Age-dependent deamidation of lifelong proteins in the human lens.

Purpose: Deamidation is a common posttranslational modification in human lens crystallins and may be a key factor in the age-related denaturation of such lifelong proteins. The aim of this study was to identify the sites of deamidation in older lenses.

Methods: High-performance liquid chromatography/mass spectrometry of tryptic digests was used to identify sites of deamidation in the major human lens crystallins. Older normal and age-matched cataractous lenses were compared with fetal lenses.

Results: Approximately equal numbers of glutamine and asparagine residues were deamidated in older lenses; however, the extent of deamidation of Asn was three times greater than that of Gln (Asn, 22.6% +/- 3.6%; Gln, 6.6% +/- 1.3%). Individual crystallins differed markedly in their extent of deamidation, and deamidated residues were typically localized within discrete regions of the polypeptides. A large percentage (42%) of the sites of deamidation were characterized by the presence of a basic amino acid one residue removed from the original Gln or Asn. At nine such sites, the extent of Asn deamidation averaged 50% in aged lenses. There were few differences in deamidation between crystallins of aged normal and nuclear cataractous lenses.

Conclusions: Equal numbers of Asn and Gln residues are deamidated in crystallins from aged normal and cataractous lenses. Deamidation of Asn/Gln in lifelong proteins, such as those in the lens, may be governed to a significant degree by base-catalyzed processes.