Change in sulfhydryl group microenvironment of calf lens alpha-crystallin by 300 nm light

In vitro non-enzymatic glycation and formation of browning products in the bovine lens alpha-crystallin

Calf lens LMW alpha-crystallin was glycated by incubating with various sugars (glucose, glucose-6-P and ribose) for 21 days. All sugars induced disulfide formation, but ribose also produced higher molecular weight cross-linked species. The ribocated protein turned yellow in color and had a strong blue fluorescence (Ex/Em = 370/450 nm) typical for a browning product. The chromophore of the browning product showed a new circular dichroism (CD) band at 320-330 nm. Conformational study indicated that the browning reaction destablized protein and may play a significant role in protein aggregation and insolubilization.

Experimental parameters affecting the ultraviolet-induced tryptophan modification in the lens

Experimental parameters affecting photolysis of tryptophan in the lens were evaluated, including: (a) the use of previously-frozen vs. freshly excised lenses, (b) the use of lenses in aqueous buffered solutions vs. in air, (c) the species source of the lens, and (d) the effects of oxygen depletion. Lenses were subjected to monochromatic ultraviolet radiation followed by monitoring of the tryptophan fluorescence spectra while exciting at two or more wavelengths. Previously frozen lenses showed faster UV-induced degradation of tryptophan, particularly in aqueous media. A rapid leakage of fluorescent components (including amino acids and peptides) from intact lenses was observed. Evidence for the artifactual production of free radicals during photometric monitoring of samples in solution was obtained. Species dependent differences were also observed.

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

The effect of riboflavin-sensitized photooxidation on calf lens beta H-crystallin has been investigated by using fluorescence and circular dichroism techniques. beta H-Crystallin showed a pronounced change in its tertiary structure (conformation) as manifested in the near-u.v. circular dichroism spectra and fluorescence yield of tryptophan residues. The rate of tryptophan photolysis was significantly diminished under anaerobic conditions, but was not affected appreciably when D2O was used in the reaction mixture instead of H2O. Ferricyanide and ferricytochrome c added to the solution prior to irradiation inhibited the rate of photolysis of tryptophan, suggesting the involvement of O2- anion in the photoreactions. Quantitative assays of O2- and H2O2 in the irradiated protein solution strongly suggest that the Type I photosensitization pathway is involved in the RF-sensitized photooxidation of beta H-crystallin. The effect of photolysis on the cysteine residues of the protein was also studied. The sulfhydryl specific fluorophore N-iodoacetyl-N'-(5-sulfonaphthyl) ethylenediamine (1,5-IAEDANS) was used to study the change in the microenvironment of the cysteine (sulfhydryl) residues of the protein by photolysis. The results indicate that there is a quantitative loss of IAEDANS labeling sites due to photooxidation as well as structural changes of the protein. Fluorescence lifetime measurements indicate that the probe is bound in two environments--the major one (95%) is exposed and the minor one (5%) hydrophobic. A decrease in the lifetimes of the bound label occurs after photooxidation. However, the relative proportion of the hydrophobic IAEDANS-labeling sites increases in the photooxidized beta H-crystallin, probably due to the formation of supra-aggregated protein by photolysis.

Spectroscopic studies on the riboflavin-sensitized conformational changes of calf lens alpha-crystallin

The change in conformation of calf lens alpha-crystallin by oxidation in the presence of the photosensitizer riboflavin and light has been investigated. Near-UV circular dichroism (CD) spectrum, absorption spectrum, tryptophan fluorescence yield and fluorescence lifetime of the SH-specific fluorescent probe, N-iodoacetyl-N'-(5-sulfo-1-naphtyl) ethylenediamine (1,5-IAEDANS), were significantly altered by irradiation in the presence of RF. In the initial stages of photolysis (1-2 hr), a slight degradation of the protein to lower molecular weight peptides was observed. Upon increased photolysis, intersubunit cross-linking to dimers and other high molecular weight species was observed. To determine the effects of cross-linking on the accessibility of the cysteine residues of the protein, lifetime quenching studies on the IAEDANS-labeled alpha-crystallin were performed. A decrease in the quenching constant (kappa q) in the photolysed sample indicates that the labeled SH groups are less susceptible to collisional quenching, which requires contact between the quencher and the excited state of the fluorophore, due to steric inhibition in the cross-linked protein. Cross-linking and the rate of loss of tryptophan fluorescence of alpha-crystallin diminished under anaerobic conditions and increased when D2O was used in the medium for irradiation. Use of inhibitors and quenchers of active species of oxygen suggests that photo-oxidation probably occurs via the action of singlet oxygen as well as substrate-sensitizer complexation.

Lanthionine, a protein cross-link in cataractous human lenses

Oxidative processes are suggested to be a main cause of the covalent cross-linking of lenticular proteins. For a major part these cross-links are disulfides (cystine). With progression of nuclear cataract, cystine may be oxidized further to cysteic acid. Another oxidative degradation product of cystine is the symmetric thioether lanthionine. In this study, we clearly demonstrate that lanthionine is a protein cross-link of cataractous human lenses. Possible mechanisms of its formation are discussed.

Structure and stability of gamma-crystallins. II. Differences in microenvironments and spatial arrangements of cysteine residues

The gamma-crystallin fractions II, III and IV from calf eye lens were treated with the thiol-specific fluorescent probe 2-(4'-maleimidylanilino)naphthalene-6-sulfonate (MIANS), in order to determine the reactivity of the seven (gamma-II) or six (gamma-III, gamma-IV) cysteine residues. Two classes of reactive cysteines were distinguished by variations in fluorescence intensity with increasing molar excess of the probe, and approximately three cysteines were nonreactive in each gamma-crystallin. From the position of the emission maximum, it is apparent that MIANS-labeled cysteines of gamma-IV are in the least hydrophobic environment. Fluorescence energy transfer was observed from tryptophan to MIANS-labeled cysteines in both gamma-II and gamma-III crystallins, with efficiencies of 86% and 89%, respectively, but not in gamma-IV crystallin. We suggest that the spatial arrangements and microenvironments of cysteine residues of gamma-crystallins are sufficiently different from each other to account for the variations in fluorescence characteristics of the MIANS-labeled proteins and the lack of energy transfer in gamma-IV crystallins.

Fluorescence study of the effects of aging and diabetes mellitus on human lens alpha-crystallin

Human alpha-crystallins were separated from fetal, young, senile nondiabetic and diabetic lenses. The effects of aging and diabetes mellitus were studied by fluorescence measurements, including emission maximum, quantum yield and polarization, using both intrinsic probes (tryptophan and non-tryptophan) and extrinsic probes [4-(N-iodoacetoxy)N-methylamino-7-nitrobenz-2-oxa-1,3-diazole (IANBD) and 6-(p-toluidinyl)naphthalene-2-sulfonate (TNS)]. Results indicate that diabetic effects (glycation and aggregation) give fluorescence change to a far greater extent than that of aging. This was demonstrated by a large decrease in tryptophan quantum yield and an increase in non-tryptophan quantum yield, and also by a decrease in polarization of non-tryptophan. The sulfhydryl (SH)-specific probe IANBD shows a blue-shift in emission maximum, a decrease in intensity and an increase in polarization. The hydrophobic probe TNS shows a decrease in both intensity and polarization. These results suggest that tryptophan oxidation, mixed disulfide formation and glycation, as well as other undetected post-translational modifications have partially unfolded the proteins, making the protein structure less rigid. The possible effect of this unfolding process is that proteins become more susceptible to aggregation.