醌诱导的蛋白质修饰：1,2

多酚氧化为醌是一种抗氧化机制，但由于醌通过迈克尔加成反应与亲核基团（如硫醇和胺）反应生成蛋白质加合物，因此所得的醌可能会损坏蛋白质。在这项研究中，通过假想一阶条件通过UV-vis停止流动分光光度法测定了4-甲基苯醌（4MBQ）与蛋白质，硫醇和胺化合物的反应速率常数。加合物的化学结构通过LC-ESI-MS / MS鉴定。

具有游离硫醇的蛋白质被4MBQ快速修饰，具有明显的二阶速率常数，牛血清白蛋白（BSA）的k 2为（3.1±0.2）×10 4  M -1  s -1和（4.8±0.2）×10 3  M pH 7.0时人血清白蛋白的-1  s -1。这些值比不含任何游离硫醇的α-乳白蛋白（4.0±0.2）×10 2  M -1  s -1至少大12倍。BSA的Cys-34与N-乙基马来酰亚胺的反应使硫醇浓度降低了约59％，导致k 2降低的百分比相近，与Cys-34的快速内吸一致。与胺4MBQ的反应（甘氨酸，Ñ α乙酰升-Lys，Ñ ε乙酰升-Lys和升-Lys）和胍基Ñ α乙酰升-Arg为至少5×10 5比用低分子量的硫醇质量慢（升-Cys，ñ α乙酰升-Cy，谷胱甘肽）。硫醇-醌相互作用通过中间加合物形成无色硫醇-苯酚产物，而胺-醌相互作用产生需要氧参与的有色胺-醌产物。这些数据为醌类物质快速修饰蛋白质硫醇提供了有力的证据，这可能对生物学和食品系统具有重要意义。

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Oxidation of polyphenols to quinones serves as an antioxidative mechanism, but the resulting quinones may induce damage to proteins as they react through a Michael addition with nucleophilic groups, such as thiols and amines to give protein adducts. In this study, rate constants for the reaction of 4-methylbenzoquinone (4MBQ) with proteins, thiol and amine compounds were determined under pseudo first-order conditions by UV–vis stopped-flow spectrophotometry. The chemical structures of the adducts were identified by LC-ESI-MS/MS.

Proteins with free thiols were rapidly modified by 4MBQ with apparent second order rate constants, k2 of (3.1±0.2)×104 M−1 s−1 for bovine serum albumin (BSA) and (4.8±0.2)×103 M−1 s−1 for human serum albumin at pH 7.0. These values are at least 12-fold greater than that for α-lactalbumin (4.0±0.2)×102 M−1 s−1, which does not contain any free thiols. Reaction of Cys-34 of BSA with N-ethylmaleimide reduced the thiol concentration by ~59%, which resulted in a decrease in k2 by a similar percentage, consistent with rapid adduction at Cys-34. Reaction of 4MBQ with amines (Gly, Nα-acetyl-l-Lys, Nε-acetyl-l-Lys and l-Lys) and the guanidine group of Nα-acetyl-l-Arg was at least 5×105 slower than with low-molecular-mass thiols (l-Cys, Nα-acetyl-l-Cys, glutathione). The thiol-quinone interactions formed colorless thiol-phenol products via an intermediate adduct, while the amine-quinone interactions generated colored amine-quinone products that require oxygen involvement. These data provide strong evidence for rapid modification of protein thiols by quinone species which may be of considerable significance for biological and food systems.