Kinetic study of racemization of aspartyl residues in model peptides of αA‐crystallin

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Abstract
We have reported that two aspartyl (Asp‐151 and Asp‐58) residues in αA‐crystallin in human eye lens were inverted to the D‐isomer and isomerized to β‐aspartyl residues with age. We report here the kinetics of the Asp racemization of three model peptides corresponding to fragments of αA‐crystallin: IQTGLD151ATHAER (T18 peptide). TVLD58SGISEVR (T6 peptide) and HFSPED84LTVK (T10 peptide, as a control). The rate constants of the racemization of Asp residues in these peptides were measured at pH 7.0. at five temperatures: 50, 60, 70, 80 and 90 °C. From the Arrhenius equation, we estimated the activation energy (E) of racemization and the time required for the Asp D/L ratio to approximate to 1.0 (D/L ratio of Asp = 0.99) at body temperature. For the peptide T18, E=21.4 kcal/mol and t=13.5 yr. For the peptide T6, E= 26.8 kcal/mol and t= 49.5 yr. For the control peptide T10, E=28.3 kcal/mol and t= 78.1 yr. The racemization rate of Asp in these three peptides is parallel to that of Asp residues in αA‐crystallin. The racemization rate of Asp in the T18 peptide was very rapid compared to that in the other peptides. This result also reflects the racemization rate in native αA‐crystallin

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