Non-oxidative modification of lens crystallins by kynurenine: a novel post-translational protein modification with possible relevance to ageing and cataract

Immunomodulatory actions of a kynurenine-derived endogenous electrophile

The up-regulation of kynurenine metabolism induces immunomodulatory responses via incompletely understood mechanisms. We report that increases in cellular and systemic kynurenine levels yield the electrophilic derivative kynurenine-carboxyketoalkene (Kyn-CKA), as evidenced by the accumulation of thiol conjugates and saturated metabolites. Kyn-CKA induces NFE2 like bZIP transcription factor 2- and aryl hydrocarbon receptor-regulated genes and inhibits nuclear factor κB- and NLR family pyrin domain containing 3-dependent proinflammatory signaling. Sickle cell disease (SCD) is a hereditary hemolytic condition characterized by basal inflammation and recurrent vaso-occlusive crises. Both transgenic SCD mice and patients with SCD exhibit increased kynurenine and Kyn-CKA metabolite levels. Plasma hemin and kynurenine concentrations are positively correlated, indicating that Kyn-CKA synthesis in SCD is up-regulated during pathogenic vascular stress. Administration of Kyn-CKA abrogated pulmonary microvasculature occlusion in SCD mice, an important factor in lung injury development. These findings demonstrate that the up-regulation of kynurenine synthesis and its metabolism to Kyn-CKA is an adaptive response that attenuates inflammation and protects tissues.

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.

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.

Optical properties of the human lens constituents

The absorption and fluorescence properties of the metabolomic (MET), water-soluble and urea-soluble protein fractions from the middle-age, aged, and cataractous human lenses have been measured. At 280nm and 300nm the major lens absorbers are crystallins, which absorb more than 90% of light in the UV-B region (280-315nm). In middle-aged lenses, the absorption at 360nm is mostly provided by UV filters contained in the MET fraction. With aging, and especially with the cataract development, the absorption of MET fraction in UV-A region (315-400nm) decreases due to the drop of the UV filter concentration, while the absorption of protein fractions increases due to the accumulation of post-translational modifications. Consequently, the contribution of the MET fraction into the total lens absorption at 360nm decays from 63% in middle-aged lenses to 25% in aged lenses to 3% in cataractous lenses. The fluorescence yield of the MET fraction from cataractous lenses also significantly increases. Therefore, the protection of the lens tissue against UV radiation in aged and cataractous lenses weakens: the absorption of UV-A light is mostly provided by modified crystallins and non-UV-filter metabolites, which are photochemically more active than the UV filters. The obtained data indicate that the aged and cataractous human lenses are more vulnerable to UV-A light than the middle-aged lenses.

Age-related changes in the kinetics of human lenses: prevention of the cataract

The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina and, by changing shape, it adjusts focal distance (accommodation). The three classes of structural proteins found in the lens are α, β, and γ crystallins. These proteins make up more than 90% of the total dry mass of the eye lens. Other components which can be found are sugars, lipids, water, several antioxidants and low weight molecules. When ageing changes occur in the lens, it causes a gradual reduction in transparency, presbyopia and an increase in the scattering and aberration of light waves as well as a degradation of the optical quality of the eye. The main changes that occur with aging are: 1) reduced diffusion of water from the outside to the inside of the lens and from its cortical to its nuclear zone; 2) crystalline change due to the accumulation of high molecular weight aggregates and insoluble proteins; 3) production of advanced glycation end products (AGEs), lipid accumulation, reduction of reduced glutathione content and destruction of ascorbic acid. Even if effective strategies in preventing cataract onset are not already known, good results have been reached in some cases with oral administration of antioxidant substances such as caffeine, pyruvic acid, epigallocatechin gallate (EGCG), α-lipoic acid and ascorbic acid. Furthermore, methionine sulfoxide reductase A (MSRA) over expression could protect lens cells both in presence and in absence of oxidative stress-induced damage. Nevertheless, promising results have been obtained by reducing ultraviolet-induced oxidative damage.

Effect of hesperetin on chaperone activity in selenite-induced cataract

BACKGROUND

Chaperone activity of α-crystallin in the lens works to prevent protein aggregation and is important to maintain the lens transparency. This study evaluated the effect of hesperetin on lens chaperone activity in selenite-induced cataracts.

METHODOLOGY

Thirteen-day-old rats were divided into four groups. Animals were given hesperetin (groups G2 and G4) or vehicle (G1 and G3) on Days 0, 1, and 2. Rats in G3 and G4 were administered selenite subcutaneously 4 hours after the first hesperetin injection. On Days 2, 4, and 6, cataract grades were evaluated using slit-lamp biomicroscopy. The amount of a-crystallin and chaperone activity in water-soluble fraction were measured after animals sacrificed.

RESULTS

G3 on day 4 had developed significant cataract, as an average cataract grading of 4.6 ± 0.2. In contrast, G4 had less severe central opacities and lower stage cataracts than G3, as an average cataract grading of 2.4 ± 0.4. The a-crystallin levels in G3 lenses were lower than in G1, but the same as G4. Additionally, chaperone activity was weaker in G3 lenses than G1, but the same as in G4.

CONCLUSIONS

Our results suggest that hesperetin can prevent the decreasing lens chaperone activity and a-crystallin water solubility by administered of selenite.

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.

Vitreous levels of oxidative stress biomarkers and the radical-scavenger α1-microglobulin/A1M in human rhegmatogenous retinal detachment

PURPOSE

To explore oxidative stress and the radical scavenger α(1)-microglobulin (A1M) in the vitreous body of human eyes with primary rhegmatogenous retinal detachment (RRD).

METHODS

Levels of carbonyl groups, a marker of oxidative stress, and A1M were measured by ELISA and RIA in 14 vitreous samples derived from patients suffering from RRD, and compared with 14 samples from macula hole (MH) patients. Carbonyl group and A1M levels in RRD samples were statistically related to detachment characteristics. Analysis of total protein level, SDS-PAGE, and Western blotting of A1M was also performed. In a separate experiment, mRNA expression of A1M was measured by RT-PCR in rat retina explants.

RESULTS

Levels of carbonyl groups and A1M varied widely in RRD vitreous samples, but were significantly higher in samples derived from eyes with large detachment area and macula-off status, while the presence of vitreous hemorrhage did not show any significant correlation. Compared with MH samples, RRD samples displayed significantly higher levels of A1M, whereas changes in total protein levels and carbonyl groups were not significant. Novel forms of A1M, not previously seen in plasma, were found in the vitreous body by Western blotting. Furthermore, A1M expression was seen in rat retina explants and was upregulated after 24 h of culturing.

CONCLUSION

Oxidative stress is a prominent feature of human eyes with primary RRD, and is directly related to detachment severity. Affected eyes can launch a protective response in the form of the radical scavenger A1M possibly derived from the retina. The results thus indicate potential therapeutic cell loss prevention in RRD by employing the endogeneous radical scavenger A1M.

Photo-oxidation of proteins

Photo-induced damage to proteins occurs via multiple pathways. Direct damage induced by UVB (λ 280-320 nm) and UVA radiation (λ 320-400 nm) is limited to a small number of amino acid residues, principally tryptophan (Trp), tyrosine (Tyr), histidine (His) and disulfide (cystine) residues, with this occurring via both excited state species and radicals. Indirect protein damage can occur via singlet oxygen ((1)O(2)(1)Δ(g)), with this resulting in damage to Trp, Tyr, His, cystine, cysteine (Cys) and methionine (Met) residues. Although initial damage is limited to these residues multiple secondary processes, that occur both during and after radiation exposure, can result in damage to other intra- and inter-molecular sites. Secondary damage can arise via radicals (e.g. Trp, Tyr and Cys radicals), from reactive intermediates generated by (1)O(2) (e.g. Trp, Tyr and His peroxides) and via molecular reactions of photo-products (e.g. reactive carbonyls). These processes can result in protein fragmentation, aggregation, altered physical and chemical properties (e.g. hydrophobicity and charge) and modulated biological turnover. Accumulating evidence implicates these events in cellular and tissue dysfunction (e.g. apoptosis, necrosis and altered cell signaling), and multiple human pathologies.

An unusual kynurenine-containing opioid tetrapeptide from the skin gland secretion of the Australian red tree frog Litoria rubella. Sequence determination by electrospray mass spectrometry

The Kyn-containing peptide FP-Kyn-L(NH(2)) is an unusual minor component of the skin peptide profile of the Australian red tree frog Litoria rubella collected from an area within a 20 kilometre radius of Alice Springs in central Australia. The structure was determined by electrospray mass spectrometry and synthesis. The major component of the skin secretion is the analogous tryptophyllin peptide FPWL(NH(2)). Both peptides show opioid activity at 10(-7) M, and are likely to act via the μ opioid receptor.

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.

Induction of indoleamine 2,3-dioxygenase by interferon-gamma in human lens epithelial cells: apoptosis through the formation of 3-hydroxykynurenine

Interferon-gamma (IFN-gamma) is known to cause apoptosis of lens epithelial cells and cataract formation, but the molecular mechanisms underlying these effects are unknown. IFN-gamma induces the expression of indoleamine 2,3-dioxygenase (IDO) and thereby enhances the production of kynurenines from l-tryptophan. The present study was designed to investigate the role of IDO and kynurenines in the IFN-gamma-mediated apoptosis of lens epithelial cells and to determine the signaling pathways involved. IFN-gamma stimulated the synthesis of IDO and activated the JAK-STAT1 signaling pathway in human lens epithelial cells (HLE-B3) in a dose-dependent manner. Meanwhile, fludarabine, an inhibitor of STAT1 activation, blocked IFN-gamma-mediated IDO expression. N-Formylkynurenine, kynurenine (Kyn) and 3-hydroxykynurenine (3OHKyn) were detected in cells, with 3OHKyn concentrations being higher than those of the other kynurenines. The intracellular production of kynurenines was completely blocked by 1-methyl-DL-tryptophan (MT), an inhibitor of IDO. Kyn- and 3OHKyn-modified proteins were detected in IFN-gamma-treated cells. The induction of IDO by IFN-gamma in HLE-B3 cells caused increases in intracellular ROS, cytosolic cytochrome c and caspase-3 activity, along with a decrease in protein-free thiol content. These changes were accompanied by apoptosis. At equimolar concentrations, 3OHKyn caused higher levels of apoptosis than the other kynurenines in HLE-B3 cells. MT and a kynurenine 3-hydroxylase inhibitor (Ro61-8048) effectively inhibited IFN-gamma-mediated apoptosis in HLE-B3 cells. Our results show that the induction of IDO by IFN-gamma is JAK-STAT1 pathway-dependent and that this induction causes 3OHKyn-mediated apoptosis in HLE-B3 cells. These data suggest that IDO-mediated kynurenine formation could play a role in cataract formation related to chronic inflammation.

Lens autofluorescence is not increased at high altitude

PURPOSE

To study the relation between ambient environmental ultraviolet radiation exposure and lens fluorescence.

METHODS

Non-invasive lens fluorometry measurements were compared in healthy Bolivian and Danish subjects. Background ultraviolet radiation was 4.5 times higher in Bolivia than in Denmark.

RESULTS

No significant differences in lens fluorescence or transmittance were found between Bolivian and Danish volunteers.

CONCLUSION

Age-corrected lens fluorescence and transmittance were comparable for healthy participants living at high altitude near the equator and healthy volunteers living at sea level at 55 degrees northern latitude. These results suggest that lens ageing, as assessed by lens autofluorometry, is independent of exposure to ultraviolet radiation.

Modulation of alpha-crystallin chaperone activity: a target to prevent or delay cataract?

Cataract, loss of eye lens transparency, is the leading cause of blindness worldwide. alpha-Crystallin, initially known as one of the major structural proteins of the eye lens, is composed of two homologous subunits alphaA- and alphaB-crystallins. It is convincingly established now that alpha-crystallin functions like a chaperone and plays a decisive role in the maintenance of eye lens transparency. The functional ability of alpha-crystallin subunits is to act in cooperation as molecular chaperones to prevent the cellular aggregation and/or inactivation of client proteins under variety of stress conditions. However, chaperone-like activity of alpha-crystallin could be deteriorated or lost during aging or under certain clinical conditions because of various genetic and environmental factors. This review will focus specifically on relevance of alpha-crystallin chaperone function to lens transparency. In particular, we reviewed the studies that demonstrate the modulation of alpha-crystallin chaperone-like activity and discussed the possibility of chaperone-like activity of alpha-crystallin as a potential target to prevent or delay the cataractogenesis.

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.

Protein aging: truncation of aquaporin 0 in human lens regions is a continuous age-dependent process

The human lens is ideal for the study of macromolecular aging because cells in the centre, along with their constituent proteins, are present for our entire lives. We examined the major membrane protein, aquaporin 0 (AQP0), in regions of the lens formed at different times during our lifespan, to determine if similar changes could be detected and if they were progressive. Membrane fractions from three concentric lens regions were examined by SDS-PAGE coupled with densitometry, and Western blotting, to assess the time course of truncation. The overall extent of modification was also examined by MALDI mass spectrometry of the undigested proteins. In all regions, AQP0 became progressively more truncated, specifically by the loss of a 2kDa intracellular C-terminal peptide. The proteolysis increased steadily in all regions such that half of the AQP0 in the barrier region (that part of the lens formed immediately after birth) had been cleaved by age 40-50. MALDI mass spectrometry revealed that in all regions, AQP0 not only was shortened, it also became progressively more heterogeneous with age. Since the lens interior is devoid of active enzymes, it is very likely that the cleavage of AQP0 is chemically induced. We speculate that the loss of this C-terminal peptide 'spacer' may allow occlusion of AQP0 pores on the cytoplasmic face of the fibre cell membranes. Once a significant proportion of AQP0 has been cleaved, this occlusion may contribute to the formation of the lens permeability barrier that develops at middle age.

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.

Cell cycle arrest by kynurenine in lens epithelial cells

PURPOSE

Indolemine 2,3-dioxygenase (IDO)-mediated oxidation of tryptophan produces kynurenines (KYNs), which may play a role in cataract formation. The molecular mechanisms by which KYNs cause cellular changes are poorly understood. The effects of KYNs on mouse lens epithelial cells by overexpression of human IDO were investigated.

METHODS

Lens epithelial cells (mLECs) derived from human IDO-overexpressing hemizygous transgenic (hemTg) and wild-type (Wt) mice were used. IDO activity was measured by quantifying kynurenine (KYN) by HPLC. KYN-mediated protein modifications were detected by immunocytochemistry and measured by ELISA. Cell proliferation and apoptosis were measured with commercially available kits. Cell distribution between cell cycle phases was examined with flow cytometric analysis. Immunoprecipitation followed by LC/MS was used to identify kynurenine-modified proteins.

RESULTS

mLECs derived from hemTg animals exhibited considerable IDO immunoreactivity and enzyme activity, which were barely detectable in Wt mLECs. KYN and KYN-mediated protein modification were detected in hemTg but not in Wt mLECs; the modified proteins were myosin II and alpha/gamma-actin. HemTg mLECs displayed reduced viability and proliferation. Cell cycle analysis of hemTg mLEC cultures showed approximately a twofold increase in cells at G(2)/M or in both phases, relative to Wt mLECs. Blocking IDO activity with 1-methyl-d,l-tryptophan in hemTg mLECs prevented KYN formation, KYN-mediated protein modification, and G(2)/M arrest.

CONCLUSIONS

Excess IDO activity in mLECs results in KYN production, KYN-mediated modification of myosin II and alpha/gamma-actin, and cell cycle perturbation. Modification of myosin II and gamma-actin by KYN may interfere with cytokinesis, leading to defective epithelial cell division and thus a decreased number of fiber cells.

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.

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).

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.

Characterisation of photo-oxidation products within photoyellowed wool proteins: tryptophan and tyrosine derived chromophores

Understanding the photodegradation of complex protein systems represents a significant goal in protein science. The photo-oxidation and resultant photoyellowing of wool in sunlight is a severe impediment to its marketability. However, although some photomodifications have been found in irradiated model amino acid systems, direct identification of the chromophoric photoproducts responsible for photoyellowing in irradiated wool itself has proved elusive. We here describe the direct characterisation and location of yellow chromophores and related photomodifications within the proteins of photoyellowed wool fabric, utilising a quasi-proteomic approach. In total, eight distinct photoproducts were characterised. Of these, five were derived from tryptophan; namely hydroxytryptophan, N-formylkynurenine, kynurenine, residues consistent with the dehydration of kynurenine, and hydroxykynurenine, while three were derived from tyrosine; namely dihydroxyphenylalanine, dityrosine, and a cross-linked residue consistent with a hydroxylated dityrosine residue. Fourteen modified peptide sequences were identified and the positions of modification for thirteen of these were located within the primary structure of known wool proteins. The nature of the photoproducts characterised offer valuable insight into the reaction pathways followed in the UV-induced photoyellowing of wool proteins.

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.

Eye lens proteomics

The eye lens is a fascinating organ as it is in essence living transparent matter. Lenticular transparency is achieved through the peculiarities of lens morphology, a semi-apoptotic process where cells elongate and loose their organelles and the precise molecular arrangement of the bulk of soluble lenticular proteins, the crystallins. The 16 crystallins ubiquitous in mammals and their modifications have been extensively characterized by 2-DE, liquid chromatography, mass spectrometry and other protein analysis techniques. The various solubility dependant fractions as well as subproteomes of lenticular morphological sections have also been explored in detail. Extensive post translational modification of the crystallins is encountered throughout the lens as a result of ageing and disease resulting in a vast number of protein species. Proteomics methodology is therefore ideal to further comprehensive understanding of this organ and the factors involved in cataractogenesis.

3-Hydroxykynurenine Oxidizes α-Crystallin: Potential Role in Cataractogenesis†

The alpha-, beta-, and gamma-crystallins are the major structural proteins of mammalian lenses. The human lens also contains tryptophan-derived UV filters, which are known to spontaneously deaminate at physiological pH and covalently attach to lens proteins. 3-Hydroxykynurenine (3OHKyn) is the third most abundant of the kynurenine UV filters in the lens, and previous studies have shown this compound to be unstable and to be oxidized under physiological conditions, producing H2O2. In this study, we show that methionine and tryptophan amino acid residues are oxidized when bovine alpha-crystallin is incubated with 3-hydroxykynurenine. We observed almost complete oxidation of methionines 1 and 138 in alphaA-crystallin and a similar extent of oxidation of methionines 1 and 68 in alphaB-crystallin after 48 h. Tryptophans 9 and 60 in alphaB-crystallin were oxidized to a lesser extent. AlphaA-crystallin was also found to have 3OHKyn bound to its single cysteine residue. Examination of normal aged human lenses revealed no evidence of oxidation of alpha-crystallin; however, oxidation was detected at methionine 1 in both alphaA- and alphaB-crystallin from human cataractous lenses. Age-related nuclear cataract is associated with coloration and insolubilization of lens proteins and extensive oxidation of cysteine and methionine residues. Our findings demonstrate that 3-hydroxykynurenine can readily catalyze the oxidation of methionine residues in both alphaB- and alphaA-crystallin, and it has been reported that alpha-crystallin modified in this way is a poorer chaperone. Thus, 3-hydroxykynurenine promotes the oxidation and modification of crystallins and may contribute to oxidative stress in the human lens.

Actions of ultraviolet light on cellular structures

Solar radiation is the primary source of human exposure to ultraviolet (UV) radiation. Overexposure without suitable protection (i.e., sunscreen and clothing) has been implicated in mutagenesis and the onset of skin cancer. These effects are believed to be initiated by UV-mediated cellular damage, with proteins and DNA as primary targets due to a combination of their UV absorption characteristics and their abundance in cells. UV radiation can mediate damage via two different mechanisms: (a) direct absorption of the incident light by the cellular components, resulting in excited state formation and subsequent chemical reaction, and (b) photosensitization mechanisms, where the light is absorbed by endogenous (or exogenous) sensitizers that are excited to their triplet states. The excited photosensitizers can induce cellular damage by two mechanisms: (a) electron transfer and hydrogen abstraction processes to yield free radicals (Type I); or (b) energy transfer with O2 to yield the reactive excited state, singlet oxygen (Type II). Direct UV absorption by DNA leads to dimers of nucleic acid bases including cyclobutane pyrimidine species and pyrimidine (6-4) pyrimidone compounds, together with their Dewar isomers. These three classes of dimers are implicated in the mutagenicity of UV radiation, which is typified by a high level of CC-->TT and C-->T transversions. Single base modifications can also occur via sensitized reactions including Type 1 and Type II processes. The main DNA product generated by (1)O2 is 8-oxo-Gua; this is a common lesion in DNA and is formed by a range of other oxidants in addition to UV. The majority of UV-induced protein damage appears to be mediated by (1)O2, which reacts preferentially with Trp, His, Tyr, Met, Cys and cystine side chains. Direct photo-oxidation reactions (particularly with short-wavelength UV) and radicals can also be formed via triplet excited states of some of these side chains. The initial products of (1)O2-mediated reactions are endoperoxides with the aromatic residues, and zwitterions with the sulfur-containing residues. These intermediates undergo a variety of further reactions, which can result in radical formation and ring-opening reactions; these result in significant yields of protein cross-links and aggregates, but little protein fragmentation. This review discusses the formation of these UV-induced modifications and their downstream consequences with particular reference to mutagenesis and alterations in protein structure and function.

Determination of Photo-oxidation Products Within Photoyellowed Bleached Wool Proteins

Photo-oxidative processes occurring in wool can lead to significant photoyellowing of the fiber. In particular, wool that has been chemically bleached photoyellows more rapidly and to a greater degree than untreated wool. Direct identification of the chromophores responsible for such yellow discoloration in irradiated wool has proven to be elusive for many years. This article describes the characterization and location of yellow photo-oxidation products within the proteins of photoyellowed bleached wool fabric, using advanced protein chemistry techniques. The discolored fabric was enzymatically digested and chromatographed by high-pressure liquid chromatography, with monitoring at 400 nm, to select out fractions containing yellow chromophoric species. Thorough tandem mass spectrometric analysis was then used to sequence peptides and, in turn, to characterize modifications to key amino acid residues that had resulted in yellow chromophore formation. In total, 11 separate yellow chromophoric species were identified, ten derived from tryptophan residues and one from tyrosine. The tryptophan-derived modifications characterized included hydroxytryptophan, N-formylkynurenine, hydroxyformylkynurenine, kynurenine, hydroxykynurenine, carbolines, tryptophandiones and nitrotryptophan. The tyrosine-derived modification of tyrosine to dopa was also identified. The range of photomodifications we observed provides insight into the photo-oxidation pathways occurring within irradiated fibrous proteins leading to the formation of yellow chromophores.

Fluorescent and compositional changes in crystallin supramolecular structures in pig lens during development

Water soluble proteins (WSPs) in Sus scrofa lenses from pigs in different developmental stages: (young (GI), young adult (GII), and middle-aged (GIII)) were separated using GF-HPLC, yielding fractions of different molecular weights. Non-tryptophan (345/420 nm) and tryptophan (280/345 nm) fluorescence was measured in these fractions. Relative non-tryptophan fluorescence increased with age at a rate directly correlated to the molecular weight of aggregates forming the different chromatographic fractions, while tryptophan fluorescence tended to decrease. The crystallins constituting each fraction were separated using 2D-electrophoresis and after development with Coomassie blue they were identified using MS-TOF. Also, the protein content of each spot was quantified by subsequent scanning and integration. The proportions of unchanged crystallins characteristically changed with age in chromatographic fractions of different molecular weights. Thus it was possible to relate these changes with those occurring in the fluorescent properties and molecular weight of supramolecular structures.

Phototransformations of advanced glycation end products in the human eye lens due to ultraviolet A light irradiation

Previous studies from this laboratory have shown that ultraviolet A (UVA) light can bleach the yellow advanced glycation end products (AGEs) of aged and cataractous human lenses. The AGEs OP-lysine and argpyrimidine are two UVA-absorbing posttranslational modifications that are abundant in the eye lens. The purpose of this study was to outline the changes in these two AGEs due to UVA irradiation. The changes of OP-lysine, OP-phenethylamine (a phenethylamine analogue of OP-lysine), and argpyrimidine due to irradiation with UVA light in the presence or absence of air and ascorbic acid were followed by different spectral methods. Aged human lenses were similarly irradiated in artificial aqueous humor. The amounts of OP-lysine in the irradiated lenses and in the corresponding dark controls were determined by HPLC. Both OP-lysine and argpyrimidine decreased 20% when irradiated with UVA light in the absence of ascorbic acid. Under the same conditions, OP-lysine was bleached 80% in the presence of ascorbic acid during irradiation experiments. In contrast, argpyrimidine UVA light bleaching was not affected by the presence of ascorbic acid. Interestingly the major product of OP-phenethylamine after UVA irradiation in the presence of ascorbic acid was phenethylamine, which indicates that the entire heterocycle of this AGE was cleaved and the initial amino group was restored. Some AGEs in the human eye lens can be transformed by UVA light.

Interactive Domains for Chaperone Activity in the Small Heat Shock Protein, Human αB Crystallin

Protein pin arrays identified seven interactive sequences for chaperone activity in human alphaB crystallin using natural lens proteins, beta(H) crystallin and gammaD crystallin, and in vitro chaperone target proteins, alcohol dehydrogenase and citrate synthase. The N-terminal domain contained two interactive sequences, (9)WIRRPFFPFHSP(20) and (43)SLSPFYLRPPSFLRAP(58). The alpha crystallin core domain contained four interactive sequences, (75)FSVNLDVK(82) (beta3), (113)FISREFHR(120), (131)LTITSSLS(138) (beta8), and (141)GVLTVNGP(148) (beta9). The C-terminal domain contained one interactive sequence, (157)RTIPITRE(164), that included the highly conserved I-X-I/V motif. Two interactive sequences, (73)DRFSVNLDVKHFS(85) and (131)LTITSSLSDGV(141), belonging to the alpha crystallin core domain were synthesized as peptides and assayed for chaperone activity in vitro. Both synthesized peptides inhibited the thermal aggregation of beta(H) crystallin, alcohol dehydrogenase, and citrate synthase in vitro. Five of the seven chaperone sequences identified by the pin arrays overlapped with sequences identified previously as sequences for subunit-subunit interactions in human alphaB crystallin. The results suggested that interactive sequences in human alphaB crystallin have dual roles in subunit-subunit assembly and chaperone activity.

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).

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.

Human α-1-Microglobulin Is Covalently Bound to Kynurenine-derived Chromophores

Alpha-1-microglobulin carries a set of covalently linked chromophores that give it a peculiar yellow-brown color, fluorescence properties, and both charge and size heterogeneity. In this report it is shown that these features are due to the adducts with the tryptophan metabolite, 3-hydroxykynurenine, and its autoxidation products and that the modification is more pronounced in the protein isolated from urine of hemodialyzed patients. The light yellow amniotic fluid alpha-1-microglobulin acquires the optical properties and charge heterogeneity of the urinary counterpart following incubation with kynurenines. The colored amino acid adducts of urinary and amniotic fluid alpha-1-microglobulins were separated by chromatography after acid hydrolysis and analyzed by mass spectrometry. Human serum albumin samples, native and treated with 3-hydroxykynurenine in the presence of oxygen, were used as a control. The retention times and mass fragmentation products were compared, and a lysyl adduct with hydroxantommathin was identified in the urinary alpha-1-microglobulin and in the modified albumin samples. The more extensive modification of the urinary protein appears to be correlated with uremia, a condition in which the catabolism of tryptophan via the kynurenine pathway is increased, and the consequent rise in the concentration of its derivatives is accompanied by the oxidative processes due to the hemodialysis treatment. The oxidative derivatives of 3-hydroxykynurenine, which are known to act as protein cross-linking agents, are the likely cause of the propensity of urinary alpha-1-microglobulin to form dimers and oligomers. This process, as well as the redox properties of these metabolites, may contribute to the toxic effects of the kynurenine species.

Protein-bound kynurenine is a photosensitizer of oxidative damage

Human lens proteins become progressively modified by tryptophan-derived UV filter compounds in an age-dependent manner. One of these compounds, kynurenine, undergoes deamination at physiological pH, and the product binds covalently to nucleophilic residues in proteins via a Michael addition. Here we demonstrate that after covalent attachment of kynurenine, lens proteins become susceptible to photo-oxidation by wavelengths of light that penetrate the cornea. H2O2 and protein-bound peroxides were found to accumulate in a time-dependent manner after exposure to UV light (lambda > 305-385 nm), with shorter-wavelength light giving more peroxides. Peroxide formation was accompanied by increases in the levels of the protein-bound tyrosine oxidation products dityrosine and 3,4-dihydroxyphenylalanine, species known to be elevated in human cataract lens proteins. Experiments using D2O, which enhances the lifetime of singlet oxygen, and azide, a potent scavenger of this species, are consistent with oxidation being mediated by singlet oxygen. These findings provide a mechanistic explanation for UV light-mediated protein oxidation in cataract lenses, and also rationalize the occurrence of age-related cataract in the nuclear region of the lens, as modification of lens proteins by UV filters occurs primarily in this region.

Carnosine disaggregates glycated alpha-crystallin: an in vitro study

Protein glycation, which promotes aggregation, involves the unwanted reaction of carbohydrate oxidation products with proteins. Glycation of lens alpha-crystallin occurs in vivo and may contribute to cataractogenesis. Anti-glycation compounds such as carnosine may be preventive, but interestingly carnosine reverses lens opacity in human trials. The mechanism for this observation may involve carnosine's ability to disaggregate glycated protein. We investigated this hypothesis using glycated alpha-crystallin as our in vitro model. Methylglyoxal-induced glycation of alpha-crystallin caused aggregation as evidenced by increased 90 degrees light scattering. After addition of carnosine, light scattering returned to baseline levels suggesting that the size of the glycation-induced aggregates decreased. Additionally, carnosine decreased tryptophan fluorescence polarization of glycated alpha-crystallin, suggesting that carnosine increased peptide chain mobility, which may contribute to the controlled unfolding of glycated protein. Comparatively, guanidine-HCl and urea had no effect. Our data support the hypothesis that carnosine disaggregates glycated alpha-crystallin.

Methylation and carbamylation of human gamma-crystallins

Accessible sulfhydryls of cysteine residues are likely sites of reaction in long-lived proteins such as human lens crystallins. Disulfide bonding between cysteines is a major contributor to intermolecular cross-linking and aggregation of crystallins. A recently reported modification of gammaS-crystallins, S-methylation of cysteine residues, can prevent disulfide formation. The aim of this study was to determine whether cysteines in gammaC-, gammaD-, and gammaB-crystallins are also S-methylated. Our data show that all the gamma-crystallins are S-methylated, but only at specific cysteines. In gammaD-crystallin, methylation is exclusively at Cys 110, whereas in gammaC- and gammaB-crystallins, the principal methylation site is Cys 22 with minor methylation at Cys 79. gammaD-crystallin is the most heavily methylated gamma-crystallin. gammaD-Crystallins from adult lenses are 37%-70% methylated, whereas gammaC and gammaB are approximately 12% methylated. The specificity of gamma-crystallin methylation and its occurrence in young clear lenses supports the idea that inhibition of disulfide bonding by S-methylation may play a protective role against cataract. Another modification, not reported previously, is carbamylation of the N termini of gammaB-, gammaC-, gammaD-crystallins. N-terminal carbamylation is likely a developmentally related modification that does not negatively impact crystallin function.

S-methylated cysteines in human lens gamma S-crystallins

The proteins of the eye lens, which do not turn over throughout life, undergo many modifications, some of which lead to senile cataract. We describe a modification, S-methylation of cysteine, that may serve to protect the lens from detrimental modifications. The modification was detected as a +14 Da peak in electrospray ionization mass spectra of human lens gammaS-crystallins. Derivatization of gammaS-crystallin with iodoacetamide showed reaction at only six of the seven cysteines, indicating the modification blocked reaction at one cysteine. Further analysis of the modified gammaS-crystallin as tryptic peptides located the modification primarily at Cys 26, with smaller amounts at Cys 24. Tandem mass spectrometry and exact mass measurements showed that the modification was S-methylation. Methylation of proteins has been documented at several other amino acid residues, but S-methylation of cysteine residues has previously been detected only as part of a methyltransferase DNA repair mechanism or at trace amounts in hemoglobin. The high levels of S-methylated cysteines in lens nuclei and the specificity for Cys 26 and Cys 24 suggest the reaction is enzymatically mediated. This modification is particularly important because it blocks disulfide bonding of gammaS-crystallins and, thereby, inhibits formation of the high-molecular weight assemblies associated with cataract. Evidence of more S-methylation in soluble than in insoluble gammaS-crystallins supports the contention that S-methylation of gammaS-crystallin inhibits protein insolubilization and may offer protection against cataract.

Phototoxicity to the retina: mechanisms of damage

Light damage to the retina occurs through three general mechanisms involving thermal, mechanical, or photochemical effects. The particular mechanism activated depends on the wavelength and exposure duration of the injuring light. The transitions between the various light damage mechanism may overlap to some extent. Energy confinement is a key concept in understanding or predicting the type of damage mechanism produced by a given light exposure. As light energy (either from a laser or an incoherent source) is deposited in the retina, its penetration through, and its absorption in, various tissue compartments is determined by its wavelength. Strongly absorbing tissue components will tend to "concentrate" the light energy. The effect of absorbed light energy largely depends on the rate of energy deposition, which is correlated with the exposure duration. If the rate of energy deposition is too low to produce an appreciable temperature increase in the tissue, then any resulting tissue damage necessarily occurs because of chemical (oxidative) reactions induced by absorption of energetic photons (photochemical damage). If the rate of energy deposition is faster than the rate of thermal diffusion (thermal confinement), then the temperature of the exposed tissue rises. If a critical temperature is reached (typically about 10 degrees C above basal), then thermal damage occurs. If the light energy is deposited faster than mechanical relaxation can occur (stress confinement), then a thermoelastic pressure wave is produced, and tissue is disrupted by shear forces or by cavitation-nonlinear effects. Very recent evidence suggests that ultrashort laser pulses can produce tissue damage through nonlinear and photochemical mechanisms; the latter because of two-photon excitation of cellular chromophores. In addition to tissue damage caused directly by light absorption, light toxicity can be produced by the presence of photosensitizing agents. Drugs excited to reactive states by ultraviolet (UV) or visible light produce damage by type I (free radical) and type II (oxygen dependent) mechanisms. Some commonly used drugs, such as certain antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and psychotherapeutic agents, as well as some popular herbal medicines, can produce ocular phototoxicity. Specific cellular effects and damage end points characteristic of light damage mechanisms are described.

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.

Identifying sites of attachment of UV filters to proteins in older human lenses

Recent results indicate that covalent modification of proteins by tryptophan-derived UV filters may explain the age-dependent coloration of human lenses, and play a role in age-related cataract. The sites of attachment of the UV filters to the lens crystallins, however, have not been determined. This study utilized a database of predicted masses of UV filter-modified tryptic peptides to target sites of UV filter attachment. Proteins were isolated from old normal lenses and digested with trypsin at pH 6, in order to preserve the integrity of the sites of modification. Peptides were separated by high-performance liquid chromatography and characterized by mass spectrometry. Major colored and fluorescent peaks in the digest were found to correspond to cysteine-containing peptides in which the sulfur atom of the sidechain was linked to the major UV filter compound, 3-hydroxykynurenine glucoside. Three of the peptides originated from gammaS-crystallin and one from betaB1-crystallin. These results show that a predicted mass database can be used to facilitate the identification of sites of UV filter modification in human lens crystallins. Furthermore, this work represents the first evidence that UV filters bind to specific residues on lens proteins in vivo, and suggests that sulfhydryl groups may be important sites for the attachment of UV filters.

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.