Elucidation of the fine structure and the generation process of modified proteins (sulfonate, nitroso, et al.) application to clinical laboratory test

蛋白质中的半胱氨酸和胱氨酸在体内外被修饰成各种衍生物。通过电喷雾质谱（ESI-MS），我们发现了血清甲状腺素运载蛋白（TTR）的衍生物，其中第10位的半胱氨酸残基被改变为甘氨酸残基和磺基半胱氨酸残基。这种从半胱氨酸到甘氨酸的变化是独特的，磺酸基团的起源也存在争议。在本文中，我们显示了包括这两种TTR衍生物的各种TTR衍生物的分子量，并且使用与TTR的包含部分半胱氨酸的序列相同的合成肽，即，SKCPLMVK。肽在pH 8.3下孵育后，产生各种衍生物，其显示半胱氨酸残基到甘氨酸、脱氢丙氨酸、S-硫代半胱氨酸和S-磺基半胱氨酸残基的变化，这通过分子质量和碰撞诱导解离光谱证实。脱氢丙氨酸可与其他氨基酸反应，并有助于形成交联原纤维，引起淀粉样变性。

Cysteine and cystine in protein are modified to various derivatives in vitro and in vivo. By electrospray ionization mass spectrometry (ESI-MS), we found derivatives of serum transthyretin (TTR) in which cysteine residue at position 10 was changed to glycine residue and sulfocysteine residue. The change, cysteine to glycine, was unique and the origin of the sulfonic acid group was controversial. In the present paper, we show the molecular masses of various TTR derivatives including these two, and the modification process was studied using a synthetic peptide with the same sequence as cysteine containing part of TTR, i.e., SKCPLMVK. After incubation of the peptide at pH 8.3, various derivatives were generated, which showed changes of cysteine residue to glycine, dehydroalanine, S-thiocysteine, and S-sulfocysteine residues, which were confirmed by molecular mass and collision-induced dissociation spectra. Dehydroalanine may react with other amino acids and contribute to form cross-linking fibril, causing amyloidosis.