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Query: UMLS:C0086543 (cataract)
 29,165 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Age-related nuclear (ARN) cataract is a major cause of world blindness. With the onset of ARN cataract, the normally transparent and colorless lens becomes opaque and can take on colors ranging from orange, brown, and even black. The molecular basis for this remarkable transformation is unknown. ARN cataract is also characterized by extensive oxidation, insolubilization, and cross-linking of polypeptides, particularly in the nucleus of the lens. It has been postulated that 3-hydroxykynurenine (3OHKyn) may be involved in these changes. This endogenous tryptophan metabolite is readily oxidized and is involved in the tanning of moth cocoons and the formation of pigments in the eyes of butterflies. 3OHKyn is a component of our primate-specific UV-filter pathway, and the brownish hue of ARN cataract lenses is also unique to humans. Because numerous colored compounds can be produced by autoxidation of 3OHKyn, this process could provide an explanation for the variety of lens colors and other changes seen in ARN cataract. For such a theory to be tenable, it needs to be demonstrated that 3OHKyn is bound to proteins in the human lens. Here, we show that all normal lenses older than 50 have 3OHKyn covalently attached to the nuclear proteins, most likely via cysteine residues. If indeed 3OHKyn is implicated in ARN cataract, a reduction in the levels that are bound in cataract, compared to normal lenses, would be expected. In agreement with this hypothesis, no bound 3OHKyn could be detected in proteins isolated from ARN cataract lenses.
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PMID:Identification of 3-hydroxykynurenine bound to proteins in the human lens. A possible role in age-related nuclear cataract. 1646 42

Aquaporin 0 (AQP0) is the major intrinsic protein of the lens and its water permeability can be modulated by changes in pH and Ca2+. The Cataract Fraser (Cat Fr) mouse accumulates an aberrant AQP0 (AQP0-LTR) in sub-cellular compartments resulting in a congenital cataract. We investigated the interference of AQP0-LTR with normal function of AQP0 in three systems. First, we created a transgenic mouse expressing AQP0 and AQP0-LTR in the lens. Expression of AQP0 did not prevent the congenital cataract but improved the size and transparency of the lens. Second, we measured water permeability of AQP0 co-expressed with AQP0-LTR in Xenopus oocytes. A low expression level of AQP0-LTR decreased the water permeability of AQP0, and a high expression level eliminated its calcium regulation. Third, we studied trafficking of AQP0 and AQP0-LTR in transfected lens epithelial cells. At low expression level, AQP0-LTR migrated with AQP0 toward the cell membrane, but at high expression level, it accumulated in sub-cellular compartments. The deleterious effect of AQP0-LTR on lens development may be explained by lowering water permeability and abolishing calcium regulation of AQP0. This study provides the first evidence that calcium regulation of AQP0 water permeability may be crucial for maintaining normal lens homeostasis and development.
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PMID:AQP0-LTR of the Cat Fr mouse alters water permeability and calcium regulation of wild type AQP0. 1651 71

We reported previously that chemical modification of human alphaA-crystallin by a metabolic dicarbonyl compound, methylglyoxal (MGO), enhances its chaperone-like function, a phenomenon which we attributed to formation of argpyrimidine at arginine residues (R) 21, 49, and 103. This structural change removes the positive charge on the arginine residues. To explore this mechanism further, we replaced these three R residues with a neutral alanine (A) residue one at a time or in combination and examined the impact on the structure and chaperone function. Measurement of intrinsic tryptophan fluorescence and near-UV CD spectra revealed alteration of the microenvironment of aromatic amino acid residues in mutant proteins. When compared to wild-type (wt) alphaA-crystallin, the chaperone function of R21A and R103A mutants increased 20% and 18% as measured by the insulin aggregation assay and increased it as much as 39% and 28% when measured by the citrate synthase (CS) aggregation assay. While the R49A mutant lost most of its chaperone function, R21A/R103A and R21A/R49A/R103A mutants had slightly better function (6-14% and 10-14%) than the wt protein in these assays. R21A and R103A mutants had higher surface hydrophobicity than wt alphaA-crystallin, but the R49A mutant had lower hydrophobicity. R21A and R103A mutants, but not the R49A mutant, were more efficient than wt protein in refolding guanidine hydrochloride-treated malate dehydrogenase to its native state. Our findings indicate that the positive charges on R21, R49, and R103 are important determinants of the chaperone function of alphaA-crystallin and suggest that chemical modification of arginine residues may play a role in protein aggregation during lens aging and cataract formation.
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PMID:Effect of site-directed mutagenesis of methylglyoxal-modifiable arginine residues on the structure and chaperone function of human alphaA-crystallin. 1658 92

L-Tryptophan (Trp) is an essential amino acid and its deficiency is involved in various pathologies. In this present investigation an attempt was made to study the role of tryptophan and its metabolites in cataract formation in wistar rats. Rats were divided and maintained in 3 groups, Group A--control; Group B--marginal-tryptophan and Group C--Tryptophan-deficient diet for 3 months. Slit lamp microscope observations indicated lenticular opacities in Group-C (tryptophan-deficient) rats. In the rats that were maintained on tryptophan deficient diet, a decrease in protein content, kynurenines, reduced glutathione (GSH), glutathione peroxidase (GPx), glutathione-s-tranferase (GSTs) and tryptophan-fluorescence intensities and an increase in lipid peroxidation indicative of oxidative stress have been observed. The above changes were normalized in the rats on supplementation of 0.05% tryptophan (Group-B) in their diets. These results suggest that tryptophan-deficiency in the diet leads to an overall significant decrease in kynurenines and levels of antioxidant enzymes (except SOD) in ocular tissue with a concomitant lenticular opacification. The results suggest that diet with adequate tryptophan has protective influence and is of immense benefit in mitigating the changes that may otherwise contribute to the lenticular opacities.
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PMID:Influence of kynurenines in pathogenesis of cataract formation in tryptophan-deficient regimen in Wistar rats. 1758 90

The urea-soluble proteins from the nucleus of two young, two aged, and two early-stage nuclear cataract lenses were subjected to tryptic digestion and analysis by 2D LC-MS/MS. Several novel post-translational modifications were identified. Deamidation was, by far, the most common modification. A number of differences were found in cataract compared to normal lenses, most notably an increase in the number of oxidized tryptophan residues. Semiquantitative analysis revealed that there appeared to be a trend toward increased levels of deamidation with age; however, there was no apparent increase upon the onset of nuclear cataract. This is in contrast to Trp oxidation, where an increase in the extent of modification was apparent in cataract lenses when compared to aged normal lenses. These findings suggest Trp oxidation may be involved in nuclear cataract development.
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PMID:Post-translational modifications in the nuclear region of young, aged, and cataract human lenses. 1782 32

The present study was performed in order to obtain structural and quantitative information regarding the modifications that take place in the human lens as a result of tryptophan oxidation. In particular, the early tryptophan oxidation product, oxindolealanine (OIA) has been detected in lyophilized and hydrolyzed cataractous lenses by mass spectrometry. OIA was confirmed in human cataract samples by observing its ion (m/z 221), fragmentation pattern and absorption spectrum. Quantitative results indicate that there are differences in the amounts of OIA in the nucleus versus the cortex in human cataractous lenses. Expressed as a ratio to the level of phenylalanine (Phe), the nucleus has more than one and a half times greater levels of OIA as compared to the cortex [nucleus=(3.7+/-0.7)x10(-2) versus cortex=(2.3+/-0.3)x10(-2)]. Furthermore, the average value for the OIA/Phe ratio in the calf lens (controls) was (0.8+/-0.2)x10(-2) as compared to (3.7+/-0.7)x10(-2) in human cataractous lens nucleus (p<0.05). The quantitative results correspond to a 4.6-fold increase of OIA in human cataractous lenses. In a separate series of experiments using HPLC with photodiode array (PDA) detection only, the differences in OIA levels in cataract nucleus versus cortex and cataracts versus controls closely matched the LC/MS data. The results suggest that OIA levels are elevated in human cataractous lenses thus providing further evidence to implicate tryptophan oxidation in this process.
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PMID:Oxindolealanine in age-related human cataracts. 1793 15

During aging, human lens proteins undergo several post-translational modifications, one of which is glycation. This process leads to the formation of advanced glycation end products (AGEs) which accumulate with time possibly leading to the formation of cataract. alphaB-Crystallin, a predominant protein in the lens, is a member of the small heat shock proteins (sHSPs) which are a ubiquitous class of molecular chaperones that interact with partially denatured proteins to prevent aggregation. This chaperone function is considered to be vital for the maintenance of lens transparency and in the prevention of cataract. In the present study, we introduced an analog of the advanced glycation end product, OP-lysine, at the 90th position of a mutated human alphaB-crystallin (K90C) by covalent modification of the cysteine residue with N-(2-bromoethyl)-3-oxidopyridinium hydrobromide. The AGE-modified K90C-alphaB-crystallin is termed as K90C-OP. We compared the structural and functional properties of K90C-OP with the original K90C mutant, with K90C chemically modified back to a lysine analog (K90C-AE), and with wild-type human alphaB-crystallin. Modified K90C-OP showed decreased intrinsic tryptophan fluorescence and bis-ANS binding without significant alterations in either the secondary, tertiary, or quaternary structure. K90C-OP, however, exhibited a reduced efficiency in the chaperoning ability with alcohol dehydrogenase, insulin, and citrate synthase as substrates compared to the other alpha-crystallin proteins. Therefore, introduction of a single AGE near the chaperone site of human alphaB-crystallin can alter the chaperoning ability of the protein with only minor changes in the local environment of the protein.
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PMID:Effect of a single AGE modification on the structure and chaperone activity of human alphaB-crystallin. 1802 13

The pathogenesis of cataract is associated with oxidative stress and with altered crystallin expression but it is still understood incompletely. In this study, the senescence-accelerated OXYS rats were used as a model. The first biomicroscopic signs of cataract in OXYS rats were registered at the age of 1.5 months; at 3 months morbidity reached 90%, and at 6 months it reached 100%. Cataract manifestation progresses: at 24 months mature cataract was detected in 90% of eyes of OXYS rats, whereas in 80% of Wistar rat eyes only initial signs of this disease were detected. Analysis of lens redox-parameters has shown that in OXYS rats the intensity of tryptophan fluorescence is higher, the GSH content being higher at 2 months but during formation of mature cataract at 13, 18, and 24 months being lower than in Wistar rats. Decrease in solubility of OXYS rat lens proteins was observed at the age of 13 months. At the age of 3 months gene expression of alphaA-crystallin and alphaB-crystallin was 3-fold and 25% lower, respectively, than in Wistar rats. At the age of 14 months there was a 27-fold decrease in expression of alphaB-crystallin in OXYS rats and it became 21-fold lower than in control. Proteins are synthesized in lens epithelial cells and dystrophic changes in senile cataract result in decrease in structural protein expression. The changes observed in OXYS rats are evidently associated with the dystrophic changes in lens epithelium, which we have described earlier, and are consistent with the model of senile cataract.
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PMID:Changes in physicochemical parameters and alpha-crystallin expression in the lens during cataract development in OXYS rats. 5. 1912 20

GammaS-crystallin, important in maintaining lens transparency, is a monomeric betagamma-crystallin comprising two paired homologous domains, each with two Greek key motifs. An autosomal dominant cortical progressive cataract has been associated with a G18V mutation in human gammaS-crystallin. To investigate the molecular mechanism of this cataract and confirm the causative nature of the G18V mutation, we examined resultant changes in conformation and stability. Human gammaS-crystallin cDNA was cloned into pET-20b(+), and the G18V mutant was generated by site-directed mutagenesis. Recombinant HgammaS-crystallins were expressed in Escherichia coli and purified by ion-exchange and size-exclusion chromatography. By analytical ultracentrifugation wild-type and mutant HgammaS-crystallins are monomers of about 21.95 +/- 0.21 and 20.89 +/- 0.18 kDa, respectively, and have similar secondary structures by far-UV CD. In increasing levels of guanidine hydrochloride (GuHCl), a sharp red shift in fluorescence lambda(max) and increase in emission correlating with exposure of tryptophans to the protein surface are detected earlier in the mutant protein. Under thermal stress, the G18V mutant begins to show changes in tryptophan fluorescence above 42 degrees C and shows a Tm of 65 degrees C as monitored by CD at 218 nm, while wild-type HgammaS-crystallin is very stable with Tm values of 75.5 and 75.0 degrees C as measured by fluorescence and CD, respectively. Equilibrium unfolding/refolding experiments as a function of GuHCl confirm the relative instability of the G18V mutant. Wild-type HgammaS-crystallin exhibits a two-state transition and reversible refolding above 1.0 M GuHCl, but the unfolding transition of mutant HgammaS-crystallin shows an intermediate state. The first transition (N --> I) shows a [GuHCl](1/2) of 0.5 M while the second transition (I --> U) has the same [GuHCl](1/2) as wild-type HgammaS-crystallin, about 2.0 M. Our present study confirms the high stability of wild-type HgammaS-crystallin and demonstrates that the G18V mutation destabilizes the protein toward heat and GuHCl-induced unfolding. These biophysical characteristics are consistent with the progressive cataract formation seen in the family members carrying this mutation.
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PMID:The G18V CRYGS mutation associated with human cataracts increases gammaS-crystallin sensitivity to thermal and chemical stress. 1955 89

Aromatic amino acids play an important role in ultraviolet (UV)-induced photochemical reactions in proteins. In this work, we aim at gaining insight into the photochemical reactions induced by near-UV light excitation of aromatic residues that lead to breakage of disulfide bridges in our model enzyme, Fusarium solani pisi cutinase, a lipolytic enzyme. With this purpose, we acquired transient absorption data of cutinase, with supplemental experimental data on tryptophan (Trp) and lysozyme as reference molecules. We here report formation kinetics and lifetimes of transient chemical species created upon UV excitation of aromatic residues in proteins. Two proteins, lysozyme and cutinase, as well as the free amino acid Trp, were studied under acidic, neutral, and alkaline conditions. The shortest-lived species is assigned to solvated electrons (lifetimes of a few microseconds to nanoseconds), whereas the longer-lived species are assigned to aromatic neutral and ionic radicals, Trp triplet states, and radical ionic disulphide bridges. The pH-dependent lifetimes of each species are reported. Solvated electrons ejected from the side chain of free Trp residues and aromatic residues in proteins were observed 12 ns after excitation, reaching a maximum yield after approximately 40 ns. It is interesting to note that the formation kinetics of solvated electrons is not pH-dependent and is similar in the different samples. On the other hand, a clear increase of the solvated electron lifetime is observed with increasing pH. This observation is correlated with H3O+ being an electron scavenger. Prolonged UV illumination of cutinase leads to a larger concentration of solvated electrons and to greater absorption at 410 nm (assigned to disulphide electron adduct RSSR *-), with concomitant faster decay kinetics and near disappearance of the Trp* radical peak at 330 nm, indicating possible additional formation of TyrO* formed upon reaction of Trp* with Tyr residues. Prolonged UV illumination of cutinase also leads to a larger concentration of free thiol groups, known to originate from the dissociation of RSSR *-. Additional mechanisms that may lead to the near disappearance of Trp(*) are discussed. Our study provides insight into one key UV-light-induced reaction in cutinase, i.e., light-induced disruption of disulphide bridges mediated by the excitation of aromatic residues. Knowledge about the nature of the formed species and their lifetimes is important for the understanding of UV-induced reactions in humans that lead to light-induced diseases, e.g., skin cancer and cataract formation.
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PMID:Flash photolysis of cutinase: identification and decay kinetics of transient intermediates formed upon UV excitation of aromatic residues. 1958 Jul 59


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