Graduate School of Science, Kyoto University, Kyoto, Japan.
Graduate School of Science, Kyoto University, Kyoto, Japan; Institute for Integrated Radiation and Nuclear Science, Kyoto University, Osaka, Japan.
Biochim Biophys Acta Proteins Proteom. 2020 Jun;1868(6):140410. doi: 10.1016/j.bbapap.2020.140410. Epub 2020 Mar 10.
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.
天冬氨酸（Asp）的异构化是一种常见的非酶促翻译后修饰。异构化残基在与年龄相关的人类疾病（如白内障）相关的蛋白质中积累，并且众所周知会影响蛋白质的结构和功能。我们之前在人血清中检测到含有 d-Asp 的肽。在这项研究中，我们通过基于非对映异构体形成和液相色谱串联质谱（LC-MS/MS）的定性 d-氨基酸分析来研究异构化 Asp 残基是否存在于人免疫球蛋白 G（IgG）κ 链中。我们还研究了来自年龄为 25、37、41、54 和 67 岁的供体血清中 IgGκ 链中 Asp 残基的 d/l 比值。结果，在 IgGκ 链中检测到两个异构化的 Asp 残基，Asp151 和 Asp170，并且发现这些残基的 d/l 比值随着年龄的增长而增加。为了评估这种异构化的影响，我们合成了四个含有 lα-、lβ-、dα-或 dβ-Asp 取代的 IgGκ 链的异构肽，并通过 CD 光谱比较了它们的二级结构。含有正常 lα-Asp170 的肽表现出 II 型 β-转角结构，而其他异构肽表现出无规卷曲结构，这清楚地表明单个 Asp 异构体的取代改变了肽的二级结构。由于 IgG 是体液免疫的主要成分，因此 IgG 中的 Asp 异构化可能反映了结构的变化和免疫功能的下降。从消旋化的角度对血清蛋白质组学进行研究可能使我们能够开发新的生物标志物并为研究衰老过程开辟新的方向。
