Products of metal exchange reactions of metallothionein.
Products of metal exchange reactions of metallothionein.
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DOI：
10.1021/bi00345a003
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发表时间：
1985-11
期刊：
影响因子：
2.9
通讯作者：
D. Nettesheim;H. Engeseth;J. Otvos
中科院分区：
文献类型：
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作者：
D. Nettesheim;H. Engeseth;J. Otvos
Cd暴露动物肝金属硫蛋白（MT）除含有Cd （4-5 mol/mol）外，还含有Zn (2-3 mol/mol)，且这两种金属在该蛋白的两个金属硫酸酯簇的7个结合位点上分布不均匀，但具有可重复性。研究了制备兔肝Cd、Zn-MT的不同方法，以深入了解为什么这种不同的混合金属簇在体内产生，以及它们是通过什么机制形成的。113Cd2+从Zn7-MT中逐步取代Zn2+的产物的核磁共振光谱显示，Cd与团簇的结合不是合作的（即，只含有Cd的团簇不会优先于混合金属Cd， Zn团簇形成），一个团簇在另一个团簇之前没有选择性占据，并且许多团簇的产生具有非天然金属分布，这表明该途径在体内可能不遵循。相反，令人惊讶的发现是，只要将适量的Cd7-MT和Zn7-MT混合在一起，并允许分子间金属交换发生，就可以精确地再现天然簇的组成及其相对浓度。这种迄今为止未知的金属交换反应很容易发生，驱动力似乎是含有Cd的三金属簇的相对热力学不稳定性。通过对Cd，Zn- mt在体内如何形成的新见解，我们能够首次对先前关于肝脏锌和金属硫蛋白水平对Cd管理的响应的观察提出合理的解释。
Hepatic metallothionein (MT) isolated from Cd-exposed animals always contains Zn (2-3 mol/mol of protein) in addition to Cd (4-5 mol/mol of protein), and the two metals are distributed in a nonuniform, but reproducible, manner among the seven binding sites of the protein's two metal-thiolate clusters. Different methodologies of preparing rabbit liver Cd, Zn-MT in vitro were investigated to provide insight into why such a distinct mixture of mixed-metal clusters is produced in vivo and by what mechanism they form. 113Cd NMR spectra of the products of stepwise displacement of Zn2+ from Zn7-MT by 113Cd2+ show that Cd binding to the clusters is not cooperative (i.e., clusters containing exclusively Cd are not formed in preference to mixed-metal Cd, Zn clusters), there is no selective occupancy of one cluster before the other, and many clusters are produced with a nonnative metal distribution indicating that this pathway is probably not followed in vivo. In contrast, the surprising discovery was made that the native cluster compositions and their relative concentrations could be reproduced exactly by simply mixing together the appropriate amounts of Cd7-MT and Zn7-MT and allowing intermolecular metal exchange to occur. This heretofore unknown metal interchange reaction occurs readily, and the driving force appears to be the relative thermodynamic instability of three-metal clusters containing Cd. With this new insight into how Cd,Zn-MT is likely to be formed in vivo we are able for the first time to postulate rational explanations for previous observations regarding the response of hepatic Zn and metallothionein levels to Cd administration.