Photochemical and thermal reactivity of kynurenine

UV light and the ocular lens: a review of exposure models and resulting biomolecular changes

UV light is known to cause damage to biomolecules in living tissue. Tissues of the eye that play highly specialised roles in forming our sense of sight are uniquely exposed to light of all wavelengths. While these tissues have evolved protective mechanisms to resist damage from UV wavelengths, prolonged exposure is thought to lead to pathological changes. In the lens, UV light exposure is a risk factor for the development of cataract, which is a condition that is characterised by opacity that impairs its function as a focusing element in the eye. Cataract can affect spatially distinct regions of the lens. Age-related nuclear cataract is the most prevalent form of cataract and is strongly associated with oxidative stress and a decrease in the antioxidant capacity of the central lens region. Since UV light can generate reactive oxygen species to induce oxidative stress, its effects on lens structure, transparency, and biochemistry have been extensively investigated in animal models in order to better understand human cataract aetiology. A review of the different light exposure models and the advances in mechanistic understanding gained from these models is presented.

Advances in white light-emitting organic crystals

In past decades, organic crystals have presented considerable potential in the field of optoelectronics due to their rich tunable physical and chemical properties and excellent optoelectronic characteristics. White-light emission, as a special application, has received widespread attention and has been applied in various fields, generating significant interest in the scientific community. By preparing white light-emitting organic crystals, a series of applications for future white-light sources can be realized. This article reviews the research progress on the molecular design and synthesis, preparation, and application of white light-emitting organic crystals in recent years. We hope that this review will help to understand and facilitate the development of white light-emitting organic crystals.

Kynurenic acid and its chromophoric core 4-hydroxyquinoline react with tryptophan via proton-coupled electron transfer, and with tyrosine via H-transfer

Kynurenic acid (KNA) and 4-hydroxyquinoline (4HQN) are photochemically active products of tryptophan catabolism that readily react with tryptophan (Trp) and tyrosine (Tyr) after optical excitation. Recently, transient absorption experiments have shown that at neutral pH Trp reacts with triplet KNA via proton-coupled electron transfer (PCET), and not via electron transfer (ET) as it was suggested before. PCET includes the stepwise transition of both electrons and protons from Trp to triplet KNA. In this work, we confirmed that PCET is the reaction mechanism by the alternative method of time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP). Further studies by TR-CIDNP revealed hydrogen transfer as the mechanism of the reaction between triplet KNA and Tyr in neutral solutions and a transition of both PCET and H-transfer mechanisms to ET under acidic conditions. 4HQN, being the chromophoric core of KNA, exhibits different spectral and photophysical properties from KNA but employs the same mechanisms for the reactions of its triplet state with Trp and Tyr at neutral and acidic pH.

Influence of pH on radical reactions between kynurenic acid and amino acids tryptophan and tyrosine. Part II. Amino acids within the protein globule of lysozyme

An acidosis, a decrease of pH within a living tissue, may alter yields of radical reactions if participating radicals undergo partial or complete protonation. One of photosensitizers found in the human eye lens, kynurenic acid (KNA^-), possesses pKa 5.5 for its radical form that is close to physiological pH 6.89 for a healthy lens. In this work we studied the influence of pH on mechanisms and products of photoinduced radical reactions between KNA^- and amino acids tryptophan (Trp) and tyrosine (Tyr) within a globule of model protein, Hen White Egg Lysozyme (HEWL). Our results show that the rate constant of back electron transfer from kynurenyl to HEWL^• radicals with the restoration of initial reagents - the major decay pathway for these radicals - does not change in the pH 3-7. The quantum yield of HEWL degradation is also pH independent, however a shift of pH from 7 to 5 completely changes the outcome of photoinduced damage to HEWL from intermolecular cross-linking to oxygenation. HPLC-MS analysis has shown that four of six Trp and all Tyr residues of HEWL are modified in different extents at all pH, but the lowering of pH from 7 to 5 significantly changes the direction of main photodamage from Trp⁶² to Trp¹⁰⁸ located at the entrance and bottom of enzymatic center, respectively. A decrease of intermolecular cross-links via Trp⁶² is followed by an increase in quantities of intramolecular cross-links Tyr²⁰-Tyr²³ and Tyr²³-Tyr⁵³. The obtained results point out the competence of cross-linking and oxygenation reactions for Trp and Tyr radicals within a protein globule and significant increase of oxygenation to the total damage of protein in the case of cross-linking deceleration by coulombic repulsion of positively charged protein globules.

Cold Atmospheric Plasma Changes the Amino Acid Composition of Solutions and Influences the Anti-Tumor Effect on Melanoma Cells

Cold Atmospheric Plasma (CAP) is an ionized gas near room temperature. Its anti-tumor effect can be transmitted either by direct treatment or mediated by a plasma-treated solution (PTS), such as treated standard cell culture medium, which contains different amino acids, inorganic salts, vitamins and other substances. Despite extensive research, the active components in PTS and its molecular or cellular mechanisms are not yet fully understood. The purpose of this study was the measurement of the reactive species in PTS and their effect on tumor cells using different plasma modes and treatment durations. The PTS analysis yielded mode- and dose-dependent differences in the production of reactive oxygen and nitrogen species (RONS), and in the decomposition and modification of the amino acids Tyrosine (Tyr) and Tryptophan (Trp). The Trp metabolites Formylkynurenine (FKyn) and Kynurenine (Kyn) were produced in PTS with the 4 kHz (oxygen) mode, inducing apoptosis in Mel Im melanoma cells. Nitrated derivatives of Trp and Tyr were formed in the 8 kHz (nitrogen) mode, elevating the p16 mRNA expression and senescence-associated ß-Galactosidase staining. In conclusion, the plasma mode has a strong impact on the composition of the active components in PTS and affects its anti-tumor mechanism. These findings are of decisive importance for the development of plasma devices and the effectiveness of tumor treatment.

Degradation Products of Tryptophan in Cell Culture Media: Contribution to Color and Toxicity

Biomanufacturing processes may be optimized by storing cell culture media at room temperature, but this is currently limited by their instability and change in color upon long-term storage. This study demonstrates that one of the critical contributing factors toward media browning is tryptophan. LC-MS technology was utilized to identify tryptophan degradation products, which are likely formed primarily from oxidation reactions. Several of the identified compounds were shown to contribute significantly to color in solutions but also to exhibit toxicity against CHO cells. A cell-culture-compatible antioxidant, a-ketoglutaric acid, was found to be an efficient cell culture media additive for stabilizing components against degradation, inhibiting the browning of media formulations, and decreasing ammonia production, thus providing a viable method for developing room-temperature stable cell culture media.

Reactivity and degradation products of tryptophan in solution and proteins

Tryptophan is one of the essential mammalian amino acids and is thus a required component in human nutrition, animal feeds, and cell culture media. However, this aromatic amino acid is highly susceptible to oxidation and is known to degrade into multiple products during manufacturing, storage, and processing. Many physical and chemical processes contribute to the degradation of this compound, primarily via oxidation or cleavage of the highly reactive indole ring. The central contributing factors are reactive oxygen species, such as singlet oxygen, hydrogen peroxide, and hydroxyl radicals; light and photosensitizers; metals; and heat. In a multi-component mixture, tryptophan also commonly reacts with carbonyl-containing compounds, leading to a wide variety of products. The purpose of this review is to summarize the current state of knowledge regarding the degradation and interaction products of tryptophan in complex liquid solutions and in proteins. For the purposes of context, a brief summary of the key pathways in tryptophan metabolism will be included, along with common methods and issues in tryptophan manufacturing. The review will focus on the conditions that lead to tryptophan degradation, the products generated in these processes, their known biological effects, and methods which may be applied to stabilize the amino acid.

UV-A induced damage to lysozyme via Type I photochemical reactions sensitized by kynurenic acid

In this work we studied the mechanisms of Type I photodamage to a model protein, hen egg white lysozyme (HEWL), sensitized by kynurenic acid (KNA) - one of the most efficient photosensitizers of the human eye lens present in trace amounts within tissue. The kynurenic acid radical, KNA^•-, formed in the quenching of triplet KNA by HEWL, can be readily oxidized by molecular oxygen with the formation of superoxide anion radical O₂^•-. This leads to two ways of damage to proteins: either via the direct reactions between KNA^•- and HEWL^• radicals (Type Ia) or via the reactions between superoxide anion O₂^•- and HEWL^• radicals (Type Ib). Our results demonstrate significant degradation of the protein during Type Ia photolysis with the formation of various oligomeric and oxygenated forms of HEWL and several deoxygenated products of KNA. Liquid chromatography-mass spectrometry analysis revealed the cross-linking of HEWL via tryptophan (Trp⁶²) and tyrosine (Tyr²³) residues and, for the first time, the covalent binding of KNA to protein via tryptophan (Trp⁶² and Trp¹²³) residues. It was found that Type Ib reactions lead to substantially smaller damage to HEWL; the degradation quantum yields (Φ_deg) of HEWL are 1.3 ± 0.3% and 0.12 ± 0.03% for Type Ia and Ib photolyses, respectively. Low Φ_deg values for both types of photolysis indicate the Back Electron Transfer (BET) with the restoration of initial reagents as the main radical decay path with significantly higher BET efficiency in the case of Type Ib reactions. Therefore, in essentially oxygen-free tissues like the eye lens, the direct radical reactions via Type Ia mechanism could induce significantly larger damage to proteins, leading to their cross-linking and oxidation. The accumulation of these modifications can cause the development of various diseases, in particular, cataracts in the eye lens.

Quantitative metabolomic analysis of changes in the lens and aqueous humor under development of age-related nuclear cataract

INTRODUCTION

Metabolites are essential for the proper functioning of the eye lens, they either enter the lens from the aqueous humor (AH), or are synthesized in the lens epithelium. Antioxidants, osmolytes and UV filters are especially important for the lens protection, and their lack may cause the development of ophthalmic diseases.

OBJECTIVES

Comparison of the metabolomic compositions of lenses and AH taken from cataract patients with that taken from human cadavers without cataract can shed light onto molecular mechanisms underlying onset of age-related nuclear cataract.

METHODS

Combined use of ¹H nuclear magnetic resonance and high performance liquid chromatography with optical and high-resolution mass spectrometric detection for the identification and quantification of metabolites in the lens and AH extracts.

RESULTS

The concentrations of 86 metabolites were determined for four groups of samples, including lenses and AH from cataract patients and from human cadavers. In cataractous lens the most abundant metabolites are (in descending order): myo-inositol, lactate, acetate, glutamate, glutathione; in AH-lactate, glucose, glutamine, alanine, valine. The concentrations of the majority of metabolites in normal post-mortem samples of both lens and AH are higher than that in samples from the cataract patients.

CONCLUSIONS

Comparison of metabolite concentrations in lens and corresponding AH reveal that the most important for the lens protection metabolites are synthesized in the lens epithelial cells. The reduced levels of antioxidants, UV filters, and osmolytes were found in the cataractous lenses what cannot be explained by post-mortem changes in normal lens; that indicates that the age-related nuclear cataract development may originate from the dysfunction of the lens epithelial cells.

Ultrafast excited state decay of natural UV filters: from intermolecular hydrogen bonds to a conical intersection

An unsaturated bond in the side chain leads to the ultrafast decay of the excited statesviaa conical intersection independent of solvent properties.

Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid

Photoinduced generation of radicals in the eye lens may play an important role in the modification of proteins leading to their coloration, aggregation, and insolubilization. The radicals can be formed via the reactions of photoexcited endogenous chromophores of the human lens with lens proteins, in particular with tryptophan residues. In the present work we studied the reactions induced by UV-A (315-400nm) light between kynurenic acid (KNA), an effective photosensitizer present in the human lens, and N-acetyl-L-tryptophan (NTrpH) under aerobic and anaerobic conditions. Our results show that the reaction mechanism strongly depends on the presence of oxygen in solution. Under aerobic conditions, the generation of singlet oxygen is the major channel of the effective NTrpH oxidation. In argon-bubbled solutions, the quenching of triplet KNA by NTrpH results in the formation of KNA^•- and NTrp^• radicals. Under laser pulse irradiation, when the radical concentration is high, the main pathway of the radical decay is the back electron transfer with the restoration of initial reagents. Other reactions include (i) the radical combination yielding NTrp dimers and (ii) the oxygen atom transfer from KNA^•- to NTrp^• with the formation of oxidized NTrp species and deoxygenated KNA products. In continuous-wave photolysis, even trace amounts of molecular oxygen are sufficient to oxidize the majority of KNA^•- radicals with the rate constant of (2.0 ± 0.2) × 10⁹M^-1s^-1, leading to the restoration of KNA and the formation of superoxide radical O₂^•-. The latter reacts with NTrp^• via either the radical combination to form oxidized NTrp (minor pathway), or the electron transfer to restore NTrpH in the ground state (major pathway). As the result, the quantum yields of the starting compound decomposition under continuous-wave anaerobic photolysis are rather low: 1.6% for NTrpH and 0.02% for KNA. The photolysis of KNA with alpha-crystallin yields the same deoxygenated KNA products as the photolysis of KNA with NTrpH, indicating the similarity of the photolysis mechanisms. Thus, inside the eye lens KNA can sensitize both protein photooxidation and protein covalent cross-linking with the minor self-degradation. This may play an important role in the lens protein modifications during the normal aging and cataract development.

Characterisation of the kynurenine pathway in skin-derived fibroblasts and keratinocytes

Acute UVB exposure triggers inflammation leading to the induction of indoleamine 2,3 dioxygenase (IDO1), one of the first enzymes in the kynurenine pathway (KP) for tryptophan degradation. However, limited studies have been undertaken to determine the catabolism of tryptophan within the skin. The aim of this study was two fold: (1) to establish if the administration of the proinflammatory cytokine interferon-gamma (IFN-γ) and/or UVB radiation elicits differential KP expression patterns in human fibroblast and keratinocytes; and (2) to evaluate the effect of KP metabolites on intracellular nicotinamide adenine dinucleotide (NAD(+) ) levels, and cell viability. Primary cultures of human fibroblasts and keratinocytes were used to examine expression of the KP at the mRNA level using qPCR, and at the protein level using immunocytochemistry. Cellular responses to KP metabolites were assessed by examining extracellular lactate dehydrogenase (LDH) activity and intracellular NAD(+) levels. Major downstream KP metabolites were analyzed using GC/MS and HPLC. Our data shows that the KP is fully expressed both in human fibroblasts and keratinocytes. Exposure to UVB radiation and/or IFN-γ causes significant changes in the expression pattern of downstream KP metabolites and enzymes. Exposure to various concentrations of KP metabolites showed marked differences in cell viability and intracellular NAD(+) production, providing support for involvement of the KP in the de novo synthesis of NAD(+) in the skin. This new information will have a significant impact on our understanding of the pathogenesis of UV related skin damage and the diagnosis of KP related disease states.

Aggregation of α-crystallins in kynurenic acid-sensitized UVA photolysis under anaerobic conditions

Extensive protein aggregation is the major outcome of kynurenic acid-sensitized photolysis of α-crystallin under anaerobic conditions. The main lens antioxidants ascorbate and glutathione effectively inhibit the protein aggregation.

Formation of fluorophores from the kynurenine pathway metabolite N-formylkynurenine and cyclic amines involves transamidation and carbon-carbon bond formation at the 2-position of the amine

BACKGROUND

Tryptophan catabolism along the kynurenine pathway is associated with a number of pathologies including cataract formation and cancer. Whilst the chemical reactions of kynurenine are well studied, less is known about the reactivity of its precursor N-formylkynurenine (NFK). We previously reported the generation of a strong fluorophore in an aqueous reaction of NFK with piperidine, and herein we describe its structure and mechanism of formation.

METHODS

Compounds were identified using NMR, mass and UV spectroscopic techniques. The products from the reaction of amines with amino acids were quantified using HPLC-MS.

RESULTS

The novel fluorophore was identified as a tetrahydroquinolone adduct (PIP-THQ), where piperidine is N-formylated and attached at its 2-position to the quinolone. NFK is initially deaminated to generate an unsaturated enone, which forms an adduct with piperidine and is subsequently converted into the fluorophore. Testing of a variety of other secondary amines showed that only cyclic amines unsubstituted at both positions adjacent to nitrogen could form fluorophores efficiently. The amino acids tryptophan and kynurenine, which lack the formamide group do not form such fluorophores.

CONCLUSIONS

NFK forms fluorophores in a not previously published reaction with cyclic amines.

GENERAL SIGNIFICANCE

Our study is the first to provide evidence for concurrent transamidation and substitution at the 2-position of a cyclic amine occurring under moderately-heated aqueous conditions with no added catalysts. The high reactivity of NFK demonstrated here could result in formation of biologically relevant metabolites yet to be characterised.

Metabolomic composition of normal aged and cataractous human lenses

Quantitative metabolomic profiles of normal and cataractous human lenses were obtained with the combined use of high-frequency nuclear magnetic resonance (NMR) and high-performance liquid chromatography with high-resolution mass-spectrometric detection (LC-MS) methods. The concentration of more than fifty metabolites in the lens cortex and nucleus has been determined. For the majority of metabolites, their concentrations in the lens cortex and nucleus are similar, which confirms low metabolic activity in the lens core. The difference between the metabolite levels in the cortex and nucleus of the normal lens is observed for antioxidants and UV filters, which demonstrates the activity of redox processes in the lens. A huge difference is found between the metabolomic compositions of normal and age-matched cataractous lenses: the concentrations of almost all metabolites in the normal lens are higher than in the cataractous one. The most pronounced difference is observed for compounds playing a key role in the lens cell protection and metabolic activity, including antioxidants, UV filters, and osmolytes. The results obtained imply that the development of the age-related cataracts might originate from the metabolic dysfunction of the lens epithelial cells.

10-Hydroxybenzo[h]quinoline: switching between single- and double-well proton transfer through structural modifications

Proton transfer in HBQ and modified compounds was investigated experimentally (steady state absorption and emission spectroscopy, NMR and chemometrics) and theoretically (DFT and TD-DFT M06-2X/TZVP calculations) in ground and excited singlet state.

Photophysical aspects of biological photosensitizer Kynurenic acid from the perspective of experimental and quantum chemical study

In the present contribution, we have explored ground and excited state spectroscopic properties of an antiexcitotoxic and anticonvulsant drug, Kynurenic acid (KA), through steady-state absorption, emission and time-resolved emission spectroscopy and quantum chemical calculations. The main focus of this article is to illustrate the effects of various parameters such as the nature of the solvents and pH of the medium on the spectral properties of KA which confirms the presence of different neutral and ionic species in the ground and excited states. The molecule KA exists mainly as keto- or anionic form in the ground state, whereas the main contribution of its emission is arising from the keto tautomer in the excited state. Quantum chemical calculations by Density Functional Theory (DFT) method has been effectively employed to correlate the experimental findings. The ground and excited state properties of KA ascertained by means of experimental and theoretical method reveal that it resembles well with other two compounds, 4-hydroxyquinoline and xanthurenic acid formed by the decomposition of UV filters.

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.

Fine tuning the energetics of excited-state intramolecular proton transfer (ESIPT): white light generation in a single ESIPT system

Using 7-hydroxy-1-indanone as a prototype (I), which exhibits excited-state intramolecular proton transfer (ESIPT), chemical modification has been performed at C(2)-C(3) positions by fusing benzene (molecule II) and naphthalene rings, (molecule III). I undergoes an ultrafast rate of ESIPT, resulting in a unique tautomer emission (λ(max) ∼530 nm), whereas excited-state equilibrium is established for both II and III, as supported by the dual emission and the associated relaxation dynamics. The forward ESIPT (normal to proton-transfer tautomer species) rates for II and III are deduced to be (30 ps)(-1) and (22 ps)(-1), respectively, while the backward ESIPT rates are (11 ps)(-1) and (48 ps)(-1). The ESIPT equilibrium constants are thus calculated to be 0.37 and 2.2 for II and III, respectively, giving a corresponding free energy change of 0.59 and -0.47 kcal/mol between normal and tautomer species. For III, normal and tautomer emissions in solid are maximized at 435 and 580 nm, respectively, achieving a white light generation with Commission Internationale de l'Eclairage (CIE) (0.30, 0.27). An organic light-emitting diode based on III is also successfully fabricated with maximum brightness of 665 cd m(-2) at 20 V (885 mA cm(-2)) and the CIE coordinates of (0.26, 0.35). The results provide the proof of concept that the white light generation can be achieved in a single ESIPT system.

Kynurenine inhibits fibroblast growth factor 2-mediated expression of crystallins and MIP26 in lens epithelial cells

Fibroblast growth factor-2 (FGF2)-mediated signaling plays an important role in fiber cell differentiation in eye lens. We had previously shown that kynurenine (KYN) produced from the overexpression of indoleamine 2,3-dioxygenase (IDO) causes defects in the differentiation of fiber cells, induces fiber cell apoptosis and cataract formation in the mouse lens, and leads to cell cycle arrest in cultured mouse lens epithelial cells (mLEC). In this study, we demonstrate that exogenous KYN reduces FGF2-mediated expression of alpha-, beta-, and gamma-crystallin and MIP26 in mLEC. We show that endogenously produced KYN in mLEC of IDO transgenic animals causes similar defects in FGF2-induced protein expression and that a competitive inhibitor of IDO prevents such defects. Our data also show that KYN inhibits FGF2-induced Akt and ERK1/2 phosphorylation in mLEC, which are required for crystallin and MIP26 expression in the lens. KYN does not inhibit FGF2 binding to cells but inhibit phosphorylation of FGFR1in mLEC. Together our data suggest that KYN might inhibit FGF2-mediated fiber cell differentiation by preventing expression of crystallins and MIP26. Our studies provide a novel mechanism by which KYN can exert deleterious effects in cells.

Kinetics and mechanism of thermal decomposition of kynurenines and biomolecular conjugates: ramifications for the modification of mammalian eye lens proteins

Thermal degradation reactions of kynurenine (KN), 3-hydroxykynurenine (3OHKN), and several adducts of KN, to amino acids and reduced glutathione (GSH) have been studied at physiological temperature. These compounds are all implicated in age-related mammalian eye lens cataract formation at the molecular level. The main reaction pathway for both KN and 3OHKN is deamination via beta-elimination to carboxyketoalkenes CKA and 3OHCKA. These reactions show a weak pH dependence below pH values of approximately 8, and a strong pH dependence above this value. The 3OHKN structure deaminates at a faster rate than KN. A mechanism for the deamination reaction is proposed, involving an aryl carbonyl enol/enolate ion, that is strongly supported by the structural, kinetic, and pH data. The degradation of Lys, His, Cys and GSH adducts of the CKA moieties was also studied. The Lys adduct was found to be relatively stable over 200 h at 37 degrees C, while significant degradation was observed for the other adducts. The results are discussed in terms of known post-translational modification reactions of the lens proteins and compared to incubation studies involving KN and related compounds in the presence of proteins.

Deaminated UV filter 3-hydroxykynurenine O-beta-D-glucoside is found in cataractous human lenses

Analysis of UV filter levels in 48 cataractous human lenses was performed with the use of HPLC. A new chromophore with the absorption maximum at 410nm and molecular mass of 369Da was detected and assigned as deaminated 3-hydroxykynurenine O-beta-D-glucoside (3OHCKAG). Cataractous lenses are characterized by the wide range of the UV filter concentrations and remarkably lower levels of UV filters and glutathione than published for the normal lenses. No correlation between the lens age and the level of UV filters has been found in cataractous lenses.

UV filter decomposition. A study of reactions of 4-(2-aminophenyl)-4-oxocrotonic acid with amino acids and antioxidants present in the human lens

Deamination of UV filters, such as kynurenine (KN), in the human lens results in protein modification. Thermal reactions of the product of kynurenine deamination, 4-(2-aminophenyl)-4-oxocrotonic acid (CKA), with amino acids (histidine, lysine, methionine, tryptophan, tyrosine, cysteine) and antioxidants (ascorbate, NADH, glutathione reduced) were studied. The rate constants of the reactions under physiological conditions were measured. The rate constants of CKA addition to cysteine k(Cys)=36+/-4M(-1)s(-1) and to glutathione k(GSH)=2.1+/-0.2M(-1)s(-1) are 4-5 orders of magnitude higher than the rate constants of CKA reactions with the other amino acids and antioxidants. The Arrhenius parameters for k(Cys) and k(GSH) were determined: A(GSH)=(1.8+/-0.7)x10(5)M(-1)s(-1), E(GSH)=29.2+/-5.6kJmol(-1), A(Cys)=(2.7+/-0.9)x10(8)M(-1)s(-1), E(Cys)=40.4+/-5.7kJmol(-1). The large difference in frequency factors for k(Cys) and k(GSH) is attributed to steric hindrance, peculiar to the bulky GSH molecule.

Development of a column-switching high-performance liquid chromatography for kynurenine enantiomers and its application to a pharmacokinetic study in rat plasma

Kynurenine (KYN), a tryptophan metabolite, is a precursor of kynurenic acid, which is an antagonist of N-methyl-d-aspartate receptor. In this study, an enantiomeric separation of d,l-KYN derivatized with the benzofurazan fluorescence reagent 4-N,N-dimethylaminosulfonyl-7-fluoro-2,1,3-benzoxadiazole (DBD-F) (DBD-d,l-KYN) was first investigated by using a high-performance liquid chromatography (HPLC) with several chiral columns. As a consequence, DBD-d,l-KYN was enantiomerically separated on a cellulose-type chiral column (CHIRALCEL OJ-RH) with a mobile phase of H(2)O/CH(3)CN/MeOH (40/50/10) containing 0.1% acetic acid. Under this condition, the separation factor and resolution were 1.48 and 1.28, respectively. Next, a column-switching HPLC consisting of both octadecylsilica and chiral columns was developed and used to determine both d- and l-KYN enantiomers in 10 microL of rat plasma following the intraperitoneal administration of d,l-KYN to rats (10 mg kg(-1)). The result revealed that the concentration of l-KYN was higher than that of d-KYN, suggesting that d-KYN was eliminated faster than l-KYN.