Of all aspartyl residues in bovine αA-crystallin, only Asp-151 exhibits pronounced racemization. Asp-151 is also one of the sites where peptide bond cleavage occurs in in vivo aging αA-crystallin. This aspartyl residue is followed by an alanyl residue and resides in a flexible carboxyl terminal extension of α-crystallin. Both in vivo and in vitro racemization studies indicate that the pronounced and site-specific racemization of Asp-151 proceeds via formation of a succinimide intermediate. The in vivo racemization of aspartyl residues in αA-crystallin is discussed with regard to the proposed tertiary structure of α-crystallin.
