Kinetics of Isomerization and Inversion of Aspartate 58 of αA-Crystallin Peptide Mimics under Physiological Conditions

Although proteins consist exclusively of L-amino acids, we have reported that aspartyl (Asp) 58 and Asp 151 residues of alpha A-crystallin of eye lenses from elderly cataract donors are highly inverted and isomerized to D-beta, D-alpha and L-beta-Asp residues through succinimide intermediates. Of these Asp isomers, large amounts of D-beta- and L-beta-isomers are present but the amount of D-alpha-isomer is not significant. The difference in abundance of the Asp isomers in the protein may be due to the rate constants for the formation of the isomers. However, the kinetics have not been well defined. Therefore, in this study, we synthesized a peptide corresponding to human alpha A-crystallin residues 55 to 65 (T(55)VLD(58)SGISEVR(65)) and its isomers in which L-alpha-Asp at position 58 was replaced with L-beta-, D-beta- and D-alpha-Asp and determined the rate of isomerization and inversion of Asp residues under physiological conditions (37 degrees C, pH7.4). The rate constant for dehydration from L-alpha-Asp peptide to L-succinimidyl peptide was 3 times higher than the rate constant for dehydration from L-beta-Asp peptide to L-succinimidyl peptide. The rate constant for hydrolysis from L-succinimidyl peptide to L-beta-Asp peptide was about 5 times higher than the rate constant for hydrolysis from L-succinimidyl peptide to L-alpha-Asp peptide. The rate constant for dehydration from L-alpha-Asp peptide to L-succinimidyl peptide was 2 times higher than the rate constant for dehydration from D-alpha-Asp peptide to D-succinimidyl peptide. The rate constants for hydrolysis from L-succinimidyl peptide to L-beta-Asp peptide and for hydrolysis from D-succinimidyl peptide to D-beta-Asp peptide were almost equal. Using these rate constants, we calculated the change in the abundance ratios of the 4 Asp isomers during a human lifespan. This result is consistent with the fact that isomerized Asp residues accumulate in proteins during the ageing process.