Nettesheim D G, Engeseth H R, Otvos J D
Biochemistry. 1985 Nov 19;24(24):6744-51. doi: 10.1021/bi00345a003.
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.
从接触镉的动物体内分离出的肝金属硫蛋白（MT），除了含有镉（每摩尔蛋白质含4 - 5摩尔）外，还总是含有锌（每摩尔蛋白质含2 - 3摩尔），这两种金属以一种不均匀但可重复的方式分布在该蛋白质两个金属硫醇盐簇的七个结合位点之间。研究了多种体外制备兔肝镉、锌 - MT的方法，以深入了解为何体内会产生这种独特的混合金属簇混合物以及它们是通过何种机制形成的。用113Cd2+逐步取代Zn7 - MT中的Zn2+所得到产物的113Cd NMR光谱表明，镉与簇的结合不具有协同性（即不会优先形成只含镉的簇而非混合金属镉、锌簇），不存在一个簇先于另一个簇被选择性占据的情况，并且产生的许多簇具有非天然的金属分布，这表明体内可能不遵循此途径。相反，令人惊讶的发现是，通过简单地将适量的Cd7 - MT和Zn7 - MT混合在一起并让分子间发生金属交换，就能精确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，其驱动力似乎是含镉三金属簇相对的热力学不稳定性。有了关于体内镉、锌 - MT可能如何形成的这一新认识，我们首次能够对先前关于肝锌和金属硫蛋白水平对镉给药反应的观察结果提出合理的解释。
