Conformational changes of beta H-crystallin in riboflavin-sensitized photooxidation

Structural and functional alteration of human αA-crystallin after exposure to full spectrum solar radiation and preventive role of lens antioxidants

The chronically exposure of eye lenses to ultra violet and visible light of the solar radiation is an important risk factor for development of the senile cataract diseases. Various photosensitizer molecules including riboflavin (RF) play a significant role in photo-oxidative damages of lens proteins underlying development of opacity in the lenticular tissues. In the current study, RF-mediated photo-oxidation of human αA-crystallin (αA-Cry) was assessed using SDS-PAGE analysis, dynamic light scattering and other spectroscopic assessments. The RF-photosensitized reactions led to non-disulfide covalent cross-linking, oligomerization and significant structural changes in αA-Cry. The photo-damaging of αA-Cry under solar radiation was also accompanied by the reduction in both Trp and Tyr fluorescence intensities which followed by the formation of new photosensitizer chromophores. The solvent exposed hydrophobic patches, secondary structures and chaperone-like activity of αA-Cry were significantly altered after exposure to the solar radiation in the presence of RF. Although glutathione and ascorbate were capable to partially protect the photo-induced structural damages of human αA-Cry, they also disrupted its chaperone function when co-exposed with this protein to the solar radiation. Also, the most promising data were obtained with cysteine which its availability in the lenticular tissues is a rate limiting factor for the biosynthesis of glutathione. Overall our results suggest that glutathione and ascorbate, as the major anti-oxidant compounds within lenticular tissues, demonstrate controversial effect on structure and chaperone-like activity of human αA-Cry. Elucidation of this effect may demand further experiments.

Gel electrophoretic analysis of corneal collagen after photodynamic cross-linking treatment

PURPOSE

Photodynamic collagen cross-linking by using ultraviolet A (UVA) irradiation and the photosensitizer riboflavin has been recently introduced as a new possible treatment of progressive keratoconus. This is the first study, to our knowledge, investigating biochemical aspects of the new procedure. Its aim was to analyze the possible changes in the electrophoretic pattern of corneal collagen type I after collagen cross-linking treatment.

METHODS

Twenty fresh postmortem porcine corneas were cross-linked; another 20 porcine corneas treated with physiologic saline were used as controls. After removal of the central 10 mm of the epithelium, the corneas were treated with the photosensitizer riboflavin and UVA irradiation for 30 minutes by using a double UVA diode (370 nm, 3 mW/cm). For biochemical analysis, the central 10-mm corneal buttons were trephined, tissue was homogenized, and collagen type I was extracted. Subsequently, the collagen extracts were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

RESULTS

In the controls, the typical collagen pattern of normal cornea was found with 1 gamma trimer band, 2 beta dimer bands, and 2 alpha monomer bands. In the cross-linked samples, there was an additional intense polymer band in the stacking gel that was resistant to mercaptoethanol, heat, and pepsin treatment. Its molecular size was estimated to be at least 1000 kDa.

CONCLUSIONS

In the cross-linked corneas, a strong band of high-molecular-weight collagen polymers was shown as the biochemical correlate of the cross-linking effect, showing the efficiency of the new cross-linking procedure. This polymer band complies well with the morphologic correlate of an increased fiber diameter after cross-linking treatment. Its chemical stability supports hopes of a long-term effect of the new treatment.

Analysis of protein-based media commonly found in paintings using synchronous fluorescence spectroscopy combined with multivariate statistical analysis

The spectrofluorimetric analysis of protein-based binding media, which are commonly found as painting materials, is based on the detection of emissions from amino acids, as well as fluorescent degradation products that develop with aging. Laser-induced fluorescence spectroscopy, fluorescence excitation emission spectroscopy, and time-resolved fluorescence spectroscopy have all been employed in efforts to discriminate between commonly found proteinaceous binding media, including egg white, egg yolk, milk, and casein, as well as collagen-based glues from rabbit skin, ox bone, parchment, and fish. However, synchronous fluorescence spectroscopy (SFS), a rapid means of recording fluorescence properties of samples, has not been reported for the differentiation between binding media. This work focuses on the analysis of a large set of naturally aged films of different protein-based binding media using SFS with a range of different offsets between excitation and emission monochromators between 30-60 nm. An interpretation of synchronous fluorescence spectra of binding media is presented and is followed by an assessment and classification of a database of recorded spectra using multivariate analysis. Importantly, following SFS analysis of films of binding media, principal component analysis is used to differentiate among all the proteinaceous media considered on the basis of clustering of data. This application is thus a novel and nondestructive means for differentiation between protein-based binding media.

Hemolysis of human red blood cells by riboflavin-Cu(II) system: enhancement by azide

Photoactivated riboflavin in the presence of Cu(II) generates reactive oxygen species (ROS) which can hemolyze human red blood cells (RBC). In the present work we examined the effect of sodium azide (NaN3) on RBC in the presence of riboflavin and Cu(II). The addition of NaN3 to the riboflavin-Cu(II) system enhanced K+ loss and hemolysis. The extent of K+ loss and hemolysis were time and concentration dependent. Bathocuproine, a Cu(I)-sequestering agent, inhibited the hemolysis completely. Among various free radical scavengers used to identify the major ROS involved in the reaction, thiourea was found to be the most effective scavenger. Thiourea caused almost 85% inhibition of hemolysis suggesting that *OH is the major ROS involved in the reaction. Using spectral studies and other observations, we propose that when NaN3 is added to the riboflavin-Cu(II) system, it inhibits the photodegradation of riboflavin resulting in increased *OH generation. Also, the possibility of azide radical formation and its involvement in the reaction could not be ruled out.

Collagen Fiber Diameter in the Rabbit Cornea After Collagen Crosslinking by Riboflavin/UVA

OBJECTIVE

Collagen crosslinking of the cornea has been developed recently as a quasiconservative treatment of keratoconus. Biomechanical in vitro measurements have demonstrated a significant increase in biomechanical stiffness of the crosslinked cornea. The aim of the present study was to evaluate the effect of this new procedure on the collagen fiber diameter of the rabbit cornea.

METHODS

The corneas of the right eyes of 10 New Zealand White albino rabbits were crosslinked by application of the photosensitizer riboflavin and exposure to UVA light (370 nm, 3 mW/cm2) for 30 minutes. The left fellow control eyes were either left untreated (rabbits 1-4), deepithelialized (rabbits 5-7), or deepithelialized and treated with riboflavin/dextran solution (rabbits 8-10) to exclude an influence of epithelial debridement or hydration changes on the fiber diameter. On ultrathin sections of samples from the anterior and posterior cornea, the collagen fiber diameter was measured semiautomatically with the help of morphometric computer software.

RESULTS

In the anterior stroma, the collagen fiber diameter in the treated corneas was significantly increased by 12.2% (3.96 nm), and in the posterior stroma by 4.6% (1.63 nm), compared with the control fellow eyes. In the crosslinked eyes, the collagen fiber diameter was also significantly increased by, on average, 9.3% (3.1 nm) in the anterior compared with the posterior stroma within the same eye.

CONCLUSIONS

Collagen crosslinking using riboflavin and UVA leads to a significant increase in corneal collagen diameter. This alteration is the morphologic correlate of the crosslinking process leading to an increase in biomechanical stability. The crosslinking effect is strongest in the anterior half of the stroma because of the rapid decrease in UVA irradiance across the corneal stroma as a result of riboflavin-enhanced UVA absorption.

Hemolysis of human red blood cells by riboflavin-Cu(II) system

The photodynamic action of riboflavin is generally considered to involve the generation of reactive oxygen species, whose production is enhanced when Cu(II) is present in the reaction. In the present study we report that photoactivated riboflavin causes K(+) loss from fresh human red blood cells (RBC) in a time dependent manner. Addition of Cu(II) further enhances the K(+) loss and also leads to significant hemolysis. Riboflavin in a 2:1 stoichiometry with Cu(II) leads to maximum K(+) loss and up to 45% hemolysis. Bathocuproine, a specific Cu(I)-sequestering agent, when present in the reaction, inhibits the hemolysis completely. Free radical scavengers like superoxide dismutase, potassium iodide and mannitol inhibited the hemolysis up to 55% or more. However, thiourea was the most effective scavenger showing 90% inhibition. These results suggest that K(+) leakage and hemolysis of human RBC are basically free radical mediated reactions.

Degradation of hyaluronic acid by photosensitized riboflavin in vitro. Modulation of the effect by transition metals, radical quenchers, and metal chelators

The effect of photoexcited riboflavin (RF) on the viscosity of hyaluronic acid (HA) solutions has been investigated. UV irradiation of RF causes under aerobic conditions fragmentation of HA and a decrease in the viscosity of its solutions. A decrease of HA viscosity occurs in PO(4)-buffered solutions and is accelerated by high pH, Fe2+ (but much less so by Fe3+), certain metal chelators, and horseradish peroxidase (HRP); it is partially inhibited by catalase and less so by superoxide dismutase (SOD). The reactivity of the system was completely blocked by Tris, ethanol, aspirin, d-manitol, dimethylthiourea (DMTU), dimethylsulfoxide (DMSO), and sodium azide. These results indicate that the most likely chemical species involved in the reaction is the hydroxyl radical. Singlet oxygen ((1)O(2)) generation is suggested by the ability of NaN3 and DMSO to completely inhibit the reactivity of the system. These two agents, however, may also interact with OH. radical, as well and suppress the reactivity of the system. H(2)O(2) and O(2).- seem also to be produced in significant amounts, because catalase and SOD partially block the reactivity of the system. The effect of HRP may be due to hydrogen subtraction from HA and H(2)O(2) reduction to water. Photoexcitation of RF may potentially occur in vitro and in vivo in the organs and tissues that are permeable to light, such as the eye or skin, and damage HA and other cell-matrix components causing inflammation and accelerating aging.

Exposure of beta H-crystallin to hydroxyl radicals enhances the transglutaminase-susceptibility of its existing amine-donor and amine-acceptor sites

beta H-crystallin was exposed to radiolytically generated hydroxyl radicals at defined radical concentrations, and its capacity to act as an amine-acceptor substrate and as an amine-donor substrate for transglutaminase were investigated. [14C]Methylamine was used as a probe for labelling amine-acceptor sites; a novel biotinylated hexapeptide was used to label amine-donor sites. The results demonstrate that both primary amine incorporation and hexapeptide incorporation by transglutaminase are considerably increased after oxidative attack on the crystallin. The identity of the labelled subunits was established, and it is shown that, in both cases, this increased incorporation is not due to the production of new substrates, but that the existing incorporation sites become more susceptible. Moreover, using the newly developed probe, we could identify, for the first time, the major crystallin subunits active as amine-donor substrates (both before and after treatment) to be beta B1-, beta A3- and beta A4-crystallin. These data support the proposal that oxidative stress and transglutaminase activity may be jointly involved in the changes found in lens crystallins with age and in the development of cataract.

Alteration of dynamic quaternary structure and calcium-binding ability of beta-crystallin by light

beta-crystallin, one of the three main constituent proteins of the eye lens, exists as an equilibrium population of oligomeric (beta H), trimeric (beta L1) and dimeric (beta L2) species. This equilibrium is dependent on various factors such as the protein concentration, ionic strength and pH of the medium. We have studied the effect of ultraviolet B radiation on the aggregational patterns of beta-crystallin, using size-exclusion chromatography. Irradiation of a solution of beta H-crystallin at 295 nm for about 30 min causes the deaggregation of the hexameric population into dimers. Irradiation for a longer time, however, produces cross-linked high molecular weight products. Irradiation of a beta L2 solution for 30 min does not perturb the elution profile, while irradiation for a longer time increases the content of beta L1 (trimeric) crystallin. Irradiation also causes a decrease in the calcium-binding affinity of the beta-crystallins.

The prevention of cataract caused by oxidative stress in cultured rat lenses. I. H2O2 and photochemically induced cataract

H2O2 stress is shown to produce cataract in cultured rat lenses. The loss of transparency begins in the equatorial region within 24 hours and the entire superficial cortex is opaque by 96 hours. No involvement of the nuclear region is observed. However after an additional 48 hours, the nuclear region becomes opaque. The loss of transparency is accompanied by a large uptake of H2O which occurs gradually over the 96 hour period, complete loss of glyceraldehyde phosphate dehydrogenase (GPD) activity, almost complete loss of non-protein thiol and a slight decrease in protein thiol. Control lenses show no change other than the establishment of a new non-protein thiol base line approximately 60% lower than 0 time levels. The Alcon glutathione peroxidase type mimic, AL-3823A, completely eliminates almost all of the H2O2 induced effects and the lens remains transparent. Utilizing a more severe photochemical model than may be anticipated physiologically with 10 microM riboflavin and exposure to daylight fluorescent lamps, significant concentrations of superoxide and low levels of OH. are produced as well as extraordinarily high concentrations of H2O2 ranging from about 400 to 1000 microM. As with the H2O2 model, opacification begins at the equator but the cataract develops more rapidly, the lens being completely opaque by 68 hours. Hydration, GPD activity, non-protein and protein thiol all decrease more rapidly than in the H2O2 model. AL-3823A prevents loss of transparency until approximately 92 hours and markedly decreases changes in other parameters. At 92 hours, slight loss of transparency is observed. Catalase is somewhat less effective. AL-3823A is shown to also significantly decrease superoxide levels. The marked delay in the onset of changes in lens biochemistry and physiology in the severe photochemical stress model and the maintenance of normal parameters in the H2O2 model in the presence of AL-3823A suggests that such compounds may prevent cataract caused by oxidative stress under physiological conditions.

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.

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 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.