Racemization of Two Proteins over Our Lifespan: Deamidation of Asparagine 76 in γS Crystallin Is Greater in Cataract than in Normal Lenses across the Age Range

A Rapid, Comprehensive Liquid Chromatography-Mass Spectrometry (LC-MS)-based Survey of the Asp Isomers in Crystallins from Human Cataract Lenses

“…The Asp-58 and Asp-151 of ␣A-crystallin and Asp-62 of ␣B-crystallin are highly inverted to the isomers in the WS fraction, which is entirely consistent with our previous results (4,5), and their isomeric ratios increased dramatically in the WI fraction. In recent work, Hooi et al (42) reported that Asp-151 of ␣A-crystallin converted to D␤, L␤, and D␣ in a ratio of 3:1:0.5 until age 15 and that the percentage of D␤-Asp-151 is 40% by age 20 with a fixed ratio to age 80. These results are consistent with the present study.…”

Biophysical chemistry of the ageing eye lens

“…This ring opening, already capable of forming structural isomer isaoaspartate, can cause the amino acid to form a D-enantiomer, with one possible product of this interconversion being a D-isoaspartate residue, equivalent to a Dβ-amino acid (Kaji et al 2007). Both αA-crystallin Asp151 and γS-crystallin Asn76 are reported to undergo gradual racemisation over their lifetime with γS-crystallin Asn76 showing particularly high modification in cataractous lenses (Hooi et al 2012). Of the 13 aspariginyl residues of αB-crystallin, Asp36 and Asp62 show extensive racemisation in aged lenses compared to young ones, a pattern repeated with βB2-crystallin Asp4 residue (Fujii et al 1994).…”

“…There are many specific modifications in cataractous lenses that are not present in normal healthy aged lenses (Hooi et al 2012). It is these modifications in particular that are of interest in order to identify specific causes for cataract formation and therefore identify potential targets for treatment or prevention.…”

“…Deamidation is one of the most common post-translational modification in the eye lens, occurring early in life and building up with age (Hains and Truscott 2010;Lampi et al 2014;Takemoto et al 2001). There are some sites of deamidation that occur more frequently in cataract-affected lenses compared to healthy lenses of the same age (Hooi et al 2012). Deamidation modifies asparagine and glutamine residues by hydrolysing the amide side chain, producing the corresponding carboxylic acid residue (Clarke 1987).…”

“…(Lyons et al 2011) Deamidation affects all of the structural proteins in aged eye lenses, but each is modified to a different extent. For example, βB2-crystallin shows deamidation at only one site and only at 3 % abundance even after 60 years of age, whereas γS-crystallin has up to 20 % deamidation at some residues, such as Asn76, by as early as 15 years of age, and up to 60 % deamidation in cataractous lenses (Hooi et al 2012).…”