A study of the photodynamic efficiencies of some eye lens constituents

Tryptophan-derived ultraviolet filter compounds covalently bound to lens proteins are photosensitizers of oxidative damage

The human eye is chronically exposed to light of wavelengths >300 nm. In the young human lens, light of wavelength 300-400 nm is predominantly absorbed by the free Trp derivatives kynurenine (Kyn), 3-hydroxykynurenine (3OHKyn), and 3-hydroxykynurenine-O-beta-D-glucoside (3OHKynG). These ultraviolet (UV) filter compounds are poor photosensitizers. With age, the levels of the free UV filters in the lens decreases and those of protein-bound UV filters increases. The photochemical behavior of these protein-bound UV filters and their role in UV damage are poorly elucidated and are examined here. UVA illumination of protein-bound UV filters generated peroxides (principally H2O2) in a metabolite-, photolysis-time-, and wavelength-dependent manner. Unmodified proteins, free Trp metabolites, and Trp metabolites that do not bind to lens proteins gave low peroxide yields. Protein-bound 3OHKyn (principally at Cys residues) yielded more peroxide than comparable Kyn and 3OHKynG adducts. Studies using D2O and sodium azide implicated 1O2 as a key intermediate. Illumination of the protein-bound adducts also yielded protein-bound Tyr oxidation products (DOPA, di-tyrosine) and protein cross-links via alternative mechanisms. These data indicate that the covalent modification of lens proteins by Kyn derivatives yields photosensitizers that may enhance oxidation in older lenses and contribute to age-related nuclear cataract.

The effect of single and repeated UVB radiation on rabbit lens

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Kinetics and Mechanism of the Sensitized Photodegradation of Uracil-Modeling the Fate of Related Herbicides in Aqueous Environments†

The dye-sensitized photodegradation of uracil (UR), the parent compound of several profusely employed herbicides, has been studied as a model of their environmental fate. In order to mimic conditions frequently found in nature, aqueous solutions of UR have been irradiated with visible light in the presence of the natural sensitizer riboflavin (Rf). The results indicate that UR is photostable in acid media, but is quickly degraded in pH 7 or pH 9 solutions, where singlet molecular oxygen [O2(1Delta(g))] and, to a lesser extent, superoxide radical anion (O2*-)-both species photogenerated from triplet excited Rf, 3Rf*-participate in the photodegradation. At pH 7, UR is slowly degraded through an O2*- -mediated mechanism, whereas Rf disappears through its reaction with O2(1Delta(g)) and, in the form of 3Rf*, with UR. On the contrary, at pH 9 Rf is photoprotected through two processes: its regeneration from the formed Rf radical species-a back electron transfer that also produces O2*- -and the elimination from the medium of O2(1Delta(g)) by its reaction with UR. The overall result of the preservation of ground state Rf is the continuity of the photosensitized process and, hence, of the UR degradation. Media with higher pH values could not be employed due to the fast photodegradation of Rf. With rose bengal (RB) as photosensitizer, the rate constants found for the overall interaction between UR and the photogenerated O2(1Delta(g)) were in the range 5 x 10(5) M(-1) s(-1) (at pH 7) to 1.3 x 10(8) M(-1) s(-1) (in 1 M NaOH aqueous solution, mainly physical quenching). The maximum O2(1Delta(g)0-mediated photooxidation efficiencies with RB were reached at pH 11, where only the O2(1Delta(g)0-reactive quenching with UR was observed.

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.

Photodynamic Action and Antimicrobial Activity of Some Excited Metabolites ofDalbergia Sissoidesand Their Ability to Cleave DNA

The photodynamic properties of two quinones, 4-methoxydalbergione (DS1) and sissoidenone (DS2), and a coumarin, dalbergin {6-hydroxy-7-methoxy-4-phenylcoumarin, (DS3)}, have been studied. Photogeneration of singlet oxygen (1O2) was monitored by both optical and EPR methods. Based on RNO bleaching, relative to Rose Bengal (RB), the singlet oxygen generating efficiencies of DS1, DS2, and DS3 were determined as 0.10, 0.051 and 0.041, respectively. Using the SOD inhibitable cytochrome c reduction assay, the photogeneration of superoxide anion (O2-•) was monitored. The formation of O2-•was enhanced in the presence of electron donors such as EDTA, DETAPAC and NADH. Photolysis of DS1 and DS3 in DMSO in the presence of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) generated a twelve line EPR spectrum characteristic of an O2-•adduct. In the photosensitization of the DS series, both Type I and Type II paths were involved. The quantum mechanically calculated lowest unoccupied molecular orbital (LUMO) energies of DS1 and DS2 were correlated with the experimental redox potential. Photoinduced DNA scission by DS1, DS2, and DS3 confirms the generation of O2-•from these metabolites.

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).

Crosslinking treatment of progressive keratoconus: new hope

PURPOSE OF REVIEW

A new method has been introduced for the treatment of progressive keratoconus using collagen crosslinking by the photosensitzer riboflavin and ultraviolet A-light. Biomechanical measurements have shown an impressive increase in corneal rigidity of 328.9% in human corneas after crosslinking.

RECENT FINDINGS

The 3 and 5-year results of the Dresden clinical study have shown that in all treated 60 eyes the progression of keratoconus was at least stopped ('freezing'). In 31 eyes there also was a slight reversal and flattening of the keratoconus by up to 2.87 diopters. Best corrected visual acuity improved slightly by 1.4 lines. So far, over 150 keratoconus patients have received crosslinking treatment in Dresden. Laboratory studies have revealed that the maximum effect of the treatment is in the anterior 300 mum of the cornea. As for the corneal endothelium, a cytotoxic level for endothelium was found to be 0.36 mW/cm which would be reached in human corneas with a stromal thickness of less than 400 mum.

SUMMARY

Collagen crosslinking by the photosensitzer riboflavin and ultraviolet A-light is an effective means for stabilizing the cornea in keratoconus. Collagen crosslinking might become the standard therapy for progressive keratoconus in the future diminishing significantly the need for corneal transplantation. Preoperative pachymetry and individual control of the ultraviolet A-irradiance before each treatment are mandatory. The treatment parameters must not be varied.

Photosensitized oxidation and inactivation of pyocyanin, a virulence factor of Pseudomonas aeruginosa

Pyocyanin (PyO-) (1-hydroxy-5-methylphenazine) is a cytotoxic compound secreted by Pseudomonas aeruginosa, an omnipresent bacterium and a human pathogen. We report that visible light illumination in the presence of rose bengal, or riboflavin, in aerated solutions (pH 7.0-7.2) induces irreversible loss of the pigment's characteristic absorption band at 690 nm, indicating its oxidation. This photobleaching was paralleled by generation of a multiline Electron Paramagnetic Resonance (EPR) spectrum attributed to a PyO(-)-derived radical. The reaction was dependent on the presence of air, sensitizers and light, was inhibited by sodium azide and was unaffected by ethanol. This suggests that PyO- was oxidized largely via singlet oxygen and that hydroxyl radicals were not involved. The photochemically modified pigment was less efficient in oxidizing NAD(P)H and generated less superoxide (by approximately 50%) than the intact PyO-, indicating its partial inactivation. 1-Methoxy-5-methylphenazine, a PyO- analog in which the -O- moiety was replaced by the methoxy group (-OMe), was resistant to oxidation, suggesting that oxidation of PyO- involves its phenolate moiety. These results also suggest that photosensitization could be a potentially useful method for inactivation of PyO- and, possibly, detoxification of superficial wounds (skin, eye) infected with P. aeruginosa.

Photoinduced oxygen uptake for 9,10-anthraquinone in air-saturated aqueous acetonitrile in the presence of formate, alcohols, ascorbic acid or amines

The photolysis of 9,10-anthraquinone (AQ), 2-methyl- and 2,3-dimethyl-AQ was studied in air-saturated acetonitrile-water in the presence of various donors: formate, ascorbic acid, alcohols, e.g. 2-propanol or methanol, and amines, e.g. ethylenediaminetetraacetate (EDTA). The photoreaction is initiated by H-atom or electron transfer from the donor to the AQ triplet state. The conversion of oxygen into hydrogen peroxide occurs via the superoxide radical and its conjugate acid. The quantum yield of oxygen uptake (Phi(-O2)) increases with increasing donor concentration. Phi(-O2) = 0.3-0.6 in the presence of 1 M 2-propanol and 3-10 mM ascorbic acid or EDTA. The properties of the quinone and donor radicals involved and the pH and concentration dependences of Phi(-O2) are described.

Oxygen Uptake and Involvement of Superoxide Radicals upon Photolysis of Ketones in Air-saturated Aqueous Alcohol, Formate, Amine or Ascorbic Acid Solutions

The photolysis of acetophenone, benzophenone, 4-carboxybenzophenone and benzil was studied in air-saturated aqueous solution in the presence of alcohols. The overall reaction is an oxidation of 2-propanol to acetone. The quantum yield of oxygen uptake (Phi(-O(2))) increases with increasing 2-propanol concentration up to 0.9. The photoreaction can also be initiated by quenching of the ketone triplet state by ascorbic acid, formate or an amine e.g. triethylamine. Subsequent reactions of the involved radicals with oxygen yield the superoxide radical and eventually hydrogen peroxide. For the ketones in the presence of 3-30 mM ascorbic acid or triethylamine Phi(-O(2)) = 0.3-0.9. The specific properties of ketones, including 4-methoxyacetophenone and 2-acetonaphthone, the radicals involved and the pH and concentration dependences of Phi(-O(2)) are discussed.

A kinetic study on the inhibitory action of sympathomimetic drugs towards photogenerated oxygen active species. The case of phenylephrine

Kinetics and mechanism of the aerobic Riboflavin (Rf, vitamin B2) sensitized photodegradation of Phenylephrine (Phen), a phenolamine belonging to the sympathomimetic drugs family, has been studied in water, employing continuous photolysis, polarographic detection of oxygen uptake, steady-state and time-resolved fluorescence spectroscopy, time-resolved IR-phosphorescence and laser flash photolysis. Results indicate the formation of a weak dark complex Rf-Phen, with an apparent association constant of 5.5+/-0.5M(-1), only detectable at Phen concentrations much higher than those employed in the photochemical experiments. Under irradiation, an intricate mechanism of competitive reactions operates. Phen quenches excited singlet and triplet states of Rf, with rate constants of 3.33+/-0.08 and 1.60+/-0.03x10(9)M(-1)s(-1), respectively. With the sympathomimetic drug in a concentration similar to that of dissolved molecular oxygen in water, Phen and oxygen competitively quench triplet excited Rf, generating superoxide radical anion and singlet molecular oxygen (O2((1)Deltag)) by processes initiated by electron- and energy-transfer mechanisms respectively. As a global result, the photodegradation of the vitamin, a known process taking place from its excited triplet state, is retarded, whereas the phenolamine, practically unreactive towards these oxidative species, behaves as a highly efficient physical deactivator of O2((1)Deltag). The phenolamine structure in Phen appears as an excellent scavenger of activated oxygen species, comparatively superior, in kinetic terms, to some commercial phenolic antioxidants.

Photodegradation of hydroxylated N-heteroaromatic derivatives in natural-like aquatic environments. A review of kinetic data of pesticide model compounds

In the present review, the results published by our group and others related with the study of the kinetic behavior and, in some cases, the mechanism of the dye-promoted photooxygenation of several hydroxypyridines, hydroxyquinolines and hydroxypyrimidines--some of them with the basic molecular structures of known pesticides, and of a few non-hydroxylated model compounds, are compiled and discussed. The main aim was to examine the experimental conditions that maximize the photodegradation efficiencies of all these compounds, under dye-sensitized photooxidation conditions similar to those frequently found in nature, with a natural dye sensitizer such as riboflavin (vitamin B2), a pigment habitually present in natural waters. The usual mechanism of action of this compound is rather complex, in many cases with the concurrent involvement of the oxidative species singlet molecular oxygen (O2(1Deltag)) and superoxide radical anion. In order to simplify the study of the processes, the results found using the synthetic dye Rose Bengal (RB), a sensitizer that generates O2(1Deltag) with high efficiency, are also discussed. RB and similar O2(1Deltag)-generators could be used for the efficient non-natural photodegradation of related pesticides in aqueous solutions under controlled conditions.

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).

Scavenging of photogenerated singlet molecular oxygen and superoxide radical anion by sulpha drugs - Kinetics and mechanism

The ability of the sulfanilic antibiotics (SDs), dapsone (DAP), sulfisoxazole (SFX), sulfadiazine (SFD), and sulfanilic acid (SFNA) to act as scavengers of the visible-light-photogenerated species superoxide radical anion (O2·–) and singlet molecular oxygen (O2(1Δg)) was studied employing the natural pigment riboflavin (Rf) and the artificial dye Rose Bengal as photosensitisers. A complex mechanism, common to all the SDs studied, was elucidated through stationary photolysis, polarographic detection of oxygen uptake, fluorescence spectroscopy, time-resolved phosphorescence detection of O2(1Δg), and laser flash photolysis. Visible-light irradiation of aqueous and aqueous methanolic solutions of Rf (ca. 0.02 mmol/L) plus SD (ca. 0.5 mmol/L) photogenerated excited singlet and triplet Rf (1Rf* and 3Rf*). Under these experimental conditions, only 3Rf* is quenched either by oxygen, giving rise to O2(1Δg) by electronic energy transfer to dissolved ground-state oxygen, or by SD, yielding semireduced Rf through an electron-transfer process. Complementary experiments performed in pure water employing superoxide dismutase and sodium azide inhibition of oxygen uptake, in parallel with laser flash photolysis data, showed that O2·– is also formed, probably due to the reaction of the anion radical from Rf with dissolved oxygen, yielding also neutral, ground-state Rf. Both active oxygen species, namely, O2·– and O2(1Δg), are quenched by the SDs and, as a result, photodegradation of the SDs — each to a different extent — and photodegradation of the sensitiser itself were observed. The SDs that kinetically behave as the better physical quenchers of O2(1Δg), which are in principle good candidates as photoprotectors, namely, DAP and SFD, suffer photooxidation, exhibiting high to moderate oxygen consumption rates due to the O2·– oxidative pathway, whereas for SFNA and SFX, oxidation predominantly occurs through an O2(1Δg)-mediated mechanism. Microbiological results for SFX, taken as a representative SD, indicate that the photodegradation of the drug, upon visible-light Rf-sensitised irradiation, is accompanied by a net loss in bacteriostatic activity.Key words: photooxidation, singlet oxygen, superoxide radical anion, sulpha drugs.

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.

Photodegradation of the herbicide Norflurazon sensitised by Riboflavin. A kinetic and mechanistic study

The kinetics and mechanism of the Riboflavin (Rf)-promoted photochemical degradation with visible light of the herbicide Norflurazon (NF) has been studied by time-resolved and stationary techniques. Using light of wavelength higher than 400 nm--a region where NF is totally transparent--and with concentrations of Rf and NF of ca. 0.02 and 1 mM, respectively, only the excited triplet state of Rf ((3)Rf*) is quenched by NF, in competition with dissolved ground state triplet oxygen, O(2)((3)Sigma(g)(-)). NF degradation mainly occurs by reaction with superoxide radical anion O(2)(-) formed through two electron transfer steps: from NF to (3)Rf*, yielding Rf radical anion, and from this anion to O(2)((3)Sigma(g)(-)), regenerating ground state Rf. Although singlet molecular oxygen is also produced, NF only quenches this oxidative species in a physical mode. The global result is the photoprotection of the sensitiser and the photodegradation of NF.

Age-dependent protein modifications and declining proteasome activity in the human lens

The proteasome is known to be the main enzymatic complex responsible for the intracellular degradation of altered proteins, and the age-related accumulation of modified lens proteins is associated to the formation of cataracts. The aim of this study was to determine whether the human lens proteasome becomes functionally impaired with age. The soluble and insoluble protein fractions of human lenses corresponding to various age-groups were characterized in terms of their levels of glyco-oxidative damage and found to show increasing anti-carboxymethyl-lysine immunoreactivity with age. Concomitantly, decreasing proteasome contents and peptidase activities were observed in the water-soluble fraction. The fact that peptidylglutamyl-peptide hydrolase activity is most severely affected with age suggests that specific changes are undergone by the proteasome itself. In particular, increasing levels of carboxymethylation were observed with age in the proteasome. It was concluded that the lower levels of soluble active enzymatic complex present in elderly lenses and the post-translational modifications affecting the proteasome may at least partly explain the decrease in proteasome activity and the concomitant accumulation of carboxymethylated and ubiquitinated proteins which occur with age.

Modelling the environmental degradation of water contaminants. Kinetics and mechanism of the riboflavin-sensitised-photooxidation of phenolic compounds

The aerobic visible-light-photosensitised irradiation of methanolic solutions of either of the phenolic-type contaminants model compounds (ArOH) p-phenylphenol (PP), p-nitrophenol (NP) and phenol (Ph), and for two additional phenolic derivatives, namely p-chlorophenol (ClP) and p-methoxyphenol (MeOP), used in some experiments, was carried out. Employing the natural pigment riboflavin (Rf) as a sensitiser, the degradation of both the ArOH and the very sensitiser was observed. A complex mechanism, common for all the ArOH studied, operates. It involves superoxide radical anion (O2-*) and singlet molecular oxygen (O2(1delta(g)) reactions. Maintaining Rf in sensitising concentrations levels (approximately 0.02 mM), the mechanism is highly dependent on the concentration of the ArOH. Kinetic experiments of oxygen and substrate consumption, static fluorescence, laser flash photolysis and time-resolved phosophorescence detection of O2(1delta(g)) demonstrate that at ArOH concentrations in the order of 10 mM, no chemical transformation occurs due to the complete quenching of Rf singlet excited state. When ArOH is present in concentrations in the order of mM or lower, O2-* is generated from the corresponding Rf radical anion, which is produced by electron transfer reaction from the ArOH to triplet excited Rf. The determined reaction rate constants for this step show a fairly good correlation with the electron-donor capabilities for Ph, PP, NP, ClP and MeOP. In this context, the main oxidative species is O2-*, since O2(1delta(g)) is quenched in an exclusive physical fashion by the ArOH. The production of O2-* regenerates Rf impeding the total degradation of the sensitiser. This kinetic scheme could partially model the fate of ArOH in aquatic media containing natural photosensitisers, under environmental conditions.

Synthesis, X-ray crystal structure, antimicrobial activity and photodynamic effects of some thiabendazole complexes

An interesting series of metal complexes of thiabendazole (tbz) is synthesized and characterized by elemental analyses and spectroscopic studies. The crystal structure of the hydrogen bonded one dimensional Co(II) complex, namely [Co(tbz)(2)(NO(3))(H(2)O)](NO(3)) is solved by single crystal X-ray diffraction. The complex crystallizes in monoclinic space group P2(1)/a with unit cell parameters, a=14.366(2), b=11.459(4), c=15.942(3) A, beta=113.78(3) degrees and z=4. The unit cell packing reveals an extensive hydrogen bonding involving a water molecule, nitrate ligands and the protonated nitrogen atoms of the tbz ligands, resulting in a one dimensional hydrogen bonding pattern. The antimicrobial activity of the complexes against selected bacteria (Escherichia coli and Bacillus subtilis) and yeast (Aspergillus flavues) is estimated. The relationship between the enzymatic production of ROS and antimicrobial activity of the complexes is examined, and a good correlation between two factors is found. Photodynamic quantum yields of singlet oxygen production (RNO bleaching assay) and rate of superoxide generation (SOD inhibitable ferricytochrome c reduction assay and EPR spin trapping experiments using 5,5-dimethyl-1-pyrroline-N-oxide as spin trap) by the metal complexes have been studied.

Scavenging of photogenerated oxidative species by antimuscarinic drugs: atropine and derivatives

The quenching ability of photogenerated oxidative species by some antimuscarinic drugs generically named atropines (e.g. atropine [I] eucatropine [II], homatropine [III] and scopolamine [IV]) have been investigated employing stationary photolysis, polarographic detection of dissolved oxygen, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Using Rose Bengal as a dye sensitiser for singlet molecular oxygen, O(2)((1)Delta(g)), generation, compounds I-IV behave as moderate chemical plus physical quenchers of the oxidative species. Correlation between kinetic and electrochemical data indicates that the process is possibly driven by a charge-transfer interaction. The situation is somewhat more complicated employing the natural pigment riboflavin (Rf) as a sensitiser. Compounds I and II complex Rf ground state, diminishing the quenching ability towards singlet and triplet excited state of the pigment. On the other hand, compounds III and IV effectively quench Rf excited states, protecting the pigment against photodegradation. Under anaerobic conditions, semireduced Rf (Rf(.-)) is formed through quenching of excited triplet Rf. Nevertheless, although Rf(.-) is a well-known generator of the reactive species superoxide radical anion by reductive quenching in the presence of oxygen, the process of O(2)((1)Delta(g)) production prevails over superoxide radical generation, due to the relatively low rate constants for the quenching of triplet Rf by the atropines (in the order of 10(7) M(-1)s(-1) for compounds III and IV) in comparison to the rate constant for the quenching by ground state oxygen, approximately two orders of magnitude higher, yielding O(2)((1)Delta(g)). Compound I is the most promising O(2)((1)Delta(g)) physical scavenger, provided that it exhibits the higher value for the overall quenching rate constant and only 11% of the quenching process leads to its own chemical damage.

Kinetics and Mechanism of the Vitamin B2–sensitized Photooxidation of Isoproterenol¶

The sensitized photooxidation promoted by daylight-absorbing compounds appears as a plausible course to produce the photodegradation of catecholamines. We report the kinetics and mechanism of vitamin B2 (riboflavin [Rf])-sensitized photooxidation of isoproterenol (Iso), a synthetic sympathomimetic drug structurally related to epinephrine, using water as a solvent. A weak dark complex Rf-Iso is formed, only detectable at relatively high Iso concentrations (>10 mM), with a mean value of 13 +/- 3 M(-1) for the apparent association constant. Under aerobic sensitizing conditions (Rf approximately 0.02 mM and Iso approximately 0.5 mM) two oxidative mechanisms operate, mediated by singlet molecular oxygen (O2(1delta g)) and superoxide radical anion (O2*-). Our analysis shows that the main reaction pathway is an electron transfer-mediated quenching of Rf excited triplet state (3Rf*) by Iso. It produces the species Iso*+ and Rf*-. The latter, in a subsequent reaction path, generates O2*-, which is mainly responsible for Iso photooxygenation. In a less-important process, energy transfer of the 3Rf* to dissolved oxygen generates O2(1delta g). The kinetic balance between chemical and physical quenching of O2(1delta g) by Iso indicates that the process is largely dominated by the physical, not chemical, interaction. The results, which can be extrapolated to an in vivo condition, show the susceptibility of Iso to undergo visible light-induced photodegradation in the presence of dye sensitizers present in the environment.

The activity of 3- and 7-hydroxyflavones as scavengers of superoxide radical anion generated from photo-excited riboflavin

The visible-light irradiation of the system Rivoflavin plus 3-hydroxyflavone or plus 7-hydroxyflavone, under aerobic conditions, produces a series of competitive processes that depend on the relative concentrations of the pigment and the flavones. The picture comprises photochemical mechanisms that potentially operate in nature. They mainly include the quenching of Rf singlet (1Rf*) and triplet (3Rf*) excited states (with bimolecular rate constants in the order of 109 M–1 s–1) and superoxide radical anion-mediated reactions. The participation of the oxidative species singlet molecular oxygen was not detected. The overall result shows chemical transformations in both Rf and 3-hydroxyflavone. No experimental evidence was found indicating any chemical reaction involving 7-hydroxyflavone. The fate of the pigment also depends on the amount of the dissolved flavonoid. At 50 mM concentrations of these compounds or higher, practically no photochemistry occurs, owing to the extensive quenching of 1Rf*. When the concentration of the flavones is in the mM range or lower, 3Rf* is photogenerated. Then, the excited triplet species can be quenched mainly by the flavones through an electron-transfer process, yielding the semireduced pigment. The latter interacts with dissolved oxygen producing O2·–, which reacts with both the pigment and 3-hydroxyflavone. In summary, 3-hydroxyflavone and 7-hydroxyflavone participate in the generation of superoxide ion in an Rf-sensitized process, and simultaneously 3-hydroxyflavone constitutes a degradable quencher of the oxidative species.Key words: flavones, riboflavin, sensitization, singlet molecular oxygen, superoxide radical anion.

Study of the interaction between triplet riboflavin and the alpha-, betaH- and betaL-crystallins of the eye lens

Time-resolved photolysis studies of riboflavin (RF) were carried out in the presence and absence of alpha-, betaH- and betaL-crystallins of bovine eye lens. The transient absorption spectra, recorded 5 micros after the laser pulse, reveal the presence of the absorption band (625-675 nm) of the RF neutral triplet state (tau = 42 micros) accompanied by the appearance of a long-lived absorption (tau = 320 micros) in the 500-600 nm region due to the formation of the semireduced RF radical. The RF excited state is quenched by the crystallin proteins through a mechanism that involves electron transfer from the proteins to the flavin, as shown by the decrease of the triplet RF band with the concomitant increase of the band of its semireduced form. Tryptophan loss on RF-sensitized photooxidation of the crystallins when irradiated with monochromatic visible light (450 nm) in a 5% oxygen atmosphere was studied. A direct correlation was found between the triplet RF quenching rate constants by the different crystallin fractions and the decomposition rate constants for the exposed and partially buried tryptophans in the proteins. The RF-sensitized photooxidation of the crystallins is accompanied by the decrease of the low molecular weight constituents giving rise to its multimeric forms. A direct correlation was observed between the initial rate of decrease of the low molecular weight bands corresponding to the irradiated alpha-, betaH- and betaL-crystallins and the quenching constant values of triplet RF by the different crystallins. The correlations found in this study confirm the importance of the Type-I photosensitizing mechanism of the crystallins, when RF acts as a sensitizer at low oxygen concentration, as can occur in the eye lens.

Comparison of the aerobic photoreactivity of A2E with its precursor retinal

A2E (2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E, 3E,5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E-hexatrienyl]pyridinium) is a blue-absorbing molecular constituent of human ocular lipofuscin and contributes to the golden-yellow emission of this pigment. Lipofuscin photoproduces toxic reactive oxygen intermediates (ROI), but the specific molecular components responsible for this phototoxicity remain unidentified. In this article the aerobic photoreactivity of A2E is quantified by comparison with its biosynthetic precursor, all-trans-retinal, and with other appropriate standards. Under blue-light exposure the efficacies for formation of cholesterol (Ch) hydroperoxides and the superoxide radical anion (O2*-) were determined using high-pressure liquid chromatography with electrochemical detection and electron spin resonance oximetry and spin trapping, respectively. Photogeneration of singlet oxygen after blue-light excitation of A2E was demonstrated unambiguously by the Ch peroxidation assay. After blue-light irradiation of A2E, O2*- were detected, but the concentration was insufficient to account for the measured production of O2*- by the solvent extract of lipofuscin granules. The collective data support the conclusion that A2E does not produce sufficient concentrations of ROI to be the primary phototoxic constituent of lipofuscin.

Photochemical Properties of Kynurenine Pathway Metabolites and Indoleamines

Photochemical damages to the biological system may occur through photodynamic action in the presence of photosensitive molecules. Photodynamic action contains the following processes; 1) photosensitisation and/or 2) electron transfer, in which singlet oxygen and superoxide radical production for each in the presence of oxygen molecules. We have studied those processes after the absorption of light by kynurenine pathway metabolites and indoleamine derivatives. We found that kynurenine and 3-hydroxykynurenine generate superoxide radical after electron transfer from their excited state molecules to oxygen molecules, and superoxide makes reduction reaction. On the other hand, it was found that kynurenic acid, melatonin, 5-methoxytryptamine and 5-methoxytryptophol work as photosensitisers with the detection of singlet oxygen production by using the N, N-dimethyl-4-nitrosoaniline bleaching method, while xanthurenic acid, serotonin and N-acetylserotonin generate no detectable amount of singlet oxygen. We have determined the photochemical quantum yields of singlet oxygen production for those photosensitisers, in which quantum yields are not so high except kynurenic acid (f3 = 0.101). In view of the multiple roles played by their metabolites in various systems, these results are relevant to taking into consideration of their photoeffect in the presence of light.