新穎的人類血清白蛋白硫尿化之研究
在本研究中，我們以 FITC 親和性層析法從人類血漿中純化出單一蛋白質。經「基質輔助雷射脫附
游離飛行時間質譜儀」 (MALDI-TOF-MS) 的鑑定，確定此蛋白質為人類血清白蛋白 (HSA) 。白 蛋白是血液循環系統中最主要的攜帶性蛋白，具有多重功能。白蛋白與 FITC 標記的分子之結合 是一種依銅型的反應。在此研究中，我們發展出一套「活性炭螢光分析法」 (CFA) 來分析白蛋白 與 FITC 標記分子的結合，並佐以膠質過濾法、親和性層析法，以及超濾技術來確認此反應複合物 的存在。白蛋白與 FITC 之間的結合是經由雙硫鍵的形成，而 FITC 分子上的硫是屬於硫尿基 ((R1 R2N)(R3R4N)C=S) 。硫尿及其衍生物廣泛的被運用在化學及製藥工業上，然而，過去並沒有報告 指出這些物質會與白蛋白形成共價鍵結。在本研究中，我們證實了銅離子會誘發硫尿物質（例如：
α-naphthylthiourea (ANTU) 、 fluorescein-5-isothiocyanate cadaverine (FTC) ）與白蛋白形成雙硫鍵鍵 結。這種反應是絕對依銅型的反應，其他金屬離子，例如鈷、鎳、鈣、鎂、鋅、鐵、錳，都不會誘 發此反應。該反應與反應物的劑量成線性關係，且受到含硫氫基（ -SH ）物質的抑制。缺乏銅離子 結合區的白蛋白也可以進行硫尿化的反應，據此推測白蛋白與銅離子的結合對反應並不重要。白蛋 白的 Cys34 氨基酸經過半胱氨酸化（ cysteinylation ）或烷基化（ alkylation ）後，會失去反應能力。
白蛋白硫尿化的最佳反應 pH 值為 6.5 ，且隨著 pH 值增高而遞減。硫尿化的白蛋白具有幾項特 性： (a) 熱不穩定性，在溫度高於 25℃ 時，雙硫鍵會逐漸斷裂； (b) 在 pH 6.0 ~ 8.0 之間穩定；
(c) 在酸性情況下其鍵結可被一價銅離子還原； (d) 其鍵結可被帶有硫氫基（ -SH ）或硫尿基的分 子還原。綜合實驗結果，我們推論白蛋白的硫尿化機轉為： (1) 二價銅離子與硫尿物質形成「硫 尿 - 銅 (II) 」複合物； (2) 「硫尿 - 銅 (II) 」複合物攻擊白蛋白 Cys34 氨基酸的硫氫基，形成「白蛋 白 -Cys34-S-S- 硫尿」複合物，並釋放一價銅離子； (3) 反應中釋出的一價銅離子會與二價銅離子 共同競爭硫尿的結合，從而抑制了二價銅離子所誘發的硫尿化反應。本論文是第一篇提出銅離子會 誘發白蛋白硫尿化反應並推論其機轉的研究。這些研究結果對於以硫尿結構所設計的藥物，在藥物 動力學分析上將有重要意義。由於白蛋白為血漿中最主要的蛋白質，因此對於銅離子代謝性疾病，
例如威爾森氏症的病人，硫尿藥物的使用應該受到進一步的評估。
Study on a Novel Thioureation of Human Serum Al bumin
In this study, a protein purified by fluorescein isothiocyanate (FITC)-affinity chromatography from human plasma was identified as albumin by MALDI-TOF-MS. Albumin (Alb) is the major carrier protein in the ci rculation and exhibits multiple functions. Albumin was found to conjugate with FITC-labeled molecules th rough a copper-dependent reaction. The formation of this complex was confirmed by methods including a newly developed “charcoal-based fluorescence assay” (CFA), gel-filtration, affinity chromatography, and u ltrafiltration. The binding was identified as disulfide bridge formation. The thiol group found in FITC mole cule is a thiourea (Tu) group ((R1R2N)(R3R4N)C=S). Thioureas (Tus), which are widely used in chemical and pharmaceutical industries, however, have not been reported to covalently conjugate with albumin. In th is study, we showed that copper induces the cross-linking of albumin with Tus such as α-naphthylthiourea (ANTU) and fluorescein-5-isothiocyanate cadaverine (FTC) through disulfide bridge formation. This reacti on was absolutely copper-dependent, whereas cobalt, nickel, calcium, magnesium, zinc, iron, and mangane se ions could not induce the same reaction. The reaction was substrate dose-dependent, and inhibited by thi ol-containing molecules. Albumin without copper-binding site exhibited the same reaction, indicating that the binding of copper with albumin is not necessary for the thioureation reaction. Albumin that was cystein ylated or alkylated in the Cys34 residue can not be thioureated. The optimal reaction occurred at pH 6.5, an d the complex formation was reduced with the increasing pH. The resulting conjugated product was heat-la bile, and stable at pH between 6.0 ~ 8.0. The linkage could be reduced by Cu(I) (in acidic pH) and thiol-co ntaining agents such as cysteine (but not cystine). The mechanism of albumin thioureation was concluded:
(i) the formation of Tus-Cu(II) complex is essential for the reaction; (ii) the thioureation is resulted from th e attack of Tus-Cu(II) at Alb-Cys34-SH to form Alb-Cys34-S-S-Tus complex accompanied with the releas e of Cu(I); (iii) the released Cu(I) would back inhibit the reaction due to its competition with Cu(II) for Tus binding. This is the first to demonstrate that copper induces a covalent binding of Tus with albumin. These phenomenons may have important implications for the pharmacokinetics of Tu-based drugs in plasma. Sin ce albumin is the major thiol-containing protein in plasma, Tu-based drugs may require special evaluation f or patients with copper metabolism disorders such as Wilson’s disease.
