Deamidation of Specific Glutamine Residues from Alpha-A Crystallin during Aging of the Human Lens

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Abstract
Although it has been hypothesized that age-dependent deamidation of glutamine and/or asparagine residues may play an important role in the turnover of proteins in vivo, surprisingly little is known concerning the extents of deamidation of biologically important proteins with very long half-lives. Alpha-A crystallin is the most abundant protein of the adult human lens, which contains long-lived proteins in the central fetal-embryonic region that were synthesized before birth of the individual. Peptides, corresponding to tryptic fragments of alpha-A crystallin, were synthesized with either the expected glutamine-6, glutamine-50, and glutamine-147 residues, or deamidated glutamic acid residues at the same positions. These synthetic peptides were used to identify and quantitate the amidated versus deamidated forms of each tryptic fragment of alpha-A crystallin from the fetal-embryonic region of lenses from donors of increasing age up to 64 years old. The results demonstrate that all three glutamine residues are very stable, with glutamine-50 undergoing a maximum of approximately 30% deamidation after 64 years postsynthesis, while glutamine-6 and glutamine-147 undergo no detectable deamidation during the same period of time. Together, these results are consistent with the hypothesis that resistance to age-dependent, nonenzymatic deamidation may be an important prerequisite for the stability of proteins in vivo.

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