Isomerization of aspartate (Asp) is a common non-enzymatic posttranslational modification. Isomerized residues accumulate in proteins associated with age-related human disorders such as cataract and are well known to affect protein structure and function. We previously detected d-Asp-containing peptides in human serum. In this study, we investigated whether isomerized Asp residues are present in human immunoglobulin G (IgG) kappa chain by a qualitative d-amino acid analysis based on diastereomer formation and liquid chromatography tandem mass spectrometry (LC-MS/MS). We also investigated the d/l ratio of Asp residues in the IgG kappa chain in serum from donors aged 25, 37, 41, 54 and 67 years. As a result, two isomerized Asp residues, Asp151 and Asp170, were detected in the IgG kappa chain, and the d/l ratio of these residues was found to increase with aging. To assess the effects of this isomerization, we synthesized four isomeric peptides of IgG kappa chain containing lα-, lβ-, dα-, or dβ-Asp at position 170, and compared their secondary structures by CD spectroscopy. Peptide containing normal lα-Asp170 showed type II β-turn structure, while the other isomeric peptides showed random structure, clearly indicating that substitution of a single Asp isomer alters the secondary structure of the peptide. Because IgG is a main component of humoral immunity, Asp isomerization in IgG may reflect changes of structure and decrease in immune function. Proteome research on serum from the standpoint of racemization might enable us to develop new kinds of biomarker and new directions to study the aging process.
Keywords: Aging process; D-amino acid; IgG; Post-translational modification; Serum proteome.
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