Identification of glutathionyl-3-hydroxykynurenine glucoside as a novel fluorophore associated with aging of the human lens

Effects of Protein-Derived Amino Acid Modification Products Present in Infant Formula on Metabolic Function, Oxidative Stress, and Intestinal Permeability in Cell Models

Proteins present in infant formulas are modified by oxidation and glycation during processing. Modified amino acid residues released from proteins may be absorbed in the gastrointestinal tract, and pose a health risk to infants. In this study, the markers of glycation furosine (1.7-3.5 μg per milligram of protein) and N^ε-(carboxymethyl)lysine (28-81 ng per milligram of protein) were quantitated in infant formulas. The effects of these species, and other amino acid modifications, at the levels detected in infant formulas, on 3T3-L1 (murine preadipocyte) and Caco-2 (human intestinal epithelial) cells were assessed. Incubation of 3T3-L1 cells for 48 h with amino acid side chain oxidation and glycation products (1 and 10 μM) resulted in a loss (up to 40%, p < 0.05) of cell thiols and decreased metabolic activity compared with those of the controls. In contrast, Caco-2 cells showed a stimulation (10-50%, p < 0.05) of cellular metabolism on exposure to these products for 24 or 48 h. A 28% ( p < 0.05) increase in protein carbonyls was detected upon incubation with 200 μM modified amino acids for 48 h, although no alteration in transepithelial electrical resistance was detected. Oxidation products were detected in the basolateral compartments of Caco-2 monolayers when modified amino acids were applied to the apical side, consistent with limited permeability (up to 3.4%) across the monolayer. These data indicate that modified amino acids present in infant formulas can induce effects on different cell types, with evidence of bioavailability and induction of cellular stress. This may lead to potential health risks for infants consistently exposed to high levels of infant formulas.

Age-related spatial differences of human lens UV filters revealed by negative ion mode MALDI imaging mass spectrometry

Tryptophan-derived UV filters are predominantly found in the lenses of primates and humans. While protective against UV radiation, aging alters the complement and spatial distributions of human lens UV filters, and a role for UV filters has been suggested in age-related cataract formation. To establish how the spatial distributions of UV filters change in normal human lens aging, matrix assisted laser desorption/ionisation-imaging mass spectrometry (MALDI-IMS) was utilised to map the locations and relative abundance of multiple UV filters simultaneously. Frozen human lenses were cryosectioned axially, and the 20 μm-thick sections coated with MALDI matrix via robotic sprayer and analysed using negative ion mode MALDI-Fourier transform-ion cyclotron resonance MS. While signal for many UV filters was detected throughout the lenses, signal intensity was generally highest in the central (embryonic) nucleus and decreased uniformly in outer (foetal, juvenile, adult) nuclear and cortical regions, and many UV filter signals declined with age. In contrast, two antioxidant-conjugated UV filters (Cys-3-OHKG and GSH-3-OHKG) were restricted to the lens nucleus and their relative signal increased with increasing lens age. The enhanced spatial resolution of MALDI-IMS over manual trephine dissection techniques and its multiplex capability allowed the spatial relationships between lens UV filters to be established and explored in relation to aging. Together these results confirmed that the complement of UV filters in each lens is dynamic and undergoes significant age-related changes. In the future, this information could be used to compare with other lens biomolecule changes to better understand the lens aging process and age-related cataract formation.

Low Serum Tryptophan Levels as an Indicator of Global Cognitive Performance in Nondemented Women over 50 Years of Age

Aging is a physiological decline process. The number of older adults is growing around the world; therefore, the incidence of cognitive impairment, dementia, and other diseases related to aging increases. The main cellular factors that converge in the aging process are mitochondrial dysfunction, antioxidant impairment, inflammation, and immune response decline, among others. In this context, these cellular changes have an influence on the kynurenine pathway (KP), the main route of tryptophan (Trp) catabolism. KP metabolites have been involved in the aging process and neurodegenerative diseases. Although there are changes in the metabolite levels with age, at this time, there is no study that has evaluated cognitive decline as a consequence of Trp catabolism fluctuation in aging. The aim of this study was to evaluate the relation between the changes in Trp catabolism and cognitive impairment associated with age through KP metabolites level alterations in women over 50 years of age. Seventy-seven nondemented women over 50 years old were examined with a standardized cognitive screening evaluation in Spanish language (Neuropsi), Beck anxiety inventory (BAI), and the geriatric depression scale (GDS). Also, serum levels of Trp, kynurenine (Kyn), kynurenic acid (KYNA), and 3-hydroykynurenine (3-HK) and the glutathione ratio (GSH/GSSG) were measured. Results showed a negative correlation between age and Trp levels and a positive correlation between age and KYNA/Trp and 3-HK/Trp ratios. The level of cognitive impairment showed a significant positive association with age and with kynurenine pathway activation and a significant negative correlation with Trp levels. The GSH/GSSG ratio correlated positively with Trp levels and negatively with Kyn/Trp and 3-HK/Trp ratios. The depression score correlated negatively with Trp and positively with the 3-HK/Trp ratio. We concluded that KP activation increases with age and it is strongly associated with the level of cognition performance in nondemented women over 50 years of age.

Evidence of glucuronidation of the glycation product LW-1: tentative structure and implications for the long-term complications of diabetes

LW-1 is a collagen-linked blue fluorophore whose skin levels increase with age, diabetes and end-stage renal disease (ESRD), and correlate with the long-term progression of microvascular disease and indices of subclinical cardiovascular disease in type 1 diabetes. The chemical structure of LW-1 is still elusive, but earlier NMR analyses showed it has a lysine residue in an aromatic ring coupled to a sugar molecule reminiscent of advanced glycation end-products (AGEs). We hypothesized and demonstrate here that the unknown sugar is a N-linked glucuronic acid. LW-1 was extracted and highly purified from ~99 g insoluble skin collagen obtained at autopsy from patients with diabetes/ESRD using multiple rounds of proteolytic digestion and purification by liquid chromatography (LC). Advanced NMR techniques (¹H-NMR, ¹³C-NMR, ¹H-¹³C HSQC, ¹H-¹H TOCSY, ¹H-¹³C HMBC) together with LC-mass spectrometry (MS) revealed a loss of 176 amu (atomic mass unit) unequivocally point to the presence of a glucuronic acid moiety in LW-1. To confirm this data, LW-1 was incubated with β-glycosidases (glucosidase, galactosidase, glucuronidase) and products were analyzed by LC-MS. Only glucuronidase could cleave the sugar from the parent molecule. These results establish LW-1 as a glucuronide, now named glucuronidine, and for the first time raise the possible existence of a "glucuronidation pathway of diabetic complications". Future research is needed to rigorously probe this concept and elucidate the molecular origin and biological source of a circulating glucuronidine aglycone.

3-Hydroxykynurenine and 3-Hydroxyanthranilic Acid Enhance the Toxicity Induced by Copper in Rat Astrocyte Culture

Copper is an integral component of various enzymes, necessary for mitochondrial respiration and other biological functions. Excess copper is related with neurodegenerative diseases as Alzheimer and is able to modify cellular redox environment, influencing its functions, signaling, and catabolic pathways. Tryptophan degradation through kynurenine pathway produces some metabolites with redox properties as 3-hydroxykynurenine (3-HK) and 3-hydroxyanthranilic acid (3-HANA). The imbalance in their production is related with some neuropathologies, where the common factors are oxidative stress, inflammation, and cell death. This study evaluated the effect of these kynurenines on the copper toxicity in astrocyte cultures. It assessed the CuSO₄ effect, alone and in combination with 3-HK or 3-HANA on MTT reduction, ROS production, mitochondrial membrane potential (MMP), GHS levels, and cell viability in primary cultured astrocytes. Also, the chelating copper effect of 3-HK and 3-HANA was evaluated. The results showed that CuSO4 decreased MTT reduction, MMP, and GSH levels while ROS production and cell death are increasing. Coincubation with 3-HK and 3-HANA enhances the toxic effect of copper in all the markers tested except in ROS production, which was abolished by these kynurenines. Data suggest that 3-HK and 3-HANA increased copper toxicity in an independent manner to ROS production.

Role of indoleamine 2,3-dioxygenase in health and disease

IDO1 (indoleamine 2,3-dioxygenase 1) is a member of a unique class of mammalian haem dioxygenases that catalyse the oxidative catabolism of the least-abundant essential amino acid, L-Trp (L-tryptophan), along the kynurenine pathway. Significant increases in knowledge have been recently gained with respect to understanding the fundamental biochemistry of IDO1 including its catalytic reaction mechanism, the scope of enzyme reactions it catalyses, the biochemical mechanisms controlling IDO1 expression and enzyme activity, and the discovery of enzyme inhibitors. Major advances in understanding the roles of IDO1 in physiology and disease have also been realised. IDO1 is recognised as a prominent immune regulatory enzyme capable of modulating immune cell activation status and phenotype via several molecular mechanisms including enzyme-dependent deprivation of L-Trp and its conversion into the aryl hydrocarbon receptor ligand kynurenine and other bioactive kynurenine pathway metabolites, or non-enzymatic cell signalling actions involving tyrosine phosphorylation of IDO1. Through these different modes of biochemical signalling, IDO1 regulates certain physiological functions (e.g. pregnancy) and modulates the pathogenesis and severity of diverse conditions including chronic inflammation, infectious disease, allergic and autoimmune disorders, transplantation, neuropathology and cancer. In the present review, we detail the current understanding of IDO1’s catalytic actions and the biochemical mechanisms regulating IDO1 expression and activity. We also discuss the biological functions of IDO1 with a focus on the enzyme's immune-modulatory function, its medical implications in diverse pathological settings and its utility as a therapeutic target.

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.

3-Hydroxykynurenine: an intriguing molecule exerting dual actions in the central nervous system

Kynurenine pathway is gaining attention due to the many metabolic processes in which it has been involved. The tryptophan conversion into several other metabolites through this pathway provides neuronal and redox modulators useful for maintenance of major functions in the brain. However, when physiopathological conditions prevail - i.e. oxidative stress, excitotoxicity, and inflammation - preferential formation and accumulation of toxic metabolites could trigger factors for degeneration in neurological disorders. 3-Hydroxykynurenine has been largely described as one of these toxic metabolites capable of inducing oxidative damage and cell death; consequently, this metabolite has been hypothesized to play a pivotal role in different neurological and psychiatric disorders. Supporting evidence has shown altered 3-hydroxykynurenine levels in samples of patients from several disorders. In contrast, some experimental studies have provided evidence of antioxidant and scavenging properties inherent to this molecule. In this review, we explored most of literature favoring one or the other concept, in order to provide an accurate vision on the real participation of this tryptophan metabolite in both experimental paradigms and human brain pathologies. Through this collected evidence, we provide an integrative hypothesis on how 3-hydroxykynurenine is exerting its dual actions in the central nervous system and what will be the course of investigations in this field for the next years.

Are ancient proteins responsible for the age-related decline in health and fitness?

There are a number of sites in the body where proteins are present for decades and sometimes for all of our lives. Over a period of many years, such proteins are subject to two types of modifications. The first results from the intrinsic instability of certain amino acid residues and leads to deamidation, racemization, and truncation. The second type can be traced to relentless covalent modification of such ancient proteins by reactive biochemicals produced during cellular metabolism.The accumulation of both types of posttranslational modifications over time may have important consequences for the properties of tissues that contain such proteins. It is proposed that the age-related decline in function of organs such as the eye, heart, brain, and lung, as well as skeletal components, comes about, in part, from the posttranslational modification of these long-lived proteins. Examples are provided in which this may be an important factor in the etiology of age-related conditions. As the properties of these proteins alter inexorably over time, the molecular changes contribute to a gradual decline in the function of individual organs and also tissues such as joints. This cumulative degeneration of old proteins at multiple sites in the body may also constrain the ultimate life span of the individual. The human lens may be particularly useful for discovering which reactive metabolites in the body are of most importance for posttranslational modification of long-lived proteins.

Presbyopia and cataract: a question of heat and time

Not only are human lenses different in many ways from those of non-primates, they also undergo dramatic changes with age. These age-dependent alterations lead to perturbations in the properties of older lenses, and ultimately to disturbances in visual function, which typically become apparent at middle age. Recent data suggest that many, if not all, of these age-dependent features can be traced to the lack of macromolecular turnover in the lens and to the inexorable modifications to proteins and membrane components over a period of decades. Exposure of lenses to heat can reproduce many of these alterations, suggesting that long-term incubation at body temperature may be an important factor in aging the human lens. Two conclusions flow from this. Firstly, the human lens may be an ideal tissue for studying macromolecular aging in man. Secondly, it will be extremely challenging to examine the origin of human age-related conditions, such as presbyopia and nuclear cataract, using traditional laboratory animals. Characterising the unfolding and decomposition of long-lived macromolecules appears to provide the key to understanding the two most common human lens disorders: presbyopia and age-related nuclear cataract.

Modulation of advanced glycation endproduct synthesis by kynurenines in human lens proteins

Human lens proteins (HLP) become chemically modified by kynurenines and advanced glycation end products (AGEs) during aging and cataractogenesis. We investigated the effects of kynurenines on AGE synthesis in HLP. We found that incubation with 5 mM ribose or 5 mM ascorbate produced significant quantities of pentosidine, and this was further enhanced in the presence of two different kynurenines (200-500 microM): N-formylkynurenine (Nfk) and kynurenine (Kyn). Another related compound, 3-hydroxykynurenine (3OH-Kyn), had disparate effects; low concentrations (10-200 microM) promoted pentosidine synthesis, but high concentrations (200-500 microM) inhibited it. 3OH-Kyn showed similar effects on pentosidine synthesis from Amadori-enriched HLP or ribated lysine. Chelex-100 treatment of phosphate buffer reduced pentosidine synthesis from Amadori-enriched HLP by approximately 90%, but it did not inhibit the stimulating effect of 3OH-Kyn and EDTA. 3OH-Kyn (100-500 microM) spontaneously produced copious amounts of H(2)O(2) (10-25 microM), but externally added H(2)O(2) had only a mild stimulating effect on pentosidine but had no effect on N(epsilon)-carboxymethyl lysine (CML) synthesis in HLP from ribose and ascorbate. Further, human lens epithelial cells incubated with ribose and 3OH-Kyn showed higher intracellular pentosidine than cells incubated with ribose alone. CML synthesis from glycating agents was inhibited 30 to 50% by 3OH-Kyn at concentrations of 100-500 microM. Argpyrimidine synthesis from 5mM methylglyoxal was slightly inhibited by all kynurenines at concentrations of 100-500 microM. These results suggest that AGE synthesis in HLP is modulated by kynurenines, and such effects indicate a mode of interplay between kynurenines and carbohydrates important for AGE formation during lens aging and cataract formation.

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.

Role of lipid peroxidation in pathogenesis of senile cataract

Background /Aim. Cataract is a structural, biochemical and optical change in the eye lens, which changes transmission and refraction of light rays reducing keenness and clarity of a figure on the retina. Its occurrence is highest in older people, over the age of 65 (45.9%), thus a certain degree of opacification exists practically in all people over the 70. Our research was directed to measuring of lipid peroxidation products in cataract lenses involved in early stages of cataractogenesis through oxidative stress and in the development of mature cataract. Methods. Clinical and biochemical research was carried out in 101 patients with cataract, 46 women and 55 men. The average age of the group was 72.47 (? = 7.98). According to the cataract maturity degree the patients were classified into two groups as follows: cataracta senilis incipiens (n = 41) and cataracta senilis matura (n = 60). Measuring of diene conjugates was carried out by spectrophotometer. Fluorescent lipid peroxidation products were measured by a spectrofluorophotometer, and malondialdehyde (MDA) concentration was measured by colorimeter as a product of a reaction with thiobarbituric acid (TBA). Result. Significantly higher diene conjugated concentration in lenses was measured in the patients with the diagnosis cataracta senilis incipiens (p < 0.001) as well as the intensity of fluorescent iminopropens (p < 0.001). Significantly higher MDA concentration in lens (p < 0.001) was measured in the patients with cataracta senilis matura. Conclusion. The lens structure changes caused by lipid peroxidation can, with other risk factors present, influence the occurrence and development of mature cataract. Some cataract types show different lipid peroxidation intensity with the most distinct changes in cataract which started as corticonuclear.

Reactions of the melatonin metabolite N1-acetyl-5-methoxykynuramine (AMK) with the tyrosine side-chain fragment, 4-ethylphenol

The melatonin metabolite N(1)-acetyl-5-methoxykynuramine (AMK) has previously been shown to interact with various free radicals. Using the ABTS cation radical [ABTS = 2,2'-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid)] as an electron abstracting reactant, which does not destroy the aromate, we found that the reactive intermediate derived from AMK strongly interacts with the benzene rings of other AMK molecules to form di- and oligomers. Since oligomerization is rather unlikely at physiological concentrations, we investigated reactions with other putative reaction partners. The incubation of tyrosine or several of its structural analogs with AMK in the presence of the ABTS cation radical led to numerous products, amongst which were compounds not detected when one of the educts was incubated with the ABTS cation radical alone. With tyrosine and most of its analogs, the number of products formed in the presence of AMK and ABTS cation radical was relatively high and included numerous oligomers. To optimize the yield of products of interest as well as their separation from other compounds, especially oligomers, we investigated the interaction with 4-ethylphenol, which represents the side chain of tyrosine lacking the carboxyl and amino residues of the amino acid, which otherwise can undergo additional reactions. A prominent product was chromatographically separated and analyzed by mass spectrometry [(+)-ESI-MS, (-)-ESI-MS, (+)-HRESI-MS], (1)H-NMR, and H,H-COSY correlations. The substance was identified as N-{3-[2'-(5''-ethyl-2''-hydroxyphenylamino)-5'-methoxyphenyl]-3-oxopropyl} acetamide. This chemically novel compound represents an adduct in which the amino nitrogen of AMK is attached to the C-2 atom of 4-ethylphenol, which corresponds to the C-3 atom in the benzene ring of tyrosine. This finding suggests that, upon interaction of AMK with an electron-abstracting radical, the kynuric intermediate may modify proteins at superficially accessible tyrosine residues. In fact, protein modification by an unidentified melatonin metabolite has been observed in an earlier study. The possibility of protein AMKylation may be of interest with regard to an eventual interference with tyrosine nitration or, more importantly, with tyrosine phosphorylation.

Presbyopia. Emerging from a blur towards an understanding of the molecular basis for this most common eye condition

All people will be presbyopic by age 50, and we now understand something of the basis for this condition. It turns out to be a direct consequence of two features; first the design of the transparent lens and the way it must change shape to enable focussing by the human eye, and second the instability of proteins over a very long time period. The incremental changes that take place in the lens to render the central region inflexible by middle age and, as a consequence the person presbyopic, may also promote the subsequent development of cataract. Based on the most recent data, heat-induced denaturation of proteins in the lens appears to be a worthy topic for future investigation. Understanding such processes may allow us to glimpse the origin both of presbyopia and age-related nuclear cataract.

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.

The effect of single and repeated UVB radiation on rabbit lens

BACKGROUND

In our previous investigations, a significant cumulative effect of ultraviolet radiation (UVR) on the corneal and aqueous humour metabolic profiles was revealed. The purpose of the present study was to monitor the alterations in the rabbit lenses under the same experimental design and thereby supplement and complete prior findings.

METHODS

Albino rabbit eyes were exposed to single (312 nm, 3.12 J/cm2) or repeated (312 nm, 3 x 1.04 J/cm2) UVB irradiations of the same overall doses. Lenticular samples were analysed by high resolution magic angle spinning proton nuclear magnetic resonance (HR-MAS 1H NMR) spectroscopy. Special grouping patterns between the UVB-irradiated and untreated control samples were evaluated using principal component analysis (PCA). Percentage alterations in the lenticular metabolite concentrations from UVR-B exposed rabbits were calculated relative to the levels in the control group.

RESULTS

UVB irradiation of the albino rabbit lenses resulted in a significant decrease in the concentrations of antioxidants (glutathione), osmolytes (taurine, myoinositol) and amino acids (alanine), and a concomitant elevation in the contents of a sugar-related compound, sorbitol. Repeated UVR-B exposure of the rabbit eye had a stronger effect on the lenticular metabolic profile than a single irradiation of the same overall dose.

CONCLUSIONS

This study reveals the cumulative effect of repeated UVB irradiations, and shows that even a 48-hour interval between subsequent UVR-B exposures is not sufficient for the healing processes to restore lenticular integrity.

Detection of kynurenine modifications in proteins using a monoclonal antibody

N-formylkynurenine and kynurenine are oxidation products of tryptophan formed from the reaction catalyzed by indoleamine 2,3-dioxygenase. These kynurenines react with proteins to produce chemical modifications in the lens. We developed a novel monoclonal antibody that detects a kynurenine modification in proteins. The antibody recognized proteins (human lens proteins, RNase A and BSA) that were modified by either kynurenine or N-formylkynurenine. The antibody also reacted strongly with N-formylkynurenine-modified N(alpha)-acetyl histidine and weakly with N-formylkynurenine-modified N(alpha)-acetyl lysine, N(alpha)-acetyl cysteine and N(alpha)-acetyl arginine. The antibody recognized kynurenine and N-formylkynurenine but not other tryptophan oxidation products. We isolated and purified a major antigen from the reaction mixture of N(alpha)-acetyl histidine and N-formylkynurenine and identified the product as N-acetyl-1-[3-(2-aminophenyl)-1-carboxy-3-oxopropyl]-histidine. We then used our purified antibody to detect kynurenine modifications in kynurenine-treated human lens epithelial cells and human lens. We found epithelial immunoreactivity in a lens from an aged donor but not in one from a very young donor. This would suggest that the antibody detects age-related changes in lens proteins altered by kynurenines. We believe that our antibody could be used to establish the importance of kynurenine modifications in diseases where tryptophan oxidation is enhanced.

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.

Protein-bound and free UV filters in cataract lenses. The concentration of UV filters is much lower than in normal lenses

In human cataract lenses the UV filters, 3-hydroxykynurenine glucoside (3OHKG) and kynurenine (Kyn) were found to be covalently bound to proteins and the levels in the nucleus were much higher than in the cortex. The levels of the bound UV filters in cataract nuclei were much lower than those in age-matched normal lenses. 3-Hydroxykynurenine could not be detected in cataract lenses. As with normal lenses, protein-bound 3OHKG in cataract lenses was found at the highest levels followed by Kyn. Free UV filter concentrations were also markedly reduced in cataract lenses. This feature may well contribute to the lower protein-bound levels; however, there was no clear relationship between free and bound UV filter contents when individual lenses were examined. We propose that since cysteine is a major site for UV filter binding, the well-documented oxidation of protein sulfhydryl groups during the progression of nuclear cataract may account, in part, for the pronounced decrease in bound UV filters in cataract lenses.

Identification of the new UV filter compound cysteine-l-3-hydroxykynurenineO-β-d-glucoside in human lenses

UV filters protect the human lens and retina from UV light-induced damage. Here, we report the identification of a new UV filter, cysteine-l-3-hydroxykynurenine O-beta-d-glucoside, which is present in older normal human lenses. Its structure was confirmed by independent synthesis. It is likely this novel UV filter is formed in the lens by nucleophilic attack of cysteine on the unsaturated ketone derived from deamination of 3-hydroxykynurenine O-beta-d-glucoside. Quantitation studies revealed considerable variation in normal lens levels that may be traced to the marked instability of the cysteine adduct. The novel UV filter was not detected in advanced nuclear cataract lenses.

Identification of 3-hydroxykynurenine bound to proteins in the human lens. A possible role in age-related nuclear cataract

Age-related nuclear (ARN) cataract is a major cause of world blindness. With the onset of ARN cataract, the normally transparent and colorless lens becomes opaque and can take on colors ranging from orange, brown, and even black. The molecular basis for this remarkable transformation is unknown. ARN cataract is also characterized by extensive oxidation, insolubilization, and cross-linking of polypeptides, particularly in the nucleus of the lens. It has been postulated that 3-hydroxykynurenine (3OHKyn) may be involved in these changes. This endogenous tryptophan metabolite is readily oxidized and is involved in the tanning of moth cocoons and the formation of pigments in the eyes of butterflies. 3OHKyn is a component of our primate-specific UV-filter pathway, and the brownish hue of ARN cataract lenses is also unique to humans. Because numerous colored compounds can be produced by autoxidation of 3OHKyn, this process could provide an explanation for the variety of lens colors and other changes seen in ARN cataract. For such a theory to be tenable, it needs to be demonstrated that 3OHKyn is bound to proteins in the human lens. Here, we show that all normal lenses older than 50 have 3OHKyn covalently attached to the nuclear proteins, most likely via cysteine residues. If indeed 3OHKyn is implicated in ARN cataract, a reduction in the levels that are bound in cataract, compared to normal lenses, would be expected. In agreement with this hypothesis, no bound 3OHKyn could be detected in proteins isolated from ARN cataract lenses.

UV filters in the lens of the thirteen lined ground squirrel (Spermophilus tridecemlineatus)

Major UV filters have been identified in the lens of the 13 lined ground squirrel (Spermophilus tridecemlineatus). These were found to be N-acetyl-3-hydroxykynurenine and N-acetyl-kynurenine, in addition to a small quantity of 3-hydroxykynurenine. The level of N-acetyl-3-hydroxykynurenine measured in the ground squirrel lens, 8.2mM, is approximately 11 times the concentration of 3-hyroxykynurenine glucoside reported previously for the human lens. Two additional UV filters of related structure were also present; however, their structures are still under investigation. HPLC elution profiles indicated that the ground squirrel lens cortex and nucleus contained comparable amounts of alpha-, beta(H)-, beta(L)-, and gamma-crystallins. Levels of GSH in the cortex and nucleus were 12.4 and 7.4mM, respectively. Such high concentrations of GSH may act to inhibit oxidation of the 3-hydroxykynurenine and N-acetyl-3-hydroxykynurenine. N-Acetylated kynurenines are less labile than those with free alpha-amino groups since N-acetyl-alpha-amino groups do not undergo spontaneous deamination. This modification thus stabilises the squirrel UV filters. In addition, because deamination is prevented, the decomposition products will not be involved in binding to lens proteins. Because of the similarity of the UV filters present in the ground squirrel to those in man, this species may be a suitable animal model for investigating the effects of UV radiation on cataract, and other ocular diseases, thought to involve exposure to light.

Impact of UVR-A on whole human lenses, supernatants of buffered human lens homogenates, and purified argpyrimidine and 3-OH-kynurenine

PURPOSE

Yellow chromophores and fluorescent compounds accumulate in the lens with age. Some of these compounds are photochemically active. The present study aimed to examine the photochemical effect of ultraviolet radiation-A (UVR-A) on the human lens.

METHODS

Intact human lenses and supernatants of buffered lens homogenates were exposed to UVR-A. The effect of UVR-A was evaluated by time-resolved and steady-state fluorescence spectroscopy, visual evaluation of colour and protein gel electrophoresis.

RESULTS

Intact lenses exposed to UVR-A showed no changes in time-resolved or steady-state fluorescence properties but the yellow coloration was visibly attenuated. The supernatants of buffered lens homogenates exposed to UVR-A demonstrated a reduction in time-resolved and steady-state fluorescent properties and protein cross-linking.

CONCLUSIONS

Exposure of the intact lens to UVR-A causes chromophore bleaching without affecting fluorescence, indicating that non-fluorescent chromophores have been destroyed. After homogenization, both chromophores and fluorophores from the lens suffer damage and proteins aggregate. This indicates that powerful mechanisms of protection against UVR-A found in the intact lens are disturbed by homogenization of the lens, suggesting that isolated lens proteins cannot be used as a model system for studying cataractogenesis. Hypothetically, the protective mechanism could be related to the rigidly packed three-dimensional structure of the lens proteins or to the abundance of antioxidative and free radical scavenging defence systems.

3-hydroxykynurenine-mediated modification of human lens proteins: structure determination of a major modification using a monoclonal antibody

Tryptophan can be oxidized in the eye lens by both enzymatic and non-enzymatic mechanisms. Oxidation products, such as kynurenines, react with proteins to form yellow-brown pigments and cause covalent cross-linking. We generated a monoclonal antibody against 3-hydroxykynurenine (3OHKYN)-modified keyhole limpet hemocyanin and characterized it using 3OHKYN-modified amino acids and proteins. This monoclonal antibody reacted with 3OHKYN-modified N(alpha)-acetyl lysine, N(alpha)-acetyl histidine, N(alpha)-acetyl arginine, and N(alpha)-acetyl cysteine. Among the several tryptophan oxidation products tested, 3OHKYN produced the highest concentration of antigen when reacted with human lens proteins. A major antigen from the reaction of 3OHKYN and N(alpha)-acetyl lysine was purified by reversed phase high pressure liquid chromatography, which was characterized by spectroscopy and identified as 2-amino-3-hydroxyl-alpha-((5S)-5-acetamino-5-carboxypentyl amino)-gamma-oxo-benzene butanoic acid. Enzyme-digested cataractous lens proteins displayed 3OHKYN-derived modifications. Immunohistochemistry revealed 3OHKYN modifications in proteins associated with the lens fiber cell plasma membrane. The low molecular products (<10,000 Da) isolated from normal lenses after reaction with glucosidase followed by incubation with proteins generated 3OHKYN-derived products. Human lens epithelial cells incubated with 3OHKYN showed intense immunoreactivity. We also investigated the effect of glycation on tryptophan oxidation and kynurenine-mediated modification of lens proteins. The results showed that glycation products failed to oxidize tryptophan or generate kynurenine modifications in proteins. Our studies indicate that 3OHKYN modifies lens proteins independent of glycation to form products that may contribute to protein aggregation and browning during cataract formation.

The photosensitiser xanthurenic acid is not present in normal human lenses

UV light has often been investigated as a risk factor for the most common cause of blindness, human age-related cataract. One mechanism whereby UV light could induce cataract is via the action of photosensitisers. In this regard, xanthurenic acid has recently been highlighted since it has been reported to be present in the human lens and, in model studies, it markedly enhances the photo-oxidation of proteins by wavelengths of light that penetrate the cornea. In this study we used HPLC and mass spectrometry to examine whether xanthurenic acid is indeed present in human lenses and, if so, the effect of age on its lenticular concentration. Xanthurenic acid could be formed artefactually by incubation of 3-hydroxykynurenine (3OHKyn) yellow, a known autoxidation product of the lenticular UV filter, 3OHKyn, in the presence of air and light, however, it could not be detected in any human lenses studied. Therefore, it appears unlikely that xanthurenic acid plays a role in lens aging or human cataract.

Age-related nuclear cataract and indoleamine 2,3-dioxygenase-initiated tryptophan metabolism in the human lens

Tryptophan-derived UV filters (kynurenine and 3-hydroxylkynurenine glucoside) have recently been shown to bind to human lens proteins. These UV filter adducts increase in amount with age and appear to be mainly responsible for the yellowing of the lens in man. On the basis of research performed in other tissues, it has been assumed that indoleamine 2,3-dioxygenase (IDO) may be the first and probably rate-limiting enzyme in UV filter biosynthesis. In this study, 25 human lenses were examined by a reliable and sensitive assay method with a monoclonal antibody specific for IDO. IDO activity was detected in all lenses ranging from 26 to 80 years, and there was no clear relationship of IDO activity with age. The mean activity was 0.85 + 0.49 nmol of kynurenine/h/lens. The level in the iris/ciliary body was negligible (<0.05 nmol of kynurenine/h). The lens IDO activity is consistent with UV filter turnover values obtained previously. These findings indicate that IDO is the first enzyme in the UV filter pathway and that UV filter biosynthesis is active even in aged lenses. Yellowing of the aged lens may therefore be preventable by drug-induced suppression of IDO activity.

UV filter instability: consequences for the human lens

Human lenses appear to become coloured with age primarily due to the covalent binding of UV filter compounds to lens proteins. These crystallin modifications result from the inherent instability of the kynurenine UV filters. Here we investigate this decomposition, the role this may have in the formation of other primate UV filters, and the interaction of the intermediates (alpha,beta-ketoalkenes) with lens components. The UV filters kynurenine, 3-hydroxykynurenine and 3-hydroxykynurenine glucoside were incubated at neutral pH in the presence or absence of NADH or NADPH. The three UV filters underwent spontaneous deamination, such that at pH 7 less than half of the starting materials (kynurenine (42%), 3-hydroxykynurenine glucoside (30%) and 3-hydroxykynurenine (21%)) remained after 7 days. In the presence of NAD(P)H, the double bond of the UV filter-derived deamination compounds, were reduced. Deamination of 3-hydroxykynurenine glucoside, followed by reduction with NAD(P)H, could thus account for the formation of the major lens UV filter 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid glucoside. beta-Benzoylacrylic acid, which possesses the same alpha,beta-ketoalkene sidechain as the deaminated kynurenine UV filters, underwent Michael addition with glutathione, was reduced (hydrogenated) by NAD(P)H, however was unreactive with ascorbate. Surprisingly, at pH 7 the UV filter-derived alpha,beta-ketoalkene intermediates, do not readily undergo intramolecular cyclization. This feature makes the double bond more available for reaction with protein nucleophilic residues and other lens components such as glutathione. On the basis of these data it is likely that glutathione and NAD(P)H, but not ascorbate, protect proteins in the lens from modification by UV filters.

Rate of formation of AGEs during ascorbate glycation and during aging in human lens tissue

The similarity of the yellow chromophores isolated from human cataracts with those from ascorbic acid modified calf lens proteins was recently published [Biochim. Biophys. Acta 1537 (2001) 14]. The data presented here additionally quantify age-dependent increases in individual yellow chromophores and fluorophores in the water-insoluble fraction of normal human lens. The water-insoluble fraction of individual normal human lens was isolated, solubilized by sonication and digested with a battery of proteolytic enzymes under argon to prevent oxidation. The level of A(330)-absorbing yellow chromophores, 350/450 nm fluorophores and total water-insoluble (WI) protein were quantified in each lens. The total yellow chromophores and fluorophores accumulated in parallel with the increase in the water-insoluble protein fraction during aging. The digest from each single human lens was then subjected to Bio-Gel P-2 size-exclusion chromatography. The fractions obtained were further separated by a semi-preparative prodigy C-18 high-performance liquid chromatography (RP-HPLC). Bio-Gel P-2 chromatography showed four major fractions, each of which increased with age. RP-HPLC of the amino acid peak resolved five major A(330)-absorbing peaks and eight fluorescent peaks, and each peak increased coordinately with age. A late-eluting peak, which contained hydrophobic amino acids increased significantly after age 60. Aliquots from an in vitro glycation of calf lens proteins by ascorbic acid were removed and subjected to the same enzymatic digestion. Ascorbic acid-modified calf lens protein digests showed an almost identical profile of chromophores, which also increased in a time-dependent manner. The late-eluting peak, however, did not increase with the time of glycation and may not be an advanced glycation endproduct (AGE) product. The data indicate that the total water-insoluble proteins, individual yellow chromophores and fluorophores increased equally both with aging in normal human lens and during ascorbate glycation in vitro. The major protein modifications, which accumulate during aging, therefore, appear to be AGEs. Whereas the late-eluting peak, which showed poor correlation to ascorbylation, may represent UV filter compounds bound to lens proteins.

Glutathione and NADH, but not ascorbate, protect lens proteins from modification by UV filters

Age-dependent human lens colouration and fluorescence may stem primarily from the covalent binding of UV filters to crystallins. The tendency of the kynurenine (Kyn) UV filters to deaminate at neutral pH, with the generation of reactive alpha,beta-ketoalkenes, underlies this phenomenon. In this study the authors examined the ability of small molecular weight antioxidants, which are known to be present in the lens, to inhibit this process. Crystallins were incubated with Kyn at pH 7 in the presence of glutathione (GSH), ascorbate or NADH. Ascorbate, even at high (15 m M) levels, was not found to significantly retard the time-dependent covalent binding of Kyn to the proteins. GSH, and to a lesser extent NADH, however, had a major impact in preventing this modification. The increase in protein UV absorbance and fluorescence was inhibited by GSH intercepting the reactive ketone intermediate, to form a GSH-Kyn adduct. NADH seemed to protect by both reduction of the reactive ketone intermediate and by competing with Kyn for presumably hydrophobic sites on the crystallins. This may indicate that the covalent attachment of aromatic Kyn molecules could be facilitated by initial hydrophobic interactions. Since GSH is present at far greater concentrations than NADH, these results show that in primate lenses, GSH is the key agent responsible for protecting the crystallins from covalent modification.

Novel protein modification by kynurenine in human lenses

It is known that human lenses increase in color and fluorescence with age, but the molecular basis for this is not well understood. We demonstrate here that proteins isolated from human lenses contain significant levels of the UV filter kynurenine covalently bound to histidine and lysine residues. Identification was confirmed by synthesis of the kynurenine amino acid adducts and comparison of the chromatographic retention times and mass spectra of these authentic standards with those of corresponding adducts isolated from human lenses following acid hydrolysis. Using calf lens proteins as a model, covalent binding of kynurenine to lens proteins has been shown to proceed via side chain deamination in a manner analogous to that observed for the related UV filter, 3-hydroxykynurenine O-beta-D-glucoside. Levels of histidylkynurenine and lysylkynurenine were low in human lenses in subjects younger than 30, but thereafter increased in concentration with the age of the individual. Post-translational modification of lens proteins by tryptophan metabolites therefore appears to be responsible, at least in part, for the age-dependent increase in coloration and fluorescence of the human lens, and this process may also be important in other tissues in which up-regulation of tryptophan catabolism occurs.

Identification of a new human lens UV filter compound

A new UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-diglucoside, has been identified in human lenses. The structure suggests that it is a further metabolic product of the second most abundant UV filter compound, 4-(2-amino-3-hydroxyphenyl)-4-oxobutanoic acid O-glucoside. Quantification studies on the new compound show that it decreases towards zero in both the nucleus and cortex as a function of age. The discovery of this novel disaccharide completes the identification of the major UV filter compounds present in the human lens.

Photo-oxidation of proteins and its role in cataractogenesis

Proteins comprise approximately 68% of the dry weight of cells and tissues and are therefore potentially major targets for photo-oxidation. Two major types of processes can occur with proteins. The first of these involves direct photo-oxidation arising from the absorption of UV radiation by the protein, or bound chromophore groups, thereby generating excited states (singlet or triplets) or radicals via photo-ionisation. The second major process involves indirect oxidation of the protein via the formation and subsequent reactions of singlet oxygen generated by the transfer of energy to ground state (triplet) molecular oxygen by either protein-bound, or other, chromophores. The basic principles behind these mechanisms of photo-oxidation of amino acids, peptides and proteins and the potential selectivity of damage are discussed. Emphasis is placed primarily on the intermediates that are generated on amino acids and proteins, and the subsequent reactions of these species, and not the identity or chemistry of the sensitizer itself, unless the sensitizing group is itself intrinsic to the protein. A particular system is then discussed--the cataractous lens--where UV photo-oxidation may play a role in the aetiology of the disease, and tryptophan-derived metabolites act as UV filters.

Kynurenine binds to the peptide binding region of the chaperone alphaB-crystallin

UV filters, such as kynurenine, are present in the human lens. They are spontaneously unstable at neutral pH and deaminate to form reactive alpha, beta unsaturated ketones. This process becomes more prominent after the lens barrier develops in middle age. Here we show that deaminated kynurenine reacts primarily with histidine residues in alphaB-crystallin: a major lens protein that lacks cysteine. Five of the nine histidines in alphaB-crystallin were found to be conjugated with kynurenine. Furthermore, a major site of covalent modification was at histidine 83, which is found in the putative peptide binding region of alphaB-crystallin; a site crucial for its role as a chaperone. We propose that modification of alphaB-crystallin by UV filters may compromise the chaperone action of this protein.

A study of kynurenine fragmentation using electrospray tandem mass spectrometry

A combination of accurate mass measurement and tandem mass spectrometry (both product ion and precursor ion scans) have been used to characterize the major fragment ions observed in the ESI mass spectrum of kynurenine. Kynurenine is a metabolite of tryptophan found in the human lens and is thought to play a role in protecting the retina from UV-induced damage. Three major fragmentation pathways were evident, following initial elimination either of ammonia, H2O and CO or the imine form of glycine. The latter is proposed to occur via the formation of an ion-molecule complex. In the case of loss of H2O and CO from deaminated kynurenine, there is evidence for an acylium ion intermediate, which is not observed for the loss of H2O and CO directly from protonated kynurenine. Product ion scans of deuterated kynurenine enabled the elucidation of structural rearrangements that were not evident in the spectra of the native compound. Since UV filter compounds can often only be isolated in small quantities from the lens, this study forms the basis for the characterization of novel UV filter compounds using mass spectrometry. The approach presented here may also be useful for the characterization of related classes of small molecules.

Photochemically modified alpha-crystallin: a model system for aging in the primate lens

The purpose of this study was to quantitatively study the changes that occur upon irradiation of 3-hydroxykynurenine (3-HK) in the presence of alpha-crystallin under conditions similar to those in the lens. The samples were prepared in 10 mM phosphate buffer at pH 7.4, bubbled with O2 or Ar and irradiated with 300-400 nm light. The amount of light absorbed by the samples (Iabs) was measured using azobenzene as an actinometer. Modifications to alpha-crystallin were monitored by ultraviolet-visible and fluorescence spectroscopy. Aerobic samples had increased absorption around 320 nm and above 400 nm while the 3-HK maximum at 368 nm decreased. The isolated modified protein showed that there was increased absorption throughout the spectrum. Changes in the anaerobic samples were similar to those of the aerobic but occurred more slowly. As irradiation time increased fluorescence emission of the isolated protein red shifted and quantum yields of fluorescence (phi f) were calculated at different irradiation time intervals by comparison to 3-HK. By comparing OD320/OD365 for the model system to values from primate lenses, Iabs can be correlated with age and transmission of the sample in the blue region of the spectrum and thus allows lenticular aging to be quantitated.

Indoleamine 2,3-dioxygenase in the human lens, the first enzyme in the synthesis of UV filters

Tryptophan-derived UV filters have recently been shown to bind to human lens proteins. These UV filter adducts increase in amount with age and appear to be mainly responsible for the yellowing of the lens in man. On the basis of research performed in other tissues, it has been assumed that indoleamine 2,3-dioxygenase (IDO) may be the first and probably rate-limiting enzyme in UV filter biosynthesis. In this study, 25 human lenses were examined by a reliable and sensitive assay method with a monoclonal antibody specific for IDO. IDO activity was detected in all lenses ranging from 26 to 80 years, and there was no clear relationship of IDO activity with age. The mean activity was 0.85 +/- 0.49 nmol of kynurenine formed hr(-1)per lens. IDO expression was found to be localized in the anterior cortex of the lens with little or no activity in the posterior cortex or nucleus. The level in the iris/ciliary body was negligible (<0.05 nmol of kynurenine formed hr(-1)). The lens IDO activity is consistent with UV filter turnover values obtained previously. These findings indicate that IDO is the first enzyme in the UV filter pathway and that UV filter biosynthesis is active even in aged lenses. Yellowing of the aged lens may therefore be preventable by drug-induced suppression of lens IDO activity.