Biol. Pharm. Bull. 28(9) 1590—1596 (2005)

Proteins and peptides are susceptible to a variety of chemical modifications that can affect their structure and biological functions. Among these modifications, the isomerization of aspartic acid and deamidation of asparagine occur in proteins and peptides during aging. The reaction proceeds spontaneously under physiological conditions through a five-membered succinimide ring intermediate formed by the nucleophilic attack of the peptide bond nitrogen atom of the following residue on the side chain carbonyl group, resulting in dehydration of aspartic acid or deamidation of asparagine (Fig. 1). The L-succinimidyl intermediate then undergoes a relatively rapid hydrolysis at either the aor b-carbonyl group to generate L-isoaspartate (L-IsoAsp) and normal L-aspartate (L-Asp) in a ratio of approximately 3 : 1 in a variety of substrates. The formation of L-succinimide can also be accompanied by enhanced racemization at the a-carbon to generate a mixture of D-succinimidyl, D-aspartyl, and D-isoaspartyl forms (Fig. 1). In the non-enzymatic reaction, the LIsoAsp form is typically the predominant product. IsoAsp forms most easily at sequences in which the side chain of the C-flanking amino acid is relatively small and hydrophilic, and is less likely to be formed where bulky or hydrophobic residues are in this position. The most favorable C-flanking amino acids are Gly, Ser, and His. In general, the halftimes of aspartyl and asparaginyl peptide degradation under physiological conditions (pH 7.4, 37 °C) vary between about 1 and 1000 d. L-Asn residues form L-succinimides about 10 times more rapidly than comparable L-Asp residues. A similar series of reactions would occur in glutamine and glutamate residues, generating Dand L-isoglutamate and glutamate residues, but these reactions are much slower than those of asparagines and aspartic acid residues.