cells in the presence of NAP led to a drastic reduction of cell viability. These results suggest that the phototoxicity is mainly due to irradiation of extracellular NAP that damages cell membranes. Moreover, we showed that NAP itself led to a low but reproducible production of reactive oxygen species (ROS), to protein modifications by lipid peroxidation-derived aldehydes, to p38 phosphorylation and to the slowing-down of DNA replication, while UVA treatment alone showed no effects. NAP photosensitization with UVA led to protein S-glutathionylation, oxidation of the proliferating cell nuclear antigen (PCNA), oxidation of cellular tryptophan, phosphorylation of Chk1 and inhibition of DNA replication. However, using small interfering RNA to down regulate Chk1 expression in cells, we showed that Chk1 is not required to slow the S-phase down. Nevertheless, inhibition of Chk1, but not of p38, sensitized the cells to the phototoxic effects of NAP. Collectively, our data suggest that the interaction of NAP with the cells triggers oxidative damage and a replication stress, which are exacerbated by UVA radiation. As oxidative and replication stress-induced genome instability are important factors in aging and tumor predisposition, it is of interest to evaluate the consequence of a non-steroidal anti-inflammatory drug, like naproxen, on genomic instability.

Synthesis of ruthenium xanthate complex and its electrocatalytic activity for tryptophan oxidation

A new ruthenium complex containing bidentate xanthate ligands was synthesized in a good yield. This complex was characterized by elemental analysis, proton nuclear magnetic resonance(1H NMR), Fourier transform infrared(FTIR) and UV-Vis spectroscopies. The cyclic voltammetry of the complex revealed one quasi-redox wave centered at Ru(III)/ Ru(II) couple, indicating its catalytic potential. So the preparation of a glass carbon electrode modified with ruthenium xanthate complex and its electrocatalytic activity toward the oxidation of tryptophan(Trp) were also studied. The experimental results show that the modified electrode had excellent electrocatalytic activity for the oxidation of tryptophan. Moreover, under the optimized conditions, the oxidation peak current was proportional to tryptophan concentration in a range of 2.5×10−7 to 5.0×10−5 mol/L with a correlation coefficient of 0.9928 and a detection limit of 8.3×10−8 mol/L(S/N=3). Using the proposed method, tryptophan was successfully determined in pharmaceutical samples with standard addition method.

Kinetics and Mechanism of Oxidation of Tryptophan by Ferrate(VI)

Kinetics of the oxidation of tryptophan (Trp) and kynurenine (Kyn), precursors of nitrogenous disinfection byproducts (N-DBP), by ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) were investigated over the acidic to basic pH range. The second-order rate constants decreased with increase in pH, which could be described by the speciation of Fe(VI) and Trp (or Kyn). The trend of pH dependence of rates for Trp (i.e., aromatic α-amino acid) differs from that for glycine (i.e., aliphatic α-amino acid). A nonlinear relationship between transformation of Trp and the added amount of Fe(VI) was found. This suggests that the formed intermediate oxidized products (OPs), identified by LC-PDA and LC-MS techniques, could possibly compete with Trp to react with Fe(VI). N-Formylkynurenine (NFK) at pH 7.0 and 4-hydroxyquinoline (4-OH Q) and kynurenic acid (Kyn-A) at pH 9.0 were the major OPs. Tryptophan radical formation during the reaction was confirmed by the rapid-freeze quench EPR experiments. The oxygen atom transfer from Fe(VI) to NFK was demonstrated by reacting Fe(18)O4(2-) ion with Trp. A proposed mechanism explains the identified OPs at both neutral and alkaline pH. Kinetics and OPs by Fe(VI) were compared with other oxidants (chlorine, ClO2(•), O3, and (•)OH).

Selective tryptophan determination using tryptophan oxidases involved in bis-indole antibiotic biosynthesis

A novel tryptophan assay was developed using tryptophan oxidases. Although many l-amino acid oxidases (LAAOs) have been reported to catalyze tryptophan oxidation, most of them have broad substrate specificity and oxidize multiple amino acids besides tryptophan. To obtain a tryptophan-specific LAAO, we focused on bis-indole antibiotic biosynthesis, a bacterial secondary metabolic pathway. A putative LAAO from Streptomyces sp. TP-A0274, StaO involved in staurosporine biosynthesis, was heterologously expressed, biochemically characterized, and shown to serve as a selective tryptophan oxidase for the first time. In addition, another LAAO, VioA involved in violacein biosynthesis in Chromobacterium violaceum, was characterized for comparison with StaO. Interestingly, StaO and VioA share similar properties, namely narrow substrate specificity and high affinity for l-tryptophan, despite the phylogenetic distance between these enzymes. Owing to these features, uncommon among known LAAOs, StaO and VioA assays can be used for selective and accurate quantification of l-tryptophan via a coupled colorimetric reaction. Indeed, StaO and VioA assays provided tryptophan concentrations in human plasma as accurately as those obtained by high-performance liquid chromatography. Therefore, these enzymes were clearly shown to offer an effective method for determining tryptophan in biological samples rapidly, inexpensively, and accurately. The results shown here also suggest the possibility of metabolism-oriented screening as a strategy to obtain enzymes highly selective for individual biomolecules.

Direct Electrochemistry of Native and Denatured Anticancer Antibody Rituximab at a Glassy Carbon Electrode

AbstractRituximab (RTX) is a human/murine chimeric monoclonal antibody (mAb) that specifically targets the transmembrane protein CD20 of B‐cells. The oxidation mechanism of native and denatured RTX was investigated on glassy carbon electrode. The denaturing agent sodium dodecyl sulfate and the redutancts tris(2‐carboxyethyl)phosphine and dithiothreitol were used. Significant differences were observed for native and denatured RTX oxidation due to morphological changes and unfolding of the RTX native structure. Native RTX presented only one oxidation peak of tyrosine and tryptophan residues, whereas in denatured RTX were detected three peaks corresponding to the oxidation of tyrosine, tryptophan and histidine residues.

Hypericin-mediated photooxidative damage of α-crystallin in human lens epithelial cells

St. John’s wort (Hypericum perforatum), a perennial herb native to Europe, is widely used for and seems to be effective in treatment of mild to moderate depression. Hypericin, a singlet oxygen-generating photosensitizer that absorbs in both the visible and the UVA range, is considered to be one of the bioactive ingredients of St. John’s wort, and commercial preparations are frequently calibrated to contain a standard concentration. Hypericin can accumulate in ocular tissues, including lenses, and can bind in vitro to α-crystallin, a major lens protein. α-crystallin is required for lens transparency and also acts as a chaperone to ensure its own integrity and the integrity of all lens proteins. Because there is no crystallin turnover, damage to α-crystallin is cumulative over the lifetime of the lens and can lead to cataracts, the principal cause of blindness worldwide. In this work we study hypericin photosensitization of α-crystallin and detect extensive polymerization of bovine α-crystallin exposed in vitro to hypericin and UVA. We use fluorescence confocal microscopy to visualize binding between hypericin and α-crystallin in a human lens epithelial (HLE) cell line. Further, we show that UVA irradiation of hypericin-treated HLE cells results in a dramatic decrease in α-crystallin detection concurrent with a dramatic accumulation of the tryptophan oxidation product N-formylkynurenine (NFK). Examination of actin in HLE cells indicates that this cytoskeleton protein accumulates NFK resulting from hypericin-mediated photosensitization. This work also shows that filtration of wavelengths <400nm provides incomplete protection against α-crystallin modification and NFK accumulation, suggesting that even by wearing UV-blocking sunglasses, routine users of St. John’s wort cannot adequately shield their lenses from hypericin-mediated photosensitized damage.

Chemical Modification of Lysozyme, Glucose 6-Phosphate Dehydrogenase, and Bovine Eye Lens Proteins Induced by Peroxyl Radicals: Role of Oxidizable Amino Acid Residues

Chemical and structural alterations to lysozyme (LYSO), glucose 6-phosphate dehydrogenase (G6PD), and bovine eye lens proteins (BLP) promoted by peroxyl radicals generated by the thermal decomposition of 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH) under aerobic conditions were investigated. SDS-PAGE analysis of the AAPH-treated proteins revealed the occurrence of protein aggregation, cross-linking, and fragmentation; BLP, which are naturally organized in globular assemblies, were the most affected proteins. Transmission electron microscopy (TEM) analysis of BLP shows the formation of complex protein aggregates after treatment with AAPH. These structural modifications were accompanied by the formation of protein carbonyl groups and protein hydroperoxides. The yield of carbonyls was lower than that for protein hydroperoxide generation and was unrelated to protein fragmentation. The oxidized proteins were also characterized by significant oxidation of Met, Trp, and Tyr (but not other) residues, and low levels of dityrosine. As the dityrosine yield is too low to account for the observed cross-linking, we propose that aggregation is associated with tryptophan oxidation and Trp-derived cross-links. It is also proposed that Trp oxidation products play a fundamental role in nonrandom fragmentation and carbonyl group formation particularly for LYSO and G6PD. These data point to a complex mechanism of peroxyl-radical mediated modification of proteins with monomeric (LYSO), dimeric (G6PD), and multimeric (BLP) structural organization, which not only results in oxidation of protein side chains but also gives rise to radical-mediated protein cross-links and fragmentation, with Trp species being critical intermediates.

A simple and novel strategy for the simultaneous determination of dopamine, acetamidophenol and tryptophan based on poly(new coccine) film modified carbon paste electrode

A polymerized film of new coccine was deposited on a carbon paste electrode (CPE) by an electropolymerization technique. Electrochemical characterization of the modified electrode was investigated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The poly(new coccine) modified CPE exhibited excellent electrocatalytic activity towards the oxidation of dopamine (DA), acetaminophenol (AP) and tryptophan (Trp) simultaneously. Well defined and separated oxidation peaks were observed by CV. Linear calibration plots for the oxidation of DA, AP and Trp were obtained in the range of 0.7–200 μM for DA, 1.5–120 μM for AP, and 0.6–50 μM for Trp. Detection limits of DA, AP and Trp were found to be 0.2, 0.3 and 0.2 μM, respectively. With good selectivity and sensitivity, the present method has been successfully applied to the determination of DA, AP and Trp in pharmaceutical samples.

Qualitative and quantitative evaluation of derivatization reagents for different types of protein-bound carbonyl groups

Mass spectrometry (MS) of 'carbonylated proteins' often involves derivatization of reactive carbonyl groups to facilitate their enrichment, identification and quantification. Among the many reported reagents, 2,4-dinitrophenylhydrazine (DNPH), biotin hydrazide (BHZ) and O-(biotinylcarbazoylmethyl) hydroxylamine (ARP) are the most frequently used. Despite their common use in carbonylation research, their reactivity towards protein-bound carbonyls has not been quantitatively evaluated in detail, to the best of our knowledge. Thus we studied the reactivity and specificity of these reagents towards different classes of reactive carbonyl groups (e.g. aldehydes, ketones and lactams), each being represented by a synthetic peptide carrying an accordingly modified residue. All three tagging reagents were selective for aliphatic aldehydes and ketones. Lactams and carbonyl-containing tryptophan oxidation products, however, were labelled only at low levels or not at all. Whereas DNPH derivatization was efficient under the published standard conditions, the derivatization conditions for BHZ and ARP had to be altered. Acidic conditions provided quantitative labelling yields for ARP. Peptides derivatized with DNPH, BHZ and ARP fragmented efficiently in tandem mass spectrometry, when the experimental conditions were chosen carefully for each reagent. Importantly, the tested carbonylated peptides did not cross-react with amino groups in other proteins present during sample preparations or enzymatic digestion. Thus, it appears favourable to digest proteins first and then derivatise the reactive carbonyl groups more efficiently at the peptide level under acidic conditions. The carbonylated model peptides used in this study might be valid internal standards for carbonylation proteomics.

Lessons Learned Regarding Symptoms of Tryptophan Deficiency and Excess from Animal Requirement Studies ,

Tryptophan is the precursor for several neurotransmitters and metabolic regulators, which, although quantitatively of little importance in determining the dietary requirement, have major importance for interpreting symptoms of dietary tryptophan deficiency and excess. The quantitative dietary tryptophan requirement appears to vary widely across species, so intakes relative to requirements are more appropriate expressions for comparison of adverse effects across species than daily intake or diet concentration. Symptoms of tryptophan deficiency may occur at intakes as little as 25% below the requirement. Symptoms include reduced feed intake and reduced growth rate but also impaired skeletal development and aberrant behavior. Older animals appear less susceptible than younger animals to tryptophan deficiency and females less than males. Symptoms of excess tryptophan intake include reduced food intake and growth rate. In growing animals, it appears that tryptophan intakes of >10 times the requirement are necessary before there are detrimental effects on growth performance. At still greater intakes, fatty liver and fibrotic changes in muscles, lung, and pancreas and the serotonin syndrome may develop. In pigs, tryptophan intake of 60 times the daily requirement did not cause mortality. The maximal tryptophan oxidation rate, measured in vivo using 13C universally labeled tryptophan, may be a possible marker of the intake above which increasing intake increases the risk of adverse effects. The advantage of the oxidation technique is that it does not necessarily rely on but still allows the identification and measurement of amino acid metabolites and is therefore simpler and more universally applicable.

Role of Surface Exposed Tryptophan as Substrate Generators for the Antibody Catalyzed Water Oxidation Pathway

The reaction of singlet oxygen with water to form hydrogen peroxide was catalyzed by antibodies and has been termed as the antibody catalyzed water oxidation pathway (ACWOP) (Nieva and Wentworth, Trends Biochem. Sci. 2004, 29, 274-278; Nieva et al. Immunol. Lett. 2006, 103, 33-38). While conserved and buried tryptophans in the antibody are thought to play a major role in this pathway, our studies with a monoclonal antibody, mAb-1 and its mutant W53A, clearly demonstrate the role of surface-exposed tryptophans in production of hydrogen peroxide, via the photo-oxidation pathway. Reactive oxygen species (ROS) such as singlet oxygen and superoxide were detected and site-specific tryptophan (Trp53) oxidation was observed under these conditions using RP-HPLC and mass spectrometry. The single mutant of the surface exposed Trp53 to Ala53 (W53A) results in a 50% reduction in hydrogen peroxide generated under these conditions, indicating that surface exposed tryptophans are highly efficient in transferring light energy to oxygen and contribute significantly to ROS generation. ACWOP potentially leads to the chemical instability of mAb-1 via the generation of ROS and is important to consider during clinical and pharmaceutical development of mAbs.

Xanthurenic Acid Binds to Neuronal G-Protein-Coupled Receptors That Secondarily Activate Cationic Channels in the Cell Line NCB-20

Xanthurenic acid (XA) is a metabolite of the tryptophan oxidation pathway through kynurenine and 3-hydroxykynurenine. XA was until now considered as a detoxification compound and dead-end product reducing accumulation of reactive radical species. Apart from a specific role for XA in the signaling cascade resulting in gamete maturation in mosquitoes, nothing was known about its functions in other species including mammals. Based upon XA distribution, transport, accumulation and release in the rat brain, we have recently suggested that XA may potentially be involved in neurotransmission/neuromodulation, assuming that neurons presumably express specific XA receptors. Recently, it has been shown that XA could act as a positive allosteric ligand for class II metabotropic glutamate receptors. This finding reinforces the proposed signaling role of XA in brain. Our present results provide several lines of evidence in favor of the existence of specific receptors for XA in the brain. First, binding experiments combined with autoradiography and time-course analysis led to the characterization of XA binding sites in the rat brain. Second, specific kinetic and pharmacological properties exhibited by these binding sites are in favor of G-protein-coupled receptors (GPCR). Finally, in patch-clamp and calcium imaging experiments using NCB-20 cells that do not express glutamate-induced calcium signals, XA elicited specific responses involving activation of cationic channels and increases in intracellular Ca2+ concentration. Altogether, these results suggest that XA, acting through a GPCR-induced cationic channel modulatory mechanism, may exert excitatory functions in various brain neuronal pathways.

The therapeutic effect of mitochondria-targeted antioxidant SkQ1 and Cistanche deserticola is associated with increased levels of tryptophan and kynurenine in the rat lens

Supplementation of senescence-accelerated OXYS rats with the mitochondria-targeted antioxidant SkQ1 and with the powder from Cistanche deserticola results in the deceleration of the cataract development and even in the improvement of lens transparency. The therapeutic effect of these preparations correlates with a significant elevation of tryptophan and kynurenine levels in the lens. This finding is attributed to a deceleration of the tryptophan and kynurenine oxidation due to antioxidant-assisted reduction of oxidative stress in the lens.

Natural inhibitors of indoleamine 3,5-dioxygenase induced by interferon-gamma in human neural stem cells

Indoleamine dioxygenase (IDO) is a heme- containing enzyme that catalyzes the oxidation of tryptophan to N-formylkynurenine, kynurenine and the downstream quinolinic acid. Though IDO is physiologically important in maintaining tissue integrity, aberrant IDO expression represses T cell function and promotes regulatory T cells (Treg) in cancer. It additionally exacerbates Alzheimer, depression, Huntington and Parkinson diseases via quinolinic acid. Inhibition of IDO has thus been recently proposed as a strategy for treating cancer and neuronal disorders. In the present study, we have developed a cell-based assay to evaluate the suppressive effect of anti-inflammatory phytochemicals on the enzyme. When stimulated by INF-γ, profound high expressions of IDO-1 mRNA as well as the protein were detected in human neural stem cells (hNSC) and verified by real-time retro-transcribed PCR and western blot analysis, respectively. The protein activity was measured by kynurenine concentration and the assay was validated by dose-responsive inhibition of IDO-1 antagonists including 1-methyltryptaphan, indomethacin and acetylsalicylic acid. Among the tested compounds, apigenin, baicalein, chrysin, and wogonin exhibit a potent repressive activity with IC50s comparable to that of indomethacin. The inhibition was further found to be independent of gene expression and protein translation because of the unaltered levels of mRNA and protein expression. Although curcumin displayed a potent inhibitory activity to the enzyme, it appeared to be cytotoxic to hNSCs. Morphological examination of hNSC revealed that baicalein and wogonin at the inhibitory concentrations induced neurite outgrowth. In conclusion, our data shows that certain phytochemicals with 2-phenyl-1-benzopyran-4-one backbone (flavones) attenuate significantly the IDO-1 protein activity without harming hNSCs. The inhibitory activity might have partially contributed to the anti-cancer and neuro-protective property of the compounds.

Light emission from tryptophan oxidation by hypobromous acid

The emission of ultraweak light from cells is a phenomenon associated with the oxidation of biomolecules by reactive oxygen species. The indole moiety present in tryptophan, serotonin and melatonin is frequently associated with the emission of light during the oxidation of these metabolites. This study presents results for hypobromous acid (HOBr) oxidation of tryptophan as a putative endogenous source of ultraweak light emission. We found that chemiluminescence elicited by the oxidation of tryptophan by HOBr was significantly higher than by hypochlorous acid (HOCl). This difference was related to secondary oxidation reactions, which were more intense using HOBr. The products identified during oxidation by HOCl, but depleted by using HOBr, were N-formylkynurenine, kynurenine, 1,2,3,3a,8,8a-hexahydro-3a-hydroxypyrrolo[2,3-b]-indole-2-carboxylic acid, oxindolylalanine and dioxindolylalanine. The emission of light is dependent on the free α-amino group of tryptophan, and hence, the indole of serotonin and melatonin, although efficiently oxidized, did not produce chemiluminescence. The emission of light was even greater using taurine monobromamine and dibromamine as the oxidant compared to HOBr. A mechanism based on bromine radical intermediates is suggested for the higher efficiency in light emission. Altogether, the experimental evidence described in the present study indicates that the oxidation of free tryptophan or tryptophan residues in proteins is an important source of ultraweak cellular emission of light. This light emission is increased in the presence of taurine, an amino acid present in large amounts in leukocytes, where this putative source of ultraweak light emission is even more relevant.