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Center for Biochemical and Biophysical Sciences and Medicine, Harvard Medical School, Cambridge, MA 02139
3 To whom correspondence should be addressed. E-mail: wolfgang_maret{at}hms.harvard.edu.
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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KEY WORDS: • metallothionein • thionein • zinc • redox • mitochondria
With 
3 g in the human body, zinc is similar to iron regarding its overall amount. Based on predictions of zinc-binding motifs, the number of zinc proteins in a given organism, so-called zinc proteomes, can be estimated from fully sequenced genomes. Thus, at least 3% of the 32,000 identified genes in the human genome encode proteins with zinc-finger domains. This approach demonstrates that prokaryotes, eukaryotes and multicellular eukaryotes increasingly acquired the potential to use zinc in their protein structures (1). Catalytic, structural and regulatory functions of zinc in proteins account for its nutritional essentiality and importance in maintaining a wide variety of biological processes (2). In addition, synaptic vesicles in zinc-containing neurons store zinc and release it upon stimulation thus endowing zinc with the characteristics of a neuromodulator (3). All of these functions require a molecular system that presides over the temporal and spatial distributions of zinc. Identification of components of such a homeostatic system reveals that "free" zinc ions are not readily available at micromolar or higher concentrations within a cell, and that their distribution is tightly controlled and regulated by multiple protein sensors, transporters and other types of proteins with novel mechanisms of action. Metallothionein (MT),4 which was discovered almost 50 y ago (4), is such a protein. Its function in cellular zinc distribution is the subject of this short account. MT binds seven zinc ions with sulfur ligands of its 20 cysteines to form two zinc/thiolate clusters. The clusters have some selectivity in binding metal ions and despite their high thermodynamic stability (Kd = 10-13 mol/L at pH 7.0), they provide reactive coordination environments to induce kinetic lability in metal-exchange reactions (5). We have suggested that the reactivity of the sulfur ligands is a key to the mechanism of action of MT (6). The clusters provide a redox-active coordination environment for the redox-inert zinc ion, and each has a different zinc-transfer potential. The cluster redox activity allows zinc to be mobilized by coupling its sulfur coordination environment to changes in the cellular redox state, which is a new functional principle for a zinc protein and for metalloproteins in general (6). Thus, oxidants release zinc, whereas reductants/antioxidants restore the full potential of the sulfur ligands to bind zinc. Biological oxidants include disulfides and selenium compounds. Either low-molecular-weight selenium compounds or selenoenzymes such as glutathione peroxidase have the capacity to form selenol(ate)s and catalytically oxidize MT or reduce the oxidized protein (7,
8). These findings suggest hitherto unrecognized interactions between the essential micronutrients selenium and zinc and mediation of at least some of the antioxidant biology of selenium through oxidative zinc release and ensuing antioxidant effects of zinc (9). Selenium catalysts couple the glutathione disulfide/glutathione and MT/thionein (T) redox pair thus controlling the release and binding of zinc. Another oxidant that has received considerable attention is nitric oxide (NO), which reacts with MT and releases zinc (10). S-Nitrosothiols react preferentially with the isoform MT-3 (growth inhibitory factor) by transnitrosation, a process in which NO is transferred between sulfhydryl groups (11). The reason for this preference is consensus motifs in the primary sequence of MT-3 that contains multiple glutamates, which can catalyze transnitrosation. NO, disulfides and selenium compounds all release zinc in vivo (12–
14), thereby demonstrating the physiological significance of this redox chemistry linked to zinc/sulfur–coordination environments. Moreover, changes in the glutathione redox state that have an effect on cell fate (15) are in the range of those affecting zinc release from MT in vitro.
Another significant advance in determining the function of MT has been the discovery that the apoprotein, T, is present in tissues in amounts commensurate with those of MT, and moreover that the MT/T ratio and the total amount MT + T varies in different rat tissues (16). The simultaneous presence of both the holo- and apoprotein is a highly unusual finding for a metalloprotein and underscores the dynamic function of zinc in this protein. Because 20 cysteine thiols constitute a significant chelating and reducing capacity, the presence of T must have significant effects on both the zinc and the redox states of the cell. Its reducing capacity suggests that antioxidant functions previously attributed to MT (17) are actually functions of T. Furthermore, in the presence of such a strong endogenous chelating agent, the amount of cellular "free" zinc must be very low. Indeed, estimates for eukaryotic cells are in the picomolar range (18– 20). T removes zinc from very tight inhibitory sites of enzymes in energy metabolism and cellular signaling, and thereby activates zinc-dependent processes (21). Previous attempts to relate MT to the amount of zinc available to the zinc fluorophore Zinquin in cells faired poorly (22). However, when the MT/T ratio and the concentration of available zinc are compared, a linear relation is observed (Yang et al., unpublished observations). Thus, the MT/T pair is a cellular zinc buffer with a buffer capacity and range that depend on the requirements of a cell. In addition to the redox modulation of MT and T, multiple signaling pathways including zinc- and redox-sensitive transcription factors control MT gene expression, and ligands such as ATP interact with MT and affect its chemical properties (23).
Biological functions of MT and T as zinc donor and acceptor, respectively, have been observed with regard to liver mitochondria (24). Mitochondria import MT into their intermembrane space, where zinc is released and inhibits mitochondrial respiration through interactions with complexes of the electron-transport chain. Translocation of MT also occurs between the cytosol and the nucleus (25, 26) as well as through the plasma membrane (27). T, on the other hand, activates zinc- and MT-inhibited respiration. These experiments demonstrate that MT translocates within the cell to compartmentalize zinc and perhaps to allow different amounts of zinc to be available in different compartments of the cell.
The chemical properties and the biological functions of the MT/T pair now seem to constitute a physiologic function in zinc metabolism. The cellular zinc and redox states converge in the MT/T pair. MT transduces oxidative signals into zinc signals via zinc release, whereas T transduces zinc signals into reductive signals via the induction of T by zinc. The system thus forms a nodal point in the signaling network of the cell. In summary, new functions of MT/T in zinc metabolism include its redox activity, translocation of zinc into cellular compartments, buffering of the cellular concentration of zinc in a controlled manner and a role of T as an endogenous chelating agent.
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FOOTNOTES |
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2 This work was supported by the Endowment for Research in Human Biology, Inc.
4 Abbreviations used: MT, metallothionein; NO, nitric oxide; T, thionein.
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LITERATURE CITED |
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TOPABSTRACTLITERATURE CITED |
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