Age-dependent deamidation of lifelong proteins in the human lens.
OBJECTIVE
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.
Hains
Peter G
PG
Save Sight Institute, University of Sydney, Sydney, NSW, Australia.
Truscott
Roger J W
RJ
D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D001216:Asparagine / Q000378:metabolism; D002386:Cataract / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003459:Crystallins / Q000378:metabolism*; D003641:Deamination; D005865:Gestational Age; D005973:Glutamine / Q000378:metabolism; D006801:Humans; D007908:Lens, Crystalline / Q000196:embryology / Q000378:metabolism*; D008875:Middle Aged; D010446:Peptide Fragments; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
D003459:Crystallins; D010446:Peptide Fragments; D005973:Glutamine; D001216:Asparagine