A study of the photodynamic efficiencies of some eye lens constituents

Role of xanthurenic acid 8-O-beta-D-glucoside, a novel fluorophore that accumulates in the brunescent human eye lens

We have been able to identify a blue fluorophore from the low-molecular weight soluble fraction of human adult nondiabetic brunescent cataract lenses as xanthurenic acid 8-O-beta-D-glucoside (XA8OG) (excitation = 338 nm and emission = 440 nm). To determine the role of this fluorophore in the lens, we have examined its photophysical and photodynamic properties. We found XA8OG to have a fluorescence quantum yield (phi) of 0.22 and a major emission lifetime of 12 ns. We found it to be a UVA-region sensitizer, capable of efficiently generating singlet oxygen species but little of superoxide. We also demonstrated that XA8OG oxidizes proteins when irradiated with UVA light, causing photodynamic covalent chemical damage to proteins. Its accumulation in the aging human lens (and the attendant decrease of its precursor O-beta-D-glucoside of 3-hydroxykynurenine) can, thus, add to the oxidative burden on the system. XA8OG, thus, appears to be an endogenous chromophore in the lens, which can act as a cataractogenic agent.

Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers: a laser flash photolysis study

The riboflavin (RF, Vitamin B2) and flavin adenine dinucleotide (FAD)-sensitized photooxidation of tryptophan (TrpH) and tyrosine (TyrOH) were studied by laser flash photolysis. TrpH and TyrOH quench triplet flavin sensitizers to produce reduced flavin radicals (FlH*) and oxidized radicals of TrpH or TyrOH (Trp* and TyrO*. Although Trp* and TyrO* cannot be observed directly by the laser flash photolysis, N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), as a probe, was added to the system to result in the formation of radical cations of TMPD (TMPD*+) via quenching of Trp* and TyrO*, which provides more definitive proof of electron transfer in the photosensitization process than only direct observation of reduced flavin radicals. Electron transfer from TrpH and TyrOH to oxidized radicals of riboflavin and FAD with similar rate constants to the triplet flavins was observed for the first time, which may be a new way of TrpH and TyrOH damage. These results may shed new light on future application of flavins in photodynamic therapy, and imply that flavins might be applied potentially to photosensitization of oxygen deficiency or under high-intensity pulsed laser irradiation.

Environmental photodegradation of pyrimidine fungicides — Kinetics of the visible-light-promoted interactions between riboflavin and 2-amino-4-hydroxy-6-methylpyrimidine

To clarify the evolution of pyrimidine fungicides under natural aquatic environmental conditions, the visible-light-promoted degradation of the model fungicide 2-amino-4-hydroxy-6-methylpyrimidine (AHMPD) has been studied in air-equilibrated pH 6 aqueous solutions in the presence of riboflavin (Rf), employing time-resolved and stationary kinetic–spectroscopic methods. AHMPD, a compound practically inert towards the attack of singlet molecular oxygen (O2(1Δg)), quenches excited singlet and triplet states of Rf with rate constants of 2.7 × 109 M–1 s–1 and 2.7 × 107 M–1 s–1, respectively. In the presence of AHMPD, the photodecomposition of Rf, which occurs from the excited triplet state of the pigment, depends on the concentration of the fungicide: at [Formula: see text]40–50 mM a limited photochemistry occurs due to the quenching of excited singlet Rf, while at ca. 5–10 mM triplet Rf is largely photogenerated and subsequently quenched either by oxygen, giving rise to O2(1Δg), or by AHMPD, yielding semireduced Rf through an electron transfer process. Flash photolysis experiments and indirect auxiliary tests confirm the presence of superoxide anion generated by the reaction of Rf anion radical with the dissolved oxygen. The neat result of this intricate scheme of competitive reactions is the photodegradation of both AHMPD and Rf, mainly through a superoxide anion-mediated oxidation, although some contribution of O2(1Δg)-mediated photooxidations cannot be disregarded.Key words: photooxidation, pyrimidine derivatives, riboflavin, singlet molecular oxygen, superoxide anion.

Ocular phototoxicity

The human eye is constantly exposed to sunlight and artificial lighting. Therefore the eye is exposed to UV-B (295-320 nm), UV-A (320-400 nm), and visible light (400-700 nm). Light is transmitted through the eye and then signals the brain directing both sight and circadian rhythm. Therefore light absorbed by the eye must be benign. Damage to the young and adult eye by intense ambient light is avoided because the eye is protected by a very efficient antioxidant system. In addition, there are protective pigments such as the kynurenines, located in the human lens, and melanin, in the uvea and retina, which absorb ambient radiation and dissipate its energy without causing damage. After middle age there is a decrease in the production of antioxidants and antioxidant enzymes. At the same time, the protective pigments are chemically modified (lenticular 3-hydroxy kynurenine pigment is enzymatically converted into the phototoxic chromophore xanthurenic acid; melanin is altered from an antioxidant to pro-oxidant) and fluorescent chromophores (lipofuscin) accumulate to concentrations high enough to produce reactive oxygen species. We have known for some time that exposure to intense artificial light and sunlight either causes or exacerbates age-related ocular diseases. We now know many of the reasons for these effects, and with this knowledge methods are being developed to interfere with these damaging processes.

Retinal photodamage

The retina represents a paradox, in that, while light and oxygen are essential for vision, these conditions also favour the formation of reactive oxygen species leading to photochemical damage to the retina. Such light damage seems to be multi-factorial and is dependent on the photoreactivity of a variety of chromophores (e.g., vitamin A metabolites, lipofuscin, melanin, flavins, porphyrins, carotenoids) endogenous to the retina. The aim of this article is to provide a detailed review of our current understanding of the photochemistry and photobiology of these chromophores and to consider how they may contribute to retinal ageing and pathology.

Photooxidation of lens proteins with xanthurenic acid: a putative chromophore for cataractogenesis

The tryptophan metabolite, xanthurenic acid (Xan), is produced through a transamination reaction in high concentrations in human lenses with age and has been isolated from aged human cataractous lenses. It has appreciable absorption between 300 and 400 nm (lambda max = 334 nm), the range absorbed by the human lens. Our recent studies have shown that unlike most tryptophan metabolites in the eye, Xan is photochemically active, producing both superoxide and singlet oxygen. To determine if Xan could act as a photosensitizer and photooxidize cytosolic lens proteins, alpha-, beta- and gamma-crystallins were irradiated (lambda > 300 nm, 12 mW/cm2) in the presence and absence of Xan. Upon irradiation and in the presence of Xan, lens proteins polymerized in the order alpha > beta > gamma as assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Further analysis of the photolyzed alpha-crystallin by mass spectrometry indicated that histidine, tryptophan and methionine residues were oxidized at specific positions in a dose-dependent (irradiation time) manner. In alpha A-crystallin two forms of oxidized histidine 154 were observed, 2-imidazolone and 2-oxohistidine. Our results suggest that naturally occurring Xan is a chromophore capable of photosensitization and photooxidation of lens proteins. Furthermore, this compound could play a role in age-related cataractogenesis.

Dark and photoinduced interactions between Trolox, a polar-solvent-soluble model for vitamin E, and riboflavin

The aerobic riboflavin (Rf)-sensitized visible-light irradiation of Trolox (TX), a polar-solvent-soluble model for vitamin E, has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Results indicate that in methanolic solution, no dark complexation exists between Rf and TX. The latter quenches singlet and triplet states of Rf, with rate constants of 6.2 x 10(9) M(-1) s(-1) and 4.7 x 10(9) M(-1) s(-1), respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved TX: at >/=30 mM very slight Rf photodecomposition occurs due to the massive quenching of excited singlet Rf, while at TX concentrations < or =1 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O(2)((1)Delta(g)), or by TX, yielding semireduced Rf through an electron transfer process. Complementary experiments performed in pure water employing superoxide dismutase and sodium azide inhibition of the oxygen uptake, in coincidence with flash photolysis data, indicate that superoxide anion and singlet molecular oxygen are generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf or by energy transfer from triplet Rf to ground-state oxygen, respectively. The final result is that both TX and Rf are photodegraded, likely through oxidation with activated oxygen species. In the absence of oxygen no degradation of TX can be detected, but Rf photodegradation is favoured because Rf regeneration is avoided.

Antioxidant properties of green and black tea, and their potential ability to retard the progression of eye lens cataract

Aqueous extracts of green and black tea are shown to quench reactive oxygen species such as singlet oxygen, superoxide and hydroxyl radicals, prevent the oxidative cross-linking of test proteins and inhibit single strand breakage of DNA in whole cells. They are also seen to be able to counteract the oxidative insult mounted by cigarette smoke. In rats in which cataract was induced by subcutaneous injection of selenite, administration of green or black tea extracts led to a retardation of the progression of lens opacity, suggesting the potential cataracto-static ability of tea.

Photochemical oxygen consumption, oxygen evolution and spectral changes during UVA irradiation of EMT6 spheroids

Remarkable rates of oxygen consumption are observed via microelectrode measurements immediately upon the onset of 325 nm irradiation of multicell tumor spheroids. Consumption is irradiance dependent over the range 20-200 mW cm-2, and its magnitude is comparable to that observed previously in the same system using exogenous photosensitizers. Oscillations in the oxygen concentrations suggest that oxygen is also being evolved during irradiation. Oxygen evolution is likely the result of enzymatic dissociation of hydrogen peroxide, which is formed through UV-induced photochemistry. Irradiation of spheroids at 442 and at 514 nm produces a much more modest but detectable oxygen consumption. The dynamics of oxygen concentration changes are quite different at these wavelengths, suggesting a different photochemical mechanism. In these cases, initial oxygen depletion is followed immediately by a more gradual, monotonic increase in the oxygen concentration, consistent with irreversible photobleaching. No oscillations in the oxygen concentration are detectable. At 662 nm, no oxygen consumption was observed over the range of irradiances studied. Fluorescence spectra of cells prior to irradiation include contributions from anthranilic acid and reduced nicotinamide adenine dinucleotide (NADH). During 325 nm irradiation, anthranilic acid is rapidly and irreversibly bleached, while NADH emission undergoes only modest reduction.

Photochemical studies on xanthurenic acid

The tryptophan metabolite xanthurenic acid (Xan) has been isolated from aged human cataractous lenses. The photophysical properties of Xan were examined to determine if it is a potential chromophore for age-related cataractogenesis. We found that Xan produces singlet oxygen (psi delta = 0.17 in CD3OD) with the same efficiency as the lenticular chromophore N-formyl kynurenine and quenches singlet oxygen at a rate similar (2.1 x 10(7); CD3OD) to other tryptophan metabolites found in the eye. As the mechanisms of induction of cataracts may also involve redox reactions, the interactions of hydrated electrons (e(aq)-), the azide radical (N3*) and hydroxyl radical (OH*) with Xan were studied using the technique of pulse radiolysis. The reaction rate constants of e(aq)-, N3* and OH* with Xan were found to be of the same order of magnitude as other tryptophan metabolites. The rate constant for reaction of Xan with e(aq)- solvated electrons was found to be diffusion controlled (k = 1.43 x 10(10) M(-1) s(-1); the reaction with N3* was very fast (k = 4.0 x 10(9) M(-1) s(-1)); and with OH* was also near diffusion controlled (k = 1.0 x 10(10) M(-1) s(-1)). Superoxide O2*- production by irradiated Xan in methanol was detected by electron paramagnetic resonance and substantiated by determining that the enhanced rate of oxygen consumption of Xan irradiated in the presence of furfuryl alcohol was lowered by superoxide dismutase.

Ultraviolet radiation and cataract

While solar radiation falling on earth comprises light in the infrared, visible, UVA, UVB, and even UVC ranges, the light incident on, and thus important to the biology of, the eye lens is essentially in the visible and UVA regions. Thus, direct photochemical damage to the lens from UVB radiation is minor, though long-term UVA (and even visible range) irradiation is seen to lead to lens malfunction. Short-term exposure of the lens in vivo to UVA light leads to compromised optical and biochemical properties which are repaired in time, while higher doses affect permanent damage. Such longer wavelength light-mediated changes in the lens occur through photodynamic means, affected by some of the compounds that accumulate in the lens over a period of time, which act as sensitizers. Isolation and chemical identification of over a dozen such compounds has been done, and their photoactive properties have been studied. While several of these are photodynamic and generate reactive oxygen species when UVA light is shone on them, other compounds that accumulate in the lens act as antioxidants.

Reactivity of monoclonal antibodies against a tryptophan-riboflavin adduct toward irradiated and non-irradiated bovine-eye-lens protein fractions: an indicator of in vivo visible-light-mediated phototransformations

We describe here the reactivity toward the soluble protein of bovine eye lens of anti-tryptophan-riboflavin (anti-Trp-RF) adduct monoclonal antibodies, which recognize the hapten tryptophan-riboflavin generated by irradiation of a solution of bovine serum albumin in the presence of riboflavin. It is demonstrated that five different anti-Trp-RF adduct monoclonal antibodies, all belonging to the IgG1 isotype, react with the total soluble proteins of bovine eye lens. The components of the soluble protein are separated by Sephadex G-200 chromatography and the isolated fractions analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). All the separated protein fractions also react by a direct ELISA with the monoclonal antibodies; this reaction is more intense when the isolated fractions have been previously irradiated with visible light in the presence of riboflavin under an atmosphere of oxygen or nitrogen. Irradiation of the total soluble protein with visible light in the presence of riboflavin produces the appearance of new bands, corresponding to compounds of higher molecular weight. Riboflavin-sensitized irradiation of the protein fractions with visible light under an oxygen or nitrogen atmosphere is accompanied by a concomitant decrease of the tryptophan fluorescence. It is postulated that the action of visible light in the presence of either the endogenous riboflavin or its derivatives could be partly responsible for the protein aggregation observed during aging.

beta-carbolines that accumulate in human tissues may serve a protective role against oxidative stress

beta-Carbolines are tricyclic nitrogen heterocycles formed in plants and animals as Maillard reaction products between amino acids and reducing sugars or aldehydes. They are being detected increasingly in human tissues, and their physiological roles need to be understood. Two beta-carboline carboxylates have been reported to accumulate in the human eye lens. We report here on the identification of another beta-carboline, namely 1-methyl-1-vinyl -2, 3,4-trihydro-beta-carboline-3-carboxylic acid, in the lenses of some cataract patients from India. Analysis of these three lenticular beta-carbolines using photodynamic and antioxidant assays shows all of them to be inert as sensitizers and effective as antioxidants; they quench singlet oxygen, superoxide and hydroxyl radicals and inhibit the oxidative formation of higher molecular weight aggregates of the test protein, eye lens gamma-crystallin. Such antioxidative ability of beta-carbolines is of particular relevance to the lens, which faces continual photic and oxidative stress. The beta-carboline diacid IV is also seen to display an unexpected ability of inhibiting the thermal coagulation of gamma-crystallin and the dithiothreitol-induced precipitation of insulin. These results offer experimental support to earlier suggestions that one of the roles that the beta-carbolines have is to offer protection against oxidative stress to the human tissues where they accumulate.

Protection against UVB inactivation (in vitro) of rat lens enzymes by natural antioxidants

Oxidative damage, through increased production of free radicals, is believed to be involved in UV-induced cataractogenesis (eye lens opacification). The possibility of UVB radiation causing damage to important lenticular enzymes was assessed by irradiating 3 months old rat lenses (in RPMI-1640 medium) at 300 nm (100 microWcm(-2)) for 24 h, in the absence and presence of ascorbic acid, alpha-tocopherol acetate and beta-carotene. UVB irradiation resulted in decreased activities of hexokinase, glucose-6-phosphate dehydrogenase, aldose reductase, and Na, K- ATPase by 42, 40, 44 and 57% respectively. While endopeptidase activity (229%) and lipid peroxidation (156%) were increased, isocitrate dehydrogenase activity was not altered on irradiation. In the presence of externally added ascorbic acid, tocopherol and beta-carotene (separately) to the medium, the changes in enzyme activities (except endopeptidase) and increased lipid peroxidation, due to UVB exposure, were prevented. These results suggest that UVB radiation exerts oxidative damage on lens enzymes and antioxidants were protective against this damage.

The molecular chaperone alphaA-crystallin enhances lens epithelial cell growth and resistance to UVA stress

alphaA-Crystallin (alphaA) is a member of the small heat shock protein (sHSP) family and has the ability to prevent denatured proteins from aggregating in vitro. Lens epithelial cells express relatively low levels of alphaA, but in differentiated fiber cells, alphaA is the most abundant soluble protein. The lenses of alphaA-knock-out mice develop opacities at an early age, implying a critical role for alphaA in the maintenance of fiber cell transparency. However, the function of alpha-crystallin in the lens epithelium is unknown. To investigate the physiological function of alphaA in lens epithelial cells, we used the following two systems: alphaA knock-out (alphaA(-/-)) mouse lens epithelial cells and human lens epithelial cells that overexpress alphaA. The growth rate of alphaA(-/-) mouse lens epithelial cells was reduced by 50% compared with wild type cells. Cell cycle kinetics, measured by fluorescence-activated cell sorter analysis of propidium iodide-stained cells, indicated a relative deficiency of alphaA(-/-) cells in the G2/M phases. Exposure of mouse lens epithelial cells to physiological levels of UVA resulted in an increase in the number of apoptotic cells in the cultures. Four hours after irradiation the fraction of apoptotic cells in the alphaA(-/-) cultures was increased 40-fold over wild type. In cells lacking alphaA, UVA exposure modified F-actin, but actin was protected in cells expressing alphaA. Stably transfected cell lines overexpressing human alphaA were generated by transfecting extended life span human lens epithelial cells with the mammalian expression vector construct pCI-neoalphaA. Cells overexpressing alphaA were resistant to UVA stress, as determined by clonogenic survival. alphaA remained cytoplasmic after exposure to either UVA or thermal stress indicating that, unlike other sHSPs, the protective effect of alphaA was not associated with its relocalization to the nucleus. These results indicate that alphaA has important cellular functions in the lens over and above its well characterized role in refraction.

Action spectrum for photocross-linking of human lens proteins

The action spectrum for photocross-linking was measured for human lens beta gamma-crystallins from young adult noncataractous lenses at wavelengths of 297, 302, 313, 334 and 365 nm. The action spectrum had a maximal effectiveness at 297 nm that sharply decreased in effectiveness up to 313 nm, then remained flat until 334 nm and decreased markedly as wavelength increased to 365 nm. Radiation at 297 nm was 36 times more effective in producing cross-linking than 302 nm radiation. The 297 nm radiation was 220, 195 and 1300 times more effective than 313, 334 and 365 nm radiation, respectively. The action spectrum had a shape similar to the absorption spectrum of the lens proteins but the response was lower than expected from the absorption data, suggesting that some of the absorptions are not effective at cross-linking. Because most animal experimentation and epidemiological studies include broadband radiation, these studies would be useful in predicting the biological response of the lens to environmental UV stress.

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

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

Singlet oxygen and superoxide characteristics of a series of novel asymmetric photosensitizers

The singlet oxygen quantum yields and superoxide quantum yields for a series of novel compounds based on an asymmetrical protoporphyrin molecule have been examined. Electron spin resonance was used to measure superoxide yield and time resolved luminescence for singlet oxygen. A comparison between these results and previously published cell survival data was carried out. A broad association was found between singlet oxygen quantum yield and clonogenic cell kill.

Fluorescence properties of isolated intact normal human corneas

We have examined the fluorescence properties of excised intact normal human corneas from over a hundred donors, using synchronous excitation fluorescence spectroscopy. In some of the corneas from the donors, a fluorophore with an excitation band centered at 330 nm was observed. This fluorophore does not seem to correspond to the dityrosine moiety or to any photoproducts of tryptophan. Isolated corneas irradiated with light of 295 nm wavelength do not produce any fluorescent photoproducts, suggesting that the intact tissue has endogenous quenchers, radical scavengers and antioxidants that inhibit its photodamage. The non-tryptophan fluorophores that accumulate in some corneas thus appear to arise largely from the nonenzymatic glycosylation (glycation) of the constituent proteins as similar fluorophores are detected in the corneas of rats in which diabetes is induced.

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

We have undertaken electron paramagnetic resonance and spin trapping investigations of the photochemistry of kynurenine (KN), a natural component of the human eye and close analog of the principal chromophore in the young human lens 3-OH-kynurenine O-glucoside (3HKG). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was employed as a spin trap. We found that upon UV irradiation (> 300 nm) KN photoreduces oxygen to superoxide radical (in DMSO) and nitromethane (CH3NO2) to a nitromethane radical anion (CH3NO2.-) (in air-free buffers, pH 7 and 9.5). KN also sensitized photooxidation of cysteine, NADH, EDTA, azide, and ascorbate; oxygen greatly accelerated this process. Oxidation of cysteine, NADH, and EDTA was accompanied by superoxide radical formation. Cysteinyl and azidyl radicals were detected as DMPO adducts. We also observed that KN undergoes photodegradation to a product(s) whose photosensitizing capacity is greater than that of KN itself. We postulate that: (i) 3HKG may be able to photoinitiate free radical reactions in vivo, and (ii) oxygen is an important factor determining the yields of free radical processes initiated by lenticular chromophores.

Deamination of 3-hydroxykynurenine in bovine lenses: a possible mechanism of cataract formation in general

BACKGROUND

3-Hydroxykynurenine, a metabolite of tryptophan, acts as UV filter in the human lens. In this study, we looked for this substance and its metabolites in young and old bovine lenses, because of their possible role in the formation of cataract.

METHODS

The substances were detected by HPLC analysis. The fluorescent substance formed from 3-hydroxykynurenine was characterized by thin-layer chromatography followed by reaction with ninhydrin, UV and fluorescence spectrum analysis, and atom bombardment for molecular mass determination. The kynurenine aminotransferase activity was determined by the method of Tobes.

RESULTS

3-Hydroxykynurenine was detected at concentrations of 0.07, 0.19, and 1.14 micrograms/g of tissue in the bovine iris/ciliary body, retina, and transparent bovine lenses respectively. 3-Hydroxykynurenine was deaminated in old bovine eyes but not in calf eyes. In old eyes, kynurenine aminotransferase activity was 2.7, 3.5, and 9.6 mumol/g of tissue per h in retina, iris/ciliary body, and lens respectively.

CONCLUSION

The deamination of 3-hydroxykynurenine resulted in the formation of a fluorescent substance which was identified as oxidized xanthurenic acid. This substance, accumulating in the bovine lens and interacting with lens proteins, could induce cataract formation.

Model studies on the photochemical production of lenticular fluorophores

With aging, human lens proteins accumulate fluorophores having blue and green emissions. Model studies were undertaken to determine the role of 3-hydroxykynurenine (3-HK) and its glucoside (3-HKG) in the photochemical production of those fluorophores. Experiments were carried out using 10(-3) M 3-HK solutions in the presence or absence of glycine (1 M), which was used to mimic the environment of the lens. The solutions were photolyzed (transmission above 295 nm) for various periods of time while the loss of starting material and the formation of fluorescent photoproducts (blue emission at 470 nm, and green emission at 520 nm) were monitored using fluorescence and UV-visible spectroscopy and thin-layer and high-pressure liquid chromatography analysis. Several parameters were varied such as oxygen tension and the addition of the free radical scavenger, penicillamine. The photolytic loss of 3-HK in the absence of glycine occurred approximately 5-10 times faster than in its presence. Conversely, blue and green fluorophores formed in irradiated solutions containing glycine but not with the photolysis of 3-HK alone. The blue fluorophore was formed first and appeared then to be photochemically converted to the green one, with the rate of formation of the latter increasing with an increase in UV dosage or oxidizing conditions. The addition of penicillamine drastically reduced the photochemical formation of both fluorophores. Both the blue and green fluorophores appear to result from the photochemically induced covalent attachment of 3-HK to glycine.(ABSTRACT TRUNCATED AT 250 WORDS)

A pulse radiolysis study of the reactions of 3-hydroxykynurenine and kynurenine with oxidizing and reducing radicals

Pulse radiolysis has been used to study the reactions of 3-hydroxykynurenine and kynurenine with solvated electrons, superoxide radicals, hydroxyl radicals and azide radicals. Both 3-hydroxykynurenine and kynurenine react with solvated electrons with diffusion controlled rate constants (k = 2.5 x 10(10) M-1 s-1 and 2.3 x 10(10) M-1s-1, respectively). Neither compound was observed to react with superoxide radicals under our experimental conditions, an upper limit of 1.2 x 10(5) M-1s-1 for the rate constant of this reaction was estimated for both compounds. However, we do observe that a stable product of autooxidation of 3-hydroxy-kynurenine reacts with superoxide radicals and we calculate a lower limit for the rate of this reaction of 5.8 x 10(6) M-1s-1. Reactions of 3-hydroxykynurenine and kynurenine with hydroxyl radicals proceed with diffusion controlled rate constants (1.2 x 10(10) M-1 s-1 and 1.3 x 10(10) M-1 s-1, respectively). The measured values for the rate constants for reaction of 3-hydroxykynurenine and kynurenine with azide radicals are 2.1 x 10(10) M-1s-1 and 4.8 x 10(9) M-1 s-1, respectively. The differences in these rate constants are attributed to differences in the measured oxidation potentials for 3-hydroxykynurenine (+1.0 V vs. NHE) and kynurenine (+1.15 V vs. NHE).

Photosensitized formation of ascorbate radicals by riboflavin: an ESR study

The riboflavin-sensitized photooxidation of ascorbate ion (HA-) to ascorbate radical (A.-) was followed by electron spin resonance (ESR) spectroscopy in conjunction with oxygen depletion measurements. In air-saturated aqueous media, steady-state amounts of A.- are rapidly established upon irradiation. The ESR signal disappears within a few seconds after the light is extinguished--more slowly under constant irradiation as oxygen is depleted. No photooxidation was observed in deaerated media. Similar results were obtained with other flavins and when ascorbyl palmitate was substituted for HA-. The effect of added superoxide dismutase, catalase, desferrioxamine, and singlet oxygen scavengers (NaN3 and tryptophan) was studied, as was replacement of water by D2O and saturation with O2. The results are indicative of ascorbate free radical production via direct reaction between ascorbate ion and triplet riboflavin in the presence of O2. While the presence of superoxide ion tends to reduce the steady-state concentration of A.-, competition from the reaction of HA- with singlet oxygen is less apparent in this system (at HA- > or = 1 mM) than in the previously studied aluminum phthalocyanine tetrasulfonate-photosensitized reaction.