Spontaneous peptide bond cleavage in aging alpha-crystallin through a succinimide intermediate

Cleavage of specific peptide bonds occurs with aging in the alpha A subunit of bovine alpha-crystallin. One of the breaks occurs at residue Asn-101. This same residue undergoes in vivo deamidation, isomerization, and racemization. Deamidation and isomerization are known to occur via succinimide ring formation of labile asparagine residues. Model studies on peptides have shown that imide formation can also lead to peptide bond cleavage (Geiger, T., and Clarke, S. (1987) J. Biol. Chem. 262, 785-794). In that case, both asparagine and aspartic acid amide would be expected as C termini of the truncated polypeptide, and this is indeed the case in the alpha A-(1-101)-chain. This thus represents a first example of nonenzymatic in vivo peptide bond cleavage in an aging protein through the formation of a succinimide intermediate. In addition, we found that in bovine lens no detectable conversion (through the action of protein-carboxyl methyltransferase) of isoaspartyl to normal aspartyl residues occurs in vivo after deamidation of Asn-101.