Mass spectrometric analysis of asparagine deamidation and aspartate isomerization in polypeptides

One of the most frequent modifications in proteins and peptides is the deamidation of asparagine, a spontaneous non‐enzymatic reaction leading to a mixture of L,D‐succinimidyl, L,D‐aspartyl, and L,D‐isoaspartyl forms, with L‐isoaspartyl dominating. Spontaneous isomerization of L‐Asp yields the same products. In vivo, these unusual forms of aspartate are repaired by the protein L‐isoaspartyl O‐methyltransferase enzyme, with the balance between isomerization and repair affecting the organism physiology. Mass spectrometric analysis of this balance involves isomer separation, iso‐Asp/Asp quantification, and iso‐Asp site identification. This review highlights the issues associated with these steps and discusses the prospects of high‐throughput iso‐Asp analysis.