Effect of Asp 96 isomerization on the properties of a lens αB-crystallin-derived short peptide

One of the major reasons for age-related cataract formation is an accumulation of insoluble lens proteins. In particular, higher-order alpha-crystallin aggregates, comprising alpha A and alpha beta subunits, are insolubilized by the build up of various post-translational modifications over time. Although we previously found an exceptional amount of Asp96 isomerization in alpha B-crystallin from aged human lens, the biological effect remains unknown. To approximate the effect of Asp 96 isomerization in alpha beta-crystallin, here residues 93-103 of alpha B-crystallin were chemically synthesized as peptides in which L-alpha-Asp was replaced with L-beta-Asp, D-alpha-Asp, or D-beta-Asp. The resulting peptides were then compared in a biological assay. The results showed that isomerization of Asp 96 altered both the local structure of peptide and its stability against enzymatic digestion. In addition, the synthesized peptides decreased the insoluble fraction of heated alpha-crystallin. The D-beta-Asp-containing peptide further decreased heat-induced precipitation of alpha-crystallin, and a chaperone assay based on heated alcohol dehydrogenase implied differential interaction of the peptides with substrate depending on the Asp isomer present in each. Our results suggest that the formation of Asp isomers is likely to affect the higher-order oligomer structure of alpha-crystallin and thereby its chaperone functions in aged lens. (C) 2015 Elsevier B.V. All rights reserved.