The glycation of bovine lens betaL-, betaS- and gamma-crystallins demonstrated by isoelectric focusing and lectin staining

Prediction of possible sites for posttranslational modifications in human gamma crystallins: Effect of glycation on the structure of human gamma-b-crystallin as analyzed by molecular modeling

Crystallins are recognized as one of the long-lived proteins of lens tissue that might serve as the target for several posttranslational modifications leading to cataract development. We have studied several such sites present in the human gamma-crystallins based either on PROSITE pattern search results or earlier experimental evidences. Their probabilities were examined on the basis of the database analysis of the gamma-crystallin sequences and on their specific locations in the constructed homology models. An N-glycosylation site in human gammaD-crystallin and several phosphorylation sites in all four human gamma-crystallins were predicted by the PROSITE search. Some of these sites were found to be strongly conserved in the gamma-crystallin sequences from different sources. An extensive analysis of these sites was performed to predict their probabilities as potential sites for protein modifications. Glycation studies were performed separately by attaching sugars to the human gammaB-crystallin model, and the effect of binding was analyzed. The studies showed that the major effect of alphaD-glucose (alphaD-G) and alphaD-glucose-6-phosphate (alphaD-G6P) binding was the disruption of charges not only at the surface but also within the molecule. Only a minor alteration in the distances of sulfhydryl groups of cysteines and on their positions in the three-dimensional models were observed, leading us to assume that glycation alone is not responsible for intra- and intermolecular disulfide bond formation.

Inhibition of ascorbic acid-induced modifications in lens proteins by peptides

The effects of three dipeptides L-phenylalanyl-glybine, glycyl-L-phenylalanine,and aspartame (L-aspartyl-L-phenylalanine, methyl ester) as inhibitors of the ascorbic acid-induced modifications in lens proteins were studied. Their efficiency was compared to that of two known inhibitors--aminoguanidine and carnosine. The tested dipeptides diminished protein carbonyl content by 32-58% and most moderated the formation of chromophores, as measured by the absorbency at 325 nm of the glycated proteins. The appearance of non-tryptophan fluorescence (excitation 340 nm/emission 410 nm) was observed for proteins glycated with ascorbic acid. All of the dipeptides examined, as well as aminoguanidine, decreased this glycation-related fluorescence. The potential inhibitors prevented the intensive formation of very high molecular weight aggregates. A competitive mechanism of their inhibitory effect was proposed, based on the reactivity of individual substances toward ascorbic acid. These findings indicate that they have a potential for use as alternatives for aminoguanidine as an anti-glycation agent.

Post-Translational Modification of βH-Crystallin of Bovine Lens with Aging

This study investigates post-translational modification of proteins of bovine lens with aging (3 year old vs. 6 month old cows). After water-soluble proteins were submitted to gel and ion exchange chromatography, betaH-crystallin, a subunit of beta-crystallin, and modified materials were isolated. These materials were then submitted to two dimensional polyacrylamide gel electrophoresis (2D-SDS PAGE) to detect and isolate the new spots. Results for lens proteins from 3 year old animals were compared to those from 6 month old animals. All spots were digested in gel with trypsin and the molecular masses of tryptic digests were measured by matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOFMS). Peptides peaks obtained from mass mapping were identified using the protein database of the MS-Fit program in the Protein prospector program of the University of California, San Francisco. We found that two post translational modifications of betaH-crystallin, acetylation and phosphorylation occurred with aging.

Comparison between modifications of lens proteins resulted from glycation with methylglyoxal, glyoxal, ascorbic acid, and fructose

Cataract is generally associated with the breakdown of the lens microarchitecture. Age-dependent chemical modifications and cross-linking of proteins are the major pathways for development of lens opacity. The specific alterations in lens proteins caused by glycation with four carbonyl metabolites, fructose, methylglyoxal, glyoxal, and ascorbic acid, were investigated. Decrease in intensity of tryptophan related fluorescence and level of reduced protein sulfhydryl groups, parameters that are indicative for changes in protein conformation, were observed after reaction with all studied carbonyl compounds. Protein carbonyl content, an index for oxidative damage to proteins, was strongly enhanced in methylglyoxal-treated proteins. Cross-linking of glycated proteins was confirmed by polyacrylamide electrophoresis. alpha-Oxoaldehydes were the most reactive in protein aggregation. They also formed specific chromophores absorbing UV light above 300 nm. Significant loss in lactate dehydrogenase activity resulted from incubation with methylglyoxal, followed by glyoxal and ascorbic acid. The results obtained showed that alterations in lens proteins do not follow the specific reactivity of studied carbonyl compounds. Despite the similarity in chemical structures of alpha-oxoaldehydes and ascorbic acid degradation products, they cause specific alterations in lens protein structure with different biological consequences.