Stillman M J, Zelazowski A J
Department of Chemistry, University of Western Ontario, London, Canada.
J Biol Chem. 1988 May 5;263(13):6128-33.
Rabbit liver Zn metallothionein-(MT) will bind cadmium readily between -26 degrees C and 70 degrees C. The binding reaction was monitored by recording the circular dichroism and magnetic circular dichroism spectra, in the region of the RS(-)----Cd2+ charge transfer transition at 250 nm, at intervals as aliquots of cadmium were added. For all temperatures, these data can be analyzed in terms of a distributed mechanism for cadmium binding when Zn-MT is used, and a domain-specific mechanism when apo-MT is used. The CD spectrum measured at -26 degrees C for Cd,Zn-MT, which was made by adding excess cadmium directly to Zn7-MT at -26 degrees C, is not the same as the CD spectrum of Cd-MT prepared at room temperature from the same Zn7-MT. Measurements of the stoichiometry of the cadmium and zinc bound to MT in the presence of excess cadmium at different temperatures indicates that below 5 degrees C at least one zinc atom remains bound to the protein. The mixed metal metallothionein, Cd/Zn-MT, that always forms below 5 degrees C, is characterized by a single maximum near 250 nm in the CD spectrum, rather than the derivative-shaped CD envelope that is diagnostic of the (Cd4-S11)alpha cluster, which indicates that the zinc occupies a site in the alpha domain. Rearrangement of the bound metals to the domain-specific distribution takes place if Cd,Zn-MT, prepared at subzero temperatures, is warmed above 30 degrees C.
兔肝锌金属硫蛋白（MT）在-26℃至70℃之间能轻易结合镉。通过记录圆二色性和磁圆二色性光谱来监测结合反应，在250nm处RS(-)----Cd2+电荷转移跃迁区域，随着镉的等分试样添加，每隔一定时间进行测量。对于所有温度，当使用锌金属硫蛋白（Zn-MT）时，这些数据可以根据镉结合的分布机制进行分析，而当使用脱辅基金属硫蛋白（apo-MT）时，则根据结构域特异性机制进行分析。在-26℃下通过直接向Zn7-MT中添加过量镉制备的Cd,Zn-MT的圆二色性光谱，与在室温下由相同的Zn7-MT制备的Cd-MT的圆二色性光谱不同。在不同温度下，在过量镉存在的情况下测量与金属硫蛋白结合的镉和锌的化学计量比表明，在5℃以下至少有一个锌原子仍与蛋白质结合。在5℃以下总是形成的混合金属金属硫蛋白Cd/Zn-MT，其圆二色性光谱在250nm附近有一个单一的最大值，而不是诊断（Cd4-S11）α簇的导数形状的圆二色性包络，这表明锌占据了α结构域中的一个位点。如果在零下温度制备的Cd,Zn-MT加热到30℃以上，结合的金属会重新排列成结构域特异性分布。
