INVITRO REACTION OF THE CARCINOGEN CHROMATE WITH CELLULAR THIOLS AND CARBOXYLIC-ACIDS
INVITRO REACTION OF THE CARCINOGEN CHROMATE WITH CELLULAR THIOLS AND CARBOXYLIC-ACIDS
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DOI：
10.1021/ja00300a035
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发表时间：
1985-01-01
影响因子：
15
通讯作者：
WETTERHAHN, KE
中科院分区：
文献类型：
--
作者：
CONNETT, PH;WETTERHAHN, KE
由于铬(VI)化合物致癌和毒性的关键步骤是细胞内还原为铬(III)，因此在生理pH条件下，研究了铬(VI)与一系列低分子量细胞还原剂和模型化合物的相互作用。在生理pH条件下，铬(VI)的还原受动力学控制，而非热力学控制。只有抗坏血酸和含硫醇基的还原剂才能在pH 7.4(1M Tris-HCl)下对六价铬有显著的还原作用。描述铬(VI)与不同硫醇反应的动力学可分为4类：谷胱甘肽在快速的预平衡步骤中生成了铬(VI)硫酸酯，然后是较慢的氧化还原步骤，包括铬(VI)硫酯与第二分子硫醇的反应；半胱氨酸乙酯、半胱氨酸、半胱胺、辅酶M、同型半胱氨酸、N-乙酰半胱氨酸、辅酶A、巯基乙醇和硫代乙酸酯的形成，随后是氧化还原步骤，涉及硫酯与第二分子硫醇的反应，动力学符合稳态近似；青霉胺、二硫苏糖醇、2，3-二硫代丁二酸、硫代乳酸和硫代马来酸单分子硫代酯的生成速度决定了铬(VI)硫代酯的形成；一硫醇和二氢硫代硫酸酯在快速的预平衡步骤中形成了铬(VI)硫代酯，随后硫代酯发生了单分子氧化还原反应，尽管没有硫代酯形成的光谱证据。可能具有生物学意义的事实是，谷胱甘肽是细胞内含量最丰富的硫醇，出现在第一类中。铬(VI)-谷胱甘肽硫酯的快速形成和缓慢还原，很可能延长其在细胞内的寿命，促进其与细胞大分子的相互作用。Bronsted图表明，硫醇-铬(VI)反应的第一步反应的二级速率常数，即生成铬(VI)硫代酯，与硫醇基团的pKa成反比。在pH 7.4时，似乎铬酸盐CrO42-受到作为限速步骤一部分的离子化的硫醇的攻击，作为限速步骤的一部分，它将质子转移到铬酸盐的氧原子，并且在含有自由氨基的硫醇的情况下，质子化有助于随后的氢氧化物配体OH-的损失。
Since a key step in the carcinogenicity and toxicity of Cr (VI) compounds is intracellular reduction to Cr(III), the interaction of Cr(VI) with a series of low molecular weight cellular reductants and model compounds were examined at physiological pH. The reduction of Cr(VI) at physiological pH was found to be under kinetic rather than thermodynamic control. Only ascorbate and those reductants containing a thiol group were capable of reducing Cr(VI) at a significant rate at pH 7.4 (1 M Tris-HCl). The kinetics describing the reaction of Cr(VI) with the various thiols could be separated into 4 categories: glutathione showed clear spectral evidence for the formation of a Cr(VI) thioester in a rapid preequilibrium step, followed by a slower redox step involving reaction of the Cr(VI) thioester with a 2nd molecule of thiol; cysteine ethyl ester, cysteine, cysteamine, coenzyme M, homocysteine, N-acetylcysteine, coenzyme A, mercaptoethanol and thioglycolate showed formation of a Cr(VI) thioester followed by a redox step involving reaction of the thioester with a 2nd molecule of thiol with kinetics consistent with the steady-state approximation; penicillamine, dithiothreitol, 2,3-dimercaptosuccinate, thiolactate and thiomalate showed rate-determining formation of the Cr(VI) thioester; and unithiol and dihydrolipoate showed the formation of a Cr(VI) thioester in a rapid preequilibrium step, followed by a unimolecular redox reaction of the thioester, although there was no spectral evidence for thioester formation. Of possible biological importance is the fact that glutathione, the most abundant intracellular thiol, appeared in the 1st category. The rapid formation of the Cr(VI) glutathione thioester, followed by its slow reduction, may well prolong the lifetime of Cr(VI) in the cell and promote its interaction with cellular macromolecules. Bronsted plots showed that the 2nd-order rate constants for the 1st step of the thiol-Cr(VI) reaction, formation of the Cr(VI) thioester, were inversely related to the pKa of the thiol groups. At pH 7.4, it appears that chromate, CrO42-, is attacked by the unionized thiol which transfers a proton to an oxygen atom of the chromate as part of the rate-limiting step and that the subsequent loss of the hydroxide ligand, OH-, is facilitated by protonation in the case of thiols containing a free amino group.