Free radical reactions photosensitized by the human lens component, kynurenine: an EPR and spin trapping investigation

Gut microbiota metabolites, redox status, and the related regulatory effects of probiotics

Oxidative stress is a state of imbalance between oxidation and antioxidation. It is caused by excess levels of free radicals and leads to the damage of DNA, proteins, and lipids. The crucial role of gut microbiota in regulating oxidative stress has been widely demonstrated. Studies have suggested that the redox regulatory effects of gut microbiota are related to gut microbiota metabolites, including fatty acids, lipopolysaccharides, tryptophan metabolites, trimethylamine-N-oxide and polyphenolic metabolites. In recent years, the potential benefits of probiotics have been gaining increasing scientific interest owing to their ability to modulate gut microbiota and oxidative stress. In this review, we summarise the adverse health effects of oxidative stress and discuss the role of the gut microbiota and its metabolites in redox regulation. Based on the influence of gut microbiota metabolites, the roles of probiotics in preventing oxidative stress are highlighted.

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

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

Cellular Localization of Kynurenine 3-Monooxygenase in the Brain: Challenging the Dogma

Kynurenine 3-monooxygenase (KMO), a key player in the kynurenine pathway (KP) of tryptophan degradation, regulates the synthesis of the neuroactive metabolites 3-hydroxykynurenine (3-HK) and kynurenic acid (KYNA). KMO activity has been implicated in several major brain diseases including Huntington's disease (HD) and schizophrenia. In the brain, KMO is widely believed to be predominantly localized in microglial cells, but verification in vivo has not been provided so far. Here, we examined KP metabolism in the brain after depleting microglial cells pharmacologically with the colony stimulating factor 1 receptor inhibitor PLX5622. Young adult mice were fed PLX5622 for 21 days and were euthanized either on the next day or after receiving normal chow for an additional 21 days. Expression of microglial marker genes was dramatically reduced on day 22 but had fully recovered by day 43. In both groups, PLX5622 treatment failed to affect Kmo expression, KMO activity or tissue levels of 3-HK and KYNA in the brain. In a parallel experiment, PLX5622 treatment also did not reduce KMO activity, 3-HK and KYNA in the brain of R6/2 mice (a model of HD with activated microglia). Finally, using freshly isolated mouse cells ex vivo, we found KMO only in microglia and neurons but not in astrocytes. Taken together, these data unexpectedly revealed that neurons contain a large proportion of functional KMO in the adult mouse brain under both physiological and pathological conditions.

On the Antioxidant Properties of L-Kynurenine: An Efficient ROS Scavenger and Enhancer of Rat Brain Antioxidant Defense

L-kynurenine (L-KYN) is an endogenous metabolite, that has been used as a neuroprotective strategy in experimental models. The protective effects of L-KYN have been attributed mainly to kynurenic acid (KYNA). However, considering that L-KYN is prone to oxidation, this redox property may play a substantial role in its protective effects. The aim of this work was to characterize the potential impact of the redox properties of L-KYN, in both synthetic and biological systems. First, we determined whether L-KYN scavenges reactive oxygen species (ROS) and prevents DNA and protein oxidative degradation in synthetic systems. The effect of L-KYN and KYNA (0.1–100 µM) on redox markers (ROS production, lipoperoxidation and cellular function) was compared in rat brain homogenates when exposed to FeSO₄ (10 µM). Then, the effect of L-KYN administration (75 mg/kg/day for 5 days) on the GSH content and the enzymatic activity of glutathione reductase (GR) and glutathione peroxidase (GPx) was determined in rat brain tissue. Finally, brain homogenates from rats pretreated with L-KYN were exposed to pro-oxidants and oxidative markers were evaluated. The results show that L-KYN is an efficient scavenger of ^●OH and ONOO⁻, but not O₂^●– or H₂O₂ and that it prevents DNA and protein oxidative degradation in synthetic systems. L-KYN diminishes the oxidative effect induced by FeSO₄ on brain homogenates at lower concentrations (1 µM) when compared to KYNA (100 µM). Furthermore, the sub-chronic administration of L-KYN increased the GSH content and the activity of both GR and GPx, and also prevented the oxidative damage induced by the ex vivo exposure to pro-oxidants. Altogether, these findings strongly suggest that L-KYN can be considered as a potential endogenous antioxidant.

Kynurenine induces an age-related phenotype in bone marrow stromal cells

Advanced age is one of the important contributing factors for musculoskeletal deterioration. Although the exact mechanism behind this degeneration is unknown, it has been previously established that nutritional signaling plays a vital role in musculoskeletal pathophysiology. Our group established the vital role of the essential amino acid, tryptophan, in aging musculoskeletal health. With advanced age, inflammatory factors activate indoleamine 2,3-dioxygenase (IDO1) and accumulate excessive intermediate tryptophan metabolites such as Kynurenine (KYN). With age, Kynurenine accumulates and suppresses osteogenic differentiation, impairs autophagy, promotes early senescence, and alters cellular bioenergetics of bone marrow stem cells. Recent studies have shown that Kynurenine negatively impacts bone marrow stromal cells (BMSCs) and, consequently, promotes bone loss. Overall, understanding the mechanism behind BMSCs losing their ability for osteogenic differentiation can provide insight into the prevention of osteoporosis and the development of targeted therapies. Therefore, in this article, we review Kynurenine and how it plays a vital role in BMSC dysfunction and bone loss with age.

Tryptophan-kynurenine pathway is dysregulated in inflammation, and immune activation

The kynurenine (Kyn) pathway is the major route for tryptophan (Trp) metabolism, and it contributes to several fundamental biological processes. Trp is constitutively oxidized by tryptophan 2, 3-dioxygenase in liver cells. In other cell types, it is catalyzed by an alternative inducible indoleamine-pyrrole 2, 3-dioxygenase (IDO) under certain pathophysiological conditions, which consequently increases the formation of Kyn metabolites. IDO is up-regulated in response to inflammatory conditions as a novel marker of immune activation in early atherosclerosis. Besides, IDO and the IDO-related pathway are important mediators of the immunoinflammatory responses in advanced atherosclerosis. In particular, Kyn, 3-hydroxykynurenine, and quinolinic acid are positively associated with inflammation, oxidative stress (SOX), endothelial dysfunction, and carotid artery intima-media thickness values in end-stage renal disease patients. Moreover, IDO is a potential novel contributor to vessel relaxation and metabolism in systemic infections, which is also activated in acute severe heart attacks. The Kyn pathway plays a key role in the increased prevalence of cardiovascular disease by regulating inflammation, SOX, and immune activation.

UVA light-excited kynurenines oxidize ascorbate and modify lens proteins through the formation of advanced glycation end products: implications for human lens aging and cataract formation

Advanced glycation end products (AGEs) contribute to lens protein pigmentation and cross-linking during aging and cataract formation. In vitro experiments have shown that ascorbate (ASC) oxidation products can form AGEs in proteins. However, the mechanisms of ASC oxidation and AGE formation in the human lens are poorly understood. Kynurenines are tryptophan oxidation products produced from the indoleamine 2,3-dioxygenase (IDO)-mediated kynurenine pathway and are present in the human lens. This study investigated the ability of UVA light-excited kynurenines to photooxidize ASC and to form AGEs in lens proteins. UVA light-excited kynurenines in both free and protein-bound forms rapidly oxidized ASC, and such oxidation occurred even in the absence of oxygen. High levels of GSH inhibited but did not completely block ASC oxidation. Upon UVA irradiation, pigmented proteins from human cataractous lenses also oxidized ASC. When exposed to UVA light (320-400 nm, 100 milliwatts/cm(2), 45 min to 2 h), young human lenses (20-36 years), which contain high levels of free kynurenines, lost a significant portion of their ASC content and accumulated AGEs. A similar formation of AGEs was observed in UVA-irradiated lenses from human IDO/human sodium-dependent vitamin C transporter-2 mice, which contain high levels of kynurenines and ASC. Our data suggest that kynurenine-mediated ASC oxidation followed by AGE formation may be an important mechanism for lens aging and the development of senile cataracts in humans.

Kynurenines with neuroactive and redox properties: relevance to aging and brain diseases

The kynurenine pathway (KP) is the main route of tryptophan degradation whose final product is NAD(+). The metabolism of tryptophan can be altered in ageing and with neurodegenerative process, leading to decreased biosynthesis of nicotinamide. This fact is very relevant considering that tryptophan is the major source of body stores of the nicotinamide-containing NAD(+) coenzymes, which is involved in almost all the bioenergetic and biosynthetic metabolism. Recently, it has been proposed that endogenous tryptophan and its metabolites can interact and/or produce reactive oxygen species in tissues and cells. This subject is of great importance due to the fact that oxidative stress, alterations in KP metabolites, energetic deficit, cell death, and inflammatory events may converge each other to enter into a feedback cycle where each one depends on the other to exert synergistic actions among them. It is worth mentioning that all these factors have been described in aging and in neurodegenerative processes; however, has so far no one established any direct link between alterations in KP and these factors. In this review, we describe each kynurenine remarking their redox properties, their effects in experimental models, their alterations in the aging process.

Excessive milk intake as a risk factor, probably associated with oxidative stress, in experimental naphthalene-initiated cataract in rats

AIM

To determine whether a diet containing excessive amounts of milk aggravates naphthalene-initiated cataracts in a common animal model of age-related human cataract.

METHODS

Ninety Sprague-Dawley rats were fed a natural diet supplemented with either water (group A), normal amounts of milk (group B), excessive amounts of milk (group C), naphthalene plus water (group D), naphthalene plus normal amounts of milk (group E), naphthalene plus excessive amounts of milk (group F). Cataract development was monitored weekly using a slit lamp and lens gray value analysis. Concentrations of reactive oxygen species (ROS), reduced glutathione (GSH) and malondialdehyde (MDA) in rat lenses were measured to determine the role of oxidative stress in cataract induction.

RESULTS

By week 4, the cortical gray value was significantly higher in group F than that in group D, and the cortical gray value was significantly higher in group D than in group A. However, by week 8, no significant differences were observed among groups C, F, B, E and A. ROS concentrations in lenses of rats of groups C and F were slightly higher than in those of groups B, E and A, but ROS concentrations in group F were significantly higher than in the other groups receiving naphthalene (i.e. groups D and E). GSH concentrations in group F were significantly lower than in the other groups. MDA concentrations in group F were significantly higher than in the other groups receiving naphthalene, indicating increased lipid peroxidation induced by naphthalene plus excessive intake of milk.

CONCLUSIONS

Our results provide quantitative evidence that excessive intake of milk aggravates naphthalene-initiated cataracts, which is probably due to oxidative damage caused by increased ROS.

Kynurenine-induced photo oxidative damage to lens in vitro: protective effect of caffeine

Photochemical generation of reactive species of oxygen in the lens and aqueous and consequent physiological damage to the tissue has been implicated in the genesis of human cataracts. The present studies were undertaken to examine the feasibility of possible prevention of such damage to the lens initiated by UV activation of kynurenine, a well-known photosensitizer in the human lens. The studies were done by organ culturing intact mouse lenses in medium containing kynurenine and exposed to UVA. Tissue damage was assessed by the inhibition of its ability to carry active transport of rubidium ions and the associated decrements in the levels of GSH and ATP. These deleterious effects were significantly prevented by caffeine, an alkaloid present in many common beverages and known to chemically deactivate the said oxygen derivatives. Further studies on the pharmacological significance of the findings are hence in progress.

Photochemical and thermal reactivity of kynurenine

The thermal and photochemical reactivity of kynurenine (KN), a tryptophan metabolite found in human lenses, has been studied in aqueous solution. The decarboxylation reaction of KN, resulting in the formation of 4-hydroxyquinoline, is reported for the first time. Rate constants for KN deamination and decarboxylation were determined in the temperature range 50-90 degrees C. The quantum yields for KN photodecomposition under argon were measured to be Phi Ar=(2.0+/-0.2) x 10(-5) and under oxygen Phi O2=(1.1+/-0.1) x 10(-4).

Photochemistry of Kynurenine, a Tryptophan Metabolite: Properties of the Triplet State

Photolysis of aqueous kynurenine (KN) solutions results in the formation of triplet kynurenine TKN. In low pH solutions, triplet formation occurs with almost 100% efficiency, while in neutral solutions the triplet quantum yield is PhiT = 0.018 +/- 0.004. The dissociation constant of TKN, which is attributed to deprotonation of the anilino group, has a pKa value of 4.7. Similar triplet absorption spectra were obtained under direct and acetone-sensitized photolysis. The large difference in quantum yields as a function of pH is attributed to excited-state properties of the first excited singlet state of KN. The rate constant quenching for TKN by oxygen is kq = 2 x 10(9) M(-1) s(-1).

Characterization of Covalent Multimers of Crystallins in Aging Human Lenses

The purpose of this study was to characterize covalent multimers with molecular mass of >90 kDa in the water-insoluble (WI) proteins of aging human lenses. The experimental approach was to first separate the multimers (molecular mass >90 kDa) as individual spots by two-dimensional gel electrophoresis and next analyze compositions of each multimers by matrix-assisted laser desorption ionization-time of flight and electrospray ionization-tandem mass spectrometric (ES-MS/MS) methods. The WI proteins from lenses of 25- and 41-year-old subjects showed distinct 5- and 16-multimer spots on two-dimensional gels, respectively, but the spots from 52- and 72-year-old lenses were non-descript and diffused. ES-MS/MS analyses showed two types of covalent multimers in 25- and 41-year-old lenses, i.e. the first type composed of fragments of eight different crystallins (i.e. alphaA, alphaB, betaA3, betaA4, betaB1, betaB2, gammaS, and gammaD), and the second type of alpha-, beta-, and gamma-crystallins (possibly fragments) and two beaded filament proteins (phakinin and filensin). The most commonly identified species in the complexes of 41-year-old lenses were: alphaA-fragment (C-terminally truncated, residues 1-157), alphaB-fragment (residues 83-90), betaB1-crystallin (residues 60-71), betaA3 (residues 33-44), betaA4 (residues 106-117), filensin (residues 78-90), and phakinin (residues 77-89). Three post-translational modifications (i.e. oxidation of Met and Trp, conversion of Ser to dehydroalanine, and formylation of His) were observed in alphaA-crystallin fragment, and the first two modifications could cross-link proteins. Together, the results suggested that covalent multimers appeared early in life (i.e. 25 years of age) and increased in number with aging, and the two beaded filament proteins form covalent complexes with crystallin fragments in vivo.

Screening for ocular phototoxicity

Normally light transmission through the eye is benign and serves to direct vision and circadian rhythm. However, with very intense light exposure, or with ambient light exposure to the aged eye and/or young or adult eye in the presence of light-activated (photosensitizing) drugs or dietary supplements, cosmetics, or diagnostic dyes, light can be hazardous, leading to blinding disorders. Light damage to the human eye is avoided because the eye is protected by a very efficient antioxidant system and the chromophores present absorb light and dissipate its energy. After middle age, there is a decrease in the production of antioxidants and antioxidant enzymes and an accumulation of endogenous chromophores that are phototoxic. The extent to which a particular exogenous photosensitizing substance is capable of producing phototoxic side effects in the eye depends on several parameters, including (1) the chemical structure; (2) the absorption spectra of the drug; (3) binding of the drug to ocular tissue (lens proteins, melanin, DNA); and (4) the ability to cross blood-ocular barriers (amphiphilic or lipophilic). For instance, compounds that have either a tricyclic, heterocyclic, or porphyrin ring structure and are incorporated into ocular tissues are potentially phototoxic agents in the eye. The extent to which these substances might damage the eye (photoefficiency) can be predicted using in vitro and photophysical techniques. With simple, inexpensive testing, compounds can be screened for their potential ocular phototoxicity at the developmental stage. It may be that a portion of the molecule can be modified to reduce phototoxicity while leaving the primary drug effect intact. Preclinical safety testing may prevent ocular side effects that can range from mild, reversible blurred vision to permanent blindness.

Electron paramagnetic resonance of radicals induced in drugs and excipients by radiation or mechanical treatments

Radiation as well as mechanical treatments induced in drugs and excipients radicals, which can be studied by electron paramagnetic resonance. A special attention is pointed about the use of electron paramagnetic resonance (EPR) to bring the proof whether or not a drug has been irradiated or not. We also discuss of other methods (thermoluminescence (TL), gas phase chromatography (GPC)) which can be used to bring the same proof in case of irradiated drugs, excipients and cosmetic products.

Oxidative DNA damage: endogenous and chemically induced

A variety of types of DNA oxidation occur endogenously and mediated by xenobiotics. Certain forms are mutagenic and carcinogenic and may lead to other pathologies.

Identification of tryptophan oxidation products in bovine alpha-crystallin

Oxidation is known to affect the structure, activity, and rate of degradation of proteins, and is believed to contribute to a variety of pathological conditions. Metal-catalyzed oxidation (MCO) is a primary oxidizing system in many cell types. In this study, the oxidative effects of a MCO system (the Fenton reaction) on the structure of the tryptophan residues of alpha-crystallin were determined. Tandem mass spectrometry (MS/MS) was utilized to identify specific tryptophan and methionine oxidation products in the bovine alpha-crystallin sequence. After oxidative exposure, alpha-crystallin was digested with trypsin, and the resulting peptides were fractionated by reverse-phase HPLC. Structural analysis by mass spectrometry revealed that tryptophan 9 of alphaA- and tryptophan 60 of alphaB-crystallin were each converted into hydroxytryptophans (HTRP), N-formylkynurenine (NFK), and kynurenine (KYN). However, only HTRP and KYN formation were detected at residue 9 of alphaB-crystallin. Oxidation of methionine 1 of alphaA- and methionine 1 and 68 of alphaB-crystallin was also detected. The products NFK and KYN are of particular importance in the lens, as they themselves are photosensitizers that can generate reactive oxygen species (ROS) upon UV light absorption. The unambiguous identification of HTRP, NFK, and KYN in intact alpha-crystallin represents the first structural proof of the formation of these products in an intact protein, and provides a basis for detailed structural analysis of oxidized proteins generated in numerous pathological conditions.

Quantitation of the singlet oxygen produced by UVA irradiation of human lens proteins

Ultraviolet irradiation of aged human lens proteins in vitro causes extensive photolytic damage of His and Trp residues. Protection by sodium azide argues for a process mediated by singlet oxygen (1O2). In the work described here, the synthesis of 1O2 was measured by the bleaching of N,N-dimethyl-4-nitrosoaniline (RNO), the oxidation of added histidine and the oxidation of furfuryl alcohol. To obtain a more accurate value for 1O2 generation, a known quantity of 1O2 was generated by the thermal dissociation of 3-(4-methyl-naphthyl)propionic acid endoperoxide, and the efficiency of each assay method to report on the 1O2 generated was determined. The values obtained were 0.003 mol of RNO bleached/mol of 1O2 generated, 0.55 mol of furfuryl alcohol oxidized/mol 1O2 and 0.5 mol of His oxidized/mol 1O2 generated. Irradiation of the human lens proteins with UVA light produced from 2.1 to 2.4 mM of 1O2 by RNO bleaching, 2.6-2.8 mM 1O2 by furfuryl alcohol oxidation and up to 1.9 mM of 1O2 by histidine oxidation during a 1 h irradiation period. The average value (2.2 mM of 1O2) corresponds to the theoretical production of 30 nmol of singlet oxygen at UVA light intensities equivalent to a 1 h exposure to sunlight at noon in the northern hemisphere.