Jacob C, Maret W, Vallee B L
Center for Biochemical and Biophysical Sciences and Medicine, Harvard Medical School, Seeley G. Mudd Building, 250 Longwood Avenue, Boston, MA 02115, USA.
Proc Natl Acad Sci U S A. 1998 Mar 31;95(7):3489-94. doi: 10.1073/pnas.95.7.3489.
Metallothionein (MT), despite its high metal binding constant (KZn = 3.2 x 10(13) M-1 at pH 7.4), can transfer zinc to the apoforms of zinc enzymes that have inherently lower stability constants. To gain insight into this paradox, we have studied zinc transfer between zinc enzymes and MT. Zinc can be transferred in both directions-i.e., from the enzymes to thionein (the apoform of MT) and from MT to the apoenzymes. Agents that mediate or enhance zinc transfer have been identified that provide kinetic pathways in either direction. MT does not transfer all of its seven zinc atoms to an apoenzyme, but apparently contains at least one that is more prone to transfer than the others. Modification of thiol ligands in MT zinc clusters increases the total number of zinc ions released and, hence, the extent of transfer. Aside from disulfide reagents, we show that selenium compounds are potential cellular enhancers of zinc transfer from MT to apoenzymes. Zinc transfer from zinc enzymes to thionein, on the other hand, is mediated by zinc-chelating agents such as Tris buffer, citrate, or glutathione. Redox agents are asymmetrically involved in both directions of zinc transfer. For example, reduced glutathione mediates zinc transfer from enzymes to thionein, whereas glutathione disulfide oxidizes MT with enhanced release of zinc and transfer of zinc to apoenzymes. Therefore, the cellular redox state as well as the concentration of other biological chelating agents might well determine the direction of zinc transfer and ultimately affect zinc distribution.
金属硫蛋白（MT），尽管其具有较高的金属结合常数（在pH 7.4时KZn = 3.2×10¹³ M⁻¹），但仍能将锌转移至稳定性常数原本较低的锌酶脱辅基形式。为深入了解这一矛盾现象，我们研究了锌在锌酶与MT之间的转移。锌可以双向转移，即从酶转移至硫蛋白（MT的脱辅基形式）以及从MT转移至脱辅基酶。已鉴定出介导或增强锌转移的试剂，它们提供了两个方向的动力学途径。MT不会将其所有七个锌原子都转移至脱辅基酶，但显然至少有一个锌原子比其他锌原子更易于转移。MT锌簇中硫醇配体的修饰增加了释放的锌离子总数，从而增加了转移程度。除了二硫试剂外，我们还表明硒化合物是锌从MT转移至脱辅基酶的潜在细胞增强剂。另一方面，锌从锌酶转移至硫蛋白是由锌螯合剂如Tris缓冲液、柠檬酸盐或谷胱甘肽介导的。氧化还原试剂在锌转移的两个方向上发挥着不对称作用。例如，还原型谷胱甘肽介导锌从酶转移至硫蛋白，而谷胱甘肽二硫化物则氧化MT，增强锌的释放并将锌转移至脱辅基酶。因此，细胞氧化还原状态以及其他生物螯合剂的浓度很可能决定锌转移的方向，并最终影响锌的分布。
