Huang Yi, Shuai Youxia, Li Hailing, Gao Zhonghong
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China; Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China.
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China; Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China.
Biochim Biophys Acta. 2014 Mar;1840(3):970-6. doi: 10.1016/j.bbagen.2013.11.011. Epub 2013 Nov 16.
Serum albumin binds avidly to heme to form heme-serum albumin complex, also called methemalbumin, and this binding is thought to protect against the potentially toxic effects of heme. However, the mechanism of detoxification has not been fully elucidated.
SDS-PAGE and Western blot were used to determine the efficiency of methemalbumin on catalyzing protein carbonylation and nitration. HPLC was used to test the formation of heme to protein cross-linked methemalbumin.
The peroxidase activity of heme increased upon human serum albumin (HSA) binding. Methemalbumin showed higher efficiency in catalyzing tyrosine oxidation than free heme in the presence of H2O2. Methemalbumin catalyzed self-nitration and significantly promoted the nitration of tyrosine in coexistent protein, but decreased the carbonylation of coexistent protein compared with heme. The heme to protein cross-linked form of methemalbumin suggested that HSA trapped the free radical accompanied by the formation of ferryl heme. When tyrosine residues in HSA were modified by iodination, HSA lost of protection effect on protein carbonylation. The low concentration of glutathione could effectively inhibit tyrosine nitration, but had no effect on protein carbonylation.
HSA protects against the toxic effect of heme by transferring the free radical to tyrosine residues in HSA, therefore protecting surrounding proteins from irreversible oxidation, rather than by direct inhibiting the peroxidase activity. The increased tyrosine radicals can be reduced by endogenic antioxidants such as GSH.
This investigation indicated the important role of tyrosine residues in heme detoxification by HSA and suggested a possible novel mechanism.
血清白蛋白能与血红素紧密结合形成血红素 - 血清白蛋白复合物，也称为高铁血红素白蛋白，这种结合被认为可防止血红素的潜在毒性作用。然而，解毒机制尚未完全阐明。
采用十二烷基硫酸钠 - 聚丙烯酰胺凝胶电泳（SDS - PAGE）和蛋白质免疫印迹法（Western blot）来测定高铁血红素白蛋白催化蛋白质羰基化和硝化的效率。使用高效液相色谱法（HPLC）检测血红素与蛋白质交联的高铁血红素白蛋白的形成。
血红素与人血清白蛋白（HSA）结合后其过氧化物酶活性增加。在过氧化氢存在的情况下，高铁血红素白蛋白在催化酪氨酸氧化方面比游离血红素具有更高的效率。高铁血红素白蛋白催化自身硝化，并显著促进共存蛋白质中酪氨酸的硝化，但与血红素相比，其降低了共存蛋白质的羰基化。血红素与蛋白质交联形式的高铁血红素白蛋白表明，HSA在形成高铁血红素的同时捕获了自由基。当HSA中的酪氨酸残基通过碘化修饰时，HSA失去了对蛋白质羰基化的保护作用。低浓度的谷胱甘肽可有效抑制酪氨酸硝化，但对蛋白质羰基化无影响。
HSA通过将自由基转移至HSA中的酪氨酸残基来防止血红素的毒性作用，从而保护周围蛋白质免受不可逆氧化，而非直接抑制过氧化物酶活性。增加的酪氨酸自由基可被内源性抗氧化剂如谷胱甘肽（GSH）还原。
本研究表明酪氨酸残基在HSA介导的血红素解毒中起重要作用，并提出了一种可能的新机制。
