Structure and stability of gamma-crystallins--IV. Aggregation and structural destabilization in photosensitized reactions

Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid

Photoinduced generation of radicals in the eye lens may play an important role in the modification of proteins leading to their coloration, aggregation, and insolubilization. The radicals can be formed via the reactions of photoexcited endogenous chromophores of the human lens with lens proteins, in particular with tryptophan residues. In the present work we studied the reactions induced by UV-A (315-400nm) light between kynurenic acid (KNA), an effective photosensitizer present in the human lens, and N-acetyl-L-tryptophan (NTrpH) under aerobic and anaerobic conditions. Our results show that the reaction mechanism strongly depends on the presence of oxygen in solution. Under aerobic conditions, the generation of singlet oxygen is the major channel of the effective NTrpH oxidation. In argon-bubbled solutions, the quenching of triplet KNA by NTrpH results in the formation of KNA^•- and NTrp^• radicals. Under laser pulse irradiation, when the radical concentration is high, the main pathway of the radical decay is the back electron transfer with the restoration of initial reagents. Other reactions include (i) the radical combination yielding NTrp dimers and (ii) the oxygen atom transfer from KNA^•- to NTrp^• with the formation of oxidized NTrp species and deoxygenated KNA products. In continuous-wave photolysis, even trace amounts of molecular oxygen are sufficient to oxidize the majority of KNA^•- radicals with the rate constant of (2.0 ± 0.2) × 10⁹M^-1s^-1, leading to the restoration of KNA and the formation of superoxide radical O₂^•-. The latter reacts with NTrp^• via either the radical combination to form oxidized NTrp (minor pathway), or the electron transfer to restore NTrpH in the ground state (major pathway). As the result, the quantum yields of the starting compound decomposition under continuous-wave anaerobic photolysis are rather low: 1.6% for NTrpH and 0.02% for KNA. The photolysis of KNA with alpha-crystallin yields the same deoxygenated KNA products as the photolysis of KNA with NTrpH, indicating the similarity of the photolysis mechanisms. Thus, inside the eye lens KNA can sensitize both protein photooxidation and protein covalent cross-linking with the minor self-degradation. This may play an important role in the lens protein modifications during the normal aging and cataract development.

Etiopathogenesis of cataract: an appraisal

Natural eye lens is a crystalline substance to produce a clear passage for light. Cataract is opacity within the clear lens of the eye and is the dominant cause of socio-medical problem i.e., blindness worldwide. The only available treatment of cataract is surgery. However, insufficient surgical facilities in poor and developing countries and post-operative complications inspire researchers to find out other modes of treatment for cataract. In this review, an attempt has been made to appraise various etiological factors of cataract to make their perception clear to build up counterpart treatment. Present study is an assortment of various available literatures and electronic information in view of cataract etiopathogenesis. Various risk factors have been identified in development of cataracts. They can be classified in to genetic factors, ageing (systemic diseases, nutritional and trace metals deficiencies, smoking, oxidative stress etc.), traumatic, complicated (inflammatory and degenerative diseases of eye), metabolic (diabetes, galactosemia etc.), toxic substances including drugs abuses, alcohol etc., radiation (ultraviolet, electromagnetic waves etc.) are implicated as significant risk factors in the development of cataract.

Tryptophan cluster protects human γD-crystallin from ultraviolet radiation-induced photoaggregation in vitro

Exposure to ultraviolet radiation (UVR) is a significant risk factor for age-related cataract, a disease of the human lens and the most prevalent cause of blindness in the world. Cataract pathology involves protein misfolding and aggregation of the primary proteins of the lens, the crystallins. Human γD-crystallin (HγD-Crys) is a major γ-crystallin in the nucleus of the human lens. We report here analysis of UVR-induced damage to HγD-Crys in vitro. Irradiation of solutions of recombinant HγD-Crys with UVA/UVB light produced a rise in solution turbidity due to polymerization of the monomeric crystallins into higher molecular weight aggregates. A significant fraction of this polymerized protein was covalently linked. Photoaggregation of HγD-Crys required oxygen and its rate was protein concentration and UVR dose dependent. To investigate the potential roles of individual tryptophan residues in photoaggregation, triple W:F mutants of HγD-Crys were irradiated. Surprisingly, despite reducing UVR absorbing capacity, multiple W:F HγD-Crys mutant proteins photoaggregated more quickly and extensively than wild type. The results reported here are consistent with previous studies that postulated that an energy transfer mechanism between the highly conserved pairs of tryptophan residues in HγD-Crys could be protective against UVR-induced photodamage.

Peptide scanning-based identification of regions of gamma-II crystallin involved in thermal aggregation: evidence of the involvement of structurally analogous, helix-containing loops from the two double Greek key domains of the molecule

Gamma crystallin is one of three structural proteins present in great abundance in the fiber cells of the vertebrate eye lens. The protein displays a tendency to aggregate readily in the course of heating, cooling, being exposed to ultraviolet radiation, or rapid refolding. To investigate the molecular mechanisms underlying such aggregation, we have employed a peptide-scanning approach aimed at identifying regions of bovine gamma-II crystallin that may be involved in intermolecular interactions leading to aggregation, using assays that measure the competitive inhibition of such aggregation by reagents drawn from a group of contiguous (overlapping) peptides derived from the sequence of the protein itself. Our results suggest that two regions, comprising residues 61-74, and 145-159, play key roles in aggregative interactions. Intriguingly, the two regions (each containing a solvent-exposed, single-turn helix in the native structure) are located in structurally analogous positions in the two homologous double Greek key (beta sheet) domains of the protein, suggesting that helix-strand conversions may operate to facilitate intermolecular beta sheet interactions during aggregation.

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.

Thermodynamic and kinetic characterization of calf lens gammaF-crystallin

Gamma-crystallin is reported to be conformationally stable because of its internal structural symmetry, and gammaF (gammaIVa) is the most stable among the various gamma-crystallin gene products. However, there is no detailed report on its thermodynamic and kinetic stability. In the present study, detailed unfolding of gammaF-crystallin was investigated by equilibrium and kinetics methods with fluorescence and far-UV CD spectroscopic measurements. The GdnHCl-induced unfolding curves probed by Trp emission maximum and intensity showed a sharp single-step transition. Upon widening the unfolding transition with the use of urea in 1.5 M GdnHCl, a more proper fit for thermodynamic analysis was obtained. GammaF-Crystallin underwent a straightforward two-state process (N <==> U) without showing any measurable amount of intermediate. The conformational stability, as measured by deltaG(D)H2O (approximately 9 kcal/mol), indicates that gammaF-crystallin is a very stable protein. The high activation energy deltaG++H2O (approximately 24 kcal/mol), calculated from unfolding kinetics monitored by far-UV CD at 218 nm, also indicates that the native and unfolded states are separated by a high activation energy barrier.

Structural perturbation of alpha-crystallin and its chaperone-like activity

alpha-Crystallin is a multimeric lenticular protein that has recently been shown to be expressed in several non-lenticular tissues as well. It is shown to prevent aggregation of non-native proteins as a molecular chaperone. By using a non-thermal aggregation model, we could show that this process is temperature-dependent. We investigated the chaperone-like activity of alpha-crystallin towards photo-induced aggregation of gamma-crystallin, aggregation of insulin and on the refolding induced aggregation of beta- and gamma-crystallins. We observed that alpha-crystallin could prevent photo-aggregation of gamma-crystallin and this chaperone-like activity of alpha-crystallin is enhanced several fold at temperatures above 30 degrees C. This enhancement parallels the exposure of its hydrophobic surfaces as a function of temperature, probed using hydrophobic fluorescent probes such as pyrene and 8-anilinonaphthalene-1-sulfonate. We, therefore, concluded that alpha-crystallin prevents the aggregation of other proteins by providing appropriately placed hydrophobic surfaces; a structural transition above 30 degrees C involving enhanced or re-organized hydrophobic surfaces of alpha-crystallin is important for its chaperone-like activity. We also addressed the issue of conformational aspects of target proteins and found that their aggregation prone molten globule states bind to alpha-crystallin. We trace these developments and discuss some new lines that suggest the role of tertiary structural aspects in the chaperone process.

Specific racemization and isomerization of the aspartyl residue of alphaA-crystallin due to UV-B irradiation

We have reported that the aspartyl (Asp)-151 residue in alphaA-crystallin in human eye lens was inverted to the D-isomer and isomerized to beta-Asp residue with age. We report here that ultraviolet (UV)-B irradiation induces the racemization and isomerization of the Asp-151 residue of alphaA-crystallin from lenses of 6-week-old rats to form D-isomer and beta-Asp residue. Simultaneous racemization and isomerization of the specific Asp residue indicate that the reaction proceeds via formation of a succinimide intermediate. This modification was not observed in UV-A irradiated and normal lenses. UV-B irradiation induced the racemization of only the Asp-151 residue and did not affect the other Asp residues in alphaA-crystallin. On the other hand, the high molecular weight fraction of the lens protein increased upon UV-B irradiation. Modification of the Asp residue would affect the three-dimensional packing array of the lens protein.

Photooxidation of tryptophan: O2(1 delta g) versus electron-transfer pathway

Tris (2,2'-bipyridyl)ruthenium(II)chloride hexahydrate (Ru[bpy]3(2+)) free in solution and adsorbed onto antimony-doped SnO2 colloidal particles was used as a photosensitizer for a comparison of the O2(1 delta g) and electron-transfer-mediated photooxidation of tryptophan (TRP), respectively. Quenching of excited Ru(bpy)3(2+) by O2(3 sigma g-) in an aerated aqueous solution leads only to the formation of O2(1 delta g) (phi delta = 0.18) and this compound was used as a type II photosensitizer. Excitation of Ru(bpy)3(2+) adsorbed onto Sb/SnO2 results in a fast injection of an electron into the conduction band of the semiconductor and accordingly to the formation of Ru(bpy)3(2+) and was used for the sensitization of the electron-transfer-mediated photooxidation. The Ru(bpy)3(3+) is reduced by TRP with a bimolecular rate constant kQ = 5.9 x 10(8) M-1 s-1, while O2(1 delta g) is quenched by TRP with kt = 7.1 x 10(7) M-1 s-1 (chemical + physical quenching). Relative rate constants for the photooxidation of TRP (kc) via both pathways were determined using fluorescence emission spectroscopy. With Np, the rate of photons absorbed, being constant for both pathways we obtained kc = (372/Np) M-1 s-1 for the O2(1 delta g) pathway and kc > or = (25,013/Np) M-1 s-1 for the electron-transfer pathway, respectively. Thus the photooxidation of Trp is more than two orders of magnitude more efficient when it is initiated by electron transfer than when initiated by O2(1 delta g).

Effect of UVB radiation on corneal aldehyde dehydrogenase

PURPOSE

A Class 3 aldehyde dehydrogenase happens to be a major soluble protein constituent of the cornea. Its role is conjectured to be manifold: to protect the tissue from oxidative damage by eliminating the toxic aldehydes produced upon lipid peroxidation under oxidative stress, to act as an UV-absorber, and to maintain the level of the coenzyme NADH in the cornea. We have studied the effect of UVB on the structure and enzyme activity of corneal aldehyde dehydrogenase.

METHODS

Aldehyde dehydrogenase was irradiated at 295 nm for varying periods of time and change in its enzyme activity assayed. The structural changes in the molecule accompanying irradiation were monitored using fluorescence and circular dichroism spectroscopy, and its hydrodynamic behavior and surface hydrophobicity studied using gel filtration chromatography and binding of the hydrophobic fluorophore ANS. The protective ability of aldehyde dehydrogenase in preventing aggregation of photolabile proteins, such as Gamma-crystallin of the eye lens, was studied by monitoring the scattering value of the test protein with irradiation by UVB.

RESULTS

Aldehyde dehydrogenase is seen to undergo photodamage with alterations in its quaternary structure, though no significant change is noticed in the peptide chain conformation. Under such conditions the molecule continues to act as a protectant by preventing aggregation of photolabile proteins such as the eye lens Gamma-crystallin.

CONCLUSIONS

Our earlier studies have shown that the free sulfhydryl groups are important for the antioxidant abilities of aldehyde dehydrogenase. Its protective ability towards photoaggregation of Gamma-crystallin seen here might arise both due to: (i) oxyradical quenching and (ii) the increased surface hydrophobicity of the molecule upon irradiation, which allows it to bind to, and thus inhibit the aggregation of interacting proteins.

Concentration dependence of transmission losses in UV-laser irradiated bovine alpha-, beta H-, beta L- and gamma-crystallin solutions

Experiments with calf lens protein fractions in aqueous buffer solutions at room temperature showed that beta H-, beta L- and gamma-crystallin fractions became opaque following ultraviolet exposure at 308 nm, while the alpha-crystallin fraction remained transparent. Transmission loss, due to UV-irradiation, for all of the crystallin samples was studied in the concentration range of 0.1 mg/mL to 1.0 mg/mL, and for alpha- and gamma-crystallin, in the range up to 5 mg/mL. With increased concentrations of beta H-, beta L- and gamma-crystallin, the rate of opacification increased. However, with alpha-crystallin, the loss of transmission was negligible for all of the concentrations and irradiation times studied. Opacification of the crystallins was accompanied by formation of higher molecular weight insoluble proteins as detected by SDS-PAGE.

Riboflavin-photosensitized anaerobic modification of rat lens proteins. A correlation with age-related changes

When rat lens homogenate or its soluble protein fractions are irradiated in the presence of riboflavin, a photo-adduct is obtained between this vitamin and the lens proteins. Irradiation of these proteins in the presence of riboflavin also leads to a modification in the chromatographic elution pattern with an increase in the high-molecular-weight fraction. In an aging study with rats, it was shown that the proportion of the high-molecular-weight protein fraction significantly increased with age, whereas the proportion of the low-molecular-weight protein fraction concomitantly decreased. It is postulated that aging produces an increase in the accessibility of the tryptophan residues of the lens proteins, as established by iodide fluorescence quenching experiments.

Melittin-induced conformational changes in human lens protein

Circular dichroism and fluorescence measurements showed a reduced conformational order in proteins of a normal human lens when they were incubated in vitro with melittin, a bee venom peptide. Since melittin is also known to react with lipids to induce a breakdown of vesicular structure, the observed denaturation of water-soluble proteins of a human lens that developed a cataract due to multiple bee stings may be accounted for by the effects of melittin to some extent. The melittin-induced decrease of conformational order, as observed in our in-vitro studies could thus be of physiological significance.

Photooxidation of specific residues in alpha-crystallin polypeptides

Singlet oxygen is a biologically important, photochemically generated species that preferentially oxidizes His, Trp, and Met residues of protein molecules. Calf alpha-crystallin was photooxidized with use of meso-tetra(p-sulfonatophenyl)porphyrin (TPPS) and uroporphyrin (UP) as singlet oxygen generators. The effects of photooxidation were monitored by analyzing the changes in alpha-crystallin peptide maps obtained by reversed-phase HPLC using a photodiode array absorbance detector. The reaction led to the loss of six specific peptides, five of which contained photooxidizable residues. Peptides containing His-97 and His-154 from the A chain and Met-68 from the B chain are preferentially photooxidized, suggesting that those residues have access to singlet oxygen. Trp residues in the N-terminal region are converted to NFK, whereas Trp-60 in the B chain is not photooxidized strongly suggesting that the former are close to the surface of alpha-crystallin while the latter Trp residue is buried. Only one peptide that is lost from the peptide maps does not contain a photooxidizable group; however, this peptide does contain an apparently undigested Lys residue. It is suggested that it forms a cross-link with a photooxidized His residue.

Mechanisms of photochemically produced turbidity in lens protein solutions

Calf alpha- and gamma-crystallin were photolyzed in 1-2 mg ml-1 aqueous solutions, using both laser and conventional UV radiation in the 297-320 nm wavelength region. Gamma-crystallin solutions became highly turbid upon UV irradiation, while alpha-crystallin developed no turbidity when irradiated under identical conditions. The photolyzed solutions were analyzed by SDS-PAGE. These gels revealed loss of normal 20 kDa polypeptide, and formation of higher molecular weight peptides, in both alpha- and gamma-crystallin, presumably as a result of photocross-linking reactions and/or protein insolubilization. Thus, although both crystallins underwent photocross-linking, significant turbidity production only occurred in gamma-crystallin. Some possible explanations for these differences are proposed, with one possibility being that most photocross-links in alpha-crystallin occur between subunits of the 1000-kDa oligomer, while in gamma-crystallin the cross-links occur between 20-kDa monomer units. Hence, cross-linking in alpha-crystallin does not affect the average size of particles in solution (or the turbidity), while cross-linking in gamma-crystallin results in a significant increase in average particle size with concomitant increase in turbidity. Another possible explanation is that UV-irradiated gamma-crystallin becomes insoluble (due to charge changes resulting in non-covalent aggregation) while alpha-crystallin does not. Other differences in the photochemical behavior of alpha- vs. gamma-crystallin were noted--gamma-crystallin photolysis rate was about 50% greater than alpha-crystallin. Alpha-crystallin photolysis yielded strong NFK-like fluorescence, while gamma-crystallin did not. One similarity was that photolyzed alpha- and gamma-crystallin lost amino acids His and Trp at about the same rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acetylation by aspirin on the thermodynamic stability of lens crystallins

To assess the effect of aspirin on cataractogenesis, we compared the stability of individual, native protein fractions alpha L, beta H, beta L, beta s, beta B2, gamma-II, gamma-III and gamma-IV with that of their acetylated counterparts. The conformational stabilities of native fractions beta B2 and beta s, which were not reported earlier, were determined first from their thermal and a thermal denaturation behaviour. Since alpha L, beta H and beta L fractions are oligomeric, no thermodynamic analysis of these fractions was attempted. The thermal stability of beta s and beta B2 is rather low; their melting temperature (T1/2) range is 58-60 degrees C compared with 67-75 degrees C for the gamma-crystallins. Furthermore, except for alpha L, which remains stable even at 100 degrees C, and beta B2, all crystallins aggregate at temperatures slightly above T1/2. The Gibbs free energy of unfolding, delta GH2OD, calculated from guanidine HCl (GdnHCl) denaturation, is surprising low (3-9 kcal mol-1) for all crystallin fractions. The low values of delta GH2OD indicate that the structural destabilization of these proteins, which may lead to cataract formation, could result from a slight disturbance of a particular kind (sugar, UV light, oxidation, and other factors). The overall effect of acetylation on the individual crystallin fractions is mixed. The thermal stability of beta B2 increased, tended to decrease in the case of gamma-crystallins, but remained virtually unchanged for other proteins. Delta GH2OD values of the native crystallin fractions do not differ significantly from those of their acetylated counterparts.

The reaction of singlet oxygen with proteins, with special reference to crystallins

Photosensitized oxidation of the eye lens proteins, the crystallins, is thought to lead to protein crosslinks and high molecular weight aggregates. Such protein modifications may be important factors in the formation of lens opacities or cataracts. We focus attention here on type 2 photo-oxidation involving the reaction of singlet oxygen (1O2) with crystallins and some "control" proteins. We find that: (1) trp residues are oxidized to N-formyl kynurenine and related products, but this in itself does not lead to the production of high molecular weight protein aggregates of the protein; (2) tyr residues react with 1O2 but we do not detect dihydroxyphenylalanine or bityrosine nor are protein crosslinks formed as a result; (3) oxidation of his residues appears necessary for high molecular weight protein covalent aggregates to form. Proteins devoid of his, e.g. melittin or bovine pancreatic trypsin inhibitor, do not form high molecular weight products upon reaction with 1O2. Prior reaction and blocking of his inhibits the crosslinking reactions. (4) The oxidized protein is seen to be more acidic than the parent and has an altered tertiary structure. (5) Among the crystallins, reactivity towards 1O2 varies in the order gamma greater than beta greater than alpha and also gamma A/E greater than gamma D greater than gamma B crystallin.

The conformational status of a protein influences the aerobic photolysis of its tryptophan residues: melittin, beta-lactoglobulin and the crystallins

We have studied the aerobic photolysis of the tryptophan residues of the proteins melittin and beta-lactoglobulin when the proteins are in ordered conformations and when they are in randomly coiled states. The results suggest that the conformational status of the protein is a factor that influences the photolysis of the constituent tryptophan residues. This point appears to be of relevance to the photo-oxidation of the tryptophan residues of the eye lens proteins crystallins.

The rates of photolysis of the four individual tryptophan residues in UV exposed calf gamma-II crystallin

Aqueous buffer solutions of the lens protein bovine gamma-II crystallin were irradiated at 295 nm in the presence of dithiothreitol to determine the individual photolysis susceptibilities of the four tryptophan residues. Reverse-phase high performance liquid chromatography was utilized to compare the tryptic peptide maps before and after irradiation. Sequence analysis of collected tryptic peptides showed that the four tryptophans in calf gamma-II crystallin. TRP-42, TRP-68, TRP-131, and TRP-157 appeared in four distinct tryptic peptides. Fluorescence and absorption (diode array) monitoring of the eluting peptides allowed assessment of the changes in peptide absorbance and fluorescence following irradiation. Tryptophan fluorescence losses of (40 +/- 15)%, (17 +/- 4)%, (35 +/- 5)% and (15 +/- 4)% were observed for the peptides containing TRP-42, TRP-68, TRP-131 and TRP-157, respectively. Thus the four tryptophans in calf gamma-II crystallin did not all photolyze at the same rate. The rate differences are presumably related to the microenvironments of the individual tryptophan residues, and this is discussed in terms of the known crystal structure of calf gamma-II crystallin.

UV laser photodamage to whole lenses

Lenses from rat or calf were exposed in vitro to UV radiation from a nitrogen laser operated at 337.1 nm or from an excimer laser operated at 3.8 nm. Visible light transmission was monitored during calf lens irradiations at 308 nm and found to decrease. Proteins were extracted from the irradiated rat or calf lenses, separated into water soluble and insoluble fractions, and analysed using SDS-PAGE. Comparison of these gels with dark controls showed that, following photolysis, there was loss of polypeptide material in the 20-30 kDa region and concomitant formation of polymers at 40 and 60 kDa, and at greater than 100 kDa in calf lens (308 nm irradiation) and rat lenses (337.1 nm irradiation) in vitro. In addition, there was evidence for formation of lower molecular weight polypeptides at 10 kDa in the protein from irradiated rat lenses. The rat SDS-PAGE gels were challenged against anti-calf gamma crystallin serum. There was clear evidence that the polymeric material, in the water insoluble protein fraction from the 337.1 nm photolyzed rat lenses was derived in part from gamma crystallin. The macromolecular changes detected in these photolyzed rat and calf lens proteins were similar to those previously reported to accompany aging in the human lens. Biochemical changes of the type observed in UV irradiated rat and calf lenses may be responsible for the loss of visible light transmission seen in calf lenses.

Photoreactions of human lens monomeric crystallins

Human lens beta s- and gamma A-crystallins exhibit very similar tryptophan fluorescence emission maxima (329 nm). gamma A isolated from infant human lenses is photo-oxidized by 300 nm irradiation and forms water-insoluble aggregates; beta s or gamma A from young human lenses form a small amount of water-soluble crosslinked species. At least part of the mechanism of photodamage by 300 nm irradiation is photogeneration of the oxidant H2O2 via the generation of O2- radical, this reaction occurs via photosensitization by the tryptophan photo-oxidation product N-formylkynurenine (N-FK) or related species. These results indicate that even though the tryptophan residues of beta s- and gamma A-crystallins are in hydrophobic (buried) microenvironments as compared to those of the alpha- and beta-crystallins, the photogeneration of N-FK is sufficient to produce O2- and H2O2.

The effects of near-UV radiation on human lens beta-crystallins: protein structural changes and the production of O2- and H2O2

beta-Crystallins (beta 1-, beta 2- and beta 3-crystallin) comprise nearly half the protein of the human lens. The effect of near-UV radiation, which is one of the possible risk factors in cataract formation, on the beta-crystallins is investigated in this study. Protein intersubunit crosslinking, change in charge of the protein subunits to more acidic species and changes in protein tertiary structure (conformation) by 300 nm irradiation are reported. The fluorescence yield of protein tryptophan residues decreases by 300 nm irradiation. There is an increase in nontryptophan fluorescence (lambda cx 340 nm, lambda cm 400-600 nm), and in protein absorption at 340 nm, due to the formation of tryptophan photooxidation products. Both tryptophan and its oxidation products can be photoexcited by 300 nm irradiation and the latter are known to be good photosensitizers. The results provide evidence for the generation of H2O2 in the irradiated human beta-crystallin solutions by the Type I photosensitizing action of the chromophores absorbing at 300 nm. The H2O2 is generated via the intermediate production of O2 anion; the latter spontaneously dismutates to H2O2, presumably via O2- protein interactions. The amount of H2O2 generated per absorbed photon is compared for various solutions of beta 1-, beta 2- and beta 3-crystallins from human lenses of different age.

Phosphorescence measurements of calf gamma-II, III, and IV crystallins at 77 and 293 K

Structural and dynamical features of bovine gamma-crystallin tryptophan residues were investigated by phosphorescence measurements at 77 and 293 K. The low temperature phosphorescence spectra and lifetimes of calf gamma-II, III, and IV crystallins did not reflect heterogeneity among the gamma-crystallins. The 0-0 bands were all at 414 +/- 1 nm and the emission lifetimes were all single-exponential with lifetimes of 5.1, 5.3 and 5.3 +/- 0.3 sec, respectively. In contrast, phosphorescence measurements at room temperature were sensitive to subtle differences in exposure, accessibility, and flexibility of gamma-crystallin tryptophan residues. Thorough deoxygenation allowed for measurement of the normally-quenched room-temperature phosphorescence, and we report the first native phosphorescence measurements of lens crystallins at ambient temperature. The emission maxima for gamma-II, III and IV were 446, 442, and 440 +/- 2 nm, respectively. The intensity decay curves were all non-single exponential, and the decays were fit to a sum of two exponentials with lifetimes of 9.1 and 93 msec (gamma-II), 11 and 75 msec (gamma-III), and 4.2 and 68 msec (gamma-IV), +/- 10%. The components of the gamma-II emission were assigned to the four tryptophans based on X-ray structural information. Quantum yields of the phosphorescence emission were in the ratio of 20:7:1 for gamma-II, III and IV, and comparison of lifetimes and quantum yields suggests that tryptophan rigidity increases in the order gamma-IV less than III less than II. Acrylamide quenching constants for the long-lived components of gamma-II and III were roughly equal, while the short-lived tryptophans of gamma-III were an order of magnitude more accessible than those of gamma-II. The wide range of phosphorescence lifetimes and quenching constants allowed for discrimination of distinct contributions to the phosphorescence emission, and we suggest that room-temperature phosphorescence measurements will be an effective tool for studying conformational changes of lens crystallins.

Heat-induced changes in the conformation of alpha- and beta-crystallins: unique thermal stability of alpha-crystallin

Of the crystallin proteins of the lens, the principal subunit of the beta-crystallin, beta B2 (beta Bp), has been considered to be the only heat-stable protein because it does not precipitate upon heating. In our recent investigations, however, we have found that the alpha-crystallin from bovine lenses is not only heat stable but also does not denature at temperatures up to 100 degrees C. Using circular dichroism and fluorescence to monitor the conformational changes of alpha- and beta B2-crystallins upon heating, we found that alpha-crystallin maintains a high degree of structure, whereas the beta B2-crystallin shows a reversible sigmoidal order-disorder transition at about 58 degrees C.