Isomerization of aspartyl residues in crystallins and its influence upon cataract.
BACKGROUND
Age-related cataracts, which probably form due to insolubilization of lens proteins, can lead to loss of vision. Although the exact reason is unknown, lens protein aggregation may be triggered by increases in PTMs such as D-β-, L-β- and D-α-Asp isomers. These isomers have been observed in aged lens; however, there have been few quantitative and site-specific studies owing to the lack of a quick and precise method for distinguishing between D- and L-Asp in a peptide or protein.


METHODS
We describe a new method for detecting peptides containing Asp isomers at individual sites in any protein by using an LC-MS/MS system combined with commercial enzymes that specifically react with different isomers. We also summarize current data on the effect of Asp isomerization on lens crystallins.


CONCLUSIONS
The new technique enabled the analysis of isomers of Asp residues in lens proteins precisely and quickly. An extensive proportion of Asp isomerization was observed at all Asp sites of crystallins in the insoluble fraction of aged lens. In addition, d-amino acid substitutions in crystallin-mimic peptides showed altered structural formation and function. These results indicate that isomerization of Asp residues affects the stability, structure and inter-subunit interaction of lens crystallins, which will induce crystallin aggregation and insolubilization, disrupt the associated functions, and ultimately contribute to the onset of senile cataract formation.


CONCLUSIONS
The mechanism underlying the onset of age-related diseases may involve isomerization, whereby D-amino acids are incorporated in the L-amino acid world of life. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan. Electronic address: nfujii@rri.kyoto-u.ac.jp.
Noriko
N
Fujii
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan.
Takumi
T
Takata
Radioisotope Research Center, Teikyo University, Kaga Itabashi-ku, Tokyo 173-8605, Japan.
Norihiko
N
Fujii
Faculty of Pharmaceutical Sciences, Himeji Dokkyo University, Kamiohno, Himeji 670-8524, Japan.
Kenzo
K
Aki
D000375:Aging / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001224:Aspartic Acid / Q000737:chemistry* / Q000378:metabolism; D002386:Cataract / Q000378:metabolism*; D003459:Crystallins / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D007536:Isomerism; D007908:Lens, Crystalline / Q000737:chemistry* / Q000378:metabolism*; D008969:Molecular Sequence Data; D013329:Structure-Activity Relationship
D003459:Crystallins; D001224:Aspartic Acid
Aging; Cataract; Isoaspartate (β-aspartate); LC–MS/MS; Protein; d-Amino acid